deep and high-temperature hydrothermal circulation in the oman … · 2007. 12. 13. · deep and...
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Deep and High-temperature HydrothermalCirculation in the Oman Ophiolite-------Petrological and Isotopic Evidence
D BOSCH1 M JAMAIS1 F BOUDIER1 A NICOLAS1J-M DAUTRIA1 AND P AGRINIER2
1LABORATOIRE DE TECTONOPHYSIQUE UMR 5568 CNRSUM2 UNIVERSITEE MONTPELLIER II PL EUGENE
BATAILLON 34095 MONTPELLIER CEDEX 5 FRANCE
2LABORATOIRE DE GEEOCHIMIE DES ISOTOPES STABLES IPG-PARIS VII 4 PLACE JUSSIEU F-75252 PARIS CEDEX 05
FRANCE
RECEIVED NOVEMBER 28 2001 ACCEPTED DECEMBER 5 2003
A systematic search for evidence of high-temperature hydrous
alteration within the gabbros of the Samail ophiolite (Oman)
shows that most of the gabbros have been affected by successive
stages of alteration starting above 975C and ending below
500C Sr and O isotopic analyses provide constraints on the
nature and origin of the fluids associated with these alteration
events Massive gabbros dykes and veins and their associated
minerals depart from mid-ocean ridge basalt (MORB)-source
magma signatures ( 87Sr86Sr 407032 and depleted d18O56) These isotopic characteristics identify seawater as the
most likely hydrothermal contaminant Samples affected by low-
temperature alteration during greenschist-facies metamorphism are
characterized by high waterrock ratios (410) A second group of
samples including massive gabbros and high-temperature dykes and
veins is characterized by lower waterrock ratios in the range of
3---5 These samples display a high-temperature hydrothermal
alteration related to seawater ingression The main fluid channels
may be sub-millimetre-sized microcracks with a dominantly vertical
attitude which constitute the recharge hydrothermal system whereas
the dykes and veins represent the discharge part This model requires
that these dykes have been generated by hydration of the crystallizing
gabbros via seawater penetration near the internal wall of the
magma chamber
KEY WORDS hydrothermal systems Oman ophiolite Sr and O isotopes
gabbros and massive dykes
INTRODUCTION
Hydrothermal circulation near fast-spreading oceanicridges is typically a greenschist-facies low-temperature(LT) system (T 5 450C) which operates at the ridgeaxis down to the base of the sheeted dyke complex andpenetrates the entire crust once it has drifted away fromthe ridge axis (Nehlig amp Juteau 1988 Wilcock ampDelaney 1996 Juteau et al 2000) The existence of asimultaneous deeper and higher-temperature hydro-thermal system also active at the ridge was firstsuspected when Gregory amp Taylor (1981) on the basisof oxygen isotope data and Lanphere (1981) andMcCulloch et al (1981) on the basis of strontium isotopedata proposed seawater contamination at high temperat-ures (HT) of the lower gabbros of the Oman ophioliteMore recently petrological studies of the upper gab-
bros drilled in the fast-spreading crust exposed in theHess Deep during Ocean Drilling Program (ODP) Leg147 (Mevel et al 1996) have revealed that solid-statereactions in hydrothermal veins occurred at 700---750C pointing to the existence of hydrothermal circula-tion at amphibolite-facies conditions within this section ofoceanic crust Manning et al (2000) conducted a similarstudy of the gabbros from one section in the Nakhl massifof the Oman ophiolite (Fig 1) They calculated temperat-ures ranging from 700C in the uppermost gabbros to825C in the deepest gabbros at the Moho Kawahata
JOURNAL OF PETROLOGY VOLUME 45 NUMBER 6 PAGES 1181ndash1208 2004 DOI 101093petrologyegh010
Corresponding author E-mail boschdstuuniv-montp2fr
Journal of Petrology 45(6) Oxford University Press 2004 all rights
reserved
et al (2001) on the basis of a Sr isotope study of the crustalunit in the Fizh massif of northern Oman found a rise inthe temperature of hydrothermal alteration from lowgreenschist facies in the basalts to amphibolite-facies con-ditions in the gabbrosBoudier et al (2000) have also suggested that seawater
could contaminate the lower gabbros during their crystal-lization as recorded by the crystallization of gabbro-norites in place of olivine gabbros This contaminationat very high temperatures (VHT) possibly as high as1200C is restricted to those parts of the Oman palaeo-ridge that display evidence of ridge-related tectonic activ-ity and the seawater contamination was attributed to thisparticular tectonic setting
Improved seismological modelling of fast-spreadingridges at the East Pacific Rise axis at 9300N has sug-gested that there may be significant narrowing of thelow-velocity zone (LVZ) below the melt lens (Dunn et al2000) with respect to earlier models This narrowingwas explicitly ascribed by Dunn et al to the existence ofdeep high-T hydrothermal circulation at modern oceanicridges This raises the question of the definition of whatshould be considered as the magma chamber below amid-ocean ridge On the basis of extensive studies of theSamail ophiolite Nicolas et al (1988) have shown that themagma chamber was tent-shaped in three dimensionsextending to the base of the crust consistent with theshape of the LVZ found below modern ridges Hence the
Fig 1 (a) Simplified map of the Oman---UAE ophiolite including the inferred location of the palaeo-ridge axes and main shear zones [modifiedfrom Nicolas et al (2000)] (b) Simplified map of the Samail ophiolite Thin numbered lines represent the log-sections of Fig 4 along wadis wherealteration facies have been studied Bold lines are the wadi sections selected for detailed petrological and geochemical work in this study
JOURNAL OF PETROLOGY VOLUME 45 NUMBER 6 JUNE 2004
1182
magma chamber is not restricted to the perched axialmelt lens that is present just below the sheeted dykecomplex of fast-spreading ridges (Detrick et al 1987)because in spite of its very low melt fraction the LVZ isstill deforming as a mush (Nicolas amp Ildefonse 1996)Building on the previous work of MacLeod amp Rothery
(1992) and Nehlig et al (1994) Nicolas et al (2001) haverecently published a structural map of the greenschist-facies hydrothermal veins throughout the Samail ophio-lite This confirms that the LT veins penetrate the entirecrust and are dominantly aligned parallel to the sheeteddyke complex Subsidiary preferred orientations arerelated to changes in the stress field during the dyingactivity of the palaeo-ridge system (Nicolas et al 2000)In a companion paper to the present study Nicolas et al
(2003) described a complete transition from these LThydrothermal veins to gabbroic dykes which suggests thepossibility of seawater-related hydrous melting of the gab-bros or entry of seawater into the partially molten magmachamber Previously these gabbroic dykes which arefairly common in the Oman gabbros were interpreted aslate crystallization products from the LVZ magma cham-ber (Pallister amp Hopson 1981) or as melt conduits relatedto its magmatic activity (Kelemen et al 1997 Kelemen amp
Aharonov 1998) If the magma that produced these gab-broic dykes originated within the magma chamber thewater should also have an internal origin the source beingeither the mantle or the melt lens In the latter case con-tamination by seawater is possible when hydrated diabasefrom the overlying sheeted dyke complex is stoped in themelt lens remelted and dewatered during subsequentsubsidence through the magma chamberAlternatively the gabbroic dykes could result from
fractures external to the magma chamber driving sea-water to great depth In the physical model proposed byNicolas et al (2003) the gabbroic dykes are generated byhydrous melting and represent the discharge system ofthe high-temperature hydrothermal circulation reachingthe inner margin of the magma chamber (Fig 2) Therecharge system is formed by a microcrack network con-trolled by the anisotropy of thermal expansion and oper-ating at falling temperatures below 1200C within adistance of 10 km from the spreading axisThe available geochemical evidence bearing on the
hydrothermal alteration of the gabbros (Gregory ampTaylor 1981 Lanphere 1981 McCulloch et al 1981Kawahata et al 2001) favours a seawater origin withseawaterrock ratios ranging from one (Gregory amp
water in Sea Floor
Magma chamber
water out
gabbroicdike
hydrous contamination
diabase dykeswith chilledmargins
diabasedikewithoutchilledmargins
750degC
500degC
1200degC
LT hydrothermal vein
hydrothermal cracks
hydrothermal cracks
VHT
plastic mantle flow
maficdike
Moho
microgabbrodike
HT
Fig 2 Scheme of hydrothermal circulation near the limit of the magma chamber below the fast-spreading ridge The downgoing seawater flow fillsthe porous network created by microcracks down to the magma chamber where it reacts with the crystallizing melt it surges back towards theseafloor along a system of hydrous gabbroic dykes Modified from Nicolas et al (2003)
BOSCH et al HYDROTHERMAL CIRCULATION IN OMAN OPHIOLITE
1183
Taylor 1981) to as high as nine (Kawahata et al 2001)Oxygen isotope data (C Lecuyer personal communica-tion 1990) on mafic dykes emplaced in the peridotites at1 km below the Moho suggest a dual source of hydrationfor the gabbros in which clinopyroxenes have a mantlesignature but associated plagioclase and brown horn-blende show evidence of seawater contamination Thisdual source was not ruled out by Benoit et al (1996) intheir Sr isotope study of similar dykes injected into themantle section Rare earth element partition coefficientsin clinopyroxene plagioclase and amphibole were usedby the above workers to constrain the temperature ofseawater contamination at the gabbro solidusThus there is growing evidence that supports a deep
penetration of high-T hydrothermal fluids into theoceanic crust at the ridge axis We have conducted asystematic study of the gabbros in the Samail ophioliteand along selected wadi sections (Fig 1) in the hydratedgabbroic dykes that cross-cut them looking for traces ofHT alterationThis study combines microtextural and petrological
observations thermometric determinations and O---Srisotope data on the same gabbroic dykes and surroundinggabbros selected with a view to determine the origin ofhydrous fluid contamination These results have import-ant consequences for thermal modelling of the oceaniccrust near fast-spreading ridges and on the budget ofchemical exchanges between the oceanic crust andseawater
HYDROUS ALTERATION OF
PRIMARY MINERALS IN THE
GABBRO SECTION
The penetrative alteration of the gabbro section has beenstudied on the basis of an extensive thin-section collec-tion which covers the entire ophiolite The studied gab-bros range from the root zone of the sheeted dykecomplex down to the Moho and the Moho transitionzone (MTZ) From the 500 specimens that have beenscreened those sites involved in tectonic complexitiessuch as the end of segments or tips of propagating rifts(Fig 1) have been rejected thus reducing the number ofstudied areas and the number of fully characterized speci-mens to 414 representative of standard gabbros at a fast-spreading palaeo-ridge In this study we mainly focus onhydrous alteration of olivine and clinopyroxene as theseare the minerals most affected by high-T alteration(above 800C) although plagioclase is stable down tothis temperature
Alteration facies
The following types of hydrous parageneses in thegabbros have been distinguished from highest to lowesttemperatures
(1) Orthopyroxene partial coronas between olivine andplagioclase which may be associated with the firstappearance of brown pargasite blebs inside clinopyrox-ene and development of pargasite between olivine andplagioclase or clinopyroxene and plagioclase (Fig 3a) Thebrown pargasite locally grades into a greenish magnesio-hornblende Although the orthopyroxene coronas couldbe assigned to a late magmatic stage we interpret themas reactive (a) being restricted to the contact betweenolivine and plagioclase thus postdating the crystallizationof plagioclase (b) being locally relayed by kelyphiticcoronas that spread into plagioclase microcracks(Fig 3b) as microphases among which diopside andlow-titanium pargasite have been identified We inferthat these reactions are related to an oxygen fugacityincrease as a result of seawater input Based on experi-ments at 2MPa pressure Koepke et al (2003) have con-strained the equilibrium olivine---orthopyroxene at lowwater content at 1030C and 950C at higher watercontent the limit at which amphibole is stabilized Byreference to this experimental work we estimate that thetemperature of equilibration for these reactions referredto here as VHT (very high temperature) hydrous altera-tion was between 900 and 1000C(2) Chlorite and pale amphibole coronas between oliv-
ine and plagioclase (Fig 3d) The coronas are composedof successive rims of clino-amphibole (tremolite) thornmagnetite Mg-chlorite (clinochlore) and a fringe of ortho-amphibole against plagioclase As described above theseminerals are associated with blebs of brown to greenishamphibole inside the pyroxenes (Fig 3c) Depending onthe degree of alteration olivine is surrounded by a thinfilm of amphibole and chlorite or is totally replaced by anassemblage of tremolite thorn magnetite thorn plagioclase An91(thorn talc) surrounded by chlorite (Fig 3e) The evolution ofthe first assemblage to Mg-chlorite indicates temperat-ures above 700C the upper breakdown of Mg-chloriteat 2 kbar (Chernosky et al 1988) The development ofpargasitic amphibole replacing clinopyroxene impliestemperatures between 550 and 900C (Liou et al 1974Spear 1981) We refer to these as HT (high temperature)(3) Iddingsite partly or totally replacing olivine inde-
pendently of orthopyroxene or chlorite---amphibole cor-onas and any significant HT alteration in clinopyroxene(Fig 3f ) Observations in thin sections show that thechlorite---amphibole coronas develop before the idding-site alteration(4) Lizardite alteration of olivine forming the familiar
mesh structure This is always accompanied by saussuritein plagioclase This alteration takes place below 500Cand is here referred to as LT (low temperature) hydrousalteration It is only at this stage that plagioclase showssigns of alterationThese various hydrous alteration facies are commonly
observed superimposed in a set of thin sections from a
JOURNAL OF PETROLOGY VOLUME 45 NUMBER 6 JUNE 2004
1184
single outcrop and even in a single thin section As a rulethe orthopyroxene reactions are more penetrative beinghomogeneously distributed from the scale of a thinsection to that of several thin sections made from a
given outcrop The distribution of chlorite---amphibolecoronas is similar although they can be restricted to themargins of microcracks filled by the same minerals whichhave been interpreted as the channels that carried the
(a) (b)
(c) (d)
(e) (f)
olopx
kel
pl
ol
hbol
amcpx
cpx
hb
chl
am+pl+mt
idg
am
hb
pl
hb
ol
cpx
opx
mt
pl
Fig 3 Hydrothermal alteration in the gabbros (scale bar represents 1mm) (a) VHT alteration marked by orthopyroxene (opx) coronas at thecontact of olivine (ol) and plagioclase (pl) and pargasitic hornblende (hb) at the contact of olivine and clinopyroxene (cpx) (sample 94 MA2)(b) Kelyphite (kel) coronas around olivine and cloudy plagioclase as a result of silicate and fluid inclusions Silicate inclusions tend to be localizedat grain boundaries and along microcracks (sample 94 MA2) (c) VHT alteration marked by brown amphibole (hb) alteration of clinopyroxene in anupper gabbro (d) HT alteration marked by chlorite (chl) (external rim) and pale amphibole (am) and magnetite (mt) (internal rim) coronas aroundolivine (e) Total replacement of olivine by an assemblage of tremolite (am)thornmagnetite (mt)thorn plagioclase (pl) (f ) Total replacement of olivine by aniddingsitic mixture (idg) inside a VHT recrystallized corona of a mosaic of orthopyroxene
BOSCH et al HYDROTHERMAL CIRCULATION IN OMAN OPHIOLITE
1185
fluids responsible for the hydrothermal alteration (Nicolaset al 2003) The iddingsite replacement is typicallyheterogeneous at the thin-section scale being preferen-tially associated with microcracks Serpentinization alsodevelops from microcracks but it can pervasively invadethe entire thin section The microcracks are commonapproximately 25 of the studied thin sections displayone or more Ignoring the cracks related to LT alterationand considering only the cracks related to the HT hydro-thermal alteration this fraction is still around 20
Distribution of high-T alteration facies
Figure 4 shows the distribution of alteration facies as afunction of depth in the crust along wadi sections throughthe Oman---UAE ophiolite offering continuous exposures(see details in Fig 1) Figure 4 excludes the LT faciesbecause (1) being based on the presence of olivine ser-pentinization only throughout our compilation its dis-tribution is biased by the decreasing amount of olivineupsection and (2) this study concerns high-temperaturealteration processes Despite the fact that the scale ofsampling may exceed the scale of heterogeneity evidencefor both VHT alteration and lsquono alterationrsquo increaseswith depth Conversely HT alteration is predominantin the middle and upper parts of the gabbro sectionThis observation is valid throughout the wadi sectionsstudied with the exception of the Wadi Tayin massifwhich shows no clear gradation with depthFigure 5 is another representation of alteration-facies
distribution with depth based on the detailed study of414 thin sections The samples are classified according totheir level in the gabbro section sheeted dyke root zonegabbros upper gabbros middle gabbros lower gabbrosand Moho gabbros (the Moho gabbros have beengrouped together with the gabbro lenses interlayered inthe dunites of the MTZ) Values represent the percent of
2 Km
3
4
5
6
7
Aswad Fizh Hilti Haylayn Samail W TayinNakhl
High T
Very High T
0 High T
Upper Crust
LID
Lower Crust
MohoMTZ
DEPTH1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27
Fig 4 Depth of penetration of hydrothermal circulation in gabbros along the wadi sections numbered in Fig 1 referring to alteration faciesdescribed in Fig 3 Depths are recorded below the palaeo-seafloor LID corresponds to the upper crust including sheeted dykes and volcanics
0
10
20
30
40
50
60
70
80
90
100
SD root
zone
upper
gabbros
middle
gabbros
lower
gabbros
MTZ
BHb-GHb CPX
Chl-Amph core OL
Chl-Amph rim OL
0
5
10
15
20
25
30
SD root
zone
upper
gabbros
middle
gabbros
lower
gabbros
MTZ
Opx OL
BHb OL
No VHTHT
Unaltered OL core
HT
VHT
(a)
(b)
Plst def
gabbros
Fig 5 Compilation of hydrothermal alteration features as a function ofdepth in the gabbro section Percentages represent the occurrence of agiven type of alteration versus the number of thin sections examined foreach selected level root of the sheeted dykes (SD) upper gabbros middlegabbros lower gabbros MTZ (Moho Transition Zone) gabbros on atotal amount of 414 thin sections (Opx orthopyroxene Amph amphi-bole OL olivine BHb brown hornblende GHb green hornblendeChl chlorite Plst def plastic deformation) (a) Distribution of the HTalteration (in ) (b) Distribution of the VHT alteration (in )
JOURNAL OF PETROLOGY VOLUME 45 NUMBER 6 JUNE 2004
1186
a given alteration facies as defined previously out of thetotal number of thin sections examined for a given levelMore refined conclusions can be drawn from this figureAs seen in Fig 5a the HT alteration decreases down-
wards whereas it affects most of the upper gabbroicsection (sheeted dyke root zone and upper gabbros)The fraction of unaltered olivine cores can be used asan index to estimate the extent of alteration This indexshows that the HT alteration is more penetrative andintense in the upper gabbros a conclusion alreadyreached by several workers (eg Gregory amp Taylor1981 Stakes amp Taylor 1992 Kawahata et al 2001)This was also noted by Manning et al (1996a 1996b)Figure 5b shows that the VHT alteration marked by
orthopyroxene or brown hornblende coronas aroundolivine increases with depth The correlation betweenthe VHT alteration and plastic deformation also suggeststhat the VHT alteration may coincide with the develop-ment of plastic flow at the expense of the magmatic flow
at temperatures just below the solidus In Fig 5b thefraction of gabbros where there is no VHT---HThydrothermal alteration increases downsection indicat-ing that the VHT---HT alteration is more localizeddownsection
Gabbroic dykes and veins
The occurrence of gabbroic dykes in the layered gabbrosof the Samail ophiolite is ubiquitous We describe gab-broic dykes and sills either plagioclase---clinopyroxeneand brown amphibole-bearing dykes or microgabbrosand amphibolitized dolerite dykes and sills correspond-ing to temperatures up to 975C (see below) Identifica-tion of gabbroic dykes in the field is not alwaysstraightforward because they may be obliterated bysuperimposition of greenschist-facies alteration (Fig 6a)Selective greenschist-facies alteration has frequently beenobserved either in the centre of a mafic dyke or alongboth margins (Fig 6b) as well as a continuous transition
(b) (c)(a)
(d) (e)
thulite
pistacite
white microvein
white vein
green vein
green vein
gabbroic dike
epidote vein
gabbroic dike
Fig 6 Field relationships of dykes and veins (scale bar represents 10 cm) (a) Thulite---pistacite vein cross-cutting a foliated gabbro matrix Reactionmargins marked by the development of secondary clinopyroxene (01 OA36b) (b) Comb-like structure in gabbroic dyke A pink epidote vein followsthe margin of the dyke (01 OA15f) (c) Along-strike transition from a gabbroic dykelet to a hydrothermal green amphibole vein (d) White prehnitevein (e) Green amphibole vein cross-cut by white prehnite vein
BOSCH et al HYDROTHERMAL CIRCULATION IN OMAN OPHIOLITE
1187
along a single crack from a mafic dyke to a hydrothermalvein (Fig 6c) We define hydrothermal veins as cracksfilled with greenschist-facies minerals that induce meta-morphic reactions in the adjacent country rocks andgabbroic dykes as planar intrusive bodies produced bythe crystallization of magma Following Nehlig amp Juteau(1988) we distinguish low-T greenschist-facies lsquowhiteveinsrsquo made of prehnite epidote chlorite and actinolite(Fig 6d) and lsquogreen veinsrsquo in which a darker greenamphibole predominates (Fig 6e) corresponding respec-tively to lower (195---410C) and higher (400---530C)temperatures (Nehlig amp Juteau 1988)The gabbroic dykes are typically a few centimetres
across composed of plagioclase clinopyroxene andbrown
hornblende They are coarse-grained to pegmatitic andcommonly display a comb-like structure (Fig 6b)Although the spacing between the dykes in the field ishighly variable the average is around 10m Gabbroicsills grade from clear intrusions to irregular and diffusepegmatitic gabbro patches up to a few metres in theirlarger dimension (Fig 7a)The distribution in the field of brownish pargasite
which appears black and glassy in outcrop is critical intracing the origin of the gabbroic dykes It is first observedat any level in the host lower and middle gabbros as largepoikilitic patches (Fig 7b) locally associated with simil-arly poikilitic patches of clino- and orthopyroxene(Fig 7a) These patches contain small aligned tablets of
(a) (b) (c)
g(e)(d)
alignment ofalignment of amphiboleamphibole oikocrystsoikocrysts
hwhite vein
hblpl
pcpx
brownbrownamphiboleamphiboleoikocrystoikocryst
eediabasedikdike
hb-plhb-plreactionreaction
imarginspbrown amphibole rich
gabbroic dikegabbroic dike
pbrown amphibole veins
Fig 7 Field relationships of dykes (scale bar is 10 cm long) (a) Irregular coarse to pegmatitic patches in a foliated gabbro matrix (01 OA42d)(b) Alignment of black amphibole oikocrysts in a foliated gabbro matrix (c) Local concentration of brown amphibole in a gabbroic dykelet Theplagioclase laths marking the foliation in the enclosing gabbro rotate into parallelism with the gabbroic dykelet (d) Two diabase dykes assimilatingthe enclosing layered gabbro with plagioclase---amphibole reaction margins (e) Pargasite-rich gabbroic dyke with wall reactions (01 OA19) (Notesmall pargasite---plagioclase veins) pl plagioclase cpx clinopyroxene hb hornblende
JOURNAL OF PETROLOGY VOLUME 45 NUMBER 6 JUNE 2004
1188
plagioclase oriented parallel to the larger plagioclaselaths defining the magmatic foliation of the gabbro(Fig 7c) Clinopyroxene oikocrysts surrounding idio-morphic plagioclase crystals are common in the upper-level gabbros they mark the end of the magmatic flowand crystallization growing before the pargasite andorthopyroxene oikocrysts as shown by their partialreplacement by pargasite Their crystallization marksthe transition at suprasolidus conditions from a dry to awet environment In the uppermost gabbros those dykesthat are typically altered in the HT hydrothermal faciesgrade upwards into the so-called lsquoisotropicrsquo or lsquovari-texturedrsquo gabbrosThe microgabbro and amphibolitized dolerite dykes
differ from the typical diabase dykes commonly cuttingthe gabbro section that have chilled margins and thusprobably intruded enclosing rocks already at temperat-ures below 450C In microgabbro and amphibolitizeddolerite dykes the absence of chilled margins and thedevelopment of dioritic reaction margins in the adjacentgabbro (Fig 7d and e) suggest that they were emplaced ingabbros that were still at temperatures above 750C(Nicolas et al 2000)
ANALYTICAL TECHNIQUES
Major and trace elements
Major element compositions were determined by elec-tron microprobe at the University of Montpellier II usinga Camebax SX100 Instrument calibration was per-formed on natural standards and operating conditionswere 20 kV accelerating potential 10 nA beam currentand 30 s counting timeRb and Sr concentrations in whole rock and separated
mineral fractions were measured by conventionalnebulization inductively coupled plasma mass spectro-metry (ICP-MS) using a VG Plasmaquad 2 (ISTEEMMontpellier) Digestion procedures followed thoseoutlined by Ionov et al (1992)
O---Sr isotopes
The samples selected for the detailed study wereextracted from gabbroic dykes or veins inside gabbrosand cut with a diamond saw into 2---3 cm fragmentsbefore being crushed into 05---1 cm fragments Forwhole-rock analysis small pieces were further crushedinto powder in an agate bowl For pure mineral analysisthe fragments were crushed with a manual agate mortarTo avoid or minimize the mixing of selected separateminerals with other phases that can bias the results astrict protocol of mineral selection was applied A smallsize fraction of 125---160 mm was used and minerals werefirst separated using a Frantz isodynamic magneticseparator Concentrates recovered (cpx plagioclase
amphibole epidote) were subsequently purified by care-ful hand-picking in alcohol under a binocular micro-scope At this stage mineral separates were very pureand only flawless minerals free of cracks and visibleinclusions were kept for isotopic analyses To ensurefurther mineral integrity these pure mineral separateswere ultrasonically washed in dilute 15N HCl and rinsedseveral times with ultra-pure water This stage potentiallyremoves adsorbed secondary micro-phases Previousstudies (eg Lecuyer amp Reynard 1996) demonstratedthat pyroxene and amphibole can be intermixed on avery fine scale with other phases (such as a range ofamphibole compositions talc Fe-oxide) Although thispossibility cannot be ruled out the procedure used inthis study makes it unlikely that complex minerals passedthrough the selection
Sr isotopic compositions
Approximately 50mg of whole-rock powder or separatedminerals were digested in a Teflon lsquobombrsquo with a mixtureof triple distilled 13N HNO3 and 48 HF with a fewdrops of HClO4 Two aliquots were separated one forthe determination of Sr isotopic composition and theother for Sr and Rb contents Sr was separated using anextraction chromatographic method modified from Pinet al (1994) Concentrations were measured by isotopedilution using 84Sr---87Rb mixed spikes To remove tracesof the late low-temperature seawater alteration and todetermine the primitive whole-rock Sr isotopic composi-tion leaching experiments were conducted on a fewsamples following the procedure described by Bosch(1991) The strontium isotopic compositions of leachateshave been analysed in the attempt to check for the leach-ing efficiency During these tests four Sr isotope composi-tions were measured which correspond to the unleachedsample the two liquids extracted after acid-leaching stepsand the final solid residue Leaching experiments wereconducted in two steps first with 6N HCl for 8min at70C second with 25N HCl for 30min at 70C Afterleaching the leachates were evaporated to dryness andthe solid residues digested similarly to the unleachedsamplesSr isotopic compositions were measured on a fully
automated VG Sector mass spectrometer housed at theUniversity of Montpellier II except for the Sr isotopicdata from the leaching experiments which have beenmeasured at the Universitee de Bretagne Occidentale(Brest) To assess the reproducibility and accuracy of Srisotopic measurements the NBS 987 standard was run12 times during this study the average value was0710245 with a precision better than 0000010 (2s)87Sr86Sr ratios are corrected for mass discrimination bynormalizing to a 86Sr88Sr value of 01194 Sr total blankconcentrations were less than 60 pg Initial 87Sr86Sr
BOSCH et al HYDROTHERMAL CIRCULATION IN OMAN OPHIOLITE
1189
ratios were calculated by correcting the measured ratiosfor accumulation of 87Sr produced by in situ 87Rb radio-active decay since 95Ma the assumed age of crystalliza-tion of the Oman ophiolite (Hacker 1994)
Oxygen isotopes
The powdered samples were introduced in Ni tubesunder dry air where they were reacted with BrF5 toform oxygen Oxygen was separated from other gasesby cooling the resulting mixture at liquid nitrogen tem-perature (77 K) that retained other volatile gases Oxygenwas further purified by trapping it on a cooled 5A mole-cular sieve at 77 K then quantified to calculate the yieldof the isotopic analyses and finally transferred to the massspectrometer (Finnigan Mat Delta E) where intensitiesassociated with masses 32 and 34 were measured todetermine d18O The isotopic composition of a sampleis given as dsample frac14 (RsampleRstandard 1) 1000 in permil unit where R is 18O16O and the standard isSMOW Analytical errors on the oxygen isotope ratioare 02 (2s)
RESULTS
Seven samples representative of the range of gabbroicdykes and five samples representative of the lower gab-bros two of them representing the margins of gabbroicdykes were selected for major element thermometricand Sr---O isotopic analyses (Tables 1---3) These samplesoriginate from wadi sections marked by bold lines inFig 1 Wadis Halhal Mayah Al Abyad Warihya andMaqsad structurally belong to a new ridge segment open-ing in slightly (1Myr) older lithosphere Wadi Gideahis in older lithosphere
Petrographic and isotopic characteristicsof the analysed samples
Olivine gabbro from the MTZ in MaqsadSamail Massif
Sample 94 MA2 is devoid of low-temperature alterationand only exhibits the discrete signature of the VHT---HTalteration The orthopyroxene corona around olivine(Fig 3a) is relayed by kelyphitic rims at the contactwith plagioclase (Fig 3b) Issuing from these rims aremicrometre-sized pale green amphiboles identified ascummingtonite and actinolite which penetrate the sur-rounding plagioclase in microcracks 004mm wide or atgrain boundaries (Fig 3b) similar to the microcrack sys-tem described by Manning et al (2000) A large numberof the plagioclase crystals have a cloudy core as a result ofthe concentration of fluid and mineral inclusions amongwhich diopside has been identified during microprobeanalysis Olivine cores are free of serpentine but theyare largely oxidized along a microcrack network Ortho-pyroxene coronas have a MgOpx number of 78---79
slightly higher than in the primary olivine cores withMgOl number of 74---75 The magmatic clinopyroxeneis totally fresh and probably not in equilibrium with thereactive orthopyroxene This sample has the lowest 87Sr86Sr initial ratio of the studied whole rocks (070322)identical to coexisting clinopyroxene (070324) and pla-gioclase (070322) This observation suggests the lack of alate superimposed LT hydrothermal alteration as it isvery unlikely that an LT hydrothermal event could havetotally equilibrated the Sr isotopic compositions of prim-ary minerals such as plagioclase and clinopyroxeneMoreover the whole rock (WR) clinopyroxene andplagioclase have similar initial Sr isotopic ratios despitevery different Sr contents Accordingly this ratio isthought to reflect the signature acquired during the crys-tallization of these minerals and is higher than the Omanmantle Sr isotopic value (McCulloch et al 1981) Thed18O values of the WR (50) and plagioclase (48) aredistinctly lower than primary magmatic values Indeedin normal mid-ocean ridge basalts (MORB) the WRd18O value is 57 02 with a corresponding plagio-clase d18O ranging from 55 to 62 ( Javoy 1980Gregory amp Taylor 1981)
Lower gabbro from Wadi Wariyah Wadi Tayin Massif
Sample 97 OA128b belongs to a layered sequence in thelower gabbro series injected by anorthositic sills Thesample was collected at the tip of one of these sillsbetween layers enriched in clinopyroxene The gabbrois composed of 60 plagioclase and 40 clinopyroxeneDespite the absence of olivine the WR has a relativelyhigh Mg content (Table 3) A series of low-T (prehnite)microveins 01mm thick cross-cut the gabbro perpendi-cular to the layering and foliation The margins of theveins are devoid of alteration This gabbro has the sameSr isotopic composition as 94 MA2 Nevertheless theWR and associated plagioclase have Sr isotope signatures(070333 and 070328) lower than the coexisting clino-pyroxene (070422) The Sr contents of the WR plagio-clase and clinopyroxene are 127 ppm 120 ppm and20 ppm respectively thus making the clinopyroxenemore sensitive to late-stage contamination Clinopyrox-ene and plagioclase oxygen isotope compositions (523and 414 respectively) are also lower than typicalmantle values Similar to the previous sample alterationat high temperature by a low d18O and high 87Sr86Srfluid is required to explain such characteristics
Amphibole gabbro patch Wadi Gideah Wadi TayinMassif
Sample 01 OA41d2 is from a laminated gabbro com-posed of millimetre-sized layers of clinopyroxeneand plagioclase Anatectic patches (Fig 7a) borderinganorthositic layers are formed of coarse-grained
JOURNAL OF PETROLOGY VOLUME 45 NUMBER 6 JUNE 2004
1190
Table1Locationoftheanalysed
samplesandsummaryoftheirpetrographicandSr---O
isotopiccharacteristics
Sam
ple
nam
eSite
Locationin
gab
broic
section
Typ
eofrock
Distance
to
Moho(km)
Parag
enesis
Alteration
87Sr
86Srinitial1
d18O
()2
94MA2
Maq
sad
From
MTZ
Layered
gab
bro
0Olivine(M
gno74---75)3
OPXco
ronas
(Mgno78---79)
WRfrac14
070322
WRfrac14
thorn495
CPX
Palegreen
Am
(cumact)
CPXfrac14
070324
nd
Plagioclase(A
n65---80)4inversezoning
Nolow-T
alteration
Plfrac14
070322
Plfrac14
thorn479
Solidfluid
inclusionsin
Pl
97OA128b
Wad
iWaryiah
From
lower
gab
bro
Layered
gab
bro
1CPX
Low-T
alteration
WRfrac14
070333
nd
Plagioclase(A
n85---87)
Prehnitemicroveins
CPXfrac14
070422
CPXfrac14
thorn523
Plfrac14
070328
Plfrac14
thorn414
01OA41d2
Wad
iGideah
From
upper
gab
bro
Gab
bro
patch
in
laminated
gab
bro
36
CPXrecrystallized
Plagioclaseidiomorphic
(An57---87)norm
alzoning
Black
pargasitic
honblende
WRfrac14
070351
CPXfrac14
070378
Plfrac14
070426
WRfrac14
thorn480
nd
Plfrac14
thorn504
Hbdfrac14
070355
nd
01OA19aamp
dWad
iWaryiah
Inlayeredgab
bro
Gab
broic
dyke
01
Olivine
OPXpoikilitic
WRfrac14
070341
WRfrac14
thorn567
CPX
Black
pargasite
Hbdfrac14
070324
Hbdfrac14
thorn287
Plagioclase(A
n73---95)
Hornblendepoikilitic
Plfrac14
nd
Plfrac14
thorn10 09
Hem
atite
WRfrac14
070418
nd
Green
Mg-hornblende
Acthornblende
Epidote
02OA90b
Wad
iAl-Abyad
Inlayeredgab
bro
Dioriticdyke
015
Green
Mg-hornblende
Hbdfrac14
070427
Hbdfrac14
thorn239
Plagioclase(A
n83---87)
Plfrac14
070387
Plfrac14
thorn498
Solidfluid
inclusionsin
plagioclase
01OA15f
Wad
iWaryiah
Inlayeredgab
bro
Comb-textured
gab
broic
dyke
025
CPXidiomorphic
Plagioclase(A
n95---99)norm
alzoning
BrownMg-H
ornblende
WRfrac14
070458
nd
02OA43a
Wad
iMayah
Inlayeredgab
bro
vein
Green
amphibole
1Palegreen
Mg-hornblendeacicular
Plagioclase(A
n91---97)in
reactionmargin
Hbdfrac14
070431
Hbdfrac14
thorn310
01OA36b
Wad
iGideah
Inlayeredgab
bro
epidosite
dyke
25
CPX
Epidote
WRfrac14
070519
nd
Plagioclase
Epfrac14
070554
nd
CPXclinopyroxene
Plplagioclase
OPXorthopyroxene
Amam
phibolecu
mcu
mmingtonite
actactinolite
WRwholerockHbdhornblende
Epep
idotend
notdetermined
187Sr
86Srinitialratioscalculatedat
95Ma(H
acker1994)Errors
ofthemeasuredratiosareindicated
inTab
le4
2Analyticalerrors
oftheoxygen
isotopevalues
are02
(2s)
3Mgnumber
frac14atomicMg(Mgthorn
Fe)
4Percentageofan
orthitein
plagioclase
BOSCH et al HYDROTHERMAL CIRCULATION IN OMAN OPHIOLITE
1191
(millimetre-sized) recrystallized clinopyroxene largelyaltered to light green magnesio-hornblende and idio-morphic plagioclase Locally primary black pargasitichornblende develops including idiomorphic normallyzoned plagioclase (Table 4) This gabbro exhibits similar87Sr86Sr ratios for the WR and pargasite (070351 and070355 respectively) These values are interpreted asrepresentative of the melt from which this amphibolecrystallized The clinopyroxene has a slightly higher Srisotope composition of 070378 consistent with the occur-rence of the light green Mg-hornblende rim Plagioclaseis significantly more radiogenic (070426) related to sec-ondary destabilization evidenced by the idiomorphichabit The Sr isotope compositions of the liquidsextracted from the acid leaching experiments of theWR (L1 and L2) are very similar 070358 and 070365respectively The 87Sr86Sr ratio of the residue obtainedafter leaching experiments is 070335 lower than boththe pargasite and unleached WR This value can beconsidered as the unaltered isotopic composition of thissample The d18O values measured for WR and coexist-ing plagioclase are lower than normal MORB magmaticvalues The 18O depletion in the plagioclase can be fairlywell explained by high-temperature hydrothermal inter-action between an oxygen-depleted fluid and the magmaAccording to the kinetics of hydrothermal exchange ofoxygen (Gregory amp Taylor 1981 Gregory et al 1989)plagioclase exchanges its oxygen isotopes with the fluidadjusting readily to the hydrothermal conditions Forhigh temperatures of exchange with seawater-derivedfluids (d18O between 0 and thorn2) the magmatic plagio-clase will have its primary d18O value lowered whereasthe d18O value for lower temperatures of exchange(5300C) will increase The amplification of the d18Oshift increases with the intensity of the hydrothermalalteration
Gabbroic dykes intrusive in lower gabbros from WadiWariyah Wadi Tayin Massif
Gabbroic dykes 01 OA19a and 01 OA19d are intrusiveinto the lower gabbros They are 3---4 cm wide discord-ant to the foliation of the host gabbros (Fig 7e) with asharp margin marked by a brown amphibole fringe Theprimary phases in the dykes (Table 1) locally preservedas elongated aggregates of a 01mm grain size mosaicassemblage at the dyke margins grade to millimetre sizein the central part of the dyke The WR major elementcomposition is Mg enriched compared with the enclosinggabbro (Table 3) Brown pargasitic amphibole oikocrystsmake up 50 of the modal composition of the dyke Indyke 01 OA19a poikilitic orthopyroxene may developinstead of brown amphibole Finally green magnesio-hornblende grows either as oikocrysts at the expense ofclinopyroxene or in association with epidote as an altera-tion product of plagioclase These low-amphibolite-faciesminerals represent the lowest-grade paragenesis recogn-ized in these dykes and are most developed in sample01OA19aTheamphibole composition showszoning fromTi-rich pargasitic hornblende to magnesio-hornblendeand actinolitic hornblende (Fig 8c) recording pro-gressive cooling of the dyke The plagioclase compositionis extremely heterogeneous (Fig 8b) varying from An95a composition similar to that of the plagioclase in the hostgabbro (Fig 8a) to An50 for the plagioclase and asso-ciated epidote inclusions in the poikilitic actinolitic horn-blende Sample 01 OA19d has an initial 87Sr86Sr ratioslightly higher for the WR (070341) than for the parga-site (070324) The Sr isotopic ratios of liquids extractedafter two steps of WR acid-leaching (L1 and L2) are070357 and 070341 respectively whereas the residueyields a 87Sr86Sr ratio of 070327 very close to thepargasite and plagioclase values The latter is similar tothe isotopic composition measured for the massive gab-bro 94 MA2 and may be taken as close to the primarymagmatic value This Sr isotopic ratio is more radiogenicthan the inferred Oman mantle-source value (McCullochet al 1981) The altered part of this sample has a radio-genic Sr isotopic composition (070418) related to thelow-amphibolite-facies alteration This gabbroic dykehas seemingly preserved a magmatic whole-rock d18Ovalue (567) but contains by far the highestd18O plagioclase (1009) of all the samples analysedThis plagioclase coexists with low d18O pargasite(thorn287) Similarly high values (d18Oplagioclase 4 8)have been previously measured for gabbroic dykes dia-bases sheeted dykes and plagiogranites from the Omanophiolite (Gregory amp Taylor 1981 Stakes amp Taylor1992) This 72 oxygen isotope fractionation betweenplagioclase and amphibole cannot possibly represent oxy-gen equilibrium at any plausible temperature This maybe explained by different exchange kinetics for these twominerals at different temperatures at high waterrock
Table 2 Average temperatures ( C 40C)
calculated for plagioclase---amphibole pairs
using the geothermometer of Holland amp
Blundy (1994)
Pargasite Magnesio-hornblende
rim core rim core
01 OA41d2 878 953 ------- -------
01 OA19a 935 974 825 869
01 OA19d 890 933 789 -------
02 OA37b ------- ------- 816 849
02 OA90b ------- ------- 859 833
JOURNAL OF PETROLOGY VOLUME 45 NUMBER 6 JUNE 2004
1192
ratios This supports the occurrence of two distincthydrothermal events one at high temperature and alater stage at lower temperature responsible for thestrong 18O enrichment in the coexisting plagioclase
Dioritic dyke associated with a diabase dyke in lowergabbros from Wadi Halhal Haylayn Massif
Sample 02 OA37b belongs to a group of brownamphibole---plagioclase lenses or reaction margins asso-ciated with diabase intrusives (Fig 7d) in lower layeredgabbros Idiomorphic brown hornblende (tschermakite)crystals are elongated in the plane of the dyke represent-ing 80 of the modal composition They are associatedwith an interstitial plagioclase matrix The browntschermakite has greenish rims Plagioclase has a cloudyappearance due to micrometre-size fluid inclusionsexcept along its margins and internal microcracks Bothhornblende and plagioclase exhibit evidence of plasticdeformation The Sr isotopic ratio for the pargasite andplagioclase is 070348 and 070420 respectively Thepargasite exhibits a Sr signature closest to that of theOman primary magmatic value (McCulloch et al 1981)Nevertheless it is too high to be attributed to anunmodified MORB-source mantle signature This sug-gests the addition of an external fluid component in goodagreement with the shift of oxygen isotopic compositionto a low value (thorn39) suggesting a high-temperatureexchange with a low d18O fluid The Sr isotope composi-tion of the plagioclase is significantly higher than that of
the coexisting pargasite probably as a result of the low-temperature alteration
Dioritic dyke cross-cutting lower layered gabbros in WadiAl-Abyad Nakhl Massif
Sample 02 OA90b is a 1 cm thick dioritic dyke cross-cutting the lower layered gabbros It grades into a greenamphibole vein as illustrated in Fig 6c The dyke is com-posed of 2 cm long dark green idiomorphic magnesio-hornblende crystals growing in the plane of the dykeand plagioclase concentrated at the margin of the dykeThe plagioclase in the dyke and in the enclosing gabbrocontains micrometre-size fluid and mineral inclusionssimilar to those observed in sample 94 MA2 The plagio-clase and green hornblende have higher Sr isotopic ratios(070387 and 070427 respectively) and lower d18Ovalues (thorn498 and thorn239 respectively) than the normalMORB-source mantle value As observed for previoussamples this supports interaction with an external fluidcomponent
Comb-textured gabbroic dykes in the lower gabbros fromWadi Wariyah Wadi Tayin Massif
Sample 01 OA15f (Fig 6b) is from Wadi Wariyah andrepresents a 2 cm wide dyke intruding the lower gabbrosThis type of comb-textured dyke developed in theclinopyroxene---plagioclase stability field It containslarge idiomorphic clinopyroxene crystals which arepartially replaced by brownish magnesio-hornblendeThe plagioclase is extremely calcic with a slight inverse
Table 3 Whole-rock major element compositions of massive gabbros and dykes determined by X-ray fluorescence
(wt )
Sample 94 Ma2 97 OA128b 01 OA41d2 01 OA19d 01 OA19a 01 OA15f 01 OA36b 01 OA36b
Rock type Gabbro Gabbro Gabbro patch Gabbroic dyke Gabbroic dyke Comb-textured
gabb dyke
Epidosite dyke Epidosite dyke
(margin)
SiO2 4830 4815 4749 4524 4339 4799 389 4371
Al2O3 2785 2032 2438 1474 1246 1192 2752 1646
Fe2O3 328 266 404 981 1204 592 381 623
MnO 003 004 005 014 016 006 008 014
MgO 405 737 427 1031 1686 1326 187 739
CaO 1289 1908 1477 1235 1156 166 2471 2339
Na2O 292 123 257 258 111 075 000 013
K2O 000 000 009 006 000 000 000 000
TiO2 005 017 063 230 064 086 013 042
P2O5 008 007 013 018 009 013 007 010
LOI 033 076 138 216 161 237 274 190
Total 9978 9985 998 9987 9992 9986 9983 9987
LOI loss on ignition gabb gabbroic
BOSCH et al HYDROTHERMAL CIRCULATION IN OMAN OPHIOLITE
1193
Table4Majorelementcomposition
aofselectedam
phibolespaired
withplagioclasecompositionsusedforthermometry
Sam
ple
nam
e1Mode2
SiO
2TiO
2Al 2O3
Cr 2O3
FeO
MnO
MgO
CaO
Na 2O
K2O
Total
Si
AlIV
AlVI
Ti
Fe3
thornCr
Mg
Fe2
thornMn
Na
KCa
XAn3
T(C)4
01OA19a
Mg-hornblende
core
45 60
26
10 78
009
860
015
16 56
12 00
212
007
96 72
6552
1448
0366
0083
0552
0004
3409
0594
0016
0611
0018
1843
0903
899
Mg-hornblende
core
47 99
263
850
002
897
011
16 61
12 10
160
020
96 36
6894
1106
0334
0028
0483
0002
3557
0594
0014
0446
0036
1862
0892
834
Mg-hornblende
rim
51 17
247
596
001
761
014
18 11
12 19
103
010
96 47
7257
0743
0253
0016
0414
0001
3828
0488
0016
0284
0018
1853
0894
820
Mg-hornblende
rim
47 50
291
920
004
880
014
16 46
11 95
174
008
96 07
6835
1165
0396
0015
0506
0005
3529
0552
0017
0486
0015
1843
0887
825
Mg-hornblende
rim
49 57
269
742
000
799
017
18 07
12 02
133
014
96 88
7013
0987
0261
0018
0575
0000
3811
0371
0020
0365
0026
1821
0860
831
Mg-hornblende
core
47 83
181
872
003
840
011
14 48
12 18
159
008
93 59
6814
1186
0279
0018
0644
0004
3711
0357
0013
0438
0015
1859
0907
874
Pargasite
rim
42 15
249
11 87
024
10 07
013
13 84
11 43
263
032
96 61
6188
1812
0242
0434
0299
0028
3028
0937
0016
0749
0059
1797
0804
972
Pargasite
rim
42 22
260
11 86
026
10 21
011
13 71
11 48
253
032
96 59
6201
1799
0254
0429
0299
0030
3001
0955
0014
0720
0060
1807
0803
962
Pargasite
core
42 05
263
11 45
030
10 19
011
13 80
11 47
246
032
96 46
6189
1811
0175
0478
0299
0035
3029
0956
0014
0702
0060
1809
0767
974
Pargasite
rim
42 36
247
11 90
016
945
013
14 16
11 76
249
026
95 98
6239
1761
0305
0366
0276
0019
3109
0888
0016
0710
0049
1856
0818
926
Pargasite
core
42 50
291
11 64
016
984
015
14 22
11 41
256
034
96 69
6219
1781
0227
0426
0322
0018
3103
0883
0018
0727
0063
1789
0812
974
Pargasite
rim
42 76
270
12 03
017
920
012
14 87
11 82
244
025
96 73
6224
1776
0880
0335
0368
0020
3226
0752
0015
0689
0047
1843
0841
941
Pargasite
rim
42 13
181
13 54
016
921
009
12 77
12 13
254
032
96 98
6172
1828
0510
0450
0000
0018
2788
1129
0011
0721
0059
1905
0841
874
01OA19d
Mg-hornblende
49 26
047
624
011
11 46
017
15 39
12 21
108
009
96 48
7137
0863
0202
0051
0437
0013
3323
0951
0021
0302
0018
1896
0768
771
Mg-hornblende
46 03
287
932
018
906
017
14 77
12 87
193
007
97 28
6672
1328
0265
0313
0046
0021
3191
1053
0021
0542
0012
1999
0758
808
Pargasite
41 89
429
11 65
008
11 03
017
13 52
11 37
259
017
96 77
6159
1841
0179
0474
0345
0010
2963
1012
0021
0739
0032
1791
0770
975
Pargasite
rim
42 80
340
11 16
009
11 01
018
13 73
11 67
240
022
96 67
6293
1707
0226
0376
0322
0010
3009
1032
0022
0684
0041
1839
0605
880
Pargasite
core
42 58
351
11 27
006
11 63
018
13 51
11 46
239
022
96 82
6257
1743
0209
0387
0391
0007
2959
1039
0023
0680
0042
1804
0595
893
Pargasite
rim
42 14
393
11 28
006
11 71
014
13 35
11 33
235
033
96 62
6214
1786
0174
0435
0391
0007
2935
1054
0018
0673
0062
1790
0519
901
Pargasite
core
41 56
420
12 05
006
11 57
016
12 51
11 35
249
027
96 24
6168
1832
0277
0469
0276
0007
2768
1160
0021
0718
0052
1805
0537
882
Pargasite
core
41 33
438
11 49
008
11 66
015
13 35
11 45
264
026
96 80
6105
1894
0107
0487
0368
0009
2941
1073
0019
0757
0048
1813
0537
942
Pargasite
41 08
446
12 39
012
11 35
017
13 13
11 36
263
015
96 85
6049
1951
0199
0494
0368
0014
2882
1030
0021
0752
0027
1792
0758
975
02OA37b
Mg-hornblende
rim
45 15
260
984
002
13 28
019
14 01
10 87
128
009
97 33
6527
1473
0203
0283
0690
0002
3018
0915
0023
0360
0016
1683
0562
828
Mg-hornblende
rim
44 72
263
988
004
13 23
021
14 01
10 78
133
009
96 92
6494
1506
0185
0287
0713
0005
3032
0894
0025
0374
0018
1677
0593
854
Mg-hornblende
rim
45 68
247
935
004
12 87
020
14 10
11 00
121
011
97 02
6621
1379
0218
0269
0598
0004
3045
0962
0024
0341
0020
1708
0562
815
Mg-hornblende
core
43 94
291
10 87
001
12 60
023
13 73
10 59
142
011
96 39
6410
1590
0278
0319
0644
0001
2985
0893
0028
0401
0021
1655
0710
860
Mg-hornblende
core
44 62
270
10 37
001
12 66
019
14 12
10 58
140
008
96 73
6479
1521
0253
0294
0644
0001
3057
0893
0023
0395
0014
1646
0674
851
Mg-hornblende
rim
47 79
181
796
002
12 09
025
15 22
11 09
100
009
97 33
6866
1134
0214
0196
0506
0002
3260
0947
0030
0280
0016
1706
0503
767
Mg-hornblende
core
45 43
249
10 3
000
11 99
022
14 50
11 14
133
008
97 48
6524
1476
0267
0269
0644
0000
3103
0796
0027
0370
0015
1715
0703
838
JOURNAL OF PETROLOGY VOLUME 45 NUMBER 6 JUNE 2004
1194
Sam
ple
nam
e1Mode2
SiO
2TiO
2Al 2O3
Cr 2O3
FeO
MnO
MgO
CaO
Na 2O
K2O
Total
Si
AlIV
AlVI
Ti
Fe3
thornCr
Mg
Fe2
thornMn
Na
KCa
XAn3
T(C)4
02OA90b
Mg-hornblende
core
48 83
038
745
005
939
010
17 29
12 24
142
006
97 21
6931
1069
0041
0041
0667
0005
3657
0448
0011
0392
0010
1862
0840
856
Mg-hornblende
core
49 49
033
731
003
916
008
17 40
12 57
139
005
97 82
6987
1013
0035
0035
0552
0004
3662
0530
0010
0379
0009
1901
0830
811
Mg-hornblende
rim
48 90
040
753
005
932
013
17 15
12 22
154
005
97 29
6948
1052
0042
0042
0575
0006
3632
0533
0015
0424
0010
1861
0880
869
Mg-hornblende
rim
48 30
044
800
007
962
010
16 86
12 32
161
005
97 35
6873
1127
0047
0047
0598
0008
3576
0546
0012
0443
0010
1879
0870
862
Mg-hornblende
rim
49 12
032
742
010
922
011
17 36
12 29
142
006
97 43
6956
1044
0034
0034
0621
0011
3665
0471
0013
0391
0010
1865
0840
845
02OA43a
Mg-hornblende
rim
48 13
040
789
007
860
007
17 41
12 19
155
006
96 36
6882
1118
0212
0043
0621
0008
3710
0407
0008
0429
0010
1867
-------
-------
Mg-hornblende
rim
50 93
035
643
001
759
006
18 50
12 24
114
003
97 28
7152
0848
0217
0037
0506
0001
3873
0385
0007
0311
0005
1841
-------
-------
Mg-hornblende
rim
50 67
014
559
009
12 33
026
15 28
12 44
068
003
97 50
7260
0740
0204
015
0483
0010
3263
0994
0032
0189
0005
1909
-------
-------
Mg-hornblende
rim
50 34
013
621
009
12 49
027
15 07
12 51
064
003
97 78
7192
0810
0240
0010
0530
0010
3210
0960
0030
0180
0010
1910
-------
-------
Mg-hornblende
rim
45 17
060
11 14
013
988
008
15 78
12 33
211
009
97 31
6473
1527
0355
0640
0644
0015
3371
0540
0010
0588
0016
1892
-------
-------
Mg-hornblende
rim
49 25
032
712
014
845
009
17 84
12 32
137
004
96 95
6983
1020
0170
0030
0620
0020
3770
0380
0010
0380
0010
1870
-------
-------
Mg-hornblende
rim
48 61
032
775
014
975
011
16 81
12 57
139
004
97 50
6905
1950
0203
0034
0621
0016
3560
0537
0013
0382
0008
1914
-------
-------
94Ma2
Anthophyllite
54 98
000
259
001
12 65
037
23 35
199
034
000
96 28
7744
0256
0174
0000
0161
0001
4903
1330
0043
0092
0001
0301
-------
-------
Anthophyllite
55 11
000
296
003
14 09
032
23 66
063
038
000
97 18
7716
0284
0205
0000
0115
0003
4938
1535
0038
0104
0000
0095
-------
-------
Actinolite
48 58
004
954
002
960
014
17 67
999
139
005
97 02
6850
0150
0435
0004
0690
0002
3713
0442
0017
0379
0009
1510
-------
-------
Actinolite
51 54
004
585
000
890
022
20 28
930
090
002
97 05
7214
0786
0178
0005
0598
0000
4230
0444
0026
0244
0004
1395
-------
-------
01OA41d2
Pargasite
rim
43 05
342
10 71
008
11 46
012
13 50
11 54
179
067
96 34
6353
1647
0216
0380
0368
0009
2967
1046
0015
0512
0127
1824
0613
863
Pargasite
rim
42 61
354
11 59
001
10 59
010
13 52
11 87
160
076
96 19
6286
1714
0302
0392
0299
0001
2972
1007
0012
0458
0143
1877
0715
844
Pargasite
core
42 54
401
11 04
006
11 81
013
13 21
11 50
202
039
96 71
6266
1734
0182
0444
0368
0007
2901
1087
0017
0576
0073
1814
0854
979
Pargasite
rim
43 03
385
10 72
002
11 70
014
13 50
11 95
146
080
97 17
6314
1686
0168
0425
0345
0002
2953
1091
0018
0415
0149
1878
0634
852
Pargasite
rim
42 65
341
10 60
006
11 97
014
13 22
11 64
183
068
96 20
6332
1668
0186
0381
0345
0007
2926
1141
0018
0528
0129
1851
0620
869
Pargasite
core
42 82
367
10 60
007
11 93
012
13 42
11 44
206
047
96 60
6315
1685
0158
0407
0391
0008
2950
1080
0016
0588
0089
1808
0615
899
Pargasite
rim
42 2
362
11 10
007
11 42
016
13 09
11 72
179
072
95 89
6282
1718
0229
0405
0299
0009
2905
1123
0020
0518
0137
1869
0715
875
Pargasite
rim
42 42
380
11 03
006
11 77
012
13 00
11 72
188
069
96 49
6283
1717
0209
0423
0299
0007
2870
1158
0015
0539
0131
1860
0817
928
Pargasite
rim
42 21
364
11 14
006
10 73
011
13 53
11 68
177
072
95 59
6277
1723
0229
0407
0322
0007
2999
1012
0014
0511
0136
1862
0736
883
Pargasite
rim
42 25
376
11 03
008
10 93
015
13 66
11 72
185
064
96 07
6257
1743
0182
0418
0345
0009
3016
1009
0019
0531
0121
1860
0766
912
Pargasite
core
42 17
396
10 91
008
11 10
010
13 57
11 49
209
038
95 85
6253
1747
0159
0442
0368
0010
3000
1009
0013
0602
0073
1826
0841
981
aMajorelem
entco
mpositionsarein
weightpercent-------noplagioclasean
alysed
andnotemperature
calculated
1Typ
eofam
phibolean
alysed
isalso
indicated
2Blankindicates
nozoningobserved
3Molefractionofan
orthitein
plagioclasead
jacentto
amphibolegrain
4Tem
perature
ofeq
uilibrium
fortheam
phibole---plagioclasepaircalculatedusingtheHollandamp
Blundythermometer
(1994)Errors
are40
C
BOSCH et al HYDROTHERMAL CIRCULATION IN OMAN OPHIOLITE
1195
zonation from An95 to An99 from core to rim (Table 1 andFig 8a) TheWR Sr isotopic ratio of the sample (070458)is significantly higher than normal MORB values Thiscould be related to the overprint of a low-temperaturealteration event consistent with the petrography of thesample
Green amphibole vein cross-cutting layered gabbros inWadi Mayah Haylayn Massif
Sample 02 OA43a belongs to a series of 1 cm thick greenamphibole veins (as in Fig 6e) cross-cutting the layeredgabbros The veins develop 15 cm thick reaction marginsin the enclosing gabbro and are exclusively composed ofacicular pale green magnesio-hornblende growing per-pendicular to the vein margin in a crack-seal mode(Fig 6f ) The reaction margins are composed of urali-tized gabbro marked by the development of the samepale green magnesio-hornblende in an inhomogeneouscalcic plagioclase matrix (An91---97) The Sr and O isotopiccompositions of this green hornblende (070431 andthorn31) do not correspond to normal MORB-sourcemantle values
Epidosite dyke from Wadi Gideah Wadi Tayin Massif
Sample 01 OA36b is representative of a series of epidoteveins cross-cutting poorly layered gabbros These 2 cmthick veins are composed of pink thulite in their centreand green pistachite at their margins (Fig 6a) Theirsharp contact with the enclosing gabbro is marked bythe development of coarse clinopyroxene crystals alteredin the epidote---greenschist facies This suggests that theepidote vein is replacive in a coarse-grained gabbro dykeThe highest 87Sr86Sr initial ratios of all the samplesstudied are recorded by the WR and coexisting epidoteof this sample (070519 and 070554 respectively)(Table 5) The Sr content of the WR and associatedepidote are also the highest of the present dataset(716 ppm and 764 ppm respectively) The Sr isotopiccomposition of this sample is consistent with a greaterdegree of exchange with a radiogenic component fromCretaceous seawater It is significantly higher than thevalue obtained for an epidosite vein from the Wadi Fizhsection of the Oman ophiolite [070435 with a Sr contentof 488 ppm reported by Kawahata et al (2001)] but ingood agreement with the average of the greenschist-faciesalteration sequence defined by Kawahata et al (2001)
GABBROS
01 km 025 km 1 km 36 km50
60
70
80
90
An
MA2
19d
19a 15f 128b
41d2
(a)
43a17b
15f
90b
37b
19a
19d
40
60
80
100
A
n
01 km 015 km 025 km 1 km
DYKES
(b)
Distance to the Moho
ACTINOLITE
MAGNESIOHORNBLENDE
TSCHERMAKITE
PARGASITEGabbros Dykes41d2MA2
19a d37b15f
90b43a
AlIV
150500
02
08
10
(Na
+ K
) A
(c)
04
06
950
900
850
800
750
70007 12 17
T degC
1000
(d)
AlIV
Fig 8 Plagioclase---amphibole compositions and results of thermometry (a) and (b) plagioclase compositions in layered gabbros and in dykesrespectively as a function of distance to the Moho (see text for sample identification) (c) amphibole compositions in layered gabbros and dykesaccording to Leakersquos (1997) nomenclature (d) equilibration temperatures calculated using the amphibole---plagioclase thermometer of Holland ampBlundy (1994) as a function of AlIV content in amphibole [same symbols as in (c)]
JOURNAL OF PETROLOGY VOLUME 45 NUMBER 6 JUNE 2004
1196
Table5RbandSrcontents87Sr
86Srandd1
8Oisotopiccompositionsofwholerocksandmineralseparatesfrom
samplesofmassivegabbrosdykes
andveinsfrom
theOman
ophiolitealsolistedarecalculated
waterrockratios(W
R)andLOIvalues
Sam
ple
Rb(ppm)
Sr(ppm)
87Rb8
6Sr
87Sr
86Sr
(87Sr
86Sr)i1
d18O
()
LOI(
)2WR
3(87Sr
86Sr)L14
(87Sr
86Sr)L25
(87Sr
86Sr)R6
01OA41d2
Whole
rock
037
1792
0006
0703516
09
070351
thorn480
138
47
0703587
0703651
0703357
Plagioclase
025
1249
0003
0704261
22
070426
thorn504
-------
-------
Pargasite
-------
-------
-------
0703548
20
070355
-------
-------
-------
Clinopyroxene
045
26 0
0050
0703845
19
070378
-------
-------
-------
01OA36b
Dykewhole
rock
0005
7164
50001
0705186
17
070519
-------
274
21 9
0705207
0705112
-------
Epidote
0003
7645
0001
0705536
12
070554
-------
-------
-------
Wallwhole
rock
005
4254
0001
0704637
09
070464
-------
191
-------
0704945
0704675
0704593
02OA43a
Green
hornblende
004
98
0012
0704323
13
070431
thorn310
-------
-------
97OA128b
Whole
rock
003
1274
50001
0703327
17
070333
-------
076
37
-------
-------
-------
Plagioclase
002
1204
50001
0703285
09
070328
thorn414
-------
-------
Clinopyroxene
003
20 0
0004
0704230
09
070422
thorn523
-------
-------
01OA15f
Whole
rock
006
1077
0002
0704582
16
070458
-------
237
13 5
-------
-------
-------
01OA19a
Whole
rock
037
1303
0008
0704189
18
070418
-------
161
96
-------
-------
-------
01OA19d
Whole
rock
058
2032
0008
0703423
08
070341
thorn567
216
415
0703574
0703408
0703271
Pargasite
091
1058
0025
0703273
11
070324
thorn287
-------
-------
Plagioclase
-------
-------
-------
-------
-------
thorn10 09
-------
-------
02OA90b
Plagioclase
004
2385
50001
0703868
11
070387
thorn498
-------
-------
Green
hornblende
010
23 8
0012
0704288
15
070427
thorn239
-------
-------
BOSCH et al HYDROTHERMAL CIRCULATION IN OMAN OPHIOLITE
1197
Table5continued
Sam
ple
Rb(ppm)
Sr(ppm)
87Rb8
6Sr
87Sr
86Sr
(87Sr
86Sr)i1
d18O
()
LOI(
)2WR
3(87Sr
86Sr)L14
(87Sr
86Sr)L25
(87Sr
86Sr)R6
02OA37b
Plagioclase
023
3121
0002
0704204
15
070420
-------
-------
-------
Pargasite
031
41 6
0021
0703505
15
070348
thorn394
-------
-------
94OAMa2
Whole
rock
0016
1750
00003
0703219
15
070322
thorn495
033
31
0703264
0703177
0703158
Plagioclase
0014
1363
00003
0703219
11
070322
thorn479
-------
-------
Clinopyroxene
-------
-------
-------
0703236
13
070324
-------
-------
-------
1Initial8
7Sr
86Srratiocalculatedat
95Ma(H
acker1994)
2LOIisloss
onignition(in)
3Water---rock
ratiocalculatedassumingaclosedsystem
TheeffectiveWR
ratiocanbeexpressed
bythefollowingeq
uation
W=Reffectivefrac14
frac12eth87Sr=
86SrrockTHORN fi
nal
eth87Sr=
86SrrockTHORN in
itial=frac12eth8
7Sr=
86SrseawaterTHORN in
itial
eth87Sr=
86SrrockTHORN fi
nal
ethCrock
initial=C
seaw
ater
initial
THORNwhere(87Sr
86Srrock) finalisthefinalmeasuredSrisotopicratioin
thesample(87Sr
86Srrock) in
itialco
rrespondingto
theinitialm
antlevalue
istakenas
070295(Lan
phere
etal1981)(87Sr
86Srseawater) in
itialistheCretaceousseaw
ater
Srisotopicco
mposition[87Sr
86Sr07073
at90
MaafterBurkeet
al(1982)]C
seaw
ater
initial
istheSrco
ntent
oflsquonorm
alrsquoseaw
ater
andis
takenas
8ppm
(deVilliers1999)
FortheCrock
initialitis
assumed
that
thepresentSrco
ncentrationmeasuredis
thesameas
theinitial
concentration
4Srisotopicratiosmeasuredforliquid
L1extractedafterthefirststep
oftheleachingexperim
ent(6NHClfor8min
at70
C)
5Srisotopicratiosmeasuredforliquid
L2extractedaftertheseco
ndstep
oftheleachingexperim
ent(2 5NHClfor30
min
at70
C)
6Srisotopicratiosmeasuredforresidueobtained
afteratotalaciddigestionachievedaftertheextractionofL1an
dL2leachates
JOURNAL OF PETROLOGY VOLUME 45 NUMBER 6 JUNE 2004
1198
Part of the same sample from the contact between thedyke and the gabbro yields a slightly lower Sr isotopiccomposition (070464) intermediate between the sur-rounding gabbro and the epidote dyke It also has anintermediate Sr content (425 ppm) Acid leaching experi-ments performed on the whole rock of the dyke yield Srisotopic compositions of 070521 and 070512 for thetwo liquids extracted Similar experiments for the samplelocated at the contact with the gabbro yield 87Sr86Srratios of 070494 and 070467 for L1 and L2 liquids TheSr isotopic composition of the residue extracted afterleaching remains high (070459) and very close to theepidosite 87Sr86Sr ratio determined by Kawahata et al(2001) In this sample the primary minerals have beentotally replaced by epidote and secondary plagioclase orquartz Thus the Sr isotopic compositions measured forthis WR sample and its constituent minerals reflect thecomposition of the fluid filling this dyke
Mineral chemistry
Plagioclase
Compared with the host gabbros the dykes show a muchlarger dispersion in their plagioclase compositions (Fig 8aand b Table 4) characterized by more calcic plagioclasethan the enclosing gabbros This tendency is mostextreme in the comb-textured dykes in which the plagio-clase is almost pure anorthite The zoning is generallynormal recording a crystal fractionation process How-ever gabbro 94 MA2 exhibits inverse zoning this tend-ency is also observed in plagioclases from dykes 01OA19a and 01 OA15f The higher calcium content ofthe plagioclase as well as the inverse zoning is consistentwith an increase of water pressure during its crystalliza-tion (Turner amp Verhoogen 1960 Carmichael et al 1974Koepke et al 2003) The particularly high An content inthe comb-textured gabbro dykes 01 OA15f may alsoresult from crystallization in a supercooled magma thisis consistent with the comb texture indicative of fast-growing crystals The lack of brown hornblende suggeststhat crystallization occurred above the temperature ofhornblende stability
Amphibole
The amphiboles analysed in the dykes (Fig 8c andTable 4) show a regular trend in a (Na thorn K)A vsAlIV diagram from titanium-rich (Ti 4 015) pargasitichornblende (brownamphibole) and tschermakite (brown---green amphibole) to low-titanium (Ti5 015) magnesio-hornblende (green amphibole) and actinolite (pale greenamphibole) The pargasitic hornblende develops aspoikilitic oikocrysts in the dykes 01 OA19a and d andin the diffuse patch 01 OA41d2 at temperatures above800C during the final stage of crystallization (see
below) The tschermakite composition corresponds tothe internal alteration of clinopyroxene in gabbro 94Ma2 occurs in the comb-textured dyke 01 OA15f andis the primary amphibole in the dioritic dyke 02 OA37bThe magnesio-hornblende develops at the edges of thepargasites or of clinopyroxenes at temperatures around800C and is the primary phase in the dioritic dyke 02OA90b and in the green vein 02 OA43a Finally actino-lite occurs as a replacement of green hornblende in dykesor in the kelyphitic rim surrounding olivine in gabbro 94Ma2 Such a large composition range at the scale of athin section has been reported in amphiboles fromamphibolite-facies oceanic gabbros (Stakes 1991 Stakesamp Taylor 1992 Gillis et al 1993) as well as in the Omanophiolite gabbros (Manning et al 2000) This variabilityhas been explained by rapid reaction rates preventinghomogenization (Manning et al 2000)
Thermometry
Plagioclase---amphibole thermometry could be appliedwhen the two minerals exhibited good contacts such asin the laminated gabbro of the middle sequence (01OA41d2) and in the intrusive dykes The temperaturesof plagioclase---amphibole equilibration (Table 2) havebeen calculated using the geothermometer lsquoBrsquo of Hollandamp Blundy (1994) Edenite thorn Albite frac14 Richterite thornAnorthite valid between 500C and 900C with anuncertainty of 40C Amphibole formulae are basedon 23 oxygen and their nomenclature is based on alkalications in the A site vs AlIV according to Leake (1997)Pressurewas assumed to be 1 kbar Forty-five plagioclase---amphibole pairs meet the compositional criteria imposedby Holland amp Blundyrsquos dataset (09 4 Xan 4 01 andAlIV 403) Electron microprobe measurements wereconstrained to contiguous plagioclase and amphiboleseparated by sharp grain boundaries assuming equili-brium of the two phases When the mineral phasesexhibit normal zoning temperatures between plagioclaseand amphibole cores and between mineral rims werecalculated assuming that the temperatures deduced fromthe mineral core chemistry provide a proxy for the high-est temperature of equilibration These data are identi-fied in the T C vs AlIV diagram (Fig 8d) Table 4presents selected major element amphibole analyses theAn content of the plagioclase adjacent to each amphibolegrain and the temperature calculated for the pairThe temperatures calculated (Fig 8d) span the entire
range of temperatures at which amphibole and plagio-clase coexist in equilibrium (ie amphibolite-facies para-geneses) This temperature interval is bracketed byorthopyroxene-bearing facies or amphibole-free facies(ie gabbro 94MA2) and epidote---actinolite facies devoidof homogeneous plagioclase (ie epidosite dyke 01OA36b and green vein 02 OA43a) The range of the
BOSCH et al HYDROTHERMAL CIRCULATION IN OMAN OPHIOLITE
1199
calculated temperatures between 4900C and 750Ccorresponds to the changing composition of amphibolesfrom pargasite to magnesio-hornblende at the scale of athin section or even at that of an individual crystal asshown by their correlation with AlIV such variabilityrecords rapid cooling in hydrous conditions The highesttemperatures are recorded by pargasite from the anatec-tic gabbro 01 OA41d2 in the gabbro dyke 01 OA19dand in the dioritic dyke 02 OA90b They were calculatedfrom minerals devoid of lower-grade alteration and arethus considered as representing the equilibrium temperat-ure of the coexisting minerals Lower temperatures cor-respond to mineral phases evolving at falling temperatureand are only indicative of the cooling history Includingthe 40C uncertainty seven pairs provide temperatureshigher than 900C the limit of validity of the Holland ampBlundy thermometer However we maintain these tem-peratures as indicative of the highest values of our data-set These values are included in the average equilibriumtemperature summarized in Table 2
DISCUSSION
Distribution of hydrothermal alteration
Two distinct petrological events are recorded in the gab-bro section of the Samail ophiolite and are documentedin the present contribution (1) the whole gabbro sectionis affected by a penetrative alteration (2) the gabbrosection is crosscut by various types of dykes and veinsincluding hydrous minerals
Penetrative alteration
The penetrative alteration developed in the gabbros atgrain boundaries and locally invading the minerals isubiquitous in the gabbro section This penetrative altera-tion has been described as very high temperature (VHT)high temperature (HT) and low temperature (LT) basedon alteration parageneses The orthopyroxene coronas inthe layered gabbro sample and the incipient appearanceof brown pargasitic amphibole mark a lower thermallimit of the VHT alteration By reference to the experi-mental work of Koepke et al (2003) the temperature ofequilibration for these reactions is estimated to bebetween 900 and 1000CThe preservation of the vertical distribution of
VHT---HT facies (Figs 4 and 5) also pointed out byManning et al (2000) indicates that the hydrothermalalteration pattern fits the distribution of isotherms withdepth at a fast-spreading ridge (ie Phipps Morgan ampChen 1993 Dunn et al 2000 Fig 2) Equilibrium tem-peratures for VHT parageneses as high as 900---1000Csuggest that VHT---HT hydrothermal alteration tookplace in a very restricted time interval at the limit ofthe cooling magma chamber It is proposed that the
penetrative alteration affecting the entire gabbro sectionis related to a pervasive network of microcracks whichact as a recharge system down to the Moho (Nicolas et al2003)
Dykes and veins
The dykes are extremely varied eg pegmatitic gabbroswith comb-texture microgabbros and amphibolitizeddolerites dioritic dykes alignments of poikilitic horn-blende and finally diffuse hornblende-bearing patchesHigh-T gabbro and diorite dykes and low-T greenamphibole---epidote veins are connectedIn the dykes textural evidence attests to suprasolidus
conditions at least for the development of the pargasiticamphibole Integrating the hornblende---plagioclase equi-libration temperatures calculated for the studied samplesthese temperatures are bracketed between 950C and875CIn the following discussion we shall consider separately
the microgabbro and dolerite dykes The latter representbasaltic melt intrusions associated upsection with thediabase sheeted dykes and downsection possibly withthe mafic dykes that are ubiquitous in the mantle section(Nicolas et al 2000) Whatever their origin the develop-ment of pargasitic amphibole during their late stage ofcrystallization creates a link with the gabbroic dykes andsuggests a crystallization process evolving from dry con-ditions to more hydrous conditionsThe other gabbroic dykes comb-textured gabbros and
hornblende-bearing dioritic dykes result from the fillingof cracks by a fluid either a melt or a silica-rich hydrousfluid These intrusive dykes which develop reaction mar-gins at their contact with the host gabbro grade verticallyinto lower-temperature green amphibole veins or align-ments of poikilitic hornblende in the layered gabbromatrix or development of pargasitic hornblende in ana-tectic gabbros (sample 01 OA41d2) In gabbros crystal-lizing in the magma chamber the development oforthopyroxene and pargasite oikocrysts enclosing the pri-mary minerals suggests a thermal evolution from thegabbro solidus to amphibolite-facies conditions Forthese reasons it is suggested that the gabbroic dykesresulted from the injection of a hydrous melt in the stillhot gabbros drifting away from the magma chamber asdiscussed below
Origin of fluids responsible for VHT---HThydrothermal alteration
The 87Sr86Sr initial ratios and d18O of whole rocks andpure mineral fractions are plotted in Fig 9 against thedistance above the Moho Transition Zone (MTZ) Theinitial Sr isotopic composition of the whole rocks apartfrom the epidosite dykes ranges from 070322 to
JOURNAL OF PETROLOGY VOLUME 45 NUMBER 6 JUNE 2004
1200
070458 All the studied samples (including whole rocksand minerals) have initial 87Sr86Sr ratios falling outsidethe field of fresh MORB which commonly have Sr ratiosbetween 07023 and 07029 with an average of 070265(ie Hart et al 1974) The lowest initial Sr isotopiccomposition (070322) from the massive gabbro 94MA2 is slightly but significantly higher than averageOman primary magmatic signatures (ie 070296McCulloch et al 1981) Similarly the d18O values ofmost whole rocks and minerals measured (Table 5)
depart from typical MORB-source mantle values( Javoy 1980 Gregory amp Taylor 1981 Kyser 1986)These differences indicate that the primary mantlesignatures in the Oman ophiolite have been modifiedby interaction with a fluid Possible origins for thisfluid are mantle components dehydration of subductedlithosphere or seawater circulation Strontium andoxygen isotopes are useful tracers for fluid---crust interac-tion as d18O and 87Sr86Sr ratios from fluids originat-ing from seawater the mantle or subducted lithosphere
-1000
1000
2000
3000
4000
5000
6000
7000
MOHO
MTZ
Dis
tan
ce a
bov
e th
e M
oh
o (
m)
pillow-basalt
sheeted-dike
gabbro
peridotite
01 OA 41d2
01 OA 36b
02 OA 43a97 OA 128b
01 OA15f
94 OA MA2
02 OA 90b01 OA 19ad
02 OA 37b
0702 0703 0704 0705 0706 0707
87Sr86Srinitial
MO
RB-s
ou
rce
Seaw
ater
Whole RockPlagioclasePargasiteGreen HornblendeClinopyroxene
Epidote
(a)
-1000
1000
2000
3000
4000
5000
6000
7000
MOHO
MTZ
Dis
tan
ce a
bov
e th
e M
oh
o (
m)
pillow-basalt
sheeted-dike
gabbro
peridotite
01 OA 41d2
01 OA 36b
02 OA 43a97 OA 128b
01 OA15f
94 OA MA2
02 OA 90b01 OA 19ad
02 OA 37b
Whole RockPlagioclasePargasiteGreen HornblendeClinopyroxene
3 5 10
δ18O (permil)
41d2
Ma2
43a
90b 19d 19d37b
128b
90b
(b)
Fig 9 (a) Variation of initial 87Sr86Sr isotopic composition as a function of the location of the studied samples in a schematic log of the crustalsection of the Oman ophiolite The field for MORB is from Hart et al (1974) and that for seawater is from Burke et al (1982) Small open squares aredata fromKawahata et al (2001) (b) Variation of d18O () as a function of the location of the studied samples in a schematic log of the crustal sectionof the Oman ophiolite Shaded curve corresponds to the data of Gregory amp Taylor (1981)
BOSCH et al HYDROTHERMAL CIRCULATION IN OMAN OPHIOLITE
1201
are significantly different Moreover the d18O valueis dependent on temperature and mineral fractiona-tion and thus yields complementary constraints to Srisotopes
Mantle origin
One model to explain the occurrence of fluids interactingwith the oceanic crust at very high temperatures is toderive them from the mantle The contribution of mantle-derived hydrous fluids linked to percolation processesor to their concentration in the final stages of gabbrocrystallization has been invoked in numerous case studies(ie Witt amp Seck 1989 Fabriees et al 1989 Dupuy et al1991 Agrinier et al 1993) O and Sr isotopic data forwhole rocks and mineral separates yield scattered valuesthat are unambiguously different from MORB mantle-source values (Fig 9a and b) With the exception of lateLT altered samples the WR data have a mean 87Sr86Srratio of 070337 and a standard deviation of 000012These surprisingly high values in mantle-derived rockscould be taken as evidence for survival of mantle isotopicheterogeneities during crystallization of the ophiolite (egLanphere 1981 McCulloch et al 1981) However the18O16O isotope ratios in the Oman gabbroic sequenceare also displaced from primary magmatic values Nogabbro in the present study has plagioclase or amphi-bole with typical mantle d18O values Thus the Sr and Oisotope data rule out the possibility of mantle-derivedfluids as a possible source for the VHT---HT hydrousalteration event recorded in the massive gabbros andgabbroic dykes and veins (Fig 10a and b)
Subducted lithosphere origin
Dehydration of subducted oceanic crust in subductionzones can generate hydrous fluids Such fluids couldpercolate through the overlying lithosphere and contam-inate it thus generating modified isotopic signatures andtrace element contents Such an explanation howeverdisagrees with the geochemical characteristics of most ofthe studied samples Fluids derived from the dehydrationof subducted slabs (fresh or altered MORB or OIB pro-toliths) should be strongly enriched in K Rb and Ba (egBecker et al 2000) whereas in Oman the Rb contentsmeasured in the gabbros and gabbroic dykes and veins arestrongly depleted Moreover the isotopic composition ofslab-derived fluids is buffered by the MORB-like oceaniccrustal protolith for Nd and Pb but not for Sr and O(Staudigel et al 1995) As a result of relatively minorfractionation of oxygen at high temperature the oxygenisotope composition of the fluids expelled during dehy-dration of the subducted slab should be high and essen-tially controlled by the oxygen composition of the uppersection of the crust altered at low temperature (with an
average of 10 and perhaps as high as 19) (Staudigelet al 1995) The Sr isotopic composition should be alsohigh (average 07046) as the fluids released by the dehy-dration of subducted oceanic crust are associated eitherwith seawater or with radiogenic Sr phases (ie smectites)The Sr and O isotopic compositions of the studied sam-ples do not fit with these signatures and so preclude asubducted lithosphere origin for the fluids involved in theVHT---HT alteration event
Seawater-derived fluids
All the analysed samples (minerals and WR) have 87Sr86Sr(T) outside the domain of primary MORB magmasand variable Sr contents Individual minerals have Srcontents reflecting their different mineralliquid partitioncoefficients Minerals characterized by a very high affi-nity for Sr such as plagioclase (ie Sr mineralliquidpartition coefficients 1) have very high Sr contentsTherefore the high abundance of plagioclase in the sam-ples analysed is responsible for the high Sr content of thestudied whole rocks The Sr content of all the whole rocksis relatively homogeneous (Table 5) without clear distinc-tion between country-rock gabbros and dykes and veinsand ranges from 110 to 200 ppm except for the epidosite(4700 ppm) Reported on a 87Sr86Sr vs 1Sr diagram(Fig 10a) most whole rocks and plagioclases analysed arecharacterized by a 1Sr ratio lower than 001 and plot inthe left part of this diagram Compared with N-MORBthe studied massive and anatectic gabbros and their con-stituent plagioclases have similar to slightly higher Srcontents but substantially higher 87Sr86Sr ratios(Fig 10a) Considering Cretaceous seawater as a possiblehigh 87Sr86Sr mixing component [87Sr86Sr 07073at 90Ma after Burke et al (1982)] responsible for theobserved geochemical modifications exchanges cannothave occurred in a closed system as seawater has too lowa Sr content (ie 8 ppm de Villiers 1999) An open-system process allowing penetration of larger quantitiesof seawater can efficiently modify the Sr isotopic ratio ofthe rocks Alternately the fluids could also be seawaterthat was modified through exchange with the surround-ing rocks during its transit through the oceanic crustalsection This modified seawater would be more enrichedin Srwith a 87Sr86Sr ratio intermediate between unmodi-fied seawater and MORBMinerals devoid of late LT alteration imprints (parga-
site green hornblende and pyroxene) and characterizedby very low Sr contents plot on the right of the diagramclose to or on the mixing line between seawater andMORB (Fig 10a) The difference between these mineralsand the other samples (WR and plagioclases) is mainlydue to the Sr fractionation coefficients of these differentminerals and to the large quantity of plagioclase inthe studied rocks The process responsible for the
JOURNAL OF PETROLOGY VOLUME 45 NUMBER 6 JUNE 2004
1202
modification of the Sr isotopic composition fromMORB-source value is however explained by the same phenom-enon As all these minerals display initial 87Sr86Sr ratiosdistinct from the MORB source and because they equili-brated at VHT or HT temperatures this supports anearly intervention of seawater during crystallization ofthese minerals Most of these samples with the exceptionof the epidosite dyke overlap the domains defined byKawahata et al (2001) for the Wadi Fizh gabbros alteredat VHT and HT conditions in the amphibolite facies(labelled lsquoVHTrsquo and lsquoHTrsquo in Fig 9a)Three samples (two epidosite dykes and an one epidote
fraction) have very high Sr contents and the highest Sr
isotopic compositions of the present dataset The twowhole-rock samples overlap the field of the Wadi Fizhsamples altered at high temperature during greenschist-facies metamorphism (Kawahata et al 2001) This typeof alteration is common at the ridge axis and formedthrough intensive exchange with seawater-derived fluidsFigure 10b indicates that all the Sr---O isotope data are
distinct from MORB compositions and suggests that thefluids reacting with the magmas could be derived fromseawater In addition the d18O values show that isotopicexchange occurred under VHT or HT conditions Thefluids could have the composition of seawater or theycould have the composition of seawater modified through
0703
0705
0707
001 01 1
1 Sr (ppm)
MORB
(87Sr
86Sr)
init
ial
Seawater
VHT
HT
MT36b
36b
36b
43a 128b90b
41d2
37b19d
15f
128b
41d2
41d2
19a
Ma2Ma2
19d
90b
37b
Whole RockPlagioclasePargasiteGreen HornblendeClinopyroxene
Epidote
(a)
0 2 4 6 8
δ18O(permil)
0703
0705
0707Seawater
MORB-mantle
128b 41d2
90b 41d2
Ma2128b
37b
43a
19d
90b
Whole RockPlagioclasePargasiteGreen HornblendeClinopyroxene
Epidote
HT LT
19d
(87Sr
86Sr)
init
ial
(b)
Fig 10 (a) Initial 87Sr86Sr isotopic ratios plotted against 1Sr for whole rocks and mineral separates from the gabbro section of the Omanophiolite Fields shown are from Kawahata et al (2001) and correspond to MT-----basalt sheeted dyke diabase altered at medium temperature(prehnite---actinolite facies sequence 3) HT-----diabase plagiogranite metagabbro and epidosite formed at high temperature (greenschist faciessequence 4) VHT-----gabbro altered at very high temperature (amphibolite facies sequence 5) The dashed line is a binary mixing line betweenMORB (Lanphere et al 1981) and Cretaceous seawater (Burke et al 1982) (b) Initial 87Sr86Sr isotopic composition plotted against d18O value forwhole rocks and minerals from the Oman ophiolite Cretaceous seawater and MORB end-members as in (a) Dashed line represents mixing curvebetweenMORB and seawater at high temperature (HT) Low-temperature (LT) exchanges between these two components would be responsible foran increase of the d18O primary MORB value
BOSCH et al HYDROTHERMAL CIRCULATION IN OMAN OPHIOLITE
1203
exchange with the surrounding rocks during its penetra-tion into the crustal section In an environment in whichfluid---rock interaction occurs at variable temperaturesthe d18O is greatly influenced by the temperature atwhich the exchange took place and by the waterrockratio It is evident from Figs 9b and 10b that none of thestudied gabbros have plagioclase and amphibole withMORB-like d18O and 87Sr86Sr values The separateminerals display a broad range of d18O values and mostexhibit extreme 18O depletion (Table 5) Hydrothermalexchange kinetics are similar in amphibole and pyroxeneand these two minerals are more resistant to oxygenexchange during water---rock interaction than coexistingplagioclase (Gregory et al 1989) The most probableexplanation of the low d18O values measured in bothamphiboles and pyroxenes is that they crystallized athigh temperature in the presence of a relatively low 18Oseawater-derived hydrothermal fluid [d18O from 01 to07 after Gregory amp Taylor (1981)] The anomalouslyhigh d18O value (thorn1009) measured for the plagioclase ofthe micro-gabbroic dyke (01 OA19d) can be interpretedas resulting from a superimposed overprint caused by alate-stage penetration of fluids at lower temperature Inthis sample the occurrence of such different 18O16Oratios between coexisting plagioclase and amphibole isnot consistent with equilibrium fractionation betweenthese two minerals at any possible temperature Thisobservation suggests that parts of the ophiolite sequencehave undergone a high-temperature hydrothermal eventprior to the later low-temperature event that producedthe strong 18O increase in coexisting plagioclaseThe depletion of the d18O values results from high-temperature hydrothermal alteration (Gregory amp Taylor1981 Stakes amp Taylor 1992) The oxygen isotopic datasupport the contention that most studied samples havebeen affected by a VHT---HT hydrothermal alteration byfluids derived from seawater Some of the analysed sam-ples presenting petrological evidence of crystallization ofLT secondary minerals have been overprinted by the latelow-temperature alteration thus swamping the earlierhigh-temperature alteration effects
Determination of waterrock ratio
To better evaluate the exchange between seawater andthe gabbroic rocks waterrock ratios (WR) have beenestimated for the whole rocks analysed during the courseof this study The different models used to constrain thisratio mainly differ by the calculation of the effective ratioor by the estimated weight ratio and by considering openor closed systems for water circulation (Albareede et al1981 McCulloch et al 1981) The WR ratios reportedin Table 5 have been calculated assuming a closed sys-tem Using this formulation these values are minimabecause the calculation always uses pristine fluid for the
initial fluid thus maximizing the value in the denomina-tor If the fluid was shifted to lower 87Sr concentrations byexchange with the rock on the downward path theinferred values would be much higher (Davis et al 2003)The calculated WR ratios for the samples from this
study range from 31 to 219 (Table 5) Two main groupsof samples can be recognized on the basis of their waterrock ratios The lowest ratios ranging from 31 to 47have been determined for massive gabbros or anatecticgabbros but also for dykes and veins devoid of late LTalteration Similar ratios have been previously calculatedfor example for the discharged hydrothermal solutions atthe 13N and 21N East Pacific Rise (Di Donato et al1990 Fouquet et al 1991) The gabbros from the Ibrasection in the Oman ophiolite gave effective WR ratiosof 05 33 and 42 (McCulloch et al 1981) in goodagreement with the values obtained in this study Theleast altered gabbro of the Ibra section yields a WR ratioof 05 in good agreement with the exceptionally fresh-ness of this rock characterized by typical mantle oxygenvalues for its minerals (McCulloch et al 1981) Samplescharacterized by waterrock effective ratios in the rangeof 3---5 can be related to penetrative alteration via thehydrothermal system Lecuyer amp Reynard (1996) havedemonstrated in oceanic gabbros from the Hess Deepaltered in similar HT conditions that WR mass ratios inthe range of 02---1 are sufficient to account for the mod-ified stable isotope signature of the gabbros Higherwaterrock values (WR 45) have been calculated forsamples affected by superimposed late hydrothermalalteration and for the epidosite samples (Table 5) Theyprobably acquired their isotopic characteristics throughinteraction with a larger volume of seawater duringgreenschist-facies metamorphism Similar or higherWR values have been published in previous studies(ie McCulloch et al 1981 Kawahata et al 2001)They correspond either to samples from the upper-crus-tal sequence (ie basalt sheeted dyke or plagiogranite) orto gabbros from the crustal section located in a particularenvironment such as for example the Wadi Fizh sectionthat is located close to a segment boundary along thepalaeo-spreading axis (Nicolas et al 2000)
Mechanism for seawater interaction withmagma
Nicolas et al (2003) have proposed that variable amountsof seawater can circulate at high temperature through-out the crustal section down to the walls of the magmachamber via a network of parallel microcracks a fractionof a millimetre wide Such microcracks and their relation-ship with the VHT---HT hydrous alteration in the sur-rounding gabbros have been described in detail byNicolas et al Via this network of microcracks largeamounts of seawater can have reacted with the magma
JOURNAL OF PETROLOGY VOLUME 45 NUMBER 6 JUNE 2004
1204
and induced variable changes of its Sr and O isotopiccomposition This regular network of parallel micro-cracks could constitute the recharge system and thehydrated gabbroic dykes the discharge system Physicalparameters and the geophysical models of Nicolas et al(2003) are in agreement with this scenario Such a pene-tration of seawater-derived hydrothermal fluids throughthe lower crust along grain boundaries and a microscopicfracture network at temperatures 4750C is consistentwith recent thermodynamic models (McCollom amp Shock1998)Seawater-altered and high-temperature recrystallized
diabases (protodykes) from the overlying sheeted dykecomplex stoped into the melt lenses could represent anindirect way for seawater to enter the system as they canremelt during their subsidence through the magmachamber A simple mass balance calculation suggeststhat this indirect contamination is not significant Assess-ments of the amount of recycled crust 1 in volumeby Nicolas et al (2000) based on the protodyke remnantsin the gabbro section or 20 by Coogan et al (2003)based on chlorine content in EPR basalts lead to a sea-waterrock ratio of the order of 104 to 103Thus following Nicolas et al (2003) we propose that
seawater has been introduced along microcracks down tothe walls of the magma chamber and has diffused alonggrain boundaries in the way described by Manning et al(2000) progressively invading the host gabbro Gabbroicdykes result from the injection of hydrous melt into thehost gabbros at falling temperatures as they drift awayfrom the magma chamber This water-saturated meltcould result from the extensive hydration of the crystal-lizing gabbros near the walls of the magma chamberwhen seawater penetrates through this wall (Fig 1)Similar plumbing systems driving seawater down to the
limit of the magma chamber and back to the surface havebeen already proposed in other environments such as theMid-Atlantic Ridge (Kelley amp Delaney 1987 Cooganet al 2001) the East Pacific Rise at Hess Deep (Gillis1995 Manning et al 1996a 1996b) the Tonga forearc(Banerjee amp Gillis 2001) and the Troodos ophiolite(Gillis amp Roberts 1999) The melting curve of wet basalthas a negative slope but is close to the dry solidus at lowpressure in the lower gabbros and the first melts areplagiogranitic (Spulber ampRutherford 1983) Such plagio-granites are ubiquitous in the highest gabbros and in theroot zone of sheeted dykes in Oman (Rothery 1983Juteau et al 1988 Nicolas amp Boudier 1991) in manyother ophiolites and in the oceanic crust where they havebeen interpreted as resulting either from wet anatexis ofhot gabbros or from the final product of crystallization ofa basaltic melt (Spulber amp Rutherford 1983) Plagio-granites are rare in the lower gabbros exceptionallyassociated with hydrous gabbro segregations inside thelayered gabbros Their constitutive minerals are also
remarkably absent along the VHTmicrocracks althoughthese gabbros were above the wet solidus A possibleexplanation has been proposed by McCollom amp Shock(1998) who wrote that at high temperature lsquoaqueousfluids may leave very little conspicuous petrological evid-ence for their presencersquo the reason being that thehigh-T conditions would be closer to batch meltingObviously more detailed studies on this specific problemare needed We conclude that the large degree of hydrousmelting locally required at the walls of the magma cham-ber to account for the generation of a gabbroic hydrousmelt may have erased the traces of the incipient plagio-granitic melt
CONCLUSIONS
Fostered by the recent discovery of high-temperatureseawater contamination in some gabbros of the Omanophiolite (Manning et al 2000) we have conducted astudy on 500 gabbro specimens from selected areas ofthe entire Samail ophiolite belt and on the hydrous gab-broic dykes that cross-cut them The highest-temperaturereactions (VHT) recorded in olivine and clinopyroxene ingabbros as orthopyroxene and pargasite coronas reach975C They are followed at lower HT conditions withreplacement of olivine by an assemblage of tremolitemagnetite and plagioclase surrounded by chlorite andby pargasite development in clinopyroxene followedfinally by the LT lizardite---olivine and saussurite---plagioclase greenschist-facies alteration With a fewexceptions the VHT alteration tends to be restricted tothe lower gabbros and to the vicinity of the Moho andthe HT alteration to the middle and upper gabbros Thepreservation of the vertical distribution of VHT---HTfacies indicates that progressive cooling during off-axisspreading did not obliterate this distribution and conse-quently that the VHT---HT hydrothermal alteration tookplace in a very restricted time interval at the limit of thecooling magma chamber In the deep and hot gabbrosthe main water channels may be sub-millimetre-sizedmicrocracks with a dominantly vertical attitude the ori-gin and dynamics of which have been investigated in aprevious paper (Nicolas et al 2003)Beyond these high-temperature alteration zones mas-
sively induced in the gabbros seawater may be respons-ible for the generation of clinopyroxene---pargasitegabbro dykes that cross-cut all gabbros and that areupsection clearly connected to the LT hydrothermalvein system Petrostructural evidence suggests that thesedykes were generated by hydration of the crystallizinggabbros near the internal wall of the LVZ---magmachamber The resultant melts were subsequently intrudedat various levels in the cooling lower and middle gabbrosPetrological observations of nearly all the gabbros ana-lysed in the present study located from the root zone of
BOSCH et al HYDROTHERMAL CIRCULATION IN OMAN OPHIOLITE
1205
the sheeted dyke complex down to the Moho emphasizethe imprint of VHT---HT hydrous alteration The VHThydrothermal event is characterized by temperatures ofequilibration around 900---1000C The temperatures atwhich the HT hydrous event occurred are estimated tobe in the range 550---900CStrontium and oxygen isotope compositions measured
on whole rocks and separated minerals significantlydepart from primary MORB values The Sr and O iso-tope data obtained for pargasite green hornblende andclinopyroxene record the participation of seawater at thetemperature of crystallization of the minerals Plagio-clases and whole rocks define a trend toward a compo-nent characterized by a high radiogenic 87Sr86Sr valueand a higher Sr content than normal seawater Seawateris clearly identified as the fluid responsible for the modi-fication of the Sr and O signatures for both massivegabbros and dykes and veins Nevertheless the high Srcontent of the extraneous component requires eitheropen-system exchange allowing penetration of a greaterquantity of seawater or penetration of seawater modifiedby interaction with the oceanic crust during its travel paththrough the crustal section The waterrock ratio calcu-lated on the basis of the Sr isotopes is between three andfive for the enclosing gabbros and for the least LT altereddykes The VHT---HT hydrothermal event affectingthese samples is related to penetrative alteration via therecharge and discharge hydrothermal system Thewaterrock ratio is 10 for dykes exhibiting evidenceof a LT hydrothermal alteration overprint and reaches22 for the epidosite dyke The depletion in d18O char-acterizes all the studied samples with the single exceptionof the micro-gabbroic dyke plagioclase This agreeswith the conclusions based on Sr isotopes suggestingthat these samples have undergone exchange with sea-water-derived fluids during a VHT---HTalteration hydro-thermal event
ACKNOWLEDGEMENTS
This work has benefited from financial support by theCentre National de la Recherche Scientifique (INSUInteerieur-Terre andDYETI programmes) and inOmanfrom the willingness of the Directorate of MineralsMinistry of Commerce and Industry and the hospitabil-ity of the people Technical help in the laboratory is alsoacknowledged including C Nevado for the thin sectionsE Ball for the illustrations B Galland for her assistancein the chemistry room and C Bassoulet for help duringthermal ionization mass spectrometric analyses of the Srresults from the leaching experiments Constructivereviews from R T Gregory C Lecuyer an anonymousreviewer and particularly from M Wilson greatlyhelped to improve an earlier version of the manuscript
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Davis A C Bickle M J amp Teagle D A H (2003) Imbalance in the
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Foucher J Erzig P Muhe R Wiedicke M Soakai S amp
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Gillis K M (1995) Controls on hydrothermal alteration in a section of
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Gillis K M amp Roberts M (1999) Cracking at the magma---hydrothermal transition evidence from the Troodos ophiolite Earth
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Gregory R T amp Taylor H P (1981) An oxygen isotope profile in a
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Ionov D A Savoyant L amp Dupuy C (1992) Application of the
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peridotites In Colloques Internationaux du CNRS 272 279---287
Juteau T Beurrier M Dahl R amp Nehlig P (1988) Segmentation
at a fossil spreading axis the plutonic sequence of the Wadi
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Juteau T Manacrsquoh G Moreau C Lecuyer C amp Ramboz C
(2000) The high temperature reaction zone of the Oman ophiolite
new field data microthermometry of fluid inclusions PIXE analyses
and oxygen isotopic ratios Marine Geophysical Research 21 351---385Kawahata H Nohara M Ishizuka H Hasebe S amp Chiba H
(2001) Sr isotope geochemistry and hydrothermal alteration of the
Oman ophiolite Journal of Geophysical Research 106 11083---11099
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fractures and generation of layered gabbros in the lower crust
beneath oceanic spreading ridges Annual Geological Union Special
Monograph 106 267---289
Kelemen P B Koga K amp Shimizu N (1997) Geochemistry of
gabbro sills in the crustmantle transition zone of the Oman
ophiolite implications for the origin of the oceanic lower crust Earth
and Planetary Science Letters 146 475---488Kelley D S amp Delaney J R (1987) Two-phase separation and
fracturing in mid-ocean ridge gabbros at temperatures greater than
700C Earth and Planetary Science Letters 83 53---66
Koepke J Feig S T Snow J amp Freise M (2003) Petrogenesis of
oceanic plagiogranites by partial melting of gabbros an experi-
mental study Contributions to Mineralogy and Petrology on line first http
wwwspringerlinkcomopenurlaspgenre=s00410-003-0511-9
Kyser T K (1986) Stable isotope variations in the mantle In
Valley J W Taylor H P amp OrsquoNeil J R (eds) Stable Isotopes in
High Temperature Geological Processes Mineralogical Society of America
Reviews in Mineralogy 16 141---164
Lanphere M A (1981) K---Ar ages of metamorphic rocks at the base
of the Samail ophiolite Oman Journal of Geophysical Research 86
2777---2782
Lanphere M A Coleman R G amp Hopson C A (1981) Sr isotopic
tracer study of the Samail ophiolite Oman Journal of Geophysical
Research 86(B4) 2709---2720
Leake B E (1997) Nomenclature of amphiboles report of the
subcommittee on amphiboles of the International Mineralogical
Association commission on new minerals and mineral names
European Journal of Mineralogy 9 623---651
Lecuyer C amp Reynard B (1996) High-temperature alteration of
oceanic gabbros by seawater (Hess Deep Ocean Drilling Program
Leg 147) evidence from oxygen isotopes and elemental fluxes
Journal of Geophysical Research 101(B7) 15883---15897
Liou J G Kuniiyoshi S amp Ito K (1974) Experimental study of the
phase relations between greenschist and amphibolite in a basaltic
system American Journal of Science 274 613---632
MacLeod C J amp Rothery D A (1992) Ridge axial segmentation in
the Oman ophiolite evidence from along-strike variations in the
sheeted dyke complex Geological Society of America Special Papers 60
39---63
Manning C E MacLeod C J amp Weston P E (1996a) Fracture-
controlled metamorphism of Hess Deep gabbros site 984
constraints on the roots of mid-ocean ridge hydrothermal systems
at fast-spreading centers In GillisM C Allan KM ampMeyer P S
(eds) Proceedings of the Ocean Drilling Program Scientific Results 147
College Station TX Ocean Drilling Program pp 189---211Manning C E Weston P E amp Mahon K I (1996b) Rapid high-
temperature metamorphism of East Pacific Rise gabbros from Hess
Deep Earth and Planetary Science Letters 144 123---132Manning C E MacLeod C J amp Weston P E (2000) Lower-crustal
cracking front at fast-spreading ridges evidence from the East Pacific
Rise and the Oman ophiolite Journal of the Geological Society London
349 261---272McCollom T M amp Shock E L (1998) Fluid---rock interactions in
the lower oceanic crust thermodynamic models of hydrothermal
alteration Journal of Geophysical Research 103 547---575
McCulloch M T Gregory R T Wasserburg G J amp Taylor H P
(1981) Sm---Nd Rb---Sr and 18O16O isotopic systematics in an
oceanic crustal section evidence from the Samail ophiolite Journal of
Geophysical Research 86(B4) 2721---2735Mevel C Gillis K M Allan J F amp Meyer P S (eds) (1996)
Proceedings of the Ocean Drilling Program Scientific Results 147 College
Station TX Ocean Drilling Program
Nehlig P amp Juteau T (1988) Flow porosities permeabilities and
preliminary data on fluid inclusions and fossil thermal gradients in
the crustal sequence of the Sumail ophiolite (Oman) Tectonophysics
151 199---221
Nehlig P Juteau T Bendel V amp Cotten J (1994) The root zone of
oceanic hydrothermal systems constraints from the Samail ophiolite
(Oman) Journal of Geophysical Research 99 4703---4713
Nicolas A amp Boudier F (1991) Rooting of the sheeted dyke complex
in Oman ophiolite In Peters Tj Nicolas A amp Coleman R (eds)
Ophiolite Genesis and Evolution of the Oceanic Lithosphere Dordrecht
Kluwer Academic pp 39---54
BOSCH et al HYDROTHERMAL CIRCULATION IN OMAN OPHIOLITE
1207
Nicolas A amp Ildefonse B (1996) Flow mechanism and viscosity in
basaltic magma chambers Geophysical Research Letters 23(16)
2013---2016
Nicolas A Ceuleneer G Boudier F amp Misseri M (1988) Structural
mapping in the Oman ophiolites mantle diapirism along an ocean
ridge Tectonophysics 151 27---56
Nicolas A Boudier F Ildefonse B amp Ball E (2000) Accretion
of Oman and United Arab Emirates ophiolite-----discussion of a
new structural map Marine Geophysical Research 21(3---4)147---179
Nicolas A Boudier F Michibayashi K amp Gerbert-Gaillard L
(2001) Aswad massif (United Arab Emirates) archetype of the
Oman---UAE ophiolite belt Geological Society of America Bulletin 349
499---451
Nicolas A Mainprice D amp Boudier F (2003) High temperature
seawater circulation through the lower crust of ocean-ridges-----amodel derived from the Oman ophiolites Journal of Geophysical
Research on line first httpwwwaguorgpubscurrentjb 108(B8)
2372
Pallister J S amp Hopson C A (1981) Samail ophiolite plutonic suite
field relations phase variation cryptic variation and layering and a
model of a spreading ridge magma chamber Journal of Geophysical
Research 86 2593---2644Phipps Morgan J amp Chen J Y (1993) Dependence of ridge-axis
morphology on magma supply and spreading rate Nature 364
706---708
Pin C Briot D Bassin C amp Poitrasson F (1994) Concomitant
extraction of Sr and Sm---Nd for isotopic analysis in silicate samples
based on specific extraction chromatography Analytica Chimica Acta
298 209---217
Rothery D A (1983) The base of a sheeted dyke complex Oman
ophiolite implications for magma chambers at oceanic spreading
axes Journal of the Geological Society London 140 287---296
Spear F S (1981) An experimental study of hornblende stability and
compositional variability in amphibolite American Journal of Science
281 697---734
Spulber S D amp Rutherford M J (1983) The origin of rhyolite and
plagiogranite in oceanic crust an experimental study Journal of
Petrology 24 1---25Stakes D S (1991) Oxygen and hydrogen isotope compositions of
oceanic plutonic rocks high-temperature deformation and meta-
morphism of oceanic layer 3 In Taylor H P OrsquoNeil J et al (eds)
Geochemical Society Special Publication 3 77---90
Stakes D S amp Taylor H P (1992) The northern Samail ophiolite an
oxygen isotope microprobe and field study Journal of Geophysical
Research 97(B5) 7043---7080Staudigel H Davies G R Hart S R Marchant M amp Smith B
M (1995) Large scale isotopic Sr Nd and O isotopic anatomy of
altered oceanic crust DSDPODP sites 417418 Earth and Planetary
Science Letters 130 169---185Turner F J amp Verhoogen J (1960) Igneous and Metamorphic Petrology
New York McGraw---Hill 694 pp
Wilcock W S D amp Delaney J R (1996) Mid-ocean ridge
sulfide deposits evidence for heat extraction from magma
chambers or cracking fronts Earth and Planetary Science Letters 145
49---64
Witt G amp Seck H A (1989) Origin of amphibole in recrystallized
and porphyroclastic mantle xenoliths from Rhenish massif implica-
tions for the nature of mantle metasomatism Earth and Planetary
Science Letters 91 327---340
JOURNAL OF PETROLOGY VOLUME 45 NUMBER 6 JUNE 2004
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et al (2001) on the basis of a Sr isotope study of the crustalunit in the Fizh massif of northern Oman found a rise inthe temperature of hydrothermal alteration from lowgreenschist facies in the basalts to amphibolite-facies con-ditions in the gabbrosBoudier et al (2000) have also suggested that seawater
could contaminate the lower gabbros during their crystal-lization as recorded by the crystallization of gabbro-norites in place of olivine gabbros This contaminationat very high temperatures (VHT) possibly as high as1200C is restricted to those parts of the Oman palaeo-ridge that display evidence of ridge-related tectonic activ-ity and the seawater contamination was attributed to thisparticular tectonic setting
Improved seismological modelling of fast-spreadingridges at the East Pacific Rise axis at 9300N has sug-gested that there may be significant narrowing of thelow-velocity zone (LVZ) below the melt lens (Dunn et al2000) with respect to earlier models This narrowingwas explicitly ascribed by Dunn et al to the existence ofdeep high-T hydrothermal circulation at modern oceanicridges This raises the question of the definition of whatshould be considered as the magma chamber below amid-ocean ridge On the basis of extensive studies of theSamail ophiolite Nicolas et al (1988) have shown that themagma chamber was tent-shaped in three dimensionsextending to the base of the crust consistent with theshape of the LVZ found below modern ridges Hence the
Fig 1 (a) Simplified map of the Oman---UAE ophiolite including the inferred location of the palaeo-ridge axes and main shear zones [modifiedfrom Nicolas et al (2000)] (b) Simplified map of the Samail ophiolite Thin numbered lines represent the log-sections of Fig 4 along wadis wherealteration facies have been studied Bold lines are the wadi sections selected for detailed petrological and geochemical work in this study
JOURNAL OF PETROLOGY VOLUME 45 NUMBER 6 JUNE 2004
1182
magma chamber is not restricted to the perched axialmelt lens that is present just below the sheeted dykecomplex of fast-spreading ridges (Detrick et al 1987)because in spite of its very low melt fraction the LVZ isstill deforming as a mush (Nicolas amp Ildefonse 1996)Building on the previous work of MacLeod amp Rothery
(1992) and Nehlig et al (1994) Nicolas et al (2001) haverecently published a structural map of the greenschist-facies hydrothermal veins throughout the Samail ophio-lite This confirms that the LT veins penetrate the entirecrust and are dominantly aligned parallel to the sheeteddyke complex Subsidiary preferred orientations arerelated to changes in the stress field during the dyingactivity of the palaeo-ridge system (Nicolas et al 2000)In a companion paper to the present study Nicolas et al
(2003) described a complete transition from these LThydrothermal veins to gabbroic dykes which suggests thepossibility of seawater-related hydrous melting of the gab-bros or entry of seawater into the partially molten magmachamber Previously these gabbroic dykes which arefairly common in the Oman gabbros were interpreted aslate crystallization products from the LVZ magma cham-ber (Pallister amp Hopson 1981) or as melt conduits relatedto its magmatic activity (Kelemen et al 1997 Kelemen amp
Aharonov 1998) If the magma that produced these gab-broic dykes originated within the magma chamber thewater should also have an internal origin the source beingeither the mantle or the melt lens In the latter case con-tamination by seawater is possible when hydrated diabasefrom the overlying sheeted dyke complex is stoped in themelt lens remelted and dewatered during subsequentsubsidence through the magma chamberAlternatively the gabbroic dykes could result from
fractures external to the magma chamber driving sea-water to great depth In the physical model proposed byNicolas et al (2003) the gabbroic dykes are generated byhydrous melting and represent the discharge system ofthe high-temperature hydrothermal circulation reachingthe inner margin of the magma chamber (Fig 2) Therecharge system is formed by a microcrack network con-trolled by the anisotropy of thermal expansion and oper-ating at falling temperatures below 1200C within adistance of 10 km from the spreading axisThe available geochemical evidence bearing on the
hydrothermal alteration of the gabbros (Gregory ampTaylor 1981 Lanphere 1981 McCulloch et al 1981Kawahata et al 2001) favours a seawater origin withseawaterrock ratios ranging from one (Gregory amp
water in Sea Floor
Magma chamber
water out
gabbroicdike
hydrous contamination
diabase dykeswith chilledmargins
diabasedikewithoutchilledmargins
750degC
500degC
1200degC
LT hydrothermal vein
hydrothermal cracks
hydrothermal cracks
VHT
plastic mantle flow
maficdike
Moho
microgabbrodike
HT
Fig 2 Scheme of hydrothermal circulation near the limit of the magma chamber below the fast-spreading ridge The downgoing seawater flow fillsthe porous network created by microcracks down to the magma chamber where it reacts with the crystallizing melt it surges back towards theseafloor along a system of hydrous gabbroic dykes Modified from Nicolas et al (2003)
BOSCH et al HYDROTHERMAL CIRCULATION IN OMAN OPHIOLITE
1183
Taylor 1981) to as high as nine (Kawahata et al 2001)Oxygen isotope data (C Lecuyer personal communica-tion 1990) on mafic dykes emplaced in the peridotites at1 km below the Moho suggest a dual source of hydrationfor the gabbros in which clinopyroxenes have a mantlesignature but associated plagioclase and brown horn-blende show evidence of seawater contamination Thisdual source was not ruled out by Benoit et al (1996) intheir Sr isotope study of similar dykes injected into themantle section Rare earth element partition coefficientsin clinopyroxene plagioclase and amphibole were usedby the above workers to constrain the temperature ofseawater contamination at the gabbro solidusThus there is growing evidence that supports a deep
penetration of high-T hydrothermal fluids into theoceanic crust at the ridge axis We have conducted asystematic study of the gabbros in the Samail ophioliteand along selected wadi sections (Fig 1) in the hydratedgabbroic dykes that cross-cut them looking for traces ofHT alterationThis study combines microtextural and petrological
observations thermometric determinations and O---Srisotope data on the same gabbroic dykes and surroundinggabbros selected with a view to determine the origin ofhydrous fluid contamination These results have import-ant consequences for thermal modelling of the oceaniccrust near fast-spreading ridges and on the budget ofchemical exchanges between the oceanic crust andseawater
HYDROUS ALTERATION OF
PRIMARY MINERALS IN THE
GABBRO SECTION
The penetrative alteration of the gabbro section has beenstudied on the basis of an extensive thin-section collec-tion which covers the entire ophiolite The studied gab-bros range from the root zone of the sheeted dykecomplex down to the Moho and the Moho transitionzone (MTZ) From the 500 specimens that have beenscreened those sites involved in tectonic complexitiessuch as the end of segments or tips of propagating rifts(Fig 1) have been rejected thus reducing the number ofstudied areas and the number of fully characterized speci-mens to 414 representative of standard gabbros at a fast-spreading palaeo-ridge In this study we mainly focus onhydrous alteration of olivine and clinopyroxene as theseare the minerals most affected by high-T alteration(above 800C) although plagioclase is stable down tothis temperature
Alteration facies
The following types of hydrous parageneses in thegabbros have been distinguished from highest to lowesttemperatures
(1) Orthopyroxene partial coronas between olivine andplagioclase which may be associated with the firstappearance of brown pargasite blebs inside clinopyrox-ene and development of pargasite between olivine andplagioclase or clinopyroxene and plagioclase (Fig 3a) Thebrown pargasite locally grades into a greenish magnesio-hornblende Although the orthopyroxene coronas couldbe assigned to a late magmatic stage we interpret themas reactive (a) being restricted to the contact betweenolivine and plagioclase thus postdating the crystallizationof plagioclase (b) being locally relayed by kelyphiticcoronas that spread into plagioclase microcracks(Fig 3b) as microphases among which diopside andlow-titanium pargasite have been identified We inferthat these reactions are related to an oxygen fugacityincrease as a result of seawater input Based on experi-ments at 2MPa pressure Koepke et al (2003) have con-strained the equilibrium olivine---orthopyroxene at lowwater content at 1030C and 950C at higher watercontent the limit at which amphibole is stabilized Byreference to this experimental work we estimate that thetemperature of equilibration for these reactions referredto here as VHT (very high temperature) hydrous altera-tion was between 900 and 1000C(2) Chlorite and pale amphibole coronas between oliv-
ine and plagioclase (Fig 3d) The coronas are composedof successive rims of clino-amphibole (tremolite) thornmagnetite Mg-chlorite (clinochlore) and a fringe of ortho-amphibole against plagioclase As described above theseminerals are associated with blebs of brown to greenishamphibole inside the pyroxenes (Fig 3c) Depending onthe degree of alteration olivine is surrounded by a thinfilm of amphibole and chlorite or is totally replaced by anassemblage of tremolite thorn magnetite thorn plagioclase An91(thorn talc) surrounded by chlorite (Fig 3e) The evolution ofthe first assemblage to Mg-chlorite indicates temperat-ures above 700C the upper breakdown of Mg-chloriteat 2 kbar (Chernosky et al 1988) The development ofpargasitic amphibole replacing clinopyroxene impliestemperatures between 550 and 900C (Liou et al 1974Spear 1981) We refer to these as HT (high temperature)(3) Iddingsite partly or totally replacing olivine inde-
pendently of orthopyroxene or chlorite---amphibole cor-onas and any significant HT alteration in clinopyroxene(Fig 3f ) Observations in thin sections show that thechlorite---amphibole coronas develop before the idding-site alteration(4) Lizardite alteration of olivine forming the familiar
mesh structure This is always accompanied by saussuritein plagioclase This alteration takes place below 500Cand is here referred to as LT (low temperature) hydrousalteration It is only at this stage that plagioclase showssigns of alterationThese various hydrous alteration facies are commonly
observed superimposed in a set of thin sections from a
JOURNAL OF PETROLOGY VOLUME 45 NUMBER 6 JUNE 2004
1184
single outcrop and even in a single thin section As a rulethe orthopyroxene reactions are more penetrative beinghomogeneously distributed from the scale of a thinsection to that of several thin sections made from a
given outcrop The distribution of chlorite---amphibolecoronas is similar although they can be restricted to themargins of microcracks filled by the same minerals whichhave been interpreted as the channels that carried the
(a) (b)
(c) (d)
(e) (f)
olopx
kel
pl
ol
hbol
amcpx
cpx
hb
chl
am+pl+mt
idg
am
hb
pl
hb
ol
cpx
opx
mt
pl
Fig 3 Hydrothermal alteration in the gabbros (scale bar represents 1mm) (a) VHT alteration marked by orthopyroxene (opx) coronas at thecontact of olivine (ol) and plagioclase (pl) and pargasitic hornblende (hb) at the contact of olivine and clinopyroxene (cpx) (sample 94 MA2)(b) Kelyphite (kel) coronas around olivine and cloudy plagioclase as a result of silicate and fluid inclusions Silicate inclusions tend to be localizedat grain boundaries and along microcracks (sample 94 MA2) (c) VHT alteration marked by brown amphibole (hb) alteration of clinopyroxene in anupper gabbro (d) HT alteration marked by chlorite (chl) (external rim) and pale amphibole (am) and magnetite (mt) (internal rim) coronas aroundolivine (e) Total replacement of olivine by an assemblage of tremolite (am)thornmagnetite (mt)thorn plagioclase (pl) (f ) Total replacement of olivine by aniddingsitic mixture (idg) inside a VHT recrystallized corona of a mosaic of orthopyroxene
BOSCH et al HYDROTHERMAL CIRCULATION IN OMAN OPHIOLITE
1185
fluids responsible for the hydrothermal alteration (Nicolaset al 2003) The iddingsite replacement is typicallyheterogeneous at the thin-section scale being preferen-tially associated with microcracks Serpentinization alsodevelops from microcracks but it can pervasively invadethe entire thin section The microcracks are commonapproximately 25 of the studied thin sections displayone or more Ignoring the cracks related to LT alterationand considering only the cracks related to the HT hydro-thermal alteration this fraction is still around 20
Distribution of high-T alteration facies
Figure 4 shows the distribution of alteration facies as afunction of depth in the crust along wadi sections throughthe Oman---UAE ophiolite offering continuous exposures(see details in Fig 1) Figure 4 excludes the LT faciesbecause (1) being based on the presence of olivine ser-pentinization only throughout our compilation its dis-tribution is biased by the decreasing amount of olivineupsection and (2) this study concerns high-temperaturealteration processes Despite the fact that the scale ofsampling may exceed the scale of heterogeneity evidencefor both VHT alteration and lsquono alterationrsquo increaseswith depth Conversely HT alteration is predominantin the middle and upper parts of the gabbro sectionThis observation is valid throughout the wadi sectionsstudied with the exception of the Wadi Tayin massifwhich shows no clear gradation with depthFigure 5 is another representation of alteration-facies
distribution with depth based on the detailed study of414 thin sections The samples are classified according totheir level in the gabbro section sheeted dyke root zonegabbros upper gabbros middle gabbros lower gabbrosand Moho gabbros (the Moho gabbros have beengrouped together with the gabbro lenses interlayered inthe dunites of the MTZ) Values represent the percent of
2 Km
3
4
5
6
7
Aswad Fizh Hilti Haylayn Samail W TayinNakhl
High T
Very High T
0 High T
Upper Crust
LID
Lower Crust
MohoMTZ
DEPTH1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27
Fig 4 Depth of penetration of hydrothermal circulation in gabbros along the wadi sections numbered in Fig 1 referring to alteration faciesdescribed in Fig 3 Depths are recorded below the palaeo-seafloor LID corresponds to the upper crust including sheeted dykes and volcanics
0
10
20
30
40
50
60
70
80
90
100
SD root
zone
upper
gabbros
middle
gabbros
lower
gabbros
MTZ
BHb-GHb CPX
Chl-Amph core OL
Chl-Amph rim OL
0
5
10
15
20
25
30
SD root
zone
upper
gabbros
middle
gabbros
lower
gabbros
MTZ
Opx OL
BHb OL
No VHTHT
Unaltered OL core
HT
VHT
(a)
(b)
Plst def
gabbros
Fig 5 Compilation of hydrothermal alteration features as a function ofdepth in the gabbro section Percentages represent the occurrence of agiven type of alteration versus the number of thin sections examined foreach selected level root of the sheeted dykes (SD) upper gabbros middlegabbros lower gabbros MTZ (Moho Transition Zone) gabbros on atotal amount of 414 thin sections (Opx orthopyroxene Amph amphi-bole OL olivine BHb brown hornblende GHb green hornblendeChl chlorite Plst def plastic deformation) (a) Distribution of the HTalteration (in ) (b) Distribution of the VHT alteration (in )
JOURNAL OF PETROLOGY VOLUME 45 NUMBER 6 JUNE 2004
1186
a given alteration facies as defined previously out of thetotal number of thin sections examined for a given levelMore refined conclusions can be drawn from this figureAs seen in Fig 5a the HT alteration decreases down-
wards whereas it affects most of the upper gabbroicsection (sheeted dyke root zone and upper gabbros)The fraction of unaltered olivine cores can be used asan index to estimate the extent of alteration This indexshows that the HT alteration is more penetrative andintense in the upper gabbros a conclusion alreadyreached by several workers (eg Gregory amp Taylor1981 Stakes amp Taylor 1992 Kawahata et al 2001)This was also noted by Manning et al (1996a 1996b)Figure 5b shows that the VHT alteration marked by
orthopyroxene or brown hornblende coronas aroundolivine increases with depth The correlation betweenthe VHT alteration and plastic deformation also suggeststhat the VHT alteration may coincide with the develop-ment of plastic flow at the expense of the magmatic flow
at temperatures just below the solidus In Fig 5b thefraction of gabbros where there is no VHT---HThydrothermal alteration increases downsection indicat-ing that the VHT---HT alteration is more localizeddownsection
Gabbroic dykes and veins
The occurrence of gabbroic dykes in the layered gabbrosof the Samail ophiolite is ubiquitous We describe gab-broic dykes and sills either plagioclase---clinopyroxeneand brown amphibole-bearing dykes or microgabbrosand amphibolitized dolerite dykes and sills correspond-ing to temperatures up to 975C (see below) Identifica-tion of gabbroic dykes in the field is not alwaysstraightforward because they may be obliterated bysuperimposition of greenschist-facies alteration (Fig 6a)Selective greenschist-facies alteration has frequently beenobserved either in the centre of a mafic dyke or alongboth margins (Fig 6b) as well as a continuous transition
(b) (c)(a)
(d) (e)
thulite
pistacite
white microvein
white vein
green vein
green vein
gabbroic dike
epidote vein
gabbroic dike
Fig 6 Field relationships of dykes and veins (scale bar represents 10 cm) (a) Thulite---pistacite vein cross-cutting a foliated gabbro matrix Reactionmargins marked by the development of secondary clinopyroxene (01 OA36b) (b) Comb-like structure in gabbroic dyke A pink epidote vein followsthe margin of the dyke (01 OA15f) (c) Along-strike transition from a gabbroic dykelet to a hydrothermal green amphibole vein (d) White prehnitevein (e) Green amphibole vein cross-cut by white prehnite vein
BOSCH et al HYDROTHERMAL CIRCULATION IN OMAN OPHIOLITE
1187
along a single crack from a mafic dyke to a hydrothermalvein (Fig 6c) We define hydrothermal veins as cracksfilled with greenschist-facies minerals that induce meta-morphic reactions in the adjacent country rocks andgabbroic dykes as planar intrusive bodies produced bythe crystallization of magma Following Nehlig amp Juteau(1988) we distinguish low-T greenschist-facies lsquowhiteveinsrsquo made of prehnite epidote chlorite and actinolite(Fig 6d) and lsquogreen veinsrsquo in which a darker greenamphibole predominates (Fig 6e) corresponding respec-tively to lower (195---410C) and higher (400---530C)temperatures (Nehlig amp Juteau 1988)The gabbroic dykes are typically a few centimetres
across composed of plagioclase clinopyroxene andbrown
hornblende They are coarse-grained to pegmatitic andcommonly display a comb-like structure (Fig 6b)Although the spacing between the dykes in the field ishighly variable the average is around 10m Gabbroicsills grade from clear intrusions to irregular and diffusepegmatitic gabbro patches up to a few metres in theirlarger dimension (Fig 7a)The distribution in the field of brownish pargasite
which appears black and glassy in outcrop is critical intracing the origin of the gabbroic dykes It is first observedat any level in the host lower and middle gabbros as largepoikilitic patches (Fig 7b) locally associated with simil-arly poikilitic patches of clino- and orthopyroxene(Fig 7a) These patches contain small aligned tablets of
(a) (b) (c)
g(e)(d)
alignment ofalignment of amphiboleamphibole oikocrystsoikocrysts
hwhite vein
hblpl
pcpx
brownbrownamphiboleamphiboleoikocrystoikocryst
eediabasedikdike
hb-plhb-plreactionreaction
imarginspbrown amphibole rich
gabbroic dikegabbroic dike
pbrown amphibole veins
Fig 7 Field relationships of dykes (scale bar is 10 cm long) (a) Irregular coarse to pegmatitic patches in a foliated gabbro matrix (01 OA42d)(b) Alignment of black amphibole oikocrysts in a foliated gabbro matrix (c) Local concentration of brown amphibole in a gabbroic dykelet Theplagioclase laths marking the foliation in the enclosing gabbro rotate into parallelism with the gabbroic dykelet (d) Two diabase dykes assimilatingthe enclosing layered gabbro with plagioclase---amphibole reaction margins (e) Pargasite-rich gabbroic dyke with wall reactions (01 OA19) (Notesmall pargasite---plagioclase veins) pl plagioclase cpx clinopyroxene hb hornblende
JOURNAL OF PETROLOGY VOLUME 45 NUMBER 6 JUNE 2004
1188
plagioclase oriented parallel to the larger plagioclaselaths defining the magmatic foliation of the gabbro(Fig 7c) Clinopyroxene oikocrysts surrounding idio-morphic plagioclase crystals are common in the upper-level gabbros they mark the end of the magmatic flowand crystallization growing before the pargasite andorthopyroxene oikocrysts as shown by their partialreplacement by pargasite Their crystallization marksthe transition at suprasolidus conditions from a dry to awet environment In the uppermost gabbros those dykesthat are typically altered in the HT hydrothermal faciesgrade upwards into the so-called lsquoisotropicrsquo or lsquovari-texturedrsquo gabbrosThe microgabbro and amphibolitized dolerite dykes
differ from the typical diabase dykes commonly cuttingthe gabbro section that have chilled margins and thusprobably intruded enclosing rocks already at temperat-ures below 450C In microgabbro and amphibolitizeddolerite dykes the absence of chilled margins and thedevelopment of dioritic reaction margins in the adjacentgabbro (Fig 7d and e) suggest that they were emplaced ingabbros that were still at temperatures above 750C(Nicolas et al 2000)
ANALYTICAL TECHNIQUES
Major and trace elements
Major element compositions were determined by elec-tron microprobe at the University of Montpellier II usinga Camebax SX100 Instrument calibration was per-formed on natural standards and operating conditionswere 20 kV accelerating potential 10 nA beam currentand 30 s counting timeRb and Sr concentrations in whole rock and separated
mineral fractions were measured by conventionalnebulization inductively coupled plasma mass spectro-metry (ICP-MS) using a VG Plasmaquad 2 (ISTEEMMontpellier) Digestion procedures followed thoseoutlined by Ionov et al (1992)
O---Sr isotopes
The samples selected for the detailed study wereextracted from gabbroic dykes or veins inside gabbrosand cut with a diamond saw into 2---3 cm fragmentsbefore being crushed into 05---1 cm fragments Forwhole-rock analysis small pieces were further crushedinto powder in an agate bowl For pure mineral analysisthe fragments were crushed with a manual agate mortarTo avoid or minimize the mixing of selected separateminerals with other phases that can bias the results astrict protocol of mineral selection was applied A smallsize fraction of 125---160 mm was used and minerals werefirst separated using a Frantz isodynamic magneticseparator Concentrates recovered (cpx plagioclase
amphibole epidote) were subsequently purified by care-ful hand-picking in alcohol under a binocular micro-scope At this stage mineral separates were very pureand only flawless minerals free of cracks and visibleinclusions were kept for isotopic analyses To ensurefurther mineral integrity these pure mineral separateswere ultrasonically washed in dilute 15N HCl and rinsedseveral times with ultra-pure water This stage potentiallyremoves adsorbed secondary micro-phases Previousstudies (eg Lecuyer amp Reynard 1996) demonstratedthat pyroxene and amphibole can be intermixed on avery fine scale with other phases (such as a range ofamphibole compositions talc Fe-oxide) Although thispossibility cannot be ruled out the procedure used inthis study makes it unlikely that complex minerals passedthrough the selection
Sr isotopic compositions
Approximately 50mg of whole-rock powder or separatedminerals were digested in a Teflon lsquobombrsquo with a mixtureof triple distilled 13N HNO3 and 48 HF with a fewdrops of HClO4 Two aliquots were separated one forthe determination of Sr isotopic composition and theother for Sr and Rb contents Sr was separated using anextraction chromatographic method modified from Pinet al (1994) Concentrations were measured by isotopedilution using 84Sr---87Rb mixed spikes To remove tracesof the late low-temperature seawater alteration and todetermine the primitive whole-rock Sr isotopic composi-tion leaching experiments were conducted on a fewsamples following the procedure described by Bosch(1991) The strontium isotopic compositions of leachateshave been analysed in the attempt to check for the leach-ing efficiency During these tests four Sr isotope composi-tions were measured which correspond to the unleachedsample the two liquids extracted after acid-leaching stepsand the final solid residue Leaching experiments wereconducted in two steps first with 6N HCl for 8min at70C second with 25N HCl for 30min at 70C Afterleaching the leachates were evaporated to dryness andthe solid residues digested similarly to the unleachedsamplesSr isotopic compositions were measured on a fully
automated VG Sector mass spectrometer housed at theUniversity of Montpellier II except for the Sr isotopicdata from the leaching experiments which have beenmeasured at the Universitee de Bretagne Occidentale(Brest) To assess the reproducibility and accuracy of Srisotopic measurements the NBS 987 standard was run12 times during this study the average value was0710245 with a precision better than 0000010 (2s)87Sr86Sr ratios are corrected for mass discrimination bynormalizing to a 86Sr88Sr value of 01194 Sr total blankconcentrations were less than 60 pg Initial 87Sr86Sr
BOSCH et al HYDROTHERMAL CIRCULATION IN OMAN OPHIOLITE
1189
ratios were calculated by correcting the measured ratiosfor accumulation of 87Sr produced by in situ 87Rb radio-active decay since 95Ma the assumed age of crystalliza-tion of the Oman ophiolite (Hacker 1994)
Oxygen isotopes
The powdered samples were introduced in Ni tubesunder dry air where they were reacted with BrF5 toform oxygen Oxygen was separated from other gasesby cooling the resulting mixture at liquid nitrogen tem-perature (77 K) that retained other volatile gases Oxygenwas further purified by trapping it on a cooled 5A mole-cular sieve at 77 K then quantified to calculate the yieldof the isotopic analyses and finally transferred to the massspectrometer (Finnigan Mat Delta E) where intensitiesassociated with masses 32 and 34 were measured todetermine d18O The isotopic composition of a sampleis given as dsample frac14 (RsampleRstandard 1) 1000 in permil unit where R is 18O16O and the standard isSMOW Analytical errors on the oxygen isotope ratioare 02 (2s)
RESULTS
Seven samples representative of the range of gabbroicdykes and five samples representative of the lower gab-bros two of them representing the margins of gabbroicdykes were selected for major element thermometricand Sr---O isotopic analyses (Tables 1---3) These samplesoriginate from wadi sections marked by bold lines inFig 1 Wadis Halhal Mayah Al Abyad Warihya andMaqsad structurally belong to a new ridge segment open-ing in slightly (1Myr) older lithosphere Wadi Gideahis in older lithosphere
Petrographic and isotopic characteristicsof the analysed samples
Olivine gabbro from the MTZ in MaqsadSamail Massif
Sample 94 MA2 is devoid of low-temperature alterationand only exhibits the discrete signature of the VHT---HTalteration The orthopyroxene corona around olivine(Fig 3a) is relayed by kelyphitic rims at the contactwith plagioclase (Fig 3b) Issuing from these rims aremicrometre-sized pale green amphiboles identified ascummingtonite and actinolite which penetrate the sur-rounding plagioclase in microcracks 004mm wide or atgrain boundaries (Fig 3b) similar to the microcrack sys-tem described by Manning et al (2000) A large numberof the plagioclase crystals have a cloudy core as a result ofthe concentration of fluid and mineral inclusions amongwhich diopside has been identified during microprobeanalysis Olivine cores are free of serpentine but theyare largely oxidized along a microcrack network Ortho-pyroxene coronas have a MgOpx number of 78---79
slightly higher than in the primary olivine cores withMgOl number of 74---75 The magmatic clinopyroxeneis totally fresh and probably not in equilibrium with thereactive orthopyroxene This sample has the lowest 87Sr86Sr initial ratio of the studied whole rocks (070322)identical to coexisting clinopyroxene (070324) and pla-gioclase (070322) This observation suggests the lack of alate superimposed LT hydrothermal alteration as it isvery unlikely that an LT hydrothermal event could havetotally equilibrated the Sr isotopic compositions of prim-ary minerals such as plagioclase and clinopyroxeneMoreover the whole rock (WR) clinopyroxene andplagioclase have similar initial Sr isotopic ratios despitevery different Sr contents Accordingly this ratio isthought to reflect the signature acquired during the crys-tallization of these minerals and is higher than the Omanmantle Sr isotopic value (McCulloch et al 1981) Thed18O values of the WR (50) and plagioclase (48) aredistinctly lower than primary magmatic values Indeedin normal mid-ocean ridge basalts (MORB) the WRd18O value is 57 02 with a corresponding plagio-clase d18O ranging from 55 to 62 ( Javoy 1980Gregory amp Taylor 1981)
Lower gabbro from Wadi Wariyah Wadi Tayin Massif
Sample 97 OA128b belongs to a layered sequence in thelower gabbro series injected by anorthositic sills Thesample was collected at the tip of one of these sillsbetween layers enriched in clinopyroxene The gabbrois composed of 60 plagioclase and 40 clinopyroxeneDespite the absence of olivine the WR has a relativelyhigh Mg content (Table 3) A series of low-T (prehnite)microveins 01mm thick cross-cut the gabbro perpendi-cular to the layering and foliation The margins of theveins are devoid of alteration This gabbro has the sameSr isotopic composition as 94 MA2 Nevertheless theWR and associated plagioclase have Sr isotope signatures(070333 and 070328) lower than the coexisting clino-pyroxene (070422) The Sr contents of the WR plagio-clase and clinopyroxene are 127 ppm 120 ppm and20 ppm respectively thus making the clinopyroxenemore sensitive to late-stage contamination Clinopyrox-ene and plagioclase oxygen isotope compositions (523and 414 respectively) are also lower than typicalmantle values Similar to the previous sample alterationat high temperature by a low d18O and high 87Sr86Srfluid is required to explain such characteristics
Amphibole gabbro patch Wadi Gideah Wadi TayinMassif
Sample 01 OA41d2 is from a laminated gabbro com-posed of millimetre-sized layers of clinopyroxeneand plagioclase Anatectic patches (Fig 7a) borderinganorthositic layers are formed of coarse-grained
JOURNAL OF PETROLOGY VOLUME 45 NUMBER 6 JUNE 2004
1190
Table1Locationoftheanalysed
samplesandsummaryoftheirpetrographicandSr---O
isotopiccharacteristics
Sam
ple
nam
eSite
Locationin
gab
broic
section
Typ
eofrock
Distance
to
Moho(km)
Parag
enesis
Alteration
87Sr
86Srinitial1
d18O
()2
94MA2
Maq
sad
From
MTZ
Layered
gab
bro
0Olivine(M
gno74---75)3
OPXco
ronas
(Mgno78---79)
WRfrac14
070322
WRfrac14
thorn495
CPX
Palegreen
Am
(cumact)
CPXfrac14
070324
nd
Plagioclase(A
n65---80)4inversezoning
Nolow-T
alteration
Plfrac14
070322
Plfrac14
thorn479
Solidfluid
inclusionsin
Pl
97OA128b
Wad
iWaryiah
From
lower
gab
bro
Layered
gab
bro
1CPX
Low-T
alteration
WRfrac14
070333
nd
Plagioclase(A
n85---87)
Prehnitemicroveins
CPXfrac14
070422
CPXfrac14
thorn523
Plfrac14
070328
Plfrac14
thorn414
01OA41d2
Wad
iGideah
From
upper
gab
bro
Gab
bro
patch
in
laminated
gab
bro
36
CPXrecrystallized
Plagioclaseidiomorphic
(An57---87)norm
alzoning
Black
pargasitic
honblende
WRfrac14
070351
CPXfrac14
070378
Plfrac14
070426
WRfrac14
thorn480
nd
Plfrac14
thorn504
Hbdfrac14
070355
nd
01OA19aamp
dWad
iWaryiah
Inlayeredgab
bro
Gab
broic
dyke
01
Olivine
OPXpoikilitic
WRfrac14
070341
WRfrac14
thorn567
CPX
Black
pargasite
Hbdfrac14
070324
Hbdfrac14
thorn287
Plagioclase(A
n73---95)
Hornblendepoikilitic
Plfrac14
nd
Plfrac14
thorn10 09
Hem
atite
WRfrac14
070418
nd
Green
Mg-hornblende
Acthornblende
Epidote
02OA90b
Wad
iAl-Abyad
Inlayeredgab
bro
Dioriticdyke
015
Green
Mg-hornblende
Hbdfrac14
070427
Hbdfrac14
thorn239
Plagioclase(A
n83---87)
Plfrac14
070387
Plfrac14
thorn498
Solidfluid
inclusionsin
plagioclase
01OA15f
Wad
iWaryiah
Inlayeredgab
bro
Comb-textured
gab
broic
dyke
025
CPXidiomorphic
Plagioclase(A
n95---99)norm
alzoning
BrownMg-H
ornblende
WRfrac14
070458
nd
02OA43a
Wad
iMayah
Inlayeredgab
bro
vein
Green
amphibole
1Palegreen
Mg-hornblendeacicular
Plagioclase(A
n91---97)in
reactionmargin
Hbdfrac14
070431
Hbdfrac14
thorn310
01OA36b
Wad
iGideah
Inlayeredgab
bro
epidosite
dyke
25
CPX
Epidote
WRfrac14
070519
nd
Plagioclase
Epfrac14
070554
nd
CPXclinopyroxene
Plplagioclase
OPXorthopyroxene
Amam
phibolecu
mcu
mmingtonite
actactinolite
WRwholerockHbdhornblende
Epep
idotend
notdetermined
187Sr
86Srinitialratioscalculatedat
95Ma(H
acker1994)Errors
ofthemeasuredratiosareindicated
inTab
le4
2Analyticalerrors
oftheoxygen
isotopevalues
are02
(2s)
3Mgnumber
frac14atomicMg(Mgthorn
Fe)
4Percentageofan
orthitein
plagioclase
BOSCH et al HYDROTHERMAL CIRCULATION IN OMAN OPHIOLITE
1191
(millimetre-sized) recrystallized clinopyroxene largelyaltered to light green magnesio-hornblende and idio-morphic plagioclase Locally primary black pargasitichornblende develops including idiomorphic normallyzoned plagioclase (Table 4) This gabbro exhibits similar87Sr86Sr ratios for the WR and pargasite (070351 and070355 respectively) These values are interpreted asrepresentative of the melt from which this amphibolecrystallized The clinopyroxene has a slightly higher Srisotope composition of 070378 consistent with the occur-rence of the light green Mg-hornblende rim Plagioclaseis significantly more radiogenic (070426) related to sec-ondary destabilization evidenced by the idiomorphichabit The Sr isotope compositions of the liquidsextracted from the acid leaching experiments of theWR (L1 and L2) are very similar 070358 and 070365respectively The 87Sr86Sr ratio of the residue obtainedafter leaching experiments is 070335 lower than boththe pargasite and unleached WR This value can beconsidered as the unaltered isotopic composition of thissample The d18O values measured for WR and coexist-ing plagioclase are lower than normal MORB magmaticvalues The 18O depletion in the plagioclase can be fairlywell explained by high-temperature hydrothermal inter-action between an oxygen-depleted fluid and the magmaAccording to the kinetics of hydrothermal exchange ofoxygen (Gregory amp Taylor 1981 Gregory et al 1989)plagioclase exchanges its oxygen isotopes with the fluidadjusting readily to the hydrothermal conditions Forhigh temperatures of exchange with seawater-derivedfluids (d18O between 0 and thorn2) the magmatic plagio-clase will have its primary d18O value lowered whereasthe d18O value for lower temperatures of exchange(5300C) will increase The amplification of the d18Oshift increases with the intensity of the hydrothermalalteration
Gabbroic dykes intrusive in lower gabbros from WadiWariyah Wadi Tayin Massif
Gabbroic dykes 01 OA19a and 01 OA19d are intrusiveinto the lower gabbros They are 3---4 cm wide discord-ant to the foliation of the host gabbros (Fig 7e) with asharp margin marked by a brown amphibole fringe Theprimary phases in the dykes (Table 1) locally preservedas elongated aggregates of a 01mm grain size mosaicassemblage at the dyke margins grade to millimetre sizein the central part of the dyke The WR major elementcomposition is Mg enriched compared with the enclosinggabbro (Table 3) Brown pargasitic amphibole oikocrystsmake up 50 of the modal composition of the dyke Indyke 01 OA19a poikilitic orthopyroxene may developinstead of brown amphibole Finally green magnesio-hornblende grows either as oikocrysts at the expense ofclinopyroxene or in association with epidote as an altera-tion product of plagioclase These low-amphibolite-faciesminerals represent the lowest-grade paragenesis recogn-ized in these dykes and are most developed in sample01OA19aTheamphibole composition showszoning fromTi-rich pargasitic hornblende to magnesio-hornblendeand actinolitic hornblende (Fig 8c) recording pro-gressive cooling of the dyke The plagioclase compositionis extremely heterogeneous (Fig 8b) varying from An95a composition similar to that of the plagioclase in the hostgabbro (Fig 8a) to An50 for the plagioclase and asso-ciated epidote inclusions in the poikilitic actinolitic horn-blende Sample 01 OA19d has an initial 87Sr86Sr ratioslightly higher for the WR (070341) than for the parga-site (070324) The Sr isotopic ratios of liquids extractedafter two steps of WR acid-leaching (L1 and L2) are070357 and 070341 respectively whereas the residueyields a 87Sr86Sr ratio of 070327 very close to thepargasite and plagioclase values The latter is similar tothe isotopic composition measured for the massive gab-bro 94 MA2 and may be taken as close to the primarymagmatic value This Sr isotopic ratio is more radiogenicthan the inferred Oman mantle-source value (McCullochet al 1981) The altered part of this sample has a radio-genic Sr isotopic composition (070418) related to thelow-amphibolite-facies alteration This gabbroic dykehas seemingly preserved a magmatic whole-rock d18Ovalue (567) but contains by far the highestd18O plagioclase (1009) of all the samples analysedThis plagioclase coexists with low d18O pargasite(thorn287) Similarly high values (d18Oplagioclase 4 8)have been previously measured for gabbroic dykes dia-bases sheeted dykes and plagiogranites from the Omanophiolite (Gregory amp Taylor 1981 Stakes amp Taylor1992) This 72 oxygen isotope fractionation betweenplagioclase and amphibole cannot possibly represent oxy-gen equilibrium at any plausible temperature This maybe explained by different exchange kinetics for these twominerals at different temperatures at high waterrock
Table 2 Average temperatures ( C 40C)
calculated for plagioclase---amphibole pairs
using the geothermometer of Holland amp
Blundy (1994)
Pargasite Magnesio-hornblende
rim core rim core
01 OA41d2 878 953 ------- -------
01 OA19a 935 974 825 869
01 OA19d 890 933 789 -------
02 OA37b ------- ------- 816 849
02 OA90b ------- ------- 859 833
JOURNAL OF PETROLOGY VOLUME 45 NUMBER 6 JUNE 2004
1192
ratios This supports the occurrence of two distincthydrothermal events one at high temperature and alater stage at lower temperature responsible for thestrong 18O enrichment in the coexisting plagioclase
Dioritic dyke associated with a diabase dyke in lowergabbros from Wadi Halhal Haylayn Massif
Sample 02 OA37b belongs to a group of brownamphibole---plagioclase lenses or reaction margins asso-ciated with diabase intrusives (Fig 7d) in lower layeredgabbros Idiomorphic brown hornblende (tschermakite)crystals are elongated in the plane of the dyke represent-ing 80 of the modal composition They are associatedwith an interstitial plagioclase matrix The browntschermakite has greenish rims Plagioclase has a cloudyappearance due to micrometre-size fluid inclusionsexcept along its margins and internal microcracks Bothhornblende and plagioclase exhibit evidence of plasticdeformation The Sr isotopic ratio for the pargasite andplagioclase is 070348 and 070420 respectively Thepargasite exhibits a Sr signature closest to that of theOman primary magmatic value (McCulloch et al 1981)Nevertheless it is too high to be attributed to anunmodified MORB-source mantle signature This sug-gests the addition of an external fluid component in goodagreement with the shift of oxygen isotopic compositionto a low value (thorn39) suggesting a high-temperatureexchange with a low d18O fluid The Sr isotope composi-tion of the plagioclase is significantly higher than that of
the coexisting pargasite probably as a result of the low-temperature alteration
Dioritic dyke cross-cutting lower layered gabbros in WadiAl-Abyad Nakhl Massif
Sample 02 OA90b is a 1 cm thick dioritic dyke cross-cutting the lower layered gabbros It grades into a greenamphibole vein as illustrated in Fig 6c The dyke is com-posed of 2 cm long dark green idiomorphic magnesio-hornblende crystals growing in the plane of the dykeand plagioclase concentrated at the margin of the dykeThe plagioclase in the dyke and in the enclosing gabbrocontains micrometre-size fluid and mineral inclusionssimilar to those observed in sample 94 MA2 The plagio-clase and green hornblende have higher Sr isotopic ratios(070387 and 070427 respectively) and lower d18Ovalues (thorn498 and thorn239 respectively) than the normalMORB-source mantle value As observed for previoussamples this supports interaction with an external fluidcomponent
Comb-textured gabbroic dykes in the lower gabbros fromWadi Wariyah Wadi Tayin Massif
Sample 01 OA15f (Fig 6b) is from Wadi Wariyah andrepresents a 2 cm wide dyke intruding the lower gabbrosThis type of comb-textured dyke developed in theclinopyroxene---plagioclase stability field It containslarge idiomorphic clinopyroxene crystals which arepartially replaced by brownish magnesio-hornblendeThe plagioclase is extremely calcic with a slight inverse
Table 3 Whole-rock major element compositions of massive gabbros and dykes determined by X-ray fluorescence
(wt )
Sample 94 Ma2 97 OA128b 01 OA41d2 01 OA19d 01 OA19a 01 OA15f 01 OA36b 01 OA36b
Rock type Gabbro Gabbro Gabbro patch Gabbroic dyke Gabbroic dyke Comb-textured
gabb dyke
Epidosite dyke Epidosite dyke
(margin)
SiO2 4830 4815 4749 4524 4339 4799 389 4371
Al2O3 2785 2032 2438 1474 1246 1192 2752 1646
Fe2O3 328 266 404 981 1204 592 381 623
MnO 003 004 005 014 016 006 008 014
MgO 405 737 427 1031 1686 1326 187 739
CaO 1289 1908 1477 1235 1156 166 2471 2339
Na2O 292 123 257 258 111 075 000 013
K2O 000 000 009 006 000 000 000 000
TiO2 005 017 063 230 064 086 013 042
P2O5 008 007 013 018 009 013 007 010
LOI 033 076 138 216 161 237 274 190
Total 9978 9985 998 9987 9992 9986 9983 9987
LOI loss on ignition gabb gabbroic
BOSCH et al HYDROTHERMAL CIRCULATION IN OMAN OPHIOLITE
1193
Table4Majorelementcomposition
aofselectedam
phibolespaired
withplagioclasecompositionsusedforthermometry
Sam
ple
nam
e1Mode2
SiO
2TiO
2Al 2O3
Cr 2O3
FeO
MnO
MgO
CaO
Na 2O
K2O
Total
Si
AlIV
AlVI
Ti
Fe3
thornCr
Mg
Fe2
thornMn
Na
KCa
XAn3
T(C)4
01OA19a
Mg-hornblende
core
45 60
26
10 78
009
860
015
16 56
12 00
212
007
96 72
6552
1448
0366
0083
0552
0004
3409
0594
0016
0611
0018
1843
0903
899
Mg-hornblende
core
47 99
263
850
002
897
011
16 61
12 10
160
020
96 36
6894
1106
0334
0028
0483
0002
3557
0594
0014
0446
0036
1862
0892
834
Mg-hornblende
rim
51 17
247
596
001
761
014
18 11
12 19
103
010
96 47
7257
0743
0253
0016
0414
0001
3828
0488
0016
0284
0018
1853
0894
820
Mg-hornblende
rim
47 50
291
920
004
880
014
16 46
11 95
174
008
96 07
6835
1165
0396
0015
0506
0005
3529
0552
0017
0486
0015
1843
0887
825
Mg-hornblende
rim
49 57
269
742
000
799
017
18 07
12 02
133
014
96 88
7013
0987
0261
0018
0575
0000
3811
0371
0020
0365
0026
1821
0860
831
Mg-hornblende
core
47 83
181
872
003
840
011
14 48
12 18
159
008
93 59
6814
1186
0279
0018
0644
0004
3711
0357
0013
0438
0015
1859
0907
874
Pargasite
rim
42 15
249
11 87
024
10 07
013
13 84
11 43
263
032
96 61
6188
1812
0242
0434
0299
0028
3028
0937
0016
0749
0059
1797
0804
972
Pargasite
rim
42 22
260
11 86
026
10 21
011
13 71
11 48
253
032
96 59
6201
1799
0254
0429
0299
0030
3001
0955
0014
0720
0060
1807
0803
962
Pargasite
core
42 05
263
11 45
030
10 19
011
13 80
11 47
246
032
96 46
6189
1811
0175
0478
0299
0035
3029
0956
0014
0702
0060
1809
0767
974
Pargasite
rim
42 36
247
11 90
016
945
013
14 16
11 76
249
026
95 98
6239
1761
0305
0366
0276
0019
3109
0888
0016
0710
0049
1856
0818
926
Pargasite
core
42 50
291
11 64
016
984
015
14 22
11 41
256
034
96 69
6219
1781
0227
0426
0322
0018
3103
0883
0018
0727
0063
1789
0812
974
Pargasite
rim
42 76
270
12 03
017
920
012
14 87
11 82
244
025
96 73
6224
1776
0880
0335
0368
0020
3226
0752
0015
0689
0047
1843
0841
941
Pargasite
rim
42 13
181
13 54
016
921
009
12 77
12 13
254
032
96 98
6172
1828
0510
0450
0000
0018
2788
1129
0011
0721
0059
1905
0841
874
01OA19d
Mg-hornblende
49 26
047
624
011
11 46
017
15 39
12 21
108
009
96 48
7137
0863
0202
0051
0437
0013
3323
0951
0021
0302
0018
1896
0768
771
Mg-hornblende
46 03
287
932
018
906
017
14 77
12 87
193
007
97 28
6672
1328
0265
0313
0046
0021
3191
1053
0021
0542
0012
1999
0758
808
Pargasite
41 89
429
11 65
008
11 03
017
13 52
11 37
259
017
96 77
6159
1841
0179
0474
0345
0010
2963
1012
0021
0739
0032
1791
0770
975
Pargasite
rim
42 80
340
11 16
009
11 01
018
13 73
11 67
240
022
96 67
6293
1707
0226
0376
0322
0010
3009
1032
0022
0684
0041
1839
0605
880
Pargasite
core
42 58
351
11 27
006
11 63
018
13 51
11 46
239
022
96 82
6257
1743
0209
0387
0391
0007
2959
1039
0023
0680
0042
1804
0595
893
Pargasite
rim
42 14
393
11 28
006
11 71
014
13 35
11 33
235
033
96 62
6214
1786
0174
0435
0391
0007
2935
1054
0018
0673
0062
1790
0519
901
Pargasite
core
41 56
420
12 05
006
11 57
016
12 51
11 35
249
027
96 24
6168
1832
0277
0469
0276
0007
2768
1160
0021
0718
0052
1805
0537
882
Pargasite
core
41 33
438
11 49
008
11 66
015
13 35
11 45
264
026
96 80
6105
1894
0107
0487
0368
0009
2941
1073
0019
0757
0048
1813
0537
942
Pargasite
41 08
446
12 39
012
11 35
017
13 13
11 36
263
015
96 85
6049
1951
0199
0494
0368
0014
2882
1030
0021
0752
0027
1792
0758
975
02OA37b
Mg-hornblende
rim
45 15
260
984
002
13 28
019
14 01
10 87
128
009
97 33
6527
1473
0203
0283
0690
0002
3018
0915
0023
0360
0016
1683
0562
828
Mg-hornblende
rim
44 72
263
988
004
13 23
021
14 01
10 78
133
009
96 92
6494
1506
0185
0287
0713
0005
3032
0894
0025
0374
0018
1677
0593
854
Mg-hornblende
rim
45 68
247
935
004
12 87
020
14 10
11 00
121
011
97 02
6621
1379
0218
0269
0598
0004
3045
0962
0024
0341
0020
1708
0562
815
Mg-hornblende
core
43 94
291
10 87
001
12 60
023
13 73
10 59
142
011
96 39
6410
1590
0278
0319
0644
0001
2985
0893
0028
0401
0021
1655
0710
860
Mg-hornblende
core
44 62
270
10 37
001
12 66
019
14 12
10 58
140
008
96 73
6479
1521
0253
0294
0644
0001
3057
0893
0023
0395
0014
1646
0674
851
Mg-hornblende
rim
47 79
181
796
002
12 09
025
15 22
11 09
100
009
97 33
6866
1134
0214
0196
0506
0002
3260
0947
0030
0280
0016
1706
0503
767
Mg-hornblende
core
45 43
249
10 3
000
11 99
022
14 50
11 14
133
008
97 48
6524
1476
0267
0269
0644
0000
3103
0796
0027
0370
0015
1715
0703
838
JOURNAL OF PETROLOGY VOLUME 45 NUMBER 6 JUNE 2004
1194
Sam
ple
nam
e1Mode2
SiO
2TiO
2Al 2O3
Cr 2O3
FeO
MnO
MgO
CaO
Na 2O
K2O
Total
Si
AlIV
AlVI
Ti
Fe3
thornCr
Mg
Fe2
thornMn
Na
KCa
XAn3
T(C)4
02OA90b
Mg-hornblende
core
48 83
038
745
005
939
010
17 29
12 24
142
006
97 21
6931
1069
0041
0041
0667
0005
3657
0448
0011
0392
0010
1862
0840
856
Mg-hornblende
core
49 49
033
731
003
916
008
17 40
12 57
139
005
97 82
6987
1013
0035
0035
0552
0004
3662
0530
0010
0379
0009
1901
0830
811
Mg-hornblende
rim
48 90
040
753
005
932
013
17 15
12 22
154
005
97 29
6948
1052
0042
0042
0575
0006
3632
0533
0015
0424
0010
1861
0880
869
Mg-hornblende
rim
48 30
044
800
007
962
010
16 86
12 32
161
005
97 35
6873
1127
0047
0047
0598
0008
3576
0546
0012
0443
0010
1879
0870
862
Mg-hornblende
rim
49 12
032
742
010
922
011
17 36
12 29
142
006
97 43
6956
1044
0034
0034
0621
0011
3665
0471
0013
0391
0010
1865
0840
845
02OA43a
Mg-hornblende
rim
48 13
040
789
007
860
007
17 41
12 19
155
006
96 36
6882
1118
0212
0043
0621
0008
3710
0407
0008
0429
0010
1867
-------
-------
Mg-hornblende
rim
50 93
035
643
001
759
006
18 50
12 24
114
003
97 28
7152
0848
0217
0037
0506
0001
3873
0385
0007
0311
0005
1841
-------
-------
Mg-hornblende
rim
50 67
014
559
009
12 33
026
15 28
12 44
068
003
97 50
7260
0740
0204
015
0483
0010
3263
0994
0032
0189
0005
1909
-------
-------
Mg-hornblende
rim
50 34
013
621
009
12 49
027
15 07
12 51
064
003
97 78
7192
0810
0240
0010
0530
0010
3210
0960
0030
0180
0010
1910
-------
-------
Mg-hornblende
rim
45 17
060
11 14
013
988
008
15 78
12 33
211
009
97 31
6473
1527
0355
0640
0644
0015
3371
0540
0010
0588
0016
1892
-------
-------
Mg-hornblende
rim
49 25
032
712
014
845
009
17 84
12 32
137
004
96 95
6983
1020
0170
0030
0620
0020
3770
0380
0010
0380
0010
1870
-------
-------
Mg-hornblende
rim
48 61
032
775
014
975
011
16 81
12 57
139
004
97 50
6905
1950
0203
0034
0621
0016
3560
0537
0013
0382
0008
1914
-------
-------
94Ma2
Anthophyllite
54 98
000
259
001
12 65
037
23 35
199
034
000
96 28
7744
0256
0174
0000
0161
0001
4903
1330
0043
0092
0001
0301
-------
-------
Anthophyllite
55 11
000
296
003
14 09
032
23 66
063
038
000
97 18
7716
0284
0205
0000
0115
0003
4938
1535
0038
0104
0000
0095
-------
-------
Actinolite
48 58
004
954
002
960
014
17 67
999
139
005
97 02
6850
0150
0435
0004
0690
0002
3713
0442
0017
0379
0009
1510
-------
-------
Actinolite
51 54
004
585
000
890
022
20 28
930
090
002
97 05
7214
0786
0178
0005
0598
0000
4230
0444
0026
0244
0004
1395
-------
-------
01OA41d2
Pargasite
rim
43 05
342
10 71
008
11 46
012
13 50
11 54
179
067
96 34
6353
1647
0216
0380
0368
0009
2967
1046
0015
0512
0127
1824
0613
863
Pargasite
rim
42 61
354
11 59
001
10 59
010
13 52
11 87
160
076
96 19
6286
1714
0302
0392
0299
0001
2972
1007
0012
0458
0143
1877
0715
844
Pargasite
core
42 54
401
11 04
006
11 81
013
13 21
11 50
202
039
96 71
6266
1734
0182
0444
0368
0007
2901
1087
0017
0576
0073
1814
0854
979
Pargasite
rim
43 03
385
10 72
002
11 70
014
13 50
11 95
146
080
97 17
6314
1686
0168
0425
0345
0002
2953
1091
0018
0415
0149
1878
0634
852
Pargasite
rim
42 65
341
10 60
006
11 97
014
13 22
11 64
183
068
96 20
6332
1668
0186
0381
0345
0007
2926
1141
0018
0528
0129
1851
0620
869
Pargasite
core
42 82
367
10 60
007
11 93
012
13 42
11 44
206
047
96 60
6315
1685
0158
0407
0391
0008
2950
1080
0016
0588
0089
1808
0615
899
Pargasite
rim
42 2
362
11 10
007
11 42
016
13 09
11 72
179
072
95 89
6282
1718
0229
0405
0299
0009
2905
1123
0020
0518
0137
1869
0715
875
Pargasite
rim
42 42
380
11 03
006
11 77
012
13 00
11 72
188
069
96 49
6283
1717
0209
0423
0299
0007
2870
1158
0015
0539
0131
1860
0817
928
Pargasite
rim
42 21
364
11 14
006
10 73
011
13 53
11 68
177
072
95 59
6277
1723
0229
0407
0322
0007
2999
1012
0014
0511
0136
1862
0736
883
Pargasite
rim
42 25
376
11 03
008
10 93
015
13 66
11 72
185
064
96 07
6257
1743
0182
0418
0345
0009
3016
1009
0019
0531
0121
1860
0766
912
Pargasite
core
42 17
396
10 91
008
11 10
010
13 57
11 49
209
038
95 85
6253
1747
0159
0442
0368
0010
3000
1009
0013
0602
0073
1826
0841
981
aMajorelem
entco
mpositionsarein
weightpercent-------noplagioclasean
alysed
andnotemperature
calculated
1Typ
eofam
phibolean
alysed
isalso
indicated
2Blankindicates
nozoningobserved
3Molefractionofan
orthitein
plagioclasead
jacentto
amphibolegrain
4Tem
perature
ofeq
uilibrium
fortheam
phibole---plagioclasepaircalculatedusingtheHollandamp
Blundythermometer
(1994)Errors
are40
C
BOSCH et al HYDROTHERMAL CIRCULATION IN OMAN OPHIOLITE
1195
zonation from An95 to An99 from core to rim (Table 1 andFig 8a) TheWR Sr isotopic ratio of the sample (070458)is significantly higher than normal MORB values Thiscould be related to the overprint of a low-temperaturealteration event consistent with the petrography of thesample
Green amphibole vein cross-cutting layered gabbros inWadi Mayah Haylayn Massif
Sample 02 OA43a belongs to a series of 1 cm thick greenamphibole veins (as in Fig 6e) cross-cutting the layeredgabbros The veins develop 15 cm thick reaction marginsin the enclosing gabbro and are exclusively composed ofacicular pale green magnesio-hornblende growing per-pendicular to the vein margin in a crack-seal mode(Fig 6f ) The reaction margins are composed of urali-tized gabbro marked by the development of the samepale green magnesio-hornblende in an inhomogeneouscalcic plagioclase matrix (An91---97) The Sr and O isotopiccompositions of this green hornblende (070431 andthorn31) do not correspond to normal MORB-sourcemantle values
Epidosite dyke from Wadi Gideah Wadi Tayin Massif
Sample 01 OA36b is representative of a series of epidoteveins cross-cutting poorly layered gabbros These 2 cmthick veins are composed of pink thulite in their centreand green pistachite at their margins (Fig 6a) Theirsharp contact with the enclosing gabbro is marked bythe development of coarse clinopyroxene crystals alteredin the epidote---greenschist facies This suggests that theepidote vein is replacive in a coarse-grained gabbro dykeThe highest 87Sr86Sr initial ratios of all the samplesstudied are recorded by the WR and coexisting epidoteof this sample (070519 and 070554 respectively)(Table 5) The Sr content of the WR and associatedepidote are also the highest of the present dataset(716 ppm and 764 ppm respectively) The Sr isotopiccomposition of this sample is consistent with a greaterdegree of exchange with a radiogenic component fromCretaceous seawater It is significantly higher than thevalue obtained for an epidosite vein from the Wadi Fizhsection of the Oman ophiolite [070435 with a Sr contentof 488 ppm reported by Kawahata et al (2001)] but ingood agreement with the average of the greenschist-faciesalteration sequence defined by Kawahata et al (2001)
GABBROS
01 km 025 km 1 km 36 km50
60
70
80
90
An
MA2
19d
19a 15f 128b
41d2
(a)
43a17b
15f
90b
37b
19a
19d
40
60
80
100
A
n
01 km 015 km 025 km 1 km
DYKES
(b)
Distance to the Moho
ACTINOLITE
MAGNESIOHORNBLENDE
TSCHERMAKITE
PARGASITEGabbros Dykes41d2MA2
19a d37b15f
90b43a
AlIV
150500
02
08
10
(Na
+ K
) A
(c)
04
06
950
900
850
800
750
70007 12 17
T degC
1000
(d)
AlIV
Fig 8 Plagioclase---amphibole compositions and results of thermometry (a) and (b) plagioclase compositions in layered gabbros and in dykesrespectively as a function of distance to the Moho (see text for sample identification) (c) amphibole compositions in layered gabbros and dykesaccording to Leakersquos (1997) nomenclature (d) equilibration temperatures calculated using the amphibole---plagioclase thermometer of Holland ampBlundy (1994) as a function of AlIV content in amphibole [same symbols as in (c)]
JOURNAL OF PETROLOGY VOLUME 45 NUMBER 6 JUNE 2004
1196
Table5RbandSrcontents87Sr
86Srandd1
8Oisotopiccompositionsofwholerocksandmineralseparatesfrom
samplesofmassivegabbrosdykes
andveinsfrom
theOman
ophiolitealsolistedarecalculated
waterrockratios(W
R)andLOIvalues
Sam
ple
Rb(ppm)
Sr(ppm)
87Rb8
6Sr
87Sr
86Sr
(87Sr
86Sr)i1
d18O
()
LOI(
)2WR
3(87Sr
86Sr)L14
(87Sr
86Sr)L25
(87Sr
86Sr)R6
01OA41d2
Whole
rock
037
1792
0006
0703516
09
070351
thorn480
138
47
0703587
0703651
0703357
Plagioclase
025
1249
0003
0704261
22
070426
thorn504
-------
-------
Pargasite
-------
-------
-------
0703548
20
070355
-------
-------
-------
Clinopyroxene
045
26 0
0050
0703845
19
070378
-------
-------
-------
01OA36b
Dykewhole
rock
0005
7164
50001
0705186
17
070519
-------
274
21 9
0705207
0705112
-------
Epidote
0003
7645
0001
0705536
12
070554
-------
-------
-------
Wallwhole
rock
005
4254
0001
0704637
09
070464
-------
191
-------
0704945
0704675
0704593
02OA43a
Green
hornblende
004
98
0012
0704323
13
070431
thorn310
-------
-------
97OA128b
Whole
rock
003
1274
50001
0703327
17
070333
-------
076
37
-------
-------
-------
Plagioclase
002
1204
50001
0703285
09
070328
thorn414
-------
-------
Clinopyroxene
003
20 0
0004
0704230
09
070422
thorn523
-------
-------
01OA15f
Whole
rock
006
1077
0002
0704582
16
070458
-------
237
13 5
-------
-------
-------
01OA19a
Whole
rock
037
1303
0008
0704189
18
070418
-------
161
96
-------
-------
-------
01OA19d
Whole
rock
058
2032
0008
0703423
08
070341
thorn567
216
415
0703574
0703408
0703271
Pargasite
091
1058
0025
0703273
11
070324
thorn287
-------
-------
Plagioclase
-------
-------
-------
-------
-------
thorn10 09
-------
-------
02OA90b
Plagioclase
004
2385
50001
0703868
11
070387
thorn498
-------
-------
Green
hornblende
010
23 8
0012
0704288
15
070427
thorn239
-------
-------
BOSCH et al HYDROTHERMAL CIRCULATION IN OMAN OPHIOLITE
1197
Table5continued
Sam
ple
Rb(ppm)
Sr(ppm)
87Rb8
6Sr
87Sr
86Sr
(87Sr
86Sr)i1
d18O
()
LOI(
)2WR
3(87Sr
86Sr)L14
(87Sr
86Sr)L25
(87Sr
86Sr)R6
02OA37b
Plagioclase
023
3121
0002
0704204
15
070420
-------
-------
-------
Pargasite
031
41 6
0021
0703505
15
070348
thorn394
-------
-------
94OAMa2
Whole
rock
0016
1750
00003
0703219
15
070322
thorn495
033
31
0703264
0703177
0703158
Plagioclase
0014
1363
00003
0703219
11
070322
thorn479
-------
-------
Clinopyroxene
-------
-------
-------
0703236
13
070324
-------
-------
-------
1Initial8
7Sr
86Srratiocalculatedat
95Ma(H
acker1994)
2LOIisloss
onignition(in)
3Water---rock
ratiocalculatedassumingaclosedsystem
TheeffectiveWR
ratiocanbeexpressed
bythefollowingeq
uation
W=Reffectivefrac14
frac12eth87Sr=
86SrrockTHORN fi
nal
eth87Sr=
86SrrockTHORN in
itial=frac12eth8
7Sr=
86SrseawaterTHORN in
itial
eth87Sr=
86SrrockTHORN fi
nal
ethCrock
initial=C
seaw
ater
initial
THORNwhere(87Sr
86Srrock) finalisthefinalmeasuredSrisotopicratioin
thesample(87Sr
86Srrock) in
itialco
rrespondingto
theinitialm
antlevalue
istakenas
070295(Lan
phere
etal1981)(87Sr
86Srseawater) in
itialistheCretaceousseaw
ater
Srisotopicco
mposition[87Sr
86Sr07073
at90
MaafterBurkeet
al(1982)]C
seaw
ater
initial
istheSrco
ntent
oflsquonorm
alrsquoseaw
ater
andis
takenas
8ppm
(deVilliers1999)
FortheCrock
initialitis
assumed
that
thepresentSrco
ncentrationmeasuredis
thesameas
theinitial
concentration
4Srisotopicratiosmeasuredforliquid
L1extractedafterthefirststep
oftheleachingexperim
ent(6NHClfor8min
at70
C)
5Srisotopicratiosmeasuredforliquid
L2extractedaftertheseco
ndstep
oftheleachingexperim
ent(2 5NHClfor30
min
at70
C)
6Srisotopicratiosmeasuredforresidueobtained
afteratotalaciddigestionachievedaftertheextractionofL1an
dL2leachates
JOURNAL OF PETROLOGY VOLUME 45 NUMBER 6 JUNE 2004
1198
Part of the same sample from the contact between thedyke and the gabbro yields a slightly lower Sr isotopiccomposition (070464) intermediate between the sur-rounding gabbro and the epidote dyke It also has anintermediate Sr content (425 ppm) Acid leaching experi-ments performed on the whole rock of the dyke yield Srisotopic compositions of 070521 and 070512 for thetwo liquids extracted Similar experiments for the samplelocated at the contact with the gabbro yield 87Sr86Srratios of 070494 and 070467 for L1 and L2 liquids TheSr isotopic composition of the residue extracted afterleaching remains high (070459) and very close to theepidosite 87Sr86Sr ratio determined by Kawahata et al(2001) In this sample the primary minerals have beentotally replaced by epidote and secondary plagioclase orquartz Thus the Sr isotopic compositions measured forthis WR sample and its constituent minerals reflect thecomposition of the fluid filling this dyke
Mineral chemistry
Plagioclase
Compared with the host gabbros the dykes show a muchlarger dispersion in their plagioclase compositions (Fig 8aand b Table 4) characterized by more calcic plagioclasethan the enclosing gabbros This tendency is mostextreme in the comb-textured dykes in which the plagio-clase is almost pure anorthite The zoning is generallynormal recording a crystal fractionation process How-ever gabbro 94 MA2 exhibits inverse zoning this tend-ency is also observed in plagioclases from dykes 01OA19a and 01 OA15f The higher calcium content ofthe plagioclase as well as the inverse zoning is consistentwith an increase of water pressure during its crystalliza-tion (Turner amp Verhoogen 1960 Carmichael et al 1974Koepke et al 2003) The particularly high An content inthe comb-textured gabbro dykes 01 OA15f may alsoresult from crystallization in a supercooled magma thisis consistent with the comb texture indicative of fast-growing crystals The lack of brown hornblende suggeststhat crystallization occurred above the temperature ofhornblende stability
Amphibole
The amphiboles analysed in the dykes (Fig 8c andTable 4) show a regular trend in a (Na thorn K)A vsAlIV diagram from titanium-rich (Ti 4 015) pargasitichornblende (brownamphibole) and tschermakite (brown---green amphibole) to low-titanium (Ti5 015) magnesio-hornblende (green amphibole) and actinolite (pale greenamphibole) The pargasitic hornblende develops aspoikilitic oikocrysts in the dykes 01 OA19a and d andin the diffuse patch 01 OA41d2 at temperatures above800C during the final stage of crystallization (see
below) The tschermakite composition corresponds tothe internal alteration of clinopyroxene in gabbro 94Ma2 occurs in the comb-textured dyke 01 OA15f andis the primary amphibole in the dioritic dyke 02 OA37bThe magnesio-hornblende develops at the edges of thepargasites or of clinopyroxenes at temperatures around800C and is the primary phase in the dioritic dyke 02OA90b and in the green vein 02 OA43a Finally actino-lite occurs as a replacement of green hornblende in dykesor in the kelyphitic rim surrounding olivine in gabbro 94Ma2 Such a large composition range at the scale of athin section has been reported in amphiboles fromamphibolite-facies oceanic gabbros (Stakes 1991 Stakesamp Taylor 1992 Gillis et al 1993) as well as in the Omanophiolite gabbros (Manning et al 2000) This variabilityhas been explained by rapid reaction rates preventinghomogenization (Manning et al 2000)
Thermometry
Plagioclase---amphibole thermometry could be appliedwhen the two minerals exhibited good contacts such asin the laminated gabbro of the middle sequence (01OA41d2) and in the intrusive dykes The temperaturesof plagioclase---amphibole equilibration (Table 2) havebeen calculated using the geothermometer lsquoBrsquo of Hollandamp Blundy (1994) Edenite thorn Albite frac14 Richterite thornAnorthite valid between 500C and 900C with anuncertainty of 40C Amphibole formulae are basedon 23 oxygen and their nomenclature is based on alkalications in the A site vs AlIV according to Leake (1997)Pressurewas assumed to be 1 kbar Forty-five plagioclase---amphibole pairs meet the compositional criteria imposedby Holland amp Blundyrsquos dataset (09 4 Xan 4 01 andAlIV 403) Electron microprobe measurements wereconstrained to contiguous plagioclase and amphiboleseparated by sharp grain boundaries assuming equili-brium of the two phases When the mineral phasesexhibit normal zoning temperatures between plagioclaseand amphibole cores and between mineral rims werecalculated assuming that the temperatures deduced fromthe mineral core chemistry provide a proxy for the high-est temperature of equilibration These data are identi-fied in the T C vs AlIV diagram (Fig 8d) Table 4presents selected major element amphibole analyses theAn content of the plagioclase adjacent to each amphibolegrain and the temperature calculated for the pairThe temperatures calculated (Fig 8d) span the entire
range of temperatures at which amphibole and plagio-clase coexist in equilibrium (ie amphibolite-facies para-geneses) This temperature interval is bracketed byorthopyroxene-bearing facies or amphibole-free facies(ie gabbro 94MA2) and epidote---actinolite facies devoidof homogeneous plagioclase (ie epidosite dyke 01OA36b and green vein 02 OA43a) The range of the
BOSCH et al HYDROTHERMAL CIRCULATION IN OMAN OPHIOLITE
1199
calculated temperatures between 4900C and 750Ccorresponds to the changing composition of amphibolesfrom pargasite to magnesio-hornblende at the scale of athin section or even at that of an individual crystal asshown by their correlation with AlIV such variabilityrecords rapid cooling in hydrous conditions The highesttemperatures are recorded by pargasite from the anatec-tic gabbro 01 OA41d2 in the gabbro dyke 01 OA19dand in the dioritic dyke 02 OA90b They were calculatedfrom minerals devoid of lower-grade alteration and arethus considered as representing the equilibrium temperat-ure of the coexisting minerals Lower temperatures cor-respond to mineral phases evolving at falling temperatureand are only indicative of the cooling history Includingthe 40C uncertainty seven pairs provide temperatureshigher than 900C the limit of validity of the Holland ampBlundy thermometer However we maintain these tem-peratures as indicative of the highest values of our data-set These values are included in the average equilibriumtemperature summarized in Table 2
DISCUSSION
Distribution of hydrothermal alteration
Two distinct petrological events are recorded in the gab-bro section of the Samail ophiolite and are documentedin the present contribution (1) the whole gabbro sectionis affected by a penetrative alteration (2) the gabbrosection is crosscut by various types of dykes and veinsincluding hydrous minerals
Penetrative alteration
The penetrative alteration developed in the gabbros atgrain boundaries and locally invading the minerals isubiquitous in the gabbro section This penetrative altera-tion has been described as very high temperature (VHT)high temperature (HT) and low temperature (LT) basedon alteration parageneses The orthopyroxene coronas inthe layered gabbro sample and the incipient appearanceof brown pargasitic amphibole mark a lower thermallimit of the VHT alteration By reference to the experi-mental work of Koepke et al (2003) the temperature ofequilibration for these reactions is estimated to bebetween 900 and 1000CThe preservation of the vertical distribution of
VHT---HT facies (Figs 4 and 5) also pointed out byManning et al (2000) indicates that the hydrothermalalteration pattern fits the distribution of isotherms withdepth at a fast-spreading ridge (ie Phipps Morgan ampChen 1993 Dunn et al 2000 Fig 2) Equilibrium tem-peratures for VHT parageneses as high as 900---1000Csuggest that VHT---HT hydrothermal alteration tookplace in a very restricted time interval at the limit ofthe cooling magma chamber It is proposed that the
penetrative alteration affecting the entire gabbro sectionis related to a pervasive network of microcracks whichact as a recharge system down to the Moho (Nicolas et al2003)
Dykes and veins
The dykes are extremely varied eg pegmatitic gabbroswith comb-texture microgabbros and amphibolitizeddolerites dioritic dykes alignments of poikilitic horn-blende and finally diffuse hornblende-bearing patchesHigh-T gabbro and diorite dykes and low-T greenamphibole---epidote veins are connectedIn the dykes textural evidence attests to suprasolidus
conditions at least for the development of the pargasiticamphibole Integrating the hornblende---plagioclase equi-libration temperatures calculated for the studied samplesthese temperatures are bracketed between 950C and875CIn the following discussion we shall consider separately
the microgabbro and dolerite dykes The latter representbasaltic melt intrusions associated upsection with thediabase sheeted dykes and downsection possibly withthe mafic dykes that are ubiquitous in the mantle section(Nicolas et al 2000) Whatever their origin the develop-ment of pargasitic amphibole during their late stage ofcrystallization creates a link with the gabbroic dykes andsuggests a crystallization process evolving from dry con-ditions to more hydrous conditionsThe other gabbroic dykes comb-textured gabbros and
hornblende-bearing dioritic dykes result from the fillingof cracks by a fluid either a melt or a silica-rich hydrousfluid These intrusive dykes which develop reaction mar-gins at their contact with the host gabbro grade verticallyinto lower-temperature green amphibole veins or align-ments of poikilitic hornblende in the layered gabbromatrix or development of pargasitic hornblende in ana-tectic gabbros (sample 01 OA41d2) In gabbros crystal-lizing in the magma chamber the development oforthopyroxene and pargasite oikocrysts enclosing the pri-mary minerals suggests a thermal evolution from thegabbro solidus to amphibolite-facies conditions Forthese reasons it is suggested that the gabbroic dykesresulted from the injection of a hydrous melt in the stillhot gabbros drifting away from the magma chamber asdiscussed below
Origin of fluids responsible for VHT---HThydrothermal alteration
The 87Sr86Sr initial ratios and d18O of whole rocks andpure mineral fractions are plotted in Fig 9 against thedistance above the Moho Transition Zone (MTZ) Theinitial Sr isotopic composition of the whole rocks apartfrom the epidosite dykes ranges from 070322 to
JOURNAL OF PETROLOGY VOLUME 45 NUMBER 6 JUNE 2004
1200
070458 All the studied samples (including whole rocksand minerals) have initial 87Sr86Sr ratios falling outsidethe field of fresh MORB which commonly have Sr ratiosbetween 07023 and 07029 with an average of 070265(ie Hart et al 1974) The lowest initial Sr isotopiccomposition (070322) from the massive gabbro 94MA2 is slightly but significantly higher than averageOman primary magmatic signatures (ie 070296McCulloch et al 1981) Similarly the d18O values ofmost whole rocks and minerals measured (Table 5)
depart from typical MORB-source mantle values( Javoy 1980 Gregory amp Taylor 1981 Kyser 1986)These differences indicate that the primary mantlesignatures in the Oman ophiolite have been modifiedby interaction with a fluid Possible origins for thisfluid are mantle components dehydration of subductedlithosphere or seawater circulation Strontium andoxygen isotopes are useful tracers for fluid---crust interac-tion as d18O and 87Sr86Sr ratios from fluids originat-ing from seawater the mantle or subducted lithosphere
-1000
1000
2000
3000
4000
5000
6000
7000
MOHO
MTZ
Dis
tan
ce a
bov
e th
e M
oh
o (
m)
pillow-basalt
sheeted-dike
gabbro
peridotite
01 OA 41d2
01 OA 36b
02 OA 43a97 OA 128b
01 OA15f
94 OA MA2
02 OA 90b01 OA 19ad
02 OA 37b
0702 0703 0704 0705 0706 0707
87Sr86Srinitial
MO
RB-s
ou
rce
Seaw
ater
Whole RockPlagioclasePargasiteGreen HornblendeClinopyroxene
Epidote
(a)
-1000
1000
2000
3000
4000
5000
6000
7000
MOHO
MTZ
Dis
tan
ce a
bov
e th
e M
oh
o (
m)
pillow-basalt
sheeted-dike
gabbro
peridotite
01 OA 41d2
01 OA 36b
02 OA 43a97 OA 128b
01 OA15f
94 OA MA2
02 OA 90b01 OA 19ad
02 OA 37b
Whole RockPlagioclasePargasiteGreen HornblendeClinopyroxene
3 5 10
δ18O (permil)
41d2
Ma2
43a
90b 19d 19d37b
128b
90b
(b)
Fig 9 (a) Variation of initial 87Sr86Sr isotopic composition as a function of the location of the studied samples in a schematic log of the crustalsection of the Oman ophiolite The field for MORB is from Hart et al (1974) and that for seawater is from Burke et al (1982) Small open squares aredata fromKawahata et al (2001) (b) Variation of d18O () as a function of the location of the studied samples in a schematic log of the crustal sectionof the Oman ophiolite Shaded curve corresponds to the data of Gregory amp Taylor (1981)
BOSCH et al HYDROTHERMAL CIRCULATION IN OMAN OPHIOLITE
1201
are significantly different Moreover the d18O valueis dependent on temperature and mineral fractiona-tion and thus yields complementary constraints to Srisotopes
Mantle origin
One model to explain the occurrence of fluids interactingwith the oceanic crust at very high temperatures is toderive them from the mantle The contribution of mantle-derived hydrous fluids linked to percolation processesor to their concentration in the final stages of gabbrocrystallization has been invoked in numerous case studies(ie Witt amp Seck 1989 Fabriees et al 1989 Dupuy et al1991 Agrinier et al 1993) O and Sr isotopic data forwhole rocks and mineral separates yield scattered valuesthat are unambiguously different from MORB mantle-source values (Fig 9a and b) With the exception of lateLT altered samples the WR data have a mean 87Sr86Srratio of 070337 and a standard deviation of 000012These surprisingly high values in mantle-derived rockscould be taken as evidence for survival of mantle isotopicheterogeneities during crystallization of the ophiolite (egLanphere 1981 McCulloch et al 1981) However the18O16O isotope ratios in the Oman gabbroic sequenceare also displaced from primary magmatic values Nogabbro in the present study has plagioclase or amphi-bole with typical mantle d18O values Thus the Sr and Oisotope data rule out the possibility of mantle-derivedfluids as a possible source for the VHT---HT hydrousalteration event recorded in the massive gabbros andgabbroic dykes and veins (Fig 10a and b)
Subducted lithosphere origin
Dehydration of subducted oceanic crust in subductionzones can generate hydrous fluids Such fluids couldpercolate through the overlying lithosphere and contam-inate it thus generating modified isotopic signatures andtrace element contents Such an explanation howeverdisagrees with the geochemical characteristics of most ofthe studied samples Fluids derived from the dehydrationof subducted slabs (fresh or altered MORB or OIB pro-toliths) should be strongly enriched in K Rb and Ba (egBecker et al 2000) whereas in Oman the Rb contentsmeasured in the gabbros and gabbroic dykes and veins arestrongly depleted Moreover the isotopic composition ofslab-derived fluids is buffered by the MORB-like oceaniccrustal protolith for Nd and Pb but not for Sr and O(Staudigel et al 1995) As a result of relatively minorfractionation of oxygen at high temperature the oxygenisotope composition of the fluids expelled during dehy-dration of the subducted slab should be high and essen-tially controlled by the oxygen composition of the uppersection of the crust altered at low temperature (with an
average of 10 and perhaps as high as 19) (Staudigelet al 1995) The Sr isotopic composition should be alsohigh (average 07046) as the fluids released by the dehy-dration of subducted oceanic crust are associated eitherwith seawater or with radiogenic Sr phases (ie smectites)The Sr and O isotopic compositions of the studied sam-ples do not fit with these signatures and so preclude asubducted lithosphere origin for the fluids involved in theVHT---HT alteration event
Seawater-derived fluids
All the analysed samples (minerals and WR) have 87Sr86Sr(T) outside the domain of primary MORB magmasand variable Sr contents Individual minerals have Srcontents reflecting their different mineralliquid partitioncoefficients Minerals characterized by a very high affi-nity for Sr such as plagioclase (ie Sr mineralliquidpartition coefficients 1) have very high Sr contentsTherefore the high abundance of plagioclase in the sam-ples analysed is responsible for the high Sr content of thestudied whole rocks The Sr content of all the whole rocksis relatively homogeneous (Table 5) without clear distinc-tion between country-rock gabbros and dykes and veinsand ranges from 110 to 200 ppm except for the epidosite(4700 ppm) Reported on a 87Sr86Sr vs 1Sr diagram(Fig 10a) most whole rocks and plagioclases analysed arecharacterized by a 1Sr ratio lower than 001 and plot inthe left part of this diagram Compared with N-MORBthe studied massive and anatectic gabbros and their con-stituent plagioclases have similar to slightly higher Srcontents but substantially higher 87Sr86Sr ratios(Fig 10a) Considering Cretaceous seawater as a possiblehigh 87Sr86Sr mixing component [87Sr86Sr 07073at 90Ma after Burke et al (1982)] responsible for theobserved geochemical modifications exchanges cannothave occurred in a closed system as seawater has too lowa Sr content (ie 8 ppm de Villiers 1999) An open-system process allowing penetration of larger quantitiesof seawater can efficiently modify the Sr isotopic ratio ofthe rocks Alternately the fluids could also be seawaterthat was modified through exchange with the surround-ing rocks during its transit through the oceanic crustalsection This modified seawater would be more enrichedin Srwith a 87Sr86Sr ratio intermediate between unmodi-fied seawater and MORBMinerals devoid of late LT alteration imprints (parga-
site green hornblende and pyroxene) and characterizedby very low Sr contents plot on the right of the diagramclose to or on the mixing line between seawater andMORB (Fig 10a) The difference between these mineralsand the other samples (WR and plagioclases) is mainlydue to the Sr fractionation coefficients of these differentminerals and to the large quantity of plagioclase inthe studied rocks The process responsible for the
JOURNAL OF PETROLOGY VOLUME 45 NUMBER 6 JUNE 2004
1202
modification of the Sr isotopic composition fromMORB-source value is however explained by the same phenom-enon As all these minerals display initial 87Sr86Sr ratiosdistinct from the MORB source and because they equili-brated at VHT or HT temperatures this supports anearly intervention of seawater during crystallization ofthese minerals Most of these samples with the exceptionof the epidosite dyke overlap the domains defined byKawahata et al (2001) for the Wadi Fizh gabbros alteredat VHT and HT conditions in the amphibolite facies(labelled lsquoVHTrsquo and lsquoHTrsquo in Fig 9a)Three samples (two epidosite dykes and an one epidote
fraction) have very high Sr contents and the highest Sr
isotopic compositions of the present dataset The twowhole-rock samples overlap the field of the Wadi Fizhsamples altered at high temperature during greenschist-facies metamorphism (Kawahata et al 2001) This typeof alteration is common at the ridge axis and formedthrough intensive exchange with seawater-derived fluidsFigure 10b indicates that all the Sr---O isotope data are
distinct from MORB compositions and suggests that thefluids reacting with the magmas could be derived fromseawater In addition the d18O values show that isotopicexchange occurred under VHT or HT conditions Thefluids could have the composition of seawater or theycould have the composition of seawater modified through
0703
0705
0707
001 01 1
1 Sr (ppm)
MORB
(87Sr
86Sr)
init
ial
Seawater
VHT
HT
MT36b
36b
36b
43a 128b90b
41d2
37b19d
15f
128b
41d2
41d2
19a
Ma2Ma2
19d
90b
37b
Whole RockPlagioclasePargasiteGreen HornblendeClinopyroxene
Epidote
(a)
0 2 4 6 8
δ18O(permil)
0703
0705
0707Seawater
MORB-mantle
128b 41d2
90b 41d2
Ma2128b
37b
43a
19d
90b
Whole RockPlagioclasePargasiteGreen HornblendeClinopyroxene
Epidote
HT LT
19d
(87Sr
86Sr)
init
ial
(b)
Fig 10 (a) Initial 87Sr86Sr isotopic ratios plotted against 1Sr for whole rocks and mineral separates from the gabbro section of the Omanophiolite Fields shown are from Kawahata et al (2001) and correspond to MT-----basalt sheeted dyke diabase altered at medium temperature(prehnite---actinolite facies sequence 3) HT-----diabase plagiogranite metagabbro and epidosite formed at high temperature (greenschist faciessequence 4) VHT-----gabbro altered at very high temperature (amphibolite facies sequence 5) The dashed line is a binary mixing line betweenMORB (Lanphere et al 1981) and Cretaceous seawater (Burke et al 1982) (b) Initial 87Sr86Sr isotopic composition plotted against d18O value forwhole rocks and minerals from the Oman ophiolite Cretaceous seawater and MORB end-members as in (a) Dashed line represents mixing curvebetweenMORB and seawater at high temperature (HT) Low-temperature (LT) exchanges between these two components would be responsible foran increase of the d18O primary MORB value
BOSCH et al HYDROTHERMAL CIRCULATION IN OMAN OPHIOLITE
1203
exchange with the surrounding rocks during its penetra-tion into the crustal section In an environment in whichfluid---rock interaction occurs at variable temperaturesthe d18O is greatly influenced by the temperature atwhich the exchange took place and by the waterrockratio It is evident from Figs 9b and 10b that none of thestudied gabbros have plagioclase and amphibole withMORB-like d18O and 87Sr86Sr values The separateminerals display a broad range of d18O values and mostexhibit extreme 18O depletion (Table 5) Hydrothermalexchange kinetics are similar in amphibole and pyroxeneand these two minerals are more resistant to oxygenexchange during water---rock interaction than coexistingplagioclase (Gregory et al 1989) The most probableexplanation of the low d18O values measured in bothamphiboles and pyroxenes is that they crystallized athigh temperature in the presence of a relatively low 18Oseawater-derived hydrothermal fluid [d18O from 01 to07 after Gregory amp Taylor (1981)] The anomalouslyhigh d18O value (thorn1009) measured for the plagioclase ofthe micro-gabbroic dyke (01 OA19d) can be interpretedas resulting from a superimposed overprint caused by alate-stage penetration of fluids at lower temperature Inthis sample the occurrence of such different 18O16Oratios between coexisting plagioclase and amphibole isnot consistent with equilibrium fractionation betweenthese two minerals at any possible temperature Thisobservation suggests that parts of the ophiolite sequencehave undergone a high-temperature hydrothermal eventprior to the later low-temperature event that producedthe strong 18O increase in coexisting plagioclaseThe depletion of the d18O values results from high-temperature hydrothermal alteration (Gregory amp Taylor1981 Stakes amp Taylor 1992) The oxygen isotopic datasupport the contention that most studied samples havebeen affected by a VHT---HT hydrothermal alteration byfluids derived from seawater Some of the analysed sam-ples presenting petrological evidence of crystallization ofLT secondary minerals have been overprinted by the latelow-temperature alteration thus swamping the earlierhigh-temperature alteration effects
Determination of waterrock ratio
To better evaluate the exchange between seawater andthe gabbroic rocks waterrock ratios (WR) have beenestimated for the whole rocks analysed during the courseof this study The different models used to constrain thisratio mainly differ by the calculation of the effective ratioor by the estimated weight ratio and by considering openor closed systems for water circulation (Albareede et al1981 McCulloch et al 1981) The WR ratios reportedin Table 5 have been calculated assuming a closed sys-tem Using this formulation these values are minimabecause the calculation always uses pristine fluid for the
initial fluid thus maximizing the value in the denomina-tor If the fluid was shifted to lower 87Sr concentrations byexchange with the rock on the downward path theinferred values would be much higher (Davis et al 2003)The calculated WR ratios for the samples from this
study range from 31 to 219 (Table 5) Two main groupsof samples can be recognized on the basis of their waterrock ratios The lowest ratios ranging from 31 to 47have been determined for massive gabbros or anatecticgabbros but also for dykes and veins devoid of late LTalteration Similar ratios have been previously calculatedfor example for the discharged hydrothermal solutions atthe 13N and 21N East Pacific Rise (Di Donato et al1990 Fouquet et al 1991) The gabbros from the Ibrasection in the Oman ophiolite gave effective WR ratiosof 05 33 and 42 (McCulloch et al 1981) in goodagreement with the values obtained in this study Theleast altered gabbro of the Ibra section yields a WR ratioof 05 in good agreement with the exceptionally fresh-ness of this rock characterized by typical mantle oxygenvalues for its minerals (McCulloch et al 1981) Samplescharacterized by waterrock effective ratios in the rangeof 3---5 can be related to penetrative alteration via thehydrothermal system Lecuyer amp Reynard (1996) havedemonstrated in oceanic gabbros from the Hess Deepaltered in similar HT conditions that WR mass ratios inthe range of 02---1 are sufficient to account for the mod-ified stable isotope signature of the gabbros Higherwaterrock values (WR 45) have been calculated forsamples affected by superimposed late hydrothermalalteration and for the epidosite samples (Table 5) Theyprobably acquired their isotopic characteristics throughinteraction with a larger volume of seawater duringgreenschist-facies metamorphism Similar or higherWR values have been published in previous studies(ie McCulloch et al 1981 Kawahata et al 2001)They correspond either to samples from the upper-crus-tal sequence (ie basalt sheeted dyke or plagiogranite) orto gabbros from the crustal section located in a particularenvironment such as for example the Wadi Fizh sectionthat is located close to a segment boundary along thepalaeo-spreading axis (Nicolas et al 2000)
Mechanism for seawater interaction withmagma
Nicolas et al (2003) have proposed that variable amountsof seawater can circulate at high temperature through-out the crustal section down to the walls of the magmachamber via a network of parallel microcracks a fractionof a millimetre wide Such microcracks and their relation-ship with the VHT---HT hydrous alteration in the sur-rounding gabbros have been described in detail byNicolas et al Via this network of microcracks largeamounts of seawater can have reacted with the magma
JOURNAL OF PETROLOGY VOLUME 45 NUMBER 6 JUNE 2004
1204
and induced variable changes of its Sr and O isotopiccomposition This regular network of parallel micro-cracks could constitute the recharge system and thehydrated gabbroic dykes the discharge system Physicalparameters and the geophysical models of Nicolas et al(2003) are in agreement with this scenario Such a pene-tration of seawater-derived hydrothermal fluids throughthe lower crust along grain boundaries and a microscopicfracture network at temperatures 4750C is consistentwith recent thermodynamic models (McCollom amp Shock1998)Seawater-altered and high-temperature recrystallized
diabases (protodykes) from the overlying sheeted dykecomplex stoped into the melt lenses could represent anindirect way for seawater to enter the system as they canremelt during their subsidence through the magmachamber A simple mass balance calculation suggeststhat this indirect contamination is not significant Assess-ments of the amount of recycled crust 1 in volumeby Nicolas et al (2000) based on the protodyke remnantsin the gabbro section or 20 by Coogan et al (2003)based on chlorine content in EPR basalts lead to a sea-waterrock ratio of the order of 104 to 103Thus following Nicolas et al (2003) we propose that
seawater has been introduced along microcracks down tothe walls of the magma chamber and has diffused alonggrain boundaries in the way described by Manning et al(2000) progressively invading the host gabbro Gabbroicdykes result from the injection of hydrous melt into thehost gabbros at falling temperatures as they drift awayfrom the magma chamber This water-saturated meltcould result from the extensive hydration of the crystal-lizing gabbros near the walls of the magma chamberwhen seawater penetrates through this wall (Fig 1)Similar plumbing systems driving seawater down to the
limit of the magma chamber and back to the surface havebeen already proposed in other environments such as theMid-Atlantic Ridge (Kelley amp Delaney 1987 Cooganet al 2001) the East Pacific Rise at Hess Deep (Gillis1995 Manning et al 1996a 1996b) the Tonga forearc(Banerjee amp Gillis 2001) and the Troodos ophiolite(Gillis amp Roberts 1999) The melting curve of wet basalthas a negative slope but is close to the dry solidus at lowpressure in the lower gabbros and the first melts areplagiogranitic (Spulber ampRutherford 1983) Such plagio-granites are ubiquitous in the highest gabbros and in theroot zone of sheeted dykes in Oman (Rothery 1983Juteau et al 1988 Nicolas amp Boudier 1991) in manyother ophiolites and in the oceanic crust where they havebeen interpreted as resulting either from wet anatexis ofhot gabbros or from the final product of crystallization ofa basaltic melt (Spulber amp Rutherford 1983) Plagio-granites are rare in the lower gabbros exceptionallyassociated with hydrous gabbro segregations inside thelayered gabbros Their constitutive minerals are also
remarkably absent along the VHTmicrocracks althoughthese gabbros were above the wet solidus A possibleexplanation has been proposed by McCollom amp Shock(1998) who wrote that at high temperature lsquoaqueousfluids may leave very little conspicuous petrological evid-ence for their presencersquo the reason being that thehigh-T conditions would be closer to batch meltingObviously more detailed studies on this specific problemare needed We conclude that the large degree of hydrousmelting locally required at the walls of the magma cham-ber to account for the generation of a gabbroic hydrousmelt may have erased the traces of the incipient plagio-granitic melt
CONCLUSIONS
Fostered by the recent discovery of high-temperatureseawater contamination in some gabbros of the Omanophiolite (Manning et al 2000) we have conducted astudy on 500 gabbro specimens from selected areas ofthe entire Samail ophiolite belt and on the hydrous gab-broic dykes that cross-cut them The highest-temperaturereactions (VHT) recorded in olivine and clinopyroxene ingabbros as orthopyroxene and pargasite coronas reach975C They are followed at lower HT conditions withreplacement of olivine by an assemblage of tremolitemagnetite and plagioclase surrounded by chlorite andby pargasite development in clinopyroxene followedfinally by the LT lizardite---olivine and saussurite---plagioclase greenschist-facies alteration With a fewexceptions the VHT alteration tends to be restricted tothe lower gabbros and to the vicinity of the Moho andthe HT alteration to the middle and upper gabbros Thepreservation of the vertical distribution of VHT---HTfacies indicates that progressive cooling during off-axisspreading did not obliterate this distribution and conse-quently that the VHT---HT hydrothermal alteration tookplace in a very restricted time interval at the limit of thecooling magma chamber In the deep and hot gabbrosthe main water channels may be sub-millimetre-sizedmicrocracks with a dominantly vertical attitude the ori-gin and dynamics of which have been investigated in aprevious paper (Nicolas et al 2003)Beyond these high-temperature alteration zones mas-
sively induced in the gabbros seawater may be respons-ible for the generation of clinopyroxene---pargasitegabbro dykes that cross-cut all gabbros and that areupsection clearly connected to the LT hydrothermalvein system Petrostructural evidence suggests that thesedykes were generated by hydration of the crystallizinggabbros near the internal wall of the LVZ---magmachamber The resultant melts were subsequently intrudedat various levels in the cooling lower and middle gabbrosPetrological observations of nearly all the gabbros ana-lysed in the present study located from the root zone of
BOSCH et al HYDROTHERMAL CIRCULATION IN OMAN OPHIOLITE
1205
the sheeted dyke complex down to the Moho emphasizethe imprint of VHT---HT hydrous alteration The VHThydrothermal event is characterized by temperatures ofequilibration around 900---1000C The temperatures atwhich the HT hydrous event occurred are estimated tobe in the range 550---900CStrontium and oxygen isotope compositions measured
on whole rocks and separated minerals significantlydepart from primary MORB values The Sr and O iso-tope data obtained for pargasite green hornblende andclinopyroxene record the participation of seawater at thetemperature of crystallization of the minerals Plagio-clases and whole rocks define a trend toward a compo-nent characterized by a high radiogenic 87Sr86Sr valueand a higher Sr content than normal seawater Seawateris clearly identified as the fluid responsible for the modi-fication of the Sr and O signatures for both massivegabbros and dykes and veins Nevertheless the high Srcontent of the extraneous component requires eitheropen-system exchange allowing penetration of a greaterquantity of seawater or penetration of seawater modifiedby interaction with the oceanic crust during its travel paththrough the crustal section The waterrock ratio calcu-lated on the basis of the Sr isotopes is between three andfive for the enclosing gabbros and for the least LT altereddykes The VHT---HT hydrothermal event affectingthese samples is related to penetrative alteration via therecharge and discharge hydrothermal system Thewaterrock ratio is 10 for dykes exhibiting evidenceof a LT hydrothermal alteration overprint and reaches22 for the epidosite dyke The depletion in d18O char-acterizes all the studied samples with the single exceptionof the micro-gabbroic dyke plagioclase This agreeswith the conclusions based on Sr isotopes suggestingthat these samples have undergone exchange with sea-water-derived fluids during a VHT---HTalteration hydro-thermal event
ACKNOWLEDGEMENTS
This work has benefited from financial support by theCentre National de la Recherche Scientifique (INSUInteerieur-Terre andDYETI programmes) and inOmanfrom the willingness of the Directorate of MineralsMinistry of Commerce and Industry and the hospitabil-ity of the people Technical help in the laboratory is alsoacknowledged including C Nevado for the thin sectionsE Ball for the illustrations B Galland for her assistancein the chemistry room and C Bassoulet for help duringthermal ionization mass spectrometric analyses of the Srresults from the leaching experiments Constructivereviews from R T Gregory C Lecuyer an anonymousreviewer and particularly from M Wilson greatlyhelped to improve an earlier version of the manuscript
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Banerjee N R amp Gillis K M (2001) Hydrothermal alteration in a
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Becker H Jochum K P amp Carlson R W (2000) Trace element
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Nelson H F amp Otto H F (1982) Variation of seawater 87Sr86Sr
throughout Phanerozoic time Geology 10 516---519Carmichael I S E Turner F J amp Verhoogen J (1974) Igneous
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Chernosky J V Berman R G amp Bryndzia L T (1988) Stability
phase relations and thermodynamic properties of chlorite and
serpentine group minerals In Bayley S W (ed) Hydrous
Phyllosilicates Mineralogical Society of America Reviews in Mineralogy 19
295---346
Coogan L A Wilson R N Gillis K M amp MacLeod C J (2001)
Near-solidus evolution of oceanic gabbros insights from amphibole
geochemistry Geochimica et Cosmochimica Acta 65 4339---4357
Coogan L A Mitchell N C amp OrsquoHara M J (2003) Roof
assimilation at fast spreading ridges an investigation combining
geophysical geochemical and field evidence Journal of Geophysical
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1171
Davis A C Bickle M J amp Teagle D A H (2003) Imbalance in the
oceanic strontium budget Earth and Planetary Science Letters 211
173---187
Detrick R S Buhl P Vera E Mutter J Orcutt J Madsen J amp
Trocher T (1987) Multi-channel seismic imaging of a crustal
magma chamber along the East Pacific Rise Nature 326 35---41
De Villiers S (1999) Seawater strontium and SrCa variability in the
Atlantic and Pacific oceans Earth and Planetary Science Letters 171
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Di Donato G Joron J L Treuil M amp Loubet M (1990)
Geochemistry of zero-age N-MORB from hole 648B ODP legs
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Proceedings of the Ocean Drilling Program Scientific Results 106109
College Station TX Ocean Drilling Program pp 57---65
Dunn R A Toomey D R amp Solomon S C (2000) Three-
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Dupuy C Mevel C Bodinier J L amp Savoyant L (1991) Zabargad
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rifting Geology 19 722---725
JOURNAL OF PETROLOGY VOLUME 45 NUMBER 6 JUNE 2004
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Fabriees J Bodinier J L Dupuy C Lorand J P amp Benkerrou C
(1989) Evidence for modal metasomatism in the orogenic spinel
lherzolite body from Caussou (northeastern Pyrenees France)
Journal of Petrology 30 199---228
Fouquet Y Von Stackelberg S U Charlou J L Donval J P
Foucher J Erzig P Muhe R Wiedicke M Soakai S amp
Whitechurch H (1991) Hydrothermal activity in the Lau back-arc
basin sulfides and water chemistry Geology 19 303---306
Gillis K M (1995) Controls on hydrothermal alteration in a section of
fast-spreading oceanic crust Earth and Planetary Science Letters 134
473---489
Gillis K M amp Roberts M (1999) Cracking at the magma---hydrothermal transition evidence from the Troodos ophiolite Earth
and Planetary Science Letters 169 227---244
Gillis K M Mevel C et al (eds) (1993) Proceedings of the Ocean Drilling
Program Initial Reports 147 College Station TX Ocean Drilling
Program
Gregory R T amp Taylor H P (1981) An oxygen isotope profile in a
section of Cretaceous oceanic crust Samail ophiolite Oman
evidence for d18O buffering of the oceans by deep (45 km)
seawater---hydrothermal circulation at mid-ocean ridges Journal of
Geophysical Research 86(B4) 2737---2755
Gregory R T Criss R E amp Taylor H P (1989) Oxygen
isotope exchange kinetics of mineral pairs in closed and open
systems applications to problems of hydrothermal alteration of
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1---42Hacker B R (1994) Rapid emplacement of young oceanic litho-
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1563---1569
Hart S R Erlank A J amp Kable J D (1974) Sea floor basalt
alteration some chemical and Sr isotopic effects Contributions to
Mineralogy and Petrology 44 219---230
Holland T amp Blundy J (1994) Non-ideal interactions in calcic
amphiboles and their bearing on amphibole---plagioclase thermo-
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Ionov D A Savoyant L amp Dupuy C (1992) Application of the
ICP-MS technique to trace element analysis of peridotites and their
minerals Geostandards Newsletter 16 311---315
Javoy M (1980) 18O16O and DH ratios in high temperature
peridotites In Colloques Internationaux du CNRS 272 279---287
Juteau T Beurrier M Dahl R amp Nehlig P (1988) Segmentation
at a fossil spreading axis the plutonic sequence of the Wadi
Haymiliyah area (Haylayn Block Sumail nappe Oman) Tectono-
physics 151 167---197
Juteau T Manacrsquoh G Moreau C Lecuyer C amp Ramboz C
(2000) The high temperature reaction zone of the Oman ophiolite
new field data microthermometry of fluid inclusions PIXE analyses
and oxygen isotopic ratios Marine Geophysical Research 21 351---385Kawahata H Nohara M Ishizuka H Hasebe S amp Chiba H
(2001) Sr isotope geochemistry and hydrothermal alteration of the
Oman ophiolite Journal of Geophysical Research 106 11083---11099
Kelemen P B amp Aharonov E (1998) Periodic formation of magma
fractures and generation of layered gabbros in the lower crust
beneath oceanic spreading ridges Annual Geological Union Special
Monograph 106 267---289
Kelemen P B Koga K amp Shimizu N (1997) Geochemistry of
gabbro sills in the crustmantle transition zone of the Oman
ophiolite implications for the origin of the oceanic lower crust Earth
and Planetary Science Letters 146 475---488Kelley D S amp Delaney J R (1987) Two-phase separation and
fracturing in mid-ocean ridge gabbros at temperatures greater than
700C Earth and Planetary Science Letters 83 53---66
Koepke J Feig S T Snow J amp Freise M (2003) Petrogenesis of
oceanic plagiogranites by partial melting of gabbros an experi-
mental study Contributions to Mineralogy and Petrology on line first http
wwwspringerlinkcomopenurlaspgenre=s00410-003-0511-9
Kyser T K (1986) Stable isotope variations in the mantle In
Valley J W Taylor H P amp OrsquoNeil J R (eds) Stable Isotopes in
High Temperature Geological Processes Mineralogical Society of America
Reviews in Mineralogy 16 141---164
Lanphere M A (1981) K---Ar ages of metamorphic rocks at the base
of the Samail ophiolite Oman Journal of Geophysical Research 86
2777---2782
Lanphere M A Coleman R G amp Hopson C A (1981) Sr isotopic
tracer study of the Samail ophiolite Oman Journal of Geophysical
Research 86(B4) 2709---2720
Leake B E (1997) Nomenclature of amphiboles report of the
subcommittee on amphiboles of the International Mineralogical
Association commission on new minerals and mineral names
European Journal of Mineralogy 9 623---651
Lecuyer C amp Reynard B (1996) High-temperature alteration of
oceanic gabbros by seawater (Hess Deep Ocean Drilling Program
Leg 147) evidence from oxygen isotopes and elemental fluxes
Journal of Geophysical Research 101(B7) 15883---15897
Liou J G Kuniiyoshi S amp Ito K (1974) Experimental study of the
phase relations between greenschist and amphibolite in a basaltic
system American Journal of Science 274 613---632
MacLeod C J amp Rothery D A (1992) Ridge axial segmentation in
the Oman ophiolite evidence from along-strike variations in the
sheeted dyke complex Geological Society of America Special Papers 60
39---63
Manning C E MacLeod C J amp Weston P E (1996a) Fracture-
controlled metamorphism of Hess Deep gabbros site 984
constraints on the roots of mid-ocean ridge hydrothermal systems
at fast-spreading centers In GillisM C Allan KM ampMeyer P S
(eds) Proceedings of the Ocean Drilling Program Scientific Results 147
College Station TX Ocean Drilling Program pp 189---211Manning C E Weston P E amp Mahon K I (1996b) Rapid high-
temperature metamorphism of East Pacific Rise gabbros from Hess
Deep Earth and Planetary Science Letters 144 123---132Manning C E MacLeod C J amp Weston P E (2000) Lower-crustal
cracking front at fast-spreading ridges evidence from the East Pacific
Rise and the Oman ophiolite Journal of the Geological Society London
349 261---272McCollom T M amp Shock E L (1998) Fluid---rock interactions in
the lower oceanic crust thermodynamic models of hydrothermal
alteration Journal of Geophysical Research 103 547---575
McCulloch M T Gregory R T Wasserburg G J amp Taylor H P
(1981) Sm---Nd Rb---Sr and 18O16O isotopic systematics in an
oceanic crustal section evidence from the Samail ophiolite Journal of
Geophysical Research 86(B4) 2721---2735Mevel C Gillis K M Allan J F amp Meyer P S (eds) (1996)
Proceedings of the Ocean Drilling Program Scientific Results 147 College
Station TX Ocean Drilling Program
Nehlig P amp Juteau T (1988) Flow porosities permeabilities and
preliminary data on fluid inclusions and fossil thermal gradients in
the crustal sequence of the Sumail ophiolite (Oman) Tectonophysics
151 199---221
Nehlig P Juteau T Bendel V amp Cotten J (1994) The root zone of
oceanic hydrothermal systems constraints from the Samail ophiolite
(Oman) Journal of Geophysical Research 99 4703---4713
Nicolas A amp Boudier F (1991) Rooting of the sheeted dyke complex
in Oman ophiolite In Peters Tj Nicolas A amp Coleman R (eds)
Ophiolite Genesis and Evolution of the Oceanic Lithosphere Dordrecht
Kluwer Academic pp 39---54
BOSCH et al HYDROTHERMAL CIRCULATION IN OMAN OPHIOLITE
1207
Nicolas A amp Ildefonse B (1996) Flow mechanism and viscosity in
basaltic magma chambers Geophysical Research Letters 23(16)
2013---2016
Nicolas A Ceuleneer G Boudier F amp Misseri M (1988) Structural
mapping in the Oman ophiolites mantle diapirism along an ocean
ridge Tectonophysics 151 27---56
Nicolas A Boudier F Ildefonse B amp Ball E (2000) Accretion
of Oman and United Arab Emirates ophiolite-----discussion of a
new structural map Marine Geophysical Research 21(3---4)147---179
Nicolas A Boudier F Michibayashi K amp Gerbert-Gaillard L
(2001) Aswad massif (United Arab Emirates) archetype of the
Oman---UAE ophiolite belt Geological Society of America Bulletin 349
499---451
Nicolas A Mainprice D amp Boudier F (2003) High temperature
seawater circulation through the lower crust of ocean-ridges-----amodel derived from the Oman ophiolites Journal of Geophysical
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2372
Pallister J S amp Hopson C A (1981) Samail ophiolite plutonic suite
field relations phase variation cryptic variation and layering and a
model of a spreading ridge magma chamber Journal of Geophysical
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morphology on magma supply and spreading rate Nature 364
706---708
Pin C Briot D Bassin C amp Poitrasson F (1994) Concomitant
extraction of Sr and Sm---Nd for isotopic analysis in silicate samples
based on specific extraction chromatography Analytica Chimica Acta
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Rothery D A (1983) The base of a sheeted dyke complex Oman
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281 697---734
Spulber S D amp Rutherford M J (1983) The origin of rhyolite and
plagiogranite in oceanic crust an experimental study Journal of
Petrology 24 1---25Stakes D S (1991) Oxygen and hydrogen isotope compositions of
oceanic plutonic rocks high-temperature deformation and meta-
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Geochemical Society Special Publication 3 77---90
Stakes D S amp Taylor H P (1992) The northern Samail ophiolite an
oxygen isotope microprobe and field study Journal of Geophysical
Research 97(B5) 7043---7080Staudigel H Davies G R Hart S R Marchant M amp Smith B
M (1995) Large scale isotopic Sr Nd and O isotopic anatomy of
altered oceanic crust DSDPODP sites 417418 Earth and Planetary
Science Letters 130 169---185Turner F J amp Verhoogen J (1960) Igneous and Metamorphic Petrology
New York McGraw---Hill 694 pp
Wilcock W S D amp Delaney J R (1996) Mid-ocean ridge
sulfide deposits evidence for heat extraction from magma
chambers or cracking fronts Earth and Planetary Science Letters 145
49---64
Witt G amp Seck H A (1989) Origin of amphibole in recrystallized
and porphyroclastic mantle xenoliths from Rhenish massif implica-
tions for the nature of mantle metasomatism Earth and Planetary
Science Letters 91 327---340
JOURNAL OF PETROLOGY VOLUME 45 NUMBER 6 JUNE 2004
1208
magma chamber is not restricted to the perched axialmelt lens that is present just below the sheeted dykecomplex of fast-spreading ridges (Detrick et al 1987)because in spite of its very low melt fraction the LVZ isstill deforming as a mush (Nicolas amp Ildefonse 1996)Building on the previous work of MacLeod amp Rothery
(1992) and Nehlig et al (1994) Nicolas et al (2001) haverecently published a structural map of the greenschist-facies hydrothermal veins throughout the Samail ophio-lite This confirms that the LT veins penetrate the entirecrust and are dominantly aligned parallel to the sheeteddyke complex Subsidiary preferred orientations arerelated to changes in the stress field during the dyingactivity of the palaeo-ridge system (Nicolas et al 2000)In a companion paper to the present study Nicolas et al
(2003) described a complete transition from these LThydrothermal veins to gabbroic dykes which suggests thepossibility of seawater-related hydrous melting of the gab-bros or entry of seawater into the partially molten magmachamber Previously these gabbroic dykes which arefairly common in the Oman gabbros were interpreted aslate crystallization products from the LVZ magma cham-ber (Pallister amp Hopson 1981) or as melt conduits relatedto its magmatic activity (Kelemen et al 1997 Kelemen amp
Aharonov 1998) If the magma that produced these gab-broic dykes originated within the magma chamber thewater should also have an internal origin the source beingeither the mantle or the melt lens In the latter case con-tamination by seawater is possible when hydrated diabasefrom the overlying sheeted dyke complex is stoped in themelt lens remelted and dewatered during subsequentsubsidence through the magma chamberAlternatively the gabbroic dykes could result from
fractures external to the magma chamber driving sea-water to great depth In the physical model proposed byNicolas et al (2003) the gabbroic dykes are generated byhydrous melting and represent the discharge system ofthe high-temperature hydrothermal circulation reachingthe inner margin of the magma chamber (Fig 2) Therecharge system is formed by a microcrack network con-trolled by the anisotropy of thermal expansion and oper-ating at falling temperatures below 1200C within adistance of 10 km from the spreading axisThe available geochemical evidence bearing on the
hydrothermal alteration of the gabbros (Gregory ampTaylor 1981 Lanphere 1981 McCulloch et al 1981Kawahata et al 2001) favours a seawater origin withseawaterrock ratios ranging from one (Gregory amp
water in Sea Floor
Magma chamber
water out
gabbroicdike
hydrous contamination
diabase dykeswith chilledmargins
diabasedikewithoutchilledmargins
750degC
500degC
1200degC
LT hydrothermal vein
hydrothermal cracks
hydrothermal cracks
VHT
plastic mantle flow
maficdike
Moho
microgabbrodike
HT
Fig 2 Scheme of hydrothermal circulation near the limit of the magma chamber below the fast-spreading ridge The downgoing seawater flow fillsthe porous network created by microcracks down to the magma chamber where it reacts with the crystallizing melt it surges back towards theseafloor along a system of hydrous gabbroic dykes Modified from Nicolas et al (2003)
BOSCH et al HYDROTHERMAL CIRCULATION IN OMAN OPHIOLITE
1183
Taylor 1981) to as high as nine (Kawahata et al 2001)Oxygen isotope data (C Lecuyer personal communica-tion 1990) on mafic dykes emplaced in the peridotites at1 km below the Moho suggest a dual source of hydrationfor the gabbros in which clinopyroxenes have a mantlesignature but associated plagioclase and brown horn-blende show evidence of seawater contamination Thisdual source was not ruled out by Benoit et al (1996) intheir Sr isotope study of similar dykes injected into themantle section Rare earth element partition coefficientsin clinopyroxene plagioclase and amphibole were usedby the above workers to constrain the temperature ofseawater contamination at the gabbro solidusThus there is growing evidence that supports a deep
penetration of high-T hydrothermal fluids into theoceanic crust at the ridge axis We have conducted asystematic study of the gabbros in the Samail ophioliteand along selected wadi sections (Fig 1) in the hydratedgabbroic dykes that cross-cut them looking for traces ofHT alterationThis study combines microtextural and petrological
observations thermometric determinations and O---Srisotope data on the same gabbroic dykes and surroundinggabbros selected with a view to determine the origin ofhydrous fluid contamination These results have import-ant consequences for thermal modelling of the oceaniccrust near fast-spreading ridges and on the budget ofchemical exchanges between the oceanic crust andseawater
HYDROUS ALTERATION OF
PRIMARY MINERALS IN THE
GABBRO SECTION
The penetrative alteration of the gabbro section has beenstudied on the basis of an extensive thin-section collec-tion which covers the entire ophiolite The studied gab-bros range from the root zone of the sheeted dykecomplex down to the Moho and the Moho transitionzone (MTZ) From the 500 specimens that have beenscreened those sites involved in tectonic complexitiessuch as the end of segments or tips of propagating rifts(Fig 1) have been rejected thus reducing the number ofstudied areas and the number of fully characterized speci-mens to 414 representative of standard gabbros at a fast-spreading palaeo-ridge In this study we mainly focus onhydrous alteration of olivine and clinopyroxene as theseare the minerals most affected by high-T alteration(above 800C) although plagioclase is stable down tothis temperature
Alteration facies
The following types of hydrous parageneses in thegabbros have been distinguished from highest to lowesttemperatures
(1) Orthopyroxene partial coronas between olivine andplagioclase which may be associated with the firstappearance of brown pargasite blebs inside clinopyrox-ene and development of pargasite between olivine andplagioclase or clinopyroxene and plagioclase (Fig 3a) Thebrown pargasite locally grades into a greenish magnesio-hornblende Although the orthopyroxene coronas couldbe assigned to a late magmatic stage we interpret themas reactive (a) being restricted to the contact betweenolivine and plagioclase thus postdating the crystallizationof plagioclase (b) being locally relayed by kelyphiticcoronas that spread into plagioclase microcracks(Fig 3b) as microphases among which diopside andlow-titanium pargasite have been identified We inferthat these reactions are related to an oxygen fugacityincrease as a result of seawater input Based on experi-ments at 2MPa pressure Koepke et al (2003) have con-strained the equilibrium olivine---orthopyroxene at lowwater content at 1030C and 950C at higher watercontent the limit at which amphibole is stabilized Byreference to this experimental work we estimate that thetemperature of equilibration for these reactions referredto here as VHT (very high temperature) hydrous altera-tion was between 900 and 1000C(2) Chlorite and pale amphibole coronas between oliv-
ine and plagioclase (Fig 3d) The coronas are composedof successive rims of clino-amphibole (tremolite) thornmagnetite Mg-chlorite (clinochlore) and a fringe of ortho-amphibole against plagioclase As described above theseminerals are associated with blebs of brown to greenishamphibole inside the pyroxenes (Fig 3c) Depending onthe degree of alteration olivine is surrounded by a thinfilm of amphibole and chlorite or is totally replaced by anassemblage of tremolite thorn magnetite thorn plagioclase An91(thorn talc) surrounded by chlorite (Fig 3e) The evolution ofthe first assemblage to Mg-chlorite indicates temperat-ures above 700C the upper breakdown of Mg-chloriteat 2 kbar (Chernosky et al 1988) The development ofpargasitic amphibole replacing clinopyroxene impliestemperatures between 550 and 900C (Liou et al 1974Spear 1981) We refer to these as HT (high temperature)(3) Iddingsite partly or totally replacing olivine inde-
pendently of orthopyroxene or chlorite---amphibole cor-onas and any significant HT alteration in clinopyroxene(Fig 3f ) Observations in thin sections show that thechlorite---amphibole coronas develop before the idding-site alteration(4) Lizardite alteration of olivine forming the familiar
mesh structure This is always accompanied by saussuritein plagioclase This alteration takes place below 500Cand is here referred to as LT (low temperature) hydrousalteration It is only at this stage that plagioclase showssigns of alterationThese various hydrous alteration facies are commonly
observed superimposed in a set of thin sections from a
JOURNAL OF PETROLOGY VOLUME 45 NUMBER 6 JUNE 2004
1184
single outcrop and even in a single thin section As a rulethe orthopyroxene reactions are more penetrative beinghomogeneously distributed from the scale of a thinsection to that of several thin sections made from a
given outcrop The distribution of chlorite---amphibolecoronas is similar although they can be restricted to themargins of microcracks filled by the same minerals whichhave been interpreted as the channels that carried the
(a) (b)
(c) (d)
(e) (f)
olopx
kel
pl
ol
hbol
amcpx
cpx
hb
chl
am+pl+mt
idg
am
hb
pl
hb
ol
cpx
opx
mt
pl
Fig 3 Hydrothermal alteration in the gabbros (scale bar represents 1mm) (a) VHT alteration marked by orthopyroxene (opx) coronas at thecontact of olivine (ol) and plagioclase (pl) and pargasitic hornblende (hb) at the contact of olivine and clinopyroxene (cpx) (sample 94 MA2)(b) Kelyphite (kel) coronas around olivine and cloudy plagioclase as a result of silicate and fluid inclusions Silicate inclusions tend to be localizedat grain boundaries and along microcracks (sample 94 MA2) (c) VHT alteration marked by brown amphibole (hb) alteration of clinopyroxene in anupper gabbro (d) HT alteration marked by chlorite (chl) (external rim) and pale amphibole (am) and magnetite (mt) (internal rim) coronas aroundolivine (e) Total replacement of olivine by an assemblage of tremolite (am)thornmagnetite (mt)thorn plagioclase (pl) (f ) Total replacement of olivine by aniddingsitic mixture (idg) inside a VHT recrystallized corona of a mosaic of orthopyroxene
BOSCH et al HYDROTHERMAL CIRCULATION IN OMAN OPHIOLITE
1185
fluids responsible for the hydrothermal alteration (Nicolaset al 2003) The iddingsite replacement is typicallyheterogeneous at the thin-section scale being preferen-tially associated with microcracks Serpentinization alsodevelops from microcracks but it can pervasively invadethe entire thin section The microcracks are commonapproximately 25 of the studied thin sections displayone or more Ignoring the cracks related to LT alterationand considering only the cracks related to the HT hydro-thermal alteration this fraction is still around 20
Distribution of high-T alteration facies
Figure 4 shows the distribution of alteration facies as afunction of depth in the crust along wadi sections throughthe Oman---UAE ophiolite offering continuous exposures(see details in Fig 1) Figure 4 excludes the LT faciesbecause (1) being based on the presence of olivine ser-pentinization only throughout our compilation its dis-tribution is biased by the decreasing amount of olivineupsection and (2) this study concerns high-temperaturealteration processes Despite the fact that the scale ofsampling may exceed the scale of heterogeneity evidencefor both VHT alteration and lsquono alterationrsquo increaseswith depth Conversely HT alteration is predominantin the middle and upper parts of the gabbro sectionThis observation is valid throughout the wadi sectionsstudied with the exception of the Wadi Tayin massifwhich shows no clear gradation with depthFigure 5 is another representation of alteration-facies
distribution with depth based on the detailed study of414 thin sections The samples are classified according totheir level in the gabbro section sheeted dyke root zonegabbros upper gabbros middle gabbros lower gabbrosand Moho gabbros (the Moho gabbros have beengrouped together with the gabbro lenses interlayered inthe dunites of the MTZ) Values represent the percent of
2 Km
3
4
5
6
7
Aswad Fizh Hilti Haylayn Samail W TayinNakhl
High T
Very High T
0 High T
Upper Crust
LID
Lower Crust
MohoMTZ
DEPTH1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27
Fig 4 Depth of penetration of hydrothermal circulation in gabbros along the wadi sections numbered in Fig 1 referring to alteration faciesdescribed in Fig 3 Depths are recorded below the palaeo-seafloor LID corresponds to the upper crust including sheeted dykes and volcanics
0
10
20
30
40
50
60
70
80
90
100
SD root
zone
upper
gabbros
middle
gabbros
lower
gabbros
MTZ
BHb-GHb CPX
Chl-Amph core OL
Chl-Amph rim OL
0
5
10
15
20
25
30
SD root
zone
upper
gabbros
middle
gabbros
lower
gabbros
MTZ
Opx OL
BHb OL
No VHTHT
Unaltered OL core
HT
VHT
(a)
(b)
Plst def
gabbros
Fig 5 Compilation of hydrothermal alteration features as a function ofdepth in the gabbro section Percentages represent the occurrence of agiven type of alteration versus the number of thin sections examined foreach selected level root of the sheeted dykes (SD) upper gabbros middlegabbros lower gabbros MTZ (Moho Transition Zone) gabbros on atotal amount of 414 thin sections (Opx orthopyroxene Amph amphi-bole OL olivine BHb brown hornblende GHb green hornblendeChl chlorite Plst def plastic deformation) (a) Distribution of the HTalteration (in ) (b) Distribution of the VHT alteration (in )
JOURNAL OF PETROLOGY VOLUME 45 NUMBER 6 JUNE 2004
1186
a given alteration facies as defined previously out of thetotal number of thin sections examined for a given levelMore refined conclusions can be drawn from this figureAs seen in Fig 5a the HT alteration decreases down-
wards whereas it affects most of the upper gabbroicsection (sheeted dyke root zone and upper gabbros)The fraction of unaltered olivine cores can be used asan index to estimate the extent of alteration This indexshows that the HT alteration is more penetrative andintense in the upper gabbros a conclusion alreadyreached by several workers (eg Gregory amp Taylor1981 Stakes amp Taylor 1992 Kawahata et al 2001)This was also noted by Manning et al (1996a 1996b)Figure 5b shows that the VHT alteration marked by
orthopyroxene or brown hornblende coronas aroundolivine increases with depth The correlation betweenthe VHT alteration and plastic deformation also suggeststhat the VHT alteration may coincide with the develop-ment of plastic flow at the expense of the magmatic flow
at temperatures just below the solidus In Fig 5b thefraction of gabbros where there is no VHT---HThydrothermal alteration increases downsection indicat-ing that the VHT---HT alteration is more localizeddownsection
Gabbroic dykes and veins
The occurrence of gabbroic dykes in the layered gabbrosof the Samail ophiolite is ubiquitous We describe gab-broic dykes and sills either plagioclase---clinopyroxeneand brown amphibole-bearing dykes or microgabbrosand amphibolitized dolerite dykes and sills correspond-ing to temperatures up to 975C (see below) Identifica-tion of gabbroic dykes in the field is not alwaysstraightforward because they may be obliterated bysuperimposition of greenschist-facies alteration (Fig 6a)Selective greenschist-facies alteration has frequently beenobserved either in the centre of a mafic dyke or alongboth margins (Fig 6b) as well as a continuous transition
(b) (c)(a)
(d) (e)
thulite
pistacite
white microvein
white vein
green vein
green vein
gabbroic dike
epidote vein
gabbroic dike
Fig 6 Field relationships of dykes and veins (scale bar represents 10 cm) (a) Thulite---pistacite vein cross-cutting a foliated gabbro matrix Reactionmargins marked by the development of secondary clinopyroxene (01 OA36b) (b) Comb-like structure in gabbroic dyke A pink epidote vein followsthe margin of the dyke (01 OA15f) (c) Along-strike transition from a gabbroic dykelet to a hydrothermal green amphibole vein (d) White prehnitevein (e) Green amphibole vein cross-cut by white prehnite vein
BOSCH et al HYDROTHERMAL CIRCULATION IN OMAN OPHIOLITE
1187
along a single crack from a mafic dyke to a hydrothermalvein (Fig 6c) We define hydrothermal veins as cracksfilled with greenschist-facies minerals that induce meta-morphic reactions in the adjacent country rocks andgabbroic dykes as planar intrusive bodies produced bythe crystallization of magma Following Nehlig amp Juteau(1988) we distinguish low-T greenschist-facies lsquowhiteveinsrsquo made of prehnite epidote chlorite and actinolite(Fig 6d) and lsquogreen veinsrsquo in which a darker greenamphibole predominates (Fig 6e) corresponding respec-tively to lower (195---410C) and higher (400---530C)temperatures (Nehlig amp Juteau 1988)The gabbroic dykes are typically a few centimetres
across composed of plagioclase clinopyroxene andbrown
hornblende They are coarse-grained to pegmatitic andcommonly display a comb-like structure (Fig 6b)Although the spacing between the dykes in the field ishighly variable the average is around 10m Gabbroicsills grade from clear intrusions to irregular and diffusepegmatitic gabbro patches up to a few metres in theirlarger dimension (Fig 7a)The distribution in the field of brownish pargasite
which appears black and glassy in outcrop is critical intracing the origin of the gabbroic dykes It is first observedat any level in the host lower and middle gabbros as largepoikilitic patches (Fig 7b) locally associated with simil-arly poikilitic patches of clino- and orthopyroxene(Fig 7a) These patches contain small aligned tablets of
(a) (b) (c)
g(e)(d)
alignment ofalignment of amphiboleamphibole oikocrystsoikocrysts
hwhite vein
hblpl
pcpx
brownbrownamphiboleamphiboleoikocrystoikocryst
eediabasedikdike
hb-plhb-plreactionreaction
imarginspbrown amphibole rich
gabbroic dikegabbroic dike
pbrown amphibole veins
Fig 7 Field relationships of dykes (scale bar is 10 cm long) (a) Irregular coarse to pegmatitic patches in a foliated gabbro matrix (01 OA42d)(b) Alignment of black amphibole oikocrysts in a foliated gabbro matrix (c) Local concentration of brown amphibole in a gabbroic dykelet Theplagioclase laths marking the foliation in the enclosing gabbro rotate into parallelism with the gabbroic dykelet (d) Two diabase dykes assimilatingthe enclosing layered gabbro with plagioclase---amphibole reaction margins (e) Pargasite-rich gabbroic dyke with wall reactions (01 OA19) (Notesmall pargasite---plagioclase veins) pl plagioclase cpx clinopyroxene hb hornblende
JOURNAL OF PETROLOGY VOLUME 45 NUMBER 6 JUNE 2004
1188
plagioclase oriented parallel to the larger plagioclaselaths defining the magmatic foliation of the gabbro(Fig 7c) Clinopyroxene oikocrysts surrounding idio-morphic plagioclase crystals are common in the upper-level gabbros they mark the end of the magmatic flowand crystallization growing before the pargasite andorthopyroxene oikocrysts as shown by their partialreplacement by pargasite Their crystallization marksthe transition at suprasolidus conditions from a dry to awet environment In the uppermost gabbros those dykesthat are typically altered in the HT hydrothermal faciesgrade upwards into the so-called lsquoisotropicrsquo or lsquovari-texturedrsquo gabbrosThe microgabbro and amphibolitized dolerite dykes
differ from the typical diabase dykes commonly cuttingthe gabbro section that have chilled margins and thusprobably intruded enclosing rocks already at temperat-ures below 450C In microgabbro and amphibolitizeddolerite dykes the absence of chilled margins and thedevelopment of dioritic reaction margins in the adjacentgabbro (Fig 7d and e) suggest that they were emplaced ingabbros that were still at temperatures above 750C(Nicolas et al 2000)
ANALYTICAL TECHNIQUES
Major and trace elements
Major element compositions were determined by elec-tron microprobe at the University of Montpellier II usinga Camebax SX100 Instrument calibration was per-formed on natural standards and operating conditionswere 20 kV accelerating potential 10 nA beam currentand 30 s counting timeRb and Sr concentrations in whole rock and separated
mineral fractions were measured by conventionalnebulization inductively coupled plasma mass spectro-metry (ICP-MS) using a VG Plasmaquad 2 (ISTEEMMontpellier) Digestion procedures followed thoseoutlined by Ionov et al (1992)
O---Sr isotopes
The samples selected for the detailed study wereextracted from gabbroic dykes or veins inside gabbrosand cut with a diamond saw into 2---3 cm fragmentsbefore being crushed into 05---1 cm fragments Forwhole-rock analysis small pieces were further crushedinto powder in an agate bowl For pure mineral analysisthe fragments were crushed with a manual agate mortarTo avoid or minimize the mixing of selected separateminerals with other phases that can bias the results astrict protocol of mineral selection was applied A smallsize fraction of 125---160 mm was used and minerals werefirst separated using a Frantz isodynamic magneticseparator Concentrates recovered (cpx plagioclase
amphibole epidote) were subsequently purified by care-ful hand-picking in alcohol under a binocular micro-scope At this stage mineral separates were very pureand only flawless minerals free of cracks and visibleinclusions were kept for isotopic analyses To ensurefurther mineral integrity these pure mineral separateswere ultrasonically washed in dilute 15N HCl and rinsedseveral times with ultra-pure water This stage potentiallyremoves adsorbed secondary micro-phases Previousstudies (eg Lecuyer amp Reynard 1996) demonstratedthat pyroxene and amphibole can be intermixed on avery fine scale with other phases (such as a range ofamphibole compositions talc Fe-oxide) Although thispossibility cannot be ruled out the procedure used inthis study makes it unlikely that complex minerals passedthrough the selection
Sr isotopic compositions
Approximately 50mg of whole-rock powder or separatedminerals were digested in a Teflon lsquobombrsquo with a mixtureof triple distilled 13N HNO3 and 48 HF with a fewdrops of HClO4 Two aliquots were separated one forthe determination of Sr isotopic composition and theother for Sr and Rb contents Sr was separated using anextraction chromatographic method modified from Pinet al (1994) Concentrations were measured by isotopedilution using 84Sr---87Rb mixed spikes To remove tracesof the late low-temperature seawater alteration and todetermine the primitive whole-rock Sr isotopic composi-tion leaching experiments were conducted on a fewsamples following the procedure described by Bosch(1991) The strontium isotopic compositions of leachateshave been analysed in the attempt to check for the leach-ing efficiency During these tests four Sr isotope composi-tions were measured which correspond to the unleachedsample the two liquids extracted after acid-leaching stepsand the final solid residue Leaching experiments wereconducted in two steps first with 6N HCl for 8min at70C second with 25N HCl for 30min at 70C Afterleaching the leachates were evaporated to dryness andthe solid residues digested similarly to the unleachedsamplesSr isotopic compositions were measured on a fully
automated VG Sector mass spectrometer housed at theUniversity of Montpellier II except for the Sr isotopicdata from the leaching experiments which have beenmeasured at the Universitee de Bretagne Occidentale(Brest) To assess the reproducibility and accuracy of Srisotopic measurements the NBS 987 standard was run12 times during this study the average value was0710245 with a precision better than 0000010 (2s)87Sr86Sr ratios are corrected for mass discrimination bynormalizing to a 86Sr88Sr value of 01194 Sr total blankconcentrations were less than 60 pg Initial 87Sr86Sr
BOSCH et al HYDROTHERMAL CIRCULATION IN OMAN OPHIOLITE
1189
ratios were calculated by correcting the measured ratiosfor accumulation of 87Sr produced by in situ 87Rb radio-active decay since 95Ma the assumed age of crystalliza-tion of the Oman ophiolite (Hacker 1994)
Oxygen isotopes
The powdered samples were introduced in Ni tubesunder dry air where they were reacted with BrF5 toform oxygen Oxygen was separated from other gasesby cooling the resulting mixture at liquid nitrogen tem-perature (77 K) that retained other volatile gases Oxygenwas further purified by trapping it on a cooled 5A mole-cular sieve at 77 K then quantified to calculate the yieldof the isotopic analyses and finally transferred to the massspectrometer (Finnigan Mat Delta E) where intensitiesassociated with masses 32 and 34 were measured todetermine d18O The isotopic composition of a sampleis given as dsample frac14 (RsampleRstandard 1) 1000 in permil unit where R is 18O16O and the standard isSMOW Analytical errors on the oxygen isotope ratioare 02 (2s)
RESULTS
Seven samples representative of the range of gabbroicdykes and five samples representative of the lower gab-bros two of them representing the margins of gabbroicdykes were selected for major element thermometricand Sr---O isotopic analyses (Tables 1---3) These samplesoriginate from wadi sections marked by bold lines inFig 1 Wadis Halhal Mayah Al Abyad Warihya andMaqsad structurally belong to a new ridge segment open-ing in slightly (1Myr) older lithosphere Wadi Gideahis in older lithosphere
Petrographic and isotopic characteristicsof the analysed samples
Olivine gabbro from the MTZ in MaqsadSamail Massif
Sample 94 MA2 is devoid of low-temperature alterationand only exhibits the discrete signature of the VHT---HTalteration The orthopyroxene corona around olivine(Fig 3a) is relayed by kelyphitic rims at the contactwith plagioclase (Fig 3b) Issuing from these rims aremicrometre-sized pale green amphiboles identified ascummingtonite and actinolite which penetrate the sur-rounding plagioclase in microcracks 004mm wide or atgrain boundaries (Fig 3b) similar to the microcrack sys-tem described by Manning et al (2000) A large numberof the plagioclase crystals have a cloudy core as a result ofthe concentration of fluid and mineral inclusions amongwhich diopside has been identified during microprobeanalysis Olivine cores are free of serpentine but theyare largely oxidized along a microcrack network Ortho-pyroxene coronas have a MgOpx number of 78---79
slightly higher than in the primary olivine cores withMgOl number of 74---75 The magmatic clinopyroxeneis totally fresh and probably not in equilibrium with thereactive orthopyroxene This sample has the lowest 87Sr86Sr initial ratio of the studied whole rocks (070322)identical to coexisting clinopyroxene (070324) and pla-gioclase (070322) This observation suggests the lack of alate superimposed LT hydrothermal alteration as it isvery unlikely that an LT hydrothermal event could havetotally equilibrated the Sr isotopic compositions of prim-ary minerals such as plagioclase and clinopyroxeneMoreover the whole rock (WR) clinopyroxene andplagioclase have similar initial Sr isotopic ratios despitevery different Sr contents Accordingly this ratio isthought to reflect the signature acquired during the crys-tallization of these minerals and is higher than the Omanmantle Sr isotopic value (McCulloch et al 1981) Thed18O values of the WR (50) and plagioclase (48) aredistinctly lower than primary magmatic values Indeedin normal mid-ocean ridge basalts (MORB) the WRd18O value is 57 02 with a corresponding plagio-clase d18O ranging from 55 to 62 ( Javoy 1980Gregory amp Taylor 1981)
Lower gabbro from Wadi Wariyah Wadi Tayin Massif
Sample 97 OA128b belongs to a layered sequence in thelower gabbro series injected by anorthositic sills Thesample was collected at the tip of one of these sillsbetween layers enriched in clinopyroxene The gabbrois composed of 60 plagioclase and 40 clinopyroxeneDespite the absence of olivine the WR has a relativelyhigh Mg content (Table 3) A series of low-T (prehnite)microveins 01mm thick cross-cut the gabbro perpendi-cular to the layering and foliation The margins of theveins are devoid of alteration This gabbro has the sameSr isotopic composition as 94 MA2 Nevertheless theWR and associated plagioclase have Sr isotope signatures(070333 and 070328) lower than the coexisting clino-pyroxene (070422) The Sr contents of the WR plagio-clase and clinopyroxene are 127 ppm 120 ppm and20 ppm respectively thus making the clinopyroxenemore sensitive to late-stage contamination Clinopyrox-ene and plagioclase oxygen isotope compositions (523and 414 respectively) are also lower than typicalmantle values Similar to the previous sample alterationat high temperature by a low d18O and high 87Sr86Srfluid is required to explain such characteristics
Amphibole gabbro patch Wadi Gideah Wadi TayinMassif
Sample 01 OA41d2 is from a laminated gabbro com-posed of millimetre-sized layers of clinopyroxeneand plagioclase Anatectic patches (Fig 7a) borderinganorthositic layers are formed of coarse-grained
JOURNAL OF PETROLOGY VOLUME 45 NUMBER 6 JUNE 2004
1190
Table1Locationoftheanalysed
samplesandsummaryoftheirpetrographicandSr---O
isotopiccharacteristics
Sam
ple
nam
eSite
Locationin
gab
broic
section
Typ
eofrock
Distance
to
Moho(km)
Parag
enesis
Alteration
87Sr
86Srinitial1
d18O
()2
94MA2
Maq
sad
From
MTZ
Layered
gab
bro
0Olivine(M
gno74---75)3
OPXco
ronas
(Mgno78---79)
WRfrac14
070322
WRfrac14
thorn495
CPX
Palegreen
Am
(cumact)
CPXfrac14
070324
nd
Plagioclase(A
n65---80)4inversezoning
Nolow-T
alteration
Plfrac14
070322
Plfrac14
thorn479
Solidfluid
inclusionsin
Pl
97OA128b
Wad
iWaryiah
From
lower
gab
bro
Layered
gab
bro
1CPX
Low-T
alteration
WRfrac14
070333
nd
Plagioclase(A
n85---87)
Prehnitemicroveins
CPXfrac14
070422
CPXfrac14
thorn523
Plfrac14
070328
Plfrac14
thorn414
01OA41d2
Wad
iGideah
From
upper
gab
bro
Gab
bro
patch
in
laminated
gab
bro
36
CPXrecrystallized
Plagioclaseidiomorphic
(An57---87)norm
alzoning
Black
pargasitic
honblende
WRfrac14
070351
CPXfrac14
070378
Plfrac14
070426
WRfrac14
thorn480
nd
Plfrac14
thorn504
Hbdfrac14
070355
nd
01OA19aamp
dWad
iWaryiah
Inlayeredgab
bro
Gab
broic
dyke
01
Olivine
OPXpoikilitic
WRfrac14
070341
WRfrac14
thorn567
CPX
Black
pargasite
Hbdfrac14
070324
Hbdfrac14
thorn287
Plagioclase(A
n73---95)
Hornblendepoikilitic
Plfrac14
nd
Plfrac14
thorn10 09
Hem
atite
WRfrac14
070418
nd
Green
Mg-hornblende
Acthornblende
Epidote
02OA90b
Wad
iAl-Abyad
Inlayeredgab
bro
Dioriticdyke
015
Green
Mg-hornblende
Hbdfrac14
070427
Hbdfrac14
thorn239
Plagioclase(A
n83---87)
Plfrac14
070387
Plfrac14
thorn498
Solidfluid
inclusionsin
plagioclase
01OA15f
Wad
iWaryiah
Inlayeredgab
bro
Comb-textured
gab
broic
dyke
025
CPXidiomorphic
Plagioclase(A
n95---99)norm
alzoning
BrownMg-H
ornblende
WRfrac14
070458
nd
02OA43a
Wad
iMayah
Inlayeredgab
bro
vein
Green
amphibole
1Palegreen
Mg-hornblendeacicular
Plagioclase(A
n91---97)in
reactionmargin
Hbdfrac14
070431
Hbdfrac14
thorn310
01OA36b
Wad
iGideah
Inlayeredgab
bro
epidosite
dyke
25
CPX
Epidote
WRfrac14
070519
nd
Plagioclase
Epfrac14
070554
nd
CPXclinopyroxene
Plplagioclase
OPXorthopyroxene
Amam
phibolecu
mcu
mmingtonite
actactinolite
WRwholerockHbdhornblende
Epep
idotend
notdetermined
187Sr
86Srinitialratioscalculatedat
95Ma(H
acker1994)Errors
ofthemeasuredratiosareindicated
inTab
le4
2Analyticalerrors
oftheoxygen
isotopevalues
are02
(2s)
3Mgnumber
frac14atomicMg(Mgthorn
Fe)
4Percentageofan
orthitein
plagioclase
BOSCH et al HYDROTHERMAL CIRCULATION IN OMAN OPHIOLITE
1191
(millimetre-sized) recrystallized clinopyroxene largelyaltered to light green magnesio-hornblende and idio-morphic plagioclase Locally primary black pargasitichornblende develops including idiomorphic normallyzoned plagioclase (Table 4) This gabbro exhibits similar87Sr86Sr ratios for the WR and pargasite (070351 and070355 respectively) These values are interpreted asrepresentative of the melt from which this amphibolecrystallized The clinopyroxene has a slightly higher Srisotope composition of 070378 consistent with the occur-rence of the light green Mg-hornblende rim Plagioclaseis significantly more radiogenic (070426) related to sec-ondary destabilization evidenced by the idiomorphichabit The Sr isotope compositions of the liquidsextracted from the acid leaching experiments of theWR (L1 and L2) are very similar 070358 and 070365respectively The 87Sr86Sr ratio of the residue obtainedafter leaching experiments is 070335 lower than boththe pargasite and unleached WR This value can beconsidered as the unaltered isotopic composition of thissample The d18O values measured for WR and coexist-ing plagioclase are lower than normal MORB magmaticvalues The 18O depletion in the plagioclase can be fairlywell explained by high-temperature hydrothermal inter-action between an oxygen-depleted fluid and the magmaAccording to the kinetics of hydrothermal exchange ofoxygen (Gregory amp Taylor 1981 Gregory et al 1989)plagioclase exchanges its oxygen isotopes with the fluidadjusting readily to the hydrothermal conditions Forhigh temperatures of exchange with seawater-derivedfluids (d18O between 0 and thorn2) the magmatic plagio-clase will have its primary d18O value lowered whereasthe d18O value for lower temperatures of exchange(5300C) will increase The amplification of the d18Oshift increases with the intensity of the hydrothermalalteration
Gabbroic dykes intrusive in lower gabbros from WadiWariyah Wadi Tayin Massif
Gabbroic dykes 01 OA19a and 01 OA19d are intrusiveinto the lower gabbros They are 3---4 cm wide discord-ant to the foliation of the host gabbros (Fig 7e) with asharp margin marked by a brown amphibole fringe Theprimary phases in the dykes (Table 1) locally preservedas elongated aggregates of a 01mm grain size mosaicassemblage at the dyke margins grade to millimetre sizein the central part of the dyke The WR major elementcomposition is Mg enriched compared with the enclosinggabbro (Table 3) Brown pargasitic amphibole oikocrystsmake up 50 of the modal composition of the dyke Indyke 01 OA19a poikilitic orthopyroxene may developinstead of brown amphibole Finally green magnesio-hornblende grows either as oikocrysts at the expense ofclinopyroxene or in association with epidote as an altera-tion product of plagioclase These low-amphibolite-faciesminerals represent the lowest-grade paragenesis recogn-ized in these dykes and are most developed in sample01OA19aTheamphibole composition showszoning fromTi-rich pargasitic hornblende to magnesio-hornblendeand actinolitic hornblende (Fig 8c) recording pro-gressive cooling of the dyke The plagioclase compositionis extremely heterogeneous (Fig 8b) varying from An95a composition similar to that of the plagioclase in the hostgabbro (Fig 8a) to An50 for the plagioclase and asso-ciated epidote inclusions in the poikilitic actinolitic horn-blende Sample 01 OA19d has an initial 87Sr86Sr ratioslightly higher for the WR (070341) than for the parga-site (070324) The Sr isotopic ratios of liquids extractedafter two steps of WR acid-leaching (L1 and L2) are070357 and 070341 respectively whereas the residueyields a 87Sr86Sr ratio of 070327 very close to thepargasite and plagioclase values The latter is similar tothe isotopic composition measured for the massive gab-bro 94 MA2 and may be taken as close to the primarymagmatic value This Sr isotopic ratio is more radiogenicthan the inferred Oman mantle-source value (McCullochet al 1981) The altered part of this sample has a radio-genic Sr isotopic composition (070418) related to thelow-amphibolite-facies alteration This gabbroic dykehas seemingly preserved a magmatic whole-rock d18Ovalue (567) but contains by far the highestd18O plagioclase (1009) of all the samples analysedThis plagioclase coexists with low d18O pargasite(thorn287) Similarly high values (d18Oplagioclase 4 8)have been previously measured for gabbroic dykes dia-bases sheeted dykes and plagiogranites from the Omanophiolite (Gregory amp Taylor 1981 Stakes amp Taylor1992) This 72 oxygen isotope fractionation betweenplagioclase and amphibole cannot possibly represent oxy-gen equilibrium at any plausible temperature This maybe explained by different exchange kinetics for these twominerals at different temperatures at high waterrock
Table 2 Average temperatures ( C 40C)
calculated for plagioclase---amphibole pairs
using the geothermometer of Holland amp
Blundy (1994)
Pargasite Magnesio-hornblende
rim core rim core
01 OA41d2 878 953 ------- -------
01 OA19a 935 974 825 869
01 OA19d 890 933 789 -------
02 OA37b ------- ------- 816 849
02 OA90b ------- ------- 859 833
JOURNAL OF PETROLOGY VOLUME 45 NUMBER 6 JUNE 2004
1192
ratios This supports the occurrence of two distincthydrothermal events one at high temperature and alater stage at lower temperature responsible for thestrong 18O enrichment in the coexisting plagioclase
Dioritic dyke associated with a diabase dyke in lowergabbros from Wadi Halhal Haylayn Massif
Sample 02 OA37b belongs to a group of brownamphibole---plagioclase lenses or reaction margins asso-ciated with diabase intrusives (Fig 7d) in lower layeredgabbros Idiomorphic brown hornblende (tschermakite)crystals are elongated in the plane of the dyke represent-ing 80 of the modal composition They are associatedwith an interstitial plagioclase matrix The browntschermakite has greenish rims Plagioclase has a cloudyappearance due to micrometre-size fluid inclusionsexcept along its margins and internal microcracks Bothhornblende and plagioclase exhibit evidence of plasticdeformation The Sr isotopic ratio for the pargasite andplagioclase is 070348 and 070420 respectively Thepargasite exhibits a Sr signature closest to that of theOman primary magmatic value (McCulloch et al 1981)Nevertheless it is too high to be attributed to anunmodified MORB-source mantle signature This sug-gests the addition of an external fluid component in goodagreement with the shift of oxygen isotopic compositionto a low value (thorn39) suggesting a high-temperatureexchange with a low d18O fluid The Sr isotope composi-tion of the plagioclase is significantly higher than that of
the coexisting pargasite probably as a result of the low-temperature alteration
Dioritic dyke cross-cutting lower layered gabbros in WadiAl-Abyad Nakhl Massif
Sample 02 OA90b is a 1 cm thick dioritic dyke cross-cutting the lower layered gabbros It grades into a greenamphibole vein as illustrated in Fig 6c The dyke is com-posed of 2 cm long dark green idiomorphic magnesio-hornblende crystals growing in the plane of the dykeand plagioclase concentrated at the margin of the dykeThe plagioclase in the dyke and in the enclosing gabbrocontains micrometre-size fluid and mineral inclusionssimilar to those observed in sample 94 MA2 The plagio-clase and green hornblende have higher Sr isotopic ratios(070387 and 070427 respectively) and lower d18Ovalues (thorn498 and thorn239 respectively) than the normalMORB-source mantle value As observed for previoussamples this supports interaction with an external fluidcomponent
Comb-textured gabbroic dykes in the lower gabbros fromWadi Wariyah Wadi Tayin Massif
Sample 01 OA15f (Fig 6b) is from Wadi Wariyah andrepresents a 2 cm wide dyke intruding the lower gabbrosThis type of comb-textured dyke developed in theclinopyroxene---plagioclase stability field It containslarge idiomorphic clinopyroxene crystals which arepartially replaced by brownish magnesio-hornblendeThe plagioclase is extremely calcic with a slight inverse
Table 3 Whole-rock major element compositions of massive gabbros and dykes determined by X-ray fluorescence
(wt )
Sample 94 Ma2 97 OA128b 01 OA41d2 01 OA19d 01 OA19a 01 OA15f 01 OA36b 01 OA36b
Rock type Gabbro Gabbro Gabbro patch Gabbroic dyke Gabbroic dyke Comb-textured
gabb dyke
Epidosite dyke Epidosite dyke
(margin)
SiO2 4830 4815 4749 4524 4339 4799 389 4371
Al2O3 2785 2032 2438 1474 1246 1192 2752 1646
Fe2O3 328 266 404 981 1204 592 381 623
MnO 003 004 005 014 016 006 008 014
MgO 405 737 427 1031 1686 1326 187 739
CaO 1289 1908 1477 1235 1156 166 2471 2339
Na2O 292 123 257 258 111 075 000 013
K2O 000 000 009 006 000 000 000 000
TiO2 005 017 063 230 064 086 013 042
P2O5 008 007 013 018 009 013 007 010
LOI 033 076 138 216 161 237 274 190
Total 9978 9985 998 9987 9992 9986 9983 9987
LOI loss on ignition gabb gabbroic
BOSCH et al HYDROTHERMAL CIRCULATION IN OMAN OPHIOLITE
1193
Table4Majorelementcomposition
aofselectedam
phibolespaired
withplagioclasecompositionsusedforthermometry
Sam
ple
nam
e1Mode2
SiO
2TiO
2Al 2O3
Cr 2O3
FeO
MnO
MgO
CaO
Na 2O
K2O
Total
Si
AlIV
AlVI
Ti
Fe3
thornCr
Mg
Fe2
thornMn
Na
KCa
XAn3
T(C)4
01OA19a
Mg-hornblende
core
45 60
26
10 78
009
860
015
16 56
12 00
212
007
96 72
6552
1448
0366
0083
0552
0004
3409
0594
0016
0611
0018
1843
0903
899
Mg-hornblende
core
47 99
263
850
002
897
011
16 61
12 10
160
020
96 36
6894
1106
0334
0028
0483
0002
3557
0594
0014
0446
0036
1862
0892
834
Mg-hornblende
rim
51 17
247
596
001
761
014
18 11
12 19
103
010
96 47
7257
0743
0253
0016
0414
0001
3828
0488
0016
0284
0018
1853
0894
820
Mg-hornblende
rim
47 50
291
920
004
880
014
16 46
11 95
174
008
96 07
6835
1165
0396
0015
0506
0005
3529
0552
0017
0486
0015
1843
0887
825
Mg-hornblende
rim
49 57
269
742
000
799
017
18 07
12 02
133
014
96 88
7013
0987
0261
0018
0575
0000
3811
0371
0020
0365
0026
1821
0860
831
Mg-hornblende
core
47 83
181
872
003
840
011
14 48
12 18
159
008
93 59
6814
1186
0279
0018
0644
0004
3711
0357
0013
0438
0015
1859
0907
874
Pargasite
rim
42 15
249
11 87
024
10 07
013
13 84
11 43
263
032
96 61
6188
1812
0242
0434
0299
0028
3028
0937
0016
0749
0059
1797
0804
972
Pargasite
rim
42 22
260
11 86
026
10 21
011
13 71
11 48
253
032
96 59
6201
1799
0254
0429
0299
0030
3001
0955
0014
0720
0060
1807
0803
962
Pargasite
core
42 05
263
11 45
030
10 19
011
13 80
11 47
246
032
96 46
6189
1811
0175
0478
0299
0035
3029
0956
0014
0702
0060
1809
0767
974
Pargasite
rim
42 36
247
11 90
016
945
013
14 16
11 76
249
026
95 98
6239
1761
0305
0366
0276
0019
3109
0888
0016
0710
0049
1856
0818
926
Pargasite
core
42 50
291
11 64
016
984
015
14 22
11 41
256
034
96 69
6219
1781
0227
0426
0322
0018
3103
0883
0018
0727
0063
1789
0812
974
Pargasite
rim
42 76
270
12 03
017
920
012
14 87
11 82
244
025
96 73
6224
1776
0880
0335
0368
0020
3226
0752
0015
0689
0047
1843
0841
941
Pargasite
rim
42 13
181
13 54
016
921
009
12 77
12 13
254
032
96 98
6172
1828
0510
0450
0000
0018
2788
1129
0011
0721
0059
1905
0841
874
01OA19d
Mg-hornblende
49 26
047
624
011
11 46
017
15 39
12 21
108
009
96 48
7137
0863
0202
0051
0437
0013
3323
0951
0021
0302
0018
1896
0768
771
Mg-hornblende
46 03
287
932
018
906
017
14 77
12 87
193
007
97 28
6672
1328
0265
0313
0046
0021
3191
1053
0021
0542
0012
1999
0758
808
Pargasite
41 89
429
11 65
008
11 03
017
13 52
11 37
259
017
96 77
6159
1841
0179
0474
0345
0010
2963
1012
0021
0739
0032
1791
0770
975
Pargasite
rim
42 80
340
11 16
009
11 01
018
13 73
11 67
240
022
96 67
6293
1707
0226
0376
0322
0010
3009
1032
0022
0684
0041
1839
0605
880
Pargasite
core
42 58
351
11 27
006
11 63
018
13 51
11 46
239
022
96 82
6257
1743
0209
0387
0391
0007
2959
1039
0023
0680
0042
1804
0595
893
Pargasite
rim
42 14
393
11 28
006
11 71
014
13 35
11 33
235
033
96 62
6214
1786
0174
0435
0391
0007
2935
1054
0018
0673
0062
1790
0519
901
Pargasite
core
41 56
420
12 05
006
11 57
016
12 51
11 35
249
027
96 24
6168
1832
0277
0469
0276
0007
2768
1160
0021
0718
0052
1805
0537
882
Pargasite
core
41 33
438
11 49
008
11 66
015
13 35
11 45
264
026
96 80
6105
1894
0107
0487
0368
0009
2941
1073
0019
0757
0048
1813
0537
942
Pargasite
41 08
446
12 39
012
11 35
017
13 13
11 36
263
015
96 85
6049
1951
0199
0494
0368
0014
2882
1030
0021
0752
0027
1792
0758
975
02OA37b
Mg-hornblende
rim
45 15
260
984
002
13 28
019
14 01
10 87
128
009
97 33
6527
1473
0203
0283
0690
0002
3018
0915
0023
0360
0016
1683
0562
828
Mg-hornblende
rim
44 72
263
988
004
13 23
021
14 01
10 78
133
009
96 92
6494
1506
0185
0287
0713
0005
3032
0894
0025
0374
0018
1677
0593
854
Mg-hornblende
rim
45 68
247
935
004
12 87
020
14 10
11 00
121
011
97 02
6621
1379
0218
0269
0598
0004
3045
0962
0024
0341
0020
1708
0562
815
Mg-hornblende
core
43 94
291
10 87
001
12 60
023
13 73
10 59
142
011
96 39
6410
1590
0278
0319
0644
0001
2985
0893
0028
0401
0021
1655
0710
860
Mg-hornblende
core
44 62
270
10 37
001
12 66
019
14 12
10 58
140
008
96 73
6479
1521
0253
0294
0644
0001
3057
0893
0023
0395
0014
1646
0674
851
Mg-hornblende
rim
47 79
181
796
002
12 09
025
15 22
11 09
100
009
97 33
6866
1134
0214
0196
0506
0002
3260
0947
0030
0280
0016
1706
0503
767
Mg-hornblende
core
45 43
249
10 3
000
11 99
022
14 50
11 14
133
008
97 48
6524
1476
0267
0269
0644
0000
3103
0796
0027
0370
0015
1715
0703
838
JOURNAL OF PETROLOGY VOLUME 45 NUMBER 6 JUNE 2004
1194
Sam
ple
nam
e1Mode2
SiO
2TiO
2Al 2O3
Cr 2O3
FeO
MnO
MgO
CaO
Na 2O
K2O
Total
Si
AlIV
AlVI
Ti
Fe3
thornCr
Mg
Fe2
thornMn
Na
KCa
XAn3
T(C)4
02OA90b
Mg-hornblende
core
48 83
038
745
005
939
010
17 29
12 24
142
006
97 21
6931
1069
0041
0041
0667
0005
3657
0448
0011
0392
0010
1862
0840
856
Mg-hornblende
core
49 49
033
731
003
916
008
17 40
12 57
139
005
97 82
6987
1013
0035
0035
0552
0004
3662
0530
0010
0379
0009
1901
0830
811
Mg-hornblende
rim
48 90
040
753
005
932
013
17 15
12 22
154
005
97 29
6948
1052
0042
0042
0575
0006
3632
0533
0015
0424
0010
1861
0880
869
Mg-hornblende
rim
48 30
044
800
007
962
010
16 86
12 32
161
005
97 35
6873
1127
0047
0047
0598
0008
3576
0546
0012
0443
0010
1879
0870
862
Mg-hornblende
rim
49 12
032
742
010
922
011
17 36
12 29
142
006
97 43
6956
1044
0034
0034
0621
0011
3665
0471
0013
0391
0010
1865
0840
845
02OA43a
Mg-hornblende
rim
48 13
040
789
007
860
007
17 41
12 19
155
006
96 36
6882
1118
0212
0043
0621
0008
3710
0407
0008
0429
0010
1867
-------
-------
Mg-hornblende
rim
50 93
035
643
001
759
006
18 50
12 24
114
003
97 28
7152
0848
0217
0037
0506
0001
3873
0385
0007
0311
0005
1841
-------
-------
Mg-hornblende
rim
50 67
014
559
009
12 33
026
15 28
12 44
068
003
97 50
7260
0740
0204
015
0483
0010
3263
0994
0032
0189
0005
1909
-------
-------
Mg-hornblende
rim
50 34
013
621
009
12 49
027
15 07
12 51
064
003
97 78
7192
0810
0240
0010
0530
0010
3210
0960
0030
0180
0010
1910
-------
-------
Mg-hornblende
rim
45 17
060
11 14
013
988
008
15 78
12 33
211
009
97 31
6473
1527
0355
0640
0644
0015
3371
0540
0010
0588
0016
1892
-------
-------
Mg-hornblende
rim
49 25
032
712
014
845
009
17 84
12 32
137
004
96 95
6983
1020
0170
0030
0620
0020
3770
0380
0010
0380
0010
1870
-------
-------
Mg-hornblende
rim
48 61
032
775
014
975
011
16 81
12 57
139
004
97 50
6905
1950
0203
0034
0621
0016
3560
0537
0013
0382
0008
1914
-------
-------
94Ma2
Anthophyllite
54 98
000
259
001
12 65
037
23 35
199
034
000
96 28
7744
0256
0174
0000
0161
0001
4903
1330
0043
0092
0001
0301
-------
-------
Anthophyllite
55 11
000
296
003
14 09
032
23 66
063
038
000
97 18
7716
0284
0205
0000
0115
0003
4938
1535
0038
0104
0000
0095
-------
-------
Actinolite
48 58
004
954
002
960
014
17 67
999
139
005
97 02
6850
0150
0435
0004
0690
0002
3713
0442
0017
0379
0009
1510
-------
-------
Actinolite
51 54
004
585
000
890
022
20 28
930
090
002
97 05
7214
0786
0178
0005
0598
0000
4230
0444
0026
0244
0004
1395
-------
-------
01OA41d2
Pargasite
rim
43 05
342
10 71
008
11 46
012
13 50
11 54
179
067
96 34
6353
1647
0216
0380
0368
0009
2967
1046
0015
0512
0127
1824
0613
863
Pargasite
rim
42 61
354
11 59
001
10 59
010
13 52
11 87
160
076
96 19
6286
1714
0302
0392
0299
0001
2972
1007
0012
0458
0143
1877
0715
844
Pargasite
core
42 54
401
11 04
006
11 81
013
13 21
11 50
202
039
96 71
6266
1734
0182
0444
0368
0007
2901
1087
0017
0576
0073
1814
0854
979
Pargasite
rim
43 03
385
10 72
002
11 70
014
13 50
11 95
146
080
97 17
6314
1686
0168
0425
0345
0002
2953
1091
0018
0415
0149
1878
0634
852
Pargasite
rim
42 65
341
10 60
006
11 97
014
13 22
11 64
183
068
96 20
6332
1668
0186
0381
0345
0007
2926
1141
0018
0528
0129
1851
0620
869
Pargasite
core
42 82
367
10 60
007
11 93
012
13 42
11 44
206
047
96 60
6315
1685
0158
0407
0391
0008
2950
1080
0016
0588
0089
1808
0615
899
Pargasite
rim
42 2
362
11 10
007
11 42
016
13 09
11 72
179
072
95 89
6282
1718
0229
0405
0299
0009
2905
1123
0020
0518
0137
1869
0715
875
Pargasite
rim
42 42
380
11 03
006
11 77
012
13 00
11 72
188
069
96 49
6283
1717
0209
0423
0299
0007
2870
1158
0015
0539
0131
1860
0817
928
Pargasite
rim
42 21
364
11 14
006
10 73
011
13 53
11 68
177
072
95 59
6277
1723
0229
0407
0322
0007
2999
1012
0014
0511
0136
1862
0736
883
Pargasite
rim
42 25
376
11 03
008
10 93
015
13 66
11 72
185
064
96 07
6257
1743
0182
0418
0345
0009
3016
1009
0019
0531
0121
1860
0766
912
Pargasite
core
42 17
396
10 91
008
11 10
010
13 57
11 49
209
038
95 85
6253
1747
0159
0442
0368
0010
3000
1009
0013
0602
0073
1826
0841
981
aMajorelem
entco
mpositionsarein
weightpercent-------noplagioclasean
alysed
andnotemperature
calculated
1Typ
eofam
phibolean
alysed
isalso
indicated
2Blankindicates
nozoningobserved
3Molefractionofan
orthitein
plagioclasead
jacentto
amphibolegrain
4Tem
perature
ofeq
uilibrium
fortheam
phibole---plagioclasepaircalculatedusingtheHollandamp
Blundythermometer
(1994)Errors
are40
C
BOSCH et al HYDROTHERMAL CIRCULATION IN OMAN OPHIOLITE
1195
zonation from An95 to An99 from core to rim (Table 1 andFig 8a) TheWR Sr isotopic ratio of the sample (070458)is significantly higher than normal MORB values Thiscould be related to the overprint of a low-temperaturealteration event consistent with the petrography of thesample
Green amphibole vein cross-cutting layered gabbros inWadi Mayah Haylayn Massif
Sample 02 OA43a belongs to a series of 1 cm thick greenamphibole veins (as in Fig 6e) cross-cutting the layeredgabbros The veins develop 15 cm thick reaction marginsin the enclosing gabbro and are exclusively composed ofacicular pale green magnesio-hornblende growing per-pendicular to the vein margin in a crack-seal mode(Fig 6f ) The reaction margins are composed of urali-tized gabbro marked by the development of the samepale green magnesio-hornblende in an inhomogeneouscalcic plagioclase matrix (An91---97) The Sr and O isotopiccompositions of this green hornblende (070431 andthorn31) do not correspond to normal MORB-sourcemantle values
Epidosite dyke from Wadi Gideah Wadi Tayin Massif
Sample 01 OA36b is representative of a series of epidoteveins cross-cutting poorly layered gabbros These 2 cmthick veins are composed of pink thulite in their centreand green pistachite at their margins (Fig 6a) Theirsharp contact with the enclosing gabbro is marked bythe development of coarse clinopyroxene crystals alteredin the epidote---greenschist facies This suggests that theepidote vein is replacive in a coarse-grained gabbro dykeThe highest 87Sr86Sr initial ratios of all the samplesstudied are recorded by the WR and coexisting epidoteof this sample (070519 and 070554 respectively)(Table 5) The Sr content of the WR and associatedepidote are also the highest of the present dataset(716 ppm and 764 ppm respectively) The Sr isotopiccomposition of this sample is consistent with a greaterdegree of exchange with a radiogenic component fromCretaceous seawater It is significantly higher than thevalue obtained for an epidosite vein from the Wadi Fizhsection of the Oman ophiolite [070435 with a Sr contentof 488 ppm reported by Kawahata et al (2001)] but ingood agreement with the average of the greenschist-faciesalteration sequence defined by Kawahata et al (2001)
GABBROS
01 km 025 km 1 km 36 km50
60
70
80
90
An
MA2
19d
19a 15f 128b
41d2
(a)
43a17b
15f
90b
37b
19a
19d
40
60
80
100
A
n
01 km 015 km 025 km 1 km
DYKES
(b)
Distance to the Moho
ACTINOLITE
MAGNESIOHORNBLENDE
TSCHERMAKITE
PARGASITEGabbros Dykes41d2MA2
19a d37b15f
90b43a
AlIV
150500
02
08
10
(Na
+ K
) A
(c)
04
06
950
900
850
800
750
70007 12 17
T degC
1000
(d)
AlIV
Fig 8 Plagioclase---amphibole compositions and results of thermometry (a) and (b) plagioclase compositions in layered gabbros and in dykesrespectively as a function of distance to the Moho (see text for sample identification) (c) amphibole compositions in layered gabbros and dykesaccording to Leakersquos (1997) nomenclature (d) equilibration temperatures calculated using the amphibole---plagioclase thermometer of Holland ampBlundy (1994) as a function of AlIV content in amphibole [same symbols as in (c)]
JOURNAL OF PETROLOGY VOLUME 45 NUMBER 6 JUNE 2004
1196
Table5RbandSrcontents87Sr
86Srandd1
8Oisotopiccompositionsofwholerocksandmineralseparatesfrom
samplesofmassivegabbrosdykes
andveinsfrom
theOman
ophiolitealsolistedarecalculated
waterrockratios(W
R)andLOIvalues
Sam
ple
Rb(ppm)
Sr(ppm)
87Rb8
6Sr
87Sr
86Sr
(87Sr
86Sr)i1
d18O
()
LOI(
)2WR
3(87Sr
86Sr)L14
(87Sr
86Sr)L25
(87Sr
86Sr)R6
01OA41d2
Whole
rock
037
1792
0006
0703516
09
070351
thorn480
138
47
0703587
0703651
0703357
Plagioclase
025
1249
0003
0704261
22
070426
thorn504
-------
-------
Pargasite
-------
-------
-------
0703548
20
070355
-------
-------
-------
Clinopyroxene
045
26 0
0050
0703845
19
070378
-------
-------
-------
01OA36b
Dykewhole
rock
0005
7164
50001
0705186
17
070519
-------
274
21 9
0705207
0705112
-------
Epidote
0003
7645
0001
0705536
12
070554
-------
-------
-------
Wallwhole
rock
005
4254
0001
0704637
09
070464
-------
191
-------
0704945
0704675
0704593
02OA43a
Green
hornblende
004
98
0012
0704323
13
070431
thorn310
-------
-------
97OA128b
Whole
rock
003
1274
50001
0703327
17
070333
-------
076
37
-------
-------
-------
Plagioclase
002
1204
50001
0703285
09
070328
thorn414
-------
-------
Clinopyroxene
003
20 0
0004
0704230
09
070422
thorn523
-------
-------
01OA15f
Whole
rock
006
1077
0002
0704582
16
070458
-------
237
13 5
-------
-------
-------
01OA19a
Whole
rock
037
1303
0008
0704189
18
070418
-------
161
96
-------
-------
-------
01OA19d
Whole
rock
058
2032
0008
0703423
08
070341
thorn567
216
415
0703574
0703408
0703271
Pargasite
091
1058
0025
0703273
11
070324
thorn287
-------
-------
Plagioclase
-------
-------
-------
-------
-------
thorn10 09
-------
-------
02OA90b
Plagioclase
004
2385
50001
0703868
11
070387
thorn498
-------
-------
Green
hornblende
010
23 8
0012
0704288
15
070427
thorn239
-------
-------
BOSCH et al HYDROTHERMAL CIRCULATION IN OMAN OPHIOLITE
1197
Table5continued
Sam
ple
Rb(ppm)
Sr(ppm)
87Rb8
6Sr
87Sr
86Sr
(87Sr
86Sr)i1
d18O
()
LOI(
)2WR
3(87Sr
86Sr)L14
(87Sr
86Sr)L25
(87Sr
86Sr)R6
02OA37b
Plagioclase
023
3121
0002
0704204
15
070420
-------
-------
-------
Pargasite
031
41 6
0021
0703505
15
070348
thorn394
-------
-------
94OAMa2
Whole
rock
0016
1750
00003
0703219
15
070322
thorn495
033
31
0703264
0703177
0703158
Plagioclase
0014
1363
00003
0703219
11
070322
thorn479
-------
-------
Clinopyroxene
-------
-------
-------
0703236
13
070324
-------
-------
-------
1Initial8
7Sr
86Srratiocalculatedat
95Ma(H
acker1994)
2LOIisloss
onignition(in)
3Water---rock
ratiocalculatedassumingaclosedsystem
TheeffectiveWR
ratiocanbeexpressed
bythefollowingeq
uation
W=Reffectivefrac14
frac12eth87Sr=
86SrrockTHORN fi
nal
eth87Sr=
86SrrockTHORN in
itial=frac12eth8
7Sr=
86SrseawaterTHORN in
itial
eth87Sr=
86SrrockTHORN fi
nal
ethCrock
initial=C
seaw
ater
initial
THORNwhere(87Sr
86Srrock) finalisthefinalmeasuredSrisotopicratioin
thesample(87Sr
86Srrock) in
itialco
rrespondingto
theinitialm
antlevalue
istakenas
070295(Lan
phere
etal1981)(87Sr
86Srseawater) in
itialistheCretaceousseaw
ater
Srisotopicco
mposition[87Sr
86Sr07073
at90
MaafterBurkeet
al(1982)]C
seaw
ater
initial
istheSrco
ntent
oflsquonorm
alrsquoseaw
ater
andis
takenas
8ppm
(deVilliers1999)
FortheCrock
initialitis
assumed
that
thepresentSrco
ncentrationmeasuredis
thesameas
theinitial
concentration
4Srisotopicratiosmeasuredforliquid
L1extractedafterthefirststep
oftheleachingexperim
ent(6NHClfor8min
at70
C)
5Srisotopicratiosmeasuredforliquid
L2extractedaftertheseco
ndstep
oftheleachingexperim
ent(2 5NHClfor30
min
at70
C)
6Srisotopicratiosmeasuredforresidueobtained
afteratotalaciddigestionachievedaftertheextractionofL1an
dL2leachates
JOURNAL OF PETROLOGY VOLUME 45 NUMBER 6 JUNE 2004
1198
Part of the same sample from the contact between thedyke and the gabbro yields a slightly lower Sr isotopiccomposition (070464) intermediate between the sur-rounding gabbro and the epidote dyke It also has anintermediate Sr content (425 ppm) Acid leaching experi-ments performed on the whole rock of the dyke yield Srisotopic compositions of 070521 and 070512 for thetwo liquids extracted Similar experiments for the samplelocated at the contact with the gabbro yield 87Sr86Srratios of 070494 and 070467 for L1 and L2 liquids TheSr isotopic composition of the residue extracted afterleaching remains high (070459) and very close to theepidosite 87Sr86Sr ratio determined by Kawahata et al(2001) In this sample the primary minerals have beentotally replaced by epidote and secondary plagioclase orquartz Thus the Sr isotopic compositions measured forthis WR sample and its constituent minerals reflect thecomposition of the fluid filling this dyke
Mineral chemistry
Plagioclase
Compared with the host gabbros the dykes show a muchlarger dispersion in their plagioclase compositions (Fig 8aand b Table 4) characterized by more calcic plagioclasethan the enclosing gabbros This tendency is mostextreme in the comb-textured dykes in which the plagio-clase is almost pure anorthite The zoning is generallynormal recording a crystal fractionation process How-ever gabbro 94 MA2 exhibits inverse zoning this tend-ency is also observed in plagioclases from dykes 01OA19a and 01 OA15f The higher calcium content ofthe plagioclase as well as the inverse zoning is consistentwith an increase of water pressure during its crystalliza-tion (Turner amp Verhoogen 1960 Carmichael et al 1974Koepke et al 2003) The particularly high An content inthe comb-textured gabbro dykes 01 OA15f may alsoresult from crystallization in a supercooled magma thisis consistent with the comb texture indicative of fast-growing crystals The lack of brown hornblende suggeststhat crystallization occurred above the temperature ofhornblende stability
Amphibole
The amphiboles analysed in the dykes (Fig 8c andTable 4) show a regular trend in a (Na thorn K)A vsAlIV diagram from titanium-rich (Ti 4 015) pargasitichornblende (brownamphibole) and tschermakite (brown---green amphibole) to low-titanium (Ti5 015) magnesio-hornblende (green amphibole) and actinolite (pale greenamphibole) The pargasitic hornblende develops aspoikilitic oikocrysts in the dykes 01 OA19a and d andin the diffuse patch 01 OA41d2 at temperatures above800C during the final stage of crystallization (see
below) The tschermakite composition corresponds tothe internal alteration of clinopyroxene in gabbro 94Ma2 occurs in the comb-textured dyke 01 OA15f andis the primary amphibole in the dioritic dyke 02 OA37bThe magnesio-hornblende develops at the edges of thepargasites or of clinopyroxenes at temperatures around800C and is the primary phase in the dioritic dyke 02OA90b and in the green vein 02 OA43a Finally actino-lite occurs as a replacement of green hornblende in dykesor in the kelyphitic rim surrounding olivine in gabbro 94Ma2 Such a large composition range at the scale of athin section has been reported in amphiboles fromamphibolite-facies oceanic gabbros (Stakes 1991 Stakesamp Taylor 1992 Gillis et al 1993) as well as in the Omanophiolite gabbros (Manning et al 2000) This variabilityhas been explained by rapid reaction rates preventinghomogenization (Manning et al 2000)
Thermometry
Plagioclase---amphibole thermometry could be appliedwhen the two minerals exhibited good contacts such asin the laminated gabbro of the middle sequence (01OA41d2) and in the intrusive dykes The temperaturesof plagioclase---amphibole equilibration (Table 2) havebeen calculated using the geothermometer lsquoBrsquo of Hollandamp Blundy (1994) Edenite thorn Albite frac14 Richterite thornAnorthite valid between 500C and 900C with anuncertainty of 40C Amphibole formulae are basedon 23 oxygen and their nomenclature is based on alkalications in the A site vs AlIV according to Leake (1997)Pressurewas assumed to be 1 kbar Forty-five plagioclase---amphibole pairs meet the compositional criteria imposedby Holland amp Blundyrsquos dataset (09 4 Xan 4 01 andAlIV 403) Electron microprobe measurements wereconstrained to contiguous plagioclase and amphiboleseparated by sharp grain boundaries assuming equili-brium of the two phases When the mineral phasesexhibit normal zoning temperatures between plagioclaseand amphibole cores and between mineral rims werecalculated assuming that the temperatures deduced fromthe mineral core chemistry provide a proxy for the high-est temperature of equilibration These data are identi-fied in the T C vs AlIV diagram (Fig 8d) Table 4presents selected major element amphibole analyses theAn content of the plagioclase adjacent to each amphibolegrain and the temperature calculated for the pairThe temperatures calculated (Fig 8d) span the entire
range of temperatures at which amphibole and plagio-clase coexist in equilibrium (ie amphibolite-facies para-geneses) This temperature interval is bracketed byorthopyroxene-bearing facies or amphibole-free facies(ie gabbro 94MA2) and epidote---actinolite facies devoidof homogeneous plagioclase (ie epidosite dyke 01OA36b and green vein 02 OA43a) The range of the
BOSCH et al HYDROTHERMAL CIRCULATION IN OMAN OPHIOLITE
1199
calculated temperatures between 4900C and 750Ccorresponds to the changing composition of amphibolesfrom pargasite to magnesio-hornblende at the scale of athin section or even at that of an individual crystal asshown by their correlation with AlIV such variabilityrecords rapid cooling in hydrous conditions The highesttemperatures are recorded by pargasite from the anatec-tic gabbro 01 OA41d2 in the gabbro dyke 01 OA19dand in the dioritic dyke 02 OA90b They were calculatedfrom minerals devoid of lower-grade alteration and arethus considered as representing the equilibrium temperat-ure of the coexisting minerals Lower temperatures cor-respond to mineral phases evolving at falling temperatureand are only indicative of the cooling history Includingthe 40C uncertainty seven pairs provide temperatureshigher than 900C the limit of validity of the Holland ampBlundy thermometer However we maintain these tem-peratures as indicative of the highest values of our data-set These values are included in the average equilibriumtemperature summarized in Table 2
DISCUSSION
Distribution of hydrothermal alteration
Two distinct petrological events are recorded in the gab-bro section of the Samail ophiolite and are documentedin the present contribution (1) the whole gabbro sectionis affected by a penetrative alteration (2) the gabbrosection is crosscut by various types of dykes and veinsincluding hydrous minerals
Penetrative alteration
The penetrative alteration developed in the gabbros atgrain boundaries and locally invading the minerals isubiquitous in the gabbro section This penetrative altera-tion has been described as very high temperature (VHT)high temperature (HT) and low temperature (LT) basedon alteration parageneses The orthopyroxene coronas inthe layered gabbro sample and the incipient appearanceof brown pargasitic amphibole mark a lower thermallimit of the VHT alteration By reference to the experi-mental work of Koepke et al (2003) the temperature ofequilibration for these reactions is estimated to bebetween 900 and 1000CThe preservation of the vertical distribution of
VHT---HT facies (Figs 4 and 5) also pointed out byManning et al (2000) indicates that the hydrothermalalteration pattern fits the distribution of isotherms withdepth at a fast-spreading ridge (ie Phipps Morgan ampChen 1993 Dunn et al 2000 Fig 2) Equilibrium tem-peratures for VHT parageneses as high as 900---1000Csuggest that VHT---HT hydrothermal alteration tookplace in a very restricted time interval at the limit ofthe cooling magma chamber It is proposed that the
penetrative alteration affecting the entire gabbro sectionis related to a pervasive network of microcracks whichact as a recharge system down to the Moho (Nicolas et al2003)
Dykes and veins
The dykes are extremely varied eg pegmatitic gabbroswith comb-texture microgabbros and amphibolitizeddolerites dioritic dykes alignments of poikilitic horn-blende and finally diffuse hornblende-bearing patchesHigh-T gabbro and diorite dykes and low-T greenamphibole---epidote veins are connectedIn the dykes textural evidence attests to suprasolidus
conditions at least for the development of the pargasiticamphibole Integrating the hornblende---plagioclase equi-libration temperatures calculated for the studied samplesthese temperatures are bracketed between 950C and875CIn the following discussion we shall consider separately
the microgabbro and dolerite dykes The latter representbasaltic melt intrusions associated upsection with thediabase sheeted dykes and downsection possibly withthe mafic dykes that are ubiquitous in the mantle section(Nicolas et al 2000) Whatever their origin the develop-ment of pargasitic amphibole during their late stage ofcrystallization creates a link with the gabbroic dykes andsuggests a crystallization process evolving from dry con-ditions to more hydrous conditionsThe other gabbroic dykes comb-textured gabbros and
hornblende-bearing dioritic dykes result from the fillingof cracks by a fluid either a melt or a silica-rich hydrousfluid These intrusive dykes which develop reaction mar-gins at their contact with the host gabbro grade verticallyinto lower-temperature green amphibole veins or align-ments of poikilitic hornblende in the layered gabbromatrix or development of pargasitic hornblende in ana-tectic gabbros (sample 01 OA41d2) In gabbros crystal-lizing in the magma chamber the development oforthopyroxene and pargasite oikocrysts enclosing the pri-mary minerals suggests a thermal evolution from thegabbro solidus to amphibolite-facies conditions Forthese reasons it is suggested that the gabbroic dykesresulted from the injection of a hydrous melt in the stillhot gabbros drifting away from the magma chamber asdiscussed below
Origin of fluids responsible for VHT---HThydrothermal alteration
The 87Sr86Sr initial ratios and d18O of whole rocks andpure mineral fractions are plotted in Fig 9 against thedistance above the Moho Transition Zone (MTZ) Theinitial Sr isotopic composition of the whole rocks apartfrom the epidosite dykes ranges from 070322 to
JOURNAL OF PETROLOGY VOLUME 45 NUMBER 6 JUNE 2004
1200
070458 All the studied samples (including whole rocksand minerals) have initial 87Sr86Sr ratios falling outsidethe field of fresh MORB which commonly have Sr ratiosbetween 07023 and 07029 with an average of 070265(ie Hart et al 1974) The lowest initial Sr isotopiccomposition (070322) from the massive gabbro 94MA2 is slightly but significantly higher than averageOman primary magmatic signatures (ie 070296McCulloch et al 1981) Similarly the d18O values ofmost whole rocks and minerals measured (Table 5)
depart from typical MORB-source mantle values( Javoy 1980 Gregory amp Taylor 1981 Kyser 1986)These differences indicate that the primary mantlesignatures in the Oman ophiolite have been modifiedby interaction with a fluid Possible origins for thisfluid are mantle components dehydration of subductedlithosphere or seawater circulation Strontium andoxygen isotopes are useful tracers for fluid---crust interac-tion as d18O and 87Sr86Sr ratios from fluids originat-ing from seawater the mantle or subducted lithosphere
-1000
1000
2000
3000
4000
5000
6000
7000
MOHO
MTZ
Dis
tan
ce a
bov
e th
e M
oh
o (
m)
pillow-basalt
sheeted-dike
gabbro
peridotite
01 OA 41d2
01 OA 36b
02 OA 43a97 OA 128b
01 OA15f
94 OA MA2
02 OA 90b01 OA 19ad
02 OA 37b
0702 0703 0704 0705 0706 0707
87Sr86Srinitial
MO
RB-s
ou
rce
Seaw
ater
Whole RockPlagioclasePargasiteGreen HornblendeClinopyroxene
Epidote
(a)
-1000
1000
2000
3000
4000
5000
6000
7000
MOHO
MTZ
Dis
tan
ce a
bov
e th
e M
oh
o (
m)
pillow-basalt
sheeted-dike
gabbro
peridotite
01 OA 41d2
01 OA 36b
02 OA 43a97 OA 128b
01 OA15f
94 OA MA2
02 OA 90b01 OA 19ad
02 OA 37b
Whole RockPlagioclasePargasiteGreen HornblendeClinopyroxene
3 5 10
δ18O (permil)
41d2
Ma2
43a
90b 19d 19d37b
128b
90b
(b)
Fig 9 (a) Variation of initial 87Sr86Sr isotopic composition as a function of the location of the studied samples in a schematic log of the crustalsection of the Oman ophiolite The field for MORB is from Hart et al (1974) and that for seawater is from Burke et al (1982) Small open squares aredata fromKawahata et al (2001) (b) Variation of d18O () as a function of the location of the studied samples in a schematic log of the crustal sectionof the Oman ophiolite Shaded curve corresponds to the data of Gregory amp Taylor (1981)
BOSCH et al HYDROTHERMAL CIRCULATION IN OMAN OPHIOLITE
1201
are significantly different Moreover the d18O valueis dependent on temperature and mineral fractiona-tion and thus yields complementary constraints to Srisotopes
Mantle origin
One model to explain the occurrence of fluids interactingwith the oceanic crust at very high temperatures is toderive them from the mantle The contribution of mantle-derived hydrous fluids linked to percolation processesor to their concentration in the final stages of gabbrocrystallization has been invoked in numerous case studies(ie Witt amp Seck 1989 Fabriees et al 1989 Dupuy et al1991 Agrinier et al 1993) O and Sr isotopic data forwhole rocks and mineral separates yield scattered valuesthat are unambiguously different from MORB mantle-source values (Fig 9a and b) With the exception of lateLT altered samples the WR data have a mean 87Sr86Srratio of 070337 and a standard deviation of 000012These surprisingly high values in mantle-derived rockscould be taken as evidence for survival of mantle isotopicheterogeneities during crystallization of the ophiolite (egLanphere 1981 McCulloch et al 1981) However the18O16O isotope ratios in the Oman gabbroic sequenceare also displaced from primary magmatic values Nogabbro in the present study has plagioclase or amphi-bole with typical mantle d18O values Thus the Sr and Oisotope data rule out the possibility of mantle-derivedfluids as a possible source for the VHT---HT hydrousalteration event recorded in the massive gabbros andgabbroic dykes and veins (Fig 10a and b)
Subducted lithosphere origin
Dehydration of subducted oceanic crust in subductionzones can generate hydrous fluids Such fluids couldpercolate through the overlying lithosphere and contam-inate it thus generating modified isotopic signatures andtrace element contents Such an explanation howeverdisagrees with the geochemical characteristics of most ofthe studied samples Fluids derived from the dehydrationof subducted slabs (fresh or altered MORB or OIB pro-toliths) should be strongly enriched in K Rb and Ba (egBecker et al 2000) whereas in Oman the Rb contentsmeasured in the gabbros and gabbroic dykes and veins arestrongly depleted Moreover the isotopic composition ofslab-derived fluids is buffered by the MORB-like oceaniccrustal protolith for Nd and Pb but not for Sr and O(Staudigel et al 1995) As a result of relatively minorfractionation of oxygen at high temperature the oxygenisotope composition of the fluids expelled during dehy-dration of the subducted slab should be high and essen-tially controlled by the oxygen composition of the uppersection of the crust altered at low temperature (with an
average of 10 and perhaps as high as 19) (Staudigelet al 1995) The Sr isotopic composition should be alsohigh (average 07046) as the fluids released by the dehy-dration of subducted oceanic crust are associated eitherwith seawater or with radiogenic Sr phases (ie smectites)The Sr and O isotopic compositions of the studied sam-ples do not fit with these signatures and so preclude asubducted lithosphere origin for the fluids involved in theVHT---HT alteration event
Seawater-derived fluids
All the analysed samples (minerals and WR) have 87Sr86Sr(T) outside the domain of primary MORB magmasand variable Sr contents Individual minerals have Srcontents reflecting their different mineralliquid partitioncoefficients Minerals characterized by a very high affi-nity for Sr such as plagioclase (ie Sr mineralliquidpartition coefficients 1) have very high Sr contentsTherefore the high abundance of plagioclase in the sam-ples analysed is responsible for the high Sr content of thestudied whole rocks The Sr content of all the whole rocksis relatively homogeneous (Table 5) without clear distinc-tion between country-rock gabbros and dykes and veinsand ranges from 110 to 200 ppm except for the epidosite(4700 ppm) Reported on a 87Sr86Sr vs 1Sr diagram(Fig 10a) most whole rocks and plagioclases analysed arecharacterized by a 1Sr ratio lower than 001 and plot inthe left part of this diagram Compared with N-MORBthe studied massive and anatectic gabbros and their con-stituent plagioclases have similar to slightly higher Srcontents but substantially higher 87Sr86Sr ratios(Fig 10a) Considering Cretaceous seawater as a possiblehigh 87Sr86Sr mixing component [87Sr86Sr 07073at 90Ma after Burke et al (1982)] responsible for theobserved geochemical modifications exchanges cannothave occurred in a closed system as seawater has too lowa Sr content (ie 8 ppm de Villiers 1999) An open-system process allowing penetration of larger quantitiesof seawater can efficiently modify the Sr isotopic ratio ofthe rocks Alternately the fluids could also be seawaterthat was modified through exchange with the surround-ing rocks during its transit through the oceanic crustalsection This modified seawater would be more enrichedin Srwith a 87Sr86Sr ratio intermediate between unmodi-fied seawater and MORBMinerals devoid of late LT alteration imprints (parga-
site green hornblende and pyroxene) and characterizedby very low Sr contents plot on the right of the diagramclose to or on the mixing line between seawater andMORB (Fig 10a) The difference between these mineralsand the other samples (WR and plagioclases) is mainlydue to the Sr fractionation coefficients of these differentminerals and to the large quantity of plagioclase inthe studied rocks The process responsible for the
JOURNAL OF PETROLOGY VOLUME 45 NUMBER 6 JUNE 2004
1202
modification of the Sr isotopic composition fromMORB-source value is however explained by the same phenom-enon As all these minerals display initial 87Sr86Sr ratiosdistinct from the MORB source and because they equili-brated at VHT or HT temperatures this supports anearly intervention of seawater during crystallization ofthese minerals Most of these samples with the exceptionof the epidosite dyke overlap the domains defined byKawahata et al (2001) for the Wadi Fizh gabbros alteredat VHT and HT conditions in the amphibolite facies(labelled lsquoVHTrsquo and lsquoHTrsquo in Fig 9a)Three samples (two epidosite dykes and an one epidote
fraction) have very high Sr contents and the highest Sr
isotopic compositions of the present dataset The twowhole-rock samples overlap the field of the Wadi Fizhsamples altered at high temperature during greenschist-facies metamorphism (Kawahata et al 2001) This typeof alteration is common at the ridge axis and formedthrough intensive exchange with seawater-derived fluidsFigure 10b indicates that all the Sr---O isotope data are
distinct from MORB compositions and suggests that thefluids reacting with the magmas could be derived fromseawater In addition the d18O values show that isotopicexchange occurred under VHT or HT conditions Thefluids could have the composition of seawater or theycould have the composition of seawater modified through
0703
0705
0707
001 01 1
1 Sr (ppm)
MORB
(87Sr
86Sr)
init
ial
Seawater
VHT
HT
MT36b
36b
36b
43a 128b90b
41d2
37b19d
15f
128b
41d2
41d2
19a
Ma2Ma2
19d
90b
37b
Whole RockPlagioclasePargasiteGreen HornblendeClinopyroxene
Epidote
(a)
0 2 4 6 8
δ18O(permil)
0703
0705
0707Seawater
MORB-mantle
128b 41d2
90b 41d2
Ma2128b
37b
43a
19d
90b
Whole RockPlagioclasePargasiteGreen HornblendeClinopyroxene
Epidote
HT LT
19d
(87Sr
86Sr)
init
ial
(b)
Fig 10 (a) Initial 87Sr86Sr isotopic ratios plotted against 1Sr for whole rocks and mineral separates from the gabbro section of the Omanophiolite Fields shown are from Kawahata et al (2001) and correspond to MT-----basalt sheeted dyke diabase altered at medium temperature(prehnite---actinolite facies sequence 3) HT-----diabase plagiogranite metagabbro and epidosite formed at high temperature (greenschist faciessequence 4) VHT-----gabbro altered at very high temperature (amphibolite facies sequence 5) The dashed line is a binary mixing line betweenMORB (Lanphere et al 1981) and Cretaceous seawater (Burke et al 1982) (b) Initial 87Sr86Sr isotopic composition plotted against d18O value forwhole rocks and minerals from the Oman ophiolite Cretaceous seawater and MORB end-members as in (a) Dashed line represents mixing curvebetweenMORB and seawater at high temperature (HT) Low-temperature (LT) exchanges between these two components would be responsible foran increase of the d18O primary MORB value
BOSCH et al HYDROTHERMAL CIRCULATION IN OMAN OPHIOLITE
1203
exchange with the surrounding rocks during its penetra-tion into the crustal section In an environment in whichfluid---rock interaction occurs at variable temperaturesthe d18O is greatly influenced by the temperature atwhich the exchange took place and by the waterrockratio It is evident from Figs 9b and 10b that none of thestudied gabbros have plagioclase and amphibole withMORB-like d18O and 87Sr86Sr values The separateminerals display a broad range of d18O values and mostexhibit extreme 18O depletion (Table 5) Hydrothermalexchange kinetics are similar in amphibole and pyroxeneand these two minerals are more resistant to oxygenexchange during water---rock interaction than coexistingplagioclase (Gregory et al 1989) The most probableexplanation of the low d18O values measured in bothamphiboles and pyroxenes is that they crystallized athigh temperature in the presence of a relatively low 18Oseawater-derived hydrothermal fluid [d18O from 01 to07 after Gregory amp Taylor (1981)] The anomalouslyhigh d18O value (thorn1009) measured for the plagioclase ofthe micro-gabbroic dyke (01 OA19d) can be interpretedas resulting from a superimposed overprint caused by alate-stage penetration of fluids at lower temperature Inthis sample the occurrence of such different 18O16Oratios between coexisting plagioclase and amphibole isnot consistent with equilibrium fractionation betweenthese two minerals at any possible temperature Thisobservation suggests that parts of the ophiolite sequencehave undergone a high-temperature hydrothermal eventprior to the later low-temperature event that producedthe strong 18O increase in coexisting plagioclaseThe depletion of the d18O values results from high-temperature hydrothermal alteration (Gregory amp Taylor1981 Stakes amp Taylor 1992) The oxygen isotopic datasupport the contention that most studied samples havebeen affected by a VHT---HT hydrothermal alteration byfluids derived from seawater Some of the analysed sam-ples presenting petrological evidence of crystallization ofLT secondary minerals have been overprinted by the latelow-temperature alteration thus swamping the earlierhigh-temperature alteration effects
Determination of waterrock ratio
To better evaluate the exchange between seawater andthe gabbroic rocks waterrock ratios (WR) have beenestimated for the whole rocks analysed during the courseof this study The different models used to constrain thisratio mainly differ by the calculation of the effective ratioor by the estimated weight ratio and by considering openor closed systems for water circulation (Albareede et al1981 McCulloch et al 1981) The WR ratios reportedin Table 5 have been calculated assuming a closed sys-tem Using this formulation these values are minimabecause the calculation always uses pristine fluid for the
initial fluid thus maximizing the value in the denomina-tor If the fluid was shifted to lower 87Sr concentrations byexchange with the rock on the downward path theinferred values would be much higher (Davis et al 2003)The calculated WR ratios for the samples from this
study range from 31 to 219 (Table 5) Two main groupsof samples can be recognized on the basis of their waterrock ratios The lowest ratios ranging from 31 to 47have been determined for massive gabbros or anatecticgabbros but also for dykes and veins devoid of late LTalteration Similar ratios have been previously calculatedfor example for the discharged hydrothermal solutions atthe 13N and 21N East Pacific Rise (Di Donato et al1990 Fouquet et al 1991) The gabbros from the Ibrasection in the Oman ophiolite gave effective WR ratiosof 05 33 and 42 (McCulloch et al 1981) in goodagreement with the values obtained in this study Theleast altered gabbro of the Ibra section yields a WR ratioof 05 in good agreement with the exceptionally fresh-ness of this rock characterized by typical mantle oxygenvalues for its minerals (McCulloch et al 1981) Samplescharacterized by waterrock effective ratios in the rangeof 3---5 can be related to penetrative alteration via thehydrothermal system Lecuyer amp Reynard (1996) havedemonstrated in oceanic gabbros from the Hess Deepaltered in similar HT conditions that WR mass ratios inthe range of 02---1 are sufficient to account for the mod-ified stable isotope signature of the gabbros Higherwaterrock values (WR 45) have been calculated forsamples affected by superimposed late hydrothermalalteration and for the epidosite samples (Table 5) Theyprobably acquired their isotopic characteristics throughinteraction with a larger volume of seawater duringgreenschist-facies metamorphism Similar or higherWR values have been published in previous studies(ie McCulloch et al 1981 Kawahata et al 2001)They correspond either to samples from the upper-crus-tal sequence (ie basalt sheeted dyke or plagiogranite) orto gabbros from the crustal section located in a particularenvironment such as for example the Wadi Fizh sectionthat is located close to a segment boundary along thepalaeo-spreading axis (Nicolas et al 2000)
Mechanism for seawater interaction withmagma
Nicolas et al (2003) have proposed that variable amountsof seawater can circulate at high temperature through-out the crustal section down to the walls of the magmachamber via a network of parallel microcracks a fractionof a millimetre wide Such microcracks and their relation-ship with the VHT---HT hydrous alteration in the sur-rounding gabbros have been described in detail byNicolas et al Via this network of microcracks largeamounts of seawater can have reacted with the magma
JOURNAL OF PETROLOGY VOLUME 45 NUMBER 6 JUNE 2004
1204
and induced variable changes of its Sr and O isotopiccomposition This regular network of parallel micro-cracks could constitute the recharge system and thehydrated gabbroic dykes the discharge system Physicalparameters and the geophysical models of Nicolas et al(2003) are in agreement with this scenario Such a pene-tration of seawater-derived hydrothermal fluids throughthe lower crust along grain boundaries and a microscopicfracture network at temperatures 4750C is consistentwith recent thermodynamic models (McCollom amp Shock1998)Seawater-altered and high-temperature recrystallized
diabases (protodykes) from the overlying sheeted dykecomplex stoped into the melt lenses could represent anindirect way for seawater to enter the system as they canremelt during their subsidence through the magmachamber A simple mass balance calculation suggeststhat this indirect contamination is not significant Assess-ments of the amount of recycled crust 1 in volumeby Nicolas et al (2000) based on the protodyke remnantsin the gabbro section or 20 by Coogan et al (2003)based on chlorine content in EPR basalts lead to a sea-waterrock ratio of the order of 104 to 103Thus following Nicolas et al (2003) we propose that
seawater has been introduced along microcracks down tothe walls of the magma chamber and has diffused alonggrain boundaries in the way described by Manning et al(2000) progressively invading the host gabbro Gabbroicdykes result from the injection of hydrous melt into thehost gabbros at falling temperatures as they drift awayfrom the magma chamber This water-saturated meltcould result from the extensive hydration of the crystal-lizing gabbros near the walls of the magma chamberwhen seawater penetrates through this wall (Fig 1)Similar plumbing systems driving seawater down to the
limit of the magma chamber and back to the surface havebeen already proposed in other environments such as theMid-Atlantic Ridge (Kelley amp Delaney 1987 Cooganet al 2001) the East Pacific Rise at Hess Deep (Gillis1995 Manning et al 1996a 1996b) the Tonga forearc(Banerjee amp Gillis 2001) and the Troodos ophiolite(Gillis amp Roberts 1999) The melting curve of wet basalthas a negative slope but is close to the dry solidus at lowpressure in the lower gabbros and the first melts areplagiogranitic (Spulber ampRutherford 1983) Such plagio-granites are ubiquitous in the highest gabbros and in theroot zone of sheeted dykes in Oman (Rothery 1983Juteau et al 1988 Nicolas amp Boudier 1991) in manyother ophiolites and in the oceanic crust where they havebeen interpreted as resulting either from wet anatexis ofhot gabbros or from the final product of crystallization ofa basaltic melt (Spulber amp Rutherford 1983) Plagio-granites are rare in the lower gabbros exceptionallyassociated with hydrous gabbro segregations inside thelayered gabbros Their constitutive minerals are also
remarkably absent along the VHTmicrocracks althoughthese gabbros were above the wet solidus A possibleexplanation has been proposed by McCollom amp Shock(1998) who wrote that at high temperature lsquoaqueousfluids may leave very little conspicuous petrological evid-ence for their presencersquo the reason being that thehigh-T conditions would be closer to batch meltingObviously more detailed studies on this specific problemare needed We conclude that the large degree of hydrousmelting locally required at the walls of the magma cham-ber to account for the generation of a gabbroic hydrousmelt may have erased the traces of the incipient plagio-granitic melt
CONCLUSIONS
Fostered by the recent discovery of high-temperatureseawater contamination in some gabbros of the Omanophiolite (Manning et al 2000) we have conducted astudy on 500 gabbro specimens from selected areas ofthe entire Samail ophiolite belt and on the hydrous gab-broic dykes that cross-cut them The highest-temperaturereactions (VHT) recorded in olivine and clinopyroxene ingabbros as orthopyroxene and pargasite coronas reach975C They are followed at lower HT conditions withreplacement of olivine by an assemblage of tremolitemagnetite and plagioclase surrounded by chlorite andby pargasite development in clinopyroxene followedfinally by the LT lizardite---olivine and saussurite---plagioclase greenschist-facies alteration With a fewexceptions the VHT alteration tends to be restricted tothe lower gabbros and to the vicinity of the Moho andthe HT alteration to the middle and upper gabbros Thepreservation of the vertical distribution of VHT---HTfacies indicates that progressive cooling during off-axisspreading did not obliterate this distribution and conse-quently that the VHT---HT hydrothermal alteration tookplace in a very restricted time interval at the limit of thecooling magma chamber In the deep and hot gabbrosthe main water channels may be sub-millimetre-sizedmicrocracks with a dominantly vertical attitude the ori-gin and dynamics of which have been investigated in aprevious paper (Nicolas et al 2003)Beyond these high-temperature alteration zones mas-
sively induced in the gabbros seawater may be respons-ible for the generation of clinopyroxene---pargasitegabbro dykes that cross-cut all gabbros and that areupsection clearly connected to the LT hydrothermalvein system Petrostructural evidence suggests that thesedykes were generated by hydration of the crystallizinggabbros near the internal wall of the LVZ---magmachamber The resultant melts were subsequently intrudedat various levels in the cooling lower and middle gabbrosPetrological observations of nearly all the gabbros ana-lysed in the present study located from the root zone of
BOSCH et al HYDROTHERMAL CIRCULATION IN OMAN OPHIOLITE
1205
the sheeted dyke complex down to the Moho emphasizethe imprint of VHT---HT hydrous alteration The VHThydrothermal event is characterized by temperatures ofequilibration around 900---1000C The temperatures atwhich the HT hydrous event occurred are estimated tobe in the range 550---900CStrontium and oxygen isotope compositions measured
on whole rocks and separated minerals significantlydepart from primary MORB values The Sr and O iso-tope data obtained for pargasite green hornblende andclinopyroxene record the participation of seawater at thetemperature of crystallization of the minerals Plagio-clases and whole rocks define a trend toward a compo-nent characterized by a high radiogenic 87Sr86Sr valueand a higher Sr content than normal seawater Seawateris clearly identified as the fluid responsible for the modi-fication of the Sr and O signatures for both massivegabbros and dykes and veins Nevertheless the high Srcontent of the extraneous component requires eitheropen-system exchange allowing penetration of a greaterquantity of seawater or penetration of seawater modifiedby interaction with the oceanic crust during its travel paththrough the crustal section The waterrock ratio calcu-lated on the basis of the Sr isotopes is between three andfive for the enclosing gabbros and for the least LT altereddykes The VHT---HT hydrothermal event affectingthese samples is related to penetrative alteration via therecharge and discharge hydrothermal system Thewaterrock ratio is 10 for dykes exhibiting evidenceof a LT hydrothermal alteration overprint and reaches22 for the epidosite dyke The depletion in d18O char-acterizes all the studied samples with the single exceptionof the micro-gabbroic dyke plagioclase This agreeswith the conclusions based on Sr isotopes suggestingthat these samples have undergone exchange with sea-water-derived fluids during a VHT---HTalteration hydro-thermal event
ACKNOWLEDGEMENTS
This work has benefited from financial support by theCentre National de la Recherche Scientifique (INSUInteerieur-Terre andDYETI programmes) and inOmanfrom the willingness of the Directorate of MineralsMinistry of Commerce and Industry and the hospitabil-ity of the people Technical help in the laboratory is alsoacknowledged including C Nevado for the thin sectionsE Ball for the illustrations B Galland for her assistancein the chemistry room and C Bassoulet for help duringthermal ionization mass spectrometric analyses of the Srresults from the leaching experiments Constructivereviews from R T Gregory C Lecuyer an anonymousreviewer and particularly from M Wilson greatlyhelped to improve an earlier version of the manuscript
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Fouquet Y Von Stackelberg S U Charlou J L Donval J P
Foucher J Erzig P Muhe R Wiedicke M Soakai S amp
Whitechurch H (1991) Hydrothermal activity in the Lau back-arc
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Gillis K M (1995) Controls on hydrothermal alteration in a section of
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Gillis K M amp Roberts M (1999) Cracking at the magma---hydrothermal transition evidence from the Troodos ophiolite Earth
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Gregory R T amp Taylor H P (1981) An oxygen isotope profile in a
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Gregory R T Criss R E amp Taylor H P (1989) Oxygen
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Hart S R Erlank A J amp Kable J D (1974) Sea floor basalt
alteration some chemical and Sr isotopic effects Contributions to
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Holland T amp Blundy J (1994) Non-ideal interactions in calcic
amphiboles and their bearing on amphibole---plagioclase thermo-
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Ionov D A Savoyant L amp Dupuy C (1992) Application of the
ICP-MS technique to trace element analysis of peridotites and their
minerals Geostandards Newsletter 16 311---315
Javoy M (1980) 18O16O and DH ratios in high temperature
peridotites In Colloques Internationaux du CNRS 272 279---287
Juteau T Beurrier M Dahl R amp Nehlig P (1988) Segmentation
at a fossil spreading axis the plutonic sequence of the Wadi
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physics 151 167---197
Juteau T Manacrsquoh G Moreau C Lecuyer C amp Ramboz C
(2000) The high temperature reaction zone of the Oman ophiolite
new field data microthermometry of fluid inclusions PIXE analyses
and oxygen isotopic ratios Marine Geophysical Research 21 351---385Kawahata H Nohara M Ishizuka H Hasebe S amp Chiba H
(2001) Sr isotope geochemistry and hydrothermal alteration of the
Oman ophiolite Journal of Geophysical Research 106 11083---11099
Kelemen P B amp Aharonov E (1998) Periodic formation of magma
fractures and generation of layered gabbros in the lower crust
beneath oceanic spreading ridges Annual Geological Union Special
Monograph 106 267---289
Kelemen P B Koga K amp Shimizu N (1997) Geochemistry of
gabbro sills in the crustmantle transition zone of the Oman
ophiolite implications for the origin of the oceanic lower crust Earth
and Planetary Science Letters 146 475---488Kelley D S amp Delaney J R (1987) Two-phase separation and
fracturing in mid-ocean ridge gabbros at temperatures greater than
700C Earth and Planetary Science Letters 83 53---66
Koepke J Feig S T Snow J amp Freise M (2003) Petrogenesis of
oceanic plagiogranites by partial melting of gabbros an experi-
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wwwspringerlinkcomopenurlaspgenre=s00410-003-0511-9
Kyser T K (1986) Stable isotope variations in the mantle In
Valley J W Taylor H P amp OrsquoNeil J R (eds) Stable Isotopes in
High Temperature Geological Processes Mineralogical Society of America
Reviews in Mineralogy 16 141---164
Lanphere M A (1981) K---Ar ages of metamorphic rocks at the base
of the Samail ophiolite Oman Journal of Geophysical Research 86
2777---2782
Lanphere M A Coleman R G amp Hopson C A (1981) Sr isotopic
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Leake B E (1997) Nomenclature of amphiboles report of the
subcommittee on amphiboles of the International Mineralogical
Association commission on new minerals and mineral names
European Journal of Mineralogy 9 623---651
Lecuyer C amp Reynard B (1996) High-temperature alteration of
oceanic gabbros by seawater (Hess Deep Ocean Drilling Program
Leg 147) evidence from oxygen isotopes and elemental fluxes
Journal of Geophysical Research 101(B7) 15883---15897
Liou J G Kuniiyoshi S amp Ito K (1974) Experimental study of the
phase relations between greenschist and amphibolite in a basaltic
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MacLeod C J amp Rothery D A (1992) Ridge axial segmentation in
the Oman ophiolite evidence from along-strike variations in the
sheeted dyke complex Geological Society of America Special Papers 60
39---63
Manning C E MacLeod C J amp Weston P E (1996a) Fracture-
controlled metamorphism of Hess Deep gabbros site 984
constraints on the roots of mid-ocean ridge hydrothermal systems
at fast-spreading centers In GillisM C Allan KM ampMeyer P S
(eds) Proceedings of the Ocean Drilling Program Scientific Results 147
College Station TX Ocean Drilling Program pp 189---211Manning C E Weston P E amp Mahon K I (1996b) Rapid high-
temperature metamorphism of East Pacific Rise gabbros from Hess
Deep Earth and Planetary Science Letters 144 123---132Manning C E MacLeod C J amp Weston P E (2000) Lower-crustal
cracking front at fast-spreading ridges evidence from the East Pacific
Rise and the Oman ophiolite Journal of the Geological Society London
349 261---272McCollom T M amp Shock E L (1998) Fluid---rock interactions in
the lower oceanic crust thermodynamic models of hydrothermal
alteration Journal of Geophysical Research 103 547---575
McCulloch M T Gregory R T Wasserburg G J amp Taylor H P
(1981) Sm---Nd Rb---Sr and 18O16O isotopic systematics in an
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Proceedings of the Ocean Drilling Program Scientific Results 147 College
Station TX Ocean Drilling Program
Nehlig P amp Juteau T (1988) Flow porosities permeabilities and
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the crustal sequence of the Sumail ophiolite (Oman) Tectonophysics
151 199---221
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1207
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Nicolas A Ceuleneer G Boudier F amp Misseri M (1988) Structural
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Nicolas A Boudier F Michibayashi K amp Gerbert-Gaillard L
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2372
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706---708
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281 697---734
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Petrology 24 1---25Stakes D S (1991) Oxygen and hydrogen isotope compositions of
oceanic plutonic rocks high-temperature deformation and meta-
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Geochemical Society Special Publication 3 77---90
Stakes D S amp Taylor H P (1992) The northern Samail ophiolite an
oxygen isotope microprobe and field study Journal of Geophysical
Research 97(B5) 7043---7080Staudigel H Davies G R Hart S R Marchant M amp Smith B
M (1995) Large scale isotopic Sr Nd and O isotopic anatomy of
altered oceanic crust DSDPODP sites 417418 Earth and Planetary
Science Letters 130 169---185Turner F J amp Verhoogen J (1960) Igneous and Metamorphic Petrology
New York McGraw---Hill 694 pp
Wilcock W S D amp Delaney J R (1996) Mid-ocean ridge
sulfide deposits evidence for heat extraction from magma
chambers or cracking fronts Earth and Planetary Science Letters 145
49---64
Witt G amp Seck H A (1989) Origin of amphibole in recrystallized
and porphyroclastic mantle xenoliths from Rhenish massif implica-
tions for the nature of mantle metasomatism Earth and Planetary
Science Letters 91 327---340
JOURNAL OF PETROLOGY VOLUME 45 NUMBER 6 JUNE 2004
1208
Taylor 1981) to as high as nine (Kawahata et al 2001)Oxygen isotope data (C Lecuyer personal communica-tion 1990) on mafic dykes emplaced in the peridotites at1 km below the Moho suggest a dual source of hydrationfor the gabbros in which clinopyroxenes have a mantlesignature but associated plagioclase and brown horn-blende show evidence of seawater contamination Thisdual source was not ruled out by Benoit et al (1996) intheir Sr isotope study of similar dykes injected into themantle section Rare earth element partition coefficientsin clinopyroxene plagioclase and amphibole were usedby the above workers to constrain the temperature ofseawater contamination at the gabbro solidusThus there is growing evidence that supports a deep
penetration of high-T hydrothermal fluids into theoceanic crust at the ridge axis We have conducted asystematic study of the gabbros in the Samail ophioliteand along selected wadi sections (Fig 1) in the hydratedgabbroic dykes that cross-cut them looking for traces ofHT alterationThis study combines microtextural and petrological
observations thermometric determinations and O---Srisotope data on the same gabbroic dykes and surroundinggabbros selected with a view to determine the origin ofhydrous fluid contamination These results have import-ant consequences for thermal modelling of the oceaniccrust near fast-spreading ridges and on the budget ofchemical exchanges between the oceanic crust andseawater
HYDROUS ALTERATION OF
PRIMARY MINERALS IN THE
GABBRO SECTION
The penetrative alteration of the gabbro section has beenstudied on the basis of an extensive thin-section collec-tion which covers the entire ophiolite The studied gab-bros range from the root zone of the sheeted dykecomplex down to the Moho and the Moho transitionzone (MTZ) From the 500 specimens that have beenscreened those sites involved in tectonic complexitiessuch as the end of segments or tips of propagating rifts(Fig 1) have been rejected thus reducing the number ofstudied areas and the number of fully characterized speci-mens to 414 representative of standard gabbros at a fast-spreading palaeo-ridge In this study we mainly focus onhydrous alteration of olivine and clinopyroxene as theseare the minerals most affected by high-T alteration(above 800C) although plagioclase is stable down tothis temperature
Alteration facies
The following types of hydrous parageneses in thegabbros have been distinguished from highest to lowesttemperatures
(1) Orthopyroxene partial coronas between olivine andplagioclase which may be associated with the firstappearance of brown pargasite blebs inside clinopyrox-ene and development of pargasite between olivine andplagioclase or clinopyroxene and plagioclase (Fig 3a) Thebrown pargasite locally grades into a greenish magnesio-hornblende Although the orthopyroxene coronas couldbe assigned to a late magmatic stage we interpret themas reactive (a) being restricted to the contact betweenolivine and plagioclase thus postdating the crystallizationof plagioclase (b) being locally relayed by kelyphiticcoronas that spread into plagioclase microcracks(Fig 3b) as microphases among which diopside andlow-titanium pargasite have been identified We inferthat these reactions are related to an oxygen fugacityincrease as a result of seawater input Based on experi-ments at 2MPa pressure Koepke et al (2003) have con-strained the equilibrium olivine---orthopyroxene at lowwater content at 1030C and 950C at higher watercontent the limit at which amphibole is stabilized Byreference to this experimental work we estimate that thetemperature of equilibration for these reactions referredto here as VHT (very high temperature) hydrous altera-tion was between 900 and 1000C(2) Chlorite and pale amphibole coronas between oliv-
ine and plagioclase (Fig 3d) The coronas are composedof successive rims of clino-amphibole (tremolite) thornmagnetite Mg-chlorite (clinochlore) and a fringe of ortho-amphibole against plagioclase As described above theseminerals are associated with blebs of brown to greenishamphibole inside the pyroxenes (Fig 3c) Depending onthe degree of alteration olivine is surrounded by a thinfilm of amphibole and chlorite or is totally replaced by anassemblage of tremolite thorn magnetite thorn plagioclase An91(thorn talc) surrounded by chlorite (Fig 3e) The evolution ofthe first assemblage to Mg-chlorite indicates temperat-ures above 700C the upper breakdown of Mg-chloriteat 2 kbar (Chernosky et al 1988) The development ofpargasitic amphibole replacing clinopyroxene impliestemperatures between 550 and 900C (Liou et al 1974Spear 1981) We refer to these as HT (high temperature)(3) Iddingsite partly or totally replacing olivine inde-
pendently of orthopyroxene or chlorite---amphibole cor-onas and any significant HT alteration in clinopyroxene(Fig 3f ) Observations in thin sections show that thechlorite---amphibole coronas develop before the idding-site alteration(4) Lizardite alteration of olivine forming the familiar
mesh structure This is always accompanied by saussuritein plagioclase This alteration takes place below 500Cand is here referred to as LT (low temperature) hydrousalteration It is only at this stage that plagioclase showssigns of alterationThese various hydrous alteration facies are commonly
observed superimposed in a set of thin sections from a
JOURNAL OF PETROLOGY VOLUME 45 NUMBER 6 JUNE 2004
1184
single outcrop and even in a single thin section As a rulethe orthopyroxene reactions are more penetrative beinghomogeneously distributed from the scale of a thinsection to that of several thin sections made from a
given outcrop The distribution of chlorite---amphibolecoronas is similar although they can be restricted to themargins of microcracks filled by the same minerals whichhave been interpreted as the channels that carried the
(a) (b)
(c) (d)
(e) (f)
olopx
kel
pl
ol
hbol
amcpx
cpx
hb
chl
am+pl+mt
idg
am
hb
pl
hb
ol
cpx
opx
mt
pl
Fig 3 Hydrothermal alteration in the gabbros (scale bar represents 1mm) (a) VHT alteration marked by orthopyroxene (opx) coronas at thecontact of olivine (ol) and plagioclase (pl) and pargasitic hornblende (hb) at the contact of olivine and clinopyroxene (cpx) (sample 94 MA2)(b) Kelyphite (kel) coronas around olivine and cloudy plagioclase as a result of silicate and fluid inclusions Silicate inclusions tend to be localizedat grain boundaries and along microcracks (sample 94 MA2) (c) VHT alteration marked by brown amphibole (hb) alteration of clinopyroxene in anupper gabbro (d) HT alteration marked by chlorite (chl) (external rim) and pale amphibole (am) and magnetite (mt) (internal rim) coronas aroundolivine (e) Total replacement of olivine by an assemblage of tremolite (am)thornmagnetite (mt)thorn plagioclase (pl) (f ) Total replacement of olivine by aniddingsitic mixture (idg) inside a VHT recrystallized corona of a mosaic of orthopyroxene
BOSCH et al HYDROTHERMAL CIRCULATION IN OMAN OPHIOLITE
1185
fluids responsible for the hydrothermal alteration (Nicolaset al 2003) The iddingsite replacement is typicallyheterogeneous at the thin-section scale being preferen-tially associated with microcracks Serpentinization alsodevelops from microcracks but it can pervasively invadethe entire thin section The microcracks are commonapproximately 25 of the studied thin sections displayone or more Ignoring the cracks related to LT alterationand considering only the cracks related to the HT hydro-thermal alteration this fraction is still around 20
Distribution of high-T alteration facies
Figure 4 shows the distribution of alteration facies as afunction of depth in the crust along wadi sections throughthe Oman---UAE ophiolite offering continuous exposures(see details in Fig 1) Figure 4 excludes the LT faciesbecause (1) being based on the presence of olivine ser-pentinization only throughout our compilation its dis-tribution is biased by the decreasing amount of olivineupsection and (2) this study concerns high-temperaturealteration processes Despite the fact that the scale ofsampling may exceed the scale of heterogeneity evidencefor both VHT alteration and lsquono alterationrsquo increaseswith depth Conversely HT alteration is predominantin the middle and upper parts of the gabbro sectionThis observation is valid throughout the wadi sectionsstudied with the exception of the Wadi Tayin massifwhich shows no clear gradation with depthFigure 5 is another representation of alteration-facies
distribution with depth based on the detailed study of414 thin sections The samples are classified according totheir level in the gabbro section sheeted dyke root zonegabbros upper gabbros middle gabbros lower gabbrosand Moho gabbros (the Moho gabbros have beengrouped together with the gabbro lenses interlayered inthe dunites of the MTZ) Values represent the percent of
2 Km
3
4
5
6
7
Aswad Fizh Hilti Haylayn Samail W TayinNakhl
High T
Very High T
0 High T
Upper Crust
LID
Lower Crust
MohoMTZ
DEPTH1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27
Fig 4 Depth of penetration of hydrothermal circulation in gabbros along the wadi sections numbered in Fig 1 referring to alteration faciesdescribed in Fig 3 Depths are recorded below the palaeo-seafloor LID corresponds to the upper crust including sheeted dykes and volcanics
0
10
20
30
40
50
60
70
80
90
100
SD root
zone
upper
gabbros
middle
gabbros
lower
gabbros
MTZ
BHb-GHb CPX
Chl-Amph core OL
Chl-Amph rim OL
0
5
10
15
20
25
30
SD root
zone
upper
gabbros
middle
gabbros
lower
gabbros
MTZ
Opx OL
BHb OL
No VHTHT
Unaltered OL core
HT
VHT
(a)
(b)
Plst def
gabbros
Fig 5 Compilation of hydrothermal alteration features as a function ofdepth in the gabbro section Percentages represent the occurrence of agiven type of alteration versus the number of thin sections examined foreach selected level root of the sheeted dykes (SD) upper gabbros middlegabbros lower gabbros MTZ (Moho Transition Zone) gabbros on atotal amount of 414 thin sections (Opx orthopyroxene Amph amphi-bole OL olivine BHb brown hornblende GHb green hornblendeChl chlorite Plst def plastic deformation) (a) Distribution of the HTalteration (in ) (b) Distribution of the VHT alteration (in )
JOURNAL OF PETROLOGY VOLUME 45 NUMBER 6 JUNE 2004
1186
a given alteration facies as defined previously out of thetotal number of thin sections examined for a given levelMore refined conclusions can be drawn from this figureAs seen in Fig 5a the HT alteration decreases down-
wards whereas it affects most of the upper gabbroicsection (sheeted dyke root zone and upper gabbros)The fraction of unaltered olivine cores can be used asan index to estimate the extent of alteration This indexshows that the HT alteration is more penetrative andintense in the upper gabbros a conclusion alreadyreached by several workers (eg Gregory amp Taylor1981 Stakes amp Taylor 1992 Kawahata et al 2001)This was also noted by Manning et al (1996a 1996b)Figure 5b shows that the VHT alteration marked by
orthopyroxene or brown hornblende coronas aroundolivine increases with depth The correlation betweenthe VHT alteration and plastic deformation also suggeststhat the VHT alteration may coincide with the develop-ment of plastic flow at the expense of the magmatic flow
at temperatures just below the solidus In Fig 5b thefraction of gabbros where there is no VHT---HThydrothermal alteration increases downsection indicat-ing that the VHT---HT alteration is more localizeddownsection
Gabbroic dykes and veins
The occurrence of gabbroic dykes in the layered gabbrosof the Samail ophiolite is ubiquitous We describe gab-broic dykes and sills either plagioclase---clinopyroxeneand brown amphibole-bearing dykes or microgabbrosand amphibolitized dolerite dykes and sills correspond-ing to temperatures up to 975C (see below) Identifica-tion of gabbroic dykes in the field is not alwaysstraightforward because they may be obliterated bysuperimposition of greenschist-facies alteration (Fig 6a)Selective greenschist-facies alteration has frequently beenobserved either in the centre of a mafic dyke or alongboth margins (Fig 6b) as well as a continuous transition
(b) (c)(a)
(d) (e)
thulite
pistacite
white microvein
white vein
green vein
green vein
gabbroic dike
epidote vein
gabbroic dike
Fig 6 Field relationships of dykes and veins (scale bar represents 10 cm) (a) Thulite---pistacite vein cross-cutting a foliated gabbro matrix Reactionmargins marked by the development of secondary clinopyroxene (01 OA36b) (b) Comb-like structure in gabbroic dyke A pink epidote vein followsthe margin of the dyke (01 OA15f) (c) Along-strike transition from a gabbroic dykelet to a hydrothermal green amphibole vein (d) White prehnitevein (e) Green amphibole vein cross-cut by white prehnite vein
BOSCH et al HYDROTHERMAL CIRCULATION IN OMAN OPHIOLITE
1187
along a single crack from a mafic dyke to a hydrothermalvein (Fig 6c) We define hydrothermal veins as cracksfilled with greenschist-facies minerals that induce meta-morphic reactions in the adjacent country rocks andgabbroic dykes as planar intrusive bodies produced bythe crystallization of magma Following Nehlig amp Juteau(1988) we distinguish low-T greenschist-facies lsquowhiteveinsrsquo made of prehnite epidote chlorite and actinolite(Fig 6d) and lsquogreen veinsrsquo in which a darker greenamphibole predominates (Fig 6e) corresponding respec-tively to lower (195---410C) and higher (400---530C)temperatures (Nehlig amp Juteau 1988)The gabbroic dykes are typically a few centimetres
across composed of plagioclase clinopyroxene andbrown
hornblende They are coarse-grained to pegmatitic andcommonly display a comb-like structure (Fig 6b)Although the spacing between the dykes in the field ishighly variable the average is around 10m Gabbroicsills grade from clear intrusions to irregular and diffusepegmatitic gabbro patches up to a few metres in theirlarger dimension (Fig 7a)The distribution in the field of brownish pargasite
which appears black and glassy in outcrop is critical intracing the origin of the gabbroic dykes It is first observedat any level in the host lower and middle gabbros as largepoikilitic patches (Fig 7b) locally associated with simil-arly poikilitic patches of clino- and orthopyroxene(Fig 7a) These patches contain small aligned tablets of
(a) (b) (c)
g(e)(d)
alignment ofalignment of amphiboleamphibole oikocrystsoikocrysts
hwhite vein
hblpl
pcpx
brownbrownamphiboleamphiboleoikocrystoikocryst
eediabasedikdike
hb-plhb-plreactionreaction
imarginspbrown amphibole rich
gabbroic dikegabbroic dike
pbrown amphibole veins
Fig 7 Field relationships of dykes (scale bar is 10 cm long) (a) Irregular coarse to pegmatitic patches in a foliated gabbro matrix (01 OA42d)(b) Alignment of black amphibole oikocrysts in a foliated gabbro matrix (c) Local concentration of brown amphibole in a gabbroic dykelet Theplagioclase laths marking the foliation in the enclosing gabbro rotate into parallelism with the gabbroic dykelet (d) Two diabase dykes assimilatingthe enclosing layered gabbro with plagioclase---amphibole reaction margins (e) Pargasite-rich gabbroic dyke with wall reactions (01 OA19) (Notesmall pargasite---plagioclase veins) pl plagioclase cpx clinopyroxene hb hornblende
JOURNAL OF PETROLOGY VOLUME 45 NUMBER 6 JUNE 2004
1188
plagioclase oriented parallel to the larger plagioclaselaths defining the magmatic foliation of the gabbro(Fig 7c) Clinopyroxene oikocrysts surrounding idio-morphic plagioclase crystals are common in the upper-level gabbros they mark the end of the magmatic flowand crystallization growing before the pargasite andorthopyroxene oikocrysts as shown by their partialreplacement by pargasite Their crystallization marksthe transition at suprasolidus conditions from a dry to awet environment In the uppermost gabbros those dykesthat are typically altered in the HT hydrothermal faciesgrade upwards into the so-called lsquoisotropicrsquo or lsquovari-texturedrsquo gabbrosThe microgabbro and amphibolitized dolerite dykes
differ from the typical diabase dykes commonly cuttingthe gabbro section that have chilled margins and thusprobably intruded enclosing rocks already at temperat-ures below 450C In microgabbro and amphibolitizeddolerite dykes the absence of chilled margins and thedevelopment of dioritic reaction margins in the adjacentgabbro (Fig 7d and e) suggest that they were emplaced ingabbros that were still at temperatures above 750C(Nicolas et al 2000)
ANALYTICAL TECHNIQUES
Major and trace elements
Major element compositions were determined by elec-tron microprobe at the University of Montpellier II usinga Camebax SX100 Instrument calibration was per-formed on natural standards and operating conditionswere 20 kV accelerating potential 10 nA beam currentand 30 s counting timeRb and Sr concentrations in whole rock and separated
mineral fractions were measured by conventionalnebulization inductively coupled plasma mass spectro-metry (ICP-MS) using a VG Plasmaquad 2 (ISTEEMMontpellier) Digestion procedures followed thoseoutlined by Ionov et al (1992)
O---Sr isotopes
The samples selected for the detailed study wereextracted from gabbroic dykes or veins inside gabbrosand cut with a diamond saw into 2---3 cm fragmentsbefore being crushed into 05---1 cm fragments Forwhole-rock analysis small pieces were further crushedinto powder in an agate bowl For pure mineral analysisthe fragments were crushed with a manual agate mortarTo avoid or minimize the mixing of selected separateminerals with other phases that can bias the results astrict protocol of mineral selection was applied A smallsize fraction of 125---160 mm was used and minerals werefirst separated using a Frantz isodynamic magneticseparator Concentrates recovered (cpx plagioclase
amphibole epidote) were subsequently purified by care-ful hand-picking in alcohol under a binocular micro-scope At this stage mineral separates were very pureand only flawless minerals free of cracks and visibleinclusions were kept for isotopic analyses To ensurefurther mineral integrity these pure mineral separateswere ultrasonically washed in dilute 15N HCl and rinsedseveral times with ultra-pure water This stage potentiallyremoves adsorbed secondary micro-phases Previousstudies (eg Lecuyer amp Reynard 1996) demonstratedthat pyroxene and amphibole can be intermixed on avery fine scale with other phases (such as a range ofamphibole compositions talc Fe-oxide) Although thispossibility cannot be ruled out the procedure used inthis study makes it unlikely that complex minerals passedthrough the selection
Sr isotopic compositions
Approximately 50mg of whole-rock powder or separatedminerals were digested in a Teflon lsquobombrsquo with a mixtureof triple distilled 13N HNO3 and 48 HF with a fewdrops of HClO4 Two aliquots were separated one forthe determination of Sr isotopic composition and theother for Sr and Rb contents Sr was separated using anextraction chromatographic method modified from Pinet al (1994) Concentrations were measured by isotopedilution using 84Sr---87Rb mixed spikes To remove tracesof the late low-temperature seawater alteration and todetermine the primitive whole-rock Sr isotopic composi-tion leaching experiments were conducted on a fewsamples following the procedure described by Bosch(1991) The strontium isotopic compositions of leachateshave been analysed in the attempt to check for the leach-ing efficiency During these tests four Sr isotope composi-tions were measured which correspond to the unleachedsample the two liquids extracted after acid-leaching stepsand the final solid residue Leaching experiments wereconducted in two steps first with 6N HCl for 8min at70C second with 25N HCl for 30min at 70C Afterleaching the leachates were evaporated to dryness andthe solid residues digested similarly to the unleachedsamplesSr isotopic compositions were measured on a fully
automated VG Sector mass spectrometer housed at theUniversity of Montpellier II except for the Sr isotopicdata from the leaching experiments which have beenmeasured at the Universitee de Bretagne Occidentale(Brest) To assess the reproducibility and accuracy of Srisotopic measurements the NBS 987 standard was run12 times during this study the average value was0710245 with a precision better than 0000010 (2s)87Sr86Sr ratios are corrected for mass discrimination bynormalizing to a 86Sr88Sr value of 01194 Sr total blankconcentrations were less than 60 pg Initial 87Sr86Sr
BOSCH et al HYDROTHERMAL CIRCULATION IN OMAN OPHIOLITE
1189
ratios were calculated by correcting the measured ratiosfor accumulation of 87Sr produced by in situ 87Rb radio-active decay since 95Ma the assumed age of crystalliza-tion of the Oman ophiolite (Hacker 1994)
Oxygen isotopes
The powdered samples were introduced in Ni tubesunder dry air where they were reacted with BrF5 toform oxygen Oxygen was separated from other gasesby cooling the resulting mixture at liquid nitrogen tem-perature (77 K) that retained other volatile gases Oxygenwas further purified by trapping it on a cooled 5A mole-cular sieve at 77 K then quantified to calculate the yieldof the isotopic analyses and finally transferred to the massspectrometer (Finnigan Mat Delta E) where intensitiesassociated with masses 32 and 34 were measured todetermine d18O The isotopic composition of a sampleis given as dsample frac14 (RsampleRstandard 1) 1000 in permil unit where R is 18O16O and the standard isSMOW Analytical errors on the oxygen isotope ratioare 02 (2s)
RESULTS
Seven samples representative of the range of gabbroicdykes and five samples representative of the lower gab-bros two of them representing the margins of gabbroicdykes were selected for major element thermometricand Sr---O isotopic analyses (Tables 1---3) These samplesoriginate from wadi sections marked by bold lines inFig 1 Wadis Halhal Mayah Al Abyad Warihya andMaqsad structurally belong to a new ridge segment open-ing in slightly (1Myr) older lithosphere Wadi Gideahis in older lithosphere
Petrographic and isotopic characteristicsof the analysed samples
Olivine gabbro from the MTZ in MaqsadSamail Massif
Sample 94 MA2 is devoid of low-temperature alterationand only exhibits the discrete signature of the VHT---HTalteration The orthopyroxene corona around olivine(Fig 3a) is relayed by kelyphitic rims at the contactwith plagioclase (Fig 3b) Issuing from these rims aremicrometre-sized pale green amphiboles identified ascummingtonite and actinolite which penetrate the sur-rounding plagioclase in microcracks 004mm wide or atgrain boundaries (Fig 3b) similar to the microcrack sys-tem described by Manning et al (2000) A large numberof the plagioclase crystals have a cloudy core as a result ofthe concentration of fluid and mineral inclusions amongwhich diopside has been identified during microprobeanalysis Olivine cores are free of serpentine but theyare largely oxidized along a microcrack network Ortho-pyroxene coronas have a MgOpx number of 78---79
slightly higher than in the primary olivine cores withMgOl number of 74---75 The magmatic clinopyroxeneis totally fresh and probably not in equilibrium with thereactive orthopyroxene This sample has the lowest 87Sr86Sr initial ratio of the studied whole rocks (070322)identical to coexisting clinopyroxene (070324) and pla-gioclase (070322) This observation suggests the lack of alate superimposed LT hydrothermal alteration as it isvery unlikely that an LT hydrothermal event could havetotally equilibrated the Sr isotopic compositions of prim-ary minerals such as plagioclase and clinopyroxeneMoreover the whole rock (WR) clinopyroxene andplagioclase have similar initial Sr isotopic ratios despitevery different Sr contents Accordingly this ratio isthought to reflect the signature acquired during the crys-tallization of these minerals and is higher than the Omanmantle Sr isotopic value (McCulloch et al 1981) Thed18O values of the WR (50) and plagioclase (48) aredistinctly lower than primary magmatic values Indeedin normal mid-ocean ridge basalts (MORB) the WRd18O value is 57 02 with a corresponding plagio-clase d18O ranging from 55 to 62 ( Javoy 1980Gregory amp Taylor 1981)
Lower gabbro from Wadi Wariyah Wadi Tayin Massif
Sample 97 OA128b belongs to a layered sequence in thelower gabbro series injected by anorthositic sills Thesample was collected at the tip of one of these sillsbetween layers enriched in clinopyroxene The gabbrois composed of 60 plagioclase and 40 clinopyroxeneDespite the absence of olivine the WR has a relativelyhigh Mg content (Table 3) A series of low-T (prehnite)microveins 01mm thick cross-cut the gabbro perpendi-cular to the layering and foliation The margins of theveins are devoid of alteration This gabbro has the sameSr isotopic composition as 94 MA2 Nevertheless theWR and associated plagioclase have Sr isotope signatures(070333 and 070328) lower than the coexisting clino-pyroxene (070422) The Sr contents of the WR plagio-clase and clinopyroxene are 127 ppm 120 ppm and20 ppm respectively thus making the clinopyroxenemore sensitive to late-stage contamination Clinopyrox-ene and plagioclase oxygen isotope compositions (523and 414 respectively) are also lower than typicalmantle values Similar to the previous sample alterationat high temperature by a low d18O and high 87Sr86Srfluid is required to explain such characteristics
Amphibole gabbro patch Wadi Gideah Wadi TayinMassif
Sample 01 OA41d2 is from a laminated gabbro com-posed of millimetre-sized layers of clinopyroxeneand plagioclase Anatectic patches (Fig 7a) borderinganorthositic layers are formed of coarse-grained
JOURNAL OF PETROLOGY VOLUME 45 NUMBER 6 JUNE 2004
1190
Table1Locationoftheanalysed
samplesandsummaryoftheirpetrographicandSr---O
isotopiccharacteristics
Sam
ple
nam
eSite
Locationin
gab
broic
section
Typ
eofrock
Distance
to
Moho(km)
Parag
enesis
Alteration
87Sr
86Srinitial1
d18O
()2
94MA2
Maq
sad
From
MTZ
Layered
gab
bro
0Olivine(M
gno74---75)3
OPXco
ronas
(Mgno78---79)
WRfrac14
070322
WRfrac14
thorn495
CPX
Palegreen
Am
(cumact)
CPXfrac14
070324
nd
Plagioclase(A
n65---80)4inversezoning
Nolow-T
alteration
Plfrac14
070322
Plfrac14
thorn479
Solidfluid
inclusionsin
Pl
97OA128b
Wad
iWaryiah
From
lower
gab
bro
Layered
gab
bro
1CPX
Low-T
alteration
WRfrac14
070333
nd
Plagioclase(A
n85---87)
Prehnitemicroveins
CPXfrac14
070422
CPXfrac14
thorn523
Plfrac14
070328
Plfrac14
thorn414
01OA41d2
Wad
iGideah
From
upper
gab
bro
Gab
bro
patch
in
laminated
gab
bro
36
CPXrecrystallized
Plagioclaseidiomorphic
(An57---87)norm
alzoning
Black
pargasitic
honblende
WRfrac14
070351
CPXfrac14
070378
Plfrac14
070426
WRfrac14
thorn480
nd
Plfrac14
thorn504
Hbdfrac14
070355
nd
01OA19aamp
dWad
iWaryiah
Inlayeredgab
bro
Gab
broic
dyke
01
Olivine
OPXpoikilitic
WRfrac14
070341
WRfrac14
thorn567
CPX
Black
pargasite
Hbdfrac14
070324
Hbdfrac14
thorn287
Plagioclase(A
n73---95)
Hornblendepoikilitic
Plfrac14
nd
Plfrac14
thorn10 09
Hem
atite
WRfrac14
070418
nd
Green
Mg-hornblende
Acthornblende
Epidote
02OA90b
Wad
iAl-Abyad
Inlayeredgab
bro
Dioriticdyke
015
Green
Mg-hornblende
Hbdfrac14
070427
Hbdfrac14
thorn239
Plagioclase(A
n83---87)
Plfrac14
070387
Plfrac14
thorn498
Solidfluid
inclusionsin
plagioclase
01OA15f
Wad
iWaryiah
Inlayeredgab
bro
Comb-textured
gab
broic
dyke
025
CPXidiomorphic
Plagioclase(A
n95---99)norm
alzoning
BrownMg-H
ornblende
WRfrac14
070458
nd
02OA43a
Wad
iMayah
Inlayeredgab
bro
vein
Green
amphibole
1Palegreen
Mg-hornblendeacicular
Plagioclase(A
n91---97)in
reactionmargin
Hbdfrac14
070431
Hbdfrac14
thorn310
01OA36b
Wad
iGideah
Inlayeredgab
bro
epidosite
dyke
25
CPX
Epidote
WRfrac14
070519
nd
Plagioclase
Epfrac14
070554
nd
CPXclinopyroxene
Plplagioclase
OPXorthopyroxene
Amam
phibolecu
mcu
mmingtonite
actactinolite
WRwholerockHbdhornblende
Epep
idotend
notdetermined
187Sr
86Srinitialratioscalculatedat
95Ma(H
acker1994)Errors
ofthemeasuredratiosareindicated
inTab
le4
2Analyticalerrors
oftheoxygen
isotopevalues
are02
(2s)
3Mgnumber
frac14atomicMg(Mgthorn
Fe)
4Percentageofan
orthitein
plagioclase
BOSCH et al HYDROTHERMAL CIRCULATION IN OMAN OPHIOLITE
1191
(millimetre-sized) recrystallized clinopyroxene largelyaltered to light green magnesio-hornblende and idio-morphic plagioclase Locally primary black pargasitichornblende develops including idiomorphic normallyzoned plagioclase (Table 4) This gabbro exhibits similar87Sr86Sr ratios for the WR and pargasite (070351 and070355 respectively) These values are interpreted asrepresentative of the melt from which this amphibolecrystallized The clinopyroxene has a slightly higher Srisotope composition of 070378 consistent with the occur-rence of the light green Mg-hornblende rim Plagioclaseis significantly more radiogenic (070426) related to sec-ondary destabilization evidenced by the idiomorphichabit The Sr isotope compositions of the liquidsextracted from the acid leaching experiments of theWR (L1 and L2) are very similar 070358 and 070365respectively The 87Sr86Sr ratio of the residue obtainedafter leaching experiments is 070335 lower than boththe pargasite and unleached WR This value can beconsidered as the unaltered isotopic composition of thissample The d18O values measured for WR and coexist-ing plagioclase are lower than normal MORB magmaticvalues The 18O depletion in the plagioclase can be fairlywell explained by high-temperature hydrothermal inter-action between an oxygen-depleted fluid and the magmaAccording to the kinetics of hydrothermal exchange ofoxygen (Gregory amp Taylor 1981 Gregory et al 1989)plagioclase exchanges its oxygen isotopes with the fluidadjusting readily to the hydrothermal conditions Forhigh temperatures of exchange with seawater-derivedfluids (d18O between 0 and thorn2) the magmatic plagio-clase will have its primary d18O value lowered whereasthe d18O value for lower temperatures of exchange(5300C) will increase The amplification of the d18Oshift increases with the intensity of the hydrothermalalteration
Gabbroic dykes intrusive in lower gabbros from WadiWariyah Wadi Tayin Massif
Gabbroic dykes 01 OA19a and 01 OA19d are intrusiveinto the lower gabbros They are 3---4 cm wide discord-ant to the foliation of the host gabbros (Fig 7e) with asharp margin marked by a brown amphibole fringe Theprimary phases in the dykes (Table 1) locally preservedas elongated aggregates of a 01mm grain size mosaicassemblage at the dyke margins grade to millimetre sizein the central part of the dyke The WR major elementcomposition is Mg enriched compared with the enclosinggabbro (Table 3) Brown pargasitic amphibole oikocrystsmake up 50 of the modal composition of the dyke Indyke 01 OA19a poikilitic orthopyroxene may developinstead of brown amphibole Finally green magnesio-hornblende grows either as oikocrysts at the expense ofclinopyroxene or in association with epidote as an altera-tion product of plagioclase These low-amphibolite-faciesminerals represent the lowest-grade paragenesis recogn-ized in these dykes and are most developed in sample01OA19aTheamphibole composition showszoning fromTi-rich pargasitic hornblende to magnesio-hornblendeand actinolitic hornblende (Fig 8c) recording pro-gressive cooling of the dyke The plagioclase compositionis extremely heterogeneous (Fig 8b) varying from An95a composition similar to that of the plagioclase in the hostgabbro (Fig 8a) to An50 for the plagioclase and asso-ciated epidote inclusions in the poikilitic actinolitic horn-blende Sample 01 OA19d has an initial 87Sr86Sr ratioslightly higher for the WR (070341) than for the parga-site (070324) The Sr isotopic ratios of liquids extractedafter two steps of WR acid-leaching (L1 and L2) are070357 and 070341 respectively whereas the residueyields a 87Sr86Sr ratio of 070327 very close to thepargasite and plagioclase values The latter is similar tothe isotopic composition measured for the massive gab-bro 94 MA2 and may be taken as close to the primarymagmatic value This Sr isotopic ratio is more radiogenicthan the inferred Oman mantle-source value (McCullochet al 1981) The altered part of this sample has a radio-genic Sr isotopic composition (070418) related to thelow-amphibolite-facies alteration This gabbroic dykehas seemingly preserved a magmatic whole-rock d18Ovalue (567) but contains by far the highestd18O plagioclase (1009) of all the samples analysedThis plagioclase coexists with low d18O pargasite(thorn287) Similarly high values (d18Oplagioclase 4 8)have been previously measured for gabbroic dykes dia-bases sheeted dykes and plagiogranites from the Omanophiolite (Gregory amp Taylor 1981 Stakes amp Taylor1992) This 72 oxygen isotope fractionation betweenplagioclase and amphibole cannot possibly represent oxy-gen equilibrium at any plausible temperature This maybe explained by different exchange kinetics for these twominerals at different temperatures at high waterrock
Table 2 Average temperatures ( C 40C)
calculated for plagioclase---amphibole pairs
using the geothermometer of Holland amp
Blundy (1994)
Pargasite Magnesio-hornblende
rim core rim core
01 OA41d2 878 953 ------- -------
01 OA19a 935 974 825 869
01 OA19d 890 933 789 -------
02 OA37b ------- ------- 816 849
02 OA90b ------- ------- 859 833
JOURNAL OF PETROLOGY VOLUME 45 NUMBER 6 JUNE 2004
1192
ratios This supports the occurrence of two distincthydrothermal events one at high temperature and alater stage at lower temperature responsible for thestrong 18O enrichment in the coexisting plagioclase
Dioritic dyke associated with a diabase dyke in lowergabbros from Wadi Halhal Haylayn Massif
Sample 02 OA37b belongs to a group of brownamphibole---plagioclase lenses or reaction margins asso-ciated with diabase intrusives (Fig 7d) in lower layeredgabbros Idiomorphic brown hornblende (tschermakite)crystals are elongated in the plane of the dyke represent-ing 80 of the modal composition They are associatedwith an interstitial plagioclase matrix The browntschermakite has greenish rims Plagioclase has a cloudyappearance due to micrometre-size fluid inclusionsexcept along its margins and internal microcracks Bothhornblende and plagioclase exhibit evidence of plasticdeformation The Sr isotopic ratio for the pargasite andplagioclase is 070348 and 070420 respectively Thepargasite exhibits a Sr signature closest to that of theOman primary magmatic value (McCulloch et al 1981)Nevertheless it is too high to be attributed to anunmodified MORB-source mantle signature This sug-gests the addition of an external fluid component in goodagreement with the shift of oxygen isotopic compositionto a low value (thorn39) suggesting a high-temperatureexchange with a low d18O fluid The Sr isotope composi-tion of the plagioclase is significantly higher than that of
the coexisting pargasite probably as a result of the low-temperature alteration
Dioritic dyke cross-cutting lower layered gabbros in WadiAl-Abyad Nakhl Massif
Sample 02 OA90b is a 1 cm thick dioritic dyke cross-cutting the lower layered gabbros It grades into a greenamphibole vein as illustrated in Fig 6c The dyke is com-posed of 2 cm long dark green idiomorphic magnesio-hornblende crystals growing in the plane of the dykeand plagioclase concentrated at the margin of the dykeThe plagioclase in the dyke and in the enclosing gabbrocontains micrometre-size fluid and mineral inclusionssimilar to those observed in sample 94 MA2 The plagio-clase and green hornblende have higher Sr isotopic ratios(070387 and 070427 respectively) and lower d18Ovalues (thorn498 and thorn239 respectively) than the normalMORB-source mantle value As observed for previoussamples this supports interaction with an external fluidcomponent
Comb-textured gabbroic dykes in the lower gabbros fromWadi Wariyah Wadi Tayin Massif
Sample 01 OA15f (Fig 6b) is from Wadi Wariyah andrepresents a 2 cm wide dyke intruding the lower gabbrosThis type of comb-textured dyke developed in theclinopyroxene---plagioclase stability field It containslarge idiomorphic clinopyroxene crystals which arepartially replaced by brownish magnesio-hornblendeThe plagioclase is extremely calcic with a slight inverse
Table 3 Whole-rock major element compositions of massive gabbros and dykes determined by X-ray fluorescence
(wt )
Sample 94 Ma2 97 OA128b 01 OA41d2 01 OA19d 01 OA19a 01 OA15f 01 OA36b 01 OA36b
Rock type Gabbro Gabbro Gabbro patch Gabbroic dyke Gabbroic dyke Comb-textured
gabb dyke
Epidosite dyke Epidosite dyke
(margin)
SiO2 4830 4815 4749 4524 4339 4799 389 4371
Al2O3 2785 2032 2438 1474 1246 1192 2752 1646
Fe2O3 328 266 404 981 1204 592 381 623
MnO 003 004 005 014 016 006 008 014
MgO 405 737 427 1031 1686 1326 187 739
CaO 1289 1908 1477 1235 1156 166 2471 2339
Na2O 292 123 257 258 111 075 000 013
K2O 000 000 009 006 000 000 000 000
TiO2 005 017 063 230 064 086 013 042
P2O5 008 007 013 018 009 013 007 010
LOI 033 076 138 216 161 237 274 190
Total 9978 9985 998 9987 9992 9986 9983 9987
LOI loss on ignition gabb gabbroic
BOSCH et al HYDROTHERMAL CIRCULATION IN OMAN OPHIOLITE
1193
Table4Majorelementcomposition
aofselectedam
phibolespaired
withplagioclasecompositionsusedforthermometry
Sam
ple
nam
e1Mode2
SiO
2TiO
2Al 2O3
Cr 2O3
FeO
MnO
MgO
CaO
Na 2O
K2O
Total
Si
AlIV
AlVI
Ti
Fe3
thornCr
Mg
Fe2
thornMn
Na
KCa
XAn3
T(C)4
01OA19a
Mg-hornblende
core
45 60
26
10 78
009
860
015
16 56
12 00
212
007
96 72
6552
1448
0366
0083
0552
0004
3409
0594
0016
0611
0018
1843
0903
899
Mg-hornblende
core
47 99
263
850
002
897
011
16 61
12 10
160
020
96 36
6894
1106
0334
0028
0483
0002
3557
0594
0014
0446
0036
1862
0892
834
Mg-hornblende
rim
51 17
247
596
001
761
014
18 11
12 19
103
010
96 47
7257
0743
0253
0016
0414
0001
3828
0488
0016
0284
0018
1853
0894
820
Mg-hornblende
rim
47 50
291
920
004
880
014
16 46
11 95
174
008
96 07
6835
1165
0396
0015
0506
0005
3529
0552
0017
0486
0015
1843
0887
825
Mg-hornblende
rim
49 57
269
742
000
799
017
18 07
12 02
133
014
96 88
7013
0987
0261
0018
0575
0000
3811
0371
0020
0365
0026
1821
0860
831
Mg-hornblende
core
47 83
181
872
003
840
011
14 48
12 18
159
008
93 59
6814
1186
0279
0018
0644
0004
3711
0357
0013
0438
0015
1859
0907
874
Pargasite
rim
42 15
249
11 87
024
10 07
013
13 84
11 43
263
032
96 61
6188
1812
0242
0434
0299
0028
3028
0937
0016
0749
0059
1797
0804
972
Pargasite
rim
42 22
260
11 86
026
10 21
011
13 71
11 48
253
032
96 59
6201
1799
0254
0429
0299
0030
3001
0955
0014
0720
0060
1807
0803
962
Pargasite
core
42 05
263
11 45
030
10 19
011
13 80
11 47
246
032
96 46
6189
1811
0175
0478
0299
0035
3029
0956
0014
0702
0060
1809
0767
974
Pargasite
rim
42 36
247
11 90
016
945
013
14 16
11 76
249
026
95 98
6239
1761
0305
0366
0276
0019
3109
0888
0016
0710
0049
1856
0818
926
Pargasite
core
42 50
291
11 64
016
984
015
14 22
11 41
256
034
96 69
6219
1781
0227
0426
0322
0018
3103
0883
0018
0727
0063
1789
0812
974
Pargasite
rim
42 76
270
12 03
017
920
012
14 87
11 82
244
025
96 73
6224
1776
0880
0335
0368
0020
3226
0752
0015
0689
0047
1843
0841
941
Pargasite
rim
42 13
181
13 54
016
921
009
12 77
12 13
254
032
96 98
6172
1828
0510
0450
0000
0018
2788
1129
0011
0721
0059
1905
0841
874
01OA19d
Mg-hornblende
49 26
047
624
011
11 46
017
15 39
12 21
108
009
96 48
7137
0863
0202
0051
0437
0013
3323
0951
0021
0302
0018
1896
0768
771
Mg-hornblende
46 03
287
932
018
906
017
14 77
12 87
193
007
97 28
6672
1328
0265
0313
0046
0021
3191
1053
0021
0542
0012
1999
0758
808
Pargasite
41 89
429
11 65
008
11 03
017
13 52
11 37
259
017
96 77
6159
1841
0179
0474
0345
0010
2963
1012
0021
0739
0032
1791
0770
975
Pargasite
rim
42 80
340
11 16
009
11 01
018
13 73
11 67
240
022
96 67
6293
1707
0226
0376
0322
0010
3009
1032
0022
0684
0041
1839
0605
880
Pargasite
core
42 58
351
11 27
006
11 63
018
13 51
11 46
239
022
96 82
6257
1743
0209
0387
0391
0007
2959
1039
0023
0680
0042
1804
0595
893
Pargasite
rim
42 14
393
11 28
006
11 71
014
13 35
11 33
235
033
96 62
6214
1786
0174
0435
0391
0007
2935
1054
0018
0673
0062
1790
0519
901
Pargasite
core
41 56
420
12 05
006
11 57
016
12 51
11 35
249
027
96 24
6168
1832
0277
0469
0276
0007
2768
1160
0021
0718
0052
1805
0537
882
Pargasite
core
41 33
438
11 49
008
11 66
015
13 35
11 45
264
026
96 80
6105
1894
0107
0487
0368
0009
2941
1073
0019
0757
0048
1813
0537
942
Pargasite
41 08
446
12 39
012
11 35
017
13 13
11 36
263
015
96 85
6049
1951
0199
0494
0368
0014
2882
1030
0021
0752
0027
1792
0758
975
02OA37b
Mg-hornblende
rim
45 15
260
984
002
13 28
019
14 01
10 87
128
009
97 33
6527
1473
0203
0283
0690
0002
3018
0915
0023
0360
0016
1683
0562
828
Mg-hornblende
rim
44 72
263
988
004
13 23
021
14 01
10 78
133
009
96 92
6494
1506
0185
0287
0713
0005
3032
0894
0025
0374
0018
1677
0593
854
Mg-hornblende
rim
45 68
247
935
004
12 87
020
14 10
11 00
121
011
97 02
6621
1379
0218
0269
0598
0004
3045
0962
0024
0341
0020
1708
0562
815
Mg-hornblende
core
43 94
291
10 87
001
12 60
023
13 73
10 59
142
011
96 39
6410
1590
0278
0319
0644
0001
2985
0893
0028
0401
0021
1655
0710
860
Mg-hornblende
core
44 62
270
10 37
001
12 66
019
14 12
10 58
140
008
96 73
6479
1521
0253
0294
0644
0001
3057
0893
0023
0395
0014
1646
0674
851
Mg-hornblende
rim
47 79
181
796
002
12 09
025
15 22
11 09
100
009
97 33
6866
1134
0214
0196
0506
0002
3260
0947
0030
0280
0016
1706
0503
767
Mg-hornblende
core
45 43
249
10 3
000
11 99
022
14 50
11 14
133
008
97 48
6524
1476
0267
0269
0644
0000
3103
0796
0027
0370
0015
1715
0703
838
JOURNAL OF PETROLOGY VOLUME 45 NUMBER 6 JUNE 2004
1194
Sam
ple
nam
e1Mode2
SiO
2TiO
2Al 2O3
Cr 2O3
FeO
MnO
MgO
CaO
Na 2O
K2O
Total
Si
AlIV
AlVI
Ti
Fe3
thornCr
Mg
Fe2
thornMn
Na
KCa
XAn3
T(C)4
02OA90b
Mg-hornblende
core
48 83
038
745
005
939
010
17 29
12 24
142
006
97 21
6931
1069
0041
0041
0667
0005
3657
0448
0011
0392
0010
1862
0840
856
Mg-hornblende
core
49 49
033
731
003
916
008
17 40
12 57
139
005
97 82
6987
1013
0035
0035
0552
0004
3662
0530
0010
0379
0009
1901
0830
811
Mg-hornblende
rim
48 90
040
753
005
932
013
17 15
12 22
154
005
97 29
6948
1052
0042
0042
0575
0006
3632
0533
0015
0424
0010
1861
0880
869
Mg-hornblende
rim
48 30
044
800
007
962
010
16 86
12 32
161
005
97 35
6873
1127
0047
0047
0598
0008
3576
0546
0012
0443
0010
1879
0870
862
Mg-hornblende
rim
49 12
032
742
010
922
011
17 36
12 29
142
006
97 43
6956
1044
0034
0034
0621
0011
3665
0471
0013
0391
0010
1865
0840
845
02OA43a
Mg-hornblende
rim
48 13
040
789
007
860
007
17 41
12 19
155
006
96 36
6882
1118
0212
0043
0621
0008
3710
0407
0008
0429
0010
1867
-------
-------
Mg-hornblende
rim
50 93
035
643
001
759
006
18 50
12 24
114
003
97 28
7152
0848
0217
0037
0506
0001
3873
0385
0007
0311
0005
1841
-------
-------
Mg-hornblende
rim
50 67
014
559
009
12 33
026
15 28
12 44
068
003
97 50
7260
0740
0204
015
0483
0010
3263
0994
0032
0189
0005
1909
-------
-------
Mg-hornblende
rim
50 34
013
621
009
12 49
027
15 07
12 51
064
003
97 78
7192
0810
0240
0010
0530
0010
3210
0960
0030
0180
0010
1910
-------
-------
Mg-hornblende
rim
45 17
060
11 14
013
988
008
15 78
12 33
211
009
97 31
6473
1527
0355
0640
0644
0015
3371
0540
0010
0588
0016
1892
-------
-------
Mg-hornblende
rim
49 25
032
712
014
845
009
17 84
12 32
137
004
96 95
6983
1020
0170
0030
0620
0020
3770
0380
0010
0380
0010
1870
-------
-------
Mg-hornblende
rim
48 61
032
775
014
975
011
16 81
12 57
139
004
97 50
6905
1950
0203
0034
0621
0016
3560
0537
0013
0382
0008
1914
-------
-------
94Ma2
Anthophyllite
54 98
000
259
001
12 65
037
23 35
199
034
000
96 28
7744
0256
0174
0000
0161
0001
4903
1330
0043
0092
0001
0301
-------
-------
Anthophyllite
55 11
000
296
003
14 09
032
23 66
063
038
000
97 18
7716
0284
0205
0000
0115
0003
4938
1535
0038
0104
0000
0095
-------
-------
Actinolite
48 58
004
954
002
960
014
17 67
999
139
005
97 02
6850
0150
0435
0004
0690
0002
3713
0442
0017
0379
0009
1510
-------
-------
Actinolite
51 54
004
585
000
890
022
20 28
930
090
002
97 05
7214
0786
0178
0005
0598
0000
4230
0444
0026
0244
0004
1395
-------
-------
01OA41d2
Pargasite
rim
43 05
342
10 71
008
11 46
012
13 50
11 54
179
067
96 34
6353
1647
0216
0380
0368
0009
2967
1046
0015
0512
0127
1824
0613
863
Pargasite
rim
42 61
354
11 59
001
10 59
010
13 52
11 87
160
076
96 19
6286
1714
0302
0392
0299
0001
2972
1007
0012
0458
0143
1877
0715
844
Pargasite
core
42 54
401
11 04
006
11 81
013
13 21
11 50
202
039
96 71
6266
1734
0182
0444
0368
0007
2901
1087
0017
0576
0073
1814
0854
979
Pargasite
rim
43 03
385
10 72
002
11 70
014
13 50
11 95
146
080
97 17
6314
1686
0168
0425
0345
0002
2953
1091
0018
0415
0149
1878
0634
852
Pargasite
rim
42 65
341
10 60
006
11 97
014
13 22
11 64
183
068
96 20
6332
1668
0186
0381
0345
0007
2926
1141
0018
0528
0129
1851
0620
869
Pargasite
core
42 82
367
10 60
007
11 93
012
13 42
11 44
206
047
96 60
6315
1685
0158
0407
0391
0008
2950
1080
0016
0588
0089
1808
0615
899
Pargasite
rim
42 2
362
11 10
007
11 42
016
13 09
11 72
179
072
95 89
6282
1718
0229
0405
0299
0009
2905
1123
0020
0518
0137
1869
0715
875
Pargasite
rim
42 42
380
11 03
006
11 77
012
13 00
11 72
188
069
96 49
6283
1717
0209
0423
0299
0007
2870
1158
0015
0539
0131
1860
0817
928
Pargasite
rim
42 21
364
11 14
006
10 73
011
13 53
11 68
177
072
95 59
6277
1723
0229
0407
0322
0007
2999
1012
0014
0511
0136
1862
0736
883
Pargasite
rim
42 25
376
11 03
008
10 93
015
13 66
11 72
185
064
96 07
6257
1743
0182
0418
0345
0009
3016
1009
0019
0531
0121
1860
0766
912
Pargasite
core
42 17
396
10 91
008
11 10
010
13 57
11 49
209
038
95 85
6253
1747
0159
0442
0368
0010
3000
1009
0013
0602
0073
1826
0841
981
aMajorelem
entco
mpositionsarein
weightpercent-------noplagioclasean
alysed
andnotemperature
calculated
1Typ
eofam
phibolean
alysed
isalso
indicated
2Blankindicates
nozoningobserved
3Molefractionofan
orthitein
plagioclasead
jacentto
amphibolegrain
4Tem
perature
ofeq
uilibrium
fortheam
phibole---plagioclasepaircalculatedusingtheHollandamp
Blundythermometer
(1994)Errors
are40
C
BOSCH et al HYDROTHERMAL CIRCULATION IN OMAN OPHIOLITE
1195
zonation from An95 to An99 from core to rim (Table 1 andFig 8a) TheWR Sr isotopic ratio of the sample (070458)is significantly higher than normal MORB values Thiscould be related to the overprint of a low-temperaturealteration event consistent with the petrography of thesample
Green amphibole vein cross-cutting layered gabbros inWadi Mayah Haylayn Massif
Sample 02 OA43a belongs to a series of 1 cm thick greenamphibole veins (as in Fig 6e) cross-cutting the layeredgabbros The veins develop 15 cm thick reaction marginsin the enclosing gabbro and are exclusively composed ofacicular pale green magnesio-hornblende growing per-pendicular to the vein margin in a crack-seal mode(Fig 6f ) The reaction margins are composed of urali-tized gabbro marked by the development of the samepale green magnesio-hornblende in an inhomogeneouscalcic plagioclase matrix (An91---97) The Sr and O isotopiccompositions of this green hornblende (070431 andthorn31) do not correspond to normal MORB-sourcemantle values
Epidosite dyke from Wadi Gideah Wadi Tayin Massif
Sample 01 OA36b is representative of a series of epidoteveins cross-cutting poorly layered gabbros These 2 cmthick veins are composed of pink thulite in their centreand green pistachite at their margins (Fig 6a) Theirsharp contact with the enclosing gabbro is marked bythe development of coarse clinopyroxene crystals alteredin the epidote---greenschist facies This suggests that theepidote vein is replacive in a coarse-grained gabbro dykeThe highest 87Sr86Sr initial ratios of all the samplesstudied are recorded by the WR and coexisting epidoteof this sample (070519 and 070554 respectively)(Table 5) The Sr content of the WR and associatedepidote are also the highest of the present dataset(716 ppm and 764 ppm respectively) The Sr isotopiccomposition of this sample is consistent with a greaterdegree of exchange with a radiogenic component fromCretaceous seawater It is significantly higher than thevalue obtained for an epidosite vein from the Wadi Fizhsection of the Oman ophiolite [070435 with a Sr contentof 488 ppm reported by Kawahata et al (2001)] but ingood agreement with the average of the greenschist-faciesalteration sequence defined by Kawahata et al (2001)
GABBROS
01 km 025 km 1 km 36 km50
60
70
80
90
An
MA2
19d
19a 15f 128b
41d2
(a)
43a17b
15f
90b
37b
19a
19d
40
60
80
100
A
n
01 km 015 km 025 km 1 km
DYKES
(b)
Distance to the Moho
ACTINOLITE
MAGNESIOHORNBLENDE
TSCHERMAKITE
PARGASITEGabbros Dykes41d2MA2
19a d37b15f
90b43a
AlIV
150500
02
08
10
(Na
+ K
) A
(c)
04
06
950
900
850
800
750
70007 12 17
T degC
1000
(d)
AlIV
Fig 8 Plagioclase---amphibole compositions and results of thermometry (a) and (b) plagioclase compositions in layered gabbros and in dykesrespectively as a function of distance to the Moho (see text for sample identification) (c) amphibole compositions in layered gabbros and dykesaccording to Leakersquos (1997) nomenclature (d) equilibration temperatures calculated using the amphibole---plagioclase thermometer of Holland ampBlundy (1994) as a function of AlIV content in amphibole [same symbols as in (c)]
JOURNAL OF PETROLOGY VOLUME 45 NUMBER 6 JUNE 2004
1196
Table5RbandSrcontents87Sr
86Srandd1
8Oisotopiccompositionsofwholerocksandmineralseparatesfrom
samplesofmassivegabbrosdykes
andveinsfrom
theOman
ophiolitealsolistedarecalculated
waterrockratios(W
R)andLOIvalues
Sam
ple
Rb(ppm)
Sr(ppm)
87Rb8
6Sr
87Sr
86Sr
(87Sr
86Sr)i1
d18O
()
LOI(
)2WR
3(87Sr
86Sr)L14
(87Sr
86Sr)L25
(87Sr
86Sr)R6
01OA41d2
Whole
rock
037
1792
0006
0703516
09
070351
thorn480
138
47
0703587
0703651
0703357
Plagioclase
025
1249
0003
0704261
22
070426
thorn504
-------
-------
Pargasite
-------
-------
-------
0703548
20
070355
-------
-------
-------
Clinopyroxene
045
26 0
0050
0703845
19
070378
-------
-------
-------
01OA36b
Dykewhole
rock
0005
7164
50001
0705186
17
070519
-------
274
21 9
0705207
0705112
-------
Epidote
0003
7645
0001
0705536
12
070554
-------
-------
-------
Wallwhole
rock
005
4254
0001
0704637
09
070464
-------
191
-------
0704945
0704675
0704593
02OA43a
Green
hornblende
004
98
0012
0704323
13
070431
thorn310
-------
-------
97OA128b
Whole
rock
003
1274
50001
0703327
17
070333
-------
076
37
-------
-------
-------
Plagioclase
002
1204
50001
0703285
09
070328
thorn414
-------
-------
Clinopyroxene
003
20 0
0004
0704230
09
070422
thorn523
-------
-------
01OA15f
Whole
rock
006
1077
0002
0704582
16
070458
-------
237
13 5
-------
-------
-------
01OA19a
Whole
rock
037
1303
0008
0704189
18
070418
-------
161
96
-------
-------
-------
01OA19d
Whole
rock
058
2032
0008
0703423
08
070341
thorn567
216
415
0703574
0703408
0703271
Pargasite
091
1058
0025
0703273
11
070324
thorn287
-------
-------
Plagioclase
-------
-------
-------
-------
-------
thorn10 09
-------
-------
02OA90b
Plagioclase
004
2385
50001
0703868
11
070387
thorn498
-------
-------
Green
hornblende
010
23 8
0012
0704288
15
070427
thorn239
-------
-------
BOSCH et al HYDROTHERMAL CIRCULATION IN OMAN OPHIOLITE
1197
Table5continued
Sam
ple
Rb(ppm)
Sr(ppm)
87Rb8
6Sr
87Sr
86Sr
(87Sr
86Sr)i1
d18O
()
LOI(
)2WR
3(87Sr
86Sr)L14
(87Sr
86Sr)L25
(87Sr
86Sr)R6
02OA37b
Plagioclase
023
3121
0002
0704204
15
070420
-------
-------
-------
Pargasite
031
41 6
0021
0703505
15
070348
thorn394
-------
-------
94OAMa2
Whole
rock
0016
1750
00003
0703219
15
070322
thorn495
033
31
0703264
0703177
0703158
Plagioclase
0014
1363
00003
0703219
11
070322
thorn479
-------
-------
Clinopyroxene
-------
-------
-------
0703236
13
070324
-------
-------
-------
1Initial8
7Sr
86Srratiocalculatedat
95Ma(H
acker1994)
2LOIisloss
onignition(in)
3Water---rock
ratiocalculatedassumingaclosedsystem
TheeffectiveWR
ratiocanbeexpressed
bythefollowingeq
uation
W=Reffectivefrac14
frac12eth87Sr=
86SrrockTHORN fi
nal
eth87Sr=
86SrrockTHORN in
itial=frac12eth8
7Sr=
86SrseawaterTHORN in
itial
eth87Sr=
86SrrockTHORN fi
nal
ethCrock
initial=C
seaw
ater
initial
THORNwhere(87Sr
86Srrock) finalisthefinalmeasuredSrisotopicratioin
thesample(87Sr
86Srrock) in
itialco
rrespondingto
theinitialm
antlevalue
istakenas
070295(Lan
phere
etal1981)(87Sr
86Srseawater) in
itialistheCretaceousseaw
ater
Srisotopicco
mposition[87Sr
86Sr07073
at90
MaafterBurkeet
al(1982)]C
seaw
ater
initial
istheSrco
ntent
oflsquonorm
alrsquoseaw
ater
andis
takenas
8ppm
(deVilliers1999)
FortheCrock
initialitis
assumed
that
thepresentSrco
ncentrationmeasuredis
thesameas
theinitial
concentration
4Srisotopicratiosmeasuredforliquid
L1extractedafterthefirststep
oftheleachingexperim
ent(6NHClfor8min
at70
C)
5Srisotopicratiosmeasuredforliquid
L2extractedaftertheseco
ndstep
oftheleachingexperim
ent(2 5NHClfor30
min
at70
C)
6Srisotopicratiosmeasuredforresidueobtained
afteratotalaciddigestionachievedaftertheextractionofL1an
dL2leachates
JOURNAL OF PETROLOGY VOLUME 45 NUMBER 6 JUNE 2004
1198
Part of the same sample from the contact between thedyke and the gabbro yields a slightly lower Sr isotopiccomposition (070464) intermediate between the sur-rounding gabbro and the epidote dyke It also has anintermediate Sr content (425 ppm) Acid leaching experi-ments performed on the whole rock of the dyke yield Srisotopic compositions of 070521 and 070512 for thetwo liquids extracted Similar experiments for the samplelocated at the contact with the gabbro yield 87Sr86Srratios of 070494 and 070467 for L1 and L2 liquids TheSr isotopic composition of the residue extracted afterleaching remains high (070459) and very close to theepidosite 87Sr86Sr ratio determined by Kawahata et al(2001) In this sample the primary minerals have beentotally replaced by epidote and secondary plagioclase orquartz Thus the Sr isotopic compositions measured forthis WR sample and its constituent minerals reflect thecomposition of the fluid filling this dyke
Mineral chemistry
Plagioclase
Compared with the host gabbros the dykes show a muchlarger dispersion in their plagioclase compositions (Fig 8aand b Table 4) characterized by more calcic plagioclasethan the enclosing gabbros This tendency is mostextreme in the comb-textured dykes in which the plagio-clase is almost pure anorthite The zoning is generallynormal recording a crystal fractionation process How-ever gabbro 94 MA2 exhibits inverse zoning this tend-ency is also observed in plagioclases from dykes 01OA19a and 01 OA15f The higher calcium content ofthe plagioclase as well as the inverse zoning is consistentwith an increase of water pressure during its crystalliza-tion (Turner amp Verhoogen 1960 Carmichael et al 1974Koepke et al 2003) The particularly high An content inthe comb-textured gabbro dykes 01 OA15f may alsoresult from crystallization in a supercooled magma thisis consistent with the comb texture indicative of fast-growing crystals The lack of brown hornblende suggeststhat crystallization occurred above the temperature ofhornblende stability
Amphibole
The amphiboles analysed in the dykes (Fig 8c andTable 4) show a regular trend in a (Na thorn K)A vsAlIV diagram from titanium-rich (Ti 4 015) pargasitichornblende (brownamphibole) and tschermakite (brown---green amphibole) to low-titanium (Ti5 015) magnesio-hornblende (green amphibole) and actinolite (pale greenamphibole) The pargasitic hornblende develops aspoikilitic oikocrysts in the dykes 01 OA19a and d andin the diffuse patch 01 OA41d2 at temperatures above800C during the final stage of crystallization (see
below) The tschermakite composition corresponds tothe internal alteration of clinopyroxene in gabbro 94Ma2 occurs in the comb-textured dyke 01 OA15f andis the primary amphibole in the dioritic dyke 02 OA37bThe magnesio-hornblende develops at the edges of thepargasites or of clinopyroxenes at temperatures around800C and is the primary phase in the dioritic dyke 02OA90b and in the green vein 02 OA43a Finally actino-lite occurs as a replacement of green hornblende in dykesor in the kelyphitic rim surrounding olivine in gabbro 94Ma2 Such a large composition range at the scale of athin section has been reported in amphiboles fromamphibolite-facies oceanic gabbros (Stakes 1991 Stakesamp Taylor 1992 Gillis et al 1993) as well as in the Omanophiolite gabbros (Manning et al 2000) This variabilityhas been explained by rapid reaction rates preventinghomogenization (Manning et al 2000)
Thermometry
Plagioclase---amphibole thermometry could be appliedwhen the two minerals exhibited good contacts such asin the laminated gabbro of the middle sequence (01OA41d2) and in the intrusive dykes The temperaturesof plagioclase---amphibole equilibration (Table 2) havebeen calculated using the geothermometer lsquoBrsquo of Hollandamp Blundy (1994) Edenite thorn Albite frac14 Richterite thornAnorthite valid between 500C and 900C with anuncertainty of 40C Amphibole formulae are basedon 23 oxygen and their nomenclature is based on alkalications in the A site vs AlIV according to Leake (1997)Pressurewas assumed to be 1 kbar Forty-five plagioclase---amphibole pairs meet the compositional criteria imposedby Holland amp Blundyrsquos dataset (09 4 Xan 4 01 andAlIV 403) Electron microprobe measurements wereconstrained to contiguous plagioclase and amphiboleseparated by sharp grain boundaries assuming equili-brium of the two phases When the mineral phasesexhibit normal zoning temperatures between plagioclaseand amphibole cores and between mineral rims werecalculated assuming that the temperatures deduced fromthe mineral core chemistry provide a proxy for the high-est temperature of equilibration These data are identi-fied in the T C vs AlIV diagram (Fig 8d) Table 4presents selected major element amphibole analyses theAn content of the plagioclase adjacent to each amphibolegrain and the temperature calculated for the pairThe temperatures calculated (Fig 8d) span the entire
range of temperatures at which amphibole and plagio-clase coexist in equilibrium (ie amphibolite-facies para-geneses) This temperature interval is bracketed byorthopyroxene-bearing facies or amphibole-free facies(ie gabbro 94MA2) and epidote---actinolite facies devoidof homogeneous plagioclase (ie epidosite dyke 01OA36b and green vein 02 OA43a) The range of the
BOSCH et al HYDROTHERMAL CIRCULATION IN OMAN OPHIOLITE
1199
calculated temperatures between 4900C and 750Ccorresponds to the changing composition of amphibolesfrom pargasite to magnesio-hornblende at the scale of athin section or even at that of an individual crystal asshown by their correlation with AlIV such variabilityrecords rapid cooling in hydrous conditions The highesttemperatures are recorded by pargasite from the anatec-tic gabbro 01 OA41d2 in the gabbro dyke 01 OA19dand in the dioritic dyke 02 OA90b They were calculatedfrom minerals devoid of lower-grade alteration and arethus considered as representing the equilibrium temperat-ure of the coexisting minerals Lower temperatures cor-respond to mineral phases evolving at falling temperatureand are only indicative of the cooling history Includingthe 40C uncertainty seven pairs provide temperatureshigher than 900C the limit of validity of the Holland ampBlundy thermometer However we maintain these tem-peratures as indicative of the highest values of our data-set These values are included in the average equilibriumtemperature summarized in Table 2
DISCUSSION
Distribution of hydrothermal alteration
Two distinct petrological events are recorded in the gab-bro section of the Samail ophiolite and are documentedin the present contribution (1) the whole gabbro sectionis affected by a penetrative alteration (2) the gabbrosection is crosscut by various types of dykes and veinsincluding hydrous minerals
Penetrative alteration
The penetrative alteration developed in the gabbros atgrain boundaries and locally invading the minerals isubiquitous in the gabbro section This penetrative altera-tion has been described as very high temperature (VHT)high temperature (HT) and low temperature (LT) basedon alteration parageneses The orthopyroxene coronas inthe layered gabbro sample and the incipient appearanceof brown pargasitic amphibole mark a lower thermallimit of the VHT alteration By reference to the experi-mental work of Koepke et al (2003) the temperature ofequilibration for these reactions is estimated to bebetween 900 and 1000CThe preservation of the vertical distribution of
VHT---HT facies (Figs 4 and 5) also pointed out byManning et al (2000) indicates that the hydrothermalalteration pattern fits the distribution of isotherms withdepth at a fast-spreading ridge (ie Phipps Morgan ampChen 1993 Dunn et al 2000 Fig 2) Equilibrium tem-peratures for VHT parageneses as high as 900---1000Csuggest that VHT---HT hydrothermal alteration tookplace in a very restricted time interval at the limit ofthe cooling magma chamber It is proposed that the
penetrative alteration affecting the entire gabbro sectionis related to a pervasive network of microcracks whichact as a recharge system down to the Moho (Nicolas et al2003)
Dykes and veins
The dykes are extremely varied eg pegmatitic gabbroswith comb-texture microgabbros and amphibolitizeddolerites dioritic dykes alignments of poikilitic horn-blende and finally diffuse hornblende-bearing patchesHigh-T gabbro and diorite dykes and low-T greenamphibole---epidote veins are connectedIn the dykes textural evidence attests to suprasolidus
conditions at least for the development of the pargasiticamphibole Integrating the hornblende---plagioclase equi-libration temperatures calculated for the studied samplesthese temperatures are bracketed between 950C and875CIn the following discussion we shall consider separately
the microgabbro and dolerite dykes The latter representbasaltic melt intrusions associated upsection with thediabase sheeted dykes and downsection possibly withthe mafic dykes that are ubiquitous in the mantle section(Nicolas et al 2000) Whatever their origin the develop-ment of pargasitic amphibole during their late stage ofcrystallization creates a link with the gabbroic dykes andsuggests a crystallization process evolving from dry con-ditions to more hydrous conditionsThe other gabbroic dykes comb-textured gabbros and
hornblende-bearing dioritic dykes result from the fillingof cracks by a fluid either a melt or a silica-rich hydrousfluid These intrusive dykes which develop reaction mar-gins at their contact with the host gabbro grade verticallyinto lower-temperature green amphibole veins or align-ments of poikilitic hornblende in the layered gabbromatrix or development of pargasitic hornblende in ana-tectic gabbros (sample 01 OA41d2) In gabbros crystal-lizing in the magma chamber the development oforthopyroxene and pargasite oikocrysts enclosing the pri-mary minerals suggests a thermal evolution from thegabbro solidus to amphibolite-facies conditions Forthese reasons it is suggested that the gabbroic dykesresulted from the injection of a hydrous melt in the stillhot gabbros drifting away from the magma chamber asdiscussed below
Origin of fluids responsible for VHT---HThydrothermal alteration
The 87Sr86Sr initial ratios and d18O of whole rocks andpure mineral fractions are plotted in Fig 9 against thedistance above the Moho Transition Zone (MTZ) Theinitial Sr isotopic composition of the whole rocks apartfrom the epidosite dykes ranges from 070322 to
JOURNAL OF PETROLOGY VOLUME 45 NUMBER 6 JUNE 2004
1200
070458 All the studied samples (including whole rocksand minerals) have initial 87Sr86Sr ratios falling outsidethe field of fresh MORB which commonly have Sr ratiosbetween 07023 and 07029 with an average of 070265(ie Hart et al 1974) The lowest initial Sr isotopiccomposition (070322) from the massive gabbro 94MA2 is slightly but significantly higher than averageOman primary magmatic signatures (ie 070296McCulloch et al 1981) Similarly the d18O values ofmost whole rocks and minerals measured (Table 5)
depart from typical MORB-source mantle values( Javoy 1980 Gregory amp Taylor 1981 Kyser 1986)These differences indicate that the primary mantlesignatures in the Oman ophiolite have been modifiedby interaction with a fluid Possible origins for thisfluid are mantle components dehydration of subductedlithosphere or seawater circulation Strontium andoxygen isotopes are useful tracers for fluid---crust interac-tion as d18O and 87Sr86Sr ratios from fluids originat-ing from seawater the mantle or subducted lithosphere
-1000
1000
2000
3000
4000
5000
6000
7000
MOHO
MTZ
Dis
tan
ce a
bov
e th
e M
oh
o (
m)
pillow-basalt
sheeted-dike
gabbro
peridotite
01 OA 41d2
01 OA 36b
02 OA 43a97 OA 128b
01 OA15f
94 OA MA2
02 OA 90b01 OA 19ad
02 OA 37b
0702 0703 0704 0705 0706 0707
87Sr86Srinitial
MO
RB-s
ou
rce
Seaw
ater
Whole RockPlagioclasePargasiteGreen HornblendeClinopyroxene
Epidote
(a)
-1000
1000
2000
3000
4000
5000
6000
7000
MOHO
MTZ
Dis
tan
ce a
bov
e th
e M
oh
o (
m)
pillow-basalt
sheeted-dike
gabbro
peridotite
01 OA 41d2
01 OA 36b
02 OA 43a97 OA 128b
01 OA15f
94 OA MA2
02 OA 90b01 OA 19ad
02 OA 37b
Whole RockPlagioclasePargasiteGreen HornblendeClinopyroxene
3 5 10
δ18O (permil)
41d2
Ma2
43a
90b 19d 19d37b
128b
90b
(b)
Fig 9 (a) Variation of initial 87Sr86Sr isotopic composition as a function of the location of the studied samples in a schematic log of the crustalsection of the Oman ophiolite The field for MORB is from Hart et al (1974) and that for seawater is from Burke et al (1982) Small open squares aredata fromKawahata et al (2001) (b) Variation of d18O () as a function of the location of the studied samples in a schematic log of the crustal sectionof the Oman ophiolite Shaded curve corresponds to the data of Gregory amp Taylor (1981)
BOSCH et al HYDROTHERMAL CIRCULATION IN OMAN OPHIOLITE
1201
are significantly different Moreover the d18O valueis dependent on temperature and mineral fractiona-tion and thus yields complementary constraints to Srisotopes
Mantle origin
One model to explain the occurrence of fluids interactingwith the oceanic crust at very high temperatures is toderive them from the mantle The contribution of mantle-derived hydrous fluids linked to percolation processesor to their concentration in the final stages of gabbrocrystallization has been invoked in numerous case studies(ie Witt amp Seck 1989 Fabriees et al 1989 Dupuy et al1991 Agrinier et al 1993) O and Sr isotopic data forwhole rocks and mineral separates yield scattered valuesthat are unambiguously different from MORB mantle-source values (Fig 9a and b) With the exception of lateLT altered samples the WR data have a mean 87Sr86Srratio of 070337 and a standard deviation of 000012These surprisingly high values in mantle-derived rockscould be taken as evidence for survival of mantle isotopicheterogeneities during crystallization of the ophiolite (egLanphere 1981 McCulloch et al 1981) However the18O16O isotope ratios in the Oman gabbroic sequenceare also displaced from primary magmatic values Nogabbro in the present study has plagioclase or amphi-bole with typical mantle d18O values Thus the Sr and Oisotope data rule out the possibility of mantle-derivedfluids as a possible source for the VHT---HT hydrousalteration event recorded in the massive gabbros andgabbroic dykes and veins (Fig 10a and b)
Subducted lithosphere origin
Dehydration of subducted oceanic crust in subductionzones can generate hydrous fluids Such fluids couldpercolate through the overlying lithosphere and contam-inate it thus generating modified isotopic signatures andtrace element contents Such an explanation howeverdisagrees with the geochemical characteristics of most ofthe studied samples Fluids derived from the dehydrationof subducted slabs (fresh or altered MORB or OIB pro-toliths) should be strongly enriched in K Rb and Ba (egBecker et al 2000) whereas in Oman the Rb contentsmeasured in the gabbros and gabbroic dykes and veins arestrongly depleted Moreover the isotopic composition ofslab-derived fluids is buffered by the MORB-like oceaniccrustal protolith for Nd and Pb but not for Sr and O(Staudigel et al 1995) As a result of relatively minorfractionation of oxygen at high temperature the oxygenisotope composition of the fluids expelled during dehy-dration of the subducted slab should be high and essen-tially controlled by the oxygen composition of the uppersection of the crust altered at low temperature (with an
average of 10 and perhaps as high as 19) (Staudigelet al 1995) The Sr isotopic composition should be alsohigh (average 07046) as the fluids released by the dehy-dration of subducted oceanic crust are associated eitherwith seawater or with radiogenic Sr phases (ie smectites)The Sr and O isotopic compositions of the studied sam-ples do not fit with these signatures and so preclude asubducted lithosphere origin for the fluids involved in theVHT---HT alteration event
Seawater-derived fluids
All the analysed samples (minerals and WR) have 87Sr86Sr(T) outside the domain of primary MORB magmasand variable Sr contents Individual minerals have Srcontents reflecting their different mineralliquid partitioncoefficients Minerals characterized by a very high affi-nity for Sr such as plagioclase (ie Sr mineralliquidpartition coefficients 1) have very high Sr contentsTherefore the high abundance of plagioclase in the sam-ples analysed is responsible for the high Sr content of thestudied whole rocks The Sr content of all the whole rocksis relatively homogeneous (Table 5) without clear distinc-tion between country-rock gabbros and dykes and veinsand ranges from 110 to 200 ppm except for the epidosite(4700 ppm) Reported on a 87Sr86Sr vs 1Sr diagram(Fig 10a) most whole rocks and plagioclases analysed arecharacterized by a 1Sr ratio lower than 001 and plot inthe left part of this diagram Compared with N-MORBthe studied massive and anatectic gabbros and their con-stituent plagioclases have similar to slightly higher Srcontents but substantially higher 87Sr86Sr ratios(Fig 10a) Considering Cretaceous seawater as a possiblehigh 87Sr86Sr mixing component [87Sr86Sr 07073at 90Ma after Burke et al (1982)] responsible for theobserved geochemical modifications exchanges cannothave occurred in a closed system as seawater has too lowa Sr content (ie 8 ppm de Villiers 1999) An open-system process allowing penetration of larger quantitiesof seawater can efficiently modify the Sr isotopic ratio ofthe rocks Alternately the fluids could also be seawaterthat was modified through exchange with the surround-ing rocks during its transit through the oceanic crustalsection This modified seawater would be more enrichedin Srwith a 87Sr86Sr ratio intermediate between unmodi-fied seawater and MORBMinerals devoid of late LT alteration imprints (parga-
site green hornblende and pyroxene) and characterizedby very low Sr contents plot on the right of the diagramclose to or on the mixing line between seawater andMORB (Fig 10a) The difference between these mineralsand the other samples (WR and plagioclases) is mainlydue to the Sr fractionation coefficients of these differentminerals and to the large quantity of plagioclase inthe studied rocks The process responsible for the
JOURNAL OF PETROLOGY VOLUME 45 NUMBER 6 JUNE 2004
1202
modification of the Sr isotopic composition fromMORB-source value is however explained by the same phenom-enon As all these minerals display initial 87Sr86Sr ratiosdistinct from the MORB source and because they equili-brated at VHT or HT temperatures this supports anearly intervention of seawater during crystallization ofthese minerals Most of these samples with the exceptionof the epidosite dyke overlap the domains defined byKawahata et al (2001) for the Wadi Fizh gabbros alteredat VHT and HT conditions in the amphibolite facies(labelled lsquoVHTrsquo and lsquoHTrsquo in Fig 9a)Three samples (two epidosite dykes and an one epidote
fraction) have very high Sr contents and the highest Sr
isotopic compositions of the present dataset The twowhole-rock samples overlap the field of the Wadi Fizhsamples altered at high temperature during greenschist-facies metamorphism (Kawahata et al 2001) This typeof alteration is common at the ridge axis and formedthrough intensive exchange with seawater-derived fluidsFigure 10b indicates that all the Sr---O isotope data are
distinct from MORB compositions and suggests that thefluids reacting with the magmas could be derived fromseawater In addition the d18O values show that isotopicexchange occurred under VHT or HT conditions Thefluids could have the composition of seawater or theycould have the composition of seawater modified through
0703
0705
0707
001 01 1
1 Sr (ppm)
MORB
(87Sr
86Sr)
init
ial
Seawater
VHT
HT
MT36b
36b
36b
43a 128b90b
41d2
37b19d
15f
128b
41d2
41d2
19a
Ma2Ma2
19d
90b
37b
Whole RockPlagioclasePargasiteGreen HornblendeClinopyroxene
Epidote
(a)
0 2 4 6 8
δ18O(permil)
0703
0705
0707Seawater
MORB-mantle
128b 41d2
90b 41d2
Ma2128b
37b
43a
19d
90b
Whole RockPlagioclasePargasiteGreen HornblendeClinopyroxene
Epidote
HT LT
19d
(87Sr
86Sr)
init
ial
(b)
Fig 10 (a) Initial 87Sr86Sr isotopic ratios plotted against 1Sr for whole rocks and mineral separates from the gabbro section of the Omanophiolite Fields shown are from Kawahata et al (2001) and correspond to MT-----basalt sheeted dyke diabase altered at medium temperature(prehnite---actinolite facies sequence 3) HT-----diabase plagiogranite metagabbro and epidosite formed at high temperature (greenschist faciessequence 4) VHT-----gabbro altered at very high temperature (amphibolite facies sequence 5) The dashed line is a binary mixing line betweenMORB (Lanphere et al 1981) and Cretaceous seawater (Burke et al 1982) (b) Initial 87Sr86Sr isotopic composition plotted against d18O value forwhole rocks and minerals from the Oman ophiolite Cretaceous seawater and MORB end-members as in (a) Dashed line represents mixing curvebetweenMORB and seawater at high temperature (HT) Low-temperature (LT) exchanges between these two components would be responsible foran increase of the d18O primary MORB value
BOSCH et al HYDROTHERMAL CIRCULATION IN OMAN OPHIOLITE
1203
exchange with the surrounding rocks during its penetra-tion into the crustal section In an environment in whichfluid---rock interaction occurs at variable temperaturesthe d18O is greatly influenced by the temperature atwhich the exchange took place and by the waterrockratio It is evident from Figs 9b and 10b that none of thestudied gabbros have plagioclase and amphibole withMORB-like d18O and 87Sr86Sr values The separateminerals display a broad range of d18O values and mostexhibit extreme 18O depletion (Table 5) Hydrothermalexchange kinetics are similar in amphibole and pyroxeneand these two minerals are more resistant to oxygenexchange during water---rock interaction than coexistingplagioclase (Gregory et al 1989) The most probableexplanation of the low d18O values measured in bothamphiboles and pyroxenes is that they crystallized athigh temperature in the presence of a relatively low 18Oseawater-derived hydrothermal fluid [d18O from 01 to07 after Gregory amp Taylor (1981)] The anomalouslyhigh d18O value (thorn1009) measured for the plagioclase ofthe micro-gabbroic dyke (01 OA19d) can be interpretedas resulting from a superimposed overprint caused by alate-stage penetration of fluids at lower temperature Inthis sample the occurrence of such different 18O16Oratios between coexisting plagioclase and amphibole isnot consistent with equilibrium fractionation betweenthese two minerals at any possible temperature Thisobservation suggests that parts of the ophiolite sequencehave undergone a high-temperature hydrothermal eventprior to the later low-temperature event that producedthe strong 18O increase in coexisting plagioclaseThe depletion of the d18O values results from high-temperature hydrothermal alteration (Gregory amp Taylor1981 Stakes amp Taylor 1992) The oxygen isotopic datasupport the contention that most studied samples havebeen affected by a VHT---HT hydrothermal alteration byfluids derived from seawater Some of the analysed sam-ples presenting petrological evidence of crystallization ofLT secondary minerals have been overprinted by the latelow-temperature alteration thus swamping the earlierhigh-temperature alteration effects
Determination of waterrock ratio
To better evaluate the exchange between seawater andthe gabbroic rocks waterrock ratios (WR) have beenestimated for the whole rocks analysed during the courseof this study The different models used to constrain thisratio mainly differ by the calculation of the effective ratioor by the estimated weight ratio and by considering openor closed systems for water circulation (Albareede et al1981 McCulloch et al 1981) The WR ratios reportedin Table 5 have been calculated assuming a closed sys-tem Using this formulation these values are minimabecause the calculation always uses pristine fluid for the
initial fluid thus maximizing the value in the denomina-tor If the fluid was shifted to lower 87Sr concentrations byexchange with the rock on the downward path theinferred values would be much higher (Davis et al 2003)The calculated WR ratios for the samples from this
study range from 31 to 219 (Table 5) Two main groupsof samples can be recognized on the basis of their waterrock ratios The lowest ratios ranging from 31 to 47have been determined for massive gabbros or anatecticgabbros but also for dykes and veins devoid of late LTalteration Similar ratios have been previously calculatedfor example for the discharged hydrothermal solutions atthe 13N and 21N East Pacific Rise (Di Donato et al1990 Fouquet et al 1991) The gabbros from the Ibrasection in the Oman ophiolite gave effective WR ratiosof 05 33 and 42 (McCulloch et al 1981) in goodagreement with the values obtained in this study Theleast altered gabbro of the Ibra section yields a WR ratioof 05 in good agreement with the exceptionally fresh-ness of this rock characterized by typical mantle oxygenvalues for its minerals (McCulloch et al 1981) Samplescharacterized by waterrock effective ratios in the rangeof 3---5 can be related to penetrative alteration via thehydrothermal system Lecuyer amp Reynard (1996) havedemonstrated in oceanic gabbros from the Hess Deepaltered in similar HT conditions that WR mass ratios inthe range of 02---1 are sufficient to account for the mod-ified stable isotope signature of the gabbros Higherwaterrock values (WR 45) have been calculated forsamples affected by superimposed late hydrothermalalteration and for the epidosite samples (Table 5) Theyprobably acquired their isotopic characteristics throughinteraction with a larger volume of seawater duringgreenschist-facies metamorphism Similar or higherWR values have been published in previous studies(ie McCulloch et al 1981 Kawahata et al 2001)They correspond either to samples from the upper-crus-tal sequence (ie basalt sheeted dyke or plagiogranite) orto gabbros from the crustal section located in a particularenvironment such as for example the Wadi Fizh sectionthat is located close to a segment boundary along thepalaeo-spreading axis (Nicolas et al 2000)
Mechanism for seawater interaction withmagma
Nicolas et al (2003) have proposed that variable amountsof seawater can circulate at high temperature through-out the crustal section down to the walls of the magmachamber via a network of parallel microcracks a fractionof a millimetre wide Such microcracks and their relation-ship with the VHT---HT hydrous alteration in the sur-rounding gabbros have been described in detail byNicolas et al Via this network of microcracks largeamounts of seawater can have reacted with the magma
JOURNAL OF PETROLOGY VOLUME 45 NUMBER 6 JUNE 2004
1204
and induced variable changes of its Sr and O isotopiccomposition This regular network of parallel micro-cracks could constitute the recharge system and thehydrated gabbroic dykes the discharge system Physicalparameters and the geophysical models of Nicolas et al(2003) are in agreement with this scenario Such a pene-tration of seawater-derived hydrothermal fluids throughthe lower crust along grain boundaries and a microscopicfracture network at temperatures 4750C is consistentwith recent thermodynamic models (McCollom amp Shock1998)Seawater-altered and high-temperature recrystallized
diabases (protodykes) from the overlying sheeted dykecomplex stoped into the melt lenses could represent anindirect way for seawater to enter the system as they canremelt during their subsidence through the magmachamber A simple mass balance calculation suggeststhat this indirect contamination is not significant Assess-ments of the amount of recycled crust 1 in volumeby Nicolas et al (2000) based on the protodyke remnantsin the gabbro section or 20 by Coogan et al (2003)based on chlorine content in EPR basalts lead to a sea-waterrock ratio of the order of 104 to 103Thus following Nicolas et al (2003) we propose that
seawater has been introduced along microcracks down tothe walls of the magma chamber and has diffused alonggrain boundaries in the way described by Manning et al(2000) progressively invading the host gabbro Gabbroicdykes result from the injection of hydrous melt into thehost gabbros at falling temperatures as they drift awayfrom the magma chamber This water-saturated meltcould result from the extensive hydration of the crystal-lizing gabbros near the walls of the magma chamberwhen seawater penetrates through this wall (Fig 1)Similar plumbing systems driving seawater down to the
limit of the magma chamber and back to the surface havebeen already proposed in other environments such as theMid-Atlantic Ridge (Kelley amp Delaney 1987 Cooganet al 2001) the East Pacific Rise at Hess Deep (Gillis1995 Manning et al 1996a 1996b) the Tonga forearc(Banerjee amp Gillis 2001) and the Troodos ophiolite(Gillis amp Roberts 1999) The melting curve of wet basalthas a negative slope but is close to the dry solidus at lowpressure in the lower gabbros and the first melts areplagiogranitic (Spulber ampRutherford 1983) Such plagio-granites are ubiquitous in the highest gabbros and in theroot zone of sheeted dykes in Oman (Rothery 1983Juteau et al 1988 Nicolas amp Boudier 1991) in manyother ophiolites and in the oceanic crust where they havebeen interpreted as resulting either from wet anatexis ofhot gabbros or from the final product of crystallization ofa basaltic melt (Spulber amp Rutherford 1983) Plagio-granites are rare in the lower gabbros exceptionallyassociated with hydrous gabbro segregations inside thelayered gabbros Their constitutive minerals are also
remarkably absent along the VHTmicrocracks althoughthese gabbros were above the wet solidus A possibleexplanation has been proposed by McCollom amp Shock(1998) who wrote that at high temperature lsquoaqueousfluids may leave very little conspicuous petrological evid-ence for their presencersquo the reason being that thehigh-T conditions would be closer to batch meltingObviously more detailed studies on this specific problemare needed We conclude that the large degree of hydrousmelting locally required at the walls of the magma cham-ber to account for the generation of a gabbroic hydrousmelt may have erased the traces of the incipient plagio-granitic melt
CONCLUSIONS
Fostered by the recent discovery of high-temperatureseawater contamination in some gabbros of the Omanophiolite (Manning et al 2000) we have conducted astudy on 500 gabbro specimens from selected areas ofthe entire Samail ophiolite belt and on the hydrous gab-broic dykes that cross-cut them The highest-temperaturereactions (VHT) recorded in olivine and clinopyroxene ingabbros as orthopyroxene and pargasite coronas reach975C They are followed at lower HT conditions withreplacement of olivine by an assemblage of tremolitemagnetite and plagioclase surrounded by chlorite andby pargasite development in clinopyroxene followedfinally by the LT lizardite---olivine and saussurite---plagioclase greenschist-facies alteration With a fewexceptions the VHT alteration tends to be restricted tothe lower gabbros and to the vicinity of the Moho andthe HT alteration to the middle and upper gabbros Thepreservation of the vertical distribution of VHT---HTfacies indicates that progressive cooling during off-axisspreading did not obliterate this distribution and conse-quently that the VHT---HT hydrothermal alteration tookplace in a very restricted time interval at the limit of thecooling magma chamber In the deep and hot gabbrosthe main water channels may be sub-millimetre-sizedmicrocracks with a dominantly vertical attitude the ori-gin and dynamics of which have been investigated in aprevious paper (Nicolas et al 2003)Beyond these high-temperature alteration zones mas-
sively induced in the gabbros seawater may be respons-ible for the generation of clinopyroxene---pargasitegabbro dykes that cross-cut all gabbros and that areupsection clearly connected to the LT hydrothermalvein system Petrostructural evidence suggests that thesedykes were generated by hydration of the crystallizinggabbros near the internal wall of the LVZ---magmachamber The resultant melts were subsequently intrudedat various levels in the cooling lower and middle gabbrosPetrological observations of nearly all the gabbros ana-lysed in the present study located from the root zone of
BOSCH et al HYDROTHERMAL CIRCULATION IN OMAN OPHIOLITE
1205
the sheeted dyke complex down to the Moho emphasizethe imprint of VHT---HT hydrous alteration The VHThydrothermal event is characterized by temperatures ofequilibration around 900---1000C The temperatures atwhich the HT hydrous event occurred are estimated tobe in the range 550---900CStrontium and oxygen isotope compositions measured
on whole rocks and separated minerals significantlydepart from primary MORB values The Sr and O iso-tope data obtained for pargasite green hornblende andclinopyroxene record the participation of seawater at thetemperature of crystallization of the minerals Plagio-clases and whole rocks define a trend toward a compo-nent characterized by a high radiogenic 87Sr86Sr valueand a higher Sr content than normal seawater Seawateris clearly identified as the fluid responsible for the modi-fication of the Sr and O signatures for both massivegabbros and dykes and veins Nevertheless the high Srcontent of the extraneous component requires eitheropen-system exchange allowing penetration of a greaterquantity of seawater or penetration of seawater modifiedby interaction with the oceanic crust during its travel paththrough the crustal section The waterrock ratio calcu-lated on the basis of the Sr isotopes is between three andfive for the enclosing gabbros and for the least LT altereddykes The VHT---HT hydrothermal event affectingthese samples is related to penetrative alteration via therecharge and discharge hydrothermal system Thewaterrock ratio is 10 for dykes exhibiting evidenceof a LT hydrothermal alteration overprint and reaches22 for the epidosite dyke The depletion in d18O char-acterizes all the studied samples with the single exceptionof the micro-gabbroic dyke plagioclase This agreeswith the conclusions based on Sr isotopes suggestingthat these samples have undergone exchange with sea-water-derived fluids during a VHT---HTalteration hydro-thermal event
ACKNOWLEDGEMENTS
This work has benefited from financial support by theCentre National de la Recherche Scientifique (INSUInteerieur-Terre andDYETI programmes) and inOmanfrom the willingness of the Directorate of MineralsMinistry of Commerce and Industry and the hospitabil-ity of the people Technical help in the laboratory is alsoacknowledged including C Nevado for the thin sectionsE Ball for the illustrations B Galland for her assistancein the chemistry room and C Bassoulet for help duringthermal ionization mass spectrometric analyses of the Srresults from the leaching experiments Constructivereviews from R T Gregory C Lecuyer an anonymousreviewer and particularly from M Wilson greatlyhelped to improve an earlier version of the manuscript
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87Sr86Sr ratios in hydrothermal waters and deposits from the East
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modern subduction zone the Tonga forearc crust Journal of
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Becker H Jochum K P amp Carlson R W (2000) Trace element
fractionation during dehydration of eclogites from high-pressure
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Bosch D (1991) Introduction drsquoeau de mer dans le diapir mantellique
de Zabargad (Mer Rouge) drsquoaprees les isotopes du Sr et du Nd
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Boudier F Godard M amp Ambruster C (2000) Significance of
noritic gabbros in the gabbro section of the Oman ophiolite Marine
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Nelson H F amp Otto H F (1982) Variation of seawater 87Sr86Sr
throughout Phanerozoic time Geology 10 516---519Carmichael I S E Turner F J amp Verhoogen J (1974) Igneous
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Chernosky J V Berman R G amp Bryndzia L T (1988) Stability
phase relations and thermodynamic properties of chlorite and
serpentine group minerals In Bayley S W (ed) Hydrous
Phyllosilicates Mineralogical Society of America Reviews in Mineralogy 19
295---346
Coogan L A Wilson R N Gillis K M amp MacLeod C J (2001)
Near-solidus evolution of oceanic gabbros insights from amphibole
geochemistry Geochimica et Cosmochimica Acta 65 4339---4357
Coogan L A Mitchell N C amp OrsquoHara M J (2003) Roof
assimilation at fast spreading ridges an investigation combining
geophysical geochemical and field evidence Journal of Geophysical
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1171
Davis A C Bickle M J amp Teagle D A H (2003) Imbalance in the
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Trocher T (1987) Multi-channel seismic imaging of a crustal
magma chamber along the East Pacific Rise Nature 326 35---41
De Villiers S (1999) Seawater strontium and SrCa variability in the
Atlantic and Pacific oceans Earth and Planetary Science Letters 171
623---634
Di Donato G Joron J L Treuil M amp Loubet M (1990)
Geochemistry of zero-age N-MORB from hole 648B ODP legs
106---109 MAR 22 In Detrick R S Honnorez J et al (eds)
Proceedings of the Ocean Drilling Program Scientific Results 106109
College Station TX Ocean Drilling Program pp 57---65
Dunn R A Toomey D R amp Solomon S C (2000) Three-
dimensional seismic structure and physical properties of shallow
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Dupuy C Mevel C Bodinier J L amp Savoyant L (1991) Zabargad
peridotite evidence for multistage metasomatism during Red Sea
rifting Geology 19 722---725
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1206
Fabriees J Bodinier J L Dupuy C Lorand J P amp Benkerrou C
(1989) Evidence for modal metasomatism in the orogenic spinel
lherzolite body from Caussou (northeastern Pyrenees France)
Journal of Petrology 30 199---228
Fouquet Y Von Stackelberg S U Charlou J L Donval J P
Foucher J Erzig P Muhe R Wiedicke M Soakai S amp
Whitechurch H (1991) Hydrothermal activity in the Lau back-arc
basin sulfides and water chemistry Geology 19 303---306
Gillis K M (1995) Controls on hydrothermal alteration in a section of
fast-spreading oceanic crust Earth and Planetary Science Letters 134
473---489
Gillis K M amp Roberts M (1999) Cracking at the magma---hydrothermal transition evidence from the Troodos ophiolite Earth
and Planetary Science Letters 169 227---244
Gillis K M Mevel C et al (eds) (1993) Proceedings of the Ocean Drilling
Program Initial Reports 147 College Station TX Ocean Drilling
Program
Gregory R T amp Taylor H P (1981) An oxygen isotope profile in a
section of Cretaceous oceanic crust Samail ophiolite Oman
evidence for d18O buffering of the oceans by deep (45 km)
seawater---hydrothermal circulation at mid-ocean ridges Journal of
Geophysical Research 86(B4) 2737---2755
Gregory R T Criss R E amp Taylor H P (1989) Oxygen
isotope exchange kinetics of mineral pairs in closed and open
systems applications to problems of hydrothermal alteration of
igneous rocks and Precambrian iron formations Chemical Geology 75
1---42Hacker B R (1994) Rapid emplacement of young oceanic litho-
sphere argon geochronology of the Oman ophiolite Science 265
1563---1569
Hart S R Erlank A J amp Kable J D (1974) Sea floor basalt
alteration some chemical and Sr isotopic effects Contributions to
Mineralogy and Petrology 44 219---230
Holland T amp Blundy J (1994) Non-ideal interactions in calcic
amphiboles and their bearing on amphibole---plagioclase thermo-
metry Contributions to Mineralogy and Petrology 116 433---447
Ionov D A Savoyant L amp Dupuy C (1992) Application of the
ICP-MS technique to trace element analysis of peridotites and their
minerals Geostandards Newsletter 16 311---315
Javoy M (1980) 18O16O and DH ratios in high temperature
peridotites In Colloques Internationaux du CNRS 272 279---287
Juteau T Beurrier M Dahl R amp Nehlig P (1988) Segmentation
at a fossil spreading axis the plutonic sequence of the Wadi
Haymiliyah area (Haylayn Block Sumail nappe Oman) Tectono-
physics 151 167---197
Juteau T Manacrsquoh G Moreau C Lecuyer C amp Ramboz C
(2000) The high temperature reaction zone of the Oman ophiolite
new field data microthermometry of fluid inclusions PIXE analyses
and oxygen isotopic ratios Marine Geophysical Research 21 351---385Kawahata H Nohara M Ishizuka H Hasebe S amp Chiba H
(2001) Sr isotope geochemistry and hydrothermal alteration of the
Oman ophiolite Journal of Geophysical Research 106 11083---11099
Kelemen P B amp Aharonov E (1998) Periodic formation of magma
fractures and generation of layered gabbros in the lower crust
beneath oceanic spreading ridges Annual Geological Union Special
Monograph 106 267---289
Kelemen P B Koga K amp Shimizu N (1997) Geochemistry of
gabbro sills in the crustmantle transition zone of the Oman
ophiolite implications for the origin of the oceanic lower crust Earth
and Planetary Science Letters 146 475---488Kelley D S amp Delaney J R (1987) Two-phase separation and
fracturing in mid-ocean ridge gabbros at temperatures greater than
700C Earth and Planetary Science Letters 83 53---66
Koepke J Feig S T Snow J amp Freise M (2003) Petrogenesis of
oceanic plagiogranites by partial melting of gabbros an experi-
mental study Contributions to Mineralogy and Petrology on line first http
wwwspringerlinkcomopenurlaspgenre=s00410-003-0511-9
Kyser T K (1986) Stable isotope variations in the mantle In
Valley J W Taylor H P amp OrsquoNeil J R (eds) Stable Isotopes in
High Temperature Geological Processes Mineralogical Society of America
Reviews in Mineralogy 16 141---164
Lanphere M A (1981) K---Ar ages of metamorphic rocks at the base
of the Samail ophiolite Oman Journal of Geophysical Research 86
2777---2782
Lanphere M A Coleman R G amp Hopson C A (1981) Sr isotopic
tracer study of the Samail ophiolite Oman Journal of Geophysical
Research 86(B4) 2709---2720
Leake B E (1997) Nomenclature of amphiboles report of the
subcommittee on amphiboles of the International Mineralogical
Association commission on new minerals and mineral names
European Journal of Mineralogy 9 623---651
Lecuyer C amp Reynard B (1996) High-temperature alteration of
oceanic gabbros by seawater (Hess Deep Ocean Drilling Program
Leg 147) evidence from oxygen isotopes and elemental fluxes
Journal of Geophysical Research 101(B7) 15883---15897
Liou J G Kuniiyoshi S amp Ito K (1974) Experimental study of the
phase relations between greenschist and amphibolite in a basaltic
system American Journal of Science 274 613---632
MacLeod C J amp Rothery D A (1992) Ridge axial segmentation in
the Oman ophiolite evidence from along-strike variations in the
sheeted dyke complex Geological Society of America Special Papers 60
39---63
Manning C E MacLeod C J amp Weston P E (1996a) Fracture-
controlled metamorphism of Hess Deep gabbros site 984
constraints on the roots of mid-ocean ridge hydrothermal systems
at fast-spreading centers In GillisM C Allan KM ampMeyer P S
(eds) Proceedings of the Ocean Drilling Program Scientific Results 147
College Station TX Ocean Drilling Program pp 189---211Manning C E Weston P E amp Mahon K I (1996b) Rapid high-
temperature metamorphism of East Pacific Rise gabbros from Hess
Deep Earth and Planetary Science Letters 144 123---132Manning C E MacLeod C J amp Weston P E (2000) Lower-crustal
cracking front at fast-spreading ridges evidence from the East Pacific
Rise and the Oman ophiolite Journal of the Geological Society London
349 261---272McCollom T M amp Shock E L (1998) Fluid---rock interactions in
the lower oceanic crust thermodynamic models of hydrothermal
alteration Journal of Geophysical Research 103 547---575
McCulloch M T Gregory R T Wasserburg G J amp Taylor H P
(1981) Sm---Nd Rb---Sr and 18O16O isotopic systematics in an
oceanic crustal section evidence from the Samail ophiolite Journal of
Geophysical Research 86(B4) 2721---2735Mevel C Gillis K M Allan J F amp Meyer P S (eds) (1996)
Proceedings of the Ocean Drilling Program Scientific Results 147 College
Station TX Ocean Drilling Program
Nehlig P amp Juteau T (1988) Flow porosities permeabilities and
preliminary data on fluid inclusions and fossil thermal gradients in
the crustal sequence of the Sumail ophiolite (Oman) Tectonophysics
151 199---221
Nehlig P Juteau T Bendel V amp Cotten J (1994) The root zone of
oceanic hydrothermal systems constraints from the Samail ophiolite
(Oman) Journal of Geophysical Research 99 4703---4713
Nicolas A amp Boudier F (1991) Rooting of the sheeted dyke complex
in Oman ophiolite In Peters Tj Nicolas A amp Coleman R (eds)
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Kluwer Academic pp 39---54
BOSCH et al HYDROTHERMAL CIRCULATION IN OMAN OPHIOLITE
1207
Nicolas A amp Ildefonse B (1996) Flow mechanism and viscosity in
basaltic magma chambers Geophysical Research Letters 23(16)
2013---2016
Nicolas A Ceuleneer G Boudier F amp Misseri M (1988) Structural
mapping in the Oman ophiolites mantle diapirism along an ocean
ridge Tectonophysics 151 27---56
Nicolas A Boudier F Ildefonse B amp Ball E (2000) Accretion
of Oman and United Arab Emirates ophiolite-----discussion of a
new structural map Marine Geophysical Research 21(3---4)147---179
Nicolas A Boudier F Michibayashi K amp Gerbert-Gaillard L
(2001) Aswad massif (United Arab Emirates) archetype of the
Oman---UAE ophiolite belt Geological Society of America Bulletin 349
499---451
Nicolas A Mainprice D amp Boudier F (2003) High temperature
seawater circulation through the lower crust of ocean-ridges-----amodel derived from the Oman ophiolites Journal of Geophysical
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2372
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morphology on magma supply and spreading rate Nature 364
706---708
Pin C Briot D Bassin C amp Poitrasson F (1994) Concomitant
extraction of Sr and Sm---Nd for isotopic analysis in silicate samples
based on specific extraction chromatography Analytica Chimica Acta
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Rothery D A (1983) The base of a sheeted dyke complex Oman
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compositional variability in amphibolite American Journal of Science
281 697---734
Spulber S D amp Rutherford M J (1983) The origin of rhyolite and
plagiogranite in oceanic crust an experimental study Journal of
Petrology 24 1---25Stakes D S (1991) Oxygen and hydrogen isotope compositions of
oceanic plutonic rocks high-temperature deformation and meta-
morphism of oceanic layer 3 In Taylor H P OrsquoNeil J et al (eds)
Geochemical Society Special Publication 3 77---90
Stakes D S amp Taylor H P (1992) The northern Samail ophiolite an
oxygen isotope microprobe and field study Journal of Geophysical
Research 97(B5) 7043---7080Staudigel H Davies G R Hart S R Marchant M amp Smith B
M (1995) Large scale isotopic Sr Nd and O isotopic anatomy of
altered oceanic crust DSDPODP sites 417418 Earth and Planetary
Science Letters 130 169---185Turner F J amp Verhoogen J (1960) Igneous and Metamorphic Petrology
New York McGraw---Hill 694 pp
Wilcock W S D amp Delaney J R (1996) Mid-ocean ridge
sulfide deposits evidence for heat extraction from magma
chambers or cracking fronts Earth and Planetary Science Letters 145
49---64
Witt G amp Seck H A (1989) Origin of amphibole in recrystallized
and porphyroclastic mantle xenoliths from Rhenish massif implica-
tions for the nature of mantle metasomatism Earth and Planetary
Science Letters 91 327---340
JOURNAL OF PETROLOGY VOLUME 45 NUMBER 6 JUNE 2004
1208
single outcrop and even in a single thin section As a rulethe orthopyroxene reactions are more penetrative beinghomogeneously distributed from the scale of a thinsection to that of several thin sections made from a
given outcrop The distribution of chlorite---amphibolecoronas is similar although they can be restricted to themargins of microcracks filled by the same minerals whichhave been interpreted as the channels that carried the
(a) (b)
(c) (d)
(e) (f)
olopx
kel
pl
ol
hbol
amcpx
cpx
hb
chl
am+pl+mt
idg
am
hb
pl
hb
ol
cpx
opx
mt
pl
Fig 3 Hydrothermal alteration in the gabbros (scale bar represents 1mm) (a) VHT alteration marked by orthopyroxene (opx) coronas at thecontact of olivine (ol) and plagioclase (pl) and pargasitic hornblende (hb) at the contact of olivine and clinopyroxene (cpx) (sample 94 MA2)(b) Kelyphite (kel) coronas around olivine and cloudy plagioclase as a result of silicate and fluid inclusions Silicate inclusions tend to be localizedat grain boundaries and along microcracks (sample 94 MA2) (c) VHT alteration marked by brown amphibole (hb) alteration of clinopyroxene in anupper gabbro (d) HT alteration marked by chlorite (chl) (external rim) and pale amphibole (am) and magnetite (mt) (internal rim) coronas aroundolivine (e) Total replacement of olivine by an assemblage of tremolite (am)thornmagnetite (mt)thorn plagioclase (pl) (f ) Total replacement of olivine by aniddingsitic mixture (idg) inside a VHT recrystallized corona of a mosaic of orthopyroxene
BOSCH et al HYDROTHERMAL CIRCULATION IN OMAN OPHIOLITE
1185
fluids responsible for the hydrothermal alteration (Nicolaset al 2003) The iddingsite replacement is typicallyheterogeneous at the thin-section scale being preferen-tially associated with microcracks Serpentinization alsodevelops from microcracks but it can pervasively invadethe entire thin section The microcracks are commonapproximately 25 of the studied thin sections displayone or more Ignoring the cracks related to LT alterationand considering only the cracks related to the HT hydro-thermal alteration this fraction is still around 20
Distribution of high-T alteration facies
Figure 4 shows the distribution of alteration facies as afunction of depth in the crust along wadi sections throughthe Oman---UAE ophiolite offering continuous exposures(see details in Fig 1) Figure 4 excludes the LT faciesbecause (1) being based on the presence of olivine ser-pentinization only throughout our compilation its dis-tribution is biased by the decreasing amount of olivineupsection and (2) this study concerns high-temperaturealteration processes Despite the fact that the scale ofsampling may exceed the scale of heterogeneity evidencefor both VHT alteration and lsquono alterationrsquo increaseswith depth Conversely HT alteration is predominantin the middle and upper parts of the gabbro sectionThis observation is valid throughout the wadi sectionsstudied with the exception of the Wadi Tayin massifwhich shows no clear gradation with depthFigure 5 is another representation of alteration-facies
distribution with depth based on the detailed study of414 thin sections The samples are classified according totheir level in the gabbro section sheeted dyke root zonegabbros upper gabbros middle gabbros lower gabbrosand Moho gabbros (the Moho gabbros have beengrouped together with the gabbro lenses interlayered inthe dunites of the MTZ) Values represent the percent of
2 Km
3
4
5
6
7
Aswad Fizh Hilti Haylayn Samail W TayinNakhl
High T
Very High T
0 High T
Upper Crust
LID
Lower Crust
MohoMTZ
DEPTH1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27
Fig 4 Depth of penetration of hydrothermal circulation in gabbros along the wadi sections numbered in Fig 1 referring to alteration faciesdescribed in Fig 3 Depths are recorded below the palaeo-seafloor LID corresponds to the upper crust including sheeted dykes and volcanics
0
10
20
30
40
50
60
70
80
90
100
SD root
zone
upper
gabbros
middle
gabbros
lower
gabbros
MTZ
BHb-GHb CPX
Chl-Amph core OL
Chl-Amph rim OL
0
5
10
15
20
25
30
SD root
zone
upper
gabbros
middle
gabbros
lower
gabbros
MTZ
Opx OL
BHb OL
No VHTHT
Unaltered OL core
HT
VHT
(a)
(b)
Plst def
gabbros
Fig 5 Compilation of hydrothermal alteration features as a function ofdepth in the gabbro section Percentages represent the occurrence of agiven type of alteration versus the number of thin sections examined foreach selected level root of the sheeted dykes (SD) upper gabbros middlegabbros lower gabbros MTZ (Moho Transition Zone) gabbros on atotal amount of 414 thin sections (Opx orthopyroxene Amph amphi-bole OL olivine BHb brown hornblende GHb green hornblendeChl chlorite Plst def plastic deformation) (a) Distribution of the HTalteration (in ) (b) Distribution of the VHT alteration (in )
JOURNAL OF PETROLOGY VOLUME 45 NUMBER 6 JUNE 2004
1186
a given alteration facies as defined previously out of thetotal number of thin sections examined for a given levelMore refined conclusions can be drawn from this figureAs seen in Fig 5a the HT alteration decreases down-
wards whereas it affects most of the upper gabbroicsection (sheeted dyke root zone and upper gabbros)The fraction of unaltered olivine cores can be used asan index to estimate the extent of alteration This indexshows that the HT alteration is more penetrative andintense in the upper gabbros a conclusion alreadyreached by several workers (eg Gregory amp Taylor1981 Stakes amp Taylor 1992 Kawahata et al 2001)This was also noted by Manning et al (1996a 1996b)Figure 5b shows that the VHT alteration marked by
orthopyroxene or brown hornblende coronas aroundolivine increases with depth The correlation betweenthe VHT alteration and plastic deformation also suggeststhat the VHT alteration may coincide with the develop-ment of plastic flow at the expense of the magmatic flow
at temperatures just below the solidus In Fig 5b thefraction of gabbros where there is no VHT---HThydrothermal alteration increases downsection indicat-ing that the VHT---HT alteration is more localizeddownsection
Gabbroic dykes and veins
The occurrence of gabbroic dykes in the layered gabbrosof the Samail ophiolite is ubiquitous We describe gab-broic dykes and sills either plagioclase---clinopyroxeneand brown amphibole-bearing dykes or microgabbrosand amphibolitized dolerite dykes and sills correspond-ing to temperatures up to 975C (see below) Identifica-tion of gabbroic dykes in the field is not alwaysstraightforward because they may be obliterated bysuperimposition of greenschist-facies alteration (Fig 6a)Selective greenschist-facies alteration has frequently beenobserved either in the centre of a mafic dyke or alongboth margins (Fig 6b) as well as a continuous transition
(b) (c)(a)
(d) (e)
thulite
pistacite
white microvein
white vein
green vein
green vein
gabbroic dike
epidote vein
gabbroic dike
Fig 6 Field relationships of dykes and veins (scale bar represents 10 cm) (a) Thulite---pistacite vein cross-cutting a foliated gabbro matrix Reactionmargins marked by the development of secondary clinopyroxene (01 OA36b) (b) Comb-like structure in gabbroic dyke A pink epidote vein followsthe margin of the dyke (01 OA15f) (c) Along-strike transition from a gabbroic dykelet to a hydrothermal green amphibole vein (d) White prehnitevein (e) Green amphibole vein cross-cut by white prehnite vein
BOSCH et al HYDROTHERMAL CIRCULATION IN OMAN OPHIOLITE
1187
along a single crack from a mafic dyke to a hydrothermalvein (Fig 6c) We define hydrothermal veins as cracksfilled with greenschist-facies minerals that induce meta-morphic reactions in the adjacent country rocks andgabbroic dykes as planar intrusive bodies produced bythe crystallization of magma Following Nehlig amp Juteau(1988) we distinguish low-T greenschist-facies lsquowhiteveinsrsquo made of prehnite epidote chlorite and actinolite(Fig 6d) and lsquogreen veinsrsquo in which a darker greenamphibole predominates (Fig 6e) corresponding respec-tively to lower (195---410C) and higher (400---530C)temperatures (Nehlig amp Juteau 1988)The gabbroic dykes are typically a few centimetres
across composed of plagioclase clinopyroxene andbrown
hornblende They are coarse-grained to pegmatitic andcommonly display a comb-like structure (Fig 6b)Although the spacing between the dykes in the field ishighly variable the average is around 10m Gabbroicsills grade from clear intrusions to irregular and diffusepegmatitic gabbro patches up to a few metres in theirlarger dimension (Fig 7a)The distribution in the field of brownish pargasite
which appears black and glassy in outcrop is critical intracing the origin of the gabbroic dykes It is first observedat any level in the host lower and middle gabbros as largepoikilitic patches (Fig 7b) locally associated with simil-arly poikilitic patches of clino- and orthopyroxene(Fig 7a) These patches contain small aligned tablets of
(a) (b) (c)
g(e)(d)
alignment ofalignment of amphiboleamphibole oikocrystsoikocrysts
hwhite vein
hblpl
pcpx
brownbrownamphiboleamphiboleoikocrystoikocryst
eediabasedikdike
hb-plhb-plreactionreaction
imarginspbrown amphibole rich
gabbroic dikegabbroic dike
pbrown amphibole veins
Fig 7 Field relationships of dykes (scale bar is 10 cm long) (a) Irregular coarse to pegmatitic patches in a foliated gabbro matrix (01 OA42d)(b) Alignment of black amphibole oikocrysts in a foliated gabbro matrix (c) Local concentration of brown amphibole in a gabbroic dykelet Theplagioclase laths marking the foliation in the enclosing gabbro rotate into parallelism with the gabbroic dykelet (d) Two diabase dykes assimilatingthe enclosing layered gabbro with plagioclase---amphibole reaction margins (e) Pargasite-rich gabbroic dyke with wall reactions (01 OA19) (Notesmall pargasite---plagioclase veins) pl plagioclase cpx clinopyroxene hb hornblende
JOURNAL OF PETROLOGY VOLUME 45 NUMBER 6 JUNE 2004
1188
plagioclase oriented parallel to the larger plagioclaselaths defining the magmatic foliation of the gabbro(Fig 7c) Clinopyroxene oikocrysts surrounding idio-morphic plagioclase crystals are common in the upper-level gabbros they mark the end of the magmatic flowand crystallization growing before the pargasite andorthopyroxene oikocrysts as shown by their partialreplacement by pargasite Their crystallization marksthe transition at suprasolidus conditions from a dry to awet environment In the uppermost gabbros those dykesthat are typically altered in the HT hydrothermal faciesgrade upwards into the so-called lsquoisotropicrsquo or lsquovari-texturedrsquo gabbrosThe microgabbro and amphibolitized dolerite dykes
differ from the typical diabase dykes commonly cuttingthe gabbro section that have chilled margins and thusprobably intruded enclosing rocks already at temperat-ures below 450C In microgabbro and amphibolitizeddolerite dykes the absence of chilled margins and thedevelopment of dioritic reaction margins in the adjacentgabbro (Fig 7d and e) suggest that they were emplaced ingabbros that were still at temperatures above 750C(Nicolas et al 2000)
ANALYTICAL TECHNIQUES
Major and trace elements
Major element compositions were determined by elec-tron microprobe at the University of Montpellier II usinga Camebax SX100 Instrument calibration was per-formed on natural standards and operating conditionswere 20 kV accelerating potential 10 nA beam currentand 30 s counting timeRb and Sr concentrations in whole rock and separated
mineral fractions were measured by conventionalnebulization inductively coupled plasma mass spectro-metry (ICP-MS) using a VG Plasmaquad 2 (ISTEEMMontpellier) Digestion procedures followed thoseoutlined by Ionov et al (1992)
O---Sr isotopes
The samples selected for the detailed study wereextracted from gabbroic dykes or veins inside gabbrosand cut with a diamond saw into 2---3 cm fragmentsbefore being crushed into 05---1 cm fragments Forwhole-rock analysis small pieces were further crushedinto powder in an agate bowl For pure mineral analysisthe fragments were crushed with a manual agate mortarTo avoid or minimize the mixing of selected separateminerals with other phases that can bias the results astrict protocol of mineral selection was applied A smallsize fraction of 125---160 mm was used and minerals werefirst separated using a Frantz isodynamic magneticseparator Concentrates recovered (cpx plagioclase
amphibole epidote) were subsequently purified by care-ful hand-picking in alcohol under a binocular micro-scope At this stage mineral separates were very pureand only flawless minerals free of cracks and visibleinclusions were kept for isotopic analyses To ensurefurther mineral integrity these pure mineral separateswere ultrasonically washed in dilute 15N HCl and rinsedseveral times with ultra-pure water This stage potentiallyremoves adsorbed secondary micro-phases Previousstudies (eg Lecuyer amp Reynard 1996) demonstratedthat pyroxene and amphibole can be intermixed on avery fine scale with other phases (such as a range ofamphibole compositions talc Fe-oxide) Although thispossibility cannot be ruled out the procedure used inthis study makes it unlikely that complex minerals passedthrough the selection
Sr isotopic compositions
Approximately 50mg of whole-rock powder or separatedminerals were digested in a Teflon lsquobombrsquo with a mixtureof triple distilled 13N HNO3 and 48 HF with a fewdrops of HClO4 Two aliquots were separated one forthe determination of Sr isotopic composition and theother for Sr and Rb contents Sr was separated using anextraction chromatographic method modified from Pinet al (1994) Concentrations were measured by isotopedilution using 84Sr---87Rb mixed spikes To remove tracesof the late low-temperature seawater alteration and todetermine the primitive whole-rock Sr isotopic composi-tion leaching experiments were conducted on a fewsamples following the procedure described by Bosch(1991) The strontium isotopic compositions of leachateshave been analysed in the attempt to check for the leach-ing efficiency During these tests four Sr isotope composi-tions were measured which correspond to the unleachedsample the two liquids extracted after acid-leaching stepsand the final solid residue Leaching experiments wereconducted in two steps first with 6N HCl for 8min at70C second with 25N HCl for 30min at 70C Afterleaching the leachates were evaporated to dryness andthe solid residues digested similarly to the unleachedsamplesSr isotopic compositions were measured on a fully
automated VG Sector mass spectrometer housed at theUniversity of Montpellier II except for the Sr isotopicdata from the leaching experiments which have beenmeasured at the Universitee de Bretagne Occidentale(Brest) To assess the reproducibility and accuracy of Srisotopic measurements the NBS 987 standard was run12 times during this study the average value was0710245 with a precision better than 0000010 (2s)87Sr86Sr ratios are corrected for mass discrimination bynormalizing to a 86Sr88Sr value of 01194 Sr total blankconcentrations were less than 60 pg Initial 87Sr86Sr
BOSCH et al HYDROTHERMAL CIRCULATION IN OMAN OPHIOLITE
1189
ratios were calculated by correcting the measured ratiosfor accumulation of 87Sr produced by in situ 87Rb radio-active decay since 95Ma the assumed age of crystalliza-tion of the Oman ophiolite (Hacker 1994)
Oxygen isotopes
The powdered samples were introduced in Ni tubesunder dry air where they were reacted with BrF5 toform oxygen Oxygen was separated from other gasesby cooling the resulting mixture at liquid nitrogen tem-perature (77 K) that retained other volatile gases Oxygenwas further purified by trapping it on a cooled 5A mole-cular sieve at 77 K then quantified to calculate the yieldof the isotopic analyses and finally transferred to the massspectrometer (Finnigan Mat Delta E) where intensitiesassociated with masses 32 and 34 were measured todetermine d18O The isotopic composition of a sampleis given as dsample frac14 (RsampleRstandard 1) 1000 in permil unit where R is 18O16O and the standard isSMOW Analytical errors on the oxygen isotope ratioare 02 (2s)
RESULTS
Seven samples representative of the range of gabbroicdykes and five samples representative of the lower gab-bros two of them representing the margins of gabbroicdykes were selected for major element thermometricand Sr---O isotopic analyses (Tables 1---3) These samplesoriginate from wadi sections marked by bold lines inFig 1 Wadis Halhal Mayah Al Abyad Warihya andMaqsad structurally belong to a new ridge segment open-ing in slightly (1Myr) older lithosphere Wadi Gideahis in older lithosphere
Petrographic and isotopic characteristicsof the analysed samples
Olivine gabbro from the MTZ in MaqsadSamail Massif
Sample 94 MA2 is devoid of low-temperature alterationand only exhibits the discrete signature of the VHT---HTalteration The orthopyroxene corona around olivine(Fig 3a) is relayed by kelyphitic rims at the contactwith plagioclase (Fig 3b) Issuing from these rims aremicrometre-sized pale green amphiboles identified ascummingtonite and actinolite which penetrate the sur-rounding plagioclase in microcracks 004mm wide or atgrain boundaries (Fig 3b) similar to the microcrack sys-tem described by Manning et al (2000) A large numberof the plagioclase crystals have a cloudy core as a result ofthe concentration of fluid and mineral inclusions amongwhich diopside has been identified during microprobeanalysis Olivine cores are free of serpentine but theyare largely oxidized along a microcrack network Ortho-pyroxene coronas have a MgOpx number of 78---79
slightly higher than in the primary olivine cores withMgOl number of 74---75 The magmatic clinopyroxeneis totally fresh and probably not in equilibrium with thereactive orthopyroxene This sample has the lowest 87Sr86Sr initial ratio of the studied whole rocks (070322)identical to coexisting clinopyroxene (070324) and pla-gioclase (070322) This observation suggests the lack of alate superimposed LT hydrothermal alteration as it isvery unlikely that an LT hydrothermal event could havetotally equilibrated the Sr isotopic compositions of prim-ary minerals such as plagioclase and clinopyroxeneMoreover the whole rock (WR) clinopyroxene andplagioclase have similar initial Sr isotopic ratios despitevery different Sr contents Accordingly this ratio isthought to reflect the signature acquired during the crys-tallization of these minerals and is higher than the Omanmantle Sr isotopic value (McCulloch et al 1981) Thed18O values of the WR (50) and plagioclase (48) aredistinctly lower than primary magmatic values Indeedin normal mid-ocean ridge basalts (MORB) the WRd18O value is 57 02 with a corresponding plagio-clase d18O ranging from 55 to 62 ( Javoy 1980Gregory amp Taylor 1981)
Lower gabbro from Wadi Wariyah Wadi Tayin Massif
Sample 97 OA128b belongs to a layered sequence in thelower gabbro series injected by anorthositic sills Thesample was collected at the tip of one of these sillsbetween layers enriched in clinopyroxene The gabbrois composed of 60 plagioclase and 40 clinopyroxeneDespite the absence of olivine the WR has a relativelyhigh Mg content (Table 3) A series of low-T (prehnite)microveins 01mm thick cross-cut the gabbro perpendi-cular to the layering and foliation The margins of theveins are devoid of alteration This gabbro has the sameSr isotopic composition as 94 MA2 Nevertheless theWR and associated plagioclase have Sr isotope signatures(070333 and 070328) lower than the coexisting clino-pyroxene (070422) The Sr contents of the WR plagio-clase and clinopyroxene are 127 ppm 120 ppm and20 ppm respectively thus making the clinopyroxenemore sensitive to late-stage contamination Clinopyrox-ene and plagioclase oxygen isotope compositions (523and 414 respectively) are also lower than typicalmantle values Similar to the previous sample alterationat high temperature by a low d18O and high 87Sr86Srfluid is required to explain such characteristics
Amphibole gabbro patch Wadi Gideah Wadi TayinMassif
Sample 01 OA41d2 is from a laminated gabbro com-posed of millimetre-sized layers of clinopyroxeneand plagioclase Anatectic patches (Fig 7a) borderinganorthositic layers are formed of coarse-grained
JOURNAL OF PETROLOGY VOLUME 45 NUMBER 6 JUNE 2004
1190
Table1Locationoftheanalysed
samplesandsummaryoftheirpetrographicandSr---O
isotopiccharacteristics
Sam
ple
nam
eSite
Locationin
gab
broic
section
Typ
eofrock
Distance
to
Moho(km)
Parag
enesis
Alteration
87Sr
86Srinitial1
d18O
()2
94MA2
Maq
sad
From
MTZ
Layered
gab
bro
0Olivine(M
gno74---75)3
OPXco
ronas
(Mgno78---79)
WRfrac14
070322
WRfrac14
thorn495
CPX
Palegreen
Am
(cumact)
CPXfrac14
070324
nd
Plagioclase(A
n65---80)4inversezoning
Nolow-T
alteration
Plfrac14
070322
Plfrac14
thorn479
Solidfluid
inclusionsin
Pl
97OA128b
Wad
iWaryiah
From
lower
gab
bro
Layered
gab
bro
1CPX
Low-T
alteration
WRfrac14
070333
nd
Plagioclase(A
n85---87)
Prehnitemicroveins
CPXfrac14
070422
CPXfrac14
thorn523
Plfrac14
070328
Plfrac14
thorn414
01OA41d2
Wad
iGideah
From
upper
gab
bro
Gab
bro
patch
in
laminated
gab
bro
36
CPXrecrystallized
Plagioclaseidiomorphic
(An57---87)norm
alzoning
Black
pargasitic
honblende
WRfrac14
070351
CPXfrac14
070378
Plfrac14
070426
WRfrac14
thorn480
nd
Plfrac14
thorn504
Hbdfrac14
070355
nd
01OA19aamp
dWad
iWaryiah
Inlayeredgab
bro
Gab
broic
dyke
01
Olivine
OPXpoikilitic
WRfrac14
070341
WRfrac14
thorn567
CPX
Black
pargasite
Hbdfrac14
070324
Hbdfrac14
thorn287
Plagioclase(A
n73---95)
Hornblendepoikilitic
Plfrac14
nd
Plfrac14
thorn10 09
Hem
atite
WRfrac14
070418
nd
Green
Mg-hornblende
Acthornblende
Epidote
02OA90b
Wad
iAl-Abyad
Inlayeredgab
bro
Dioriticdyke
015
Green
Mg-hornblende
Hbdfrac14
070427
Hbdfrac14
thorn239
Plagioclase(A
n83---87)
Plfrac14
070387
Plfrac14
thorn498
Solidfluid
inclusionsin
plagioclase
01OA15f
Wad
iWaryiah
Inlayeredgab
bro
Comb-textured
gab
broic
dyke
025
CPXidiomorphic
Plagioclase(A
n95---99)norm
alzoning
BrownMg-H
ornblende
WRfrac14
070458
nd
02OA43a
Wad
iMayah
Inlayeredgab
bro
vein
Green
amphibole
1Palegreen
Mg-hornblendeacicular
Plagioclase(A
n91---97)in
reactionmargin
Hbdfrac14
070431
Hbdfrac14
thorn310
01OA36b
Wad
iGideah
Inlayeredgab
bro
epidosite
dyke
25
CPX
Epidote
WRfrac14
070519
nd
Plagioclase
Epfrac14
070554
nd
CPXclinopyroxene
Plplagioclase
OPXorthopyroxene
Amam
phibolecu
mcu
mmingtonite
actactinolite
WRwholerockHbdhornblende
Epep
idotend
notdetermined
187Sr
86Srinitialratioscalculatedat
95Ma(H
acker1994)Errors
ofthemeasuredratiosareindicated
inTab
le4
2Analyticalerrors
oftheoxygen
isotopevalues
are02
(2s)
3Mgnumber
frac14atomicMg(Mgthorn
Fe)
4Percentageofan
orthitein
plagioclase
BOSCH et al HYDROTHERMAL CIRCULATION IN OMAN OPHIOLITE
1191
(millimetre-sized) recrystallized clinopyroxene largelyaltered to light green magnesio-hornblende and idio-morphic plagioclase Locally primary black pargasitichornblende develops including idiomorphic normallyzoned plagioclase (Table 4) This gabbro exhibits similar87Sr86Sr ratios for the WR and pargasite (070351 and070355 respectively) These values are interpreted asrepresentative of the melt from which this amphibolecrystallized The clinopyroxene has a slightly higher Srisotope composition of 070378 consistent with the occur-rence of the light green Mg-hornblende rim Plagioclaseis significantly more radiogenic (070426) related to sec-ondary destabilization evidenced by the idiomorphichabit The Sr isotope compositions of the liquidsextracted from the acid leaching experiments of theWR (L1 and L2) are very similar 070358 and 070365respectively The 87Sr86Sr ratio of the residue obtainedafter leaching experiments is 070335 lower than boththe pargasite and unleached WR This value can beconsidered as the unaltered isotopic composition of thissample The d18O values measured for WR and coexist-ing plagioclase are lower than normal MORB magmaticvalues The 18O depletion in the plagioclase can be fairlywell explained by high-temperature hydrothermal inter-action between an oxygen-depleted fluid and the magmaAccording to the kinetics of hydrothermal exchange ofoxygen (Gregory amp Taylor 1981 Gregory et al 1989)plagioclase exchanges its oxygen isotopes with the fluidadjusting readily to the hydrothermal conditions Forhigh temperatures of exchange with seawater-derivedfluids (d18O between 0 and thorn2) the magmatic plagio-clase will have its primary d18O value lowered whereasthe d18O value for lower temperatures of exchange(5300C) will increase The amplification of the d18Oshift increases with the intensity of the hydrothermalalteration
Gabbroic dykes intrusive in lower gabbros from WadiWariyah Wadi Tayin Massif
Gabbroic dykes 01 OA19a and 01 OA19d are intrusiveinto the lower gabbros They are 3---4 cm wide discord-ant to the foliation of the host gabbros (Fig 7e) with asharp margin marked by a brown amphibole fringe Theprimary phases in the dykes (Table 1) locally preservedas elongated aggregates of a 01mm grain size mosaicassemblage at the dyke margins grade to millimetre sizein the central part of the dyke The WR major elementcomposition is Mg enriched compared with the enclosinggabbro (Table 3) Brown pargasitic amphibole oikocrystsmake up 50 of the modal composition of the dyke Indyke 01 OA19a poikilitic orthopyroxene may developinstead of brown amphibole Finally green magnesio-hornblende grows either as oikocrysts at the expense ofclinopyroxene or in association with epidote as an altera-tion product of plagioclase These low-amphibolite-faciesminerals represent the lowest-grade paragenesis recogn-ized in these dykes and are most developed in sample01OA19aTheamphibole composition showszoning fromTi-rich pargasitic hornblende to magnesio-hornblendeand actinolitic hornblende (Fig 8c) recording pro-gressive cooling of the dyke The plagioclase compositionis extremely heterogeneous (Fig 8b) varying from An95a composition similar to that of the plagioclase in the hostgabbro (Fig 8a) to An50 for the plagioclase and asso-ciated epidote inclusions in the poikilitic actinolitic horn-blende Sample 01 OA19d has an initial 87Sr86Sr ratioslightly higher for the WR (070341) than for the parga-site (070324) The Sr isotopic ratios of liquids extractedafter two steps of WR acid-leaching (L1 and L2) are070357 and 070341 respectively whereas the residueyields a 87Sr86Sr ratio of 070327 very close to thepargasite and plagioclase values The latter is similar tothe isotopic composition measured for the massive gab-bro 94 MA2 and may be taken as close to the primarymagmatic value This Sr isotopic ratio is more radiogenicthan the inferred Oman mantle-source value (McCullochet al 1981) The altered part of this sample has a radio-genic Sr isotopic composition (070418) related to thelow-amphibolite-facies alteration This gabbroic dykehas seemingly preserved a magmatic whole-rock d18Ovalue (567) but contains by far the highestd18O plagioclase (1009) of all the samples analysedThis plagioclase coexists with low d18O pargasite(thorn287) Similarly high values (d18Oplagioclase 4 8)have been previously measured for gabbroic dykes dia-bases sheeted dykes and plagiogranites from the Omanophiolite (Gregory amp Taylor 1981 Stakes amp Taylor1992) This 72 oxygen isotope fractionation betweenplagioclase and amphibole cannot possibly represent oxy-gen equilibrium at any plausible temperature This maybe explained by different exchange kinetics for these twominerals at different temperatures at high waterrock
Table 2 Average temperatures ( C 40C)
calculated for plagioclase---amphibole pairs
using the geothermometer of Holland amp
Blundy (1994)
Pargasite Magnesio-hornblende
rim core rim core
01 OA41d2 878 953 ------- -------
01 OA19a 935 974 825 869
01 OA19d 890 933 789 -------
02 OA37b ------- ------- 816 849
02 OA90b ------- ------- 859 833
JOURNAL OF PETROLOGY VOLUME 45 NUMBER 6 JUNE 2004
1192
ratios This supports the occurrence of two distincthydrothermal events one at high temperature and alater stage at lower temperature responsible for thestrong 18O enrichment in the coexisting plagioclase
Dioritic dyke associated with a diabase dyke in lowergabbros from Wadi Halhal Haylayn Massif
Sample 02 OA37b belongs to a group of brownamphibole---plagioclase lenses or reaction margins asso-ciated with diabase intrusives (Fig 7d) in lower layeredgabbros Idiomorphic brown hornblende (tschermakite)crystals are elongated in the plane of the dyke represent-ing 80 of the modal composition They are associatedwith an interstitial plagioclase matrix The browntschermakite has greenish rims Plagioclase has a cloudyappearance due to micrometre-size fluid inclusionsexcept along its margins and internal microcracks Bothhornblende and plagioclase exhibit evidence of plasticdeformation The Sr isotopic ratio for the pargasite andplagioclase is 070348 and 070420 respectively Thepargasite exhibits a Sr signature closest to that of theOman primary magmatic value (McCulloch et al 1981)Nevertheless it is too high to be attributed to anunmodified MORB-source mantle signature This sug-gests the addition of an external fluid component in goodagreement with the shift of oxygen isotopic compositionto a low value (thorn39) suggesting a high-temperatureexchange with a low d18O fluid The Sr isotope composi-tion of the plagioclase is significantly higher than that of
the coexisting pargasite probably as a result of the low-temperature alteration
Dioritic dyke cross-cutting lower layered gabbros in WadiAl-Abyad Nakhl Massif
Sample 02 OA90b is a 1 cm thick dioritic dyke cross-cutting the lower layered gabbros It grades into a greenamphibole vein as illustrated in Fig 6c The dyke is com-posed of 2 cm long dark green idiomorphic magnesio-hornblende crystals growing in the plane of the dykeand plagioclase concentrated at the margin of the dykeThe plagioclase in the dyke and in the enclosing gabbrocontains micrometre-size fluid and mineral inclusionssimilar to those observed in sample 94 MA2 The plagio-clase and green hornblende have higher Sr isotopic ratios(070387 and 070427 respectively) and lower d18Ovalues (thorn498 and thorn239 respectively) than the normalMORB-source mantle value As observed for previoussamples this supports interaction with an external fluidcomponent
Comb-textured gabbroic dykes in the lower gabbros fromWadi Wariyah Wadi Tayin Massif
Sample 01 OA15f (Fig 6b) is from Wadi Wariyah andrepresents a 2 cm wide dyke intruding the lower gabbrosThis type of comb-textured dyke developed in theclinopyroxene---plagioclase stability field It containslarge idiomorphic clinopyroxene crystals which arepartially replaced by brownish magnesio-hornblendeThe plagioclase is extremely calcic with a slight inverse
Table 3 Whole-rock major element compositions of massive gabbros and dykes determined by X-ray fluorescence
(wt )
Sample 94 Ma2 97 OA128b 01 OA41d2 01 OA19d 01 OA19a 01 OA15f 01 OA36b 01 OA36b
Rock type Gabbro Gabbro Gabbro patch Gabbroic dyke Gabbroic dyke Comb-textured
gabb dyke
Epidosite dyke Epidosite dyke
(margin)
SiO2 4830 4815 4749 4524 4339 4799 389 4371
Al2O3 2785 2032 2438 1474 1246 1192 2752 1646
Fe2O3 328 266 404 981 1204 592 381 623
MnO 003 004 005 014 016 006 008 014
MgO 405 737 427 1031 1686 1326 187 739
CaO 1289 1908 1477 1235 1156 166 2471 2339
Na2O 292 123 257 258 111 075 000 013
K2O 000 000 009 006 000 000 000 000
TiO2 005 017 063 230 064 086 013 042
P2O5 008 007 013 018 009 013 007 010
LOI 033 076 138 216 161 237 274 190
Total 9978 9985 998 9987 9992 9986 9983 9987
LOI loss on ignition gabb gabbroic
BOSCH et al HYDROTHERMAL CIRCULATION IN OMAN OPHIOLITE
1193
Table4Majorelementcomposition
aofselectedam
phibolespaired
withplagioclasecompositionsusedforthermometry
Sam
ple
nam
e1Mode2
SiO
2TiO
2Al 2O3
Cr 2O3
FeO
MnO
MgO
CaO
Na 2O
K2O
Total
Si
AlIV
AlVI
Ti
Fe3
thornCr
Mg
Fe2
thornMn
Na
KCa
XAn3
T(C)4
01OA19a
Mg-hornblende
core
45 60
26
10 78
009
860
015
16 56
12 00
212
007
96 72
6552
1448
0366
0083
0552
0004
3409
0594
0016
0611
0018
1843
0903
899
Mg-hornblende
core
47 99
263
850
002
897
011
16 61
12 10
160
020
96 36
6894
1106
0334
0028
0483
0002
3557
0594
0014
0446
0036
1862
0892
834
Mg-hornblende
rim
51 17
247
596
001
761
014
18 11
12 19
103
010
96 47
7257
0743
0253
0016
0414
0001
3828
0488
0016
0284
0018
1853
0894
820
Mg-hornblende
rim
47 50
291
920
004
880
014
16 46
11 95
174
008
96 07
6835
1165
0396
0015
0506
0005
3529
0552
0017
0486
0015
1843
0887
825
Mg-hornblende
rim
49 57
269
742
000
799
017
18 07
12 02
133
014
96 88
7013
0987
0261
0018
0575
0000
3811
0371
0020
0365
0026
1821
0860
831
Mg-hornblende
core
47 83
181
872
003
840
011
14 48
12 18
159
008
93 59
6814
1186
0279
0018
0644
0004
3711
0357
0013
0438
0015
1859
0907
874
Pargasite
rim
42 15
249
11 87
024
10 07
013
13 84
11 43
263
032
96 61
6188
1812
0242
0434
0299
0028
3028
0937
0016
0749
0059
1797
0804
972
Pargasite
rim
42 22
260
11 86
026
10 21
011
13 71
11 48
253
032
96 59
6201
1799
0254
0429
0299
0030
3001
0955
0014
0720
0060
1807
0803
962
Pargasite
core
42 05
263
11 45
030
10 19
011
13 80
11 47
246
032
96 46
6189
1811
0175
0478
0299
0035
3029
0956
0014
0702
0060
1809
0767
974
Pargasite
rim
42 36
247
11 90
016
945
013
14 16
11 76
249
026
95 98
6239
1761
0305
0366
0276
0019
3109
0888
0016
0710
0049
1856
0818
926
Pargasite
core
42 50
291
11 64
016
984
015
14 22
11 41
256
034
96 69
6219
1781
0227
0426
0322
0018
3103
0883
0018
0727
0063
1789
0812
974
Pargasite
rim
42 76
270
12 03
017
920
012
14 87
11 82
244
025
96 73
6224
1776
0880
0335
0368
0020
3226
0752
0015
0689
0047
1843
0841
941
Pargasite
rim
42 13
181
13 54
016
921
009
12 77
12 13
254
032
96 98
6172
1828
0510
0450
0000
0018
2788
1129
0011
0721
0059
1905
0841
874
01OA19d
Mg-hornblende
49 26
047
624
011
11 46
017
15 39
12 21
108
009
96 48
7137
0863
0202
0051
0437
0013
3323
0951
0021
0302
0018
1896
0768
771
Mg-hornblende
46 03
287
932
018
906
017
14 77
12 87
193
007
97 28
6672
1328
0265
0313
0046
0021
3191
1053
0021
0542
0012
1999
0758
808
Pargasite
41 89
429
11 65
008
11 03
017
13 52
11 37
259
017
96 77
6159
1841
0179
0474
0345
0010
2963
1012
0021
0739
0032
1791
0770
975
Pargasite
rim
42 80
340
11 16
009
11 01
018
13 73
11 67
240
022
96 67
6293
1707
0226
0376
0322
0010
3009
1032
0022
0684
0041
1839
0605
880
Pargasite
core
42 58
351
11 27
006
11 63
018
13 51
11 46
239
022
96 82
6257
1743
0209
0387
0391
0007
2959
1039
0023
0680
0042
1804
0595
893
Pargasite
rim
42 14
393
11 28
006
11 71
014
13 35
11 33
235
033
96 62
6214
1786
0174
0435
0391
0007
2935
1054
0018
0673
0062
1790
0519
901
Pargasite
core
41 56
420
12 05
006
11 57
016
12 51
11 35
249
027
96 24
6168
1832
0277
0469
0276
0007
2768
1160
0021
0718
0052
1805
0537
882
Pargasite
core
41 33
438
11 49
008
11 66
015
13 35
11 45
264
026
96 80
6105
1894
0107
0487
0368
0009
2941
1073
0019
0757
0048
1813
0537
942
Pargasite
41 08
446
12 39
012
11 35
017
13 13
11 36
263
015
96 85
6049
1951
0199
0494
0368
0014
2882
1030
0021
0752
0027
1792
0758
975
02OA37b
Mg-hornblende
rim
45 15
260
984
002
13 28
019
14 01
10 87
128
009
97 33
6527
1473
0203
0283
0690
0002
3018
0915
0023
0360
0016
1683
0562
828
Mg-hornblende
rim
44 72
263
988
004
13 23
021
14 01
10 78
133
009
96 92
6494
1506
0185
0287
0713
0005
3032
0894
0025
0374
0018
1677
0593
854
Mg-hornblende
rim
45 68
247
935
004
12 87
020
14 10
11 00
121
011
97 02
6621
1379
0218
0269
0598
0004
3045
0962
0024
0341
0020
1708
0562
815
Mg-hornblende
core
43 94
291
10 87
001
12 60
023
13 73
10 59
142
011
96 39
6410
1590
0278
0319
0644
0001
2985
0893
0028
0401
0021
1655
0710
860
Mg-hornblende
core
44 62
270
10 37
001
12 66
019
14 12
10 58
140
008
96 73
6479
1521
0253
0294
0644
0001
3057
0893
0023
0395
0014
1646
0674
851
Mg-hornblende
rim
47 79
181
796
002
12 09
025
15 22
11 09
100
009
97 33
6866
1134
0214
0196
0506
0002
3260
0947
0030
0280
0016
1706
0503
767
Mg-hornblende
core
45 43
249
10 3
000
11 99
022
14 50
11 14
133
008
97 48
6524
1476
0267
0269
0644
0000
3103
0796
0027
0370
0015
1715
0703
838
JOURNAL OF PETROLOGY VOLUME 45 NUMBER 6 JUNE 2004
1194
Sam
ple
nam
e1Mode2
SiO
2TiO
2Al 2O3
Cr 2O3
FeO
MnO
MgO
CaO
Na 2O
K2O
Total
Si
AlIV
AlVI
Ti
Fe3
thornCr
Mg
Fe2
thornMn
Na
KCa
XAn3
T(C)4
02OA90b
Mg-hornblende
core
48 83
038
745
005
939
010
17 29
12 24
142
006
97 21
6931
1069
0041
0041
0667
0005
3657
0448
0011
0392
0010
1862
0840
856
Mg-hornblende
core
49 49
033
731
003
916
008
17 40
12 57
139
005
97 82
6987
1013
0035
0035
0552
0004
3662
0530
0010
0379
0009
1901
0830
811
Mg-hornblende
rim
48 90
040
753
005
932
013
17 15
12 22
154
005
97 29
6948
1052
0042
0042
0575
0006
3632
0533
0015
0424
0010
1861
0880
869
Mg-hornblende
rim
48 30
044
800
007
962
010
16 86
12 32
161
005
97 35
6873
1127
0047
0047
0598
0008
3576
0546
0012
0443
0010
1879
0870
862
Mg-hornblende
rim
49 12
032
742
010
922
011
17 36
12 29
142
006
97 43
6956
1044
0034
0034
0621
0011
3665
0471
0013
0391
0010
1865
0840
845
02OA43a
Mg-hornblende
rim
48 13
040
789
007
860
007
17 41
12 19
155
006
96 36
6882
1118
0212
0043
0621
0008
3710
0407
0008
0429
0010
1867
-------
-------
Mg-hornblende
rim
50 93
035
643
001
759
006
18 50
12 24
114
003
97 28
7152
0848
0217
0037
0506
0001
3873
0385
0007
0311
0005
1841
-------
-------
Mg-hornblende
rim
50 67
014
559
009
12 33
026
15 28
12 44
068
003
97 50
7260
0740
0204
015
0483
0010
3263
0994
0032
0189
0005
1909
-------
-------
Mg-hornblende
rim
50 34
013
621
009
12 49
027
15 07
12 51
064
003
97 78
7192
0810
0240
0010
0530
0010
3210
0960
0030
0180
0010
1910
-------
-------
Mg-hornblende
rim
45 17
060
11 14
013
988
008
15 78
12 33
211
009
97 31
6473
1527
0355
0640
0644
0015
3371
0540
0010
0588
0016
1892
-------
-------
Mg-hornblende
rim
49 25
032
712
014
845
009
17 84
12 32
137
004
96 95
6983
1020
0170
0030
0620
0020
3770
0380
0010
0380
0010
1870
-------
-------
Mg-hornblende
rim
48 61
032
775
014
975
011
16 81
12 57
139
004
97 50
6905
1950
0203
0034
0621
0016
3560
0537
0013
0382
0008
1914
-------
-------
94Ma2
Anthophyllite
54 98
000
259
001
12 65
037
23 35
199
034
000
96 28
7744
0256
0174
0000
0161
0001
4903
1330
0043
0092
0001
0301
-------
-------
Anthophyllite
55 11
000
296
003
14 09
032
23 66
063
038
000
97 18
7716
0284
0205
0000
0115
0003
4938
1535
0038
0104
0000
0095
-------
-------
Actinolite
48 58
004
954
002
960
014
17 67
999
139
005
97 02
6850
0150
0435
0004
0690
0002
3713
0442
0017
0379
0009
1510
-------
-------
Actinolite
51 54
004
585
000
890
022
20 28
930
090
002
97 05
7214
0786
0178
0005
0598
0000
4230
0444
0026
0244
0004
1395
-------
-------
01OA41d2
Pargasite
rim
43 05
342
10 71
008
11 46
012
13 50
11 54
179
067
96 34
6353
1647
0216
0380
0368
0009
2967
1046
0015
0512
0127
1824
0613
863
Pargasite
rim
42 61
354
11 59
001
10 59
010
13 52
11 87
160
076
96 19
6286
1714
0302
0392
0299
0001
2972
1007
0012
0458
0143
1877
0715
844
Pargasite
core
42 54
401
11 04
006
11 81
013
13 21
11 50
202
039
96 71
6266
1734
0182
0444
0368
0007
2901
1087
0017
0576
0073
1814
0854
979
Pargasite
rim
43 03
385
10 72
002
11 70
014
13 50
11 95
146
080
97 17
6314
1686
0168
0425
0345
0002
2953
1091
0018
0415
0149
1878
0634
852
Pargasite
rim
42 65
341
10 60
006
11 97
014
13 22
11 64
183
068
96 20
6332
1668
0186
0381
0345
0007
2926
1141
0018
0528
0129
1851
0620
869
Pargasite
core
42 82
367
10 60
007
11 93
012
13 42
11 44
206
047
96 60
6315
1685
0158
0407
0391
0008
2950
1080
0016
0588
0089
1808
0615
899
Pargasite
rim
42 2
362
11 10
007
11 42
016
13 09
11 72
179
072
95 89
6282
1718
0229
0405
0299
0009
2905
1123
0020
0518
0137
1869
0715
875
Pargasite
rim
42 42
380
11 03
006
11 77
012
13 00
11 72
188
069
96 49
6283
1717
0209
0423
0299
0007
2870
1158
0015
0539
0131
1860
0817
928
Pargasite
rim
42 21
364
11 14
006
10 73
011
13 53
11 68
177
072
95 59
6277
1723
0229
0407
0322
0007
2999
1012
0014
0511
0136
1862
0736
883
Pargasite
rim
42 25
376
11 03
008
10 93
015
13 66
11 72
185
064
96 07
6257
1743
0182
0418
0345
0009
3016
1009
0019
0531
0121
1860
0766
912
Pargasite
core
42 17
396
10 91
008
11 10
010
13 57
11 49
209
038
95 85
6253
1747
0159
0442
0368
0010
3000
1009
0013
0602
0073
1826
0841
981
aMajorelem
entco
mpositionsarein
weightpercent-------noplagioclasean
alysed
andnotemperature
calculated
1Typ
eofam
phibolean
alysed
isalso
indicated
2Blankindicates
nozoningobserved
3Molefractionofan
orthitein
plagioclasead
jacentto
amphibolegrain
4Tem
perature
ofeq
uilibrium
fortheam
phibole---plagioclasepaircalculatedusingtheHollandamp
Blundythermometer
(1994)Errors
are40
C
BOSCH et al HYDROTHERMAL CIRCULATION IN OMAN OPHIOLITE
1195
zonation from An95 to An99 from core to rim (Table 1 andFig 8a) TheWR Sr isotopic ratio of the sample (070458)is significantly higher than normal MORB values Thiscould be related to the overprint of a low-temperaturealteration event consistent with the petrography of thesample
Green amphibole vein cross-cutting layered gabbros inWadi Mayah Haylayn Massif
Sample 02 OA43a belongs to a series of 1 cm thick greenamphibole veins (as in Fig 6e) cross-cutting the layeredgabbros The veins develop 15 cm thick reaction marginsin the enclosing gabbro and are exclusively composed ofacicular pale green magnesio-hornblende growing per-pendicular to the vein margin in a crack-seal mode(Fig 6f ) The reaction margins are composed of urali-tized gabbro marked by the development of the samepale green magnesio-hornblende in an inhomogeneouscalcic plagioclase matrix (An91---97) The Sr and O isotopiccompositions of this green hornblende (070431 andthorn31) do not correspond to normal MORB-sourcemantle values
Epidosite dyke from Wadi Gideah Wadi Tayin Massif
Sample 01 OA36b is representative of a series of epidoteveins cross-cutting poorly layered gabbros These 2 cmthick veins are composed of pink thulite in their centreand green pistachite at their margins (Fig 6a) Theirsharp contact with the enclosing gabbro is marked bythe development of coarse clinopyroxene crystals alteredin the epidote---greenschist facies This suggests that theepidote vein is replacive in a coarse-grained gabbro dykeThe highest 87Sr86Sr initial ratios of all the samplesstudied are recorded by the WR and coexisting epidoteof this sample (070519 and 070554 respectively)(Table 5) The Sr content of the WR and associatedepidote are also the highest of the present dataset(716 ppm and 764 ppm respectively) The Sr isotopiccomposition of this sample is consistent with a greaterdegree of exchange with a radiogenic component fromCretaceous seawater It is significantly higher than thevalue obtained for an epidosite vein from the Wadi Fizhsection of the Oman ophiolite [070435 with a Sr contentof 488 ppm reported by Kawahata et al (2001)] but ingood agreement with the average of the greenschist-faciesalteration sequence defined by Kawahata et al (2001)
GABBROS
01 km 025 km 1 km 36 km50
60
70
80
90
An
MA2
19d
19a 15f 128b
41d2
(a)
43a17b
15f
90b
37b
19a
19d
40
60
80
100
A
n
01 km 015 km 025 km 1 km
DYKES
(b)
Distance to the Moho
ACTINOLITE
MAGNESIOHORNBLENDE
TSCHERMAKITE
PARGASITEGabbros Dykes41d2MA2
19a d37b15f
90b43a
AlIV
150500
02
08
10
(Na
+ K
) A
(c)
04
06
950
900
850
800
750
70007 12 17
T degC
1000
(d)
AlIV
Fig 8 Plagioclase---amphibole compositions and results of thermometry (a) and (b) plagioclase compositions in layered gabbros and in dykesrespectively as a function of distance to the Moho (see text for sample identification) (c) amphibole compositions in layered gabbros and dykesaccording to Leakersquos (1997) nomenclature (d) equilibration temperatures calculated using the amphibole---plagioclase thermometer of Holland ampBlundy (1994) as a function of AlIV content in amphibole [same symbols as in (c)]
JOURNAL OF PETROLOGY VOLUME 45 NUMBER 6 JUNE 2004
1196
Table5RbandSrcontents87Sr
86Srandd1
8Oisotopiccompositionsofwholerocksandmineralseparatesfrom
samplesofmassivegabbrosdykes
andveinsfrom
theOman
ophiolitealsolistedarecalculated
waterrockratios(W
R)andLOIvalues
Sam
ple
Rb(ppm)
Sr(ppm)
87Rb8
6Sr
87Sr
86Sr
(87Sr
86Sr)i1
d18O
()
LOI(
)2WR
3(87Sr
86Sr)L14
(87Sr
86Sr)L25
(87Sr
86Sr)R6
01OA41d2
Whole
rock
037
1792
0006
0703516
09
070351
thorn480
138
47
0703587
0703651
0703357
Plagioclase
025
1249
0003
0704261
22
070426
thorn504
-------
-------
Pargasite
-------
-------
-------
0703548
20
070355
-------
-------
-------
Clinopyroxene
045
26 0
0050
0703845
19
070378
-------
-------
-------
01OA36b
Dykewhole
rock
0005
7164
50001
0705186
17
070519
-------
274
21 9
0705207
0705112
-------
Epidote
0003
7645
0001
0705536
12
070554
-------
-------
-------
Wallwhole
rock
005
4254
0001
0704637
09
070464
-------
191
-------
0704945
0704675
0704593
02OA43a
Green
hornblende
004
98
0012
0704323
13
070431
thorn310
-------
-------
97OA128b
Whole
rock
003
1274
50001
0703327
17
070333
-------
076
37
-------
-------
-------
Plagioclase
002
1204
50001
0703285
09
070328
thorn414
-------
-------
Clinopyroxene
003
20 0
0004
0704230
09
070422
thorn523
-------
-------
01OA15f
Whole
rock
006
1077
0002
0704582
16
070458
-------
237
13 5
-------
-------
-------
01OA19a
Whole
rock
037
1303
0008
0704189
18
070418
-------
161
96
-------
-------
-------
01OA19d
Whole
rock
058
2032
0008
0703423
08
070341
thorn567
216
415
0703574
0703408
0703271
Pargasite
091
1058
0025
0703273
11
070324
thorn287
-------
-------
Plagioclase
-------
-------
-------
-------
-------
thorn10 09
-------
-------
02OA90b
Plagioclase
004
2385
50001
0703868
11
070387
thorn498
-------
-------
Green
hornblende
010
23 8
0012
0704288
15
070427
thorn239
-------
-------
BOSCH et al HYDROTHERMAL CIRCULATION IN OMAN OPHIOLITE
1197
Table5continued
Sam
ple
Rb(ppm)
Sr(ppm)
87Rb8
6Sr
87Sr
86Sr
(87Sr
86Sr)i1
d18O
()
LOI(
)2WR
3(87Sr
86Sr)L14
(87Sr
86Sr)L25
(87Sr
86Sr)R6
02OA37b
Plagioclase
023
3121
0002
0704204
15
070420
-------
-------
-------
Pargasite
031
41 6
0021
0703505
15
070348
thorn394
-------
-------
94OAMa2
Whole
rock
0016
1750
00003
0703219
15
070322
thorn495
033
31
0703264
0703177
0703158
Plagioclase
0014
1363
00003
0703219
11
070322
thorn479
-------
-------
Clinopyroxene
-------
-------
-------
0703236
13
070324
-------
-------
-------
1Initial8
7Sr
86Srratiocalculatedat
95Ma(H
acker1994)
2LOIisloss
onignition(in)
3Water---rock
ratiocalculatedassumingaclosedsystem
TheeffectiveWR
ratiocanbeexpressed
bythefollowingeq
uation
W=Reffectivefrac14
frac12eth87Sr=
86SrrockTHORN fi
nal
eth87Sr=
86SrrockTHORN in
itial=frac12eth8
7Sr=
86SrseawaterTHORN in
itial
eth87Sr=
86SrrockTHORN fi
nal
ethCrock
initial=C
seaw
ater
initial
THORNwhere(87Sr
86Srrock) finalisthefinalmeasuredSrisotopicratioin
thesample(87Sr
86Srrock) in
itialco
rrespondingto
theinitialm
antlevalue
istakenas
070295(Lan
phere
etal1981)(87Sr
86Srseawater) in
itialistheCretaceousseaw
ater
Srisotopicco
mposition[87Sr
86Sr07073
at90
MaafterBurkeet
al(1982)]C
seaw
ater
initial
istheSrco
ntent
oflsquonorm
alrsquoseaw
ater
andis
takenas
8ppm
(deVilliers1999)
FortheCrock
initialitis
assumed
that
thepresentSrco
ncentrationmeasuredis
thesameas
theinitial
concentration
4Srisotopicratiosmeasuredforliquid
L1extractedafterthefirststep
oftheleachingexperim
ent(6NHClfor8min
at70
C)
5Srisotopicratiosmeasuredforliquid
L2extractedaftertheseco
ndstep
oftheleachingexperim
ent(2 5NHClfor30
min
at70
C)
6Srisotopicratiosmeasuredforresidueobtained
afteratotalaciddigestionachievedaftertheextractionofL1an
dL2leachates
JOURNAL OF PETROLOGY VOLUME 45 NUMBER 6 JUNE 2004
1198
Part of the same sample from the contact between thedyke and the gabbro yields a slightly lower Sr isotopiccomposition (070464) intermediate between the sur-rounding gabbro and the epidote dyke It also has anintermediate Sr content (425 ppm) Acid leaching experi-ments performed on the whole rock of the dyke yield Srisotopic compositions of 070521 and 070512 for thetwo liquids extracted Similar experiments for the samplelocated at the contact with the gabbro yield 87Sr86Srratios of 070494 and 070467 for L1 and L2 liquids TheSr isotopic composition of the residue extracted afterleaching remains high (070459) and very close to theepidosite 87Sr86Sr ratio determined by Kawahata et al(2001) In this sample the primary minerals have beentotally replaced by epidote and secondary plagioclase orquartz Thus the Sr isotopic compositions measured forthis WR sample and its constituent minerals reflect thecomposition of the fluid filling this dyke
Mineral chemistry
Plagioclase
Compared with the host gabbros the dykes show a muchlarger dispersion in their plagioclase compositions (Fig 8aand b Table 4) characterized by more calcic plagioclasethan the enclosing gabbros This tendency is mostextreme in the comb-textured dykes in which the plagio-clase is almost pure anorthite The zoning is generallynormal recording a crystal fractionation process How-ever gabbro 94 MA2 exhibits inverse zoning this tend-ency is also observed in plagioclases from dykes 01OA19a and 01 OA15f The higher calcium content ofthe plagioclase as well as the inverse zoning is consistentwith an increase of water pressure during its crystalliza-tion (Turner amp Verhoogen 1960 Carmichael et al 1974Koepke et al 2003) The particularly high An content inthe comb-textured gabbro dykes 01 OA15f may alsoresult from crystallization in a supercooled magma thisis consistent with the comb texture indicative of fast-growing crystals The lack of brown hornblende suggeststhat crystallization occurred above the temperature ofhornblende stability
Amphibole
The amphiboles analysed in the dykes (Fig 8c andTable 4) show a regular trend in a (Na thorn K)A vsAlIV diagram from titanium-rich (Ti 4 015) pargasitichornblende (brownamphibole) and tschermakite (brown---green amphibole) to low-titanium (Ti5 015) magnesio-hornblende (green amphibole) and actinolite (pale greenamphibole) The pargasitic hornblende develops aspoikilitic oikocrysts in the dykes 01 OA19a and d andin the diffuse patch 01 OA41d2 at temperatures above800C during the final stage of crystallization (see
below) The tschermakite composition corresponds tothe internal alteration of clinopyroxene in gabbro 94Ma2 occurs in the comb-textured dyke 01 OA15f andis the primary amphibole in the dioritic dyke 02 OA37bThe magnesio-hornblende develops at the edges of thepargasites or of clinopyroxenes at temperatures around800C and is the primary phase in the dioritic dyke 02OA90b and in the green vein 02 OA43a Finally actino-lite occurs as a replacement of green hornblende in dykesor in the kelyphitic rim surrounding olivine in gabbro 94Ma2 Such a large composition range at the scale of athin section has been reported in amphiboles fromamphibolite-facies oceanic gabbros (Stakes 1991 Stakesamp Taylor 1992 Gillis et al 1993) as well as in the Omanophiolite gabbros (Manning et al 2000) This variabilityhas been explained by rapid reaction rates preventinghomogenization (Manning et al 2000)
Thermometry
Plagioclase---amphibole thermometry could be appliedwhen the two minerals exhibited good contacts such asin the laminated gabbro of the middle sequence (01OA41d2) and in the intrusive dykes The temperaturesof plagioclase---amphibole equilibration (Table 2) havebeen calculated using the geothermometer lsquoBrsquo of Hollandamp Blundy (1994) Edenite thorn Albite frac14 Richterite thornAnorthite valid between 500C and 900C with anuncertainty of 40C Amphibole formulae are basedon 23 oxygen and their nomenclature is based on alkalications in the A site vs AlIV according to Leake (1997)Pressurewas assumed to be 1 kbar Forty-five plagioclase---amphibole pairs meet the compositional criteria imposedby Holland amp Blundyrsquos dataset (09 4 Xan 4 01 andAlIV 403) Electron microprobe measurements wereconstrained to contiguous plagioclase and amphiboleseparated by sharp grain boundaries assuming equili-brium of the two phases When the mineral phasesexhibit normal zoning temperatures between plagioclaseand amphibole cores and between mineral rims werecalculated assuming that the temperatures deduced fromthe mineral core chemistry provide a proxy for the high-est temperature of equilibration These data are identi-fied in the T C vs AlIV diagram (Fig 8d) Table 4presents selected major element amphibole analyses theAn content of the plagioclase adjacent to each amphibolegrain and the temperature calculated for the pairThe temperatures calculated (Fig 8d) span the entire
range of temperatures at which amphibole and plagio-clase coexist in equilibrium (ie amphibolite-facies para-geneses) This temperature interval is bracketed byorthopyroxene-bearing facies or amphibole-free facies(ie gabbro 94MA2) and epidote---actinolite facies devoidof homogeneous plagioclase (ie epidosite dyke 01OA36b and green vein 02 OA43a) The range of the
BOSCH et al HYDROTHERMAL CIRCULATION IN OMAN OPHIOLITE
1199
calculated temperatures between 4900C and 750Ccorresponds to the changing composition of amphibolesfrom pargasite to magnesio-hornblende at the scale of athin section or even at that of an individual crystal asshown by their correlation with AlIV such variabilityrecords rapid cooling in hydrous conditions The highesttemperatures are recorded by pargasite from the anatec-tic gabbro 01 OA41d2 in the gabbro dyke 01 OA19dand in the dioritic dyke 02 OA90b They were calculatedfrom minerals devoid of lower-grade alteration and arethus considered as representing the equilibrium temperat-ure of the coexisting minerals Lower temperatures cor-respond to mineral phases evolving at falling temperatureand are only indicative of the cooling history Includingthe 40C uncertainty seven pairs provide temperatureshigher than 900C the limit of validity of the Holland ampBlundy thermometer However we maintain these tem-peratures as indicative of the highest values of our data-set These values are included in the average equilibriumtemperature summarized in Table 2
DISCUSSION
Distribution of hydrothermal alteration
Two distinct petrological events are recorded in the gab-bro section of the Samail ophiolite and are documentedin the present contribution (1) the whole gabbro sectionis affected by a penetrative alteration (2) the gabbrosection is crosscut by various types of dykes and veinsincluding hydrous minerals
Penetrative alteration
The penetrative alteration developed in the gabbros atgrain boundaries and locally invading the minerals isubiquitous in the gabbro section This penetrative altera-tion has been described as very high temperature (VHT)high temperature (HT) and low temperature (LT) basedon alteration parageneses The orthopyroxene coronas inthe layered gabbro sample and the incipient appearanceof brown pargasitic amphibole mark a lower thermallimit of the VHT alteration By reference to the experi-mental work of Koepke et al (2003) the temperature ofequilibration for these reactions is estimated to bebetween 900 and 1000CThe preservation of the vertical distribution of
VHT---HT facies (Figs 4 and 5) also pointed out byManning et al (2000) indicates that the hydrothermalalteration pattern fits the distribution of isotherms withdepth at a fast-spreading ridge (ie Phipps Morgan ampChen 1993 Dunn et al 2000 Fig 2) Equilibrium tem-peratures for VHT parageneses as high as 900---1000Csuggest that VHT---HT hydrothermal alteration tookplace in a very restricted time interval at the limit ofthe cooling magma chamber It is proposed that the
penetrative alteration affecting the entire gabbro sectionis related to a pervasive network of microcracks whichact as a recharge system down to the Moho (Nicolas et al2003)
Dykes and veins
The dykes are extremely varied eg pegmatitic gabbroswith comb-texture microgabbros and amphibolitizeddolerites dioritic dykes alignments of poikilitic horn-blende and finally diffuse hornblende-bearing patchesHigh-T gabbro and diorite dykes and low-T greenamphibole---epidote veins are connectedIn the dykes textural evidence attests to suprasolidus
conditions at least for the development of the pargasiticamphibole Integrating the hornblende---plagioclase equi-libration temperatures calculated for the studied samplesthese temperatures are bracketed between 950C and875CIn the following discussion we shall consider separately
the microgabbro and dolerite dykes The latter representbasaltic melt intrusions associated upsection with thediabase sheeted dykes and downsection possibly withthe mafic dykes that are ubiquitous in the mantle section(Nicolas et al 2000) Whatever their origin the develop-ment of pargasitic amphibole during their late stage ofcrystallization creates a link with the gabbroic dykes andsuggests a crystallization process evolving from dry con-ditions to more hydrous conditionsThe other gabbroic dykes comb-textured gabbros and
hornblende-bearing dioritic dykes result from the fillingof cracks by a fluid either a melt or a silica-rich hydrousfluid These intrusive dykes which develop reaction mar-gins at their contact with the host gabbro grade verticallyinto lower-temperature green amphibole veins or align-ments of poikilitic hornblende in the layered gabbromatrix or development of pargasitic hornblende in ana-tectic gabbros (sample 01 OA41d2) In gabbros crystal-lizing in the magma chamber the development oforthopyroxene and pargasite oikocrysts enclosing the pri-mary minerals suggests a thermal evolution from thegabbro solidus to amphibolite-facies conditions Forthese reasons it is suggested that the gabbroic dykesresulted from the injection of a hydrous melt in the stillhot gabbros drifting away from the magma chamber asdiscussed below
Origin of fluids responsible for VHT---HThydrothermal alteration
The 87Sr86Sr initial ratios and d18O of whole rocks andpure mineral fractions are plotted in Fig 9 against thedistance above the Moho Transition Zone (MTZ) Theinitial Sr isotopic composition of the whole rocks apartfrom the epidosite dykes ranges from 070322 to
JOURNAL OF PETROLOGY VOLUME 45 NUMBER 6 JUNE 2004
1200
070458 All the studied samples (including whole rocksand minerals) have initial 87Sr86Sr ratios falling outsidethe field of fresh MORB which commonly have Sr ratiosbetween 07023 and 07029 with an average of 070265(ie Hart et al 1974) The lowest initial Sr isotopiccomposition (070322) from the massive gabbro 94MA2 is slightly but significantly higher than averageOman primary magmatic signatures (ie 070296McCulloch et al 1981) Similarly the d18O values ofmost whole rocks and minerals measured (Table 5)
depart from typical MORB-source mantle values( Javoy 1980 Gregory amp Taylor 1981 Kyser 1986)These differences indicate that the primary mantlesignatures in the Oman ophiolite have been modifiedby interaction with a fluid Possible origins for thisfluid are mantle components dehydration of subductedlithosphere or seawater circulation Strontium andoxygen isotopes are useful tracers for fluid---crust interac-tion as d18O and 87Sr86Sr ratios from fluids originat-ing from seawater the mantle or subducted lithosphere
-1000
1000
2000
3000
4000
5000
6000
7000
MOHO
MTZ
Dis
tan
ce a
bov
e th
e M
oh
o (
m)
pillow-basalt
sheeted-dike
gabbro
peridotite
01 OA 41d2
01 OA 36b
02 OA 43a97 OA 128b
01 OA15f
94 OA MA2
02 OA 90b01 OA 19ad
02 OA 37b
0702 0703 0704 0705 0706 0707
87Sr86Srinitial
MO
RB-s
ou
rce
Seaw
ater
Whole RockPlagioclasePargasiteGreen HornblendeClinopyroxene
Epidote
(a)
-1000
1000
2000
3000
4000
5000
6000
7000
MOHO
MTZ
Dis
tan
ce a
bov
e th
e M
oh
o (
m)
pillow-basalt
sheeted-dike
gabbro
peridotite
01 OA 41d2
01 OA 36b
02 OA 43a97 OA 128b
01 OA15f
94 OA MA2
02 OA 90b01 OA 19ad
02 OA 37b
Whole RockPlagioclasePargasiteGreen HornblendeClinopyroxene
3 5 10
δ18O (permil)
41d2
Ma2
43a
90b 19d 19d37b
128b
90b
(b)
Fig 9 (a) Variation of initial 87Sr86Sr isotopic composition as a function of the location of the studied samples in a schematic log of the crustalsection of the Oman ophiolite The field for MORB is from Hart et al (1974) and that for seawater is from Burke et al (1982) Small open squares aredata fromKawahata et al (2001) (b) Variation of d18O () as a function of the location of the studied samples in a schematic log of the crustal sectionof the Oman ophiolite Shaded curve corresponds to the data of Gregory amp Taylor (1981)
BOSCH et al HYDROTHERMAL CIRCULATION IN OMAN OPHIOLITE
1201
are significantly different Moreover the d18O valueis dependent on temperature and mineral fractiona-tion and thus yields complementary constraints to Srisotopes
Mantle origin
One model to explain the occurrence of fluids interactingwith the oceanic crust at very high temperatures is toderive them from the mantle The contribution of mantle-derived hydrous fluids linked to percolation processesor to their concentration in the final stages of gabbrocrystallization has been invoked in numerous case studies(ie Witt amp Seck 1989 Fabriees et al 1989 Dupuy et al1991 Agrinier et al 1993) O and Sr isotopic data forwhole rocks and mineral separates yield scattered valuesthat are unambiguously different from MORB mantle-source values (Fig 9a and b) With the exception of lateLT altered samples the WR data have a mean 87Sr86Srratio of 070337 and a standard deviation of 000012These surprisingly high values in mantle-derived rockscould be taken as evidence for survival of mantle isotopicheterogeneities during crystallization of the ophiolite (egLanphere 1981 McCulloch et al 1981) However the18O16O isotope ratios in the Oman gabbroic sequenceare also displaced from primary magmatic values Nogabbro in the present study has plagioclase or amphi-bole with typical mantle d18O values Thus the Sr and Oisotope data rule out the possibility of mantle-derivedfluids as a possible source for the VHT---HT hydrousalteration event recorded in the massive gabbros andgabbroic dykes and veins (Fig 10a and b)
Subducted lithosphere origin
Dehydration of subducted oceanic crust in subductionzones can generate hydrous fluids Such fluids couldpercolate through the overlying lithosphere and contam-inate it thus generating modified isotopic signatures andtrace element contents Such an explanation howeverdisagrees with the geochemical characteristics of most ofthe studied samples Fluids derived from the dehydrationof subducted slabs (fresh or altered MORB or OIB pro-toliths) should be strongly enriched in K Rb and Ba (egBecker et al 2000) whereas in Oman the Rb contentsmeasured in the gabbros and gabbroic dykes and veins arestrongly depleted Moreover the isotopic composition ofslab-derived fluids is buffered by the MORB-like oceaniccrustal protolith for Nd and Pb but not for Sr and O(Staudigel et al 1995) As a result of relatively minorfractionation of oxygen at high temperature the oxygenisotope composition of the fluids expelled during dehy-dration of the subducted slab should be high and essen-tially controlled by the oxygen composition of the uppersection of the crust altered at low temperature (with an
average of 10 and perhaps as high as 19) (Staudigelet al 1995) The Sr isotopic composition should be alsohigh (average 07046) as the fluids released by the dehy-dration of subducted oceanic crust are associated eitherwith seawater or with radiogenic Sr phases (ie smectites)The Sr and O isotopic compositions of the studied sam-ples do not fit with these signatures and so preclude asubducted lithosphere origin for the fluids involved in theVHT---HT alteration event
Seawater-derived fluids
All the analysed samples (minerals and WR) have 87Sr86Sr(T) outside the domain of primary MORB magmasand variable Sr contents Individual minerals have Srcontents reflecting their different mineralliquid partitioncoefficients Minerals characterized by a very high affi-nity for Sr such as plagioclase (ie Sr mineralliquidpartition coefficients 1) have very high Sr contentsTherefore the high abundance of plagioclase in the sam-ples analysed is responsible for the high Sr content of thestudied whole rocks The Sr content of all the whole rocksis relatively homogeneous (Table 5) without clear distinc-tion between country-rock gabbros and dykes and veinsand ranges from 110 to 200 ppm except for the epidosite(4700 ppm) Reported on a 87Sr86Sr vs 1Sr diagram(Fig 10a) most whole rocks and plagioclases analysed arecharacterized by a 1Sr ratio lower than 001 and plot inthe left part of this diagram Compared with N-MORBthe studied massive and anatectic gabbros and their con-stituent plagioclases have similar to slightly higher Srcontents but substantially higher 87Sr86Sr ratios(Fig 10a) Considering Cretaceous seawater as a possiblehigh 87Sr86Sr mixing component [87Sr86Sr 07073at 90Ma after Burke et al (1982)] responsible for theobserved geochemical modifications exchanges cannothave occurred in a closed system as seawater has too lowa Sr content (ie 8 ppm de Villiers 1999) An open-system process allowing penetration of larger quantitiesof seawater can efficiently modify the Sr isotopic ratio ofthe rocks Alternately the fluids could also be seawaterthat was modified through exchange with the surround-ing rocks during its transit through the oceanic crustalsection This modified seawater would be more enrichedin Srwith a 87Sr86Sr ratio intermediate between unmodi-fied seawater and MORBMinerals devoid of late LT alteration imprints (parga-
site green hornblende and pyroxene) and characterizedby very low Sr contents plot on the right of the diagramclose to or on the mixing line between seawater andMORB (Fig 10a) The difference between these mineralsand the other samples (WR and plagioclases) is mainlydue to the Sr fractionation coefficients of these differentminerals and to the large quantity of plagioclase inthe studied rocks The process responsible for the
JOURNAL OF PETROLOGY VOLUME 45 NUMBER 6 JUNE 2004
1202
modification of the Sr isotopic composition fromMORB-source value is however explained by the same phenom-enon As all these minerals display initial 87Sr86Sr ratiosdistinct from the MORB source and because they equili-brated at VHT or HT temperatures this supports anearly intervention of seawater during crystallization ofthese minerals Most of these samples with the exceptionof the epidosite dyke overlap the domains defined byKawahata et al (2001) for the Wadi Fizh gabbros alteredat VHT and HT conditions in the amphibolite facies(labelled lsquoVHTrsquo and lsquoHTrsquo in Fig 9a)Three samples (two epidosite dykes and an one epidote
fraction) have very high Sr contents and the highest Sr
isotopic compositions of the present dataset The twowhole-rock samples overlap the field of the Wadi Fizhsamples altered at high temperature during greenschist-facies metamorphism (Kawahata et al 2001) This typeof alteration is common at the ridge axis and formedthrough intensive exchange with seawater-derived fluidsFigure 10b indicates that all the Sr---O isotope data are
distinct from MORB compositions and suggests that thefluids reacting with the magmas could be derived fromseawater In addition the d18O values show that isotopicexchange occurred under VHT or HT conditions Thefluids could have the composition of seawater or theycould have the composition of seawater modified through
0703
0705
0707
001 01 1
1 Sr (ppm)
MORB
(87Sr
86Sr)
init
ial
Seawater
VHT
HT
MT36b
36b
36b
43a 128b90b
41d2
37b19d
15f
128b
41d2
41d2
19a
Ma2Ma2
19d
90b
37b
Whole RockPlagioclasePargasiteGreen HornblendeClinopyroxene
Epidote
(a)
0 2 4 6 8
δ18O(permil)
0703
0705
0707Seawater
MORB-mantle
128b 41d2
90b 41d2
Ma2128b
37b
43a
19d
90b
Whole RockPlagioclasePargasiteGreen HornblendeClinopyroxene
Epidote
HT LT
19d
(87Sr
86Sr)
init
ial
(b)
Fig 10 (a) Initial 87Sr86Sr isotopic ratios plotted against 1Sr for whole rocks and mineral separates from the gabbro section of the Omanophiolite Fields shown are from Kawahata et al (2001) and correspond to MT-----basalt sheeted dyke diabase altered at medium temperature(prehnite---actinolite facies sequence 3) HT-----diabase plagiogranite metagabbro and epidosite formed at high temperature (greenschist faciessequence 4) VHT-----gabbro altered at very high temperature (amphibolite facies sequence 5) The dashed line is a binary mixing line betweenMORB (Lanphere et al 1981) and Cretaceous seawater (Burke et al 1982) (b) Initial 87Sr86Sr isotopic composition plotted against d18O value forwhole rocks and minerals from the Oman ophiolite Cretaceous seawater and MORB end-members as in (a) Dashed line represents mixing curvebetweenMORB and seawater at high temperature (HT) Low-temperature (LT) exchanges between these two components would be responsible foran increase of the d18O primary MORB value
BOSCH et al HYDROTHERMAL CIRCULATION IN OMAN OPHIOLITE
1203
exchange with the surrounding rocks during its penetra-tion into the crustal section In an environment in whichfluid---rock interaction occurs at variable temperaturesthe d18O is greatly influenced by the temperature atwhich the exchange took place and by the waterrockratio It is evident from Figs 9b and 10b that none of thestudied gabbros have plagioclase and amphibole withMORB-like d18O and 87Sr86Sr values The separateminerals display a broad range of d18O values and mostexhibit extreme 18O depletion (Table 5) Hydrothermalexchange kinetics are similar in amphibole and pyroxeneand these two minerals are more resistant to oxygenexchange during water---rock interaction than coexistingplagioclase (Gregory et al 1989) The most probableexplanation of the low d18O values measured in bothamphiboles and pyroxenes is that they crystallized athigh temperature in the presence of a relatively low 18Oseawater-derived hydrothermal fluid [d18O from 01 to07 after Gregory amp Taylor (1981)] The anomalouslyhigh d18O value (thorn1009) measured for the plagioclase ofthe micro-gabbroic dyke (01 OA19d) can be interpretedas resulting from a superimposed overprint caused by alate-stage penetration of fluids at lower temperature Inthis sample the occurrence of such different 18O16Oratios between coexisting plagioclase and amphibole isnot consistent with equilibrium fractionation betweenthese two minerals at any possible temperature Thisobservation suggests that parts of the ophiolite sequencehave undergone a high-temperature hydrothermal eventprior to the later low-temperature event that producedthe strong 18O increase in coexisting plagioclaseThe depletion of the d18O values results from high-temperature hydrothermal alteration (Gregory amp Taylor1981 Stakes amp Taylor 1992) The oxygen isotopic datasupport the contention that most studied samples havebeen affected by a VHT---HT hydrothermal alteration byfluids derived from seawater Some of the analysed sam-ples presenting petrological evidence of crystallization ofLT secondary minerals have been overprinted by the latelow-temperature alteration thus swamping the earlierhigh-temperature alteration effects
Determination of waterrock ratio
To better evaluate the exchange between seawater andthe gabbroic rocks waterrock ratios (WR) have beenestimated for the whole rocks analysed during the courseof this study The different models used to constrain thisratio mainly differ by the calculation of the effective ratioor by the estimated weight ratio and by considering openor closed systems for water circulation (Albareede et al1981 McCulloch et al 1981) The WR ratios reportedin Table 5 have been calculated assuming a closed sys-tem Using this formulation these values are minimabecause the calculation always uses pristine fluid for the
initial fluid thus maximizing the value in the denomina-tor If the fluid was shifted to lower 87Sr concentrations byexchange with the rock on the downward path theinferred values would be much higher (Davis et al 2003)The calculated WR ratios for the samples from this
study range from 31 to 219 (Table 5) Two main groupsof samples can be recognized on the basis of their waterrock ratios The lowest ratios ranging from 31 to 47have been determined for massive gabbros or anatecticgabbros but also for dykes and veins devoid of late LTalteration Similar ratios have been previously calculatedfor example for the discharged hydrothermal solutions atthe 13N and 21N East Pacific Rise (Di Donato et al1990 Fouquet et al 1991) The gabbros from the Ibrasection in the Oman ophiolite gave effective WR ratiosof 05 33 and 42 (McCulloch et al 1981) in goodagreement with the values obtained in this study Theleast altered gabbro of the Ibra section yields a WR ratioof 05 in good agreement with the exceptionally fresh-ness of this rock characterized by typical mantle oxygenvalues for its minerals (McCulloch et al 1981) Samplescharacterized by waterrock effective ratios in the rangeof 3---5 can be related to penetrative alteration via thehydrothermal system Lecuyer amp Reynard (1996) havedemonstrated in oceanic gabbros from the Hess Deepaltered in similar HT conditions that WR mass ratios inthe range of 02---1 are sufficient to account for the mod-ified stable isotope signature of the gabbros Higherwaterrock values (WR 45) have been calculated forsamples affected by superimposed late hydrothermalalteration and for the epidosite samples (Table 5) Theyprobably acquired their isotopic characteristics throughinteraction with a larger volume of seawater duringgreenschist-facies metamorphism Similar or higherWR values have been published in previous studies(ie McCulloch et al 1981 Kawahata et al 2001)They correspond either to samples from the upper-crus-tal sequence (ie basalt sheeted dyke or plagiogranite) orto gabbros from the crustal section located in a particularenvironment such as for example the Wadi Fizh sectionthat is located close to a segment boundary along thepalaeo-spreading axis (Nicolas et al 2000)
Mechanism for seawater interaction withmagma
Nicolas et al (2003) have proposed that variable amountsof seawater can circulate at high temperature through-out the crustal section down to the walls of the magmachamber via a network of parallel microcracks a fractionof a millimetre wide Such microcracks and their relation-ship with the VHT---HT hydrous alteration in the sur-rounding gabbros have been described in detail byNicolas et al Via this network of microcracks largeamounts of seawater can have reacted with the magma
JOURNAL OF PETROLOGY VOLUME 45 NUMBER 6 JUNE 2004
1204
and induced variable changes of its Sr and O isotopiccomposition This regular network of parallel micro-cracks could constitute the recharge system and thehydrated gabbroic dykes the discharge system Physicalparameters and the geophysical models of Nicolas et al(2003) are in agreement with this scenario Such a pene-tration of seawater-derived hydrothermal fluids throughthe lower crust along grain boundaries and a microscopicfracture network at temperatures 4750C is consistentwith recent thermodynamic models (McCollom amp Shock1998)Seawater-altered and high-temperature recrystallized
diabases (protodykes) from the overlying sheeted dykecomplex stoped into the melt lenses could represent anindirect way for seawater to enter the system as they canremelt during their subsidence through the magmachamber A simple mass balance calculation suggeststhat this indirect contamination is not significant Assess-ments of the amount of recycled crust 1 in volumeby Nicolas et al (2000) based on the protodyke remnantsin the gabbro section or 20 by Coogan et al (2003)based on chlorine content in EPR basalts lead to a sea-waterrock ratio of the order of 104 to 103Thus following Nicolas et al (2003) we propose that
seawater has been introduced along microcracks down tothe walls of the magma chamber and has diffused alonggrain boundaries in the way described by Manning et al(2000) progressively invading the host gabbro Gabbroicdykes result from the injection of hydrous melt into thehost gabbros at falling temperatures as they drift awayfrom the magma chamber This water-saturated meltcould result from the extensive hydration of the crystal-lizing gabbros near the walls of the magma chamberwhen seawater penetrates through this wall (Fig 1)Similar plumbing systems driving seawater down to the
limit of the magma chamber and back to the surface havebeen already proposed in other environments such as theMid-Atlantic Ridge (Kelley amp Delaney 1987 Cooganet al 2001) the East Pacific Rise at Hess Deep (Gillis1995 Manning et al 1996a 1996b) the Tonga forearc(Banerjee amp Gillis 2001) and the Troodos ophiolite(Gillis amp Roberts 1999) The melting curve of wet basalthas a negative slope but is close to the dry solidus at lowpressure in the lower gabbros and the first melts areplagiogranitic (Spulber ampRutherford 1983) Such plagio-granites are ubiquitous in the highest gabbros and in theroot zone of sheeted dykes in Oman (Rothery 1983Juteau et al 1988 Nicolas amp Boudier 1991) in manyother ophiolites and in the oceanic crust where they havebeen interpreted as resulting either from wet anatexis ofhot gabbros or from the final product of crystallization ofa basaltic melt (Spulber amp Rutherford 1983) Plagio-granites are rare in the lower gabbros exceptionallyassociated with hydrous gabbro segregations inside thelayered gabbros Their constitutive minerals are also
remarkably absent along the VHTmicrocracks althoughthese gabbros were above the wet solidus A possibleexplanation has been proposed by McCollom amp Shock(1998) who wrote that at high temperature lsquoaqueousfluids may leave very little conspicuous petrological evid-ence for their presencersquo the reason being that thehigh-T conditions would be closer to batch meltingObviously more detailed studies on this specific problemare needed We conclude that the large degree of hydrousmelting locally required at the walls of the magma cham-ber to account for the generation of a gabbroic hydrousmelt may have erased the traces of the incipient plagio-granitic melt
CONCLUSIONS
Fostered by the recent discovery of high-temperatureseawater contamination in some gabbros of the Omanophiolite (Manning et al 2000) we have conducted astudy on 500 gabbro specimens from selected areas ofthe entire Samail ophiolite belt and on the hydrous gab-broic dykes that cross-cut them The highest-temperaturereactions (VHT) recorded in olivine and clinopyroxene ingabbros as orthopyroxene and pargasite coronas reach975C They are followed at lower HT conditions withreplacement of olivine by an assemblage of tremolitemagnetite and plagioclase surrounded by chlorite andby pargasite development in clinopyroxene followedfinally by the LT lizardite---olivine and saussurite---plagioclase greenschist-facies alteration With a fewexceptions the VHT alteration tends to be restricted tothe lower gabbros and to the vicinity of the Moho andthe HT alteration to the middle and upper gabbros Thepreservation of the vertical distribution of VHT---HTfacies indicates that progressive cooling during off-axisspreading did not obliterate this distribution and conse-quently that the VHT---HT hydrothermal alteration tookplace in a very restricted time interval at the limit of thecooling magma chamber In the deep and hot gabbrosthe main water channels may be sub-millimetre-sizedmicrocracks with a dominantly vertical attitude the ori-gin and dynamics of which have been investigated in aprevious paper (Nicolas et al 2003)Beyond these high-temperature alteration zones mas-
sively induced in the gabbros seawater may be respons-ible for the generation of clinopyroxene---pargasitegabbro dykes that cross-cut all gabbros and that areupsection clearly connected to the LT hydrothermalvein system Petrostructural evidence suggests that thesedykes were generated by hydration of the crystallizinggabbros near the internal wall of the LVZ---magmachamber The resultant melts were subsequently intrudedat various levels in the cooling lower and middle gabbrosPetrological observations of nearly all the gabbros ana-lysed in the present study located from the root zone of
BOSCH et al HYDROTHERMAL CIRCULATION IN OMAN OPHIOLITE
1205
the sheeted dyke complex down to the Moho emphasizethe imprint of VHT---HT hydrous alteration The VHThydrothermal event is characterized by temperatures ofequilibration around 900---1000C The temperatures atwhich the HT hydrous event occurred are estimated tobe in the range 550---900CStrontium and oxygen isotope compositions measured
on whole rocks and separated minerals significantlydepart from primary MORB values The Sr and O iso-tope data obtained for pargasite green hornblende andclinopyroxene record the participation of seawater at thetemperature of crystallization of the minerals Plagio-clases and whole rocks define a trend toward a compo-nent characterized by a high radiogenic 87Sr86Sr valueand a higher Sr content than normal seawater Seawateris clearly identified as the fluid responsible for the modi-fication of the Sr and O signatures for both massivegabbros and dykes and veins Nevertheless the high Srcontent of the extraneous component requires eitheropen-system exchange allowing penetration of a greaterquantity of seawater or penetration of seawater modifiedby interaction with the oceanic crust during its travel paththrough the crustal section The waterrock ratio calcu-lated on the basis of the Sr isotopes is between three andfive for the enclosing gabbros and for the least LT altereddykes The VHT---HT hydrothermal event affectingthese samples is related to penetrative alteration via therecharge and discharge hydrothermal system Thewaterrock ratio is 10 for dykes exhibiting evidenceof a LT hydrothermal alteration overprint and reaches22 for the epidosite dyke The depletion in d18O char-acterizes all the studied samples with the single exceptionof the micro-gabbroic dyke plagioclase This agreeswith the conclusions based on Sr isotopes suggestingthat these samples have undergone exchange with sea-water-derived fluids during a VHT---HTalteration hydro-thermal event
ACKNOWLEDGEMENTS
This work has benefited from financial support by theCentre National de la Recherche Scientifique (INSUInteerieur-Terre andDYETI programmes) and inOmanfrom the willingness of the Directorate of MineralsMinistry of Commerce and Industry and the hospitabil-ity of the people Technical help in the laboratory is alsoacknowledged including C Nevado for the thin sectionsE Ball for the illustrations B Galland for her assistancein the chemistry room and C Bassoulet for help duringthermal ionization mass spectrometric analyses of the Srresults from the leaching experiments Constructivereviews from R T Gregory C Lecuyer an anonymousreviewer and particularly from M Wilson greatlyhelped to improve an earlier version of the manuscript
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Banerjee N R amp Gillis K M (2001) Hydrothermal alteration in a
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Becker H Jochum K P amp Carlson R W (2000) Trace element
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Nelson H F amp Otto H F (1982) Variation of seawater 87Sr86Sr
throughout Phanerozoic time Geology 10 516---519Carmichael I S E Turner F J amp Verhoogen J (1974) Igneous
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Chernosky J V Berman R G amp Bryndzia L T (1988) Stability
phase relations and thermodynamic properties of chlorite and
serpentine group minerals In Bayley S W (ed) Hydrous
Phyllosilicates Mineralogical Society of America Reviews in Mineralogy 19
295---346
Coogan L A Wilson R N Gillis K M amp MacLeod C J (2001)
Near-solidus evolution of oceanic gabbros insights from amphibole
geochemistry Geochimica et Cosmochimica Acta 65 4339---4357
Coogan L A Mitchell N C amp OrsquoHara M J (2003) Roof
assimilation at fast spreading ridges an investigation combining
geophysical geochemical and field evidence Journal of Geophysical
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1171
Davis A C Bickle M J amp Teagle D A H (2003) Imbalance in the
oceanic strontium budget Earth and Planetary Science Letters 211
173---187
Detrick R S Buhl P Vera E Mutter J Orcutt J Madsen J amp
Trocher T (1987) Multi-channel seismic imaging of a crustal
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De Villiers S (1999) Seawater strontium and SrCa variability in the
Atlantic and Pacific oceans Earth and Planetary Science Letters 171
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Di Donato G Joron J L Treuil M amp Loubet M (1990)
Geochemistry of zero-age N-MORB from hole 648B ODP legs
106---109 MAR 22 In Detrick R S Honnorez J et al (eds)
Proceedings of the Ocean Drilling Program Scientific Results 106109
College Station TX Ocean Drilling Program pp 57---65
Dunn R A Toomey D R amp Solomon S C (2000) Three-
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mantle beneath the East Pacific Rise at 9300N Journal of Geophysical
Research 105 23537---23555
Dupuy C Mevel C Bodinier J L amp Savoyant L (1991) Zabargad
peridotite evidence for multistage metasomatism during Red Sea
rifting Geology 19 722---725
JOURNAL OF PETROLOGY VOLUME 45 NUMBER 6 JUNE 2004
1206
Fabriees J Bodinier J L Dupuy C Lorand J P amp Benkerrou C
(1989) Evidence for modal metasomatism in the orogenic spinel
lherzolite body from Caussou (northeastern Pyrenees France)
Journal of Petrology 30 199---228
Fouquet Y Von Stackelberg S U Charlou J L Donval J P
Foucher J Erzig P Muhe R Wiedicke M Soakai S amp
Whitechurch H (1991) Hydrothermal activity in the Lau back-arc
basin sulfides and water chemistry Geology 19 303---306
Gillis K M (1995) Controls on hydrothermal alteration in a section of
fast-spreading oceanic crust Earth and Planetary Science Letters 134
473---489
Gillis K M amp Roberts M (1999) Cracking at the magma---hydrothermal transition evidence from the Troodos ophiolite Earth
and Planetary Science Letters 169 227---244
Gillis K M Mevel C et al (eds) (1993) Proceedings of the Ocean Drilling
Program Initial Reports 147 College Station TX Ocean Drilling
Program
Gregory R T amp Taylor H P (1981) An oxygen isotope profile in a
section of Cretaceous oceanic crust Samail ophiolite Oman
evidence for d18O buffering of the oceans by deep (45 km)
seawater---hydrothermal circulation at mid-ocean ridges Journal of
Geophysical Research 86(B4) 2737---2755
Gregory R T Criss R E amp Taylor H P (1989) Oxygen
isotope exchange kinetics of mineral pairs in closed and open
systems applications to problems of hydrothermal alteration of
igneous rocks and Precambrian iron formations Chemical Geology 75
1---42Hacker B R (1994) Rapid emplacement of young oceanic litho-
sphere argon geochronology of the Oman ophiolite Science 265
1563---1569
Hart S R Erlank A J amp Kable J D (1974) Sea floor basalt
alteration some chemical and Sr isotopic effects Contributions to
Mineralogy and Petrology 44 219---230
Holland T amp Blundy J (1994) Non-ideal interactions in calcic
amphiboles and their bearing on amphibole---plagioclase thermo-
metry Contributions to Mineralogy and Petrology 116 433---447
Ionov D A Savoyant L amp Dupuy C (1992) Application of the
ICP-MS technique to trace element analysis of peridotites and their
minerals Geostandards Newsletter 16 311---315
Javoy M (1980) 18O16O and DH ratios in high temperature
peridotites In Colloques Internationaux du CNRS 272 279---287
Juteau T Beurrier M Dahl R amp Nehlig P (1988) Segmentation
at a fossil spreading axis the plutonic sequence of the Wadi
Haymiliyah area (Haylayn Block Sumail nappe Oman) Tectono-
physics 151 167---197
Juteau T Manacrsquoh G Moreau C Lecuyer C amp Ramboz C
(2000) The high temperature reaction zone of the Oman ophiolite
new field data microthermometry of fluid inclusions PIXE analyses
and oxygen isotopic ratios Marine Geophysical Research 21 351---385Kawahata H Nohara M Ishizuka H Hasebe S amp Chiba H
(2001) Sr isotope geochemistry and hydrothermal alteration of the
Oman ophiolite Journal of Geophysical Research 106 11083---11099
Kelemen P B amp Aharonov E (1998) Periodic formation of magma
fractures and generation of layered gabbros in the lower crust
beneath oceanic spreading ridges Annual Geological Union Special
Monograph 106 267---289
Kelemen P B Koga K amp Shimizu N (1997) Geochemistry of
gabbro sills in the crustmantle transition zone of the Oman
ophiolite implications for the origin of the oceanic lower crust Earth
and Planetary Science Letters 146 475---488Kelley D S amp Delaney J R (1987) Two-phase separation and
fracturing in mid-ocean ridge gabbros at temperatures greater than
700C Earth and Planetary Science Letters 83 53---66
Koepke J Feig S T Snow J amp Freise M (2003) Petrogenesis of
oceanic plagiogranites by partial melting of gabbros an experi-
mental study Contributions to Mineralogy and Petrology on line first http
wwwspringerlinkcomopenurlaspgenre=s00410-003-0511-9
Kyser T K (1986) Stable isotope variations in the mantle In
Valley J W Taylor H P amp OrsquoNeil J R (eds) Stable Isotopes in
High Temperature Geological Processes Mineralogical Society of America
Reviews in Mineralogy 16 141---164
Lanphere M A (1981) K---Ar ages of metamorphic rocks at the base
of the Samail ophiolite Oman Journal of Geophysical Research 86
2777---2782
Lanphere M A Coleman R G amp Hopson C A (1981) Sr isotopic
tracer study of the Samail ophiolite Oman Journal of Geophysical
Research 86(B4) 2709---2720
Leake B E (1997) Nomenclature of amphiboles report of the
subcommittee on amphiboles of the International Mineralogical
Association commission on new minerals and mineral names
European Journal of Mineralogy 9 623---651
Lecuyer C amp Reynard B (1996) High-temperature alteration of
oceanic gabbros by seawater (Hess Deep Ocean Drilling Program
Leg 147) evidence from oxygen isotopes and elemental fluxes
Journal of Geophysical Research 101(B7) 15883---15897
Liou J G Kuniiyoshi S amp Ito K (1974) Experimental study of the
phase relations between greenschist and amphibolite in a basaltic
system American Journal of Science 274 613---632
MacLeod C J amp Rothery D A (1992) Ridge axial segmentation in
the Oman ophiolite evidence from along-strike variations in the
sheeted dyke complex Geological Society of America Special Papers 60
39---63
Manning C E MacLeod C J amp Weston P E (1996a) Fracture-
controlled metamorphism of Hess Deep gabbros site 984
constraints on the roots of mid-ocean ridge hydrothermal systems
at fast-spreading centers In GillisM C Allan KM ampMeyer P S
(eds) Proceedings of the Ocean Drilling Program Scientific Results 147
College Station TX Ocean Drilling Program pp 189---211Manning C E Weston P E amp Mahon K I (1996b) Rapid high-
temperature metamorphism of East Pacific Rise gabbros from Hess
Deep Earth and Planetary Science Letters 144 123---132Manning C E MacLeod C J amp Weston P E (2000) Lower-crustal
cracking front at fast-spreading ridges evidence from the East Pacific
Rise and the Oman ophiolite Journal of the Geological Society London
349 261---272McCollom T M amp Shock E L (1998) Fluid---rock interactions in
the lower oceanic crust thermodynamic models of hydrothermal
alteration Journal of Geophysical Research 103 547---575
McCulloch M T Gregory R T Wasserburg G J amp Taylor H P
(1981) Sm---Nd Rb---Sr and 18O16O isotopic systematics in an
oceanic crustal section evidence from the Samail ophiolite Journal of
Geophysical Research 86(B4) 2721---2735Mevel C Gillis K M Allan J F amp Meyer P S (eds) (1996)
Proceedings of the Ocean Drilling Program Scientific Results 147 College
Station TX Ocean Drilling Program
Nehlig P amp Juteau T (1988) Flow porosities permeabilities and
preliminary data on fluid inclusions and fossil thermal gradients in
the crustal sequence of the Sumail ophiolite (Oman) Tectonophysics
151 199---221
Nehlig P Juteau T Bendel V amp Cotten J (1994) The root zone of
oceanic hydrothermal systems constraints from the Samail ophiolite
(Oman) Journal of Geophysical Research 99 4703---4713
Nicolas A amp Boudier F (1991) Rooting of the sheeted dyke complex
in Oman ophiolite In Peters Tj Nicolas A amp Coleman R (eds)
Ophiolite Genesis and Evolution of the Oceanic Lithosphere Dordrecht
Kluwer Academic pp 39---54
BOSCH et al HYDROTHERMAL CIRCULATION IN OMAN OPHIOLITE
1207
Nicolas A amp Ildefonse B (1996) Flow mechanism and viscosity in
basaltic magma chambers Geophysical Research Letters 23(16)
2013---2016
Nicolas A Ceuleneer G Boudier F amp Misseri M (1988) Structural
mapping in the Oman ophiolites mantle diapirism along an ocean
ridge Tectonophysics 151 27---56
Nicolas A Boudier F Ildefonse B amp Ball E (2000) Accretion
of Oman and United Arab Emirates ophiolite-----discussion of a
new structural map Marine Geophysical Research 21(3---4)147---179
Nicolas A Boudier F Michibayashi K amp Gerbert-Gaillard L
(2001) Aswad massif (United Arab Emirates) archetype of the
Oman---UAE ophiolite belt Geological Society of America Bulletin 349
499---451
Nicolas A Mainprice D amp Boudier F (2003) High temperature
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2372
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field relations phase variation cryptic variation and layering and a
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morphology on magma supply and spreading rate Nature 364
706---708
Pin C Briot D Bassin C amp Poitrasson F (1994) Concomitant
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based on specific extraction chromatography Analytica Chimica Acta
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Rothery D A (1983) The base of a sheeted dyke complex Oman
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Spear F S (1981) An experimental study of hornblende stability and
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281 697---734
Spulber S D amp Rutherford M J (1983) The origin of rhyolite and
plagiogranite in oceanic crust an experimental study Journal of
Petrology 24 1---25Stakes D S (1991) Oxygen and hydrogen isotope compositions of
oceanic plutonic rocks high-temperature deformation and meta-
morphism of oceanic layer 3 In Taylor H P OrsquoNeil J et al (eds)
Geochemical Society Special Publication 3 77---90
Stakes D S amp Taylor H P (1992) The northern Samail ophiolite an
oxygen isotope microprobe and field study Journal of Geophysical
Research 97(B5) 7043---7080Staudigel H Davies G R Hart S R Marchant M amp Smith B
M (1995) Large scale isotopic Sr Nd and O isotopic anatomy of
altered oceanic crust DSDPODP sites 417418 Earth and Planetary
Science Letters 130 169---185Turner F J amp Verhoogen J (1960) Igneous and Metamorphic Petrology
New York McGraw---Hill 694 pp
Wilcock W S D amp Delaney J R (1996) Mid-ocean ridge
sulfide deposits evidence for heat extraction from magma
chambers or cracking fronts Earth and Planetary Science Letters 145
49---64
Witt G amp Seck H A (1989) Origin of amphibole in recrystallized
and porphyroclastic mantle xenoliths from Rhenish massif implica-
tions for the nature of mantle metasomatism Earth and Planetary
Science Letters 91 327---340
JOURNAL OF PETROLOGY VOLUME 45 NUMBER 6 JUNE 2004
1208
fluids responsible for the hydrothermal alteration (Nicolaset al 2003) The iddingsite replacement is typicallyheterogeneous at the thin-section scale being preferen-tially associated with microcracks Serpentinization alsodevelops from microcracks but it can pervasively invadethe entire thin section The microcracks are commonapproximately 25 of the studied thin sections displayone or more Ignoring the cracks related to LT alterationand considering only the cracks related to the HT hydro-thermal alteration this fraction is still around 20
Distribution of high-T alteration facies
Figure 4 shows the distribution of alteration facies as afunction of depth in the crust along wadi sections throughthe Oman---UAE ophiolite offering continuous exposures(see details in Fig 1) Figure 4 excludes the LT faciesbecause (1) being based on the presence of olivine ser-pentinization only throughout our compilation its dis-tribution is biased by the decreasing amount of olivineupsection and (2) this study concerns high-temperaturealteration processes Despite the fact that the scale ofsampling may exceed the scale of heterogeneity evidencefor both VHT alteration and lsquono alterationrsquo increaseswith depth Conversely HT alteration is predominantin the middle and upper parts of the gabbro sectionThis observation is valid throughout the wadi sectionsstudied with the exception of the Wadi Tayin massifwhich shows no clear gradation with depthFigure 5 is another representation of alteration-facies
distribution with depth based on the detailed study of414 thin sections The samples are classified according totheir level in the gabbro section sheeted dyke root zonegabbros upper gabbros middle gabbros lower gabbrosand Moho gabbros (the Moho gabbros have beengrouped together with the gabbro lenses interlayered inthe dunites of the MTZ) Values represent the percent of
2 Km
3
4
5
6
7
Aswad Fizh Hilti Haylayn Samail W TayinNakhl
High T
Very High T
0 High T
Upper Crust
LID
Lower Crust
MohoMTZ
DEPTH1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27
Fig 4 Depth of penetration of hydrothermal circulation in gabbros along the wadi sections numbered in Fig 1 referring to alteration faciesdescribed in Fig 3 Depths are recorded below the palaeo-seafloor LID corresponds to the upper crust including sheeted dykes and volcanics
0
10
20
30
40
50
60
70
80
90
100
SD root
zone
upper
gabbros
middle
gabbros
lower
gabbros
MTZ
BHb-GHb CPX
Chl-Amph core OL
Chl-Amph rim OL
0
5
10
15
20
25
30
SD root
zone
upper
gabbros
middle
gabbros
lower
gabbros
MTZ
Opx OL
BHb OL
No VHTHT
Unaltered OL core
HT
VHT
(a)
(b)
Plst def
gabbros
Fig 5 Compilation of hydrothermal alteration features as a function ofdepth in the gabbro section Percentages represent the occurrence of agiven type of alteration versus the number of thin sections examined foreach selected level root of the sheeted dykes (SD) upper gabbros middlegabbros lower gabbros MTZ (Moho Transition Zone) gabbros on atotal amount of 414 thin sections (Opx orthopyroxene Amph amphi-bole OL olivine BHb brown hornblende GHb green hornblendeChl chlorite Plst def plastic deformation) (a) Distribution of the HTalteration (in ) (b) Distribution of the VHT alteration (in )
JOURNAL OF PETROLOGY VOLUME 45 NUMBER 6 JUNE 2004
1186
a given alteration facies as defined previously out of thetotal number of thin sections examined for a given levelMore refined conclusions can be drawn from this figureAs seen in Fig 5a the HT alteration decreases down-
wards whereas it affects most of the upper gabbroicsection (sheeted dyke root zone and upper gabbros)The fraction of unaltered olivine cores can be used asan index to estimate the extent of alteration This indexshows that the HT alteration is more penetrative andintense in the upper gabbros a conclusion alreadyreached by several workers (eg Gregory amp Taylor1981 Stakes amp Taylor 1992 Kawahata et al 2001)This was also noted by Manning et al (1996a 1996b)Figure 5b shows that the VHT alteration marked by
orthopyroxene or brown hornblende coronas aroundolivine increases with depth The correlation betweenthe VHT alteration and plastic deformation also suggeststhat the VHT alteration may coincide with the develop-ment of plastic flow at the expense of the magmatic flow
at temperatures just below the solidus In Fig 5b thefraction of gabbros where there is no VHT---HThydrothermal alteration increases downsection indicat-ing that the VHT---HT alteration is more localizeddownsection
Gabbroic dykes and veins
The occurrence of gabbroic dykes in the layered gabbrosof the Samail ophiolite is ubiquitous We describe gab-broic dykes and sills either plagioclase---clinopyroxeneand brown amphibole-bearing dykes or microgabbrosand amphibolitized dolerite dykes and sills correspond-ing to temperatures up to 975C (see below) Identifica-tion of gabbroic dykes in the field is not alwaysstraightforward because they may be obliterated bysuperimposition of greenschist-facies alteration (Fig 6a)Selective greenschist-facies alteration has frequently beenobserved either in the centre of a mafic dyke or alongboth margins (Fig 6b) as well as a continuous transition
(b) (c)(a)
(d) (e)
thulite
pistacite
white microvein
white vein
green vein
green vein
gabbroic dike
epidote vein
gabbroic dike
Fig 6 Field relationships of dykes and veins (scale bar represents 10 cm) (a) Thulite---pistacite vein cross-cutting a foliated gabbro matrix Reactionmargins marked by the development of secondary clinopyroxene (01 OA36b) (b) Comb-like structure in gabbroic dyke A pink epidote vein followsthe margin of the dyke (01 OA15f) (c) Along-strike transition from a gabbroic dykelet to a hydrothermal green amphibole vein (d) White prehnitevein (e) Green amphibole vein cross-cut by white prehnite vein
BOSCH et al HYDROTHERMAL CIRCULATION IN OMAN OPHIOLITE
1187
along a single crack from a mafic dyke to a hydrothermalvein (Fig 6c) We define hydrothermal veins as cracksfilled with greenschist-facies minerals that induce meta-morphic reactions in the adjacent country rocks andgabbroic dykes as planar intrusive bodies produced bythe crystallization of magma Following Nehlig amp Juteau(1988) we distinguish low-T greenschist-facies lsquowhiteveinsrsquo made of prehnite epidote chlorite and actinolite(Fig 6d) and lsquogreen veinsrsquo in which a darker greenamphibole predominates (Fig 6e) corresponding respec-tively to lower (195---410C) and higher (400---530C)temperatures (Nehlig amp Juteau 1988)The gabbroic dykes are typically a few centimetres
across composed of plagioclase clinopyroxene andbrown
hornblende They are coarse-grained to pegmatitic andcommonly display a comb-like structure (Fig 6b)Although the spacing between the dykes in the field ishighly variable the average is around 10m Gabbroicsills grade from clear intrusions to irregular and diffusepegmatitic gabbro patches up to a few metres in theirlarger dimension (Fig 7a)The distribution in the field of brownish pargasite
which appears black and glassy in outcrop is critical intracing the origin of the gabbroic dykes It is first observedat any level in the host lower and middle gabbros as largepoikilitic patches (Fig 7b) locally associated with simil-arly poikilitic patches of clino- and orthopyroxene(Fig 7a) These patches contain small aligned tablets of
(a) (b) (c)
g(e)(d)
alignment ofalignment of amphiboleamphibole oikocrystsoikocrysts
hwhite vein
hblpl
pcpx
brownbrownamphiboleamphiboleoikocrystoikocryst
eediabasedikdike
hb-plhb-plreactionreaction
imarginspbrown amphibole rich
gabbroic dikegabbroic dike
pbrown amphibole veins
Fig 7 Field relationships of dykes (scale bar is 10 cm long) (a) Irregular coarse to pegmatitic patches in a foliated gabbro matrix (01 OA42d)(b) Alignment of black amphibole oikocrysts in a foliated gabbro matrix (c) Local concentration of brown amphibole in a gabbroic dykelet Theplagioclase laths marking the foliation in the enclosing gabbro rotate into parallelism with the gabbroic dykelet (d) Two diabase dykes assimilatingthe enclosing layered gabbro with plagioclase---amphibole reaction margins (e) Pargasite-rich gabbroic dyke with wall reactions (01 OA19) (Notesmall pargasite---plagioclase veins) pl plagioclase cpx clinopyroxene hb hornblende
JOURNAL OF PETROLOGY VOLUME 45 NUMBER 6 JUNE 2004
1188
plagioclase oriented parallel to the larger plagioclaselaths defining the magmatic foliation of the gabbro(Fig 7c) Clinopyroxene oikocrysts surrounding idio-morphic plagioclase crystals are common in the upper-level gabbros they mark the end of the magmatic flowand crystallization growing before the pargasite andorthopyroxene oikocrysts as shown by their partialreplacement by pargasite Their crystallization marksthe transition at suprasolidus conditions from a dry to awet environment In the uppermost gabbros those dykesthat are typically altered in the HT hydrothermal faciesgrade upwards into the so-called lsquoisotropicrsquo or lsquovari-texturedrsquo gabbrosThe microgabbro and amphibolitized dolerite dykes
differ from the typical diabase dykes commonly cuttingthe gabbro section that have chilled margins and thusprobably intruded enclosing rocks already at temperat-ures below 450C In microgabbro and amphibolitizeddolerite dykes the absence of chilled margins and thedevelopment of dioritic reaction margins in the adjacentgabbro (Fig 7d and e) suggest that they were emplaced ingabbros that were still at temperatures above 750C(Nicolas et al 2000)
ANALYTICAL TECHNIQUES
Major and trace elements
Major element compositions were determined by elec-tron microprobe at the University of Montpellier II usinga Camebax SX100 Instrument calibration was per-formed on natural standards and operating conditionswere 20 kV accelerating potential 10 nA beam currentand 30 s counting timeRb and Sr concentrations in whole rock and separated
mineral fractions were measured by conventionalnebulization inductively coupled plasma mass spectro-metry (ICP-MS) using a VG Plasmaquad 2 (ISTEEMMontpellier) Digestion procedures followed thoseoutlined by Ionov et al (1992)
O---Sr isotopes
The samples selected for the detailed study wereextracted from gabbroic dykes or veins inside gabbrosand cut with a diamond saw into 2---3 cm fragmentsbefore being crushed into 05---1 cm fragments Forwhole-rock analysis small pieces were further crushedinto powder in an agate bowl For pure mineral analysisthe fragments were crushed with a manual agate mortarTo avoid or minimize the mixing of selected separateminerals with other phases that can bias the results astrict protocol of mineral selection was applied A smallsize fraction of 125---160 mm was used and minerals werefirst separated using a Frantz isodynamic magneticseparator Concentrates recovered (cpx plagioclase
amphibole epidote) were subsequently purified by care-ful hand-picking in alcohol under a binocular micro-scope At this stage mineral separates were very pureand only flawless minerals free of cracks and visibleinclusions were kept for isotopic analyses To ensurefurther mineral integrity these pure mineral separateswere ultrasonically washed in dilute 15N HCl and rinsedseveral times with ultra-pure water This stage potentiallyremoves adsorbed secondary micro-phases Previousstudies (eg Lecuyer amp Reynard 1996) demonstratedthat pyroxene and amphibole can be intermixed on avery fine scale with other phases (such as a range ofamphibole compositions talc Fe-oxide) Although thispossibility cannot be ruled out the procedure used inthis study makes it unlikely that complex minerals passedthrough the selection
Sr isotopic compositions
Approximately 50mg of whole-rock powder or separatedminerals were digested in a Teflon lsquobombrsquo with a mixtureof triple distilled 13N HNO3 and 48 HF with a fewdrops of HClO4 Two aliquots were separated one forthe determination of Sr isotopic composition and theother for Sr and Rb contents Sr was separated using anextraction chromatographic method modified from Pinet al (1994) Concentrations were measured by isotopedilution using 84Sr---87Rb mixed spikes To remove tracesof the late low-temperature seawater alteration and todetermine the primitive whole-rock Sr isotopic composi-tion leaching experiments were conducted on a fewsamples following the procedure described by Bosch(1991) The strontium isotopic compositions of leachateshave been analysed in the attempt to check for the leach-ing efficiency During these tests four Sr isotope composi-tions were measured which correspond to the unleachedsample the two liquids extracted after acid-leaching stepsand the final solid residue Leaching experiments wereconducted in two steps first with 6N HCl for 8min at70C second with 25N HCl for 30min at 70C Afterleaching the leachates were evaporated to dryness andthe solid residues digested similarly to the unleachedsamplesSr isotopic compositions were measured on a fully
automated VG Sector mass spectrometer housed at theUniversity of Montpellier II except for the Sr isotopicdata from the leaching experiments which have beenmeasured at the Universitee de Bretagne Occidentale(Brest) To assess the reproducibility and accuracy of Srisotopic measurements the NBS 987 standard was run12 times during this study the average value was0710245 with a precision better than 0000010 (2s)87Sr86Sr ratios are corrected for mass discrimination bynormalizing to a 86Sr88Sr value of 01194 Sr total blankconcentrations were less than 60 pg Initial 87Sr86Sr
BOSCH et al HYDROTHERMAL CIRCULATION IN OMAN OPHIOLITE
1189
ratios were calculated by correcting the measured ratiosfor accumulation of 87Sr produced by in situ 87Rb radio-active decay since 95Ma the assumed age of crystalliza-tion of the Oman ophiolite (Hacker 1994)
Oxygen isotopes
The powdered samples were introduced in Ni tubesunder dry air where they were reacted with BrF5 toform oxygen Oxygen was separated from other gasesby cooling the resulting mixture at liquid nitrogen tem-perature (77 K) that retained other volatile gases Oxygenwas further purified by trapping it on a cooled 5A mole-cular sieve at 77 K then quantified to calculate the yieldof the isotopic analyses and finally transferred to the massspectrometer (Finnigan Mat Delta E) where intensitiesassociated with masses 32 and 34 were measured todetermine d18O The isotopic composition of a sampleis given as dsample frac14 (RsampleRstandard 1) 1000 in permil unit where R is 18O16O and the standard isSMOW Analytical errors on the oxygen isotope ratioare 02 (2s)
RESULTS
Seven samples representative of the range of gabbroicdykes and five samples representative of the lower gab-bros two of them representing the margins of gabbroicdykes were selected for major element thermometricand Sr---O isotopic analyses (Tables 1---3) These samplesoriginate from wadi sections marked by bold lines inFig 1 Wadis Halhal Mayah Al Abyad Warihya andMaqsad structurally belong to a new ridge segment open-ing in slightly (1Myr) older lithosphere Wadi Gideahis in older lithosphere
Petrographic and isotopic characteristicsof the analysed samples
Olivine gabbro from the MTZ in MaqsadSamail Massif
Sample 94 MA2 is devoid of low-temperature alterationand only exhibits the discrete signature of the VHT---HTalteration The orthopyroxene corona around olivine(Fig 3a) is relayed by kelyphitic rims at the contactwith plagioclase (Fig 3b) Issuing from these rims aremicrometre-sized pale green amphiboles identified ascummingtonite and actinolite which penetrate the sur-rounding plagioclase in microcracks 004mm wide or atgrain boundaries (Fig 3b) similar to the microcrack sys-tem described by Manning et al (2000) A large numberof the plagioclase crystals have a cloudy core as a result ofthe concentration of fluid and mineral inclusions amongwhich diopside has been identified during microprobeanalysis Olivine cores are free of serpentine but theyare largely oxidized along a microcrack network Ortho-pyroxene coronas have a MgOpx number of 78---79
slightly higher than in the primary olivine cores withMgOl number of 74---75 The magmatic clinopyroxeneis totally fresh and probably not in equilibrium with thereactive orthopyroxene This sample has the lowest 87Sr86Sr initial ratio of the studied whole rocks (070322)identical to coexisting clinopyroxene (070324) and pla-gioclase (070322) This observation suggests the lack of alate superimposed LT hydrothermal alteration as it isvery unlikely that an LT hydrothermal event could havetotally equilibrated the Sr isotopic compositions of prim-ary minerals such as plagioclase and clinopyroxeneMoreover the whole rock (WR) clinopyroxene andplagioclase have similar initial Sr isotopic ratios despitevery different Sr contents Accordingly this ratio isthought to reflect the signature acquired during the crys-tallization of these minerals and is higher than the Omanmantle Sr isotopic value (McCulloch et al 1981) Thed18O values of the WR (50) and plagioclase (48) aredistinctly lower than primary magmatic values Indeedin normal mid-ocean ridge basalts (MORB) the WRd18O value is 57 02 with a corresponding plagio-clase d18O ranging from 55 to 62 ( Javoy 1980Gregory amp Taylor 1981)
Lower gabbro from Wadi Wariyah Wadi Tayin Massif
Sample 97 OA128b belongs to a layered sequence in thelower gabbro series injected by anorthositic sills Thesample was collected at the tip of one of these sillsbetween layers enriched in clinopyroxene The gabbrois composed of 60 plagioclase and 40 clinopyroxeneDespite the absence of olivine the WR has a relativelyhigh Mg content (Table 3) A series of low-T (prehnite)microveins 01mm thick cross-cut the gabbro perpendi-cular to the layering and foliation The margins of theveins are devoid of alteration This gabbro has the sameSr isotopic composition as 94 MA2 Nevertheless theWR and associated plagioclase have Sr isotope signatures(070333 and 070328) lower than the coexisting clino-pyroxene (070422) The Sr contents of the WR plagio-clase and clinopyroxene are 127 ppm 120 ppm and20 ppm respectively thus making the clinopyroxenemore sensitive to late-stage contamination Clinopyrox-ene and plagioclase oxygen isotope compositions (523and 414 respectively) are also lower than typicalmantle values Similar to the previous sample alterationat high temperature by a low d18O and high 87Sr86Srfluid is required to explain such characteristics
Amphibole gabbro patch Wadi Gideah Wadi TayinMassif
Sample 01 OA41d2 is from a laminated gabbro com-posed of millimetre-sized layers of clinopyroxeneand plagioclase Anatectic patches (Fig 7a) borderinganorthositic layers are formed of coarse-grained
JOURNAL OF PETROLOGY VOLUME 45 NUMBER 6 JUNE 2004
1190
Table1Locationoftheanalysed
samplesandsummaryoftheirpetrographicandSr---O
isotopiccharacteristics
Sam
ple
nam
eSite
Locationin
gab
broic
section
Typ
eofrock
Distance
to
Moho(km)
Parag
enesis
Alteration
87Sr
86Srinitial1
d18O
()2
94MA2
Maq
sad
From
MTZ
Layered
gab
bro
0Olivine(M
gno74---75)3
OPXco
ronas
(Mgno78---79)
WRfrac14
070322
WRfrac14
thorn495
CPX
Palegreen
Am
(cumact)
CPXfrac14
070324
nd
Plagioclase(A
n65---80)4inversezoning
Nolow-T
alteration
Plfrac14
070322
Plfrac14
thorn479
Solidfluid
inclusionsin
Pl
97OA128b
Wad
iWaryiah
From
lower
gab
bro
Layered
gab
bro
1CPX
Low-T
alteration
WRfrac14
070333
nd
Plagioclase(A
n85---87)
Prehnitemicroveins
CPXfrac14
070422
CPXfrac14
thorn523
Plfrac14
070328
Plfrac14
thorn414
01OA41d2
Wad
iGideah
From
upper
gab
bro
Gab
bro
patch
in
laminated
gab
bro
36
CPXrecrystallized
Plagioclaseidiomorphic
(An57---87)norm
alzoning
Black
pargasitic
honblende
WRfrac14
070351
CPXfrac14
070378
Plfrac14
070426
WRfrac14
thorn480
nd
Plfrac14
thorn504
Hbdfrac14
070355
nd
01OA19aamp
dWad
iWaryiah
Inlayeredgab
bro
Gab
broic
dyke
01
Olivine
OPXpoikilitic
WRfrac14
070341
WRfrac14
thorn567
CPX
Black
pargasite
Hbdfrac14
070324
Hbdfrac14
thorn287
Plagioclase(A
n73---95)
Hornblendepoikilitic
Plfrac14
nd
Plfrac14
thorn10 09
Hem
atite
WRfrac14
070418
nd
Green
Mg-hornblende
Acthornblende
Epidote
02OA90b
Wad
iAl-Abyad
Inlayeredgab
bro
Dioriticdyke
015
Green
Mg-hornblende
Hbdfrac14
070427
Hbdfrac14
thorn239
Plagioclase(A
n83---87)
Plfrac14
070387
Plfrac14
thorn498
Solidfluid
inclusionsin
plagioclase
01OA15f
Wad
iWaryiah
Inlayeredgab
bro
Comb-textured
gab
broic
dyke
025
CPXidiomorphic
Plagioclase(A
n95---99)norm
alzoning
BrownMg-H
ornblende
WRfrac14
070458
nd
02OA43a
Wad
iMayah
Inlayeredgab
bro
vein
Green
amphibole
1Palegreen
Mg-hornblendeacicular
Plagioclase(A
n91---97)in
reactionmargin
Hbdfrac14
070431
Hbdfrac14
thorn310
01OA36b
Wad
iGideah
Inlayeredgab
bro
epidosite
dyke
25
CPX
Epidote
WRfrac14
070519
nd
Plagioclase
Epfrac14
070554
nd
CPXclinopyroxene
Plplagioclase
OPXorthopyroxene
Amam
phibolecu
mcu
mmingtonite
actactinolite
WRwholerockHbdhornblende
Epep
idotend
notdetermined
187Sr
86Srinitialratioscalculatedat
95Ma(H
acker1994)Errors
ofthemeasuredratiosareindicated
inTab
le4
2Analyticalerrors
oftheoxygen
isotopevalues
are02
(2s)
3Mgnumber
frac14atomicMg(Mgthorn
Fe)
4Percentageofan
orthitein
plagioclase
BOSCH et al HYDROTHERMAL CIRCULATION IN OMAN OPHIOLITE
1191
(millimetre-sized) recrystallized clinopyroxene largelyaltered to light green magnesio-hornblende and idio-morphic plagioclase Locally primary black pargasitichornblende develops including idiomorphic normallyzoned plagioclase (Table 4) This gabbro exhibits similar87Sr86Sr ratios for the WR and pargasite (070351 and070355 respectively) These values are interpreted asrepresentative of the melt from which this amphibolecrystallized The clinopyroxene has a slightly higher Srisotope composition of 070378 consistent with the occur-rence of the light green Mg-hornblende rim Plagioclaseis significantly more radiogenic (070426) related to sec-ondary destabilization evidenced by the idiomorphichabit The Sr isotope compositions of the liquidsextracted from the acid leaching experiments of theWR (L1 and L2) are very similar 070358 and 070365respectively The 87Sr86Sr ratio of the residue obtainedafter leaching experiments is 070335 lower than boththe pargasite and unleached WR This value can beconsidered as the unaltered isotopic composition of thissample The d18O values measured for WR and coexist-ing plagioclase are lower than normal MORB magmaticvalues The 18O depletion in the plagioclase can be fairlywell explained by high-temperature hydrothermal inter-action between an oxygen-depleted fluid and the magmaAccording to the kinetics of hydrothermal exchange ofoxygen (Gregory amp Taylor 1981 Gregory et al 1989)plagioclase exchanges its oxygen isotopes with the fluidadjusting readily to the hydrothermal conditions Forhigh temperatures of exchange with seawater-derivedfluids (d18O between 0 and thorn2) the magmatic plagio-clase will have its primary d18O value lowered whereasthe d18O value for lower temperatures of exchange(5300C) will increase The amplification of the d18Oshift increases with the intensity of the hydrothermalalteration
Gabbroic dykes intrusive in lower gabbros from WadiWariyah Wadi Tayin Massif
Gabbroic dykes 01 OA19a and 01 OA19d are intrusiveinto the lower gabbros They are 3---4 cm wide discord-ant to the foliation of the host gabbros (Fig 7e) with asharp margin marked by a brown amphibole fringe Theprimary phases in the dykes (Table 1) locally preservedas elongated aggregates of a 01mm grain size mosaicassemblage at the dyke margins grade to millimetre sizein the central part of the dyke The WR major elementcomposition is Mg enriched compared with the enclosinggabbro (Table 3) Brown pargasitic amphibole oikocrystsmake up 50 of the modal composition of the dyke Indyke 01 OA19a poikilitic orthopyroxene may developinstead of brown amphibole Finally green magnesio-hornblende grows either as oikocrysts at the expense ofclinopyroxene or in association with epidote as an altera-tion product of plagioclase These low-amphibolite-faciesminerals represent the lowest-grade paragenesis recogn-ized in these dykes and are most developed in sample01OA19aTheamphibole composition showszoning fromTi-rich pargasitic hornblende to magnesio-hornblendeand actinolitic hornblende (Fig 8c) recording pro-gressive cooling of the dyke The plagioclase compositionis extremely heterogeneous (Fig 8b) varying from An95a composition similar to that of the plagioclase in the hostgabbro (Fig 8a) to An50 for the plagioclase and asso-ciated epidote inclusions in the poikilitic actinolitic horn-blende Sample 01 OA19d has an initial 87Sr86Sr ratioslightly higher for the WR (070341) than for the parga-site (070324) The Sr isotopic ratios of liquids extractedafter two steps of WR acid-leaching (L1 and L2) are070357 and 070341 respectively whereas the residueyields a 87Sr86Sr ratio of 070327 very close to thepargasite and plagioclase values The latter is similar tothe isotopic composition measured for the massive gab-bro 94 MA2 and may be taken as close to the primarymagmatic value This Sr isotopic ratio is more radiogenicthan the inferred Oman mantle-source value (McCullochet al 1981) The altered part of this sample has a radio-genic Sr isotopic composition (070418) related to thelow-amphibolite-facies alteration This gabbroic dykehas seemingly preserved a magmatic whole-rock d18Ovalue (567) but contains by far the highestd18O plagioclase (1009) of all the samples analysedThis plagioclase coexists with low d18O pargasite(thorn287) Similarly high values (d18Oplagioclase 4 8)have been previously measured for gabbroic dykes dia-bases sheeted dykes and plagiogranites from the Omanophiolite (Gregory amp Taylor 1981 Stakes amp Taylor1992) This 72 oxygen isotope fractionation betweenplagioclase and amphibole cannot possibly represent oxy-gen equilibrium at any plausible temperature This maybe explained by different exchange kinetics for these twominerals at different temperatures at high waterrock
Table 2 Average temperatures ( C 40C)
calculated for plagioclase---amphibole pairs
using the geothermometer of Holland amp
Blundy (1994)
Pargasite Magnesio-hornblende
rim core rim core
01 OA41d2 878 953 ------- -------
01 OA19a 935 974 825 869
01 OA19d 890 933 789 -------
02 OA37b ------- ------- 816 849
02 OA90b ------- ------- 859 833
JOURNAL OF PETROLOGY VOLUME 45 NUMBER 6 JUNE 2004
1192
ratios This supports the occurrence of two distincthydrothermal events one at high temperature and alater stage at lower temperature responsible for thestrong 18O enrichment in the coexisting plagioclase
Dioritic dyke associated with a diabase dyke in lowergabbros from Wadi Halhal Haylayn Massif
Sample 02 OA37b belongs to a group of brownamphibole---plagioclase lenses or reaction margins asso-ciated with diabase intrusives (Fig 7d) in lower layeredgabbros Idiomorphic brown hornblende (tschermakite)crystals are elongated in the plane of the dyke represent-ing 80 of the modal composition They are associatedwith an interstitial plagioclase matrix The browntschermakite has greenish rims Plagioclase has a cloudyappearance due to micrometre-size fluid inclusionsexcept along its margins and internal microcracks Bothhornblende and plagioclase exhibit evidence of plasticdeformation The Sr isotopic ratio for the pargasite andplagioclase is 070348 and 070420 respectively Thepargasite exhibits a Sr signature closest to that of theOman primary magmatic value (McCulloch et al 1981)Nevertheless it is too high to be attributed to anunmodified MORB-source mantle signature This sug-gests the addition of an external fluid component in goodagreement with the shift of oxygen isotopic compositionto a low value (thorn39) suggesting a high-temperatureexchange with a low d18O fluid The Sr isotope composi-tion of the plagioclase is significantly higher than that of
the coexisting pargasite probably as a result of the low-temperature alteration
Dioritic dyke cross-cutting lower layered gabbros in WadiAl-Abyad Nakhl Massif
Sample 02 OA90b is a 1 cm thick dioritic dyke cross-cutting the lower layered gabbros It grades into a greenamphibole vein as illustrated in Fig 6c The dyke is com-posed of 2 cm long dark green idiomorphic magnesio-hornblende crystals growing in the plane of the dykeand plagioclase concentrated at the margin of the dykeThe plagioclase in the dyke and in the enclosing gabbrocontains micrometre-size fluid and mineral inclusionssimilar to those observed in sample 94 MA2 The plagio-clase and green hornblende have higher Sr isotopic ratios(070387 and 070427 respectively) and lower d18Ovalues (thorn498 and thorn239 respectively) than the normalMORB-source mantle value As observed for previoussamples this supports interaction with an external fluidcomponent
Comb-textured gabbroic dykes in the lower gabbros fromWadi Wariyah Wadi Tayin Massif
Sample 01 OA15f (Fig 6b) is from Wadi Wariyah andrepresents a 2 cm wide dyke intruding the lower gabbrosThis type of comb-textured dyke developed in theclinopyroxene---plagioclase stability field It containslarge idiomorphic clinopyroxene crystals which arepartially replaced by brownish magnesio-hornblendeThe plagioclase is extremely calcic with a slight inverse
Table 3 Whole-rock major element compositions of massive gabbros and dykes determined by X-ray fluorescence
(wt )
Sample 94 Ma2 97 OA128b 01 OA41d2 01 OA19d 01 OA19a 01 OA15f 01 OA36b 01 OA36b
Rock type Gabbro Gabbro Gabbro patch Gabbroic dyke Gabbroic dyke Comb-textured
gabb dyke
Epidosite dyke Epidosite dyke
(margin)
SiO2 4830 4815 4749 4524 4339 4799 389 4371
Al2O3 2785 2032 2438 1474 1246 1192 2752 1646
Fe2O3 328 266 404 981 1204 592 381 623
MnO 003 004 005 014 016 006 008 014
MgO 405 737 427 1031 1686 1326 187 739
CaO 1289 1908 1477 1235 1156 166 2471 2339
Na2O 292 123 257 258 111 075 000 013
K2O 000 000 009 006 000 000 000 000
TiO2 005 017 063 230 064 086 013 042
P2O5 008 007 013 018 009 013 007 010
LOI 033 076 138 216 161 237 274 190
Total 9978 9985 998 9987 9992 9986 9983 9987
LOI loss on ignition gabb gabbroic
BOSCH et al HYDROTHERMAL CIRCULATION IN OMAN OPHIOLITE
1193
Table4Majorelementcomposition
aofselectedam
phibolespaired
withplagioclasecompositionsusedforthermometry
Sam
ple
nam
e1Mode2
SiO
2TiO
2Al 2O3
Cr 2O3
FeO
MnO
MgO
CaO
Na 2O
K2O
Total
Si
AlIV
AlVI
Ti
Fe3
thornCr
Mg
Fe2
thornMn
Na
KCa
XAn3
T(C)4
01OA19a
Mg-hornblende
core
45 60
26
10 78
009
860
015
16 56
12 00
212
007
96 72
6552
1448
0366
0083
0552
0004
3409
0594
0016
0611
0018
1843
0903
899
Mg-hornblende
core
47 99
263
850
002
897
011
16 61
12 10
160
020
96 36
6894
1106
0334
0028
0483
0002
3557
0594
0014
0446
0036
1862
0892
834
Mg-hornblende
rim
51 17
247
596
001
761
014
18 11
12 19
103
010
96 47
7257
0743
0253
0016
0414
0001
3828
0488
0016
0284
0018
1853
0894
820
Mg-hornblende
rim
47 50
291
920
004
880
014
16 46
11 95
174
008
96 07
6835
1165
0396
0015
0506
0005
3529
0552
0017
0486
0015
1843
0887
825
Mg-hornblende
rim
49 57
269
742
000
799
017
18 07
12 02
133
014
96 88
7013
0987
0261
0018
0575
0000
3811
0371
0020
0365
0026
1821
0860
831
Mg-hornblende
core
47 83
181
872
003
840
011
14 48
12 18
159
008
93 59
6814
1186
0279
0018
0644
0004
3711
0357
0013
0438
0015
1859
0907
874
Pargasite
rim
42 15
249
11 87
024
10 07
013
13 84
11 43
263
032
96 61
6188
1812
0242
0434
0299
0028
3028
0937
0016
0749
0059
1797
0804
972
Pargasite
rim
42 22
260
11 86
026
10 21
011
13 71
11 48
253
032
96 59
6201
1799
0254
0429
0299
0030
3001
0955
0014
0720
0060
1807
0803
962
Pargasite
core
42 05
263
11 45
030
10 19
011
13 80
11 47
246
032
96 46
6189
1811
0175
0478
0299
0035
3029
0956
0014
0702
0060
1809
0767
974
Pargasite
rim
42 36
247
11 90
016
945
013
14 16
11 76
249
026
95 98
6239
1761
0305
0366
0276
0019
3109
0888
0016
0710
0049
1856
0818
926
Pargasite
core
42 50
291
11 64
016
984
015
14 22
11 41
256
034
96 69
6219
1781
0227
0426
0322
0018
3103
0883
0018
0727
0063
1789
0812
974
Pargasite
rim
42 76
270
12 03
017
920
012
14 87
11 82
244
025
96 73
6224
1776
0880
0335
0368
0020
3226
0752
0015
0689
0047
1843
0841
941
Pargasite
rim
42 13
181
13 54
016
921
009
12 77
12 13
254
032
96 98
6172
1828
0510
0450
0000
0018
2788
1129
0011
0721
0059
1905
0841
874
01OA19d
Mg-hornblende
49 26
047
624
011
11 46
017
15 39
12 21
108
009
96 48
7137
0863
0202
0051
0437
0013
3323
0951
0021
0302
0018
1896
0768
771
Mg-hornblende
46 03
287
932
018
906
017
14 77
12 87
193
007
97 28
6672
1328
0265
0313
0046
0021
3191
1053
0021
0542
0012
1999
0758
808
Pargasite
41 89
429
11 65
008
11 03
017
13 52
11 37
259
017
96 77
6159
1841
0179
0474
0345
0010
2963
1012
0021
0739
0032
1791
0770
975
Pargasite
rim
42 80
340
11 16
009
11 01
018
13 73
11 67
240
022
96 67
6293
1707
0226
0376
0322
0010
3009
1032
0022
0684
0041
1839
0605
880
Pargasite
core
42 58
351
11 27
006
11 63
018
13 51
11 46
239
022
96 82
6257
1743
0209
0387
0391
0007
2959
1039
0023
0680
0042
1804
0595
893
Pargasite
rim
42 14
393
11 28
006
11 71
014
13 35
11 33
235
033
96 62
6214
1786
0174
0435
0391
0007
2935
1054
0018
0673
0062
1790
0519
901
Pargasite
core
41 56
420
12 05
006
11 57
016
12 51
11 35
249
027
96 24
6168
1832
0277
0469
0276
0007
2768
1160
0021
0718
0052
1805
0537
882
Pargasite
core
41 33
438
11 49
008
11 66
015
13 35
11 45
264
026
96 80
6105
1894
0107
0487
0368
0009
2941
1073
0019
0757
0048
1813
0537
942
Pargasite
41 08
446
12 39
012
11 35
017
13 13
11 36
263
015
96 85
6049
1951
0199
0494
0368
0014
2882
1030
0021
0752
0027
1792
0758
975
02OA37b
Mg-hornblende
rim
45 15
260
984
002
13 28
019
14 01
10 87
128
009
97 33
6527
1473
0203
0283
0690
0002
3018
0915
0023
0360
0016
1683
0562
828
Mg-hornblende
rim
44 72
263
988
004
13 23
021
14 01
10 78
133
009
96 92
6494
1506
0185
0287
0713
0005
3032
0894
0025
0374
0018
1677
0593
854
Mg-hornblende
rim
45 68
247
935
004
12 87
020
14 10
11 00
121
011
97 02
6621
1379
0218
0269
0598
0004
3045
0962
0024
0341
0020
1708
0562
815
Mg-hornblende
core
43 94
291
10 87
001
12 60
023
13 73
10 59
142
011
96 39
6410
1590
0278
0319
0644
0001
2985
0893
0028
0401
0021
1655
0710
860
Mg-hornblende
core
44 62
270
10 37
001
12 66
019
14 12
10 58
140
008
96 73
6479
1521
0253
0294
0644
0001
3057
0893
0023
0395
0014
1646
0674
851
Mg-hornblende
rim
47 79
181
796
002
12 09
025
15 22
11 09
100
009
97 33
6866
1134
0214
0196
0506
0002
3260
0947
0030
0280
0016
1706
0503
767
Mg-hornblende
core
45 43
249
10 3
000
11 99
022
14 50
11 14
133
008
97 48
6524
1476
0267
0269
0644
0000
3103
0796
0027
0370
0015
1715
0703
838
JOURNAL OF PETROLOGY VOLUME 45 NUMBER 6 JUNE 2004
1194
Sam
ple
nam
e1Mode2
SiO
2TiO
2Al 2O3
Cr 2O3
FeO
MnO
MgO
CaO
Na 2O
K2O
Total
Si
AlIV
AlVI
Ti
Fe3
thornCr
Mg
Fe2
thornMn
Na
KCa
XAn3
T(C)4
02OA90b
Mg-hornblende
core
48 83
038
745
005
939
010
17 29
12 24
142
006
97 21
6931
1069
0041
0041
0667
0005
3657
0448
0011
0392
0010
1862
0840
856
Mg-hornblende
core
49 49
033
731
003
916
008
17 40
12 57
139
005
97 82
6987
1013
0035
0035
0552
0004
3662
0530
0010
0379
0009
1901
0830
811
Mg-hornblende
rim
48 90
040
753
005
932
013
17 15
12 22
154
005
97 29
6948
1052
0042
0042
0575
0006
3632
0533
0015
0424
0010
1861
0880
869
Mg-hornblende
rim
48 30
044
800
007
962
010
16 86
12 32
161
005
97 35
6873
1127
0047
0047
0598
0008
3576
0546
0012
0443
0010
1879
0870
862
Mg-hornblende
rim
49 12
032
742
010
922
011
17 36
12 29
142
006
97 43
6956
1044
0034
0034
0621
0011
3665
0471
0013
0391
0010
1865
0840
845
02OA43a
Mg-hornblende
rim
48 13
040
789
007
860
007
17 41
12 19
155
006
96 36
6882
1118
0212
0043
0621
0008
3710
0407
0008
0429
0010
1867
-------
-------
Mg-hornblende
rim
50 93
035
643
001
759
006
18 50
12 24
114
003
97 28
7152
0848
0217
0037
0506
0001
3873
0385
0007
0311
0005
1841
-------
-------
Mg-hornblende
rim
50 67
014
559
009
12 33
026
15 28
12 44
068
003
97 50
7260
0740
0204
015
0483
0010
3263
0994
0032
0189
0005
1909
-------
-------
Mg-hornblende
rim
50 34
013
621
009
12 49
027
15 07
12 51
064
003
97 78
7192
0810
0240
0010
0530
0010
3210
0960
0030
0180
0010
1910
-------
-------
Mg-hornblende
rim
45 17
060
11 14
013
988
008
15 78
12 33
211
009
97 31
6473
1527
0355
0640
0644
0015
3371
0540
0010
0588
0016
1892
-------
-------
Mg-hornblende
rim
49 25
032
712
014
845
009
17 84
12 32
137
004
96 95
6983
1020
0170
0030
0620
0020
3770
0380
0010
0380
0010
1870
-------
-------
Mg-hornblende
rim
48 61
032
775
014
975
011
16 81
12 57
139
004
97 50
6905
1950
0203
0034
0621
0016
3560
0537
0013
0382
0008
1914
-------
-------
94Ma2
Anthophyllite
54 98
000
259
001
12 65
037
23 35
199
034
000
96 28
7744
0256
0174
0000
0161
0001
4903
1330
0043
0092
0001
0301
-------
-------
Anthophyllite
55 11
000
296
003
14 09
032
23 66
063
038
000
97 18
7716
0284
0205
0000
0115
0003
4938
1535
0038
0104
0000
0095
-------
-------
Actinolite
48 58
004
954
002
960
014
17 67
999
139
005
97 02
6850
0150
0435
0004
0690
0002
3713
0442
0017
0379
0009
1510
-------
-------
Actinolite
51 54
004
585
000
890
022
20 28
930
090
002
97 05
7214
0786
0178
0005
0598
0000
4230
0444
0026
0244
0004
1395
-------
-------
01OA41d2
Pargasite
rim
43 05
342
10 71
008
11 46
012
13 50
11 54
179
067
96 34
6353
1647
0216
0380
0368
0009
2967
1046
0015
0512
0127
1824
0613
863
Pargasite
rim
42 61
354
11 59
001
10 59
010
13 52
11 87
160
076
96 19
6286
1714
0302
0392
0299
0001
2972
1007
0012
0458
0143
1877
0715
844
Pargasite
core
42 54
401
11 04
006
11 81
013
13 21
11 50
202
039
96 71
6266
1734
0182
0444
0368
0007
2901
1087
0017
0576
0073
1814
0854
979
Pargasite
rim
43 03
385
10 72
002
11 70
014
13 50
11 95
146
080
97 17
6314
1686
0168
0425
0345
0002
2953
1091
0018
0415
0149
1878
0634
852
Pargasite
rim
42 65
341
10 60
006
11 97
014
13 22
11 64
183
068
96 20
6332
1668
0186
0381
0345
0007
2926
1141
0018
0528
0129
1851
0620
869
Pargasite
core
42 82
367
10 60
007
11 93
012
13 42
11 44
206
047
96 60
6315
1685
0158
0407
0391
0008
2950
1080
0016
0588
0089
1808
0615
899
Pargasite
rim
42 2
362
11 10
007
11 42
016
13 09
11 72
179
072
95 89
6282
1718
0229
0405
0299
0009
2905
1123
0020
0518
0137
1869
0715
875
Pargasite
rim
42 42
380
11 03
006
11 77
012
13 00
11 72
188
069
96 49
6283
1717
0209
0423
0299
0007
2870
1158
0015
0539
0131
1860
0817
928
Pargasite
rim
42 21
364
11 14
006
10 73
011
13 53
11 68
177
072
95 59
6277
1723
0229
0407
0322
0007
2999
1012
0014
0511
0136
1862
0736
883
Pargasite
rim
42 25
376
11 03
008
10 93
015
13 66
11 72
185
064
96 07
6257
1743
0182
0418
0345
0009
3016
1009
0019
0531
0121
1860
0766
912
Pargasite
core
42 17
396
10 91
008
11 10
010
13 57
11 49
209
038
95 85
6253
1747
0159
0442
0368
0010
3000
1009
0013
0602
0073
1826
0841
981
aMajorelem
entco
mpositionsarein
weightpercent-------noplagioclasean
alysed
andnotemperature
calculated
1Typ
eofam
phibolean
alysed
isalso
indicated
2Blankindicates
nozoningobserved
3Molefractionofan
orthitein
plagioclasead
jacentto
amphibolegrain
4Tem
perature
ofeq
uilibrium
fortheam
phibole---plagioclasepaircalculatedusingtheHollandamp
Blundythermometer
(1994)Errors
are40
C
BOSCH et al HYDROTHERMAL CIRCULATION IN OMAN OPHIOLITE
1195
zonation from An95 to An99 from core to rim (Table 1 andFig 8a) TheWR Sr isotopic ratio of the sample (070458)is significantly higher than normal MORB values Thiscould be related to the overprint of a low-temperaturealteration event consistent with the petrography of thesample
Green amphibole vein cross-cutting layered gabbros inWadi Mayah Haylayn Massif
Sample 02 OA43a belongs to a series of 1 cm thick greenamphibole veins (as in Fig 6e) cross-cutting the layeredgabbros The veins develop 15 cm thick reaction marginsin the enclosing gabbro and are exclusively composed ofacicular pale green magnesio-hornblende growing per-pendicular to the vein margin in a crack-seal mode(Fig 6f ) The reaction margins are composed of urali-tized gabbro marked by the development of the samepale green magnesio-hornblende in an inhomogeneouscalcic plagioclase matrix (An91---97) The Sr and O isotopiccompositions of this green hornblende (070431 andthorn31) do not correspond to normal MORB-sourcemantle values
Epidosite dyke from Wadi Gideah Wadi Tayin Massif
Sample 01 OA36b is representative of a series of epidoteveins cross-cutting poorly layered gabbros These 2 cmthick veins are composed of pink thulite in their centreand green pistachite at their margins (Fig 6a) Theirsharp contact with the enclosing gabbro is marked bythe development of coarse clinopyroxene crystals alteredin the epidote---greenschist facies This suggests that theepidote vein is replacive in a coarse-grained gabbro dykeThe highest 87Sr86Sr initial ratios of all the samplesstudied are recorded by the WR and coexisting epidoteof this sample (070519 and 070554 respectively)(Table 5) The Sr content of the WR and associatedepidote are also the highest of the present dataset(716 ppm and 764 ppm respectively) The Sr isotopiccomposition of this sample is consistent with a greaterdegree of exchange with a radiogenic component fromCretaceous seawater It is significantly higher than thevalue obtained for an epidosite vein from the Wadi Fizhsection of the Oman ophiolite [070435 with a Sr contentof 488 ppm reported by Kawahata et al (2001)] but ingood agreement with the average of the greenschist-faciesalteration sequence defined by Kawahata et al (2001)
GABBROS
01 km 025 km 1 km 36 km50
60
70
80
90
An
MA2
19d
19a 15f 128b
41d2
(a)
43a17b
15f
90b
37b
19a
19d
40
60
80
100
A
n
01 km 015 km 025 km 1 km
DYKES
(b)
Distance to the Moho
ACTINOLITE
MAGNESIOHORNBLENDE
TSCHERMAKITE
PARGASITEGabbros Dykes41d2MA2
19a d37b15f
90b43a
AlIV
150500
02
08
10
(Na
+ K
) A
(c)
04
06
950
900
850
800
750
70007 12 17
T degC
1000
(d)
AlIV
Fig 8 Plagioclase---amphibole compositions and results of thermometry (a) and (b) plagioclase compositions in layered gabbros and in dykesrespectively as a function of distance to the Moho (see text for sample identification) (c) amphibole compositions in layered gabbros and dykesaccording to Leakersquos (1997) nomenclature (d) equilibration temperatures calculated using the amphibole---plagioclase thermometer of Holland ampBlundy (1994) as a function of AlIV content in amphibole [same symbols as in (c)]
JOURNAL OF PETROLOGY VOLUME 45 NUMBER 6 JUNE 2004
1196
Table5RbandSrcontents87Sr
86Srandd1
8Oisotopiccompositionsofwholerocksandmineralseparatesfrom
samplesofmassivegabbrosdykes
andveinsfrom
theOman
ophiolitealsolistedarecalculated
waterrockratios(W
R)andLOIvalues
Sam
ple
Rb(ppm)
Sr(ppm)
87Rb8
6Sr
87Sr
86Sr
(87Sr
86Sr)i1
d18O
()
LOI(
)2WR
3(87Sr
86Sr)L14
(87Sr
86Sr)L25
(87Sr
86Sr)R6
01OA41d2
Whole
rock
037
1792
0006
0703516
09
070351
thorn480
138
47
0703587
0703651
0703357
Plagioclase
025
1249
0003
0704261
22
070426
thorn504
-------
-------
Pargasite
-------
-------
-------
0703548
20
070355
-------
-------
-------
Clinopyroxene
045
26 0
0050
0703845
19
070378
-------
-------
-------
01OA36b
Dykewhole
rock
0005
7164
50001
0705186
17
070519
-------
274
21 9
0705207
0705112
-------
Epidote
0003
7645
0001
0705536
12
070554
-------
-------
-------
Wallwhole
rock
005
4254
0001
0704637
09
070464
-------
191
-------
0704945
0704675
0704593
02OA43a
Green
hornblende
004
98
0012
0704323
13
070431
thorn310
-------
-------
97OA128b
Whole
rock
003
1274
50001
0703327
17
070333
-------
076
37
-------
-------
-------
Plagioclase
002
1204
50001
0703285
09
070328
thorn414
-------
-------
Clinopyroxene
003
20 0
0004
0704230
09
070422
thorn523
-------
-------
01OA15f
Whole
rock
006
1077
0002
0704582
16
070458
-------
237
13 5
-------
-------
-------
01OA19a
Whole
rock
037
1303
0008
0704189
18
070418
-------
161
96
-------
-------
-------
01OA19d
Whole
rock
058
2032
0008
0703423
08
070341
thorn567
216
415
0703574
0703408
0703271
Pargasite
091
1058
0025
0703273
11
070324
thorn287
-------
-------
Plagioclase
-------
-------
-------
-------
-------
thorn10 09
-------
-------
02OA90b
Plagioclase
004
2385
50001
0703868
11
070387
thorn498
-------
-------
Green
hornblende
010
23 8
0012
0704288
15
070427
thorn239
-------
-------
BOSCH et al HYDROTHERMAL CIRCULATION IN OMAN OPHIOLITE
1197
Table5continued
Sam
ple
Rb(ppm)
Sr(ppm)
87Rb8
6Sr
87Sr
86Sr
(87Sr
86Sr)i1
d18O
()
LOI(
)2WR
3(87Sr
86Sr)L14
(87Sr
86Sr)L25
(87Sr
86Sr)R6
02OA37b
Plagioclase
023
3121
0002
0704204
15
070420
-------
-------
-------
Pargasite
031
41 6
0021
0703505
15
070348
thorn394
-------
-------
94OAMa2
Whole
rock
0016
1750
00003
0703219
15
070322
thorn495
033
31
0703264
0703177
0703158
Plagioclase
0014
1363
00003
0703219
11
070322
thorn479
-------
-------
Clinopyroxene
-------
-------
-------
0703236
13
070324
-------
-------
-------
1Initial8
7Sr
86Srratiocalculatedat
95Ma(H
acker1994)
2LOIisloss
onignition(in)
3Water---rock
ratiocalculatedassumingaclosedsystem
TheeffectiveWR
ratiocanbeexpressed
bythefollowingeq
uation
W=Reffectivefrac14
frac12eth87Sr=
86SrrockTHORN fi
nal
eth87Sr=
86SrrockTHORN in
itial=frac12eth8
7Sr=
86SrseawaterTHORN in
itial
eth87Sr=
86SrrockTHORN fi
nal
ethCrock
initial=C
seaw
ater
initial
THORNwhere(87Sr
86Srrock) finalisthefinalmeasuredSrisotopicratioin
thesample(87Sr
86Srrock) in
itialco
rrespondingto
theinitialm
antlevalue
istakenas
070295(Lan
phere
etal1981)(87Sr
86Srseawater) in
itialistheCretaceousseaw
ater
Srisotopicco
mposition[87Sr
86Sr07073
at90
MaafterBurkeet
al(1982)]C
seaw
ater
initial
istheSrco
ntent
oflsquonorm
alrsquoseaw
ater
andis
takenas
8ppm
(deVilliers1999)
FortheCrock
initialitis
assumed
that
thepresentSrco
ncentrationmeasuredis
thesameas
theinitial
concentration
4Srisotopicratiosmeasuredforliquid
L1extractedafterthefirststep
oftheleachingexperim
ent(6NHClfor8min
at70
C)
5Srisotopicratiosmeasuredforliquid
L2extractedaftertheseco
ndstep
oftheleachingexperim
ent(2 5NHClfor30
min
at70
C)
6Srisotopicratiosmeasuredforresidueobtained
afteratotalaciddigestionachievedaftertheextractionofL1an
dL2leachates
JOURNAL OF PETROLOGY VOLUME 45 NUMBER 6 JUNE 2004
1198
Part of the same sample from the contact between thedyke and the gabbro yields a slightly lower Sr isotopiccomposition (070464) intermediate between the sur-rounding gabbro and the epidote dyke It also has anintermediate Sr content (425 ppm) Acid leaching experi-ments performed on the whole rock of the dyke yield Srisotopic compositions of 070521 and 070512 for thetwo liquids extracted Similar experiments for the samplelocated at the contact with the gabbro yield 87Sr86Srratios of 070494 and 070467 for L1 and L2 liquids TheSr isotopic composition of the residue extracted afterleaching remains high (070459) and very close to theepidosite 87Sr86Sr ratio determined by Kawahata et al(2001) In this sample the primary minerals have beentotally replaced by epidote and secondary plagioclase orquartz Thus the Sr isotopic compositions measured forthis WR sample and its constituent minerals reflect thecomposition of the fluid filling this dyke
Mineral chemistry
Plagioclase
Compared with the host gabbros the dykes show a muchlarger dispersion in their plagioclase compositions (Fig 8aand b Table 4) characterized by more calcic plagioclasethan the enclosing gabbros This tendency is mostextreme in the comb-textured dykes in which the plagio-clase is almost pure anorthite The zoning is generallynormal recording a crystal fractionation process How-ever gabbro 94 MA2 exhibits inverse zoning this tend-ency is also observed in plagioclases from dykes 01OA19a and 01 OA15f The higher calcium content ofthe plagioclase as well as the inverse zoning is consistentwith an increase of water pressure during its crystalliza-tion (Turner amp Verhoogen 1960 Carmichael et al 1974Koepke et al 2003) The particularly high An content inthe comb-textured gabbro dykes 01 OA15f may alsoresult from crystallization in a supercooled magma thisis consistent with the comb texture indicative of fast-growing crystals The lack of brown hornblende suggeststhat crystallization occurred above the temperature ofhornblende stability
Amphibole
The amphiboles analysed in the dykes (Fig 8c andTable 4) show a regular trend in a (Na thorn K)A vsAlIV diagram from titanium-rich (Ti 4 015) pargasitichornblende (brownamphibole) and tschermakite (brown---green amphibole) to low-titanium (Ti5 015) magnesio-hornblende (green amphibole) and actinolite (pale greenamphibole) The pargasitic hornblende develops aspoikilitic oikocrysts in the dykes 01 OA19a and d andin the diffuse patch 01 OA41d2 at temperatures above800C during the final stage of crystallization (see
below) The tschermakite composition corresponds tothe internal alteration of clinopyroxene in gabbro 94Ma2 occurs in the comb-textured dyke 01 OA15f andis the primary amphibole in the dioritic dyke 02 OA37bThe magnesio-hornblende develops at the edges of thepargasites or of clinopyroxenes at temperatures around800C and is the primary phase in the dioritic dyke 02OA90b and in the green vein 02 OA43a Finally actino-lite occurs as a replacement of green hornblende in dykesor in the kelyphitic rim surrounding olivine in gabbro 94Ma2 Such a large composition range at the scale of athin section has been reported in amphiboles fromamphibolite-facies oceanic gabbros (Stakes 1991 Stakesamp Taylor 1992 Gillis et al 1993) as well as in the Omanophiolite gabbros (Manning et al 2000) This variabilityhas been explained by rapid reaction rates preventinghomogenization (Manning et al 2000)
Thermometry
Plagioclase---amphibole thermometry could be appliedwhen the two minerals exhibited good contacts such asin the laminated gabbro of the middle sequence (01OA41d2) and in the intrusive dykes The temperaturesof plagioclase---amphibole equilibration (Table 2) havebeen calculated using the geothermometer lsquoBrsquo of Hollandamp Blundy (1994) Edenite thorn Albite frac14 Richterite thornAnorthite valid between 500C and 900C with anuncertainty of 40C Amphibole formulae are basedon 23 oxygen and their nomenclature is based on alkalications in the A site vs AlIV according to Leake (1997)Pressurewas assumed to be 1 kbar Forty-five plagioclase---amphibole pairs meet the compositional criteria imposedby Holland amp Blundyrsquos dataset (09 4 Xan 4 01 andAlIV 403) Electron microprobe measurements wereconstrained to contiguous plagioclase and amphiboleseparated by sharp grain boundaries assuming equili-brium of the two phases When the mineral phasesexhibit normal zoning temperatures between plagioclaseand amphibole cores and between mineral rims werecalculated assuming that the temperatures deduced fromthe mineral core chemistry provide a proxy for the high-est temperature of equilibration These data are identi-fied in the T C vs AlIV diagram (Fig 8d) Table 4presents selected major element amphibole analyses theAn content of the plagioclase adjacent to each amphibolegrain and the temperature calculated for the pairThe temperatures calculated (Fig 8d) span the entire
range of temperatures at which amphibole and plagio-clase coexist in equilibrium (ie amphibolite-facies para-geneses) This temperature interval is bracketed byorthopyroxene-bearing facies or amphibole-free facies(ie gabbro 94MA2) and epidote---actinolite facies devoidof homogeneous plagioclase (ie epidosite dyke 01OA36b and green vein 02 OA43a) The range of the
BOSCH et al HYDROTHERMAL CIRCULATION IN OMAN OPHIOLITE
1199
calculated temperatures between 4900C and 750Ccorresponds to the changing composition of amphibolesfrom pargasite to magnesio-hornblende at the scale of athin section or even at that of an individual crystal asshown by their correlation with AlIV such variabilityrecords rapid cooling in hydrous conditions The highesttemperatures are recorded by pargasite from the anatec-tic gabbro 01 OA41d2 in the gabbro dyke 01 OA19dand in the dioritic dyke 02 OA90b They were calculatedfrom minerals devoid of lower-grade alteration and arethus considered as representing the equilibrium temperat-ure of the coexisting minerals Lower temperatures cor-respond to mineral phases evolving at falling temperatureand are only indicative of the cooling history Includingthe 40C uncertainty seven pairs provide temperatureshigher than 900C the limit of validity of the Holland ampBlundy thermometer However we maintain these tem-peratures as indicative of the highest values of our data-set These values are included in the average equilibriumtemperature summarized in Table 2
DISCUSSION
Distribution of hydrothermal alteration
Two distinct petrological events are recorded in the gab-bro section of the Samail ophiolite and are documentedin the present contribution (1) the whole gabbro sectionis affected by a penetrative alteration (2) the gabbrosection is crosscut by various types of dykes and veinsincluding hydrous minerals
Penetrative alteration
The penetrative alteration developed in the gabbros atgrain boundaries and locally invading the minerals isubiquitous in the gabbro section This penetrative altera-tion has been described as very high temperature (VHT)high temperature (HT) and low temperature (LT) basedon alteration parageneses The orthopyroxene coronas inthe layered gabbro sample and the incipient appearanceof brown pargasitic amphibole mark a lower thermallimit of the VHT alteration By reference to the experi-mental work of Koepke et al (2003) the temperature ofequilibration for these reactions is estimated to bebetween 900 and 1000CThe preservation of the vertical distribution of
VHT---HT facies (Figs 4 and 5) also pointed out byManning et al (2000) indicates that the hydrothermalalteration pattern fits the distribution of isotherms withdepth at a fast-spreading ridge (ie Phipps Morgan ampChen 1993 Dunn et al 2000 Fig 2) Equilibrium tem-peratures for VHT parageneses as high as 900---1000Csuggest that VHT---HT hydrothermal alteration tookplace in a very restricted time interval at the limit ofthe cooling magma chamber It is proposed that the
penetrative alteration affecting the entire gabbro sectionis related to a pervasive network of microcracks whichact as a recharge system down to the Moho (Nicolas et al2003)
Dykes and veins
The dykes are extremely varied eg pegmatitic gabbroswith comb-texture microgabbros and amphibolitizeddolerites dioritic dykes alignments of poikilitic horn-blende and finally diffuse hornblende-bearing patchesHigh-T gabbro and diorite dykes and low-T greenamphibole---epidote veins are connectedIn the dykes textural evidence attests to suprasolidus
conditions at least for the development of the pargasiticamphibole Integrating the hornblende---plagioclase equi-libration temperatures calculated for the studied samplesthese temperatures are bracketed between 950C and875CIn the following discussion we shall consider separately
the microgabbro and dolerite dykes The latter representbasaltic melt intrusions associated upsection with thediabase sheeted dykes and downsection possibly withthe mafic dykes that are ubiquitous in the mantle section(Nicolas et al 2000) Whatever their origin the develop-ment of pargasitic amphibole during their late stage ofcrystallization creates a link with the gabbroic dykes andsuggests a crystallization process evolving from dry con-ditions to more hydrous conditionsThe other gabbroic dykes comb-textured gabbros and
hornblende-bearing dioritic dykes result from the fillingof cracks by a fluid either a melt or a silica-rich hydrousfluid These intrusive dykes which develop reaction mar-gins at their contact with the host gabbro grade verticallyinto lower-temperature green amphibole veins or align-ments of poikilitic hornblende in the layered gabbromatrix or development of pargasitic hornblende in ana-tectic gabbros (sample 01 OA41d2) In gabbros crystal-lizing in the magma chamber the development oforthopyroxene and pargasite oikocrysts enclosing the pri-mary minerals suggests a thermal evolution from thegabbro solidus to amphibolite-facies conditions Forthese reasons it is suggested that the gabbroic dykesresulted from the injection of a hydrous melt in the stillhot gabbros drifting away from the magma chamber asdiscussed below
Origin of fluids responsible for VHT---HThydrothermal alteration
The 87Sr86Sr initial ratios and d18O of whole rocks andpure mineral fractions are plotted in Fig 9 against thedistance above the Moho Transition Zone (MTZ) Theinitial Sr isotopic composition of the whole rocks apartfrom the epidosite dykes ranges from 070322 to
JOURNAL OF PETROLOGY VOLUME 45 NUMBER 6 JUNE 2004
1200
070458 All the studied samples (including whole rocksand minerals) have initial 87Sr86Sr ratios falling outsidethe field of fresh MORB which commonly have Sr ratiosbetween 07023 and 07029 with an average of 070265(ie Hart et al 1974) The lowest initial Sr isotopiccomposition (070322) from the massive gabbro 94MA2 is slightly but significantly higher than averageOman primary magmatic signatures (ie 070296McCulloch et al 1981) Similarly the d18O values ofmost whole rocks and minerals measured (Table 5)
depart from typical MORB-source mantle values( Javoy 1980 Gregory amp Taylor 1981 Kyser 1986)These differences indicate that the primary mantlesignatures in the Oman ophiolite have been modifiedby interaction with a fluid Possible origins for thisfluid are mantle components dehydration of subductedlithosphere or seawater circulation Strontium andoxygen isotopes are useful tracers for fluid---crust interac-tion as d18O and 87Sr86Sr ratios from fluids originat-ing from seawater the mantle or subducted lithosphere
-1000
1000
2000
3000
4000
5000
6000
7000
MOHO
MTZ
Dis
tan
ce a
bov
e th
e M
oh
o (
m)
pillow-basalt
sheeted-dike
gabbro
peridotite
01 OA 41d2
01 OA 36b
02 OA 43a97 OA 128b
01 OA15f
94 OA MA2
02 OA 90b01 OA 19ad
02 OA 37b
0702 0703 0704 0705 0706 0707
87Sr86Srinitial
MO
RB-s
ou
rce
Seaw
ater
Whole RockPlagioclasePargasiteGreen HornblendeClinopyroxene
Epidote
(a)
-1000
1000
2000
3000
4000
5000
6000
7000
MOHO
MTZ
Dis
tan
ce a
bov
e th
e M
oh
o (
m)
pillow-basalt
sheeted-dike
gabbro
peridotite
01 OA 41d2
01 OA 36b
02 OA 43a97 OA 128b
01 OA15f
94 OA MA2
02 OA 90b01 OA 19ad
02 OA 37b
Whole RockPlagioclasePargasiteGreen HornblendeClinopyroxene
3 5 10
δ18O (permil)
41d2
Ma2
43a
90b 19d 19d37b
128b
90b
(b)
Fig 9 (a) Variation of initial 87Sr86Sr isotopic composition as a function of the location of the studied samples in a schematic log of the crustalsection of the Oman ophiolite The field for MORB is from Hart et al (1974) and that for seawater is from Burke et al (1982) Small open squares aredata fromKawahata et al (2001) (b) Variation of d18O () as a function of the location of the studied samples in a schematic log of the crustal sectionof the Oman ophiolite Shaded curve corresponds to the data of Gregory amp Taylor (1981)
BOSCH et al HYDROTHERMAL CIRCULATION IN OMAN OPHIOLITE
1201
are significantly different Moreover the d18O valueis dependent on temperature and mineral fractiona-tion and thus yields complementary constraints to Srisotopes
Mantle origin
One model to explain the occurrence of fluids interactingwith the oceanic crust at very high temperatures is toderive them from the mantle The contribution of mantle-derived hydrous fluids linked to percolation processesor to their concentration in the final stages of gabbrocrystallization has been invoked in numerous case studies(ie Witt amp Seck 1989 Fabriees et al 1989 Dupuy et al1991 Agrinier et al 1993) O and Sr isotopic data forwhole rocks and mineral separates yield scattered valuesthat are unambiguously different from MORB mantle-source values (Fig 9a and b) With the exception of lateLT altered samples the WR data have a mean 87Sr86Srratio of 070337 and a standard deviation of 000012These surprisingly high values in mantle-derived rockscould be taken as evidence for survival of mantle isotopicheterogeneities during crystallization of the ophiolite (egLanphere 1981 McCulloch et al 1981) However the18O16O isotope ratios in the Oman gabbroic sequenceare also displaced from primary magmatic values Nogabbro in the present study has plagioclase or amphi-bole with typical mantle d18O values Thus the Sr and Oisotope data rule out the possibility of mantle-derivedfluids as a possible source for the VHT---HT hydrousalteration event recorded in the massive gabbros andgabbroic dykes and veins (Fig 10a and b)
Subducted lithosphere origin
Dehydration of subducted oceanic crust in subductionzones can generate hydrous fluids Such fluids couldpercolate through the overlying lithosphere and contam-inate it thus generating modified isotopic signatures andtrace element contents Such an explanation howeverdisagrees with the geochemical characteristics of most ofthe studied samples Fluids derived from the dehydrationof subducted slabs (fresh or altered MORB or OIB pro-toliths) should be strongly enriched in K Rb and Ba (egBecker et al 2000) whereas in Oman the Rb contentsmeasured in the gabbros and gabbroic dykes and veins arestrongly depleted Moreover the isotopic composition ofslab-derived fluids is buffered by the MORB-like oceaniccrustal protolith for Nd and Pb but not for Sr and O(Staudigel et al 1995) As a result of relatively minorfractionation of oxygen at high temperature the oxygenisotope composition of the fluids expelled during dehy-dration of the subducted slab should be high and essen-tially controlled by the oxygen composition of the uppersection of the crust altered at low temperature (with an
average of 10 and perhaps as high as 19) (Staudigelet al 1995) The Sr isotopic composition should be alsohigh (average 07046) as the fluids released by the dehy-dration of subducted oceanic crust are associated eitherwith seawater or with radiogenic Sr phases (ie smectites)The Sr and O isotopic compositions of the studied sam-ples do not fit with these signatures and so preclude asubducted lithosphere origin for the fluids involved in theVHT---HT alteration event
Seawater-derived fluids
All the analysed samples (minerals and WR) have 87Sr86Sr(T) outside the domain of primary MORB magmasand variable Sr contents Individual minerals have Srcontents reflecting their different mineralliquid partitioncoefficients Minerals characterized by a very high affi-nity for Sr such as plagioclase (ie Sr mineralliquidpartition coefficients 1) have very high Sr contentsTherefore the high abundance of plagioclase in the sam-ples analysed is responsible for the high Sr content of thestudied whole rocks The Sr content of all the whole rocksis relatively homogeneous (Table 5) without clear distinc-tion between country-rock gabbros and dykes and veinsand ranges from 110 to 200 ppm except for the epidosite(4700 ppm) Reported on a 87Sr86Sr vs 1Sr diagram(Fig 10a) most whole rocks and plagioclases analysed arecharacterized by a 1Sr ratio lower than 001 and plot inthe left part of this diagram Compared with N-MORBthe studied massive and anatectic gabbros and their con-stituent plagioclases have similar to slightly higher Srcontents but substantially higher 87Sr86Sr ratios(Fig 10a) Considering Cretaceous seawater as a possiblehigh 87Sr86Sr mixing component [87Sr86Sr 07073at 90Ma after Burke et al (1982)] responsible for theobserved geochemical modifications exchanges cannothave occurred in a closed system as seawater has too lowa Sr content (ie 8 ppm de Villiers 1999) An open-system process allowing penetration of larger quantitiesof seawater can efficiently modify the Sr isotopic ratio ofthe rocks Alternately the fluids could also be seawaterthat was modified through exchange with the surround-ing rocks during its transit through the oceanic crustalsection This modified seawater would be more enrichedin Srwith a 87Sr86Sr ratio intermediate between unmodi-fied seawater and MORBMinerals devoid of late LT alteration imprints (parga-
site green hornblende and pyroxene) and characterizedby very low Sr contents plot on the right of the diagramclose to or on the mixing line between seawater andMORB (Fig 10a) The difference between these mineralsand the other samples (WR and plagioclases) is mainlydue to the Sr fractionation coefficients of these differentminerals and to the large quantity of plagioclase inthe studied rocks The process responsible for the
JOURNAL OF PETROLOGY VOLUME 45 NUMBER 6 JUNE 2004
1202
modification of the Sr isotopic composition fromMORB-source value is however explained by the same phenom-enon As all these minerals display initial 87Sr86Sr ratiosdistinct from the MORB source and because they equili-brated at VHT or HT temperatures this supports anearly intervention of seawater during crystallization ofthese minerals Most of these samples with the exceptionof the epidosite dyke overlap the domains defined byKawahata et al (2001) for the Wadi Fizh gabbros alteredat VHT and HT conditions in the amphibolite facies(labelled lsquoVHTrsquo and lsquoHTrsquo in Fig 9a)Three samples (two epidosite dykes and an one epidote
fraction) have very high Sr contents and the highest Sr
isotopic compositions of the present dataset The twowhole-rock samples overlap the field of the Wadi Fizhsamples altered at high temperature during greenschist-facies metamorphism (Kawahata et al 2001) This typeof alteration is common at the ridge axis and formedthrough intensive exchange with seawater-derived fluidsFigure 10b indicates that all the Sr---O isotope data are
distinct from MORB compositions and suggests that thefluids reacting with the magmas could be derived fromseawater In addition the d18O values show that isotopicexchange occurred under VHT or HT conditions Thefluids could have the composition of seawater or theycould have the composition of seawater modified through
0703
0705
0707
001 01 1
1 Sr (ppm)
MORB
(87Sr
86Sr)
init
ial
Seawater
VHT
HT
MT36b
36b
36b
43a 128b90b
41d2
37b19d
15f
128b
41d2
41d2
19a
Ma2Ma2
19d
90b
37b
Whole RockPlagioclasePargasiteGreen HornblendeClinopyroxene
Epidote
(a)
0 2 4 6 8
δ18O(permil)
0703
0705
0707Seawater
MORB-mantle
128b 41d2
90b 41d2
Ma2128b
37b
43a
19d
90b
Whole RockPlagioclasePargasiteGreen HornblendeClinopyroxene
Epidote
HT LT
19d
(87Sr
86Sr)
init
ial
(b)
Fig 10 (a) Initial 87Sr86Sr isotopic ratios plotted against 1Sr for whole rocks and mineral separates from the gabbro section of the Omanophiolite Fields shown are from Kawahata et al (2001) and correspond to MT-----basalt sheeted dyke diabase altered at medium temperature(prehnite---actinolite facies sequence 3) HT-----diabase plagiogranite metagabbro and epidosite formed at high temperature (greenschist faciessequence 4) VHT-----gabbro altered at very high temperature (amphibolite facies sequence 5) The dashed line is a binary mixing line betweenMORB (Lanphere et al 1981) and Cretaceous seawater (Burke et al 1982) (b) Initial 87Sr86Sr isotopic composition plotted against d18O value forwhole rocks and minerals from the Oman ophiolite Cretaceous seawater and MORB end-members as in (a) Dashed line represents mixing curvebetweenMORB and seawater at high temperature (HT) Low-temperature (LT) exchanges between these two components would be responsible foran increase of the d18O primary MORB value
BOSCH et al HYDROTHERMAL CIRCULATION IN OMAN OPHIOLITE
1203
exchange with the surrounding rocks during its penetra-tion into the crustal section In an environment in whichfluid---rock interaction occurs at variable temperaturesthe d18O is greatly influenced by the temperature atwhich the exchange took place and by the waterrockratio It is evident from Figs 9b and 10b that none of thestudied gabbros have plagioclase and amphibole withMORB-like d18O and 87Sr86Sr values The separateminerals display a broad range of d18O values and mostexhibit extreme 18O depletion (Table 5) Hydrothermalexchange kinetics are similar in amphibole and pyroxeneand these two minerals are more resistant to oxygenexchange during water---rock interaction than coexistingplagioclase (Gregory et al 1989) The most probableexplanation of the low d18O values measured in bothamphiboles and pyroxenes is that they crystallized athigh temperature in the presence of a relatively low 18Oseawater-derived hydrothermal fluid [d18O from 01 to07 after Gregory amp Taylor (1981)] The anomalouslyhigh d18O value (thorn1009) measured for the plagioclase ofthe micro-gabbroic dyke (01 OA19d) can be interpretedas resulting from a superimposed overprint caused by alate-stage penetration of fluids at lower temperature Inthis sample the occurrence of such different 18O16Oratios between coexisting plagioclase and amphibole isnot consistent with equilibrium fractionation betweenthese two minerals at any possible temperature Thisobservation suggests that parts of the ophiolite sequencehave undergone a high-temperature hydrothermal eventprior to the later low-temperature event that producedthe strong 18O increase in coexisting plagioclaseThe depletion of the d18O values results from high-temperature hydrothermal alteration (Gregory amp Taylor1981 Stakes amp Taylor 1992) The oxygen isotopic datasupport the contention that most studied samples havebeen affected by a VHT---HT hydrothermal alteration byfluids derived from seawater Some of the analysed sam-ples presenting petrological evidence of crystallization ofLT secondary minerals have been overprinted by the latelow-temperature alteration thus swamping the earlierhigh-temperature alteration effects
Determination of waterrock ratio
To better evaluate the exchange between seawater andthe gabbroic rocks waterrock ratios (WR) have beenestimated for the whole rocks analysed during the courseof this study The different models used to constrain thisratio mainly differ by the calculation of the effective ratioor by the estimated weight ratio and by considering openor closed systems for water circulation (Albareede et al1981 McCulloch et al 1981) The WR ratios reportedin Table 5 have been calculated assuming a closed sys-tem Using this formulation these values are minimabecause the calculation always uses pristine fluid for the
initial fluid thus maximizing the value in the denomina-tor If the fluid was shifted to lower 87Sr concentrations byexchange with the rock on the downward path theinferred values would be much higher (Davis et al 2003)The calculated WR ratios for the samples from this
study range from 31 to 219 (Table 5) Two main groupsof samples can be recognized on the basis of their waterrock ratios The lowest ratios ranging from 31 to 47have been determined for massive gabbros or anatecticgabbros but also for dykes and veins devoid of late LTalteration Similar ratios have been previously calculatedfor example for the discharged hydrothermal solutions atthe 13N and 21N East Pacific Rise (Di Donato et al1990 Fouquet et al 1991) The gabbros from the Ibrasection in the Oman ophiolite gave effective WR ratiosof 05 33 and 42 (McCulloch et al 1981) in goodagreement with the values obtained in this study Theleast altered gabbro of the Ibra section yields a WR ratioof 05 in good agreement with the exceptionally fresh-ness of this rock characterized by typical mantle oxygenvalues for its minerals (McCulloch et al 1981) Samplescharacterized by waterrock effective ratios in the rangeof 3---5 can be related to penetrative alteration via thehydrothermal system Lecuyer amp Reynard (1996) havedemonstrated in oceanic gabbros from the Hess Deepaltered in similar HT conditions that WR mass ratios inthe range of 02---1 are sufficient to account for the mod-ified stable isotope signature of the gabbros Higherwaterrock values (WR 45) have been calculated forsamples affected by superimposed late hydrothermalalteration and for the epidosite samples (Table 5) Theyprobably acquired their isotopic characteristics throughinteraction with a larger volume of seawater duringgreenschist-facies metamorphism Similar or higherWR values have been published in previous studies(ie McCulloch et al 1981 Kawahata et al 2001)They correspond either to samples from the upper-crus-tal sequence (ie basalt sheeted dyke or plagiogranite) orto gabbros from the crustal section located in a particularenvironment such as for example the Wadi Fizh sectionthat is located close to a segment boundary along thepalaeo-spreading axis (Nicolas et al 2000)
Mechanism for seawater interaction withmagma
Nicolas et al (2003) have proposed that variable amountsof seawater can circulate at high temperature through-out the crustal section down to the walls of the magmachamber via a network of parallel microcracks a fractionof a millimetre wide Such microcracks and their relation-ship with the VHT---HT hydrous alteration in the sur-rounding gabbros have been described in detail byNicolas et al Via this network of microcracks largeamounts of seawater can have reacted with the magma
JOURNAL OF PETROLOGY VOLUME 45 NUMBER 6 JUNE 2004
1204
and induced variable changes of its Sr and O isotopiccomposition This regular network of parallel micro-cracks could constitute the recharge system and thehydrated gabbroic dykes the discharge system Physicalparameters and the geophysical models of Nicolas et al(2003) are in agreement with this scenario Such a pene-tration of seawater-derived hydrothermal fluids throughthe lower crust along grain boundaries and a microscopicfracture network at temperatures 4750C is consistentwith recent thermodynamic models (McCollom amp Shock1998)Seawater-altered and high-temperature recrystallized
diabases (protodykes) from the overlying sheeted dykecomplex stoped into the melt lenses could represent anindirect way for seawater to enter the system as they canremelt during their subsidence through the magmachamber A simple mass balance calculation suggeststhat this indirect contamination is not significant Assess-ments of the amount of recycled crust 1 in volumeby Nicolas et al (2000) based on the protodyke remnantsin the gabbro section or 20 by Coogan et al (2003)based on chlorine content in EPR basalts lead to a sea-waterrock ratio of the order of 104 to 103Thus following Nicolas et al (2003) we propose that
seawater has been introduced along microcracks down tothe walls of the magma chamber and has diffused alonggrain boundaries in the way described by Manning et al(2000) progressively invading the host gabbro Gabbroicdykes result from the injection of hydrous melt into thehost gabbros at falling temperatures as they drift awayfrom the magma chamber This water-saturated meltcould result from the extensive hydration of the crystal-lizing gabbros near the walls of the magma chamberwhen seawater penetrates through this wall (Fig 1)Similar plumbing systems driving seawater down to the
limit of the magma chamber and back to the surface havebeen already proposed in other environments such as theMid-Atlantic Ridge (Kelley amp Delaney 1987 Cooganet al 2001) the East Pacific Rise at Hess Deep (Gillis1995 Manning et al 1996a 1996b) the Tonga forearc(Banerjee amp Gillis 2001) and the Troodos ophiolite(Gillis amp Roberts 1999) The melting curve of wet basalthas a negative slope but is close to the dry solidus at lowpressure in the lower gabbros and the first melts areplagiogranitic (Spulber ampRutherford 1983) Such plagio-granites are ubiquitous in the highest gabbros and in theroot zone of sheeted dykes in Oman (Rothery 1983Juteau et al 1988 Nicolas amp Boudier 1991) in manyother ophiolites and in the oceanic crust where they havebeen interpreted as resulting either from wet anatexis ofhot gabbros or from the final product of crystallization ofa basaltic melt (Spulber amp Rutherford 1983) Plagio-granites are rare in the lower gabbros exceptionallyassociated with hydrous gabbro segregations inside thelayered gabbros Their constitutive minerals are also
remarkably absent along the VHTmicrocracks althoughthese gabbros were above the wet solidus A possibleexplanation has been proposed by McCollom amp Shock(1998) who wrote that at high temperature lsquoaqueousfluids may leave very little conspicuous petrological evid-ence for their presencersquo the reason being that thehigh-T conditions would be closer to batch meltingObviously more detailed studies on this specific problemare needed We conclude that the large degree of hydrousmelting locally required at the walls of the magma cham-ber to account for the generation of a gabbroic hydrousmelt may have erased the traces of the incipient plagio-granitic melt
CONCLUSIONS
Fostered by the recent discovery of high-temperatureseawater contamination in some gabbros of the Omanophiolite (Manning et al 2000) we have conducted astudy on 500 gabbro specimens from selected areas ofthe entire Samail ophiolite belt and on the hydrous gab-broic dykes that cross-cut them The highest-temperaturereactions (VHT) recorded in olivine and clinopyroxene ingabbros as orthopyroxene and pargasite coronas reach975C They are followed at lower HT conditions withreplacement of olivine by an assemblage of tremolitemagnetite and plagioclase surrounded by chlorite andby pargasite development in clinopyroxene followedfinally by the LT lizardite---olivine and saussurite---plagioclase greenschist-facies alteration With a fewexceptions the VHT alteration tends to be restricted tothe lower gabbros and to the vicinity of the Moho andthe HT alteration to the middle and upper gabbros Thepreservation of the vertical distribution of VHT---HTfacies indicates that progressive cooling during off-axisspreading did not obliterate this distribution and conse-quently that the VHT---HT hydrothermal alteration tookplace in a very restricted time interval at the limit of thecooling magma chamber In the deep and hot gabbrosthe main water channels may be sub-millimetre-sizedmicrocracks with a dominantly vertical attitude the ori-gin and dynamics of which have been investigated in aprevious paper (Nicolas et al 2003)Beyond these high-temperature alteration zones mas-
sively induced in the gabbros seawater may be respons-ible for the generation of clinopyroxene---pargasitegabbro dykes that cross-cut all gabbros and that areupsection clearly connected to the LT hydrothermalvein system Petrostructural evidence suggests that thesedykes were generated by hydration of the crystallizinggabbros near the internal wall of the LVZ---magmachamber The resultant melts were subsequently intrudedat various levels in the cooling lower and middle gabbrosPetrological observations of nearly all the gabbros ana-lysed in the present study located from the root zone of
BOSCH et al HYDROTHERMAL CIRCULATION IN OMAN OPHIOLITE
1205
the sheeted dyke complex down to the Moho emphasizethe imprint of VHT---HT hydrous alteration The VHThydrothermal event is characterized by temperatures ofequilibration around 900---1000C The temperatures atwhich the HT hydrous event occurred are estimated tobe in the range 550---900CStrontium and oxygen isotope compositions measured
on whole rocks and separated minerals significantlydepart from primary MORB values The Sr and O iso-tope data obtained for pargasite green hornblende andclinopyroxene record the participation of seawater at thetemperature of crystallization of the minerals Plagio-clases and whole rocks define a trend toward a compo-nent characterized by a high radiogenic 87Sr86Sr valueand a higher Sr content than normal seawater Seawateris clearly identified as the fluid responsible for the modi-fication of the Sr and O signatures for both massivegabbros and dykes and veins Nevertheless the high Srcontent of the extraneous component requires eitheropen-system exchange allowing penetration of a greaterquantity of seawater or penetration of seawater modifiedby interaction with the oceanic crust during its travel paththrough the crustal section The waterrock ratio calcu-lated on the basis of the Sr isotopes is between three andfive for the enclosing gabbros and for the least LT altereddykes The VHT---HT hydrothermal event affectingthese samples is related to penetrative alteration via therecharge and discharge hydrothermal system Thewaterrock ratio is 10 for dykes exhibiting evidenceof a LT hydrothermal alteration overprint and reaches22 for the epidosite dyke The depletion in d18O char-acterizes all the studied samples with the single exceptionof the micro-gabbroic dyke plagioclase This agreeswith the conclusions based on Sr isotopes suggestingthat these samples have undergone exchange with sea-water-derived fluids during a VHT---HTalteration hydro-thermal event
ACKNOWLEDGEMENTS
This work has benefited from financial support by theCentre National de la Recherche Scientifique (INSUInteerieur-Terre andDYETI programmes) and inOmanfrom the willingness of the Directorate of MineralsMinistry of Commerce and Industry and the hospitabil-ity of the people Technical help in the laboratory is alsoacknowledged including C Nevado for the thin sectionsE Ball for the illustrations B Galland for her assistancein the chemistry room and C Bassoulet for help duringthermal ionization mass spectrometric analyses of the Srresults from the leaching experiments Constructivereviews from R T Gregory C Lecuyer an anonymousreviewer and particularly from M Wilson greatlyhelped to improve an earlier version of the manuscript
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Kyser T K (1986) Stable isotope variations in the mantle In
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Lanphere M A Coleman R G amp Hopson C A (1981) Sr isotopic
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Liou J G Kuniiyoshi S amp Ito K (1974) Experimental study of the
phase relations between greenschist and amphibolite in a basaltic
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sheeted dyke complex Geological Society of America Special Papers 60
39---63
Manning C E MacLeod C J amp Weston P E (1996a) Fracture-
controlled metamorphism of Hess Deep gabbros site 984
constraints on the roots of mid-ocean ridge hydrothermal systems
at fast-spreading centers In GillisM C Allan KM ampMeyer P S
(eds) Proceedings of the Ocean Drilling Program Scientific Results 147
College Station TX Ocean Drilling Program pp 189---211Manning C E Weston P E amp Mahon K I (1996b) Rapid high-
temperature metamorphism of East Pacific Rise gabbros from Hess
Deep Earth and Planetary Science Letters 144 123---132Manning C E MacLeod C J amp Weston P E (2000) Lower-crustal
cracking front at fast-spreading ridges evidence from the East Pacific
Rise and the Oman ophiolite Journal of the Geological Society London
349 261---272McCollom T M amp Shock E L (1998) Fluid---rock interactions in
the lower oceanic crust thermodynamic models of hydrothermal
alteration Journal of Geophysical Research 103 547---575
McCulloch M T Gregory R T Wasserburg G J amp Taylor H P
(1981) Sm---Nd Rb---Sr and 18O16O isotopic systematics in an
oceanic crustal section evidence from the Samail ophiolite Journal of
Geophysical Research 86(B4) 2721---2735Mevel C Gillis K M Allan J F amp Meyer P S (eds) (1996)
Proceedings of the Ocean Drilling Program Scientific Results 147 College
Station TX Ocean Drilling Program
Nehlig P amp Juteau T (1988) Flow porosities permeabilities and
preliminary data on fluid inclusions and fossil thermal gradients in
the crustal sequence of the Sumail ophiolite (Oman) Tectonophysics
151 199---221
Nehlig P Juteau T Bendel V amp Cotten J (1994) The root zone of
oceanic hydrothermal systems constraints from the Samail ophiolite
(Oman) Journal of Geophysical Research 99 4703---4713
Nicolas A amp Boudier F (1991) Rooting of the sheeted dyke complex
in Oman ophiolite In Peters Tj Nicolas A amp Coleman R (eds)
Ophiolite Genesis and Evolution of the Oceanic Lithosphere Dordrecht
Kluwer Academic pp 39---54
BOSCH et al HYDROTHERMAL CIRCULATION IN OMAN OPHIOLITE
1207
Nicolas A amp Ildefonse B (1996) Flow mechanism and viscosity in
basaltic magma chambers Geophysical Research Letters 23(16)
2013---2016
Nicolas A Ceuleneer G Boudier F amp Misseri M (1988) Structural
mapping in the Oman ophiolites mantle diapirism along an ocean
ridge Tectonophysics 151 27---56
Nicolas A Boudier F Ildefonse B amp Ball E (2000) Accretion
of Oman and United Arab Emirates ophiolite-----discussion of a
new structural map Marine Geophysical Research 21(3---4)147---179
Nicolas A Boudier F Michibayashi K amp Gerbert-Gaillard L
(2001) Aswad massif (United Arab Emirates) archetype of the
Oman---UAE ophiolite belt Geological Society of America Bulletin 349
499---451
Nicolas A Mainprice D amp Boudier F (2003) High temperature
seawater circulation through the lower crust of ocean-ridges-----amodel derived from the Oman ophiolites Journal of Geophysical
Research on line first httpwwwaguorgpubscurrentjb 108(B8)
2372
Pallister J S amp Hopson C A (1981) Samail ophiolite plutonic suite
field relations phase variation cryptic variation and layering and a
model of a spreading ridge magma chamber Journal of Geophysical
Research 86 2593---2644Phipps Morgan J amp Chen J Y (1993) Dependence of ridge-axis
morphology on magma supply and spreading rate Nature 364
706---708
Pin C Briot D Bassin C amp Poitrasson F (1994) Concomitant
extraction of Sr and Sm---Nd for isotopic analysis in silicate samples
based on specific extraction chromatography Analytica Chimica Acta
298 209---217
Rothery D A (1983) The base of a sheeted dyke complex Oman
ophiolite implications for magma chambers at oceanic spreading
axes Journal of the Geological Society London 140 287---296
Spear F S (1981) An experimental study of hornblende stability and
compositional variability in amphibolite American Journal of Science
281 697---734
Spulber S D amp Rutherford M J (1983) The origin of rhyolite and
plagiogranite in oceanic crust an experimental study Journal of
Petrology 24 1---25Stakes D S (1991) Oxygen and hydrogen isotope compositions of
oceanic plutonic rocks high-temperature deformation and meta-
morphism of oceanic layer 3 In Taylor H P OrsquoNeil J et al (eds)
Geochemical Society Special Publication 3 77---90
Stakes D S amp Taylor H P (1992) The northern Samail ophiolite an
oxygen isotope microprobe and field study Journal of Geophysical
Research 97(B5) 7043---7080Staudigel H Davies G R Hart S R Marchant M amp Smith B
M (1995) Large scale isotopic Sr Nd and O isotopic anatomy of
altered oceanic crust DSDPODP sites 417418 Earth and Planetary
Science Letters 130 169---185Turner F J amp Verhoogen J (1960) Igneous and Metamorphic Petrology
New York McGraw---Hill 694 pp
Wilcock W S D amp Delaney J R (1996) Mid-ocean ridge
sulfide deposits evidence for heat extraction from magma
chambers or cracking fronts Earth and Planetary Science Letters 145
49---64
Witt G amp Seck H A (1989) Origin of amphibole in recrystallized
and porphyroclastic mantle xenoliths from Rhenish massif implica-
tions for the nature of mantle metasomatism Earth and Planetary
Science Letters 91 327---340
JOURNAL OF PETROLOGY VOLUME 45 NUMBER 6 JUNE 2004
1208
a given alteration facies as defined previously out of thetotal number of thin sections examined for a given levelMore refined conclusions can be drawn from this figureAs seen in Fig 5a the HT alteration decreases down-
wards whereas it affects most of the upper gabbroicsection (sheeted dyke root zone and upper gabbros)The fraction of unaltered olivine cores can be used asan index to estimate the extent of alteration This indexshows that the HT alteration is more penetrative andintense in the upper gabbros a conclusion alreadyreached by several workers (eg Gregory amp Taylor1981 Stakes amp Taylor 1992 Kawahata et al 2001)This was also noted by Manning et al (1996a 1996b)Figure 5b shows that the VHT alteration marked by
orthopyroxene or brown hornblende coronas aroundolivine increases with depth The correlation betweenthe VHT alteration and plastic deformation also suggeststhat the VHT alteration may coincide with the develop-ment of plastic flow at the expense of the magmatic flow
at temperatures just below the solidus In Fig 5b thefraction of gabbros where there is no VHT---HThydrothermal alteration increases downsection indicat-ing that the VHT---HT alteration is more localizeddownsection
Gabbroic dykes and veins
The occurrence of gabbroic dykes in the layered gabbrosof the Samail ophiolite is ubiquitous We describe gab-broic dykes and sills either plagioclase---clinopyroxeneand brown amphibole-bearing dykes or microgabbrosand amphibolitized dolerite dykes and sills correspond-ing to temperatures up to 975C (see below) Identifica-tion of gabbroic dykes in the field is not alwaysstraightforward because they may be obliterated bysuperimposition of greenschist-facies alteration (Fig 6a)Selective greenschist-facies alteration has frequently beenobserved either in the centre of a mafic dyke or alongboth margins (Fig 6b) as well as a continuous transition
(b) (c)(a)
(d) (e)
thulite
pistacite
white microvein
white vein
green vein
green vein
gabbroic dike
epidote vein
gabbroic dike
Fig 6 Field relationships of dykes and veins (scale bar represents 10 cm) (a) Thulite---pistacite vein cross-cutting a foliated gabbro matrix Reactionmargins marked by the development of secondary clinopyroxene (01 OA36b) (b) Comb-like structure in gabbroic dyke A pink epidote vein followsthe margin of the dyke (01 OA15f) (c) Along-strike transition from a gabbroic dykelet to a hydrothermal green amphibole vein (d) White prehnitevein (e) Green amphibole vein cross-cut by white prehnite vein
BOSCH et al HYDROTHERMAL CIRCULATION IN OMAN OPHIOLITE
1187
along a single crack from a mafic dyke to a hydrothermalvein (Fig 6c) We define hydrothermal veins as cracksfilled with greenschist-facies minerals that induce meta-morphic reactions in the adjacent country rocks andgabbroic dykes as planar intrusive bodies produced bythe crystallization of magma Following Nehlig amp Juteau(1988) we distinguish low-T greenschist-facies lsquowhiteveinsrsquo made of prehnite epidote chlorite and actinolite(Fig 6d) and lsquogreen veinsrsquo in which a darker greenamphibole predominates (Fig 6e) corresponding respec-tively to lower (195---410C) and higher (400---530C)temperatures (Nehlig amp Juteau 1988)The gabbroic dykes are typically a few centimetres
across composed of plagioclase clinopyroxene andbrown
hornblende They are coarse-grained to pegmatitic andcommonly display a comb-like structure (Fig 6b)Although the spacing between the dykes in the field ishighly variable the average is around 10m Gabbroicsills grade from clear intrusions to irregular and diffusepegmatitic gabbro patches up to a few metres in theirlarger dimension (Fig 7a)The distribution in the field of brownish pargasite
which appears black and glassy in outcrop is critical intracing the origin of the gabbroic dykes It is first observedat any level in the host lower and middle gabbros as largepoikilitic patches (Fig 7b) locally associated with simil-arly poikilitic patches of clino- and orthopyroxene(Fig 7a) These patches contain small aligned tablets of
(a) (b) (c)
g(e)(d)
alignment ofalignment of amphiboleamphibole oikocrystsoikocrysts
hwhite vein
hblpl
pcpx
brownbrownamphiboleamphiboleoikocrystoikocryst
eediabasedikdike
hb-plhb-plreactionreaction
imarginspbrown amphibole rich
gabbroic dikegabbroic dike
pbrown amphibole veins
Fig 7 Field relationships of dykes (scale bar is 10 cm long) (a) Irregular coarse to pegmatitic patches in a foliated gabbro matrix (01 OA42d)(b) Alignment of black amphibole oikocrysts in a foliated gabbro matrix (c) Local concentration of brown amphibole in a gabbroic dykelet Theplagioclase laths marking the foliation in the enclosing gabbro rotate into parallelism with the gabbroic dykelet (d) Two diabase dykes assimilatingthe enclosing layered gabbro with plagioclase---amphibole reaction margins (e) Pargasite-rich gabbroic dyke with wall reactions (01 OA19) (Notesmall pargasite---plagioclase veins) pl plagioclase cpx clinopyroxene hb hornblende
JOURNAL OF PETROLOGY VOLUME 45 NUMBER 6 JUNE 2004
1188
plagioclase oriented parallel to the larger plagioclaselaths defining the magmatic foliation of the gabbro(Fig 7c) Clinopyroxene oikocrysts surrounding idio-morphic plagioclase crystals are common in the upper-level gabbros they mark the end of the magmatic flowand crystallization growing before the pargasite andorthopyroxene oikocrysts as shown by their partialreplacement by pargasite Their crystallization marksthe transition at suprasolidus conditions from a dry to awet environment In the uppermost gabbros those dykesthat are typically altered in the HT hydrothermal faciesgrade upwards into the so-called lsquoisotropicrsquo or lsquovari-texturedrsquo gabbrosThe microgabbro and amphibolitized dolerite dykes
differ from the typical diabase dykes commonly cuttingthe gabbro section that have chilled margins and thusprobably intruded enclosing rocks already at temperat-ures below 450C In microgabbro and amphibolitizeddolerite dykes the absence of chilled margins and thedevelopment of dioritic reaction margins in the adjacentgabbro (Fig 7d and e) suggest that they were emplaced ingabbros that were still at temperatures above 750C(Nicolas et al 2000)
ANALYTICAL TECHNIQUES
Major and trace elements
Major element compositions were determined by elec-tron microprobe at the University of Montpellier II usinga Camebax SX100 Instrument calibration was per-formed on natural standards and operating conditionswere 20 kV accelerating potential 10 nA beam currentand 30 s counting timeRb and Sr concentrations in whole rock and separated
mineral fractions were measured by conventionalnebulization inductively coupled plasma mass spectro-metry (ICP-MS) using a VG Plasmaquad 2 (ISTEEMMontpellier) Digestion procedures followed thoseoutlined by Ionov et al (1992)
O---Sr isotopes
The samples selected for the detailed study wereextracted from gabbroic dykes or veins inside gabbrosand cut with a diamond saw into 2---3 cm fragmentsbefore being crushed into 05---1 cm fragments Forwhole-rock analysis small pieces were further crushedinto powder in an agate bowl For pure mineral analysisthe fragments were crushed with a manual agate mortarTo avoid or minimize the mixing of selected separateminerals with other phases that can bias the results astrict protocol of mineral selection was applied A smallsize fraction of 125---160 mm was used and minerals werefirst separated using a Frantz isodynamic magneticseparator Concentrates recovered (cpx plagioclase
amphibole epidote) were subsequently purified by care-ful hand-picking in alcohol under a binocular micro-scope At this stage mineral separates were very pureand only flawless minerals free of cracks and visibleinclusions were kept for isotopic analyses To ensurefurther mineral integrity these pure mineral separateswere ultrasonically washed in dilute 15N HCl and rinsedseveral times with ultra-pure water This stage potentiallyremoves adsorbed secondary micro-phases Previousstudies (eg Lecuyer amp Reynard 1996) demonstratedthat pyroxene and amphibole can be intermixed on avery fine scale with other phases (such as a range ofamphibole compositions talc Fe-oxide) Although thispossibility cannot be ruled out the procedure used inthis study makes it unlikely that complex minerals passedthrough the selection
Sr isotopic compositions
Approximately 50mg of whole-rock powder or separatedminerals were digested in a Teflon lsquobombrsquo with a mixtureof triple distilled 13N HNO3 and 48 HF with a fewdrops of HClO4 Two aliquots were separated one forthe determination of Sr isotopic composition and theother for Sr and Rb contents Sr was separated using anextraction chromatographic method modified from Pinet al (1994) Concentrations were measured by isotopedilution using 84Sr---87Rb mixed spikes To remove tracesof the late low-temperature seawater alteration and todetermine the primitive whole-rock Sr isotopic composi-tion leaching experiments were conducted on a fewsamples following the procedure described by Bosch(1991) The strontium isotopic compositions of leachateshave been analysed in the attempt to check for the leach-ing efficiency During these tests four Sr isotope composi-tions were measured which correspond to the unleachedsample the two liquids extracted after acid-leaching stepsand the final solid residue Leaching experiments wereconducted in two steps first with 6N HCl for 8min at70C second with 25N HCl for 30min at 70C Afterleaching the leachates were evaporated to dryness andthe solid residues digested similarly to the unleachedsamplesSr isotopic compositions were measured on a fully
automated VG Sector mass spectrometer housed at theUniversity of Montpellier II except for the Sr isotopicdata from the leaching experiments which have beenmeasured at the Universitee de Bretagne Occidentale(Brest) To assess the reproducibility and accuracy of Srisotopic measurements the NBS 987 standard was run12 times during this study the average value was0710245 with a precision better than 0000010 (2s)87Sr86Sr ratios are corrected for mass discrimination bynormalizing to a 86Sr88Sr value of 01194 Sr total blankconcentrations were less than 60 pg Initial 87Sr86Sr
BOSCH et al HYDROTHERMAL CIRCULATION IN OMAN OPHIOLITE
1189
ratios were calculated by correcting the measured ratiosfor accumulation of 87Sr produced by in situ 87Rb radio-active decay since 95Ma the assumed age of crystalliza-tion of the Oman ophiolite (Hacker 1994)
Oxygen isotopes
The powdered samples were introduced in Ni tubesunder dry air where they were reacted with BrF5 toform oxygen Oxygen was separated from other gasesby cooling the resulting mixture at liquid nitrogen tem-perature (77 K) that retained other volatile gases Oxygenwas further purified by trapping it on a cooled 5A mole-cular sieve at 77 K then quantified to calculate the yieldof the isotopic analyses and finally transferred to the massspectrometer (Finnigan Mat Delta E) where intensitiesassociated with masses 32 and 34 were measured todetermine d18O The isotopic composition of a sampleis given as dsample frac14 (RsampleRstandard 1) 1000 in permil unit where R is 18O16O and the standard isSMOW Analytical errors on the oxygen isotope ratioare 02 (2s)
RESULTS
Seven samples representative of the range of gabbroicdykes and five samples representative of the lower gab-bros two of them representing the margins of gabbroicdykes were selected for major element thermometricand Sr---O isotopic analyses (Tables 1---3) These samplesoriginate from wadi sections marked by bold lines inFig 1 Wadis Halhal Mayah Al Abyad Warihya andMaqsad structurally belong to a new ridge segment open-ing in slightly (1Myr) older lithosphere Wadi Gideahis in older lithosphere
Petrographic and isotopic characteristicsof the analysed samples
Olivine gabbro from the MTZ in MaqsadSamail Massif
Sample 94 MA2 is devoid of low-temperature alterationand only exhibits the discrete signature of the VHT---HTalteration The orthopyroxene corona around olivine(Fig 3a) is relayed by kelyphitic rims at the contactwith plagioclase (Fig 3b) Issuing from these rims aremicrometre-sized pale green amphiboles identified ascummingtonite and actinolite which penetrate the sur-rounding plagioclase in microcracks 004mm wide or atgrain boundaries (Fig 3b) similar to the microcrack sys-tem described by Manning et al (2000) A large numberof the plagioclase crystals have a cloudy core as a result ofthe concentration of fluid and mineral inclusions amongwhich diopside has been identified during microprobeanalysis Olivine cores are free of serpentine but theyare largely oxidized along a microcrack network Ortho-pyroxene coronas have a MgOpx number of 78---79
slightly higher than in the primary olivine cores withMgOl number of 74---75 The magmatic clinopyroxeneis totally fresh and probably not in equilibrium with thereactive orthopyroxene This sample has the lowest 87Sr86Sr initial ratio of the studied whole rocks (070322)identical to coexisting clinopyroxene (070324) and pla-gioclase (070322) This observation suggests the lack of alate superimposed LT hydrothermal alteration as it isvery unlikely that an LT hydrothermal event could havetotally equilibrated the Sr isotopic compositions of prim-ary minerals such as plagioclase and clinopyroxeneMoreover the whole rock (WR) clinopyroxene andplagioclase have similar initial Sr isotopic ratios despitevery different Sr contents Accordingly this ratio isthought to reflect the signature acquired during the crys-tallization of these minerals and is higher than the Omanmantle Sr isotopic value (McCulloch et al 1981) Thed18O values of the WR (50) and plagioclase (48) aredistinctly lower than primary magmatic values Indeedin normal mid-ocean ridge basalts (MORB) the WRd18O value is 57 02 with a corresponding plagio-clase d18O ranging from 55 to 62 ( Javoy 1980Gregory amp Taylor 1981)
Lower gabbro from Wadi Wariyah Wadi Tayin Massif
Sample 97 OA128b belongs to a layered sequence in thelower gabbro series injected by anorthositic sills Thesample was collected at the tip of one of these sillsbetween layers enriched in clinopyroxene The gabbrois composed of 60 plagioclase and 40 clinopyroxeneDespite the absence of olivine the WR has a relativelyhigh Mg content (Table 3) A series of low-T (prehnite)microveins 01mm thick cross-cut the gabbro perpendi-cular to the layering and foliation The margins of theveins are devoid of alteration This gabbro has the sameSr isotopic composition as 94 MA2 Nevertheless theWR and associated plagioclase have Sr isotope signatures(070333 and 070328) lower than the coexisting clino-pyroxene (070422) The Sr contents of the WR plagio-clase and clinopyroxene are 127 ppm 120 ppm and20 ppm respectively thus making the clinopyroxenemore sensitive to late-stage contamination Clinopyrox-ene and plagioclase oxygen isotope compositions (523and 414 respectively) are also lower than typicalmantle values Similar to the previous sample alterationat high temperature by a low d18O and high 87Sr86Srfluid is required to explain such characteristics
Amphibole gabbro patch Wadi Gideah Wadi TayinMassif
Sample 01 OA41d2 is from a laminated gabbro com-posed of millimetre-sized layers of clinopyroxeneand plagioclase Anatectic patches (Fig 7a) borderinganorthositic layers are formed of coarse-grained
JOURNAL OF PETROLOGY VOLUME 45 NUMBER 6 JUNE 2004
1190
Table1Locationoftheanalysed
samplesandsummaryoftheirpetrographicandSr---O
isotopiccharacteristics
Sam
ple
nam
eSite
Locationin
gab
broic
section
Typ
eofrock
Distance
to
Moho(km)
Parag
enesis
Alteration
87Sr
86Srinitial1
d18O
()2
94MA2
Maq
sad
From
MTZ
Layered
gab
bro
0Olivine(M
gno74---75)3
OPXco
ronas
(Mgno78---79)
WRfrac14
070322
WRfrac14
thorn495
CPX
Palegreen
Am
(cumact)
CPXfrac14
070324
nd
Plagioclase(A
n65---80)4inversezoning
Nolow-T
alteration
Plfrac14
070322
Plfrac14
thorn479
Solidfluid
inclusionsin
Pl
97OA128b
Wad
iWaryiah
From
lower
gab
bro
Layered
gab
bro
1CPX
Low-T
alteration
WRfrac14
070333
nd
Plagioclase(A
n85---87)
Prehnitemicroveins
CPXfrac14
070422
CPXfrac14
thorn523
Plfrac14
070328
Plfrac14
thorn414
01OA41d2
Wad
iGideah
From
upper
gab
bro
Gab
bro
patch
in
laminated
gab
bro
36
CPXrecrystallized
Plagioclaseidiomorphic
(An57---87)norm
alzoning
Black
pargasitic
honblende
WRfrac14
070351
CPXfrac14
070378
Plfrac14
070426
WRfrac14
thorn480
nd
Plfrac14
thorn504
Hbdfrac14
070355
nd
01OA19aamp
dWad
iWaryiah
Inlayeredgab
bro
Gab
broic
dyke
01
Olivine
OPXpoikilitic
WRfrac14
070341
WRfrac14
thorn567
CPX
Black
pargasite
Hbdfrac14
070324
Hbdfrac14
thorn287
Plagioclase(A
n73---95)
Hornblendepoikilitic
Plfrac14
nd
Plfrac14
thorn10 09
Hem
atite
WRfrac14
070418
nd
Green
Mg-hornblende
Acthornblende
Epidote
02OA90b
Wad
iAl-Abyad
Inlayeredgab
bro
Dioriticdyke
015
Green
Mg-hornblende
Hbdfrac14
070427
Hbdfrac14
thorn239
Plagioclase(A
n83---87)
Plfrac14
070387
Plfrac14
thorn498
Solidfluid
inclusionsin
plagioclase
01OA15f
Wad
iWaryiah
Inlayeredgab
bro
Comb-textured
gab
broic
dyke
025
CPXidiomorphic
Plagioclase(A
n95---99)norm
alzoning
BrownMg-H
ornblende
WRfrac14
070458
nd
02OA43a
Wad
iMayah
Inlayeredgab
bro
vein
Green
amphibole
1Palegreen
Mg-hornblendeacicular
Plagioclase(A
n91---97)in
reactionmargin
Hbdfrac14
070431
Hbdfrac14
thorn310
01OA36b
Wad
iGideah
Inlayeredgab
bro
epidosite
dyke
25
CPX
Epidote
WRfrac14
070519
nd
Plagioclase
Epfrac14
070554
nd
CPXclinopyroxene
Plplagioclase
OPXorthopyroxene
Amam
phibolecu
mcu
mmingtonite
actactinolite
WRwholerockHbdhornblende
Epep
idotend
notdetermined
187Sr
86Srinitialratioscalculatedat
95Ma(H
acker1994)Errors
ofthemeasuredratiosareindicated
inTab
le4
2Analyticalerrors
oftheoxygen
isotopevalues
are02
(2s)
3Mgnumber
frac14atomicMg(Mgthorn
Fe)
4Percentageofan
orthitein
plagioclase
BOSCH et al HYDROTHERMAL CIRCULATION IN OMAN OPHIOLITE
1191
(millimetre-sized) recrystallized clinopyroxene largelyaltered to light green magnesio-hornblende and idio-morphic plagioclase Locally primary black pargasitichornblende develops including idiomorphic normallyzoned plagioclase (Table 4) This gabbro exhibits similar87Sr86Sr ratios for the WR and pargasite (070351 and070355 respectively) These values are interpreted asrepresentative of the melt from which this amphibolecrystallized The clinopyroxene has a slightly higher Srisotope composition of 070378 consistent with the occur-rence of the light green Mg-hornblende rim Plagioclaseis significantly more radiogenic (070426) related to sec-ondary destabilization evidenced by the idiomorphichabit The Sr isotope compositions of the liquidsextracted from the acid leaching experiments of theWR (L1 and L2) are very similar 070358 and 070365respectively The 87Sr86Sr ratio of the residue obtainedafter leaching experiments is 070335 lower than boththe pargasite and unleached WR This value can beconsidered as the unaltered isotopic composition of thissample The d18O values measured for WR and coexist-ing plagioclase are lower than normal MORB magmaticvalues The 18O depletion in the plagioclase can be fairlywell explained by high-temperature hydrothermal inter-action between an oxygen-depleted fluid and the magmaAccording to the kinetics of hydrothermal exchange ofoxygen (Gregory amp Taylor 1981 Gregory et al 1989)plagioclase exchanges its oxygen isotopes with the fluidadjusting readily to the hydrothermal conditions Forhigh temperatures of exchange with seawater-derivedfluids (d18O between 0 and thorn2) the magmatic plagio-clase will have its primary d18O value lowered whereasthe d18O value for lower temperatures of exchange(5300C) will increase The amplification of the d18Oshift increases with the intensity of the hydrothermalalteration
Gabbroic dykes intrusive in lower gabbros from WadiWariyah Wadi Tayin Massif
Gabbroic dykes 01 OA19a and 01 OA19d are intrusiveinto the lower gabbros They are 3---4 cm wide discord-ant to the foliation of the host gabbros (Fig 7e) with asharp margin marked by a brown amphibole fringe Theprimary phases in the dykes (Table 1) locally preservedas elongated aggregates of a 01mm grain size mosaicassemblage at the dyke margins grade to millimetre sizein the central part of the dyke The WR major elementcomposition is Mg enriched compared with the enclosinggabbro (Table 3) Brown pargasitic amphibole oikocrystsmake up 50 of the modal composition of the dyke Indyke 01 OA19a poikilitic orthopyroxene may developinstead of brown amphibole Finally green magnesio-hornblende grows either as oikocrysts at the expense ofclinopyroxene or in association with epidote as an altera-tion product of plagioclase These low-amphibolite-faciesminerals represent the lowest-grade paragenesis recogn-ized in these dykes and are most developed in sample01OA19aTheamphibole composition showszoning fromTi-rich pargasitic hornblende to magnesio-hornblendeand actinolitic hornblende (Fig 8c) recording pro-gressive cooling of the dyke The plagioclase compositionis extremely heterogeneous (Fig 8b) varying from An95a composition similar to that of the plagioclase in the hostgabbro (Fig 8a) to An50 for the plagioclase and asso-ciated epidote inclusions in the poikilitic actinolitic horn-blende Sample 01 OA19d has an initial 87Sr86Sr ratioslightly higher for the WR (070341) than for the parga-site (070324) The Sr isotopic ratios of liquids extractedafter two steps of WR acid-leaching (L1 and L2) are070357 and 070341 respectively whereas the residueyields a 87Sr86Sr ratio of 070327 very close to thepargasite and plagioclase values The latter is similar tothe isotopic composition measured for the massive gab-bro 94 MA2 and may be taken as close to the primarymagmatic value This Sr isotopic ratio is more radiogenicthan the inferred Oman mantle-source value (McCullochet al 1981) The altered part of this sample has a radio-genic Sr isotopic composition (070418) related to thelow-amphibolite-facies alteration This gabbroic dykehas seemingly preserved a magmatic whole-rock d18Ovalue (567) but contains by far the highestd18O plagioclase (1009) of all the samples analysedThis plagioclase coexists with low d18O pargasite(thorn287) Similarly high values (d18Oplagioclase 4 8)have been previously measured for gabbroic dykes dia-bases sheeted dykes and plagiogranites from the Omanophiolite (Gregory amp Taylor 1981 Stakes amp Taylor1992) This 72 oxygen isotope fractionation betweenplagioclase and amphibole cannot possibly represent oxy-gen equilibrium at any plausible temperature This maybe explained by different exchange kinetics for these twominerals at different temperatures at high waterrock
Table 2 Average temperatures ( C 40C)
calculated for plagioclase---amphibole pairs
using the geothermometer of Holland amp
Blundy (1994)
Pargasite Magnesio-hornblende
rim core rim core
01 OA41d2 878 953 ------- -------
01 OA19a 935 974 825 869
01 OA19d 890 933 789 -------
02 OA37b ------- ------- 816 849
02 OA90b ------- ------- 859 833
JOURNAL OF PETROLOGY VOLUME 45 NUMBER 6 JUNE 2004
1192
ratios This supports the occurrence of two distincthydrothermal events one at high temperature and alater stage at lower temperature responsible for thestrong 18O enrichment in the coexisting plagioclase
Dioritic dyke associated with a diabase dyke in lowergabbros from Wadi Halhal Haylayn Massif
Sample 02 OA37b belongs to a group of brownamphibole---plagioclase lenses or reaction margins asso-ciated with diabase intrusives (Fig 7d) in lower layeredgabbros Idiomorphic brown hornblende (tschermakite)crystals are elongated in the plane of the dyke represent-ing 80 of the modal composition They are associatedwith an interstitial plagioclase matrix The browntschermakite has greenish rims Plagioclase has a cloudyappearance due to micrometre-size fluid inclusionsexcept along its margins and internal microcracks Bothhornblende and plagioclase exhibit evidence of plasticdeformation The Sr isotopic ratio for the pargasite andplagioclase is 070348 and 070420 respectively Thepargasite exhibits a Sr signature closest to that of theOman primary magmatic value (McCulloch et al 1981)Nevertheless it is too high to be attributed to anunmodified MORB-source mantle signature This sug-gests the addition of an external fluid component in goodagreement with the shift of oxygen isotopic compositionto a low value (thorn39) suggesting a high-temperatureexchange with a low d18O fluid The Sr isotope composi-tion of the plagioclase is significantly higher than that of
the coexisting pargasite probably as a result of the low-temperature alteration
Dioritic dyke cross-cutting lower layered gabbros in WadiAl-Abyad Nakhl Massif
Sample 02 OA90b is a 1 cm thick dioritic dyke cross-cutting the lower layered gabbros It grades into a greenamphibole vein as illustrated in Fig 6c The dyke is com-posed of 2 cm long dark green idiomorphic magnesio-hornblende crystals growing in the plane of the dykeand plagioclase concentrated at the margin of the dykeThe plagioclase in the dyke and in the enclosing gabbrocontains micrometre-size fluid and mineral inclusionssimilar to those observed in sample 94 MA2 The plagio-clase and green hornblende have higher Sr isotopic ratios(070387 and 070427 respectively) and lower d18Ovalues (thorn498 and thorn239 respectively) than the normalMORB-source mantle value As observed for previoussamples this supports interaction with an external fluidcomponent
Comb-textured gabbroic dykes in the lower gabbros fromWadi Wariyah Wadi Tayin Massif
Sample 01 OA15f (Fig 6b) is from Wadi Wariyah andrepresents a 2 cm wide dyke intruding the lower gabbrosThis type of comb-textured dyke developed in theclinopyroxene---plagioclase stability field It containslarge idiomorphic clinopyroxene crystals which arepartially replaced by brownish magnesio-hornblendeThe plagioclase is extremely calcic with a slight inverse
Table 3 Whole-rock major element compositions of massive gabbros and dykes determined by X-ray fluorescence
(wt )
Sample 94 Ma2 97 OA128b 01 OA41d2 01 OA19d 01 OA19a 01 OA15f 01 OA36b 01 OA36b
Rock type Gabbro Gabbro Gabbro patch Gabbroic dyke Gabbroic dyke Comb-textured
gabb dyke
Epidosite dyke Epidosite dyke
(margin)
SiO2 4830 4815 4749 4524 4339 4799 389 4371
Al2O3 2785 2032 2438 1474 1246 1192 2752 1646
Fe2O3 328 266 404 981 1204 592 381 623
MnO 003 004 005 014 016 006 008 014
MgO 405 737 427 1031 1686 1326 187 739
CaO 1289 1908 1477 1235 1156 166 2471 2339
Na2O 292 123 257 258 111 075 000 013
K2O 000 000 009 006 000 000 000 000
TiO2 005 017 063 230 064 086 013 042
P2O5 008 007 013 018 009 013 007 010
LOI 033 076 138 216 161 237 274 190
Total 9978 9985 998 9987 9992 9986 9983 9987
LOI loss on ignition gabb gabbroic
BOSCH et al HYDROTHERMAL CIRCULATION IN OMAN OPHIOLITE
1193
Table4Majorelementcomposition
aofselectedam
phibolespaired
withplagioclasecompositionsusedforthermometry
Sam
ple
nam
e1Mode2
SiO
2TiO
2Al 2O3
Cr 2O3
FeO
MnO
MgO
CaO
Na 2O
K2O
Total
Si
AlIV
AlVI
Ti
Fe3
thornCr
Mg
Fe2
thornMn
Na
KCa
XAn3
T(C)4
01OA19a
Mg-hornblende
core
45 60
26
10 78
009
860
015
16 56
12 00
212
007
96 72
6552
1448
0366
0083
0552
0004
3409
0594
0016
0611
0018
1843
0903
899
Mg-hornblende
core
47 99
263
850
002
897
011
16 61
12 10
160
020
96 36
6894
1106
0334
0028
0483
0002
3557
0594
0014
0446
0036
1862
0892
834
Mg-hornblende
rim
51 17
247
596
001
761
014
18 11
12 19
103
010
96 47
7257
0743
0253
0016
0414
0001
3828
0488
0016
0284
0018
1853
0894
820
Mg-hornblende
rim
47 50
291
920
004
880
014
16 46
11 95
174
008
96 07
6835
1165
0396
0015
0506
0005
3529
0552
0017
0486
0015
1843
0887
825
Mg-hornblende
rim
49 57
269
742
000
799
017
18 07
12 02
133
014
96 88
7013
0987
0261
0018
0575
0000
3811
0371
0020
0365
0026
1821
0860
831
Mg-hornblende
core
47 83
181
872
003
840
011
14 48
12 18
159
008
93 59
6814
1186
0279
0018
0644
0004
3711
0357
0013
0438
0015
1859
0907
874
Pargasite
rim
42 15
249
11 87
024
10 07
013
13 84
11 43
263
032
96 61
6188
1812
0242
0434
0299
0028
3028
0937
0016
0749
0059
1797
0804
972
Pargasite
rim
42 22
260
11 86
026
10 21
011
13 71
11 48
253
032
96 59
6201
1799
0254
0429
0299
0030
3001
0955
0014
0720
0060
1807
0803
962
Pargasite
core
42 05
263
11 45
030
10 19
011
13 80
11 47
246
032
96 46
6189
1811
0175
0478
0299
0035
3029
0956
0014
0702
0060
1809
0767
974
Pargasite
rim
42 36
247
11 90
016
945
013
14 16
11 76
249
026
95 98
6239
1761
0305
0366
0276
0019
3109
0888
0016
0710
0049
1856
0818
926
Pargasite
core
42 50
291
11 64
016
984
015
14 22
11 41
256
034
96 69
6219
1781
0227
0426
0322
0018
3103
0883
0018
0727
0063
1789
0812
974
Pargasite
rim
42 76
270
12 03
017
920
012
14 87
11 82
244
025
96 73
6224
1776
0880
0335
0368
0020
3226
0752
0015
0689
0047
1843
0841
941
Pargasite
rim
42 13
181
13 54
016
921
009
12 77
12 13
254
032
96 98
6172
1828
0510
0450
0000
0018
2788
1129
0011
0721
0059
1905
0841
874
01OA19d
Mg-hornblende
49 26
047
624
011
11 46
017
15 39
12 21
108
009
96 48
7137
0863
0202
0051
0437
0013
3323
0951
0021
0302
0018
1896
0768
771
Mg-hornblende
46 03
287
932
018
906
017
14 77
12 87
193
007
97 28
6672
1328
0265
0313
0046
0021
3191
1053
0021
0542
0012
1999
0758
808
Pargasite
41 89
429
11 65
008
11 03
017
13 52
11 37
259
017
96 77
6159
1841
0179
0474
0345
0010
2963
1012
0021
0739
0032
1791
0770
975
Pargasite
rim
42 80
340
11 16
009
11 01
018
13 73
11 67
240
022
96 67
6293
1707
0226
0376
0322
0010
3009
1032
0022
0684
0041
1839
0605
880
Pargasite
core
42 58
351
11 27
006
11 63
018
13 51
11 46
239
022
96 82
6257
1743
0209
0387
0391
0007
2959
1039
0023
0680
0042
1804
0595
893
Pargasite
rim
42 14
393
11 28
006
11 71
014
13 35
11 33
235
033
96 62
6214
1786
0174
0435
0391
0007
2935
1054
0018
0673
0062
1790
0519
901
Pargasite
core
41 56
420
12 05
006
11 57
016
12 51
11 35
249
027
96 24
6168
1832
0277
0469
0276
0007
2768
1160
0021
0718
0052
1805
0537
882
Pargasite
core
41 33
438
11 49
008
11 66
015
13 35
11 45
264
026
96 80
6105
1894
0107
0487
0368
0009
2941
1073
0019
0757
0048
1813
0537
942
Pargasite
41 08
446
12 39
012
11 35
017
13 13
11 36
263
015
96 85
6049
1951
0199
0494
0368
0014
2882
1030
0021
0752
0027
1792
0758
975
02OA37b
Mg-hornblende
rim
45 15
260
984
002
13 28
019
14 01
10 87
128
009
97 33
6527
1473
0203
0283
0690
0002
3018
0915
0023
0360
0016
1683
0562
828
Mg-hornblende
rim
44 72
263
988
004
13 23
021
14 01
10 78
133
009
96 92
6494
1506
0185
0287
0713
0005
3032
0894
0025
0374
0018
1677
0593
854
Mg-hornblende
rim
45 68
247
935
004
12 87
020
14 10
11 00
121
011
97 02
6621
1379
0218
0269
0598
0004
3045
0962
0024
0341
0020
1708
0562
815
Mg-hornblende
core
43 94
291
10 87
001
12 60
023
13 73
10 59
142
011
96 39
6410
1590
0278
0319
0644
0001
2985
0893
0028
0401
0021
1655
0710
860
Mg-hornblende
core
44 62
270
10 37
001
12 66
019
14 12
10 58
140
008
96 73
6479
1521
0253
0294
0644
0001
3057
0893
0023
0395
0014
1646
0674
851
Mg-hornblende
rim
47 79
181
796
002
12 09
025
15 22
11 09
100
009
97 33
6866
1134
0214
0196
0506
0002
3260
0947
0030
0280
0016
1706
0503
767
Mg-hornblende
core
45 43
249
10 3
000
11 99
022
14 50
11 14
133
008
97 48
6524
1476
0267
0269
0644
0000
3103
0796
0027
0370
0015
1715
0703
838
JOURNAL OF PETROLOGY VOLUME 45 NUMBER 6 JUNE 2004
1194
Sam
ple
nam
e1Mode2
SiO
2TiO
2Al 2O3
Cr 2O3
FeO
MnO
MgO
CaO
Na 2O
K2O
Total
Si
AlIV
AlVI
Ti
Fe3
thornCr
Mg
Fe2
thornMn
Na
KCa
XAn3
T(C)4
02OA90b
Mg-hornblende
core
48 83
038
745
005
939
010
17 29
12 24
142
006
97 21
6931
1069
0041
0041
0667
0005
3657
0448
0011
0392
0010
1862
0840
856
Mg-hornblende
core
49 49
033
731
003
916
008
17 40
12 57
139
005
97 82
6987
1013
0035
0035
0552
0004
3662
0530
0010
0379
0009
1901
0830
811
Mg-hornblende
rim
48 90
040
753
005
932
013
17 15
12 22
154
005
97 29
6948
1052
0042
0042
0575
0006
3632
0533
0015
0424
0010
1861
0880
869
Mg-hornblende
rim
48 30
044
800
007
962
010
16 86
12 32
161
005
97 35
6873
1127
0047
0047
0598
0008
3576
0546
0012
0443
0010
1879
0870
862
Mg-hornblende
rim
49 12
032
742
010
922
011
17 36
12 29
142
006
97 43
6956
1044
0034
0034
0621
0011
3665
0471
0013
0391
0010
1865
0840
845
02OA43a
Mg-hornblende
rim
48 13
040
789
007
860
007
17 41
12 19
155
006
96 36
6882
1118
0212
0043
0621
0008
3710
0407
0008
0429
0010
1867
-------
-------
Mg-hornblende
rim
50 93
035
643
001
759
006
18 50
12 24
114
003
97 28
7152
0848
0217
0037
0506
0001
3873
0385
0007
0311
0005
1841
-------
-------
Mg-hornblende
rim
50 67
014
559
009
12 33
026
15 28
12 44
068
003
97 50
7260
0740
0204
015
0483
0010
3263
0994
0032
0189
0005
1909
-------
-------
Mg-hornblende
rim
50 34
013
621
009
12 49
027
15 07
12 51
064
003
97 78
7192
0810
0240
0010
0530
0010
3210
0960
0030
0180
0010
1910
-------
-------
Mg-hornblende
rim
45 17
060
11 14
013
988
008
15 78
12 33
211
009
97 31
6473
1527
0355
0640
0644
0015
3371
0540
0010
0588
0016
1892
-------
-------
Mg-hornblende
rim
49 25
032
712
014
845
009
17 84
12 32
137
004
96 95
6983
1020
0170
0030
0620
0020
3770
0380
0010
0380
0010
1870
-------
-------
Mg-hornblende
rim
48 61
032
775
014
975
011
16 81
12 57
139
004
97 50
6905
1950
0203
0034
0621
0016
3560
0537
0013
0382
0008
1914
-------
-------
94Ma2
Anthophyllite
54 98
000
259
001
12 65
037
23 35
199
034
000
96 28
7744
0256
0174
0000
0161
0001
4903
1330
0043
0092
0001
0301
-------
-------
Anthophyllite
55 11
000
296
003
14 09
032
23 66
063
038
000
97 18
7716
0284
0205
0000
0115
0003
4938
1535
0038
0104
0000
0095
-------
-------
Actinolite
48 58
004
954
002
960
014
17 67
999
139
005
97 02
6850
0150
0435
0004
0690
0002
3713
0442
0017
0379
0009
1510
-------
-------
Actinolite
51 54
004
585
000
890
022
20 28
930
090
002
97 05
7214
0786
0178
0005
0598
0000
4230
0444
0026
0244
0004
1395
-------
-------
01OA41d2
Pargasite
rim
43 05
342
10 71
008
11 46
012
13 50
11 54
179
067
96 34
6353
1647
0216
0380
0368
0009
2967
1046
0015
0512
0127
1824
0613
863
Pargasite
rim
42 61
354
11 59
001
10 59
010
13 52
11 87
160
076
96 19
6286
1714
0302
0392
0299
0001
2972
1007
0012
0458
0143
1877
0715
844
Pargasite
core
42 54
401
11 04
006
11 81
013
13 21
11 50
202
039
96 71
6266
1734
0182
0444
0368
0007
2901
1087
0017
0576
0073
1814
0854
979
Pargasite
rim
43 03
385
10 72
002
11 70
014
13 50
11 95
146
080
97 17
6314
1686
0168
0425
0345
0002
2953
1091
0018
0415
0149
1878
0634
852
Pargasite
rim
42 65
341
10 60
006
11 97
014
13 22
11 64
183
068
96 20
6332
1668
0186
0381
0345
0007
2926
1141
0018
0528
0129
1851
0620
869
Pargasite
core
42 82
367
10 60
007
11 93
012
13 42
11 44
206
047
96 60
6315
1685
0158
0407
0391
0008
2950
1080
0016
0588
0089
1808
0615
899
Pargasite
rim
42 2
362
11 10
007
11 42
016
13 09
11 72
179
072
95 89
6282
1718
0229
0405
0299
0009
2905
1123
0020
0518
0137
1869
0715
875
Pargasite
rim
42 42
380
11 03
006
11 77
012
13 00
11 72
188
069
96 49
6283
1717
0209
0423
0299
0007
2870
1158
0015
0539
0131
1860
0817
928
Pargasite
rim
42 21
364
11 14
006
10 73
011
13 53
11 68
177
072
95 59
6277
1723
0229
0407
0322
0007
2999
1012
0014
0511
0136
1862
0736
883
Pargasite
rim
42 25
376
11 03
008
10 93
015
13 66
11 72
185
064
96 07
6257
1743
0182
0418
0345
0009
3016
1009
0019
0531
0121
1860
0766
912
Pargasite
core
42 17
396
10 91
008
11 10
010
13 57
11 49
209
038
95 85
6253
1747
0159
0442
0368
0010
3000
1009
0013
0602
0073
1826
0841
981
aMajorelem
entco
mpositionsarein
weightpercent-------noplagioclasean
alysed
andnotemperature
calculated
1Typ
eofam
phibolean
alysed
isalso
indicated
2Blankindicates
nozoningobserved
3Molefractionofan
orthitein
plagioclasead
jacentto
amphibolegrain
4Tem
perature
ofeq
uilibrium
fortheam
phibole---plagioclasepaircalculatedusingtheHollandamp
Blundythermometer
(1994)Errors
are40
C
BOSCH et al HYDROTHERMAL CIRCULATION IN OMAN OPHIOLITE
1195
zonation from An95 to An99 from core to rim (Table 1 andFig 8a) TheWR Sr isotopic ratio of the sample (070458)is significantly higher than normal MORB values Thiscould be related to the overprint of a low-temperaturealteration event consistent with the petrography of thesample
Green amphibole vein cross-cutting layered gabbros inWadi Mayah Haylayn Massif
Sample 02 OA43a belongs to a series of 1 cm thick greenamphibole veins (as in Fig 6e) cross-cutting the layeredgabbros The veins develop 15 cm thick reaction marginsin the enclosing gabbro and are exclusively composed ofacicular pale green magnesio-hornblende growing per-pendicular to the vein margin in a crack-seal mode(Fig 6f ) The reaction margins are composed of urali-tized gabbro marked by the development of the samepale green magnesio-hornblende in an inhomogeneouscalcic plagioclase matrix (An91---97) The Sr and O isotopiccompositions of this green hornblende (070431 andthorn31) do not correspond to normal MORB-sourcemantle values
Epidosite dyke from Wadi Gideah Wadi Tayin Massif
Sample 01 OA36b is representative of a series of epidoteveins cross-cutting poorly layered gabbros These 2 cmthick veins are composed of pink thulite in their centreand green pistachite at their margins (Fig 6a) Theirsharp contact with the enclosing gabbro is marked bythe development of coarse clinopyroxene crystals alteredin the epidote---greenschist facies This suggests that theepidote vein is replacive in a coarse-grained gabbro dykeThe highest 87Sr86Sr initial ratios of all the samplesstudied are recorded by the WR and coexisting epidoteof this sample (070519 and 070554 respectively)(Table 5) The Sr content of the WR and associatedepidote are also the highest of the present dataset(716 ppm and 764 ppm respectively) The Sr isotopiccomposition of this sample is consistent with a greaterdegree of exchange with a radiogenic component fromCretaceous seawater It is significantly higher than thevalue obtained for an epidosite vein from the Wadi Fizhsection of the Oman ophiolite [070435 with a Sr contentof 488 ppm reported by Kawahata et al (2001)] but ingood agreement with the average of the greenschist-faciesalteration sequence defined by Kawahata et al (2001)
GABBROS
01 km 025 km 1 km 36 km50
60
70
80
90
An
MA2
19d
19a 15f 128b
41d2
(a)
43a17b
15f
90b
37b
19a
19d
40
60
80
100
A
n
01 km 015 km 025 km 1 km
DYKES
(b)
Distance to the Moho
ACTINOLITE
MAGNESIOHORNBLENDE
TSCHERMAKITE
PARGASITEGabbros Dykes41d2MA2
19a d37b15f
90b43a
AlIV
150500
02
08
10
(Na
+ K
) A
(c)
04
06
950
900
850
800
750
70007 12 17
T degC
1000
(d)
AlIV
Fig 8 Plagioclase---amphibole compositions and results of thermometry (a) and (b) plagioclase compositions in layered gabbros and in dykesrespectively as a function of distance to the Moho (see text for sample identification) (c) amphibole compositions in layered gabbros and dykesaccording to Leakersquos (1997) nomenclature (d) equilibration temperatures calculated using the amphibole---plagioclase thermometer of Holland ampBlundy (1994) as a function of AlIV content in amphibole [same symbols as in (c)]
JOURNAL OF PETROLOGY VOLUME 45 NUMBER 6 JUNE 2004
1196
Table5RbandSrcontents87Sr
86Srandd1
8Oisotopiccompositionsofwholerocksandmineralseparatesfrom
samplesofmassivegabbrosdykes
andveinsfrom
theOman
ophiolitealsolistedarecalculated
waterrockratios(W
R)andLOIvalues
Sam
ple
Rb(ppm)
Sr(ppm)
87Rb8
6Sr
87Sr
86Sr
(87Sr
86Sr)i1
d18O
()
LOI(
)2WR
3(87Sr
86Sr)L14
(87Sr
86Sr)L25
(87Sr
86Sr)R6
01OA41d2
Whole
rock
037
1792
0006
0703516
09
070351
thorn480
138
47
0703587
0703651
0703357
Plagioclase
025
1249
0003
0704261
22
070426
thorn504
-------
-------
Pargasite
-------
-------
-------
0703548
20
070355
-------
-------
-------
Clinopyroxene
045
26 0
0050
0703845
19
070378
-------
-------
-------
01OA36b
Dykewhole
rock
0005
7164
50001
0705186
17
070519
-------
274
21 9
0705207
0705112
-------
Epidote
0003
7645
0001
0705536
12
070554
-------
-------
-------
Wallwhole
rock
005
4254
0001
0704637
09
070464
-------
191
-------
0704945
0704675
0704593
02OA43a
Green
hornblende
004
98
0012
0704323
13
070431
thorn310
-------
-------
97OA128b
Whole
rock
003
1274
50001
0703327
17
070333
-------
076
37
-------
-------
-------
Plagioclase
002
1204
50001
0703285
09
070328
thorn414
-------
-------
Clinopyroxene
003
20 0
0004
0704230
09
070422
thorn523
-------
-------
01OA15f
Whole
rock
006
1077
0002
0704582
16
070458
-------
237
13 5
-------
-------
-------
01OA19a
Whole
rock
037
1303
0008
0704189
18
070418
-------
161
96
-------
-------
-------
01OA19d
Whole
rock
058
2032
0008
0703423
08
070341
thorn567
216
415
0703574
0703408
0703271
Pargasite
091
1058
0025
0703273
11
070324
thorn287
-------
-------
Plagioclase
-------
-------
-------
-------
-------
thorn10 09
-------
-------
02OA90b
Plagioclase
004
2385
50001
0703868
11
070387
thorn498
-------
-------
Green
hornblende
010
23 8
0012
0704288
15
070427
thorn239
-------
-------
BOSCH et al HYDROTHERMAL CIRCULATION IN OMAN OPHIOLITE
1197
Table5continued
Sam
ple
Rb(ppm)
Sr(ppm)
87Rb8
6Sr
87Sr
86Sr
(87Sr
86Sr)i1
d18O
()
LOI(
)2WR
3(87Sr
86Sr)L14
(87Sr
86Sr)L25
(87Sr
86Sr)R6
02OA37b
Plagioclase
023
3121
0002
0704204
15
070420
-------
-------
-------
Pargasite
031
41 6
0021
0703505
15
070348
thorn394
-------
-------
94OAMa2
Whole
rock
0016
1750
00003
0703219
15
070322
thorn495
033
31
0703264
0703177
0703158
Plagioclase
0014
1363
00003
0703219
11
070322
thorn479
-------
-------
Clinopyroxene
-------
-------
-------
0703236
13
070324
-------
-------
-------
1Initial8
7Sr
86Srratiocalculatedat
95Ma(H
acker1994)
2LOIisloss
onignition(in)
3Water---rock
ratiocalculatedassumingaclosedsystem
TheeffectiveWR
ratiocanbeexpressed
bythefollowingeq
uation
W=Reffectivefrac14
frac12eth87Sr=
86SrrockTHORN fi
nal
eth87Sr=
86SrrockTHORN in
itial=frac12eth8
7Sr=
86SrseawaterTHORN in
itial
eth87Sr=
86SrrockTHORN fi
nal
ethCrock
initial=C
seaw
ater
initial
THORNwhere(87Sr
86Srrock) finalisthefinalmeasuredSrisotopicratioin
thesample(87Sr
86Srrock) in
itialco
rrespondingto
theinitialm
antlevalue
istakenas
070295(Lan
phere
etal1981)(87Sr
86Srseawater) in
itialistheCretaceousseaw
ater
Srisotopicco
mposition[87Sr
86Sr07073
at90
MaafterBurkeet
al(1982)]C
seaw
ater
initial
istheSrco
ntent
oflsquonorm
alrsquoseaw
ater
andis
takenas
8ppm
(deVilliers1999)
FortheCrock
initialitis
assumed
that
thepresentSrco
ncentrationmeasuredis
thesameas
theinitial
concentration
4Srisotopicratiosmeasuredforliquid
L1extractedafterthefirststep
oftheleachingexperim
ent(6NHClfor8min
at70
C)
5Srisotopicratiosmeasuredforliquid
L2extractedaftertheseco
ndstep
oftheleachingexperim
ent(2 5NHClfor30
min
at70
C)
6Srisotopicratiosmeasuredforresidueobtained
afteratotalaciddigestionachievedaftertheextractionofL1an
dL2leachates
JOURNAL OF PETROLOGY VOLUME 45 NUMBER 6 JUNE 2004
1198
Part of the same sample from the contact between thedyke and the gabbro yields a slightly lower Sr isotopiccomposition (070464) intermediate between the sur-rounding gabbro and the epidote dyke It also has anintermediate Sr content (425 ppm) Acid leaching experi-ments performed on the whole rock of the dyke yield Srisotopic compositions of 070521 and 070512 for thetwo liquids extracted Similar experiments for the samplelocated at the contact with the gabbro yield 87Sr86Srratios of 070494 and 070467 for L1 and L2 liquids TheSr isotopic composition of the residue extracted afterleaching remains high (070459) and very close to theepidosite 87Sr86Sr ratio determined by Kawahata et al(2001) In this sample the primary minerals have beentotally replaced by epidote and secondary plagioclase orquartz Thus the Sr isotopic compositions measured forthis WR sample and its constituent minerals reflect thecomposition of the fluid filling this dyke
Mineral chemistry
Plagioclase
Compared with the host gabbros the dykes show a muchlarger dispersion in their plagioclase compositions (Fig 8aand b Table 4) characterized by more calcic plagioclasethan the enclosing gabbros This tendency is mostextreme in the comb-textured dykes in which the plagio-clase is almost pure anorthite The zoning is generallynormal recording a crystal fractionation process How-ever gabbro 94 MA2 exhibits inverse zoning this tend-ency is also observed in plagioclases from dykes 01OA19a and 01 OA15f The higher calcium content ofthe plagioclase as well as the inverse zoning is consistentwith an increase of water pressure during its crystalliza-tion (Turner amp Verhoogen 1960 Carmichael et al 1974Koepke et al 2003) The particularly high An content inthe comb-textured gabbro dykes 01 OA15f may alsoresult from crystallization in a supercooled magma thisis consistent with the comb texture indicative of fast-growing crystals The lack of brown hornblende suggeststhat crystallization occurred above the temperature ofhornblende stability
Amphibole
The amphiboles analysed in the dykes (Fig 8c andTable 4) show a regular trend in a (Na thorn K)A vsAlIV diagram from titanium-rich (Ti 4 015) pargasitichornblende (brownamphibole) and tschermakite (brown---green amphibole) to low-titanium (Ti5 015) magnesio-hornblende (green amphibole) and actinolite (pale greenamphibole) The pargasitic hornblende develops aspoikilitic oikocrysts in the dykes 01 OA19a and d andin the diffuse patch 01 OA41d2 at temperatures above800C during the final stage of crystallization (see
below) The tschermakite composition corresponds tothe internal alteration of clinopyroxene in gabbro 94Ma2 occurs in the comb-textured dyke 01 OA15f andis the primary amphibole in the dioritic dyke 02 OA37bThe magnesio-hornblende develops at the edges of thepargasites or of clinopyroxenes at temperatures around800C and is the primary phase in the dioritic dyke 02OA90b and in the green vein 02 OA43a Finally actino-lite occurs as a replacement of green hornblende in dykesor in the kelyphitic rim surrounding olivine in gabbro 94Ma2 Such a large composition range at the scale of athin section has been reported in amphiboles fromamphibolite-facies oceanic gabbros (Stakes 1991 Stakesamp Taylor 1992 Gillis et al 1993) as well as in the Omanophiolite gabbros (Manning et al 2000) This variabilityhas been explained by rapid reaction rates preventinghomogenization (Manning et al 2000)
Thermometry
Plagioclase---amphibole thermometry could be appliedwhen the two minerals exhibited good contacts such asin the laminated gabbro of the middle sequence (01OA41d2) and in the intrusive dykes The temperaturesof plagioclase---amphibole equilibration (Table 2) havebeen calculated using the geothermometer lsquoBrsquo of Hollandamp Blundy (1994) Edenite thorn Albite frac14 Richterite thornAnorthite valid between 500C and 900C with anuncertainty of 40C Amphibole formulae are basedon 23 oxygen and their nomenclature is based on alkalications in the A site vs AlIV according to Leake (1997)Pressurewas assumed to be 1 kbar Forty-five plagioclase---amphibole pairs meet the compositional criteria imposedby Holland amp Blundyrsquos dataset (09 4 Xan 4 01 andAlIV 403) Electron microprobe measurements wereconstrained to contiguous plagioclase and amphiboleseparated by sharp grain boundaries assuming equili-brium of the two phases When the mineral phasesexhibit normal zoning temperatures between plagioclaseand amphibole cores and between mineral rims werecalculated assuming that the temperatures deduced fromthe mineral core chemistry provide a proxy for the high-est temperature of equilibration These data are identi-fied in the T C vs AlIV diagram (Fig 8d) Table 4presents selected major element amphibole analyses theAn content of the plagioclase adjacent to each amphibolegrain and the temperature calculated for the pairThe temperatures calculated (Fig 8d) span the entire
range of temperatures at which amphibole and plagio-clase coexist in equilibrium (ie amphibolite-facies para-geneses) This temperature interval is bracketed byorthopyroxene-bearing facies or amphibole-free facies(ie gabbro 94MA2) and epidote---actinolite facies devoidof homogeneous plagioclase (ie epidosite dyke 01OA36b and green vein 02 OA43a) The range of the
BOSCH et al HYDROTHERMAL CIRCULATION IN OMAN OPHIOLITE
1199
calculated temperatures between 4900C and 750Ccorresponds to the changing composition of amphibolesfrom pargasite to magnesio-hornblende at the scale of athin section or even at that of an individual crystal asshown by their correlation with AlIV such variabilityrecords rapid cooling in hydrous conditions The highesttemperatures are recorded by pargasite from the anatec-tic gabbro 01 OA41d2 in the gabbro dyke 01 OA19dand in the dioritic dyke 02 OA90b They were calculatedfrom minerals devoid of lower-grade alteration and arethus considered as representing the equilibrium temperat-ure of the coexisting minerals Lower temperatures cor-respond to mineral phases evolving at falling temperatureand are only indicative of the cooling history Includingthe 40C uncertainty seven pairs provide temperatureshigher than 900C the limit of validity of the Holland ampBlundy thermometer However we maintain these tem-peratures as indicative of the highest values of our data-set These values are included in the average equilibriumtemperature summarized in Table 2
DISCUSSION
Distribution of hydrothermal alteration
Two distinct petrological events are recorded in the gab-bro section of the Samail ophiolite and are documentedin the present contribution (1) the whole gabbro sectionis affected by a penetrative alteration (2) the gabbrosection is crosscut by various types of dykes and veinsincluding hydrous minerals
Penetrative alteration
The penetrative alteration developed in the gabbros atgrain boundaries and locally invading the minerals isubiquitous in the gabbro section This penetrative altera-tion has been described as very high temperature (VHT)high temperature (HT) and low temperature (LT) basedon alteration parageneses The orthopyroxene coronas inthe layered gabbro sample and the incipient appearanceof brown pargasitic amphibole mark a lower thermallimit of the VHT alteration By reference to the experi-mental work of Koepke et al (2003) the temperature ofequilibration for these reactions is estimated to bebetween 900 and 1000CThe preservation of the vertical distribution of
VHT---HT facies (Figs 4 and 5) also pointed out byManning et al (2000) indicates that the hydrothermalalteration pattern fits the distribution of isotherms withdepth at a fast-spreading ridge (ie Phipps Morgan ampChen 1993 Dunn et al 2000 Fig 2) Equilibrium tem-peratures for VHT parageneses as high as 900---1000Csuggest that VHT---HT hydrothermal alteration tookplace in a very restricted time interval at the limit ofthe cooling magma chamber It is proposed that the
penetrative alteration affecting the entire gabbro sectionis related to a pervasive network of microcracks whichact as a recharge system down to the Moho (Nicolas et al2003)
Dykes and veins
The dykes are extremely varied eg pegmatitic gabbroswith comb-texture microgabbros and amphibolitizeddolerites dioritic dykes alignments of poikilitic horn-blende and finally diffuse hornblende-bearing patchesHigh-T gabbro and diorite dykes and low-T greenamphibole---epidote veins are connectedIn the dykes textural evidence attests to suprasolidus
conditions at least for the development of the pargasiticamphibole Integrating the hornblende---plagioclase equi-libration temperatures calculated for the studied samplesthese temperatures are bracketed between 950C and875CIn the following discussion we shall consider separately
the microgabbro and dolerite dykes The latter representbasaltic melt intrusions associated upsection with thediabase sheeted dykes and downsection possibly withthe mafic dykes that are ubiquitous in the mantle section(Nicolas et al 2000) Whatever their origin the develop-ment of pargasitic amphibole during their late stage ofcrystallization creates a link with the gabbroic dykes andsuggests a crystallization process evolving from dry con-ditions to more hydrous conditionsThe other gabbroic dykes comb-textured gabbros and
hornblende-bearing dioritic dykes result from the fillingof cracks by a fluid either a melt or a silica-rich hydrousfluid These intrusive dykes which develop reaction mar-gins at their contact with the host gabbro grade verticallyinto lower-temperature green amphibole veins or align-ments of poikilitic hornblende in the layered gabbromatrix or development of pargasitic hornblende in ana-tectic gabbros (sample 01 OA41d2) In gabbros crystal-lizing in the magma chamber the development oforthopyroxene and pargasite oikocrysts enclosing the pri-mary minerals suggests a thermal evolution from thegabbro solidus to amphibolite-facies conditions Forthese reasons it is suggested that the gabbroic dykesresulted from the injection of a hydrous melt in the stillhot gabbros drifting away from the magma chamber asdiscussed below
Origin of fluids responsible for VHT---HThydrothermal alteration
The 87Sr86Sr initial ratios and d18O of whole rocks andpure mineral fractions are plotted in Fig 9 against thedistance above the Moho Transition Zone (MTZ) Theinitial Sr isotopic composition of the whole rocks apartfrom the epidosite dykes ranges from 070322 to
JOURNAL OF PETROLOGY VOLUME 45 NUMBER 6 JUNE 2004
1200
070458 All the studied samples (including whole rocksand minerals) have initial 87Sr86Sr ratios falling outsidethe field of fresh MORB which commonly have Sr ratiosbetween 07023 and 07029 with an average of 070265(ie Hart et al 1974) The lowest initial Sr isotopiccomposition (070322) from the massive gabbro 94MA2 is slightly but significantly higher than averageOman primary magmatic signatures (ie 070296McCulloch et al 1981) Similarly the d18O values ofmost whole rocks and minerals measured (Table 5)
depart from typical MORB-source mantle values( Javoy 1980 Gregory amp Taylor 1981 Kyser 1986)These differences indicate that the primary mantlesignatures in the Oman ophiolite have been modifiedby interaction with a fluid Possible origins for thisfluid are mantle components dehydration of subductedlithosphere or seawater circulation Strontium andoxygen isotopes are useful tracers for fluid---crust interac-tion as d18O and 87Sr86Sr ratios from fluids originat-ing from seawater the mantle or subducted lithosphere
-1000
1000
2000
3000
4000
5000
6000
7000
MOHO
MTZ
Dis
tan
ce a
bov
e th
e M
oh
o (
m)
pillow-basalt
sheeted-dike
gabbro
peridotite
01 OA 41d2
01 OA 36b
02 OA 43a97 OA 128b
01 OA15f
94 OA MA2
02 OA 90b01 OA 19ad
02 OA 37b
0702 0703 0704 0705 0706 0707
87Sr86Srinitial
MO
RB-s
ou
rce
Seaw
ater
Whole RockPlagioclasePargasiteGreen HornblendeClinopyroxene
Epidote
(a)
-1000
1000
2000
3000
4000
5000
6000
7000
MOHO
MTZ
Dis
tan
ce a
bov
e th
e M
oh
o (
m)
pillow-basalt
sheeted-dike
gabbro
peridotite
01 OA 41d2
01 OA 36b
02 OA 43a97 OA 128b
01 OA15f
94 OA MA2
02 OA 90b01 OA 19ad
02 OA 37b
Whole RockPlagioclasePargasiteGreen HornblendeClinopyroxene
3 5 10
δ18O (permil)
41d2
Ma2
43a
90b 19d 19d37b
128b
90b
(b)
Fig 9 (a) Variation of initial 87Sr86Sr isotopic composition as a function of the location of the studied samples in a schematic log of the crustalsection of the Oman ophiolite The field for MORB is from Hart et al (1974) and that for seawater is from Burke et al (1982) Small open squares aredata fromKawahata et al (2001) (b) Variation of d18O () as a function of the location of the studied samples in a schematic log of the crustal sectionof the Oman ophiolite Shaded curve corresponds to the data of Gregory amp Taylor (1981)
BOSCH et al HYDROTHERMAL CIRCULATION IN OMAN OPHIOLITE
1201
are significantly different Moreover the d18O valueis dependent on temperature and mineral fractiona-tion and thus yields complementary constraints to Srisotopes
Mantle origin
One model to explain the occurrence of fluids interactingwith the oceanic crust at very high temperatures is toderive them from the mantle The contribution of mantle-derived hydrous fluids linked to percolation processesor to their concentration in the final stages of gabbrocrystallization has been invoked in numerous case studies(ie Witt amp Seck 1989 Fabriees et al 1989 Dupuy et al1991 Agrinier et al 1993) O and Sr isotopic data forwhole rocks and mineral separates yield scattered valuesthat are unambiguously different from MORB mantle-source values (Fig 9a and b) With the exception of lateLT altered samples the WR data have a mean 87Sr86Srratio of 070337 and a standard deviation of 000012These surprisingly high values in mantle-derived rockscould be taken as evidence for survival of mantle isotopicheterogeneities during crystallization of the ophiolite (egLanphere 1981 McCulloch et al 1981) However the18O16O isotope ratios in the Oman gabbroic sequenceare also displaced from primary magmatic values Nogabbro in the present study has plagioclase or amphi-bole with typical mantle d18O values Thus the Sr and Oisotope data rule out the possibility of mantle-derivedfluids as a possible source for the VHT---HT hydrousalteration event recorded in the massive gabbros andgabbroic dykes and veins (Fig 10a and b)
Subducted lithosphere origin
Dehydration of subducted oceanic crust in subductionzones can generate hydrous fluids Such fluids couldpercolate through the overlying lithosphere and contam-inate it thus generating modified isotopic signatures andtrace element contents Such an explanation howeverdisagrees with the geochemical characteristics of most ofthe studied samples Fluids derived from the dehydrationof subducted slabs (fresh or altered MORB or OIB pro-toliths) should be strongly enriched in K Rb and Ba (egBecker et al 2000) whereas in Oman the Rb contentsmeasured in the gabbros and gabbroic dykes and veins arestrongly depleted Moreover the isotopic composition ofslab-derived fluids is buffered by the MORB-like oceaniccrustal protolith for Nd and Pb but not for Sr and O(Staudigel et al 1995) As a result of relatively minorfractionation of oxygen at high temperature the oxygenisotope composition of the fluids expelled during dehy-dration of the subducted slab should be high and essen-tially controlled by the oxygen composition of the uppersection of the crust altered at low temperature (with an
average of 10 and perhaps as high as 19) (Staudigelet al 1995) The Sr isotopic composition should be alsohigh (average 07046) as the fluids released by the dehy-dration of subducted oceanic crust are associated eitherwith seawater or with radiogenic Sr phases (ie smectites)The Sr and O isotopic compositions of the studied sam-ples do not fit with these signatures and so preclude asubducted lithosphere origin for the fluids involved in theVHT---HT alteration event
Seawater-derived fluids
All the analysed samples (minerals and WR) have 87Sr86Sr(T) outside the domain of primary MORB magmasand variable Sr contents Individual minerals have Srcontents reflecting their different mineralliquid partitioncoefficients Minerals characterized by a very high affi-nity for Sr such as plagioclase (ie Sr mineralliquidpartition coefficients 1) have very high Sr contentsTherefore the high abundance of plagioclase in the sam-ples analysed is responsible for the high Sr content of thestudied whole rocks The Sr content of all the whole rocksis relatively homogeneous (Table 5) without clear distinc-tion between country-rock gabbros and dykes and veinsand ranges from 110 to 200 ppm except for the epidosite(4700 ppm) Reported on a 87Sr86Sr vs 1Sr diagram(Fig 10a) most whole rocks and plagioclases analysed arecharacterized by a 1Sr ratio lower than 001 and plot inthe left part of this diagram Compared with N-MORBthe studied massive and anatectic gabbros and their con-stituent plagioclases have similar to slightly higher Srcontents but substantially higher 87Sr86Sr ratios(Fig 10a) Considering Cretaceous seawater as a possiblehigh 87Sr86Sr mixing component [87Sr86Sr 07073at 90Ma after Burke et al (1982)] responsible for theobserved geochemical modifications exchanges cannothave occurred in a closed system as seawater has too lowa Sr content (ie 8 ppm de Villiers 1999) An open-system process allowing penetration of larger quantitiesof seawater can efficiently modify the Sr isotopic ratio ofthe rocks Alternately the fluids could also be seawaterthat was modified through exchange with the surround-ing rocks during its transit through the oceanic crustalsection This modified seawater would be more enrichedin Srwith a 87Sr86Sr ratio intermediate between unmodi-fied seawater and MORBMinerals devoid of late LT alteration imprints (parga-
site green hornblende and pyroxene) and characterizedby very low Sr contents plot on the right of the diagramclose to or on the mixing line between seawater andMORB (Fig 10a) The difference between these mineralsand the other samples (WR and plagioclases) is mainlydue to the Sr fractionation coefficients of these differentminerals and to the large quantity of plagioclase inthe studied rocks The process responsible for the
JOURNAL OF PETROLOGY VOLUME 45 NUMBER 6 JUNE 2004
1202
modification of the Sr isotopic composition fromMORB-source value is however explained by the same phenom-enon As all these minerals display initial 87Sr86Sr ratiosdistinct from the MORB source and because they equili-brated at VHT or HT temperatures this supports anearly intervention of seawater during crystallization ofthese minerals Most of these samples with the exceptionof the epidosite dyke overlap the domains defined byKawahata et al (2001) for the Wadi Fizh gabbros alteredat VHT and HT conditions in the amphibolite facies(labelled lsquoVHTrsquo and lsquoHTrsquo in Fig 9a)Three samples (two epidosite dykes and an one epidote
fraction) have very high Sr contents and the highest Sr
isotopic compositions of the present dataset The twowhole-rock samples overlap the field of the Wadi Fizhsamples altered at high temperature during greenschist-facies metamorphism (Kawahata et al 2001) This typeof alteration is common at the ridge axis and formedthrough intensive exchange with seawater-derived fluidsFigure 10b indicates that all the Sr---O isotope data are
distinct from MORB compositions and suggests that thefluids reacting with the magmas could be derived fromseawater In addition the d18O values show that isotopicexchange occurred under VHT or HT conditions Thefluids could have the composition of seawater or theycould have the composition of seawater modified through
0703
0705
0707
001 01 1
1 Sr (ppm)
MORB
(87Sr
86Sr)
init
ial
Seawater
VHT
HT
MT36b
36b
36b
43a 128b90b
41d2
37b19d
15f
128b
41d2
41d2
19a
Ma2Ma2
19d
90b
37b
Whole RockPlagioclasePargasiteGreen HornblendeClinopyroxene
Epidote
(a)
0 2 4 6 8
δ18O(permil)
0703
0705
0707Seawater
MORB-mantle
128b 41d2
90b 41d2
Ma2128b
37b
43a
19d
90b
Whole RockPlagioclasePargasiteGreen HornblendeClinopyroxene
Epidote
HT LT
19d
(87Sr
86Sr)
init
ial
(b)
Fig 10 (a) Initial 87Sr86Sr isotopic ratios plotted against 1Sr for whole rocks and mineral separates from the gabbro section of the Omanophiolite Fields shown are from Kawahata et al (2001) and correspond to MT-----basalt sheeted dyke diabase altered at medium temperature(prehnite---actinolite facies sequence 3) HT-----diabase plagiogranite metagabbro and epidosite formed at high temperature (greenschist faciessequence 4) VHT-----gabbro altered at very high temperature (amphibolite facies sequence 5) The dashed line is a binary mixing line betweenMORB (Lanphere et al 1981) and Cretaceous seawater (Burke et al 1982) (b) Initial 87Sr86Sr isotopic composition plotted against d18O value forwhole rocks and minerals from the Oman ophiolite Cretaceous seawater and MORB end-members as in (a) Dashed line represents mixing curvebetweenMORB and seawater at high temperature (HT) Low-temperature (LT) exchanges between these two components would be responsible foran increase of the d18O primary MORB value
BOSCH et al HYDROTHERMAL CIRCULATION IN OMAN OPHIOLITE
1203
exchange with the surrounding rocks during its penetra-tion into the crustal section In an environment in whichfluid---rock interaction occurs at variable temperaturesthe d18O is greatly influenced by the temperature atwhich the exchange took place and by the waterrockratio It is evident from Figs 9b and 10b that none of thestudied gabbros have plagioclase and amphibole withMORB-like d18O and 87Sr86Sr values The separateminerals display a broad range of d18O values and mostexhibit extreme 18O depletion (Table 5) Hydrothermalexchange kinetics are similar in amphibole and pyroxeneand these two minerals are more resistant to oxygenexchange during water---rock interaction than coexistingplagioclase (Gregory et al 1989) The most probableexplanation of the low d18O values measured in bothamphiboles and pyroxenes is that they crystallized athigh temperature in the presence of a relatively low 18Oseawater-derived hydrothermal fluid [d18O from 01 to07 after Gregory amp Taylor (1981)] The anomalouslyhigh d18O value (thorn1009) measured for the plagioclase ofthe micro-gabbroic dyke (01 OA19d) can be interpretedas resulting from a superimposed overprint caused by alate-stage penetration of fluids at lower temperature Inthis sample the occurrence of such different 18O16Oratios between coexisting plagioclase and amphibole isnot consistent with equilibrium fractionation betweenthese two minerals at any possible temperature Thisobservation suggests that parts of the ophiolite sequencehave undergone a high-temperature hydrothermal eventprior to the later low-temperature event that producedthe strong 18O increase in coexisting plagioclaseThe depletion of the d18O values results from high-temperature hydrothermal alteration (Gregory amp Taylor1981 Stakes amp Taylor 1992) The oxygen isotopic datasupport the contention that most studied samples havebeen affected by a VHT---HT hydrothermal alteration byfluids derived from seawater Some of the analysed sam-ples presenting petrological evidence of crystallization ofLT secondary minerals have been overprinted by the latelow-temperature alteration thus swamping the earlierhigh-temperature alteration effects
Determination of waterrock ratio
To better evaluate the exchange between seawater andthe gabbroic rocks waterrock ratios (WR) have beenestimated for the whole rocks analysed during the courseof this study The different models used to constrain thisratio mainly differ by the calculation of the effective ratioor by the estimated weight ratio and by considering openor closed systems for water circulation (Albareede et al1981 McCulloch et al 1981) The WR ratios reportedin Table 5 have been calculated assuming a closed sys-tem Using this formulation these values are minimabecause the calculation always uses pristine fluid for the
initial fluid thus maximizing the value in the denomina-tor If the fluid was shifted to lower 87Sr concentrations byexchange with the rock on the downward path theinferred values would be much higher (Davis et al 2003)The calculated WR ratios for the samples from this
study range from 31 to 219 (Table 5) Two main groupsof samples can be recognized on the basis of their waterrock ratios The lowest ratios ranging from 31 to 47have been determined for massive gabbros or anatecticgabbros but also for dykes and veins devoid of late LTalteration Similar ratios have been previously calculatedfor example for the discharged hydrothermal solutions atthe 13N and 21N East Pacific Rise (Di Donato et al1990 Fouquet et al 1991) The gabbros from the Ibrasection in the Oman ophiolite gave effective WR ratiosof 05 33 and 42 (McCulloch et al 1981) in goodagreement with the values obtained in this study Theleast altered gabbro of the Ibra section yields a WR ratioof 05 in good agreement with the exceptionally fresh-ness of this rock characterized by typical mantle oxygenvalues for its minerals (McCulloch et al 1981) Samplescharacterized by waterrock effective ratios in the rangeof 3---5 can be related to penetrative alteration via thehydrothermal system Lecuyer amp Reynard (1996) havedemonstrated in oceanic gabbros from the Hess Deepaltered in similar HT conditions that WR mass ratios inthe range of 02---1 are sufficient to account for the mod-ified stable isotope signature of the gabbros Higherwaterrock values (WR 45) have been calculated forsamples affected by superimposed late hydrothermalalteration and for the epidosite samples (Table 5) Theyprobably acquired their isotopic characteristics throughinteraction with a larger volume of seawater duringgreenschist-facies metamorphism Similar or higherWR values have been published in previous studies(ie McCulloch et al 1981 Kawahata et al 2001)They correspond either to samples from the upper-crus-tal sequence (ie basalt sheeted dyke or plagiogranite) orto gabbros from the crustal section located in a particularenvironment such as for example the Wadi Fizh sectionthat is located close to a segment boundary along thepalaeo-spreading axis (Nicolas et al 2000)
Mechanism for seawater interaction withmagma
Nicolas et al (2003) have proposed that variable amountsof seawater can circulate at high temperature through-out the crustal section down to the walls of the magmachamber via a network of parallel microcracks a fractionof a millimetre wide Such microcracks and their relation-ship with the VHT---HT hydrous alteration in the sur-rounding gabbros have been described in detail byNicolas et al Via this network of microcracks largeamounts of seawater can have reacted with the magma
JOURNAL OF PETROLOGY VOLUME 45 NUMBER 6 JUNE 2004
1204
and induced variable changes of its Sr and O isotopiccomposition This regular network of parallel micro-cracks could constitute the recharge system and thehydrated gabbroic dykes the discharge system Physicalparameters and the geophysical models of Nicolas et al(2003) are in agreement with this scenario Such a pene-tration of seawater-derived hydrothermal fluids throughthe lower crust along grain boundaries and a microscopicfracture network at temperatures 4750C is consistentwith recent thermodynamic models (McCollom amp Shock1998)Seawater-altered and high-temperature recrystallized
diabases (protodykes) from the overlying sheeted dykecomplex stoped into the melt lenses could represent anindirect way for seawater to enter the system as they canremelt during their subsidence through the magmachamber A simple mass balance calculation suggeststhat this indirect contamination is not significant Assess-ments of the amount of recycled crust 1 in volumeby Nicolas et al (2000) based on the protodyke remnantsin the gabbro section or 20 by Coogan et al (2003)based on chlorine content in EPR basalts lead to a sea-waterrock ratio of the order of 104 to 103Thus following Nicolas et al (2003) we propose that
seawater has been introduced along microcracks down tothe walls of the magma chamber and has diffused alonggrain boundaries in the way described by Manning et al(2000) progressively invading the host gabbro Gabbroicdykes result from the injection of hydrous melt into thehost gabbros at falling temperatures as they drift awayfrom the magma chamber This water-saturated meltcould result from the extensive hydration of the crystal-lizing gabbros near the walls of the magma chamberwhen seawater penetrates through this wall (Fig 1)Similar plumbing systems driving seawater down to the
limit of the magma chamber and back to the surface havebeen already proposed in other environments such as theMid-Atlantic Ridge (Kelley amp Delaney 1987 Cooganet al 2001) the East Pacific Rise at Hess Deep (Gillis1995 Manning et al 1996a 1996b) the Tonga forearc(Banerjee amp Gillis 2001) and the Troodos ophiolite(Gillis amp Roberts 1999) The melting curve of wet basalthas a negative slope but is close to the dry solidus at lowpressure in the lower gabbros and the first melts areplagiogranitic (Spulber ampRutherford 1983) Such plagio-granites are ubiquitous in the highest gabbros and in theroot zone of sheeted dykes in Oman (Rothery 1983Juteau et al 1988 Nicolas amp Boudier 1991) in manyother ophiolites and in the oceanic crust where they havebeen interpreted as resulting either from wet anatexis ofhot gabbros or from the final product of crystallization ofa basaltic melt (Spulber amp Rutherford 1983) Plagio-granites are rare in the lower gabbros exceptionallyassociated with hydrous gabbro segregations inside thelayered gabbros Their constitutive minerals are also
remarkably absent along the VHTmicrocracks althoughthese gabbros were above the wet solidus A possibleexplanation has been proposed by McCollom amp Shock(1998) who wrote that at high temperature lsquoaqueousfluids may leave very little conspicuous petrological evid-ence for their presencersquo the reason being that thehigh-T conditions would be closer to batch meltingObviously more detailed studies on this specific problemare needed We conclude that the large degree of hydrousmelting locally required at the walls of the magma cham-ber to account for the generation of a gabbroic hydrousmelt may have erased the traces of the incipient plagio-granitic melt
CONCLUSIONS
Fostered by the recent discovery of high-temperatureseawater contamination in some gabbros of the Omanophiolite (Manning et al 2000) we have conducted astudy on 500 gabbro specimens from selected areas ofthe entire Samail ophiolite belt and on the hydrous gab-broic dykes that cross-cut them The highest-temperaturereactions (VHT) recorded in olivine and clinopyroxene ingabbros as orthopyroxene and pargasite coronas reach975C They are followed at lower HT conditions withreplacement of olivine by an assemblage of tremolitemagnetite and plagioclase surrounded by chlorite andby pargasite development in clinopyroxene followedfinally by the LT lizardite---olivine and saussurite---plagioclase greenschist-facies alteration With a fewexceptions the VHT alteration tends to be restricted tothe lower gabbros and to the vicinity of the Moho andthe HT alteration to the middle and upper gabbros Thepreservation of the vertical distribution of VHT---HTfacies indicates that progressive cooling during off-axisspreading did not obliterate this distribution and conse-quently that the VHT---HT hydrothermal alteration tookplace in a very restricted time interval at the limit of thecooling magma chamber In the deep and hot gabbrosthe main water channels may be sub-millimetre-sizedmicrocracks with a dominantly vertical attitude the ori-gin and dynamics of which have been investigated in aprevious paper (Nicolas et al 2003)Beyond these high-temperature alteration zones mas-
sively induced in the gabbros seawater may be respons-ible for the generation of clinopyroxene---pargasitegabbro dykes that cross-cut all gabbros and that areupsection clearly connected to the LT hydrothermalvein system Petrostructural evidence suggests that thesedykes were generated by hydration of the crystallizinggabbros near the internal wall of the LVZ---magmachamber The resultant melts were subsequently intrudedat various levels in the cooling lower and middle gabbrosPetrological observations of nearly all the gabbros ana-lysed in the present study located from the root zone of
BOSCH et al HYDROTHERMAL CIRCULATION IN OMAN OPHIOLITE
1205
the sheeted dyke complex down to the Moho emphasizethe imprint of VHT---HT hydrous alteration The VHThydrothermal event is characterized by temperatures ofequilibration around 900---1000C The temperatures atwhich the HT hydrous event occurred are estimated tobe in the range 550---900CStrontium and oxygen isotope compositions measured
on whole rocks and separated minerals significantlydepart from primary MORB values The Sr and O iso-tope data obtained for pargasite green hornblende andclinopyroxene record the participation of seawater at thetemperature of crystallization of the minerals Plagio-clases and whole rocks define a trend toward a compo-nent characterized by a high radiogenic 87Sr86Sr valueand a higher Sr content than normal seawater Seawateris clearly identified as the fluid responsible for the modi-fication of the Sr and O signatures for both massivegabbros and dykes and veins Nevertheless the high Srcontent of the extraneous component requires eitheropen-system exchange allowing penetration of a greaterquantity of seawater or penetration of seawater modifiedby interaction with the oceanic crust during its travel paththrough the crustal section The waterrock ratio calcu-lated on the basis of the Sr isotopes is between three andfive for the enclosing gabbros and for the least LT altereddykes The VHT---HT hydrothermal event affectingthese samples is related to penetrative alteration via therecharge and discharge hydrothermal system Thewaterrock ratio is 10 for dykes exhibiting evidenceof a LT hydrothermal alteration overprint and reaches22 for the epidosite dyke The depletion in d18O char-acterizes all the studied samples with the single exceptionof the micro-gabbroic dyke plagioclase This agreeswith the conclusions based on Sr isotopes suggestingthat these samples have undergone exchange with sea-water-derived fluids during a VHT---HTalteration hydro-thermal event
ACKNOWLEDGEMENTS
This work has benefited from financial support by theCentre National de la Recherche Scientifique (INSUInteerieur-Terre andDYETI programmes) and inOmanfrom the willingness of the Directorate of MineralsMinistry of Commerce and Industry and the hospitabil-ity of the people Technical help in the laboratory is alsoacknowledged including C Nevado for the thin sectionsE Ball for the illustrations B Galland for her assistancein the chemistry room and C Bassoulet for help duringthermal ionization mass spectrometric analyses of the Srresults from the leaching experiments Constructivereviews from R T Gregory C Lecuyer an anonymousreviewer and particularly from M Wilson greatlyhelped to improve an earlier version of the manuscript
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Boudier F Godard M amp Ambruster C (2000) Significance of
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Coogan L A Wilson R N Gillis K M amp MacLeod C J (2001)
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1171
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Di Donato G Joron J L Treuil M amp Loubet M (1990)
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College Station TX Ocean Drilling Program pp 57---65
Dunn R A Toomey D R amp Solomon S C (2000) Three-
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peridotite evidence for multistage metasomatism during Red Sea
rifting Geology 19 722---725
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Fabriees J Bodinier J L Dupuy C Lorand J P amp Benkerrou C
(1989) Evidence for modal metasomatism in the orogenic spinel
lherzolite body from Caussou (northeastern Pyrenees France)
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Fouquet Y Von Stackelberg S U Charlou J L Donval J P
Foucher J Erzig P Muhe R Wiedicke M Soakai S amp
Whitechurch H (1991) Hydrothermal activity in the Lau back-arc
basin sulfides and water chemistry Geology 19 303---306
Gillis K M (1995) Controls on hydrothermal alteration in a section of
fast-spreading oceanic crust Earth and Planetary Science Letters 134
473---489
Gillis K M amp Roberts M (1999) Cracking at the magma---hydrothermal transition evidence from the Troodos ophiolite Earth
and Planetary Science Letters 169 227---244
Gillis K M Mevel C et al (eds) (1993) Proceedings of the Ocean Drilling
Program Initial Reports 147 College Station TX Ocean Drilling
Program
Gregory R T amp Taylor H P (1981) An oxygen isotope profile in a
section of Cretaceous oceanic crust Samail ophiolite Oman
evidence for d18O buffering of the oceans by deep (45 km)
seawater---hydrothermal circulation at mid-ocean ridges Journal of
Geophysical Research 86(B4) 2737---2755
Gregory R T Criss R E amp Taylor H P (1989) Oxygen
isotope exchange kinetics of mineral pairs in closed and open
systems applications to problems of hydrothermal alteration of
igneous rocks and Precambrian iron formations Chemical Geology 75
1---42Hacker B R (1994) Rapid emplacement of young oceanic litho-
sphere argon geochronology of the Oman ophiolite Science 265
1563---1569
Hart S R Erlank A J amp Kable J D (1974) Sea floor basalt
alteration some chemical and Sr isotopic effects Contributions to
Mineralogy and Petrology 44 219---230
Holland T amp Blundy J (1994) Non-ideal interactions in calcic
amphiboles and their bearing on amphibole---plagioclase thermo-
metry Contributions to Mineralogy and Petrology 116 433---447
Ionov D A Savoyant L amp Dupuy C (1992) Application of the
ICP-MS technique to trace element analysis of peridotites and their
minerals Geostandards Newsletter 16 311---315
Javoy M (1980) 18O16O and DH ratios in high temperature
peridotites In Colloques Internationaux du CNRS 272 279---287
Juteau T Beurrier M Dahl R amp Nehlig P (1988) Segmentation
at a fossil spreading axis the plutonic sequence of the Wadi
Haymiliyah area (Haylayn Block Sumail nappe Oman) Tectono-
physics 151 167---197
Juteau T Manacrsquoh G Moreau C Lecuyer C amp Ramboz C
(2000) The high temperature reaction zone of the Oman ophiolite
new field data microthermometry of fluid inclusions PIXE analyses
and oxygen isotopic ratios Marine Geophysical Research 21 351---385Kawahata H Nohara M Ishizuka H Hasebe S amp Chiba H
(2001) Sr isotope geochemistry and hydrothermal alteration of the
Oman ophiolite Journal of Geophysical Research 106 11083---11099
Kelemen P B amp Aharonov E (1998) Periodic formation of magma
fractures and generation of layered gabbros in the lower crust
beneath oceanic spreading ridges Annual Geological Union Special
Monograph 106 267---289
Kelemen P B Koga K amp Shimizu N (1997) Geochemistry of
gabbro sills in the crustmantle transition zone of the Oman
ophiolite implications for the origin of the oceanic lower crust Earth
and Planetary Science Letters 146 475---488Kelley D S amp Delaney J R (1987) Two-phase separation and
fracturing in mid-ocean ridge gabbros at temperatures greater than
700C Earth and Planetary Science Letters 83 53---66
Koepke J Feig S T Snow J amp Freise M (2003) Petrogenesis of
oceanic plagiogranites by partial melting of gabbros an experi-
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wwwspringerlinkcomopenurlaspgenre=s00410-003-0511-9
Kyser T K (1986) Stable isotope variations in the mantle In
Valley J W Taylor H P amp OrsquoNeil J R (eds) Stable Isotopes in
High Temperature Geological Processes Mineralogical Society of America
Reviews in Mineralogy 16 141---164
Lanphere M A (1981) K---Ar ages of metamorphic rocks at the base
of the Samail ophiolite Oman Journal of Geophysical Research 86
2777---2782
Lanphere M A Coleman R G amp Hopson C A (1981) Sr isotopic
tracer study of the Samail ophiolite Oman Journal of Geophysical
Research 86(B4) 2709---2720
Leake B E (1997) Nomenclature of amphiboles report of the
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Association commission on new minerals and mineral names
European Journal of Mineralogy 9 623---651
Lecuyer C amp Reynard B (1996) High-temperature alteration of
oceanic gabbros by seawater (Hess Deep Ocean Drilling Program
Leg 147) evidence from oxygen isotopes and elemental fluxes
Journal of Geophysical Research 101(B7) 15883---15897
Liou J G Kuniiyoshi S amp Ito K (1974) Experimental study of the
phase relations between greenschist and amphibolite in a basaltic
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MacLeod C J amp Rothery D A (1992) Ridge axial segmentation in
the Oman ophiolite evidence from along-strike variations in the
sheeted dyke complex Geological Society of America Special Papers 60
39---63
Manning C E MacLeod C J amp Weston P E (1996a) Fracture-
controlled metamorphism of Hess Deep gabbros site 984
constraints on the roots of mid-ocean ridge hydrothermal systems
at fast-spreading centers In GillisM C Allan KM ampMeyer P S
(eds) Proceedings of the Ocean Drilling Program Scientific Results 147
College Station TX Ocean Drilling Program pp 189---211Manning C E Weston P E amp Mahon K I (1996b) Rapid high-
temperature metamorphism of East Pacific Rise gabbros from Hess
Deep Earth and Planetary Science Letters 144 123---132Manning C E MacLeod C J amp Weston P E (2000) Lower-crustal
cracking front at fast-spreading ridges evidence from the East Pacific
Rise and the Oman ophiolite Journal of the Geological Society London
349 261---272McCollom T M amp Shock E L (1998) Fluid---rock interactions in
the lower oceanic crust thermodynamic models of hydrothermal
alteration Journal of Geophysical Research 103 547---575
McCulloch M T Gregory R T Wasserburg G J amp Taylor H P
(1981) Sm---Nd Rb---Sr and 18O16O isotopic systematics in an
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Proceedings of the Ocean Drilling Program Scientific Results 147 College
Station TX Ocean Drilling Program
Nehlig P amp Juteau T (1988) Flow porosities permeabilities and
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1207
Nicolas A amp Ildefonse B (1996) Flow mechanism and viscosity in
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Nicolas A Ceuleneer G Boudier F amp Misseri M (1988) Structural
mapping in the Oman ophiolites mantle diapirism along an ocean
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of Oman and United Arab Emirates ophiolite-----discussion of a
new structural map Marine Geophysical Research 21(3---4)147---179
Nicolas A Boudier F Michibayashi K amp Gerbert-Gaillard L
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Nicolas A Mainprice D amp Boudier F (2003) High temperature
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morphology on magma supply and spreading rate Nature 364
706---708
Pin C Briot D Bassin C amp Poitrasson F (1994) Concomitant
extraction of Sr and Sm---Nd for isotopic analysis in silicate samples
based on specific extraction chromatography Analytica Chimica Acta
298 209---217
Rothery D A (1983) The base of a sheeted dyke complex Oman
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281 697---734
Spulber S D amp Rutherford M J (1983) The origin of rhyolite and
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Petrology 24 1---25Stakes D S (1991) Oxygen and hydrogen isotope compositions of
oceanic plutonic rocks high-temperature deformation and meta-
morphism of oceanic layer 3 In Taylor H P OrsquoNeil J et al (eds)
Geochemical Society Special Publication 3 77---90
Stakes D S amp Taylor H P (1992) The northern Samail ophiolite an
oxygen isotope microprobe and field study Journal of Geophysical
Research 97(B5) 7043---7080Staudigel H Davies G R Hart S R Marchant M amp Smith B
M (1995) Large scale isotopic Sr Nd and O isotopic anatomy of
altered oceanic crust DSDPODP sites 417418 Earth and Planetary
Science Letters 130 169---185Turner F J amp Verhoogen J (1960) Igneous and Metamorphic Petrology
New York McGraw---Hill 694 pp
Wilcock W S D amp Delaney J R (1996) Mid-ocean ridge
sulfide deposits evidence for heat extraction from magma
chambers or cracking fronts Earth and Planetary Science Letters 145
49---64
Witt G amp Seck H A (1989) Origin of amphibole in recrystallized
and porphyroclastic mantle xenoliths from Rhenish massif implica-
tions for the nature of mantle metasomatism Earth and Planetary
Science Letters 91 327---340
JOURNAL OF PETROLOGY VOLUME 45 NUMBER 6 JUNE 2004
1208
along a single crack from a mafic dyke to a hydrothermalvein (Fig 6c) We define hydrothermal veins as cracksfilled with greenschist-facies minerals that induce meta-morphic reactions in the adjacent country rocks andgabbroic dykes as planar intrusive bodies produced bythe crystallization of magma Following Nehlig amp Juteau(1988) we distinguish low-T greenschist-facies lsquowhiteveinsrsquo made of prehnite epidote chlorite and actinolite(Fig 6d) and lsquogreen veinsrsquo in which a darker greenamphibole predominates (Fig 6e) corresponding respec-tively to lower (195---410C) and higher (400---530C)temperatures (Nehlig amp Juteau 1988)The gabbroic dykes are typically a few centimetres
across composed of plagioclase clinopyroxene andbrown
hornblende They are coarse-grained to pegmatitic andcommonly display a comb-like structure (Fig 6b)Although the spacing between the dykes in the field ishighly variable the average is around 10m Gabbroicsills grade from clear intrusions to irregular and diffusepegmatitic gabbro patches up to a few metres in theirlarger dimension (Fig 7a)The distribution in the field of brownish pargasite
which appears black and glassy in outcrop is critical intracing the origin of the gabbroic dykes It is first observedat any level in the host lower and middle gabbros as largepoikilitic patches (Fig 7b) locally associated with simil-arly poikilitic patches of clino- and orthopyroxene(Fig 7a) These patches contain small aligned tablets of
(a) (b) (c)
g(e)(d)
alignment ofalignment of amphiboleamphibole oikocrystsoikocrysts
hwhite vein
hblpl
pcpx
brownbrownamphiboleamphiboleoikocrystoikocryst
eediabasedikdike
hb-plhb-plreactionreaction
imarginspbrown amphibole rich
gabbroic dikegabbroic dike
pbrown amphibole veins
Fig 7 Field relationships of dykes (scale bar is 10 cm long) (a) Irregular coarse to pegmatitic patches in a foliated gabbro matrix (01 OA42d)(b) Alignment of black amphibole oikocrysts in a foliated gabbro matrix (c) Local concentration of brown amphibole in a gabbroic dykelet Theplagioclase laths marking the foliation in the enclosing gabbro rotate into parallelism with the gabbroic dykelet (d) Two diabase dykes assimilatingthe enclosing layered gabbro with plagioclase---amphibole reaction margins (e) Pargasite-rich gabbroic dyke with wall reactions (01 OA19) (Notesmall pargasite---plagioclase veins) pl plagioclase cpx clinopyroxene hb hornblende
JOURNAL OF PETROLOGY VOLUME 45 NUMBER 6 JUNE 2004
1188
plagioclase oriented parallel to the larger plagioclaselaths defining the magmatic foliation of the gabbro(Fig 7c) Clinopyroxene oikocrysts surrounding idio-morphic plagioclase crystals are common in the upper-level gabbros they mark the end of the magmatic flowand crystallization growing before the pargasite andorthopyroxene oikocrysts as shown by their partialreplacement by pargasite Their crystallization marksthe transition at suprasolidus conditions from a dry to awet environment In the uppermost gabbros those dykesthat are typically altered in the HT hydrothermal faciesgrade upwards into the so-called lsquoisotropicrsquo or lsquovari-texturedrsquo gabbrosThe microgabbro and amphibolitized dolerite dykes
differ from the typical diabase dykes commonly cuttingthe gabbro section that have chilled margins and thusprobably intruded enclosing rocks already at temperat-ures below 450C In microgabbro and amphibolitizeddolerite dykes the absence of chilled margins and thedevelopment of dioritic reaction margins in the adjacentgabbro (Fig 7d and e) suggest that they were emplaced ingabbros that were still at temperatures above 750C(Nicolas et al 2000)
ANALYTICAL TECHNIQUES
Major and trace elements
Major element compositions were determined by elec-tron microprobe at the University of Montpellier II usinga Camebax SX100 Instrument calibration was per-formed on natural standards and operating conditionswere 20 kV accelerating potential 10 nA beam currentand 30 s counting timeRb and Sr concentrations in whole rock and separated
mineral fractions were measured by conventionalnebulization inductively coupled plasma mass spectro-metry (ICP-MS) using a VG Plasmaquad 2 (ISTEEMMontpellier) Digestion procedures followed thoseoutlined by Ionov et al (1992)
O---Sr isotopes
The samples selected for the detailed study wereextracted from gabbroic dykes or veins inside gabbrosand cut with a diamond saw into 2---3 cm fragmentsbefore being crushed into 05---1 cm fragments Forwhole-rock analysis small pieces were further crushedinto powder in an agate bowl For pure mineral analysisthe fragments were crushed with a manual agate mortarTo avoid or minimize the mixing of selected separateminerals with other phases that can bias the results astrict protocol of mineral selection was applied A smallsize fraction of 125---160 mm was used and minerals werefirst separated using a Frantz isodynamic magneticseparator Concentrates recovered (cpx plagioclase
amphibole epidote) were subsequently purified by care-ful hand-picking in alcohol under a binocular micro-scope At this stage mineral separates were very pureand only flawless minerals free of cracks and visibleinclusions were kept for isotopic analyses To ensurefurther mineral integrity these pure mineral separateswere ultrasonically washed in dilute 15N HCl and rinsedseveral times with ultra-pure water This stage potentiallyremoves adsorbed secondary micro-phases Previousstudies (eg Lecuyer amp Reynard 1996) demonstratedthat pyroxene and amphibole can be intermixed on avery fine scale with other phases (such as a range ofamphibole compositions talc Fe-oxide) Although thispossibility cannot be ruled out the procedure used inthis study makes it unlikely that complex minerals passedthrough the selection
Sr isotopic compositions
Approximately 50mg of whole-rock powder or separatedminerals were digested in a Teflon lsquobombrsquo with a mixtureof triple distilled 13N HNO3 and 48 HF with a fewdrops of HClO4 Two aliquots were separated one forthe determination of Sr isotopic composition and theother for Sr and Rb contents Sr was separated using anextraction chromatographic method modified from Pinet al (1994) Concentrations were measured by isotopedilution using 84Sr---87Rb mixed spikes To remove tracesof the late low-temperature seawater alteration and todetermine the primitive whole-rock Sr isotopic composi-tion leaching experiments were conducted on a fewsamples following the procedure described by Bosch(1991) The strontium isotopic compositions of leachateshave been analysed in the attempt to check for the leach-ing efficiency During these tests four Sr isotope composi-tions were measured which correspond to the unleachedsample the two liquids extracted after acid-leaching stepsand the final solid residue Leaching experiments wereconducted in two steps first with 6N HCl for 8min at70C second with 25N HCl for 30min at 70C Afterleaching the leachates were evaporated to dryness andthe solid residues digested similarly to the unleachedsamplesSr isotopic compositions were measured on a fully
automated VG Sector mass spectrometer housed at theUniversity of Montpellier II except for the Sr isotopicdata from the leaching experiments which have beenmeasured at the Universitee de Bretagne Occidentale(Brest) To assess the reproducibility and accuracy of Srisotopic measurements the NBS 987 standard was run12 times during this study the average value was0710245 with a precision better than 0000010 (2s)87Sr86Sr ratios are corrected for mass discrimination bynormalizing to a 86Sr88Sr value of 01194 Sr total blankconcentrations were less than 60 pg Initial 87Sr86Sr
BOSCH et al HYDROTHERMAL CIRCULATION IN OMAN OPHIOLITE
1189
ratios were calculated by correcting the measured ratiosfor accumulation of 87Sr produced by in situ 87Rb radio-active decay since 95Ma the assumed age of crystalliza-tion of the Oman ophiolite (Hacker 1994)
Oxygen isotopes
The powdered samples were introduced in Ni tubesunder dry air where they were reacted with BrF5 toform oxygen Oxygen was separated from other gasesby cooling the resulting mixture at liquid nitrogen tem-perature (77 K) that retained other volatile gases Oxygenwas further purified by trapping it on a cooled 5A mole-cular sieve at 77 K then quantified to calculate the yieldof the isotopic analyses and finally transferred to the massspectrometer (Finnigan Mat Delta E) where intensitiesassociated with masses 32 and 34 were measured todetermine d18O The isotopic composition of a sampleis given as dsample frac14 (RsampleRstandard 1) 1000 in permil unit where R is 18O16O and the standard isSMOW Analytical errors on the oxygen isotope ratioare 02 (2s)
RESULTS
Seven samples representative of the range of gabbroicdykes and five samples representative of the lower gab-bros two of them representing the margins of gabbroicdykes were selected for major element thermometricand Sr---O isotopic analyses (Tables 1---3) These samplesoriginate from wadi sections marked by bold lines inFig 1 Wadis Halhal Mayah Al Abyad Warihya andMaqsad structurally belong to a new ridge segment open-ing in slightly (1Myr) older lithosphere Wadi Gideahis in older lithosphere
Petrographic and isotopic characteristicsof the analysed samples
Olivine gabbro from the MTZ in MaqsadSamail Massif
Sample 94 MA2 is devoid of low-temperature alterationand only exhibits the discrete signature of the VHT---HTalteration The orthopyroxene corona around olivine(Fig 3a) is relayed by kelyphitic rims at the contactwith plagioclase (Fig 3b) Issuing from these rims aremicrometre-sized pale green amphiboles identified ascummingtonite and actinolite which penetrate the sur-rounding plagioclase in microcracks 004mm wide or atgrain boundaries (Fig 3b) similar to the microcrack sys-tem described by Manning et al (2000) A large numberof the plagioclase crystals have a cloudy core as a result ofthe concentration of fluid and mineral inclusions amongwhich diopside has been identified during microprobeanalysis Olivine cores are free of serpentine but theyare largely oxidized along a microcrack network Ortho-pyroxene coronas have a MgOpx number of 78---79
slightly higher than in the primary olivine cores withMgOl number of 74---75 The magmatic clinopyroxeneis totally fresh and probably not in equilibrium with thereactive orthopyroxene This sample has the lowest 87Sr86Sr initial ratio of the studied whole rocks (070322)identical to coexisting clinopyroxene (070324) and pla-gioclase (070322) This observation suggests the lack of alate superimposed LT hydrothermal alteration as it isvery unlikely that an LT hydrothermal event could havetotally equilibrated the Sr isotopic compositions of prim-ary minerals such as plagioclase and clinopyroxeneMoreover the whole rock (WR) clinopyroxene andplagioclase have similar initial Sr isotopic ratios despitevery different Sr contents Accordingly this ratio isthought to reflect the signature acquired during the crys-tallization of these minerals and is higher than the Omanmantle Sr isotopic value (McCulloch et al 1981) Thed18O values of the WR (50) and plagioclase (48) aredistinctly lower than primary magmatic values Indeedin normal mid-ocean ridge basalts (MORB) the WRd18O value is 57 02 with a corresponding plagio-clase d18O ranging from 55 to 62 ( Javoy 1980Gregory amp Taylor 1981)
Lower gabbro from Wadi Wariyah Wadi Tayin Massif
Sample 97 OA128b belongs to a layered sequence in thelower gabbro series injected by anorthositic sills Thesample was collected at the tip of one of these sillsbetween layers enriched in clinopyroxene The gabbrois composed of 60 plagioclase and 40 clinopyroxeneDespite the absence of olivine the WR has a relativelyhigh Mg content (Table 3) A series of low-T (prehnite)microveins 01mm thick cross-cut the gabbro perpendi-cular to the layering and foliation The margins of theveins are devoid of alteration This gabbro has the sameSr isotopic composition as 94 MA2 Nevertheless theWR and associated plagioclase have Sr isotope signatures(070333 and 070328) lower than the coexisting clino-pyroxene (070422) The Sr contents of the WR plagio-clase and clinopyroxene are 127 ppm 120 ppm and20 ppm respectively thus making the clinopyroxenemore sensitive to late-stage contamination Clinopyrox-ene and plagioclase oxygen isotope compositions (523and 414 respectively) are also lower than typicalmantle values Similar to the previous sample alterationat high temperature by a low d18O and high 87Sr86Srfluid is required to explain such characteristics
Amphibole gabbro patch Wadi Gideah Wadi TayinMassif
Sample 01 OA41d2 is from a laminated gabbro com-posed of millimetre-sized layers of clinopyroxeneand plagioclase Anatectic patches (Fig 7a) borderinganorthositic layers are formed of coarse-grained
JOURNAL OF PETROLOGY VOLUME 45 NUMBER 6 JUNE 2004
1190
Table1Locationoftheanalysed
samplesandsummaryoftheirpetrographicandSr---O
isotopiccharacteristics
Sam
ple
nam
eSite
Locationin
gab
broic
section
Typ
eofrock
Distance
to
Moho(km)
Parag
enesis
Alteration
87Sr
86Srinitial1
d18O
()2
94MA2
Maq
sad
From
MTZ
Layered
gab
bro
0Olivine(M
gno74---75)3
OPXco
ronas
(Mgno78---79)
WRfrac14
070322
WRfrac14
thorn495
CPX
Palegreen
Am
(cumact)
CPXfrac14
070324
nd
Plagioclase(A
n65---80)4inversezoning
Nolow-T
alteration
Plfrac14
070322
Plfrac14
thorn479
Solidfluid
inclusionsin
Pl
97OA128b
Wad
iWaryiah
From
lower
gab
bro
Layered
gab
bro
1CPX
Low-T
alteration
WRfrac14
070333
nd
Plagioclase(A
n85---87)
Prehnitemicroveins
CPXfrac14
070422
CPXfrac14
thorn523
Plfrac14
070328
Plfrac14
thorn414
01OA41d2
Wad
iGideah
From
upper
gab
bro
Gab
bro
patch
in
laminated
gab
bro
36
CPXrecrystallized
Plagioclaseidiomorphic
(An57---87)norm
alzoning
Black
pargasitic
honblende
WRfrac14
070351
CPXfrac14
070378
Plfrac14
070426
WRfrac14
thorn480
nd
Plfrac14
thorn504
Hbdfrac14
070355
nd
01OA19aamp
dWad
iWaryiah
Inlayeredgab
bro
Gab
broic
dyke
01
Olivine
OPXpoikilitic
WRfrac14
070341
WRfrac14
thorn567
CPX
Black
pargasite
Hbdfrac14
070324
Hbdfrac14
thorn287
Plagioclase(A
n73---95)
Hornblendepoikilitic
Plfrac14
nd
Plfrac14
thorn10 09
Hem
atite
WRfrac14
070418
nd
Green
Mg-hornblende
Acthornblende
Epidote
02OA90b
Wad
iAl-Abyad
Inlayeredgab
bro
Dioriticdyke
015
Green
Mg-hornblende
Hbdfrac14
070427
Hbdfrac14
thorn239
Plagioclase(A
n83---87)
Plfrac14
070387
Plfrac14
thorn498
Solidfluid
inclusionsin
plagioclase
01OA15f
Wad
iWaryiah
Inlayeredgab
bro
Comb-textured
gab
broic
dyke
025
CPXidiomorphic
Plagioclase(A
n95---99)norm
alzoning
BrownMg-H
ornblende
WRfrac14
070458
nd
02OA43a
Wad
iMayah
Inlayeredgab
bro
vein
Green
amphibole
1Palegreen
Mg-hornblendeacicular
Plagioclase(A
n91---97)in
reactionmargin
Hbdfrac14
070431
Hbdfrac14
thorn310
01OA36b
Wad
iGideah
Inlayeredgab
bro
epidosite
dyke
25
CPX
Epidote
WRfrac14
070519
nd
Plagioclase
Epfrac14
070554
nd
CPXclinopyroxene
Plplagioclase
OPXorthopyroxene
Amam
phibolecu
mcu
mmingtonite
actactinolite
WRwholerockHbdhornblende
Epep
idotend
notdetermined
187Sr
86Srinitialratioscalculatedat
95Ma(H
acker1994)Errors
ofthemeasuredratiosareindicated
inTab
le4
2Analyticalerrors
oftheoxygen
isotopevalues
are02
(2s)
3Mgnumber
frac14atomicMg(Mgthorn
Fe)
4Percentageofan
orthitein
plagioclase
BOSCH et al HYDROTHERMAL CIRCULATION IN OMAN OPHIOLITE
1191
(millimetre-sized) recrystallized clinopyroxene largelyaltered to light green magnesio-hornblende and idio-morphic plagioclase Locally primary black pargasitichornblende develops including idiomorphic normallyzoned plagioclase (Table 4) This gabbro exhibits similar87Sr86Sr ratios for the WR and pargasite (070351 and070355 respectively) These values are interpreted asrepresentative of the melt from which this amphibolecrystallized The clinopyroxene has a slightly higher Srisotope composition of 070378 consistent with the occur-rence of the light green Mg-hornblende rim Plagioclaseis significantly more radiogenic (070426) related to sec-ondary destabilization evidenced by the idiomorphichabit The Sr isotope compositions of the liquidsextracted from the acid leaching experiments of theWR (L1 and L2) are very similar 070358 and 070365respectively The 87Sr86Sr ratio of the residue obtainedafter leaching experiments is 070335 lower than boththe pargasite and unleached WR This value can beconsidered as the unaltered isotopic composition of thissample The d18O values measured for WR and coexist-ing plagioclase are lower than normal MORB magmaticvalues The 18O depletion in the plagioclase can be fairlywell explained by high-temperature hydrothermal inter-action between an oxygen-depleted fluid and the magmaAccording to the kinetics of hydrothermal exchange ofoxygen (Gregory amp Taylor 1981 Gregory et al 1989)plagioclase exchanges its oxygen isotopes with the fluidadjusting readily to the hydrothermal conditions Forhigh temperatures of exchange with seawater-derivedfluids (d18O between 0 and thorn2) the magmatic plagio-clase will have its primary d18O value lowered whereasthe d18O value for lower temperatures of exchange(5300C) will increase The amplification of the d18Oshift increases with the intensity of the hydrothermalalteration
Gabbroic dykes intrusive in lower gabbros from WadiWariyah Wadi Tayin Massif
Gabbroic dykes 01 OA19a and 01 OA19d are intrusiveinto the lower gabbros They are 3---4 cm wide discord-ant to the foliation of the host gabbros (Fig 7e) with asharp margin marked by a brown amphibole fringe Theprimary phases in the dykes (Table 1) locally preservedas elongated aggregates of a 01mm grain size mosaicassemblage at the dyke margins grade to millimetre sizein the central part of the dyke The WR major elementcomposition is Mg enriched compared with the enclosinggabbro (Table 3) Brown pargasitic amphibole oikocrystsmake up 50 of the modal composition of the dyke Indyke 01 OA19a poikilitic orthopyroxene may developinstead of brown amphibole Finally green magnesio-hornblende grows either as oikocrysts at the expense ofclinopyroxene or in association with epidote as an altera-tion product of plagioclase These low-amphibolite-faciesminerals represent the lowest-grade paragenesis recogn-ized in these dykes and are most developed in sample01OA19aTheamphibole composition showszoning fromTi-rich pargasitic hornblende to magnesio-hornblendeand actinolitic hornblende (Fig 8c) recording pro-gressive cooling of the dyke The plagioclase compositionis extremely heterogeneous (Fig 8b) varying from An95a composition similar to that of the plagioclase in the hostgabbro (Fig 8a) to An50 for the plagioclase and asso-ciated epidote inclusions in the poikilitic actinolitic horn-blende Sample 01 OA19d has an initial 87Sr86Sr ratioslightly higher for the WR (070341) than for the parga-site (070324) The Sr isotopic ratios of liquids extractedafter two steps of WR acid-leaching (L1 and L2) are070357 and 070341 respectively whereas the residueyields a 87Sr86Sr ratio of 070327 very close to thepargasite and plagioclase values The latter is similar tothe isotopic composition measured for the massive gab-bro 94 MA2 and may be taken as close to the primarymagmatic value This Sr isotopic ratio is more radiogenicthan the inferred Oman mantle-source value (McCullochet al 1981) The altered part of this sample has a radio-genic Sr isotopic composition (070418) related to thelow-amphibolite-facies alteration This gabbroic dykehas seemingly preserved a magmatic whole-rock d18Ovalue (567) but contains by far the highestd18O plagioclase (1009) of all the samples analysedThis plagioclase coexists with low d18O pargasite(thorn287) Similarly high values (d18Oplagioclase 4 8)have been previously measured for gabbroic dykes dia-bases sheeted dykes and plagiogranites from the Omanophiolite (Gregory amp Taylor 1981 Stakes amp Taylor1992) This 72 oxygen isotope fractionation betweenplagioclase and amphibole cannot possibly represent oxy-gen equilibrium at any plausible temperature This maybe explained by different exchange kinetics for these twominerals at different temperatures at high waterrock
Table 2 Average temperatures ( C 40C)
calculated for plagioclase---amphibole pairs
using the geothermometer of Holland amp
Blundy (1994)
Pargasite Magnesio-hornblende
rim core rim core
01 OA41d2 878 953 ------- -------
01 OA19a 935 974 825 869
01 OA19d 890 933 789 -------
02 OA37b ------- ------- 816 849
02 OA90b ------- ------- 859 833
JOURNAL OF PETROLOGY VOLUME 45 NUMBER 6 JUNE 2004
1192
ratios This supports the occurrence of two distincthydrothermal events one at high temperature and alater stage at lower temperature responsible for thestrong 18O enrichment in the coexisting plagioclase
Dioritic dyke associated with a diabase dyke in lowergabbros from Wadi Halhal Haylayn Massif
Sample 02 OA37b belongs to a group of brownamphibole---plagioclase lenses or reaction margins asso-ciated with diabase intrusives (Fig 7d) in lower layeredgabbros Idiomorphic brown hornblende (tschermakite)crystals are elongated in the plane of the dyke represent-ing 80 of the modal composition They are associatedwith an interstitial plagioclase matrix The browntschermakite has greenish rims Plagioclase has a cloudyappearance due to micrometre-size fluid inclusionsexcept along its margins and internal microcracks Bothhornblende and plagioclase exhibit evidence of plasticdeformation The Sr isotopic ratio for the pargasite andplagioclase is 070348 and 070420 respectively Thepargasite exhibits a Sr signature closest to that of theOman primary magmatic value (McCulloch et al 1981)Nevertheless it is too high to be attributed to anunmodified MORB-source mantle signature This sug-gests the addition of an external fluid component in goodagreement with the shift of oxygen isotopic compositionto a low value (thorn39) suggesting a high-temperatureexchange with a low d18O fluid The Sr isotope composi-tion of the plagioclase is significantly higher than that of
the coexisting pargasite probably as a result of the low-temperature alteration
Dioritic dyke cross-cutting lower layered gabbros in WadiAl-Abyad Nakhl Massif
Sample 02 OA90b is a 1 cm thick dioritic dyke cross-cutting the lower layered gabbros It grades into a greenamphibole vein as illustrated in Fig 6c The dyke is com-posed of 2 cm long dark green idiomorphic magnesio-hornblende crystals growing in the plane of the dykeand plagioclase concentrated at the margin of the dykeThe plagioclase in the dyke and in the enclosing gabbrocontains micrometre-size fluid and mineral inclusionssimilar to those observed in sample 94 MA2 The plagio-clase and green hornblende have higher Sr isotopic ratios(070387 and 070427 respectively) and lower d18Ovalues (thorn498 and thorn239 respectively) than the normalMORB-source mantle value As observed for previoussamples this supports interaction with an external fluidcomponent
Comb-textured gabbroic dykes in the lower gabbros fromWadi Wariyah Wadi Tayin Massif
Sample 01 OA15f (Fig 6b) is from Wadi Wariyah andrepresents a 2 cm wide dyke intruding the lower gabbrosThis type of comb-textured dyke developed in theclinopyroxene---plagioclase stability field It containslarge idiomorphic clinopyroxene crystals which arepartially replaced by brownish magnesio-hornblendeThe plagioclase is extremely calcic with a slight inverse
Table 3 Whole-rock major element compositions of massive gabbros and dykes determined by X-ray fluorescence
(wt )
Sample 94 Ma2 97 OA128b 01 OA41d2 01 OA19d 01 OA19a 01 OA15f 01 OA36b 01 OA36b
Rock type Gabbro Gabbro Gabbro patch Gabbroic dyke Gabbroic dyke Comb-textured
gabb dyke
Epidosite dyke Epidosite dyke
(margin)
SiO2 4830 4815 4749 4524 4339 4799 389 4371
Al2O3 2785 2032 2438 1474 1246 1192 2752 1646
Fe2O3 328 266 404 981 1204 592 381 623
MnO 003 004 005 014 016 006 008 014
MgO 405 737 427 1031 1686 1326 187 739
CaO 1289 1908 1477 1235 1156 166 2471 2339
Na2O 292 123 257 258 111 075 000 013
K2O 000 000 009 006 000 000 000 000
TiO2 005 017 063 230 064 086 013 042
P2O5 008 007 013 018 009 013 007 010
LOI 033 076 138 216 161 237 274 190
Total 9978 9985 998 9987 9992 9986 9983 9987
LOI loss on ignition gabb gabbroic
BOSCH et al HYDROTHERMAL CIRCULATION IN OMAN OPHIOLITE
1193
Table4Majorelementcomposition
aofselectedam
phibolespaired
withplagioclasecompositionsusedforthermometry
Sam
ple
nam
e1Mode2
SiO
2TiO
2Al 2O3
Cr 2O3
FeO
MnO
MgO
CaO
Na 2O
K2O
Total
Si
AlIV
AlVI
Ti
Fe3
thornCr
Mg
Fe2
thornMn
Na
KCa
XAn3
T(C)4
01OA19a
Mg-hornblende
core
45 60
26
10 78
009
860
015
16 56
12 00
212
007
96 72
6552
1448
0366
0083
0552
0004
3409
0594
0016
0611
0018
1843
0903
899
Mg-hornblende
core
47 99
263
850
002
897
011
16 61
12 10
160
020
96 36
6894
1106
0334
0028
0483
0002
3557
0594
0014
0446
0036
1862
0892
834
Mg-hornblende
rim
51 17
247
596
001
761
014
18 11
12 19
103
010
96 47
7257
0743
0253
0016
0414
0001
3828
0488
0016
0284
0018
1853
0894
820
Mg-hornblende
rim
47 50
291
920
004
880
014
16 46
11 95
174
008
96 07
6835
1165
0396
0015
0506
0005
3529
0552
0017
0486
0015
1843
0887
825
Mg-hornblende
rim
49 57
269
742
000
799
017
18 07
12 02
133
014
96 88
7013
0987
0261
0018
0575
0000
3811
0371
0020
0365
0026
1821
0860
831
Mg-hornblende
core
47 83
181
872
003
840
011
14 48
12 18
159
008
93 59
6814
1186
0279
0018
0644
0004
3711
0357
0013
0438
0015
1859
0907
874
Pargasite
rim
42 15
249
11 87
024
10 07
013
13 84
11 43
263
032
96 61
6188
1812
0242
0434
0299
0028
3028
0937
0016
0749
0059
1797
0804
972
Pargasite
rim
42 22
260
11 86
026
10 21
011
13 71
11 48
253
032
96 59
6201
1799
0254
0429
0299
0030
3001
0955
0014
0720
0060
1807
0803
962
Pargasite
core
42 05
263
11 45
030
10 19
011
13 80
11 47
246
032
96 46
6189
1811
0175
0478
0299
0035
3029
0956
0014
0702
0060
1809
0767
974
Pargasite
rim
42 36
247
11 90
016
945
013
14 16
11 76
249
026
95 98
6239
1761
0305
0366
0276
0019
3109
0888
0016
0710
0049
1856
0818
926
Pargasite
core
42 50
291
11 64
016
984
015
14 22
11 41
256
034
96 69
6219
1781
0227
0426
0322
0018
3103
0883
0018
0727
0063
1789
0812
974
Pargasite
rim
42 76
270
12 03
017
920
012
14 87
11 82
244
025
96 73
6224
1776
0880
0335
0368
0020
3226
0752
0015
0689
0047
1843
0841
941
Pargasite
rim
42 13
181
13 54
016
921
009
12 77
12 13
254
032
96 98
6172
1828
0510
0450
0000
0018
2788
1129
0011
0721
0059
1905
0841
874
01OA19d
Mg-hornblende
49 26
047
624
011
11 46
017
15 39
12 21
108
009
96 48
7137
0863
0202
0051
0437
0013
3323
0951
0021
0302
0018
1896
0768
771
Mg-hornblende
46 03
287
932
018
906
017
14 77
12 87
193
007
97 28
6672
1328
0265
0313
0046
0021
3191
1053
0021
0542
0012
1999
0758
808
Pargasite
41 89
429
11 65
008
11 03
017
13 52
11 37
259
017
96 77
6159
1841
0179
0474
0345
0010
2963
1012
0021
0739
0032
1791
0770
975
Pargasite
rim
42 80
340
11 16
009
11 01
018
13 73
11 67
240
022
96 67
6293
1707
0226
0376
0322
0010
3009
1032
0022
0684
0041
1839
0605
880
Pargasite
core
42 58
351
11 27
006
11 63
018
13 51
11 46
239
022
96 82
6257
1743
0209
0387
0391
0007
2959
1039
0023
0680
0042
1804
0595
893
Pargasite
rim
42 14
393
11 28
006
11 71
014
13 35
11 33
235
033
96 62
6214
1786
0174
0435
0391
0007
2935
1054
0018
0673
0062
1790
0519
901
Pargasite
core
41 56
420
12 05
006
11 57
016
12 51
11 35
249
027
96 24
6168
1832
0277
0469
0276
0007
2768
1160
0021
0718
0052
1805
0537
882
Pargasite
core
41 33
438
11 49
008
11 66
015
13 35
11 45
264
026
96 80
6105
1894
0107
0487
0368
0009
2941
1073
0019
0757
0048
1813
0537
942
Pargasite
41 08
446
12 39
012
11 35
017
13 13
11 36
263
015
96 85
6049
1951
0199
0494
0368
0014
2882
1030
0021
0752
0027
1792
0758
975
02OA37b
Mg-hornblende
rim
45 15
260
984
002
13 28
019
14 01
10 87
128
009
97 33
6527
1473
0203
0283
0690
0002
3018
0915
0023
0360
0016
1683
0562
828
Mg-hornblende
rim
44 72
263
988
004
13 23
021
14 01
10 78
133
009
96 92
6494
1506
0185
0287
0713
0005
3032
0894
0025
0374
0018
1677
0593
854
Mg-hornblende
rim
45 68
247
935
004
12 87
020
14 10
11 00
121
011
97 02
6621
1379
0218
0269
0598
0004
3045
0962
0024
0341
0020
1708
0562
815
Mg-hornblende
core
43 94
291
10 87
001
12 60
023
13 73
10 59
142
011
96 39
6410
1590
0278
0319
0644
0001
2985
0893
0028
0401
0021
1655
0710
860
Mg-hornblende
core
44 62
270
10 37
001
12 66
019
14 12
10 58
140
008
96 73
6479
1521
0253
0294
0644
0001
3057
0893
0023
0395
0014
1646
0674
851
Mg-hornblende
rim
47 79
181
796
002
12 09
025
15 22
11 09
100
009
97 33
6866
1134
0214
0196
0506
0002
3260
0947
0030
0280
0016
1706
0503
767
Mg-hornblende
core
45 43
249
10 3
000
11 99
022
14 50
11 14
133
008
97 48
6524
1476
0267
0269
0644
0000
3103
0796
0027
0370
0015
1715
0703
838
JOURNAL OF PETROLOGY VOLUME 45 NUMBER 6 JUNE 2004
1194
Sam
ple
nam
e1Mode2
SiO
2TiO
2Al 2O3
Cr 2O3
FeO
MnO
MgO
CaO
Na 2O
K2O
Total
Si
AlIV
AlVI
Ti
Fe3
thornCr
Mg
Fe2
thornMn
Na
KCa
XAn3
T(C)4
02OA90b
Mg-hornblende
core
48 83
038
745
005
939
010
17 29
12 24
142
006
97 21
6931
1069
0041
0041
0667
0005
3657
0448
0011
0392
0010
1862
0840
856
Mg-hornblende
core
49 49
033
731
003
916
008
17 40
12 57
139
005
97 82
6987
1013
0035
0035
0552
0004
3662
0530
0010
0379
0009
1901
0830
811
Mg-hornblende
rim
48 90
040
753
005
932
013
17 15
12 22
154
005
97 29
6948
1052
0042
0042
0575
0006
3632
0533
0015
0424
0010
1861
0880
869
Mg-hornblende
rim
48 30
044
800
007
962
010
16 86
12 32
161
005
97 35
6873
1127
0047
0047
0598
0008
3576
0546
0012
0443
0010
1879
0870
862
Mg-hornblende
rim
49 12
032
742
010
922
011
17 36
12 29
142
006
97 43
6956
1044
0034
0034
0621
0011
3665
0471
0013
0391
0010
1865
0840
845
02OA43a
Mg-hornblende
rim
48 13
040
789
007
860
007
17 41
12 19
155
006
96 36
6882
1118
0212
0043
0621
0008
3710
0407
0008
0429
0010
1867
-------
-------
Mg-hornblende
rim
50 93
035
643
001
759
006
18 50
12 24
114
003
97 28
7152
0848
0217
0037
0506
0001
3873
0385
0007
0311
0005
1841
-------
-------
Mg-hornblende
rim
50 67
014
559
009
12 33
026
15 28
12 44
068
003
97 50
7260
0740
0204
015
0483
0010
3263
0994
0032
0189
0005
1909
-------
-------
Mg-hornblende
rim
50 34
013
621
009
12 49
027
15 07
12 51
064
003
97 78
7192
0810
0240
0010
0530
0010
3210
0960
0030
0180
0010
1910
-------
-------
Mg-hornblende
rim
45 17
060
11 14
013
988
008
15 78
12 33
211
009
97 31
6473
1527
0355
0640
0644
0015
3371
0540
0010
0588
0016
1892
-------
-------
Mg-hornblende
rim
49 25
032
712
014
845
009
17 84
12 32
137
004
96 95
6983
1020
0170
0030
0620
0020
3770
0380
0010
0380
0010
1870
-------
-------
Mg-hornblende
rim
48 61
032
775
014
975
011
16 81
12 57
139
004
97 50
6905
1950
0203
0034
0621
0016
3560
0537
0013
0382
0008
1914
-------
-------
94Ma2
Anthophyllite
54 98
000
259
001
12 65
037
23 35
199
034
000
96 28
7744
0256
0174
0000
0161
0001
4903
1330
0043
0092
0001
0301
-------
-------
Anthophyllite
55 11
000
296
003
14 09
032
23 66
063
038
000
97 18
7716
0284
0205
0000
0115
0003
4938
1535
0038
0104
0000
0095
-------
-------
Actinolite
48 58
004
954
002
960
014
17 67
999
139
005
97 02
6850
0150
0435
0004
0690
0002
3713
0442
0017
0379
0009
1510
-------
-------
Actinolite
51 54
004
585
000
890
022
20 28
930
090
002
97 05
7214
0786
0178
0005
0598
0000
4230
0444
0026
0244
0004
1395
-------
-------
01OA41d2
Pargasite
rim
43 05
342
10 71
008
11 46
012
13 50
11 54
179
067
96 34
6353
1647
0216
0380
0368
0009
2967
1046
0015
0512
0127
1824
0613
863
Pargasite
rim
42 61
354
11 59
001
10 59
010
13 52
11 87
160
076
96 19
6286
1714
0302
0392
0299
0001
2972
1007
0012
0458
0143
1877
0715
844
Pargasite
core
42 54
401
11 04
006
11 81
013
13 21
11 50
202
039
96 71
6266
1734
0182
0444
0368
0007
2901
1087
0017
0576
0073
1814
0854
979
Pargasite
rim
43 03
385
10 72
002
11 70
014
13 50
11 95
146
080
97 17
6314
1686
0168
0425
0345
0002
2953
1091
0018
0415
0149
1878
0634
852
Pargasite
rim
42 65
341
10 60
006
11 97
014
13 22
11 64
183
068
96 20
6332
1668
0186
0381
0345
0007
2926
1141
0018
0528
0129
1851
0620
869
Pargasite
core
42 82
367
10 60
007
11 93
012
13 42
11 44
206
047
96 60
6315
1685
0158
0407
0391
0008
2950
1080
0016
0588
0089
1808
0615
899
Pargasite
rim
42 2
362
11 10
007
11 42
016
13 09
11 72
179
072
95 89
6282
1718
0229
0405
0299
0009
2905
1123
0020
0518
0137
1869
0715
875
Pargasite
rim
42 42
380
11 03
006
11 77
012
13 00
11 72
188
069
96 49
6283
1717
0209
0423
0299
0007
2870
1158
0015
0539
0131
1860
0817
928
Pargasite
rim
42 21
364
11 14
006
10 73
011
13 53
11 68
177
072
95 59
6277
1723
0229
0407
0322
0007
2999
1012
0014
0511
0136
1862
0736
883
Pargasite
rim
42 25
376
11 03
008
10 93
015
13 66
11 72
185
064
96 07
6257
1743
0182
0418
0345
0009
3016
1009
0019
0531
0121
1860
0766
912
Pargasite
core
42 17
396
10 91
008
11 10
010
13 57
11 49
209
038
95 85
6253
1747
0159
0442
0368
0010
3000
1009
0013
0602
0073
1826
0841
981
aMajorelem
entco
mpositionsarein
weightpercent-------noplagioclasean
alysed
andnotemperature
calculated
1Typ
eofam
phibolean
alysed
isalso
indicated
2Blankindicates
nozoningobserved
3Molefractionofan
orthitein
plagioclasead
jacentto
amphibolegrain
4Tem
perature
ofeq
uilibrium
fortheam
phibole---plagioclasepaircalculatedusingtheHollandamp
Blundythermometer
(1994)Errors
are40
C
BOSCH et al HYDROTHERMAL CIRCULATION IN OMAN OPHIOLITE
1195
zonation from An95 to An99 from core to rim (Table 1 andFig 8a) TheWR Sr isotopic ratio of the sample (070458)is significantly higher than normal MORB values Thiscould be related to the overprint of a low-temperaturealteration event consistent with the petrography of thesample
Green amphibole vein cross-cutting layered gabbros inWadi Mayah Haylayn Massif
Sample 02 OA43a belongs to a series of 1 cm thick greenamphibole veins (as in Fig 6e) cross-cutting the layeredgabbros The veins develop 15 cm thick reaction marginsin the enclosing gabbro and are exclusively composed ofacicular pale green magnesio-hornblende growing per-pendicular to the vein margin in a crack-seal mode(Fig 6f ) The reaction margins are composed of urali-tized gabbro marked by the development of the samepale green magnesio-hornblende in an inhomogeneouscalcic plagioclase matrix (An91---97) The Sr and O isotopiccompositions of this green hornblende (070431 andthorn31) do not correspond to normal MORB-sourcemantle values
Epidosite dyke from Wadi Gideah Wadi Tayin Massif
Sample 01 OA36b is representative of a series of epidoteveins cross-cutting poorly layered gabbros These 2 cmthick veins are composed of pink thulite in their centreand green pistachite at their margins (Fig 6a) Theirsharp contact with the enclosing gabbro is marked bythe development of coarse clinopyroxene crystals alteredin the epidote---greenschist facies This suggests that theepidote vein is replacive in a coarse-grained gabbro dykeThe highest 87Sr86Sr initial ratios of all the samplesstudied are recorded by the WR and coexisting epidoteof this sample (070519 and 070554 respectively)(Table 5) The Sr content of the WR and associatedepidote are also the highest of the present dataset(716 ppm and 764 ppm respectively) The Sr isotopiccomposition of this sample is consistent with a greaterdegree of exchange with a radiogenic component fromCretaceous seawater It is significantly higher than thevalue obtained for an epidosite vein from the Wadi Fizhsection of the Oman ophiolite [070435 with a Sr contentof 488 ppm reported by Kawahata et al (2001)] but ingood agreement with the average of the greenschist-faciesalteration sequence defined by Kawahata et al (2001)
GABBROS
01 km 025 km 1 km 36 km50
60
70
80
90
An
MA2
19d
19a 15f 128b
41d2
(a)
43a17b
15f
90b
37b
19a
19d
40
60
80
100
A
n
01 km 015 km 025 km 1 km
DYKES
(b)
Distance to the Moho
ACTINOLITE
MAGNESIOHORNBLENDE
TSCHERMAKITE
PARGASITEGabbros Dykes41d2MA2
19a d37b15f
90b43a
AlIV
150500
02
08
10
(Na
+ K
) A
(c)
04
06
950
900
850
800
750
70007 12 17
T degC
1000
(d)
AlIV
Fig 8 Plagioclase---amphibole compositions and results of thermometry (a) and (b) plagioclase compositions in layered gabbros and in dykesrespectively as a function of distance to the Moho (see text for sample identification) (c) amphibole compositions in layered gabbros and dykesaccording to Leakersquos (1997) nomenclature (d) equilibration temperatures calculated using the amphibole---plagioclase thermometer of Holland ampBlundy (1994) as a function of AlIV content in amphibole [same symbols as in (c)]
JOURNAL OF PETROLOGY VOLUME 45 NUMBER 6 JUNE 2004
1196
Table5RbandSrcontents87Sr
86Srandd1
8Oisotopiccompositionsofwholerocksandmineralseparatesfrom
samplesofmassivegabbrosdykes
andveinsfrom
theOman
ophiolitealsolistedarecalculated
waterrockratios(W
R)andLOIvalues
Sam
ple
Rb(ppm)
Sr(ppm)
87Rb8
6Sr
87Sr
86Sr
(87Sr
86Sr)i1
d18O
()
LOI(
)2WR
3(87Sr
86Sr)L14
(87Sr
86Sr)L25
(87Sr
86Sr)R6
01OA41d2
Whole
rock
037
1792
0006
0703516
09
070351
thorn480
138
47
0703587
0703651
0703357
Plagioclase
025
1249
0003
0704261
22
070426
thorn504
-------
-------
Pargasite
-------
-------
-------
0703548
20
070355
-------
-------
-------
Clinopyroxene
045
26 0
0050
0703845
19
070378
-------
-------
-------
01OA36b
Dykewhole
rock
0005
7164
50001
0705186
17
070519
-------
274
21 9
0705207
0705112
-------
Epidote
0003
7645
0001
0705536
12
070554
-------
-------
-------
Wallwhole
rock
005
4254
0001
0704637
09
070464
-------
191
-------
0704945
0704675
0704593
02OA43a
Green
hornblende
004
98
0012
0704323
13
070431
thorn310
-------
-------
97OA128b
Whole
rock
003
1274
50001
0703327
17
070333
-------
076
37
-------
-------
-------
Plagioclase
002
1204
50001
0703285
09
070328
thorn414
-------
-------
Clinopyroxene
003
20 0
0004
0704230
09
070422
thorn523
-------
-------
01OA15f
Whole
rock
006
1077
0002
0704582
16
070458
-------
237
13 5
-------
-------
-------
01OA19a
Whole
rock
037
1303
0008
0704189
18
070418
-------
161
96
-------
-------
-------
01OA19d
Whole
rock
058
2032
0008
0703423
08
070341
thorn567
216
415
0703574
0703408
0703271
Pargasite
091
1058
0025
0703273
11
070324
thorn287
-------
-------
Plagioclase
-------
-------
-------
-------
-------
thorn10 09
-------
-------
02OA90b
Plagioclase
004
2385
50001
0703868
11
070387
thorn498
-------
-------
Green
hornblende
010
23 8
0012
0704288
15
070427
thorn239
-------
-------
BOSCH et al HYDROTHERMAL CIRCULATION IN OMAN OPHIOLITE
1197
Table5continued
Sam
ple
Rb(ppm)
Sr(ppm)
87Rb8
6Sr
87Sr
86Sr
(87Sr
86Sr)i1
d18O
()
LOI(
)2WR
3(87Sr
86Sr)L14
(87Sr
86Sr)L25
(87Sr
86Sr)R6
02OA37b
Plagioclase
023
3121
0002
0704204
15
070420
-------
-------
-------
Pargasite
031
41 6
0021
0703505
15
070348
thorn394
-------
-------
94OAMa2
Whole
rock
0016
1750
00003
0703219
15
070322
thorn495
033
31
0703264
0703177
0703158
Plagioclase
0014
1363
00003
0703219
11
070322
thorn479
-------
-------
Clinopyroxene
-------
-------
-------
0703236
13
070324
-------
-------
-------
1Initial8
7Sr
86Srratiocalculatedat
95Ma(H
acker1994)
2LOIisloss
onignition(in)
3Water---rock
ratiocalculatedassumingaclosedsystem
TheeffectiveWR
ratiocanbeexpressed
bythefollowingeq
uation
W=Reffectivefrac14
frac12eth87Sr=
86SrrockTHORN fi
nal
eth87Sr=
86SrrockTHORN in
itial=frac12eth8
7Sr=
86SrseawaterTHORN in
itial
eth87Sr=
86SrrockTHORN fi
nal
ethCrock
initial=C
seaw
ater
initial
THORNwhere(87Sr
86Srrock) finalisthefinalmeasuredSrisotopicratioin
thesample(87Sr
86Srrock) in
itialco
rrespondingto
theinitialm
antlevalue
istakenas
070295(Lan
phere
etal1981)(87Sr
86Srseawater) in
itialistheCretaceousseaw
ater
Srisotopicco
mposition[87Sr
86Sr07073
at90
MaafterBurkeet
al(1982)]C
seaw
ater
initial
istheSrco
ntent
oflsquonorm
alrsquoseaw
ater
andis
takenas
8ppm
(deVilliers1999)
FortheCrock
initialitis
assumed
that
thepresentSrco
ncentrationmeasuredis
thesameas
theinitial
concentration
4Srisotopicratiosmeasuredforliquid
L1extractedafterthefirststep
oftheleachingexperim
ent(6NHClfor8min
at70
C)
5Srisotopicratiosmeasuredforliquid
L2extractedaftertheseco
ndstep
oftheleachingexperim
ent(2 5NHClfor30
min
at70
C)
6Srisotopicratiosmeasuredforresidueobtained
afteratotalaciddigestionachievedaftertheextractionofL1an
dL2leachates
JOURNAL OF PETROLOGY VOLUME 45 NUMBER 6 JUNE 2004
1198
Part of the same sample from the contact between thedyke and the gabbro yields a slightly lower Sr isotopiccomposition (070464) intermediate between the sur-rounding gabbro and the epidote dyke It also has anintermediate Sr content (425 ppm) Acid leaching experi-ments performed on the whole rock of the dyke yield Srisotopic compositions of 070521 and 070512 for thetwo liquids extracted Similar experiments for the samplelocated at the contact with the gabbro yield 87Sr86Srratios of 070494 and 070467 for L1 and L2 liquids TheSr isotopic composition of the residue extracted afterleaching remains high (070459) and very close to theepidosite 87Sr86Sr ratio determined by Kawahata et al(2001) In this sample the primary minerals have beentotally replaced by epidote and secondary plagioclase orquartz Thus the Sr isotopic compositions measured forthis WR sample and its constituent minerals reflect thecomposition of the fluid filling this dyke
Mineral chemistry
Plagioclase
Compared with the host gabbros the dykes show a muchlarger dispersion in their plagioclase compositions (Fig 8aand b Table 4) characterized by more calcic plagioclasethan the enclosing gabbros This tendency is mostextreme in the comb-textured dykes in which the plagio-clase is almost pure anorthite The zoning is generallynormal recording a crystal fractionation process How-ever gabbro 94 MA2 exhibits inverse zoning this tend-ency is also observed in plagioclases from dykes 01OA19a and 01 OA15f The higher calcium content ofthe plagioclase as well as the inverse zoning is consistentwith an increase of water pressure during its crystalliza-tion (Turner amp Verhoogen 1960 Carmichael et al 1974Koepke et al 2003) The particularly high An content inthe comb-textured gabbro dykes 01 OA15f may alsoresult from crystallization in a supercooled magma thisis consistent with the comb texture indicative of fast-growing crystals The lack of brown hornblende suggeststhat crystallization occurred above the temperature ofhornblende stability
Amphibole
The amphiboles analysed in the dykes (Fig 8c andTable 4) show a regular trend in a (Na thorn K)A vsAlIV diagram from titanium-rich (Ti 4 015) pargasitichornblende (brownamphibole) and tschermakite (brown---green amphibole) to low-titanium (Ti5 015) magnesio-hornblende (green amphibole) and actinolite (pale greenamphibole) The pargasitic hornblende develops aspoikilitic oikocrysts in the dykes 01 OA19a and d andin the diffuse patch 01 OA41d2 at temperatures above800C during the final stage of crystallization (see
below) The tschermakite composition corresponds tothe internal alteration of clinopyroxene in gabbro 94Ma2 occurs in the comb-textured dyke 01 OA15f andis the primary amphibole in the dioritic dyke 02 OA37bThe magnesio-hornblende develops at the edges of thepargasites or of clinopyroxenes at temperatures around800C and is the primary phase in the dioritic dyke 02OA90b and in the green vein 02 OA43a Finally actino-lite occurs as a replacement of green hornblende in dykesor in the kelyphitic rim surrounding olivine in gabbro 94Ma2 Such a large composition range at the scale of athin section has been reported in amphiboles fromamphibolite-facies oceanic gabbros (Stakes 1991 Stakesamp Taylor 1992 Gillis et al 1993) as well as in the Omanophiolite gabbros (Manning et al 2000) This variabilityhas been explained by rapid reaction rates preventinghomogenization (Manning et al 2000)
Thermometry
Plagioclase---amphibole thermometry could be appliedwhen the two minerals exhibited good contacts such asin the laminated gabbro of the middle sequence (01OA41d2) and in the intrusive dykes The temperaturesof plagioclase---amphibole equilibration (Table 2) havebeen calculated using the geothermometer lsquoBrsquo of Hollandamp Blundy (1994) Edenite thorn Albite frac14 Richterite thornAnorthite valid between 500C and 900C with anuncertainty of 40C Amphibole formulae are basedon 23 oxygen and their nomenclature is based on alkalications in the A site vs AlIV according to Leake (1997)Pressurewas assumed to be 1 kbar Forty-five plagioclase---amphibole pairs meet the compositional criteria imposedby Holland amp Blundyrsquos dataset (09 4 Xan 4 01 andAlIV 403) Electron microprobe measurements wereconstrained to contiguous plagioclase and amphiboleseparated by sharp grain boundaries assuming equili-brium of the two phases When the mineral phasesexhibit normal zoning temperatures between plagioclaseand amphibole cores and between mineral rims werecalculated assuming that the temperatures deduced fromthe mineral core chemistry provide a proxy for the high-est temperature of equilibration These data are identi-fied in the T C vs AlIV diagram (Fig 8d) Table 4presents selected major element amphibole analyses theAn content of the plagioclase adjacent to each amphibolegrain and the temperature calculated for the pairThe temperatures calculated (Fig 8d) span the entire
range of temperatures at which amphibole and plagio-clase coexist in equilibrium (ie amphibolite-facies para-geneses) This temperature interval is bracketed byorthopyroxene-bearing facies or amphibole-free facies(ie gabbro 94MA2) and epidote---actinolite facies devoidof homogeneous plagioclase (ie epidosite dyke 01OA36b and green vein 02 OA43a) The range of the
BOSCH et al HYDROTHERMAL CIRCULATION IN OMAN OPHIOLITE
1199
calculated temperatures between 4900C and 750Ccorresponds to the changing composition of amphibolesfrom pargasite to magnesio-hornblende at the scale of athin section or even at that of an individual crystal asshown by their correlation with AlIV such variabilityrecords rapid cooling in hydrous conditions The highesttemperatures are recorded by pargasite from the anatec-tic gabbro 01 OA41d2 in the gabbro dyke 01 OA19dand in the dioritic dyke 02 OA90b They were calculatedfrom minerals devoid of lower-grade alteration and arethus considered as representing the equilibrium temperat-ure of the coexisting minerals Lower temperatures cor-respond to mineral phases evolving at falling temperatureand are only indicative of the cooling history Includingthe 40C uncertainty seven pairs provide temperatureshigher than 900C the limit of validity of the Holland ampBlundy thermometer However we maintain these tem-peratures as indicative of the highest values of our data-set These values are included in the average equilibriumtemperature summarized in Table 2
DISCUSSION
Distribution of hydrothermal alteration
Two distinct petrological events are recorded in the gab-bro section of the Samail ophiolite and are documentedin the present contribution (1) the whole gabbro sectionis affected by a penetrative alteration (2) the gabbrosection is crosscut by various types of dykes and veinsincluding hydrous minerals
Penetrative alteration
The penetrative alteration developed in the gabbros atgrain boundaries and locally invading the minerals isubiquitous in the gabbro section This penetrative altera-tion has been described as very high temperature (VHT)high temperature (HT) and low temperature (LT) basedon alteration parageneses The orthopyroxene coronas inthe layered gabbro sample and the incipient appearanceof brown pargasitic amphibole mark a lower thermallimit of the VHT alteration By reference to the experi-mental work of Koepke et al (2003) the temperature ofequilibration for these reactions is estimated to bebetween 900 and 1000CThe preservation of the vertical distribution of
VHT---HT facies (Figs 4 and 5) also pointed out byManning et al (2000) indicates that the hydrothermalalteration pattern fits the distribution of isotherms withdepth at a fast-spreading ridge (ie Phipps Morgan ampChen 1993 Dunn et al 2000 Fig 2) Equilibrium tem-peratures for VHT parageneses as high as 900---1000Csuggest that VHT---HT hydrothermal alteration tookplace in a very restricted time interval at the limit ofthe cooling magma chamber It is proposed that the
penetrative alteration affecting the entire gabbro sectionis related to a pervasive network of microcracks whichact as a recharge system down to the Moho (Nicolas et al2003)
Dykes and veins
The dykes are extremely varied eg pegmatitic gabbroswith comb-texture microgabbros and amphibolitizeddolerites dioritic dykes alignments of poikilitic horn-blende and finally diffuse hornblende-bearing patchesHigh-T gabbro and diorite dykes and low-T greenamphibole---epidote veins are connectedIn the dykes textural evidence attests to suprasolidus
conditions at least for the development of the pargasiticamphibole Integrating the hornblende---plagioclase equi-libration temperatures calculated for the studied samplesthese temperatures are bracketed between 950C and875CIn the following discussion we shall consider separately
the microgabbro and dolerite dykes The latter representbasaltic melt intrusions associated upsection with thediabase sheeted dykes and downsection possibly withthe mafic dykes that are ubiquitous in the mantle section(Nicolas et al 2000) Whatever their origin the develop-ment of pargasitic amphibole during their late stage ofcrystallization creates a link with the gabbroic dykes andsuggests a crystallization process evolving from dry con-ditions to more hydrous conditionsThe other gabbroic dykes comb-textured gabbros and
hornblende-bearing dioritic dykes result from the fillingof cracks by a fluid either a melt or a silica-rich hydrousfluid These intrusive dykes which develop reaction mar-gins at their contact with the host gabbro grade verticallyinto lower-temperature green amphibole veins or align-ments of poikilitic hornblende in the layered gabbromatrix or development of pargasitic hornblende in ana-tectic gabbros (sample 01 OA41d2) In gabbros crystal-lizing in the magma chamber the development oforthopyroxene and pargasite oikocrysts enclosing the pri-mary minerals suggests a thermal evolution from thegabbro solidus to amphibolite-facies conditions Forthese reasons it is suggested that the gabbroic dykesresulted from the injection of a hydrous melt in the stillhot gabbros drifting away from the magma chamber asdiscussed below
Origin of fluids responsible for VHT---HThydrothermal alteration
The 87Sr86Sr initial ratios and d18O of whole rocks andpure mineral fractions are plotted in Fig 9 against thedistance above the Moho Transition Zone (MTZ) Theinitial Sr isotopic composition of the whole rocks apartfrom the epidosite dykes ranges from 070322 to
JOURNAL OF PETROLOGY VOLUME 45 NUMBER 6 JUNE 2004
1200
070458 All the studied samples (including whole rocksand minerals) have initial 87Sr86Sr ratios falling outsidethe field of fresh MORB which commonly have Sr ratiosbetween 07023 and 07029 with an average of 070265(ie Hart et al 1974) The lowest initial Sr isotopiccomposition (070322) from the massive gabbro 94MA2 is slightly but significantly higher than averageOman primary magmatic signatures (ie 070296McCulloch et al 1981) Similarly the d18O values ofmost whole rocks and minerals measured (Table 5)
depart from typical MORB-source mantle values( Javoy 1980 Gregory amp Taylor 1981 Kyser 1986)These differences indicate that the primary mantlesignatures in the Oman ophiolite have been modifiedby interaction with a fluid Possible origins for thisfluid are mantle components dehydration of subductedlithosphere or seawater circulation Strontium andoxygen isotopes are useful tracers for fluid---crust interac-tion as d18O and 87Sr86Sr ratios from fluids originat-ing from seawater the mantle or subducted lithosphere
-1000
1000
2000
3000
4000
5000
6000
7000
MOHO
MTZ
Dis
tan
ce a
bov
e th
e M
oh
o (
m)
pillow-basalt
sheeted-dike
gabbro
peridotite
01 OA 41d2
01 OA 36b
02 OA 43a97 OA 128b
01 OA15f
94 OA MA2
02 OA 90b01 OA 19ad
02 OA 37b
0702 0703 0704 0705 0706 0707
87Sr86Srinitial
MO
RB-s
ou
rce
Seaw
ater
Whole RockPlagioclasePargasiteGreen HornblendeClinopyroxene
Epidote
(a)
-1000
1000
2000
3000
4000
5000
6000
7000
MOHO
MTZ
Dis
tan
ce a
bov
e th
e M
oh
o (
m)
pillow-basalt
sheeted-dike
gabbro
peridotite
01 OA 41d2
01 OA 36b
02 OA 43a97 OA 128b
01 OA15f
94 OA MA2
02 OA 90b01 OA 19ad
02 OA 37b
Whole RockPlagioclasePargasiteGreen HornblendeClinopyroxene
3 5 10
δ18O (permil)
41d2
Ma2
43a
90b 19d 19d37b
128b
90b
(b)
Fig 9 (a) Variation of initial 87Sr86Sr isotopic composition as a function of the location of the studied samples in a schematic log of the crustalsection of the Oman ophiolite The field for MORB is from Hart et al (1974) and that for seawater is from Burke et al (1982) Small open squares aredata fromKawahata et al (2001) (b) Variation of d18O () as a function of the location of the studied samples in a schematic log of the crustal sectionof the Oman ophiolite Shaded curve corresponds to the data of Gregory amp Taylor (1981)
BOSCH et al HYDROTHERMAL CIRCULATION IN OMAN OPHIOLITE
1201
are significantly different Moreover the d18O valueis dependent on temperature and mineral fractiona-tion and thus yields complementary constraints to Srisotopes
Mantle origin
One model to explain the occurrence of fluids interactingwith the oceanic crust at very high temperatures is toderive them from the mantle The contribution of mantle-derived hydrous fluids linked to percolation processesor to their concentration in the final stages of gabbrocrystallization has been invoked in numerous case studies(ie Witt amp Seck 1989 Fabriees et al 1989 Dupuy et al1991 Agrinier et al 1993) O and Sr isotopic data forwhole rocks and mineral separates yield scattered valuesthat are unambiguously different from MORB mantle-source values (Fig 9a and b) With the exception of lateLT altered samples the WR data have a mean 87Sr86Srratio of 070337 and a standard deviation of 000012These surprisingly high values in mantle-derived rockscould be taken as evidence for survival of mantle isotopicheterogeneities during crystallization of the ophiolite (egLanphere 1981 McCulloch et al 1981) However the18O16O isotope ratios in the Oman gabbroic sequenceare also displaced from primary magmatic values Nogabbro in the present study has plagioclase or amphi-bole with typical mantle d18O values Thus the Sr and Oisotope data rule out the possibility of mantle-derivedfluids as a possible source for the VHT---HT hydrousalteration event recorded in the massive gabbros andgabbroic dykes and veins (Fig 10a and b)
Subducted lithosphere origin
Dehydration of subducted oceanic crust in subductionzones can generate hydrous fluids Such fluids couldpercolate through the overlying lithosphere and contam-inate it thus generating modified isotopic signatures andtrace element contents Such an explanation howeverdisagrees with the geochemical characteristics of most ofthe studied samples Fluids derived from the dehydrationof subducted slabs (fresh or altered MORB or OIB pro-toliths) should be strongly enriched in K Rb and Ba (egBecker et al 2000) whereas in Oman the Rb contentsmeasured in the gabbros and gabbroic dykes and veins arestrongly depleted Moreover the isotopic composition ofslab-derived fluids is buffered by the MORB-like oceaniccrustal protolith for Nd and Pb but not for Sr and O(Staudigel et al 1995) As a result of relatively minorfractionation of oxygen at high temperature the oxygenisotope composition of the fluids expelled during dehy-dration of the subducted slab should be high and essen-tially controlled by the oxygen composition of the uppersection of the crust altered at low temperature (with an
average of 10 and perhaps as high as 19) (Staudigelet al 1995) The Sr isotopic composition should be alsohigh (average 07046) as the fluids released by the dehy-dration of subducted oceanic crust are associated eitherwith seawater or with radiogenic Sr phases (ie smectites)The Sr and O isotopic compositions of the studied sam-ples do not fit with these signatures and so preclude asubducted lithosphere origin for the fluids involved in theVHT---HT alteration event
Seawater-derived fluids
All the analysed samples (minerals and WR) have 87Sr86Sr(T) outside the domain of primary MORB magmasand variable Sr contents Individual minerals have Srcontents reflecting their different mineralliquid partitioncoefficients Minerals characterized by a very high affi-nity for Sr such as plagioclase (ie Sr mineralliquidpartition coefficients 1) have very high Sr contentsTherefore the high abundance of plagioclase in the sam-ples analysed is responsible for the high Sr content of thestudied whole rocks The Sr content of all the whole rocksis relatively homogeneous (Table 5) without clear distinc-tion between country-rock gabbros and dykes and veinsand ranges from 110 to 200 ppm except for the epidosite(4700 ppm) Reported on a 87Sr86Sr vs 1Sr diagram(Fig 10a) most whole rocks and plagioclases analysed arecharacterized by a 1Sr ratio lower than 001 and plot inthe left part of this diagram Compared with N-MORBthe studied massive and anatectic gabbros and their con-stituent plagioclases have similar to slightly higher Srcontents but substantially higher 87Sr86Sr ratios(Fig 10a) Considering Cretaceous seawater as a possiblehigh 87Sr86Sr mixing component [87Sr86Sr 07073at 90Ma after Burke et al (1982)] responsible for theobserved geochemical modifications exchanges cannothave occurred in a closed system as seawater has too lowa Sr content (ie 8 ppm de Villiers 1999) An open-system process allowing penetration of larger quantitiesof seawater can efficiently modify the Sr isotopic ratio ofthe rocks Alternately the fluids could also be seawaterthat was modified through exchange with the surround-ing rocks during its transit through the oceanic crustalsection This modified seawater would be more enrichedin Srwith a 87Sr86Sr ratio intermediate between unmodi-fied seawater and MORBMinerals devoid of late LT alteration imprints (parga-
site green hornblende and pyroxene) and characterizedby very low Sr contents plot on the right of the diagramclose to or on the mixing line between seawater andMORB (Fig 10a) The difference between these mineralsand the other samples (WR and plagioclases) is mainlydue to the Sr fractionation coefficients of these differentminerals and to the large quantity of plagioclase inthe studied rocks The process responsible for the
JOURNAL OF PETROLOGY VOLUME 45 NUMBER 6 JUNE 2004
1202
modification of the Sr isotopic composition fromMORB-source value is however explained by the same phenom-enon As all these minerals display initial 87Sr86Sr ratiosdistinct from the MORB source and because they equili-brated at VHT or HT temperatures this supports anearly intervention of seawater during crystallization ofthese minerals Most of these samples with the exceptionof the epidosite dyke overlap the domains defined byKawahata et al (2001) for the Wadi Fizh gabbros alteredat VHT and HT conditions in the amphibolite facies(labelled lsquoVHTrsquo and lsquoHTrsquo in Fig 9a)Three samples (two epidosite dykes and an one epidote
fraction) have very high Sr contents and the highest Sr
isotopic compositions of the present dataset The twowhole-rock samples overlap the field of the Wadi Fizhsamples altered at high temperature during greenschist-facies metamorphism (Kawahata et al 2001) This typeof alteration is common at the ridge axis and formedthrough intensive exchange with seawater-derived fluidsFigure 10b indicates that all the Sr---O isotope data are
distinct from MORB compositions and suggests that thefluids reacting with the magmas could be derived fromseawater In addition the d18O values show that isotopicexchange occurred under VHT or HT conditions Thefluids could have the composition of seawater or theycould have the composition of seawater modified through
0703
0705
0707
001 01 1
1 Sr (ppm)
MORB
(87Sr
86Sr)
init
ial
Seawater
VHT
HT
MT36b
36b
36b
43a 128b90b
41d2
37b19d
15f
128b
41d2
41d2
19a
Ma2Ma2
19d
90b
37b
Whole RockPlagioclasePargasiteGreen HornblendeClinopyroxene
Epidote
(a)
0 2 4 6 8
δ18O(permil)
0703
0705
0707Seawater
MORB-mantle
128b 41d2
90b 41d2
Ma2128b
37b
43a
19d
90b
Whole RockPlagioclasePargasiteGreen HornblendeClinopyroxene
Epidote
HT LT
19d
(87Sr
86Sr)
init
ial
(b)
Fig 10 (a) Initial 87Sr86Sr isotopic ratios plotted against 1Sr for whole rocks and mineral separates from the gabbro section of the Omanophiolite Fields shown are from Kawahata et al (2001) and correspond to MT-----basalt sheeted dyke diabase altered at medium temperature(prehnite---actinolite facies sequence 3) HT-----diabase plagiogranite metagabbro and epidosite formed at high temperature (greenschist faciessequence 4) VHT-----gabbro altered at very high temperature (amphibolite facies sequence 5) The dashed line is a binary mixing line betweenMORB (Lanphere et al 1981) and Cretaceous seawater (Burke et al 1982) (b) Initial 87Sr86Sr isotopic composition plotted against d18O value forwhole rocks and minerals from the Oman ophiolite Cretaceous seawater and MORB end-members as in (a) Dashed line represents mixing curvebetweenMORB and seawater at high temperature (HT) Low-temperature (LT) exchanges between these two components would be responsible foran increase of the d18O primary MORB value
BOSCH et al HYDROTHERMAL CIRCULATION IN OMAN OPHIOLITE
1203
exchange with the surrounding rocks during its penetra-tion into the crustal section In an environment in whichfluid---rock interaction occurs at variable temperaturesthe d18O is greatly influenced by the temperature atwhich the exchange took place and by the waterrockratio It is evident from Figs 9b and 10b that none of thestudied gabbros have plagioclase and amphibole withMORB-like d18O and 87Sr86Sr values The separateminerals display a broad range of d18O values and mostexhibit extreme 18O depletion (Table 5) Hydrothermalexchange kinetics are similar in amphibole and pyroxeneand these two minerals are more resistant to oxygenexchange during water---rock interaction than coexistingplagioclase (Gregory et al 1989) The most probableexplanation of the low d18O values measured in bothamphiboles and pyroxenes is that they crystallized athigh temperature in the presence of a relatively low 18Oseawater-derived hydrothermal fluid [d18O from 01 to07 after Gregory amp Taylor (1981)] The anomalouslyhigh d18O value (thorn1009) measured for the plagioclase ofthe micro-gabbroic dyke (01 OA19d) can be interpretedas resulting from a superimposed overprint caused by alate-stage penetration of fluids at lower temperature Inthis sample the occurrence of such different 18O16Oratios between coexisting plagioclase and amphibole isnot consistent with equilibrium fractionation betweenthese two minerals at any possible temperature Thisobservation suggests that parts of the ophiolite sequencehave undergone a high-temperature hydrothermal eventprior to the later low-temperature event that producedthe strong 18O increase in coexisting plagioclaseThe depletion of the d18O values results from high-temperature hydrothermal alteration (Gregory amp Taylor1981 Stakes amp Taylor 1992) The oxygen isotopic datasupport the contention that most studied samples havebeen affected by a VHT---HT hydrothermal alteration byfluids derived from seawater Some of the analysed sam-ples presenting petrological evidence of crystallization ofLT secondary minerals have been overprinted by the latelow-temperature alteration thus swamping the earlierhigh-temperature alteration effects
Determination of waterrock ratio
To better evaluate the exchange between seawater andthe gabbroic rocks waterrock ratios (WR) have beenestimated for the whole rocks analysed during the courseof this study The different models used to constrain thisratio mainly differ by the calculation of the effective ratioor by the estimated weight ratio and by considering openor closed systems for water circulation (Albareede et al1981 McCulloch et al 1981) The WR ratios reportedin Table 5 have been calculated assuming a closed sys-tem Using this formulation these values are minimabecause the calculation always uses pristine fluid for the
initial fluid thus maximizing the value in the denomina-tor If the fluid was shifted to lower 87Sr concentrations byexchange with the rock on the downward path theinferred values would be much higher (Davis et al 2003)The calculated WR ratios for the samples from this
study range from 31 to 219 (Table 5) Two main groupsof samples can be recognized on the basis of their waterrock ratios The lowest ratios ranging from 31 to 47have been determined for massive gabbros or anatecticgabbros but also for dykes and veins devoid of late LTalteration Similar ratios have been previously calculatedfor example for the discharged hydrothermal solutions atthe 13N and 21N East Pacific Rise (Di Donato et al1990 Fouquet et al 1991) The gabbros from the Ibrasection in the Oman ophiolite gave effective WR ratiosof 05 33 and 42 (McCulloch et al 1981) in goodagreement with the values obtained in this study Theleast altered gabbro of the Ibra section yields a WR ratioof 05 in good agreement with the exceptionally fresh-ness of this rock characterized by typical mantle oxygenvalues for its minerals (McCulloch et al 1981) Samplescharacterized by waterrock effective ratios in the rangeof 3---5 can be related to penetrative alteration via thehydrothermal system Lecuyer amp Reynard (1996) havedemonstrated in oceanic gabbros from the Hess Deepaltered in similar HT conditions that WR mass ratios inthe range of 02---1 are sufficient to account for the mod-ified stable isotope signature of the gabbros Higherwaterrock values (WR 45) have been calculated forsamples affected by superimposed late hydrothermalalteration and for the epidosite samples (Table 5) Theyprobably acquired their isotopic characteristics throughinteraction with a larger volume of seawater duringgreenschist-facies metamorphism Similar or higherWR values have been published in previous studies(ie McCulloch et al 1981 Kawahata et al 2001)They correspond either to samples from the upper-crus-tal sequence (ie basalt sheeted dyke or plagiogranite) orto gabbros from the crustal section located in a particularenvironment such as for example the Wadi Fizh sectionthat is located close to a segment boundary along thepalaeo-spreading axis (Nicolas et al 2000)
Mechanism for seawater interaction withmagma
Nicolas et al (2003) have proposed that variable amountsof seawater can circulate at high temperature through-out the crustal section down to the walls of the magmachamber via a network of parallel microcracks a fractionof a millimetre wide Such microcracks and their relation-ship with the VHT---HT hydrous alteration in the sur-rounding gabbros have been described in detail byNicolas et al Via this network of microcracks largeamounts of seawater can have reacted with the magma
JOURNAL OF PETROLOGY VOLUME 45 NUMBER 6 JUNE 2004
1204
and induced variable changes of its Sr and O isotopiccomposition This regular network of parallel micro-cracks could constitute the recharge system and thehydrated gabbroic dykes the discharge system Physicalparameters and the geophysical models of Nicolas et al(2003) are in agreement with this scenario Such a pene-tration of seawater-derived hydrothermal fluids throughthe lower crust along grain boundaries and a microscopicfracture network at temperatures 4750C is consistentwith recent thermodynamic models (McCollom amp Shock1998)Seawater-altered and high-temperature recrystallized
diabases (protodykes) from the overlying sheeted dykecomplex stoped into the melt lenses could represent anindirect way for seawater to enter the system as they canremelt during their subsidence through the magmachamber A simple mass balance calculation suggeststhat this indirect contamination is not significant Assess-ments of the amount of recycled crust 1 in volumeby Nicolas et al (2000) based on the protodyke remnantsin the gabbro section or 20 by Coogan et al (2003)based on chlorine content in EPR basalts lead to a sea-waterrock ratio of the order of 104 to 103Thus following Nicolas et al (2003) we propose that
seawater has been introduced along microcracks down tothe walls of the magma chamber and has diffused alonggrain boundaries in the way described by Manning et al(2000) progressively invading the host gabbro Gabbroicdykes result from the injection of hydrous melt into thehost gabbros at falling temperatures as they drift awayfrom the magma chamber This water-saturated meltcould result from the extensive hydration of the crystal-lizing gabbros near the walls of the magma chamberwhen seawater penetrates through this wall (Fig 1)Similar plumbing systems driving seawater down to the
limit of the magma chamber and back to the surface havebeen already proposed in other environments such as theMid-Atlantic Ridge (Kelley amp Delaney 1987 Cooganet al 2001) the East Pacific Rise at Hess Deep (Gillis1995 Manning et al 1996a 1996b) the Tonga forearc(Banerjee amp Gillis 2001) and the Troodos ophiolite(Gillis amp Roberts 1999) The melting curve of wet basalthas a negative slope but is close to the dry solidus at lowpressure in the lower gabbros and the first melts areplagiogranitic (Spulber ampRutherford 1983) Such plagio-granites are ubiquitous in the highest gabbros and in theroot zone of sheeted dykes in Oman (Rothery 1983Juteau et al 1988 Nicolas amp Boudier 1991) in manyother ophiolites and in the oceanic crust where they havebeen interpreted as resulting either from wet anatexis ofhot gabbros or from the final product of crystallization ofa basaltic melt (Spulber amp Rutherford 1983) Plagio-granites are rare in the lower gabbros exceptionallyassociated with hydrous gabbro segregations inside thelayered gabbros Their constitutive minerals are also
remarkably absent along the VHTmicrocracks althoughthese gabbros were above the wet solidus A possibleexplanation has been proposed by McCollom amp Shock(1998) who wrote that at high temperature lsquoaqueousfluids may leave very little conspicuous petrological evid-ence for their presencersquo the reason being that thehigh-T conditions would be closer to batch meltingObviously more detailed studies on this specific problemare needed We conclude that the large degree of hydrousmelting locally required at the walls of the magma cham-ber to account for the generation of a gabbroic hydrousmelt may have erased the traces of the incipient plagio-granitic melt
CONCLUSIONS
Fostered by the recent discovery of high-temperatureseawater contamination in some gabbros of the Omanophiolite (Manning et al 2000) we have conducted astudy on 500 gabbro specimens from selected areas ofthe entire Samail ophiolite belt and on the hydrous gab-broic dykes that cross-cut them The highest-temperaturereactions (VHT) recorded in olivine and clinopyroxene ingabbros as orthopyroxene and pargasite coronas reach975C They are followed at lower HT conditions withreplacement of olivine by an assemblage of tremolitemagnetite and plagioclase surrounded by chlorite andby pargasite development in clinopyroxene followedfinally by the LT lizardite---olivine and saussurite---plagioclase greenschist-facies alteration With a fewexceptions the VHT alteration tends to be restricted tothe lower gabbros and to the vicinity of the Moho andthe HT alteration to the middle and upper gabbros Thepreservation of the vertical distribution of VHT---HTfacies indicates that progressive cooling during off-axisspreading did not obliterate this distribution and conse-quently that the VHT---HT hydrothermal alteration tookplace in a very restricted time interval at the limit of thecooling magma chamber In the deep and hot gabbrosthe main water channels may be sub-millimetre-sizedmicrocracks with a dominantly vertical attitude the ori-gin and dynamics of which have been investigated in aprevious paper (Nicolas et al 2003)Beyond these high-temperature alteration zones mas-
sively induced in the gabbros seawater may be respons-ible for the generation of clinopyroxene---pargasitegabbro dykes that cross-cut all gabbros and that areupsection clearly connected to the LT hydrothermalvein system Petrostructural evidence suggests that thesedykes were generated by hydration of the crystallizinggabbros near the internal wall of the LVZ---magmachamber The resultant melts were subsequently intrudedat various levels in the cooling lower and middle gabbrosPetrological observations of nearly all the gabbros ana-lysed in the present study located from the root zone of
BOSCH et al HYDROTHERMAL CIRCULATION IN OMAN OPHIOLITE
1205
the sheeted dyke complex down to the Moho emphasizethe imprint of VHT---HT hydrous alteration The VHThydrothermal event is characterized by temperatures ofequilibration around 900---1000C The temperatures atwhich the HT hydrous event occurred are estimated tobe in the range 550---900CStrontium and oxygen isotope compositions measured
on whole rocks and separated minerals significantlydepart from primary MORB values The Sr and O iso-tope data obtained for pargasite green hornblende andclinopyroxene record the participation of seawater at thetemperature of crystallization of the minerals Plagio-clases and whole rocks define a trend toward a compo-nent characterized by a high radiogenic 87Sr86Sr valueand a higher Sr content than normal seawater Seawateris clearly identified as the fluid responsible for the modi-fication of the Sr and O signatures for both massivegabbros and dykes and veins Nevertheless the high Srcontent of the extraneous component requires eitheropen-system exchange allowing penetration of a greaterquantity of seawater or penetration of seawater modifiedby interaction with the oceanic crust during its travel paththrough the crustal section The waterrock ratio calcu-lated on the basis of the Sr isotopes is between three andfive for the enclosing gabbros and for the least LT altereddykes The VHT---HT hydrothermal event affectingthese samples is related to penetrative alteration via therecharge and discharge hydrothermal system Thewaterrock ratio is 10 for dykes exhibiting evidenceof a LT hydrothermal alteration overprint and reaches22 for the epidosite dyke The depletion in d18O char-acterizes all the studied samples with the single exceptionof the micro-gabbroic dyke plagioclase This agreeswith the conclusions based on Sr isotopes suggestingthat these samples have undergone exchange with sea-water-derived fluids during a VHT---HTalteration hydro-thermal event
ACKNOWLEDGEMENTS
This work has benefited from financial support by theCentre National de la Recherche Scientifique (INSUInteerieur-Terre andDYETI programmes) and inOmanfrom the willingness of the Directorate of MineralsMinistry of Commerce and Industry and the hospitabil-ity of the people Technical help in the laboratory is alsoacknowledged including C Nevado for the thin sectionsE Ball for the illustrations B Galland for her assistancein the chemistry room and C Bassoulet for help duringthermal ionization mass spectrometric analyses of the Srresults from the leaching experiments Constructivereviews from R T Gregory C Lecuyer an anonymousreviewer and particularly from M Wilson greatlyhelped to improve an earlier version of the manuscript
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Banerjee N R amp Gillis K M (2001) Hydrothermal alteration in a
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Becker H Jochum K P amp Carlson R W (2000) Trace element
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Nelson H F amp Otto H F (1982) Variation of seawater 87Sr86Sr
throughout Phanerozoic time Geology 10 516---519Carmichael I S E Turner F J amp Verhoogen J (1974) Igneous
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Chernosky J V Berman R G amp Bryndzia L T (1988) Stability
phase relations and thermodynamic properties of chlorite and
serpentine group minerals In Bayley S W (ed) Hydrous
Phyllosilicates Mineralogical Society of America Reviews in Mineralogy 19
295---346
Coogan L A Wilson R N Gillis K M amp MacLeod C J (2001)
Near-solidus evolution of oceanic gabbros insights from amphibole
geochemistry Geochimica et Cosmochimica Acta 65 4339---4357
Coogan L A Mitchell N C amp OrsquoHara M J (2003) Roof
assimilation at fast spreading ridges an investigation combining
geophysical geochemical and field evidence Journal of Geophysical
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1171
Davis A C Bickle M J amp Teagle D A H (2003) Imbalance in the
oceanic strontium budget Earth and Planetary Science Letters 211
173---187
Detrick R S Buhl P Vera E Mutter J Orcutt J Madsen J amp
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Journal of Petrology 30 199---228
Fouquet Y Von Stackelberg S U Charlou J L Donval J P
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Gregory R T amp Taylor H P (1981) An oxygen isotope profile in a
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Holland T amp Blundy J (1994) Non-ideal interactions in calcic
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Javoy M (1980) 18O16O and DH ratios in high temperature
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Juteau T Beurrier M Dahl R amp Nehlig P (1988) Segmentation
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Juteau T Manacrsquoh G Moreau C Lecuyer C amp Ramboz C
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Kelemen P B Koga K amp Shimizu N (1997) Geochemistry of
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Manning C E MacLeod C J amp Weston P E (1996a) Fracture-
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constraints on the roots of mid-ocean ridge hydrothermal systems
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the lower oceanic crust thermodynamic models of hydrothermal
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McCulloch M T Gregory R T Wasserburg G J amp Taylor H P
(1981) Sm---Nd Rb---Sr and 18O16O isotopic systematics in an
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Proceedings of the Ocean Drilling Program Scientific Results 147 College
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Nehlig P amp Juteau T (1988) Flow porosities permeabilities and
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BOSCH et al HYDROTHERMAL CIRCULATION IN OMAN OPHIOLITE
1207
Nicolas A amp Ildefonse B (1996) Flow mechanism and viscosity in
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Nicolas A Ceuleneer G Boudier F amp Misseri M (1988) Structural
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Nicolas A Boudier F Michibayashi K amp Gerbert-Gaillard L
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Nicolas A Mainprice D amp Boudier F (2003) High temperature
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altered oceanic crust DSDPODP sites 417418 Earth and Planetary
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New York McGraw---Hill 694 pp
Wilcock W S D amp Delaney J R (1996) Mid-ocean ridge
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tions for the nature of mantle metasomatism Earth and Planetary
Science Letters 91 327---340
JOURNAL OF PETROLOGY VOLUME 45 NUMBER 6 JUNE 2004
1208
plagioclase oriented parallel to the larger plagioclaselaths defining the magmatic foliation of the gabbro(Fig 7c) Clinopyroxene oikocrysts surrounding idio-morphic plagioclase crystals are common in the upper-level gabbros they mark the end of the magmatic flowand crystallization growing before the pargasite andorthopyroxene oikocrysts as shown by their partialreplacement by pargasite Their crystallization marksthe transition at suprasolidus conditions from a dry to awet environment In the uppermost gabbros those dykesthat are typically altered in the HT hydrothermal faciesgrade upwards into the so-called lsquoisotropicrsquo or lsquovari-texturedrsquo gabbrosThe microgabbro and amphibolitized dolerite dykes
differ from the typical diabase dykes commonly cuttingthe gabbro section that have chilled margins and thusprobably intruded enclosing rocks already at temperat-ures below 450C In microgabbro and amphibolitizeddolerite dykes the absence of chilled margins and thedevelopment of dioritic reaction margins in the adjacentgabbro (Fig 7d and e) suggest that they were emplaced ingabbros that were still at temperatures above 750C(Nicolas et al 2000)
ANALYTICAL TECHNIQUES
Major and trace elements
Major element compositions were determined by elec-tron microprobe at the University of Montpellier II usinga Camebax SX100 Instrument calibration was per-formed on natural standards and operating conditionswere 20 kV accelerating potential 10 nA beam currentand 30 s counting timeRb and Sr concentrations in whole rock and separated
mineral fractions were measured by conventionalnebulization inductively coupled plasma mass spectro-metry (ICP-MS) using a VG Plasmaquad 2 (ISTEEMMontpellier) Digestion procedures followed thoseoutlined by Ionov et al (1992)
O---Sr isotopes
The samples selected for the detailed study wereextracted from gabbroic dykes or veins inside gabbrosand cut with a diamond saw into 2---3 cm fragmentsbefore being crushed into 05---1 cm fragments Forwhole-rock analysis small pieces were further crushedinto powder in an agate bowl For pure mineral analysisthe fragments were crushed with a manual agate mortarTo avoid or minimize the mixing of selected separateminerals with other phases that can bias the results astrict protocol of mineral selection was applied A smallsize fraction of 125---160 mm was used and minerals werefirst separated using a Frantz isodynamic magneticseparator Concentrates recovered (cpx plagioclase
amphibole epidote) were subsequently purified by care-ful hand-picking in alcohol under a binocular micro-scope At this stage mineral separates were very pureand only flawless minerals free of cracks and visibleinclusions were kept for isotopic analyses To ensurefurther mineral integrity these pure mineral separateswere ultrasonically washed in dilute 15N HCl and rinsedseveral times with ultra-pure water This stage potentiallyremoves adsorbed secondary micro-phases Previousstudies (eg Lecuyer amp Reynard 1996) demonstratedthat pyroxene and amphibole can be intermixed on avery fine scale with other phases (such as a range ofamphibole compositions talc Fe-oxide) Although thispossibility cannot be ruled out the procedure used inthis study makes it unlikely that complex minerals passedthrough the selection
Sr isotopic compositions
Approximately 50mg of whole-rock powder or separatedminerals were digested in a Teflon lsquobombrsquo with a mixtureof triple distilled 13N HNO3 and 48 HF with a fewdrops of HClO4 Two aliquots were separated one forthe determination of Sr isotopic composition and theother for Sr and Rb contents Sr was separated using anextraction chromatographic method modified from Pinet al (1994) Concentrations were measured by isotopedilution using 84Sr---87Rb mixed spikes To remove tracesof the late low-temperature seawater alteration and todetermine the primitive whole-rock Sr isotopic composi-tion leaching experiments were conducted on a fewsamples following the procedure described by Bosch(1991) The strontium isotopic compositions of leachateshave been analysed in the attempt to check for the leach-ing efficiency During these tests four Sr isotope composi-tions were measured which correspond to the unleachedsample the two liquids extracted after acid-leaching stepsand the final solid residue Leaching experiments wereconducted in two steps first with 6N HCl for 8min at70C second with 25N HCl for 30min at 70C Afterleaching the leachates were evaporated to dryness andthe solid residues digested similarly to the unleachedsamplesSr isotopic compositions were measured on a fully
automated VG Sector mass spectrometer housed at theUniversity of Montpellier II except for the Sr isotopicdata from the leaching experiments which have beenmeasured at the Universitee de Bretagne Occidentale(Brest) To assess the reproducibility and accuracy of Srisotopic measurements the NBS 987 standard was run12 times during this study the average value was0710245 with a precision better than 0000010 (2s)87Sr86Sr ratios are corrected for mass discrimination bynormalizing to a 86Sr88Sr value of 01194 Sr total blankconcentrations were less than 60 pg Initial 87Sr86Sr
BOSCH et al HYDROTHERMAL CIRCULATION IN OMAN OPHIOLITE
1189
ratios were calculated by correcting the measured ratiosfor accumulation of 87Sr produced by in situ 87Rb radio-active decay since 95Ma the assumed age of crystalliza-tion of the Oman ophiolite (Hacker 1994)
Oxygen isotopes
The powdered samples were introduced in Ni tubesunder dry air where they were reacted with BrF5 toform oxygen Oxygen was separated from other gasesby cooling the resulting mixture at liquid nitrogen tem-perature (77 K) that retained other volatile gases Oxygenwas further purified by trapping it on a cooled 5A mole-cular sieve at 77 K then quantified to calculate the yieldof the isotopic analyses and finally transferred to the massspectrometer (Finnigan Mat Delta E) where intensitiesassociated with masses 32 and 34 were measured todetermine d18O The isotopic composition of a sampleis given as dsample frac14 (RsampleRstandard 1) 1000 in permil unit where R is 18O16O and the standard isSMOW Analytical errors on the oxygen isotope ratioare 02 (2s)
RESULTS
Seven samples representative of the range of gabbroicdykes and five samples representative of the lower gab-bros two of them representing the margins of gabbroicdykes were selected for major element thermometricand Sr---O isotopic analyses (Tables 1---3) These samplesoriginate from wadi sections marked by bold lines inFig 1 Wadis Halhal Mayah Al Abyad Warihya andMaqsad structurally belong to a new ridge segment open-ing in slightly (1Myr) older lithosphere Wadi Gideahis in older lithosphere
Petrographic and isotopic characteristicsof the analysed samples
Olivine gabbro from the MTZ in MaqsadSamail Massif
Sample 94 MA2 is devoid of low-temperature alterationand only exhibits the discrete signature of the VHT---HTalteration The orthopyroxene corona around olivine(Fig 3a) is relayed by kelyphitic rims at the contactwith plagioclase (Fig 3b) Issuing from these rims aremicrometre-sized pale green amphiboles identified ascummingtonite and actinolite which penetrate the sur-rounding plagioclase in microcracks 004mm wide or atgrain boundaries (Fig 3b) similar to the microcrack sys-tem described by Manning et al (2000) A large numberof the plagioclase crystals have a cloudy core as a result ofthe concentration of fluid and mineral inclusions amongwhich diopside has been identified during microprobeanalysis Olivine cores are free of serpentine but theyare largely oxidized along a microcrack network Ortho-pyroxene coronas have a MgOpx number of 78---79
slightly higher than in the primary olivine cores withMgOl number of 74---75 The magmatic clinopyroxeneis totally fresh and probably not in equilibrium with thereactive orthopyroxene This sample has the lowest 87Sr86Sr initial ratio of the studied whole rocks (070322)identical to coexisting clinopyroxene (070324) and pla-gioclase (070322) This observation suggests the lack of alate superimposed LT hydrothermal alteration as it isvery unlikely that an LT hydrothermal event could havetotally equilibrated the Sr isotopic compositions of prim-ary minerals such as plagioclase and clinopyroxeneMoreover the whole rock (WR) clinopyroxene andplagioclase have similar initial Sr isotopic ratios despitevery different Sr contents Accordingly this ratio isthought to reflect the signature acquired during the crys-tallization of these minerals and is higher than the Omanmantle Sr isotopic value (McCulloch et al 1981) Thed18O values of the WR (50) and plagioclase (48) aredistinctly lower than primary magmatic values Indeedin normal mid-ocean ridge basalts (MORB) the WRd18O value is 57 02 with a corresponding plagio-clase d18O ranging from 55 to 62 ( Javoy 1980Gregory amp Taylor 1981)
Lower gabbro from Wadi Wariyah Wadi Tayin Massif
Sample 97 OA128b belongs to a layered sequence in thelower gabbro series injected by anorthositic sills Thesample was collected at the tip of one of these sillsbetween layers enriched in clinopyroxene The gabbrois composed of 60 plagioclase and 40 clinopyroxeneDespite the absence of olivine the WR has a relativelyhigh Mg content (Table 3) A series of low-T (prehnite)microveins 01mm thick cross-cut the gabbro perpendi-cular to the layering and foliation The margins of theveins are devoid of alteration This gabbro has the sameSr isotopic composition as 94 MA2 Nevertheless theWR and associated plagioclase have Sr isotope signatures(070333 and 070328) lower than the coexisting clino-pyroxene (070422) The Sr contents of the WR plagio-clase and clinopyroxene are 127 ppm 120 ppm and20 ppm respectively thus making the clinopyroxenemore sensitive to late-stage contamination Clinopyrox-ene and plagioclase oxygen isotope compositions (523and 414 respectively) are also lower than typicalmantle values Similar to the previous sample alterationat high temperature by a low d18O and high 87Sr86Srfluid is required to explain such characteristics
Amphibole gabbro patch Wadi Gideah Wadi TayinMassif
Sample 01 OA41d2 is from a laminated gabbro com-posed of millimetre-sized layers of clinopyroxeneand plagioclase Anatectic patches (Fig 7a) borderinganorthositic layers are formed of coarse-grained
JOURNAL OF PETROLOGY VOLUME 45 NUMBER 6 JUNE 2004
1190
Table1Locationoftheanalysed
samplesandsummaryoftheirpetrographicandSr---O
isotopiccharacteristics
Sam
ple
nam
eSite
Locationin
gab
broic
section
Typ
eofrock
Distance
to
Moho(km)
Parag
enesis
Alteration
87Sr
86Srinitial1
d18O
()2
94MA2
Maq
sad
From
MTZ
Layered
gab
bro
0Olivine(M
gno74---75)3
OPXco
ronas
(Mgno78---79)
WRfrac14
070322
WRfrac14
thorn495
CPX
Palegreen
Am
(cumact)
CPXfrac14
070324
nd
Plagioclase(A
n65---80)4inversezoning
Nolow-T
alteration
Plfrac14
070322
Plfrac14
thorn479
Solidfluid
inclusionsin
Pl
97OA128b
Wad
iWaryiah
From
lower
gab
bro
Layered
gab
bro
1CPX
Low-T
alteration
WRfrac14
070333
nd
Plagioclase(A
n85---87)
Prehnitemicroveins
CPXfrac14
070422
CPXfrac14
thorn523
Plfrac14
070328
Plfrac14
thorn414
01OA41d2
Wad
iGideah
From
upper
gab
bro
Gab
bro
patch
in
laminated
gab
bro
36
CPXrecrystallized
Plagioclaseidiomorphic
(An57---87)norm
alzoning
Black
pargasitic
honblende
WRfrac14
070351
CPXfrac14
070378
Plfrac14
070426
WRfrac14
thorn480
nd
Plfrac14
thorn504
Hbdfrac14
070355
nd
01OA19aamp
dWad
iWaryiah
Inlayeredgab
bro
Gab
broic
dyke
01
Olivine
OPXpoikilitic
WRfrac14
070341
WRfrac14
thorn567
CPX
Black
pargasite
Hbdfrac14
070324
Hbdfrac14
thorn287
Plagioclase(A
n73---95)
Hornblendepoikilitic
Plfrac14
nd
Plfrac14
thorn10 09
Hem
atite
WRfrac14
070418
nd
Green
Mg-hornblende
Acthornblende
Epidote
02OA90b
Wad
iAl-Abyad
Inlayeredgab
bro
Dioriticdyke
015
Green
Mg-hornblende
Hbdfrac14
070427
Hbdfrac14
thorn239
Plagioclase(A
n83---87)
Plfrac14
070387
Plfrac14
thorn498
Solidfluid
inclusionsin
plagioclase
01OA15f
Wad
iWaryiah
Inlayeredgab
bro
Comb-textured
gab
broic
dyke
025
CPXidiomorphic
Plagioclase(A
n95---99)norm
alzoning
BrownMg-H
ornblende
WRfrac14
070458
nd
02OA43a
Wad
iMayah
Inlayeredgab
bro
vein
Green
amphibole
1Palegreen
Mg-hornblendeacicular
Plagioclase(A
n91---97)in
reactionmargin
Hbdfrac14
070431
Hbdfrac14
thorn310
01OA36b
Wad
iGideah
Inlayeredgab
bro
epidosite
dyke
25
CPX
Epidote
WRfrac14
070519
nd
Plagioclase
Epfrac14
070554
nd
CPXclinopyroxene
Plplagioclase
OPXorthopyroxene
Amam
phibolecu
mcu
mmingtonite
actactinolite
WRwholerockHbdhornblende
Epep
idotend
notdetermined
187Sr
86Srinitialratioscalculatedat
95Ma(H
acker1994)Errors
ofthemeasuredratiosareindicated
inTab
le4
2Analyticalerrors
oftheoxygen
isotopevalues
are02
(2s)
3Mgnumber
frac14atomicMg(Mgthorn
Fe)
4Percentageofan
orthitein
plagioclase
BOSCH et al HYDROTHERMAL CIRCULATION IN OMAN OPHIOLITE
1191
(millimetre-sized) recrystallized clinopyroxene largelyaltered to light green magnesio-hornblende and idio-morphic plagioclase Locally primary black pargasitichornblende develops including idiomorphic normallyzoned plagioclase (Table 4) This gabbro exhibits similar87Sr86Sr ratios for the WR and pargasite (070351 and070355 respectively) These values are interpreted asrepresentative of the melt from which this amphibolecrystallized The clinopyroxene has a slightly higher Srisotope composition of 070378 consistent with the occur-rence of the light green Mg-hornblende rim Plagioclaseis significantly more radiogenic (070426) related to sec-ondary destabilization evidenced by the idiomorphichabit The Sr isotope compositions of the liquidsextracted from the acid leaching experiments of theWR (L1 and L2) are very similar 070358 and 070365respectively The 87Sr86Sr ratio of the residue obtainedafter leaching experiments is 070335 lower than boththe pargasite and unleached WR This value can beconsidered as the unaltered isotopic composition of thissample The d18O values measured for WR and coexist-ing plagioclase are lower than normal MORB magmaticvalues The 18O depletion in the plagioclase can be fairlywell explained by high-temperature hydrothermal inter-action between an oxygen-depleted fluid and the magmaAccording to the kinetics of hydrothermal exchange ofoxygen (Gregory amp Taylor 1981 Gregory et al 1989)plagioclase exchanges its oxygen isotopes with the fluidadjusting readily to the hydrothermal conditions Forhigh temperatures of exchange with seawater-derivedfluids (d18O between 0 and thorn2) the magmatic plagio-clase will have its primary d18O value lowered whereasthe d18O value for lower temperatures of exchange(5300C) will increase The amplification of the d18Oshift increases with the intensity of the hydrothermalalteration
Gabbroic dykes intrusive in lower gabbros from WadiWariyah Wadi Tayin Massif
Gabbroic dykes 01 OA19a and 01 OA19d are intrusiveinto the lower gabbros They are 3---4 cm wide discord-ant to the foliation of the host gabbros (Fig 7e) with asharp margin marked by a brown amphibole fringe Theprimary phases in the dykes (Table 1) locally preservedas elongated aggregates of a 01mm grain size mosaicassemblage at the dyke margins grade to millimetre sizein the central part of the dyke The WR major elementcomposition is Mg enriched compared with the enclosinggabbro (Table 3) Brown pargasitic amphibole oikocrystsmake up 50 of the modal composition of the dyke Indyke 01 OA19a poikilitic orthopyroxene may developinstead of brown amphibole Finally green magnesio-hornblende grows either as oikocrysts at the expense ofclinopyroxene or in association with epidote as an altera-tion product of plagioclase These low-amphibolite-faciesminerals represent the lowest-grade paragenesis recogn-ized in these dykes and are most developed in sample01OA19aTheamphibole composition showszoning fromTi-rich pargasitic hornblende to magnesio-hornblendeand actinolitic hornblende (Fig 8c) recording pro-gressive cooling of the dyke The plagioclase compositionis extremely heterogeneous (Fig 8b) varying from An95a composition similar to that of the plagioclase in the hostgabbro (Fig 8a) to An50 for the plagioclase and asso-ciated epidote inclusions in the poikilitic actinolitic horn-blende Sample 01 OA19d has an initial 87Sr86Sr ratioslightly higher for the WR (070341) than for the parga-site (070324) The Sr isotopic ratios of liquids extractedafter two steps of WR acid-leaching (L1 and L2) are070357 and 070341 respectively whereas the residueyields a 87Sr86Sr ratio of 070327 very close to thepargasite and plagioclase values The latter is similar tothe isotopic composition measured for the massive gab-bro 94 MA2 and may be taken as close to the primarymagmatic value This Sr isotopic ratio is more radiogenicthan the inferred Oman mantle-source value (McCullochet al 1981) The altered part of this sample has a radio-genic Sr isotopic composition (070418) related to thelow-amphibolite-facies alteration This gabbroic dykehas seemingly preserved a magmatic whole-rock d18Ovalue (567) but contains by far the highestd18O plagioclase (1009) of all the samples analysedThis plagioclase coexists with low d18O pargasite(thorn287) Similarly high values (d18Oplagioclase 4 8)have been previously measured for gabbroic dykes dia-bases sheeted dykes and plagiogranites from the Omanophiolite (Gregory amp Taylor 1981 Stakes amp Taylor1992) This 72 oxygen isotope fractionation betweenplagioclase and amphibole cannot possibly represent oxy-gen equilibrium at any plausible temperature This maybe explained by different exchange kinetics for these twominerals at different temperatures at high waterrock
Table 2 Average temperatures ( C 40C)
calculated for plagioclase---amphibole pairs
using the geothermometer of Holland amp
Blundy (1994)
Pargasite Magnesio-hornblende
rim core rim core
01 OA41d2 878 953 ------- -------
01 OA19a 935 974 825 869
01 OA19d 890 933 789 -------
02 OA37b ------- ------- 816 849
02 OA90b ------- ------- 859 833
JOURNAL OF PETROLOGY VOLUME 45 NUMBER 6 JUNE 2004
1192
ratios This supports the occurrence of two distincthydrothermal events one at high temperature and alater stage at lower temperature responsible for thestrong 18O enrichment in the coexisting plagioclase
Dioritic dyke associated with a diabase dyke in lowergabbros from Wadi Halhal Haylayn Massif
Sample 02 OA37b belongs to a group of brownamphibole---plagioclase lenses or reaction margins asso-ciated with diabase intrusives (Fig 7d) in lower layeredgabbros Idiomorphic brown hornblende (tschermakite)crystals are elongated in the plane of the dyke represent-ing 80 of the modal composition They are associatedwith an interstitial plagioclase matrix The browntschermakite has greenish rims Plagioclase has a cloudyappearance due to micrometre-size fluid inclusionsexcept along its margins and internal microcracks Bothhornblende and plagioclase exhibit evidence of plasticdeformation The Sr isotopic ratio for the pargasite andplagioclase is 070348 and 070420 respectively Thepargasite exhibits a Sr signature closest to that of theOman primary magmatic value (McCulloch et al 1981)Nevertheless it is too high to be attributed to anunmodified MORB-source mantle signature This sug-gests the addition of an external fluid component in goodagreement with the shift of oxygen isotopic compositionto a low value (thorn39) suggesting a high-temperatureexchange with a low d18O fluid The Sr isotope composi-tion of the plagioclase is significantly higher than that of
the coexisting pargasite probably as a result of the low-temperature alteration
Dioritic dyke cross-cutting lower layered gabbros in WadiAl-Abyad Nakhl Massif
Sample 02 OA90b is a 1 cm thick dioritic dyke cross-cutting the lower layered gabbros It grades into a greenamphibole vein as illustrated in Fig 6c The dyke is com-posed of 2 cm long dark green idiomorphic magnesio-hornblende crystals growing in the plane of the dykeand plagioclase concentrated at the margin of the dykeThe plagioclase in the dyke and in the enclosing gabbrocontains micrometre-size fluid and mineral inclusionssimilar to those observed in sample 94 MA2 The plagio-clase and green hornblende have higher Sr isotopic ratios(070387 and 070427 respectively) and lower d18Ovalues (thorn498 and thorn239 respectively) than the normalMORB-source mantle value As observed for previoussamples this supports interaction with an external fluidcomponent
Comb-textured gabbroic dykes in the lower gabbros fromWadi Wariyah Wadi Tayin Massif
Sample 01 OA15f (Fig 6b) is from Wadi Wariyah andrepresents a 2 cm wide dyke intruding the lower gabbrosThis type of comb-textured dyke developed in theclinopyroxene---plagioclase stability field It containslarge idiomorphic clinopyroxene crystals which arepartially replaced by brownish magnesio-hornblendeThe plagioclase is extremely calcic with a slight inverse
Table 3 Whole-rock major element compositions of massive gabbros and dykes determined by X-ray fluorescence
(wt )
Sample 94 Ma2 97 OA128b 01 OA41d2 01 OA19d 01 OA19a 01 OA15f 01 OA36b 01 OA36b
Rock type Gabbro Gabbro Gabbro patch Gabbroic dyke Gabbroic dyke Comb-textured
gabb dyke
Epidosite dyke Epidosite dyke
(margin)
SiO2 4830 4815 4749 4524 4339 4799 389 4371
Al2O3 2785 2032 2438 1474 1246 1192 2752 1646
Fe2O3 328 266 404 981 1204 592 381 623
MnO 003 004 005 014 016 006 008 014
MgO 405 737 427 1031 1686 1326 187 739
CaO 1289 1908 1477 1235 1156 166 2471 2339
Na2O 292 123 257 258 111 075 000 013
K2O 000 000 009 006 000 000 000 000
TiO2 005 017 063 230 064 086 013 042
P2O5 008 007 013 018 009 013 007 010
LOI 033 076 138 216 161 237 274 190
Total 9978 9985 998 9987 9992 9986 9983 9987
LOI loss on ignition gabb gabbroic
BOSCH et al HYDROTHERMAL CIRCULATION IN OMAN OPHIOLITE
1193
Table4Majorelementcomposition
aofselectedam
phibolespaired
withplagioclasecompositionsusedforthermometry
Sam
ple
nam
e1Mode2
SiO
2TiO
2Al 2O3
Cr 2O3
FeO
MnO
MgO
CaO
Na 2O
K2O
Total
Si
AlIV
AlVI
Ti
Fe3
thornCr
Mg
Fe2
thornMn
Na
KCa
XAn3
T(C)4
01OA19a
Mg-hornblende
core
45 60
26
10 78
009
860
015
16 56
12 00
212
007
96 72
6552
1448
0366
0083
0552
0004
3409
0594
0016
0611
0018
1843
0903
899
Mg-hornblende
core
47 99
263
850
002
897
011
16 61
12 10
160
020
96 36
6894
1106
0334
0028
0483
0002
3557
0594
0014
0446
0036
1862
0892
834
Mg-hornblende
rim
51 17
247
596
001
761
014
18 11
12 19
103
010
96 47
7257
0743
0253
0016
0414
0001
3828
0488
0016
0284
0018
1853
0894
820
Mg-hornblende
rim
47 50
291
920
004
880
014
16 46
11 95
174
008
96 07
6835
1165
0396
0015
0506
0005
3529
0552
0017
0486
0015
1843
0887
825
Mg-hornblende
rim
49 57
269
742
000
799
017
18 07
12 02
133
014
96 88
7013
0987
0261
0018
0575
0000
3811
0371
0020
0365
0026
1821
0860
831
Mg-hornblende
core
47 83
181
872
003
840
011
14 48
12 18
159
008
93 59
6814
1186
0279
0018
0644
0004
3711
0357
0013
0438
0015
1859
0907
874
Pargasite
rim
42 15
249
11 87
024
10 07
013
13 84
11 43
263
032
96 61
6188
1812
0242
0434
0299
0028
3028
0937
0016
0749
0059
1797
0804
972
Pargasite
rim
42 22
260
11 86
026
10 21
011
13 71
11 48
253
032
96 59
6201
1799
0254
0429
0299
0030
3001
0955
0014
0720
0060
1807
0803
962
Pargasite
core
42 05
263
11 45
030
10 19
011
13 80
11 47
246
032
96 46
6189
1811
0175
0478
0299
0035
3029
0956
0014
0702
0060
1809
0767
974
Pargasite
rim
42 36
247
11 90
016
945
013
14 16
11 76
249
026
95 98
6239
1761
0305
0366
0276
0019
3109
0888
0016
0710
0049
1856
0818
926
Pargasite
core
42 50
291
11 64
016
984
015
14 22
11 41
256
034
96 69
6219
1781
0227
0426
0322
0018
3103
0883
0018
0727
0063
1789
0812
974
Pargasite
rim
42 76
270
12 03
017
920
012
14 87
11 82
244
025
96 73
6224
1776
0880
0335
0368
0020
3226
0752
0015
0689
0047
1843
0841
941
Pargasite
rim
42 13
181
13 54
016
921
009
12 77
12 13
254
032
96 98
6172
1828
0510
0450
0000
0018
2788
1129
0011
0721
0059
1905
0841
874
01OA19d
Mg-hornblende
49 26
047
624
011
11 46
017
15 39
12 21
108
009
96 48
7137
0863
0202
0051
0437
0013
3323
0951
0021
0302
0018
1896
0768
771
Mg-hornblende
46 03
287
932
018
906
017
14 77
12 87
193
007
97 28
6672
1328
0265
0313
0046
0021
3191
1053
0021
0542
0012
1999
0758
808
Pargasite
41 89
429
11 65
008
11 03
017
13 52
11 37
259
017
96 77
6159
1841
0179
0474
0345
0010
2963
1012
0021
0739
0032
1791
0770
975
Pargasite
rim
42 80
340
11 16
009
11 01
018
13 73
11 67
240
022
96 67
6293
1707
0226
0376
0322
0010
3009
1032
0022
0684
0041
1839
0605
880
Pargasite
core
42 58
351
11 27
006
11 63
018
13 51
11 46
239
022
96 82
6257
1743
0209
0387
0391
0007
2959
1039
0023
0680
0042
1804
0595
893
Pargasite
rim
42 14
393
11 28
006
11 71
014
13 35
11 33
235
033
96 62
6214
1786
0174
0435
0391
0007
2935
1054
0018
0673
0062
1790
0519
901
Pargasite
core
41 56
420
12 05
006
11 57
016
12 51
11 35
249
027
96 24
6168
1832
0277
0469
0276
0007
2768
1160
0021
0718
0052
1805
0537
882
Pargasite
core
41 33
438
11 49
008
11 66
015
13 35
11 45
264
026
96 80
6105
1894
0107
0487
0368
0009
2941
1073
0019
0757
0048
1813
0537
942
Pargasite
41 08
446
12 39
012
11 35
017
13 13
11 36
263
015
96 85
6049
1951
0199
0494
0368
0014
2882
1030
0021
0752
0027
1792
0758
975
02OA37b
Mg-hornblende
rim
45 15
260
984
002
13 28
019
14 01
10 87
128
009
97 33
6527
1473
0203
0283
0690
0002
3018
0915
0023
0360
0016
1683
0562
828
Mg-hornblende
rim
44 72
263
988
004
13 23
021
14 01
10 78
133
009
96 92
6494
1506
0185
0287
0713
0005
3032
0894
0025
0374
0018
1677
0593
854
Mg-hornblende
rim
45 68
247
935
004
12 87
020
14 10
11 00
121
011
97 02
6621
1379
0218
0269
0598
0004
3045
0962
0024
0341
0020
1708
0562
815
Mg-hornblende
core
43 94
291
10 87
001
12 60
023
13 73
10 59
142
011
96 39
6410
1590
0278
0319
0644
0001
2985
0893
0028
0401
0021
1655
0710
860
Mg-hornblende
core
44 62
270
10 37
001
12 66
019
14 12
10 58
140
008
96 73
6479
1521
0253
0294
0644
0001
3057
0893
0023
0395
0014
1646
0674
851
Mg-hornblende
rim
47 79
181
796
002
12 09
025
15 22
11 09
100
009
97 33
6866
1134
0214
0196
0506
0002
3260
0947
0030
0280
0016
1706
0503
767
Mg-hornblende
core
45 43
249
10 3
000
11 99
022
14 50
11 14
133
008
97 48
6524
1476
0267
0269
0644
0000
3103
0796
0027
0370
0015
1715
0703
838
JOURNAL OF PETROLOGY VOLUME 45 NUMBER 6 JUNE 2004
1194
Sam
ple
nam
e1Mode2
SiO
2TiO
2Al 2O3
Cr 2O3
FeO
MnO
MgO
CaO
Na 2O
K2O
Total
Si
AlIV
AlVI
Ti
Fe3
thornCr
Mg
Fe2
thornMn
Na
KCa
XAn3
T(C)4
02OA90b
Mg-hornblende
core
48 83
038
745
005
939
010
17 29
12 24
142
006
97 21
6931
1069
0041
0041
0667
0005
3657
0448
0011
0392
0010
1862
0840
856
Mg-hornblende
core
49 49
033
731
003
916
008
17 40
12 57
139
005
97 82
6987
1013
0035
0035
0552
0004
3662
0530
0010
0379
0009
1901
0830
811
Mg-hornblende
rim
48 90
040
753
005
932
013
17 15
12 22
154
005
97 29
6948
1052
0042
0042
0575
0006
3632
0533
0015
0424
0010
1861
0880
869
Mg-hornblende
rim
48 30
044
800
007
962
010
16 86
12 32
161
005
97 35
6873
1127
0047
0047
0598
0008
3576
0546
0012
0443
0010
1879
0870
862
Mg-hornblende
rim
49 12
032
742
010
922
011
17 36
12 29
142
006
97 43
6956
1044
0034
0034
0621
0011
3665
0471
0013
0391
0010
1865
0840
845
02OA43a
Mg-hornblende
rim
48 13
040
789
007
860
007
17 41
12 19
155
006
96 36
6882
1118
0212
0043
0621
0008
3710
0407
0008
0429
0010
1867
-------
-------
Mg-hornblende
rim
50 93
035
643
001
759
006
18 50
12 24
114
003
97 28
7152
0848
0217
0037
0506
0001
3873
0385
0007
0311
0005
1841
-------
-------
Mg-hornblende
rim
50 67
014
559
009
12 33
026
15 28
12 44
068
003
97 50
7260
0740
0204
015
0483
0010
3263
0994
0032
0189
0005
1909
-------
-------
Mg-hornblende
rim
50 34
013
621
009
12 49
027
15 07
12 51
064
003
97 78
7192
0810
0240
0010
0530
0010
3210
0960
0030
0180
0010
1910
-------
-------
Mg-hornblende
rim
45 17
060
11 14
013
988
008
15 78
12 33
211
009
97 31
6473
1527
0355
0640
0644
0015
3371
0540
0010
0588
0016
1892
-------
-------
Mg-hornblende
rim
49 25
032
712
014
845
009
17 84
12 32
137
004
96 95
6983
1020
0170
0030
0620
0020
3770
0380
0010
0380
0010
1870
-------
-------
Mg-hornblende
rim
48 61
032
775
014
975
011
16 81
12 57
139
004
97 50
6905
1950
0203
0034
0621
0016
3560
0537
0013
0382
0008
1914
-------
-------
94Ma2
Anthophyllite
54 98
000
259
001
12 65
037
23 35
199
034
000
96 28
7744
0256
0174
0000
0161
0001
4903
1330
0043
0092
0001
0301
-------
-------
Anthophyllite
55 11
000
296
003
14 09
032
23 66
063
038
000
97 18
7716
0284
0205
0000
0115
0003
4938
1535
0038
0104
0000
0095
-------
-------
Actinolite
48 58
004
954
002
960
014
17 67
999
139
005
97 02
6850
0150
0435
0004
0690
0002
3713
0442
0017
0379
0009
1510
-------
-------
Actinolite
51 54
004
585
000
890
022
20 28
930
090
002
97 05
7214
0786
0178
0005
0598
0000
4230
0444
0026
0244
0004
1395
-------
-------
01OA41d2
Pargasite
rim
43 05
342
10 71
008
11 46
012
13 50
11 54
179
067
96 34
6353
1647
0216
0380
0368
0009
2967
1046
0015
0512
0127
1824
0613
863
Pargasite
rim
42 61
354
11 59
001
10 59
010
13 52
11 87
160
076
96 19
6286
1714
0302
0392
0299
0001
2972
1007
0012
0458
0143
1877
0715
844
Pargasite
core
42 54
401
11 04
006
11 81
013
13 21
11 50
202
039
96 71
6266
1734
0182
0444
0368
0007
2901
1087
0017
0576
0073
1814
0854
979
Pargasite
rim
43 03
385
10 72
002
11 70
014
13 50
11 95
146
080
97 17
6314
1686
0168
0425
0345
0002
2953
1091
0018
0415
0149
1878
0634
852
Pargasite
rim
42 65
341
10 60
006
11 97
014
13 22
11 64
183
068
96 20
6332
1668
0186
0381
0345
0007
2926
1141
0018
0528
0129
1851
0620
869
Pargasite
core
42 82
367
10 60
007
11 93
012
13 42
11 44
206
047
96 60
6315
1685
0158
0407
0391
0008
2950
1080
0016
0588
0089
1808
0615
899
Pargasite
rim
42 2
362
11 10
007
11 42
016
13 09
11 72
179
072
95 89
6282
1718
0229
0405
0299
0009
2905
1123
0020
0518
0137
1869
0715
875
Pargasite
rim
42 42
380
11 03
006
11 77
012
13 00
11 72
188
069
96 49
6283
1717
0209
0423
0299
0007
2870
1158
0015
0539
0131
1860
0817
928
Pargasite
rim
42 21
364
11 14
006
10 73
011
13 53
11 68
177
072
95 59
6277
1723
0229
0407
0322
0007
2999
1012
0014
0511
0136
1862
0736
883
Pargasite
rim
42 25
376
11 03
008
10 93
015
13 66
11 72
185
064
96 07
6257
1743
0182
0418
0345
0009
3016
1009
0019
0531
0121
1860
0766
912
Pargasite
core
42 17
396
10 91
008
11 10
010
13 57
11 49
209
038
95 85
6253
1747
0159
0442
0368
0010
3000
1009
0013
0602
0073
1826
0841
981
aMajorelem
entco
mpositionsarein
weightpercent-------noplagioclasean
alysed
andnotemperature
calculated
1Typ
eofam
phibolean
alysed
isalso
indicated
2Blankindicates
nozoningobserved
3Molefractionofan
orthitein
plagioclasead
jacentto
amphibolegrain
4Tem
perature
ofeq
uilibrium
fortheam
phibole---plagioclasepaircalculatedusingtheHollandamp
Blundythermometer
(1994)Errors
are40
C
BOSCH et al HYDROTHERMAL CIRCULATION IN OMAN OPHIOLITE
1195
zonation from An95 to An99 from core to rim (Table 1 andFig 8a) TheWR Sr isotopic ratio of the sample (070458)is significantly higher than normal MORB values Thiscould be related to the overprint of a low-temperaturealteration event consistent with the petrography of thesample
Green amphibole vein cross-cutting layered gabbros inWadi Mayah Haylayn Massif
Sample 02 OA43a belongs to a series of 1 cm thick greenamphibole veins (as in Fig 6e) cross-cutting the layeredgabbros The veins develop 15 cm thick reaction marginsin the enclosing gabbro and are exclusively composed ofacicular pale green magnesio-hornblende growing per-pendicular to the vein margin in a crack-seal mode(Fig 6f ) The reaction margins are composed of urali-tized gabbro marked by the development of the samepale green magnesio-hornblende in an inhomogeneouscalcic plagioclase matrix (An91---97) The Sr and O isotopiccompositions of this green hornblende (070431 andthorn31) do not correspond to normal MORB-sourcemantle values
Epidosite dyke from Wadi Gideah Wadi Tayin Massif
Sample 01 OA36b is representative of a series of epidoteveins cross-cutting poorly layered gabbros These 2 cmthick veins are composed of pink thulite in their centreand green pistachite at their margins (Fig 6a) Theirsharp contact with the enclosing gabbro is marked bythe development of coarse clinopyroxene crystals alteredin the epidote---greenschist facies This suggests that theepidote vein is replacive in a coarse-grained gabbro dykeThe highest 87Sr86Sr initial ratios of all the samplesstudied are recorded by the WR and coexisting epidoteof this sample (070519 and 070554 respectively)(Table 5) The Sr content of the WR and associatedepidote are also the highest of the present dataset(716 ppm and 764 ppm respectively) The Sr isotopiccomposition of this sample is consistent with a greaterdegree of exchange with a radiogenic component fromCretaceous seawater It is significantly higher than thevalue obtained for an epidosite vein from the Wadi Fizhsection of the Oman ophiolite [070435 with a Sr contentof 488 ppm reported by Kawahata et al (2001)] but ingood agreement with the average of the greenschist-faciesalteration sequence defined by Kawahata et al (2001)
GABBROS
01 km 025 km 1 km 36 km50
60
70
80
90
An
MA2
19d
19a 15f 128b
41d2
(a)
43a17b
15f
90b
37b
19a
19d
40
60
80
100
A
n
01 km 015 km 025 km 1 km
DYKES
(b)
Distance to the Moho
ACTINOLITE
MAGNESIOHORNBLENDE
TSCHERMAKITE
PARGASITEGabbros Dykes41d2MA2
19a d37b15f
90b43a
AlIV
150500
02
08
10
(Na
+ K
) A
(c)
04
06
950
900
850
800
750
70007 12 17
T degC
1000
(d)
AlIV
Fig 8 Plagioclase---amphibole compositions and results of thermometry (a) and (b) plagioclase compositions in layered gabbros and in dykesrespectively as a function of distance to the Moho (see text for sample identification) (c) amphibole compositions in layered gabbros and dykesaccording to Leakersquos (1997) nomenclature (d) equilibration temperatures calculated using the amphibole---plagioclase thermometer of Holland ampBlundy (1994) as a function of AlIV content in amphibole [same symbols as in (c)]
JOURNAL OF PETROLOGY VOLUME 45 NUMBER 6 JUNE 2004
1196
Table5RbandSrcontents87Sr
86Srandd1
8Oisotopiccompositionsofwholerocksandmineralseparatesfrom
samplesofmassivegabbrosdykes
andveinsfrom
theOman
ophiolitealsolistedarecalculated
waterrockratios(W
R)andLOIvalues
Sam
ple
Rb(ppm)
Sr(ppm)
87Rb8
6Sr
87Sr
86Sr
(87Sr
86Sr)i1
d18O
()
LOI(
)2WR
3(87Sr
86Sr)L14
(87Sr
86Sr)L25
(87Sr
86Sr)R6
01OA41d2
Whole
rock
037
1792
0006
0703516
09
070351
thorn480
138
47
0703587
0703651
0703357
Plagioclase
025
1249
0003
0704261
22
070426
thorn504
-------
-------
Pargasite
-------
-------
-------
0703548
20
070355
-------
-------
-------
Clinopyroxene
045
26 0
0050
0703845
19
070378
-------
-------
-------
01OA36b
Dykewhole
rock
0005
7164
50001
0705186
17
070519
-------
274
21 9
0705207
0705112
-------
Epidote
0003
7645
0001
0705536
12
070554
-------
-------
-------
Wallwhole
rock
005
4254
0001
0704637
09
070464
-------
191
-------
0704945
0704675
0704593
02OA43a
Green
hornblende
004
98
0012
0704323
13
070431
thorn310
-------
-------
97OA128b
Whole
rock
003
1274
50001
0703327
17
070333
-------
076
37
-------
-------
-------
Plagioclase
002
1204
50001
0703285
09
070328
thorn414
-------
-------
Clinopyroxene
003
20 0
0004
0704230
09
070422
thorn523
-------
-------
01OA15f
Whole
rock
006
1077
0002
0704582
16
070458
-------
237
13 5
-------
-------
-------
01OA19a
Whole
rock
037
1303
0008
0704189
18
070418
-------
161
96
-------
-------
-------
01OA19d
Whole
rock
058
2032
0008
0703423
08
070341
thorn567
216
415
0703574
0703408
0703271
Pargasite
091
1058
0025
0703273
11
070324
thorn287
-------
-------
Plagioclase
-------
-------
-------
-------
-------
thorn10 09
-------
-------
02OA90b
Plagioclase
004
2385
50001
0703868
11
070387
thorn498
-------
-------
Green
hornblende
010
23 8
0012
0704288
15
070427
thorn239
-------
-------
BOSCH et al HYDROTHERMAL CIRCULATION IN OMAN OPHIOLITE
1197
Table5continued
Sam
ple
Rb(ppm)
Sr(ppm)
87Rb8
6Sr
87Sr
86Sr
(87Sr
86Sr)i1
d18O
()
LOI(
)2WR
3(87Sr
86Sr)L14
(87Sr
86Sr)L25
(87Sr
86Sr)R6
02OA37b
Plagioclase
023
3121
0002
0704204
15
070420
-------
-------
-------
Pargasite
031
41 6
0021
0703505
15
070348
thorn394
-------
-------
94OAMa2
Whole
rock
0016
1750
00003
0703219
15
070322
thorn495
033
31
0703264
0703177
0703158
Plagioclase
0014
1363
00003
0703219
11
070322
thorn479
-------
-------
Clinopyroxene
-------
-------
-------
0703236
13
070324
-------
-------
-------
1Initial8
7Sr
86Srratiocalculatedat
95Ma(H
acker1994)
2LOIisloss
onignition(in)
3Water---rock
ratiocalculatedassumingaclosedsystem
TheeffectiveWR
ratiocanbeexpressed
bythefollowingeq
uation
W=Reffectivefrac14
frac12eth87Sr=
86SrrockTHORN fi
nal
eth87Sr=
86SrrockTHORN in
itial=frac12eth8
7Sr=
86SrseawaterTHORN in
itial
eth87Sr=
86SrrockTHORN fi
nal
ethCrock
initial=C
seaw
ater
initial
THORNwhere(87Sr
86Srrock) finalisthefinalmeasuredSrisotopicratioin
thesample(87Sr
86Srrock) in
itialco
rrespondingto
theinitialm
antlevalue
istakenas
070295(Lan
phere
etal1981)(87Sr
86Srseawater) in
itialistheCretaceousseaw
ater
Srisotopicco
mposition[87Sr
86Sr07073
at90
MaafterBurkeet
al(1982)]C
seaw
ater
initial
istheSrco
ntent
oflsquonorm
alrsquoseaw
ater
andis
takenas
8ppm
(deVilliers1999)
FortheCrock
initialitis
assumed
that
thepresentSrco
ncentrationmeasuredis
thesameas
theinitial
concentration
4Srisotopicratiosmeasuredforliquid
L1extractedafterthefirststep
oftheleachingexperim
ent(6NHClfor8min
at70
C)
5Srisotopicratiosmeasuredforliquid
L2extractedaftertheseco
ndstep
oftheleachingexperim
ent(2 5NHClfor30
min
at70
C)
6Srisotopicratiosmeasuredforresidueobtained
afteratotalaciddigestionachievedaftertheextractionofL1an
dL2leachates
JOURNAL OF PETROLOGY VOLUME 45 NUMBER 6 JUNE 2004
1198
Part of the same sample from the contact between thedyke and the gabbro yields a slightly lower Sr isotopiccomposition (070464) intermediate between the sur-rounding gabbro and the epidote dyke It also has anintermediate Sr content (425 ppm) Acid leaching experi-ments performed on the whole rock of the dyke yield Srisotopic compositions of 070521 and 070512 for thetwo liquids extracted Similar experiments for the samplelocated at the contact with the gabbro yield 87Sr86Srratios of 070494 and 070467 for L1 and L2 liquids TheSr isotopic composition of the residue extracted afterleaching remains high (070459) and very close to theepidosite 87Sr86Sr ratio determined by Kawahata et al(2001) In this sample the primary minerals have beentotally replaced by epidote and secondary plagioclase orquartz Thus the Sr isotopic compositions measured forthis WR sample and its constituent minerals reflect thecomposition of the fluid filling this dyke
Mineral chemistry
Plagioclase
Compared with the host gabbros the dykes show a muchlarger dispersion in their plagioclase compositions (Fig 8aand b Table 4) characterized by more calcic plagioclasethan the enclosing gabbros This tendency is mostextreme in the comb-textured dykes in which the plagio-clase is almost pure anorthite The zoning is generallynormal recording a crystal fractionation process How-ever gabbro 94 MA2 exhibits inverse zoning this tend-ency is also observed in plagioclases from dykes 01OA19a and 01 OA15f The higher calcium content ofthe plagioclase as well as the inverse zoning is consistentwith an increase of water pressure during its crystalliza-tion (Turner amp Verhoogen 1960 Carmichael et al 1974Koepke et al 2003) The particularly high An content inthe comb-textured gabbro dykes 01 OA15f may alsoresult from crystallization in a supercooled magma thisis consistent with the comb texture indicative of fast-growing crystals The lack of brown hornblende suggeststhat crystallization occurred above the temperature ofhornblende stability
Amphibole
The amphiboles analysed in the dykes (Fig 8c andTable 4) show a regular trend in a (Na thorn K)A vsAlIV diagram from titanium-rich (Ti 4 015) pargasitichornblende (brownamphibole) and tschermakite (brown---green amphibole) to low-titanium (Ti5 015) magnesio-hornblende (green amphibole) and actinolite (pale greenamphibole) The pargasitic hornblende develops aspoikilitic oikocrysts in the dykes 01 OA19a and d andin the diffuse patch 01 OA41d2 at temperatures above800C during the final stage of crystallization (see
below) The tschermakite composition corresponds tothe internal alteration of clinopyroxene in gabbro 94Ma2 occurs in the comb-textured dyke 01 OA15f andis the primary amphibole in the dioritic dyke 02 OA37bThe magnesio-hornblende develops at the edges of thepargasites or of clinopyroxenes at temperatures around800C and is the primary phase in the dioritic dyke 02OA90b and in the green vein 02 OA43a Finally actino-lite occurs as a replacement of green hornblende in dykesor in the kelyphitic rim surrounding olivine in gabbro 94Ma2 Such a large composition range at the scale of athin section has been reported in amphiboles fromamphibolite-facies oceanic gabbros (Stakes 1991 Stakesamp Taylor 1992 Gillis et al 1993) as well as in the Omanophiolite gabbros (Manning et al 2000) This variabilityhas been explained by rapid reaction rates preventinghomogenization (Manning et al 2000)
Thermometry
Plagioclase---amphibole thermometry could be appliedwhen the two minerals exhibited good contacts such asin the laminated gabbro of the middle sequence (01OA41d2) and in the intrusive dykes The temperaturesof plagioclase---amphibole equilibration (Table 2) havebeen calculated using the geothermometer lsquoBrsquo of Hollandamp Blundy (1994) Edenite thorn Albite frac14 Richterite thornAnorthite valid between 500C and 900C with anuncertainty of 40C Amphibole formulae are basedon 23 oxygen and their nomenclature is based on alkalications in the A site vs AlIV according to Leake (1997)Pressurewas assumed to be 1 kbar Forty-five plagioclase---amphibole pairs meet the compositional criteria imposedby Holland amp Blundyrsquos dataset (09 4 Xan 4 01 andAlIV 403) Electron microprobe measurements wereconstrained to contiguous plagioclase and amphiboleseparated by sharp grain boundaries assuming equili-brium of the two phases When the mineral phasesexhibit normal zoning temperatures between plagioclaseand amphibole cores and between mineral rims werecalculated assuming that the temperatures deduced fromthe mineral core chemistry provide a proxy for the high-est temperature of equilibration These data are identi-fied in the T C vs AlIV diagram (Fig 8d) Table 4presents selected major element amphibole analyses theAn content of the plagioclase adjacent to each amphibolegrain and the temperature calculated for the pairThe temperatures calculated (Fig 8d) span the entire
range of temperatures at which amphibole and plagio-clase coexist in equilibrium (ie amphibolite-facies para-geneses) This temperature interval is bracketed byorthopyroxene-bearing facies or amphibole-free facies(ie gabbro 94MA2) and epidote---actinolite facies devoidof homogeneous plagioclase (ie epidosite dyke 01OA36b and green vein 02 OA43a) The range of the
BOSCH et al HYDROTHERMAL CIRCULATION IN OMAN OPHIOLITE
1199
calculated temperatures between 4900C and 750Ccorresponds to the changing composition of amphibolesfrom pargasite to magnesio-hornblende at the scale of athin section or even at that of an individual crystal asshown by their correlation with AlIV such variabilityrecords rapid cooling in hydrous conditions The highesttemperatures are recorded by pargasite from the anatec-tic gabbro 01 OA41d2 in the gabbro dyke 01 OA19dand in the dioritic dyke 02 OA90b They were calculatedfrom minerals devoid of lower-grade alteration and arethus considered as representing the equilibrium temperat-ure of the coexisting minerals Lower temperatures cor-respond to mineral phases evolving at falling temperatureand are only indicative of the cooling history Includingthe 40C uncertainty seven pairs provide temperatureshigher than 900C the limit of validity of the Holland ampBlundy thermometer However we maintain these tem-peratures as indicative of the highest values of our data-set These values are included in the average equilibriumtemperature summarized in Table 2
DISCUSSION
Distribution of hydrothermal alteration
Two distinct petrological events are recorded in the gab-bro section of the Samail ophiolite and are documentedin the present contribution (1) the whole gabbro sectionis affected by a penetrative alteration (2) the gabbrosection is crosscut by various types of dykes and veinsincluding hydrous minerals
Penetrative alteration
The penetrative alteration developed in the gabbros atgrain boundaries and locally invading the minerals isubiquitous in the gabbro section This penetrative altera-tion has been described as very high temperature (VHT)high temperature (HT) and low temperature (LT) basedon alteration parageneses The orthopyroxene coronas inthe layered gabbro sample and the incipient appearanceof brown pargasitic amphibole mark a lower thermallimit of the VHT alteration By reference to the experi-mental work of Koepke et al (2003) the temperature ofequilibration for these reactions is estimated to bebetween 900 and 1000CThe preservation of the vertical distribution of
VHT---HT facies (Figs 4 and 5) also pointed out byManning et al (2000) indicates that the hydrothermalalteration pattern fits the distribution of isotherms withdepth at a fast-spreading ridge (ie Phipps Morgan ampChen 1993 Dunn et al 2000 Fig 2) Equilibrium tem-peratures for VHT parageneses as high as 900---1000Csuggest that VHT---HT hydrothermal alteration tookplace in a very restricted time interval at the limit ofthe cooling magma chamber It is proposed that the
penetrative alteration affecting the entire gabbro sectionis related to a pervasive network of microcracks whichact as a recharge system down to the Moho (Nicolas et al2003)
Dykes and veins
The dykes are extremely varied eg pegmatitic gabbroswith comb-texture microgabbros and amphibolitizeddolerites dioritic dykes alignments of poikilitic horn-blende and finally diffuse hornblende-bearing patchesHigh-T gabbro and diorite dykes and low-T greenamphibole---epidote veins are connectedIn the dykes textural evidence attests to suprasolidus
conditions at least for the development of the pargasiticamphibole Integrating the hornblende---plagioclase equi-libration temperatures calculated for the studied samplesthese temperatures are bracketed between 950C and875CIn the following discussion we shall consider separately
the microgabbro and dolerite dykes The latter representbasaltic melt intrusions associated upsection with thediabase sheeted dykes and downsection possibly withthe mafic dykes that are ubiquitous in the mantle section(Nicolas et al 2000) Whatever their origin the develop-ment of pargasitic amphibole during their late stage ofcrystallization creates a link with the gabbroic dykes andsuggests a crystallization process evolving from dry con-ditions to more hydrous conditionsThe other gabbroic dykes comb-textured gabbros and
hornblende-bearing dioritic dykes result from the fillingof cracks by a fluid either a melt or a silica-rich hydrousfluid These intrusive dykes which develop reaction mar-gins at their contact with the host gabbro grade verticallyinto lower-temperature green amphibole veins or align-ments of poikilitic hornblende in the layered gabbromatrix or development of pargasitic hornblende in ana-tectic gabbros (sample 01 OA41d2) In gabbros crystal-lizing in the magma chamber the development oforthopyroxene and pargasite oikocrysts enclosing the pri-mary minerals suggests a thermal evolution from thegabbro solidus to amphibolite-facies conditions Forthese reasons it is suggested that the gabbroic dykesresulted from the injection of a hydrous melt in the stillhot gabbros drifting away from the magma chamber asdiscussed below
Origin of fluids responsible for VHT---HThydrothermal alteration
The 87Sr86Sr initial ratios and d18O of whole rocks andpure mineral fractions are plotted in Fig 9 against thedistance above the Moho Transition Zone (MTZ) Theinitial Sr isotopic composition of the whole rocks apartfrom the epidosite dykes ranges from 070322 to
JOURNAL OF PETROLOGY VOLUME 45 NUMBER 6 JUNE 2004
1200
070458 All the studied samples (including whole rocksand minerals) have initial 87Sr86Sr ratios falling outsidethe field of fresh MORB which commonly have Sr ratiosbetween 07023 and 07029 with an average of 070265(ie Hart et al 1974) The lowest initial Sr isotopiccomposition (070322) from the massive gabbro 94MA2 is slightly but significantly higher than averageOman primary magmatic signatures (ie 070296McCulloch et al 1981) Similarly the d18O values ofmost whole rocks and minerals measured (Table 5)
depart from typical MORB-source mantle values( Javoy 1980 Gregory amp Taylor 1981 Kyser 1986)These differences indicate that the primary mantlesignatures in the Oman ophiolite have been modifiedby interaction with a fluid Possible origins for thisfluid are mantle components dehydration of subductedlithosphere or seawater circulation Strontium andoxygen isotopes are useful tracers for fluid---crust interac-tion as d18O and 87Sr86Sr ratios from fluids originat-ing from seawater the mantle or subducted lithosphere
-1000
1000
2000
3000
4000
5000
6000
7000
MOHO
MTZ
Dis
tan
ce a
bov
e th
e M
oh
o (
m)
pillow-basalt
sheeted-dike
gabbro
peridotite
01 OA 41d2
01 OA 36b
02 OA 43a97 OA 128b
01 OA15f
94 OA MA2
02 OA 90b01 OA 19ad
02 OA 37b
0702 0703 0704 0705 0706 0707
87Sr86Srinitial
MO
RB-s
ou
rce
Seaw
ater
Whole RockPlagioclasePargasiteGreen HornblendeClinopyroxene
Epidote
(a)
-1000
1000
2000
3000
4000
5000
6000
7000
MOHO
MTZ
Dis
tan
ce a
bov
e th
e M
oh
o (
m)
pillow-basalt
sheeted-dike
gabbro
peridotite
01 OA 41d2
01 OA 36b
02 OA 43a97 OA 128b
01 OA15f
94 OA MA2
02 OA 90b01 OA 19ad
02 OA 37b
Whole RockPlagioclasePargasiteGreen HornblendeClinopyroxene
3 5 10
δ18O (permil)
41d2
Ma2
43a
90b 19d 19d37b
128b
90b
(b)
Fig 9 (a) Variation of initial 87Sr86Sr isotopic composition as a function of the location of the studied samples in a schematic log of the crustalsection of the Oman ophiolite The field for MORB is from Hart et al (1974) and that for seawater is from Burke et al (1982) Small open squares aredata fromKawahata et al (2001) (b) Variation of d18O () as a function of the location of the studied samples in a schematic log of the crustal sectionof the Oman ophiolite Shaded curve corresponds to the data of Gregory amp Taylor (1981)
BOSCH et al HYDROTHERMAL CIRCULATION IN OMAN OPHIOLITE
1201
are significantly different Moreover the d18O valueis dependent on temperature and mineral fractiona-tion and thus yields complementary constraints to Srisotopes
Mantle origin
One model to explain the occurrence of fluids interactingwith the oceanic crust at very high temperatures is toderive them from the mantle The contribution of mantle-derived hydrous fluids linked to percolation processesor to their concentration in the final stages of gabbrocrystallization has been invoked in numerous case studies(ie Witt amp Seck 1989 Fabriees et al 1989 Dupuy et al1991 Agrinier et al 1993) O and Sr isotopic data forwhole rocks and mineral separates yield scattered valuesthat are unambiguously different from MORB mantle-source values (Fig 9a and b) With the exception of lateLT altered samples the WR data have a mean 87Sr86Srratio of 070337 and a standard deviation of 000012These surprisingly high values in mantle-derived rockscould be taken as evidence for survival of mantle isotopicheterogeneities during crystallization of the ophiolite (egLanphere 1981 McCulloch et al 1981) However the18O16O isotope ratios in the Oman gabbroic sequenceare also displaced from primary magmatic values Nogabbro in the present study has plagioclase or amphi-bole with typical mantle d18O values Thus the Sr and Oisotope data rule out the possibility of mantle-derivedfluids as a possible source for the VHT---HT hydrousalteration event recorded in the massive gabbros andgabbroic dykes and veins (Fig 10a and b)
Subducted lithosphere origin
Dehydration of subducted oceanic crust in subductionzones can generate hydrous fluids Such fluids couldpercolate through the overlying lithosphere and contam-inate it thus generating modified isotopic signatures andtrace element contents Such an explanation howeverdisagrees with the geochemical characteristics of most ofthe studied samples Fluids derived from the dehydrationof subducted slabs (fresh or altered MORB or OIB pro-toliths) should be strongly enriched in K Rb and Ba (egBecker et al 2000) whereas in Oman the Rb contentsmeasured in the gabbros and gabbroic dykes and veins arestrongly depleted Moreover the isotopic composition ofslab-derived fluids is buffered by the MORB-like oceaniccrustal protolith for Nd and Pb but not for Sr and O(Staudigel et al 1995) As a result of relatively minorfractionation of oxygen at high temperature the oxygenisotope composition of the fluids expelled during dehy-dration of the subducted slab should be high and essen-tially controlled by the oxygen composition of the uppersection of the crust altered at low temperature (with an
average of 10 and perhaps as high as 19) (Staudigelet al 1995) The Sr isotopic composition should be alsohigh (average 07046) as the fluids released by the dehy-dration of subducted oceanic crust are associated eitherwith seawater or with radiogenic Sr phases (ie smectites)The Sr and O isotopic compositions of the studied sam-ples do not fit with these signatures and so preclude asubducted lithosphere origin for the fluids involved in theVHT---HT alteration event
Seawater-derived fluids
All the analysed samples (minerals and WR) have 87Sr86Sr(T) outside the domain of primary MORB magmasand variable Sr contents Individual minerals have Srcontents reflecting their different mineralliquid partitioncoefficients Minerals characterized by a very high affi-nity for Sr such as plagioclase (ie Sr mineralliquidpartition coefficients 1) have very high Sr contentsTherefore the high abundance of plagioclase in the sam-ples analysed is responsible for the high Sr content of thestudied whole rocks The Sr content of all the whole rocksis relatively homogeneous (Table 5) without clear distinc-tion between country-rock gabbros and dykes and veinsand ranges from 110 to 200 ppm except for the epidosite(4700 ppm) Reported on a 87Sr86Sr vs 1Sr diagram(Fig 10a) most whole rocks and plagioclases analysed arecharacterized by a 1Sr ratio lower than 001 and plot inthe left part of this diagram Compared with N-MORBthe studied massive and anatectic gabbros and their con-stituent plagioclases have similar to slightly higher Srcontents but substantially higher 87Sr86Sr ratios(Fig 10a) Considering Cretaceous seawater as a possiblehigh 87Sr86Sr mixing component [87Sr86Sr 07073at 90Ma after Burke et al (1982)] responsible for theobserved geochemical modifications exchanges cannothave occurred in a closed system as seawater has too lowa Sr content (ie 8 ppm de Villiers 1999) An open-system process allowing penetration of larger quantitiesof seawater can efficiently modify the Sr isotopic ratio ofthe rocks Alternately the fluids could also be seawaterthat was modified through exchange with the surround-ing rocks during its transit through the oceanic crustalsection This modified seawater would be more enrichedin Srwith a 87Sr86Sr ratio intermediate between unmodi-fied seawater and MORBMinerals devoid of late LT alteration imprints (parga-
site green hornblende and pyroxene) and characterizedby very low Sr contents plot on the right of the diagramclose to or on the mixing line between seawater andMORB (Fig 10a) The difference between these mineralsand the other samples (WR and plagioclases) is mainlydue to the Sr fractionation coefficients of these differentminerals and to the large quantity of plagioclase inthe studied rocks The process responsible for the
JOURNAL OF PETROLOGY VOLUME 45 NUMBER 6 JUNE 2004
1202
modification of the Sr isotopic composition fromMORB-source value is however explained by the same phenom-enon As all these minerals display initial 87Sr86Sr ratiosdistinct from the MORB source and because they equili-brated at VHT or HT temperatures this supports anearly intervention of seawater during crystallization ofthese minerals Most of these samples with the exceptionof the epidosite dyke overlap the domains defined byKawahata et al (2001) for the Wadi Fizh gabbros alteredat VHT and HT conditions in the amphibolite facies(labelled lsquoVHTrsquo and lsquoHTrsquo in Fig 9a)Three samples (two epidosite dykes and an one epidote
fraction) have very high Sr contents and the highest Sr
isotopic compositions of the present dataset The twowhole-rock samples overlap the field of the Wadi Fizhsamples altered at high temperature during greenschist-facies metamorphism (Kawahata et al 2001) This typeof alteration is common at the ridge axis and formedthrough intensive exchange with seawater-derived fluidsFigure 10b indicates that all the Sr---O isotope data are
distinct from MORB compositions and suggests that thefluids reacting with the magmas could be derived fromseawater In addition the d18O values show that isotopicexchange occurred under VHT or HT conditions Thefluids could have the composition of seawater or theycould have the composition of seawater modified through
0703
0705
0707
001 01 1
1 Sr (ppm)
MORB
(87Sr
86Sr)
init
ial
Seawater
VHT
HT
MT36b
36b
36b
43a 128b90b
41d2
37b19d
15f
128b
41d2
41d2
19a
Ma2Ma2
19d
90b
37b
Whole RockPlagioclasePargasiteGreen HornblendeClinopyroxene
Epidote
(a)
0 2 4 6 8
δ18O(permil)
0703
0705
0707Seawater
MORB-mantle
128b 41d2
90b 41d2
Ma2128b
37b
43a
19d
90b
Whole RockPlagioclasePargasiteGreen HornblendeClinopyroxene
Epidote
HT LT
19d
(87Sr
86Sr)
init
ial
(b)
Fig 10 (a) Initial 87Sr86Sr isotopic ratios plotted against 1Sr for whole rocks and mineral separates from the gabbro section of the Omanophiolite Fields shown are from Kawahata et al (2001) and correspond to MT-----basalt sheeted dyke diabase altered at medium temperature(prehnite---actinolite facies sequence 3) HT-----diabase plagiogranite metagabbro and epidosite formed at high temperature (greenschist faciessequence 4) VHT-----gabbro altered at very high temperature (amphibolite facies sequence 5) The dashed line is a binary mixing line betweenMORB (Lanphere et al 1981) and Cretaceous seawater (Burke et al 1982) (b) Initial 87Sr86Sr isotopic composition plotted against d18O value forwhole rocks and minerals from the Oman ophiolite Cretaceous seawater and MORB end-members as in (a) Dashed line represents mixing curvebetweenMORB and seawater at high temperature (HT) Low-temperature (LT) exchanges between these two components would be responsible foran increase of the d18O primary MORB value
BOSCH et al HYDROTHERMAL CIRCULATION IN OMAN OPHIOLITE
1203
exchange with the surrounding rocks during its penetra-tion into the crustal section In an environment in whichfluid---rock interaction occurs at variable temperaturesthe d18O is greatly influenced by the temperature atwhich the exchange took place and by the waterrockratio It is evident from Figs 9b and 10b that none of thestudied gabbros have plagioclase and amphibole withMORB-like d18O and 87Sr86Sr values The separateminerals display a broad range of d18O values and mostexhibit extreme 18O depletion (Table 5) Hydrothermalexchange kinetics are similar in amphibole and pyroxeneand these two minerals are more resistant to oxygenexchange during water---rock interaction than coexistingplagioclase (Gregory et al 1989) The most probableexplanation of the low d18O values measured in bothamphiboles and pyroxenes is that they crystallized athigh temperature in the presence of a relatively low 18Oseawater-derived hydrothermal fluid [d18O from 01 to07 after Gregory amp Taylor (1981)] The anomalouslyhigh d18O value (thorn1009) measured for the plagioclase ofthe micro-gabbroic dyke (01 OA19d) can be interpretedas resulting from a superimposed overprint caused by alate-stage penetration of fluids at lower temperature Inthis sample the occurrence of such different 18O16Oratios between coexisting plagioclase and amphibole isnot consistent with equilibrium fractionation betweenthese two minerals at any possible temperature Thisobservation suggests that parts of the ophiolite sequencehave undergone a high-temperature hydrothermal eventprior to the later low-temperature event that producedthe strong 18O increase in coexisting plagioclaseThe depletion of the d18O values results from high-temperature hydrothermal alteration (Gregory amp Taylor1981 Stakes amp Taylor 1992) The oxygen isotopic datasupport the contention that most studied samples havebeen affected by a VHT---HT hydrothermal alteration byfluids derived from seawater Some of the analysed sam-ples presenting petrological evidence of crystallization ofLT secondary minerals have been overprinted by the latelow-temperature alteration thus swamping the earlierhigh-temperature alteration effects
Determination of waterrock ratio
To better evaluate the exchange between seawater andthe gabbroic rocks waterrock ratios (WR) have beenestimated for the whole rocks analysed during the courseof this study The different models used to constrain thisratio mainly differ by the calculation of the effective ratioor by the estimated weight ratio and by considering openor closed systems for water circulation (Albareede et al1981 McCulloch et al 1981) The WR ratios reportedin Table 5 have been calculated assuming a closed sys-tem Using this formulation these values are minimabecause the calculation always uses pristine fluid for the
initial fluid thus maximizing the value in the denomina-tor If the fluid was shifted to lower 87Sr concentrations byexchange with the rock on the downward path theinferred values would be much higher (Davis et al 2003)The calculated WR ratios for the samples from this
study range from 31 to 219 (Table 5) Two main groupsof samples can be recognized on the basis of their waterrock ratios The lowest ratios ranging from 31 to 47have been determined for massive gabbros or anatecticgabbros but also for dykes and veins devoid of late LTalteration Similar ratios have been previously calculatedfor example for the discharged hydrothermal solutions atthe 13N and 21N East Pacific Rise (Di Donato et al1990 Fouquet et al 1991) The gabbros from the Ibrasection in the Oman ophiolite gave effective WR ratiosof 05 33 and 42 (McCulloch et al 1981) in goodagreement with the values obtained in this study Theleast altered gabbro of the Ibra section yields a WR ratioof 05 in good agreement with the exceptionally fresh-ness of this rock characterized by typical mantle oxygenvalues for its minerals (McCulloch et al 1981) Samplescharacterized by waterrock effective ratios in the rangeof 3---5 can be related to penetrative alteration via thehydrothermal system Lecuyer amp Reynard (1996) havedemonstrated in oceanic gabbros from the Hess Deepaltered in similar HT conditions that WR mass ratios inthe range of 02---1 are sufficient to account for the mod-ified stable isotope signature of the gabbros Higherwaterrock values (WR 45) have been calculated forsamples affected by superimposed late hydrothermalalteration and for the epidosite samples (Table 5) Theyprobably acquired their isotopic characteristics throughinteraction with a larger volume of seawater duringgreenschist-facies metamorphism Similar or higherWR values have been published in previous studies(ie McCulloch et al 1981 Kawahata et al 2001)They correspond either to samples from the upper-crus-tal sequence (ie basalt sheeted dyke or plagiogranite) orto gabbros from the crustal section located in a particularenvironment such as for example the Wadi Fizh sectionthat is located close to a segment boundary along thepalaeo-spreading axis (Nicolas et al 2000)
Mechanism for seawater interaction withmagma
Nicolas et al (2003) have proposed that variable amountsof seawater can circulate at high temperature through-out the crustal section down to the walls of the magmachamber via a network of parallel microcracks a fractionof a millimetre wide Such microcracks and their relation-ship with the VHT---HT hydrous alteration in the sur-rounding gabbros have been described in detail byNicolas et al Via this network of microcracks largeamounts of seawater can have reacted with the magma
JOURNAL OF PETROLOGY VOLUME 45 NUMBER 6 JUNE 2004
1204
and induced variable changes of its Sr and O isotopiccomposition This regular network of parallel micro-cracks could constitute the recharge system and thehydrated gabbroic dykes the discharge system Physicalparameters and the geophysical models of Nicolas et al(2003) are in agreement with this scenario Such a pene-tration of seawater-derived hydrothermal fluids throughthe lower crust along grain boundaries and a microscopicfracture network at temperatures 4750C is consistentwith recent thermodynamic models (McCollom amp Shock1998)Seawater-altered and high-temperature recrystallized
diabases (protodykes) from the overlying sheeted dykecomplex stoped into the melt lenses could represent anindirect way for seawater to enter the system as they canremelt during their subsidence through the magmachamber A simple mass balance calculation suggeststhat this indirect contamination is not significant Assess-ments of the amount of recycled crust 1 in volumeby Nicolas et al (2000) based on the protodyke remnantsin the gabbro section or 20 by Coogan et al (2003)based on chlorine content in EPR basalts lead to a sea-waterrock ratio of the order of 104 to 103Thus following Nicolas et al (2003) we propose that
seawater has been introduced along microcracks down tothe walls of the magma chamber and has diffused alonggrain boundaries in the way described by Manning et al(2000) progressively invading the host gabbro Gabbroicdykes result from the injection of hydrous melt into thehost gabbros at falling temperatures as they drift awayfrom the magma chamber This water-saturated meltcould result from the extensive hydration of the crystal-lizing gabbros near the walls of the magma chamberwhen seawater penetrates through this wall (Fig 1)Similar plumbing systems driving seawater down to the
limit of the magma chamber and back to the surface havebeen already proposed in other environments such as theMid-Atlantic Ridge (Kelley amp Delaney 1987 Cooganet al 2001) the East Pacific Rise at Hess Deep (Gillis1995 Manning et al 1996a 1996b) the Tonga forearc(Banerjee amp Gillis 2001) and the Troodos ophiolite(Gillis amp Roberts 1999) The melting curve of wet basalthas a negative slope but is close to the dry solidus at lowpressure in the lower gabbros and the first melts areplagiogranitic (Spulber ampRutherford 1983) Such plagio-granites are ubiquitous in the highest gabbros and in theroot zone of sheeted dykes in Oman (Rothery 1983Juteau et al 1988 Nicolas amp Boudier 1991) in manyother ophiolites and in the oceanic crust where they havebeen interpreted as resulting either from wet anatexis ofhot gabbros or from the final product of crystallization ofa basaltic melt (Spulber amp Rutherford 1983) Plagio-granites are rare in the lower gabbros exceptionallyassociated with hydrous gabbro segregations inside thelayered gabbros Their constitutive minerals are also
remarkably absent along the VHTmicrocracks althoughthese gabbros were above the wet solidus A possibleexplanation has been proposed by McCollom amp Shock(1998) who wrote that at high temperature lsquoaqueousfluids may leave very little conspicuous petrological evid-ence for their presencersquo the reason being that thehigh-T conditions would be closer to batch meltingObviously more detailed studies on this specific problemare needed We conclude that the large degree of hydrousmelting locally required at the walls of the magma cham-ber to account for the generation of a gabbroic hydrousmelt may have erased the traces of the incipient plagio-granitic melt
CONCLUSIONS
Fostered by the recent discovery of high-temperatureseawater contamination in some gabbros of the Omanophiolite (Manning et al 2000) we have conducted astudy on 500 gabbro specimens from selected areas ofthe entire Samail ophiolite belt and on the hydrous gab-broic dykes that cross-cut them The highest-temperaturereactions (VHT) recorded in olivine and clinopyroxene ingabbros as orthopyroxene and pargasite coronas reach975C They are followed at lower HT conditions withreplacement of olivine by an assemblage of tremolitemagnetite and plagioclase surrounded by chlorite andby pargasite development in clinopyroxene followedfinally by the LT lizardite---olivine and saussurite---plagioclase greenschist-facies alteration With a fewexceptions the VHT alteration tends to be restricted tothe lower gabbros and to the vicinity of the Moho andthe HT alteration to the middle and upper gabbros Thepreservation of the vertical distribution of VHT---HTfacies indicates that progressive cooling during off-axisspreading did not obliterate this distribution and conse-quently that the VHT---HT hydrothermal alteration tookplace in a very restricted time interval at the limit of thecooling magma chamber In the deep and hot gabbrosthe main water channels may be sub-millimetre-sizedmicrocracks with a dominantly vertical attitude the ori-gin and dynamics of which have been investigated in aprevious paper (Nicolas et al 2003)Beyond these high-temperature alteration zones mas-
sively induced in the gabbros seawater may be respons-ible for the generation of clinopyroxene---pargasitegabbro dykes that cross-cut all gabbros and that areupsection clearly connected to the LT hydrothermalvein system Petrostructural evidence suggests that thesedykes were generated by hydration of the crystallizinggabbros near the internal wall of the LVZ---magmachamber The resultant melts were subsequently intrudedat various levels in the cooling lower and middle gabbrosPetrological observations of nearly all the gabbros ana-lysed in the present study located from the root zone of
BOSCH et al HYDROTHERMAL CIRCULATION IN OMAN OPHIOLITE
1205
the sheeted dyke complex down to the Moho emphasizethe imprint of VHT---HT hydrous alteration The VHThydrothermal event is characterized by temperatures ofequilibration around 900---1000C The temperatures atwhich the HT hydrous event occurred are estimated tobe in the range 550---900CStrontium and oxygen isotope compositions measured
on whole rocks and separated minerals significantlydepart from primary MORB values The Sr and O iso-tope data obtained for pargasite green hornblende andclinopyroxene record the participation of seawater at thetemperature of crystallization of the minerals Plagio-clases and whole rocks define a trend toward a compo-nent characterized by a high radiogenic 87Sr86Sr valueand a higher Sr content than normal seawater Seawateris clearly identified as the fluid responsible for the modi-fication of the Sr and O signatures for both massivegabbros and dykes and veins Nevertheless the high Srcontent of the extraneous component requires eitheropen-system exchange allowing penetration of a greaterquantity of seawater or penetration of seawater modifiedby interaction with the oceanic crust during its travel paththrough the crustal section The waterrock ratio calcu-lated on the basis of the Sr isotopes is between three andfive for the enclosing gabbros and for the least LT altereddykes The VHT---HT hydrothermal event affectingthese samples is related to penetrative alteration via therecharge and discharge hydrothermal system Thewaterrock ratio is 10 for dykes exhibiting evidenceof a LT hydrothermal alteration overprint and reaches22 for the epidosite dyke The depletion in d18O char-acterizes all the studied samples with the single exceptionof the micro-gabbroic dyke plagioclase This agreeswith the conclusions based on Sr isotopes suggestingthat these samples have undergone exchange with sea-water-derived fluids during a VHT---HTalteration hydro-thermal event
ACKNOWLEDGEMENTS
This work has benefited from financial support by theCentre National de la Recherche Scientifique (INSUInteerieur-Terre andDYETI programmes) and inOmanfrom the willingness of the Directorate of MineralsMinistry of Commerce and Industry and the hospitabil-ity of the people Technical help in the laboratory is alsoacknowledged including C Nevado for the thin sectionsE Ball for the illustrations B Galland for her assistancein the chemistry room and C Bassoulet for help duringthermal ionization mass spectrometric analyses of the Srresults from the leaching experiments Constructivereviews from R T Gregory C Lecuyer an anonymousreviewer and particularly from M Wilson greatlyhelped to improve an earlier version of the manuscript
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1207
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JOURNAL OF PETROLOGY VOLUME 45 NUMBER 6 JUNE 2004
1208
ratios were calculated by correcting the measured ratiosfor accumulation of 87Sr produced by in situ 87Rb radio-active decay since 95Ma the assumed age of crystalliza-tion of the Oman ophiolite (Hacker 1994)
Oxygen isotopes
The powdered samples were introduced in Ni tubesunder dry air where they were reacted with BrF5 toform oxygen Oxygen was separated from other gasesby cooling the resulting mixture at liquid nitrogen tem-perature (77 K) that retained other volatile gases Oxygenwas further purified by trapping it on a cooled 5A mole-cular sieve at 77 K then quantified to calculate the yieldof the isotopic analyses and finally transferred to the massspectrometer (Finnigan Mat Delta E) where intensitiesassociated with masses 32 and 34 were measured todetermine d18O The isotopic composition of a sampleis given as dsample frac14 (RsampleRstandard 1) 1000 in permil unit where R is 18O16O and the standard isSMOW Analytical errors on the oxygen isotope ratioare 02 (2s)
RESULTS
Seven samples representative of the range of gabbroicdykes and five samples representative of the lower gab-bros two of them representing the margins of gabbroicdykes were selected for major element thermometricand Sr---O isotopic analyses (Tables 1---3) These samplesoriginate from wadi sections marked by bold lines inFig 1 Wadis Halhal Mayah Al Abyad Warihya andMaqsad structurally belong to a new ridge segment open-ing in slightly (1Myr) older lithosphere Wadi Gideahis in older lithosphere
Petrographic and isotopic characteristicsof the analysed samples
Olivine gabbro from the MTZ in MaqsadSamail Massif
Sample 94 MA2 is devoid of low-temperature alterationand only exhibits the discrete signature of the VHT---HTalteration The orthopyroxene corona around olivine(Fig 3a) is relayed by kelyphitic rims at the contactwith plagioclase (Fig 3b) Issuing from these rims aremicrometre-sized pale green amphiboles identified ascummingtonite and actinolite which penetrate the sur-rounding plagioclase in microcracks 004mm wide or atgrain boundaries (Fig 3b) similar to the microcrack sys-tem described by Manning et al (2000) A large numberof the plagioclase crystals have a cloudy core as a result ofthe concentration of fluid and mineral inclusions amongwhich diopside has been identified during microprobeanalysis Olivine cores are free of serpentine but theyare largely oxidized along a microcrack network Ortho-pyroxene coronas have a MgOpx number of 78---79
slightly higher than in the primary olivine cores withMgOl number of 74---75 The magmatic clinopyroxeneis totally fresh and probably not in equilibrium with thereactive orthopyroxene This sample has the lowest 87Sr86Sr initial ratio of the studied whole rocks (070322)identical to coexisting clinopyroxene (070324) and pla-gioclase (070322) This observation suggests the lack of alate superimposed LT hydrothermal alteration as it isvery unlikely that an LT hydrothermal event could havetotally equilibrated the Sr isotopic compositions of prim-ary minerals such as plagioclase and clinopyroxeneMoreover the whole rock (WR) clinopyroxene andplagioclase have similar initial Sr isotopic ratios despitevery different Sr contents Accordingly this ratio isthought to reflect the signature acquired during the crys-tallization of these minerals and is higher than the Omanmantle Sr isotopic value (McCulloch et al 1981) Thed18O values of the WR (50) and plagioclase (48) aredistinctly lower than primary magmatic values Indeedin normal mid-ocean ridge basalts (MORB) the WRd18O value is 57 02 with a corresponding plagio-clase d18O ranging from 55 to 62 ( Javoy 1980Gregory amp Taylor 1981)
Lower gabbro from Wadi Wariyah Wadi Tayin Massif
Sample 97 OA128b belongs to a layered sequence in thelower gabbro series injected by anorthositic sills Thesample was collected at the tip of one of these sillsbetween layers enriched in clinopyroxene The gabbrois composed of 60 plagioclase and 40 clinopyroxeneDespite the absence of olivine the WR has a relativelyhigh Mg content (Table 3) A series of low-T (prehnite)microveins 01mm thick cross-cut the gabbro perpendi-cular to the layering and foliation The margins of theveins are devoid of alteration This gabbro has the sameSr isotopic composition as 94 MA2 Nevertheless theWR and associated plagioclase have Sr isotope signatures(070333 and 070328) lower than the coexisting clino-pyroxene (070422) The Sr contents of the WR plagio-clase and clinopyroxene are 127 ppm 120 ppm and20 ppm respectively thus making the clinopyroxenemore sensitive to late-stage contamination Clinopyrox-ene and plagioclase oxygen isotope compositions (523and 414 respectively) are also lower than typicalmantle values Similar to the previous sample alterationat high temperature by a low d18O and high 87Sr86Srfluid is required to explain such characteristics
Amphibole gabbro patch Wadi Gideah Wadi TayinMassif
Sample 01 OA41d2 is from a laminated gabbro com-posed of millimetre-sized layers of clinopyroxeneand plagioclase Anatectic patches (Fig 7a) borderinganorthositic layers are formed of coarse-grained
JOURNAL OF PETROLOGY VOLUME 45 NUMBER 6 JUNE 2004
1190
Table1Locationoftheanalysed
samplesandsummaryoftheirpetrographicandSr---O
isotopiccharacteristics
Sam
ple
nam
eSite
Locationin
gab
broic
section
Typ
eofrock
Distance
to
Moho(km)
Parag
enesis
Alteration
87Sr
86Srinitial1
d18O
()2
94MA2
Maq
sad
From
MTZ
Layered
gab
bro
0Olivine(M
gno74---75)3
OPXco
ronas
(Mgno78---79)
WRfrac14
070322
WRfrac14
thorn495
CPX
Palegreen
Am
(cumact)
CPXfrac14
070324
nd
Plagioclase(A
n65---80)4inversezoning
Nolow-T
alteration
Plfrac14
070322
Plfrac14
thorn479
Solidfluid
inclusionsin
Pl
97OA128b
Wad
iWaryiah
From
lower
gab
bro
Layered
gab
bro
1CPX
Low-T
alteration
WRfrac14
070333
nd
Plagioclase(A
n85---87)
Prehnitemicroveins
CPXfrac14
070422
CPXfrac14
thorn523
Plfrac14
070328
Plfrac14
thorn414
01OA41d2
Wad
iGideah
From
upper
gab
bro
Gab
bro
patch
in
laminated
gab
bro
36
CPXrecrystallized
Plagioclaseidiomorphic
(An57---87)norm
alzoning
Black
pargasitic
honblende
WRfrac14
070351
CPXfrac14
070378
Plfrac14
070426
WRfrac14
thorn480
nd
Plfrac14
thorn504
Hbdfrac14
070355
nd
01OA19aamp
dWad
iWaryiah
Inlayeredgab
bro
Gab
broic
dyke
01
Olivine
OPXpoikilitic
WRfrac14
070341
WRfrac14
thorn567
CPX
Black
pargasite
Hbdfrac14
070324
Hbdfrac14
thorn287
Plagioclase(A
n73---95)
Hornblendepoikilitic
Plfrac14
nd
Plfrac14
thorn10 09
Hem
atite
WRfrac14
070418
nd
Green
Mg-hornblende
Acthornblende
Epidote
02OA90b
Wad
iAl-Abyad
Inlayeredgab
bro
Dioriticdyke
015
Green
Mg-hornblende
Hbdfrac14
070427
Hbdfrac14
thorn239
Plagioclase(A
n83---87)
Plfrac14
070387
Plfrac14
thorn498
Solidfluid
inclusionsin
plagioclase
01OA15f
Wad
iWaryiah
Inlayeredgab
bro
Comb-textured
gab
broic
dyke
025
CPXidiomorphic
Plagioclase(A
n95---99)norm
alzoning
BrownMg-H
ornblende
WRfrac14
070458
nd
02OA43a
Wad
iMayah
Inlayeredgab
bro
vein
Green
amphibole
1Palegreen
Mg-hornblendeacicular
Plagioclase(A
n91---97)in
reactionmargin
Hbdfrac14
070431
Hbdfrac14
thorn310
01OA36b
Wad
iGideah
Inlayeredgab
bro
epidosite
dyke
25
CPX
Epidote
WRfrac14
070519
nd
Plagioclase
Epfrac14
070554
nd
CPXclinopyroxene
Plplagioclase
OPXorthopyroxene
Amam
phibolecu
mcu
mmingtonite
actactinolite
WRwholerockHbdhornblende
Epep
idotend
notdetermined
187Sr
86Srinitialratioscalculatedat
95Ma(H
acker1994)Errors
ofthemeasuredratiosareindicated
inTab
le4
2Analyticalerrors
oftheoxygen
isotopevalues
are02
(2s)
3Mgnumber
frac14atomicMg(Mgthorn
Fe)
4Percentageofan
orthitein
plagioclase
BOSCH et al HYDROTHERMAL CIRCULATION IN OMAN OPHIOLITE
1191
(millimetre-sized) recrystallized clinopyroxene largelyaltered to light green magnesio-hornblende and idio-morphic plagioclase Locally primary black pargasitichornblende develops including idiomorphic normallyzoned plagioclase (Table 4) This gabbro exhibits similar87Sr86Sr ratios for the WR and pargasite (070351 and070355 respectively) These values are interpreted asrepresentative of the melt from which this amphibolecrystallized The clinopyroxene has a slightly higher Srisotope composition of 070378 consistent with the occur-rence of the light green Mg-hornblende rim Plagioclaseis significantly more radiogenic (070426) related to sec-ondary destabilization evidenced by the idiomorphichabit The Sr isotope compositions of the liquidsextracted from the acid leaching experiments of theWR (L1 and L2) are very similar 070358 and 070365respectively The 87Sr86Sr ratio of the residue obtainedafter leaching experiments is 070335 lower than boththe pargasite and unleached WR This value can beconsidered as the unaltered isotopic composition of thissample The d18O values measured for WR and coexist-ing plagioclase are lower than normal MORB magmaticvalues The 18O depletion in the plagioclase can be fairlywell explained by high-temperature hydrothermal inter-action between an oxygen-depleted fluid and the magmaAccording to the kinetics of hydrothermal exchange ofoxygen (Gregory amp Taylor 1981 Gregory et al 1989)plagioclase exchanges its oxygen isotopes with the fluidadjusting readily to the hydrothermal conditions Forhigh temperatures of exchange with seawater-derivedfluids (d18O between 0 and thorn2) the magmatic plagio-clase will have its primary d18O value lowered whereasthe d18O value for lower temperatures of exchange(5300C) will increase The amplification of the d18Oshift increases with the intensity of the hydrothermalalteration
Gabbroic dykes intrusive in lower gabbros from WadiWariyah Wadi Tayin Massif
Gabbroic dykes 01 OA19a and 01 OA19d are intrusiveinto the lower gabbros They are 3---4 cm wide discord-ant to the foliation of the host gabbros (Fig 7e) with asharp margin marked by a brown amphibole fringe Theprimary phases in the dykes (Table 1) locally preservedas elongated aggregates of a 01mm grain size mosaicassemblage at the dyke margins grade to millimetre sizein the central part of the dyke The WR major elementcomposition is Mg enriched compared with the enclosinggabbro (Table 3) Brown pargasitic amphibole oikocrystsmake up 50 of the modal composition of the dyke Indyke 01 OA19a poikilitic orthopyroxene may developinstead of brown amphibole Finally green magnesio-hornblende grows either as oikocrysts at the expense ofclinopyroxene or in association with epidote as an altera-tion product of plagioclase These low-amphibolite-faciesminerals represent the lowest-grade paragenesis recogn-ized in these dykes and are most developed in sample01OA19aTheamphibole composition showszoning fromTi-rich pargasitic hornblende to magnesio-hornblendeand actinolitic hornblende (Fig 8c) recording pro-gressive cooling of the dyke The plagioclase compositionis extremely heterogeneous (Fig 8b) varying from An95a composition similar to that of the plagioclase in the hostgabbro (Fig 8a) to An50 for the plagioclase and asso-ciated epidote inclusions in the poikilitic actinolitic horn-blende Sample 01 OA19d has an initial 87Sr86Sr ratioslightly higher for the WR (070341) than for the parga-site (070324) The Sr isotopic ratios of liquids extractedafter two steps of WR acid-leaching (L1 and L2) are070357 and 070341 respectively whereas the residueyields a 87Sr86Sr ratio of 070327 very close to thepargasite and plagioclase values The latter is similar tothe isotopic composition measured for the massive gab-bro 94 MA2 and may be taken as close to the primarymagmatic value This Sr isotopic ratio is more radiogenicthan the inferred Oman mantle-source value (McCullochet al 1981) The altered part of this sample has a radio-genic Sr isotopic composition (070418) related to thelow-amphibolite-facies alteration This gabbroic dykehas seemingly preserved a magmatic whole-rock d18Ovalue (567) but contains by far the highestd18O plagioclase (1009) of all the samples analysedThis plagioclase coexists with low d18O pargasite(thorn287) Similarly high values (d18Oplagioclase 4 8)have been previously measured for gabbroic dykes dia-bases sheeted dykes and plagiogranites from the Omanophiolite (Gregory amp Taylor 1981 Stakes amp Taylor1992) This 72 oxygen isotope fractionation betweenplagioclase and amphibole cannot possibly represent oxy-gen equilibrium at any plausible temperature This maybe explained by different exchange kinetics for these twominerals at different temperatures at high waterrock
Table 2 Average temperatures ( C 40C)
calculated for plagioclase---amphibole pairs
using the geothermometer of Holland amp
Blundy (1994)
Pargasite Magnesio-hornblende
rim core rim core
01 OA41d2 878 953 ------- -------
01 OA19a 935 974 825 869
01 OA19d 890 933 789 -------
02 OA37b ------- ------- 816 849
02 OA90b ------- ------- 859 833
JOURNAL OF PETROLOGY VOLUME 45 NUMBER 6 JUNE 2004
1192
ratios This supports the occurrence of two distincthydrothermal events one at high temperature and alater stage at lower temperature responsible for thestrong 18O enrichment in the coexisting plagioclase
Dioritic dyke associated with a diabase dyke in lowergabbros from Wadi Halhal Haylayn Massif
Sample 02 OA37b belongs to a group of brownamphibole---plagioclase lenses or reaction margins asso-ciated with diabase intrusives (Fig 7d) in lower layeredgabbros Idiomorphic brown hornblende (tschermakite)crystals are elongated in the plane of the dyke represent-ing 80 of the modal composition They are associatedwith an interstitial plagioclase matrix The browntschermakite has greenish rims Plagioclase has a cloudyappearance due to micrometre-size fluid inclusionsexcept along its margins and internal microcracks Bothhornblende and plagioclase exhibit evidence of plasticdeformation The Sr isotopic ratio for the pargasite andplagioclase is 070348 and 070420 respectively Thepargasite exhibits a Sr signature closest to that of theOman primary magmatic value (McCulloch et al 1981)Nevertheless it is too high to be attributed to anunmodified MORB-source mantle signature This sug-gests the addition of an external fluid component in goodagreement with the shift of oxygen isotopic compositionto a low value (thorn39) suggesting a high-temperatureexchange with a low d18O fluid The Sr isotope composi-tion of the plagioclase is significantly higher than that of
the coexisting pargasite probably as a result of the low-temperature alteration
Dioritic dyke cross-cutting lower layered gabbros in WadiAl-Abyad Nakhl Massif
Sample 02 OA90b is a 1 cm thick dioritic dyke cross-cutting the lower layered gabbros It grades into a greenamphibole vein as illustrated in Fig 6c The dyke is com-posed of 2 cm long dark green idiomorphic magnesio-hornblende crystals growing in the plane of the dykeand plagioclase concentrated at the margin of the dykeThe plagioclase in the dyke and in the enclosing gabbrocontains micrometre-size fluid and mineral inclusionssimilar to those observed in sample 94 MA2 The plagio-clase and green hornblende have higher Sr isotopic ratios(070387 and 070427 respectively) and lower d18Ovalues (thorn498 and thorn239 respectively) than the normalMORB-source mantle value As observed for previoussamples this supports interaction with an external fluidcomponent
Comb-textured gabbroic dykes in the lower gabbros fromWadi Wariyah Wadi Tayin Massif
Sample 01 OA15f (Fig 6b) is from Wadi Wariyah andrepresents a 2 cm wide dyke intruding the lower gabbrosThis type of comb-textured dyke developed in theclinopyroxene---plagioclase stability field It containslarge idiomorphic clinopyroxene crystals which arepartially replaced by brownish magnesio-hornblendeThe plagioclase is extremely calcic with a slight inverse
Table 3 Whole-rock major element compositions of massive gabbros and dykes determined by X-ray fluorescence
(wt )
Sample 94 Ma2 97 OA128b 01 OA41d2 01 OA19d 01 OA19a 01 OA15f 01 OA36b 01 OA36b
Rock type Gabbro Gabbro Gabbro patch Gabbroic dyke Gabbroic dyke Comb-textured
gabb dyke
Epidosite dyke Epidosite dyke
(margin)
SiO2 4830 4815 4749 4524 4339 4799 389 4371
Al2O3 2785 2032 2438 1474 1246 1192 2752 1646
Fe2O3 328 266 404 981 1204 592 381 623
MnO 003 004 005 014 016 006 008 014
MgO 405 737 427 1031 1686 1326 187 739
CaO 1289 1908 1477 1235 1156 166 2471 2339
Na2O 292 123 257 258 111 075 000 013
K2O 000 000 009 006 000 000 000 000
TiO2 005 017 063 230 064 086 013 042
P2O5 008 007 013 018 009 013 007 010
LOI 033 076 138 216 161 237 274 190
Total 9978 9985 998 9987 9992 9986 9983 9987
LOI loss on ignition gabb gabbroic
BOSCH et al HYDROTHERMAL CIRCULATION IN OMAN OPHIOLITE
1193
Table4Majorelementcomposition
aofselectedam
phibolespaired
withplagioclasecompositionsusedforthermometry
Sam
ple
nam
e1Mode2
SiO
2TiO
2Al 2O3
Cr 2O3
FeO
MnO
MgO
CaO
Na 2O
K2O
Total
Si
AlIV
AlVI
Ti
Fe3
thornCr
Mg
Fe2
thornMn
Na
KCa
XAn3
T(C)4
01OA19a
Mg-hornblende
core
45 60
26
10 78
009
860
015
16 56
12 00
212
007
96 72
6552
1448
0366
0083
0552
0004
3409
0594
0016
0611
0018
1843
0903
899
Mg-hornblende
core
47 99
263
850
002
897
011
16 61
12 10
160
020
96 36
6894
1106
0334
0028
0483
0002
3557
0594
0014
0446
0036
1862
0892
834
Mg-hornblende
rim
51 17
247
596
001
761
014
18 11
12 19
103
010
96 47
7257
0743
0253
0016
0414
0001
3828
0488
0016
0284
0018
1853
0894
820
Mg-hornblende
rim
47 50
291
920
004
880
014
16 46
11 95
174
008
96 07
6835
1165
0396
0015
0506
0005
3529
0552
0017
0486
0015
1843
0887
825
Mg-hornblende
rim
49 57
269
742
000
799
017
18 07
12 02
133
014
96 88
7013
0987
0261
0018
0575
0000
3811
0371
0020
0365
0026
1821
0860
831
Mg-hornblende
core
47 83
181
872
003
840
011
14 48
12 18
159
008
93 59
6814
1186
0279
0018
0644
0004
3711
0357
0013
0438
0015
1859
0907
874
Pargasite
rim
42 15
249
11 87
024
10 07
013
13 84
11 43
263
032
96 61
6188
1812
0242
0434
0299
0028
3028
0937
0016
0749
0059
1797
0804
972
Pargasite
rim
42 22
260
11 86
026
10 21
011
13 71
11 48
253
032
96 59
6201
1799
0254
0429
0299
0030
3001
0955
0014
0720
0060
1807
0803
962
Pargasite
core
42 05
263
11 45
030
10 19
011
13 80
11 47
246
032
96 46
6189
1811
0175
0478
0299
0035
3029
0956
0014
0702
0060
1809
0767
974
Pargasite
rim
42 36
247
11 90
016
945
013
14 16
11 76
249
026
95 98
6239
1761
0305
0366
0276
0019
3109
0888
0016
0710
0049
1856
0818
926
Pargasite
core
42 50
291
11 64
016
984
015
14 22
11 41
256
034
96 69
6219
1781
0227
0426
0322
0018
3103
0883
0018
0727
0063
1789
0812
974
Pargasite
rim
42 76
270
12 03
017
920
012
14 87
11 82
244
025
96 73
6224
1776
0880
0335
0368
0020
3226
0752
0015
0689
0047
1843
0841
941
Pargasite
rim
42 13
181
13 54
016
921
009
12 77
12 13
254
032
96 98
6172
1828
0510
0450
0000
0018
2788
1129
0011
0721
0059
1905
0841
874
01OA19d
Mg-hornblende
49 26
047
624
011
11 46
017
15 39
12 21
108
009
96 48
7137
0863
0202
0051
0437
0013
3323
0951
0021
0302
0018
1896
0768
771
Mg-hornblende
46 03
287
932
018
906
017
14 77
12 87
193
007
97 28
6672
1328
0265
0313
0046
0021
3191
1053
0021
0542
0012
1999
0758
808
Pargasite
41 89
429
11 65
008
11 03
017
13 52
11 37
259
017
96 77
6159
1841
0179
0474
0345
0010
2963
1012
0021
0739
0032
1791
0770
975
Pargasite
rim
42 80
340
11 16
009
11 01
018
13 73
11 67
240
022
96 67
6293
1707
0226
0376
0322
0010
3009
1032
0022
0684
0041
1839
0605
880
Pargasite
core
42 58
351
11 27
006
11 63
018
13 51
11 46
239
022
96 82
6257
1743
0209
0387
0391
0007
2959
1039
0023
0680
0042
1804
0595
893
Pargasite
rim
42 14
393
11 28
006
11 71
014
13 35
11 33
235
033
96 62
6214
1786
0174
0435
0391
0007
2935
1054
0018
0673
0062
1790
0519
901
Pargasite
core
41 56
420
12 05
006
11 57
016
12 51
11 35
249
027
96 24
6168
1832
0277
0469
0276
0007
2768
1160
0021
0718
0052
1805
0537
882
Pargasite
core
41 33
438
11 49
008
11 66
015
13 35
11 45
264
026
96 80
6105
1894
0107
0487
0368
0009
2941
1073
0019
0757
0048
1813
0537
942
Pargasite
41 08
446
12 39
012
11 35
017
13 13
11 36
263
015
96 85
6049
1951
0199
0494
0368
0014
2882
1030
0021
0752
0027
1792
0758
975
02OA37b
Mg-hornblende
rim
45 15
260
984
002
13 28
019
14 01
10 87
128
009
97 33
6527
1473
0203
0283
0690
0002
3018
0915
0023
0360
0016
1683
0562
828
Mg-hornblende
rim
44 72
263
988
004
13 23
021
14 01
10 78
133
009
96 92
6494
1506
0185
0287
0713
0005
3032
0894
0025
0374
0018
1677
0593
854
Mg-hornblende
rim
45 68
247
935
004
12 87
020
14 10
11 00
121
011
97 02
6621
1379
0218
0269
0598
0004
3045
0962
0024
0341
0020
1708
0562
815
Mg-hornblende
core
43 94
291
10 87
001
12 60
023
13 73
10 59
142
011
96 39
6410
1590
0278
0319
0644
0001
2985
0893
0028
0401
0021
1655
0710
860
Mg-hornblende
core
44 62
270
10 37
001
12 66
019
14 12
10 58
140
008
96 73
6479
1521
0253
0294
0644
0001
3057
0893
0023
0395
0014
1646
0674
851
Mg-hornblende
rim
47 79
181
796
002
12 09
025
15 22
11 09
100
009
97 33
6866
1134
0214
0196
0506
0002
3260
0947
0030
0280
0016
1706
0503
767
Mg-hornblende
core
45 43
249
10 3
000
11 99
022
14 50
11 14
133
008
97 48
6524
1476
0267
0269
0644
0000
3103
0796
0027
0370
0015
1715
0703
838
JOURNAL OF PETROLOGY VOLUME 45 NUMBER 6 JUNE 2004
1194
Sam
ple
nam
e1Mode2
SiO
2TiO
2Al 2O3
Cr 2O3
FeO
MnO
MgO
CaO
Na 2O
K2O
Total
Si
AlIV
AlVI
Ti
Fe3
thornCr
Mg
Fe2
thornMn
Na
KCa
XAn3
T(C)4
02OA90b
Mg-hornblende
core
48 83
038
745
005
939
010
17 29
12 24
142
006
97 21
6931
1069
0041
0041
0667
0005
3657
0448
0011
0392
0010
1862
0840
856
Mg-hornblende
core
49 49
033
731
003
916
008
17 40
12 57
139
005
97 82
6987
1013
0035
0035
0552
0004
3662
0530
0010
0379
0009
1901
0830
811
Mg-hornblende
rim
48 90
040
753
005
932
013
17 15
12 22
154
005
97 29
6948
1052
0042
0042
0575
0006
3632
0533
0015
0424
0010
1861
0880
869
Mg-hornblende
rim
48 30
044
800
007
962
010
16 86
12 32
161
005
97 35
6873
1127
0047
0047
0598
0008
3576
0546
0012
0443
0010
1879
0870
862
Mg-hornblende
rim
49 12
032
742
010
922
011
17 36
12 29
142
006
97 43
6956
1044
0034
0034
0621
0011
3665
0471
0013
0391
0010
1865
0840
845
02OA43a
Mg-hornblende
rim
48 13
040
789
007
860
007
17 41
12 19
155
006
96 36
6882
1118
0212
0043
0621
0008
3710
0407
0008
0429
0010
1867
-------
-------
Mg-hornblende
rim
50 93
035
643
001
759
006
18 50
12 24
114
003
97 28
7152
0848
0217
0037
0506
0001
3873
0385
0007
0311
0005
1841
-------
-------
Mg-hornblende
rim
50 67
014
559
009
12 33
026
15 28
12 44
068
003
97 50
7260
0740
0204
015
0483
0010
3263
0994
0032
0189
0005
1909
-------
-------
Mg-hornblende
rim
50 34
013
621
009
12 49
027
15 07
12 51
064
003
97 78
7192
0810
0240
0010
0530
0010
3210
0960
0030
0180
0010
1910
-------
-------
Mg-hornblende
rim
45 17
060
11 14
013
988
008
15 78
12 33
211
009
97 31
6473
1527
0355
0640
0644
0015
3371
0540
0010
0588
0016
1892
-------
-------
Mg-hornblende
rim
49 25
032
712
014
845
009
17 84
12 32
137
004
96 95
6983
1020
0170
0030
0620
0020
3770
0380
0010
0380
0010
1870
-------
-------
Mg-hornblende
rim
48 61
032
775
014
975
011
16 81
12 57
139
004
97 50
6905
1950
0203
0034
0621
0016
3560
0537
0013
0382
0008
1914
-------
-------
94Ma2
Anthophyllite
54 98
000
259
001
12 65
037
23 35
199
034
000
96 28
7744
0256
0174
0000
0161
0001
4903
1330
0043
0092
0001
0301
-------
-------
Anthophyllite
55 11
000
296
003
14 09
032
23 66
063
038
000
97 18
7716
0284
0205
0000
0115
0003
4938
1535
0038
0104
0000
0095
-------
-------
Actinolite
48 58
004
954
002
960
014
17 67
999
139
005
97 02
6850
0150
0435
0004
0690
0002
3713
0442
0017
0379
0009
1510
-------
-------
Actinolite
51 54
004
585
000
890
022
20 28
930
090
002
97 05
7214
0786
0178
0005
0598
0000
4230
0444
0026
0244
0004
1395
-------
-------
01OA41d2
Pargasite
rim
43 05
342
10 71
008
11 46
012
13 50
11 54
179
067
96 34
6353
1647
0216
0380
0368
0009
2967
1046
0015
0512
0127
1824
0613
863
Pargasite
rim
42 61
354
11 59
001
10 59
010
13 52
11 87
160
076
96 19
6286
1714
0302
0392
0299
0001
2972
1007
0012
0458
0143
1877
0715
844
Pargasite
core
42 54
401
11 04
006
11 81
013
13 21
11 50
202
039
96 71
6266
1734
0182
0444
0368
0007
2901
1087
0017
0576
0073
1814
0854
979
Pargasite
rim
43 03
385
10 72
002
11 70
014
13 50
11 95
146
080
97 17
6314
1686
0168
0425
0345
0002
2953
1091
0018
0415
0149
1878
0634
852
Pargasite
rim
42 65
341
10 60
006
11 97
014
13 22
11 64
183
068
96 20
6332
1668
0186
0381
0345
0007
2926
1141
0018
0528
0129
1851
0620
869
Pargasite
core
42 82
367
10 60
007
11 93
012
13 42
11 44
206
047
96 60
6315
1685
0158
0407
0391
0008
2950
1080
0016
0588
0089
1808
0615
899
Pargasite
rim
42 2
362
11 10
007
11 42
016
13 09
11 72
179
072
95 89
6282
1718
0229
0405
0299
0009
2905
1123
0020
0518
0137
1869
0715
875
Pargasite
rim
42 42
380
11 03
006
11 77
012
13 00
11 72
188
069
96 49
6283
1717
0209
0423
0299
0007
2870
1158
0015
0539
0131
1860
0817
928
Pargasite
rim
42 21
364
11 14
006
10 73
011
13 53
11 68
177
072
95 59
6277
1723
0229
0407
0322
0007
2999
1012
0014
0511
0136
1862
0736
883
Pargasite
rim
42 25
376
11 03
008
10 93
015
13 66
11 72
185
064
96 07
6257
1743
0182
0418
0345
0009
3016
1009
0019
0531
0121
1860
0766
912
Pargasite
core
42 17
396
10 91
008
11 10
010
13 57
11 49
209
038
95 85
6253
1747
0159
0442
0368
0010
3000
1009
0013
0602
0073
1826
0841
981
aMajorelem
entco
mpositionsarein
weightpercent-------noplagioclasean
alysed
andnotemperature
calculated
1Typ
eofam
phibolean
alysed
isalso
indicated
2Blankindicates
nozoningobserved
3Molefractionofan
orthitein
plagioclasead
jacentto
amphibolegrain
4Tem
perature
ofeq
uilibrium
fortheam
phibole---plagioclasepaircalculatedusingtheHollandamp
Blundythermometer
(1994)Errors
are40
C
BOSCH et al HYDROTHERMAL CIRCULATION IN OMAN OPHIOLITE
1195
zonation from An95 to An99 from core to rim (Table 1 andFig 8a) TheWR Sr isotopic ratio of the sample (070458)is significantly higher than normal MORB values Thiscould be related to the overprint of a low-temperaturealteration event consistent with the petrography of thesample
Green amphibole vein cross-cutting layered gabbros inWadi Mayah Haylayn Massif
Sample 02 OA43a belongs to a series of 1 cm thick greenamphibole veins (as in Fig 6e) cross-cutting the layeredgabbros The veins develop 15 cm thick reaction marginsin the enclosing gabbro and are exclusively composed ofacicular pale green magnesio-hornblende growing per-pendicular to the vein margin in a crack-seal mode(Fig 6f ) The reaction margins are composed of urali-tized gabbro marked by the development of the samepale green magnesio-hornblende in an inhomogeneouscalcic plagioclase matrix (An91---97) The Sr and O isotopiccompositions of this green hornblende (070431 andthorn31) do not correspond to normal MORB-sourcemantle values
Epidosite dyke from Wadi Gideah Wadi Tayin Massif
Sample 01 OA36b is representative of a series of epidoteveins cross-cutting poorly layered gabbros These 2 cmthick veins are composed of pink thulite in their centreand green pistachite at their margins (Fig 6a) Theirsharp contact with the enclosing gabbro is marked bythe development of coarse clinopyroxene crystals alteredin the epidote---greenschist facies This suggests that theepidote vein is replacive in a coarse-grained gabbro dykeThe highest 87Sr86Sr initial ratios of all the samplesstudied are recorded by the WR and coexisting epidoteof this sample (070519 and 070554 respectively)(Table 5) The Sr content of the WR and associatedepidote are also the highest of the present dataset(716 ppm and 764 ppm respectively) The Sr isotopiccomposition of this sample is consistent with a greaterdegree of exchange with a radiogenic component fromCretaceous seawater It is significantly higher than thevalue obtained for an epidosite vein from the Wadi Fizhsection of the Oman ophiolite [070435 with a Sr contentof 488 ppm reported by Kawahata et al (2001)] but ingood agreement with the average of the greenschist-faciesalteration sequence defined by Kawahata et al (2001)
GABBROS
01 km 025 km 1 km 36 km50
60
70
80
90
An
MA2
19d
19a 15f 128b
41d2
(a)
43a17b
15f
90b
37b
19a
19d
40
60
80
100
A
n
01 km 015 km 025 km 1 km
DYKES
(b)
Distance to the Moho
ACTINOLITE
MAGNESIOHORNBLENDE
TSCHERMAKITE
PARGASITEGabbros Dykes41d2MA2
19a d37b15f
90b43a
AlIV
150500
02
08
10
(Na
+ K
) A
(c)
04
06
950
900
850
800
750
70007 12 17
T degC
1000
(d)
AlIV
Fig 8 Plagioclase---amphibole compositions and results of thermometry (a) and (b) plagioclase compositions in layered gabbros and in dykesrespectively as a function of distance to the Moho (see text for sample identification) (c) amphibole compositions in layered gabbros and dykesaccording to Leakersquos (1997) nomenclature (d) equilibration temperatures calculated using the amphibole---plagioclase thermometer of Holland ampBlundy (1994) as a function of AlIV content in amphibole [same symbols as in (c)]
JOURNAL OF PETROLOGY VOLUME 45 NUMBER 6 JUNE 2004
1196
Table5RbandSrcontents87Sr
86Srandd1
8Oisotopiccompositionsofwholerocksandmineralseparatesfrom
samplesofmassivegabbrosdykes
andveinsfrom
theOman
ophiolitealsolistedarecalculated
waterrockratios(W
R)andLOIvalues
Sam
ple
Rb(ppm)
Sr(ppm)
87Rb8
6Sr
87Sr
86Sr
(87Sr
86Sr)i1
d18O
()
LOI(
)2WR
3(87Sr
86Sr)L14
(87Sr
86Sr)L25
(87Sr
86Sr)R6
01OA41d2
Whole
rock
037
1792
0006
0703516
09
070351
thorn480
138
47
0703587
0703651
0703357
Plagioclase
025
1249
0003
0704261
22
070426
thorn504
-------
-------
Pargasite
-------
-------
-------
0703548
20
070355
-------
-------
-------
Clinopyroxene
045
26 0
0050
0703845
19
070378
-------
-------
-------
01OA36b
Dykewhole
rock
0005
7164
50001
0705186
17
070519
-------
274
21 9
0705207
0705112
-------
Epidote
0003
7645
0001
0705536
12
070554
-------
-------
-------
Wallwhole
rock
005
4254
0001
0704637
09
070464
-------
191
-------
0704945
0704675
0704593
02OA43a
Green
hornblende
004
98
0012
0704323
13
070431
thorn310
-------
-------
97OA128b
Whole
rock
003
1274
50001
0703327
17
070333
-------
076
37
-------
-------
-------
Plagioclase
002
1204
50001
0703285
09
070328
thorn414
-------
-------
Clinopyroxene
003
20 0
0004
0704230
09
070422
thorn523
-------
-------
01OA15f
Whole
rock
006
1077
0002
0704582
16
070458
-------
237
13 5
-------
-------
-------
01OA19a
Whole
rock
037
1303
0008
0704189
18
070418
-------
161
96
-------
-------
-------
01OA19d
Whole
rock
058
2032
0008
0703423
08
070341
thorn567
216
415
0703574
0703408
0703271
Pargasite
091
1058
0025
0703273
11
070324
thorn287
-------
-------
Plagioclase
-------
-------
-------
-------
-------
thorn10 09
-------
-------
02OA90b
Plagioclase
004
2385
50001
0703868
11
070387
thorn498
-------
-------
Green
hornblende
010
23 8
0012
0704288
15
070427
thorn239
-------
-------
BOSCH et al HYDROTHERMAL CIRCULATION IN OMAN OPHIOLITE
1197
Table5continued
Sam
ple
Rb(ppm)
Sr(ppm)
87Rb8
6Sr
87Sr
86Sr
(87Sr
86Sr)i1
d18O
()
LOI(
)2WR
3(87Sr
86Sr)L14
(87Sr
86Sr)L25
(87Sr
86Sr)R6
02OA37b
Plagioclase
023
3121
0002
0704204
15
070420
-------
-------
-------
Pargasite
031
41 6
0021
0703505
15
070348
thorn394
-------
-------
94OAMa2
Whole
rock
0016
1750
00003
0703219
15
070322
thorn495
033
31
0703264
0703177
0703158
Plagioclase
0014
1363
00003
0703219
11
070322
thorn479
-------
-------
Clinopyroxene
-------
-------
-------
0703236
13
070324
-------
-------
-------
1Initial8
7Sr
86Srratiocalculatedat
95Ma(H
acker1994)
2LOIisloss
onignition(in)
3Water---rock
ratiocalculatedassumingaclosedsystem
TheeffectiveWR
ratiocanbeexpressed
bythefollowingeq
uation
W=Reffectivefrac14
frac12eth87Sr=
86SrrockTHORN fi
nal
eth87Sr=
86SrrockTHORN in
itial=frac12eth8
7Sr=
86SrseawaterTHORN in
itial
eth87Sr=
86SrrockTHORN fi
nal
ethCrock
initial=C
seaw
ater
initial
THORNwhere(87Sr
86Srrock) finalisthefinalmeasuredSrisotopicratioin
thesample(87Sr
86Srrock) in
itialco
rrespondingto
theinitialm
antlevalue
istakenas
070295(Lan
phere
etal1981)(87Sr
86Srseawater) in
itialistheCretaceousseaw
ater
Srisotopicco
mposition[87Sr
86Sr07073
at90
MaafterBurkeet
al(1982)]C
seaw
ater
initial
istheSrco
ntent
oflsquonorm
alrsquoseaw
ater
andis
takenas
8ppm
(deVilliers1999)
FortheCrock
initialitis
assumed
that
thepresentSrco
ncentrationmeasuredis
thesameas
theinitial
concentration
4Srisotopicratiosmeasuredforliquid
L1extractedafterthefirststep
oftheleachingexperim
ent(6NHClfor8min
at70
C)
5Srisotopicratiosmeasuredforliquid
L2extractedaftertheseco
ndstep
oftheleachingexperim
ent(2 5NHClfor30
min
at70
C)
6Srisotopicratiosmeasuredforresidueobtained
afteratotalaciddigestionachievedaftertheextractionofL1an
dL2leachates
JOURNAL OF PETROLOGY VOLUME 45 NUMBER 6 JUNE 2004
1198
Part of the same sample from the contact between thedyke and the gabbro yields a slightly lower Sr isotopiccomposition (070464) intermediate between the sur-rounding gabbro and the epidote dyke It also has anintermediate Sr content (425 ppm) Acid leaching experi-ments performed on the whole rock of the dyke yield Srisotopic compositions of 070521 and 070512 for thetwo liquids extracted Similar experiments for the samplelocated at the contact with the gabbro yield 87Sr86Srratios of 070494 and 070467 for L1 and L2 liquids TheSr isotopic composition of the residue extracted afterleaching remains high (070459) and very close to theepidosite 87Sr86Sr ratio determined by Kawahata et al(2001) In this sample the primary minerals have beentotally replaced by epidote and secondary plagioclase orquartz Thus the Sr isotopic compositions measured forthis WR sample and its constituent minerals reflect thecomposition of the fluid filling this dyke
Mineral chemistry
Plagioclase
Compared with the host gabbros the dykes show a muchlarger dispersion in their plagioclase compositions (Fig 8aand b Table 4) characterized by more calcic plagioclasethan the enclosing gabbros This tendency is mostextreme in the comb-textured dykes in which the plagio-clase is almost pure anorthite The zoning is generallynormal recording a crystal fractionation process How-ever gabbro 94 MA2 exhibits inverse zoning this tend-ency is also observed in plagioclases from dykes 01OA19a and 01 OA15f The higher calcium content ofthe plagioclase as well as the inverse zoning is consistentwith an increase of water pressure during its crystalliza-tion (Turner amp Verhoogen 1960 Carmichael et al 1974Koepke et al 2003) The particularly high An content inthe comb-textured gabbro dykes 01 OA15f may alsoresult from crystallization in a supercooled magma thisis consistent with the comb texture indicative of fast-growing crystals The lack of brown hornblende suggeststhat crystallization occurred above the temperature ofhornblende stability
Amphibole
The amphiboles analysed in the dykes (Fig 8c andTable 4) show a regular trend in a (Na thorn K)A vsAlIV diagram from titanium-rich (Ti 4 015) pargasitichornblende (brownamphibole) and tschermakite (brown---green amphibole) to low-titanium (Ti5 015) magnesio-hornblende (green amphibole) and actinolite (pale greenamphibole) The pargasitic hornblende develops aspoikilitic oikocrysts in the dykes 01 OA19a and d andin the diffuse patch 01 OA41d2 at temperatures above800C during the final stage of crystallization (see
below) The tschermakite composition corresponds tothe internal alteration of clinopyroxene in gabbro 94Ma2 occurs in the comb-textured dyke 01 OA15f andis the primary amphibole in the dioritic dyke 02 OA37bThe magnesio-hornblende develops at the edges of thepargasites or of clinopyroxenes at temperatures around800C and is the primary phase in the dioritic dyke 02OA90b and in the green vein 02 OA43a Finally actino-lite occurs as a replacement of green hornblende in dykesor in the kelyphitic rim surrounding olivine in gabbro 94Ma2 Such a large composition range at the scale of athin section has been reported in amphiboles fromamphibolite-facies oceanic gabbros (Stakes 1991 Stakesamp Taylor 1992 Gillis et al 1993) as well as in the Omanophiolite gabbros (Manning et al 2000) This variabilityhas been explained by rapid reaction rates preventinghomogenization (Manning et al 2000)
Thermometry
Plagioclase---amphibole thermometry could be appliedwhen the two minerals exhibited good contacts such asin the laminated gabbro of the middle sequence (01OA41d2) and in the intrusive dykes The temperaturesof plagioclase---amphibole equilibration (Table 2) havebeen calculated using the geothermometer lsquoBrsquo of Hollandamp Blundy (1994) Edenite thorn Albite frac14 Richterite thornAnorthite valid between 500C and 900C with anuncertainty of 40C Amphibole formulae are basedon 23 oxygen and their nomenclature is based on alkalications in the A site vs AlIV according to Leake (1997)Pressurewas assumed to be 1 kbar Forty-five plagioclase---amphibole pairs meet the compositional criteria imposedby Holland amp Blundyrsquos dataset (09 4 Xan 4 01 andAlIV 403) Electron microprobe measurements wereconstrained to contiguous plagioclase and amphiboleseparated by sharp grain boundaries assuming equili-brium of the two phases When the mineral phasesexhibit normal zoning temperatures between plagioclaseand amphibole cores and between mineral rims werecalculated assuming that the temperatures deduced fromthe mineral core chemistry provide a proxy for the high-est temperature of equilibration These data are identi-fied in the T C vs AlIV diagram (Fig 8d) Table 4presents selected major element amphibole analyses theAn content of the plagioclase adjacent to each amphibolegrain and the temperature calculated for the pairThe temperatures calculated (Fig 8d) span the entire
range of temperatures at which amphibole and plagio-clase coexist in equilibrium (ie amphibolite-facies para-geneses) This temperature interval is bracketed byorthopyroxene-bearing facies or amphibole-free facies(ie gabbro 94MA2) and epidote---actinolite facies devoidof homogeneous plagioclase (ie epidosite dyke 01OA36b and green vein 02 OA43a) The range of the
BOSCH et al HYDROTHERMAL CIRCULATION IN OMAN OPHIOLITE
1199
calculated temperatures between 4900C and 750Ccorresponds to the changing composition of amphibolesfrom pargasite to magnesio-hornblende at the scale of athin section or even at that of an individual crystal asshown by their correlation with AlIV such variabilityrecords rapid cooling in hydrous conditions The highesttemperatures are recorded by pargasite from the anatec-tic gabbro 01 OA41d2 in the gabbro dyke 01 OA19dand in the dioritic dyke 02 OA90b They were calculatedfrom minerals devoid of lower-grade alteration and arethus considered as representing the equilibrium temperat-ure of the coexisting minerals Lower temperatures cor-respond to mineral phases evolving at falling temperatureand are only indicative of the cooling history Includingthe 40C uncertainty seven pairs provide temperatureshigher than 900C the limit of validity of the Holland ampBlundy thermometer However we maintain these tem-peratures as indicative of the highest values of our data-set These values are included in the average equilibriumtemperature summarized in Table 2
DISCUSSION
Distribution of hydrothermal alteration
Two distinct petrological events are recorded in the gab-bro section of the Samail ophiolite and are documentedin the present contribution (1) the whole gabbro sectionis affected by a penetrative alteration (2) the gabbrosection is crosscut by various types of dykes and veinsincluding hydrous minerals
Penetrative alteration
The penetrative alteration developed in the gabbros atgrain boundaries and locally invading the minerals isubiquitous in the gabbro section This penetrative altera-tion has been described as very high temperature (VHT)high temperature (HT) and low temperature (LT) basedon alteration parageneses The orthopyroxene coronas inthe layered gabbro sample and the incipient appearanceof brown pargasitic amphibole mark a lower thermallimit of the VHT alteration By reference to the experi-mental work of Koepke et al (2003) the temperature ofequilibration for these reactions is estimated to bebetween 900 and 1000CThe preservation of the vertical distribution of
VHT---HT facies (Figs 4 and 5) also pointed out byManning et al (2000) indicates that the hydrothermalalteration pattern fits the distribution of isotherms withdepth at a fast-spreading ridge (ie Phipps Morgan ampChen 1993 Dunn et al 2000 Fig 2) Equilibrium tem-peratures for VHT parageneses as high as 900---1000Csuggest that VHT---HT hydrothermal alteration tookplace in a very restricted time interval at the limit ofthe cooling magma chamber It is proposed that the
penetrative alteration affecting the entire gabbro sectionis related to a pervasive network of microcracks whichact as a recharge system down to the Moho (Nicolas et al2003)
Dykes and veins
The dykes are extremely varied eg pegmatitic gabbroswith comb-texture microgabbros and amphibolitizeddolerites dioritic dykes alignments of poikilitic horn-blende and finally diffuse hornblende-bearing patchesHigh-T gabbro and diorite dykes and low-T greenamphibole---epidote veins are connectedIn the dykes textural evidence attests to suprasolidus
conditions at least for the development of the pargasiticamphibole Integrating the hornblende---plagioclase equi-libration temperatures calculated for the studied samplesthese temperatures are bracketed between 950C and875CIn the following discussion we shall consider separately
the microgabbro and dolerite dykes The latter representbasaltic melt intrusions associated upsection with thediabase sheeted dykes and downsection possibly withthe mafic dykes that are ubiquitous in the mantle section(Nicolas et al 2000) Whatever their origin the develop-ment of pargasitic amphibole during their late stage ofcrystallization creates a link with the gabbroic dykes andsuggests a crystallization process evolving from dry con-ditions to more hydrous conditionsThe other gabbroic dykes comb-textured gabbros and
hornblende-bearing dioritic dykes result from the fillingof cracks by a fluid either a melt or a silica-rich hydrousfluid These intrusive dykes which develop reaction mar-gins at their contact with the host gabbro grade verticallyinto lower-temperature green amphibole veins or align-ments of poikilitic hornblende in the layered gabbromatrix or development of pargasitic hornblende in ana-tectic gabbros (sample 01 OA41d2) In gabbros crystal-lizing in the magma chamber the development oforthopyroxene and pargasite oikocrysts enclosing the pri-mary minerals suggests a thermal evolution from thegabbro solidus to amphibolite-facies conditions Forthese reasons it is suggested that the gabbroic dykesresulted from the injection of a hydrous melt in the stillhot gabbros drifting away from the magma chamber asdiscussed below
Origin of fluids responsible for VHT---HThydrothermal alteration
The 87Sr86Sr initial ratios and d18O of whole rocks andpure mineral fractions are plotted in Fig 9 against thedistance above the Moho Transition Zone (MTZ) Theinitial Sr isotopic composition of the whole rocks apartfrom the epidosite dykes ranges from 070322 to
JOURNAL OF PETROLOGY VOLUME 45 NUMBER 6 JUNE 2004
1200
070458 All the studied samples (including whole rocksand minerals) have initial 87Sr86Sr ratios falling outsidethe field of fresh MORB which commonly have Sr ratiosbetween 07023 and 07029 with an average of 070265(ie Hart et al 1974) The lowest initial Sr isotopiccomposition (070322) from the massive gabbro 94MA2 is slightly but significantly higher than averageOman primary magmatic signatures (ie 070296McCulloch et al 1981) Similarly the d18O values ofmost whole rocks and minerals measured (Table 5)
depart from typical MORB-source mantle values( Javoy 1980 Gregory amp Taylor 1981 Kyser 1986)These differences indicate that the primary mantlesignatures in the Oman ophiolite have been modifiedby interaction with a fluid Possible origins for thisfluid are mantle components dehydration of subductedlithosphere or seawater circulation Strontium andoxygen isotopes are useful tracers for fluid---crust interac-tion as d18O and 87Sr86Sr ratios from fluids originat-ing from seawater the mantle or subducted lithosphere
-1000
1000
2000
3000
4000
5000
6000
7000
MOHO
MTZ
Dis
tan
ce a
bov
e th
e M
oh
o (
m)
pillow-basalt
sheeted-dike
gabbro
peridotite
01 OA 41d2
01 OA 36b
02 OA 43a97 OA 128b
01 OA15f
94 OA MA2
02 OA 90b01 OA 19ad
02 OA 37b
0702 0703 0704 0705 0706 0707
87Sr86Srinitial
MO
RB-s
ou
rce
Seaw
ater
Whole RockPlagioclasePargasiteGreen HornblendeClinopyroxene
Epidote
(a)
-1000
1000
2000
3000
4000
5000
6000
7000
MOHO
MTZ
Dis
tan
ce a
bov
e th
e M
oh
o (
m)
pillow-basalt
sheeted-dike
gabbro
peridotite
01 OA 41d2
01 OA 36b
02 OA 43a97 OA 128b
01 OA15f
94 OA MA2
02 OA 90b01 OA 19ad
02 OA 37b
Whole RockPlagioclasePargasiteGreen HornblendeClinopyroxene
3 5 10
δ18O (permil)
41d2
Ma2
43a
90b 19d 19d37b
128b
90b
(b)
Fig 9 (a) Variation of initial 87Sr86Sr isotopic composition as a function of the location of the studied samples in a schematic log of the crustalsection of the Oman ophiolite The field for MORB is from Hart et al (1974) and that for seawater is from Burke et al (1982) Small open squares aredata fromKawahata et al (2001) (b) Variation of d18O () as a function of the location of the studied samples in a schematic log of the crustal sectionof the Oman ophiolite Shaded curve corresponds to the data of Gregory amp Taylor (1981)
BOSCH et al HYDROTHERMAL CIRCULATION IN OMAN OPHIOLITE
1201
are significantly different Moreover the d18O valueis dependent on temperature and mineral fractiona-tion and thus yields complementary constraints to Srisotopes
Mantle origin
One model to explain the occurrence of fluids interactingwith the oceanic crust at very high temperatures is toderive them from the mantle The contribution of mantle-derived hydrous fluids linked to percolation processesor to their concentration in the final stages of gabbrocrystallization has been invoked in numerous case studies(ie Witt amp Seck 1989 Fabriees et al 1989 Dupuy et al1991 Agrinier et al 1993) O and Sr isotopic data forwhole rocks and mineral separates yield scattered valuesthat are unambiguously different from MORB mantle-source values (Fig 9a and b) With the exception of lateLT altered samples the WR data have a mean 87Sr86Srratio of 070337 and a standard deviation of 000012These surprisingly high values in mantle-derived rockscould be taken as evidence for survival of mantle isotopicheterogeneities during crystallization of the ophiolite (egLanphere 1981 McCulloch et al 1981) However the18O16O isotope ratios in the Oman gabbroic sequenceare also displaced from primary magmatic values Nogabbro in the present study has plagioclase or amphi-bole with typical mantle d18O values Thus the Sr and Oisotope data rule out the possibility of mantle-derivedfluids as a possible source for the VHT---HT hydrousalteration event recorded in the massive gabbros andgabbroic dykes and veins (Fig 10a and b)
Subducted lithosphere origin
Dehydration of subducted oceanic crust in subductionzones can generate hydrous fluids Such fluids couldpercolate through the overlying lithosphere and contam-inate it thus generating modified isotopic signatures andtrace element contents Such an explanation howeverdisagrees with the geochemical characteristics of most ofthe studied samples Fluids derived from the dehydrationof subducted slabs (fresh or altered MORB or OIB pro-toliths) should be strongly enriched in K Rb and Ba (egBecker et al 2000) whereas in Oman the Rb contentsmeasured in the gabbros and gabbroic dykes and veins arestrongly depleted Moreover the isotopic composition ofslab-derived fluids is buffered by the MORB-like oceaniccrustal protolith for Nd and Pb but not for Sr and O(Staudigel et al 1995) As a result of relatively minorfractionation of oxygen at high temperature the oxygenisotope composition of the fluids expelled during dehy-dration of the subducted slab should be high and essen-tially controlled by the oxygen composition of the uppersection of the crust altered at low temperature (with an
average of 10 and perhaps as high as 19) (Staudigelet al 1995) The Sr isotopic composition should be alsohigh (average 07046) as the fluids released by the dehy-dration of subducted oceanic crust are associated eitherwith seawater or with radiogenic Sr phases (ie smectites)The Sr and O isotopic compositions of the studied sam-ples do not fit with these signatures and so preclude asubducted lithosphere origin for the fluids involved in theVHT---HT alteration event
Seawater-derived fluids
All the analysed samples (minerals and WR) have 87Sr86Sr(T) outside the domain of primary MORB magmasand variable Sr contents Individual minerals have Srcontents reflecting their different mineralliquid partitioncoefficients Minerals characterized by a very high affi-nity for Sr such as plagioclase (ie Sr mineralliquidpartition coefficients 1) have very high Sr contentsTherefore the high abundance of plagioclase in the sam-ples analysed is responsible for the high Sr content of thestudied whole rocks The Sr content of all the whole rocksis relatively homogeneous (Table 5) without clear distinc-tion between country-rock gabbros and dykes and veinsand ranges from 110 to 200 ppm except for the epidosite(4700 ppm) Reported on a 87Sr86Sr vs 1Sr diagram(Fig 10a) most whole rocks and plagioclases analysed arecharacterized by a 1Sr ratio lower than 001 and plot inthe left part of this diagram Compared with N-MORBthe studied massive and anatectic gabbros and their con-stituent plagioclases have similar to slightly higher Srcontents but substantially higher 87Sr86Sr ratios(Fig 10a) Considering Cretaceous seawater as a possiblehigh 87Sr86Sr mixing component [87Sr86Sr 07073at 90Ma after Burke et al (1982)] responsible for theobserved geochemical modifications exchanges cannothave occurred in a closed system as seawater has too lowa Sr content (ie 8 ppm de Villiers 1999) An open-system process allowing penetration of larger quantitiesof seawater can efficiently modify the Sr isotopic ratio ofthe rocks Alternately the fluids could also be seawaterthat was modified through exchange with the surround-ing rocks during its transit through the oceanic crustalsection This modified seawater would be more enrichedin Srwith a 87Sr86Sr ratio intermediate between unmodi-fied seawater and MORBMinerals devoid of late LT alteration imprints (parga-
site green hornblende and pyroxene) and characterizedby very low Sr contents plot on the right of the diagramclose to or on the mixing line between seawater andMORB (Fig 10a) The difference between these mineralsand the other samples (WR and plagioclases) is mainlydue to the Sr fractionation coefficients of these differentminerals and to the large quantity of plagioclase inthe studied rocks The process responsible for the
JOURNAL OF PETROLOGY VOLUME 45 NUMBER 6 JUNE 2004
1202
modification of the Sr isotopic composition fromMORB-source value is however explained by the same phenom-enon As all these minerals display initial 87Sr86Sr ratiosdistinct from the MORB source and because they equili-brated at VHT or HT temperatures this supports anearly intervention of seawater during crystallization ofthese minerals Most of these samples with the exceptionof the epidosite dyke overlap the domains defined byKawahata et al (2001) for the Wadi Fizh gabbros alteredat VHT and HT conditions in the amphibolite facies(labelled lsquoVHTrsquo and lsquoHTrsquo in Fig 9a)Three samples (two epidosite dykes and an one epidote
fraction) have very high Sr contents and the highest Sr
isotopic compositions of the present dataset The twowhole-rock samples overlap the field of the Wadi Fizhsamples altered at high temperature during greenschist-facies metamorphism (Kawahata et al 2001) This typeof alteration is common at the ridge axis and formedthrough intensive exchange with seawater-derived fluidsFigure 10b indicates that all the Sr---O isotope data are
distinct from MORB compositions and suggests that thefluids reacting with the magmas could be derived fromseawater In addition the d18O values show that isotopicexchange occurred under VHT or HT conditions Thefluids could have the composition of seawater or theycould have the composition of seawater modified through
0703
0705
0707
001 01 1
1 Sr (ppm)
MORB
(87Sr
86Sr)
init
ial
Seawater
VHT
HT
MT36b
36b
36b
43a 128b90b
41d2
37b19d
15f
128b
41d2
41d2
19a
Ma2Ma2
19d
90b
37b
Whole RockPlagioclasePargasiteGreen HornblendeClinopyroxene
Epidote
(a)
0 2 4 6 8
δ18O(permil)
0703
0705
0707Seawater
MORB-mantle
128b 41d2
90b 41d2
Ma2128b
37b
43a
19d
90b
Whole RockPlagioclasePargasiteGreen HornblendeClinopyroxene
Epidote
HT LT
19d
(87Sr
86Sr)
init
ial
(b)
Fig 10 (a) Initial 87Sr86Sr isotopic ratios plotted against 1Sr for whole rocks and mineral separates from the gabbro section of the Omanophiolite Fields shown are from Kawahata et al (2001) and correspond to MT-----basalt sheeted dyke diabase altered at medium temperature(prehnite---actinolite facies sequence 3) HT-----diabase plagiogranite metagabbro and epidosite formed at high temperature (greenschist faciessequence 4) VHT-----gabbro altered at very high temperature (amphibolite facies sequence 5) The dashed line is a binary mixing line betweenMORB (Lanphere et al 1981) and Cretaceous seawater (Burke et al 1982) (b) Initial 87Sr86Sr isotopic composition plotted against d18O value forwhole rocks and minerals from the Oman ophiolite Cretaceous seawater and MORB end-members as in (a) Dashed line represents mixing curvebetweenMORB and seawater at high temperature (HT) Low-temperature (LT) exchanges between these two components would be responsible foran increase of the d18O primary MORB value
BOSCH et al HYDROTHERMAL CIRCULATION IN OMAN OPHIOLITE
1203
exchange with the surrounding rocks during its penetra-tion into the crustal section In an environment in whichfluid---rock interaction occurs at variable temperaturesthe d18O is greatly influenced by the temperature atwhich the exchange took place and by the waterrockratio It is evident from Figs 9b and 10b that none of thestudied gabbros have plagioclase and amphibole withMORB-like d18O and 87Sr86Sr values The separateminerals display a broad range of d18O values and mostexhibit extreme 18O depletion (Table 5) Hydrothermalexchange kinetics are similar in amphibole and pyroxeneand these two minerals are more resistant to oxygenexchange during water---rock interaction than coexistingplagioclase (Gregory et al 1989) The most probableexplanation of the low d18O values measured in bothamphiboles and pyroxenes is that they crystallized athigh temperature in the presence of a relatively low 18Oseawater-derived hydrothermal fluid [d18O from 01 to07 after Gregory amp Taylor (1981)] The anomalouslyhigh d18O value (thorn1009) measured for the plagioclase ofthe micro-gabbroic dyke (01 OA19d) can be interpretedas resulting from a superimposed overprint caused by alate-stage penetration of fluids at lower temperature Inthis sample the occurrence of such different 18O16Oratios between coexisting plagioclase and amphibole isnot consistent with equilibrium fractionation betweenthese two minerals at any possible temperature Thisobservation suggests that parts of the ophiolite sequencehave undergone a high-temperature hydrothermal eventprior to the later low-temperature event that producedthe strong 18O increase in coexisting plagioclaseThe depletion of the d18O values results from high-temperature hydrothermal alteration (Gregory amp Taylor1981 Stakes amp Taylor 1992) The oxygen isotopic datasupport the contention that most studied samples havebeen affected by a VHT---HT hydrothermal alteration byfluids derived from seawater Some of the analysed sam-ples presenting petrological evidence of crystallization ofLT secondary minerals have been overprinted by the latelow-temperature alteration thus swamping the earlierhigh-temperature alteration effects
Determination of waterrock ratio
To better evaluate the exchange between seawater andthe gabbroic rocks waterrock ratios (WR) have beenestimated for the whole rocks analysed during the courseof this study The different models used to constrain thisratio mainly differ by the calculation of the effective ratioor by the estimated weight ratio and by considering openor closed systems for water circulation (Albareede et al1981 McCulloch et al 1981) The WR ratios reportedin Table 5 have been calculated assuming a closed sys-tem Using this formulation these values are minimabecause the calculation always uses pristine fluid for the
initial fluid thus maximizing the value in the denomina-tor If the fluid was shifted to lower 87Sr concentrations byexchange with the rock on the downward path theinferred values would be much higher (Davis et al 2003)The calculated WR ratios for the samples from this
study range from 31 to 219 (Table 5) Two main groupsof samples can be recognized on the basis of their waterrock ratios The lowest ratios ranging from 31 to 47have been determined for massive gabbros or anatecticgabbros but also for dykes and veins devoid of late LTalteration Similar ratios have been previously calculatedfor example for the discharged hydrothermal solutions atthe 13N and 21N East Pacific Rise (Di Donato et al1990 Fouquet et al 1991) The gabbros from the Ibrasection in the Oman ophiolite gave effective WR ratiosof 05 33 and 42 (McCulloch et al 1981) in goodagreement with the values obtained in this study Theleast altered gabbro of the Ibra section yields a WR ratioof 05 in good agreement with the exceptionally fresh-ness of this rock characterized by typical mantle oxygenvalues for its minerals (McCulloch et al 1981) Samplescharacterized by waterrock effective ratios in the rangeof 3---5 can be related to penetrative alteration via thehydrothermal system Lecuyer amp Reynard (1996) havedemonstrated in oceanic gabbros from the Hess Deepaltered in similar HT conditions that WR mass ratios inthe range of 02---1 are sufficient to account for the mod-ified stable isotope signature of the gabbros Higherwaterrock values (WR 45) have been calculated forsamples affected by superimposed late hydrothermalalteration and for the epidosite samples (Table 5) Theyprobably acquired their isotopic characteristics throughinteraction with a larger volume of seawater duringgreenschist-facies metamorphism Similar or higherWR values have been published in previous studies(ie McCulloch et al 1981 Kawahata et al 2001)They correspond either to samples from the upper-crus-tal sequence (ie basalt sheeted dyke or plagiogranite) orto gabbros from the crustal section located in a particularenvironment such as for example the Wadi Fizh sectionthat is located close to a segment boundary along thepalaeo-spreading axis (Nicolas et al 2000)
Mechanism for seawater interaction withmagma
Nicolas et al (2003) have proposed that variable amountsof seawater can circulate at high temperature through-out the crustal section down to the walls of the magmachamber via a network of parallel microcracks a fractionof a millimetre wide Such microcracks and their relation-ship with the VHT---HT hydrous alteration in the sur-rounding gabbros have been described in detail byNicolas et al Via this network of microcracks largeamounts of seawater can have reacted with the magma
JOURNAL OF PETROLOGY VOLUME 45 NUMBER 6 JUNE 2004
1204
and induced variable changes of its Sr and O isotopiccomposition This regular network of parallel micro-cracks could constitute the recharge system and thehydrated gabbroic dykes the discharge system Physicalparameters and the geophysical models of Nicolas et al(2003) are in agreement with this scenario Such a pene-tration of seawater-derived hydrothermal fluids throughthe lower crust along grain boundaries and a microscopicfracture network at temperatures 4750C is consistentwith recent thermodynamic models (McCollom amp Shock1998)Seawater-altered and high-temperature recrystallized
diabases (protodykes) from the overlying sheeted dykecomplex stoped into the melt lenses could represent anindirect way for seawater to enter the system as they canremelt during their subsidence through the magmachamber A simple mass balance calculation suggeststhat this indirect contamination is not significant Assess-ments of the amount of recycled crust 1 in volumeby Nicolas et al (2000) based on the protodyke remnantsin the gabbro section or 20 by Coogan et al (2003)based on chlorine content in EPR basalts lead to a sea-waterrock ratio of the order of 104 to 103Thus following Nicolas et al (2003) we propose that
seawater has been introduced along microcracks down tothe walls of the magma chamber and has diffused alonggrain boundaries in the way described by Manning et al(2000) progressively invading the host gabbro Gabbroicdykes result from the injection of hydrous melt into thehost gabbros at falling temperatures as they drift awayfrom the magma chamber This water-saturated meltcould result from the extensive hydration of the crystal-lizing gabbros near the walls of the magma chamberwhen seawater penetrates through this wall (Fig 1)Similar plumbing systems driving seawater down to the
limit of the magma chamber and back to the surface havebeen already proposed in other environments such as theMid-Atlantic Ridge (Kelley amp Delaney 1987 Cooganet al 2001) the East Pacific Rise at Hess Deep (Gillis1995 Manning et al 1996a 1996b) the Tonga forearc(Banerjee amp Gillis 2001) and the Troodos ophiolite(Gillis amp Roberts 1999) The melting curve of wet basalthas a negative slope but is close to the dry solidus at lowpressure in the lower gabbros and the first melts areplagiogranitic (Spulber ampRutherford 1983) Such plagio-granites are ubiquitous in the highest gabbros and in theroot zone of sheeted dykes in Oman (Rothery 1983Juteau et al 1988 Nicolas amp Boudier 1991) in manyother ophiolites and in the oceanic crust where they havebeen interpreted as resulting either from wet anatexis ofhot gabbros or from the final product of crystallization ofa basaltic melt (Spulber amp Rutherford 1983) Plagio-granites are rare in the lower gabbros exceptionallyassociated with hydrous gabbro segregations inside thelayered gabbros Their constitutive minerals are also
remarkably absent along the VHTmicrocracks althoughthese gabbros were above the wet solidus A possibleexplanation has been proposed by McCollom amp Shock(1998) who wrote that at high temperature lsquoaqueousfluids may leave very little conspicuous petrological evid-ence for their presencersquo the reason being that thehigh-T conditions would be closer to batch meltingObviously more detailed studies on this specific problemare needed We conclude that the large degree of hydrousmelting locally required at the walls of the magma cham-ber to account for the generation of a gabbroic hydrousmelt may have erased the traces of the incipient plagio-granitic melt
CONCLUSIONS
Fostered by the recent discovery of high-temperatureseawater contamination in some gabbros of the Omanophiolite (Manning et al 2000) we have conducted astudy on 500 gabbro specimens from selected areas ofthe entire Samail ophiolite belt and on the hydrous gab-broic dykes that cross-cut them The highest-temperaturereactions (VHT) recorded in olivine and clinopyroxene ingabbros as orthopyroxene and pargasite coronas reach975C They are followed at lower HT conditions withreplacement of olivine by an assemblage of tremolitemagnetite and plagioclase surrounded by chlorite andby pargasite development in clinopyroxene followedfinally by the LT lizardite---olivine and saussurite---plagioclase greenschist-facies alteration With a fewexceptions the VHT alteration tends to be restricted tothe lower gabbros and to the vicinity of the Moho andthe HT alteration to the middle and upper gabbros Thepreservation of the vertical distribution of VHT---HTfacies indicates that progressive cooling during off-axisspreading did not obliterate this distribution and conse-quently that the VHT---HT hydrothermal alteration tookplace in a very restricted time interval at the limit of thecooling magma chamber In the deep and hot gabbrosthe main water channels may be sub-millimetre-sizedmicrocracks with a dominantly vertical attitude the ori-gin and dynamics of which have been investigated in aprevious paper (Nicolas et al 2003)Beyond these high-temperature alteration zones mas-
sively induced in the gabbros seawater may be respons-ible for the generation of clinopyroxene---pargasitegabbro dykes that cross-cut all gabbros and that areupsection clearly connected to the LT hydrothermalvein system Petrostructural evidence suggests that thesedykes were generated by hydration of the crystallizinggabbros near the internal wall of the LVZ---magmachamber The resultant melts were subsequently intrudedat various levels in the cooling lower and middle gabbrosPetrological observations of nearly all the gabbros ana-lysed in the present study located from the root zone of
BOSCH et al HYDROTHERMAL CIRCULATION IN OMAN OPHIOLITE
1205
the sheeted dyke complex down to the Moho emphasizethe imprint of VHT---HT hydrous alteration The VHThydrothermal event is characterized by temperatures ofequilibration around 900---1000C The temperatures atwhich the HT hydrous event occurred are estimated tobe in the range 550---900CStrontium and oxygen isotope compositions measured
on whole rocks and separated minerals significantlydepart from primary MORB values The Sr and O iso-tope data obtained for pargasite green hornblende andclinopyroxene record the participation of seawater at thetemperature of crystallization of the minerals Plagio-clases and whole rocks define a trend toward a compo-nent characterized by a high radiogenic 87Sr86Sr valueand a higher Sr content than normal seawater Seawateris clearly identified as the fluid responsible for the modi-fication of the Sr and O signatures for both massivegabbros and dykes and veins Nevertheless the high Srcontent of the extraneous component requires eitheropen-system exchange allowing penetration of a greaterquantity of seawater or penetration of seawater modifiedby interaction with the oceanic crust during its travel paththrough the crustal section The waterrock ratio calcu-lated on the basis of the Sr isotopes is between three andfive for the enclosing gabbros and for the least LT altereddykes The VHT---HT hydrothermal event affectingthese samples is related to penetrative alteration via therecharge and discharge hydrothermal system Thewaterrock ratio is 10 for dykes exhibiting evidenceof a LT hydrothermal alteration overprint and reaches22 for the epidosite dyke The depletion in d18O char-acterizes all the studied samples with the single exceptionof the micro-gabbroic dyke plagioclase This agreeswith the conclusions based on Sr isotopes suggestingthat these samples have undergone exchange with sea-water-derived fluids during a VHT---HTalteration hydro-thermal event
ACKNOWLEDGEMENTS
This work has benefited from financial support by theCentre National de la Recherche Scientifique (INSUInteerieur-Terre andDYETI programmes) and inOmanfrom the willingness of the Directorate of MineralsMinistry of Commerce and Industry and the hospitabil-ity of the people Technical help in the laboratory is alsoacknowledged including C Nevado for the thin sectionsE Ball for the illustrations B Galland for her assistancein the chemistry room and C Bassoulet for help duringthermal ionization mass spectrometric analyses of the Srresults from the leaching experiments Constructivereviews from R T Gregory C Lecuyer an anonymousreviewer and particularly from M Wilson greatlyhelped to improve an earlier version of the manuscript
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Banerjee N R amp Gillis K M (2001) Hydrothermal alteration in a
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295---346
Coogan L A Wilson R N Gillis K M amp MacLeod C J (2001)
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Coogan L A Mitchell N C amp OrsquoHara M J (2003) Roof
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Davis A C Bickle M J amp Teagle D A H (2003) Imbalance in the
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Detrick R S Buhl P Vera E Mutter J Orcutt J Madsen J amp
Trocher T (1987) Multi-channel seismic imaging of a crustal
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De Villiers S (1999) Seawater strontium and SrCa variability in the
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Di Donato G Joron J L Treuil M amp Loubet M (1990)
Geochemistry of zero-age N-MORB from hole 648B ODP legs
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Dunn R A Toomey D R amp Solomon S C (2000) Three-
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1206
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Fouquet Y Von Stackelberg S U Charlou J L Donval J P
Foucher J Erzig P Muhe R Wiedicke M Soakai S amp
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Gillis K M (1995) Controls on hydrothermal alteration in a section of
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Gillis K M amp Roberts M (1999) Cracking at the magma---hydrothermal transition evidence from the Troodos ophiolite Earth
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Gregory R T amp Taylor H P (1981) An oxygen isotope profile in a
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Javoy M (1980) 18O16O and DH ratios in high temperature
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Juteau T Beurrier M Dahl R amp Nehlig P (1988) Segmentation
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Juteau T Manacrsquoh G Moreau C Lecuyer C amp Ramboz C
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(2001) Sr isotope geochemistry and hydrothermal alteration of the
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fractures and generation of layered gabbros in the lower crust
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Monograph 106 267---289
Kelemen P B Koga K amp Shimizu N (1997) Geochemistry of
gabbro sills in the crustmantle transition zone of the Oman
ophiolite implications for the origin of the oceanic lower crust Earth
and Planetary Science Letters 146 475---488Kelley D S amp Delaney J R (1987) Two-phase separation and
fracturing in mid-ocean ridge gabbros at temperatures greater than
700C Earth and Planetary Science Letters 83 53---66
Koepke J Feig S T Snow J amp Freise M (2003) Petrogenesis of
oceanic plagiogranites by partial melting of gabbros an experi-
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wwwspringerlinkcomopenurlaspgenre=s00410-003-0511-9
Kyser T K (1986) Stable isotope variations in the mantle In
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Lanphere M A (1981) K---Ar ages of metamorphic rocks at the base
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Lanphere M A Coleman R G amp Hopson C A (1981) Sr isotopic
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European Journal of Mineralogy 9 623---651
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McCulloch M T Gregory R T Wasserburg G J amp Taylor H P
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1207
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Wilcock W S D amp Delaney J R (1996) Mid-ocean ridge
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Science Letters 91 327---340
JOURNAL OF PETROLOGY VOLUME 45 NUMBER 6 JUNE 2004
1208
Table1Locationoftheanalysed
samplesandsummaryoftheirpetrographicandSr---O
isotopiccharacteristics
Sam
ple
nam
eSite
Locationin
gab
broic
section
Typ
eofrock
Distance
to
Moho(km)
Parag
enesis
Alteration
87Sr
86Srinitial1
d18O
()2
94MA2
Maq
sad
From
MTZ
Layered
gab
bro
0Olivine(M
gno74---75)3
OPXco
ronas
(Mgno78---79)
WRfrac14
070322
WRfrac14
thorn495
CPX
Palegreen
Am
(cumact)
CPXfrac14
070324
nd
Plagioclase(A
n65---80)4inversezoning
Nolow-T
alteration
Plfrac14
070322
Plfrac14
thorn479
Solidfluid
inclusionsin
Pl
97OA128b
Wad
iWaryiah
From
lower
gab
bro
Layered
gab
bro
1CPX
Low-T
alteration
WRfrac14
070333
nd
Plagioclase(A
n85---87)
Prehnitemicroveins
CPXfrac14
070422
CPXfrac14
thorn523
Plfrac14
070328
Plfrac14
thorn414
01OA41d2
Wad
iGideah
From
upper
gab
bro
Gab
bro
patch
in
laminated
gab
bro
36
CPXrecrystallized
Plagioclaseidiomorphic
(An57---87)norm
alzoning
Black
pargasitic
honblende
WRfrac14
070351
CPXfrac14
070378
Plfrac14
070426
WRfrac14
thorn480
nd
Plfrac14
thorn504
Hbdfrac14
070355
nd
01OA19aamp
dWad
iWaryiah
Inlayeredgab
bro
Gab
broic
dyke
01
Olivine
OPXpoikilitic
WRfrac14
070341
WRfrac14
thorn567
CPX
Black
pargasite
Hbdfrac14
070324
Hbdfrac14
thorn287
Plagioclase(A
n73---95)
Hornblendepoikilitic
Plfrac14
nd
Plfrac14
thorn10 09
Hem
atite
WRfrac14
070418
nd
Green
Mg-hornblende
Acthornblende
Epidote
02OA90b
Wad
iAl-Abyad
Inlayeredgab
bro
Dioriticdyke
015
Green
Mg-hornblende
Hbdfrac14
070427
Hbdfrac14
thorn239
Plagioclase(A
n83---87)
Plfrac14
070387
Plfrac14
thorn498
Solidfluid
inclusionsin
plagioclase
01OA15f
Wad
iWaryiah
Inlayeredgab
bro
Comb-textured
gab
broic
dyke
025
CPXidiomorphic
Plagioclase(A
n95---99)norm
alzoning
BrownMg-H
ornblende
WRfrac14
070458
nd
02OA43a
Wad
iMayah
Inlayeredgab
bro
vein
Green
amphibole
1Palegreen
Mg-hornblendeacicular
Plagioclase(A
n91---97)in
reactionmargin
Hbdfrac14
070431
Hbdfrac14
thorn310
01OA36b
Wad
iGideah
Inlayeredgab
bro
epidosite
dyke
25
CPX
Epidote
WRfrac14
070519
nd
Plagioclase
Epfrac14
070554
nd
CPXclinopyroxene
Plplagioclase
OPXorthopyroxene
Amam
phibolecu
mcu
mmingtonite
actactinolite
WRwholerockHbdhornblende
Epep
idotend
notdetermined
187Sr
86Srinitialratioscalculatedat
95Ma(H
acker1994)Errors
ofthemeasuredratiosareindicated
inTab
le4
2Analyticalerrors
oftheoxygen
isotopevalues
are02
(2s)
3Mgnumber
frac14atomicMg(Mgthorn
Fe)
4Percentageofan
orthitein
plagioclase
BOSCH et al HYDROTHERMAL CIRCULATION IN OMAN OPHIOLITE
1191
(millimetre-sized) recrystallized clinopyroxene largelyaltered to light green magnesio-hornblende and idio-morphic plagioclase Locally primary black pargasitichornblende develops including idiomorphic normallyzoned plagioclase (Table 4) This gabbro exhibits similar87Sr86Sr ratios for the WR and pargasite (070351 and070355 respectively) These values are interpreted asrepresentative of the melt from which this amphibolecrystallized The clinopyroxene has a slightly higher Srisotope composition of 070378 consistent with the occur-rence of the light green Mg-hornblende rim Plagioclaseis significantly more radiogenic (070426) related to sec-ondary destabilization evidenced by the idiomorphichabit The Sr isotope compositions of the liquidsextracted from the acid leaching experiments of theWR (L1 and L2) are very similar 070358 and 070365respectively The 87Sr86Sr ratio of the residue obtainedafter leaching experiments is 070335 lower than boththe pargasite and unleached WR This value can beconsidered as the unaltered isotopic composition of thissample The d18O values measured for WR and coexist-ing plagioclase are lower than normal MORB magmaticvalues The 18O depletion in the plagioclase can be fairlywell explained by high-temperature hydrothermal inter-action between an oxygen-depleted fluid and the magmaAccording to the kinetics of hydrothermal exchange ofoxygen (Gregory amp Taylor 1981 Gregory et al 1989)plagioclase exchanges its oxygen isotopes with the fluidadjusting readily to the hydrothermal conditions Forhigh temperatures of exchange with seawater-derivedfluids (d18O between 0 and thorn2) the magmatic plagio-clase will have its primary d18O value lowered whereasthe d18O value for lower temperatures of exchange(5300C) will increase The amplification of the d18Oshift increases with the intensity of the hydrothermalalteration
Gabbroic dykes intrusive in lower gabbros from WadiWariyah Wadi Tayin Massif
Gabbroic dykes 01 OA19a and 01 OA19d are intrusiveinto the lower gabbros They are 3---4 cm wide discord-ant to the foliation of the host gabbros (Fig 7e) with asharp margin marked by a brown amphibole fringe Theprimary phases in the dykes (Table 1) locally preservedas elongated aggregates of a 01mm grain size mosaicassemblage at the dyke margins grade to millimetre sizein the central part of the dyke The WR major elementcomposition is Mg enriched compared with the enclosinggabbro (Table 3) Brown pargasitic amphibole oikocrystsmake up 50 of the modal composition of the dyke Indyke 01 OA19a poikilitic orthopyroxene may developinstead of brown amphibole Finally green magnesio-hornblende grows either as oikocrysts at the expense ofclinopyroxene or in association with epidote as an altera-tion product of plagioclase These low-amphibolite-faciesminerals represent the lowest-grade paragenesis recogn-ized in these dykes and are most developed in sample01OA19aTheamphibole composition showszoning fromTi-rich pargasitic hornblende to magnesio-hornblendeand actinolitic hornblende (Fig 8c) recording pro-gressive cooling of the dyke The plagioclase compositionis extremely heterogeneous (Fig 8b) varying from An95a composition similar to that of the plagioclase in the hostgabbro (Fig 8a) to An50 for the plagioclase and asso-ciated epidote inclusions in the poikilitic actinolitic horn-blende Sample 01 OA19d has an initial 87Sr86Sr ratioslightly higher for the WR (070341) than for the parga-site (070324) The Sr isotopic ratios of liquids extractedafter two steps of WR acid-leaching (L1 and L2) are070357 and 070341 respectively whereas the residueyields a 87Sr86Sr ratio of 070327 very close to thepargasite and plagioclase values The latter is similar tothe isotopic composition measured for the massive gab-bro 94 MA2 and may be taken as close to the primarymagmatic value This Sr isotopic ratio is more radiogenicthan the inferred Oman mantle-source value (McCullochet al 1981) The altered part of this sample has a radio-genic Sr isotopic composition (070418) related to thelow-amphibolite-facies alteration This gabbroic dykehas seemingly preserved a magmatic whole-rock d18Ovalue (567) but contains by far the highestd18O plagioclase (1009) of all the samples analysedThis plagioclase coexists with low d18O pargasite(thorn287) Similarly high values (d18Oplagioclase 4 8)have been previously measured for gabbroic dykes dia-bases sheeted dykes and plagiogranites from the Omanophiolite (Gregory amp Taylor 1981 Stakes amp Taylor1992) This 72 oxygen isotope fractionation betweenplagioclase and amphibole cannot possibly represent oxy-gen equilibrium at any plausible temperature This maybe explained by different exchange kinetics for these twominerals at different temperatures at high waterrock
Table 2 Average temperatures ( C 40C)
calculated for plagioclase---amphibole pairs
using the geothermometer of Holland amp
Blundy (1994)
Pargasite Magnesio-hornblende
rim core rim core
01 OA41d2 878 953 ------- -------
01 OA19a 935 974 825 869
01 OA19d 890 933 789 -------
02 OA37b ------- ------- 816 849
02 OA90b ------- ------- 859 833
JOURNAL OF PETROLOGY VOLUME 45 NUMBER 6 JUNE 2004
1192
ratios This supports the occurrence of two distincthydrothermal events one at high temperature and alater stage at lower temperature responsible for thestrong 18O enrichment in the coexisting plagioclase
Dioritic dyke associated with a diabase dyke in lowergabbros from Wadi Halhal Haylayn Massif
Sample 02 OA37b belongs to a group of brownamphibole---plagioclase lenses or reaction margins asso-ciated with diabase intrusives (Fig 7d) in lower layeredgabbros Idiomorphic brown hornblende (tschermakite)crystals are elongated in the plane of the dyke represent-ing 80 of the modal composition They are associatedwith an interstitial plagioclase matrix The browntschermakite has greenish rims Plagioclase has a cloudyappearance due to micrometre-size fluid inclusionsexcept along its margins and internal microcracks Bothhornblende and plagioclase exhibit evidence of plasticdeformation The Sr isotopic ratio for the pargasite andplagioclase is 070348 and 070420 respectively Thepargasite exhibits a Sr signature closest to that of theOman primary magmatic value (McCulloch et al 1981)Nevertheless it is too high to be attributed to anunmodified MORB-source mantle signature This sug-gests the addition of an external fluid component in goodagreement with the shift of oxygen isotopic compositionto a low value (thorn39) suggesting a high-temperatureexchange with a low d18O fluid The Sr isotope composi-tion of the plagioclase is significantly higher than that of
the coexisting pargasite probably as a result of the low-temperature alteration
Dioritic dyke cross-cutting lower layered gabbros in WadiAl-Abyad Nakhl Massif
Sample 02 OA90b is a 1 cm thick dioritic dyke cross-cutting the lower layered gabbros It grades into a greenamphibole vein as illustrated in Fig 6c The dyke is com-posed of 2 cm long dark green idiomorphic magnesio-hornblende crystals growing in the plane of the dykeand plagioclase concentrated at the margin of the dykeThe plagioclase in the dyke and in the enclosing gabbrocontains micrometre-size fluid and mineral inclusionssimilar to those observed in sample 94 MA2 The plagio-clase and green hornblende have higher Sr isotopic ratios(070387 and 070427 respectively) and lower d18Ovalues (thorn498 and thorn239 respectively) than the normalMORB-source mantle value As observed for previoussamples this supports interaction with an external fluidcomponent
Comb-textured gabbroic dykes in the lower gabbros fromWadi Wariyah Wadi Tayin Massif
Sample 01 OA15f (Fig 6b) is from Wadi Wariyah andrepresents a 2 cm wide dyke intruding the lower gabbrosThis type of comb-textured dyke developed in theclinopyroxene---plagioclase stability field It containslarge idiomorphic clinopyroxene crystals which arepartially replaced by brownish magnesio-hornblendeThe plagioclase is extremely calcic with a slight inverse
Table 3 Whole-rock major element compositions of massive gabbros and dykes determined by X-ray fluorescence
(wt )
Sample 94 Ma2 97 OA128b 01 OA41d2 01 OA19d 01 OA19a 01 OA15f 01 OA36b 01 OA36b
Rock type Gabbro Gabbro Gabbro patch Gabbroic dyke Gabbroic dyke Comb-textured
gabb dyke
Epidosite dyke Epidosite dyke
(margin)
SiO2 4830 4815 4749 4524 4339 4799 389 4371
Al2O3 2785 2032 2438 1474 1246 1192 2752 1646
Fe2O3 328 266 404 981 1204 592 381 623
MnO 003 004 005 014 016 006 008 014
MgO 405 737 427 1031 1686 1326 187 739
CaO 1289 1908 1477 1235 1156 166 2471 2339
Na2O 292 123 257 258 111 075 000 013
K2O 000 000 009 006 000 000 000 000
TiO2 005 017 063 230 064 086 013 042
P2O5 008 007 013 018 009 013 007 010
LOI 033 076 138 216 161 237 274 190
Total 9978 9985 998 9987 9992 9986 9983 9987
LOI loss on ignition gabb gabbroic
BOSCH et al HYDROTHERMAL CIRCULATION IN OMAN OPHIOLITE
1193
Table4Majorelementcomposition
aofselectedam
phibolespaired
withplagioclasecompositionsusedforthermometry
Sam
ple
nam
e1Mode2
SiO
2TiO
2Al 2O3
Cr 2O3
FeO
MnO
MgO
CaO
Na 2O
K2O
Total
Si
AlIV
AlVI
Ti
Fe3
thornCr
Mg
Fe2
thornMn
Na
KCa
XAn3
T(C)4
01OA19a
Mg-hornblende
core
45 60
26
10 78
009
860
015
16 56
12 00
212
007
96 72
6552
1448
0366
0083
0552
0004
3409
0594
0016
0611
0018
1843
0903
899
Mg-hornblende
core
47 99
263
850
002
897
011
16 61
12 10
160
020
96 36
6894
1106
0334
0028
0483
0002
3557
0594
0014
0446
0036
1862
0892
834
Mg-hornblende
rim
51 17
247
596
001
761
014
18 11
12 19
103
010
96 47
7257
0743
0253
0016
0414
0001
3828
0488
0016
0284
0018
1853
0894
820
Mg-hornblende
rim
47 50
291
920
004
880
014
16 46
11 95
174
008
96 07
6835
1165
0396
0015
0506
0005
3529
0552
0017
0486
0015
1843
0887
825
Mg-hornblende
rim
49 57
269
742
000
799
017
18 07
12 02
133
014
96 88
7013
0987
0261
0018
0575
0000
3811
0371
0020
0365
0026
1821
0860
831
Mg-hornblende
core
47 83
181
872
003
840
011
14 48
12 18
159
008
93 59
6814
1186
0279
0018
0644
0004
3711
0357
0013
0438
0015
1859
0907
874
Pargasite
rim
42 15
249
11 87
024
10 07
013
13 84
11 43
263
032
96 61
6188
1812
0242
0434
0299
0028
3028
0937
0016
0749
0059
1797
0804
972
Pargasite
rim
42 22
260
11 86
026
10 21
011
13 71
11 48
253
032
96 59
6201
1799
0254
0429
0299
0030
3001
0955
0014
0720
0060
1807
0803
962
Pargasite
core
42 05
263
11 45
030
10 19
011
13 80
11 47
246
032
96 46
6189
1811
0175
0478
0299
0035
3029
0956
0014
0702
0060
1809
0767
974
Pargasite
rim
42 36
247
11 90
016
945
013
14 16
11 76
249
026
95 98
6239
1761
0305
0366
0276
0019
3109
0888
0016
0710
0049
1856
0818
926
Pargasite
core
42 50
291
11 64
016
984
015
14 22
11 41
256
034
96 69
6219
1781
0227
0426
0322
0018
3103
0883
0018
0727
0063
1789
0812
974
Pargasite
rim
42 76
270
12 03
017
920
012
14 87
11 82
244
025
96 73
6224
1776
0880
0335
0368
0020
3226
0752
0015
0689
0047
1843
0841
941
Pargasite
rim
42 13
181
13 54
016
921
009
12 77
12 13
254
032
96 98
6172
1828
0510
0450
0000
0018
2788
1129
0011
0721
0059
1905
0841
874
01OA19d
Mg-hornblende
49 26
047
624
011
11 46
017
15 39
12 21
108
009
96 48
7137
0863
0202
0051
0437
0013
3323
0951
0021
0302
0018
1896
0768
771
Mg-hornblende
46 03
287
932
018
906
017
14 77
12 87
193
007
97 28
6672
1328
0265
0313
0046
0021
3191
1053
0021
0542
0012
1999
0758
808
Pargasite
41 89
429
11 65
008
11 03
017
13 52
11 37
259
017
96 77
6159
1841
0179
0474
0345
0010
2963
1012
0021
0739
0032
1791
0770
975
Pargasite
rim
42 80
340
11 16
009
11 01
018
13 73
11 67
240
022
96 67
6293
1707
0226
0376
0322
0010
3009
1032
0022
0684
0041
1839
0605
880
Pargasite
core
42 58
351
11 27
006
11 63
018
13 51
11 46
239
022
96 82
6257
1743
0209
0387
0391
0007
2959
1039
0023
0680
0042
1804
0595
893
Pargasite
rim
42 14
393
11 28
006
11 71
014
13 35
11 33
235
033
96 62
6214
1786
0174
0435
0391
0007
2935
1054
0018
0673
0062
1790
0519
901
Pargasite
core
41 56
420
12 05
006
11 57
016
12 51
11 35
249
027
96 24
6168
1832
0277
0469
0276
0007
2768
1160
0021
0718
0052
1805
0537
882
Pargasite
core
41 33
438
11 49
008
11 66
015
13 35
11 45
264
026
96 80
6105
1894
0107
0487
0368
0009
2941
1073
0019
0757
0048
1813
0537
942
Pargasite
41 08
446
12 39
012
11 35
017
13 13
11 36
263
015
96 85
6049
1951
0199
0494
0368
0014
2882
1030
0021
0752
0027
1792
0758
975
02OA37b
Mg-hornblende
rim
45 15
260
984
002
13 28
019
14 01
10 87
128
009
97 33
6527
1473
0203
0283
0690
0002
3018
0915
0023
0360
0016
1683
0562
828
Mg-hornblende
rim
44 72
263
988
004
13 23
021
14 01
10 78
133
009
96 92
6494
1506
0185
0287
0713
0005
3032
0894
0025
0374
0018
1677
0593
854
Mg-hornblende
rim
45 68
247
935
004
12 87
020
14 10
11 00
121
011
97 02
6621
1379
0218
0269
0598
0004
3045
0962
0024
0341
0020
1708
0562
815
Mg-hornblende
core
43 94
291
10 87
001
12 60
023
13 73
10 59
142
011
96 39
6410
1590
0278
0319
0644
0001
2985
0893
0028
0401
0021
1655
0710
860
Mg-hornblende
core
44 62
270
10 37
001
12 66
019
14 12
10 58
140
008
96 73
6479
1521
0253
0294
0644
0001
3057
0893
0023
0395
0014
1646
0674
851
Mg-hornblende
rim
47 79
181
796
002
12 09
025
15 22
11 09
100
009
97 33
6866
1134
0214
0196
0506
0002
3260
0947
0030
0280
0016
1706
0503
767
Mg-hornblende
core
45 43
249
10 3
000
11 99
022
14 50
11 14
133
008
97 48
6524
1476
0267
0269
0644
0000
3103
0796
0027
0370
0015
1715
0703
838
JOURNAL OF PETROLOGY VOLUME 45 NUMBER 6 JUNE 2004
1194
Sam
ple
nam
e1Mode2
SiO
2TiO
2Al 2O3
Cr 2O3
FeO
MnO
MgO
CaO
Na 2O
K2O
Total
Si
AlIV
AlVI
Ti
Fe3
thornCr
Mg
Fe2
thornMn
Na
KCa
XAn3
T(C)4
02OA90b
Mg-hornblende
core
48 83
038
745
005
939
010
17 29
12 24
142
006
97 21
6931
1069
0041
0041
0667
0005
3657
0448
0011
0392
0010
1862
0840
856
Mg-hornblende
core
49 49
033
731
003
916
008
17 40
12 57
139
005
97 82
6987
1013
0035
0035
0552
0004
3662
0530
0010
0379
0009
1901
0830
811
Mg-hornblende
rim
48 90
040
753
005
932
013
17 15
12 22
154
005
97 29
6948
1052
0042
0042
0575
0006
3632
0533
0015
0424
0010
1861
0880
869
Mg-hornblende
rim
48 30
044
800
007
962
010
16 86
12 32
161
005
97 35
6873
1127
0047
0047
0598
0008
3576
0546
0012
0443
0010
1879
0870
862
Mg-hornblende
rim
49 12
032
742
010
922
011
17 36
12 29
142
006
97 43
6956
1044
0034
0034
0621
0011
3665
0471
0013
0391
0010
1865
0840
845
02OA43a
Mg-hornblende
rim
48 13
040
789
007
860
007
17 41
12 19
155
006
96 36
6882
1118
0212
0043
0621
0008
3710
0407
0008
0429
0010
1867
-------
-------
Mg-hornblende
rim
50 93
035
643
001
759
006
18 50
12 24
114
003
97 28
7152
0848
0217
0037
0506
0001
3873
0385
0007
0311
0005
1841
-------
-------
Mg-hornblende
rim
50 67
014
559
009
12 33
026
15 28
12 44
068
003
97 50
7260
0740
0204
015
0483
0010
3263
0994
0032
0189
0005
1909
-------
-------
Mg-hornblende
rim
50 34
013
621
009
12 49
027
15 07
12 51
064
003
97 78
7192
0810
0240
0010
0530
0010
3210
0960
0030
0180
0010
1910
-------
-------
Mg-hornblende
rim
45 17
060
11 14
013
988
008
15 78
12 33
211
009
97 31
6473
1527
0355
0640
0644
0015
3371
0540
0010
0588
0016
1892
-------
-------
Mg-hornblende
rim
49 25
032
712
014
845
009
17 84
12 32
137
004
96 95
6983
1020
0170
0030
0620
0020
3770
0380
0010
0380
0010
1870
-------
-------
Mg-hornblende
rim
48 61
032
775
014
975
011
16 81
12 57
139
004
97 50
6905
1950
0203
0034
0621
0016
3560
0537
0013
0382
0008
1914
-------
-------
94Ma2
Anthophyllite
54 98
000
259
001
12 65
037
23 35
199
034
000
96 28
7744
0256
0174
0000
0161
0001
4903
1330
0043
0092
0001
0301
-------
-------
Anthophyllite
55 11
000
296
003
14 09
032
23 66
063
038
000
97 18
7716
0284
0205
0000
0115
0003
4938
1535
0038
0104
0000
0095
-------
-------
Actinolite
48 58
004
954
002
960
014
17 67
999
139
005
97 02
6850
0150
0435
0004
0690
0002
3713
0442
0017
0379
0009
1510
-------
-------
Actinolite
51 54
004
585
000
890
022
20 28
930
090
002
97 05
7214
0786
0178
0005
0598
0000
4230
0444
0026
0244
0004
1395
-------
-------
01OA41d2
Pargasite
rim
43 05
342
10 71
008
11 46
012
13 50
11 54
179
067
96 34
6353
1647
0216
0380
0368
0009
2967
1046
0015
0512
0127
1824
0613
863
Pargasite
rim
42 61
354
11 59
001
10 59
010
13 52
11 87
160
076
96 19
6286
1714
0302
0392
0299
0001
2972
1007
0012
0458
0143
1877
0715
844
Pargasite
core
42 54
401
11 04
006
11 81
013
13 21
11 50
202
039
96 71
6266
1734
0182
0444
0368
0007
2901
1087
0017
0576
0073
1814
0854
979
Pargasite
rim
43 03
385
10 72
002
11 70
014
13 50
11 95
146
080
97 17
6314
1686
0168
0425
0345
0002
2953
1091
0018
0415
0149
1878
0634
852
Pargasite
rim
42 65
341
10 60
006
11 97
014
13 22
11 64
183
068
96 20
6332
1668
0186
0381
0345
0007
2926
1141
0018
0528
0129
1851
0620
869
Pargasite
core
42 82
367
10 60
007
11 93
012
13 42
11 44
206
047
96 60
6315
1685
0158
0407
0391
0008
2950
1080
0016
0588
0089
1808
0615
899
Pargasite
rim
42 2
362
11 10
007
11 42
016
13 09
11 72
179
072
95 89
6282
1718
0229
0405
0299
0009
2905
1123
0020
0518
0137
1869
0715
875
Pargasite
rim
42 42
380
11 03
006
11 77
012
13 00
11 72
188
069
96 49
6283
1717
0209
0423
0299
0007
2870
1158
0015
0539
0131
1860
0817
928
Pargasite
rim
42 21
364
11 14
006
10 73
011
13 53
11 68
177
072
95 59
6277
1723
0229
0407
0322
0007
2999
1012
0014
0511
0136
1862
0736
883
Pargasite
rim
42 25
376
11 03
008
10 93
015
13 66
11 72
185
064
96 07
6257
1743
0182
0418
0345
0009
3016
1009
0019
0531
0121
1860
0766
912
Pargasite
core
42 17
396
10 91
008
11 10
010
13 57
11 49
209
038
95 85
6253
1747
0159
0442
0368
0010
3000
1009
0013
0602
0073
1826
0841
981
aMajorelem
entco
mpositionsarein
weightpercent-------noplagioclasean
alysed
andnotemperature
calculated
1Typ
eofam
phibolean
alysed
isalso
indicated
2Blankindicates
nozoningobserved
3Molefractionofan
orthitein
plagioclasead
jacentto
amphibolegrain
4Tem
perature
ofeq
uilibrium
fortheam
phibole---plagioclasepaircalculatedusingtheHollandamp
Blundythermometer
(1994)Errors
are40
C
BOSCH et al HYDROTHERMAL CIRCULATION IN OMAN OPHIOLITE
1195
zonation from An95 to An99 from core to rim (Table 1 andFig 8a) TheWR Sr isotopic ratio of the sample (070458)is significantly higher than normal MORB values Thiscould be related to the overprint of a low-temperaturealteration event consistent with the petrography of thesample
Green amphibole vein cross-cutting layered gabbros inWadi Mayah Haylayn Massif
Sample 02 OA43a belongs to a series of 1 cm thick greenamphibole veins (as in Fig 6e) cross-cutting the layeredgabbros The veins develop 15 cm thick reaction marginsin the enclosing gabbro and are exclusively composed ofacicular pale green magnesio-hornblende growing per-pendicular to the vein margin in a crack-seal mode(Fig 6f ) The reaction margins are composed of urali-tized gabbro marked by the development of the samepale green magnesio-hornblende in an inhomogeneouscalcic plagioclase matrix (An91---97) The Sr and O isotopiccompositions of this green hornblende (070431 andthorn31) do not correspond to normal MORB-sourcemantle values
Epidosite dyke from Wadi Gideah Wadi Tayin Massif
Sample 01 OA36b is representative of a series of epidoteveins cross-cutting poorly layered gabbros These 2 cmthick veins are composed of pink thulite in their centreand green pistachite at their margins (Fig 6a) Theirsharp contact with the enclosing gabbro is marked bythe development of coarse clinopyroxene crystals alteredin the epidote---greenschist facies This suggests that theepidote vein is replacive in a coarse-grained gabbro dykeThe highest 87Sr86Sr initial ratios of all the samplesstudied are recorded by the WR and coexisting epidoteof this sample (070519 and 070554 respectively)(Table 5) The Sr content of the WR and associatedepidote are also the highest of the present dataset(716 ppm and 764 ppm respectively) The Sr isotopiccomposition of this sample is consistent with a greaterdegree of exchange with a radiogenic component fromCretaceous seawater It is significantly higher than thevalue obtained for an epidosite vein from the Wadi Fizhsection of the Oman ophiolite [070435 with a Sr contentof 488 ppm reported by Kawahata et al (2001)] but ingood agreement with the average of the greenschist-faciesalteration sequence defined by Kawahata et al (2001)
GABBROS
01 km 025 km 1 km 36 km50
60
70
80
90
An
MA2
19d
19a 15f 128b
41d2
(a)
43a17b
15f
90b
37b
19a
19d
40
60
80
100
A
n
01 km 015 km 025 km 1 km
DYKES
(b)
Distance to the Moho
ACTINOLITE
MAGNESIOHORNBLENDE
TSCHERMAKITE
PARGASITEGabbros Dykes41d2MA2
19a d37b15f
90b43a
AlIV
150500
02
08
10
(Na
+ K
) A
(c)
04
06
950
900
850
800
750
70007 12 17
T degC
1000
(d)
AlIV
Fig 8 Plagioclase---amphibole compositions and results of thermometry (a) and (b) plagioclase compositions in layered gabbros and in dykesrespectively as a function of distance to the Moho (see text for sample identification) (c) amphibole compositions in layered gabbros and dykesaccording to Leakersquos (1997) nomenclature (d) equilibration temperatures calculated using the amphibole---plagioclase thermometer of Holland ampBlundy (1994) as a function of AlIV content in amphibole [same symbols as in (c)]
JOURNAL OF PETROLOGY VOLUME 45 NUMBER 6 JUNE 2004
1196
Table5RbandSrcontents87Sr
86Srandd1
8Oisotopiccompositionsofwholerocksandmineralseparatesfrom
samplesofmassivegabbrosdykes
andveinsfrom
theOman
ophiolitealsolistedarecalculated
waterrockratios(W
R)andLOIvalues
Sam
ple
Rb(ppm)
Sr(ppm)
87Rb8
6Sr
87Sr
86Sr
(87Sr
86Sr)i1
d18O
()
LOI(
)2WR
3(87Sr
86Sr)L14
(87Sr
86Sr)L25
(87Sr
86Sr)R6
01OA41d2
Whole
rock
037
1792
0006
0703516
09
070351
thorn480
138
47
0703587
0703651
0703357
Plagioclase
025
1249
0003
0704261
22
070426
thorn504
-------
-------
Pargasite
-------
-------
-------
0703548
20
070355
-------
-------
-------
Clinopyroxene
045
26 0
0050
0703845
19
070378
-------
-------
-------
01OA36b
Dykewhole
rock
0005
7164
50001
0705186
17
070519
-------
274
21 9
0705207
0705112
-------
Epidote
0003
7645
0001
0705536
12
070554
-------
-------
-------
Wallwhole
rock
005
4254
0001
0704637
09
070464
-------
191
-------
0704945
0704675
0704593
02OA43a
Green
hornblende
004
98
0012
0704323
13
070431
thorn310
-------
-------
97OA128b
Whole
rock
003
1274
50001
0703327
17
070333
-------
076
37
-------
-------
-------
Plagioclase
002
1204
50001
0703285
09
070328
thorn414
-------
-------
Clinopyroxene
003
20 0
0004
0704230
09
070422
thorn523
-------
-------
01OA15f
Whole
rock
006
1077
0002
0704582
16
070458
-------
237
13 5
-------
-------
-------
01OA19a
Whole
rock
037
1303
0008
0704189
18
070418
-------
161
96
-------
-------
-------
01OA19d
Whole
rock
058
2032
0008
0703423
08
070341
thorn567
216
415
0703574
0703408
0703271
Pargasite
091
1058
0025
0703273
11
070324
thorn287
-------
-------
Plagioclase
-------
-------
-------
-------
-------
thorn10 09
-------
-------
02OA90b
Plagioclase
004
2385
50001
0703868
11
070387
thorn498
-------
-------
Green
hornblende
010
23 8
0012
0704288
15
070427
thorn239
-------
-------
BOSCH et al HYDROTHERMAL CIRCULATION IN OMAN OPHIOLITE
1197
Table5continued
Sam
ple
Rb(ppm)
Sr(ppm)
87Rb8
6Sr
87Sr
86Sr
(87Sr
86Sr)i1
d18O
()
LOI(
)2WR
3(87Sr
86Sr)L14
(87Sr
86Sr)L25
(87Sr
86Sr)R6
02OA37b
Plagioclase
023
3121
0002
0704204
15
070420
-------
-------
-------
Pargasite
031
41 6
0021
0703505
15
070348
thorn394
-------
-------
94OAMa2
Whole
rock
0016
1750
00003
0703219
15
070322
thorn495
033
31
0703264
0703177
0703158
Plagioclase
0014
1363
00003
0703219
11
070322
thorn479
-------
-------
Clinopyroxene
-------
-------
-------
0703236
13
070324
-------
-------
-------
1Initial8
7Sr
86Srratiocalculatedat
95Ma(H
acker1994)
2LOIisloss
onignition(in)
3Water---rock
ratiocalculatedassumingaclosedsystem
TheeffectiveWR
ratiocanbeexpressed
bythefollowingeq
uation
W=Reffectivefrac14
frac12eth87Sr=
86SrrockTHORN fi
nal
eth87Sr=
86SrrockTHORN in
itial=frac12eth8
7Sr=
86SrseawaterTHORN in
itial
eth87Sr=
86SrrockTHORN fi
nal
ethCrock
initial=C
seaw
ater
initial
THORNwhere(87Sr
86Srrock) finalisthefinalmeasuredSrisotopicratioin
thesample(87Sr
86Srrock) in
itialco
rrespondingto
theinitialm
antlevalue
istakenas
070295(Lan
phere
etal1981)(87Sr
86Srseawater) in
itialistheCretaceousseaw
ater
Srisotopicco
mposition[87Sr
86Sr07073
at90
MaafterBurkeet
al(1982)]C
seaw
ater
initial
istheSrco
ntent
oflsquonorm
alrsquoseaw
ater
andis
takenas
8ppm
(deVilliers1999)
FortheCrock
initialitis
assumed
that
thepresentSrco
ncentrationmeasuredis
thesameas
theinitial
concentration
4Srisotopicratiosmeasuredforliquid
L1extractedafterthefirststep
oftheleachingexperim
ent(6NHClfor8min
at70
C)
5Srisotopicratiosmeasuredforliquid
L2extractedaftertheseco
ndstep
oftheleachingexperim
ent(2 5NHClfor30
min
at70
C)
6Srisotopicratiosmeasuredforresidueobtained
afteratotalaciddigestionachievedaftertheextractionofL1an
dL2leachates
JOURNAL OF PETROLOGY VOLUME 45 NUMBER 6 JUNE 2004
1198
Part of the same sample from the contact between thedyke and the gabbro yields a slightly lower Sr isotopiccomposition (070464) intermediate between the sur-rounding gabbro and the epidote dyke It also has anintermediate Sr content (425 ppm) Acid leaching experi-ments performed on the whole rock of the dyke yield Srisotopic compositions of 070521 and 070512 for thetwo liquids extracted Similar experiments for the samplelocated at the contact with the gabbro yield 87Sr86Srratios of 070494 and 070467 for L1 and L2 liquids TheSr isotopic composition of the residue extracted afterleaching remains high (070459) and very close to theepidosite 87Sr86Sr ratio determined by Kawahata et al(2001) In this sample the primary minerals have beentotally replaced by epidote and secondary plagioclase orquartz Thus the Sr isotopic compositions measured forthis WR sample and its constituent minerals reflect thecomposition of the fluid filling this dyke
Mineral chemistry
Plagioclase
Compared with the host gabbros the dykes show a muchlarger dispersion in their plagioclase compositions (Fig 8aand b Table 4) characterized by more calcic plagioclasethan the enclosing gabbros This tendency is mostextreme in the comb-textured dykes in which the plagio-clase is almost pure anorthite The zoning is generallynormal recording a crystal fractionation process How-ever gabbro 94 MA2 exhibits inverse zoning this tend-ency is also observed in plagioclases from dykes 01OA19a and 01 OA15f The higher calcium content ofthe plagioclase as well as the inverse zoning is consistentwith an increase of water pressure during its crystalliza-tion (Turner amp Verhoogen 1960 Carmichael et al 1974Koepke et al 2003) The particularly high An content inthe comb-textured gabbro dykes 01 OA15f may alsoresult from crystallization in a supercooled magma thisis consistent with the comb texture indicative of fast-growing crystals The lack of brown hornblende suggeststhat crystallization occurred above the temperature ofhornblende stability
Amphibole
The amphiboles analysed in the dykes (Fig 8c andTable 4) show a regular trend in a (Na thorn K)A vsAlIV diagram from titanium-rich (Ti 4 015) pargasitichornblende (brownamphibole) and tschermakite (brown---green amphibole) to low-titanium (Ti5 015) magnesio-hornblende (green amphibole) and actinolite (pale greenamphibole) The pargasitic hornblende develops aspoikilitic oikocrysts in the dykes 01 OA19a and d andin the diffuse patch 01 OA41d2 at temperatures above800C during the final stage of crystallization (see
below) The tschermakite composition corresponds tothe internal alteration of clinopyroxene in gabbro 94Ma2 occurs in the comb-textured dyke 01 OA15f andis the primary amphibole in the dioritic dyke 02 OA37bThe magnesio-hornblende develops at the edges of thepargasites or of clinopyroxenes at temperatures around800C and is the primary phase in the dioritic dyke 02OA90b and in the green vein 02 OA43a Finally actino-lite occurs as a replacement of green hornblende in dykesor in the kelyphitic rim surrounding olivine in gabbro 94Ma2 Such a large composition range at the scale of athin section has been reported in amphiboles fromamphibolite-facies oceanic gabbros (Stakes 1991 Stakesamp Taylor 1992 Gillis et al 1993) as well as in the Omanophiolite gabbros (Manning et al 2000) This variabilityhas been explained by rapid reaction rates preventinghomogenization (Manning et al 2000)
Thermometry
Plagioclase---amphibole thermometry could be appliedwhen the two minerals exhibited good contacts such asin the laminated gabbro of the middle sequence (01OA41d2) and in the intrusive dykes The temperaturesof plagioclase---amphibole equilibration (Table 2) havebeen calculated using the geothermometer lsquoBrsquo of Hollandamp Blundy (1994) Edenite thorn Albite frac14 Richterite thornAnorthite valid between 500C and 900C with anuncertainty of 40C Amphibole formulae are basedon 23 oxygen and their nomenclature is based on alkalications in the A site vs AlIV according to Leake (1997)Pressurewas assumed to be 1 kbar Forty-five plagioclase---amphibole pairs meet the compositional criteria imposedby Holland amp Blundyrsquos dataset (09 4 Xan 4 01 andAlIV 403) Electron microprobe measurements wereconstrained to contiguous plagioclase and amphiboleseparated by sharp grain boundaries assuming equili-brium of the two phases When the mineral phasesexhibit normal zoning temperatures between plagioclaseand amphibole cores and between mineral rims werecalculated assuming that the temperatures deduced fromthe mineral core chemistry provide a proxy for the high-est temperature of equilibration These data are identi-fied in the T C vs AlIV diagram (Fig 8d) Table 4presents selected major element amphibole analyses theAn content of the plagioclase adjacent to each amphibolegrain and the temperature calculated for the pairThe temperatures calculated (Fig 8d) span the entire
range of temperatures at which amphibole and plagio-clase coexist in equilibrium (ie amphibolite-facies para-geneses) This temperature interval is bracketed byorthopyroxene-bearing facies or amphibole-free facies(ie gabbro 94MA2) and epidote---actinolite facies devoidof homogeneous plagioclase (ie epidosite dyke 01OA36b and green vein 02 OA43a) The range of the
BOSCH et al HYDROTHERMAL CIRCULATION IN OMAN OPHIOLITE
1199
calculated temperatures between 4900C and 750Ccorresponds to the changing composition of amphibolesfrom pargasite to magnesio-hornblende at the scale of athin section or even at that of an individual crystal asshown by their correlation with AlIV such variabilityrecords rapid cooling in hydrous conditions The highesttemperatures are recorded by pargasite from the anatec-tic gabbro 01 OA41d2 in the gabbro dyke 01 OA19dand in the dioritic dyke 02 OA90b They were calculatedfrom minerals devoid of lower-grade alteration and arethus considered as representing the equilibrium temperat-ure of the coexisting minerals Lower temperatures cor-respond to mineral phases evolving at falling temperatureand are only indicative of the cooling history Includingthe 40C uncertainty seven pairs provide temperatureshigher than 900C the limit of validity of the Holland ampBlundy thermometer However we maintain these tem-peratures as indicative of the highest values of our data-set These values are included in the average equilibriumtemperature summarized in Table 2
DISCUSSION
Distribution of hydrothermal alteration
Two distinct petrological events are recorded in the gab-bro section of the Samail ophiolite and are documentedin the present contribution (1) the whole gabbro sectionis affected by a penetrative alteration (2) the gabbrosection is crosscut by various types of dykes and veinsincluding hydrous minerals
Penetrative alteration
The penetrative alteration developed in the gabbros atgrain boundaries and locally invading the minerals isubiquitous in the gabbro section This penetrative altera-tion has been described as very high temperature (VHT)high temperature (HT) and low temperature (LT) basedon alteration parageneses The orthopyroxene coronas inthe layered gabbro sample and the incipient appearanceof brown pargasitic amphibole mark a lower thermallimit of the VHT alteration By reference to the experi-mental work of Koepke et al (2003) the temperature ofequilibration for these reactions is estimated to bebetween 900 and 1000CThe preservation of the vertical distribution of
VHT---HT facies (Figs 4 and 5) also pointed out byManning et al (2000) indicates that the hydrothermalalteration pattern fits the distribution of isotherms withdepth at a fast-spreading ridge (ie Phipps Morgan ampChen 1993 Dunn et al 2000 Fig 2) Equilibrium tem-peratures for VHT parageneses as high as 900---1000Csuggest that VHT---HT hydrothermal alteration tookplace in a very restricted time interval at the limit ofthe cooling magma chamber It is proposed that the
penetrative alteration affecting the entire gabbro sectionis related to a pervasive network of microcracks whichact as a recharge system down to the Moho (Nicolas et al2003)
Dykes and veins
The dykes are extremely varied eg pegmatitic gabbroswith comb-texture microgabbros and amphibolitizeddolerites dioritic dykes alignments of poikilitic horn-blende and finally diffuse hornblende-bearing patchesHigh-T gabbro and diorite dykes and low-T greenamphibole---epidote veins are connectedIn the dykes textural evidence attests to suprasolidus
conditions at least for the development of the pargasiticamphibole Integrating the hornblende---plagioclase equi-libration temperatures calculated for the studied samplesthese temperatures are bracketed between 950C and875CIn the following discussion we shall consider separately
the microgabbro and dolerite dykes The latter representbasaltic melt intrusions associated upsection with thediabase sheeted dykes and downsection possibly withthe mafic dykes that are ubiquitous in the mantle section(Nicolas et al 2000) Whatever their origin the develop-ment of pargasitic amphibole during their late stage ofcrystallization creates a link with the gabbroic dykes andsuggests a crystallization process evolving from dry con-ditions to more hydrous conditionsThe other gabbroic dykes comb-textured gabbros and
hornblende-bearing dioritic dykes result from the fillingof cracks by a fluid either a melt or a silica-rich hydrousfluid These intrusive dykes which develop reaction mar-gins at their contact with the host gabbro grade verticallyinto lower-temperature green amphibole veins or align-ments of poikilitic hornblende in the layered gabbromatrix or development of pargasitic hornblende in ana-tectic gabbros (sample 01 OA41d2) In gabbros crystal-lizing in the magma chamber the development oforthopyroxene and pargasite oikocrysts enclosing the pri-mary minerals suggests a thermal evolution from thegabbro solidus to amphibolite-facies conditions Forthese reasons it is suggested that the gabbroic dykesresulted from the injection of a hydrous melt in the stillhot gabbros drifting away from the magma chamber asdiscussed below
Origin of fluids responsible for VHT---HThydrothermal alteration
The 87Sr86Sr initial ratios and d18O of whole rocks andpure mineral fractions are plotted in Fig 9 against thedistance above the Moho Transition Zone (MTZ) Theinitial Sr isotopic composition of the whole rocks apartfrom the epidosite dykes ranges from 070322 to
JOURNAL OF PETROLOGY VOLUME 45 NUMBER 6 JUNE 2004
1200
070458 All the studied samples (including whole rocksand minerals) have initial 87Sr86Sr ratios falling outsidethe field of fresh MORB which commonly have Sr ratiosbetween 07023 and 07029 with an average of 070265(ie Hart et al 1974) The lowest initial Sr isotopiccomposition (070322) from the massive gabbro 94MA2 is slightly but significantly higher than averageOman primary magmatic signatures (ie 070296McCulloch et al 1981) Similarly the d18O values ofmost whole rocks and minerals measured (Table 5)
depart from typical MORB-source mantle values( Javoy 1980 Gregory amp Taylor 1981 Kyser 1986)These differences indicate that the primary mantlesignatures in the Oman ophiolite have been modifiedby interaction with a fluid Possible origins for thisfluid are mantle components dehydration of subductedlithosphere or seawater circulation Strontium andoxygen isotopes are useful tracers for fluid---crust interac-tion as d18O and 87Sr86Sr ratios from fluids originat-ing from seawater the mantle or subducted lithosphere
-1000
1000
2000
3000
4000
5000
6000
7000
MOHO
MTZ
Dis
tan
ce a
bov
e th
e M
oh
o (
m)
pillow-basalt
sheeted-dike
gabbro
peridotite
01 OA 41d2
01 OA 36b
02 OA 43a97 OA 128b
01 OA15f
94 OA MA2
02 OA 90b01 OA 19ad
02 OA 37b
0702 0703 0704 0705 0706 0707
87Sr86Srinitial
MO
RB-s
ou
rce
Seaw
ater
Whole RockPlagioclasePargasiteGreen HornblendeClinopyroxene
Epidote
(a)
-1000
1000
2000
3000
4000
5000
6000
7000
MOHO
MTZ
Dis
tan
ce a
bov
e th
e M
oh
o (
m)
pillow-basalt
sheeted-dike
gabbro
peridotite
01 OA 41d2
01 OA 36b
02 OA 43a97 OA 128b
01 OA15f
94 OA MA2
02 OA 90b01 OA 19ad
02 OA 37b
Whole RockPlagioclasePargasiteGreen HornblendeClinopyroxene
3 5 10
δ18O (permil)
41d2
Ma2
43a
90b 19d 19d37b
128b
90b
(b)
Fig 9 (a) Variation of initial 87Sr86Sr isotopic composition as a function of the location of the studied samples in a schematic log of the crustalsection of the Oman ophiolite The field for MORB is from Hart et al (1974) and that for seawater is from Burke et al (1982) Small open squares aredata fromKawahata et al (2001) (b) Variation of d18O () as a function of the location of the studied samples in a schematic log of the crustal sectionof the Oman ophiolite Shaded curve corresponds to the data of Gregory amp Taylor (1981)
BOSCH et al HYDROTHERMAL CIRCULATION IN OMAN OPHIOLITE
1201
are significantly different Moreover the d18O valueis dependent on temperature and mineral fractiona-tion and thus yields complementary constraints to Srisotopes
Mantle origin
One model to explain the occurrence of fluids interactingwith the oceanic crust at very high temperatures is toderive them from the mantle The contribution of mantle-derived hydrous fluids linked to percolation processesor to their concentration in the final stages of gabbrocrystallization has been invoked in numerous case studies(ie Witt amp Seck 1989 Fabriees et al 1989 Dupuy et al1991 Agrinier et al 1993) O and Sr isotopic data forwhole rocks and mineral separates yield scattered valuesthat are unambiguously different from MORB mantle-source values (Fig 9a and b) With the exception of lateLT altered samples the WR data have a mean 87Sr86Srratio of 070337 and a standard deviation of 000012These surprisingly high values in mantle-derived rockscould be taken as evidence for survival of mantle isotopicheterogeneities during crystallization of the ophiolite (egLanphere 1981 McCulloch et al 1981) However the18O16O isotope ratios in the Oman gabbroic sequenceare also displaced from primary magmatic values Nogabbro in the present study has plagioclase or amphi-bole with typical mantle d18O values Thus the Sr and Oisotope data rule out the possibility of mantle-derivedfluids as a possible source for the VHT---HT hydrousalteration event recorded in the massive gabbros andgabbroic dykes and veins (Fig 10a and b)
Subducted lithosphere origin
Dehydration of subducted oceanic crust in subductionzones can generate hydrous fluids Such fluids couldpercolate through the overlying lithosphere and contam-inate it thus generating modified isotopic signatures andtrace element contents Such an explanation howeverdisagrees with the geochemical characteristics of most ofthe studied samples Fluids derived from the dehydrationof subducted slabs (fresh or altered MORB or OIB pro-toliths) should be strongly enriched in K Rb and Ba (egBecker et al 2000) whereas in Oman the Rb contentsmeasured in the gabbros and gabbroic dykes and veins arestrongly depleted Moreover the isotopic composition ofslab-derived fluids is buffered by the MORB-like oceaniccrustal protolith for Nd and Pb but not for Sr and O(Staudigel et al 1995) As a result of relatively minorfractionation of oxygen at high temperature the oxygenisotope composition of the fluids expelled during dehy-dration of the subducted slab should be high and essen-tially controlled by the oxygen composition of the uppersection of the crust altered at low temperature (with an
average of 10 and perhaps as high as 19) (Staudigelet al 1995) The Sr isotopic composition should be alsohigh (average 07046) as the fluids released by the dehy-dration of subducted oceanic crust are associated eitherwith seawater or with radiogenic Sr phases (ie smectites)The Sr and O isotopic compositions of the studied sam-ples do not fit with these signatures and so preclude asubducted lithosphere origin for the fluids involved in theVHT---HT alteration event
Seawater-derived fluids
All the analysed samples (minerals and WR) have 87Sr86Sr(T) outside the domain of primary MORB magmasand variable Sr contents Individual minerals have Srcontents reflecting their different mineralliquid partitioncoefficients Minerals characterized by a very high affi-nity for Sr such as plagioclase (ie Sr mineralliquidpartition coefficients 1) have very high Sr contentsTherefore the high abundance of plagioclase in the sam-ples analysed is responsible for the high Sr content of thestudied whole rocks The Sr content of all the whole rocksis relatively homogeneous (Table 5) without clear distinc-tion between country-rock gabbros and dykes and veinsand ranges from 110 to 200 ppm except for the epidosite(4700 ppm) Reported on a 87Sr86Sr vs 1Sr diagram(Fig 10a) most whole rocks and plagioclases analysed arecharacterized by a 1Sr ratio lower than 001 and plot inthe left part of this diagram Compared with N-MORBthe studied massive and anatectic gabbros and their con-stituent plagioclases have similar to slightly higher Srcontents but substantially higher 87Sr86Sr ratios(Fig 10a) Considering Cretaceous seawater as a possiblehigh 87Sr86Sr mixing component [87Sr86Sr 07073at 90Ma after Burke et al (1982)] responsible for theobserved geochemical modifications exchanges cannothave occurred in a closed system as seawater has too lowa Sr content (ie 8 ppm de Villiers 1999) An open-system process allowing penetration of larger quantitiesof seawater can efficiently modify the Sr isotopic ratio ofthe rocks Alternately the fluids could also be seawaterthat was modified through exchange with the surround-ing rocks during its transit through the oceanic crustalsection This modified seawater would be more enrichedin Srwith a 87Sr86Sr ratio intermediate between unmodi-fied seawater and MORBMinerals devoid of late LT alteration imprints (parga-
site green hornblende and pyroxene) and characterizedby very low Sr contents plot on the right of the diagramclose to or on the mixing line between seawater andMORB (Fig 10a) The difference between these mineralsand the other samples (WR and plagioclases) is mainlydue to the Sr fractionation coefficients of these differentminerals and to the large quantity of plagioclase inthe studied rocks The process responsible for the
JOURNAL OF PETROLOGY VOLUME 45 NUMBER 6 JUNE 2004
1202
modification of the Sr isotopic composition fromMORB-source value is however explained by the same phenom-enon As all these minerals display initial 87Sr86Sr ratiosdistinct from the MORB source and because they equili-brated at VHT or HT temperatures this supports anearly intervention of seawater during crystallization ofthese minerals Most of these samples with the exceptionof the epidosite dyke overlap the domains defined byKawahata et al (2001) for the Wadi Fizh gabbros alteredat VHT and HT conditions in the amphibolite facies(labelled lsquoVHTrsquo and lsquoHTrsquo in Fig 9a)Three samples (two epidosite dykes and an one epidote
fraction) have very high Sr contents and the highest Sr
isotopic compositions of the present dataset The twowhole-rock samples overlap the field of the Wadi Fizhsamples altered at high temperature during greenschist-facies metamorphism (Kawahata et al 2001) This typeof alteration is common at the ridge axis and formedthrough intensive exchange with seawater-derived fluidsFigure 10b indicates that all the Sr---O isotope data are
distinct from MORB compositions and suggests that thefluids reacting with the magmas could be derived fromseawater In addition the d18O values show that isotopicexchange occurred under VHT or HT conditions Thefluids could have the composition of seawater or theycould have the composition of seawater modified through
0703
0705
0707
001 01 1
1 Sr (ppm)
MORB
(87Sr
86Sr)
init
ial
Seawater
VHT
HT
MT36b
36b
36b
43a 128b90b
41d2
37b19d
15f
128b
41d2
41d2
19a
Ma2Ma2
19d
90b
37b
Whole RockPlagioclasePargasiteGreen HornblendeClinopyroxene
Epidote
(a)
0 2 4 6 8
δ18O(permil)
0703
0705
0707Seawater
MORB-mantle
128b 41d2
90b 41d2
Ma2128b
37b
43a
19d
90b
Whole RockPlagioclasePargasiteGreen HornblendeClinopyroxene
Epidote
HT LT
19d
(87Sr
86Sr)
init
ial
(b)
Fig 10 (a) Initial 87Sr86Sr isotopic ratios plotted against 1Sr for whole rocks and mineral separates from the gabbro section of the Omanophiolite Fields shown are from Kawahata et al (2001) and correspond to MT-----basalt sheeted dyke diabase altered at medium temperature(prehnite---actinolite facies sequence 3) HT-----diabase plagiogranite metagabbro and epidosite formed at high temperature (greenschist faciessequence 4) VHT-----gabbro altered at very high temperature (amphibolite facies sequence 5) The dashed line is a binary mixing line betweenMORB (Lanphere et al 1981) and Cretaceous seawater (Burke et al 1982) (b) Initial 87Sr86Sr isotopic composition plotted against d18O value forwhole rocks and minerals from the Oman ophiolite Cretaceous seawater and MORB end-members as in (a) Dashed line represents mixing curvebetweenMORB and seawater at high temperature (HT) Low-temperature (LT) exchanges between these two components would be responsible foran increase of the d18O primary MORB value
BOSCH et al HYDROTHERMAL CIRCULATION IN OMAN OPHIOLITE
1203
exchange with the surrounding rocks during its penetra-tion into the crustal section In an environment in whichfluid---rock interaction occurs at variable temperaturesthe d18O is greatly influenced by the temperature atwhich the exchange took place and by the waterrockratio It is evident from Figs 9b and 10b that none of thestudied gabbros have plagioclase and amphibole withMORB-like d18O and 87Sr86Sr values The separateminerals display a broad range of d18O values and mostexhibit extreme 18O depletion (Table 5) Hydrothermalexchange kinetics are similar in amphibole and pyroxeneand these two minerals are more resistant to oxygenexchange during water---rock interaction than coexistingplagioclase (Gregory et al 1989) The most probableexplanation of the low d18O values measured in bothamphiboles and pyroxenes is that they crystallized athigh temperature in the presence of a relatively low 18Oseawater-derived hydrothermal fluid [d18O from 01 to07 after Gregory amp Taylor (1981)] The anomalouslyhigh d18O value (thorn1009) measured for the plagioclase ofthe micro-gabbroic dyke (01 OA19d) can be interpretedas resulting from a superimposed overprint caused by alate-stage penetration of fluids at lower temperature Inthis sample the occurrence of such different 18O16Oratios between coexisting plagioclase and amphibole isnot consistent with equilibrium fractionation betweenthese two minerals at any possible temperature Thisobservation suggests that parts of the ophiolite sequencehave undergone a high-temperature hydrothermal eventprior to the later low-temperature event that producedthe strong 18O increase in coexisting plagioclaseThe depletion of the d18O values results from high-temperature hydrothermal alteration (Gregory amp Taylor1981 Stakes amp Taylor 1992) The oxygen isotopic datasupport the contention that most studied samples havebeen affected by a VHT---HT hydrothermal alteration byfluids derived from seawater Some of the analysed sam-ples presenting petrological evidence of crystallization ofLT secondary minerals have been overprinted by the latelow-temperature alteration thus swamping the earlierhigh-temperature alteration effects
Determination of waterrock ratio
To better evaluate the exchange between seawater andthe gabbroic rocks waterrock ratios (WR) have beenestimated for the whole rocks analysed during the courseof this study The different models used to constrain thisratio mainly differ by the calculation of the effective ratioor by the estimated weight ratio and by considering openor closed systems for water circulation (Albareede et al1981 McCulloch et al 1981) The WR ratios reportedin Table 5 have been calculated assuming a closed sys-tem Using this formulation these values are minimabecause the calculation always uses pristine fluid for the
initial fluid thus maximizing the value in the denomina-tor If the fluid was shifted to lower 87Sr concentrations byexchange with the rock on the downward path theinferred values would be much higher (Davis et al 2003)The calculated WR ratios for the samples from this
study range from 31 to 219 (Table 5) Two main groupsof samples can be recognized on the basis of their waterrock ratios The lowest ratios ranging from 31 to 47have been determined for massive gabbros or anatecticgabbros but also for dykes and veins devoid of late LTalteration Similar ratios have been previously calculatedfor example for the discharged hydrothermal solutions atthe 13N and 21N East Pacific Rise (Di Donato et al1990 Fouquet et al 1991) The gabbros from the Ibrasection in the Oman ophiolite gave effective WR ratiosof 05 33 and 42 (McCulloch et al 1981) in goodagreement with the values obtained in this study Theleast altered gabbro of the Ibra section yields a WR ratioof 05 in good agreement with the exceptionally fresh-ness of this rock characterized by typical mantle oxygenvalues for its minerals (McCulloch et al 1981) Samplescharacterized by waterrock effective ratios in the rangeof 3---5 can be related to penetrative alteration via thehydrothermal system Lecuyer amp Reynard (1996) havedemonstrated in oceanic gabbros from the Hess Deepaltered in similar HT conditions that WR mass ratios inthe range of 02---1 are sufficient to account for the mod-ified stable isotope signature of the gabbros Higherwaterrock values (WR 45) have been calculated forsamples affected by superimposed late hydrothermalalteration and for the epidosite samples (Table 5) Theyprobably acquired their isotopic characteristics throughinteraction with a larger volume of seawater duringgreenschist-facies metamorphism Similar or higherWR values have been published in previous studies(ie McCulloch et al 1981 Kawahata et al 2001)They correspond either to samples from the upper-crus-tal sequence (ie basalt sheeted dyke or plagiogranite) orto gabbros from the crustal section located in a particularenvironment such as for example the Wadi Fizh sectionthat is located close to a segment boundary along thepalaeo-spreading axis (Nicolas et al 2000)
Mechanism for seawater interaction withmagma
Nicolas et al (2003) have proposed that variable amountsof seawater can circulate at high temperature through-out the crustal section down to the walls of the magmachamber via a network of parallel microcracks a fractionof a millimetre wide Such microcracks and their relation-ship with the VHT---HT hydrous alteration in the sur-rounding gabbros have been described in detail byNicolas et al Via this network of microcracks largeamounts of seawater can have reacted with the magma
JOURNAL OF PETROLOGY VOLUME 45 NUMBER 6 JUNE 2004
1204
and induced variable changes of its Sr and O isotopiccomposition This regular network of parallel micro-cracks could constitute the recharge system and thehydrated gabbroic dykes the discharge system Physicalparameters and the geophysical models of Nicolas et al(2003) are in agreement with this scenario Such a pene-tration of seawater-derived hydrothermal fluids throughthe lower crust along grain boundaries and a microscopicfracture network at temperatures 4750C is consistentwith recent thermodynamic models (McCollom amp Shock1998)Seawater-altered and high-temperature recrystallized
diabases (protodykes) from the overlying sheeted dykecomplex stoped into the melt lenses could represent anindirect way for seawater to enter the system as they canremelt during their subsidence through the magmachamber A simple mass balance calculation suggeststhat this indirect contamination is not significant Assess-ments of the amount of recycled crust 1 in volumeby Nicolas et al (2000) based on the protodyke remnantsin the gabbro section or 20 by Coogan et al (2003)based on chlorine content in EPR basalts lead to a sea-waterrock ratio of the order of 104 to 103Thus following Nicolas et al (2003) we propose that
seawater has been introduced along microcracks down tothe walls of the magma chamber and has diffused alonggrain boundaries in the way described by Manning et al(2000) progressively invading the host gabbro Gabbroicdykes result from the injection of hydrous melt into thehost gabbros at falling temperatures as they drift awayfrom the magma chamber This water-saturated meltcould result from the extensive hydration of the crystal-lizing gabbros near the walls of the magma chamberwhen seawater penetrates through this wall (Fig 1)Similar plumbing systems driving seawater down to the
limit of the magma chamber and back to the surface havebeen already proposed in other environments such as theMid-Atlantic Ridge (Kelley amp Delaney 1987 Cooganet al 2001) the East Pacific Rise at Hess Deep (Gillis1995 Manning et al 1996a 1996b) the Tonga forearc(Banerjee amp Gillis 2001) and the Troodos ophiolite(Gillis amp Roberts 1999) The melting curve of wet basalthas a negative slope but is close to the dry solidus at lowpressure in the lower gabbros and the first melts areplagiogranitic (Spulber ampRutherford 1983) Such plagio-granites are ubiquitous in the highest gabbros and in theroot zone of sheeted dykes in Oman (Rothery 1983Juteau et al 1988 Nicolas amp Boudier 1991) in manyother ophiolites and in the oceanic crust where they havebeen interpreted as resulting either from wet anatexis ofhot gabbros or from the final product of crystallization ofa basaltic melt (Spulber amp Rutherford 1983) Plagio-granites are rare in the lower gabbros exceptionallyassociated with hydrous gabbro segregations inside thelayered gabbros Their constitutive minerals are also
remarkably absent along the VHTmicrocracks althoughthese gabbros were above the wet solidus A possibleexplanation has been proposed by McCollom amp Shock(1998) who wrote that at high temperature lsquoaqueousfluids may leave very little conspicuous petrological evid-ence for their presencersquo the reason being that thehigh-T conditions would be closer to batch meltingObviously more detailed studies on this specific problemare needed We conclude that the large degree of hydrousmelting locally required at the walls of the magma cham-ber to account for the generation of a gabbroic hydrousmelt may have erased the traces of the incipient plagio-granitic melt
CONCLUSIONS
Fostered by the recent discovery of high-temperatureseawater contamination in some gabbros of the Omanophiolite (Manning et al 2000) we have conducted astudy on 500 gabbro specimens from selected areas ofthe entire Samail ophiolite belt and on the hydrous gab-broic dykes that cross-cut them The highest-temperaturereactions (VHT) recorded in olivine and clinopyroxene ingabbros as orthopyroxene and pargasite coronas reach975C They are followed at lower HT conditions withreplacement of olivine by an assemblage of tremolitemagnetite and plagioclase surrounded by chlorite andby pargasite development in clinopyroxene followedfinally by the LT lizardite---olivine and saussurite---plagioclase greenschist-facies alteration With a fewexceptions the VHT alteration tends to be restricted tothe lower gabbros and to the vicinity of the Moho andthe HT alteration to the middle and upper gabbros Thepreservation of the vertical distribution of VHT---HTfacies indicates that progressive cooling during off-axisspreading did not obliterate this distribution and conse-quently that the VHT---HT hydrothermal alteration tookplace in a very restricted time interval at the limit of thecooling magma chamber In the deep and hot gabbrosthe main water channels may be sub-millimetre-sizedmicrocracks with a dominantly vertical attitude the ori-gin and dynamics of which have been investigated in aprevious paper (Nicolas et al 2003)Beyond these high-temperature alteration zones mas-
sively induced in the gabbros seawater may be respons-ible for the generation of clinopyroxene---pargasitegabbro dykes that cross-cut all gabbros and that areupsection clearly connected to the LT hydrothermalvein system Petrostructural evidence suggests that thesedykes were generated by hydration of the crystallizinggabbros near the internal wall of the LVZ---magmachamber The resultant melts were subsequently intrudedat various levels in the cooling lower and middle gabbrosPetrological observations of nearly all the gabbros ana-lysed in the present study located from the root zone of
BOSCH et al HYDROTHERMAL CIRCULATION IN OMAN OPHIOLITE
1205
the sheeted dyke complex down to the Moho emphasizethe imprint of VHT---HT hydrous alteration The VHThydrothermal event is characterized by temperatures ofequilibration around 900---1000C The temperatures atwhich the HT hydrous event occurred are estimated tobe in the range 550---900CStrontium and oxygen isotope compositions measured
on whole rocks and separated minerals significantlydepart from primary MORB values The Sr and O iso-tope data obtained for pargasite green hornblende andclinopyroxene record the participation of seawater at thetemperature of crystallization of the minerals Plagio-clases and whole rocks define a trend toward a compo-nent characterized by a high radiogenic 87Sr86Sr valueand a higher Sr content than normal seawater Seawateris clearly identified as the fluid responsible for the modi-fication of the Sr and O signatures for both massivegabbros and dykes and veins Nevertheless the high Srcontent of the extraneous component requires eitheropen-system exchange allowing penetration of a greaterquantity of seawater or penetration of seawater modifiedby interaction with the oceanic crust during its travel paththrough the crustal section The waterrock ratio calcu-lated on the basis of the Sr isotopes is between three andfive for the enclosing gabbros and for the least LT altereddykes The VHT---HT hydrothermal event affectingthese samples is related to penetrative alteration via therecharge and discharge hydrothermal system Thewaterrock ratio is 10 for dykes exhibiting evidenceof a LT hydrothermal alteration overprint and reaches22 for the epidosite dyke The depletion in d18O char-acterizes all the studied samples with the single exceptionof the micro-gabbroic dyke plagioclase This agreeswith the conclusions based on Sr isotopes suggestingthat these samples have undergone exchange with sea-water-derived fluids during a VHT---HTalteration hydro-thermal event
ACKNOWLEDGEMENTS
This work has benefited from financial support by theCentre National de la Recherche Scientifique (INSUInteerieur-Terre andDYETI programmes) and inOmanfrom the willingness of the Directorate of MineralsMinistry of Commerce and Industry and the hospitabil-ity of the people Technical help in the laboratory is alsoacknowledged including C Nevado for the thin sectionsE Ball for the illustrations B Galland for her assistancein the chemistry room and C Bassoulet for help duringthermal ionization mass spectrometric analyses of the Srresults from the leaching experiments Constructivereviews from R T Gregory C Lecuyer an anonymousreviewer and particularly from M Wilson greatlyhelped to improve an earlier version of the manuscript
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Juteau T Manacrsquoh G Moreau C Lecuyer C amp Ramboz C
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cracking front at fast-spreading ridges evidence from the East Pacific
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349 261---272McCollom T M amp Shock E L (1998) Fluid---rock interactions in
the lower oceanic crust thermodynamic models of hydrothermal
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1207
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Geochemical Society Special Publication 3 77---90
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oxygen isotope microprobe and field study Journal of Geophysical
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altered oceanic crust DSDPODP sites 417418 Earth and Planetary
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New York McGraw---Hill 694 pp
Wilcock W S D amp Delaney J R (1996) Mid-ocean ridge
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tions for the nature of mantle metasomatism Earth and Planetary
Science Letters 91 327---340
JOURNAL OF PETROLOGY VOLUME 45 NUMBER 6 JUNE 2004
1208
(millimetre-sized) recrystallized clinopyroxene largelyaltered to light green magnesio-hornblende and idio-morphic plagioclase Locally primary black pargasitichornblende develops including idiomorphic normallyzoned plagioclase (Table 4) This gabbro exhibits similar87Sr86Sr ratios for the WR and pargasite (070351 and070355 respectively) These values are interpreted asrepresentative of the melt from which this amphibolecrystallized The clinopyroxene has a slightly higher Srisotope composition of 070378 consistent with the occur-rence of the light green Mg-hornblende rim Plagioclaseis significantly more radiogenic (070426) related to sec-ondary destabilization evidenced by the idiomorphichabit The Sr isotope compositions of the liquidsextracted from the acid leaching experiments of theWR (L1 and L2) are very similar 070358 and 070365respectively The 87Sr86Sr ratio of the residue obtainedafter leaching experiments is 070335 lower than boththe pargasite and unleached WR This value can beconsidered as the unaltered isotopic composition of thissample The d18O values measured for WR and coexist-ing plagioclase are lower than normal MORB magmaticvalues The 18O depletion in the plagioclase can be fairlywell explained by high-temperature hydrothermal inter-action between an oxygen-depleted fluid and the magmaAccording to the kinetics of hydrothermal exchange ofoxygen (Gregory amp Taylor 1981 Gregory et al 1989)plagioclase exchanges its oxygen isotopes with the fluidadjusting readily to the hydrothermal conditions Forhigh temperatures of exchange with seawater-derivedfluids (d18O between 0 and thorn2) the magmatic plagio-clase will have its primary d18O value lowered whereasthe d18O value for lower temperatures of exchange(5300C) will increase The amplification of the d18Oshift increases with the intensity of the hydrothermalalteration
Gabbroic dykes intrusive in lower gabbros from WadiWariyah Wadi Tayin Massif
Gabbroic dykes 01 OA19a and 01 OA19d are intrusiveinto the lower gabbros They are 3---4 cm wide discord-ant to the foliation of the host gabbros (Fig 7e) with asharp margin marked by a brown amphibole fringe Theprimary phases in the dykes (Table 1) locally preservedas elongated aggregates of a 01mm grain size mosaicassemblage at the dyke margins grade to millimetre sizein the central part of the dyke The WR major elementcomposition is Mg enriched compared with the enclosinggabbro (Table 3) Brown pargasitic amphibole oikocrystsmake up 50 of the modal composition of the dyke Indyke 01 OA19a poikilitic orthopyroxene may developinstead of brown amphibole Finally green magnesio-hornblende grows either as oikocrysts at the expense ofclinopyroxene or in association with epidote as an altera-tion product of plagioclase These low-amphibolite-faciesminerals represent the lowest-grade paragenesis recogn-ized in these dykes and are most developed in sample01OA19aTheamphibole composition showszoning fromTi-rich pargasitic hornblende to magnesio-hornblendeand actinolitic hornblende (Fig 8c) recording pro-gressive cooling of the dyke The plagioclase compositionis extremely heterogeneous (Fig 8b) varying from An95a composition similar to that of the plagioclase in the hostgabbro (Fig 8a) to An50 for the plagioclase and asso-ciated epidote inclusions in the poikilitic actinolitic horn-blende Sample 01 OA19d has an initial 87Sr86Sr ratioslightly higher for the WR (070341) than for the parga-site (070324) The Sr isotopic ratios of liquids extractedafter two steps of WR acid-leaching (L1 and L2) are070357 and 070341 respectively whereas the residueyields a 87Sr86Sr ratio of 070327 very close to thepargasite and plagioclase values The latter is similar tothe isotopic composition measured for the massive gab-bro 94 MA2 and may be taken as close to the primarymagmatic value This Sr isotopic ratio is more radiogenicthan the inferred Oman mantle-source value (McCullochet al 1981) The altered part of this sample has a radio-genic Sr isotopic composition (070418) related to thelow-amphibolite-facies alteration This gabbroic dykehas seemingly preserved a magmatic whole-rock d18Ovalue (567) but contains by far the highestd18O plagioclase (1009) of all the samples analysedThis plagioclase coexists with low d18O pargasite(thorn287) Similarly high values (d18Oplagioclase 4 8)have been previously measured for gabbroic dykes dia-bases sheeted dykes and plagiogranites from the Omanophiolite (Gregory amp Taylor 1981 Stakes amp Taylor1992) This 72 oxygen isotope fractionation betweenplagioclase and amphibole cannot possibly represent oxy-gen equilibrium at any plausible temperature This maybe explained by different exchange kinetics for these twominerals at different temperatures at high waterrock
Table 2 Average temperatures ( C 40C)
calculated for plagioclase---amphibole pairs
using the geothermometer of Holland amp
Blundy (1994)
Pargasite Magnesio-hornblende
rim core rim core
01 OA41d2 878 953 ------- -------
01 OA19a 935 974 825 869
01 OA19d 890 933 789 -------
02 OA37b ------- ------- 816 849
02 OA90b ------- ------- 859 833
JOURNAL OF PETROLOGY VOLUME 45 NUMBER 6 JUNE 2004
1192
ratios This supports the occurrence of two distincthydrothermal events one at high temperature and alater stage at lower temperature responsible for thestrong 18O enrichment in the coexisting plagioclase
Dioritic dyke associated with a diabase dyke in lowergabbros from Wadi Halhal Haylayn Massif
Sample 02 OA37b belongs to a group of brownamphibole---plagioclase lenses or reaction margins asso-ciated with diabase intrusives (Fig 7d) in lower layeredgabbros Idiomorphic brown hornblende (tschermakite)crystals are elongated in the plane of the dyke represent-ing 80 of the modal composition They are associatedwith an interstitial plagioclase matrix The browntschermakite has greenish rims Plagioclase has a cloudyappearance due to micrometre-size fluid inclusionsexcept along its margins and internal microcracks Bothhornblende and plagioclase exhibit evidence of plasticdeformation The Sr isotopic ratio for the pargasite andplagioclase is 070348 and 070420 respectively Thepargasite exhibits a Sr signature closest to that of theOman primary magmatic value (McCulloch et al 1981)Nevertheless it is too high to be attributed to anunmodified MORB-source mantle signature This sug-gests the addition of an external fluid component in goodagreement with the shift of oxygen isotopic compositionto a low value (thorn39) suggesting a high-temperatureexchange with a low d18O fluid The Sr isotope composi-tion of the plagioclase is significantly higher than that of
the coexisting pargasite probably as a result of the low-temperature alteration
Dioritic dyke cross-cutting lower layered gabbros in WadiAl-Abyad Nakhl Massif
Sample 02 OA90b is a 1 cm thick dioritic dyke cross-cutting the lower layered gabbros It grades into a greenamphibole vein as illustrated in Fig 6c The dyke is com-posed of 2 cm long dark green idiomorphic magnesio-hornblende crystals growing in the plane of the dykeand plagioclase concentrated at the margin of the dykeThe plagioclase in the dyke and in the enclosing gabbrocontains micrometre-size fluid and mineral inclusionssimilar to those observed in sample 94 MA2 The plagio-clase and green hornblende have higher Sr isotopic ratios(070387 and 070427 respectively) and lower d18Ovalues (thorn498 and thorn239 respectively) than the normalMORB-source mantle value As observed for previoussamples this supports interaction with an external fluidcomponent
Comb-textured gabbroic dykes in the lower gabbros fromWadi Wariyah Wadi Tayin Massif
Sample 01 OA15f (Fig 6b) is from Wadi Wariyah andrepresents a 2 cm wide dyke intruding the lower gabbrosThis type of comb-textured dyke developed in theclinopyroxene---plagioclase stability field It containslarge idiomorphic clinopyroxene crystals which arepartially replaced by brownish magnesio-hornblendeThe plagioclase is extremely calcic with a slight inverse
Table 3 Whole-rock major element compositions of massive gabbros and dykes determined by X-ray fluorescence
(wt )
Sample 94 Ma2 97 OA128b 01 OA41d2 01 OA19d 01 OA19a 01 OA15f 01 OA36b 01 OA36b
Rock type Gabbro Gabbro Gabbro patch Gabbroic dyke Gabbroic dyke Comb-textured
gabb dyke
Epidosite dyke Epidosite dyke
(margin)
SiO2 4830 4815 4749 4524 4339 4799 389 4371
Al2O3 2785 2032 2438 1474 1246 1192 2752 1646
Fe2O3 328 266 404 981 1204 592 381 623
MnO 003 004 005 014 016 006 008 014
MgO 405 737 427 1031 1686 1326 187 739
CaO 1289 1908 1477 1235 1156 166 2471 2339
Na2O 292 123 257 258 111 075 000 013
K2O 000 000 009 006 000 000 000 000
TiO2 005 017 063 230 064 086 013 042
P2O5 008 007 013 018 009 013 007 010
LOI 033 076 138 216 161 237 274 190
Total 9978 9985 998 9987 9992 9986 9983 9987
LOI loss on ignition gabb gabbroic
BOSCH et al HYDROTHERMAL CIRCULATION IN OMAN OPHIOLITE
1193
Table4Majorelementcomposition
aofselectedam
phibolespaired
withplagioclasecompositionsusedforthermometry
Sam
ple
nam
e1Mode2
SiO
2TiO
2Al 2O3
Cr 2O3
FeO
MnO
MgO
CaO
Na 2O
K2O
Total
Si
AlIV
AlVI
Ti
Fe3
thornCr
Mg
Fe2
thornMn
Na
KCa
XAn3
T(C)4
01OA19a
Mg-hornblende
core
45 60
26
10 78
009
860
015
16 56
12 00
212
007
96 72
6552
1448
0366
0083
0552
0004
3409
0594
0016
0611
0018
1843
0903
899
Mg-hornblende
core
47 99
263
850
002
897
011
16 61
12 10
160
020
96 36
6894
1106
0334
0028
0483
0002
3557
0594
0014
0446
0036
1862
0892
834
Mg-hornblende
rim
51 17
247
596
001
761
014
18 11
12 19
103
010
96 47
7257
0743
0253
0016
0414
0001
3828
0488
0016
0284
0018
1853
0894
820
Mg-hornblende
rim
47 50
291
920
004
880
014
16 46
11 95
174
008
96 07
6835
1165
0396
0015
0506
0005
3529
0552
0017
0486
0015
1843
0887
825
Mg-hornblende
rim
49 57
269
742
000
799
017
18 07
12 02
133
014
96 88
7013
0987
0261
0018
0575
0000
3811
0371
0020
0365
0026
1821
0860
831
Mg-hornblende
core
47 83
181
872
003
840
011
14 48
12 18
159
008
93 59
6814
1186
0279
0018
0644
0004
3711
0357
0013
0438
0015
1859
0907
874
Pargasite
rim
42 15
249
11 87
024
10 07
013
13 84
11 43
263
032
96 61
6188
1812
0242
0434
0299
0028
3028
0937
0016
0749
0059
1797
0804
972
Pargasite
rim
42 22
260
11 86
026
10 21
011
13 71
11 48
253
032
96 59
6201
1799
0254
0429
0299
0030
3001
0955
0014
0720
0060
1807
0803
962
Pargasite
core
42 05
263
11 45
030
10 19
011
13 80
11 47
246
032
96 46
6189
1811
0175
0478
0299
0035
3029
0956
0014
0702
0060
1809
0767
974
Pargasite
rim
42 36
247
11 90
016
945
013
14 16
11 76
249
026
95 98
6239
1761
0305
0366
0276
0019
3109
0888
0016
0710
0049
1856
0818
926
Pargasite
core
42 50
291
11 64
016
984
015
14 22
11 41
256
034
96 69
6219
1781
0227
0426
0322
0018
3103
0883
0018
0727
0063
1789
0812
974
Pargasite
rim
42 76
270
12 03
017
920
012
14 87
11 82
244
025
96 73
6224
1776
0880
0335
0368
0020
3226
0752
0015
0689
0047
1843
0841
941
Pargasite
rim
42 13
181
13 54
016
921
009
12 77
12 13
254
032
96 98
6172
1828
0510
0450
0000
0018
2788
1129
0011
0721
0059
1905
0841
874
01OA19d
Mg-hornblende
49 26
047
624
011
11 46
017
15 39
12 21
108
009
96 48
7137
0863
0202
0051
0437
0013
3323
0951
0021
0302
0018
1896
0768
771
Mg-hornblende
46 03
287
932
018
906
017
14 77
12 87
193
007
97 28
6672
1328
0265
0313
0046
0021
3191
1053
0021
0542
0012
1999
0758
808
Pargasite
41 89
429
11 65
008
11 03
017
13 52
11 37
259
017
96 77
6159
1841
0179
0474
0345
0010
2963
1012
0021
0739
0032
1791
0770
975
Pargasite
rim
42 80
340
11 16
009
11 01
018
13 73
11 67
240
022
96 67
6293
1707
0226
0376
0322
0010
3009
1032
0022
0684
0041
1839
0605
880
Pargasite
core
42 58
351
11 27
006
11 63
018
13 51
11 46
239
022
96 82
6257
1743
0209
0387
0391
0007
2959
1039
0023
0680
0042
1804
0595
893
Pargasite
rim
42 14
393
11 28
006
11 71
014
13 35
11 33
235
033
96 62
6214
1786
0174
0435
0391
0007
2935
1054
0018
0673
0062
1790
0519
901
Pargasite
core
41 56
420
12 05
006
11 57
016
12 51
11 35
249
027
96 24
6168
1832
0277
0469
0276
0007
2768
1160
0021
0718
0052
1805
0537
882
Pargasite
core
41 33
438
11 49
008
11 66
015
13 35
11 45
264
026
96 80
6105
1894
0107
0487
0368
0009
2941
1073
0019
0757
0048
1813
0537
942
Pargasite
41 08
446
12 39
012
11 35
017
13 13
11 36
263
015
96 85
6049
1951
0199
0494
0368
0014
2882
1030
0021
0752
0027
1792
0758
975
02OA37b
Mg-hornblende
rim
45 15
260
984
002
13 28
019
14 01
10 87
128
009
97 33
6527
1473
0203
0283
0690
0002
3018
0915
0023
0360
0016
1683
0562
828
Mg-hornblende
rim
44 72
263
988
004
13 23
021
14 01
10 78
133
009
96 92
6494
1506
0185
0287
0713
0005
3032
0894
0025
0374
0018
1677
0593
854
Mg-hornblende
rim
45 68
247
935
004
12 87
020
14 10
11 00
121
011
97 02
6621
1379
0218
0269
0598
0004
3045
0962
0024
0341
0020
1708
0562
815
Mg-hornblende
core
43 94
291
10 87
001
12 60
023
13 73
10 59
142
011
96 39
6410
1590
0278
0319
0644
0001
2985
0893
0028
0401
0021
1655
0710
860
Mg-hornblende
core
44 62
270
10 37
001
12 66
019
14 12
10 58
140
008
96 73
6479
1521
0253
0294
0644
0001
3057
0893
0023
0395
0014
1646
0674
851
Mg-hornblende
rim
47 79
181
796
002
12 09
025
15 22
11 09
100
009
97 33
6866
1134
0214
0196
0506
0002
3260
0947
0030
0280
0016
1706
0503
767
Mg-hornblende
core
45 43
249
10 3
000
11 99
022
14 50
11 14
133
008
97 48
6524
1476
0267
0269
0644
0000
3103
0796
0027
0370
0015
1715
0703
838
JOURNAL OF PETROLOGY VOLUME 45 NUMBER 6 JUNE 2004
1194
Sam
ple
nam
e1Mode2
SiO
2TiO
2Al 2O3
Cr 2O3
FeO
MnO
MgO
CaO
Na 2O
K2O
Total
Si
AlIV
AlVI
Ti
Fe3
thornCr
Mg
Fe2
thornMn
Na
KCa
XAn3
T(C)4
02OA90b
Mg-hornblende
core
48 83
038
745
005
939
010
17 29
12 24
142
006
97 21
6931
1069
0041
0041
0667
0005
3657
0448
0011
0392
0010
1862
0840
856
Mg-hornblende
core
49 49
033
731
003
916
008
17 40
12 57
139
005
97 82
6987
1013
0035
0035
0552
0004
3662
0530
0010
0379
0009
1901
0830
811
Mg-hornblende
rim
48 90
040
753
005
932
013
17 15
12 22
154
005
97 29
6948
1052
0042
0042
0575
0006
3632
0533
0015
0424
0010
1861
0880
869
Mg-hornblende
rim
48 30
044
800
007
962
010
16 86
12 32
161
005
97 35
6873
1127
0047
0047
0598
0008
3576
0546
0012
0443
0010
1879
0870
862
Mg-hornblende
rim
49 12
032
742
010
922
011
17 36
12 29
142
006
97 43
6956
1044
0034
0034
0621
0011
3665
0471
0013
0391
0010
1865
0840
845
02OA43a
Mg-hornblende
rim
48 13
040
789
007
860
007
17 41
12 19
155
006
96 36
6882
1118
0212
0043
0621
0008
3710
0407
0008
0429
0010
1867
-------
-------
Mg-hornblende
rim
50 93
035
643
001
759
006
18 50
12 24
114
003
97 28
7152
0848
0217
0037
0506
0001
3873
0385
0007
0311
0005
1841
-------
-------
Mg-hornblende
rim
50 67
014
559
009
12 33
026
15 28
12 44
068
003
97 50
7260
0740
0204
015
0483
0010
3263
0994
0032
0189
0005
1909
-------
-------
Mg-hornblende
rim
50 34
013
621
009
12 49
027
15 07
12 51
064
003
97 78
7192
0810
0240
0010
0530
0010
3210
0960
0030
0180
0010
1910
-------
-------
Mg-hornblende
rim
45 17
060
11 14
013
988
008
15 78
12 33
211
009
97 31
6473
1527
0355
0640
0644
0015
3371
0540
0010
0588
0016
1892
-------
-------
Mg-hornblende
rim
49 25
032
712
014
845
009
17 84
12 32
137
004
96 95
6983
1020
0170
0030
0620
0020
3770
0380
0010
0380
0010
1870
-------
-------
Mg-hornblende
rim
48 61
032
775
014
975
011
16 81
12 57
139
004
97 50
6905
1950
0203
0034
0621
0016
3560
0537
0013
0382
0008
1914
-------
-------
94Ma2
Anthophyllite
54 98
000
259
001
12 65
037
23 35
199
034
000
96 28
7744
0256
0174
0000
0161
0001
4903
1330
0043
0092
0001
0301
-------
-------
Anthophyllite
55 11
000
296
003
14 09
032
23 66
063
038
000
97 18
7716
0284
0205
0000
0115
0003
4938
1535
0038
0104
0000
0095
-------
-------
Actinolite
48 58
004
954
002
960
014
17 67
999
139
005
97 02
6850
0150
0435
0004
0690
0002
3713
0442
0017
0379
0009
1510
-------
-------
Actinolite
51 54
004
585
000
890
022
20 28
930
090
002
97 05
7214
0786
0178
0005
0598
0000
4230
0444
0026
0244
0004
1395
-------
-------
01OA41d2
Pargasite
rim
43 05
342
10 71
008
11 46
012
13 50
11 54
179
067
96 34
6353
1647
0216
0380
0368
0009
2967
1046
0015
0512
0127
1824
0613
863
Pargasite
rim
42 61
354
11 59
001
10 59
010
13 52
11 87
160
076
96 19
6286
1714
0302
0392
0299
0001
2972
1007
0012
0458
0143
1877
0715
844
Pargasite
core
42 54
401
11 04
006
11 81
013
13 21
11 50
202
039
96 71
6266
1734
0182
0444
0368
0007
2901
1087
0017
0576
0073
1814
0854
979
Pargasite
rim
43 03
385
10 72
002
11 70
014
13 50
11 95
146
080
97 17
6314
1686
0168
0425
0345
0002
2953
1091
0018
0415
0149
1878
0634
852
Pargasite
rim
42 65
341
10 60
006
11 97
014
13 22
11 64
183
068
96 20
6332
1668
0186
0381
0345
0007
2926
1141
0018
0528
0129
1851
0620
869
Pargasite
core
42 82
367
10 60
007
11 93
012
13 42
11 44
206
047
96 60
6315
1685
0158
0407
0391
0008
2950
1080
0016
0588
0089
1808
0615
899
Pargasite
rim
42 2
362
11 10
007
11 42
016
13 09
11 72
179
072
95 89
6282
1718
0229
0405
0299
0009
2905
1123
0020
0518
0137
1869
0715
875
Pargasite
rim
42 42
380
11 03
006
11 77
012
13 00
11 72
188
069
96 49
6283
1717
0209
0423
0299
0007
2870
1158
0015
0539
0131
1860
0817
928
Pargasite
rim
42 21
364
11 14
006
10 73
011
13 53
11 68
177
072
95 59
6277
1723
0229
0407
0322
0007
2999
1012
0014
0511
0136
1862
0736
883
Pargasite
rim
42 25
376
11 03
008
10 93
015
13 66
11 72
185
064
96 07
6257
1743
0182
0418
0345
0009
3016
1009
0019
0531
0121
1860
0766
912
Pargasite
core
42 17
396
10 91
008
11 10
010
13 57
11 49
209
038
95 85
6253
1747
0159
0442
0368
0010
3000
1009
0013
0602
0073
1826
0841
981
aMajorelem
entco
mpositionsarein
weightpercent-------noplagioclasean
alysed
andnotemperature
calculated
1Typ
eofam
phibolean
alysed
isalso
indicated
2Blankindicates
nozoningobserved
3Molefractionofan
orthitein
plagioclasead
jacentto
amphibolegrain
4Tem
perature
ofeq
uilibrium
fortheam
phibole---plagioclasepaircalculatedusingtheHollandamp
Blundythermometer
(1994)Errors
are40
C
BOSCH et al HYDROTHERMAL CIRCULATION IN OMAN OPHIOLITE
1195
zonation from An95 to An99 from core to rim (Table 1 andFig 8a) TheWR Sr isotopic ratio of the sample (070458)is significantly higher than normal MORB values Thiscould be related to the overprint of a low-temperaturealteration event consistent with the petrography of thesample
Green amphibole vein cross-cutting layered gabbros inWadi Mayah Haylayn Massif
Sample 02 OA43a belongs to a series of 1 cm thick greenamphibole veins (as in Fig 6e) cross-cutting the layeredgabbros The veins develop 15 cm thick reaction marginsin the enclosing gabbro and are exclusively composed ofacicular pale green magnesio-hornblende growing per-pendicular to the vein margin in a crack-seal mode(Fig 6f ) The reaction margins are composed of urali-tized gabbro marked by the development of the samepale green magnesio-hornblende in an inhomogeneouscalcic plagioclase matrix (An91---97) The Sr and O isotopiccompositions of this green hornblende (070431 andthorn31) do not correspond to normal MORB-sourcemantle values
Epidosite dyke from Wadi Gideah Wadi Tayin Massif
Sample 01 OA36b is representative of a series of epidoteveins cross-cutting poorly layered gabbros These 2 cmthick veins are composed of pink thulite in their centreand green pistachite at their margins (Fig 6a) Theirsharp contact with the enclosing gabbro is marked bythe development of coarse clinopyroxene crystals alteredin the epidote---greenschist facies This suggests that theepidote vein is replacive in a coarse-grained gabbro dykeThe highest 87Sr86Sr initial ratios of all the samplesstudied are recorded by the WR and coexisting epidoteof this sample (070519 and 070554 respectively)(Table 5) The Sr content of the WR and associatedepidote are also the highest of the present dataset(716 ppm and 764 ppm respectively) The Sr isotopiccomposition of this sample is consistent with a greaterdegree of exchange with a radiogenic component fromCretaceous seawater It is significantly higher than thevalue obtained for an epidosite vein from the Wadi Fizhsection of the Oman ophiolite [070435 with a Sr contentof 488 ppm reported by Kawahata et al (2001)] but ingood agreement with the average of the greenschist-faciesalteration sequence defined by Kawahata et al (2001)
GABBROS
01 km 025 km 1 km 36 km50
60
70
80
90
An
MA2
19d
19a 15f 128b
41d2
(a)
43a17b
15f
90b
37b
19a
19d
40
60
80
100
A
n
01 km 015 km 025 km 1 km
DYKES
(b)
Distance to the Moho
ACTINOLITE
MAGNESIOHORNBLENDE
TSCHERMAKITE
PARGASITEGabbros Dykes41d2MA2
19a d37b15f
90b43a
AlIV
150500
02
08
10
(Na
+ K
) A
(c)
04
06
950
900
850
800
750
70007 12 17
T degC
1000
(d)
AlIV
Fig 8 Plagioclase---amphibole compositions and results of thermometry (a) and (b) plagioclase compositions in layered gabbros and in dykesrespectively as a function of distance to the Moho (see text for sample identification) (c) amphibole compositions in layered gabbros and dykesaccording to Leakersquos (1997) nomenclature (d) equilibration temperatures calculated using the amphibole---plagioclase thermometer of Holland ampBlundy (1994) as a function of AlIV content in amphibole [same symbols as in (c)]
JOURNAL OF PETROLOGY VOLUME 45 NUMBER 6 JUNE 2004
1196
Table5RbandSrcontents87Sr
86Srandd1
8Oisotopiccompositionsofwholerocksandmineralseparatesfrom
samplesofmassivegabbrosdykes
andveinsfrom
theOman
ophiolitealsolistedarecalculated
waterrockratios(W
R)andLOIvalues
Sam
ple
Rb(ppm)
Sr(ppm)
87Rb8
6Sr
87Sr
86Sr
(87Sr
86Sr)i1
d18O
()
LOI(
)2WR
3(87Sr
86Sr)L14
(87Sr
86Sr)L25
(87Sr
86Sr)R6
01OA41d2
Whole
rock
037
1792
0006
0703516
09
070351
thorn480
138
47
0703587
0703651
0703357
Plagioclase
025
1249
0003
0704261
22
070426
thorn504
-------
-------
Pargasite
-------
-------
-------
0703548
20
070355
-------
-------
-------
Clinopyroxene
045
26 0
0050
0703845
19
070378
-------
-------
-------
01OA36b
Dykewhole
rock
0005
7164
50001
0705186
17
070519
-------
274
21 9
0705207
0705112
-------
Epidote
0003
7645
0001
0705536
12
070554
-------
-------
-------
Wallwhole
rock
005
4254
0001
0704637
09
070464
-------
191
-------
0704945
0704675
0704593
02OA43a
Green
hornblende
004
98
0012
0704323
13
070431
thorn310
-------
-------
97OA128b
Whole
rock
003
1274
50001
0703327
17
070333
-------
076
37
-------
-------
-------
Plagioclase
002
1204
50001
0703285
09
070328
thorn414
-------
-------
Clinopyroxene
003
20 0
0004
0704230
09
070422
thorn523
-------
-------
01OA15f
Whole
rock
006
1077
0002
0704582
16
070458
-------
237
13 5
-------
-------
-------
01OA19a
Whole
rock
037
1303
0008
0704189
18
070418
-------
161
96
-------
-------
-------
01OA19d
Whole
rock
058
2032
0008
0703423
08
070341
thorn567
216
415
0703574
0703408
0703271
Pargasite
091
1058
0025
0703273
11
070324
thorn287
-------
-------
Plagioclase
-------
-------
-------
-------
-------
thorn10 09
-------
-------
02OA90b
Plagioclase
004
2385
50001
0703868
11
070387
thorn498
-------
-------
Green
hornblende
010
23 8
0012
0704288
15
070427
thorn239
-------
-------
BOSCH et al HYDROTHERMAL CIRCULATION IN OMAN OPHIOLITE
1197
Table5continued
Sam
ple
Rb(ppm)
Sr(ppm)
87Rb8
6Sr
87Sr
86Sr
(87Sr
86Sr)i1
d18O
()
LOI(
)2WR
3(87Sr
86Sr)L14
(87Sr
86Sr)L25
(87Sr
86Sr)R6
02OA37b
Plagioclase
023
3121
0002
0704204
15
070420
-------
-------
-------
Pargasite
031
41 6
0021
0703505
15
070348
thorn394
-------
-------
94OAMa2
Whole
rock
0016
1750
00003
0703219
15
070322
thorn495
033
31
0703264
0703177
0703158
Plagioclase
0014
1363
00003
0703219
11
070322
thorn479
-------
-------
Clinopyroxene
-------
-------
-------
0703236
13
070324
-------
-------
-------
1Initial8
7Sr
86Srratiocalculatedat
95Ma(H
acker1994)
2LOIisloss
onignition(in)
3Water---rock
ratiocalculatedassumingaclosedsystem
TheeffectiveWR
ratiocanbeexpressed
bythefollowingeq
uation
W=Reffectivefrac14
frac12eth87Sr=
86SrrockTHORN fi
nal
eth87Sr=
86SrrockTHORN in
itial=frac12eth8
7Sr=
86SrseawaterTHORN in
itial
eth87Sr=
86SrrockTHORN fi
nal
ethCrock
initial=C
seaw
ater
initial
THORNwhere(87Sr
86Srrock) finalisthefinalmeasuredSrisotopicratioin
thesample(87Sr
86Srrock) in
itialco
rrespondingto
theinitialm
antlevalue
istakenas
070295(Lan
phere
etal1981)(87Sr
86Srseawater) in
itialistheCretaceousseaw
ater
Srisotopicco
mposition[87Sr
86Sr07073
at90
MaafterBurkeet
al(1982)]C
seaw
ater
initial
istheSrco
ntent
oflsquonorm
alrsquoseaw
ater
andis
takenas
8ppm
(deVilliers1999)
FortheCrock
initialitis
assumed
that
thepresentSrco
ncentrationmeasuredis
thesameas
theinitial
concentration
4Srisotopicratiosmeasuredforliquid
L1extractedafterthefirststep
oftheleachingexperim
ent(6NHClfor8min
at70
C)
5Srisotopicratiosmeasuredforliquid
L2extractedaftertheseco
ndstep
oftheleachingexperim
ent(2 5NHClfor30
min
at70
C)
6Srisotopicratiosmeasuredforresidueobtained
afteratotalaciddigestionachievedaftertheextractionofL1an
dL2leachates
JOURNAL OF PETROLOGY VOLUME 45 NUMBER 6 JUNE 2004
1198
Part of the same sample from the contact between thedyke and the gabbro yields a slightly lower Sr isotopiccomposition (070464) intermediate between the sur-rounding gabbro and the epidote dyke It also has anintermediate Sr content (425 ppm) Acid leaching experi-ments performed on the whole rock of the dyke yield Srisotopic compositions of 070521 and 070512 for thetwo liquids extracted Similar experiments for the samplelocated at the contact with the gabbro yield 87Sr86Srratios of 070494 and 070467 for L1 and L2 liquids TheSr isotopic composition of the residue extracted afterleaching remains high (070459) and very close to theepidosite 87Sr86Sr ratio determined by Kawahata et al(2001) In this sample the primary minerals have beentotally replaced by epidote and secondary plagioclase orquartz Thus the Sr isotopic compositions measured forthis WR sample and its constituent minerals reflect thecomposition of the fluid filling this dyke
Mineral chemistry
Plagioclase
Compared with the host gabbros the dykes show a muchlarger dispersion in their plagioclase compositions (Fig 8aand b Table 4) characterized by more calcic plagioclasethan the enclosing gabbros This tendency is mostextreme in the comb-textured dykes in which the plagio-clase is almost pure anorthite The zoning is generallynormal recording a crystal fractionation process How-ever gabbro 94 MA2 exhibits inverse zoning this tend-ency is also observed in plagioclases from dykes 01OA19a and 01 OA15f The higher calcium content ofthe plagioclase as well as the inverse zoning is consistentwith an increase of water pressure during its crystalliza-tion (Turner amp Verhoogen 1960 Carmichael et al 1974Koepke et al 2003) The particularly high An content inthe comb-textured gabbro dykes 01 OA15f may alsoresult from crystallization in a supercooled magma thisis consistent with the comb texture indicative of fast-growing crystals The lack of brown hornblende suggeststhat crystallization occurred above the temperature ofhornblende stability
Amphibole
The amphiboles analysed in the dykes (Fig 8c andTable 4) show a regular trend in a (Na thorn K)A vsAlIV diagram from titanium-rich (Ti 4 015) pargasitichornblende (brownamphibole) and tschermakite (brown---green amphibole) to low-titanium (Ti5 015) magnesio-hornblende (green amphibole) and actinolite (pale greenamphibole) The pargasitic hornblende develops aspoikilitic oikocrysts in the dykes 01 OA19a and d andin the diffuse patch 01 OA41d2 at temperatures above800C during the final stage of crystallization (see
below) The tschermakite composition corresponds tothe internal alteration of clinopyroxene in gabbro 94Ma2 occurs in the comb-textured dyke 01 OA15f andis the primary amphibole in the dioritic dyke 02 OA37bThe magnesio-hornblende develops at the edges of thepargasites or of clinopyroxenes at temperatures around800C and is the primary phase in the dioritic dyke 02OA90b and in the green vein 02 OA43a Finally actino-lite occurs as a replacement of green hornblende in dykesor in the kelyphitic rim surrounding olivine in gabbro 94Ma2 Such a large composition range at the scale of athin section has been reported in amphiboles fromamphibolite-facies oceanic gabbros (Stakes 1991 Stakesamp Taylor 1992 Gillis et al 1993) as well as in the Omanophiolite gabbros (Manning et al 2000) This variabilityhas been explained by rapid reaction rates preventinghomogenization (Manning et al 2000)
Thermometry
Plagioclase---amphibole thermometry could be appliedwhen the two minerals exhibited good contacts such asin the laminated gabbro of the middle sequence (01OA41d2) and in the intrusive dykes The temperaturesof plagioclase---amphibole equilibration (Table 2) havebeen calculated using the geothermometer lsquoBrsquo of Hollandamp Blundy (1994) Edenite thorn Albite frac14 Richterite thornAnorthite valid between 500C and 900C with anuncertainty of 40C Amphibole formulae are basedon 23 oxygen and their nomenclature is based on alkalications in the A site vs AlIV according to Leake (1997)Pressurewas assumed to be 1 kbar Forty-five plagioclase---amphibole pairs meet the compositional criteria imposedby Holland amp Blundyrsquos dataset (09 4 Xan 4 01 andAlIV 403) Electron microprobe measurements wereconstrained to contiguous plagioclase and amphiboleseparated by sharp grain boundaries assuming equili-brium of the two phases When the mineral phasesexhibit normal zoning temperatures between plagioclaseand amphibole cores and between mineral rims werecalculated assuming that the temperatures deduced fromthe mineral core chemistry provide a proxy for the high-est temperature of equilibration These data are identi-fied in the T C vs AlIV diagram (Fig 8d) Table 4presents selected major element amphibole analyses theAn content of the plagioclase adjacent to each amphibolegrain and the temperature calculated for the pairThe temperatures calculated (Fig 8d) span the entire
range of temperatures at which amphibole and plagio-clase coexist in equilibrium (ie amphibolite-facies para-geneses) This temperature interval is bracketed byorthopyroxene-bearing facies or amphibole-free facies(ie gabbro 94MA2) and epidote---actinolite facies devoidof homogeneous plagioclase (ie epidosite dyke 01OA36b and green vein 02 OA43a) The range of the
BOSCH et al HYDROTHERMAL CIRCULATION IN OMAN OPHIOLITE
1199
calculated temperatures between 4900C and 750Ccorresponds to the changing composition of amphibolesfrom pargasite to magnesio-hornblende at the scale of athin section or even at that of an individual crystal asshown by their correlation with AlIV such variabilityrecords rapid cooling in hydrous conditions The highesttemperatures are recorded by pargasite from the anatec-tic gabbro 01 OA41d2 in the gabbro dyke 01 OA19dand in the dioritic dyke 02 OA90b They were calculatedfrom minerals devoid of lower-grade alteration and arethus considered as representing the equilibrium temperat-ure of the coexisting minerals Lower temperatures cor-respond to mineral phases evolving at falling temperatureand are only indicative of the cooling history Includingthe 40C uncertainty seven pairs provide temperatureshigher than 900C the limit of validity of the Holland ampBlundy thermometer However we maintain these tem-peratures as indicative of the highest values of our data-set These values are included in the average equilibriumtemperature summarized in Table 2
DISCUSSION
Distribution of hydrothermal alteration
Two distinct petrological events are recorded in the gab-bro section of the Samail ophiolite and are documentedin the present contribution (1) the whole gabbro sectionis affected by a penetrative alteration (2) the gabbrosection is crosscut by various types of dykes and veinsincluding hydrous minerals
Penetrative alteration
The penetrative alteration developed in the gabbros atgrain boundaries and locally invading the minerals isubiquitous in the gabbro section This penetrative altera-tion has been described as very high temperature (VHT)high temperature (HT) and low temperature (LT) basedon alteration parageneses The orthopyroxene coronas inthe layered gabbro sample and the incipient appearanceof brown pargasitic amphibole mark a lower thermallimit of the VHT alteration By reference to the experi-mental work of Koepke et al (2003) the temperature ofequilibration for these reactions is estimated to bebetween 900 and 1000CThe preservation of the vertical distribution of
VHT---HT facies (Figs 4 and 5) also pointed out byManning et al (2000) indicates that the hydrothermalalteration pattern fits the distribution of isotherms withdepth at a fast-spreading ridge (ie Phipps Morgan ampChen 1993 Dunn et al 2000 Fig 2) Equilibrium tem-peratures for VHT parageneses as high as 900---1000Csuggest that VHT---HT hydrothermal alteration tookplace in a very restricted time interval at the limit ofthe cooling magma chamber It is proposed that the
penetrative alteration affecting the entire gabbro sectionis related to a pervasive network of microcracks whichact as a recharge system down to the Moho (Nicolas et al2003)
Dykes and veins
The dykes are extremely varied eg pegmatitic gabbroswith comb-texture microgabbros and amphibolitizeddolerites dioritic dykes alignments of poikilitic horn-blende and finally diffuse hornblende-bearing patchesHigh-T gabbro and diorite dykes and low-T greenamphibole---epidote veins are connectedIn the dykes textural evidence attests to suprasolidus
conditions at least for the development of the pargasiticamphibole Integrating the hornblende---plagioclase equi-libration temperatures calculated for the studied samplesthese temperatures are bracketed between 950C and875CIn the following discussion we shall consider separately
the microgabbro and dolerite dykes The latter representbasaltic melt intrusions associated upsection with thediabase sheeted dykes and downsection possibly withthe mafic dykes that are ubiquitous in the mantle section(Nicolas et al 2000) Whatever their origin the develop-ment of pargasitic amphibole during their late stage ofcrystallization creates a link with the gabbroic dykes andsuggests a crystallization process evolving from dry con-ditions to more hydrous conditionsThe other gabbroic dykes comb-textured gabbros and
hornblende-bearing dioritic dykes result from the fillingof cracks by a fluid either a melt or a silica-rich hydrousfluid These intrusive dykes which develop reaction mar-gins at their contact with the host gabbro grade verticallyinto lower-temperature green amphibole veins or align-ments of poikilitic hornblende in the layered gabbromatrix or development of pargasitic hornblende in ana-tectic gabbros (sample 01 OA41d2) In gabbros crystal-lizing in the magma chamber the development oforthopyroxene and pargasite oikocrysts enclosing the pri-mary minerals suggests a thermal evolution from thegabbro solidus to amphibolite-facies conditions Forthese reasons it is suggested that the gabbroic dykesresulted from the injection of a hydrous melt in the stillhot gabbros drifting away from the magma chamber asdiscussed below
Origin of fluids responsible for VHT---HThydrothermal alteration
The 87Sr86Sr initial ratios and d18O of whole rocks andpure mineral fractions are plotted in Fig 9 against thedistance above the Moho Transition Zone (MTZ) Theinitial Sr isotopic composition of the whole rocks apartfrom the epidosite dykes ranges from 070322 to
JOURNAL OF PETROLOGY VOLUME 45 NUMBER 6 JUNE 2004
1200
070458 All the studied samples (including whole rocksand minerals) have initial 87Sr86Sr ratios falling outsidethe field of fresh MORB which commonly have Sr ratiosbetween 07023 and 07029 with an average of 070265(ie Hart et al 1974) The lowest initial Sr isotopiccomposition (070322) from the massive gabbro 94MA2 is slightly but significantly higher than averageOman primary magmatic signatures (ie 070296McCulloch et al 1981) Similarly the d18O values ofmost whole rocks and minerals measured (Table 5)
depart from typical MORB-source mantle values( Javoy 1980 Gregory amp Taylor 1981 Kyser 1986)These differences indicate that the primary mantlesignatures in the Oman ophiolite have been modifiedby interaction with a fluid Possible origins for thisfluid are mantle components dehydration of subductedlithosphere or seawater circulation Strontium andoxygen isotopes are useful tracers for fluid---crust interac-tion as d18O and 87Sr86Sr ratios from fluids originat-ing from seawater the mantle or subducted lithosphere
-1000
1000
2000
3000
4000
5000
6000
7000
MOHO
MTZ
Dis
tan
ce a
bov
e th
e M
oh
o (
m)
pillow-basalt
sheeted-dike
gabbro
peridotite
01 OA 41d2
01 OA 36b
02 OA 43a97 OA 128b
01 OA15f
94 OA MA2
02 OA 90b01 OA 19ad
02 OA 37b
0702 0703 0704 0705 0706 0707
87Sr86Srinitial
MO
RB-s
ou
rce
Seaw
ater
Whole RockPlagioclasePargasiteGreen HornblendeClinopyroxene
Epidote
(a)
-1000
1000
2000
3000
4000
5000
6000
7000
MOHO
MTZ
Dis
tan
ce a
bov
e th
e M
oh
o (
m)
pillow-basalt
sheeted-dike
gabbro
peridotite
01 OA 41d2
01 OA 36b
02 OA 43a97 OA 128b
01 OA15f
94 OA MA2
02 OA 90b01 OA 19ad
02 OA 37b
Whole RockPlagioclasePargasiteGreen HornblendeClinopyroxene
3 5 10
δ18O (permil)
41d2
Ma2
43a
90b 19d 19d37b
128b
90b
(b)
Fig 9 (a) Variation of initial 87Sr86Sr isotopic composition as a function of the location of the studied samples in a schematic log of the crustalsection of the Oman ophiolite The field for MORB is from Hart et al (1974) and that for seawater is from Burke et al (1982) Small open squares aredata fromKawahata et al (2001) (b) Variation of d18O () as a function of the location of the studied samples in a schematic log of the crustal sectionof the Oman ophiolite Shaded curve corresponds to the data of Gregory amp Taylor (1981)
BOSCH et al HYDROTHERMAL CIRCULATION IN OMAN OPHIOLITE
1201
are significantly different Moreover the d18O valueis dependent on temperature and mineral fractiona-tion and thus yields complementary constraints to Srisotopes
Mantle origin
One model to explain the occurrence of fluids interactingwith the oceanic crust at very high temperatures is toderive them from the mantle The contribution of mantle-derived hydrous fluids linked to percolation processesor to their concentration in the final stages of gabbrocrystallization has been invoked in numerous case studies(ie Witt amp Seck 1989 Fabriees et al 1989 Dupuy et al1991 Agrinier et al 1993) O and Sr isotopic data forwhole rocks and mineral separates yield scattered valuesthat are unambiguously different from MORB mantle-source values (Fig 9a and b) With the exception of lateLT altered samples the WR data have a mean 87Sr86Srratio of 070337 and a standard deviation of 000012These surprisingly high values in mantle-derived rockscould be taken as evidence for survival of mantle isotopicheterogeneities during crystallization of the ophiolite (egLanphere 1981 McCulloch et al 1981) However the18O16O isotope ratios in the Oman gabbroic sequenceare also displaced from primary magmatic values Nogabbro in the present study has plagioclase or amphi-bole with typical mantle d18O values Thus the Sr and Oisotope data rule out the possibility of mantle-derivedfluids as a possible source for the VHT---HT hydrousalteration event recorded in the massive gabbros andgabbroic dykes and veins (Fig 10a and b)
Subducted lithosphere origin
Dehydration of subducted oceanic crust in subductionzones can generate hydrous fluids Such fluids couldpercolate through the overlying lithosphere and contam-inate it thus generating modified isotopic signatures andtrace element contents Such an explanation howeverdisagrees with the geochemical characteristics of most ofthe studied samples Fluids derived from the dehydrationof subducted slabs (fresh or altered MORB or OIB pro-toliths) should be strongly enriched in K Rb and Ba (egBecker et al 2000) whereas in Oman the Rb contentsmeasured in the gabbros and gabbroic dykes and veins arestrongly depleted Moreover the isotopic composition ofslab-derived fluids is buffered by the MORB-like oceaniccrustal protolith for Nd and Pb but not for Sr and O(Staudigel et al 1995) As a result of relatively minorfractionation of oxygen at high temperature the oxygenisotope composition of the fluids expelled during dehy-dration of the subducted slab should be high and essen-tially controlled by the oxygen composition of the uppersection of the crust altered at low temperature (with an
average of 10 and perhaps as high as 19) (Staudigelet al 1995) The Sr isotopic composition should be alsohigh (average 07046) as the fluids released by the dehy-dration of subducted oceanic crust are associated eitherwith seawater or with radiogenic Sr phases (ie smectites)The Sr and O isotopic compositions of the studied sam-ples do not fit with these signatures and so preclude asubducted lithosphere origin for the fluids involved in theVHT---HT alteration event
Seawater-derived fluids
All the analysed samples (minerals and WR) have 87Sr86Sr(T) outside the domain of primary MORB magmasand variable Sr contents Individual minerals have Srcontents reflecting their different mineralliquid partitioncoefficients Minerals characterized by a very high affi-nity for Sr such as plagioclase (ie Sr mineralliquidpartition coefficients 1) have very high Sr contentsTherefore the high abundance of plagioclase in the sam-ples analysed is responsible for the high Sr content of thestudied whole rocks The Sr content of all the whole rocksis relatively homogeneous (Table 5) without clear distinc-tion between country-rock gabbros and dykes and veinsand ranges from 110 to 200 ppm except for the epidosite(4700 ppm) Reported on a 87Sr86Sr vs 1Sr diagram(Fig 10a) most whole rocks and plagioclases analysed arecharacterized by a 1Sr ratio lower than 001 and plot inthe left part of this diagram Compared with N-MORBthe studied massive and anatectic gabbros and their con-stituent plagioclases have similar to slightly higher Srcontents but substantially higher 87Sr86Sr ratios(Fig 10a) Considering Cretaceous seawater as a possiblehigh 87Sr86Sr mixing component [87Sr86Sr 07073at 90Ma after Burke et al (1982)] responsible for theobserved geochemical modifications exchanges cannothave occurred in a closed system as seawater has too lowa Sr content (ie 8 ppm de Villiers 1999) An open-system process allowing penetration of larger quantitiesof seawater can efficiently modify the Sr isotopic ratio ofthe rocks Alternately the fluids could also be seawaterthat was modified through exchange with the surround-ing rocks during its transit through the oceanic crustalsection This modified seawater would be more enrichedin Srwith a 87Sr86Sr ratio intermediate between unmodi-fied seawater and MORBMinerals devoid of late LT alteration imprints (parga-
site green hornblende and pyroxene) and characterizedby very low Sr contents plot on the right of the diagramclose to or on the mixing line between seawater andMORB (Fig 10a) The difference between these mineralsand the other samples (WR and plagioclases) is mainlydue to the Sr fractionation coefficients of these differentminerals and to the large quantity of plagioclase inthe studied rocks The process responsible for the
JOURNAL OF PETROLOGY VOLUME 45 NUMBER 6 JUNE 2004
1202
modification of the Sr isotopic composition fromMORB-source value is however explained by the same phenom-enon As all these minerals display initial 87Sr86Sr ratiosdistinct from the MORB source and because they equili-brated at VHT or HT temperatures this supports anearly intervention of seawater during crystallization ofthese minerals Most of these samples with the exceptionof the epidosite dyke overlap the domains defined byKawahata et al (2001) for the Wadi Fizh gabbros alteredat VHT and HT conditions in the amphibolite facies(labelled lsquoVHTrsquo and lsquoHTrsquo in Fig 9a)Three samples (two epidosite dykes and an one epidote
fraction) have very high Sr contents and the highest Sr
isotopic compositions of the present dataset The twowhole-rock samples overlap the field of the Wadi Fizhsamples altered at high temperature during greenschist-facies metamorphism (Kawahata et al 2001) This typeof alteration is common at the ridge axis and formedthrough intensive exchange with seawater-derived fluidsFigure 10b indicates that all the Sr---O isotope data are
distinct from MORB compositions and suggests that thefluids reacting with the magmas could be derived fromseawater In addition the d18O values show that isotopicexchange occurred under VHT or HT conditions Thefluids could have the composition of seawater or theycould have the composition of seawater modified through
0703
0705
0707
001 01 1
1 Sr (ppm)
MORB
(87Sr
86Sr)
init
ial
Seawater
VHT
HT
MT36b
36b
36b
43a 128b90b
41d2
37b19d
15f
128b
41d2
41d2
19a
Ma2Ma2
19d
90b
37b
Whole RockPlagioclasePargasiteGreen HornblendeClinopyroxene
Epidote
(a)
0 2 4 6 8
δ18O(permil)
0703
0705
0707Seawater
MORB-mantle
128b 41d2
90b 41d2
Ma2128b
37b
43a
19d
90b
Whole RockPlagioclasePargasiteGreen HornblendeClinopyroxene
Epidote
HT LT
19d
(87Sr
86Sr)
init
ial
(b)
Fig 10 (a) Initial 87Sr86Sr isotopic ratios plotted against 1Sr for whole rocks and mineral separates from the gabbro section of the Omanophiolite Fields shown are from Kawahata et al (2001) and correspond to MT-----basalt sheeted dyke diabase altered at medium temperature(prehnite---actinolite facies sequence 3) HT-----diabase plagiogranite metagabbro and epidosite formed at high temperature (greenschist faciessequence 4) VHT-----gabbro altered at very high temperature (amphibolite facies sequence 5) The dashed line is a binary mixing line betweenMORB (Lanphere et al 1981) and Cretaceous seawater (Burke et al 1982) (b) Initial 87Sr86Sr isotopic composition plotted against d18O value forwhole rocks and minerals from the Oman ophiolite Cretaceous seawater and MORB end-members as in (a) Dashed line represents mixing curvebetweenMORB and seawater at high temperature (HT) Low-temperature (LT) exchanges between these two components would be responsible foran increase of the d18O primary MORB value
BOSCH et al HYDROTHERMAL CIRCULATION IN OMAN OPHIOLITE
1203
exchange with the surrounding rocks during its penetra-tion into the crustal section In an environment in whichfluid---rock interaction occurs at variable temperaturesthe d18O is greatly influenced by the temperature atwhich the exchange took place and by the waterrockratio It is evident from Figs 9b and 10b that none of thestudied gabbros have plagioclase and amphibole withMORB-like d18O and 87Sr86Sr values The separateminerals display a broad range of d18O values and mostexhibit extreme 18O depletion (Table 5) Hydrothermalexchange kinetics are similar in amphibole and pyroxeneand these two minerals are more resistant to oxygenexchange during water---rock interaction than coexistingplagioclase (Gregory et al 1989) The most probableexplanation of the low d18O values measured in bothamphiboles and pyroxenes is that they crystallized athigh temperature in the presence of a relatively low 18Oseawater-derived hydrothermal fluid [d18O from 01 to07 after Gregory amp Taylor (1981)] The anomalouslyhigh d18O value (thorn1009) measured for the plagioclase ofthe micro-gabbroic dyke (01 OA19d) can be interpretedas resulting from a superimposed overprint caused by alate-stage penetration of fluids at lower temperature Inthis sample the occurrence of such different 18O16Oratios between coexisting plagioclase and amphibole isnot consistent with equilibrium fractionation betweenthese two minerals at any possible temperature Thisobservation suggests that parts of the ophiolite sequencehave undergone a high-temperature hydrothermal eventprior to the later low-temperature event that producedthe strong 18O increase in coexisting plagioclaseThe depletion of the d18O values results from high-temperature hydrothermal alteration (Gregory amp Taylor1981 Stakes amp Taylor 1992) The oxygen isotopic datasupport the contention that most studied samples havebeen affected by a VHT---HT hydrothermal alteration byfluids derived from seawater Some of the analysed sam-ples presenting petrological evidence of crystallization ofLT secondary minerals have been overprinted by the latelow-temperature alteration thus swamping the earlierhigh-temperature alteration effects
Determination of waterrock ratio
To better evaluate the exchange between seawater andthe gabbroic rocks waterrock ratios (WR) have beenestimated for the whole rocks analysed during the courseof this study The different models used to constrain thisratio mainly differ by the calculation of the effective ratioor by the estimated weight ratio and by considering openor closed systems for water circulation (Albareede et al1981 McCulloch et al 1981) The WR ratios reportedin Table 5 have been calculated assuming a closed sys-tem Using this formulation these values are minimabecause the calculation always uses pristine fluid for the
initial fluid thus maximizing the value in the denomina-tor If the fluid was shifted to lower 87Sr concentrations byexchange with the rock on the downward path theinferred values would be much higher (Davis et al 2003)The calculated WR ratios for the samples from this
study range from 31 to 219 (Table 5) Two main groupsof samples can be recognized on the basis of their waterrock ratios The lowest ratios ranging from 31 to 47have been determined for massive gabbros or anatecticgabbros but also for dykes and veins devoid of late LTalteration Similar ratios have been previously calculatedfor example for the discharged hydrothermal solutions atthe 13N and 21N East Pacific Rise (Di Donato et al1990 Fouquet et al 1991) The gabbros from the Ibrasection in the Oman ophiolite gave effective WR ratiosof 05 33 and 42 (McCulloch et al 1981) in goodagreement with the values obtained in this study Theleast altered gabbro of the Ibra section yields a WR ratioof 05 in good agreement with the exceptionally fresh-ness of this rock characterized by typical mantle oxygenvalues for its minerals (McCulloch et al 1981) Samplescharacterized by waterrock effective ratios in the rangeof 3---5 can be related to penetrative alteration via thehydrothermal system Lecuyer amp Reynard (1996) havedemonstrated in oceanic gabbros from the Hess Deepaltered in similar HT conditions that WR mass ratios inthe range of 02---1 are sufficient to account for the mod-ified stable isotope signature of the gabbros Higherwaterrock values (WR 45) have been calculated forsamples affected by superimposed late hydrothermalalteration and for the epidosite samples (Table 5) Theyprobably acquired their isotopic characteristics throughinteraction with a larger volume of seawater duringgreenschist-facies metamorphism Similar or higherWR values have been published in previous studies(ie McCulloch et al 1981 Kawahata et al 2001)They correspond either to samples from the upper-crus-tal sequence (ie basalt sheeted dyke or plagiogranite) orto gabbros from the crustal section located in a particularenvironment such as for example the Wadi Fizh sectionthat is located close to a segment boundary along thepalaeo-spreading axis (Nicolas et al 2000)
Mechanism for seawater interaction withmagma
Nicolas et al (2003) have proposed that variable amountsof seawater can circulate at high temperature through-out the crustal section down to the walls of the magmachamber via a network of parallel microcracks a fractionof a millimetre wide Such microcracks and their relation-ship with the VHT---HT hydrous alteration in the sur-rounding gabbros have been described in detail byNicolas et al Via this network of microcracks largeamounts of seawater can have reacted with the magma
JOURNAL OF PETROLOGY VOLUME 45 NUMBER 6 JUNE 2004
1204
and induced variable changes of its Sr and O isotopiccomposition This regular network of parallel micro-cracks could constitute the recharge system and thehydrated gabbroic dykes the discharge system Physicalparameters and the geophysical models of Nicolas et al(2003) are in agreement with this scenario Such a pene-tration of seawater-derived hydrothermal fluids throughthe lower crust along grain boundaries and a microscopicfracture network at temperatures 4750C is consistentwith recent thermodynamic models (McCollom amp Shock1998)Seawater-altered and high-temperature recrystallized
diabases (protodykes) from the overlying sheeted dykecomplex stoped into the melt lenses could represent anindirect way for seawater to enter the system as they canremelt during their subsidence through the magmachamber A simple mass balance calculation suggeststhat this indirect contamination is not significant Assess-ments of the amount of recycled crust 1 in volumeby Nicolas et al (2000) based on the protodyke remnantsin the gabbro section or 20 by Coogan et al (2003)based on chlorine content in EPR basalts lead to a sea-waterrock ratio of the order of 104 to 103Thus following Nicolas et al (2003) we propose that
seawater has been introduced along microcracks down tothe walls of the magma chamber and has diffused alonggrain boundaries in the way described by Manning et al(2000) progressively invading the host gabbro Gabbroicdykes result from the injection of hydrous melt into thehost gabbros at falling temperatures as they drift awayfrom the magma chamber This water-saturated meltcould result from the extensive hydration of the crystal-lizing gabbros near the walls of the magma chamberwhen seawater penetrates through this wall (Fig 1)Similar plumbing systems driving seawater down to the
limit of the magma chamber and back to the surface havebeen already proposed in other environments such as theMid-Atlantic Ridge (Kelley amp Delaney 1987 Cooganet al 2001) the East Pacific Rise at Hess Deep (Gillis1995 Manning et al 1996a 1996b) the Tonga forearc(Banerjee amp Gillis 2001) and the Troodos ophiolite(Gillis amp Roberts 1999) The melting curve of wet basalthas a negative slope but is close to the dry solidus at lowpressure in the lower gabbros and the first melts areplagiogranitic (Spulber ampRutherford 1983) Such plagio-granites are ubiquitous in the highest gabbros and in theroot zone of sheeted dykes in Oman (Rothery 1983Juteau et al 1988 Nicolas amp Boudier 1991) in manyother ophiolites and in the oceanic crust where they havebeen interpreted as resulting either from wet anatexis ofhot gabbros or from the final product of crystallization ofa basaltic melt (Spulber amp Rutherford 1983) Plagio-granites are rare in the lower gabbros exceptionallyassociated with hydrous gabbro segregations inside thelayered gabbros Their constitutive minerals are also
remarkably absent along the VHTmicrocracks althoughthese gabbros were above the wet solidus A possibleexplanation has been proposed by McCollom amp Shock(1998) who wrote that at high temperature lsquoaqueousfluids may leave very little conspicuous petrological evid-ence for their presencersquo the reason being that thehigh-T conditions would be closer to batch meltingObviously more detailed studies on this specific problemare needed We conclude that the large degree of hydrousmelting locally required at the walls of the magma cham-ber to account for the generation of a gabbroic hydrousmelt may have erased the traces of the incipient plagio-granitic melt
CONCLUSIONS
Fostered by the recent discovery of high-temperatureseawater contamination in some gabbros of the Omanophiolite (Manning et al 2000) we have conducted astudy on 500 gabbro specimens from selected areas ofthe entire Samail ophiolite belt and on the hydrous gab-broic dykes that cross-cut them The highest-temperaturereactions (VHT) recorded in olivine and clinopyroxene ingabbros as orthopyroxene and pargasite coronas reach975C They are followed at lower HT conditions withreplacement of olivine by an assemblage of tremolitemagnetite and plagioclase surrounded by chlorite andby pargasite development in clinopyroxene followedfinally by the LT lizardite---olivine and saussurite---plagioclase greenschist-facies alteration With a fewexceptions the VHT alteration tends to be restricted tothe lower gabbros and to the vicinity of the Moho andthe HT alteration to the middle and upper gabbros Thepreservation of the vertical distribution of VHT---HTfacies indicates that progressive cooling during off-axisspreading did not obliterate this distribution and conse-quently that the VHT---HT hydrothermal alteration tookplace in a very restricted time interval at the limit of thecooling magma chamber In the deep and hot gabbrosthe main water channels may be sub-millimetre-sizedmicrocracks with a dominantly vertical attitude the ori-gin and dynamics of which have been investigated in aprevious paper (Nicolas et al 2003)Beyond these high-temperature alteration zones mas-
sively induced in the gabbros seawater may be respons-ible for the generation of clinopyroxene---pargasitegabbro dykes that cross-cut all gabbros and that areupsection clearly connected to the LT hydrothermalvein system Petrostructural evidence suggests that thesedykes were generated by hydration of the crystallizinggabbros near the internal wall of the LVZ---magmachamber The resultant melts were subsequently intrudedat various levels in the cooling lower and middle gabbrosPetrological observations of nearly all the gabbros ana-lysed in the present study located from the root zone of
BOSCH et al HYDROTHERMAL CIRCULATION IN OMAN OPHIOLITE
1205
the sheeted dyke complex down to the Moho emphasizethe imprint of VHT---HT hydrous alteration The VHThydrothermal event is characterized by temperatures ofequilibration around 900---1000C The temperatures atwhich the HT hydrous event occurred are estimated tobe in the range 550---900CStrontium and oxygen isotope compositions measured
on whole rocks and separated minerals significantlydepart from primary MORB values The Sr and O iso-tope data obtained for pargasite green hornblende andclinopyroxene record the participation of seawater at thetemperature of crystallization of the minerals Plagio-clases and whole rocks define a trend toward a compo-nent characterized by a high radiogenic 87Sr86Sr valueand a higher Sr content than normal seawater Seawateris clearly identified as the fluid responsible for the modi-fication of the Sr and O signatures for both massivegabbros and dykes and veins Nevertheless the high Srcontent of the extraneous component requires eitheropen-system exchange allowing penetration of a greaterquantity of seawater or penetration of seawater modifiedby interaction with the oceanic crust during its travel paththrough the crustal section The waterrock ratio calcu-lated on the basis of the Sr isotopes is between three andfive for the enclosing gabbros and for the least LT altereddykes The VHT---HT hydrothermal event affectingthese samples is related to penetrative alteration via therecharge and discharge hydrothermal system Thewaterrock ratio is 10 for dykes exhibiting evidenceof a LT hydrothermal alteration overprint and reaches22 for the epidosite dyke The depletion in d18O char-acterizes all the studied samples with the single exceptionof the micro-gabbroic dyke plagioclase This agreeswith the conclusions based on Sr isotopes suggestingthat these samples have undergone exchange with sea-water-derived fluids during a VHT---HTalteration hydro-thermal event
ACKNOWLEDGEMENTS
This work has benefited from financial support by theCentre National de la Recherche Scientifique (INSUInteerieur-Terre andDYETI programmes) and inOmanfrom the willingness of the Directorate of MineralsMinistry of Commerce and Industry and the hospitabil-ity of the people Technical help in the laboratory is alsoacknowledged including C Nevado for the thin sectionsE Ball for the illustrations B Galland for her assistancein the chemistry room and C Bassoulet for help duringthermal ionization mass spectrometric analyses of the Srresults from the leaching experiments Constructivereviews from R T Gregory C Lecuyer an anonymousreviewer and particularly from M Wilson greatlyhelped to improve an earlier version of the manuscript
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Fouquet Y Von Stackelberg S U Charlou J L Donval J P
Foucher J Erzig P Muhe R Wiedicke M Soakai S amp
Whitechurch H (1991) Hydrothermal activity in the Lau back-arc
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Gregory R T amp Taylor H P (1981) An oxygen isotope profile in a
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Gregory R T Criss R E amp Taylor H P (1989) Oxygen
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Hart S R Erlank A J amp Kable J D (1974) Sea floor basalt
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Holland T amp Blundy J (1994) Non-ideal interactions in calcic
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Ionov D A Savoyant L amp Dupuy C (1992) Application of the
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minerals Geostandards Newsletter 16 311---315
Javoy M (1980) 18O16O and DH ratios in high temperature
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Juteau T Beurrier M Dahl R amp Nehlig P (1988) Segmentation
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Juteau T Manacrsquoh G Moreau C Lecuyer C amp Ramboz C
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Koepke J Feig S T Snow J amp Freise M (2003) Petrogenesis of
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the lower oceanic crust thermodynamic models of hydrothermal
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1207
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Science Letters 91 327---340
JOURNAL OF PETROLOGY VOLUME 45 NUMBER 6 JUNE 2004
1208
ratios This supports the occurrence of two distincthydrothermal events one at high temperature and alater stage at lower temperature responsible for thestrong 18O enrichment in the coexisting plagioclase
Dioritic dyke associated with a diabase dyke in lowergabbros from Wadi Halhal Haylayn Massif
Sample 02 OA37b belongs to a group of brownamphibole---plagioclase lenses or reaction margins asso-ciated with diabase intrusives (Fig 7d) in lower layeredgabbros Idiomorphic brown hornblende (tschermakite)crystals are elongated in the plane of the dyke represent-ing 80 of the modal composition They are associatedwith an interstitial plagioclase matrix The browntschermakite has greenish rims Plagioclase has a cloudyappearance due to micrometre-size fluid inclusionsexcept along its margins and internal microcracks Bothhornblende and plagioclase exhibit evidence of plasticdeformation The Sr isotopic ratio for the pargasite andplagioclase is 070348 and 070420 respectively Thepargasite exhibits a Sr signature closest to that of theOman primary magmatic value (McCulloch et al 1981)Nevertheless it is too high to be attributed to anunmodified MORB-source mantle signature This sug-gests the addition of an external fluid component in goodagreement with the shift of oxygen isotopic compositionto a low value (thorn39) suggesting a high-temperatureexchange with a low d18O fluid The Sr isotope composi-tion of the plagioclase is significantly higher than that of
the coexisting pargasite probably as a result of the low-temperature alteration
Dioritic dyke cross-cutting lower layered gabbros in WadiAl-Abyad Nakhl Massif
Sample 02 OA90b is a 1 cm thick dioritic dyke cross-cutting the lower layered gabbros It grades into a greenamphibole vein as illustrated in Fig 6c The dyke is com-posed of 2 cm long dark green idiomorphic magnesio-hornblende crystals growing in the plane of the dykeand plagioclase concentrated at the margin of the dykeThe plagioclase in the dyke and in the enclosing gabbrocontains micrometre-size fluid and mineral inclusionssimilar to those observed in sample 94 MA2 The plagio-clase and green hornblende have higher Sr isotopic ratios(070387 and 070427 respectively) and lower d18Ovalues (thorn498 and thorn239 respectively) than the normalMORB-source mantle value As observed for previoussamples this supports interaction with an external fluidcomponent
Comb-textured gabbroic dykes in the lower gabbros fromWadi Wariyah Wadi Tayin Massif
Sample 01 OA15f (Fig 6b) is from Wadi Wariyah andrepresents a 2 cm wide dyke intruding the lower gabbrosThis type of comb-textured dyke developed in theclinopyroxene---plagioclase stability field It containslarge idiomorphic clinopyroxene crystals which arepartially replaced by brownish magnesio-hornblendeThe plagioclase is extremely calcic with a slight inverse
Table 3 Whole-rock major element compositions of massive gabbros and dykes determined by X-ray fluorescence
(wt )
Sample 94 Ma2 97 OA128b 01 OA41d2 01 OA19d 01 OA19a 01 OA15f 01 OA36b 01 OA36b
Rock type Gabbro Gabbro Gabbro patch Gabbroic dyke Gabbroic dyke Comb-textured
gabb dyke
Epidosite dyke Epidosite dyke
(margin)
SiO2 4830 4815 4749 4524 4339 4799 389 4371
Al2O3 2785 2032 2438 1474 1246 1192 2752 1646
Fe2O3 328 266 404 981 1204 592 381 623
MnO 003 004 005 014 016 006 008 014
MgO 405 737 427 1031 1686 1326 187 739
CaO 1289 1908 1477 1235 1156 166 2471 2339
Na2O 292 123 257 258 111 075 000 013
K2O 000 000 009 006 000 000 000 000
TiO2 005 017 063 230 064 086 013 042
P2O5 008 007 013 018 009 013 007 010
LOI 033 076 138 216 161 237 274 190
Total 9978 9985 998 9987 9992 9986 9983 9987
LOI loss on ignition gabb gabbroic
BOSCH et al HYDROTHERMAL CIRCULATION IN OMAN OPHIOLITE
1193
Table4Majorelementcomposition
aofselectedam
phibolespaired
withplagioclasecompositionsusedforthermometry
Sam
ple
nam
e1Mode2
SiO
2TiO
2Al 2O3
Cr 2O3
FeO
MnO
MgO
CaO
Na 2O
K2O
Total
Si
AlIV
AlVI
Ti
Fe3
thornCr
Mg
Fe2
thornMn
Na
KCa
XAn3
T(C)4
01OA19a
Mg-hornblende
core
45 60
26
10 78
009
860
015
16 56
12 00
212
007
96 72
6552
1448
0366
0083
0552
0004
3409
0594
0016
0611
0018
1843
0903
899
Mg-hornblende
core
47 99
263
850
002
897
011
16 61
12 10
160
020
96 36
6894
1106
0334
0028
0483
0002
3557
0594
0014
0446
0036
1862
0892
834
Mg-hornblende
rim
51 17
247
596
001
761
014
18 11
12 19
103
010
96 47
7257
0743
0253
0016
0414
0001
3828
0488
0016
0284
0018
1853
0894
820
Mg-hornblende
rim
47 50
291
920
004
880
014
16 46
11 95
174
008
96 07
6835
1165
0396
0015
0506
0005
3529
0552
0017
0486
0015
1843
0887
825
Mg-hornblende
rim
49 57
269
742
000
799
017
18 07
12 02
133
014
96 88
7013
0987
0261
0018
0575
0000
3811
0371
0020
0365
0026
1821
0860
831
Mg-hornblende
core
47 83
181
872
003
840
011
14 48
12 18
159
008
93 59
6814
1186
0279
0018
0644
0004
3711
0357
0013
0438
0015
1859
0907
874
Pargasite
rim
42 15
249
11 87
024
10 07
013
13 84
11 43
263
032
96 61
6188
1812
0242
0434
0299
0028
3028
0937
0016
0749
0059
1797
0804
972
Pargasite
rim
42 22
260
11 86
026
10 21
011
13 71
11 48
253
032
96 59
6201
1799
0254
0429
0299
0030
3001
0955
0014
0720
0060
1807
0803
962
Pargasite
core
42 05
263
11 45
030
10 19
011
13 80
11 47
246
032
96 46
6189
1811
0175
0478
0299
0035
3029
0956
0014
0702
0060
1809
0767
974
Pargasite
rim
42 36
247
11 90
016
945
013
14 16
11 76
249
026
95 98
6239
1761
0305
0366
0276
0019
3109
0888
0016
0710
0049
1856
0818
926
Pargasite
core
42 50
291
11 64
016
984
015
14 22
11 41
256
034
96 69
6219
1781
0227
0426
0322
0018
3103
0883
0018
0727
0063
1789
0812
974
Pargasite
rim
42 76
270
12 03
017
920
012
14 87
11 82
244
025
96 73
6224
1776
0880
0335
0368
0020
3226
0752
0015
0689
0047
1843
0841
941
Pargasite
rim
42 13
181
13 54
016
921
009
12 77
12 13
254
032
96 98
6172
1828
0510
0450
0000
0018
2788
1129
0011
0721
0059
1905
0841
874
01OA19d
Mg-hornblende
49 26
047
624
011
11 46
017
15 39
12 21
108
009
96 48
7137
0863
0202
0051
0437
0013
3323
0951
0021
0302
0018
1896
0768
771
Mg-hornblende
46 03
287
932
018
906
017
14 77
12 87
193
007
97 28
6672
1328
0265
0313
0046
0021
3191
1053
0021
0542
0012
1999
0758
808
Pargasite
41 89
429
11 65
008
11 03
017
13 52
11 37
259
017
96 77
6159
1841
0179
0474
0345
0010
2963
1012
0021
0739
0032
1791
0770
975
Pargasite
rim
42 80
340
11 16
009
11 01
018
13 73
11 67
240
022
96 67
6293
1707
0226
0376
0322
0010
3009
1032
0022
0684
0041
1839
0605
880
Pargasite
core
42 58
351
11 27
006
11 63
018
13 51
11 46
239
022
96 82
6257
1743
0209
0387
0391
0007
2959
1039
0023
0680
0042
1804
0595
893
Pargasite
rim
42 14
393
11 28
006
11 71
014
13 35
11 33
235
033
96 62
6214
1786
0174
0435
0391
0007
2935
1054
0018
0673
0062
1790
0519
901
Pargasite
core
41 56
420
12 05
006
11 57
016
12 51
11 35
249
027
96 24
6168
1832
0277
0469
0276
0007
2768
1160
0021
0718
0052
1805
0537
882
Pargasite
core
41 33
438
11 49
008
11 66
015
13 35
11 45
264
026
96 80
6105
1894
0107
0487
0368
0009
2941
1073
0019
0757
0048
1813
0537
942
Pargasite
41 08
446
12 39
012
11 35
017
13 13
11 36
263
015
96 85
6049
1951
0199
0494
0368
0014
2882
1030
0021
0752
0027
1792
0758
975
02OA37b
Mg-hornblende
rim
45 15
260
984
002
13 28
019
14 01
10 87
128
009
97 33
6527
1473
0203
0283
0690
0002
3018
0915
0023
0360
0016
1683
0562
828
Mg-hornblende
rim
44 72
263
988
004
13 23
021
14 01
10 78
133
009
96 92
6494
1506
0185
0287
0713
0005
3032
0894
0025
0374
0018
1677
0593
854
Mg-hornblende
rim
45 68
247
935
004
12 87
020
14 10
11 00
121
011
97 02
6621
1379
0218
0269
0598
0004
3045
0962
0024
0341
0020
1708
0562
815
Mg-hornblende
core
43 94
291
10 87
001
12 60
023
13 73
10 59
142
011
96 39
6410
1590
0278
0319
0644
0001
2985
0893
0028
0401
0021
1655
0710
860
Mg-hornblende
core
44 62
270
10 37
001
12 66
019
14 12
10 58
140
008
96 73
6479
1521
0253
0294
0644
0001
3057
0893
0023
0395
0014
1646
0674
851
Mg-hornblende
rim
47 79
181
796
002
12 09
025
15 22
11 09
100
009
97 33
6866
1134
0214
0196
0506
0002
3260
0947
0030
0280
0016
1706
0503
767
Mg-hornblende
core
45 43
249
10 3
000
11 99
022
14 50
11 14
133
008
97 48
6524
1476
0267
0269
0644
0000
3103
0796
0027
0370
0015
1715
0703
838
JOURNAL OF PETROLOGY VOLUME 45 NUMBER 6 JUNE 2004
1194
Sam
ple
nam
e1Mode2
SiO
2TiO
2Al 2O3
Cr 2O3
FeO
MnO
MgO
CaO
Na 2O
K2O
Total
Si
AlIV
AlVI
Ti
Fe3
thornCr
Mg
Fe2
thornMn
Na
KCa
XAn3
T(C)4
02OA90b
Mg-hornblende
core
48 83
038
745
005
939
010
17 29
12 24
142
006
97 21
6931
1069
0041
0041
0667
0005
3657
0448
0011
0392
0010
1862
0840
856
Mg-hornblende
core
49 49
033
731
003
916
008
17 40
12 57
139
005
97 82
6987
1013
0035
0035
0552
0004
3662
0530
0010
0379
0009
1901
0830
811
Mg-hornblende
rim
48 90
040
753
005
932
013
17 15
12 22
154
005
97 29
6948
1052
0042
0042
0575
0006
3632
0533
0015
0424
0010
1861
0880
869
Mg-hornblende
rim
48 30
044
800
007
962
010
16 86
12 32
161
005
97 35
6873
1127
0047
0047
0598
0008
3576
0546
0012
0443
0010
1879
0870
862
Mg-hornblende
rim
49 12
032
742
010
922
011
17 36
12 29
142
006
97 43
6956
1044
0034
0034
0621
0011
3665
0471
0013
0391
0010
1865
0840
845
02OA43a
Mg-hornblende
rim
48 13
040
789
007
860
007
17 41
12 19
155
006
96 36
6882
1118
0212
0043
0621
0008
3710
0407
0008
0429
0010
1867
-------
-------
Mg-hornblende
rim
50 93
035
643
001
759
006
18 50
12 24
114
003
97 28
7152
0848
0217
0037
0506
0001
3873
0385
0007
0311
0005
1841
-------
-------
Mg-hornblende
rim
50 67
014
559
009
12 33
026
15 28
12 44
068
003
97 50
7260
0740
0204
015
0483
0010
3263
0994
0032
0189
0005
1909
-------
-------
Mg-hornblende
rim
50 34
013
621
009
12 49
027
15 07
12 51
064
003
97 78
7192
0810
0240
0010
0530
0010
3210
0960
0030
0180
0010
1910
-------
-------
Mg-hornblende
rim
45 17
060
11 14
013
988
008
15 78
12 33
211
009
97 31
6473
1527
0355
0640
0644
0015
3371
0540
0010
0588
0016
1892
-------
-------
Mg-hornblende
rim
49 25
032
712
014
845
009
17 84
12 32
137
004
96 95
6983
1020
0170
0030
0620
0020
3770
0380
0010
0380
0010
1870
-------
-------
Mg-hornblende
rim
48 61
032
775
014
975
011
16 81
12 57
139
004
97 50
6905
1950
0203
0034
0621
0016
3560
0537
0013
0382
0008
1914
-------
-------
94Ma2
Anthophyllite
54 98
000
259
001
12 65
037
23 35
199
034
000
96 28
7744
0256
0174
0000
0161
0001
4903
1330
0043
0092
0001
0301
-------
-------
Anthophyllite
55 11
000
296
003
14 09
032
23 66
063
038
000
97 18
7716
0284
0205
0000
0115
0003
4938
1535
0038
0104
0000
0095
-------
-------
Actinolite
48 58
004
954
002
960
014
17 67
999
139
005
97 02
6850
0150
0435
0004
0690
0002
3713
0442
0017
0379
0009
1510
-------
-------
Actinolite
51 54
004
585
000
890
022
20 28
930
090
002
97 05
7214
0786
0178
0005
0598
0000
4230
0444
0026
0244
0004
1395
-------
-------
01OA41d2
Pargasite
rim
43 05
342
10 71
008
11 46
012
13 50
11 54
179
067
96 34
6353
1647
0216
0380
0368
0009
2967
1046
0015
0512
0127
1824
0613
863
Pargasite
rim
42 61
354
11 59
001
10 59
010
13 52
11 87
160
076
96 19
6286
1714
0302
0392
0299
0001
2972
1007
0012
0458
0143
1877
0715
844
Pargasite
core
42 54
401
11 04
006
11 81
013
13 21
11 50
202
039
96 71
6266
1734
0182
0444
0368
0007
2901
1087
0017
0576
0073
1814
0854
979
Pargasite
rim
43 03
385
10 72
002
11 70
014
13 50
11 95
146
080
97 17
6314
1686
0168
0425
0345
0002
2953
1091
0018
0415
0149
1878
0634
852
Pargasite
rim
42 65
341
10 60
006
11 97
014
13 22
11 64
183
068
96 20
6332
1668
0186
0381
0345
0007
2926
1141
0018
0528
0129
1851
0620
869
Pargasite
core
42 82
367
10 60
007
11 93
012
13 42
11 44
206
047
96 60
6315
1685
0158
0407
0391
0008
2950
1080
0016
0588
0089
1808
0615
899
Pargasite
rim
42 2
362
11 10
007
11 42
016
13 09
11 72
179
072
95 89
6282
1718
0229
0405
0299
0009
2905
1123
0020
0518
0137
1869
0715
875
Pargasite
rim
42 42
380
11 03
006
11 77
012
13 00
11 72
188
069
96 49
6283
1717
0209
0423
0299
0007
2870
1158
0015
0539
0131
1860
0817
928
Pargasite
rim
42 21
364
11 14
006
10 73
011
13 53
11 68
177
072
95 59
6277
1723
0229
0407
0322
0007
2999
1012
0014
0511
0136
1862
0736
883
Pargasite
rim
42 25
376
11 03
008
10 93
015
13 66
11 72
185
064
96 07
6257
1743
0182
0418
0345
0009
3016
1009
0019
0531
0121
1860
0766
912
Pargasite
core
42 17
396
10 91
008
11 10
010
13 57
11 49
209
038
95 85
6253
1747
0159
0442
0368
0010
3000
1009
0013
0602
0073
1826
0841
981
aMajorelem
entco
mpositionsarein
weightpercent-------noplagioclasean
alysed
andnotemperature
calculated
1Typ
eofam
phibolean
alysed
isalso
indicated
2Blankindicates
nozoningobserved
3Molefractionofan
orthitein
plagioclasead
jacentto
amphibolegrain
4Tem
perature
ofeq
uilibrium
fortheam
phibole---plagioclasepaircalculatedusingtheHollandamp
Blundythermometer
(1994)Errors
are40
C
BOSCH et al HYDROTHERMAL CIRCULATION IN OMAN OPHIOLITE
1195
zonation from An95 to An99 from core to rim (Table 1 andFig 8a) TheWR Sr isotopic ratio of the sample (070458)is significantly higher than normal MORB values Thiscould be related to the overprint of a low-temperaturealteration event consistent with the petrography of thesample
Green amphibole vein cross-cutting layered gabbros inWadi Mayah Haylayn Massif
Sample 02 OA43a belongs to a series of 1 cm thick greenamphibole veins (as in Fig 6e) cross-cutting the layeredgabbros The veins develop 15 cm thick reaction marginsin the enclosing gabbro and are exclusively composed ofacicular pale green magnesio-hornblende growing per-pendicular to the vein margin in a crack-seal mode(Fig 6f ) The reaction margins are composed of urali-tized gabbro marked by the development of the samepale green magnesio-hornblende in an inhomogeneouscalcic plagioclase matrix (An91---97) The Sr and O isotopiccompositions of this green hornblende (070431 andthorn31) do not correspond to normal MORB-sourcemantle values
Epidosite dyke from Wadi Gideah Wadi Tayin Massif
Sample 01 OA36b is representative of a series of epidoteveins cross-cutting poorly layered gabbros These 2 cmthick veins are composed of pink thulite in their centreand green pistachite at their margins (Fig 6a) Theirsharp contact with the enclosing gabbro is marked bythe development of coarse clinopyroxene crystals alteredin the epidote---greenschist facies This suggests that theepidote vein is replacive in a coarse-grained gabbro dykeThe highest 87Sr86Sr initial ratios of all the samplesstudied are recorded by the WR and coexisting epidoteof this sample (070519 and 070554 respectively)(Table 5) The Sr content of the WR and associatedepidote are also the highest of the present dataset(716 ppm and 764 ppm respectively) The Sr isotopiccomposition of this sample is consistent with a greaterdegree of exchange with a radiogenic component fromCretaceous seawater It is significantly higher than thevalue obtained for an epidosite vein from the Wadi Fizhsection of the Oman ophiolite [070435 with a Sr contentof 488 ppm reported by Kawahata et al (2001)] but ingood agreement with the average of the greenschist-faciesalteration sequence defined by Kawahata et al (2001)
GABBROS
01 km 025 km 1 km 36 km50
60
70
80
90
An
MA2
19d
19a 15f 128b
41d2
(a)
43a17b
15f
90b
37b
19a
19d
40
60
80
100
A
n
01 km 015 km 025 km 1 km
DYKES
(b)
Distance to the Moho
ACTINOLITE
MAGNESIOHORNBLENDE
TSCHERMAKITE
PARGASITEGabbros Dykes41d2MA2
19a d37b15f
90b43a
AlIV
150500
02
08
10
(Na
+ K
) A
(c)
04
06
950
900
850
800
750
70007 12 17
T degC
1000
(d)
AlIV
Fig 8 Plagioclase---amphibole compositions and results of thermometry (a) and (b) plagioclase compositions in layered gabbros and in dykesrespectively as a function of distance to the Moho (see text for sample identification) (c) amphibole compositions in layered gabbros and dykesaccording to Leakersquos (1997) nomenclature (d) equilibration temperatures calculated using the amphibole---plagioclase thermometer of Holland ampBlundy (1994) as a function of AlIV content in amphibole [same symbols as in (c)]
JOURNAL OF PETROLOGY VOLUME 45 NUMBER 6 JUNE 2004
1196
Table5RbandSrcontents87Sr
86Srandd1
8Oisotopiccompositionsofwholerocksandmineralseparatesfrom
samplesofmassivegabbrosdykes
andveinsfrom
theOman
ophiolitealsolistedarecalculated
waterrockratios(W
R)andLOIvalues
Sam
ple
Rb(ppm)
Sr(ppm)
87Rb8
6Sr
87Sr
86Sr
(87Sr
86Sr)i1
d18O
()
LOI(
)2WR
3(87Sr
86Sr)L14
(87Sr
86Sr)L25
(87Sr
86Sr)R6
01OA41d2
Whole
rock
037
1792
0006
0703516
09
070351
thorn480
138
47
0703587
0703651
0703357
Plagioclase
025
1249
0003
0704261
22
070426
thorn504
-------
-------
Pargasite
-------
-------
-------
0703548
20
070355
-------
-------
-------
Clinopyroxene
045
26 0
0050
0703845
19
070378
-------
-------
-------
01OA36b
Dykewhole
rock
0005
7164
50001
0705186
17
070519
-------
274
21 9
0705207
0705112
-------
Epidote
0003
7645
0001
0705536
12
070554
-------
-------
-------
Wallwhole
rock
005
4254
0001
0704637
09
070464
-------
191
-------
0704945
0704675
0704593
02OA43a
Green
hornblende
004
98
0012
0704323
13
070431
thorn310
-------
-------
97OA128b
Whole
rock
003
1274
50001
0703327
17
070333
-------
076
37
-------
-------
-------
Plagioclase
002
1204
50001
0703285
09
070328
thorn414
-------
-------
Clinopyroxene
003
20 0
0004
0704230
09
070422
thorn523
-------
-------
01OA15f
Whole
rock
006
1077
0002
0704582
16
070458
-------
237
13 5
-------
-------
-------
01OA19a
Whole
rock
037
1303
0008
0704189
18
070418
-------
161
96
-------
-------
-------
01OA19d
Whole
rock
058
2032
0008
0703423
08
070341
thorn567
216
415
0703574
0703408
0703271
Pargasite
091
1058
0025
0703273
11
070324
thorn287
-------
-------
Plagioclase
-------
-------
-------
-------
-------
thorn10 09
-------
-------
02OA90b
Plagioclase
004
2385
50001
0703868
11
070387
thorn498
-------
-------
Green
hornblende
010
23 8
0012
0704288
15
070427
thorn239
-------
-------
BOSCH et al HYDROTHERMAL CIRCULATION IN OMAN OPHIOLITE
1197
Table5continued
Sam
ple
Rb(ppm)
Sr(ppm)
87Rb8
6Sr
87Sr
86Sr
(87Sr
86Sr)i1
d18O
()
LOI(
)2WR
3(87Sr
86Sr)L14
(87Sr
86Sr)L25
(87Sr
86Sr)R6
02OA37b
Plagioclase
023
3121
0002
0704204
15
070420
-------
-------
-------
Pargasite
031
41 6
0021
0703505
15
070348
thorn394
-------
-------
94OAMa2
Whole
rock
0016
1750
00003
0703219
15
070322
thorn495
033
31
0703264
0703177
0703158
Plagioclase
0014
1363
00003
0703219
11
070322
thorn479
-------
-------
Clinopyroxene
-------
-------
-------
0703236
13
070324
-------
-------
-------
1Initial8
7Sr
86Srratiocalculatedat
95Ma(H
acker1994)
2LOIisloss
onignition(in)
3Water---rock
ratiocalculatedassumingaclosedsystem
TheeffectiveWR
ratiocanbeexpressed
bythefollowingeq
uation
W=Reffectivefrac14
frac12eth87Sr=
86SrrockTHORN fi
nal
eth87Sr=
86SrrockTHORN in
itial=frac12eth8
7Sr=
86SrseawaterTHORN in
itial
eth87Sr=
86SrrockTHORN fi
nal
ethCrock
initial=C
seaw
ater
initial
THORNwhere(87Sr
86Srrock) finalisthefinalmeasuredSrisotopicratioin
thesample(87Sr
86Srrock) in
itialco
rrespondingto
theinitialm
antlevalue
istakenas
070295(Lan
phere
etal1981)(87Sr
86Srseawater) in
itialistheCretaceousseaw
ater
Srisotopicco
mposition[87Sr
86Sr07073
at90
MaafterBurkeet
al(1982)]C
seaw
ater
initial
istheSrco
ntent
oflsquonorm
alrsquoseaw
ater
andis
takenas
8ppm
(deVilliers1999)
FortheCrock
initialitis
assumed
that
thepresentSrco
ncentrationmeasuredis
thesameas
theinitial
concentration
4Srisotopicratiosmeasuredforliquid
L1extractedafterthefirststep
oftheleachingexperim
ent(6NHClfor8min
at70
C)
5Srisotopicratiosmeasuredforliquid
L2extractedaftertheseco
ndstep
oftheleachingexperim
ent(2 5NHClfor30
min
at70
C)
6Srisotopicratiosmeasuredforresidueobtained
afteratotalaciddigestionachievedaftertheextractionofL1an
dL2leachates
JOURNAL OF PETROLOGY VOLUME 45 NUMBER 6 JUNE 2004
1198
Part of the same sample from the contact between thedyke and the gabbro yields a slightly lower Sr isotopiccomposition (070464) intermediate between the sur-rounding gabbro and the epidote dyke It also has anintermediate Sr content (425 ppm) Acid leaching experi-ments performed on the whole rock of the dyke yield Srisotopic compositions of 070521 and 070512 for thetwo liquids extracted Similar experiments for the samplelocated at the contact with the gabbro yield 87Sr86Srratios of 070494 and 070467 for L1 and L2 liquids TheSr isotopic composition of the residue extracted afterleaching remains high (070459) and very close to theepidosite 87Sr86Sr ratio determined by Kawahata et al(2001) In this sample the primary minerals have beentotally replaced by epidote and secondary plagioclase orquartz Thus the Sr isotopic compositions measured forthis WR sample and its constituent minerals reflect thecomposition of the fluid filling this dyke
Mineral chemistry
Plagioclase
Compared with the host gabbros the dykes show a muchlarger dispersion in their plagioclase compositions (Fig 8aand b Table 4) characterized by more calcic plagioclasethan the enclosing gabbros This tendency is mostextreme in the comb-textured dykes in which the plagio-clase is almost pure anorthite The zoning is generallynormal recording a crystal fractionation process How-ever gabbro 94 MA2 exhibits inverse zoning this tend-ency is also observed in plagioclases from dykes 01OA19a and 01 OA15f The higher calcium content ofthe plagioclase as well as the inverse zoning is consistentwith an increase of water pressure during its crystalliza-tion (Turner amp Verhoogen 1960 Carmichael et al 1974Koepke et al 2003) The particularly high An content inthe comb-textured gabbro dykes 01 OA15f may alsoresult from crystallization in a supercooled magma thisis consistent with the comb texture indicative of fast-growing crystals The lack of brown hornblende suggeststhat crystallization occurred above the temperature ofhornblende stability
Amphibole
The amphiboles analysed in the dykes (Fig 8c andTable 4) show a regular trend in a (Na thorn K)A vsAlIV diagram from titanium-rich (Ti 4 015) pargasitichornblende (brownamphibole) and tschermakite (brown---green amphibole) to low-titanium (Ti5 015) magnesio-hornblende (green amphibole) and actinolite (pale greenamphibole) The pargasitic hornblende develops aspoikilitic oikocrysts in the dykes 01 OA19a and d andin the diffuse patch 01 OA41d2 at temperatures above800C during the final stage of crystallization (see
below) The tschermakite composition corresponds tothe internal alteration of clinopyroxene in gabbro 94Ma2 occurs in the comb-textured dyke 01 OA15f andis the primary amphibole in the dioritic dyke 02 OA37bThe magnesio-hornblende develops at the edges of thepargasites or of clinopyroxenes at temperatures around800C and is the primary phase in the dioritic dyke 02OA90b and in the green vein 02 OA43a Finally actino-lite occurs as a replacement of green hornblende in dykesor in the kelyphitic rim surrounding olivine in gabbro 94Ma2 Such a large composition range at the scale of athin section has been reported in amphiboles fromamphibolite-facies oceanic gabbros (Stakes 1991 Stakesamp Taylor 1992 Gillis et al 1993) as well as in the Omanophiolite gabbros (Manning et al 2000) This variabilityhas been explained by rapid reaction rates preventinghomogenization (Manning et al 2000)
Thermometry
Plagioclase---amphibole thermometry could be appliedwhen the two minerals exhibited good contacts such asin the laminated gabbro of the middle sequence (01OA41d2) and in the intrusive dykes The temperaturesof plagioclase---amphibole equilibration (Table 2) havebeen calculated using the geothermometer lsquoBrsquo of Hollandamp Blundy (1994) Edenite thorn Albite frac14 Richterite thornAnorthite valid between 500C and 900C with anuncertainty of 40C Amphibole formulae are basedon 23 oxygen and their nomenclature is based on alkalications in the A site vs AlIV according to Leake (1997)Pressurewas assumed to be 1 kbar Forty-five plagioclase---amphibole pairs meet the compositional criteria imposedby Holland amp Blundyrsquos dataset (09 4 Xan 4 01 andAlIV 403) Electron microprobe measurements wereconstrained to contiguous plagioclase and amphiboleseparated by sharp grain boundaries assuming equili-brium of the two phases When the mineral phasesexhibit normal zoning temperatures between plagioclaseand amphibole cores and between mineral rims werecalculated assuming that the temperatures deduced fromthe mineral core chemistry provide a proxy for the high-est temperature of equilibration These data are identi-fied in the T C vs AlIV diagram (Fig 8d) Table 4presents selected major element amphibole analyses theAn content of the plagioclase adjacent to each amphibolegrain and the temperature calculated for the pairThe temperatures calculated (Fig 8d) span the entire
range of temperatures at which amphibole and plagio-clase coexist in equilibrium (ie amphibolite-facies para-geneses) This temperature interval is bracketed byorthopyroxene-bearing facies or amphibole-free facies(ie gabbro 94MA2) and epidote---actinolite facies devoidof homogeneous plagioclase (ie epidosite dyke 01OA36b and green vein 02 OA43a) The range of the
BOSCH et al HYDROTHERMAL CIRCULATION IN OMAN OPHIOLITE
1199
calculated temperatures between 4900C and 750Ccorresponds to the changing composition of amphibolesfrom pargasite to magnesio-hornblende at the scale of athin section or even at that of an individual crystal asshown by their correlation with AlIV such variabilityrecords rapid cooling in hydrous conditions The highesttemperatures are recorded by pargasite from the anatec-tic gabbro 01 OA41d2 in the gabbro dyke 01 OA19dand in the dioritic dyke 02 OA90b They were calculatedfrom minerals devoid of lower-grade alteration and arethus considered as representing the equilibrium temperat-ure of the coexisting minerals Lower temperatures cor-respond to mineral phases evolving at falling temperatureand are only indicative of the cooling history Includingthe 40C uncertainty seven pairs provide temperatureshigher than 900C the limit of validity of the Holland ampBlundy thermometer However we maintain these tem-peratures as indicative of the highest values of our data-set These values are included in the average equilibriumtemperature summarized in Table 2
DISCUSSION
Distribution of hydrothermal alteration
Two distinct petrological events are recorded in the gab-bro section of the Samail ophiolite and are documentedin the present contribution (1) the whole gabbro sectionis affected by a penetrative alteration (2) the gabbrosection is crosscut by various types of dykes and veinsincluding hydrous minerals
Penetrative alteration
The penetrative alteration developed in the gabbros atgrain boundaries and locally invading the minerals isubiquitous in the gabbro section This penetrative altera-tion has been described as very high temperature (VHT)high temperature (HT) and low temperature (LT) basedon alteration parageneses The orthopyroxene coronas inthe layered gabbro sample and the incipient appearanceof brown pargasitic amphibole mark a lower thermallimit of the VHT alteration By reference to the experi-mental work of Koepke et al (2003) the temperature ofequilibration for these reactions is estimated to bebetween 900 and 1000CThe preservation of the vertical distribution of
VHT---HT facies (Figs 4 and 5) also pointed out byManning et al (2000) indicates that the hydrothermalalteration pattern fits the distribution of isotherms withdepth at a fast-spreading ridge (ie Phipps Morgan ampChen 1993 Dunn et al 2000 Fig 2) Equilibrium tem-peratures for VHT parageneses as high as 900---1000Csuggest that VHT---HT hydrothermal alteration tookplace in a very restricted time interval at the limit ofthe cooling magma chamber It is proposed that the
penetrative alteration affecting the entire gabbro sectionis related to a pervasive network of microcracks whichact as a recharge system down to the Moho (Nicolas et al2003)
Dykes and veins
The dykes are extremely varied eg pegmatitic gabbroswith comb-texture microgabbros and amphibolitizeddolerites dioritic dykes alignments of poikilitic horn-blende and finally diffuse hornblende-bearing patchesHigh-T gabbro and diorite dykes and low-T greenamphibole---epidote veins are connectedIn the dykes textural evidence attests to suprasolidus
conditions at least for the development of the pargasiticamphibole Integrating the hornblende---plagioclase equi-libration temperatures calculated for the studied samplesthese temperatures are bracketed between 950C and875CIn the following discussion we shall consider separately
the microgabbro and dolerite dykes The latter representbasaltic melt intrusions associated upsection with thediabase sheeted dykes and downsection possibly withthe mafic dykes that are ubiquitous in the mantle section(Nicolas et al 2000) Whatever their origin the develop-ment of pargasitic amphibole during their late stage ofcrystallization creates a link with the gabbroic dykes andsuggests a crystallization process evolving from dry con-ditions to more hydrous conditionsThe other gabbroic dykes comb-textured gabbros and
hornblende-bearing dioritic dykes result from the fillingof cracks by a fluid either a melt or a silica-rich hydrousfluid These intrusive dykes which develop reaction mar-gins at their contact with the host gabbro grade verticallyinto lower-temperature green amphibole veins or align-ments of poikilitic hornblende in the layered gabbromatrix or development of pargasitic hornblende in ana-tectic gabbros (sample 01 OA41d2) In gabbros crystal-lizing in the magma chamber the development oforthopyroxene and pargasite oikocrysts enclosing the pri-mary minerals suggests a thermal evolution from thegabbro solidus to amphibolite-facies conditions Forthese reasons it is suggested that the gabbroic dykesresulted from the injection of a hydrous melt in the stillhot gabbros drifting away from the magma chamber asdiscussed below
Origin of fluids responsible for VHT---HThydrothermal alteration
The 87Sr86Sr initial ratios and d18O of whole rocks andpure mineral fractions are plotted in Fig 9 against thedistance above the Moho Transition Zone (MTZ) Theinitial Sr isotopic composition of the whole rocks apartfrom the epidosite dykes ranges from 070322 to
JOURNAL OF PETROLOGY VOLUME 45 NUMBER 6 JUNE 2004
1200
070458 All the studied samples (including whole rocksand minerals) have initial 87Sr86Sr ratios falling outsidethe field of fresh MORB which commonly have Sr ratiosbetween 07023 and 07029 with an average of 070265(ie Hart et al 1974) The lowest initial Sr isotopiccomposition (070322) from the massive gabbro 94MA2 is slightly but significantly higher than averageOman primary magmatic signatures (ie 070296McCulloch et al 1981) Similarly the d18O values ofmost whole rocks and minerals measured (Table 5)
depart from typical MORB-source mantle values( Javoy 1980 Gregory amp Taylor 1981 Kyser 1986)These differences indicate that the primary mantlesignatures in the Oman ophiolite have been modifiedby interaction with a fluid Possible origins for thisfluid are mantle components dehydration of subductedlithosphere or seawater circulation Strontium andoxygen isotopes are useful tracers for fluid---crust interac-tion as d18O and 87Sr86Sr ratios from fluids originat-ing from seawater the mantle or subducted lithosphere
-1000
1000
2000
3000
4000
5000
6000
7000
MOHO
MTZ
Dis
tan
ce a
bov
e th
e M
oh
o (
m)
pillow-basalt
sheeted-dike
gabbro
peridotite
01 OA 41d2
01 OA 36b
02 OA 43a97 OA 128b
01 OA15f
94 OA MA2
02 OA 90b01 OA 19ad
02 OA 37b
0702 0703 0704 0705 0706 0707
87Sr86Srinitial
MO
RB-s
ou
rce
Seaw
ater
Whole RockPlagioclasePargasiteGreen HornblendeClinopyroxene
Epidote
(a)
-1000
1000
2000
3000
4000
5000
6000
7000
MOHO
MTZ
Dis
tan
ce a
bov
e th
e M
oh
o (
m)
pillow-basalt
sheeted-dike
gabbro
peridotite
01 OA 41d2
01 OA 36b
02 OA 43a97 OA 128b
01 OA15f
94 OA MA2
02 OA 90b01 OA 19ad
02 OA 37b
Whole RockPlagioclasePargasiteGreen HornblendeClinopyroxene
3 5 10
δ18O (permil)
41d2
Ma2
43a
90b 19d 19d37b
128b
90b
(b)
Fig 9 (a) Variation of initial 87Sr86Sr isotopic composition as a function of the location of the studied samples in a schematic log of the crustalsection of the Oman ophiolite The field for MORB is from Hart et al (1974) and that for seawater is from Burke et al (1982) Small open squares aredata fromKawahata et al (2001) (b) Variation of d18O () as a function of the location of the studied samples in a schematic log of the crustal sectionof the Oman ophiolite Shaded curve corresponds to the data of Gregory amp Taylor (1981)
BOSCH et al HYDROTHERMAL CIRCULATION IN OMAN OPHIOLITE
1201
are significantly different Moreover the d18O valueis dependent on temperature and mineral fractiona-tion and thus yields complementary constraints to Srisotopes
Mantle origin
One model to explain the occurrence of fluids interactingwith the oceanic crust at very high temperatures is toderive them from the mantle The contribution of mantle-derived hydrous fluids linked to percolation processesor to their concentration in the final stages of gabbrocrystallization has been invoked in numerous case studies(ie Witt amp Seck 1989 Fabriees et al 1989 Dupuy et al1991 Agrinier et al 1993) O and Sr isotopic data forwhole rocks and mineral separates yield scattered valuesthat are unambiguously different from MORB mantle-source values (Fig 9a and b) With the exception of lateLT altered samples the WR data have a mean 87Sr86Srratio of 070337 and a standard deviation of 000012These surprisingly high values in mantle-derived rockscould be taken as evidence for survival of mantle isotopicheterogeneities during crystallization of the ophiolite (egLanphere 1981 McCulloch et al 1981) However the18O16O isotope ratios in the Oman gabbroic sequenceare also displaced from primary magmatic values Nogabbro in the present study has plagioclase or amphi-bole with typical mantle d18O values Thus the Sr and Oisotope data rule out the possibility of mantle-derivedfluids as a possible source for the VHT---HT hydrousalteration event recorded in the massive gabbros andgabbroic dykes and veins (Fig 10a and b)
Subducted lithosphere origin
Dehydration of subducted oceanic crust in subductionzones can generate hydrous fluids Such fluids couldpercolate through the overlying lithosphere and contam-inate it thus generating modified isotopic signatures andtrace element contents Such an explanation howeverdisagrees with the geochemical characteristics of most ofthe studied samples Fluids derived from the dehydrationof subducted slabs (fresh or altered MORB or OIB pro-toliths) should be strongly enriched in K Rb and Ba (egBecker et al 2000) whereas in Oman the Rb contentsmeasured in the gabbros and gabbroic dykes and veins arestrongly depleted Moreover the isotopic composition ofslab-derived fluids is buffered by the MORB-like oceaniccrustal protolith for Nd and Pb but not for Sr and O(Staudigel et al 1995) As a result of relatively minorfractionation of oxygen at high temperature the oxygenisotope composition of the fluids expelled during dehy-dration of the subducted slab should be high and essen-tially controlled by the oxygen composition of the uppersection of the crust altered at low temperature (with an
average of 10 and perhaps as high as 19) (Staudigelet al 1995) The Sr isotopic composition should be alsohigh (average 07046) as the fluids released by the dehy-dration of subducted oceanic crust are associated eitherwith seawater or with radiogenic Sr phases (ie smectites)The Sr and O isotopic compositions of the studied sam-ples do not fit with these signatures and so preclude asubducted lithosphere origin for the fluids involved in theVHT---HT alteration event
Seawater-derived fluids
All the analysed samples (minerals and WR) have 87Sr86Sr(T) outside the domain of primary MORB magmasand variable Sr contents Individual minerals have Srcontents reflecting their different mineralliquid partitioncoefficients Minerals characterized by a very high affi-nity for Sr such as plagioclase (ie Sr mineralliquidpartition coefficients 1) have very high Sr contentsTherefore the high abundance of plagioclase in the sam-ples analysed is responsible for the high Sr content of thestudied whole rocks The Sr content of all the whole rocksis relatively homogeneous (Table 5) without clear distinc-tion between country-rock gabbros and dykes and veinsand ranges from 110 to 200 ppm except for the epidosite(4700 ppm) Reported on a 87Sr86Sr vs 1Sr diagram(Fig 10a) most whole rocks and plagioclases analysed arecharacterized by a 1Sr ratio lower than 001 and plot inthe left part of this diagram Compared with N-MORBthe studied massive and anatectic gabbros and their con-stituent plagioclases have similar to slightly higher Srcontents but substantially higher 87Sr86Sr ratios(Fig 10a) Considering Cretaceous seawater as a possiblehigh 87Sr86Sr mixing component [87Sr86Sr 07073at 90Ma after Burke et al (1982)] responsible for theobserved geochemical modifications exchanges cannothave occurred in a closed system as seawater has too lowa Sr content (ie 8 ppm de Villiers 1999) An open-system process allowing penetration of larger quantitiesof seawater can efficiently modify the Sr isotopic ratio ofthe rocks Alternately the fluids could also be seawaterthat was modified through exchange with the surround-ing rocks during its transit through the oceanic crustalsection This modified seawater would be more enrichedin Srwith a 87Sr86Sr ratio intermediate between unmodi-fied seawater and MORBMinerals devoid of late LT alteration imprints (parga-
site green hornblende and pyroxene) and characterizedby very low Sr contents plot on the right of the diagramclose to or on the mixing line between seawater andMORB (Fig 10a) The difference between these mineralsand the other samples (WR and plagioclases) is mainlydue to the Sr fractionation coefficients of these differentminerals and to the large quantity of plagioclase inthe studied rocks The process responsible for the
JOURNAL OF PETROLOGY VOLUME 45 NUMBER 6 JUNE 2004
1202
modification of the Sr isotopic composition fromMORB-source value is however explained by the same phenom-enon As all these minerals display initial 87Sr86Sr ratiosdistinct from the MORB source and because they equili-brated at VHT or HT temperatures this supports anearly intervention of seawater during crystallization ofthese minerals Most of these samples with the exceptionof the epidosite dyke overlap the domains defined byKawahata et al (2001) for the Wadi Fizh gabbros alteredat VHT and HT conditions in the amphibolite facies(labelled lsquoVHTrsquo and lsquoHTrsquo in Fig 9a)Three samples (two epidosite dykes and an one epidote
fraction) have very high Sr contents and the highest Sr
isotopic compositions of the present dataset The twowhole-rock samples overlap the field of the Wadi Fizhsamples altered at high temperature during greenschist-facies metamorphism (Kawahata et al 2001) This typeof alteration is common at the ridge axis and formedthrough intensive exchange with seawater-derived fluidsFigure 10b indicates that all the Sr---O isotope data are
distinct from MORB compositions and suggests that thefluids reacting with the magmas could be derived fromseawater In addition the d18O values show that isotopicexchange occurred under VHT or HT conditions Thefluids could have the composition of seawater or theycould have the composition of seawater modified through
0703
0705
0707
001 01 1
1 Sr (ppm)
MORB
(87Sr
86Sr)
init
ial
Seawater
VHT
HT
MT36b
36b
36b
43a 128b90b
41d2
37b19d
15f
128b
41d2
41d2
19a
Ma2Ma2
19d
90b
37b
Whole RockPlagioclasePargasiteGreen HornblendeClinopyroxene
Epidote
(a)
0 2 4 6 8
δ18O(permil)
0703
0705
0707Seawater
MORB-mantle
128b 41d2
90b 41d2
Ma2128b
37b
43a
19d
90b
Whole RockPlagioclasePargasiteGreen HornblendeClinopyroxene
Epidote
HT LT
19d
(87Sr
86Sr)
init
ial
(b)
Fig 10 (a) Initial 87Sr86Sr isotopic ratios plotted against 1Sr for whole rocks and mineral separates from the gabbro section of the Omanophiolite Fields shown are from Kawahata et al (2001) and correspond to MT-----basalt sheeted dyke diabase altered at medium temperature(prehnite---actinolite facies sequence 3) HT-----diabase plagiogranite metagabbro and epidosite formed at high temperature (greenschist faciessequence 4) VHT-----gabbro altered at very high temperature (amphibolite facies sequence 5) The dashed line is a binary mixing line betweenMORB (Lanphere et al 1981) and Cretaceous seawater (Burke et al 1982) (b) Initial 87Sr86Sr isotopic composition plotted against d18O value forwhole rocks and minerals from the Oman ophiolite Cretaceous seawater and MORB end-members as in (a) Dashed line represents mixing curvebetweenMORB and seawater at high temperature (HT) Low-temperature (LT) exchanges between these two components would be responsible foran increase of the d18O primary MORB value
BOSCH et al HYDROTHERMAL CIRCULATION IN OMAN OPHIOLITE
1203
exchange with the surrounding rocks during its penetra-tion into the crustal section In an environment in whichfluid---rock interaction occurs at variable temperaturesthe d18O is greatly influenced by the temperature atwhich the exchange took place and by the waterrockratio It is evident from Figs 9b and 10b that none of thestudied gabbros have plagioclase and amphibole withMORB-like d18O and 87Sr86Sr values The separateminerals display a broad range of d18O values and mostexhibit extreme 18O depletion (Table 5) Hydrothermalexchange kinetics are similar in amphibole and pyroxeneand these two minerals are more resistant to oxygenexchange during water---rock interaction than coexistingplagioclase (Gregory et al 1989) The most probableexplanation of the low d18O values measured in bothamphiboles and pyroxenes is that they crystallized athigh temperature in the presence of a relatively low 18Oseawater-derived hydrothermal fluid [d18O from 01 to07 after Gregory amp Taylor (1981)] The anomalouslyhigh d18O value (thorn1009) measured for the plagioclase ofthe micro-gabbroic dyke (01 OA19d) can be interpretedas resulting from a superimposed overprint caused by alate-stage penetration of fluids at lower temperature Inthis sample the occurrence of such different 18O16Oratios between coexisting plagioclase and amphibole isnot consistent with equilibrium fractionation betweenthese two minerals at any possible temperature Thisobservation suggests that parts of the ophiolite sequencehave undergone a high-temperature hydrothermal eventprior to the later low-temperature event that producedthe strong 18O increase in coexisting plagioclaseThe depletion of the d18O values results from high-temperature hydrothermal alteration (Gregory amp Taylor1981 Stakes amp Taylor 1992) The oxygen isotopic datasupport the contention that most studied samples havebeen affected by a VHT---HT hydrothermal alteration byfluids derived from seawater Some of the analysed sam-ples presenting petrological evidence of crystallization ofLT secondary minerals have been overprinted by the latelow-temperature alteration thus swamping the earlierhigh-temperature alteration effects
Determination of waterrock ratio
To better evaluate the exchange between seawater andthe gabbroic rocks waterrock ratios (WR) have beenestimated for the whole rocks analysed during the courseof this study The different models used to constrain thisratio mainly differ by the calculation of the effective ratioor by the estimated weight ratio and by considering openor closed systems for water circulation (Albareede et al1981 McCulloch et al 1981) The WR ratios reportedin Table 5 have been calculated assuming a closed sys-tem Using this formulation these values are minimabecause the calculation always uses pristine fluid for the
initial fluid thus maximizing the value in the denomina-tor If the fluid was shifted to lower 87Sr concentrations byexchange with the rock on the downward path theinferred values would be much higher (Davis et al 2003)The calculated WR ratios for the samples from this
study range from 31 to 219 (Table 5) Two main groupsof samples can be recognized on the basis of their waterrock ratios The lowest ratios ranging from 31 to 47have been determined for massive gabbros or anatecticgabbros but also for dykes and veins devoid of late LTalteration Similar ratios have been previously calculatedfor example for the discharged hydrothermal solutions atthe 13N and 21N East Pacific Rise (Di Donato et al1990 Fouquet et al 1991) The gabbros from the Ibrasection in the Oman ophiolite gave effective WR ratiosof 05 33 and 42 (McCulloch et al 1981) in goodagreement with the values obtained in this study Theleast altered gabbro of the Ibra section yields a WR ratioof 05 in good agreement with the exceptionally fresh-ness of this rock characterized by typical mantle oxygenvalues for its minerals (McCulloch et al 1981) Samplescharacterized by waterrock effective ratios in the rangeof 3---5 can be related to penetrative alteration via thehydrothermal system Lecuyer amp Reynard (1996) havedemonstrated in oceanic gabbros from the Hess Deepaltered in similar HT conditions that WR mass ratios inthe range of 02---1 are sufficient to account for the mod-ified stable isotope signature of the gabbros Higherwaterrock values (WR 45) have been calculated forsamples affected by superimposed late hydrothermalalteration and for the epidosite samples (Table 5) Theyprobably acquired their isotopic characteristics throughinteraction with a larger volume of seawater duringgreenschist-facies metamorphism Similar or higherWR values have been published in previous studies(ie McCulloch et al 1981 Kawahata et al 2001)They correspond either to samples from the upper-crus-tal sequence (ie basalt sheeted dyke or plagiogranite) orto gabbros from the crustal section located in a particularenvironment such as for example the Wadi Fizh sectionthat is located close to a segment boundary along thepalaeo-spreading axis (Nicolas et al 2000)
Mechanism for seawater interaction withmagma
Nicolas et al (2003) have proposed that variable amountsof seawater can circulate at high temperature through-out the crustal section down to the walls of the magmachamber via a network of parallel microcracks a fractionof a millimetre wide Such microcracks and their relation-ship with the VHT---HT hydrous alteration in the sur-rounding gabbros have been described in detail byNicolas et al Via this network of microcracks largeamounts of seawater can have reacted with the magma
JOURNAL OF PETROLOGY VOLUME 45 NUMBER 6 JUNE 2004
1204
and induced variable changes of its Sr and O isotopiccomposition This regular network of parallel micro-cracks could constitute the recharge system and thehydrated gabbroic dykes the discharge system Physicalparameters and the geophysical models of Nicolas et al(2003) are in agreement with this scenario Such a pene-tration of seawater-derived hydrothermal fluids throughthe lower crust along grain boundaries and a microscopicfracture network at temperatures 4750C is consistentwith recent thermodynamic models (McCollom amp Shock1998)Seawater-altered and high-temperature recrystallized
diabases (protodykes) from the overlying sheeted dykecomplex stoped into the melt lenses could represent anindirect way for seawater to enter the system as they canremelt during their subsidence through the magmachamber A simple mass balance calculation suggeststhat this indirect contamination is not significant Assess-ments of the amount of recycled crust 1 in volumeby Nicolas et al (2000) based on the protodyke remnantsin the gabbro section or 20 by Coogan et al (2003)based on chlorine content in EPR basalts lead to a sea-waterrock ratio of the order of 104 to 103Thus following Nicolas et al (2003) we propose that
seawater has been introduced along microcracks down tothe walls of the magma chamber and has diffused alonggrain boundaries in the way described by Manning et al(2000) progressively invading the host gabbro Gabbroicdykes result from the injection of hydrous melt into thehost gabbros at falling temperatures as they drift awayfrom the magma chamber This water-saturated meltcould result from the extensive hydration of the crystal-lizing gabbros near the walls of the magma chamberwhen seawater penetrates through this wall (Fig 1)Similar plumbing systems driving seawater down to the
limit of the magma chamber and back to the surface havebeen already proposed in other environments such as theMid-Atlantic Ridge (Kelley amp Delaney 1987 Cooganet al 2001) the East Pacific Rise at Hess Deep (Gillis1995 Manning et al 1996a 1996b) the Tonga forearc(Banerjee amp Gillis 2001) and the Troodos ophiolite(Gillis amp Roberts 1999) The melting curve of wet basalthas a negative slope but is close to the dry solidus at lowpressure in the lower gabbros and the first melts areplagiogranitic (Spulber ampRutherford 1983) Such plagio-granites are ubiquitous in the highest gabbros and in theroot zone of sheeted dykes in Oman (Rothery 1983Juteau et al 1988 Nicolas amp Boudier 1991) in manyother ophiolites and in the oceanic crust where they havebeen interpreted as resulting either from wet anatexis ofhot gabbros or from the final product of crystallization ofa basaltic melt (Spulber amp Rutherford 1983) Plagio-granites are rare in the lower gabbros exceptionallyassociated with hydrous gabbro segregations inside thelayered gabbros Their constitutive minerals are also
remarkably absent along the VHTmicrocracks althoughthese gabbros were above the wet solidus A possibleexplanation has been proposed by McCollom amp Shock(1998) who wrote that at high temperature lsquoaqueousfluids may leave very little conspicuous petrological evid-ence for their presencersquo the reason being that thehigh-T conditions would be closer to batch meltingObviously more detailed studies on this specific problemare needed We conclude that the large degree of hydrousmelting locally required at the walls of the magma cham-ber to account for the generation of a gabbroic hydrousmelt may have erased the traces of the incipient plagio-granitic melt
CONCLUSIONS
Fostered by the recent discovery of high-temperatureseawater contamination in some gabbros of the Omanophiolite (Manning et al 2000) we have conducted astudy on 500 gabbro specimens from selected areas ofthe entire Samail ophiolite belt and on the hydrous gab-broic dykes that cross-cut them The highest-temperaturereactions (VHT) recorded in olivine and clinopyroxene ingabbros as orthopyroxene and pargasite coronas reach975C They are followed at lower HT conditions withreplacement of olivine by an assemblage of tremolitemagnetite and plagioclase surrounded by chlorite andby pargasite development in clinopyroxene followedfinally by the LT lizardite---olivine and saussurite---plagioclase greenschist-facies alteration With a fewexceptions the VHT alteration tends to be restricted tothe lower gabbros and to the vicinity of the Moho andthe HT alteration to the middle and upper gabbros Thepreservation of the vertical distribution of VHT---HTfacies indicates that progressive cooling during off-axisspreading did not obliterate this distribution and conse-quently that the VHT---HT hydrothermal alteration tookplace in a very restricted time interval at the limit of thecooling magma chamber In the deep and hot gabbrosthe main water channels may be sub-millimetre-sizedmicrocracks with a dominantly vertical attitude the ori-gin and dynamics of which have been investigated in aprevious paper (Nicolas et al 2003)Beyond these high-temperature alteration zones mas-
sively induced in the gabbros seawater may be respons-ible for the generation of clinopyroxene---pargasitegabbro dykes that cross-cut all gabbros and that areupsection clearly connected to the LT hydrothermalvein system Petrostructural evidence suggests that thesedykes were generated by hydration of the crystallizinggabbros near the internal wall of the LVZ---magmachamber The resultant melts were subsequently intrudedat various levels in the cooling lower and middle gabbrosPetrological observations of nearly all the gabbros ana-lysed in the present study located from the root zone of
BOSCH et al HYDROTHERMAL CIRCULATION IN OMAN OPHIOLITE
1205
the sheeted dyke complex down to the Moho emphasizethe imprint of VHT---HT hydrous alteration The VHThydrothermal event is characterized by temperatures ofequilibration around 900---1000C The temperatures atwhich the HT hydrous event occurred are estimated tobe in the range 550---900CStrontium and oxygen isotope compositions measured
on whole rocks and separated minerals significantlydepart from primary MORB values The Sr and O iso-tope data obtained for pargasite green hornblende andclinopyroxene record the participation of seawater at thetemperature of crystallization of the minerals Plagio-clases and whole rocks define a trend toward a compo-nent characterized by a high radiogenic 87Sr86Sr valueand a higher Sr content than normal seawater Seawateris clearly identified as the fluid responsible for the modi-fication of the Sr and O signatures for both massivegabbros and dykes and veins Nevertheless the high Srcontent of the extraneous component requires eitheropen-system exchange allowing penetration of a greaterquantity of seawater or penetration of seawater modifiedby interaction with the oceanic crust during its travel paththrough the crustal section The waterrock ratio calcu-lated on the basis of the Sr isotopes is between three andfive for the enclosing gabbros and for the least LT altereddykes The VHT---HT hydrothermal event affectingthese samples is related to penetrative alteration via therecharge and discharge hydrothermal system Thewaterrock ratio is 10 for dykes exhibiting evidenceof a LT hydrothermal alteration overprint and reaches22 for the epidosite dyke The depletion in d18O char-acterizes all the studied samples with the single exceptionof the micro-gabbroic dyke plagioclase This agreeswith the conclusions based on Sr isotopes suggestingthat these samples have undergone exchange with sea-water-derived fluids during a VHT---HTalteration hydro-thermal event
ACKNOWLEDGEMENTS
This work has benefited from financial support by theCentre National de la Recherche Scientifique (INSUInteerieur-Terre andDYETI programmes) and inOmanfrom the willingness of the Directorate of MineralsMinistry of Commerce and Industry and the hospitabil-ity of the people Technical help in the laboratory is alsoacknowledged including C Nevado for the thin sectionsE Ball for the illustrations B Galland for her assistancein the chemistry room and C Bassoulet for help duringthermal ionization mass spectrometric analyses of the Srresults from the leaching experiments Constructivereviews from R T Gregory C Lecuyer an anonymousreviewer and particularly from M Wilson greatlyhelped to improve an earlier version of the manuscript
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Banerjee N R amp Gillis K M (2001) Hydrothermal alteration in a
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Becker H Jochum K P amp Carlson R W (2000) Trace element
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Nelson H F amp Otto H F (1982) Variation of seawater 87Sr86Sr
throughout Phanerozoic time Geology 10 516---519Carmichael I S E Turner F J amp Verhoogen J (1974) Igneous
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Chernosky J V Berman R G amp Bryndzia L T (1988) Stability
phase relations and thermodynamic properties of chlorite and
serpentine group minerals In Bayley S W (ed) Hydrous
Phyllosilicates Mineralogical Society of America Reviews in Mineralogy 19
295---346
Coogan L A Wilson R N Gillis K M amp MacLeod C J (2001)
Near-solidus evolution of oceanic gabbros insights from amphibole
geochemistry Geochimica et Cosmochimica Acta 65 4339---4357
Coogan L A Mitchell N C amp OrsquoHara M J (2003) Roof
assimilation at fast spreading ridges an investigation combining
geophysical geochemical and field evidence Journal of Geophysical
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1171
Davis A C Bickle M J amp Teagle D A H (2003) Imbalance in the
oceanic strontium budget Earth and Planetary Science Letters 211
173---187
Detrick R S Buhl P Vera E Mutter J Orcutt J Madsen J amp
Trocher T (1987) Multi-channel seismic imaging of a crustal
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De Villiers S (1999) Seawater strontium and SrCa variability in the
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Di Donato G Joron J L Treuil M amp Loubet M (1990)
Geochemistry of zero-age N-MORB from hole 648B ODP legs
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College Station TX Ocean Drilling Program pp 57---65
Dunn R A Toomey D R amp Solomon S C (2000) Three-
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Fabriees J Bodinier J L Dupuy C Lorand J P amp Benkerrou C
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lherzolite body from Caussou (northeastern Pyrenees France)
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Fouquet Y Von Stackelberg S U Charlou J L Donval J P
Foucher J Erzig P Muhe R Wiedicke M Soakai S amp
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Gillis K M (1995) Controls on hydrothermal alteration in a section of
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Gillis K M amp Roberts M (1999) Cracking at the magma---hydrothermal transition evidence from the Troodos ophiolite Earth
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Gregory R T amp Taylor H P (1981) An oxygen isotope profile in a
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Gregory R T Criss R E amp Taylor H P (1989) Oxygen
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Hart S R Erlank A J amp Kable J D (1974) Sea floor basalt
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Holland T amp Blundy J (1994) Non-ideal interactions in calcic
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Ionov D A Savoyant L amp Dupuy C (1992) Application of the
ICP-MS technique to trace element analysis of peridotites and their
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Javoy M (1980) 18O16O and DH ratios in high temperature
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Juteau T Manacrsquoh G Moreau C Lecuyer C amp Ramboz C
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and oxygen isotopic ratios Marine Geophysical Research 21 351---385Kawahata H Nohara M Ishizuka H Hasebe S amp Chiba H
(2001) Sr isotope geochemistry and hydrothermal alteration of the
Oman ophiolite Journal of Geophysical Research 106 11083---11099
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Kelemen P B Koga K amp Shimizu N (1997) Geochemistry of
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and Planetary Science Letters 146 475---488Kelley D S amp Delaney J R (1987) Two-phase separation and
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Koepke J Feig S T Snow J amp Freise M (2003) Petrogenesis of
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Science Letters 91 327---340
JOURNAL OF PETROLOGY VOLUME 45 NUMBER 6 JUNE 2004
1208
Table4Majorelementcomposition
aofselectedam
phibolespaired
withplagioclasecompositionsusedforthermometry
Sam
ple
nam
e1Mode2
SiO
2TiO
2Al 2O3
Cr 2O3
FeO
MnO
MgO
CaO
Na 2O
K2O
Total
Si
AlIV
AlVI
Ti
Fe3
thornCr
Mg
Fe2
thornMn
Na
KCa
XAn3
T(C)4
01OA19a
Mg-hornblende
core
45 60
26
10 78
009
860
015
16 56
12 00
212
007
96 72
6552
1448
0366
0083
0552
0004
3409
0594
0016
0611
0018
1843
0903
899
Mg-hornblende
core
47 99
263
850
002
897
011
16 61
12 10
160
020
96 36
6894
1106
0334
0028
0483
0002
3557
0594
0014
0446
0036
1862
0892
834
Mg-hornblende
rim
51 17
247
596
001
761
014
18 11
12 19
103
010
96 47
7257
0743
0253
0016
0414
0001
3828
0488
0016
0284
0018
1853
0894
820
Mg-hornblende
rim
47 50
291
920
004
880
014
16 46
11 95
174
008
96 07
6835
1165
0396
0015
0506
0005
3529
0552
0017
0486
0015
1843
0887
825
Mg-hornblende
rim
49 57
269
742
000
799
017
18 07
12 02
133
014
96 88
7013
0987
0261
0018
0575
0000
3811
0371
0020
0365
0026
1821
0860
831
Mg-hornblende
core
47 83
181
872
003
840
011
14 48
12 18
159
008
93 59
6814
1186
0279
0018
0644
0004
3711
0357
0013
0438
0015
1859
0907
874
Pargasite
rim
42 15
249
11 87
024
10 07
013
13 84
11 43
263
032
96 61
6188
1812
0242
0434
0299
0028
3028
0937
0016
0749
0059
1797
0804
972
Pargasite
rim
42 22
260
11 86
026
10 21
011
13 71
11 48
253
032
96 59
6201
1799
0254
0429
0299
0030
3001
0955
0014
0720
0060
1807
0803
962
Pargasite
core
42 05
263
11 45
030
10 19
011
13 80
11 47
246
032
96 46
6189
1811
0175
0478
0299
0035
3029
0956
0014
0702
0060
1809
0767
974
Pargasite
rim
42 36
247
11 90
016
945
013
14 16
11 76
249
026
95 98
6239
1761
0305
0366
0276
0019
3109
0888
0016
0710
0049
1856
0818
926
Pargasite
core
42 50
291
11 64
016
984
015
14 22
11 41
256
034
96 69
6219
1781
0227
0426
0322
0018
3103
0883
0018
0727
0063
1789
0812
974
Pargasite
rim
42 76
270
12 03
017
920
012
14 87
11 82
244
025
96 73
6224
1776
0880
0335
0368
0020
3226
0752
0015
0689
0047
1843
0841
941
Pargasite
rim
42 13
181
13 54
016
921
009
12 77
12 13
254
032
96 98
6172
1828
0510
0450
0000
0018
2788
1129
0011
0721
0059
1905
0841
874
01OA19d
Mg-hornblende
49 26
047
624
011
11 46
017
15 39
12 21
108
009
96 48
7137
0863
0202
0051
0437
0013
3323
0951
0021
0302
0018
1896
0768
771
Mg-hornblende
46 03
287
932
018
906
017
14 77
12 87
193
007
97 28
6672
1328
0265
0313
0046
0021
3191
1053
0021
0542
0012
1999
0758
808
Pargasite
41 89
429
11 65
008
11 03
017
13 52
11 37
259
017
96 77
6159
1841
0179
0474
0345
0010
2963
1012
0021
0739
0032
1791
0770
975
Pargasite
rim
42 80
340
11 16
009
11 01
018
13 73
11 67
240
022
96 67
6293
1707
0226
0376
0322
0010
3009
1032
0022
0684
0041
1839
0605
880
Pargasite
core
42 58
351
11 27
006
11 63
018
13 51
11 46
239
022
96 82
6257
1743
0209
0387
0391
0007
2959
1039
0023
0680
0042
1804
0595
893
Pargasite
rim
42 14
393
11 28
006
11 71
014
13 35
11 33
235
033
96 62
6214
1786
0174
0435
0391
0007
2935
1054
0018
0673
0062
1790
0519
901
Pargasite
core
41 56
420
12 05
006
11 57
016
12 51
11 35
249
027
96 24
6168
1832
0277
0469
0276
0007
2768
1160
0021
0718
0052
1805
0537
882
Pargasite
core
41 33
438
11 49
008
11 66
015
13 35
11 45
264
026
96 80
6105
1894
0107
0487
0368
0009
2941
1073
0019
0757
0048
1813
0537
942
Pargasite
41 08
446
12 39
012
11 35
017
13 13
11 36
263
015
96 85
6049
1951
0199
0494
0368
0014
2882
1030
0021
0752
0027
1792
0758
975
02OA37b
Mg-hornblende
rim
45 15
260
984
002
13 28
019
14 01
10 87
128
009
97 33
6527
1473
0203
0283
0690
0002
3018
0915
0023
0360
0016
1683
0562
828
Mg-hornblende
rim
44 72
263
988
004
13 23
021
14 01
10 78
133
009
96 92
6494
1506
0185
0287
0713
0005
3032
0894
0025
0374
0018
1677
0593
854
Mg-hornblende
rim
45 68
247
935
004
12 87
020
14 10
11 00
121
011
97 02
6621
1379
0218
0269
0598
0004
3045
0962
0024
0341
0020
1708
0562
815
Mg-hornblende
core
43 94
291
10 87
001
12 60
023
13 73
10 59
142
011
96 39
6410
1590
0278
0319
0644
0001
2985
0893
0028
0401
0021
1655
0710
860
Mg-hornblende
core
44 62
270
10 37
001
12 66
019
14 12
10 58
140
008
96 73
6479
1521
0253
0294
0644
0001
3057
0893
0023
0395
0014
1646
0674
851
Mg-hornblende
rim
47 79
181
796
002
12 09
025
15 22
11 09
100
009
97 33
6866
1134
0214
0196
0506
0002
3260
0947
0030
0280
0016
1706
0503
767
Mg-hornblende
core
45 43
249
10 3
000
11 99
022
14 50
11 14
133
008
97 48
6524
1476
0267
0269
0644
0000
3103
0796
0027
0370
0015
1715
0703
838
JOURNAL OF PETROLOGY VOLUME 45 NUMBER 6 JUNE 2004
1194
Sam
ple
nam
e1Mode2
SiO
2TiO
2Al 2O3
Cr 2O3
FeO
MnO
MgO
CaO
Na 2O
K2O
Total
Si
AlIV
AlVI
Ti
Fe3
thornCr
Mg
Fe2
thornMn
Na
KCa
XAn3
T(C)4
02OA90b
Mg-hornblende
core
48 83
038
745
005
939
010
17 29
12 24
142
006
97 21
6931
1069
0041
0041
0667
0005
3657
0448
0011
0392
0010
1862
0840
856
Mg-hornblende
core
49 49
033
731
003
916
008
17 40
12 57
139
005
97 82
6987
1013
0035
0035
0552
0004
3662
0530
0010
0379
0009
1901
0830
811
Mg-hornblende
rim
48 90
040
753
005
932
013
17 15
12 22
154
005
97 29
6948
1052
0042
0042
0575
0006
3632
0533
0015
0424
0010
1861
0880
869
Mg-hornblende
rim
48 30
044
800
007
962
010
16 86
12 32
161
005
97 35
6873
1127
0047
0047
0598
0008
3576
0546
0012
0443
0010
1879
0870
862
Mg-hornblende
rim
49 12
032
742
010
922
011
17 36
12 29
142
006
97 43
6956
1044
0034
0034
0621
0011
3665
0471
0013
0391
0010
1865
0840
845
02OA43a
Mg-hornblende
rim
48 13
040
789
007
860
007
17 41
12 19
155
006
96 36
6882
1118
0212
0043
0621
0008
3710
0407
0008
0429
0010
1867
-------
-------
Mg-hornblende
rim
50 93
035
643
001
759
006
18 50
12 24
114
003
97 28
7152
0848
0217
0037
0506
0001
3873
0385
0007
0311
0005
1841
-------
-------
Mg-hornblende
rim
50 67
014
559
009
12 33
026
15 28
12 44
068
003
97 50
7260
0740
0204
015
0483
0010
3263
0994
0032
0189
0005
1909
-------
-------
Mg-hornblende
rim
50 34
013
621
009
12 49
027
15 07
12 51
064
003
97 78
7192
0810
0240
0010
0530
0010
3210
0960
0030
0180
0010
1910
-------
-------
Mg-hornblende
rim
45 17
060
11 14
013
988
008
15 78
12 33
211
009
97 31
6473
1527
0355
0640
0644
0015
3371
0540
0010
0588
0016
1892
-------
-------
Mg-hornblende
rim
49 25
032
712
014
845
009
17 84
12 32
137
004
96 95
6983
1020
0170
0030
0620
0020
3770
0380
0010
0380
0010
1870
-------
-------
Mg-hornblende
rim
48 61
032
775
014
975
011
16 81
12 57
139
004
97 50
6905
1950
0203
0034
0621
0016
3560
0537
0013
0382
0008
1914
-------
-------
94Ma2
Anthophyllite
54 98
000
259
001
12 65
037
23 35
199
034
000
96 28
7744
0256
0174
0000
0161
0001
4903
1330
0043
0092
0001
0301
-------
-------
Anthophyllite
55 11
000
296
003
14 09
032
23 66
063
038
000
97 18
7716
0284
0205
0000
0115
0003
4938
1535
0038
0104
0000
0095
-------
-------
Actinolite
48 58
004
954
002
960
014
17 67
999
139
005
97 02
6850
0150
0435
0004
0690
0002
3713
0442
0017
0379
0009
1510
-------
-------
Actinolite
51 54
004
585
000
890
022
20 28
930
090
002
97 05
7214
0786
0178
0005
0598
0000
4230
0444
0026
0244
0004
1395
-------
-------
01OA41d2
Pargasite
rim
43 05
342
10 71
008
11 46
012
13 50
11 54
179
067
96 34
6353
1647
0216
0380
0368
0009
2967
1046
0015
0512
0127
1824
0613
863
Pargasite
rim
42 61
354
11 59
001
10 59
010
13 52
11 87
160
076
96 19
6286
1714
0302
0392
0299
0001
2972
1007
0012
0458
0143
1877
0715
844
Pargasite
core
42 54
401
11 04
006
11 81
013
13 21
11 50
202
039
96 71
6266
1734
0182
0444
0368
0007
2901
1087
0017
0576
0073
1814
0854
979
Pargasite
rim
43 03
385
10 72
002
11 70
014
13 50
11 95
146
080
97 17
6314
1686
0168
0425
0345
0002
2953
1091
0018
0415
0149
1878
0634
852
Pargasite
rim
42 65
341
10 60
006
11 97
014
13 22
11 64
183
068
96 20
6332
1668
0186
0381
0345
0007
2926
1141
0018
0528
0129
1851
0620
869
Pargasite
core
42 82
367
10 60
007
11 93
012
13 42
11 44
206
047
96 60
6315
1685
0158
0407
0391
0008
2950
1080
0016
0588
0089
1808
0615
899
Pargasite
rim
42 2
362
11 10
007
11 42
016
13 09
11 72
179
072
95 89
6282
1718
0229
0405
0299
0009
2905
1123
0020
0518
0137
1869
0715
875
Pargasite
rim
42 42
380
11 03
006
11 77
012
13 00
11 72
188
069
96 49
6283
1717
0209
0423
0299
0007
2870
1158
0015
0539
0131
1860
0817
928
Pargasite
rim
42 21
364
11 14
006
10 73
011
13 53
11 68
177
072
95 59
6277
1723
0229
0407
0322
0007
2999
1012
0014
0511
0136
1862
0736
883
Pargasite
rim
42 25
376
11 03
008
10 93
015
13 66
11 72
185
064
96 07
6257
1743
0182
0418
0345
0009
3016
1009
0019
0531
0121
1860
0766
912
Pargasite
core
42 17
396
10 91
008
11 10
010
13 57
11 49
209
038
95 85
6253
1747
0159
0442
0368
0010
3000
1009
0013
0602
0073
1826
0841
981
aMajorelem
entco
mpositionsarein
weightpercent-------noplagioclasean
alysed
andnotemperature
calculated
1Typ
eofam
phibolean
alysed
isalso
indicated
2Blankindicates
nozoningobserved
3Molefractionofan
orthitein
plagioclasead
jacentto
amphibolegrain
4Tem
perature
ofeq
uilibrium
fortheam
phibole---plagioclasepaircalculatedusingtheHollandamp
Blundythermometer
(1994)Errors
are40
C
BOSCH et al HYDROTHERMAL CIRCULATION IN OMAN OPHIOLITE
1195
zonation from An95 to An99 from core to rim (Table 1 andFig 8a) TheWR Sr isotopic ratio of the sample (070458)is significantly higher than normal MORB values Thiscould be related to the overprint of a low-temperaturealteration event consistent with the petrography of thesample
Green amphibole vein cross-cutting layered gabbros inWadi Mayah Haylayn Massif
Sample 02 OA43a belongs to a series of 1 cm thick greenamphibole veins (as in Fig 6e) cross-cutting the layeredgabbros The veins develop 15 cm thick reaction marginsin the enclosing gabbro and are exclusively composed ofacicular pale green magnesio-hornblende growing per-pendicular to the vein margin in a crack-seal mode(Fig 6f ) The reaction margins are composed of urali-tized gabbro marked by the development of the samepale green magnesio-hornblende in an inhomogeneouscalcic plagioclase matrix (An91---97) The Sr and O isotopiccompositions of this green hornblende (070431 andthorn31) do not correspond to normal MORB-sourcemantle values
Epidosite dyke from Wadi Gideah Wadi Tayin Massif
Sample 01 OA36b is representative of a series of epidoteveins cross-cutting poorly layered gabbros These 2 cmthick veins are composed of pink thulite in their centreand green pistachite at their margins (Fig 6a) Theirsharp contact with the enclosing gabbro is marked bythe development of coarse clinopyroxene crystals alteredin the epidote---greenschist facies This suggests that theepidote vein is replacive in a coarse-grained gabbro dykeThe highest 87Sr86Sr initial ratios of all the samplesstudied are recorded by the WR and coexisting epidoteof this sample (070519 and 070554 respectively)(Table 5) The Sr content of the WR and associatedepidote are also the highest of the present dataset(716 ppm and 764 ppm respectively) The Sr isotopiccomposition of this sample is consistent with a greaterdegree of exchange with a radiogenic component fromCretaceous seawater It is significantly higher than thevalue obtained for an epidosite vein from the Wadi Fizhsection of the Oman ophiolite [070435 with a Sr contentof 488 ppm reported by Kawahata et al (2001)] but ingood agreement with the average of the greenschist-faciesalteration sequence defined by Kawahata et al (2001)
GABBROS
01 km 025 km 1 km 36 km50
60
70
80
90
An
MA2
19d
19a 15f 128b
41d2
(a)
43a17b
15f
90b
37b
19a
19d
40
60
80
100
A
n
01 km 015 km 025 km 1 km
DYKES
(b)
Distance to the Moho
ACTINOLITE
MAGNESIOHORNBLENDE
TSCHERMAKITE
PARGASITEGabbros Dykes41d2MA2
19a d37b15f
90b43a
AlIV
150500
02
08
10
(Na
+ K
) A
(c)
04
06
950
900
850
800
750
70007 12 17
T degC
1000
(d)
AlIV
Fig 8 Plagioclase---amphibole compositions and results of thermometry (a) and (b) plagioclase compositions in layered gabbros and in dykesrespectively as a function of distance to the Moho (see text for sample identification) (c) amphibole compositions in layered gabbros and dykesaccording to Leakersquos (1997) nomenclature (d) equilibration temperatures calculated using the amphibole---plagioclase thermometer of Holland ampBlundy (1994) as a function of AlIV content in amphibole [same symbols as in (c)]
JOURNAL OF PETROLOGY VOLUME 45 NUMBER 6 JUNE 2004
1196
Table5RbandSrcontents87Sr
86Srandd1
8Oisotopiccompositionsofwholerocksandmineralseparatesfrom
samplesofmassivegabbrosdykes
andveinsfrom
theOman
ophiolitealsolistedarecalculated
waterrockratios(W
R)andLOIvalues
Sam
ple
Rb(ppm)
Sr(ppm)
87Rb8
6Sr
87Sr
86Sr
(87Sr
86Sr)i1
d18O
()
LOI(
)2WR
3(87Sr
86Sr)L14
(87Sr
86Sr)L25
(87Sr
86Sr)R6
01OA41d2
Whole
rock
037
1792
0006
0703516
09
070351
thorn480
138
47
0703587
0703651
0703357
Plagioclase
025
1249
0003
0704261
22
070426
thorn504
-------
-------
Pargasite
-------
-------
-------
0703548
20
070355
-------
-------
-------
Clinopyroxene
045
26 0
0050
0703845
19
070378
-------
-------
-------
01OA36b
Dykewhole
rock
0005
7164
50001
0705186
17
070519
-------
274
21 9
0705207
0705112
-------
Epidote
0003
7645
0001
0705536
12
070554
-------
-------
-------
Wallwhole
rock
005
4254
0001
0704637
09
070464
-------
191
-------
0704945
0704675
0704593
02OA43a
Green
hornblende
004
98
0012
0704323
13
070431
thorn310
-------
-------
97OA128b
Whole
rock
003
1274
50001
0703327
17
070333
-------
076
37
-------
-------
-------
Plagioclase
002
1204
50001
0703285
09
070328
thorn414
-------
-------
Clinopyroxene
003
20 0
0004
0704230
09
070422
thorn523
-------
-------
01OA15f
Whole
rock
006
1077
0002
0704582
16
070458
-------
237
13 5
-------
-------
-------
01OA19a
Whole
rock
037
1303
0008
0704189
18
070418
-------
161
96
-------
-------
-------
01OA19d
Whole
rock
058
2032
0008
0703423
08
070341
thorn567
216
415
0703574
0703408
0703271
Pargasite
091
1058
0025
0703273
11
070324
thorn287
-------
-------
Plagioclase
-------
-------
-------
-------
-------
thorn10 09
-------
-------
02OA90b
Plagioclase
004
2385
50001
0703868
11
070387
thorn498
-------
-------
Green
hornblende
010
23 8
0012
0704288
15
070427
thorn239
-------
-------
BOSCH et al HYDROTHERMAL CIRCULATION IN OMAN OPHIOLITE
1197
Table5continued
Sam
ple
Rb(ppm)
Sr(ppm)
87Rb8
6Sr
87Sr
86Sr
(87Sr
86Sr)i1
d18O
()
LOI(
)2WR
3(87Sr
86Sr)L14
(87Sr
86Sr)L25
(87Sr
86Sr)R6
02OA37b
Plagioclase
023
3121
0002
0704204
15
070420
-------
-------
-------
Pargasite
031
41 6
0021
0703505
15
070348
thorn394
-------
-------
94OAMa2
Whole
rock
0016
1750
00003
0703219
15
070322
thorn495
033
31
0703264
0703177
0703158
Plagioclase
0014
1363
00003
0703219
11
070322
thorn479
-------
-------
Clinopyroxene
-------
-------
-------
0703236
13
070324
-------
-------
-------
1Initial8
7Sr
86Srratiocalculatedat
95Ma(H
acker1994)
2LOIisloss
onignition(in)
3Water---rock
ratiocalculatedassumingaclosedsystem
TheeffectiveWR
ratiocanbeexpressed
bythefollowingeq
uation
W=Reffectivefrac14
frac12eth87Sr=
86SrrockTHORN fi
nal
eth87Sr=
86SrrockTHORN in
itial=frac12eth8
7Sr=
86SrseawaterTHORN in
itial
eth87Sr=
86SrrockTHORN fi
nal
ethCrock
initial=C
seaw
ater
initial
THORNwhere(87Sr
86Srrock) finalisthefinalmeasuredSrisotopicratioin
thesample(87Sr
86Srrock) in
itialco
rrespondingto
theinitialm
antlevalue
istakenas
070295(Lan
phere
etal1981)(87Sr
86Srseawater) in
itialistheCretaceousseaw
ater
Srisotopicco
mposition[87Sr
86Sr07073
at90
MaafterBurkeet
al(1982)]C
seaw
ater
initial
istheSrco
ntent
oflsquonorm
alrsquoseaw
ater
andis
takenas
8ppm
(deVilliers1999)
FortheCrock
initialitis
assumed
that
thepresentSrco
ncentrationmeasuredis
thesameas
theinitial
concentration
4Srisotopicratiosmeasuredforliquid
L1extractedafterthefirststep
oftheleachingexperim
ent(6NHClfor8min
at70
C)
5Srisotopicratiosmeasuredforliquid
L2extractedaftertheseco
ndstep
oftheleachingexperim
ent(2 5NHClfor30
min
at70
C)
6Srisotopicratiosmeasuredforresidueobtained
afteratotalaciddigestionachievedaftertheextractionofL1an
dL2leachates
JOURNAL OF PETROLOGY VOLUME 45 NUMBER 6 JUNE 2004
1198
Part of the same sample from the contact between thedyke and the gabbro yields a slightly lower Sr isotopiccomposition (070464) intermediate between the sur-rounding gabbro and the epidote dyke It also has anintermediate Sr content (425 ppm) Acid leaching experi-ments performed on the whole rock of the dyke yield Srisotopic compositions of 070521 and 070512 for thetwo liquids extracted Similar experiments for the samplelocated at the contact with the gabbro yield 87Sr86Srratios of 070494 and 070467 for L1 and L2 liquids TheSr isotopic composition of the residue extracted afterleaching remains high (070459) and very close to theepidosite 87Sr86Sr ratio determined by Kawahata et al(2001) In this sample the primary minerals have beentotally replaced by epidote and secondary plagioclase orquartz Thus the Sr isotopic compositions measured forthis WR sample and its constituent minerals reflect thecomposition of the fluid filling this dyke
Mineral chemistry
Plagioclase
Compared with the host gabbros the dykes show a muchlarger dispersion in their plagioclase compositions (Fig 8aand b Table 4) characterized by more calcic plagioclasethan the enclosing gabbros This tendency is mostextreme in the comb-textured dykes in which the plagio-clase is almost pure anorthite The zoning is generallynormal recording a crystal fractionation process How-ever gabbro 94 MA2 exhibits inverse zoning this tend-ency is also observed in plagioclases from dykes 01OA19a and 01 OA15f The higher calcium content ofthe plagioclase as well as the inverse zoning is consistentwith an increase of water pressure during its crystalliza-tion (Turner amp Verhoogen 1960 Carmichael et al 1974Koepke et al 2003) The particularly high An content inthe comb-textured gabbro dykes 01 OA15f may alsoresult from crystallization in a supercooled magma thisis consistent with the comb texture indicative of fast-growing crystals The lack of brown hornblende suggeststhat crystallization occurred above the temperature ofhornblende stability
Amphibole
The amphiboles analysed in the dykes (Fig 8c andTable 4) show a regular trend in a (Na thorn K)A vsAlIV diagram from titanium-rich (Ti 4 015) pargasitichornblende (brownamphibole) and tschermakite (brown---green amphibole) to low-titanium (Ti5 015) magnesio-hornblende (green amphibole) and actinolite (pale greenamphibole) The pargasitic hornblende develops aspoikilitic oikocrysts in the dykes 01 OA19a and d andin the diffuse patch 01 OA41d2 at temperatures above800C during the final stage of crystallization (see
below) The tschermakite composition corresponds tothe internal alteration of clinopyroxene in gabbro 94Ma2 occurs in the comb-textured dyke 01 OA15f andis the primary amphibole in the dioritic dyke 02 OA37bThe magnesio-hornblende develops at the edges of thepargasites or of clinopyroxenes at temperatures around800C and is the primary phase in the dioritic dyke 02OA90b and in the green vein 02 OA43a Finally actino-lite occurs as a replacement of green hornblende in dykesor in the kelyphitic rim surrounding olivine in gabbro 94Ma2 Such a large composition range at the scale of athin section has been reported in amphiboles fromamphibolite-facies oceanic gabbros (Stakes 1991 Stakesamp Taylor 1992 Gillis et al 1993) as well as in the Omanophiolite gabbros (Manning et al 2000) This variabilityhas been explained by rapid reaction rates preventinghomogenization (Manning et al 2000)
Thermometry
Plagioclase---amphibole thermometry could be appliedwhen the two minerals exhibited good contacts such asin the laminated gabbro of the middle sequence (01OA41d2) and in the intrusive dykes The temperaturesof plagioclase---amphibole equilibration (Table 2) havebeen calculated using the geothermometer lsquoBrsquo of Hollandamp Blundy (1994) Edenite thorn Albite frac14 Richterite thornAnorthite valid between 500C and 900C with anuncertainty of 40C Amphibole formulae are basedon 23 oxygen and their nomenclature is based on alkalications in the A site vs AlIV according to Leake (1997)Pressurewas assumed to be 1 kbar Forty-five plagioclase---amphibole pairs meet the compositional criteria imposedby Holland amp Blundyrsquos dataset (09 4 Xan 4 01 andAlIV 403) Electron microprobe measurements wereconstrained to contiguous plagioclase and amphiboleseparated by sharp grain boundaries assuming equili-brium of the two phases When the mineral phasesexhibit normal zoning temperatures between plagioclaseand amphibole cores and between mineral rims werecalculated assuming that the temperatures deduced fromthe mineral core chemistry provide a proxy for the high-est temperature of equilibration These data are identi-fied in the T C vs AlIV diagram (Fig 8d) Table 4presents selected major element amphibole analyses theAn content of the plagioclase adjacent to each amphibolegrain and the temperature calculated for the pairThe temperatures calculated (Fig 8d) span the entire
range of temperatures at which amphibole and plagio-clase coexist in equilibrium (ie amphibolite-facies para-geneses) This temperature interval is bracketed byorthopyroxene-bearing facies or amphibole-free facies(ie gabbro 94MA2) and epidote---actinolite facies devoidof homogeneous plagioclase (ie epidosite dyke 01OA36b and green vein 02 OA43a) The range of the
BOSCH et al HYDROTHERMAL CIRCULATION IN OMAN OPHIOLITE
1199
calculated temperatures between 4900C and 750Ccorresponds to the changing composition of amphibolesfrom pargasite to magnesio-hornblende at the scale of athin section or even at that of an individual crystal asshown by their correlation with AlIV such variabilityrecords rapid cooling in hydrous conditions The highesttemperatures are recorded by pargasite from the anatec-tic gabbro 01 OA41d2 in the gabbro dyke 01 OA19dand in the dioritic dyke 02 OA90b They were calculatedfrom minerals devoid of lower-grade alteration and arethus considered as representing the equilibrium temperat-ure of the coexisting minerals Lower temperatures cor-respond to mineral phases evolving at falling temperatureand are only indicative of the cooling history Includingthe 40C uncertainty seven pairs provide temperatureshigher than 900C the limit of validity of the Holland ampBlundy thermometer However we maintain these tem-peratures as indicative of the highest values of our data-set These values are included in the average equilibriumtemperature summarized in Table 2
DISCUSSION
Distribution of hydrothermal alteration
Two distinct petrological events are recorded in the gab-bro section of the Samail ophiolite and are documentedin the present contribution (1) the whole gabbro sectionis affected by a penetrative alteration (2) the gabbrosection is crosscut by various types of dykes and veinsincluding hydrous minerals
Penetrative alteration
The penetrative alteration developed in the gabbros atgrain boundaries and locally invading the minerals isubiquitous in the gabbro section This penetrative altera-tion has been described as very high temperature (VHT)high temperature (HT) and low temperature (LT) basedon alteration parageneses The orthopyroxene coronas inthe layered gabbro sample and the incipient appearanceof brown pargasitic amphibole mark a lower thermallimit of the VHT alteration By reference to the experi-mental work of Koepke et al (2003) the temperature ofequilibration for these reactions is estimated to bebetween 900 and 1000CThe preservation of the vertical distribution of
VHT---HT facies (Figs 4 and 5) also pointed out byManning et al (2000) indicates that the hydrothermalalteration pattern fits the distribution of isotherms withdepth at a fast-spreading ridge (ie Phipps Morgan ampChen 1993 Dunn et al 2000 Fig 2) Equilibrium tem-peratures for VHT parageneses as high as 900---1000Csuggest that VHT---HT hydrothermal alteration tookplace in a very restricted time interval at the limit ofthe cooling magma chamber It is proposed that the
penetrative alteration affecting the entire gabbro sectionis related to a pervasive network of microcracks whichact as a recharge system down to the Moho (Nicolas et al2003)
Dykes and veins
The dykes are extremely varied eg pegmatitic gabbroswith comb-texture microgabbros and amphibolitizeddolerites dioritic dykes alignments of poikilitic horn-blende and finally diffuse hornblende-bearing patchesHigh-T gabbro and diorite dykes and low-T greenamphibole---epidote veins are connectedIn the dykes textural evidence attests to suprasolidus
conditions at least for the development of the pargasiticamphibole Integrating the hornblende---plagioclase equi-libration temperatures calculated for the studied samplesthese temperatures are bracketed between 950C and875CIn the following discussion we shall consider separately
the microgabbro and dolerite dykes The latter representbasaltic melt intrusions associated upsection with thediabase sheeted dykes and downsection possibly withthe mafic dykes that are ubiquitous in the mantle section(Nicolas et al 2000) Whatever their origin the develop-ment of pargasitic amphibole during their late stage ofcrystallization creates a link with the gabbroic dykes andsuggests a crystallization process evolving from dry con-ditions to more hydrous conditionsThe other gabbroic dykes comb-textured gabbros and
hornblende-bearing dioritic dykes result from the fillingof cracks by a fluid either a melt or a silica-rich hydrousfluid These intrusive dykes which develop reaction mar-gins at their contact with the host gabbro grade verticallyinto lower-temperature green amphibole veins or align-ments of poikilitic hornblende in the layered gabbromatrix or development of pargasitic hornblende in ana-tectic gabbros (sample 01 OA41d2) In gabbros crystal-lizing in the magma chamber the development oforthopyroxene and pargasite oikocrysts enclosing the pri-mary minerals suggests a thermal evolution from thegabbro solidus to amphibolite-facies conditions Forthese reasons it is suggested that the gabbroic dykesresulted from the injection of a hydrous melt in the stillhot gabbros drifting away from the magma chamber asdiscussed below
Origin of fluids responsible for VHT---HThydrothermal alteration
The 87Sr86Sr initial ratios and d18O of whole rocks andpure mineral fractions are plotted in Fig 9 against thedistance above the Moho Transition Zone (MTZ) Theinitial Sr isotopic composition of the whole rocks apartfrom the epidosite dykes ranges from 070322 to
JOURNAL OF PETROLOGY VOLUME 45 NUMBER 6 JUNE 2004
1200
070458 All the studied samples (including whole rocksand minerals) have initial 87Sr86Sr ratios falling outsidethe field of fresh MORB which commonly have Sr ratiosbetween 07023 and 07029 with an average of 070265(ie Hart et al 1974) The lowest initial Sr isotopiccomposition (070322) from the massive gabbro 94MA2 is slightly but significantly higher than averageOman primary magmatic signatures (ie 070296McCulloch et al 1981) Similarly the d18O values ofmost whole rocks and minerals measured (Table 5)
depart from typical MORB-source mantle values( Javoy 1980 Gregory amp Taylor 1981 Kyser 1986)These differences indicate that the primary mantlesignatures in the Oman ophiolite have been modifiedby interaction with a fluid Possible origins for thisfluid are mantle components dehydration of subductedlithosphere or seawater circulation Strontium andoxygen isotopes are useful tracers for fluid---crust interac-tion as d18O and 87Sr86Sr ratios from fluids originat-ing from seawater the mantle or subducted lithosphere
-1000
1000
2000
3000
4000
5000
6000
7000
MOHO
MTZ
Dis
tan
ce a
bov
e th
e M
oh
o (
m)
pillow-basalt
sheeted-dike
gabbro
peridotite
01 OA 41d2
01 OA 36b
02 OA 43a97 OA 128b
01 OA15f
94 OA MA2
02 OA 90b01 OA 19ad
02 OA 37b
0702 0703 0704 0705 0706 0707
87Sr86Srinitial
MO
RB-s
ou
rce
Seaw
ater
Whole RockPlagioclasePargasiteGreen HornblendeClinopyroxene
Epidote
(a)
-1000
1000
2000
3000
4000
5000
6000
7000
MOHO
MTZ
Dis
tan
ce a
bov
e th
e M
oh
o (
m)
pillow-basalt
sheeted-dike
gabbro
peridotite
01 OA 41d2
01 OA 36b
02 OA 43a97 OA 128b
01 OA15f
94 OA MA2
02 OA 90b01 OA 19ad
02 OA 37b
Whole RockPlagioclasePargasiteGreen HornblendeClinopyroxene
3 5 10
δ18O (permil)
41d2
Ma2
43a
90b 19d 19d37b
128b
90b
(b)
Fig 9 (a) Variation of initial 87Sr86Sr isotopic composition as a function of the location of the studied samples in a schematic log of the crustalsection of the Oman ophiolite The field for MORB is from Hart et al (1974) and that for seawater is from Burke et al (1982) Small open squares aredata fromKawahata et al (2001) (b) Variation of d18O () as a function of the location of the studied samples in a schematic log of the crustal sectionof the Oman ophiolite Shaded curve corresponds to the data of Gregory amp Taylor (1981)
BOSCH et al HYDROTHERMAL CIRCULATION IN OMAN OPHIOLITE
1201
are significantly different Moreover the d18O valueis dependent on temperature and mineral fractiona-tion and thus yields complementary constraints to Srisotopes
Mantle origin
One model to explain the occurrence of fluids interactingwith the oceanic crust at very high temperatures is toderive them from the mantle The contribution of mantle-derived hydrous fluids linked to percolation processesor to their concentration in the final stages of gabbrocrystallization has been invoked in numerous case studies(ie Witt amp Seck 1989 Fabriees et al 1989 Dupuy et al1991 Agrinier et al 1993) O and Sr isotopic data forwhole rocks and mineral separates yield scattered valuesthat are unambiguously different from MORB mantle-source values (Fig 9a and b) With the exception of lateLT altered samples the WR data have a mean 87Sr86Srratio of 070337 and a standard deviation of 000012These surprisingly high values in mantle-derived rockscould be taken as evidence for survival of mantle isotopicheterogeneities during crystallization of the ophiolite (egLanphere 1981 McCulloch et al 1981) However the18O16O isotope ratios in the Oman gabbroic sequenceare also displaced from primary magmatic values Nogabbro in the present study has plagioclase or amphi-bole with typical mantle d18O values Thus the Sr and Oisotope data rule out the possibility of mantle-derivedfluids as a possible source for the VHT---HT hydrousalteration event recorded in the massive gabbros andgabbroic dykes and veins (Fig 10a and b)
Subducted lithosphere origin
Dehydration of subducted oceanic crust in subductionzones can generate hydrous fluids Such fluids couldpercolate through the overlying lithosphere and contam-inate it thus generating modified isotopic signatures andtrace element contents Such an explanation howeverdisagrees with the geochemical characteristics of most ofthe studied samples Fluids derived from the dehydrationof subducted slabs (fresh or altered MORB or OIB pro-toliths) should be strongly enriched in K Rb and Ba (egBecker et al 2000) whereas in Oman the Rb contentsmeasured in the gabbros and gabbroic dykes and veins arestrongly depleted Moreover the isotopic composition ofslab-derived fluids is buffered by the MORB-like oceaniccrustal protolith for Nd and Pb but not for Sr and O(Staudigel et al 1995) As a result of relatively minorfractionation of oxygen at high temperature the oxygenisotope composition of the fluids expelled during dehy-dration of the subducted slab should be high and essen-tially controlled by the oxygen composition of the uppersection of the crust altered at low temperature (with an
average of 10 and perhaps as high as 19) (Staudigelet al 1995) The Sr isotopic composition should be alsohigh (average 07046) as the fluids released by the dehy-dration of subducted oceanic crust are associated eitherwith seawater or with radiogenic Sr phases (ie smectites)The Sr and O isotopic compositions of the studied sam-ples do not fit with these signatures and so preclude asubducted lithosphere origin for the fluids involved in theVHT---HT alteration event
Seawater-derived fluids
All the analysed samples (minerals and WR) have 87Sr86Sr(T) outside the domain of primary MORB magmasand variable Sr contents Individual minerals have Srcontents reflecting their different mineralliquid partitioncoefficients Minerals characterized by a very high affi-nity for Sr such as plagioclase (ie Sr mineralliquidpartition coefficients 1) have very high Sr contentsTherefore the high abundance of plagioclase in the sam-ples analysed is responsible for the high Sr content of thestudied whole rocks The Sr content of all the whole rocksis relatively homogeneous (Table 5) without clear distinc-tion between country-rock gabbros and dykes and veinsand ranges from 110 to 200 ppm except for the epidosite(4700 ppm) Reported on a 87Sr86Sr vs 1Sr diagram(Fig 10a) most whole rocks and plagioclases analysed arecharacterized by a 1Sr ratio lower than 001 and plot inthe left part of this diagram Compared with N-MORBthe studied massive and anatectic gabbros and their con-stituent plagioclases have similar to slightly higher Srcontents but substantially higher 87Sr86Sr ratios(Fig 10a) Considering Cretaceous seawater as a possiblehigh 87Sr86Sr mixing component [87Sr86Sr 07073at 90Ma after Burke et al (1982)] responsible for theobserved geochemical modifications exchanges cannothave occurred in a closed system as seawater has too lowa Sr content (ie 8 ppm de Villiers 1999) An open-system process allowing penetration of larger quantitiesof seawater can efficiently modify the Sr isotopic ratio ofthe rocks Alternately the fluids could also be seawaterthat was modified through exchange with the surround-ing rocks during its transit through the oceanic crustalsection This modified seawater would be more enrichedin Srwith a 87Sr86Sr ratio intermediate between unmodi-fied seawater and MORBMinerals devoid of late LT alteration imprints (parga-
site green hornblende and pyroxene) and characterizedby very low Sr contents plot on the right of the diagramclose to or on the mixing line between seawater andMORB (Fig 10a) The difference between these mineralsand the other samples (WR and plagioclases) is mainlydue to the Sr fractionation coefficients of these differentminerals and to the large quantity of plagioclase inthe studied rocks The process responsible for the
JOURNAL OF PETROLOGY VOLUME 45 NUMBER 6 JUNE 2004
1202
modification of the Sr isotopic composition fromMORB-source value is however explained by the same phenom-enon As all these minerals display initial 87Sr86Sr ratiosdistinct from the MORB source and because they equili-brated at VHT or HT temperatures this supports anearly intervention of seawater during crystallization ofthese minerals Most of these samples with the exceptionof the epidosite dyke overlap the domains defined byKawahata et al (2001) for the Wadi Fizh gabbros alteredat VHT and HT conditions in the amphibolite facies(labelled lsquoVHTrsquo and lsquoHTrsquo in Fig 9a)Three samples (two epidosite dykes and an one epidote
fraction) have very high Sr contents and the highest Sr
isotopic compositions of the present dataset The twowhole-rock samples overlap the field of the Wadi Fizhsamples altered at high temperature during greenschist-facies metamorphism (Kawahata et al 2001) This typeof alteration is common at the ridge axis and formedthrough intensive exchange with seawater-derived fluidsFigure 10b indicates that all the Sr---O isotope data are
distinct from MORB compositions and suggests that thefluids reacting with the magmas could be derived fromseawater In addition the d18O values show that isotopicexchange occurred under VHT or HT conditions Thefluids could have the composition of seawater or theycould have the composition of seawater modified through
0703
0705
0707
001 01 1
1 Sr (ppm)
MORB
(87Sr
86Sr)
init
ial
Seawater
VHT
HT
MT36b
36b
36b
43a 128b90b
41d2
37b19d
15f
128b
41d2
41d2
19a
Ma2Ma2
19d
90b
37b
Whole RockPlagioclasePargasiteGreen HornblendeClinopyroxene
Epidote
(a)
0 2 4 6 8
δ18O(permil)
0703
0705
0707Seawater
MORB-mantle
128b 41d2
90b 41d2
Ma2128b
37b
43a
19d
90b
Whole RockPlagioclasePargasiteGreen HornblendeClinopyroxene
Epidote
HT LT
19d
(87Sr
86Sr)
init
ial
(b)
Fig 10 (a) Initial 87Sr86Sr isotopic ratios plotted against 1Sr for whole rocks and mineral separates from the gabbro section of the Omanophiolite Fields shown are from Kawahata et al (2001) and correspond to MT-----basalt sheeted dyke diabase altered at medium temperature(prehnite---actinolite facies sequence 3) HT-----diabase plagiogranite metagabbro and epidosite formed at high temperature (greenschist faciessequence 4) VHT-----gabbro altered at very high temperature (amphibolite facies sequence 5) The dashed line is a binary mixing line betweenMORB (Lanphere et al 1981) and Cretaceous seawater (Burke et al 1982) (b) Initial 87Sr86Sr isotopic composition plotted against d18O value forwhole rocks and minerals from the Oman ophiolite Cretaceous seawater and MORB end-members as in (a) Dashed line represents mixing curvebetweenMORB and seawater at high temperature (HT) Low-temperature (LT) exchanges between these two components would be responsible foran increase of the d18O primary MORB value
BOSCH et al HYDROTHERMAL CIRCULATION IN OMAN OPHIOLITE
1203
exchange with the surrounding rocks during its penetra-tion into the crustal section In an environment in whichfluid---rock interaction occurs at variable temperaturesthe d18O is greatly influenced by the temperature atwhich the exchange took place and by the waterrockratio It is evident from Figs 9b and 10b that none of thestudied gabbros have plagioclase and amphibole withMORB-like d18O and 87Sr86Sr values The separateminerals display a broad range of d18O values and mostexhibit extreme 18O depletion (Table 5) Hydrothermalexchange kinetics are similar in amphibole and pyroxeneand these two minerals are more resistant to oxygenexchange during water---rock interaction than coexistingplagioclase (Gregory et al 1989) The most probableexplanation of the low d18O values measured in bothamphiboles and pyroxenes is that they crystallized athigh temperature in the presence of a relatively low 18Oseawater-derived hydrothermal fluid [d18O from 01 to07 after Gregory amp Taylor (1981)] The anomalouslyhigh d18O value (thorn1009) measured for the plagioclase ofthe micro-gabbroic dyke (01 OA19d) can be interpretedas resulting from a superimposed overprint caused by alate-stage penetration of fluids at lower temperature Inthis sample the occurrence of such different 18O16Oratios between coexisting plagioclase and amphibole isnot consistent with equilibrium fractionation betweenthese two minerals at any possible temperature Thisobservation suggests that parts of the ophiolite sequencehave undergone a high-temperature hydrothermal eventprior to the later low-temperature event that producedthe strong 18O increase in coexisting plagioclaseThe depletion of the d18O values results from high-temperature hydrothermal alteration (Gregory amp Taylor1981 Stakes amp Taylor 1992) The oxygen isotopic datasupport the contention that most studied samples havebeen affected by a VHT---HT hydrothermal alteration byfluids derived from seawater Some of the analysed sam-ples presenting petrological evidence of crystallization ofLT secondary minerals have been overprinted by the latelow-temperature alteration thus swamping the earlierhigh-temperature alteration effects
Determination of waterrock ratio
To better evaluate the exchange between seawater andthe gabbroic rocks waterrock ratios (WR) have beenestimated for the whole rocks analysed during the courseof this study The different models used to constrain thisratio mainly differ by the calculation of the effective ratioor by the estimated weight ratio and by considering openor closed systems for water circulation (Albareede et al1981 McCulloch et al 1981) The WR ratios reportedin Table 5 have been calculated assuming a closed sys-tem Using this formulation these values are minimabecause the calculation always uses pristine fluid for the
initial fluid thus maximizing the value in the denomina-tor If the fluid was shifted to lower 87Sr concentrations byexchange with the rock on the downward path theinferred values would be much higher (Davis et al 2003)The calculated WR ratios for the samples from this
study range from 31 to 219 (Table 5) Two main groupsof samples can be recognized on the basis of their waterrock ratios The lowest ratios ranging from 31 to 47have been determined for massive gabbros or anatecticgabbros but also for dykes and veins devoid of late LTalteration Similar ratios have been previously calculatedfor example for the discharged hydrothermal solutions atthe 13N and 21N East Pacific Rise (Di Donato et al1990 Fouquet et al 1991) The gabbros from the Ibrasection in the Oman ophiolite gave effective WR ratiosof 05 33 and 42 (McCulloch et al 1981) in goodagreement with the values obtained in this study Theleast altered gabbro of the Ibra section yields a WR ratioof 05 in good agreement with the exceptionally fresh-ness of this rock characterized by typical mantle oxygenvalues for its minerals (McCulloch et al 1981) Samplescharacterized by waterrock effective ratios in the rangeof 3---5 can be related to penetrative alteration via thehydrothermal system Lecuyer amp Reynard (1996) havedemonstrated in oceanic gabbros from the Hess Deepaltered in similar HT conditions that WR mass ratios inthe range of 02---1 are sufficient to account for the mod-ified stable isotope signature of the gabbros Higherwaterrock values (WR 45) have been calculated forsamples affected by superimposed late hydrothermalalteration and for the epidosite samples (Table 5) Theyprobably acquired their isotopic characteristics throughinteraction with a larger volume of seawater duringgreenschist-facies metamorphism Similar or higherWR values have been published in previous studies(ie McCulloch et al 1981 Kawahata et al 2001)They correspond either to samples from the upper-crus-tal sequence (ie basalt sheeted dyke or plagiogranite) orto gabbros from the crustal section located in a particularenvironment such as for example the Wadi Fizh sectionthat is located close to a segment boundary along thepalaeo-spreading axis (Nicolas et al 2000)
Mechanism for seawater interaction withmagma
Nicolas et al (2003) have proposed that variable amountsof seawater can circulate at high temperature through-out the crustal section down to the walls of the magmachamber via a network of parallel microcracks a fractionof a millimetre wide Such microcracks and their relation-ship with the VHT---HT hydrous alteration in the sur-rounding gabbros have been described in detail byNicolas et al Via this network of microcracks largeamounts of seawater can have reacted with the magma
JOURNAL OF PETROLOGY VOLUME 45 NUMBER 6 JUNE 2004
1204
and induced variable changes of its Sr and O isotopiccomposition This regular network of parallel micro-cracks could constitute the recharge system and thehydrated gabbroic dykes the discharge system Physicalparameters and the geophysical models of Nicolas et al(2003) are in agreement with this scenario Such a pene-tration of seawater-derived hydrothermal fluids throughthe lower crust along grain boundaries and a microscopicfracture network at temperatures 4750C is consistentwith recent thermodynamic models (McCollom amp Shock1998)Seawater-altered and high-temperature recrystallized
diabases (protodykes) from the overlying sheeted dykecomplex stoped into the melt lenses could represent anindirect way for seawater to enter the system as they canremelt during their subsidence through the magmachamber A simple mass balance calculation suggeststhat this indirect contamination is not significant Assess-ments of the amount of recycled crust 1 in volumeby Nicolas et al (2000) based on the protodyke remnantsin the gabbro section or 20 by Coogan et al (2003)based on chlorine content in EPR basalts lead to a sea-waterrock ratio of the order of 104 to 103Thus following Nicolas et al (2003) we propose that
seawater has been introduced along microcracks down tothe walls of the magma chamber and has diffused alonggrain boundaries in the way described by Manning et al(2000) progressively invading the host gabbro Gabbroicdykes result from the injection of hydrous melt into thehost gabbros at falling temperatures as they drift awayfrom the magma chamber This water-saturated meltcould result from the extensive hydration of the crystal-lizing gabbros near the walls of the magma chamberwhen seawater penetrates through this wall (Fig 1)Similar plumbing systems driving seawater down to the
limit of the magma chamber and back to the surface havebeen already proposed in other environments such as theMid-Atlantic Ridge (Kelley amp Delaney 1987 Cooganet al 2001) the East Pacific Rise at Hess Deep (Gillis1995 Manning et al 1996a 1996b) the Tonga forearc(Banerjee amp Gillis 2001) and the Troodos ophiolite(Gillis amp Roberts 1999) The melting curve of wet basalthas a negative slope but is close to the dry solidus at lowpressure in the lower gabbros and the first melts areplagiogranitic (Spulber ampRutherford 1983) Such plagio-granites are ubiquitous in the highest gabbros and in theroot zone of sheeted dykes in Oman (Rothery 1983Juteau et al 1988 Nicolas amp Boudier 1991) in manyother ophiolites and in the oceanic crust where they havebeen interpreted as resulting either from wet anatexis ofhot gabbros or from the final product of crystallization ofa basaltic melt (Spulber amp Rutherford 1983) Plagio-granites are rare in the lower gabbros exceptionallyassociated with hydrous gabbro segregations inside thelayered gabbros Their constitutive minerals are also
remarkably absent along the VHTmicrocracks althoughthese gabbros were above the wet solidus A possibleexplanation has been proposed by McCollom amp Shock(1998) who wrote that at high temperature lsquoaqueousfluids may leave very little conspicuous petrological evid-ence for their presencersquo the reason being that thehigh-T conditions would be closer to batch meltingObviously more detailed studies on this specific problemare needed We conclude that the large degree of hydrousmelting locally required at the walls of the magma cham-ber to account for the generation of a gabbroic hydrousmelt may have erased the traces of the incipient plagio-granitic melt
CONCLUSIONS
Fostered by the recent discovery of high-temperatureseawater contamination in some gabbros of the Omanophiolite (Manning et al 2000) we have conducted astudy on 500 gabbro specimens from selected areas ofthe entire Samail ophiolite belt and on the hydrous gab-broic dykes that cross-cut them The highest-temperaturereactions (VHT) recorded in olivine and clinopyroxene ingabbros as orthopyroxene and pargasite coronas reach975C They are followed at lower HT conditions withreplacement of olivine by an assemblage of tremolitemagnetite and plagioclase surrounded by chlorite andby pargasite development in clinopyroxene followedfinally by the LT lizardite---olivine and saussurite---plagioclase greenschist-facies alteration With a fewexceptions the VHT alteration tends to be restricted tothe lower gabbros and to the vicinity of the Moho andthe HT alteration to the middle and upper gabbros Thepreservation of the vertical distribution of VHT---HTfacies indicates that progressive cooling during off-axisspreading did not obliterate this distribution and conse-quently that the VHT---HT hydrothermal alteration tookplace in a very restricted time interval at the limit of thecooling magma chamber In the deep and hot gabbrosthe main water channels may be sub-millimetre-sizedmicrocracks with a dominantly vertical attitude the ori-gin and dynamics of which have been investigated in aprevious paper (Nicolas et al 2003)Beyond these high-temperature alteration zones mas-
sively induced in the gabbros seawater may be respons-ible for the generation of clinopyroxene---pargasitegabbro dykes that cross-cut all gabbros and that areupsection clearly connected to the LT hydrothermalvein system Petrostructural evidence suggests that thesedykes were generated by hydration of the crystallizinggabbros near the internal wall of the LVZ---magmachamber The resultant melts were subsequently intrudedat various levels in the cooling lower and middle gabbrosPetrological observations of nearly all the gabbros ana-lysed in the present study located from the root zone of
BOSCH et al HYDROTHERMAL CIRCULATION IN OMAN OPHIOLITE
1205
the sheeted dyke complex down to the Moho emphasizethe imprint of VHT---HT hydrous alteration The VHThydrothermal event is characterized by temperatures ofequilibration around 900---1000C The temperatures atwhich the HT hydrous event occurred are estimated tobe in the range 550---900CStrontium and oxygen isotope compositions measured
on whole rocks and separated minerals significantlydepart from primary MORB values The Sr and O iso-tope data obtained for pargasite green hornblende andclinopyroxene record the participation of seawater at thetemperature of crystallization of the minerals Plagio-clases and whole rocks define a trend toward a compo-nent characterized by a high radiogenic 87Sr86Sr valueand a higher Sr content than normal seawater Seawateris clearly identified as the fluid responsible for the modi-fication of the Sr and O signatures for both massivegabbros and dykes and veins Nevertheless the high Srcontent of the extraneous component requires eitheropen-system exchange allowing penetration of a greaterquantity of seawater or penetration of seawater modifiedby interaction with the oceanic crust during its travel paththrough the crustal section The waterrock ratio calcu-lated on the basis of the Sr isotopes is between three andfive for the enclosing gabbros and for the least LT altereddykes The VHT---HT hydrothermal event affectingthese samples is related to penetrative alteration via therecharge and discharge hydrothermal system Thewaterrock ratio is 10 for dykes exhibiting evidenceof a LT hydrothermal alteration overprint and reaches22 for the epidosite dyke The depletion in d18O char-acterizes all the studied samples with the single exceptionof the micro-gabbroic dyke plagioclase This agreeswith the conclusions based on Sr isotopes suggestingthat these samples have undergone exchange with sea-water-derived fluids during a VHT---HTalteration hydro-thermal event
ACKNOWLEDGEMENTS
This work has benefited from financial support by theCentre National de la Recherche Scientifique (INSUInteerieur-Terre andDYETI programmes) and inOmanfrom the willingness of the Directorate of MineralsMinistry of Commerce and Industry and the hospitabil-ity of the people Technical help in the laboratory is alsoacknowledged including C Nevado for the thin sectionsE Ball for the illustrations B Galland for her assistancein the chemistry room and C Bassoulet for help duringthermal ionization mass spectrometric analyses of the Srresults from the leaching experiments Constructivereviews from R T Gregory C Lecuyer an anonymousreviewer and particularly from M Wilson greatlyhelped to improve an earlier version of the manuscript
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controlled metamorphism of Hess Deep gabbros site 984
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349 261---272McCollom T M amp Shock E L (1998) Fluid---rock interactions in
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(1981) Sm---Nd Rb---Sr and 18O16O isotopic systematics in an
oceanic crustal section evidence from the Samail ophiolite Journal of
Geophysical Research 86(B4) 2721---2735Mevel C Gillis K M Allan J F amp Meyer P S (eds) (1996)
Proceedings of the Ocean Drilling Program Scientific Results 147 College
Station TX Ocean Drilling Program
Nehlig P amp Juteau T (1988) Flow porosities permeabilities and
preliminary data on fluid inclusions and fossil thermal gradients in
the crustal sequence of the Sumail ophiolite (Oman) Tectonophysics
151 199---221
Nehlig P Juteau T Bendel V amp Cotten J (1994) The root zone of
oceanic hydrothermal systems constraints from the Samail ophiolite
(Oman) Journal of Geophysical Research 99 4703---4713
Nicolas A amp Boudier F (1991) Rooting of the sheeted dyke complex
in Oman ophiolite In Peters Tj Nicolas A amp Coleman R (eds)
Ophiolite Genesis and Evolution of the Oceanic Lithosphere Dordrecht
Kluwer Academic pp 39---54
BOSCH et al HYDROTHERMAL CIRCULATION IN OMAN OPHIOLITE
1207
Nicolas A amp Ildefonse B (1996) Flow mechanism and viscosity in
basaltic magma chambers Geophysical Research Letters 23(16)
2013---2016
Nicolas A Ceuleneer G Boudier F amp Misseri M (1988) Structural
mapping in the Oman ophiolites mantle diapirism along an ocean
ridge Tectonophysics 151 27---56
Nicolas A Boudier F Ildefonse B amp Ball E (2000) Accretion
of Oman and United Arab Emirates ophiolite-----discussion of a
new structural map Marine Geophysical Research 21(3---4)147---179
Nicolas A Boudier F Michibayashi K amp Gerbert-Gaillard L
(2001) Aswad massif (United Arab Emirates) archetype of the
Oman---UAE ophiolite belt Geological Society of America Bulletin 349
499---451
Nicolas A Mainprice D amp Boudier F (2003) High temperature
seawater circulation through the lower crust of ocean-ridges-----amodel derived from the Oman ophiolites Journal of Geophysical
Research on line first httpwwwaguorgpubscurrentjb 108(B8)
2372
Pallister J S amp Hopson C A (1981) Samail ophiolite plutonic suite
field relations phase variation cryptic variation and layering and a
model of a spreading ridge magma chamber Journal of Geophysical
Research 86 2593---2644Phipps Morgan J amp Chen J Y (1993) Dependence of ridge-axis
morphology on magma supply and spreading rate Nature 364
706---708
Pin C Briot D Bassin C amp Poitrasson F (1994) Concomitant
extraction of Sr and Sm---Nd for isotopic analysis in silicate samples
based on specific extraction chromatography Analytica Chimica Acta
298 209---217
Rothery D A (1983) The base of a sheeted dyke complex Oman
ophiolite implications for magma chambers at oceanic spreading
axes Journal of the Geological Society London 140 287---296
Spear F S (1981) An experimental study of hornblende stability and
compositional variability in amphibolite American Journal of Science
281 697---734
Spulber S D amp Rutherford M J (1983) The origin of rhyolite and
plagiogranite in oceanic crust an experimental study Journal of
Petrology 24 1---25Stakes D S (1991) Oxygen and hydrogen isotope compositions of
oceanic plutonic rocks high-temperature deformation and meta-
morphism of oceanic layer 3 In Taylor H P OrsquoNeil J et al (eds)
Geochemical Society Special Publication 3 77---90
Stakes D S amp Taylor H P (1992) The northern Samail ophiolite an
oxygen isotope microprobe and field study Journal of Geophysical
Research 97(B5) 7043---7080Staudigel H Davies G R Hart S R Marchant M amp Smith B
M (1995) Large scale isotopic Sr Nd and O isotopic anatomy of
altered oceanic crust DSDPODP sites 417418 Earth and Planetary
Science Letters 130 169---185Turner F J amp Verhoogen J (1960) Igneous and Metamorphic Petrology
New York McGraw---Hill 694 pp
Wilcock W S D amp Delaney J R (1996) Mid-ocean ridge
sulfide deposits evidence for heat extraction from magma
chambers or cracking fronts Earth and Planetary Science Letters 145
49---64
Witt G amp Seck H A (1989) Origin of amphibole in recrystallized
and porphyroclastic mantle xenoliths from Rhenish massif implica-
tions for the nature of mantle metasomatism Earth and Planetary
Science Letters 91 327---340
JOURNAL OF PETROLOGY VOLUME 45 NUMBER 6 JUNE 2004
1208
Sam
ple
nam
e1Mode2
SiO
2TiO
2Al 2O3
Cr 2O3
FeO
MnO
MgO
CaO
Na 2O
K2O
Total
Si
AlIV
AlVI
Ti
Fe3
thornCr
Mg
Fe2
thornMn
Na
KCa
XAn3
T(C)4
02OA90b
Mg-hornblende
core
48 83
038
745
005
939
010
17 29
12 24
142
006
97 21
6931
1069
0041
0041
0667
0005
3657
0448
0011
0392
0010
1862
0840
856
Mg-hornblende
core
49 49
033
731
003
916
008
17 40
12 57
139
005
97 82
6987
1013
0035
0035
0552
0004
3662
0530
0010
0379
0009
1901
0830
811
Mg-hornblende
rim
48 90
040
753
005
932
013
17 15
12 22
154
005
97 29
6948
1052
0042
0042
0575
0006
3632
0533
0015
0424
0010
1861
0880
869
Mg-hornblende
rim
48 30
044
800
007
962
010
16 86
12 32
161
005
97 35
6873
1127
0047
0047
0598
0008
3576
0546
0012
0443
0010
1879
0870
862
Mg-hornblende
rim
49 12
032
742
010
922
011
17 36
12 29
142
006
97 43
6956
1044
0034
0034
0621
0011
3665
0471
0013
0391
0010
1865
0840
845
02OA43a
Mg-hornblende
rim
48 13
040
789
007
860
007
17 41
12 19
155
006
96 36
6882
1118
0212
0043
0621
0008
3710
0407
0008
0429
0010
1867
-------
-------
Mg-hornblende
rim
50 93
035
643
001
759
006
18 50
12 24
114
003
97 28
7152
0848
0217
0037
0506
0001
3873
0385
0007
0311
0005
1841
-------
-------
Mg-hornblende
rim
50 67
014
559
009
12 33
026
15 28
12 44
068
003
97 50
7260
0740
0204
015
0483
0010
3263
0994
0032
0189
0005
1909
-------
-------
Mg-hornblende
rim
50 34
013
621
009
12 49
027
15 07
12 51
064
003
97 78
7192
0810
0240
0010
0530
0010
3210
0960
0030
0180
0010
1910
-------
-------
Mg-hornblende
rim
45 17
060
11 14
013
988
008
15 78
12 33
211
009
97 31
6473
1527
0355
0640
0644
0015
3371
0540
0010
0588
0016
1892
-------
-------
Mg-hornblende
rim
49 25
032
712
014
845
009
17 84
12 32
137
004
96 95
6983
1020
0170
0030
0620
0020
3770
0380
0010
0380
0010
1870
-------
-------
Mg-hornblende
rim
48 61
032
775
014
975
011
16 81
12 57
139
004
97 50
6905
1950
0203
0034
0621
0016
3560
0537
0013
0382
0008
1914
-------
-------
94Ma2
Anthophyllite
54 98
000
259
001
12 65
037
23 35
199
034
000
96 28
7744
0256
0174
0000
0161
0001
4903
1330
0043
0092
0001
0301
-------
-------
Anthophyllite
55 11
000
296
003
14 09
032
23 66
063
038
000
97 18
7716
0284
0205
0000
0115
0003
4938
1535
0038
0104
0000
0095
-------
-------
Actinolite
48 58
004
954
002
960
014
17 67
999
139
005
97 02
6850
0150
0435
0004
0690
0002
3713
0442
0017
0379
0009
1510
-------
-------
Actinolite
51 54
004
585
000
890
022
20 28
930
090
002
97 05
7214
0786
0178
0005
0598
0000
4230
0444
0026
0244
0004
1395
-------
-------
01OA41d2
Pargasite
rim
43 05
342
10 71
008
11 46
012
13 50
11 54
179
067
96 34
6353
1647
0216
0380
0368
0009
2967
1046
0015
0512
0127
1824
0613
863
Pargasite
rim
42 61
354
11 59
001
10 59
010
13 52
11 87
160
076
96 19
6286
1714
0302
0392
0299
0001
2972
1007
0012
0458
0143
1877
0715
844
Pargasite
core
42 54
401
11 04
006
11 81
013
13 21
11 50
202
039
96 71
6266
1734
0182
0444
0368
0007
2901
1087
0017
0576
0073
1814
0854
979
Pargasite
rim
43 03
385
10 72
002
11 70
014
13 50
11 95
146
080
97 17
6314
1686
0168
0425
0345
0002
2953
1091
0018
0415
0149
1878
0634
852
Pargasite
rim
42 65
341
10 60
006
11 97
014
13 22
11 64
183
068
96 20
6332
1668
0186
0381
0345
0007
2926
1141
0018
0528
0129
1851
0620
869
Pargasite
core
42 82
367
10 60
007
11 93
012
13 42
11 44
206
047
96 60
6315
1685
0158
0407
0391
0008
2950
1080
0016
0588
0089
1808
0615
899
Pargasite
rim
42 2
362
11 10
007
11 42
016
13 09
11 72
179
072
95 89
6282
1718
0229
0405
0299
0009
2905
1123
0020
0518
0137
1869
0715
875
Pargasite
rim
42 42
380
11 03
006
11 77
012
13 00
11 72
188
069
96 49
6283
1717
0209
0423
0299
0007
2870
1158
0015
0539
0131
1860
0817
928
Pargasite
rim
42 21
364
11 14
006
10 73
011
13 53
11 68
177
072
95 59
6277
1723
0229
0407
0322
0007
2999
1012
0014
0511
0136
1862
0736
883
Pargasite
rim
42 25
376
11 03
008
10 93
015
13 66
11 72
185
064
96 07
6257
1743
0182
0418
0345
0009
3016
1009
0019
0531
0121
1860
0766
912
Pargasite
core
42 17
396
10 91
008
11 10
010
13 57
11 49
209
038
95 85
6253
1747
0159
0442
0368
0010
3000
1009
0013
0602
0073
1826
0841
981
aMajorelem
entco
mpositionsarein
weightpercent-------noplagioclasean
alysed
andnotemperature
calculated
1Typ
eofam
phibolean
alysed
isalso
indicated
2Blankindicates
nozoningobserved
3Molefractionofan
orthitein
plagioclasead
jacentto
amphibolegrain
4Tem
perature
ofeq
uilibrium
fortheam
phibole---plagioclasepaircalculatedusingtheHollandamp
Blundythermometer
(1994)Errors
are40
C
BOSCH et al HYDROTHERMAL CIRCULATION IN OMAN OPHIOLITE
1195
zonation from An95 to An99 from core to rim (Table 1 andFig 8a) TheWR Sr isotopic ratio of the sample (070458)is significantly higher than normal MORB values Thiscould be related to the overprint of a low-temperaturealteration event consistent with the petrography of thesample
Green amphibole vein cross-cutting layered gabbros inWadi Mayah Haylayn Massif
Sample 02 OA43a belongs to a series of 1 cm thick greenamphibole veins (as in Fig 6e) cross-cutting the layeredgabbros The veins develop 15 cm thick reaction marginsin the enclosing gabbro and are exclusively composed ofacicular pale green magnesio-hornblende growing per-pendicular to the vein margin in a crack-seal mode(Fig 6f ) The reaction margins are composed of urali-tized gabbro marked by the development of the samepale green magnesio-hornblende in an inhomogeneouscalcic plagioclase matrix (An91---97) The Sr and O isotopiccompositions of this green hornblende (070431 andthorn31) do not correspond to normal MORB-sourcemantle values
Epidosite dyke from Wadi Gideah Wadi Tayin Massif
Sample 01 OA36b is representative of a series of epidoteveins cross-cutting poorly layered gabbros These 2 cmthick veins are composed of pink thulite in their centreand green pistachite at their margins (Fig 6a) Theirsharp contact with the enclosing gabbro is marked bythe development of coarse clinopyroxene crystals alteredin the epidote---greenschist facies This suggests that theepidote vein is replacive in a coarse-grained gabbro dykeThe highest 87Sr86Sr initial ratios of all the samplesstudied are recorded by the WR and coexisting epidoteof this sample (070519 and 070554 respectively)(Table 5) The Sr content of the WR and associatedepidote are also the highest of the present dataset(716 ppm and 764 ppm respectively) The Sr isotopiccomposition of this sample is consistent with a greaterdegree of exchange with a radiogenic component fromCretaceous seawater It is significantly higher than thevalue obtained for an epidosite vein from the Wadi Fizhsection of the Oman ophiolite [070435 with a Sr contentof 488 ppm reported by Kawahata et al (2001)] but ingood agreement with the average of the greenschist-faciesalteration sequence defined by Kawahata et al (2001)
GABBROS
01 km 025 km 1 km 36 km50
60
70
80
90
An
MA2
19d
19a 15f 128b
41d2
(a)
43a17b
15f
90b
37b
19a
19d
40
60
80
100
A
n
01 km 015 km 025 km 1 km
DYKES
(b)
Distance to the Moho
ACTINOLITE
MAGNESIOHORNBLENDE
TSCHERMAKITE
PARGASITEGabbros Dykes41d2MA2
19a d37b15f
90b43a
AlIV
150500
02
08
10
(Na
+ K
) A
(c)
04
06
950
900
850
800
750
70007 12 17
T degC
1000
(d)
AlIV
Fig 8 Plagioclase---amphibole compositions and results of thermometry (a) and (b) plagioclase compositions in layered gabbros and in dykesrespectively as a function of distance to the Moho (see text for sample identification) (c) amphibole compositions in layered gabbros and dykesaccording to Leakersquos (1997) nomenclature (d) equilibration temperatures calculated using the amphibole---plagioclase thermometer of Holland ampBlundy (1994) as a function of AlIV content in amphibole [same symbols as in (c)]
JOURNAL OF PETROLOGY VOLUME 45 NUMBER 6 JUNE 2004
1196
Table5RbandSrcontents87Sr
86Srandd1
8Oisotopiccompositionsofwholerocksandmineralseparatesfrom
samplesofmassivegabbrosdykes
andveinsfrom
theOman
ophiolitealsolistedarecalculated
waterrockratios(W
R)andLOIvalues
Sam
ple
Rb(ppm)
Sr(ppm)
87Rb8
6Sr
87Sr
86Sr
(87Sr
86Sr)i1
d18O
()
LOI(
)2WR
3(87Sr
86Sr)L14
(87Sr
86Sr)L25
(87Sr
86Sr)R6
01OA41d2
Whole
rock
037
1792
0006
0703516
09
070351
thorn480
138
47
0703587
0703651
0703357
Plagioclase
025
1249
0003
0704261
22
070426
thorn504
-------
-------
Pargasite
-------
-------
-------
0703548
20
070355
-------
-------
-------
Clinopyroxene
045
26 0
0050
0703845
19
070378
-------
-------
-------
01OA36b
Dykewhole
rock
0005
7164
50001
0705186
17
070519
-------
274
21 9
0705207
0705112
-------
Epidote
0003
7645
0001
0705536
12
070554
-------
-------
-------
Wallwhole
rock
005
4254
0001
0704637
09
070464
-------
191
-------
0704945
0704675
0704593
02OA43a
Green
hornblende
004
98
0012
0704323
13
070431
thorn310
-------
-------
97OA128b
Whole
rock
003
1274
50001
0703327
17
070333
-------
076
37
-------
-------
-------
Plagioclase
002
1204
50001
0703285
09
070328
thorn414
-------
-------
Clinopyroxene
003
20 0
0004
0704230
09
070422
thorn523
-------
-------
01OA15f
Whole
rock
006
1077
0002
0704582
16
070458
-------
237
13 5
-------
-------
-------
01OA19a
Whole
rock
037
1303
0008
0704189
18
070418
-------
161
96
-------
-------
-------
01OA19d
Whole
rock
058
2032
0008
0703423
08
070341
thorn567
216
415
0703574
0703408
0703271
Pargasite
091
1058
0025
0703273
11
070324
thorn287
-------
-------
Plagioclase
-------
-------
-------
-------
-------
thorn10 09
-------
-------
02OA90b
Plagioclase
004
2385
50001
0703868
11
070387
thorn498
-------
-------
Green
hornblende
010
23 8
0012
0704288
15
070427
thorn239
-------
-------
BOSCH et al HYDROTHERMAL CIRCULATION IN OMAN OPHIOLITE
1197
Table5continued
Sam
ple
Rb(ppm)
Sr(ppm)
87Rb8
6Sr
87Sr
86Sr
(87Sr
86Sr)i1
d18O
()
LOI(
)2WR
3(87Sr
86Sr)L14
(87Sr
86Sr)L25
(87Sr
86Sr)R6
02OA37b
Plagioclase
023
3121
0002
0704204
15
070420
-------
-------
-------
Pargasite
031
41 6
0021
0703505
15
070348
thorn394
-------
-------
94OAMa2
Whole
rock
0016
1750
00003
0703219
15
070322
thorn495
033
31
0703264
0703177
0703158
Plagioclase
0014
1363
00003
0703219
11
070322
thorn479
-------
-------
Clinopyroxene
-------
-------
-------
0703236
13
070324
-------
-------
-------
1Initial8
7Sr
86Srratiocalculatedat
95Ma(H
acker1994)
2LOIisloss
onignition(in)
3Water---rock
ratiocalculatedassumingaclosedsystem
TheeffectiveWR
ratiocanbeexpressed
bythefollowingeq
uation
W=Reffectivefrac14
frac12eth87Sr=
86SrrockTHORN fi
nal
eth87Sr=
86SrrockTHORN in
itial=frac12eth8
7Sr=
86SrseawaterTHORN in
itial
eth87Sr=
86SrrockTHORN fi
nal
ethCrock
initial=C
seaw
ater
initial
THORNwhere(87Sr
86Srrock) finalisthefinalmeasuredSrisotopicratioin
thesample(87Sr
86Srrock) in
itialco
rrespondingto
theinitialm
antlevalue
istakenas
070295(Lan
phere
etal1981)(87Sr
86Srseawater) in
itialistheCretaceousseaw
ater
Srisotopicco
mposition[87Sr
86Sr07073
at90
MaafterBurkeet
al(1982)]C
seaw
ater
initial
istheSrco
ntent
oflsquonorm
alrsquoseaw
ater
andis
takenas
8ppm
(deVilliers1999)
FortheCrock
initialitis
assumed
that
thepresentSrco
ncentrationmeasuredis
thesameas
theinitial
concentration
4Srisotopicratiosmeasuredforliquid
L1extractedafterthefirststep
oftheleachingexperim
ent(6NHClfor8min
at70
C)
5Srisotopicratiosmeasuredforliquid
L2extractedaftertheseco
ndstep
oftheleachingexperim
ent(2 5NHClfor30
min
at70
C)
6Srisotopicratiosmeasuredforresidueobtained
afteratotalaciddigestionachievedaftertheextractionofL1an
dL2leachates
JOURNAL OF PETROLOGY VOLUME 45 NUMBER 6 JUNE 2004
1198
Part of the same sample from the contact between thedyke and the gabbro yields a slightly lower Sr isotopiccomposition (070464) intermediate between the sur-rounding gabbro and the epidote dyke It also has anintermediate Sr content (425 ppm) Acid leaching experi-ments performed on the whole rock of the dyke yield Srisotopic compositions of 070521 and 070512 for thetwo liquids extracted Similar experiments for the samplelocated at the contact with the gabbro yield 87Sr86Srratios of 070494 and 070467 for L1 and L2 liquids TheSr isotopic composition of the residue extracted afterleaching remains high (070459) and very close to theepidosite 87Sr86Sr ratio determined by Kawahata et al(2001) In this sample the primary minerals have beentotally replaced by epidote and secondary plagioclase orquartz Thus the Sr isotopic compositions measured forthis WR sample and its constituent minerals reflect thecomposition of the fluid filling this dyke
Mineral chemistry
Plagioclase
Compared with the host gabbros the dykes show a muchlarger dispersion in their plagioclase compositions (Fig 8aand b Table 4) characterized by more calcic plagioclasethan the enclosing gabbros This tendency is mostextreme in the comb-textured dykes in which the plagio-clase is almost pure anorthite The zoning is generallynormal recording a crystal fractionation process How-ever gabbro 94 MA2 exhibits inverse zoning this tend-ency is also observed in plagioclases from dykes 01OA19a and 01 OA15f The higher calcium content ofthe plagioclase as well as the inverse zoning is consistentwith an increase of water pressure during its crystalliza-tion (Turner amp Verhoogen 1960 Carmichael et al 1974Koepke et al 2003) The particularly high An content inthe comb-textured gabbro dykes 01 OA15f may alsoresult from crystallization in a supercooled magma thisis consistent with the comb texture indicative of fast-growing crystals The lack of brown hornblende suggeststhat crystallization occurred above the temperature ofhornblende stability
Amphibole
The amphiboles analysed in the dykes (Fig 8c andTable 4) show a regular trend in a (Na thorn K)A vsAlIV diagram from titanium-rich (Ti 4 015) pargasitichornblende (brownamphibole) and tschermakite (brown---green amphibole) to low-titanium (Ti5 015) magnesio-hornblende (green amphibole) and actinolite (pale greenamphibole) The pargasitic hornblende develops aspoikilitic oikocrysts in the dykes 01 OA19a and d andin the diffuse patch 01 OA41d2 at temperatures above800C during the final stage of crystallization (see
below) The tschermakite composition corresponds tothe internal alteration of clinopyroxene in gabbro 94Ma2 occurs in the comb-textured dyke 01 OA15f andis the primary amphibole in the dioritic dyke 02 OA37bThe magnesio-hornblende develops at the edges of thepargasites or of clinopyroxenes at temperatures around800C and is the primary phase in the dioritic dyke 02OA90b and in the green vein 02 OA43a Finally actino-lite occurs as a replacement of green hornblende in dykesor in the kelyphitic rim surrounding olivine in gabbro 94Ma2 Such a large composition range at the scale of athin section has been reported in amphiboles fromamphibolite-facies oceanic gabbros (Stakes 1991 Stakesamp Taylor 1992 Gillis et al 1993) as well as in the Omanophiolite gabbros (Manning et al 2000) This variabilityhas been explained by rapid reaction rates preventinghomogenization (Manning et al 2000)
Thermometry
Plagioclase---amphibole thermometry could be appliedwhen the two minerals exhibited good contacts such asin the laminated gabbro of the middle sequence (01OA41d2) and in the intrusive dykes The temperaturesof plagioclase---amphibole equilibration (Table 2) havebeen calculated using the geothermometer lsquoBrsquo of Hollandamp Blundy (1994) Edenite thorn Albite frac14 Richterite thornAnorthite valid between 500C and 900C with anuncertainty of 40C Amphibole formulae are basedon 23 oxygen and their nomenclature is based on alkalications in the A site vs AlIV according to Leake (1997)Pressurewas assumed to be 1 kbar Forty-five plagioclase---amphibole pairs meet the compositional criteria imposedby Holland amp Blundyrsquos dataset (09 4 Xan 4 01 andAlIV 403) Electron microprobe measurements wereconstrained to contiguous plagioclase and amphiboleseparated by sharp grain boundaries assuming equili-brium of the two phases When the mineral phasesexhibit normal zoning temperatures between plagioclaseand amphibole cores and between mineral rims werecalculated assuming that the temperatures deduced fromthe mineral core chemistry provide a proxy for the high-est temperature of equilibration These data are identi-fied in the T C vs AlIV diagram (Fig 8d) Table 4presents selected major element amphibole analyses theAn content of the plagioclase adjacent to each amphibolegrain and the temperature calculated for the pairThe temperatures calculated (Fig 8d) span the entire
range of temperatures at which amphibole and plagio-clase coexist in equilibrium (ie amphibolite-facies para-geneses) This temperature interval is bracketed byorthopyroxene-bearing facies or amphibole-free facies(ie gabbro 94MA2) and epidote---actinolite facies devoidof homogeneous plagioclase (ie epidosite dyke 01OA36b and green vein 02 OA43a) The range of the
BOSCH et al HYDROTHERMAL CIRCULATION IN OMAN OPHIOLITE
1199
calculated temperatures between 4900C and 750Ccorresponds to the changing composition of amphibolesfrom pargasite to magnesio-hornblende at the scale of athin section or even at that of an individual crystal asshown by their correlation with AlIV such variabilityrecords rapid cooling in hydrous conditions The highesttemperatures are recorded by pargasite from the anatec-tic gabbro 01 OA41d2 in the gabbro dyke 01 OA19dand in the dioritic dyke 02 OA90b They were calculatedfrom minerals devoid of lower-grade alteration and arethus considered as representing the equilibrium temperat-ure of the coexisting minerals Lower temperatures cor-respond to mineral phases evolving at falling temperatureand are only indicative of the cooling history Includingthe 40C uncertainty seven pairs provide temperatureshigher than 900C the limit of validity of the Holland ampBlundy thermometer However we maintain these tem-peratures as indicative of the highest values of our data-set These values are included in the average equilibriumtemperature summarized in Table 2
DISCUSSION
Distribution of hydrothermal alteration
Two distinct petrological events are recorded in the gab-bro section of the Samail ophiolite and are documentedin the present contribution (1) the whole gabbro sectionis affected by a penetrative alteration (2) the gabbrosection is crosscut by various types of dykes and veinsincluding hydrous minerals
Penetrative alteration
The penetrative alteration developed in the gabbros atgrain boundaries and locally invading the minerals isubiquitous in the gabbro section This penetrative altera-tion has been described as very high temperature (VHT)high temperature (HT) and low temperature (LT) basedon alteration parageneses The orthopyroxene coronas inthe layered gabbro sample and the incipient appearanceof brown pargasitic amphibole mark a lower thermallimit of the VHT alteration By reference to the experi-mental work of Koepke et al (2003) the temperature ofequilibration for these reactions is estimated to bebetween 900 and 1000CThe preservation of the vertical distribution of
VHT---HT facies (Figs 4 and 5) also pointed out byManning et al (2000) indicates that the hydrothermalalteration pattern fits the distribution of isotherms withdepth at a fast-spreading ridge (ie Phipps Morgan ampChen 1993 Dunn et al 2000 Fig 2) Equilibrium tem-peratures for VHT parageneses as high as 900---1000Csuggest that VHT---HT hydrothermal alteration tookplace in a very restricted time interval at the limit ofthe cooling magma chamber It is proposed that the
penetrative alteration affecting the entire gabbro sectionis related to a pervasive network of microcracks whichact as a recharge system down to the Moho (Nicolas et al2003)
Dykes and veins
The dykes are extremely varied eg pegmatitic gabbroswith comb-texture microgabbros and amphibolitizeddolerites dioritic dykes alignments of poikilitic horn-blende and finally diffuse hornblende-bearing patchesHigh-T gabbro and diorite dykes and low-T greenamphibole---epidote veins are connectedIn the dykes textural evidence attests to suprasolidus
conditions at least for the development of the pargasiticamphibole Integrating the hornblende---plagioclase equi-libration temperatures calculated for the studied samplesthese temperatures are bracketed between 950C and875CIn the following discussion we shall consider separately
the microgabbro and dolerite dykes The latter representbasaltic melt intrusions associated upsection with thediabase sheeted dykes and downsection possibly withthe mafic dykes that are ubiquitous in the mantle section(Nicolas et al 2000) Whatever their origin the develop-ment of pargasitic amphibole during their late stage ofcrystallization creates a link with the gabbroic dykes andsuggests a crystallization process evolving from dry con-ditions to more hydrous conditionsThe other gabbroic dykes comb-textured gabbros and
hornblende-bearing dioritic dykes result from the fillingof cracks by a fluid either a melt or a silica-rich hydrousfluid These intrusive dykes which develop reaction mar-gins at their contact with the host gabbro grade verticallyinto lower-temperature green amphibole veins or align-ments of poikilitic hornblende in the layered gabbromatrix or development of pargasitic hornblende in ana-tectic gabbros (sample 01 OA41d2) In gabbros crystal-lizing in the magma chamber the development oforthopyroxene and pargasite oikocrysts enclosing the pri-mary minerals suggests a thermal evolution from thegabbro solidus to amphibolite-facies conditions Forthese reasons it is suggested that the gabbroic dykesresulted from the injection of a hydrous melt in the stillhot gabbros drifting away from the magma chamber asdiscussed below
Origin of fluids responsible for VHT---HThydrothermal alteration
The 87Sr86Sr initial ratios and d18O of whole rocks andpure mineral fractions are plotted in Fig 9 against thedistance above the Moho Transition Zone (MTZ) Theinitial Sr isotopic composition of the whole rocks apartfrom the epidosite dykes ranges from 070322 to
JOURNAL OF PETROLOGY VOLUME 45 NUMBER 6 JUNE 2004
1200
070458 All the studied samples (including whole rocksand minerals) have initial 87Sr86Sr ratios falling outsidethe field of fresh MORB which commonly have Sr ratiosbetween 07023 and 07029 with an average of 070265(ie Hart et al 1974) The lowest initial Sr isotopiccomposition (070322) from the massive gabbro 94MA2 is slightly but significantly higher than averageOman primary magmatic signatures (ie 070296McCulloch et al 1981) Similarly the d18O values ofmost whole rocks and minerals measured (Table 5)
depart from typical MORB-source mantle values( Javoy 1980 Gregory amp Taylor 1981 Kyser 1986)These differences indicate that the primary mantlesignatures in the Oman ophiolite have been modifiedby interaction with a fluid Possible origins for thisfluid are mantle components dehydration of subductedlithosphere or seawater circulation Strontium andoxygen isotopes are useful tracers for fluid---crust interac-tion as d18O and 87Sr86Sr ratios from fluids originat-ing from seawater the mantle or subducted lithosphere
-1000
1000
2000
3000
4000
5000
6000
7000
MOHO
MTZ
Dis
tan
ce a
bov
e th
e M
oh
o (
m)
pillow-basalt
sheeted-dike
gabbro
peridotite
01 OA 41d2
01 OA 36b
02 OA 43a97 OA 128b
01 OA15f
94 OA MA2
02 OA 90b01 OA 19ad
02 OA 37b
0702 0703 0704 0705 0706 0707
87Sr86Srinitial
MO
RB-s
ou
rce
Seaw
ater
Whole RockPlagioclasePargasiteGreen HornblendeClinopyroxene
Epidote
(a)
-1000
1000
2000
3000
4000
5000
6000
7000
MOHO
MTZ
Dis
tan
ce a
bov
e th
e M
oh
o (
m)
pillow-basalt
sheeted-dike
gabbro
peridotite
01 OA 41d2
01 OA 36b
02 OA 43a97 OA 128b
01 OA15f
94 OA MA2
02 OA 90b01 OA 19ad
02 OA 37b
Whole RockPlagioclasePargasiteGreen HornblendeClinopyroxene
3 5 10
δ18O (permil)
41d2
Ma2
43a
90b 19d 19d37b
128b
90b
(b)
Fig 9 (a) Variation of initial 87Sr86Sr isotopic composition as a function of the location of the studied samples in a schematic log of the crustalsection of the Oman ophiolite The field for MORB is from Hart et al (1974) and that for seawater is from Burke et al (1982) Small open squares aredata fromKawahata et al (2001) (b) Variation of d18O () as a function of the location of the studied samples in a schematic log of the crustal sectionof the Oman ophiolite Shaded curve corresponds to the data of Gregory amp Taylor (1981)
BOSCH et al HYDROTHERMAL CIRCULATION IN OMAN OPHIOLITE
1201
are significantly different Moreover the d18O valueis dependent on temperature and mineral fractiona-tion and thus yields complementary constraints to Srisotopes
Mantle origin
One model to explain the occurrence of fluids interactingwith the oceanic crust at very high temperatures is toderive them from the mantle The contribution of mantle-derived hydrous fluids linked to percolation processesor to their concentration in the final stages of gabbrocrystallization has been invoked in numerous case studies(ie Witt amp Seck 1989 Fabriees et al 1989 Dupuy et al1991 Agrinier et al 1993) O and Sr isotopic data forwhole rocks and mineral separates yield scattered valuesthat are unambiguously different from MORB mantle-source values (Fig 9a and b) With the exception of lateLT altered samples the WR data have a mean 87Sr86Srratio of 070337 and a standard deviation of 000012These surprisingly high values in mantle-derived rockscould be taken as evidence for survival of mantle isotopicheterogeneities during crystallization of the ophiolite (egLanphere 1981 McCulloch et al 1981) However the18O16O isotope ratios in the Oman gabbroic sequenceare also displaced from primary magmatic values Nogabbro in the present study has plagioclase or amphi-bole with typical mantle d18O values Thus the Sr and Oisotope data rule out the possibility of mantle-derivedfluids as a possible source for the VHT---HT hydrousalteration event recorded in the massive gabbros andgabbroic dykes and veins (Fig 10a and b)
Subducted lithosphere origin
Dehydration of subducted oceanic crust in subductionzones can generate hydrous fluids Such fluids couldpercolate through the overlying lithosphere and contam-inate it thus generating modified isotopic signatures andtrace element contents Such an explanation howeverdisagrees with the geochemical characteristics of most ofthe studied samples Fluids derived from the dehydrationof subducted slabs (fresh or altered MORB or OIB pro-toliths) should be strongly enriched in K Rb and Ba (egBecker et al 2000) whereas in Oman the Rb contentsmeasured in the gabbros and gabbroic dykes and veins arestrongly depleted Moreover the isotopic composition ofslab-derived fluids is buffered by the MORB-like oceaniccrustal protolith for Nd and Pb but not for Sr and O(Staudigel et al 1995) As a result of relatively minorfractionation of oxygen at high temperature the oxygenisotope composition of the fluids expelled during dehy-dration of the subducted slab should be high and essen-tially controlled by the oxygen composition of the uppersection of the crust altered at low temperature (with an
average of 10 and perhaps as high as 19) (Staudigelet al 1995) The Sr isotopic composition should be alsohigh (average 07046) as the fluids released by the dehy-dration of subducted oceanic crust are associated eitherwith seawater or with radiogenic Sr phases (ie smectites)The Sr and O isotopic compositions of the studied sam-ples do not fit with these signatures and so preclude asubducted lithosphere origin for the fluids involved in theVHT---HT alteration event
Seawater-derived fluids
All the analysed samples (minerals and WR) have 87Sr86Sr(T) outside the domain of primary MORB magmasand variable Sr contents Individual minerals have Srcontents reflecting their different mineralliquid partitioncoefficients Minerals characterized by a very high affi-nity for Sr such as plagioclase (ie Sr mineralliquidpartition coefficients 1) have very high Sr contentsTherefore the high abundance of plagioclase in the sam-ples analysed is responsible for the high Sr content of thestudied whole rocks The Sr content of all the whole rocksis relatively homogeneous (Table 5) without clear distinc-tion between country-rock gabbros and dykes and veinsand ranges from 110 to 200 ppm except for the epidosite(4700 ppm) Reported on a 87Sr86Sr vs 1Sr diagram(Fig 10a) most whole rocks and plagioclases analysed arecharacterized by a 1Sr ratio lower than 001 and plot inthe left part of this diagram Compared with N-MORBthe studied massive and anatectic gabbros and their con-stituent plagioclases have similar to slightly higher Srcontents but substantially higher 87Sr86Sr ratios(Fig 10a) Considering Cretaceous seawater as a possiblehigh 87Sr86Sr mixing component [87Sr86Sr 07073at 90Ma after Burke et al (1982)] responsible for theobserved geochemical modifications exchanges cannothave occurred in a closed system as seawater has too lowa Sr content (ie 8 ppm de Villiers 1999) An open-system process allowing penetration of larger quantitiesof seawater can efficiently modify the Sr isotopic ratio ofthe rocks Alternately the fluids could also be seawaterthat was modified through exchange with the surround-ing rocks during its transit through the oceanic crustalsection This modified seawater would be more enrichedin Srwith a 87Sr86Sr ratio intermediate between unmodi-fied seawater and MORBMinerals devoid of late LT alteration imprints (parga-
site green hornblende and pyroxene) and characterizedby very low Sr contents plot on the right of the diagramclose to or on the mixing line between seawater andMORB (Fig 10a) The difference between these mineralsand the other samples (WR and plagioclases) is mainlydue to the Sr fractionation coefficients of these differentminerals and to the large quantity of plagioclase inthe studied rocks The process responsible for the
JOURNAL OF PETROLOGY VOLUME 45 NUMBER 6 JUNE 2004
1202
modification of the Sr isotopic composition fromMORB-source value is however explained by the same phenom-enon As all these minerals display initial 87Sr86Sr ratiosdistinct from the MORB source and because they equili-brated at VHT or HT temperatures this supports anearly intervention of seawater during crystallization ofthese minerals Most of these samples with the exceptionof the epidosite dyke overlap the domains defined byKawahata et al (2001) for the Wadi Fizh gabbros alteredat VHT and HT conditions in the amphibolite facies(labelled lsquoVHTrsquo and lsquoHTrsquo in Fig 9a)Three samples (two epidosite dykes and an one epidote
fraction) have very high Sr contents and the highest Sr
isotopic compositions of the present dataset The twowhole-rock samples overlap the field of the Wadi Fizhsamples altered at high temperature during greenschist-facies metamorphism (Kawahata et al 2001) This typeof alteration is common at the ridge axis and formedthrough intensive exchange with seawater-derived fluidsFigure 10b indicates that all the Sr---O isotope data are
distinct from MORB compositions and suggests that thefluids reacting with the magmas could be derived fromseawater In addition the d18O values show that isotopicexchange occurred under VHT or HT conditions Thefluids could have the composition of seawater or theycould have the composition of seawater modified through
0703
0705
0707
001 01 1
1 Sr (ppm)
MORB
(87Sr
86Sr)
init
ial
Seawater
VHT
HT
MT36b
36b
36b
43a 128b90b
41d2
37b19d
15f
128b
41d2
41d2
19a
Ma2Ma2
19d
90b
37b
Whole RockPlagioclasePargasiteGreen HornblendeClinopyroxene
Epidote
(a)
0 2 4 6 8
δ18O(permil)
0703
0705
0707Seawater
MORB-mantle
128b 41d2
90b 41d2
Ma2128b
37b
43a
19d
90b
Whole RockPlagioclasePargasiteGreen HornblendeClinopyroxene
Epidote
HT LT
19d
(87Sr
86Sr)
init
ial
(b)
Fig 10 (a) Initial 87Sr86Sr isotopic ratios plotted against 1Sr for whole rocks and mineral separates from the gabbro section of the Omanophiolite Fields shown are from Kawahata et al (2001) and correspond to MT-----basalt sheeted dyke diabase altered at medium temperature(prehnite---actinolite facies sequence 3) HT-----diabase plagiogranite metagabbro and epidosite formed at high temperature (greenschist faciessequence 4) VHT-----gabbro altered at very high temperature (amphibolite facies sequence 5) The dashed line is a binary mixing line betweenMORB (Lanphere et al 1981) and Cretaceous seawater (Burke et al 1982) (b) Initial 87Sr86Sr isotopic composition plotted against d18O value forwhole rocks and minerals from the Oman ophiolite Cretaceous seawater and MORB end-members as in (a) Dashed line represents mixing curvebetweenMORB and seawater at high temperature (HT) Low-temperature (LT) exchanges between these two components would be responsible foran increase of the d18O primary MORB value
BOSCH et al HYDROTHERMAL CIRCULATION IN OMAN OPHIOLITE
1203
exchange with the surrounding rocks during its penetra-tion into the crustal section In an environment in whichfluid---rock interaction occurs at variable temperaturesthe d18O is greatly influenced by the temperature atwhich the exchange took place and by the waterrockratio It is evident from Figs 9b and 10b that none of thestudied gabbros have plagioclase and amphibole withMORB-like d18O and 87Sr86Sr values The separateminerals display a broad range of d18O values and mostexhibit extreme 18O depletion (Table 5) Hydrothermalexchange kinetics are similar in amphibole and pyroxeneand these two minerals are more resistant to oxygenexchange during water---rock interaction than coexistingplagioclase (Gregory et al 1989) The most probableexplanation of the low d18O values measured in bothamphiboles and pyroxenes is that they crystallized athigh temperature in the presence of a relatively low 18Oseawater-derived hydrothermal fluid [d18O from 01 to07 after Gregory amp Taylor (1981)] The anomalouslyhigh d18O value (thorn1009) measured for the plagioclase ofthe micro-gabbroic dyke (01 OA19d) can be interpretedas resulting from a superimposed overprint caused by alate-stage penetration of fluids at lower temperature Inthis sample the occurrence of such different 18O16Oratios between coexisting plagioclase and amphibole isnot consistent with equilibrium fractionation betweenthese two minerals at any possible temperature Thisobservation suggests that parts of the ophiolite sequencehave undergone a high-temperature hydrothermal eventprior to the later low-temperature event that producedthe strong 18O increase in coexisting plagioclaseThe depletion of the d18O values results from high-temperature hydrothermal alteration (Gregory amp Taylor1981 Stakes amp Taylor 1992) The oxygen isotopic datasupport the contention that most studied samples havebeen affected by a VHT---HT hydrothermal alteration byfluids derived from seawater Some of the analysed sam-ples presenting petrological evidence of crystallization ofLT secondary minerals have been overprinted by the latelow-temperature alteration thus swamping the earlierhigh-temperature alteration effects
Determination of waterrock ratio
To better evaluate the exchange between seawater andthe gabbroic rocks waterrock ratios (WR) have beenestimated for the whole rocks analysed during the courseof this study The different models used to constrain thisratio mainly differ by the calculation of the effective ratioor by the estimated weight ratio and by considering openor closed systems for water circulation (Albareede et al1981 McCulloch et al 1981) The WR ratios reportedin Table 5 have been calculated assuming a closed sys-tem Using this formulation these values are minimabecause the calculation always uses pristine fluid for the
initial fluid thus maximizing the value in the denomina-tor If the fluid was shifted to lower 87Sr concentrations byexchange with the rock on the downward path theinferred values would be much higher (Davis et al 2003)The calculated WR ratios for the samples from this
study range from 31 to 219 (Table 5) Two main groupsof samples can be recognized on the basis of their waterrock ratios The lowest ratios ranging from 31 to 47have been determined for massive gabbros or anatecticgabbros but also for dykes and veins devoid of late LTalteration Similar ratios have been previously calculatedfor example for the discharged hydrothermal solutions atthe 13N and 21N East Pacific Rise (Di Donato et al1990 Fouquet et al 1991) The gabbros from the Ibrasection in the Oman ophiolite gave effective WR ratiosof 05 33 and 42 (McCulloch et al 1981) in goodagreement with the values obtained in this study Theleast altered gabbro of the Ibra section yields a WR ratioof 05 in good agreement with the exceptionally fresh-ness of this rock characterized by typical mantle oxygenvalues for its minerals (McCulloch et al 1981) Samplescharacterized by waterrock effective ratios in the rangeof 3---5 can be related to penetrative alteration via thehydrothermal system Lecuyer amp Reynard (1996) havedemonstrated in oceanic gabbros from the Hess Deepaltered in similar HT conditions that WR mass ratios inthe range of 02---1 are sufficient to account for the mod-ified stable isotope signature of the gabbros Higherwaterrock values (WR 45) have been calculated forsamples affected by superimposed late hydrothermalalteration and for the epidosite samples (Table 5) Theyprobably acquired their isotopic characteristics throughinteraction with a larger volume of seawater duringgreenschist-facies metamorphism Similar or higherWR values have been published in previous studies(ie McCulloch et al 1981 Kawahata et al 2001)They correspond either to samples from the upper-crus-tal sequence (ie basalt sheeted dyke or plagiogranite) orto gabbros from the crustal section located in a particularenvironment such as for example the Wadi Fizh sectionthat is located close to a segment boundary along thepalaeo-spreading axis (Nicolas et al 2000)
Mechanism for seawater interaction withmagma
Nicolas et al (2003) have proposed that variable amountsof seawater can circulate at high temperature through-out the crustal section down to the walls of the magmachamber via a network of parallel microcracks a fractionof a millimetre wide Such microcracks and their relation-ship with the VHT---HT hydrous alteration in the sur-rounding gabbros have been described in detail byNicolas et al Via this network of microcracks largeamounts of seawater can have reacted with the magma
JOURNAL OF PETROLOGY VOLUME 45 NUMBER 6 JUNE 2004
1204
and induced variable changes of its Sr and O isotopiccomposition This regular network of parallel micro-cracks could constitute the recharge system and thehydrated gabbroic dykes the discharge system Physicalparameters and the geophysical models of Nicolas et al(2003) are in agreement with this scenario Such a pene-tration of seawater-derived hydrothermal fluids throughthe lower crust along grain boundaries and a microscopicfracture network at temperatures 4750C is consistentwith recent thermodynamic models (McCollom amp Shock1998)Seawater-altered and high-temperature recrystallized
diabases (protodykes) from the overlying sheeted dykecomplex stoped into the melt lenses could represent anindirect way for seawater to enter the system as they canremelt during their subsidence through the magmachamber A simple mass balance calculation suggeststhat this indirect contamination is not significant Assess-ments of the amount of recycled crust 1 in volumeby Nicolas et al (2000) based on the protodyke remnantsin the gabbro section or 20 by Coogan et al (2003)based on chlorine content in EPR basalts lead to a sea-waterrock ratio of the order of 104 to 103Thus following Nicolas et al (2003) we propose that
seawater has been introduced along microcracks down tothe walls of the magma chamber and has diffused alonggrain boundaries in the way described by Manning et al(2000) progressively invading the host gabbro Gabbroicdykes result from the injection of hydrous melt into thehost gabbros at falling temperatures as they drift awayfrom the magma chamber This water-saturated meltcould result from the extensive hydration of the crystal-lizing gabbros near the walls of the magma chamberwhen seawater penetrates through this wall (Fig 1)Similar plumbing systems driving seawater down to the
limit of the magma chamber and back to the surface havebeen already proposed in other environments such as theMid-Atlantic Ridge (Kelley amp Delaney 1987 Cooganet al 2001) the East Pacific Rise at Hess Deep (Gillis1995 Manning et al 1996a 1996b) the Tonga forearc(Banerjee amp Gillis 2001) and the Troodos ophiolite(Gillis amp Roberts 1999) The melting curve of wet basalthas a negative slope but is close to the dry solidus at lowpressure in the lower gabbros and the first melts areplagiogranitic (Spulber ampRutherford 1983) Such plagio-granites are ubiquitous in the highest gabbros and in theroot zone of sheeted dykes in Oman (Rothery 1983Juteau et al 1988 Nicolas amp Boudier 1991) in manyother ophiolites and in the oceanic crust where they havebeen interpreted as resulting either from wet anatexis ofhot gabbros or from the final product of crystallization ofa basaltic melt (Spulber amp Rutherford 1983) Plagio-granites are rare in the lower gabbros exceptionallyassociated with hydrous gabbro segregations inside thelayered gabbros Their constitutive minerals are also
remarkably absent along the VHTmicrocracks althoughthese gabbros were above the wet solidus A possibleexplanation has been proposed by McCollom amp Shock(1998) who wrote that at high temperature lsquoaqueousfluids may leave very little conspicuous petrological evid-ence for their presencersquo the reason being that thehigh-T conditions would be closer to batch meltingObviously more detailed studies on this specific problemare needed We conclude that the large degree of hydrousmelting locally required at the walls of the magma cham-ber to account for the generation of a gabbroic hydrousmelt may have erased the traces of the incipient plagio-granitic melt
CONCLUSIONS
Fostered by the recent discovery of high-temperatureseawater contamination in some gabbros of the Omanophiolite (Manning et al 2000) we have conducted astudy on 500 gabbro specimens from selected areas ofthe entire Samail ophiolite belt and on the hydrous gab-broic dykes that cross-cut them The highest-temperaturereactions (VHT) recorded in olivine and clinopyroxene ingabbros as orthopyroxene and pargasite coronas reach975C They are followed at lower HT conditions withreplacement of olivine by an assemblage of tremolitemagnetite and plagioclase surrounded by chlorite andby pargasite development in clinopyroxene followedfinally by the LT lizardite---olivine and saussurite---plagioclase greenschist-facies alteration With a fewexceptions the VHT alteration tends to be restricted tothe lower gabbros and to the vicinity of the Moho andthe HT alteration to the middle and upper gabbros Thepreservation of the vertical distribution of VHT---HTfacies indicates that progressive cooling during off-axisspreading did not obliterate this distribution and conse-quently that the VHT---HT hydrothermal alteration tookplace in a very restricted time interval at the limit of thecooling magma chamber In the deep and hot gabbrosthe main water channels may be sub-millimetre-sizedmicrocracks with a dominantly vertical attitude the ori-gin and dynamics of which have been investigated in aprevious paper (Nicolas et al 2003)Beyond these high-temperature alteration zones mas-
sively induced in the gabbros seawater may be respons-ible for the generation of clinopyroxene---pargasitegabbro dykes that cross-cut all gabbros and that areupsection clearly connected to the LT hydrothermalvein system Petrostructural evidence suggests that thesedykes were generated by hydration of the crystallizinggabbros near the internal wall of the LVZ---magmachamber The resultant melts were subsequently intrudedat various levels in the cooling lower and middle gabbrosPetrological observations of nearly all the gabbros ana-lysed in the present study located from the root zone of
BOSCH et al HYDROTHERMAL CIRCULATION IN OMAN OPHIOLITE
1205
the sheeted dyke complex down to the Moho emphasizethe imprint of VHT---HT hydrous alteration The VHThydrothermal event is characterized by temperatures ofequilibration around 900---1000C The temperatures atwhich the HT hydrous event occurred are estimated tobe in the range 550---900CStrontium and oxygen isotope compositions measured
on whole rocks and separated minerals significantlydepart from primary MORB values The Sr and O iso-tope data obtained for pargasite green hornblende andclinopyroxene record the participation of seawater at thetemperature of crystallization of the minerals Plagio-clases and whole rocks define a trend toward a compo-nent characterized by a high radiogenic 87Sr86Sr valueand a higher Sr content than normal seawater Seawateris clearly identified as the fluid responsible for the modi-fication of the Sr and O signatures for both massivegabbros and dykes and veins Nevertheless the high Srcontent of the extraneous component requires eitheropen-system exchange allowing penetration of a greaterquantity of seawater or penetration of seawater modifiedby interaction with the oceanic crust during its travel paththrough the crustal section The waterrock ratio calcu-lated on the basis of the Sr isotopes is between three andfive for the enclosing gabbros and for the least LT altereddykes The VHT---HT hydrothermal event affectingthese samples is related to penetrative alteration via therecharge and discharge hydrothermal system Thewaterrock ratio is 10 for dykes exhibiting evidenceof a LT hydrothermal alteration overprint and reaches22 for the epidosite dyke The depletion in d18O char-acterizes all the studied samples with the single exceptionof the micro-gabbroic dyke plagioclase This agreeswith the conclusions based on Sr isotopes suggestingthat these samples have undergone exchange with sea-water-derived fluids during a VHT---HTalteration hydro-thermal event
ACKNOWLEDGEMENTS
This work has benefited from financial support by theCentre National de la Recherche Scientifique (INSUInteerieur-Terre andDYETI programmes) and inOmanfrom the willingness of the Directorate of MineralsMinistry of Commerce and Industry and the hospitabil-ity of the people Technical help in the laboratory is alsoacknowledged including C Nevado for the thin sectionsE Ball for the illustrations B Galland for her assistancein the chemistry room and C Bassoulet for help duringthermal ionization mass spectrometric analyses of the Srresults from the leaching experiments Constructivereviews from R T Gregory C Lecuyer an anonymousreviewer and particularly from M Wilson greatlyhelped to improve an earlier version of the manuscript
REFERENCESAgrinier P Mevel C Bosch D amp Javoy M (1993) Metasomatic
hydrous fluids in amphibole peridotites from Zabargad Island (Red
Sea) Earth and Planetary Science Letters 120 187---205Albareede F A Michard A Minster J F amp Michard G (1981)
87Sr86Sr ratios in hydrothermal waters and deposits from the East
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Gillis K M (1995) Controls on hydrothermal alteration in a section of
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Gregory R T amp Taylor H P (1981) An oxygen isotope profile in a
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Gregory R T Criss R E amp Taylor H P (1989) Oxygen
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Hart S R Erlank A J amp Kable J D (1974) Sea floor basalt
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Ionov D A Savoyant L amp Dupuy C (1992) Application of the
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Javoy M (1980) 18O16O and DH ratios in high temperature
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Juteau T Beurrier M Dahl R amp Nehlig P (1988) Segmentation
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Juteau T Manacrsquoh G Moreau C Lecuyer C amp Ramboz C
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Oman ophiolite Journal of Geophysical Research 106 11083---11099
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beneath oceanic spreading ridges Annual Geological Union Special
Monograph 106 267---289
Kelemen P B Koga K amp Shimizu N (1997) Geochemistry of
gabbro sills in the crustmantle transition zone of the Oman
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and Planetary Science Letters 146 475---488Kelley D S amp Delaney J R (1987) Two-phase separation and
fracturing in mid-ocean ridge gabbros at temperatures greater than
700C Earth and Planetary Science Letters 83 53---66
Koepke J Feig S T Snow J amp Freise M (2003) Petrogenesis of
oceanic plagiogranites by partial melting of gabbros an experi-
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Kyser T K (1986) Stable isotope variations in the mantle In
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High Temperature Geological Processes Mineralogical Society of America
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Lanphere M A (1981) K---Ar ages of metamorphic rocks at the base
of the Samail ophiolite Oman Journal of Geophysical Research 86
2777---2782
Lanphere M A Coleman R G amp Hopson C A (1981) Sr isotopic
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Leake B E (1997) Nomenclature of amphiboles report of the
subcommittee on amphiboles of the International Mineralogical
Association commission on new minerals and mineral names
European Journal of Mineralogy 9 623---651
Lecuyer C amp Reynard B (1996) High-temperature alteration of
oceanic gabbros by seawater (Hess Deep Ocean Drilling Program
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Journal of Geophysical Research 101(B7) 15883---15897
Liou J G Kuniiyoshi S amp Ito K (1974) Experimental study of the
phase relations between greenschist and amphibolite in a basaltic
system American Journal of Science 274 613---632
MacLeod C J amp Rothery D A (1992) Ridge axial segmentation in
the Oman ophiolite evidence from along-strike variations in the
sheeted dyke complex Geological Society of America Special Papers 60
39---63
Manning C E MacLeod C J amp Weston P E (1996a) Fracture-
controlled metamorphism of Hess Deep gabbros site 984
constraints on the roots of mid-ocean ridge hydrothermal systems
at fast-spreading centers In GillisM C Allan KM ampMeyer P S
(eds) Proceedings of the Ocean Drilling Program Scientific Results 147
College Station TX Ocean Drilling Program pp 189---211Manning C E Weston P E amp Mahon K I (1996b) Rapid high-
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cracking front at fast-spreading ridges evidence from the East Pacific
Rise and the Oman ophiolite Journal of the Geological Society London
349 261---272McCollom T M amp Shock E L (1998) Fluid---rock interactions in
the lower oceanic crust thermodynamic models of hydrothermal
alteration Journal of Geophysical Research 103 547---575
McCulloch M T Gregory R T Wasserburg G J amp Taylor H P
(1981) Sm---Nd Rb---Sr and 18O16O isotopic systematics in an
oceanic crustal section evidence from the Samail ophiolite Journal of
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Proceedings of the Ocean Drilling Program Scientific Results 147 College
Station TX Ocean Drilling Program
Nehlig P amp Juteau T (1988) Flow porosities permeabilities and
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BOSCH et al HYDROTHERMAL CIRCULATION IN OMAN OPHIOLITE
1207
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Nicolas A Ceuleneer G Boudier F amp Misseri M (1988) Structural
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M (1995) Large scale isotopic Sr Nd and O isotopic anatomy of
altered oceanic crust DSDPODP sites 417418 Earth and Planetary
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New York McGraw---Hill 694 pp
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tions for the nature of mantle metasomatism Earth and Planetary
Science Letters 91 327---340
JOURNAL OF PETROLOGY VOLUME 45 NUMBER 6 JUNE 2004
1208
zonation from An95 to An99 from core to rim (Table 1 andFig 8a) TheWR Sr isotopic ratio of the sample (070458)is significantly higher than normal MORB values Thiscould be related to the overprint of a low-temperaturealteration event consistent with the petrography of thesample
Green amphibole vein cross-cutting layered gabbros inWadi Mayah Haylayn Massif
Sample 02 OA43a belongs to a series of 1 cm thick greenamphibole veins (as in Fig 6e) cross-cutting the layeredgabbros The veins develop 15 cm thick reaction marginsin the enclosing gabbro and are exclusively composed ofacicular pale green magnesio-hornblende growing per-pendicular to the vein margin in a crack-seal mode(Fig 6f ) The reaction margins are composed of urali-tized gabbro marked by the development of the samepale green magnesio-hornblende in an inhomogeneouscalcic plagioclase matrix (An91---97) The Sr and O isotopiccompositions of this green hornblende (070431 andthorn31) do not correspond to normal MORB-sourcemantle values
Epidosite dyke from Wadi Gideah Wadi Tayin Massif
Sample 01 OA36b is representative of a series of epidoteveins cross-cutting poorly layered gabbros These 2 cmthick veins are composed of pink thulite in their centreand green pistachite at their margins (Fig 6a) Theirsharp contact with the enclosing gabbro is marked bythe development of coarse clinopyroxene crystals alteredin the epidote---greenschist facies This suggests that theepidote vein is replacive in a coarse-grained gabbro dykeThe highest 87Sr86Sr initial ratios of all the samplesstudied are recorded by the WR and coexisting epidoteof this sample (070519 and 070554 respectively)(Table 5) The Sr content of the WR and associatedepidote are also the highest of the present dataset(716 ppm and 764 ppm respectively) The Sr isotopiccomposition of this sample is consistent with a greaterdegree of exchange with a radiogenic component fromCretaceous seawater It is significantly higher than thevalue obtained for an epidosite vein from the Wadi Fizhsection of the Oman ophiolite [070435 with a Sr contentof 488 ppm reported by Kawahata et al (2001)] but ingood agreement with the average of the greenschist-faciesalteration sequence defined by Kawahata et al (2001)
GABBROS
01 km 025 km 1 km 36 km50
60
70
80
90
An
MA2
19d
19a 15f 128b
41d2
(a)
43a17b
15f
90b
37b
19a
19d
40
60
80
100
A
n
01 km 015 km 025 km 1 km
DYKES
(b)
Distance to the Moho
ACTINOLITE
MAGNESIOHORNBLENDE
TSCHERMAKITE
PARGASITEGabbros Dykes41d2MA2
19a d37b15f
90b43a
AlIV
150500
02
08
10
(Na
+ K
) A
(c)
04
06
950
900
850
800
750
70007 12 17
T degC
1000
(d)
AlIV
Fig 8 Plagioclase---amphibole compositions and results of thermometry (a) and (b) plagioclase compositions in layered gabbros and in dykesrespectively as a function of distance to the Moho (see text for sample identification) (c) amphibole compositions in layered gabbros and dykesaccording to Leakersquos (1997) nomenclature (d) equilibration temperatures calculated using the amphibole---plagioclase thermometer of Holland ampBlundy (1994) as a function of AlIV content in amphibole [same symbols as in (c)]
JOURNAL OF PETROLOGY VOLUME 45 NUMBER 6 JUNE 2004
1196
Table5RbandSrcontents87Sr
86Srandd1
8Oisotopiccompositionsofwholerocksandmineralseparatesfrom
samplesofmassivegabbrosdykes
andveinsfrom
theOman
ophiolitealsolistedarecalculated
waterrockratios(W
R)andLOIvalues
Sam
ple
Rb(ppm)
Sr(ppm)
87Rb8
6Sr
87Sr
86Sr
(87Sr
86Sr)i1
d18O
()
LOI(
)2WR
3(87Sr
86Sr)L14
(87Sr
86Sr)L25
(87Sr
86Sr)R6
01OA41d2
Whole
rock
037
1792
0006
0703516
09
070351
thorn480
138
47
0703587
0703651
0703357
Plagioclase
025
1249
0003
0704261
22
070426
thorn504
-------
-------
Pargasite
-------
-------
-------
0703548
20
070355
-------
-------
-------
Clinopyroxene
045
26 0
0050
0703845
19
070378
-------
-------
-------
01OA36b
Dykewhole
rock
0005
7164
50001
0705186
17
070519
-------
274
21 9
0705207
0705112
-------
Epidote
0003
7645
0001
0705536
12
070554
-------
-------
-------
Wallwhole
rock
005
4254
0001
0704637
09
070464
-------
191
-------
0704945
0704675
0704593
02OA43a
Green
hornblende
004
98
0012
0704323
13
070431
thorn310
-------
-------
97OA128b
Whole
rock
003
1274
50001
0703327
17
070333
-------
076
37
-------
-------
-------
Plagioclase
002
1204
50001
0703285
09
070328
thorn414
-------
-------
Clinopyroxene
003
20 0
0004
0704230
09
070422
thorn523
-------
-------
01OA15f
Whole
rock
006
1077
0002
0704582
16
070458
-------
237
13 5
-------
-------
-------
01OA19a
Whole
rock
037
1303
0008
0704189
18
070418
-------
161
96
-------
-------
-------
01OA19d
Whole
rock
058
2032
0008
0703423
08
070341
thorn567
216
415
0703574
0703408
0703271
Pargasite
091
1058
0025
0703273
11
070324
thorn287
-------
-------
Plagioclase
-------
-------
-------
-------
-------
thorn10 09
-------
-------
02OA90b
Plagioclase
004
2385
50001
0703868
11
070387
thorn498
-------
-------
Green
hornblende
010
23 8
0012
0704288
15
070427
thorn239
-------
-------
BOSCH et al HYDROTHERMAL CIRCULATION IN OMAN OPHIOLITE
1197
Table5continued
Sam
ple
Rb(ppm)
Sr(ppm)
87Rb8
6Sr
87Sr
86Sr
(87Sr
86Sr)i1
d18O
()
LOI(
)2WR
3(87Sr
86Sr)L14
(87Sr
86Sr)L25
(87Sr
86Sr)R6
02OA37b
Plagioclase
023
3121
0002
0704204
15
070420
-------
-------
-------
Pargasite
031
41 6
0021
0703505
15
070348
thorn394
-------
-------
94OAMa2
Whole
rock
0016
1750
00003
0703219
15
070322
thorn495
033
31
0703264
0703177
0703158
Plagioclase
0014
1363
00003
0703219
11
070322
thorn479
-------
-------
Clinopyroxene
-------
-------
-------
0703236
13
070324
-------
-------
-------
1Initial8
7Sr
86Srratiocalculatedat
95Ma(H
acker1994)
2LOIisloss
onignition(in)
3Water---rock
ratiocalculatedassumingaclosedsystem
TheeffectiveWR
ratiocanbeexpressed
bythefollowingeq
uation
W=Reffectivefrac14
frac12eth87Sr=
86SrrockTHORN fi
nal
eth87Sr=
86SrrockTHORN in
itial=frac12eth8
7Sr=
86SrseawaterTHORN in
itial
eth87Sr=
86SrrockTHORN fi
nal
ethCrock
initial=C
seaw
ater
initial
THORNwhere(87Sr
86Srrock) finalisthefinalmeasuredSrisotopicratioin
thesample(87Sr
86Srrock) in
itialco
rrespondingto
theinitialm
antlevalue
istakenas
070295(Lan
phere
etal1981)(87Sr
86Srseawater) in
itialistheCretaceousseaw
ater
Srisotopicco
mposition[87Sr
86Sr07073
at90
MaafterBurkeet
al(1982)]C
seaw
ater
initial
istheSrco
ntent
oflsquonorm
alrsquoseaw
ater
andis
takenas
8ppm
(deVilliers1999)
FortheCrock
initialitis
assumed
that
thepresentSrco
ncentrationmeasuredis
thesameas
theinitial
concentration
4Srisotopicratiosmeasuredforliquid
L1extractedafterthefirststep
oftheleachingexperim
ent(6NHClfor8min
at70
C)
5Srisotopicratiosmeasuredforliquid
L2extractedaftertheseco
ndstep
oftheleachingexperim
ent(2 5NHClfor30
min
at70
C)
6Srisotopicratiosmeasuredforresidueobtained
afteratotalaciddigestionachievedaftertheextractionofL1an
dL2leachates
JOURNAL OF PETROLOGY VOLUME 45 NUMBER 6 JUNE 2004
1198
Part of the same sample from the contact between thedyke and the gabbro yields a slightly lower Sr isotopiccomposition (070464) intermediate between the sur-rounding gabbro and the epidote dyke It also has anintermediate Sr content (425 ppm) Acid leaching experi-ments performed on the whole rock of the dyke yield Srisotopic compositions of 070521 and 070512 for thetwo liquids extracted Similar experiments for the samplelocated at the contact with the gabbro yield 87Sr86Srratios of 070494 and 070467 for L1 and L2 liquids TheSr isotopic composition of the residue extracted afterleaching remains high (070459) and very close to theepidosite 87Sr86Sr ratio determined by Kawahata et al(2001) In this sample the primary minerals have beentotally replaced by epidote and secondary plagioclase orquartz Thus the Sr isotopic compositions measured forthis WR sample and its constituent minerals reflect thecomposition of the fluid filling this dyke
Mineral chemistry
Plagioclase
Compared with the host gabbros the dykes show a muchlarger dispersion in their plagioclase compositions (Fig 8aand b Table 4) characterized by more calcic plagioclasethan the enclosing gabbros This tendency is mostextreme in the comb-textured dykes in which the plagio-clase is almost pure anorthite The zoning is generallynormal recording a crystal fractionation process How-ever gabbro 94 MA2 exhibits inverse zoning this tend-ency is also observed in plagioclases from dykes 01OA19a and 01 OA15f The higher calcium content ofthe plagioclase as well as the inverse zoning is consistentwith an increase of water pressure during its crystalliza-tion (Turner amp Verhoogen 1960 Carmichael et al 1974Koepke et al 2003) The particularly high An content inthe comb-textured gabbro dykes 01 OA15f may alsoresult from crystallization in a supercooled magma thisis consistent with the comb texture indicative of fast-growing crystals The lack of brown hornblende suggeststhat crystallization occurred above the temperature ofhornblende stability
Amphibole
The amphiboles analysed in the dykes (Fig 8c andTable 4) show a regular trend in a (Na thorn K)A vsAlIV diagram from titanium-rich (Ti 4 015) pargasitichornblende (brownamphibole) and tschermakite (brown---green amphibole) to low-titanium (Ti5 015) magnesio-hornblende (green amphibole) and actinolite (pale greenamphibole) The pargasitic hornblende develops aspoikilitic oikocrysts in the dykes 01 OA19a and d andin the diffuse patch 01 OA41d2 at temperatures above800C during the final stage of crystallization (see
below) The tschermakite composition corresponds tothe internal alteration of clinopyroxene in gabbro 94Ma2 occurs in the comb-textured dyke 01 OA15f andis the primary amphibole in the dioritic dyke 02 OA37bThe magnesio-hornblende develops at the edges of thepargasites or of clinopyroxenes at temperatures around800C and is the primary phase in the dioritic dyke 02OA90b and in the green vein 02 OA43a Finally actino-lite occurs as a replacement of green hornblende in dykesor in the kelyphitic rim surrounding olivine in gabbro 94Ma2 Such a large composition range at the scale of athin section has been reported in amphiboles fromamphibolite-facies oceanic gabbros (Stakes 1991 Stakesamp Taylor 1992 Gillis et al 1993) as well as in the Omanophiolite gabbros (Manning et al 2000) This variabilityhas been explained by rapid reaction rates preventinghomogenization (Manning et al 2000)
Thermometry
Plagioclase---amphibole thermometry could be appliedwhen the two minerals exhibited good contacts such asin the laminated gabbro of the middle sequence (01OA41d2) and in the intrusive dykes The temperaturesof plagioclase---amphibole equilibration (Table 2) havebeen calculated using the geothermometer lsquoBrsquo of Hollandamp Blundy (1994) Edenite thorn Albite frac14 Richterite thornAnorthite valid between 500C and 900C with anuncertainty of 40C Amphibole formulae are basedon 23 oxygen and their nomenclature is based on alkalications in the A site vs AlIV according to Leake (1997)Pressurewas assumed to be 1 kbar Forty-five plagioclase---amphibole pairs meet the compositional criteria imposedby Holland amp Blundyrsquos dataset (09 4 Xan 4 01 andAlIV 403) Electron microprobe measurements wereconstrained to contiguous plagioclase and amphiboleseparated by sharp grain boundaries assuming equili-brium of the two phases When the mineral phasesexhibit normal zoning temperatures between plagioclaseand amphibole cores and between mineral rims werecalculated assuming that the temperatures deduced fromthe mineral core chemistry provide a proxy for the high-est temperature of equilibration These data are identi-fied in the T C vs AlIV diagram (Fig 8d) Table 4presents selected major element amphibole analyses theAn content of the plagioclase adjacent to each amphibolegrain and the temperature calculated for the pairThe temperatures calculated (Fig 8d) span the entire
range of temperatures at which amphibole and plagio-clase coexist in equilibrium (ie amphibolite-facies para-geneses) This temperature interval is bracketed byorthopyroxene-bearing facies or amphibole-free facies(ie gabbro 94MA2) and epidote---actinolite facies devoidof homogeneous plagioclase (ie epidosite dyke 01OA36b and green vein 02 OA43a) The range of the
BOSCH et al HYDROTHERMAL CIRCULATION IN OMAN OPHIOLITE
1199
calculated temperatures between 4900C and 750Ccorresponds to the changing composition of amphibolesfrom pargasite to magnesio-hornblende at the scale of athin section or even at that of an individual crystal asshown by their correlation with AlIV such variabilityrecords rapid cooling in hydrous conditions The highesttemperatures are recorded by pargasite from the anatec-tic gabbro 01 OA41d2 in the gabbro dyke 01 OA19dand in the dioritic dyke 02 OA90b They were calculatedfrom minerals devoid of lower-grade alteration and arethus considered as representing the equilibrium temperat-ure of the coexisting minerals Lower temperatures cor-respond to mineral phases evolving at falling temperatureand are only indicative of the cooling history Includingthe 40C uncertainty seven pairs provide temperatureshigher than 900C the limit of validity of the Holland ampBlundy thermometer However we maintain these tem-peratures as indicative of the highest values of our data-set These values are included in the average equilibriumtemperature summarized in Table 2
DISCUSSION
Distribution of hydrothermal alteration
Two distinct petrological events are recorded in the gab-bro section of the Samail ophiolite and are documentedin the present contribution (1) the whole gabbro sectionis affected by a penetrative alteration (2) the gabbrosection is crosscut by various types of dykes and veinsincluding hydrous minerals
Penetrative alteration
The penetrative alteration developed in the gabbros atgrain boundaries and locally invading the minerals isubiquitous in the gabbro section This penetrative altera-tion has been described as very high temperature (VHT)high temperature (HT) and low temperature (LT) basedon alteration parageneses The orthopyroxene coronas inthe layered gabbro sample and the incipient appearanceof brown pargasitic amphibole mark a lower thermallimit of the VHT alteration By reference to the experi-mental work of Koepke et al (2003) the temperature ofequilibration for these reactions is estimated to bebetween 900 and 1000CThe preservation of the vertical distribution of
VHT---HT facies (Figs 4 and 5) also pointed out byManning et al (2000) indicates that the hydrothermalalteration pattern fits the distribution of isotherms withdepth at a fast-spreading ridge (ie Phipps Morgan ampChen 1993 Dunn et al 2000 Fig 2) Equilibrium tem-peratures for VHT parageneses as high as 900---1000Csuggest that VHT---HT hydrothermal alteration tookplace in a very restricted time interval at the limit ofthe cooling magma chamber It is proposed that the
penetrative alteration affecting the entire gabbro sectionis related to a pervasive network of microcracks whichact as a recharge system down to the Moho (Nicolas et al2003)
Dykes and veins
The dykes are extremely varied eg pegmatitic gabbroswith comb-texture microgabbros and amphibolitizeddolerites dioritic dykes alignments of poikilitic horn-blende and finally diffuse hornblende-bearing patchesHigh-T gabbro and diorite dykes and low-T greenamphibole---epidote veins are connectedIn the dykes textural evidence attests to suprasolidus
conditions at least for the development of the pargasiticamphibole Integrating the hornblende---plagioclase equi-libration temperatures calculated for the studied samplesthese temperatures are bracketed between 950C and875CIn the following discussion we shall consider separately
the microgabbro and dolerite dykes The latter representbasaltic melt intrusions associated upsection with thediabase sheeted dykes and downsection possibly withthe mafic dykes that are ubiquitous in the mantle section(Nicolas et al 2000) Whatever their origin the develop-ment of pargasitic amphibole during their late stage ofcrystallization creates a link with the gabbroic dykes andsuggests a crystallization process evolving from dry con-ditions to more hydrous conditionsThe other gabbroic dykes comb-textured gabbros and
hornblende-bearing dioritic dykes result from the fillingof cracks by a fluid either a melt or a silica-rich hydrousfluid These intrusive dykes which develop reaction mar-gins at their contact with the host gabbro grade verticallyinto lower-temperature green amphibole veins or align-ments of poikilitic hornblende in the layered gabbromatrix or development of pargasitic hornblende in ana-tectic gabbros (sample 01 OA41d2) In gabbros crystal-lizing in the magma chamber the development oforthopyroxene and pargasite oikocrysts enclosing the pri-mary minerals suggests a thermal evolution from thegabbro solidus to amphibolite-facies conditions Forthese reasons it is suggested that the gabbroic dykesresulted from the injection of a hydrous melt in the stillhot gabbros drifting away from the magma chamber asdiscussed below
Origin of fluids responsible for VHT---HThydrothermal alteration
The 87Sr86Sr initial ratios and d18O of whole rocks andpure mineral fractions are plotted in Fig 9 against thedistance above the Moho Transition Zone (MTZ) Theinitial Sr isotopic composition of the whole rocks apartfrom the epidosite dykes ranges from 070322 to
JOURNAL OF PETROLOGY VOLUME 45 NUMBER 6 JUNE 2004
1200
070458 All the studied samples (including whole rocksand minerals) have initial 87Sr86Sr ratios falling outsidethe field of fresh MORB which commonly have Sr ratiosbetween 07023 and 07029 with an average of 070265(ie Hart et al 1974) The lowest initial Sr isotopiccomposition (070322) from the massive gabbro 94MA2 is slightly but significantly higher than averageOman primary magmatic signatures (ie 070296McCulloch et al 1981) Similarly the d18O values ofmost whole rocks and minerals measured (Table 5)
depart from typical MORB-source mantle values( Javoy 1980 Gregory amp Taylor 1981 Kyser 1986)These differences indicate that the primary mantlesignatures in the Oman ophiolite have been modifiedby interaction with a fluid Possible origins for thisfluid are mantle components dehydration of subductedlithosphere or seawater circulation Strontium andoxygen isotopes are useful tracers for fluid---crust interac-tion as d18O and 87Sr86Sr ratios from fluids originat-ing from seawater the mantle or subducted lithosphere
-1000
1000
2000
3000
4000
5000
6000
7000
MOHO
MTZ
Dis
tan
ce a
bov
e th
e M
oh
o (
m)
pillow-basalt
sheeted-dike
gabbro
peridotite
01 OA 41d2
01 OA 36b
02 OA 43a97 OA 128b
01 OA15f
94 OA MA2
02 OA 90b01 OA 19ad
02 OA 37b
0702 0703 0704 0705 0706 0707
87Sr86Srinitial
MO
RB-s
ou
rce
Seaw
ater
Whole RockPlagioclasePargasiteGreen HornblendeClinopyroxene
Epidote
(a)
-1000
1000
2000
3000
4000
5000
6000
7000
MOHO
MTZ
Dis
tan
ce a
bov
e th
e M
oh
o (
m)
pillow-basalt
sheeted-dike
gabbro
peridotite
01 OA 41d2
01 OA 36b
02 OA 43a97 OA 128b
01 OA15f
94 OA MA2
02 OA 90b01 OA 19ad
02 OA 37b
Whole RockPlagioclasePargasiteGreen HornblendeClinopyroxene
3 5 10
δ18O (permil)
41d2
Ma2
43a
90b 19d 19d37b
128b
90b
(b)
Fig 9 (a) Variation of initial 87Sr86Sr isotopic composition as a function of the location of the studied samples in a schematic log of the crustalsection of the Oman ophiolite The field for MORB is from Hart et al (1974) and that for seawater is from Burke et al (1982) Small open squares aredata fromKawahata et al (2001) (b) Variation of d18O () as a function of the location of the studied samples in a schematic log of the crustal sectionof the Oman ophiolite Shaded curve corresponds to the data of Gregory amp Taylor (1981)
BOSCH et al HYDROTHERMAL CIRCULATION IN OMAN OPHIOLITE
1201
are significantly different Moreover the d18O valueis dependent on temperature and mineral fractiona-tion and thus yields complementary constraints to Srisotopes
Mantle origin
One model to explain the occurrence of fluids interactingwith the oceanic crust at very high temperatures is toderive them from the mantle The contribution of mantle-derived hydrous fluids linked to percolation processesor to their concentration in the final stages of gabbrocrystallization has been invoked in numerous case studies(ie Witt amp Seck 1989 Fabriees et al 1989 Dupuy et al1991 Agrinier et al 1993) O and Sr isotopic data forwhole rocks and mineral separates yield scattered valuesthat are unambiguously different from MORB mantle-source values (Fig 9a and b) With the exception of lateLT altered samples the WR data have a mean 87Sr86Srratio of 070337 and a standard deviation of 000012These surprisingly high values in mantle-derived rockscould be taken as evidence for survival of mantle isotopicheterogeneities during crystallization of the ophiolite (egLanphere 1981 McCulloch et al 1981) However the18O16O isotope ratios in the Oman gabbroic sequenceare also displaced from primary magmatic values Nogabbro in the present study has plagioclase or amphi-bole with typical mantle d18O values Thus the Sr and Oisotope data rule out the possibility of mantle-derivedfluids as a possible source for the VHT---HT hydrousalteration event recorded in the massive gabbros andgabbroic dykes and veins (Fig 10a and b)
Subducted lithosphere origin
Dehydration of subducted oceanic crust in subductionzones can generate hydrous fluids Such fluids couldpercolate through the overlying lithosphere and contam-inate it thus generating modified isotopic signatures andtrace element contents Such an explanation howeverdisagrees with the geochemical characteristics of most ofthe studied samples Fluids derived from the dehydrationof subducted slabs (fresh or altered MORB or OIB pro-toliths) should be strongly enriched in K Rb and Ba (egBecker et al 2000) whereas in Oman the Rb contentsmeasured in the gabbros and gabbroic dykes and veins arestrongly depleted Moreover the isotopic composition ofslab-derived fluids is buffered by the MORB-like oceaniccrustal protolith for Nd and Pb but not for Sr and O(Staudigel et al 1995) As a result of relatively minorfractionation of oxygen at high temperature the oxygenisotope composition of the fluids expelled during dehy-dration of the subducted slab should be high and essen-tially controlled by the oxygen composition of the uppersection of the crust altered at low temperature (with an
average of 10 and perhaps as high as 19) (Staudigelet al 1995) The Sr isotopic composition should be alsohigh (average 07046) as the fluids released by the dehy-dration of subducted oceanic crust are associated eitherwith seawater or with radiogenic Sr phases (ie smectites)The Sr and O isotopic compositions of the studied sam-ples do not fit with these signatures and so preclude asubducted lithosphere origin for the fluids involved in theVHT---HT alteration event
Seawater-derived fluids
All the analysed samples (minerals and WR) have 87Sr86Sr(T) outside the domain of primary MORB magmasand variable Sr contents Individual minerals have Srcontents reflecting their different mineralliquid partitioncoefficients Minerals characterized by a very high affi-nity for Sr such as plagioclase (ie Sr mineralliquidpartition coefficients 1) have very high Sr contentsTherefore the high abundance of plagioclase in the sam-ples analysed is responsible for the high Sr content of thestudied whole rocks The Sr content of all the whole rocksis relatively homogeneous (Table 5) without clear distinc-tion between country-rock gabbros and dykes and veinsand ranges from 110 to 200 ppm except for the epidosite(4700 ppm) Reported on a 87Sr86Sr vs 1Sr diagram(Fig 10a) most whole rocks and plagioclases analysed arecharacterized by a 1Sr ratio lower than 001 and plot inthe left part of this diagram Compared with N-MORBthe studied massive and anatectic gabbros and their con-stituent plagioclases have similar to slightly higher Srcontents but substantially higher 87Sr86Sr ratios(Fig 10a) Considering Cretaceous seawater as a possiblehigh 87Sr86Sr mixing component [87Sr86Sr 07073at 90Ma after Burke et al (1982)] responsible for theobserved geochemical modifications exchanges cannothave occurred in a closed system as seawater has too lowa Sr content (ie 8 ppm de Villiers 1999) An open-system process allowing penetration of larger quantitiesof seawater can efficiently modify the Sr isotopic ratio ofthe rocks Alternately the fluids could also be seawaterthat was modified through exchange with the surround-ing rocks during its transit through the oceanic crustalsection This modified seawater would be more enrichedin Srwith a 87Sr86Sr ratio intermediate between unmodi-fied seawater and MORBMinerals devoid of late LT alteration imprints (parga-
site green hornblende and pyroxene) and characterizedby very low Sr contents plot on the right of the diagramclose to or on the mixing line between seawater andMORB (Fig 10a) The difference between these mineralsand the other samples (WR and plagioclases) is mainlydue to the Sr fractionation coefficients of these differentminerals and to the large quantity of plagioclase inthe studied rocks The process responsible for the
JOURNAL OF PETROLOGY VOLUME 45 NUMBER 6 JUNE 2004
1202
modification of the Sr isotopic composition fromMORB-source value is however explained by the same phenom-enon As all these minerals display initial 87Sr86Sr ratiosdistinct from the MORB source and because they equili-brated at VHT or HT temperatures this supports anearly intervention of seawater during crystallization ofthese minerals Most of these samples with the exceptionof the epidosite dyke overlap the domains defined byKawahata et al (2001) for the Wadi Fizh gabbros alteredat VHT and HT conditions in the amphibolite facies(labelled lsquoVHTrsquo and lsquoHTrsquo in Fig 9a)Three samples (two epidosite dykes and an one epidote
fraction) have very high Sr contents and the highest Sr
isotopic compositions of the present dataset The twowhole-rock samples overlap the field of the Wadi Fizhsamples altered at high temperature during greenschist-facies metamorphism (Kawahata et al 2001) This typeof alteration is common at the ridge axis and formedthrough intensive exchange with seawater-derived fluidsFigure 10b indicates that all the Sr---O isotope data are
distinct from MORB compositions and suggests that thefluids reacting with the magmas could be derived fromseawater In addition the d18O values show that isotopicexchange occurred under VHT or HT conditions Thefluids could have the composition of seawater or theycould have the composition of seawater modified through
0703
0705
0707
001 01 1
1 Sr (ppm)
MORB
(87Sr
86Sr)
init
ial
Seawater
VHT
HT
MT36b
36b
36b
43a 128b90b
41d2
37b19d
15f
128b
41d2
41d2
19a
Ma2Ma2
19d
90b
37b
Whole RockPlagioclasePargasiteGreen HornblendeClinopyroxene
Epidote
(a)
0 2 4 6 8
δ18O(permil)
0703
0705
0707Seawater
MORB-mantle
128b 41d2
90b 41d2
Ma2128b
37b
43a
19d
90b
Whole RockPlagioclasePargasiteGreen HornblendeClinopyroxene
Epidote
HT LT
19d
(87Sr
86Sr)
init
ial
(b)
Fig 10 (a) Initial 87Sr86Sr isotopic ratios plotted against 1Sr for whole rocks and mineral separates from the gabbro section of the Omanophiolite Fields shown are from Kawahata et al (2001) and correspond to MT-----basalt sheeted dyke diabase altered at medium temperature(prehnite---actinolite facies sequence 3) HT-----diabase plagiogranite metagabbro and epidosite formed at high temperature (greenschist faciessequence 4) VHT-----gabbro altered at very high temperature (amphibolite facies sequence 5) The dashed line is a binary mixing line betweenMORB (Lanphere et al 1981) and Cretaceous seawater (Burke et al 1982) (b) Initial 87Sr86Sr isotopic composition plotted against d18O value forwhole rocks and minerals from the Oman ophiolite Cretaceous seawater and MORB end-members as in (a) Dashed line represents mixing curvebetweenMORB and seawater at high temperature (HT) Low-temperature (LT) exchanges between these two components would be responsible foran increase of the d18O primary MORB value
BOSCH et al HYDROTHERMAL CIRCULATION IN OMAN OPHIOLITE
1203
exchange with the surrounding rocks during its penetra-tion into the crustal section In an environment in whichfluid---rock interaction occurs at variable temperaturesthe d18O is greatly influenced by the temperature atwhich the exchange took place and by the waterrockratio It is evident from Figs 9b and 10b that none of thestudied gabbros have plagioclase and amphibole withMORB-like d18O and 87Sr86Sr values The separateminerals display a broad range of d18O values and mostexhibit extreme 18O depletion (Table 5) Hydrothermalexchange kinetics are similar in amphibole and pyroxeneand these two minerals are more resistant to oxygenexchange during water---rock interaction than coexistingplagioclase (Gregory et al 1989) The most probableexplanation of the low d18O values measured in bothamphiboles and pyroxenes is that they crystallized athigh temperature in the presence of a relatively low 18Oseawater-derived hydrothermal fluid [d18O from 01 to07 after Gregory amp Taylor (1981)] The anomalouslyhigh d18O value (thorn1009) measured for the plagioclase ofthe micro-gabbroic dyke (01 OA19d) can be interpretedas resulting from a superimposed overprint caused by alate-stage penetration of fluids at lower temperature Inthis sample the occurrence of such different 18O16Oratios between coexisting plagioclase and amphibole isnot consistent with equilibrium fractionation betweenthese two minerals at any possible temperature Thisobservation suggests that parts of the ophiolite sequencehave undergone a high-temperature hydrothermal eventprior to the later low-temperature event that producedthe strong 18O increase in coexisting plagioclaseThe depletion of the d18O values results from high-temperature hydrothermal alteration (Gregory amp Taylor1981 Stakes amp Taylor 1992) The oxygen isotopic datasupport the contention that most studied samples havebeen affected by a VHT---HT hydrothermal alteration byfluids derived from seawater Some of the analysed sam-ples presenting petrological evidence of crystallization ofLT secondary minerals have been overprinted by the latelow-temperature alteration thus swamping the earlierhigh-temperature alteration effects
Determination of waterrock ratio
To better evaluate the exchange between seawater andthe gabbroic rocks waterrock ratios (WR) have beenestimated for the whole rocks analysed during the courseof this study The different models used to constrain thisratio mainly differ by the calculation of the effective ratioor by the estimated weight ratio and by considering openor closed systems for water circulation (Albareede et al1981 McCulloch et al 1981) The WR ratios reportedin Table 5 have been calculated assuming a closed sys-tem Using this formulation these values are minimabecause the calculation always uses pristine fluid for the
initial fluid thus maximizing the value in the denomina-tor If the fluid was shifted to lower 87Sr concentrations byexchange with the rock on the downward path theinferred values would be much higher (Davis et al 2003)The calculated WR ratios for the samples from this
study range from 31 to 219 (Table 5) Two main groupsof samples can be recognized on the basis of their waterrock ratios The lowest ratios ranging from 31 to 47have been determined for massive gabbros or anatecticgabbros but also for dykes and veins devoid of late LTalteration Similar ratios have been previously calculatedfor example for the discharged hydrothermal solutions atthe 13N and 21N East Pacific Rise (Di Donato et al1990 Fouquet et al 1991) The gabbros from the Ibrasection in the Oman ophiolite gave effective WR ratiosof 05 33 and 42 (McCulloch et al 1981) in goodagreement with the values obtained in this study Theleast altered gabbro of the Ibra section yields a WR ratioof 05 in good agreement with the exceptionally fresh-ness of this rock characterized by typical mantle oxygenvalues for its minerals (McCulloch et al 1981) Samplescharacterized by waterrock effective ratios in the rangeof 3---5 can be related to penetrative alteration via thehydrothermal system Lecuyer amp Reynard (1996) havedemonstrated in oceanic gabbros from the Hess Deepaltered in similar HT conditions that WR mass ratios inthe range of 02---1 are sufficient to account for the mod-ified stable isotope signature of the gabbros Higherwaterrock values (WR 45) have been calculated forsamples affected by superimposed late hydrothermalalteration and for the epidosite samples (Table 5) Theyprobably acquired their isotopic characteristics throughinteraction with a larger volume of seawater duringgreenschist-facies metamorphism Similar or higherWR values have been published in previous studies(ie McCulloch et al 1981 Kawahata et al 2001)They correspond either to samples from the upper-crus-tal sequence (ie basalt sheeted dyke or plagiogranite) orto gabbros from the crustal section located in a particularenvironment such as for example the Wadi Fizh sectionthat is located close to a segment boundary along thepalaeo-spreading axis (Nicolas et al 2000)
Mechanism for seawater interaction withmagma
Nicolas et al (2003) have proposed that variable amountsof seawater can circulate at high temperature through-out the crustal section down to the walls of the magmachamber via a network of parallel microcracks a fractionof a millimetre wide Such microcracks and their relation-ship with the VHT---HT hydrous alteration in the sur-rounding gabbros have been described in detail byNicolas et al Via this network of microcracks largeamounts of seawater can have reacted with the magma
JOURNAL OF PETROLOGY VOLUME 45 NUMBER 6 JUNE 2004
1204
and induced variable changes of its Sr and O isotopiccomposition This regular network of parallel micro-cracks could constitute the recharge system and thehydrated gabbroic dykes the discharge system Physicalparameters and the geophysical models of Nicolas et al(2003) are in agreement with this scenario Such a pene-tration of seawater-derived hydrothermal fluids throughthe lower crust along grain boundaries and a microscopicfracture network at temperatures 4750C is consistentwith recent thermodynamic models (McCollom amp Shock1998)Seawater-altered and high-temperature recrystallized
diabases (protodykes) from the overlying sheeted dykecomplex stoped into the melt lenses could represent anindirect way for seawater to enter the system as they canremelt during their subsidence through the magmachamber A simple mass balance calculation suggeststhat this indirect contamination is not significant Assess-ments of the amount of recycled crust 1 in volumeby Nicolas et al (2000) based on the protodyke remnantsin the gabbro section or 20 by Coogan et al (2003)based on chlorine content in EPR basalts lead to a sea-waterrock ratio of the order of 104 to 103Thus following Nicolas et al (2003) we propose that
seawater has been introduced along microcracks down tothe walls of the magma chamber and has diffused alonggrain boundaries in the way described by Manning et al(2000) progressively invading the host gabbro Gabbroicdykes result from the injection of hydrous melt into thehost gabbros at falling temperatures as they drift awayfrom the magma chamber This water-saturated meltcould result from the extensive hydration of the crystal-lizing gabbros near the walls of the magma chamberwhen seawater penetrates through this wall (Fig 1)Similar plumbing systems driving seawater down to the
limit of the magma chamber and back to the surface havebeen already proposed in other environments such as theMid-Atlantic Ridge (Kelley amp Delaney 1987 Cooganet al 2001) the East Pacific Rise at Hess Deep (Gillis1995 Manning et al 1996a 1996b) the Tonga forearc(Banerjee amp Gillis 2001) and the Troodos ophiolite(Gillis amp Roberts 1999) The melting curve of wet basalthas a negative slope but is close to the dry solidus at lowpressure in the lower gabbros and the first melts areplagiogranitic (Spulber ampRutherford 1983) Such plagio-granites are ubiquitous in the highest gabbros and in theroot zone of sheeted dykes in Oman (Rothery 1983Juteau et al 1988 Nicolas amp Boudier 1991) in manyother ophiolites and in the oceanic crust where they havebeen interpreted as resulting either from wet anatexis ofhot gabbros or from the final product of crystallization ofa basaltic melt (Spulber amp Rutherford 1983) Plagio-granites are rare in the lower gabbros exceptionallyassociated with hydrous gabbro segregations inside thelayered gabbros Their constitutive minerals are also
remarkably absent along the VHTmicrocracks althoughthese gabbros were above the wet solidus A possibleexplanation has been proposed by McCollom amp Shock(1998) who wrote that at high temperature lsquoaqueousfluids may leave very little conspicuous petrological evid-ence for their presencersquo the reason being that thehigh-T conditions would be closer to batch meltingObviously more detailed studies on this specific problemare needed We conclude that the large degree of hydrousmelting locally required at the walls of the magma cham-ber to account for the generation of a gabbroic hydrousmelt may have erased the traces of the incipient plagio-granitic melt
CONCLUSIONS
Fostered by the recent discovery of high-temperatureseawater contamination in some gabbros of the Omanophiolite (Manning et al 2000) we have conducted astudy on 500 gabbro specimens from selected areas ofthe entire Samail ophiolite belt and on the hydrous gab-broic dykes that cross-cut them The highest-temperaturereactions (VHT) recorded in olivine and clinopyroxene ingabbros as orthopyroxene and pargasite coronas reach975C They are followed at lower HT conditions withreplacement of olivine by an assemblage of tremolitemagnetite and plagioclase surrounded by chlorite andby pargasite development in clinopyroxene followedfinally by the LT lizardite---olivine and saussurite---plagioclase greenschist-facies alteration With a fewexceptions the VHT alteration tends to be restricted tothe lower gabbros and to the vicinity of the Moho andthe HT alteration to the middle and upper gabbros Thepreservation of the vertical distribution of VHT---HTfacies indicates that progressive cooling during off-axisspreading did not obliterate this distribution and conse-quently that the VHT---HT hydrothermal alteration tookplace in a very restricted time interval at the limit of thecooling magma chamber In the deep and hot gabbrosthe main water channels may be sub-millimetre-sizedmicrocracks with a dominantly vertical attitude the ori-gin and dynamics of which have been investigated in aprevious paper (Nicolas et al 2003)Beyond these high-temperature alteration zones mas-
sively induced in the gabbros seawater may be respons-ible for the generation of clinopyroxene---pargasitegabbro dykes that cross-cut all gabbros and that areupsection clearly connected to the LT hydrothermalvein system Petrostructural evidence suggests that thesedykes were generated by hydration of the crystallizinggabbros near the internal wall of the LVZ---magmachamber The resultant melts were subsequently intrudedat various levels in the cooling lower and middle gabbrosPetrological observations of nearly all the gabbros ana-lysed in the present study located from the root zone of
BOSCH et al HYDROTHERMAL CIRCULATION IN OMAN OPHIOLITE
1205
the sheeted dyke complex down to the Moho emphasizethe imprint of VHT---HT hydrous alteration The VHThydrothermal event is characterized by temperatures ofequilibration around 900---1000C The temperatures atwhich the HT hydrous event occurred are estimated tobe in the range 550---900CStrontium and oxygen isotope compositions measured
on whole rocks and separated minerals significantlydepart from primary MORB values The Sr and O iso-tope data obtained for pargasite green hornblende andclinopyroxene record the participation of seawater at thetemperature of crystallization of the minerals Plagio-clases and whole rocks define a trend toward a compo-nent characterized by a high radiogenic 87Sr86Sr valueand a higher Sr content than normal seawater Seawateris clearly identified as the fluid responsible for the modi-fication of the Sr and O signatures for both massivegabbros and dykes and veins Nevertheless the high Srcontent of the extraneous component requires eitheropen-system exchange allowing penetration of a greaterquantity of seawater or penetration of seawater modifiedby interaction with the oceanic crust during its travel paththrough the crustal section The waterrock ratio calcu-lated on the basis of the Sr isotopes is between three andfive for the enclosing gabbros and for the least LT altereddykes The VHT---HT hydrothermal event affectingthese samples is related to penetrative alteration via therecharge and discharge hydrothermal system Thewaterrock ratio is 10 for dykes exhibiting evidenceof a LT hydrothermal alteration overprint and reaches22 for the epidosite dyke The depletion in d18O char-acterizes all the studied samples with the single exceptionof the micro-gabbroic dyke plagioclase This agreeswith the conclusions based on Sr isotopes suggestingthat these samples have undergone exchange with sea-water-derived fluids during a VHT---HTalteration hydro-thermal event
ACKNOWLEDGEMENTS
This work has benefited from financial support by theCentre National de la Recherche Scientifique (INSUInteerieur-Terre andDYETI programmes) and inOmanfrom the willingness of the Directorate of MineralsMinistry of Commerce and Industry and the hospitabil-ity of the people Technical help in the laboratory is alsoacknowledged including C Nevado for the thin sectionsE Ball for the illustrations B Galland for her assistancein the chemistry room and C Bassoulet for help duringthermal ionization mass spectrometric analyses of the Srresults from the leaching experiments Constructivereviews from R T Gregory C Lecuyer an anonymousreviewer and particularly from M Wilson greatlyhelped to improve an earlier version of the manuscript
REFERENCESAgrinier P Mevel C Bosch D amp Javoy M (1993) Metasomatic
hydrous fluids in amphibole peridotites from Zabargad Island (Red
Sea) Earth and Planetary Science Letters 120 187---205Albareede F A Michard A Minster J F amp Michard G (1981)
87Sr86Sr ratios in hydrothermal waters and deposits from the East
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Banerjee N R amp Gillis K M (2001) Hydrothermal alteration in a
modern subduction zone the Tonga forearc crust Journal of
Geophysical Research 106(B10) 21737---21750
Becker H Jochum K P amp Carlson R W (2000) Trace element
fractionation during dehydration of eclogites from high-pressure
terranes and the implications for element fluxes in subduction zones
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(Oman ophiolite) Chemical Geology 134 199---214
Bosch D (1991) Introduction drsquoeau de mer dans le diapir mantellique
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Comptes Rendus de lrsquoAcadeemie des Sciences 313 49---56
Boudier F Godard M amp Ambruster C (2000) Significance of
noritic gabbros in the gabbro section of the Oman ophiolite Marine
Geophysical Research 21(3---4) 307---326Burke W H Denison R E Hetherington R B Koepnick R B
Nelson H F amp Otto H F (1982) Variation of seawater 87Sr86Sr
throughout Phanerozoic time Geology 10 516---519Carmichael I S E Turner F J amp Verhoogen J (1974) Igneous
Petrology Berkeley CA University of California Berkeley 739 pp
Chernosky J V Berman R G amp Bryndzia L T (1988) Stability
phase relations and thermodynamic properties of chlorite and
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295---346
Coogan L A Wilson R N Gillis K M amp MacLeod C J (2001)
Near-solidus evolution of oceanic gabbros insights from amphibole
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Coogan L A Mitchell N C amp OrsquoHara M J (2003) Roof
assimilation at fast spreading ridges an investigation combining
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1171
Davis A C Bickle M J amp Teagle D A H (2003) Imbalance in the
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Detrick R S Buhl P Vera E Mutter J Orcutt J Madsen J amp
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De Villiers S (1999) Seawater strontium and SrCa variability in the
Atlantic and Pacific oceans Earth and Planetary Science Letters 171
623---634
Di Donato G Joron J L Treuil M amp Loubet M (1990)
Geochemistry of zero-age N-MORB from hole 648B ODP legs
106---109 MAR 22 In Detrick R S Honnorez J et al (eds)
Proceedings of the Ocean Drilling Program Scientific Results 106109
College Station TX Ocean Drilling Program pp 57---65
Dunn R A Toomey D R amp Solomon S C (2000) Three-
dimensional seismic structure and physical properties of shallow
mantle beneath the East Pacific Rise at 9300N Journal of Geophysical
Research 105 23537---23555
Dupuy C Mevel C Bodinier J L amp Savoyant L (1991) Zabargad
peridotite evidence for multistage metasomatism during Red Sea
rifting Geology 19 722---725
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1206
Fabriees J Bodinier J L Dupuy C Lorand J P amp Benkerrou C
(1989) Evidence for modal metasomatism in the orogenic spinel
lherzolite body from Caussou (northeastern Pyrenees France)
Journal of Petrology 30 199---228
Fouquet Y Von Stackelberg S U Charlou J L Donval J P
Foucher J Erzig P Muhe R Wiedicke M Soakai S amp
Whitechurch H (1991) Hydrothermal activity in the Lau back-arc
basin sulfides and water chemistry Geology 19 303---306
Gillis K M (1995) Controls on hydrothermal alteration in a section of
fast-spreading oceanic crust Earth and Planetary Science Letters 134
473---489
Gillis K M amp Roberts M (1999) Cracking at the magma---hydrothermal transition evidence from the Troodos ophiolite Earth
and Planetary Science Letters 169 227---244
Gillis K M Mevel C et al (eds) (1993) Proceedings of the Ocean Drilling
Program Initial Reports 147 College Station TX Ocean Drilling
Program
Gregory R T amp Taylor H P (1981) An oxygen isotope profile in a
section of Cretaceous oceanic crust Samail ophiolite Oman
evidence for d18O buffering of the oceans by deep (45 km)
seawater---hydrothermal circulation at mid-ocean ridges Journal of
Geophysical Research 86(B4) 2737---2755
Gregory R T Criss R E amp Taylor H P (1989) Oxygen
isotope exchange kinetics of mineral pairs in closed and open
systems applications to problems of hydrothermal alteration of
igneous rocks and Precambrian iron formations Chemical Geology 75
1---42Hacker B R (1994) Rapid emplacement of young oceanic litho-
sphere argon geochronology of the Oman ophiolite Science 265
1563---1569
Hart S R Erlank A J amp Kable J D (1974) Sea floor basalt
alteration some chemical and Sr isotopic effects Contributions to
Mineralogy and Petrology 44 219---230
Holland T amp Blundy J (1994) Non-ideal interactions in calcic
amphiboles and their bearing on amphibole---plagioclase thermo-
metry Contributions to Mineralogy and Petrology 116 433---447
Ionov D A Savoyant L amp Dupuy C (1992) Application of the
ICP-MS technique to trace element analysis of peridotites and their
minerals Geostandards Newsletter 16 311---315
Javoy M (1980) 18O16O and DH ratios in high temperature
peridotites In Colloques Internationaux du CNRS 272 279---287
Juteau T Beurrier M Dahl R amp Nehlig P (1988) Segmentation
at a fossil spreading axis the plutonic sequence of the Wadi
Haymiliyah area (Haylayn Block Sumail nappe Oman) Tectono-
physics 151 167---197
Juteau T Manacrsquoh G Moreau C Lecuyer C amp Ramboz C
(2000) The high temperature reaction zone of the Oman ophiolite
new field data microthermometry of fluid inclusions PIXE analyses
and oxygen isotopic ratios Marine Geophysical Research 21 351---385Kawahata H Nohara M Ishizuka H Hasebe S amp Chiba H
(2001) Sr isotope geochemistry and hydrothermal alteration of the
Oman ophiolite Journal of Geophysical Research 106 11083---11099
Kelemen P B amp Aharonov E (1998) Periodic formation of magma
fractures and generation of layered gabbros in the lower crust
beneath oceanic spreading ridges Annual Geological Union Special
Monograph 106 267---289
Kelemen P B Koga K amp Shimizu N (1997) Geochemistry of
gabbro sills in the crustmantle transition zone of the Oman
ophiolite implications for the origin of the oceanic lower crust Earth
and Planetary Science Letters 146 475---488Kelley D S amp Delaney J R (1987) Two-phase separation and
fracturing in mid-ocean ridge gabbros at temperatures greater than
700C Earth and Planetary Science Letters 83 53---66
Koepke J Feig S T Snow J amp Freise M (2003) Petrogenesis of
oceanic plagiogranites by partial melting of gabbros an experi-
mental study Contributions to Mineralogy and Petrology on line first http
wwwspringerlinkcomopenurlaspgenre=s00410-003-0511-9
Kyser T K (1986) Stable isotope variations in the mantle In
Valley J W Taylor H P amp OrsquoNeil J R (eds) Stable Isotopes in
High Temperature Geological Processes Mineralogical Society of America
Reviews in Mineralogy 16 141---164
Lanphere M A (1981) K---Ar ages of metamorphic rocks at the base
of the Samail ophiolite Oman Journal of Geophysical Research 86
2777---2782
Lanphere M A Coleman R G amp Hopson C A (1981) Sr isotopic
tracer study of the Samail ophiolite Oman Journal of Geophysical
Research 86(B4) 2709---2720
Leake B E (1997) Nomenclature of amphiboles report of the
subcommittee on amphiboles of the International Mineralogical
Association commission on new minerals and mineral names
European Journal of Mineralogy 9 623---651
Lecuyer C amp Reynard B (1996) High-temperature alteration of
oceanic gabbros by seawater (Hess Deep Ocean Drilling Program
Leg 147) evidence from oxygen isotopes and elemental fluxes
Journal of Geophysical Research 101(B7) 15883---15897
Liou J G Kuniiyoshi S amp Ito K (1974) Experimental study of the
phase relations between greenschist and amphibolite in a basaltic
system American Journal of Science 274 613---632
MacLeod C J amp Rothery D A (1992) Ridge axial segmentation in
the Oman ophiolite evidence from along-strike variations in the
sheeted dyke complex Geological Society of America Special Papers 60
39---63
Manning C E MacLeod C J amp Weston P E (1996a) Fracture-
controlled metamorphism of Hess Deep gabbros site 984
constraints on the roots of mid-ocean ridge hydrothermal systems
at fast-spreading centers In GillisM C Allan KM ampMeyer P S
(eds) Proceedings of the Ocean Drilling Program Scientific Results 147
College Station TX Ocean Drilling Program pp 189---211Manning C E Weston P E amp Mahon K I (1996b) Rapid high-
temperature metamorphism of East Pacific Rise gabbros from Hess
Deep Earth and Planetary Science Letters 144 123---132Manning C E MacLeod C J amp Weston P E (2000) Lower-crustal
cracking front at fast-spreading ridges evidence from the East Pacific
Rise and the Oman ophiolite Journal of the Geological Society London
349 261---272McCollom T M amp Shock E L (1998) Fluid---rock interactions in
the lower oceanic crust thermodynamic models of hydrothermal
alteration Journal of Geophysical Research 103 547---575
McCulloch M T Gregory R T Wasserburg G J amp Taylor H P
(1981) Sm---Nd Rb---Sr and 18O16O isotopic systematics in an
oceanic crustal section evidence from the Samail ophiolite Journal of
Geophysical Research 86(B4) 2721---2735Mevel C Gillis K M Allan J F amp Meyer P S (eds) (1996)
Proceedings of the Ocean Drilling Program Scientific Results 147 College
Station TX Ocean Drilling Program
Nehlig P amp Juteau T (1988) Flow porosities permeabilities and
preliminary data on fluid inclusions and fossil thermal gradients in
the crustal sequence of the Sumail ophiolite (Oman) Tectonophysics
151 199---221
Nehlig P Juteau T Bendel V amp Cotten J (1994) The root zone of
oceanic hydrothermal systems constraints from the Samail ophiolite
(Oman) Journal of Geophysical Research 99 4703---4713
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in Oman ophiolite In Peters Tj Nicolas A amp Coleman R (eds)
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Kluwer Academic pp 39---54
BOSCH et al HYDROTHERMAL CIRCULATION IN OMAN OPHIOLITE
1207
Nicolas A amp Ildefonse B (1996) Flow mechanism and viscosity in
basaltic magma chambers Geophysical Research Letters 23(16)
2013---2016
Nicolas A Ceuleneer G Boudier F amp Misseri M (1988) Structural
mapping in the Oman ophiolites mantle diapirism along an ocean
ridge Tectonophysics 151 27---56
Nicolas A Boudier F Ildefonse B amp Ball E (2000) Accretion
of Oman and United Arab Emirates ophiolite-----discussion of a
new structural map Marine Geophysical Research 21(3---4)147---179
Nicolas A Boudier F Michibayashi K amp Gerbert-Gaillard L
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Oman---UAE ophiolite belt Geological Society of America Bulletin 349
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Nicolas A Mainprice D amp Boudier F (2003) High temperature
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2372
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706---708
Pin C Briot D Bassin C amp Poitrasson F (1994) Concomitant
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Rothery D A (1983) The base of a sheeted dyke complex Oman
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Spear F S (1981) An experimental study of hornblende stability and
compositional variability in amphibolite American Journal of Science
281 697---734
Spulber S D amp Rutherford M J (1983) The origin of rhyolite and
plagiogranite in oceanic crust an experimental study Journal of
Petrology 24 1---25Stakes D S (1991) Oxygen and hydrogen isotope compositions of
oceanic plutonic rocks high-temperature deformation and meta-
morphism of oceanic layer 3 In Taylor H P OrsquoNeil J et al (eds)
Geochemical Society Special Publication 3 77---90
Stakes D S amp Taylor H P (1992) The northern Samail ophiolite an
oxygen isotope microprobe and field study Journal of Geophysical
Research 97(B5) 7043---7080Staudigel H Davies G R Hart S R Marchant M amp Smith B
M (1995) Large scale isotopic Sr Nd and O isotopic anatomy of
altered oceanic crust DSDPODP sites 417418 Earth and Planetary
Science Letters 130 169---185Turner F J amp Verhoogen J (1960) Igneous and Metamorphic Petrology
New York McGraw---Hill 694 pp
Wilcock W S D amp Delaney J R (1996) Mid-ocean ridge
sulfide deposits evidence for heat extraction from magma
chambers or cracking fronts Earth and Planetary Science Letters 145
49---64
Witt G amp Seck H A (1989) Origin of amphibole in recrystallized
and porphyroclastic mantle xenoliths from Rhenish massif implica-
tions for the nature of mantle metasomatism Earth and Planetary
Science Letters 91 327---340
JOURNAL OF PETROLOGY VOLUME 45 NUMBER 6 JUNE 2004
1208
Table5RbandSrcontents87Sr
86Srandd1
8Oisotopiccompositionsofwholerocksandmineralseparatesfrom
samplesofmassivegabbrosdykes
andveinsfrom
theOman
ophiolitealsolistedarecalculated
waterrockratios(W
R)andLOIvalues
Sam
ple
Rb(ppm)
Sr(ppm)
87Rb8
6Sr
87Sr
86Sr
(87Sr
86Sr)i1
d18O
()
LOI(
)2WR
3(87Sr
86Sr)L14
(87Sr
86Sr)L25
(87Sr
86Sr)R6
01OA41d2
Whole
rock
037
1792
0006
0703516
09
070351
thorn480
138
47
0703587
0703651
0703357
Plagioclase
025
1249
0003
0704261
22
070426
thorn504
-------
-------
Pargasite
-------
-------
-------
0703548
20
070355
-------
-------
-------
Clinopyroxene
045
26 0
0050
0703845
19
070378
-------
-------
-------
01OA36b
Dykewhole
rock
0005
7164
50001
0705186
17
070519
-------
274
21 9
0705207
0705112
-------
Epidote
0003
7645
0001
0705536
12
070554
-------
-------
-------
Wallwhole
rock
005
4254
0001
0704637
09
070464
-------
191
-------
0704945
0704675
0704593
02OA43a
Green
hornblende
004
98
0012
0704323
13
070431
thorn310
-------
-------
97OA128b
Whole
rock
003
1274
50001
0703327
17
070333
-------
076
37
-------
-------
-------
Plagioclase
002
1204
50001
0703285
09
070328
thorn414
-------
-------
Clinopyroxene
003
20 0
0004
0704230
09
070422
thorn523
-------
-------
01OA15f
Whole
rock
006
1077
0002
0704582
16
070458
-------
237
13 5
-------
-------
-------
01OA19a
Whole
rock
037
1303
0008
0704189
18
070418
-------
161
96
-------
-------
-------
01OA19d
Whole
rock
058
2032
0008
0703423
08
070341
thorn567
216
415
0703574
0703408
0703271
Pargasite
091
1058
0025
0703273
11
070324
thorn287
-------
-------
Plagioclase
-------
-------
-------
-------
-------
thorn10 09
-------
-------
02OA90b
Plagioclase
004
2385
50001
0703868
11
070387
thorn498
-------
-------
Green
hornblende
010
23 8
0012
0704288
15
070427
thorn239
-------
-------
BOSCH et al HYDROTHERMAL CIRCULATION IN OMAN OPHIOLITE
1197
Table5continued
Sam
ple
Rb(ppm)
Sr(ppm)
87Rb8
6Sr
87Sr
86Sr
(87Sr
86Sr)i1
d18O
()
LOI(
)2WR
3(87Sr
86Sr)L14
(87Sr
86Sr)L25
(87Sr
86Sr)R6
02OA37b
Plagioclase
023
3121
0002
0704204
15
070420
-------
-------
-------
Pargasite
031
41 6
0021
0703505
15
070348
thorn394
-------
-------
94OAMa2
Whole
rock
0016
1750
00003
0703219
15
070322
thorn495
033
31
0703264
0703177
0703158
Plagioclase
0014
1363
00003
0703219
11
070322
thorn479
-------
-------
Clinopyroxene
-------
-------
-------
0703236
13
070324
-------
-------
-------
1Initial8
7Sr
86Srratiocalculatedat
95Ma(H
acker1994)
2LOIisloss
onignition(in)
3Water---rock
ratiocalculatedassumingaclosedsystem
TheeffectiveWR
ratiocanbeexpressed
bythefollowingeq
uation
W=Reffectivefrac14
frac12eth87Sr=
86SrrockTHORN fi
nal
eth87Sr=
86SrrockTHORN in
itial=frac12eth8
7Sr=
86SrseawaterTHORN in
itial
eth87Sr=
86SrrockTHORN fi
nal
ethCrock
initial=C
seaw
ater
initial
THORNwhere(87Sr
86Srrock) finalisthefinalmeasuredSrisotopicratioin
thesample(87Sr
86Srrock) in
itialco
rrespondingto
theinitialm
antlevalue
istakenas
070295(Lan
phere
etal1981)(87Sr
86Srseawater) in
itialistheCretaceousseaw
ater
Srisotopicco
mposition[87Sr
86Sr07073
at90
MaafterBurkeet
al(1982)]C
seaw
ater
initial
istheSrco
ntent
oflsquonorm
alrsquoseaw
ater
andis
takenas
8ppm
(deVilliers1999)
FortheCrock
initialitis
assumed
that
thepresentSrco
ncentrationmeasuredis
thesameas
theinitial
concentration
4Srisotopicratiosmeasuredforliquid
L1extractedafterthefirststep
oftheleachingexperim
ent(6NHClfor8min
at70
C)
5Srisotopicratiosmeasuredforliquid
L2extractedaftertheseco
ndstep
oftheleachingexperim
ent(2 5NHClfor30
min
at70
C)
6Srisotopicratiosmeasuredforresidueobtained
afteratotalaciddigestionachievedaftertheextractionofL1an
dL2leachates
JOURNAL OF PETROLOGY VOLUME 45 NUMBER 6 JUNE 2004
1198
Part of the same sample from the contact between thedyke and the gabbro yields a slightly lower Sr isotopiccomposition (070464) intermediate between the sur-rounding gabbro and the epidote dyke It also has anintermediate Sr content (425 ppm) Acid leaching experi-ments performed on the whole rock of the dyke yield Srisotopic compositions of 070521 and 070512 for thetwo liquids extracted Similar experiments for the samplelocated at the contact with the gabbro yield 87Sr86Srratios of 070494 and 070467 for L1 and L2 liquids TheSr isotopic composition of the residue extracted afterleaching remains high (070459) and very close to theepidosite 87Sr86Sr ratio determined by Kawahata et al(2001) In this sample the primary minerals have beentotally replaced by epidote and secondary plagioclase orquartz Thus the Sr isotopic compositions measured forthis WR sample and its constituent minerals reflect thecomposition of the fluid filling this dyke
Mineral chemistry
Plagioclase
Compared with the host gabbros the dykes show a muchlarger dispersion in their plagioclase compositions (Fig 8aand b Table 4) characterized by more calcic plagioclasethan the enclosing gabbros This tendency is mostextreme in the comb-textured dykes in which the plagio-clase is almost pure anorthite The zoning is generallynormal recording a crystal fractionation process How-ever gabbro 94 MA2 exhibits inverse zoning this tend-ency is also observed in plagioclases from dykes 01OA19a and 01 OA15f The higher calcium content ofthe plagioclase as well as the inverse zoning is consistentwith an increase of water pressure during its crystalliza-tion (Turner amp Verhoogen 1960 Carmichael et al 1974Koepke et al 2003) The particularly high An content inthe comb-textured gabbro dykes 01 OA15f may alsoresult from crystallization in a supercooled magma thisis consistent with the comb texture indicative of fast-growing crystals The lack of brown hornblende suggeststhat crystallization occurred above the temperature ofhornblende stability
Amphibole
The amphiboles analysed in the dykes (Fig 8c andTable 4) show a regular trend in a (Na thorn K)A vsAlIV diagram from titanium-rich (Ti 4 015) pargasitichornblende (brownamphibole) and tschermakite (brown---green amphibole) to low-titanium (Ti5 015) magnesio-hornblende (green amphibole) and actinolite (pale greenamphibole) The pargasitic hornblende develops aspoikilitic oikocrysts in the dykes 01 OA19a and d andin the diffuse patch 01 OA41d2 at temperatures above800C during the final stage of crystallization (see
below) The tschermakite composition corresponds tothe internal alteration of clinopyroxene in gabbro 94Ma2 occurs in the comb-textured dyke 01 OA15f andis the primary amphibole in the dioritic dyke 02 OA37bThe magnesio-hornblende develops at the edges of thepargasites or of clinopyroxenes at temperatures around800C and is the primary phase in the dioritic dyke 02OA90b and in the green vein 02 OA43a Finally actino-lite occurs as a replacement of green hornblende in dykesor in the kelyphitic rim surrounding olivine in gabbro 94Ma2 Such a large composition range at the scale of athin section has been reported in amphiboles fromamphibolite-facies oceanic gabbros (Stakes 1991 Stakesamp Taylor 1992 Gillis et al 1993) as well as in the Omanophiolite gabbros (Manning et al 2000) This variabilityhas been explained by rapid reaction rates preventinghomogenization (Manning et al 2000)
Thermometry
Plagioclase---amphibole thermometry could be appliedwhen the two minerals exhibited good contacts such asin the laminated gabbro of the middle sequence (01OA41d2) and in the intrusive dykes The temperaturesof plagioclase---amphibole equilibration (Table 2) havebeen calculated using the geothermometer lsquoBrsquo of Hollandamp Blundy (1994) Edenite thorn Albite frac14 Richterite thornAnorthite valid between 500C and 900C with anuncertainty of 40C Amphibole formulae are basedon 23 oxygen and their nomenclature is based on alkalications in the A site vs AlIV according to Leake (1997)Pressurewas assumed to be 1 kbar Forty-five plagioclase---amphibole pairs meet the compositional criteria imposedby Holland amp Blundyrsquos dataset (09 4 Xan 4 01 andAlIV 403) Electron microprobe measurements wereconstrained to contiguous plagioclase and amphiboleseparated by sharp grain boundaries assuming equili-brium of the two phases When the mineral phasesexhibit normal zoning temperatures between plagioclaseand amphibole cores and between mineral rims werecalculated assuming that the temperatures deduced fromthe mineral core chemistry provide a proxy for the high-est temperature of equilibration These data are identi-fied in the T C vs AlIV diagram (Fig 8d) Table 4presents selected major element amphibole analyses theAn content of the plagioclase adjacent to each amphibolegrain and the temperature calculated for the pairThe temperatures calculated (Fig 8d) span the entire
range of temperatures at which amphibole and plagio-clase coexist in equilibrium (ie amphibolite-facies para-geneses) This temperature interval is bracketed byorthopyroxene-bearing facies or amphibole-free facies(ie gabbro 94MA2) and epidote---actinolite facies devoidof homogeneous plagioclase (ie epidosite dyke 01OA36b and green vein 02 OA43a) The range of the
BOSCH et al HYDROTHERMAL CIRCULATION IN OMAN OPHIOLITE
1199
calculated temperatures between 4900C and 750Ccorresponds to the changing composition of amphibolesfrom pargasite to magnesio-hornblende at the scale of athin section or even at that of an individual crystal asshown by their correlation with AlIV such variabilityrecords rapid cooling in hydrous conditions The highesttemperatures are recorded by pargasite from the anatec-tic gabbro 01 OA41d2 in the gabbro dyke 01 OA19dand in the dioritic dyke 02 OA90b They were calculatedfrom minerals devoid of lower-grade alteration and arethus considered as representing the equilibrium temperat-ure of the coexisting minerals Lower temperatures cor-respond to mineral phases evolving at falling temperatureand are only indicative of the cooling history Includingthe 40C uncertainty seven pairs provide temperatureshigher than 900C the limit of validity of the Holland ampBlundy thermometer However we maintain these tem-peratures as indicative of the highest values of our data-set These values are included in the average equilibriumtemperature summarized in Table 2
DISCUSSION
Distribution of hydrothermal alteration
Two distinct petrological events are recorded in the gab-bro section of the Samail ophiolite and are documentedin the present contribution (1) the whole gabbro sectionis affected by a penetrative alteration (2) the gabbrosection is crosscut by various types of dykes and veinsincluding hydrous minerals
Penetrative alteration
The penetrative alteration developed in the gabbros atgrain boundaries and locally invading the minerals isubiquitous in the gabbro section This penetrative altera-tion has been described as very high temperature (VHT)high temperature (HT) and low temperature (LT) basedon alteration parageneses The orthopyroxene coronas inthe layered gabbro sample and the incipient appearanceof brown pargasitic amphibole mark a lower thermallimit of the VHT alteration By reference to the experi-mental work of Koepke et al (2003) the temperature ofequilibration for these reactions is estimated to bebetween 900 and 1000CThe preservation of the vertical distribution of
VHT---HT facies (Figs 4 and 5) also pointed out byManning et al (2000) indicates that the hydrothermalalteration pattern fits the distribution of isotherms withdepth at a fast-spreading ridge (ie Phipps Morgan ampChen 1993 Dunn et al 2000 Fig 2) Equilibrium tem-peratures for VHT parageneses as high as 900---1000Csuggest that VHT---HT hydrothermal alteration tookplace in a very restricted time interval at the limit ofthe cooling magma chamber It is proposed that the
penetrative alteration affecting the entire gabbro sectionis related to a pervasive network of microcracks whichact as a recharge system down to the Moho (Nicolas et al2003)
Dykes and veins
The dykes are extremely varied eg pegmatitic gabbroswith comb-texture microgabbros and amphibolitizeddolerites dioritic dykes alignments of poikilitic horn-blende and finally diffuse hornblende-bearing patchesHigh-T gabbro and diorite dykes and low-T greenamphibole---epidote veins are connectedIn the dykes textural evidence attests to suprasolidus
conditions at least for the development of the pargasiticamphibole Integrating the hornblende---plagioclase equi-libration temperatures calculated for the studied samplesthese temperatures are bracketed between 950C and875CIn the following discussion we shall consider separately
the microgabbro and dolerite dykes The latter representbasaltic melt intrusions associated upsection with thediabase sheeted dykes and downsection possibly withthe mafic dykes that are ubiquitous in the mantle section(Nicolas et al 2000) Whatever their origin the develop-ment of pargasitic amphibole during their late stage ofcrystallization creates a link with the gabbroic dykes andsuggests a crystallization process evolving from dry con-ditions to more hydrous conditionsThe other gabbroic dykes comb-textured gabbros and
hornblende-bearing dioritic dykes result from the fillingof cracks by a fluid either a melt or a silica-rich hydrousfluid These intrusive dykes which develop reaction mar-gins at their contact with the host gabbro grade verticallyinto lower-temperature green amphibole veins or align-ments of poikilitic hornblende in the layered gabbromatrix or development of pargasitic hornblende in ana-tectic gabbros (sample 01 OA41d2) In gabbros crystal-lizing in the magma chamber the development oforthopyroxene and pargasite oikocrysts enclosing the pri-mary minerals suggests a thermal evolution from thegabbro solidus to amphibolite-facies conditions Forthese reasons it is suggested that the gabbroic dykesresulted from the injection of a hydrous melt in the stillhot gabbros drifting away from the magma chamber asdiscussed below
Origin of fluids responsible for VHT---HThydrothermal alteration
The 87Sr86Sr initial ratios and d18O of whole rocks andpure mineral fractions are plotted in Fig 9 against thedistance above the Moho Transition Zone (MTZ) Theinitial Sr isotopic composition of the whole rocks apartfrom the epidosite dykes ranges from 070322 to
JOURNAL OF PETROLOGY VOLUME 45 NUMBER 6 JUNE 2004
1200
070458 All the studied samples (including whole rocksand minerals) have initial 87Sr86Sr ratios falling outsidethe field of fresh MORB which commonly have Sr ratiosbetween 07023 and 07029 with an average of 070265(ie Hart et al 1974) The lowest initial Sr isotopiccomposition (070322) from the massive gabbro 94MA2 is slightly but significantly higher than averageOman primary magmatic signatures (ie 070296McCulloch et al 1981) Similarly the d18O values ofmost whole rocks and minerals measured (Table 5)
depart from typical MORB-source mantle values( Javoy 1980 Gregory amp Taylor 1981 Kyser 1986)These differences indicate that the primary mantlesignatures in the Oman ophiolite have been modifiedby interaction with a fluid Possible origins for thisfluid are mantle components dehydration of subductedlithosphere or seawater circulation Strontium andoxygen isotopes are useful tracers for fluid---crust interac-tion as d18O and 87Sr86Sr ratios from fluids originat-ing from seawater the mantle or subducted lithosphere
-1000
1000
2000
3000
4000
5000
6000
7000
MOHO
MTZ
Dis
tan
ce a
bov
e th
e M
oh
o (
m)
pillow-basalt
sheeted-dike
gabbro
peridotite
01 OA 41d2
01 OA 36b
02 OA 43a97 OA 128b
01 OA15f
94 OA MA2
02 OA 90b01 OA 19ad
02 OA 37b
0702 0703 0704 0705 0706 0707
87Sr86Srinitial
MO
RB-s
ou
rce
Seaw
ater
Whole RockPlagioclasePargasiteGreen HornblendeClinopyroxene
Epidote
(a)
-1000
1000
2000
3000
4000
5000
6000
7000
MOHO
MTZ
Dis
tan
ce a
bov
e th
e M
oh
o (
m)
pillow-basalt
sheeted-dike
gabbro
peridotite
01 OA 41d2
01 OA 36b
02 OA 43a97 OA 128b
01 OA15f
94 OA MA2
02 OA 90b01 OA 19ad
02 OA 37b
Whole RockPlagioclasePargasiteGreen HornblendeClinopyroxene
3 5 10
δ18O (permil)
41d2
Ma2
43a
90b 19d 19d37b
128b
90b
(b)
Fig 9 (a) Variation of initial 87Sr86Sr isotopic composition as a function of the location of the studied samples in a schematic log of the crustalsection of the Oman ophiolite The field for MORB is from Hart et al (1974) and that for seawater is from Burke et al (1982) Small open squares aredata fromKawahata et al (2001) (b) Variation of d18O () as a function of the location of the studied samples in a schematic log of the crustal sectionof the Oman ophiolite Shaded curve corresponds to the data of Gregory amp Taylor (1981)
BOSCH et al HYDROTHERMAL CIRCULATION IN OMAN OPHIOLITE
1201
are significantly different Moreover the d18O valueis dependent on temperature and mineral fractiona-tion and thus yields complementary constraints to Srisotopes
Mantle origin
One model to explain the occurrence of fluids interactingwith the oceanic crust at very high temperatures is toderive them from the mantle The contribution of mantle-derived hydrous fluids linked to percolation processesor to their concentration in the final stages of gabbrocrystallization has been invoked in numerous case studies(ie Witt amp Seck 1989 Fabriees et al 1989 Dupuy et al1991 Agrinier et al 1993) O and Sr isotopic data forwhole rocks and mineral separates yield scattered valuesthat are unambiguously different from MORB mantle-source values (Fig 9a and b) With the exception of lateLT altered samples the WR data have a mean 87Sr86Srratio of 070337 and a standard deviation of 000012These surprisingly high values in mantle-derived rockscould be taken as evidence for survival of mantle isotopicheterogeneities during crystallization of the ophiolite (egLanphere 1981 McCulloch et al 1981) However the18O16O isotope ratios in the Oman gabbroic sequenceare also displaced from primary magmatic values Nogabbro in the present study has plagioclase or amphi-bole with typical mantle d18O values Thus the Sr and Oisotope data rule out the possibility of mantle-derivedfluids as a possible source for the VHT---HT hydrousalteration event recorded in the massive gabbros andgabbroic dykes and veins (Fig 10a and b)
Subducted lithosphere origin
Dehydration of subducted oceanic crust in subductionzones can generate hydrous fluids Such fluids couldpercolate through the overlying lithosphere and contam-inate it thus generating modified isotopic signatures andtrace element contents Such an explanation howeverdisagrees with the geochemical characteristics of most ofthe studied samples Fluids derived from the dehydrationof subducted slabs (fresh or altered MORB or OIB pro-toliths) should be strongly enriched in K Rb and Ba (egBecker et al 2000) whereas in Oman the Rb contentsmeasured in the gabbros and gabbroic dykes and veins arestrongly depleted Moreover the isotopic composition ofslab-derived fluids is buffered by the MORB-like oceaniccrustal protolith for Nd and Pb but not for Sr and O(Staudigel et al 1995) As a result of relatively minorfractionation of oxygen at high temperature the oxygenisotope composition of the fluids expelled during dehy-dration of the subducted slab should be high and essen-tially controlled by the oxygen composition of the uppersection of the crust altered at low temperature (with an
average of 10 and perhaps as high as 19) (Staudigelet al 1995) The Sr isotopic composition should be alsohigh (average 07046) as the fluids released by the dehy-dration of subducted oceanic crust are associated eitherwith seawater or with radiogenic Sr phases (ie smectites)The Sr and O isotopic compositions of the studied sam-ples do not fit with these signatures and so preclude asubducted lithosphere origin for the fluids involved in theVHT---HT alteration event
Seawater-derived fluids
All the analysed samples (minerals and WR) have 87Sr86Sr(T) outside the domain of primary MORB magmasand variable Sr contents Individual minerals have Srcontents reflecting their different mineralliquid partitioncoefficients Minerals characterized by a very high affi-nity for Sr such as plagioclase (ie Sr mineralliquidpartition coefficients 1) have very high Sr contentsTherefore the high abundance of plagioclase in the sam-ples analysed is responsible for the high Sr content of thestudied whole rocks The Sr content of all the whole rocksis relatively homogeneous (Table 5) without clear distinc-tion between country-rock gabbros and dykes and veinsand ranges from 110 to 200 ppm except for the epidosite(4700 ppm) Reported on a 87Sr86Sr vs 1Sr diagram(Fig 10a) most whole rocks and plagioclases analysed arecharacterized by a 1Sr ratio lower than 001 and plot inthe left part of this diagram Compared with N-MORBthe studied massive and anatectic gabbros and their con-stituent plagioclases have similar to slightly higher Srcontents but substantially higher 87Sr86Sr ratios(Fig 10a) Considering Cretaceous seawater as a possiblehigh 87Sr86Sr mixing component [87Sr86Sr 07073at 90Ma after Burke et al (1982)] responsible for theobserved geochemical modifications exchanges cannothave occurred in a closed system as seawater has too lowa Sr content (ie 8 ppm de Villiers 1999) An open-system process allowing penetration of larger quantitiesof seawater can efficiently modify the Sr isotopic ratio ofthe rocks Alternately the fluids could also be seawaterthat was modified through exchange with the surround-ing rocks during its transit through the oceanic crustalsection This modified seawater would be more enrichedin Srwith a 87Sr86Sr ratio intermediate between unmodi-fied seawater and MORBMinerals devoid of late LT alteration imprints (parga-
site green hornblende and pyroxene) and characterizedby very low Sr contents plot on the right of the diagramclose to or on the mixing line between seawater andMORB (Fig 10a) The difference between these mineralsand the other samples (WR and plagioclases) is mainlydue to the Sr fractionation coefficients of these differentminerals and to the large quantity of plagioclase inthe studied rocks The process responsible for the
JOURNAL OF PETROLOGY VOLUME 45 NUMBER 6 JUNE 2004
1202
modification of the Sr isotopic composition fromMORB-source value is however explained by the same phenom-enon As all these minerals display initial 87Sr86Sr ratiosdistinct from the MORB source and because they equili-brated at VHT or HT temperatures this supports anearly intervention of seawater during crystallization ofthese minerals Most of these samples with the exceptionof the epidosite dyke overlap the domains defined byKawahata et al (2001) for the Wadi Fizh gabbros alteredat VHT and HT conditions in the amphibolite facies(labelled lsquoVHTrsquo and lsquoHTrsquo in Fig 9a)Three samples (two epidosite dykes and an one epidote
fraction) have very high Sr contents and the highest Sr
isotopic compositions of the present dataset The twowhole-rock samples overlap the field of the Wadi Fizhsamples altered at high temperature during greenschist-facies metamorphism (Kawahata et al 2001) This typeof alteration is common at the ridge axis and formedthrough intensive exchange with seawater-derived fluidsFigure 10b indicates that all the Sr---O isotope data are
distinct from MORB compositions and suggests that thefluids reacting with the magmas could be derived fromseawater In addition the d18O values show that isotopicexchange occurred under VHT or HT conditions Thefluids could have the composition of seawater or theycould have the composition of seawater modified through
0703
0705
0707
001 01 1
1 Sr (ppm)
MORB
(87Sr
86Sr)
init
ial
Seawater
VHT
HT
MT36b
36b
36b
43a 128b90b
41d2
37b19d
15f
128b
41d2
41d2
19a
Ma2Ma2
19d
90b
37b
Whole RockPlagioclasePargasiteGreen HornblendeClinopyroxene
Epidote
(a)
0 2 4 6 8
δ18O(permil)
0703
0705
0707Seawater
MORB-mantle
128b 41d2
90b 41d2
Ma2128b
37b
43a
19d
90b
Whole RockPlagioclasePargasiteGreen HornblendeClinopyroxene
Epidote
HT LT
19d
(87Sr
86Sr)
init
ial
(b)
Fig 10 (a) Initial 87Sr86Sr isotopic ratios plotted against 1Sr for whole rocks and mineral separates from the gabbro section of the Omanophiolite Fields shown are from Kawahata et al (2001) and correspond to MT-----basalt sheeted dyke diabase altered at medium temperature(prehnite---actinolite facies sequence 3) HT-----diabase plagiogranite metagabbro and epidosite formed at high temperature (greenschist faciessequence 4) VHT-----gabbro altered at very high temperature (amphibolite facies sequence 5) The dashed line is a binary mixing line betweenMORB (Lanphere et al 1981) and Cretaceous seawater (Burke et al 1982) (b) Initial 87Sr86Sr isotopic composition plotted against d18O value forwhole rocks and minerals from the Oman ophiolite Cretaceous seawater and MORB end-members as in (a) Dashed line represents mixing curvebetweenMORB and seawater at high temperature (HT) Low-temperature (LT) exchanges between these two components would be responsible foran increase of the d18O primary MORB value
BOSCH et al HYDROTHERMAL CIRCULATION IN OMAN OPHIOLITE
1203
exchange with the surrounding rocks during its penetra-tion into the crustal section In an environment in whichfluid---rock interaction occurs at variable temperaturesthe d18O is greatly influenced by the temperature atwhich the exchange took place and by the waterrockratio It is evident from Figs 9b and 10b that none of thestudied gabbros have plagioclase and amphibole withMORB-like d18O and 87Sr86Sr values The separateminerals display a broad range of d18O values and mostexhibit extreme 18O depletion (Table 5) Hydrothermalexchange kinetics are similar in amphibole and pyroxeneand these two minerals are more resistant to oxygenexchange during water---rock interaction than coexistingplagioclase (Gregory et al 1989) The most probableexplanation of the low d18O values measured in bothamphiboles and pyroxenes is that they crystallized athigh temperature in the presence of a relatively low 18Oseawater-derived hydrothermal fluid [d18O from 01 to07 after Gregory amp Taylor (1981)] The anomalouslyhigh d18O value (thorn1009) measured for the plagioclase ofthe micro-gabbroic dyke (01 OA19d) can be interpretedas resulting from a superimposed overprint caused by alate-stage penetration of fluids at lower temperature Inthis sample the occurrence of such different 18O16Oratios between coexisting plagioclase and amphibole isnot consistent with equilibrium fractionation betweenthese two minerals at any possible temperature Thisobservation suggests that parts of the ophiolite sequencehave undergone a high-temperature hydrothermal eventprior to the later low-temperature event that producedthe strong 18O increase in coexisting plagioclaseThe depletion of the d18O values results from high-temperature hydrothermal alteration (Gregory amp Taylor1981 Stakes amp Taylor 1992) The oxygen isotopic datasupport the contention that most studied samples havebeen affected by a VHT---HT hydrothermal alteration byfluids derived from seawater Some of the analysed sam-ples presenting petrological evidence of crystallization ofLT secondary minerals have been overprinted by the latelow-temperature alteration thus swamping the earlierhigh-temperature alteration effects
Determination of waterrock ratio
To better evaluate the exchange between seawater andthe gabbroic rocks waterrock ratios (WR) have beenestimated for the whole rocks analysed during the courseof this study The different models used to constrain thisratio mainly differ by the calculation of the effective ratioor by the estimated weight ratio and by considering openor closed systems for water circulation (Albareede et al1981 McCulloch et al 1981) The WR ratios reportedin Table 5 have been calculated assuming a closed sys-tem Using this formulation these values are minimabecause the calculation always uses pristine fluid for the
initial fluid thus maximizing the value in the denomina-tor If the fluid was shifted to lower 87Sr concentrations byexchange with the rock on the downward path theinferred values would be much higher (Davis et al 2003)The calculated WR ratios for the samples from this
study range from 31 to 219 (Table 5) Two main groupsof samples can be recognized on the basis of their waterrock ratios The lowest ratios ranging from 31 to 47have been determined for massive gabbros or anatecticgabbros but also for dykes and veins devoid of late LTalteration Similar ratios have been previously calculatedfor example for the discharged hydrothermal solutions atthe 13N and 21N East Pacific Rise (Di Donato et al1990 Fouquet et al 1991) The gabbros from the Ibrasection in the Oman ophiolite gave effective WR ratiosof 05 33 and 42 (McCulloch et al 1981) in goodagreement with the values obtained in this study Theleast altered gabbro of the Ibra section yields a WR ratioof 05 in good agreement with the exceptionally fresh-ness of this rock characterized by typical mantle oxygenvalues for its minerals (McCulloch et al 1981) Samplescharacterized by waterrock effective ratios in the rangeof 3---5 can be related to penetrative alteration via thehydrothermal system Lecuyer amp Reynard (1996) havedemonstrated in oceanic gabbros from the Hess Deepaltered in similar HT conditions that WR mass ratios inthe range of 02---1 are sufficient to account for the mod-ified stable isotope signature of the gabbros Higherwaterrock values (WR 45) have been calculated forsamples affected by superimposed late hydrothermalalteration and for the epidosite samples (Table 5) Theyprobably acquired their isotopic characteristics throughinteraction with a larger volume of seawater duringgreenschist-facies metamorphism Similar or higherWR values have been published in previous studies(ie McCulloch et al 1981 Kawahata et al 2001)They correspond either to samples from the upper-crus-tal sequence (ie basalt sheeted dyke or plagiogranite) orto gabbros from the crustal section located in a particularenvironment such as for example the Wadi Fizh sectionthat is located close to a segment boundary along thepalaeo-spreading axis (Nicolas et al 2000)
Mechanism for seawater interaction withmagma
Nicolas et al (2003) have proposed that variable amountsof seawater can circulate at high temperature through-out the crustal section down to the walls of the magmachamber via a network of parallel microcracks a fractionof a millimetre wide Such microcracks and their relation-ship with the VHT---HT hydrous alteration in the sur-rounding gabbros have been described in detail byNicolas et al Via this network of microcracks largeamounts of seawater can have reacted with the magma
JOURNAL OF PETROLOGY VOLUME 45 NUMBER 6 JUNE 2004
1204
and induced variable changes of its Sr and O isotopiccomposition This regular network of parallel micro-cracks could constitute the recharge system and thehydrated gabbroic dykes the discharge system Physicalparameters and the geophysical models of Nicolas et al(2003) are in agreement with this scenario Such a pene-tration of seawater-derived hydrothermal fluids throughthe lower crust along grain boundaries and a microscopicfracture network at temperatures 4750C is consistentwith recent thermodynamic models (McCollom amp Shock1998)Seawater-altered and high-temperature recrystallized
diabases (protodykes) from the overlying sheeted dykecomplex stoped into the melt lenses could represent anindirect way for seawater to enter the system as they canremelt during their subsidence through the magmachamber A simple mass balance calculation suggeststhat this indirect contamination is not significant Assess-ments of the amount of recycled crust 1 in volumeby Nicolas et al (2000) based on the protodyke remnantsin the gabbro section or 20 by Coogan et al (2003)based on chlorine content in EPR basalts lead to a sea-waterrock ratio of the order of 104 to 103Thus following Nicolas et al (2003) we propose that
seawater has been introduced along microcracks down tothe walls of the magma chamber and has diffused alonggrain boundaries in the way described by Manning et al(2000) progressively invading the host gabbro Gabbroicdykes result from the injection of hydrous melt into thehost gabbros at falling temperatures as they drift awayfrom the magma chamber This water-saturated meltcould result from the extensive hydration of the crystal-lizing gabbros near the walls of the magma chamberwhen seawater penetrates through this wall (Fig 1)Similar plumbing systems driving seawater down to the
limit of the magma chamber and back to the surface havebeen already proposed in other environments such as theMid-Atlantic Ridge (Kelley amp Delaney 1987 Cooganet al 2001) the East Pacific Rise at Hess Deep (Gillis1995 Manning et al 1996a 1996b) the Tonga forearc(Banerjee amp Gillis 2001) and the Troodos ophiolite(Gillis amp Roberts 1999) The melting curve of wet basalthas a negative slope but is close to the dry solidus at lowpressure in the lower gabbros and the first melts areplagiogranitic (Spulber ampRutherford 1983) Such plagio-granites are ubiquitous in the highest gabbros and in theroot zone of sheeted dykes in Oman (Rothery 1983Juteau et al 1988 Nicolas amp Boudier 1991) in manyother ophiolites and in the oceanic crust where they havebeen interpreted as resulting either from wet anatexis ofhot gabbros or from the final product of crystallization ofa basaltic melt (Spulber amp Rutherford 1983) Plagio-granites are rare in the lower gabbros exceptionallyassociated with hydrous gabbro segregations inside thelayered gabbros Their constitutive minerals are also
remarkably absent along the VHTmicrocracks althoughthese gabbros were above the wet solidus A possibleexplanation has been proposed by McCollom amp Shock(1998) who wrote that at high temperature lsquoaqueousfluids may leave very little conspicuous petrological evid-ence for their presencersquo the reason being that thehigh-T conditions would be closer to batch meltingObviously more detailed studies on this specific problemare needed We conclude that the large degree of hydrousmelting locally required at the walls of the magma cham-ber to account for the generation of a gabbroic hydrousmelt may have erased the traces of the incipient plagio-granitic melt
CONCLUSIONS
Fostered by the recent discovery of high-temperatureseawater contamination in some gabbros of the Omanophiolite (Manning et al 2000) we have conducted astudy on 500 gabbro specimens from selected areas ofthe entire Samail ophiolite belt and on the hydrous gab-broic dykes that cross-cut them The highest-temperaturereactions (VHT) recorded in olivine and clinopyroxene ingabbros as orthopyroxene and pargasite coronas reach975C They are followed at lower HT conditions withreplacement of olivine by an assemblage of tremolitemagnetite and plagioclase surrounded by chlorite andby pargasite development in clinopyroxene followedfinally by the LT lizardite---olivine and saussurite---plagioclase greenschist-facies alteration With a fewexceptions the VHT alteration tends to be restricted tothe lower gabbros and to the vicinity of the Moho andthe HT alteration to the middle and upper gabbros Thepreservation of the vertical distribution of VHT---HTfacies indicates that progressive cooling during off-axisspreading did not obliterate this distribution and conse-quently that the VHT---HT hydrothermal alteration tookplace in a very restricted time interval at the limit of thecooling magma chamber In the deep and hot gabbrosthe main water channels may be sub-millimetre-sizedmicrocracks with a dominantly vertical attitude the ori-gin and dynamics of which have been investigated in aprevious paper (Nicolas et al 2003)Beyond these high-temperature alteration zones mas-
sively induced in the gabbros seawater may be respons-ible for the generation of clinopyroxene---pargasitegabbro dykes that cross-cut all gabbros and that areupsection clearly connected to the LT hydrothermalvein system Petrostructural evidence suggests that thesedykes were generated by hydration of the crystallizinggabbros near the internal wall of the LVZ---magmachamber The resultant melts were subsequently intrudedat various levels in the cooling lower and middle gabbrosPetrological observations of nearly all the gabbros ana-lysed in the present study located from the root zone of
BOSCH et al HYDROTHERMAL CIRCULATION IN OMAN OPHIOLITE
1205
the sheeted dyke complex down to the Moho emphasizethe imprint of VHT---HT hydrous alteration The VHThydrothermal event is characterized by temperatures ofequilibration around 900---1000C The temperatures atwhich the HT hydrous event occurred are estimated tobe in the range 550---900CStrontium and oxygen isotope compositions measured
on whole rocks and separated minerals significantlydepart from primary MORB values The Sr and O iso-tope data obtained for pargasite green hornblende andclinopyroxene record the participation of seawater at thetemperature of crystallization of the minerals Plagio-clases and whole rocks define a trend toward a compo-nent characterized by a high radiogenic 87Sr86Sr valueand a higher Sr content than normal seawater Seawateris clearly identified as the fluid responsible for the modi-fication of the Sr and O signatures for both massivegabbros and dykes and veins Nevertheless the high Srcontent of the extraneous component requires eitheropen-system exchange allowing penetration of a greaterquantity of seawater or penetration of seawater modifiedby interaction with the oceanic crust during its travel paththrough the crustal section The waterrock ratio calcu-lated on the basis of the Sr isotopes is between three andfive for the enclosing gabbros and for the least LT altereddykes The VHT---HT hydrothermal event affectingthese samples is related to penetrative alteration via therecharge and discharge hydrothermal system Thewaterrock ratio is 10 for dykes exhibiting evidenceof a LT hydrothermal alteration overprint and reaches22 for the epidosite dyke The depletion in d18O char-acterizes all the studied samples with the single exceptionof the micro-gabbroic dyke plagioclase This agreeswith the conclusions based on Sr isotopes suggestingthat these samples have undergone exchange with sea-water-derived fluids during a VHT---HTalteration hydro-thermal event
ACKNOWLEDGEMENTS
This work has benefited from financial support by theCentre National de la Recherche Scientifique (INSUInteerieur-Terre andDYETI programmes) and inOmanfrom the willingness of the Directorate of MineralsMinistry of Commerce and Industry and the hospitabil-ity of the people Technical help in the laboratory is alsoacknowledged including C Nevado for the thin sectionsE Ball for the illustrations B Galland for her assistancein the chemistry room and C Bassoulet for help duringthermal ionization mass spectrometric analyses of the Srresults from the leaching experiments Constructivereviews from R T Gregory C Lecuyer an anonymousreviewer and particularly from M Wilson greatlyhelped to improve an earlier version of the manuscript
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Banerjee N R amp Gillis K M (2001) Hydrothermal alteration in a
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295---346
Coogan L A Wilson R N Gillis K M amp MacLeod C J (2001)
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Coogan L A Mitchell N C amp OrsquoHara M J (2003) Roof
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Davis A C Bickle M J amp Teagle D A H (2003) Imbalance in the
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Detrick R S Buhl P Vera E Mutter J Orcutt J Madsen J amp
Trocher T (1987) Multi-channel seismic imaging of a crustal
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De Villiers S (1999) Seawater strontium and SrCa variability in the
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Di Donato G Joron J L Treuil M amp Loubet M (1990)
Geochemistry of zero-age N-MORB from hole 648B ODP legs
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Dunn R A Toomey D R amp Solomon S C (2000) Three-
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1206
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Fouquet Y Von Stackelberg S U Charlou J L Donval J P
Foucher J Erzig P Muhe R Wiedicke M Soakai S amp
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Gillis K M (1995) Controls on hydrothermal alteration in a section of
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Gillis K M amp Roberts M (1999) Cracking at the magma---hydrothermal transition evidence from the Troodos ophiolite Earth
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Gregory R T amp Taylor H P (1981) An oxygen isotope profile in a
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seawater---hydrothermal circulation at mid-ocean ridges Journal of
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Gregory R T Criss R E amp Taylor H P (1989) Oxygen
isotope exchange kinetics of mineral pairs in closed and open
systems applications to problems of hydrothermal alteration of
igneous rocks and Precambrian iron formations Chemical Geology 75
1---42Hacker B R (1994) Rapid emplacement of young oceanic litho-
sphere argon geochronology of the Oman ophiolite Science 265
1563---1569
Hart S R Erlank A J amp Kable J D (1974) Sea floor basalt
alteration some chemical and Sr isotopic effects Contributions to
Mineralogy and Petrology 44 219---230
Holland T amp Blundy J (1994) Non-ideal interactions in calcic
amphiboles and their bearing on amphibole---plagioclase thermo-
metry Contributions to Mineralogy and Petrology 116 433---447
Ionov D A Savoyant L amp Dupuy C (1992) Application of the
ICP-MS technique to trace element analysis of peridotites and their
minerals Geostandards Newsletter 16 311---315
Javoy M (1980) 18O16O and DH ratios in high temperature
peridotites In Colloques Internationaux du CNRS 272 279---287
Juteau T Beurrier M Dahl R amp Nehlig P (1988) Segmentation
at a fossil spreading axis the plutonic sequence of the Wadi
Haymiliyah area (Haylayn Block Sumail nappe Oman) Tectono-
physics 151 167---197
Juteau T Manacrsquoh G Moreau C Lecuyer C amp Ramboz C
(2000) The high temperature reaction zone of the Oman ophiolite
new field data microthermometry of fluid inclusions PIXE analyses
and oxygen isotopic ratios Marine Geophysical Research 21 351---385Kawahata H Nohara M Ishizuka H Hasebe S amp Chiba H
(2001) Sr isotope geochemistry and hydrothermal alteration of the
Oman ophiolite Journal of Geophysical Research 106 11083---11099
Kelemen P B amp Aharonov E (1998) Periodic formation of magma
fractures and generation of layered gabbros in the lower crust
beneath oceanic spreading ridges Annual Geological Union Special
Monograph 106 267---289
Kelemen P B Koga K amp Shimizu N (1997) Geochemistry of
gabbro sills in the crustmantle transition zone of the Oman
ophiolite implications for the origin of the oceanic lower crust Earth
and Planetary Science Letters 146 475---488Kelley D S amp Delaney J R (1987) Two-phase separation and
fracturing in mid-ocean ridge gabbros at temperatures greater than
700C Earth and Planetary Science Letters 83 53---66
Koepke J Feig S T Snow J amp Freise M (2003) Petrogenesis of
oceanic plagiogranites by partial melting of gabbros an experi-
mental study Contributions to Mineralogy and Petrology on line first http
wwwspringerlinkcomopenurlaspgenre=s00410-003-0511-9
Kyser T K (1986) Stable isotope variations in the mantle In
Valley J W Taylor H P amp OrsquoNeil J R (eds) Stable Isotopes in
High Temperature Geological Processes Mineralogical Society of America
Reviews in Mineralogy 16 141---164
Lanphere M A (1981) K---Ar ages of metamorphic rocks at the base
of the Samail ophiolite Oman Journal of Geophysical Research 86
2777---2782
Lanphere M A Coleman R G amp Hopson C A (1981) Sr isotopic
tracer study of the Samail ophiolite Oman Journal of Geophysical
Research 86(B4) 2709---2720
Leake B E (1997) Nomenclature of amphiboles report of the
subcommittee on amphiboles of the International Mineralogical
Association commission on new minerals and mineral names
European Journal of Mineralogy 9 623---651
Lecuyer C amp Reynard B (1996) High-temperature alteration of
oceanic gabbros by seawater (Hess Deep Ocean Drilling Program
Leg 147) evidence from oxygen isotopes and elemental fluxes
Journal of Geophysical Research 101(B7) 15883---15897
Liou J G Kuniiyoshi S amp Ito K (1974) Experimental study of the
phase relations between greenschist and amphibolite in a basaltic
system American Journal of Science 274 613---632
MacLeod C J amp Rothery D A (1992) Ridge axial segmentation in
the Oman ophiolite evidence from along-strike variations in the
sheeted dyke complex Geological Society of America Special Papers 60
39---63
Manning C E MacLeod C J amp Weston P E (1996a) Fracture-
controlled metamorphism of Hess Deep gabbros site 984
constraints on the roots of mid-ocean ridge hydrothermal systems
at fast-spreading centers In GillisM C Allan KM ampMeyer P S
(eds) Proceedings of the Ocean Drilling Program Scientific Results 147
College Station TX Ocean Drilling Program pp 189---211Manning C E Weston P E amp Mahon K I (1996b) Rapid high-
temperature metamorphism of East Pacific Rise gabbros from Hess
Deep Earth and Planetary Science Letters 144 123---132Manning C E MacLeod C J amp Weston P E (2000) Lower-crustal
cracking front at fast-spreading ridges evidence from the East Pacific
Rise and the Oman ophiolite Journal of the Geological Society London
349 261---272McCollom T M amp Shock E L (1998) Fluid---rock interactions in
the lower oceanic crust thermodynamic models of hydrothermal
alteration Journal of Geophysical Research 103 547---575
McCulloch M T Gregory R T Wasserburg G J amp Taylor H P
(1981) Sm---Nd Rb---Sr and 18O16O isotopic systematics in an
oceanic crustal section evidence from the Samail ophiolite Journal of
Geophysical Research 86(B4) 2721---2735Mevel C Gillis K M Allan J F amp Meyer P S (eds) (1996)
Proceedings of the Ocean Drilling Program Scientific Results 147 College
Station TX Ocean Drilling Program
Nehlig P amp Juteau T (1988) Flow porosities permeabilities and
preliminary data on fluid inclusions and fossil thermal gradients in
the crustal sequence of the Sumail ophiolite (Oman) Tectonophysics
151 199---221
Nehlig P Juteau T Bendel V amp Cotten J (1994) The root zone of
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(Oman) Journal of Geophysical Research 99 4703---4713
Nicolas A amp Boudier F (1991) Rooting of the sheeted dyke complex
in Oman ophiolite In Peters Tj Nicolas A amp Coleman R (eds)
Ophiolite Genesis and Evolution of the Oceanic Lithosphere Dordrecht
Kluwer Academic pp 39---54
BOSCH et al HYDROTHERMAL CIRCULATION IN OMAN OPHIOLITE
1207
Nicolas A amp Ildefonse B (1996) Flow mechanism and viscosity in
basaltic magma chambers Geophysical Research Letters 23(16)
2013---2016
Nicolas A Ceuleneer G Boudier F amp Misseri M (1988) Structural
mapping in the Oman ophiolites mantle diapirism along an ocean
ridge Tectonophysics 151 27---56
Nicolas A Boudier F Ildefonse B amp Ball E (2000) Accretion
of Oman and United Arab Emirates ophiolite-----discussion of a
new structural map Marine Geophysical Research 21(3---4)147---179
Nicolas A Boudier F Michibayashi K amp Gerbert-Gaillard L
(2001) Aswad massif (United Arab Emirates) archetype of the
Oman---UAE ophiolite belt Geological Society of America Bulletin 349
499---451
Nicolas A Mainprice D amp Boudier F (2003) High temperature
seawater circulation through the lower crust of ocean-ridges-----amodel derived from the Oman ophiolites Journal of Geophysical
Research on line first httpwwwaguorgpubscurrentjb 108(B8)
2372
Pallister J S amp Hopson C A (1981) Samail ophiolite plutonic suite
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morphology on magma supply and spreading rate Nature 364
706---708
Pin C Briot D Bassin C amp Poitrasson F (1994) Concomitant
extraction of Sr and Sm---Nd for isotopic analysis in silicate samples
based on specific extraction chromatography Analytica Chimica Acta
298 209---217
Rothery D A (1983) The base of a sheeted dyke complex Oman
ophiolite implications for magma chambers at oceanic spreading
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Spear F S (1981) An experimental study of hornblende stability and
compositional variability in amphibolite American Journal of Science
281 697---734
Spulber S D amp Rutherford M J (1983) The origin of rhyolite and
plagiogranite in oceanic crust an experimental study Journal of
Petrology 24 1---25Stakes D S (1991) Oxygen and hydrogen isotope compositions of
oceanic plutonic rocks high-temperature deformation and meta-
morphism of oceanic layer 3 In Taylor H P OrsquoNeil J et al (eds)
Geochemical Society Special Publication 3 77---90
Stakes D S amp Taylor H P (1992) The northern Samail ophiolite an
oxygen isotope microprobe and field study Journal of Geophysical
Research 97(B5) 7043---7080Staudigel H Davies G R Hart S R Marchant M amp Smith B
M (1995) Large scale isotopic Sr Nd and O isotopic anatomy of
altered oceanic crust DSDPODP sites 417418 Earth and Planetary
Science Letters 130 169---185Turner F J amp Verhoogen J (1960) Igneous and Metamorphic Petrology
New York McGraw---Hill 694 pp
Wilcock W S D amp Delaney J R (1996) Mid-ocean ridge
sulfide deposits evidence for heat extraction from magma
chambers or cracking fronts Earth and Planetary Science Letters 145
49---64
Witt G amp Seck H A (1989) Origin of amphibole in recrystallized
and porphyroclastic mantle xenoliths from Rhenish massif implica-
tions for the nature of mantle metasomatism Earth and Planetary
Science Letters 91 327---340
JOURNAL OF PETROLOGY VOLUME 45 NUMBER 6 JUNE 2004
1208
Table5continued
Sam
ple
Rb(ppm)
Sr(ppm)
87Rb8
6Sr
87Sr
86Sr
(87Sr
86Sr)i1
d18O
()
LOI(
)2WR
3(87Sr
86Sr)L14
(87Sr
86Sr)L25
(87Sr
86Sr)R6
02OA37b
Plagioclase
023
3121
0002
0704204
15
070420
-------
-------
-------
Pargasite
031
41 6
0021
0703505
15
070348
thorn394
-------
-------
94OAMa2
Whole
rock
0016
1750
00003
0703219
15
070322
thorn495
033
31
0703264
0703177
0703158
Plagioclase
0014
1363
00003
0703219
11
070322
thorn479
-------
-------
Clinopyroxene
-------
-------
-------
0703236
13
070324
-------
-------
-------
1Initial8
7Sr
86Srratiocalculatedat
95Ma(H
acker1994)
2LOIisloss
onignition(in)
3Water---rock
ratiocalculatedassumingaclosedsystem
TheeffectiveWR
ratiocanbeexpressed
bythefollowingeq
uation
W=Reffectivefrac14
frac12eth87Sr=
86SrrockTHORN fi
nal
eth87Sr=
86SrrockTHORN in
itial=frac12eth8
7Sr=
86SrseawaterTHORN in
itial
eth87Sr=
86SrrockTHORN fi
nal
ethCrock
initial=C
seaw
ater
initial
THORNwhere(87Sr
86Srrock) finalisthefinalmeasuredSrisotopicratioin
thesample(87Sr
86Srrock) in
itialco
rrespondingto
theinitialm
antlevalue
istakenas
070295(Lan
phere
etal1981)(87Sr
86Srseawater) in
itialistheCretaceousseaw
ater
Srisotopicco
mposition[87Sr
86Sr07073
at90
MaafterBurkeet
al(1982)]C
seaw
ater
initial
istheSrco
ntent
oflsquonorm
alrsquoseaw
ater
andis
takenas
8ppm
(deVilliers1999)
FortheCrock
initialitis
assumed
that
thepresentSrco
ncentrationmeasuredis
thesameas
theinitial
concentration
4Srisotopicratiosmeasuredforliquid
L1extractedafterthefirststep
oftheleachingexperim
ent(6NHClfor8min
at70
C)
5Srisotopicratiosmeasuredforliquid
L2extractedaftertheseco
ndstep
oftheleachingexperim
ent(2 5NHClfor30
min
at70
C)
6Srisotopicratiosmeasuredforresidueobtained
afteratotalaciddigestionachievedaftertheextractionofL1an
dL2leachates
JOURNAL OF PETROLOGY VOLUME 45 NUMBER 6 JUNE 2004
1198
Part of the same sample from the contact between thedyke and the gabbro yields a slightly lower Sr isotopiccomposition (070464) intermediate between the sur-rounding gabbro and the epidote dyke It also has anintermediate Sr content (425 ppm) Acid leaching experi-ments performed on the whole rock of the dyke yield Srisotopic compositions of 070521 and 070512 for thetwo liquids extracted Similar experiments for the samplelocated at the contact with the gabbro yield 87Sr86Srratios of 070494 and 070467 for L1 and L2 liquids TheSr isotopic composition of the residue extracted afterleaching remains high (070459) and very close to theepidosite 87Sr86Sr ratio determined by Kawahata et al(2001) In this sample the primary minerals have beentotally replaced by epidote and secondary plagioclase orquartz Thus the Sr isotopic compositions measured forthis WR sample and its constituent minerals reflect thecomposition of the fluid filling this dyke
Mineral chemistry
Plagioclase
Compared with the host gabbros the dykes show a muchlarger dispersion in their plagioclase compositions (Fig 8aand b Table 4) characterized by more calcic plagioclasethan the enclosing gabbros This tendency is mostextreme in the comb-textured dykes in which the plagio-clase is almost pure anorthite The zoning is generallynormal recording a crystal fractionation process How-ever gabbro 94 MA2 exhibits inverse zoning this tend-ency is also observed in plagioclases from dykes 01OA19a and 01 OA15f The higher calcium content ofthe plagioclase as well as the inverse zoning is consistentwith an increase of water pressure during its crystalliza-tion (Turner amp Verhoogen 1960 Carmichael et al 1974Koepke et al 2003) The particularly high An content inthe comb-textured gabbro dykes 01 OA15f may alsoresult from crystallization in a supercooled magma thisis consistent with the comb texture indicative of fast-growing crystals The lack of brown hornblende suggeststhat crystallization occurred above the temperature ofhornblende stability
Amphibole
The amphiboles analysed in the dykes (Fig 8c andTable 4) show a regular trend in a (Na thorn K)A vsAlIV diagram from titanium-rich (Ti 4 015) pargasitichornblende (brownamphibole) and tschermakite (brown---green amphibole) to low-titanium (Ti5 015) magnesio-hornblende (green amphibole) and actinolite (pale greenamphibole) The pargasitic hornblende develops aspoikilitic oikocrysts in the dykes 01 OA19a and d andin the diffuse patch 01 OA41d2 at temperatures above800C during the final stage of crystallization (see
below) The tschermakite composition corresponds tothe internal alteration of clinopyroxene in gabbro 94Ma2 occurs in the comb-textured dyke 01 OA15f andis the primary amphibole in the dioritic dyke 02 OA37bThe magnesio-hornblende develops at the edges of thepargasites or of clinopyroxenes at temperatures around800C and is the primary phase in the dioritic dyke 02OA90b and in the green vein 02 OA43a Finally actino-lite occurs as a replacement of green hornblende in dykesor in the kelyphitic rim surrounding olivine in gabbro 94Ma2 Such a large composition range at the scale of athin section has been reported in amphiboles fromamphibolite-facies oceanic gabbros (Stakes 1991 Stakesamp Taylor 1992 Gillis et al 1993) as well as in the Omanophiolite gabbros (Manning et al 2000) This variabilityhas been explained by rapid reaction rates preventinghomogenization (Manning et al 2000)
Thermometry
Plagioclase---amphibole thermometry could be appliedwhen the two minerals exhibited good contacts such asin the laminated gabbro of the middle sequence (01OA41d2) and in the intrusive dykes The temperaturesof plagioclase---amphibole equilibration (Table 2) havebeen calculated using the geothermometer lsquoBrsquo of Hollandamp Blundy (1994) Edenite thorn Albite frac14 Richterite thornAnorthite valid between 500C and 900C with anuncertainty of 40C Amphibole formulae are basedon 23 oxygen and their nomenclature is based on alkalications in the A site vs AlIV according to Leake (1997)Pressurewas assumed to be 1 kbar Forty-five plagioclase---amphibole pairs meet the compositional criteria imposedby Holland amp Blundyrsquos dataset (09 4 Xan 4 01 andAlIV 403) Electron microprobe measurements wereconstrained to contiguous plagioclase and amphiboleseparated by sharp grain boundaries assuming equili-brium of the two phases When the mineral phasesexhibit normal zoning temperatures between plagioclaseand amphibole cores and between mineral rims werecalculated assuming that the temperatures deduced fromthe mineral core chemistry provide a proxy for the high-est temperature of equilibration These data are identi-fied in the T C vs AlIV diagram (Fig 8d) Table 4presents selected major element amphibole analyses theAn content of the plagioclase adjacent to each amphibolegrain and the temperature calculated for the pairThe temperatures calculated (Fig 8d) span the entire
range of temperatures at which amphibole and plagio-clase coexist in equilibrium (ie amphibolite-facies para-geneses) This temperature interval is bracketed byorthopyroxene-bearing facies or amphibole-free facies(ie gabbro 94MA2) and epidote---actinolite facies devoidof homogeneous plagioclase (ie epidosite dyke 01OA36b and green vein 02 OA43a) The range of the
BOSCH et al HYDROTHERMAL CIRCULATION IN OMAN OPHIOLITE
1199
calculated temperatures between 4900C and 750Ccorresponds to the changing composition of amphibolesfrom pargasite to magnesio-hornblende at the scale of athin section or even at that of an individual crystal asshown by their correlation with AlIV such variabilityrecords rapid cooling in hydrous conditions The highesttemperatures are recorded by pargasite from the anatec-tic gabbro 01 OA41d2 in the gabbro dyke 01 OA19dand in the dioritic dyke 02 OA90b They were calculatedfrom minerals devoid of lower-grade alteration and arethus considered as representing the equilibrium temperat-ure of the coexisting minerals Lower temperatures cor-respond to mineral phases evolving at falling temperatureand are only indicative of the cooling history Includingthe 40C uncertainty seven pairs provide temperatureshigher than 900C the limit of validity of the Holland ampBlundy thermometer However we maintain these tem-peratures as indicative of the highest values of our data-set These values are included in the average equilibriumtemperature summarized in Table 2
DISCUSSION
Distribution of hydrothermal alteration
Two distinct petrological events are recorded in the gab-bro section of the Samail ophiolite and are documentedin the present contribution (1) the whole gabbro sectionis affected by a penetrative alteration (2) the gabbrosection is crosscut by various types of dykes and veinsincluding hydrous minerals
Penetrative alteration
The penetrative alteration developed in the gabbros atgrain boundaries and locally invading the minerals isubiquitous in the gabbro section This penetrative altera-tion has been described as very high temperature (VHT)high temperature (HT) and low temperature (LT) basedon alteration parageneses The orthopyroxene coronas inthe layered gabbro sample and the incipient appearanceof brown pargasitic amphibole mark a lower thermallimit of the VHT alteration By reference to the experi-mental work of Koepke et al (2003) the temperature ofequilibration for these reactions is estimated to bebetween 900 and 1000CThe preservation of the vertical distribution of
VHT---HT facies (Figs 4 and 5) also pointed out byManning et al (2000) indicates that the hydrothermalalteration pattern fits the distribution of isotherms withdepth at a fast-spreading ridge (ie Phipps Morgan ampChen 1993 Dunn et al 2000 Fig 2) Equilibrium tem-peratures for VHT parageneses as high as 900---1000Csuggest that VHT---HT hydrothermal alteration tookplace in a very restricted time interval at the limit ofthe cooling magma chamber It is proposed that the
penetrative alteration affecting the entire gabbro sectionis related to a pervasive network of microcracks whichact as a recharge system down to the Moho (Nicolas et al2003)
Dykes and veins
The dykes are extremely varied eg pegmatitic gabbroswith comb-texture microgabbros and amphibolitizeddolerites dioritic dykes alignments of poikilitic horn-blende and finally diffuse hornblende-bearing patchesHigh-T gabbro and diorite dykes and low-T greenamphibole---epidote veins are connectedIn the dykes textural evidence attests to suprasolidus
conditions at least for the development of the pargasiticamphibole Integrating the hornblende---plagioclase equi-libration temperatures calculated for the studied samplesthese temperatures are bracketed between 950C and875CIn the following discussion we shall consider separately
the microgabbro and dolerite dykes The latter representbasaltic melt intrusions associated upsection with thediabase sheeted dykes and downsection possibly withthe mafic dykes that are ubiquitous in the mantle section(Nicolas et al 2000) Whatever their origin the develop-ment of pargasitic amphibole during their late stage ofcrystallization creates a link with the gabbroic dykes andsuggests a crystallization process evolving from dry con-ditions to more hydrous conditionsThe other gabbroic dykes comb-textured gabbros and
hornblende-bearing dioritic dykes result from the fillingof cracks by a fluid either a melt or a silica-rich hydrousfluid These intrusive dykes which develop reaction mar-gins at their contact with the host gabbro grade verticallyinto lower-temperature green amphibole veins or align-ments of poikilitic hornblende in the layered gabbromatrix or development of pargasitic hornblende in ana-tectic gabbros (sample 01 OA41d2) In gabbros crystal-lizing in the magma chamber the development oforthopyroxene and pargasite oikocrysts enclosing the pri-mary minerals suggests a thermal evolution from thegabbro solidus to amphibolite-facies conditions Forthese reasons it is suggested that the gabbroic dykesresulted from the injection of a hydrous melt in the stillhot gabbros drifting away from the magma chamber asdiscussed below
Origin of fluids responsible for VHT---HThydrothermal alteration
The 87Sr86Sr initial ratios and d18O of whole rocks andpure mineral fractions are plotted in Fig 9 against thedistance above the Moho Transition Zone (MTZ) Theinitial Sr isotopic composition of the whole rocks apartfrom the epidosite dykes ranges from 070322 to
JOURNAL OF PETROLOGY VOLUME 45 NUMBER 6 JUNE 2004
1200
070458 All the studied samples (including whole rocksand minerals) have initial 87Sr86Sr ratios falling outsidethe field of fresh MORB which commonly have Sr ratiosbetween 07023 and 07029 with an average of 070265(ie Hart et al 1974) The lowest initial Sr isotopiccomposition (070322) from the massive gabbro 94MA2 is slightly but significantly higher than averageOman primary magmatic signatures (ie 070296McCulloch et al 1981) Similarly the d18O values ofmost whole rocks and minerals measured (Table 5)
depart from typical MORB-source mantle values( Javoy 1980 Gregory amp Taylor 1981 Kyser 1986)These differences indicate that the primary mantlesignatures in the Oman ophiolite have been modifiedby interaction with a fluid Possible origins for thisfluid are mantle components dehydration of subductedlithosphere or seawater circulation Strontium andoxygen isotopes are useful tracers for fluid---crust interac-tion as d18O and 87Sr86Sr ratios from fluids originat-ing from seawater the mantle or subducted lithosphere
-1000
1000
2000
3000
4000
5000
6000
7000
MOHO
MTZ
Dis
tan
ce a
bov
e th
e M
oh
o (
m)
pillow-basalt
sheeted-dike
gabbro
peridotite
01 OA 41d2
01 OA 36b
02 OA 43a97 OA 128b
01 OA15f
94 OA MA2
02 OA 90b01 OA 19ad
02 OA 37b
0702 0703 0704 0705 0706 0707
87Sr86Srinitial
MO
RB-s
ou
rce
Seaw
ater
Whole RockPlagioclasePargasiteGreen HornblendeClinopyroxene
Epidote
(a)
-1000
1000
2000
3000
4000
5000
6000
7000
MOHO
MTZ
Dis
tan
ce a
bov
e th
e M
oh
o (
m)
pillow-basalt
sheeted-dike
gabbro
peridotite
01 OA 41d2
01 OA 36b
02 OA 43a97 OA 128b
01 OA15f
94 OA MA2
02 OA 90b01 OA 19ad
02 OA 37b
Whole RockPlagioclasePargasiteGreen HornblendeClinopyroxene
3 5 10
δ18O (permil)
41d2
Ma2
43a
90b 19d 19d37b
128b
90b
(b)
Fig 9 (a) Variation of initial 87Sr86Sr isotopic composition as a function of the location of the studied samples in a schematic log of the crustalsection of the Oman ophiolite The field for MORB is from Hart et al (1974) and that for seawater is from Burke et al (1982) Small open squares aredata fromKawahata et al (2001) (b) Variation of d18O () as a function of the location of the studied samples in a schematic log of the crustal sectionof the Oman ophiolite Shaded curve corresponds to the data of Gregory amp Taylor (1981)
BOSCH et al HYDROTHERMAL CIRCULATION IN OMAN OPHIOLITE
1201
are significantly different Moreover the d18O valueis dependent on temperature and mineral fractiona-tion and thus yields complementary constraints to Srisotopes
Mantle origin
One model to explain the occurrence of fluids interactingwith the oceanic crust at very high temperatures is toderive them from the mantle The contribution of mantle-derived hydrous fluids linked to percolation processesor to their concentration in the final stages of gabbrocrystallization has been invoked in numerous case studies(ie Witt amp Seck 1989 Fabriees et al 1989 Dupuy et al1991 Agrinier et al 1993) O and Sr isotopic data forwhole rocks and mineral separates yield scattered valuesthat are unambiguously different from MORB mantle-source values (Fig 9a and b) With the exception of lateLT altered samples the WR data have a mean 87Sr86Srratio of 070337 and a standard deviation of 000012These surprisingly high values in mantle-derived rockscould be taken as evidence for survival of mantle isotopicheterogeneities during crystallization of the ophiolite (egLanphere 1981 McCulloch et al 1981) However the18O16O isotope ratios in the Oman gabbroic sequenceare also displaced from primary magmatic values Nogabbro in the present study has plagioclase or amphi-bole with typical mantle d18O values Thus the Sr and Oisotope data rule out the possibility of mantle-derivedfluids as a possible source for the VHT---HT hydrousalteration event recorded in the massive gabbros andgabbroic dykes and veins (Fig 10a and b)
Subducted lithosphere origin
Dehydration of subducted oceanic crust in subductionzones can generate hydrous fluids Such fluids couldpercolate through the overlying lithosphere and contam-inate it thus generating modified isotopic signatures andtrace element contents Such an explanation howeverdisagrees with the geochemical characteristics of most ofthe studied samples Fluids derived from the dehydrationof subducted slabs (fresh or altered MORB or OIB pro-toliths) should be strongly enriched in K Rb and Ba (egBecker et al 2000) whereas in Oman the Rb contentsmeasured in the gabbros and gabbroic dykes and veins arestrongly depleted Moreover the isotopic composition ofslab-derived fluids is buffered by the MORB-like oceaniccrustal protolith for Nd and Pb but not for Sr and O(Staudigel et al 1995) As a result of relatively minorfractionation of oxygen at high temperature the oxygenisotope composition of the fluids expelled during dehy-dration of the subducted slab should be high and essen-tially controlled by the oxygen composition of the uppersection of the crust altered at low temperature (with an
average of 10 and perhaps as high as 19) (Staudigelet al 1995) The Sr isotopic composition should be alsohigh (average 07046) as the fluids released by the dehy-dration of subducted oceanic crust are associated eitherwith seawater or with radiogenic Sr phases (ie smectites)The Sr and O isotopic compositions of the studied sam-ples do not fit with these signatures and so preclude asubducted lithosphere origin for the fluids involved in theVHT---HT alteration event
Seawater-derived fluids
All the analysed samples (minerals and WR) have 87Sr86Sr(T) outside the domain of primary MORB magmasand variable Sr contents Individual minerals have Srcontents reflecting their different mineralliquid partitioncoefficients Minerals characterized by a very high affi-nity for Sr such as plagioclase (ie Sr mineralliquidpartition coefficients 1) have very high Sr contentsTherefore the high abundance of plagioclase in the sam-ples analysed is responsible for the high Sr content of thestudied whole rocks The Sr content of all the whole rocksis relatively homogeneous (Table 5) without clear distinc-tion between country-rock gabbros and dykes and veinsand ranges from 110 to 200 ppm except for the epidosite(4700 ppm) Reported on a 87Sr86Sr vs 1Sr diagram(Fig 10a) most whole rocks and plagioclases analysed arecharacterized by a 1Sr ratio lower than 001 and plot inthe left part of this diagram Compared with N-MORBthe studied massive and anatectic gabbros and their con-stituent plagioclases have similar to slightly higher Srcontents but substantially higher 87Sr86Sr ratios(Fig 10a) Considering Cretaceous seawater as a possiblehigh 87Sr86Sr mixing component [87Sr86Sr 07073at 90Ma after Burke et al (1982)] responsible for theobserved geochemical modifications exchanges cannothave occurred in a closed system as seawater has too lowa Sr content (ie 8 ppm de Villiers 1999) An open-system process allowing penetration of larger quantitiesof seawater can efficiently modify the Sr isotopic ratio ofthe rocks Alternately the fluids could also be seawaterthat was modified through exchange with the surround-ing rocks during its transit through the oceanic crustalsection This modified seawater would be more enrichedin Srwith a 87Sr86Sr ratio intermediate between unmodi-fied seawater and MORBMinerals devoid of late LT alteration imprints (parga-
site green hornblende and pyroxene) and characterizedby very low Sr contents plot on the right of the diagramclose to or on the mixing line between seawater andMORB (Fig 10a) The difference between these mineralsand the other samples (WR and plagioclases) is mainlydue to the Sr fractionation coefficients of these differentminerals and to the large quantity of plagioclase inthe studied rocks The process responsible for the
JOURNAL OF PETROLOGY VOLUME 45 NUMBER 6 JUNE 2004
1202
modification of the Sr isotopic composition fromMORB-source value is however explained by the same phenom-enon As all these minerals display initial 87Sr86Sr ratiosdistinct from the MORB source and because they equili-brated at VHT or HT temperatures this supports anearly intervention of seawater during crystallization ofthese minerals Most of these samples with the exceptionof the epidosite dyke overlap the domains defined byKawahata et al (2001) for the Wadi Fizh gabbros alteredat VHT and HT conditions in the amphibolite facies(labelled lsquoVHTrsquo and lsquoHTrsquo in Fig 9a)Three samples (two epidosite dykes and an one epidote
fraction) have very high Sr contents and the highest Sr
isotopic compositions of the present dataset The twowhole-rock samples overlap the field of the Wadi Fizhsamples altered at high temperature during greenschist-facies metamorphism (Kawahata et al 2001) This typeof alteration is common at the ridge axis and formedthrough intensive exchange with seawater-derived fluidsFigure 10b indicates that all the Sr---O isotope data are
distinct from MORB compositions and suggests that thefluids reacting with the magmas could be derived fromseawater In addition the d18O values show that isotopicexchange occurred under VHT or HT conditions Thefluids could have the composition of seawater or theycould have the composition of seawater modified through
0703
0705
0707
001 01 1
1 Sr (ppm)
MORB
(87Sr
86Sr)
init
ial
Seawater
VHT
HT
MT36b
36b
36b
43a 128b90b
41d2
37b19d
15f
128b
41d2
41d2
19a
Ma2Ma2
19d
90b
37b
Whole RockPlagioclasePargasiteGreen HornblendeClinopyroxene
Epidote
(a)
0 2 4 6 8
δ18O(permil)
0703
0705
0707Seawater
MORB-mantle
128b 41d2
90b 41d2
Ma2128b
37b
43a
19d
90b
Whole RockPlagioclasePargasiteGreen HornblendeClinopyroxene
Epidote
HT LT
19d
(87Sr
86Sr)
init
ial
(b)
Fig 10 (a) Initial 87Sr86Sr isotopic ratios plotted against 1Sr for whole rocks and mineral separates from the gabbro section of the Omanophiolite Fields shown are from Kawahata et al (2001) and correspond to MT-----basalt sheeted dyke diabase altered at medium temperature(prehnite---actinolite facies sequence 3) HT-----diabase plagiogranite metagabbro and epidosite formed at high temperature (greenschist faciessequence 4) VHT-----gabbro altered at very high temperature (amphibolite facies sequence 5) The dashed line is a binary mixing line betweenMORB (Lanphere et al 1981) and Cretaceous seawater (Burke et al 1982) (b) Initial 87Sr86Sr isotopic composition plotted against d18O value forwhole rocks and minerals from the Oman ophiolite Cretaceous seawater and MORB end-members as in (a) Dashed line represents mixing curvebetweenMORB and seawater at high temperature (HT) Low-temperature (LT) exchanges between these two components would be responsible foran increase of the d18O primary MORB value
BOSCH et al HYDROTHERMAL CIRCULATION IN OMAN OPHIOLITE
1203
exchange with the surrounding rocks during its penetra-tion into the crustal section In an environment in whichfluid---rock interaction occurs at variable temperaturesthe d18O is greatly influenced by the temperature atwhich the exchange took place and by the waterrockratio It is evident from Figs 9b and 10b that none of thestudied gabbros have plagioclase and amphibole withMORB-like d18O and 87Sr86Sr values The separateminerals display a broad range of d18O values and mostexhibit extreme 18O depletion (Table 5) Hydrothermalexchange kinetics are similar in amphibole and pyroxeneand these two minerals are more resistant to oxygenexchange during water---rock interaction than coexistingplagioclase (Gregory et al 1989) The most probableexplanation of the low d18O values measured in bothamphiboles and pyroxenes is that they crystallized athigh temperature in the presence of a relatively low 18Oseawater-derived hydrothermal fluid [d18O from 01 to07 after Gregory amp Taylor (1981)] The anomalouslyhigh d18O value (thorn1009) measured for the plagioclase ofthe micro-gabbroic dyke (01 OA19d) can be interpretedas resulting from a superimposed overprint caused by alate-stage penetration of fluids at lower temperature Inthis sample the occurrence of such different 18O16Oratios between coexisting plagioclase and amphibole isnot consistent with equilibrium fractionation betweenthese two minerals at any possible temperature Thisobservation suggests that parts of the ophiolite sequencehave undergone a high-temperature hydrothermal eventprior to the later low-temperature event that producedthe strong 18O increase in coexisting plagioclaseThe depletion of the d18O values results from high-temperature hydrothermal alteration (Gregory amp Taylor1981 Stakes amp Taylor 1992) The oxygen isotopic datasupport the contention that most studied samples havebeen affected by a VHT---HT hydrothermal alteration byfluids derived from seawater Some of the analysed sam-ples presenting petrological evidence of crystallization ofLT secondary minerals have been overprinted by the latelow-temperature alteration thus swamping the earlierhigh-temperature alteration effects
Determination of waterrock ratio
To better evaluate the exchange between seawater andthe gabbroic rocks waterrock ratios (WR) have beenestimated for the whole rocks analysed during the courseof this study The different models used to constrain thisratio mainly differ by the calculation of the effective ratioor by the estimated weight ratio and by considering openor closed systems for water circulation (Albareede et al1981 McCulloch et al 1981) The WR ratios reportedin Table 5 have been calculated assuming a closed sys-tem Using this formulation these values are minimabecause the calculation always uses pristine fluid for the
initial fluid thus maximizing the value in the denomina-tor If the fluid was shifted to lower 87Sr concentrations byexchange with the rock on the downward path theinferred values would be much higher (Davis et al 2003)The calculated WR ratios for the samples from this
study range from 31 to 219 (Table 5) Two main groupsof samples can be recognized on the basis of their waterrock ratios The lowest ratios ranging from 31 to 47have been determined for massive gabbros or anatecticgabbros but also for dykes and veins devoid of late LTalteration Similar ratios have been previously calculatedfor example for the discharged hydrothermal solutions atthe 13N and 21N East Pacific Rise (Di Donato et al1990 Fouquet et al 1991) The gabbros from the Ibrasection in the Oman ophiolite gave effective WR ratiosof 05 33 and 42 (McCulloch et al 1981) in goodagreement with the values obtained in this study Theleast altered gabbro of the Ibra section yields a WR ratioof 05 in good agreement with the exceptionally fresh-ness of this rock characterized by typical mantle oxygenvalues for its minerals (McCulloch et al 1981) Samplescharacterized by waterrock effective ratios in the rangeof 3---5 can be related to penetrative alteration via thehydrothermal system Lecuyer amp Reynard (1996) havedemonstrated in oceanic gabbros from the Hess Deepaltered in similar HT conditions that WR mass ratios inthe range of 02---1 are sufficient to account for the mod-ified stable isotope signature of the gabbros Higherwaterrock values (WR 45) have been calculated forsamples affected by superimposed late hydrothermalalteration and for the epidosite samples (Table 5) Theyprobably acquired their isotopic characteristics throughinteraction with a larger volume of seawater duringgreenschist-facies metamorphism Similar or higherWR values have been published in previous studies(ie McCulloch et al 1981 Kawahata et al 2001)They correspond either to samples from the upper-crus-tal sequence (ie basalt sheeted dyke or plagiogranite) orto gabbros from the crustal section located in a particularenvironment such as for example the Wadi Fizh sectionthat is located close to a segment boundary along thepalaeo-spreading axis (Nicolas et al 2000)
Mechanism for seawater interaction withmagma
Nicolas et al (2003) have proposed that variable amountsof seawater can circulate at high temperature through-out the crustal section down to the walls of the magmachamber via a network of parallel microcracks a fractionof a millimetre wide Such microcracks and their relation-ship with the VHT---HT hydrous alteration in the sur-rounding gabbros have been described in detail byNicolas et al Via this network of microcracks largeamounts of seawater can have reacted with the magma
JOURNAL OF PETROLOGY VOLUME 45 NUMBER 6 JUNE 2004
1204
and induced variable changes of its Sr and O isotopiccomposition This regular network of parallel micro-cracks could constitute the recharge system and thehydrated gabbroic dykes the discharge system Physicalparameters and the geophysical models of Nicolas et al(2003) are in agreement with this scenario Such a pene-tration of seawater-derived hydrothermal fluids throughthe lower crust along grain boundaries and a microscopicfracture network at temperatures 4750C is consistentwith recent thermodynamic models (McCollom amp Shock1998)Seawater-altered and high-temperature recrystallized
diabases (protodykes) from the overlying sheeted dykecomplex stoped into the melt lenses could represent anindirect way for seawater to enter the system as they canremelt during their subsidence through the magmachamber A simple mass balance calculation suggeststhat this indirect contamination is not significant Assess-ments of the amount of recycled crust 1 in volumeby Nicolas et al (2000) based on the protodyke remnantsin the gabbro section or 20 by Coogan et al (2003)based on chlorine content in EPR basalts lead to a sea-waterrock ratio of the order of 104 to 103Thus following Nicolas et al (2003) we propose that
seawater has been introduced along microcracks down tothe walls of the magma chamber and has diffused alonggrain boundaries in the way described by Manning et al(2000) progressively invading the host gabbro Gabbroicdykes result from the injection of hydrous melt into thehost gabbros at falling temperatures as they drift awayfrom the magma chamber This water-saturated meltcould result from the extensive hydration of the crystal-lizing gabbros near the walls of the magma chamberwhen seawater penetrates through this wall (Fig 1)Similar plumbing systems driving seawater down to the
limit of the magma chamber and back to the surface havebeen already proposed in other environments such as theMid-Atlantic Ridge (Kelley amp Delaney 1987 Cooganet al 2001) the East Pacific Rise at Hess Deep (Gillis1995 Manning et al 1996a 1996b) the Tonga forearc(Banerjee amp Gillis 2001) and the Troodos ophiolite(Gillis amp Roberts 1999) The melting curve of wet basalthas a negative slope but is close to the dry solidus at lowpressure in the lower gabbros and the first melts areplagiogranitic (Spulber ampRutherford 1983) Such plagio-granites are ubiquitous in the highest gabbros and in theroot zone of sheeted dykes in Oman (Rothery 1983Juteau et al 1988 Nicolas amp Boudier 1991) in manyother ophiolites and in the oceanic crust where they havebeen interpreted as resulting either from wet anatexis ofhot gabbros or from the final product of crystallization ofa basaltic melt (Spulber amp Rutherford 1983) Plagio-granites are rare in the lower gabbros exceptionallyassociated with hydrous gabbro segregations inside thelayered gabbros Their constitutive minerals are also
remarkably absent along the VHTmicrocracks althoughthese gabbros were above the wet solidus A possibleexplanation has been proposed by McCollom amp Shock(1998) who wrote that at high temperature lsquoaqueousfluids may leave very little conspicuous petrological evid-ence for their presencersquo the reason being that thehigh-T conditions would be closer to batch meltingObviously more detailed studies on this specific problemare needed We conclude that the large degree of hydrousmelting locally required at the walls of the magma cham-ber to account for the generation of a gabbroic hydrousmelt may have erased the traces of the incipient plagio-granitic melt
CONCLUSIONS
Fostered by the recent discovery of high-temperatureseawater contamination in some gabbros of the Omanophiolite (Manning et al 2000) we have conducted astudy on 500 gabbro specimens from selected areas ofthe entire Samail ophiolite belt and on the hydrous gab-broic dykes that cross-cut them The highest-temperaturereactions (VHT) recorded in olivine and clinopyroxene ingabbros as orthopyroxene and pargasite coronas reach975C They are followed at lower HT conditions withreplacement of olivine by an assemblage of tremolitemagnetite and plagioclase surrounded by chlorite andby pargasite development in clinopyroxene followedfinally by the LT lizardite---olivine and saussurite---plagioclase greenschist-facies alteration With a fewexceptions the VHT alteration tends to be restricted tothe lower gabbros and to the vicinity of the Moho andthe HT alteration to the middle and upper gabbros Thepreservation of the vertical distribution of VHT---HTfacies indicates that progressive cooling during off-axisspreading did not obliterate this distribution and conse-quently that the VHT---HT hydrothermal alteration tookplace in a very restricted time interval at the limit of thecooling magma chamber In the deep and hot gabbrosthe main water channels may be sub-millimetre-sizedmicrocracks with a dominantly vertical attitude the ori-gin and dynamics of which have been investigated in aprevious paper (Nicolas et al 2003)Beyond these high-temperature alteration zones mas-
sively induced in the gabbros seawater may be respons-ible for the generation of clinopyroxene---pargasitegabbro dykes that cross-cut all gabbros and that areupsection clearly connected to the LT hydrothermalvein system Petrostructural evidence suggests that thesedykes were generated by hydration of the crystallizinggabbros near the internal wall of the LVZ---magmachamber The resultant melts were subsequently intrudedat various levels in the cooling lower and middle gabbrosPetrological observations of nearly all the gabbros ana-lysed in the present study located from the root zone of
BOSCH et al HYDROTHERMAL CIRCULATION IN OMAN OPHIOLITE
1205
the sheeted dyke complex down to the Moho emphasizethe imprint of VHT---HT hydrous alteration The VHThydrothermal event is characterized by temperatures ofequilibration around 900---1000C The temperatures atwhich the HT hydrous event occurred are estimated tobe in the range 550---900CStrontium and oxygen isotope compositions measured
on whole rocks and separated minerals significantlydepart from primary MORB values The Sr and O iso-tope data obtained for pargasite green hornblende andclinopyroxene record the participation of seawater at thetemperature of crystallization of the minerals Plagio-clases and whole rocks define a trend toward a compo-nent characterized by a high radiogenic 87Sr86Sr valueand a higher Sr content than normal seawater Seawateris clearly identified as the fluid responsible for the modi-fication of the Sr and O signatures for both massivegabbros and dykes and veins Nevertheless the high Srcontent of the extraneous component requires eitheropen-system exchange allowing penetration of a greaterquantity of seawater or penetration of seawater modifiedby interaction with the oceanic crust during its travel paththrough the crustal section The waterrock ratio calcu-lated on the basis of the Sr isotopes is between three andfive for the enclosing gabbros and for the least LT altereddykes The VHT---HT hydrothermal event affectingthese samples is related to penetrative alteration via therecharge and discharge hydrothermal system Thewaterrock ratio is 10 for dykes exhibiting evidenceof a LT hydrothermal alteration overprint and reaches22 for the epidosite dyke The depletion in d18O char-acterizes all the studied samples with the single exceptionof the micro-gabbroic dyke plagioclase This agreeswith the conclusions based on Sr isotopes suggestingthat these samples have undergone exchange with sea-water-derived fluids during a VHT---HTalteration hydro-thermal event
ACKNOWLEDGEMENTS
This work has benefited from financial support by theCentre National de la Recherche Scientifique (INSUInteerieur-Terre andDYETI programmes) and inOmanfrom the willingness of the Directorate of MineralsMinistry of Commerce and Industry and the hospitabil-ity of the people Technical help in the laboratory is alsoacknowledged including C Nevado for the thin sectionsE Ball for the illustrations B Galland for her assistancein the chemistry room and C Bassoulet for help duringthermal ionization mass spectrometric analyses of the Srresults from the leaching experiments Constructivereviews from R T Gregory C Lecuyer an anonymousreviewer and particularly from M Wilson greatlyhelped to improve an earlier version of the manuscript
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1207
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706---708
Pin C Briot D Bassin C amp Poitrasson F (1994) Concomitant
extraction of Sr and Sm---Nd for isotopic analysis in silicate samples
based on specific extraction chromatography Analytica Chimica Acta
298 209---217
Rothery D A (1983) The base of a sheeted dyke complex Oman
ophiolite implications for magma chambers at oceanic spreading
axes Journal of the Geological Society London 140 287---296
Spear F S (1981) An experimental study of hornblende stability and
compositional variability in amphibolite American Journal of Science
281 697---734
Spulber S D amp Rutherford M J (1983) The origin of rhyolite and
plagiogranite in oceanic crust an experimental study Journal of
Petrology 24 1---25Stakes D S (1991) Oxygen and hydrogen isotope compositions of
oceanic plutonic rocks high-temperature deformation and meta-
morphism of oceanic layer 3 In Taylor H P OrsquoNeil J et al (eds)
Geochemical Society Special Publication 3 77---90
Stakes D S amp Taylor H P (1992) The northern Samail ophiolite an
oxygen isotope microprobe and field study Journal of Geophysical
Research 97(B5) 7043---7080Staudigel H Davies G R Hart S R Marchant M amp Smith B
M (1995) Large scale isotopic Sr Nd and O isotopic anatomy of
altered oceanic crust DSDPODP sites 417418 Earth and Planetary
Science Letters 130 169---185Turner F J amp Verhoogen J (1960) Igneous and Metamorphic Petrology
New York McGraw---Hill 694 pp
Wilcock W S D amp Delaney J R (1996) Mid-ocean ridge
sulfide deposits evidence for heat extraction from magma
chambers or cracking fronts Earth and Planetary Science Letters 145
49---64
Witt G amp Seck H A (1989) Origin of amphibole in recrystallized
and porphyroclastic mantle xenoliths from Rhenish massif implica-
tions for the nature of mantle metasomatism Earth and Planetary
Science Letters 91 327---340
JOURNAL OF PETROLOGY VOLUME 45 NUMBER 6 JUNE 2004
1208
Part of the same sample from the contact between thedyke and the gabbro yields a slightly lower Sr isotopiccomposition (070464) intermediate between the sur-rounding gabbro and the epidote dyke It also has anintermediate Sr content (425 ppm) Acid leaching experi-ments performed on the whole rock of the dyke yield Srisotopic compositions of 070521 and 070512 for thetwo liquids extracted Similar experiments for the samplelocated at the contact with the gabbro yield 87Sr86Srratios of 070494 and 070467 for L1 and L2 liquids TheSr isotopic composition of the residue extracted afterleaching remains high (070459) and very close to theepidosite 87Sr86Sr ratio determined by Kawahata et al(2001) In this sample the primary minerals have beentotally replaced by epidote and secondary plagioclase orquartz Thus the Sr isotopic compositions measured forthis WR sample and its constituent minerals reflect thecomposition of the fluid filling this dyke
Mineral chemistry
Plagioclase
Compared with the host gabbros the dykes show a muchlarger dispersion in their plagioclase compositions (Fig 8aand b Table 4) characterized by more calcic plagioclasethan the enclosing gabbros This tendency is mostextreme in the comb-textured dykes in which the plagio-clase is almost pure anorthite The zoning is generallynormal recording a crystal fractionation process How-ever gabbro 94 MA2 exhibits inverse zoning this tend-ency is also observed in plagioclases from dykes 01OA19a and 01 OA15f The higher calcium content ofthe plagioclase as well as the inverse zoning is consistentwith an increase of water pressure during its crystalliza-tion (Turner amp Verhoogen 1960 Carmichael et al 1974Koepke et al 2003) The particularly high An content inthe comb-textured gabbro dykes 01 OA15f may alsoresult from crystallization in a supercooled magma thisis consistent with the comb texture indicative of fast-growing crystals The lack of brown hornblende suggeststhat crystallization occurred above the temperature ofhornblende stability
Amphibole
The amphiboles analysed in the dykes (Fig 8c andTable 4) show a regular trend in a (Na thorn K)A vsAlIV diagram from titanium-rich (Ti 4 015) pargasitichornblende (brownamphibole) and tschermakite (brown---green amphibole) to low-titanium (Ti5 015) magnesio-hornblende (green amphibole) and actinolite (pale greenamphibole) The pargasitic hornblende develops aspoikilitic oikocrysts in the dykes 01 OA19a and d andin the diffuse patch 01 OA41d2 at temperatures above800C during the final stage of crystallization (see
below) The tschermakite composition corresponds tothe internal alteration of clinopyroxene in gabbro 94Ma2 occurs in the comb-textured dyke 01 OA15f andis the primary amphibole in the dioritic dyke 02 OA37bThe magnesio-hornblende develops at the edges of thepargasites or of clinopyroxenes at temperatures around800C and is the primary phase in the dioritic dyke 02OA90b and in the green vein 02 OA43a Finally actino-lite occurs as a replacement of green hornblende in dykesor in the kelyphitic rim surrounding olivine in gabbro 94Ma2 Such a large composition range at the scale of athin section has been reported in amphiboles fromamphibolite-facies oceanic gabbros (Stakes 1991 Stakesamp Taylor 1992 Gillis et al 1993) as well as in the Omanophiolite gabbros (Manning et al 2000) This variabilityhas been explained by rapid reaction rates preventinghomogenization (Manning et al 2000)
Thermometry
Plagioclase---amphibole thermometry could be appliedwhen the two minerals exhibited good contacts such asin the laminated gabbro of the middle sequence (01OA41d2) and in the intrusive dykes The temperaturesof plagioclase---amphibole equilibration (Table 2) havebeen calculated using the geothermometer lsquoBrsquo of Hollandamp Blundy (1994) Edenite thorn Albite frac14 Richterite thornAnorthite valid between 500C and 900C with anuncertainty of 40C Amphibole formulae are basedon 23 oxygen and their nomenclature is based on alkalications in the A site vs AlIV according to Leake (1997)Pressurewas assumed to be 1 kbar Forty-five plagioclase---amphibole pairs meet the compositional criteria imposedby Holland amp Blundyrsquos dataset (09 4 Xan 4 01 andAlIV 403) Electron microprobe measurements wereconstrained to contiguous plagioclase and amphiboleseparated by sharp grain boundaries assuming equili-brium of the two phases When the mineral phasesexhibit normal zoning temperatures between plagioclaseand amphibole cores and between mineral rims werecalculated assuming that the temperatures deduced fromthe mineral core chemistry provide a proxy for the high-est temperature of equilibration These data are identi-fied in the T C vs AlIV diagram (Fig 8d) Table 4presents selected major element amphibole analyses theAn content of the plagioclase adjacent to each amphibolegrain and the temperature calculated for the pairThe temperatures calculated (Fig 8d) span the entire
range of temperatures at which amphibole and plagio-clase coexist in equilibrium (ie amphibolite-facies para-geneses) This temperature interval is bracketed byorthopyroxene-bearing facies or amphibole-free facies(ie gabbro 94MA2) and epidote---actinolite facies devoidof homogeneous plagioclase (ie epidosite dyke 01OA36b and green vein 02 OA43a) The range of the
BOSCH et al HYDROTHERMAL CIRCULATION IN OMAN OPHIOLITE
1199
calculated temperatures between 4900C and 750Ccorresponds to the changing composition of amphibolesfrom pargasite to magnesio-hornblende at the scale of athin section or even at that of an individual crystal asshown by their correlation with AlIV such variabilityrecords rapid cooling in hydrous conditions The highesttemperatures are recorded by pargasite from the anatec-tic gabbro 01 OA41d2 in the gabbro dyke 01 OA19dand in the dioritic dyke 02 OA90b They were calculatedfrom minerals devoid of lower-grade alteration and arethus considered as representing the equilibrium temperat-ure of the coexisting minerals Lower temperatures cor-respond to mineral phases evolving at falling temperatureand are only indicative of the cooling history Includingthe 40C uncertainty seven pairs provide temperatureshigher than 900C the limit of validity of the Holland ampBlundy thermometer However we maintain these tem-peratures as indicative of the highest values of our data-set These values are included in the average equilibriumtemperature summarized in Table 2
DISCUSSION
Distribution of hydrothermal alteration
Two distinct petrological events are recorded in the gab-bro section of the Samail ophiolite and are documentedin the present contribution (1) the whole gabbro sectionis affected by a penetrative alteration (2) the gabbrosection is crosscut by various types of dykes and veinsincluding hydrous minerals
Penetrative alteration
The penetrative alteration developed in the gabbros atgrain boundaries and locally invading the minerals isubiquitous in the gabbro section This penetrative altera-tion has been described as very high temperature (VHT)high temperature (HT) and low temperature (LT) basedon alteration parageneses The orthopyroxene coronas inthe layered gabbro sample and the incipient appearanceof brown pargasitic amphibole mark a lower thermallimit of the VHT alteration By reference to the experi-mental work of Koepke et al (2003) the temperature ofequilibration for these reactions is estimated to bebetween 900 and 1000CThe preservation of the vertical distribution of
VHT---HT facies (Figs 4 and 5) also pointed out byManning et al (2000) indicates that the hydrothermalalteration pattern fits the distribution of isotherms withdepth at a fast-spreading ridge (ie Phipps Morgan ampChen 1993 Dunn et al 2000 Fig 2) Equilibrium tem-peratures for VHT parageneses as high as 900---1000Csuggest that VHT---HT hydrothermal alteration tookplace in a very restricted time interval at the limit ofthe cooling magma chamber It is proposed that the
penetrative alteration affecting the entire gabbro sectionis related to a pervasive network of microcracks whichact as a recharge system down to the Moho (Nicolas et al2003)
Dykes and veins
The dykes are extremely varied eg pegmatitic gabbroswith comb-texture microgabbros and amphibolitizeddolerites dioritic dykes alignments of poikilitic horn-blende and finally diffuse hornblende-bearing patchesHigh-T gabbro and diorite dykes and low-T greenamphibole---epidote veins are connectedIn the dykes textural evidence attests to suprasolidus
conditions at least for the development of the pargasiticamphibole Integrating the hornblende---plagioclase equi-libration temperatures calculated for the studied samplesthese temperatures are bracketed between 950C and875CIn the following discussion we shall consider separately
the microgabbro and dolerite dykes The latter representbasaltic melt intrusions associated upsection with thediabase sheeted dykes and downsection possibly withthe mafic dykes that are ubiquitous in the mantle section(Nicolas et al 2000) Whatever their origin the develop-ment of pargasitic amphibole during their late stage ofcrystallization creates a link with the gabbroic dykes andsuggests a crystallization process evolving from dry con-ditions to more hydrous conditionsThe other gabbroic dykes comb-textured gabbros and
hornblende-bearing dioritic dykes result from the fillingof cracks by a fluid either a melt or a silica-rich hydrousfluid These intrusive dykes which develop reaction mar-gins at their contact with the host gabbro grade verticallyinto lower-temperature green amphibole veins or align-ments of poikilitic hornblende in the layered gabbromatrix or development of pargasitic hornblende in ana-tectic gabbros (sample 01 OA41d2) In gabbros crystal-lizing in the magma chamber the development oforthopyroxene and pargasite oikocrysts enclosing the pri-mary minerals suggests a thermal evolution from thegabbro solidus to amphibolite-facies conditions Forthese reasons it is suggested that the gabbroic dykesresulted from the injection of a hydrous melt in the stillhot gabbros drifting away from the magma chamber asdiscussed below
Origin of fluids responsible for VHT---HThydrothermal alteration
The 87Sr86Sr initial ratios and d18O of whole rocks andpure mineral fractions are plotted in Fig 9 against thedistance above the Moho Transition Zone (MTZ) Theinitial Sr isotopic composition of the whole rocks apartfrom the epidosite dykes ranges from 070322 to
JOURNAL OF PETROLOGY VOLUME 45 NUMBER 6 JUNE 2004
1200
070458 All the studied samples (including whole rocksand minerals) have initial 87Sr86Sr ratios falling outsidethe field of fresh MORB which commonly have Sr ratiosbetween 07023 and 07029 with an average of 070265(ie Hart et al 1974) The lowest initial Sr isotopiccomposition (070322) from the massive gabbro 94MA2 is slightly but significantly higher than averageOman primary magmatic signatures (ie 070296McCulloch et al 1981) Similarly the d18O values ofmost whole rocks and minerals measured (Table 5)
depart from typical MORB-source mantle values( Javoy 1980 Gregory amp Taylor 1981 Kyser 1986)These differences indicate that the primary mantlesignatures in the Oman ophiolite have been modifiedby interaction with a fluid Possible origins for thisfluid are mantle components dehydration of subductedlithosphere or seawater circulation Strontium andoxygen isotopes are useful tracers for fluid---crust interac-tion as d18O and 87Sr86Sr ratios from fluids originat-ing from seawater the mantle or subducted lithosphere
-1000
1000
2000
3000
4000
5000
6000
7000
MOHO
MTZ
Dis
tan
ce a
bov
e th
e M
oh
o (
m)
pillow-basalt
sheeted-dike
gabbro
peridotite
01 OA 41d2
01 OA 36b
02 OA 43a97 OA 128b
01 OA15f
94 OA MA2
02 OA 90b01 OA 19ad
02 OA 37b
0702 0703 0704 0705 0706 0707
87Sr86Srinitial
MO
RB-s
ou
rce
Seaw
ater
Whole RockPlagioclasePargasiteGreen HornblendeClinopyroxene
Epidote
(a)
-1000
1000
2000
3000
4000
5000
6000
7000
MOHO
MTZ
Dis
tan
ce a
bov
e th
e M
oh
o (
m)
pillow-basalt
sheeted-dike
gabbro
peridotite
01 OA 41d2
01 OA 36b
02 OA 43a97 OA 128b
01 OA15f
94 OA MA2
02 OA 90b01 OA 19ad
02 OA 37b
Whole RockPlagioclasePargasiteGreen HornblendeClinopyroxene
3 5 10
δ18O (permil)
41d2
Ma2
43a
90b 19d 19d37b
128b
90b
(b)
Fig 9 (a) Variation of initial 87Sr86Sr isotopic composition as a function of the location of the studied samples in a schematic log of the crustalsection of the Oman ophiolite The field for MORB is from Hart et al (1974) and that for seawater is from Burke et al (1982) Small open squares aredata fromKawahata et al (2001) (b) Variation of d18O () as a function of the location of the studied samples in a schematic log of the crustal sectionof the Oman ophiolite Shaded curve corresponds to the data of Gregory amp Taylor (1981)
BOSCH et al HYDROTHERMAL CIRCULATION IN OMAN OPHIOLITE
1201
are significantly different Moreover the d18O valueis dependent on temperature and mineral fractiona-tion and thus yields complementary constraints to Srisotopes
Mantle origin
One model to explain the occurrence of fluids interactingwith the oceanic crust at very high temperatures is toderive them from the mantle The contribution of mantle-derived hydrous fluids linked to percolation processesor to their concentration in the final stages of gabbrocrystallization has been invoked in numerous case studies(ie Witt amp Seck 1989 Fabriees et al 1989 Dupuy et al1991 Agrinier et al 1993) O and Sr isotopic data forwhole rocks and mineral separates yield scattered valuesthat are unambiguously different from MORB mantle-source values (Fig 9a and b) With the exception of lateLT altered samples the WR data have a mean 87Sr86Srratio of 070337 and a standard deviation of 000012These surprisingly high values in mantle-derived rockscould be taken as evidence for survival of mantle isotopicheterogeneities during crystallization of the ophiolite (egLanphere 1981 McCulloch et al 1981) However the18O16O isotope ratios in the Oman gabbroic sequenceare also displaced from primary magmatic values Nogabbro in the present study has plagioclase or amphi-bole with typical mantle d18O values Thus the Sr and Oisotope data rule out the possibility of mantle-derivedfluids as a possible source for the VHT---HT hydrousalteration event recorded in the massive gabbros andgabbroic dykes and veins (Fig 10a and b)
Subducted lithosphere origin
Dehydration of subducted oceanic crust in subductionzones can generate hydrous fluids Such fluids couldpercolate through the overlying lithosphere and contam-inate it thus generating modified isotopic signatures andtrace element contents Such an explanation howeverdisagrees with the geochemical characteristics of most ofthe studied samples Fluids derived from the dehydrationof subducted slabs (fresh or altered MORB or OIB pro-toliths) should be strongly enriched in K Rb and Ba (egBecker et al 2000) whereas in Oman the Rb contentsmeasured in the gabbros and gabbroic dykes and veins arestrongly depleted Moreover the isotopic composition ofslab-derived fluids is buffered by the MORB-like oceaniccrustal protolith for Nd and Pb but not for Sr and O(Staudigel et al 1995) As a result of relatively minorfractionation of oxygen at high temperature the oxygenisotope composition of the fluids expelled during dehy-dration of the subducted slab should be high and essen-tially controlled by the oxygen composition of the uppersection of the crust altered at low temperature (with an
average of 10 and perhaps as high as 19) (Staudigelet al 1995) The Sr isotopic composition should be alsohigh (average 07046) as the fluids released by the dehy-dration of subducted oceanic crust are associated eitherwith seawater or with radiogenic Sr phases (ie smectites)The Sr and O isotopic compositions of the studied sam-ples do not fit with these signatures and so preclude asubducted lithosphere origin for the fluids involved in theVHT---HT alteration event
Seawater-derived fluids
All the analysed samples (minerals and WR) have 87Sr86Sr(T) outside the domain of primary MORB magmasand variable Sr contents Individual minerals have Srcontents reflecting their different mineralliquid partitioncoefficients Minerals characterized by a very high affi-nity for Sr such as plagioclase (ie Sr mineralliquidpartition coefficients 1) have very high Sr contentsTherefore the high abundance of plagioclase in the sam-ples analysed is responsible for the high Sr content of thestudied whole rocks The Sr content of all the whole rocksis relatively homogeneous (Table 5) without clear distinc-tion between country-rock gabbros and dykes and veinsand ranges from 110 to 200 ppm except for the epidosite(4700 ppm) Reported on a 87Sr86Sr vs 1Sr diagram(Fig 10a) most whole rocks and plagioclases analysed arecharacterized by a 1Sr ratio lower than 001 and plot inthe left part of this diagram Compared with N-MORBthe studied massive and anatectic gabbros and their con-stituent plagioclases have similar to slightly higher Srcontents but substantially higher 87Sr86Sr ratios(Fig 10a) Considering Cretaceous seawater as a possiblehigh 87Sr86Sr mixing component [87Sr86Sr 07073at 90Ma after Burke et al (1982)] responsible for theobserved geochemical modifications exchanges cannothave occurred in a closed system as seawater has too lowa Sr content (ie 8 ppm de Villiers 1999) An open-system process allowing penetration of larger quantitiesof seawater can efficiently modify the Sr isotopic ratio ofthe rocks Alternately the fluids could also be seawaterthat was modified through exchange with the surround-ing rocks during its transit through the oceanic crustalsection This modified seawater would be more enrichedin Srwith a 87Sr86Sr ratio intermediate between unmodi-fied seawater and MORBMinerals devoid of late LT alteration imprints (parga-
site green hornblende and pyroxene) and characterizedby very low Sr contents plot on the right of the diagramclose to or on the mixing line between seawater andMORB (Fig 10a) The difference between these mineralsand the other samples (WR and plagioclases) is mainlydue to the Sr fractionation coefficients of these differentminerals and to the large quantity of plagioclase inthe studied rocks The process responsible for the
JOURNAL OF PETROLOGY VOLUME 45 NUMBER 6 JUNE 2004
1202
modification of the Sr isotopic composition fromMORB-source value is however explained by the same phenom-enon As all these minerals display initial 87Sr86Sr ratiosdistinct from the MORB source and because they equili-brated at VHT or HT temperatures this supports anearly intervention of seawater during crystallization ofthese minerals Most of these samples with the exceptionof the epidosite dyke overlap the domains defined byKawahata et al (2001) for the Wadi Fizh gabbros alteredat VHT and HT conditions in the amphibolite facies(labelled lsquoVHTrsquo and lsquoHTrsquo in Fig 9a)Three samples (two epidosite dykes and an one epidote
fraction) have very high Sr contents and the highest Sr
isotopic compositions of the present dataset The twowhole-rock samples overlap the field of the Wadi Fizhsamples altered at high temperature during greenschist-facies metamorphism (Kawahata et al 2001) This typeof alteration is common at the ridge axis and formedthrough intensive exchange with seawater-derived fluidsFigure 10b indicates that all the Sr---O isotope data are
distinct from MORB compositions and suggests that thefluids reacting with the magmas could be derived fromseawater In addition the d18O values show that isotopicexchange occurred under VHT or HT conditions Thefluids could have the composition of seawater or theycould have the composition of seawater modified through
0703
0705
0707
001 01 1
1 Sr (ppm)
MORB
(87Sr
86Sr)
init
ial
Seawater
VHT
HT
MT36b
36b
36b
43a 128b90b
41d2
37b19d
15f
128b
41d2
41d2
19a
Ma2Ma2
19d
90b
37b
Whole RockPlagioclasePargasiteGreen HornblendeClinopyroxene
Epidote
(a)
0 2 4 6 8
δ18O(permil)
0703
0705
0707Seawater
MORB-mantle
128b 41d2
90b 41d2
Ma2128b
37b
43a
19d
90b
Whole RockPlagioclasePargasiteGreen HornblendeClinopyroxene
Epidote
HT LT
19d
(87Sr
86Sr)
init
ial
(b)
Fig 10 (a) Initial 87Sr86Sr isotopic ratios plotted against 1Sr for whole rocks and mineral separates from the gabbro section of the Omanophiolite Fields shown are from Kawahata et al (2001) and correspond to MT-----basalt sheeted dyke diabase altered at medium temperature(prehnite---actinolite facies sequence 3) HT-----diabase plagiogranite metagabbro and epidosite formed at high temperature (greenschist faciessequence 4) VHT-----gabbro altered at very high temperature (amphibolite facies sequence 5) The dashed line is a binary mixing line betweenMORB (Lanphere et al 1981) and Cretaceous seawater (Burke et al 1982) (b) Initial 87Sr86Sr isotopic composition plotted against d18O value forwhole rocks and minerals from the Oman ophiolite Cretaceous seawater and MORB end-members as in (a) Dashed line represents mixing curvebetweenMORB and seawater at high temperature (HT) Low-temperature (LT) exchanges between these two components would be responsible foran increase of the d18O primary MORB value
BOSCH et al HYDROTHERMAL CIRCULATION IN OMAN OPHIOLITE
1203
exchange with the surrounding rocks during its penetra-tion into the crustal section In an environment in whichfluid---rock interaction occurs at variable temperaturesthe d18O is greatly influenced by the temperature atwhich the exchange took place and by the waterrockratio It is evident from Figs 9b and 10b that none of thestudied gabbros have plagioclase and amphibole withMORB-like d18O and 87Sr86Sr values The separateminerals display a broad range of d18O values and mostexhibit extreme 18O depletion (Table 5) Hydrothermalexchange kinetics are similar in amphibole and pyroxeneand these two minerals are more resistant to oxygenexchange during water---rock interaction than coexistingplagioclase (Gregory et al 1989) The most probableexplanation of the low d18O values measured in bothamphiboles and pyroxenes is that they crystallized athigh temperature in the presence of a relatively low 18Oseawater-derived hydrothermal fluid [d18O from 01 to07 after Gregory amp Taylor (1981)] The anomalouslyhigh d18O value (thorn1009) measured for the plagioclase ofthe micro-gabbroic dyke (01 OA19d) can be interpretedas resulting from a superimposed overprint caused by alate-stage penetration of fluids at lower temperature Inthis sample the occurrence of such different 18O16Oratios between coexisting plagioclase and amphibole isnot consistent with equilibrium fractionation betweenthese two minerals at any possible temperature Thisobservation suggests that parts of the ophiolite sequencehave undergone a high-temperature hydrothermal eventprior to the later low-temperature event that producedthe strong 18O increase in coexisting plagioclaseThe depletion of the d18O values results from high-temperature hydrothermal alteration (Gregory amp Taylor1981 Stakes amp Taylor 1992) The oxygen isotopic datasupport the contention that most studied samples havebeen affected by a VHT---HT hydrothermal alteration byfluids derived from seawater Some of the analysed sam-ples presenting petrological evidence of crystallization ofLT secondary minerals have been overprinted by the latelow-temperature alteration thus swamping the earlierhigh-temperature alteration effects
Determination of waterrock ratio
To better evaluate the exchange between seawater andthe gabbroic rocks waterrock ratios (WR) have beenestimated for the whole rocks analysed during the courseof this study The different models used to constrain thisratio mainly differ by the calculation of the effective ratioor by the estimated weight ratio and by considering openor closed systems for water circulation (Albareede et al1981 McCulloch et al 1981) The WR ratios reportedin Table 5 have been calculated assuming a closed sys-tem Using this formulation these values are minimabecause the calculation always uses pristine fluid for the
initial fluid thus maximizing the value in the denomina-tor If the fluid was shifted to lower 87Sr concentrations byexchange with the rock on the downward path theinferred values would be much higher (Davis et al 2003)The calculated WR ratios for the samples from this
study range from 31 to 219 (Table 5) Two main groupsof samples can be recognized on the basis of their waterrock ratios The lowest ratios ranging from 31 to 47have been determined for massive gabbros or anatecticgabbros but also for dykes and veins devoid of late LTalteration Similar ratios have been previously calculatedfor example for the discharged hydrothermal solutions atthe 13N and 21N East Pacific Rise (Di Donato et al1990 Fouquet et al 1991) The gabbros from the Ibrasection in the Oman ophiolite gave effective WR ratiosof 05 33 and 42 (McCulloch et al 1981) in goodagreement with the values obtained in this study Theleast altered gabbro of the Ibra section yields a WR ratioof 05 in good agreement with the exceptionally fresh-ness of this rock characterized by typical mantle oxygenvalues for its minerals (McCulloch et al 1981) Samplescharacterized by waterrock effective ratios in the rangeof 3---5 can be related to penetrative alteration via thehydrothermal system Lecuyer amp Reynard (1996) havedemonstrated in oceanic gabbros from the Hess Deepaltered in similar HT conditions that WR mass ratios inthe range of 02---1 are sufficient to account for the mod-ified stable isotope signature of the gabbros Higherwaterrock values (WR 45) have been calculated forsamples affected by superimposed late hydrothermalalteration and for the epidosite samples (Table 5) Theyprobably acquired their isotopic characteristics throughinteraction with a larger volume of seawater duringgreenschist-facies metamorphism Similar or higherWR values have been published in previous studies(ie McCulloch et al 1981 Kawahata et al 2001)They correspond either to samples from the upper-crus-tal sequence (ie basalt sheeted dyke or plagiogranite) orto gabbros from the crustal section located in a particularenvironment such as for example the Wadi Fizh sectionthat is located close to a segment boundary along thepalaeo-spreading axis (Nicolas et al 2000)
Mechanism for seawater interaction withmagma
Nicolas et al (2003) have proposed that variable amountsof seawater can circulate at high temperature through-out the crustal section down to the walls of the magmachamber via a network of parallel microcracks a fractionof a millimetre wide Such microcracks and their relation-ship with the VHT---HT hydrous alteration in the sur-rounding gabbros have been described in detail byNicolas et al Via this network of microcracks largeamounts of seawater can have reacted with the magma
JOURNAL OF PETROLOGY VOLUME 45 NUMBER 6 JUNE 2004
1204
and induced variable changes of its Sr and O isotopiccomposition This regular network of parallel micro-cracks could constitute the recharge system and thehydrated gabbroic dykes the discharge system Physicalparameters and the geophysical models of Nicolas et al(2003) are in agreement with this scenario Such a pene-tration of seawater-derived hydrothermal fluids throughthe lower crust along grain boundaries and a microscopicfracture network at temperatures 4750C is consistentwith recent thermodynamic models (McCollom amp Shock1998)Seawater-altered and high-temperature recrystallized
diabases (protodykes) from the overlying sheeted dykecomplex stoped into the melt lenses could represent anindirect way for seawater to enter the system as they canremelt during their subsidence through the magmachamber A simple mass balance calculation suggeststhat this indirect contamination is not significant Assess-ments of the amount of recycled crust 1 in volumeby Nicolas et al (2000) based on the protodyke remnantsin the gabbro section or 20 by Coogan et al (2003)based on chlorine content in EPR basalts lead to a sea-waterrock ratio of the order of 104 to 103Thus following Nicolas et al (2003) we propose that
seawater has been introduced along microcracks down tothe walls of the magma chamber and has diffused alonggrain boundaries in the way described by Manning et al(2000) progressively invading the host gabbro Gabbroicdykes result from the injection of hydrous melt into thehost gabbros at falling temperatures as they drift awayfrom the magma chamber This water-saturated meltcould result from the extensive hydration of the crystal-lizing gabbros near the walls of the magma chamberwhen seawater penetrates through this wall (Fig 1)Similar plumbing systems driving seawater down to the
limit of the magma chamber and back to the surface havebeen already proposed in other environments such as theMid-Atlantic Ridge (Kelley amp Delaney 1987 Cooganet al 2001) the East Pacific Rise at Hess Deep (Gillis1995 Manning et al 1996a 1996b) the Tonga forearc(Banerjee amp Gillis 2001) and the Troodos ophiolite(Gillis amp Roberts 1999) The melting curve of wet basalthas a negative slope but is close to the dry solidus at lowpressure in the lower gabbros and the first melts areplagiogranitic (Spulber ampRutherford 1983) Such plagio-granites are ubiquitous in the highest gabbros and in theroot zone of sheeted dykes in Oman (Rothery 1983Juteau et al 1988 Nicolas amp Boudier 1991) in manyother ophiolites and in the oceanic crust where they havebeen interpreted as resulting either from wet anatexis ofhot gabbros or from the final product of crystallization ofa basaltic melt (Spulber amp Rutherford 1983) Plagio-granites are rare in the lower gabbros exceptionallyassociated with hydrous gabbro segregations inside thelayered gabbros Their constitutive minerals are also
remarkably absent along the VHTmicrocracks althoughthese gabbros were above the wet solidus A possibleexplanation has been proposed by McCollom amp Shock(1998) who wrote that at high temperature lsquoaqueousfluids may leave very little conspicuous petrological evid-ence for their presencersquo the reason being that thehigh-T conditions would be closer to batch meltingObviously more detailed studies on this specific problemare needed We conclude that the large degree of hydrousmelting locally required at the walls of the magma cham-ber to account for the generation of a gabbroic hydrousmelt may have erased the traces of the incipient plagio-granitic melt
CONCLUSIONS
Fostered by the recent discovery of high-temperatureseawater contamination in some gabbros of the Omanophiolite (Manning et al 2000) we have conducted astudy on 500 gabbro specimens from selected areas ofthe entire Samail ophiolite belt and on the hydrous gab-broic dykes that cross-cut them The highest-temperaturereactions (VHT) recorded in olivine and clinopyroxene ingabbros as orthopyroxene and pargasite coronas reach975C They are followed at lower HT conditions withreplacement of olivine by an assemblage of tremolitemagnetite and plagioclase surrounded by chlorite andby pargasite development in clinopyroxene followedfinally by the LT lizardite---olivine and saussurite---plagioclase greenschist-facies alteration With a fewexceptions the VHT alteration tends to be restricted tothe lower gabbros and to the vicinity of the Moho andthe HT alteration to the middle and upper gabbros Thepreservation of the vertical distribution of VHT---HTfacies indicates that progressive cooling during off-axisspreading did not obliterate this distribution and conse-quently that the VHT---HT hydrothermal alteration tookplace in a very restricted time interval at the limit of thecooling magma chamber In the deep and hot gabbrosthe main water channels may be sub-millimetre-sizedmicrocracks with a dominantly vertical attitude the ori-gin and dynamics of which have been investigated in aprevious paper (Nicolas et al 2003)Beyond these high-temperature alteration zones mas-
sively induced in the gabbros seawater may be respons-ible for the generation of clinopyroxene---pargasitegabbro dykes that cross-cut all gabbros and that areupsection clearly connected to the LT hydrothermalvein system Petrostructural evidence suggests that thesedykes were generated by hydration of the crystallizinggabbros near the internal wall of the LVZ---magmachamber The resultant melts were subsequently intrudedat various levels in the cooling lower and middle gabbrosPetrological observations of nearly all the gabbros ana-lysed in the present study located from the root zone of
BOSCH et al HYDROTHERMAL CIRCULATION IN OMAN OPHIOLITE
1205
the sheeted dyke complex down to the Moho emphasizethe imprint of VHT---HT hydrous alteration The VHThydrothermal event is characterized by temperatures ofequilibration around 900---1000C The temperatures atwhich the HT hydrous event occurred are estimated tobe in the range 550---900CStrontium and oxygen isotope compositions measured
on whole rocks and separated minerals significantlydepart from primary MORB values The Sr and O iso-tope data obtained for pargasite green hornblende andclinopyroxene record the participation of seawater at thetemperature of crystallization of the minerals Plagio-clases and whole rocks define a trend toward a compo-nent characterized by a high radiogenic 87Sr86Sr valueand a higher Sr content than normal seawater Seawateris clearly identified as the fluid responsible for the modi-fication of the Sr and O signatures for both massivegabbros and dykes and veins Nevertheless the high Srcontent of the extraneous component requires eitheropen-system exchange allowing penetration of a greaterquantity of seawater or penetration of seawater modifiedby interaction with the oceanic crust during its travel paththrough the crustal section The waterrock ratio calcu-lated on the basis of the Sr isotopes is between three andfive for the enclosing gabbros and for the least LT altereddykes The VHT---HT hydrothermal event affectingthese samples is related to penetrative alteration via therecharge and discharge hydrothermal system Thewaterrock ratio is 10 for dykes exhibiting evidenceof a LT hydrothermal alteration overprint and reaches22 for the epidosite dyke The depletion in d18O char-acterizes all the studied samples with the single exceptionof the micro-gabbroic dyke plagioclase This agreeswith the conclusions based on Sr isotopes suggestingthat these samples have undergone exchange with sea-water-derived fluids during a VHT---HTalteration hydro-thermal event
ACKNOWLEDGEMENTS
This work has benefited from financial support by theCentre National de la Recherche Scientifique (INSUInteerieur-Terre andDYETI programmes) and inOmanfrom the willingness of the Directorate of MineralsMinistry of Commerce and Industry and the hospitabil-ity of the people Technical help in the laboratory is alsoacknowledged including C Nevado for the thin sectionsE Ball for the illustrations B Galland for her assistancein the chemistry room and C Bassoulet for help duringthermal ionization mass spectrometric analyses of the Srresults from the leaching experiments Constructivereviews from R T Gregory C Lecuyer an anonymousreviewer and particularly from M Wilson greatlyhelped to improve an earlier version of the manuscript
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Gregory R T Criss R E amp Taylor H P (1989) Oxygen
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Ionov D A Savoyant L amp Dupuy C (1992) Application of the
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minerals Geostandards Newsletter 16 311---315
Javoy M (1980) 18O16O and DH ratios in high temperature
peridotites In Colloques Internationaux du CNRS 272 279---287
Juteau T Beurrier M Dahl R amp Nehlig P (1988) Segmentation
at a fossil spreading axis the plutonic sequence of the Wadi
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Juteau T Manacrsquoh G Moreau C Lecuyer C amp Ramboz C
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Kelemen P B Koga K amp Shimizu N (1997) Geochemistry of
gabbro sills in the crustmantle transition zone of the Oman
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and Planetary Science Letters 146 475---488Kelley D S amp Delaney J R (1987) Two-phase separation and
fracturing in mid-ocean ridge gabbros at temperatures greater than
700C Earth and Planetary Science Letters 83 53---66
Koepke J Feig S T Snow J amp Freise M (2003) Petrogenesis of
oceanic plagiogranites by partial melting of gabbros an experi-
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wwwspringerlinkcomopenurlaspgenre=s00410-003-0511-9
Kyser T K (1986) Stable isotope variations in the mantle In
Valley J W Taylor H P amp OrsquoNeil J R (eds) Stable Isotopes in
High Temperature Geological Processes Mineralogical Society of America
Reviews in Mineralogy 16 141---164
Lanphere M A (1981) K---Ar ages of metamorphic rocks at the base
of the Samail ophiolite Oman Journal of Geophysical Research 86
2777---2782
Lanphere M A Coleman R G amp Hopson C A (1981) Sr isotopic
tracer study of the Samail ophiolite Oman Journal of Geophysical
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Leake B E (1997) Nomenclature of amphiboles report of the
subcommittee on amphiboles of the International Mineralogical
Association commission on new minerals and mineral names
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Lecuyer C amp Reynard B (1996) High-temperature alteration of
oceanic gabbros by seawater (Hess Deep Ocean Drilling Program
Leg 147) evidence from oxygen isotopes and elemental fluxes
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Liou J G Kuniiyoshi S amp Ito K (1974) Experimental study of the
phase relations between greenschist and amphibolite in a basaltic
system American Journal of Science 274 613---632
MacLeod C J amp Rothery D A (1992) Ridge axial segmentation in
the Oman ophiolite evidence from along-strike variations in the
sheeted dyke complex Geological Society of America Special Papers 60
39---63
Manning C E MacLeod C J amp Weston P E (1996a) Fracture-
controlled metamorphism of Hess Deep gabbros site 984
constraints on the roots of mid-ocean ridge hydrothermal systems
at fast-spreading centers In GillisM C Allan KM ampMeyer P S
(eds) Proceedings of the Ocean Drilling Program Scientific Results 147
College Station TX Ocean Drilling Program pp 189---211Manning C E Weston P E amp Mahon K I (1996b) Rapid high-
temperature metamorphism of East Pacific Rise gabbros from Hess
Deep Earth and Planetary Science Letters 144 123---132Manning C E MacLeod C J amp Weston P E (2000) Lower-crustal
cracking front at fast-spreading ridges evidence from the East Pacific
Rise and the Oman ophiolite Journal of the Geological Society London
349 261---272McCollom T M amp Shock E L (1998) Fluid---rock interactions in
the lower oceanic crust thermodynamic models of hydrothermal
alteration Journal of Geophysical Research 103 547---575
McCulloch M T Gregory R T Wasserburg G J amp Taylor H P
(1981) Sm---Nd Rb---Sr and 18O16O isotopic systematics in an
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Proceedings of the Ocean Drilling Program Scientific Results 147 College
Station TX Ocean Drilling Program
Nehlig P amp Juteau T (1988) Flow porosities permeabilities and
preliminary data on fluid inclusions and fossil thermal gradients in
the crustal sequence of the Sumail ophiolite (Oman) Tectonophysics
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1207
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Geochemical Society Special Publication 3 77---90
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New York McGraw---Hill 694 pp
Wilcock W S D amp Delaney J R (1996) Mid-ocean ridge
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49---64
Witt G amp Seck H A (1989) Origin of amphibole in recrystallized
and porphyroclastic mantle xenoliths from Rhenish massif implica-
tions for the nature of mantle metasomatism Earth and Planetary
Science Letters 91 327---340
JOURNAL OF PETROLOGY VOLUME 45 NUMBER 6 JUNE 2004
1208
calculated temperatures between 4900C and 750Ccorresponds to the changing composition of amphibolesfrom pargasite to magnesio-hornblende at the scale of athin section or even at that of an individual crystal asshown by their correlation with AlIV such variabilityrecords rapid cooling in hydrous conditions The highesttemperatures are recorded by pargasite from the anatec-tic gabbro 01 OA41d2 in the gabbro dyke 01 OA19dand in the dioritic dyke 02 OA90b They were calculatedfrom minerals devoid of lower-grade alteration and arethus considered as representing the equilibrium temperat-ure of the coexisting minerals Lower temperatures cor-respond to mineral phases evolving at falling temperatureand are only indicative of the cooling history Includingthe 40C uncertainty seven pairs provide temperatureshigher than 900C the limit of validity of the Holland ampBlundy thermometer However we maintain these tem-peratures as indicative of the highest values of our data-set These values are included in the average equilibriumtemperature summarized in Table 2
DISCUSSION
Distribution of hydrothermal alteration
Two distinct petrological events are recorded in the gab-bro section of the Samail ophiolite and are documentedin the present contribution (1) the whole gabbro sectionis affected by a penetrative alteration (2) the gabbrosection is crosscut by various types of dykes and veinsincluding hydrous minerals
Penetrative alteration
The penetrative alteration developed in the gabbros atgrain boundaries and locally invading the minerals isubiquitous in the gabbro section This penetrative altera-tion has been described as very high temperature (VHT)high temperature (HT) and low temperature (LT) basedon alteration parageneses The orthopyroxene coronas inthe layered gabbro sample and the incipient appearanceof brown pargasitic amphibole mark a lower thermallimit of the VHT alteration By reference to the experi-mental work of Koepke et al (2003) the temperature ofequilibration for these reactions is estimated to bebetween 900 and 1000CThe preservation of the vertical distribution of
VHT---HT facies (Figs 4 and 5) also pointed out byManning et al (2000) indicates that the hydrothermalalteration pattern fits the distribution of isotherms withdepth at a fast-spreading ridge (ie Phipps Morgan ampChen 1993 Dunn et al 2000 Fig 2) Equilibrium tem-peratures for VHT parageneses as high as 900---1000Csuggest that VHT---HT hydrothermal alteration tookplace in a very restricted time interval at the limit ofthe cooling magma chamber It is proposed that the
penetrative alteration affecting the entire gabbro sectionis related to a pervasive network of microcracks whichact as a recharge system down to the Moho (Nicolas et al2003)
Dykes and veins
The dykes are extremely varied eg pegmatitic gabbroswith comb-texture microgabbros and amphibolitizeddolerites dioritic dykes alignments of poikilitic horn-blende and finally diffuse hornblende-bearing patchesHigh-T gabbro and diorite dykes and low-T greenamphibole---epidote veins are connectedIn the dykes textural evidence attests to suprasolidus
conditions at least for the development of the pargasiticamphibole Integrating the hornblende---plagioclase equi-libration temperatures calculated for the studied samplesthese temperatures are bracketed between 950C and875CIn the following discussion we shall consider separately
the microgabbro and dolerite dykes The latter representbasaltic melt intrusions associated upsection with thediabase sheeted dykes and downsection possibly withthe mafic dykes that are ubiquitous in the mantle section(Nicolas et al 2000) Whatever their origin the develop-ment of pargasitic amphibole during their late stage ofcrystallization creates a link with the gabbroic dykes andsuggests a crystallization process evolving from dry con-ditions to more hydrous conditionsThe other gabbroic dykes comb-textured gabbros and
hornblende-bearing dioritic dykes result from the fillingof cracks by a fluid either a melt or a silica-rich hydrousfluid These intrusive dykes which develop reaction mar-gins at their contact with the host gabbro grade verticallyinto lower-temperature green amphibole veins or align-ments of poikilitic hornblende in the layered gabbromatrix or development of pargasitic hornblende in ana-tectic gabbros (sample 01 OA41d2) In gabbros crystal-lizing in the magma chamber the development oforthopyroxene and pargasite oikocrysts enclosing the pri-mary minerals suggests a thermal evolution from thegabbro solidus to amphibolite-facies conditions Forthese reasons it is suggested that the gabbroic dykesresulted from the injection of a hydrous melt in the stillhot gabbros drifting away from the magma chamber asdiscussed below
Origin of fluids responsible for VHT---HThydrothermal alteration
The 87Sr86Sr initial ratios and d18O of whole rocks andpure mineral fractions are plotted in Fig 9 against thedistance above the Moho Transition Zone (MTZ) Theinitial Sr isotopic composition of the whole rocks apartfrom the epidosite dykes ranges from 070322 to
JOURNAL OF PETROLOGY VOLUME 45 NUMBER 6 JUNE 2004
1200
070458 All the studied samples (including whole rocksand minerals) have initial 87Sr86Sr ratios falling outsidethe field of fresh MORB which commonly have Sr ratiosbetween 07023 and 07029 with an average of 070265(ie Hart et al 1974) The lowest initial Sr isotopiccomposition (070322) from the massive gabbro 94MA2 is slightly but significantly higher than averageOman primary magmatic signatures (ie 070296McCulloch et al 1981) Similarly the d18O values ofmost whole rocks and minerals measured (Table 5)
depart from typical MORB-source mantle values( Javoy 1980 Gregory amp Taylor 1981 Kyser 1986)These differences indicate that the primary mantlesignatures in the Oman ophiolite have been modifiedby interaction with a fluid Possible origins for thisfluid are mantle components dehydration of subductedlithosphere or seawater circulation Strontium andoxygen isotopes are useful tracers for fluid---crust interac-tion as d18O and 87Sr86Sr ratios from fluids originat-ing from seawater the mantle or subducted lithosphere
-1000
1000
2000
3000
4000
5000
6000
7000
MOHO
MTZ
Dis
tan
ce a
bov
e th
e M
oh
o (
m)
pillow-basalt
sheeted-dike
gabbro
peridotite
01 OA 41d2
01 OA 36b
02 OA 43a97 OA 128b
01 OA15f
94 OA MA2
02 OA 90b01 OA 19ad
02 OA 37b
0702 0703 0704 0705 0706 0707
87Sr86Srinitial
MO
RB-s
ou
rce
Seaw
ater
Whole RockPlagioclasePargasiteGreen HornblendeClinopyroxene
Epidote
(a)
-1000
1000
2000
3000
4000
5000
6000
7000
MOHO
MTZ
Dis
tan
ce a
bov
e th
e M
oh
o (
m)
pillow-basalt
sheeted-dike
gabbro
peridotite
01 OA 41d2
01 OA 36b
02 OA 43a97 OA 128b
01 OA15f
94 OA MA2
02 OA 90b01 OA 19ad
02 OA 37b
Whole RockPlagioclasePargasiteGreen HornblendeClinopyroxene
3 5 10
δ18O (permil)
41d2
Ma2
43a
90b 19d 19d37b
128b
90b
(b)
Fig 9 (a) Variation of initial 87Sr86Sr isotopic composition as a function of the location of the studied samples in a schematic log of the crustalsection of the Oman ophiolite The field for MORB is from Hart et al (1974) and that for seawater is from Burke et al (1982) Small open squares aredata fromKawahata et al (2001) (b) Variation of d18O () as a function of the location of the studied samples in a schematic log of the crustal sectionof the Oman ophiolite Shaded curve corresponds to the data of Gregory amp Taylor (1981)
BOSCH et al HYDROTHERMAL CIRCULATION IN OMAN OPHIOLITE
1201
are significantly different Moreover the d18O valueis dependent on temperature and mineral fractiona-tion and thus yields complementary constraints to Srisotopes
Mantle origin
One model to explain the occurrence of fluids interactingwith the oceanic crust at very high temperatures is toderive them from the mantle The contribution of mantle-derived hydrous fluids linked to percolation processesor to their concentration in the final stages of gabbrocrystallization has been invoked in numerous case studies(ie Witt amp Seck 1989 Fabriees et al 1989 Dupuy et al1991 Agrinier et al 1993) O and Sr isotopic data forwhole rocks and mineral separates yield scattered valuesthat are unambiguously different from MORB mantle-source values (Fig 9a and b) With the exception of lateLT altered samples the WR data have a mean 87Sr86Srratio of 070337 and a standard deviation of 000012These surprisingly high values in mantle-derived rockscould be taken as evidence for survival of mantle isotopicheterogeneities during crystallization of the ophiolite (egLanphere 1981 McCulloch et al 1981) However the18O16O isotope ratios in the Oman gabbroic sequenceare also displaced from primary magmatic values Nogabbro in the present study has plagioclase or amphi-bole with typical mantle d18O values Thus the Sr and Oisotope data rule out the possibility of mantle-derivedfluids as a possible source for the VHT---HT hydrousalteration event recorded in the massive gabbros andgabbroic dykes and veins (Fig 10a and b)
Subducted lithosphere origin
Dehydration of subducted oceanic crust in subductionzones can generate hydrous fluids Such fluids couldpercolate through the overlying lithosphere and contam-inate it thus generating modified isotopic signatures andtrace element contents Such an explanation howeverdisagrees with the geochemical characteristics of most ofthe studied samples Fluids derived from the dehydrationof subducted slabs (fresh or altered MORB or OIB pro-toliths) should be strongly enriched in K Rb and Ba (egBecker et al 2000) whereas in Oman the Rb contentsmeasured in the gabbros and gabbroic dykes and veins arestrongly depleted Moreover the isotopic composition ofslab-derived fluids is buffered by the MORB-like oceaniccrustal protolith for Nd and Pb but not for Sr and O(Staudigel et al 1995) As a result of relatively minorfractionation of oxygen at high temperature the oxygenisotope composition of the fluids expelled during dehy-dration of the subducted slab should be high and essen-tially controlled by the oxygen composition of the uppersection of the crust altered at low temperature (with an
average of 10 and perhaps as high as 19) (Staudigelet al 1995) The Sr isotopic composition should be alsohigh (average 07046) as the fluids released by the dehy-dration of subducted oceanic crust are associated eitherwith seawater or with radiogenic Sr phases (ie smectites)The Sr and O isotopic compositions of the studied sam-ples do not fit with these signatures and so preclude asubducted lithosphere origin for the fluids involved in theVHT---HT alteration event
Seawater-derived fluids
All the analysed samples (minerals and WR) have 87Sr86Sr(T) outside the domain of primary MORB magmasand variable Sr contents Individual minerals have Srcontents reflecting their different mineralliquid partitioncoefficients Minerals characterized by a very high affi-nity for Sr such as plagioclase (ie Sr mineralliquidpartition coefficients 1) have very high Sr contentsTherefore the high abundance of plagioclase in the sam-ples analysed is responsible for the high Sr content of thestudied whole rocks The Sr content of all the whole rocksis relatively homogeneous (Table 5) without clear distinc-tion between country-rock gabbros and dykes and veinsand ranges from 110 to 200 ppm except for the epidosite(4700 ppm) Reported on a 87Sr86Sr vs 1Sr diagram(Fig 10a) most whole rocks and plagioclases analysed arecharacterized by a 1Sr ratio lower than 001 and plot inthe left part of this diagram Compared with N-MORBthe studied massive and anatectic gabbros and their con-stituent plagioclases have similar to slightly higher Srcontents but substantially higher 87Sr86Sr ratios(Fig 10a) Considering Cretaceous seawater as a possiblehigh 87Sr86Sr mixing component [87Sr86Sr 07073at 90Ma after Burke et al (1982)] responsible for theobserved geochemical modifications exchanges cannothave occurred in a closed system as seawater has too lowa Sr content (ie 8 ppm de Villiers 1999) An open-system process allowing penetration of larger quantitiesof seawater can efficiently modify the Sr isotopic ratio ofthe rocks Alternately the fluids could also be seawaterthat was modified through exchange with the surround-ing rocks during its transit through the oceanic crustalsection This modified seawater would be more enrichedin Srwith a 87Sr86Sr ratio intermediate between unmodi-fied seawater and MORBMinerals devoid of late LT alteration imprints (parga-
site green hornblende and pyroxene) and characterizedby very low Sr contents plot on the right of the diagramclose to or on the mixing line between seawater andMORB (Fig 10a) The difference between these mineralsand the other samples (WR and plagioclases) is mainlydue to the Sr fractionation coefficients of these differentminerals and to the large quantity of plagioclase inthe studied rocks The process responsible for the
JOURNAL OF PETROLOGY VOLUME 45 NUMBER 6 JUNE 2004
1202
modification of the Sr isotopic composition fromMORB-source value is however explained by the same phenom-enon As all these minerals display initial 87Sr86Sr ratiosdistinct from the MORB source and because they equili-brated at VHT or HT temperatures this supports anearly intervention of seawater during crystallization ofthese minerals Most of these samples with the exceptionof the epidosite dyke overlap the domains defined byKawahata et al (2001) for the Wadi Fizh gabbros alteredat VHT and HT conditions in the amphibolite facies(labelled lsquoVHTrsquo and lsquoHTrsquo in Fig 9a)Three samples (two epidosite dykes and an one epidote
fraction) have very high Sr contents and the highest Sr
isotopic compositions of the present dataset The twowhole-rock samples overlap the field of the Wadi Fizhsamples altered at high temperature during greenschist-facies metamorphism (Kawahata et al 2001) This typeof alteration is common at the ridge axis and formedthrough intensive exchange with seawater-derived fluidsFigure 10b indicates that all the Sr---O isotope data are
distinct from MORB compositions and suggests that thefluids reacting with the magmas could be derived fromseawater In addition the d18O values show that isotopicexchange occurred under VHT or HT conditions Thefluids could have the composition of seawater or theycould have the composition of seawater modified through
0703
0705
0707
001 01 1
1 Sr (ppm)
MORB
(87Sr
86Sr)
init
ial
Seawater
VHT
HT
MT36b
36b
36b
43a 128b90b
41d2
37b19d
15f
128b
41d2
41d2
19a
Ma2Ma2
19d
90b
37b
Whole RockPlagioclasePargasiteGreen HornblendeClinopyroxene
Epidote
(a)
0 2 4 6 8
δ18O(permil)
0703
0705
0707Seawater
MORB-mantle
128b 41d2
90b 41d2
Ma2128b
37b
43a
19d
90b
Whole RockPlagioclasePargasiteGreen HornblendeClinopyroxene
Epidote
HT LT
19d
(87Sr
86Sr)
init
ial
(b)
Fig 10 (a) Initial 87Sr86Sr isotopic ratios plotted against 1Sr for whole rocks and mineral separates from the gabbro section of the Omanophiolite Fields shown are from Kawahata et al (2001) and correspond to MT-----basalt sheeted dyke diabase altered at medium temperature(prehnite---actinolite facies sequence 3) HT-----diabase plagiogranite metagabbro and epidosite formed at high temperature (greenschist faciessequence 4) VHT-----gabbro altered at very high temperature (amphibolite facies sequence 5) The dashed line is a binary mixing line betweenMORB (Lanphere et al 1981) and Cretaceous seawater (Burke et al 1982) (b) Initial 87Sr86Sr isotopic composition plotted against d18O value forwhole rocks and minerals from the Oman ophiolite Cretaceous seawater and MORB end-members as in (a) Dashed line represents mixing curvebetweenMORB and seawater at high temperature (HT) Low-temperature (LT) exchanges between these two components would be responsible foran increase of the d18O primary MORB value
BOSCH et al HYDROTHERMAL CIRCULATION IN OMAN OPHIOLITE
1203
exchange with the surrounding rocks during its penetra-tion into the crustal section In an environment in whichfluid---rock interaction occurs at variable temperaturesthe d18O is greatly influenced by the temperature atwhich the exchange took place and by the waterrockratio It is evident from Figs 9b and 10b that none of thestudied gabbros have plagioclase and amphibole withMORB-like d18O and 87Sr86Sr values The separateminerals display a broad range of d18O values and mostexhibit extreme 18O depletion (Table 5) Hydrothermalexchange kinetics are similar in amphibole and pyroxeneand these two minerals are more resistant to oxygenexchange during water---rock interaction than coexistingplagioclase (Gregory et al 1989) The most probableexplanation of the low d18O values measured in bothamphiboles and pyroxenes is that they crystallized athigh temperature in the presence of a relatively low 18Oseawater-derived hydrothermal fluid [d18O from 01 to07 after Gregory amp Taylor (1981)] The anomalouslyhigh d18O value (thorn1009) measured for the plagioclase ofthe micro-gabbroic dyke (01 OA19d) can be interpretedas resulting from a superimposed overprint caused by alate-stage penetration of fluids at lower temperature Inthis sample the occurrence of such different 18O16Oratios between coexisting plagioclase and amphibole isnot consistent with equilibrium fractionation betweenthese two minerals at any possible temperature Thisobservation suggests that parts of the ophiolite sequencehave undergone a high-temperature hydrothermal eventprior to the later low-temperature event that producedthe strong 18O increase in coexisting plagioclaseThe depletion of the d18O values results from high-temperature hydrothermal alteration (Gregory amp Taylor1981 Stakes amp Taylor 1992) The oxygen isotopic datasupport the contention that most studied samples havebeen affected by a VHT---HT hydrothermal alteration byfluids derived from seawater Some of the analysed sam-ples presenting petrological evidence of crystallization ofLT secondary minerals have been overprinted by the latelow-temperature alteration thus swamping the earlierhigh-temperature alteration effects
Determination of waterrock ratio
To better evaluate the exchange between seawater andthe gabbroic rocks waterrock ratios (WR) have beenestimated for the whole rocks analysed during the courseof this study The different models used to constrain thisratio mainly differ by the calculation of the effective ratioor by the estimated weight ratio and by considering openor closed systems for water circulation (Albareede et al1981 McCulloch et al 1981) The WR ratios reportedin Table 5 have been calculated assuming a closed sys-tem Using this formulation these values are minimabecause the calculation always uses pristine fluid for the
initial fluid thus maximizing the value in the denomina-tor If the fluid was shifted to lower 87Sr concentrations byexchange with the rock on the downward path theinferred values would be much higher (Davis et al 2003)The calculated WR ratios for the samples from this
study range from 31 to 219 (Table 5) Two main groupsof samples can be recognized on the basis of their waterrock ratios The lowest ratios ranging from 31 to 47have been determined for massive gabbros or anatecticgabbros but also for dykes and veins devoid of late LTalteration Similar ratios have been previously calculatedfor example for the discharged hydrothermal solutions atthe 13N and 21N East Pacific Rise (Di Donato et al1990 Fouquet et al 1991) The gabbros from the Ibrasection in the Oman ophiolite gave effective WR ratiosof 05 33 and 42 (McCulloch et al 1981) in goodagreement with the values obtained in this study Theleast altered gabbro of the Ibra section yields a WR ratioof 05 in good agreement with the exceptionally fresh-ness of this rock characterized by typical mantle oxygenvalues for its minerals (McCulloch et al 1981) Samplescharacterized by waterrock effective ratios in the rangeof 3---5 can be related to penetrative alteration via thehydrothermal system Lecuyer amp Reynard (1996) havedemonstrated in oceanic gabbros from the Hess Deepaltered in similar HT conditions that WR mass ratios inthe range of 02---1 are sufficient to account for the mod-ified stable isotope signature of the gabbros Higherwaterrock values (WR 45) have been calculated forsamples affected by superimposed late hydrothermalalteration and for the epidosite samples (Table 5) Theyprobably acquired their isotopic characteristics throughinteraction with a larger volume of seawater duringgreenschist-facies metamorphism Similar or higherWR values have been published in previous studies(ie McCulloch et al 1981 Kawahata et al 2001)They correspond either to samples from the upper-crus-tal sequence (ie basalt sheeted dyke or plagiogranite) orto gabbros from the crustal section located in a particularenvironment such as for example the Wadi Fizh sectionthat is located close to a segment boundary along thepalaeo-spreading axis (Nicolas et al 2000)
Mechanism for seawater interaction withmagma
Nicolas et al (2003) have proposed that variable amountsof seawater can circulate at high temperature through-out the crustal section down to the walls of the magmachamber via a network of parallel microcracks a fractionof a millimetre wide Such microcracks and their relation-ship with the VHT---HT hydrous alteration in the sur-rounding gabbros have been described in detail byNicolas et al Via this network of microcracks largeamounts of seawater can have reacted with the magma
JOURNAL OF PETROLOGY VOLUME 45 NUMBER 6 JUNE 2004
1204
and induced variable changes of its Sr and O isotopiccomposition This regular network of parallel micro-cracks could constitute the recharge system and thehydrated gabbroic dykes the discharge system Physicalparameters and the geophysical models of Nicolas et al(2003) are in agreement with this scenario Such a pene-tration of seawater-derived hydrothermal fluids throughthe lower crust along grain boundaries and a microscopicfracture network at temperatures 4750C is consistentwith recent thermodynamic models (McCollom amp Shock1998)Seawater-altered and high-temperature recrystallized
diabases (protodykes) from the overlying sheeted dykecomplex stoped into the melt lenses could represent anindirect way for seawater to enter the system as they canremelt during their subsidence through the magmachamber A simple mass balance calculation suggeststhat this indirect contamination is not significant Assess-ments of the amount of recycled crust 1 in volumeby Nicolas et al (2000) based on the protodyke remnantsin the gabbro section or 20 by Coogan et al (2003)based on chlorine content in EPR basalts lead to a sea-waterrock ratio of the order of 104 to 103Thus following Nicolas et al (2003) we propose that
seawater has been introduced along microcracks down tothe walls of the magma chamber and has diffused alonggrain boundaries in the way described by Manning et al(2000) progressively invading the host gabbro Gabbroicdykes result from the injection of hydrous melt into thehost gabbros at falling temperatures as they drift awayfrom the magma chamber This water-saturated meltcould result from the extensive hydration of the crystal-lizing gabbros near the walls of the magma chamberwhen seawater penetrates through this wall (Fig 1)Similar plumbing systems driving seawater down to the
limit of the magma chamber and back to the surface havebeen already proposed in other environments such as theMid-Atlantic Ridge (Kelley amp Delaney 1987 Cooganet al 2001) the East Pacific Rise at Hess Deep (Gillis1995 Manning et al 1996a 1996b) the Tonga forearc(Banerjee amp Gillis 2001) and the Troodos ophiolite(Gillis amp Roberts 1999) The melting curve of wet basalthas a negative slope but is close to the dry solidus at lowpressure in the lower gabbros and the first melts areplagiogranitic (Spulber ampRutherford 1983) Such plagio-granites are ubiquitous in the highest gabbros and in theroot zone of sheeted dykes in Oman (Rothery 1983Juteau et al 1988 Nicolas amp Boudier 1991) in manyother ophiolites and in the oceanic crust where they havebeen interpreted as resulting either from wet anatexis ofhot gabbros or from the final product of crystallization ofa basaltic melt (Spulber amp Rutherford 1983) Plagio-granites are rare in the lower gabbros exceptionallyassociated with hydrous gabbro segregations inside thelayered gabbros Their constitutive minerals are also
remarkably absent along the VHTmicrocracks althoughthese gabbros were above the wet solidus A possibleexplanation has been proposed by McCollom amp Shock(1998) who wrote that at high temperature lsquoaqueousfluids may leave very little conspicuous petrological evid-ence for their presencersquo the reason being that thehigh-T conditions would be closer to batch meltingObviously more detailed studies on this specific problemare needed We conclude that the large degree of hydrousmelting locally required at the walls of the magma cham-ber to account for the generation of a gabbroic hydrousmelt may have erased the traces of the incipient plagio-granitic melt
CONCLUSIONS
Fostered by the recent discovery of high-temperatureseawater contamination in some gabbros of the Omanophiolite (Manning et al 2000) we have conducted astudy on 500 gabbro specimens from selected areas ofthe entire Samail ophiolite belt and on the hydrous gab-broic dykes that cross-cut them The highest-temperaturereactions (VHT) recorded in olivine and clinopyroxene ingabbros as orthopyroxene and pargasite coronas reach975C They are followed at lower HT conditions withreplacement of olivine by an assemblage of tremolitemagnetite and plagioclase surrounded by chlorite andby pargasite development in clinopyroxene followedfinally by the LT lizardite---olivine and saussurite---plagioclase greenschist-facies alteration With a fewexceptions the VHT alteration tends to be restricted tothe lower gabbros and to the vicinity of the Moho andthe HT alteration to the middle and upper gabbros Thepreservation of the vertical distribution of VHT---HTfacies indicates that progressive cooling during off-axisspreading did not obliterate this distribution and conse-quently that the VHT---HT hydrothermal alteration tookplace in a very restricted time interval at the limit of thecooling magma chamber In the deep and hot gabbrosthe main water channels may be sub-millimetre-sizedmicrocracks with a dominantly vertical attitude the ori-gin and dynamics of which have been investigated in aprevious paper (Nicolas et al 2003)Beyond these high-temperature alteration zones mas-
sively induced in the gabbros seawater may be respons-ible for the generation of clinopyroxene---pargasitegabbro dykes that cross-cut all gabbros and that areupsection clearly connected to the LT hydrothermalvein system Petrostructural evidence suggests that thesedykes were generated by hydration of the crystallizinggabbros near the internal wall of the LVZ---magmachamber The resultant melts were subsequently intrudedat various levels in the cooling lower and middle gabbrosPetrological observations of nearly all the gabbros ana-lysed in the present study located from the root zone of
BOSCH et al HYDROTHERMAL CIRCULATION IN OMAN OPHIOLITE
1205
the sheeted dyke complex down to the Moho emphasizethe imprint of VHT---HT hydrous alteration The VHThydrothermal event is characterized by temperatures ofequilibration around 900---1000C The temperatures atwhich the HT hydrous event occurred are estimated tobe in the range 550---900CStrontium and oxygen isotope compositions measured
on whole rocks and separated minerals significantlydepart from primary MORB values The Sr and O iso-tope data obtained for pargasite green hornblende andclinopyroxene record the participation of seawater at thetemperature of crystallization of the minerals Plagio-clases and whole rocks define a trend toward a compo-nent characterized by a high radiogenic 87Sr86Sr valueand a higher Sr content than normal seawater Seawateris clearly identified as the fluid responsible for the modi-fication of the Sr and O signatures for both massivegabbros and dykes and veins Nevertheless the high Srcontent of the extraneous component requires eitheropen-system exchange allowing penetration of a greaterquantity of seawater or penetration of seawater modifiedby interaction with the oceanic crust during its travel paththrough the crustal section The waterrock ratio calcu-lated on the basis of the Sr isotopes is between three andfive for the enclosing gabbros and for the least LT altereddykes The VHT---HT hydrothermal event affectingthese samples is related to penetrative alteration via therecharge and discharge hydrothermal system Thewaterrock ratio is 10 for dykes exhibiting evidenceof a LT hydrothermal alteration overprint and reaches22 for the epidosite dyke The depletion in d18O char-acterizes all the studied samples with the single exceptionof the micro-gabbroic dyke plagioclase This agreeswith the conclusions based on Sr isotopes suggestingthat these samples have undergone exchange with sea-water-derived fluids during a VHT---HTalteration hydro-thermal event
ACKNOWLEDGEMENTS
This work has benefited from financial support by theCentre National de la Recherche Scientifique (INSUInteerieur-Terre andDYETI programmes) and inOmanfrom the willingness of the Directorate of MineralsMinistry of Commerce and Industry and the hospitabil-ity of the people Technical help in the laboratory is alsoacknowledged including C Nevado for the thin sectionsE Ball for the illustrations B Galland for her assistancein the chemistry room and C Bassoulet for help duringthermal ionization mass spectrometric analyses of the Srresults from the leaching experiments Constructivereviews from R T Gregory C Lecuyer an anonymousreviewer and particularly from M Wilson greatlyhelped to improve an earlier version of the manuscript
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controlled metamorphism of Hess Deep gabbros site 984
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349 261---272McCollom T M amp Shock E L (1998) Fluid---rock interactions in
the lower oceanic crust thermodynamic models of hydrothermal
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BOSCH et al HYDROTHERMAL CIRCULATION IN OMAN OPHIOLITE
1207
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Research on line first httpwwwaguorgpubscurrentjb 108(B8)
2372
Pallister J S amp Hopson C A (1981) Samail ophiolite plutonic suite
field relations phase variation cryptic variation and layering and a
model of a spreading ridge magma chamber Journal of Geophysical
Research 86 2593---2644Phipps Morgan J amp Chen J Y (1993) Dependence of ridge-axis
morphology on magma supply and spreading rate Nature 364
706---708
Pin C Briot D Bassin C amp Poitrasson F (1994) Concomitant
extraction of Sr and Sm---Nd for isotopic analysis in silicate samples
based on specific extraction chromatography Analytica Chimica Acta
298 209---217
Rothery D A (1983) The base of a sheeted dyke complex Oman
ophiolite implications for magma chambers at oceanic spreading
axes Journal of the Geological Society London 140 287---296
Spear F S (1981) An experimental study of hornblende stability and
compositional variability in amphibolite American Journal of Science
281 697---734
Spulber S D amp Rutherford M J (1983) The origin of rhyolite and
plagiogranite in oceanic crust an experimental study Journal of
Petrology 24 1---25Stakes D S (1991) Oxygen and hydrogen isotope compositions of
oceanic plutonic rocks high-temperature deformation and meta-
morphism of oceanic layer 3 In Taylor H P OrsquoNeil J et al (eds)
Geochemical Society Special Publication 3 77---90
Stakes D S amp Taylor H P (1992) The northern Samail ophiolite an
oxygen isotope microprobe and field study Journal of Geophysical
Research 97(B5) 7043---7080Staudigel H Davies G R Hart S R Marchant M amp Smith B
M (1995) Large scale isotopic Sr Nd and O isotopic anatomy of
altered oceanic crust DSDPODP sites 417418 Earth and Planetary
Science Letters 130 169---185Turner F J amp Verhoogen J (1960) Igneous and Metamorphic Petrology
New York McGraw---Hill 694 pp
Wilcock W S D amp Delaney J R (1996) Mid-ocean ridge
sulfide deposits evidence for heat extraction from magma
chambers or cracking fronts Earth and Planetary Science Letters 145
49---64
Witt G amp Seck H A (1989) Origin of amphibole in recrystallized
and porphyroclastic mantle xenoliths from Rhenish massif implica-
tions for the nature of mantle metasomatism Earth and Planetary
Science Letters 91 327---340
JOURNAL OF PETROLOGY VOLUME 45 NUMBER 6 JUNE 2004
1208
070458 All the studied samples (including whole rocksand minerals) have initial 87Sr86Sr ratios falling outsidethe field of fresh MORB which commonly have Sr ratiosbetween 07023 and 07029 with an average of 070265(ie Hart et al 1974) The lowest initial Sr isotopiccomposition (070322) from the massive gabbro 94MA2 is slightly but significantly higher than averageOman primary magmatic signatures (ie 070296McCulloch et al 1981) Similarly the d18O values ofmost whole rocks and minerals measured (Table 5)
depart from typical MORB-source mantle values( Javoy 1980 Gregory amp Taylor 1981 Kyser 1986)These differences indicate that the primary mantlesignatures in the Oman ophiolite have been modifiedby interaction with a fluid Possible origins for thisfluid are mantle components dehydration of subductedlithosphere or seawater circulation Strontium andoxygen isotopes are useful tracers for fluid---crust interac-tion as d18O and 87Sr86Sr ratios from fluids originat-ing from seawater the mantle or subducted lithosphere
-1000
1000
2000
3000
4000
5000
6000
7000
MOHO
MTZ
Dis
tan
ce a
bov
e th
e M
oh
o (
m)
pillow-basalt
sheeted-dike
gabbro
peridotite
01 OA 41d2
01 OA 36b
02 OA 43a97 OA 128b
01 OA15f
94 OA MA2
02 OA 90b01 OA 19ad
02 OA 37b
0702 0703 0704 0705 0706 0707
87Sr86Srinitial
MO
RB-s
ou
rce
Seaw
ater
Whole RockPlagioclasePargasiteGreen HornblendeClinopyroxene
Epidote
(a)
-1000
1000
2000
3000
4000
5000
6000
7000
MOHO
MTZ
Dis
tan
ce a
bov
e th
e M
oh
o (
m)
pillow-basalt
sheeted-dike
gabbro
peridotite
01 OA 41d2
01 OA 36b
02 OA 43a97 OA 128b
01 OA15f
94 OA MA2
02 OA 90b01 OA 19ad
02 OA 37b
Whole RockPlagioclasePargasiteGreen HornblendeClinopyroxene
3 5 10
δ18O (permil)
41d2
Ma2
43a
90b 19d 19d37b
128b
90b
(b)
Fig 9 (a) Variation of initial 87Sr86Sr isotopic composition as a function of the location of the studied samples in a schematic log of the crustalsection of the Oman ophiolite The field for MORB is from Hart et al (1974) and that for seawater is from Burke et al (1982) Small open squares aredata fromKawahata et al (2001) (b) Variation of d18O () as a function of the location of the studied samples in a schematic log of the crustal sectionof the Oman ophiolite Shaded curve corresponds to the data of Gregory amp Taylor (1981)
BOSCH et al HYDROTHERMAL CIRCULATION IN OMAN OPHIOLITE
1201
are significantly different Moreover the d18O valueis dependent on temperature and mineral fractiona-tion and thus yields complementary constraints to Srisotopes
Mantle origin
One model to explain the occurrence of fluids interactingwith the oceanic crust at very high temperatures is toderive them from the mantle The contribution of mantle-derived hydrous fluids linked to percolation processesor to their concentration in the final stages of gabbrocrystallization has been invoked in numerous case studies(ie Witt amp Seck 1989 Fabriees et al 1989 Dupuy et al1991 Agrinier et al 1993) O and Sr isotopic data forwhole rocks and mineral separates yield scattered valuesthat are unambiguously different from MORB mantle-source values (Fig 9a and b) With the exception of lateLT altered samples the WR data have a mean 87Sr86Srratio of 070337 and a standard deviation of 000012These surprisingly high values in mantle-derived rockscould be taken as evidence for survival of mantle isotopicheterogeneities during crystallization of the ophiolite (egLanphere 1981 McCulloch et al 1981) However the18O16O isotope ratios in the Oman gabbroic sequenceare also displaced from primary magmatic values Nogabbro in the present study has plagioclase or amphi-bole with typical mantle d18O values Thus the Sr and Oisotope data rule out the possibility of mantle-derivedfluids as a possible source for the VHT---HT hydrousalteration event recorded in the massive gabbros andgabbroic dykes and veins (Fig 10a and b)
Subducted lithosphere origin
Dehydration of subducted oceanic crust in subductionzones can generate hydrous fluids Such fluids couldpercolate through the overlying lithosphere and contam-inate it thus generating modified isotopic signatures andtrace element contents Such an explanation howeverdisagrees with the geochemical characteristics of most ofthe studied samples Fluids derived from the dehydrationof subducted slabs (fresh or altered MORB or OIB pro-toliths) should be strongly enriched in K Rb and Ba (egBecker et al 2000) whereas in Oman the Rb contentsmeasured in the gabbros and gabbroic dykes and veins arestrongly depleted Moreover the isotopic composition ofslab-derived fluids is buffered by the MORB-like oceaniccrustal protolith for Nd and Pb but not for Sr and O(Staudigel et al 1995) As a result of relatively minorfractionation of oxygen at high temperature the oxygenisotope composition of the fluids expelled during dehy-dration of the subducted slab should be high and essen-tially controlled by the oxygen composition of the uppersection of the crust altered at low temperature (with an
average of 10 and perhaps as high as 19) (Staudigelet al 1995) The Sr isotopic composition should be alsohigh (average 07046) as the fluids released by the dehy-dration of subducted oceanic crust are associated eitherwith seawater or with radiogenic Sr phases (ie smectites)The Sr and O isotopic compositions of the studied sam-ples do not fit with these signatures and so preclude asubducted lithosphere origin for the fluids involved in theVHT---HT alteration event
Seawater-derived fluids
All the analysed samples (minerals and WR) have 87Sr86Sr(T) outside the domain of primary MORB magmasand variable Sr contents Individual minerals have Srcontents reflecting their different mineralliquid partitioncoefficients Minerals characterized by a very high affi-nity for Sr such as plagioclase (ie Sr mineralliquidpartition coefficients 1) have very high Sr contentsTherefore the high abundance of plagioclase in the sam-ples analysed is responsible for the high Sr content of thestudied whole rocks The Sr content of all the whole rocksis relatively homogeneous (Table 5) without clear distinc-tion between country-rock gabbros and dykes and veinsand ranges from 110 to 200 ppm except for the epidosite(4700 ppm) Reported on a 87Sr86Sr vs 1Sr diagram(Fig 10a) most whole rocks and plagioclases analysed arecharacterized by a 1Sr ratio lower than 001 and plot inthe left part of this diagram Compared with N-MORBthe studied massive and anatectic gabbros and their con-stituent plagioclases have similar to slightly higher Srcontents but substantially higher 87Sr86Sr ratios(Fig 10a) Considering Cretaceous seawater as a possiblehigh 87Sr86Sr mixing component [87Sr86Sr 07073at 90Ma after Burke et al (1982)] responsible for theobserved geochemical modifications exchanges cannothave occurred in a closed system as seawater has too lowa Sr content (ie 8 ppm de Villiers 1999) An open-system process allowing penetration of larger quantitiesof seawater can efficiently modify the Sr isotopic ratio ofthe rocks Alternately the fluids could also be seawaterthat was modified through exchange with the surround-ing rocks during its transit through the oceanic crustalsection This modified seawater would be more enrichedin Srwith a 87Sr86Sr ratio intermediate between unmodi-fied seawater and MORBMinerals devoid of late LT alteration imprints (parga-
site green hornblende and pyroxene) and characterizedby very low Sr contents plot on the right of the diagramclose to or on the mixing line between seawater andMORB (Fig 10a) The difference between these mineralsand the other samples (WR and plagioclases) is mainlydue to the Sr fractionation coefficients of these differentminerals and to the large quantity of plagioclase inthe studied rocks The process responsible for the
JOURNAL OF PETROLOGY VOLUME 45 NUMBER 6 JUNE 2004
1202
modification of the Sr isotopic composition fromMORB-source value is however explained by the same phenom-enon As all these minerals display initial 87Sr86Sr ratiosdistinct from the MORB source and because they equili-brated at VHT or HT temperatures this supports anearly intervention of seawater during crystallization ofthese minerals Most of these samples with the exceptionof the epidosite dyke overlap the domains defined byKawahata et al (2001) for the Wadi Fizh gabbros alteredat VHT and HT conditions in the amphibolite facies(labelled lsquoVHTrsquo and lsquoHTrsquo in Fig 9a)Three samples (two epidosite dykes and an one epidote
fraction) have very high Sr contents and the highest Sr
isotopic compositions of the present dataset The twowhole-rock samples overlap the field of the Wadi Fizhsamples altered at high temperature during greenschist-facies metamorphism (Kawahata et al 2001) This typeof alteration is common at the ridge axis and formedthrough intensive exchange with seawater-derived fluidsFigure 10b indicates that all the Sr---O isotope data are
distinct from MORB compositions and suggests that thefluids reacting with the magmas could be derived fromseawater In addition the d18O values show that isotopicexchange occurred under VHT or HT conditions Thefluids could have the composition of seawater or theycould have the composition of seawater modified through
0703
0705
0707
001 01 1
1 Sr (ppm)
MORB
(87Sr
86Sr)
init
ial
Seawater
VHT
HT
MT36b
36b
36b
43a 128b90b
41d2
37b19d
15f
128b
41d2
41d2
19a
Ma2Ma2
19d
90b
37b
Whole RockPlagioclasePargasiteGreen HornblendeClinopyroxene
Epidote
(a)
0 2 4 6 8
δ18O(permil)
0703
0705
0707Seawater
MORB-mantle
128b 41d2
90b 41d2
Ma2128b
37b
43a
19d
90b
Whole RockPlagioclasePargasiteGreen HornblendeClinopyroxene
Epidote
HT LT
19d
(87Sr
86Sr)
init
ial
(b)
Fig 10 (a) Initial 87Sr86Sr isotopic ratios plotted against 1Sr for whole rocks and mineral separates from the gabbro section of the Omanophiolite Fields shown are from Kawahata et al (2001) and correspond to MT-----basalt sheeted dyke diabase altered at medium temperature(prehnite---actinolite facies sequence 3) HT-----diabase plagiogranite metagabbro and epidosite formed at high temperature (greenschist faciessequence 4) VHT-----gabbro altered at very high temperature (amphibolite facies sequence 5) The dashed line is a binary mixing line betweenMORB (Lanphere et al 1981) and Cretaceous seawater (Burke et al 1982) (b) Initial 87Sr86Sr isotopic composition plotted against d18O value forwhole rocks and minerals from the Oman ophiolite Cretaceous seawater and MORB end-members as in (a) Dashed line represents mixing curvebetweenMORB and seawater at high temperature (HT) Low-temperature (LT) exchanges between these two components would be responsible foran increase of the d18O primary MORB value
BOSCH et al HYDROTHERMAL CIRCULATION IN OMAN OPHIOLITE
1203
exchange with the surrounding rocks during its penetra-tion into the crustal section In an environment in whichfluid---rock interaction occurs at variable temperaturesthe d18O is greatly influenced by the temperature atwhich the exchange took place and by the waterrockratio It is evident from Figs 9b and 10b that none of thestudied gabbros have plagioclase and amphibole withMORB-like d18O and 87Sr86Sr values The separateminerals display a broad range of d18O values and mostexhibit extreme 18O depletion (Table 5) Hydrothermalexchange kinetics are similar in amphibole and pyroxeneand these two minerals are more resistant to oxygenexchange during water---rock interaction than coexistingplagioclase (Gregory et al 1989) The most probableexplanation of the low d18O values measured in bothamphiboles and pyroxenes is that they crystallized athigh temperature in the presence of a relatively low 18Oseawater-derived hydrothermal fluid [d18O from 01 to07 after Gregory amp Taylor (1981)] The anomalouslyhigh d18O value (thorn1009) measured for the plagioclase ofthe micro-gabbroic dyke (01 OA19d) can be interpretedas resulting from a superimposed overprint caused by alate-stage penetration of fluids at lower temperature Inthis sample the occurrence of such different 18O16Oratios between coexisting plagioclase and amphibole isnot consistent with equilibrium fractionation betweenthese two minerals at any possible temperature Thisobservation suggests that parts of the ophiolite sequencehave undergone a high-temperature hydrothermal eventprior to the later low-temperature event that producedthe strong 18O increase in coexisting plagioclaseThe depletion of the d18O values results from high-temperature hydrothermal alteration (Gregory amp Taylor1981 Stakes amp Taylor 1992) The oxygen isotopic datasupport the contention that most studied samples havebeen affected by a VHT---HT hydrothermal alteration byfluids derived from seawater Some of the analysed sam-ples presenting petrological evidence of crystallization ofLT secondary minerals have been overprinted by the latelow-temperature alteration thus swamping the earlierhigh-temperature alteration effects
Determination of waterrock ratio
To better evaluate the exchange between seawater andthe gabbroic rocks waterrock ratios (WR) have beenestimated for the whole rocks analysed during the courseof this study The different models used to constrain thisratio mainly differ by the calculation of the effective ratioor by the estimated weight ratio and by considering openor closed systems for water circulation (Albareede et al1981 McCulloch et al 1981) The WR ratios reportedin Table 5 have been calculated assuming a closed sys-tem Using this formulation these values are minimabecause the calculation always uses pristine fluid for the
initial fluid thus maximizing the value in the denomina-tor If the fluid was shifted to lower 87Sr concentrations byexchange with the rock on the downward path theinferred values would be much higher (Davis et al 2003)The calculated WR ratios for the samples from this
study range from 31 to 219 (Table 5) Two main groupsof samples can be recognized on the basis of their waterrock ratios The lowest ratios ranging from 31 to 47have been determined for massive gabbros or anatecticgabbros but also for dykes and veins devoid of late LTalteration Similar ratios have been previously calculatedfor example for the discharged hydrothermal solutions atthe 13N and 21N East Pacific Rise (Di Donato et al1990 Fouquet et al 1991) The gabbros from the Ibrasection in the Oman ophiolite gave effective WR ratiosof 05 33 and 42 (McCulloch et al 1981) in goodagreement with the values obtained in this study Theleast altered gabbro of the Ibra section yields a WR ratioof 05 in good agreement with the exceptionally fresh-ness of this rock characterized by typical mantle oxygenvalues for its minerals (McCulloch et al 1981) Samplescharacterized by waterrock effective ratios in the rangeof 3---5 can be related to penetrative alteration via thehydrothermal system Lecuyer amp Reynard (1996) havedemonstrated in oceanic gabbros from the Hess Deepaltered in similar HT conditions that WR mass ratios inthe range of 02---1 are sufficient to account for the mod-ified stable isotope signature of the gabbros Higherwaterrock values (WR 45) have been calculated forsamples affected by superimposed late hydrothermalalteration and for the epidosite samples (Table 5) Theyprobably acquired their isotopic characteristics throughinteraction with a larger volume of seawater duringgreenschist-facies metamorphism Similar or higherWR values have been published in previous studies(ie McCulloch et al 1981 Kawahata et al 2001)They correspond either to samples from the upper-crus-tal sequence (ie basalt sheeted dyke or plagiogranite) orto gabbros from the crustal section located in a particularenvironment such as for example the Wadi Fizh sectionthat is located close to a segment boundary along thepalaeo-spreading axis (Nicolas et al 2000)
Mechanism for seawater interaction withmagma
Nicolas et al (2003) have proposed that variable amountsof seawater can circulate at high temperature through-out the crustal section down to the walls of the magmachamber via a network of parallel microcracks a fractionof a millimetre wide Such microcracks and their relation-ship with the VHT---HT hydrous alteration in the sur-rounding gabbros have been described in detail byNicolas et al Via this network of microcracks largeamounts of seawater can have reacted with the magma
JOURNAL OF PETROLOGY VOLUME 45 NUMBER 6 JUNE 2004
1204
and induced variable changes of its Sr and O isotopiccomposition This regular network of parallel micro-cracks could constitute the recharge system and thehydrated gabbroic dykes the discharge system Physicalparameters and the geophysical models of Nicolas et al(2003) are in agreement with this scenario Such a pene-tration of seawater-derived hydrothermal fluids throughthe lower crust along grain boundaries and a microscopicfracture network at temperatures 4750C is consistentwith recent thermodynamic models (McCollom amp Shock1998)Seawater-altered and high-temperature recrystallized
diabases (protodykes) from the overlying sheeted dykecomplex stoped into the melt lenses could represent anindirect way for seawater to enter the system as they canremelt during their subsidence through the magmachamber A simple mass balance calculation suggeststhat this indirect contamination is not significant Assess-ments of the amount of recycled crust 1 in volumeby Nicolas et al (2000) based on the protodyke remnantsin the gabbro section or 20 by Coogan et al (2003)based on chlorine content in EPR basalts lead to a sea-waterrock ratio of the order of 104 to 103Thus following Nicolas et al (2003) we propose that
seawater has been introduced along microcracks down tothe walls of the magma chamber and has diffused alonggrain boundaries in the way described by Manning et al(2000) progressively invading the host gabbro Gabbroicdykes result from the injection of hydrous melt into thehost gabbros at falling temperatures as they drift awayfrom the magma chamber This water-saturated meltcould result from the extensive hydration of the crystal-lizing gabbros near the walls of the magma chamberwhen seawater penetrates through this wall (Fig 1)Similar plumbing systems driving seawater down to the
limit of the magma chamber and back to the surface havebeen already proposed in other environments such as theMid-Atlantic Ridge (Kelley amp Delaney 1987 Cooganet al 2001) the East Pacific Rise at Hess Deep (Gillis1995 Manning et al 1996a 1996b) the Tonga forearc(Banerjee amp Gillis 2001) and the Troodos ophiolite(Gillis amp Roberts 1999) The melting curve of wet basalthas a negative slope but is close to the dry solidus at lowpressure in the lower gabbros and the first melts areplagiogranitic (Spulber ampRutherford 1983) Such plagio-granites are ubiquitous in the highest gabbros and in theroot zone of sheeted dykes in Oman (Rothery 1983Juteau et al 1988 Nicolas amp Boudier 1991) in manyother ophiolites and in the oceanic crust where they havebeen interpreted as resulting either from wet anatexis ofhot gabbros or from the final product of crystallization ofa basaltic melt (Spulber amp Rutherford 1983) Plagio-granites are rare in the lower gabbros exceptionallyassociated with hydrous gabbro segregations inside thelayered gabbros Their constitutive minerals are also
remarkably absent along the VHTmicrocracks althoughthese gabbros were above the wet solidus A possibleexplanation has been proposed by McCollom amp Shock(1998) who wrote that at high temperature lsquoaqueousfluids may leave very little conspicuous petrological evid-ence for their presencersquo the reason being that thehigh-T conditions would be closer to batch meltingObviously more detailed studies on this specific problemare needed We conclude that the large degree of hydrousmelting locally required at the walls of the magma cham-ber to account for the generation of a gabbroic hydrousmelt may have erased the traces of the incipient plagio-granitic melt
CONCLUSIONS
Fostered by the recent discovery of high-temperatureseawater contamination in some gabbros of the Omanophiolite (Manning et al 2000) we have conducted astudy on 500 gabbro specimens from selected areas ofthe entire Samail ophiolite belt and on the hydrous gab-broic dykes that cross-cut them The highest-temperaturereactions (VHT) recorded in olivine and clinopyroxene ingabbros as orthopyroxene and pargasite coronas reach975C They are followed at lower HT conditions withreplacement of olivine by an assemblage of tremolitemagnetite and plagioclase surrounded by chlorite andby pargasite development in clinopyroxene followedfinally by the LT lizardite---olivine and saussurite---plagioclase greenschist-facies alteration With a fewexceptions the VHT alteration tends to be restricted tothe lower gabbros and to the vicinity of the Moho andthe HT alteration to the middle and upper gabbros Thepreservation of the vertical distribution of VHT---HTfacies indicates that progressive cooling during off-axisspreading did not obliterate this distribution and conse-quently that the VHT---HT hydrothermal alteration tookplace in a very restricted time interval at the limit of thecooling magma chamber In the deep and hot gabbrosthe main water channels may be sub-millimetre-sizedmicrocracks with a dominantly vertical attitude the ori-gin and dynamics of which have been investigated in aprevious paper (Nicolas et al 2003)Beyond these high-temperature alteration zones mas-
sively induced in the gabbros seawater may be respons-ible for the generation of clinopyroxene---pargasitegabbro dykes that cross-cut all gabbros and that areupsection clearly connected to the LT hydrothermalvein system Petrostructural evidence suggests that thesedykes were generated by hydration of the crystallizinggabbros near the internal wall of the LVZ---magmachamber The resultant melts were subsequently intrudedat various levels in the cooling lower and middle gabbrosPetrological observations of nearly all the gabbros ana-lysed in the present study located from the root zone of
BOSCH et al HYDROTHERMAL CIRCULATION IN OMAN OPHIOLITE
1205
the sheeted dyke complex down to the Moho emphasizethe imprint of VHT---HT hydrous alteration The VHThydrothermal event is characterized by temperatures ofequilibration around 900---1000C The temperatures atwhich the HT hydrous event occurred are estimated tobe in the range 550---900CStrontium and oxygen isotope compositions measured
on whole rocks and separated minerals significantlydepart from primary MORB values The Sr and O iso-tope data obtained for pargasite green hornblende andclinopyroxene record the participation of seawater at thetemperature of crystallization of the minerals Plagio-clases and whole rocks define a trend toward a compo-nent characterized by a high radiogenic 87Sr86Sr valueand a higher Sr content than normal seawater Seawateris clearly identified as the fluid responsible for the modi-fication of the Sr and O signatures for both massivegabbros and dykes and veins Nevertheless the high Srcontent of the extraneous component requires eitheropen-system exchange allowing penetration of a greaterquantity of seawater or penetration of seawater modifiedby interaction with the oceanic crust during its travel paththrough the crustal section The waterrock ratio calcu-lated on the basis of the Sr isotopes is between three andfive for the enclosing gabbros and for the least LT altereddykes The VHT---HT hydrothermal event affectingthese samples is related to penetrative alteration via therecharge and discharge hydrothermal system Thewaterrock ratio is 10 for dykes exhibiting evidenceof a LT hydrothermal alteration overprint and reaches22 for the epidosite dyke The depletion in d18O char-acterizes all the studied samples with the single exceptionof the micro-gabbroic dyke plagioclase This agreeswith the conclusions based on Sr isotopes suggestingthat these samples have undergone exchange with sea-water-derived fluids during a VHT---HTalteration hydro-thermal event
ACKNOWLEDGEMENTS
This work has benefited from financial support by theCentre National de la Recherche Scientifique (INSUInteerieur-Terre andDYETI programmes) and inOmanfrom the willingness of the Directorate of MineralsMinistry of Commerce and Industry and the hospitabil-ity of the people Technical help in the laboratory is alsoacknowledged including C Nevado for the thin sectionsE Ball for the illustrations B Galland for her assistancein the chemistry room and C Bassoulet for help duringthermal ionization mass spectrometric analyses of the Srresults from the leaching experiments Constructivereviews from R T Gregory C Lecuyer an anonymousreviewer and particularly from M Wilson greatlyhelped to improve an earlier version of the manuscript
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Banerjee N R amp Gillis K M (2001) Hydrothermal alteration in a
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Becker H Jochum K P amp Carlson R W (2000) Trace element
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Bosch D (1991) Introduction drsquoeau de mer dans le diapir mantellique
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Chernosky J V Berman R G amp Bryndzia L T (1988) Stability
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295---346
Coogan L A Wilson R N Gillis K M amp MacLeod C J (2001)
Near-solidus evolution of oceanic gabbros insights from amphibole
geochemistry Geochimica et Cosmochimica Acta 65 4339---4357
Coogan L A Mitchell N C amp OrsquoHara M J (2003) Roof
assimilation at fast spreading ridges an investigation combining
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Davis A C Bickle M J amp Teagle D A H (2003) Imbalance in the
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Detrick R S Buhl P Vera E Mutter J Orcutt J Madsen J amp
Trocher T (1987) Multi-channel seismic imaging of a crustal
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De Villiers S (1999) Seawater strontium and SrCa variability in the
Atlantic and Pacific oceans Earth and Planetary Science Letters 171
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Di Donato G Joron J L Treuil M amp Loubet M (1990)
Geochemistry of zero-age N-MORB from hole 648B ODP legs
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Dunn R A Toomey D R amp Solomon S C (2000) Three-
dimensional seismic structure and physical properties of shallow
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Dupuy C Mevel C Bodinier J L amp Savoyant L (1991) Zabargad
peridotite evidence for multistage metasomatism during Red Sea
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1206
Fabriees J Bodinier J L Dupuy C Lorand J P amp Benkerrou C
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Journal of Petrology 30 199---228
Fouquet Y Von Stackelberg S U Charlou J L Donval J P
Foucher J Erzig P Muhe R Wiedicke M Soakai S amp
Whitechurch H (1991) Hydrothermal activity in the Lau back-arc
basin sulfides and water chemistry Geology 19 303---306
Gillis K M (1995) Controls on hydrothermal alteration in a section of
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Gillis K M amp Roberts M (1999) Cracking at the magma---hydrothermal transition evidence from the Troodos ophiolite Earth
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Gillis K M Mevel C et al (eds) (1993) Proceedings of the Ocean Drilling
Program Initial Reports 147 College Station TX Ocean Drilling
Program
Gregory R T amp Taylor H P (1981) An oxygen isotope profile in a
section of Cretaceous oceanic crust Samail ophiolite Oman
evidence for d18O buffering of the oceans by deep (45 km)
seawater---hydrothermal circulation at mid-ocean ridges Journal of
Geophysical Research 86(B4) 2737---2755
Gregory R T Criss R E amp Taylor H P (1989) Oxygen
isotope exchange kinetics of mineral pairs in closed and open
systems applications to problems of hydrothermal alteration of
igneous rocks and Precambrian iron formations Chemical Geology 75
1---42Hacker B R (1994) Rapid emplacement of young oceanic litho-
sphere argon geochronology of the Oman ophiolite Science 265
1563---1569
Hart S R Erlank A J amp Kable J D (1974) Sea floor basalt
alteration some chemical and Sr isotopic effects Contributions to
Mineralogy and Petrology 44 219---230
Holland T amp Blundy J (1994) Non-ideal interactions in calcic
amphiboles and their bearing on amphibole---plagioclase thermo-
metry Contributions to Mineralogy and Petrology 116 433---447
Ionov D A Savoyant L amp Dupuy C (1992) Application of the
ICP-MS technique to trace element analysis of peridotites and their
minerals Geostandards Newsletter 16 311---315
Javoy M (1980) 18O16O and DH ratios in high temperature
peridotites In Colloques Internationaux du CNRS 272 279---287
Juteau T Beurrier M Dahl R amp Nehlig P (1988) Segmentation
at a fossil spreading axis the plutonic sequence of the Wadi
Haymiliyah area (Haylayn Block Sumail nappe Oman) Tectono-
physics 151 167---197
Juteau T Manacrsquoh G Moreau C Lecuyer C amp Ramboz C
(2000) The high temperature reaction zone of the Oman ophiolite
new field data microthermometry of fluid inclusions PIXE analyses
and oxygen isotopic ratios Marine Geophysical Research 21 351---385Kawahata H Nohara M Ishizuka H Hasebe S amp Chiba H
(2001) Sr isotope geochemistry and hydrothermal alteration of the
Oman ophiolite Journal of Geophysical Research 106 11083---11099
Kelemen P B amp Aharonov E (1998) Periodic formation of magma
fractures and generation of layered gabbros in the lower crust
beneath oceanic spreading ridges Annual Geological Union Special
Monograph 106 267---289
Kelemen P B Koga K amp Shimizu N (1997) Geochemistry of
gabbro sills in the crustmantle transition zone of the Oman
ophiolite implications for the origin of the oceanic lower crust Earth
and Planetary Science Letters 146 475---488Kelley D S amp Delaney J R (1987) Two-phase separation and
fracturing in mid-ocean ridge gabbros at temperatures greater than
700C Earth and Planetary Science Letters 83 53---66
Koepke J Feig S T Snow J amp Freise M (2003) Petrogenesis of
oceanic plagiogranites by partial melting of gabbros an experi-
mental study Contributions to Mineralogy and Petrology on line first http
wwwspringerlinkcomopenurlaspgenre=s00410-003-0511-9
Kyser T K (1986) Stable isotope variations in the mantle In
Valley J W Taylor H P amp OrsquoNeil J R (eds) Stable Isotopes in
High Temperature Geological Processes Mineralogical Society of America
Reviews in Mineralogy 16 141---164
Lanphere M A (1981) K---Ar ages of metamorphic rocks at the base
of the Samail ophiolite Oman Journal of Geophysical Research 86
2777---2782
Lanphere M A Coleman R G amp Hopson C A (1981) Sr isotopic
tracer study of the Samail ophiolite Oman Journal of Geophysical
Research 86(B4) 2709---2720
Leake B E (1997) Nomenclature of amphiboles report of the
subcommittee on amphiboles of the International Mineralogical
Association commission on new minerals and mineral names
European Journal of Mineralogy 9 623---651
Lecuyer C amp Reynard B (1996) High-temperature alteration of
oceanic gabbros by seawater (Hess Deep Ocean Drilling Program
Leg 147) evidence from oxygen isotopes and elemental fluxes
Journal of Geophysical Research 101(B7) 15883---15897
Liou J G Kuniiyoshi S amp Ito K (1974) Experimental study of the
phase relations between greenschist and amphibolite in a basaltic
system American Journal of Science 274 613---632
MacLeod C J amp Rothery D A (1992) Ridge axial segmentation in
the Oman ophiolite evidence from along-strike variations in the
sheeted dyke complex Geological Society of America Special Papers 60
39---63
Manning C E MacLeod C J amp Weston P E (1996a) Fracture-
controlled metamorphism of Hess Deep gabbros site 984
constraints on the roots of mid-ocean ridge hydrothermal systems
at fast-spreading centers In GillisM C Allan KM ampMeyer P S
(eds) Proceedings of the Ocean Drilling Program Scientific Results 147
College Station TX Ocean Drilling Program pp 189---211Manning C E Weston P E amp Mahon K I (1996b) Rapid high-
temperature metamorphism of East Pacific Rise gabbros from Hess
Deep Earth and Planetary Science Letters 144 123---132Manning C E MacLeod C J amp Weston P E (2000) Lower-crustal
cracking front at fast-spreading ridges evidence from the East Pacific
Rise and the Oman ophiolite Journal of the Geological Society London
349 261---272McCollom T M amp Shock E L (1998) Fluid---rock interactions in
the lower oceanic crust thermodynamic models of hydrothermal
alteration Journal of Geophysical Research 103 547---575
McCulloch M T Gregory R T Wasserburg G J amp Taylor H P
(1981) Sm---Nd Rb---Sr and 18O16O isotopic systematics in an
oceanic crustal section evidence from the Samail ophiolite Journal of
Geophysical Research 86(B4) 2721---2735Mevel C Gillis K M Allan J F amp Meyer P S (eds) (1996)
Proceedings of the Ocean Drilling Program Scientific Results 147 College
Station TX Ocean Drilling Program
Nehlig P amp Juteau T (1988) Flow porosities permeabilities and
preliminary data on fluid inclusions and fossil thermal gradients in
the crustal sequence of the Sumail ophiolite (Oman) Tectonophysics
151 199---221
Nehlig P Juteau T Bendel V amp Cotten J (1994) The root zone of
oceanic hydrothermal systems constraints from the Samail ophiolite
(Oman) Journal of Geophysical Research 99 4703---4713
Nicolas A amp Boudier F (1991) Rooting of the sheeted dyke complex
in Oman ophiolite In Peters Tj Nicolas A amp Coleman R (eds)
Ophiolite Genesis and Evolution of the Oceanic Lithosphere Dordrecht
Kluwer Academic pp 39---54
BOSCH et al HYDROTHERMAL CIRCULATION IN OMAN OPHIOLITE
1207
Nicolas A amp Ildefonse B (1996) Flow mechanism and viscosity in
basaltic magma chambers Geophysical Research Letters 23(16)
2013---2016
Nicolas A Ceuleneer G Boudier F amp Misseri M (1988) Structural
mapping in the Oman ophiolites mantle diapirism along an ocean
ridge Tectonophysics 151 27---56
Nicolas A Boudier F Ildefonse B amp Ball E (2000) Accretion
of Oman and United Arab Emirates ophiolite-----discussion of a
new structural map Marine Geophysical Research 21(3---4)147---179
Nicolas A Boudier F Michibayashi K amp Gerbert-Gaillard L
(2001) Aswad massif (United Arab Emirates) archetype of the
Oman---UAE ophiolite belt Geological Society of America Bulletin 349
499---451
Nicolas A Mainprice D amp Boudier F (2003) High temperature
seawater circulation through the lower crust of ocean-ridges-----amodel derived from the Oman ophiolites Journal of Geophysical
Research on line first httpwwwaguorgpubscurrentjb 108(B8)
2372
Pallister J S amp Hopson C A (1981) Samail ophiolite plutonic suite
field relations phase variation cryptic variation and layering and a
model of a spreading ridge magma chamber Journal of Geophysical
Research 86 2593---2644Phipps Morgan J amp Chen J Y (1993) Dependence of ridge-axis
morphology on magma supply and spreading rate Nature 364
706---708
Pin C Briot D Bassin C amp Poitrasson F (1994) Concomitant
extraction of Sr and Sm---Nd for isotopic analysis in silicate samples
based on specific extraction chromatography Analytica Chimica Acta
298 209---217
Rothery D A (1983) The base of a sheeted dyke complex Oman
ophiolite implications for magma chambers at oceanic spreading
axes Journal of the Geological Society London 140 287---296
Spear F S (1981) An experimental study of hornblende stability and
compositional variability in amphibolite American Journal of Science
281 697---734
Spulber S D amp Rutherford M J (1983) The origin of rhyolite and
plagiogranite in oceanic crust an experimental study Journal of
Petrology 24 1---25Stakes D S (1991) Oxygen and hydrogen isotope compositions of
oceanic plutonic rocks high-temperature deformation and meta-
morphism of oceanic layer 3 In Taylor H P OrsquoNeil J et al (eds)
Geochemical Society Special Publication 3 77---90
Stakes D S amp Taylor H P (1992) The northern Samail ophiolite an
oxygen isotope microprobe and field study Journal of Geophysical
Research 97(B5) 7043---7080Staudigel H Davies G R Hart S R Marchant M amp Smith B
M (1995) Large scale isotopic Sr Nd and O isotopic anatomy of
altered oceanic crust DSDPODP sites 417418 Earth and Planetary
Science Letters 130 169---185Turner F J amp Verhoogen J (1960) Igneous and Metamorphic Petrology
New York McGraw---Hill 694 pp
Wilcock W S D amp Delaney J R (1996) Mid-ocean ridge
sulfide deposits evidence for heat extraction from magma
chambers or cracking fronts Earth and Planetary Science Letters 145
49---64
Witt G amp Seck H A (1989) Origin of amphibole in recrystallized
and porphyroclastic mantle xenoliths from Rhenish massif implica-
tions for the nature of mantle metasomatism Earth and Planetary
Science Letters 91 327---340
JOURNAL OF PETROLOGY VOLUME 45 NUMBER 6 JUNE 2004
1208
are significantly different Moreover the d18O valueis dependent on temperature and mineral fractiona-tion and thus yields complementary constraints to Srisotopes
Mantle origin
One model to explain the occurrence of fluids interactingwith the oceanic crust at very high temperatures is toderive them from the mantle The contribution of mantle-derived hydrous fluids linked to percolation processesor to their concentration in the final stages of gabbrocrystallization has been invoked in numerous case studies(ie Witt amp Seck 1989 Fabriees et al 1989 Dupuy et al1991 Agrinier et al 1993) O and Sr isotopic data forwhole rocks and mineral separates yield scattered valuesthat are unambiguously different from MORB mantle-source values (Fig 9a and b) With the exception of lateLT altered samples the WR data have a mean 87Sr86Srratio of 070337 and a standard deviation of 000012These surprisingly high values in mantle-derived rockscould be taken as evidence for survival of mantle isotopicheterogeneities during crystallization of the ophiolite (egLanphere 1981 McCulloch et al 1981) However the18O16O isotope ratios in the Oman gabbroic sequenceare also displaced from primary magmatic values Nogabbro in the present study has plagioclase or amphi-bole with typical mantle d18O values Thus the Sr and Oisotope data rule out the possibility of mantle-derivedfluids as a possible source for the VHT---HT hydrousalteration event recorded in the massive gabbros andgabbroic dykes and veins (Fig 10a and b)
Subducted lithosphere origin
Dehydration of subducted oceanic crust in subductionzones can generate hydrous fluids Such fluids couldpercolate through the overlying lithosphere and contam-inate it thus generating modified isotopic signatures andtrace element contents Such an explanation howeverdisagrees with the geochemical characteristics of most ofthe studied samples Fluids derived from the dehydrationof subducted slabs (fresh or altered MORB or OIB pro-toliths) should be strongly enriched in K Rb and Ba (egBecker et al 2000) whereas in Oman the Rb contentsmeasured in the gabbros and gabbroic dykes and veins arestrongly depleted Moreover the isotopic composition ofslab-derived fluids is buffered by the MORB-like oceaniccrustal protolith for Nd and Pb but not for Sr and O(Staudigel et al 1995) As a result of relatively minorfractionation of oxygen at high temperature the oxygenisotope composition of the fluids expelled during dehy-dration of the subducted slab should be high and essen-tially controlled by the oxygen composition of the uppersection of the crust altered at low temperature (with an
average of 10 and perhaps as high as 19) (Staudigelet al 1995) The Sr isotopic composition should be alsohigh (average 07046) as the fluids released by the dehy-dration of subducted oceanic crust are associated eitherwith seawater or with radiogenic Sr phases (ie smectites)The Sr and O isotopic compositions of the studied sam-ples do not fit with these signatures and so preclude asubducted lithosphere origin for the fluids involved in theVHT---HT alteration event
Seawater-derived fluids
All the analysed samples (minerals and WR) have 87Sr86Sr(T) outside the domain of primary MORB magmasand variable Sr contents Individual minerals have Srcontents reflecting their different mineralliquid partitioncoefficients Minerals characterized by a very high affi-nity for Sr such as plagioclase (ie Sr mineralliquidpartition coefficients 1) have very high Sr contentsTherefore the high abundance of plagioclase in the sam-ples analysed is responsible for the high Sr content of thestudied whole rocks The Sr content of all the whole rocksis relatively homogeneous (Table 5) without clear distinc-tion between country-rock gabbros and dykes and veinsand ranges from 110 to 200 ppm except for the epidosite(4700 ppm) Reported on a 87Sr86Sr vs 1Sr diagram(Fig 10a) most whole rocks and plagioclases analysed arecharacterized by a 1Sr ratio lower than 001 and plot inthe left part of this diagram Compared with N-MORBthe studied massive and anatectic gabbros and their con-stituent plagioclases have similar to slightly higher Srcontents but substantially higher 87Sr86Sr ratios(Fig 10a) Considering Cretaceous seawater as a possiblehigh 87Sr86Sr mixing component [87Sr86Sr 07073at 90Ma after Burke et al (1982)] responsible for theobserved geochemical modifications exchanges cannothave occurred in a closed system as seawater has too lowa Sr content (ie 8 ppm de Villiers 1999) An open-system process allowing penetration of larger quantitiesof seawater can efficiently modify the Sr isotopic ratio ofthe rocks Alternately the fluids could also be seawaterthat was modified through exchange with the surround-ing rocks during its transit through the oceanic crustalsection This modified seawater would be more enrichedin Srwith a 87Sr86Sr ratio intermediate between unmodi-fied seawater and MORBMinerals devoid of late LT alteration imprints (parga-
site green hornblende and pyroxene) and characterizedby very low Sr contents plot on the right of the diagramclose to or on the mixing line between seawater andMORB (Fig 10a) The difference between these mineralsand the other samples (WR and plagioclases) is mainlydue to the Sr fractionation coefficients of these differentminerals and to the large quantity of plagioclase inthe studied rocks The process responsible for the
JOURNAL OF PETROLOGY VOLUME 45 NUMBER 6 JUNE 2004
1202
modification of the Sr isotopic composition fromMORB-source value is however explained by the same phenom-enon As all these minerals display initial 87Sr86Sr ratiosdistinct from the MORB source and because they equili-brated at VHT or HT temperatures this supports anearly intervention of seawater during crystallization ofthese minerals Most of these samples with the exceptionof the epidosite dyke overlap the domains defined byKawahata et al (2001) for the Wadi Fizh gabbros alteredat VHT and HT conditions in the amphibolite facies(labelled lsquoVHTrsquo and lsquoHTrsquo in Fig 9a)Three samples (two epidosite dykes and an one epidote
fraction) have very high Sr contents and the highest Sr
isotopic compositions of the present dataset The twowhole-rock samples overlap the field of the Wadi Fizhsamples altered at high temperature during greenschist-facies metamorphism (Kawahata et al 2001) This typeof alteration is common at the ridge axis and formedthrough intensive exchange with seawater-derived fluidsFigure 10b indicates that all the Sr---O isotope data are
distinct from MORB compositions and suggests that thefluids reacting with the magmas could be derived fromseawater In addition the d18O values show that isotopicexchange occurred under VHT or HT conditions Thefluids could have the composition of seawater or theycould have the composition of seawater modified through
0703
0705
0707
001 01 1
1 Sr (ppm)
MORB
(87Sr
86Sr)
init
ial
Seawater
VHT
HT
MT36b
36b
36b
43a 128b90b
41d2
37b19d
15f
128b
41d2
41d2
19a
Ma2Ma2
19d
90b
37b
Whole RockPlagioclasePargasiteGreen HornblendeClinopyroxene
Epidote
(a)
0 2 4 6 8
δ18O(permil)
0703
0705
0707Seawater
MORB-mantle
128b 41d2
90b 41d2
Ma2128b
37b
43a
19d
90b
Whole RockPlagioclasePargasiteGreen HornblendeClinopyroxene
Epidote
HT LT
19d
(87Sr
86Sr)
init
ial
(b)
Fig 10 (a) Initial 87Sr86Sr isotopic ratios plotted against 1Sr for whole rocks and mineral separates from the gabbro section of the Omanophiolite Fields shown are from Kawahata et al (2001) and correspond to MT-----basalt sheeted dyke diabase altered at medium temperature(prehnite---actinolite facies sequence 3) HT-----diabase plagiogranite metagabbro and epidosite formed at high temperature (greenschist faciessequence 4) VHT-----gabbro altered at very high temperature (amphibolite facies sequence 5) The dashed line is a binary mixing line betweenMORB (Lanphere et al 1981) and Cretaceous seawater (Burke et al 1982) (b) Initial 87Sr86Sr isotopic composition plotted against d18O value forwhole rocks and minerals from the Oman ophiolite Cretaceous seawater and MORB end-members as in (a) Dashed line represents mixing curvebetweenMORB and seawater at high temperature (HT) Low-temperature (LT) exchanges between these two components would be responsible foran increase of the d18O primary MORB value
BOSCH et al HYDROTHERMAL CIRCULATION IN OMAN OPHIOLITE
1203
exchange with the surrounding rocks during its penetra-tion into the crustal section In an environment in whichfluid---rock interaction occurs at variable temperaturesthe d18O is greatly influenced by the temperature atwhich the exchange took place and by the waterrockratio It is evident from Figs 9b and 10b that none of thestudied gabbros have plagioclase and amphibole withMORB-like d18O and 87Sr86Sr values The separateminerals display a broad range of d18O values and mostexhibit extreme 18O depletion (Table 5) Hydrothermalexchange kinetics are similar in amphibole and pyroxeneand these two minerals are more resistant to oxygenexchange during water---rock interaction than coexistingplagioclase (Gregory et al 1989) The most probableexplanation of the low d18O values measured in bothamphiboles and pyroxenes is that they crystallized athigh temperature in the presence of a relatively low 18Oseawater-derived hydrothermal fluid [d18O from 01 to07 after Gregory amp Taylor (1981)] The anomalouslyhigh d18O value (thorn1009) measured for the plagioclase ofthe micro-gabbroic dyke (01 OA19d) can be interpretedas resulting from a superimposed overprint caused by alate-stage penetration of fluids at lower temperature Inthis sample the occurrence of such different 18O16Oratios between coexisting plagioclase and amphibole isnot consistent with equilibrium fractionation betweenthese two minerals at any possible temperature Thisobservation suggests that parts of the ophiolite sequencehave undergone a high-temperature hydrothermal eventprior to the later low-temperature event that producedthe strong 18O increase in coexisting plagioclaseThe depletion of the d18O values results from high-temperature hydrothermal alteration (Gregory amp Taylor1981 Stakes amp Taylor 1992) The oxygen isotopic datasupport the contention that most studied samples havebeen affected by a VHT---HT hydrothermal alteration byfluids derived from seawater Some of the analysed sam-ples presenting petrological evidence of crystallization ofLT secondary minerals have been overprinted by the latelow-temperature alteration thus swamping the earlierhigh-temperature alteration effects
Determination of waterrock ratio
To better evaluate the exchange between seawater andthe gabbroic rocks waterrock ratios (WR) have beenestimated for the whole rocks analysed during the courseof this study The different models used to constrain thisratio mainly differ by the calculation of the effective ratioor by the estimated weight ratio and by considering openor closed systems for water circulation (Albareede et al1981 McCulloch et al 1981) The WR ratios reportedin Table 5 have been calculated assuming a closed sys-tem Using this formulation these values are minimabecause the calculation always uses pristine fluid for the
initial fluid thus maximizing the value in the denomina-tor If the fluid was shifted to lower 87Sr concentrations byexchange with the rock on the downward path theinferred values would be much higher (Davis et al 2003)The calculated WR ratios for the samples from this
study range from 31 to 219 (Table 5) Two main groupsof samples can be recognized on the basis of their waterrock ratios The lowest ratios ranging from 31 to 47have been determined for massive gabbros or anatecticgabbros but also for dykes and veins devoid of late LTalteration Similar ratios have been previously calculatedfor example for the discharged hydrothermal solutions atthe 13N and 21N East Pacific Rise (Di Donato et al1990 Fouquet et al 1991) The gabbros from the Ibrasection in the Oman ophiolite gave effective WR ratiosof 05 33 and 42 (McCulloch et al 1981) in goodagreement with the values obtained in this study Theleast altered gabbro of the Ibra section yields a WR ratioof 05 in good agreement with the exceptionally fresh-ness of this rock characterized by typical mantle oxygenvalues for its minerals (McCulloch et al 1981) Samplescharacterized by waterrock effective ratios in the rangeof 3---5 can be related to penetrative alteration via thehydrothermal system Lecuyer amp Reynard (1996) havedemonstrated in oceanic gabbros from the Hess Deepaltered in similar HT conditions that WR mass ratios inthe range of 02---1 are sufficient to account for the mod-ified stable isotope signature of the gabbros Higherwaterrock values (WR 45) have been calculated forsamples affected by superimposed late hydrothermalalteration and for the epidosite samples (Table 5) Theyprobably acquired their isotopic characteristics throughinteraction with a larger volume of seawater duringgreenschist-facies metamorphism Similar or higherWR values have been published in previous studies(ie McCulloch et al 1981 Kawahata et al 2001)They correspond either to samples from the upper-crus-tal sequence (ie basalt sheeted dyke or plagiogranite) orto gabbros from the crustal section located in a particularenvironment such as for example the Wadi Fizh sectionthat is located close to a segment boundary along thepalaeo-spreading axis (Nicolas et al 2000)
Mechanism for seawater interaction withmagma
Nicolas et al (2003) have proposed that variable amountsof seawater can circulate at high temperature through-out the crustal section down to the walls of the magmachamber via a network of parallel microcracks a fractionof a millimetre wide Such microcracks and their relation-ship with the VHT---HT hydrous alteration in the sur-rounding gabbros have been described in detail byNicolas et al Via this network of microcracks largeamounts of seawater can have reacted with the magma
JOURNAL OF PETROLOGY VOLUME 45 NUMBER 6 JUNE 2004
1204
and induced variable changes of its Sr and O isotopiccomposition This regular network of parallel micro-cracks could constitute the recharge system and thehydrated gabbroic dykes the discharge system Physicalparameters and the geophysical models of Nicolas et al(2003) are in agreement with this scenario Such a pene-tration of seawater-derived hydrothermal fluids throughthe lower crust along grain boundaries and a microscopicfracture network at temperatures 4750C is consistentwith recent thermodynamic models (McCollom amp Shock1998)Seawater-altered and high-temperature recrystallized
diabases (protodykes) from the overlying sheeted dykecomplex stoped into the melt lenses could represent anindirect way for seawater to enter the system as they canremelt during their subsidence through the magmachamber A simple mass balance calculation suggeststhat this indirect contamination is not significant Assess-ments of the amount of recycled crust 1 in volumeby Nicolas et al (2000) based on the protodyke remnantsin the gabbro section or 20 by Coogan et al (2003)based on chlorine content in EPR basalts lead to a sea-waterrock ratio of the order of 104 to 103Thus following Nicolas et al (2003) we propose that
seawater has been introduced along microcracks down tothe walls of the magma chamber and has diffused alonggrain boundaries in the way described by Manning et al(2000) progressively invading the host gabbro Gabbroicdykes result from the injection of hydrous melt into thehost gabbros at falling temperatures as they drift awayfrom the magma chamber This water-saturated meltcould result from the extensive hydration of the crystal-lizing gabbros near the walls of the magma chamberwhen seawater penetrates through this wall (Fig 1)Similar plumbing systems driving seawater down to the
limit of the magma chamber and back to the surface havebeen already proposed in other environments such as theMid-Atlantic Ridge (Kelley amp Delaney 1987 Cooganet al 2001) the East Pacific Rise at Hess Deep (Gillis1995 Manning et al 1996a 1996b) the Tonga forearc(Banerjee amp Gillis 2001) and the Troodos ophiolite(Gillis amp Roberts 1999) The melting curve of wet basalthas a negative slope but is close to the dry solidus at lowpressure in the lower gabbros and the first melts areplagiogranitic (Spulber ampRutherford 1983) Such plagio-granites are ubiquitous in the highest gabbros and in theroot zone of sheeted dykes in Oman (Rothery 1983Juteau et al 1988 Nicolas amp Boudier 1991) in manyother ophiolites and in the oceanic crust where they havebeen interpreted as resulting either from wet anatexis ofhot gabbros or from the final product of crystallization ofa basaltic melt (Spulber amp Rutherford 1983) Plagio-granites are rare in the lower gabbros exceptionallyassociated with hydrous gabbro segregations inside thelayered gabbros Their constitutive minerals are also
remarkably absent along the VHTmicrocracks althoughthese gabbros were above the wet solidus A possibleexplanation has been proposed by McCollom amp Shock(1998) who wrote that at high temperature lsquoaqueousfluids may leave very little conspicuous petrological evid-ence for their presencersquo the reason being that thehigh-T conditions would be closer to batch meltingObviously more detailed studies on this specific problemare needed We conclude that the large degree of hydrousmelting locally required at the walls of the magma cham-ber to account for the generation of a gabbroic hydrousmelt may have erased the traces of the incipient plagio-granitic melt
CONCLUSIONS
Fostered by the recent discovery of high-temperatureseawater contamination in some gabbros of the Omanophiolite (Manning et al 2000) we have conducted astudy on 500 gabbro specimens from selected areas ofthe entire Samail ophiolite belt and on the hydrous gab-broic dykes that cross-cut them The highest-temperaturereactions (VHT) recorded in olivine and clinopyroxene ingabbros as orthopyroxene and pargasite coronas reach975C They are followed at lower HT conditions withreplacement of olivine by an assemblage of tremolitemagnetite and plagioclase surrounded by chlorite andby pargasite development in clinopyroxene followedfinally by the LT lizardite---olivine and saussurite---plagioclase greenschist-facies alteration With a fewexceptions the VHT alteration tends to be restricted tothe lower gabbros and to the vicinity of the Moho andthe HT alteration to the middle and upper gabbros Thepreservation of the vertical distribution of VHT---HTfacies indicates that progressive cooling during off-axisspreading did not obliterate this distribution and conse-quently that the VHT---HT hydrothermal alteration tookplace in a very restricted time interval at the limit of thecooling magma chamber In the deep and hot gabbrosthe main water channels may be sub-millimetre-sizedmicrocracks with a dominantly vertical attitude the ori-gin and dynamics of which have been investigated in aprevious paper (Nicolas et al 2003)Beyond these high-temperature alteration zones mas-
sively induced in the gabbros seawater may be respons-ible for the generation of clinopyroxene---pargasitegabbro dykes that cross-cut all gabbros and that areupsection clearly connected to the LT hydrothermalvein system Petrostructural evidence suggests that thesedykes were generated by hydration of the crystallizinggabbros near the internal wall of the LVZ---magmachamber The resultant melts were subsequently intrudedat various levels in the cooling lower and middle gabbrosPetrological observations of nearly all the gabbros ana-lysed in the present study located from the root zone of
BOSCH et al HYDROTHERMAL CIRCULATION IN OMAN OPHIOLITE
1205
the sheeted dyke complex down to the Moho emphasizethe imprint of VHT---HT hydrous alteration The VHThydrothermal event is characterized by temperatures ofequilibration around 900---1000C The temperatures atwhich the HT hydrous event occurred are estimated tobe in the range 550---900CStrontium and oxygen isotope compositions measured
on whole rocks and separated minerals significantlydepart from primary MORB values The Sr and O iso-tope data obtained for pargasite green hornblende andclinopyroxene record the participation of seawater at thetemperature of crystallization of the minerals Plagio-clases and whole rocks define a trend toward a compo-nent characterized by a high radiogenic 87Sr86Sr valueand a higher Sr content than normal seawater Seawateris clearly identified as the fluid responsible for the modi-fication of the Sr and O signatures for both massivegabbros and dykes and veins Nevertheless the high Srcontent of the extraneous component requires eitheropen-system exchange allowing penetration of a greaterquantity of seawater or penetration of seawater modifiedby interaction with the oceanic crust during its travel paththrough the crustal section The waterrock ratio calcu-lated on the basis of the Sr isotopes is between three andfive for the enclosing gabbros and for the least LT altereddykes The VHT---HT hydrothermal event affectingthese samples is related to penetrative alteration via therecharge and discharge hydrothermal system Thewaterrock ratio is 10 for dykes exhibiting evidenceof a LT hydrothermal alteration overprint and reaches22 for the epidosite dyke The depletion in d18O char-acterizes all the studied samples with the single exceptionof the micro-gabbroic dyke plagioclase This agreeswith the conclusions based on Sr isotopes suggestingthat these samples have undergone exchange with sea-water-derived fluids during a VHT---HTalteration hydro-thermal event
ACKNOWLEDGEMENTS
This work has benefited from financial support by theCentre National de la Recherche Scientifique (INSUInteerieur-Terre andDYETI programmes) and inOmanfrom the willingness of the Directorate of MineralsMinistry of Commerce and Industry and the hospitabil-ity of the people Technical help in the laboratory is alsoacknowledged including C Nevado for the thin sectionsE Ball for the illustrations B Galland for her assistancein the chemistry room and C Bassoulet for help duringthermal ionization mass spectrometric analyses of the Srresults from the leaching experiments Constructivereviews from R T Gregory C Lecuyer an anonymousreviewer and particularly from M Wilson greatlyhelped to improve an earlier version of the manuscript
REFERENCESAgrinier P Mevel C Bosch D amp Javoy M (1993) Metasomatic
hydrous fluids in amphibole peridotites from Zabargad Island (Red
Sea) Earth and Planetary Science Letters 120 187---205Albareede F A Michard A Minster J F amp Michard G (1981)
87Sr86Sr ratios in hydrothermal waters and deposits from the East
Pacific Rise at 21N Earth and Planetary Science Letters 55 229---236
Banerjee N R amp Gillis K M (2001) Hydrothermal alteration in a
modern subduction zone the Tonga forearc crust Journal of
Geophysical Research 106(B10) 21737---21750
Becker H Jochum K P amp Carlson R W (2000) Trace element
fractionation during dehydration of eclogites from high-pressure
terranes and the implications for element fluxes in subduction zones
Chemical Geology 163 65---99
Benoit M Polve M amp Ceulener G (1996) Trace element and
isotopic characterization of mafic cumulates in a fossil mantle diapir
(Oman ophiolite) Chemical Geology 134 199---214
Bosch D (1991) Introduction drsquoeau de mer dans le diapir mantellique
de Zabargad (Mer Rouge) drsquoaprees les isotopes du Sr et du Nd
Comptes Rendus de lrsquoAcadeemie des Sciences 313 49---56
Boudier F Godard M amp Ambruster C (2000) Significance of
noritic gabbros in the gabbro section of the Oman ophiolite Marine
Geophysical Research 21(3---4) 307---326Burke W H Denison R E Hetherington R B Koepnick R B
Nelson H F amp Otto H F (1982) Variation of seawater 87Sr86Sr
throughout Phanerozoic time Geology 10 516---519Carmichael I S E Turner F J amp Verhoogen J (1974) Igneous
Petrology Berkeley CA University of California Berkeley 739 pp
Chernosky J V Berman R G amp Bryndzia L T (1988) Stability
phase relations and thermodynamic properties of chlorite and
serpentine group minerals In Bayley S W (ed) Hydrous
Phyllosilicates Mineralogical Society of America Reviews in Mineralogy 19
295---346
Coogan L A Wilson R N Gillis K M amp MacLeod C J (2001)
Near-solidus evolution of oceanic gabbros insights from amphibole
geochemistry Geochimica et Cosmochimica Acta 65 4339---4357
Coogan L A Mitchell N C amp OrsquoHara M J (2003) Roof
assimilation at fast spreading ridges an investigation combining
geophysical geochemical and field evidence Journal of Geophysical
Research on line first httpwwwaguorgpubscurrentjb 108(B1)
1171
Davis A C Bickle M J amp Teagle D A H (2003) Imbalance in the
oceanic strontium budget Earth and Planetary Science Letters 211
173---187
Detrick R S Buhl P Vera E Mutter J Orcutt J Madsen J amp
Trocher T (1987) Multi-channel seismic imaging of a crustal
magma chamber along the East Pacific Rise Nature 326 35---41
De Villiers S (1999) Seawater strontium and SrCa variability in the
Atlantic and Pacific oceans Earth and Planetary Science Letters 171
623---634
Di Donato G Joron J L Treuil M amp Loubet M (1990)
Geochemistry of zero-age N-MORB from hole 648B ODP legs
106---109 MAR 22 In Detrick R S Honnorez J et al (eds)
Proceedings of the Ocean Drilling Program Scientific Results 106109
College Station TX Ocean Drilling Program pp 57---65
Dunn R A Toomey D R amp Solomon S C (2000) Three-
dimensional seismic structure and physical properties of shallow
mantle beneath the East Pacific Rise at 9300N Journal of Geophysical
Research 105 23537---23555
Dupuy C Mevel C Bodinier J L amp Savoyant L (1991) Zabargad
peridotite evidence for multistage metasomatism during Red Sea
rifting Geology 19 722---725
JOURNAL OF PETROLOGY VOLUME 45 NUMBER 6 JUNE 2004
1206
Fabriees J Bodinier J L Dupuy C Lorand J P amp Benkerrou C
(1989) Evidence for modal metasomatism in the orogenic spinel
lherzolite body from Caussou (northeastern Pyrenees France)
Journal of Petrology 30 199---228
Fouquet Y Von Stackelberg S U Charlou J L Donval J P
Foucher J Erzig P Muhe R Wiedicke M Soakai S amp
Whitechurch H (1991) Hydrothermal activity in the Lau back-arc
basin sulfides and water chemistry Geology 19 303---306
Gillis K M (1995) Controls on hydrothermal alteration in a section of
fast-spreading oceanic crust Earth and Planetary Science Letters 134
473---489
Gillis K M amp Roberts M (1999) Cracking at the magma---hydrothermal transition evidence from the Troodos ophiolite Earth
and Planetary Science Letters 169 227---244
Gillis K M Mevel C et al (eds) (1993) Proceedings of the Ocean Drilling
Program Initial Reports 147 College Station TX Ocean Drilling
Program
Gregory R T amp Taylor H P (1981) An oxygen isotope profile in a
section of Cretaceous oceanic crust Samail ophiolite Oman
evidence for d18O buffering of the oceans by deep (45 km)
seawater---hydrothermal circulation at mid-ocean ridges Journal of
Geophysical Research 86(B4) 2737---2755
Gregory R T Criss R E amp Taylor H P (1989) Oxygen
isotope exchange kinetics of mineral pairs in closed and open
systems applications to problems of hydrothermal alteration of
igneous rocks and Precambrian iron formations Chemical Geology 75
1---42Hacker B R (1994) Rapid emplacement of young oceanic litho-
sphere argon geochronology of the Oman ophiolite Science 265
1563---1569
Hart S R Erlank A J amp Kable J D (1974) Sea floor basalt
alteration some chemical and Sr isotopic effects Contributions to
Mineralogy and Petrology 44 219---230
Holland T amp Blundy J (1994) Non-ideal interactions in calcic
amphiboles and their bearing on amphibole---plagioclase thermo-
metry Contributions to Mineralogy and Petrology 116 433---447
Ionov D A Savoyant L amp Dupuy C (1992) Application of the
ICP-MS technique to trace element analysis of peridotites and their
minerals Geostandards Newsletter 16 311---315
Javoy M (1980) 18O16O and DH ratios in high temperature
peridotites In Colloques Internationaux du CNRS 272 279---287
Juteau T Beurrier M Dahl R amp Nehlig P (1988) Segmentation
at a fossil spreading axis the plutonic sequence of the Wadi
Haymiliyah area (Haylayn Block Sumail nappe Oman) Tectono-
physics 151 167---197
Juteau T Manacrsquoh G Moreau C Lecuyer C amp Ramboz C
(2000) The high temperature reaction zone of the Oman ophiolite
new field data microthermometry of fluid inclusions PIXE analyses
and oxygen isotopic ratios Marine Geophysical Research 21 351---385Kawahata H Nohara M Ishizuka H Hasebe S amp Chiba H
(2001) Sr isotope geochemistry and hydrothermal alteration of the
Oman ophiolite Journal of Geophysical Research 106 11083---11099
Kelemen P B amp Aharonov E (1998) Periodic formation of magma
fractures and generation of layered gabbros in the lower crust
beneath oceanic spreading ridges Annual Geological Union Special
Monograph 106 267---289
Kelemen P B Koga K amp Shimizu N (1997) Geochemistry of
gabbro sills in the crustmantle transition zone of the Oman
ophiolite implications for the origin of the oceanic lower crust Earth
and Planetary Science Letters 146 475---488Kelley D S amp Delaney J R (1987) Two-phase separation and
fracturing in mid-ocean ridge gabbros at temperatures greater than
700C Earth and Planetary Science Letters 83 53---66
Koepke J Feig S T Snow J amp Freise M (2003) Petrogenesis of
oceanic plagiogranites by partial melting of gabbros an experi-
mental study Contributions to Mineralogy and Petrology on line first http
wwwspringerlinkcomopenurlaspgenre=s00410-003-0511-9
Kyser T K (1986) Stable isotope variations in the mantle In
Valley J W Taylor H P amp OrsquoNeil J R (eds) Stable Isotopes in
High Temperature Geological Processes Mineralogical Society of America
Reviews in Mineralogy 16 141---164
Lanphere M A (1981) K---Ar ages of metamorphic rocks at the base
of the Samail ophiolite Oman Journal of Geophysical Research 86
2777---2782
Lanphere M A Coleman R G amp Hopson C A (1981) Sr isotopic
tracer study of the Samail ophiolite Oman Journal of Geophysical
Research 86(B4) 2709---2720
Leake B E (1997) Nomenclature of amphiboles report of the
subcommittee on amphiboles of the International Mineralogical
Association commission on new minerals and mineral names
European Journal of Mineralogy 9 623---651
Lecuyer C amp Reynard B (1996) High-temperature alteration of
oceanic gabbros by seawater (Hess Deep Ocean Drilling Program
Leg 147) evidence from oxygen isotopes and elemental fluxes
Journal of Geophysical Research 101(B7) 15883---15897
Liou J G Kuniiyoshi S amp Ito K (1974) Experimental study of the
phase relations between greenschist and amphibolite in a basaltic
system American Journal of Science 274 613---632
MacLeod C J amp Rothery D A (1992) Ridge axial segmentation in
the Oman ophiolite evidence from along-strike variations in the
sheeted dyke complex Geological Society of America Special Papers 60
39---63
Manning C E MacLeod C J amp Weston P E (1996a) Fracture-
controlled metamorphism of Hess Deep gabbros site 984
constraints on the roots of mid-ocean ridge hydrothermal systems
at fast-spreading centers In GillisM C Allan KM ampMeyer P S
(eds) Proceedings of the Ocean Drilling Program Scientific Results 147
College Station TX Ocean Drilling Program pp 189---211Manning C E Weston P E amp Mahon K I (1996b) Rapid high-
temperature metamorphism of East Pacific Rise gabbros from Hess
Deep Earth and Planetary Science Letters 144 123---132Manning C E MacLeod C J amp Weston P E (2000) Lower-crustal
cracking front at fast-spreading ridges evidence from the East Pacific
Rise and the Oman ophiolite Journal of the Geological Society London
349 261---272McCollom T M amp Shock E L (1998) Fluid---rock interactions in
the lower oceanic crust thermodynamic models of hydrothermal
alteration Journal of Geophysical Research 103 547---575
McCulloch M T Gregory R T Wasserburg G J amp Taylor H P
(1981) Sm---Nd Rb---Sr and 18O16O isotopic systematics in an
oceanic crustal section evidence from the Samail ophiolite Journal of
Geophysical Research 86(B4) 2721---2735Mevel C Gillis K M Allan J F amp Meyer P S (eds) (1996)
Proceedings of the Ocean Drilling Program Scientific Results 147 College
Station TX Ocean Drilling Program
Nehlig P amp Juteau T (1988) Flow porosities permeabilities and
preliminary data on fluid inclusions and fossil thermal gradients in
the crustal sequence of the Sumail ophiolite (Oman) Tectonophysics
151 199---221
Nehlig P Juteau T Bendel V amp Cotten J (1994) The root zone of
oceanic hydrothermal systems constraints from the Samail ophiolite
(Oman) Journal of Geophysical Research 99 4703---4713
Nicolas A amp Boudier F (1991) Rooting of the sheeted dyke complex
in Oman ophiolite In Peters Tj Nicolas A amp Coleman R (eds)
Ophiolite Genesis and Evolution of the Oceanic Lithosphere Dordrecht
Kluwer Academic pp 39---54
BOSCH et al HYDROTHERMAL CIRCULATION IN OMAN OPHIOLITE
1207
Nicolas A amp Ildefonse B (1996) Flow mechanism and viscosity in
basaltic magma chambers Geophysical Research Letters 23(16)
2013---2016
Nicolas A Ceuleneer G Boudier F amp Misseri M (1988) Structural
mapping in the Oman ophiolites mantle diapirism along an ocean
ridge Tectonophysics 151 27---56
Nicolas A Boudier F Ildefonse B amp Ball E (2000) Accretion
of Oman and United Arab Emirates ophiolite-----discussion of a
new structural map Marine Geophysical Research 21(3---4)147---179
Nicolas A Boudier F Michibayashi K amp Gerbert-Gaillard L
(2001) Aswad massif (United Arab Emirates) archetype of the
Oman---UAE ophiolite belt Geological Society of America Bulletin 349
499---451
Nicolas A Mainprice D amp Boudier F (2003) High temperature
seawater circulation through the lower crust of ocean-ridges-----amodel derived from the Oman ophiolites Journal of Geophysical
Research on line first httpwwwaguorgpubscurrentjb 108(B8)
2372
Pallister J S amp Hopson C A (1981) Samail ophiolite plutonic suite
field relations phase variation cryptic variation and layering and a
model of a spreading ridge magma chamber Journal of Geophysical
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morphology on magma supply and spreading rate Nature 364
706---708
Pin C Briot D Bassin C amp Poitrasson F (1994) Concomitant
extraction of Sr and Sm---Nd for isotopic analysis in silicate samples
based on specific extraction chromatography Analytica Chimica Acta
298 209---217
Rothery D A (1983) The base of a sheeted dyke complex Oman
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axes Journal of the Geological Society London 140 287---296
Spear F S (1981) An experimental study of hornblende stability and
compositional variability in amphibolite American Journal of Science
281 697---734
Spulber S D amp Rutherford M J (1983) The origin of rhyolite and
plagiogranite in oceanic crust an experimental study Journal of
Petrology 24 1---25Stakes D S (1991) Oxygen and hydrogen isotope compositions of
oceanic plutonic rocks high-temperature deformation and meta-
morphism of oceanic layer 3 In Taylor H P OrsquoNeil J et al (eds)
Geochemical Society Special Publication 3 77---90
Stakes D S amp Taylor H P (1992) The northern Samail ophiolite an
oxygen isotope microprobe and field study Journal of Geophysical
Research 97(B5) 7043---7080Staudigel H Davies G R Hart S R Marchant M amp Smith B
M (1995) Large scale isotopic Sr Nd and O isotopic anatomy of
altered oceanic crust DSDPODP sites 417418 Earth and Planetary
Science Letters 130 169---185Turner F J amp Verhoogen J (1960) Igneous and Metamorphic Petrology
New York McGraw---Hill 694 pp
Wilcock W S D amp Delaney J R (1996) Mid-ocean ridge
sulfide deposits evidence for heat extraction from magma
chambers or cracking fronts Earth and Planetary Science Letters 145
49---64
Witt G amp Seck H A (1989) Origin of amphibole in recrystallized
and porphyroclastic mantle xenoliths from Rhenish massif implica-
tions for the nature of mantle metasomatism Earth and Planetary
Science Letters 91 327---340
JOURNAL OF PETROLOGY VOLUME 45 NUMBER 6 JUNE 2004
1208
modification of the Sr isotopic composition fromMORB-source value is however explained by the same phenom-enon As all these minerals display initial 87Sr86Sr ratiosdistinct from the MORB source and because they equili-brated at VHT or HT temperatures this supports anearly intervention of seawater during crystallization ofthese minerals Most of these samples with the exceptionof the epidosite dyke overlap the domains defined byKawahata et al (2001) for the Wadi Fizh gabbros alteredat VHT and HT conditions in the amphibolite facies(labelled lsquoVHTrsquo and lsquoHTrsquo in Fig 9a)Three samples (two epidosite dykes and an one epidote
fraction) have very high Sr contents and the highest Sr
isotopic compositions of the present dataset The twowhole-rock samples overlap the field of the Wadi Fizhsamples altered at high temperature during greenschist-facies metamorphism (Kawahata et al 2001) This typeof alteration is common at the ridge axis and formedthrough intensive exchange with seawater-derived fluidsFigure 10b indicates that all the Sr---O isotope data are
distinct from MORB compositions and suggests that thefluids reacting with the magmas could be derived fromseawater In addition the d18O values show that isotopicexchange occurred under VHT or HT conditions Thefluids could have the composition of seawater or theycould have the composition of seawater modified through
0703
0705
0707
001 01 1
1 Sr (ppm)
MORB
(87Sr
86Sr)
init
ial
Seawater
VHT
HT
MT36b
36b
36b
43a 128b90b
41d2
37b19d
15f
128b
41d2
41d2
19a
Ma2Ma2
19d
90b
37b
Whole RockPlagioclasePargasiteGreen HornblendeClinopyroxene
Epidote
(a)
0 2 4 6 8
δ18O(permil)
0703
0705
0707Seawater
MORB-mantle
128b 41d2
90b 41d2
Ma2128b
37b
43a
19d
90b
Whole RockPlagioclasePargasiteGreen HornblendeClinopyroxene
Epidote
HT LT
19d
(87Sr
86Sr)
init
ial
(b)
Fig 10 (a) Initial 87Sr86Sr isotopic ratios plotted against 1Sr for whole rocks and mineral separates from the gabbro section of the Omanophiolite Fields shown are from Kawahata et al (2001) and correspond to MT-----basalt sheeted dyke diabase altered at medium temperature(prehnite---actinolite facies sequence 3) HT-----diabase plagiogranite metagabbro and epidosite formed at high temperature (greenschist faciessequence 4) VHT-----gabbro altered at very high temperature (amphibolite facies sequence 5) The dashed line is a binary mixing line betweenMORB (Lanphere et al 1981) and Cretaceous seawater (Burke et al 1982) (b) Initial 87Sr86Sr isotopic composition plotted against d18O value forwhole rocks and minerals from the Oman ophiolite Cretaceous seawater and MORB end-members as in (a) Dashed line represents mixing curvebetweenMORB and seawater at high temperature (HT) Low-temperature (LT) exchanges between these two components would be responsible foran increase of the d18O primary MORB value
BOSCH et al HYDROTHERMAL CIRCULATION IN OMAN OPHIOLITE
1203
exchange with the surrounding rocks during its penetra-tion into the crustal section In an environment in whichfluid---rock interaction occurs at variable temperaturesthe d18O is greatly influenced by the temperature atwhich the exchange took place and by the waterrockratio It is evident from Figs 9b and 10b that none of thestudied gabbros have plagioclase and amphibole withMORB-like d18O and 87Sr86Sr values The separateminerals display a broad range of d18O values and mostexhibit extreme 18O depletion (Table 5) Hydrothermalexchange kinetics are similar in amphibole and pyroxeneand these two minerals are more resistant to oxygenexchange during water---rock interaction than coexistingplagioclase (Gregory et al 1989) The most probableexplanation of the low d18O values measured in bothamphiboles and pyroxenes is that they crystallized athigh temperature in the presence of a relatively low 18Oseawater-derived hydrothermal fluid [d18O from 01 to07 after Gregory amp Taylor (1981)] The anomalouslyhigh d18O value (thorn1009) measured for the plagioclase ofthe micro-gabbroic dyke (01 OA19d) can be interpretedas resulting from a superimposed overprint caused by alate-stage penetration of fluids at lower temperature Inthis sample the occurrence of such different 18O16Oratios between coexisting plagioclase and amphibole isnot consistent with equilibrium fractionation betweenthese two minerals at any possible temperature Thisobservation suggests that parts of the ophiolite sequencehave undergone a high-temperature hydrothermal eventprior to the later low-temperature event that producedthe strong 18O increase in coexisting plagioclaseThe depletion of the d18O values results from high-temperature hydrothermal alteration (Gregory amp Taylor1981 Stakes amp Taylor 1992) The oxygen isotopic datasupport the contention that most studied samples havebeen affected by a VHT---HT hydrothermal alteration byfluids derived from seawater Some of the analysed sam-ples presenting petrological evidence of crystallization ofLT secondary minerals have been overprinted by the latelow-temperature alteration thus swamping the earlierhigh-temperature alteration effects
Determination of waterrock ratio
To better evaluate the exchange between seawater andthe gabbroic rocks waterrock ratios (WR) have beenestimated for the whole rocks analysed during the courseof this study The different models used to constrain thisratio mainly differ by the calculation of the effective ratioor by the estimated weight ratio and by considering openor closed systems for water circulation (Albareede et al1981 McCulloch et al 1981) The WR ratios reportedin Table 5 have been calculated assuming a closed sys-tem Using this formulation these values are minimabecause the calculation always uses pristine fluid for the
initial fluid thus maximizing the value in the denomina-tor If the fluid was shifted to lower 87Sr concentrations byexchange with the rock on the downward path theinferred values would be much higher (Davis et al 2003)The calculated WR ratios for the samples from this
study range from 31 to 219 (Table 5) Two main groupsof samples can be recognized on the basis of their waterrock ratios The lowest ratios ranging from 31 to 47have been determined for massive gabbros or anatecticgabbros but also for dykes and veins devoid of late LTalteration Similar ratios have been previously calculatedfor example for the discharged hydrothermal solutions atthe 13N and 21N East Pacific Rise (Di Donato et al1990 Fouquet et al 1991) The gabbros from the Ibrasection in the Oman ophiolite gave effective WR ratiosof 05 33 and 42 (McCulloch et al 1981) in goodagreement with the values obtained in this study Theleast altered gabbro of the Ibra section yields a WR ratioof 05 in good agreement with the exceptionally fresh-ness of this rock characterized by typical mantle oxygenvalues for its minerals (McCulloch et al 1981) Samplescharacterized by waterrock effective ratios in the rangeof 3---5 can be related to penetrative alteration via thehydrothermal system Lecuyer amp Reynard (1996) havedemonstrated in oceanic gabbros from the Hess Deepaltered in similar HT conditions that WR mass ratios inthe range of 02---1 are sufficient to account for the mod-ified stable isotope signature of the gabbros Higherwaterrock values (WR 45) have been calculated forsamples affected by superimposed late hydrothermalalteration and for the epidosite samples (Table 5) Theyprobably acquired their isotopic characteristics throughinteraction with a larger volume of seawater duringgreenschist-facies metamorphism Similar or higherWR values have been published in previous studies(ie McCulloch et al 1981 Kawahata et al 2001)They correspond either to samples from the upper-crus-tal sequence (ie basalt sheeted dyke or plagiogranite) orto gabbros from the crustal section located in a particularenvironment such as for example the Wadi Fizh sectionthat is located close to a segment boundary along thepalaeo-spreading axis (Nicolas et al 2000)
Mechanism for seawater interaction withmagma
Nicolas et al (2003) have proposed that variable amountsof seawater can circulate at high temperature through-out the crustal section down to the walls of the magmachamber via a network of parallel microcracks a fractionof a millimetre wide Such microcracks and their relation-ship with the VHT---HT hydrous alteration in the sur-rounding gabbros have been described in detail byNicolas et al Via this network of microcracks largeamounts of seawater can have reacted with the magma
JOURNAL OF PETROLOGY VOLUME 45 NUMBER 6 JUNE 2004
1204
and induced variable changes of its Sr and O isotopiccomposition This regular network of parallel micro-cracks could constitute the recharge system and thehydrated gabbroic dykes the discharge system Physicalparameters and the geophysical models of Nicolas et al(2003) are in agreement with this scenario Such a pene-tration of seawater-derived hydrothermal fluids throughthe lower crust along grain boundaries and a microscopicfracture network at temperatures 4750C is consistentwith recent thermodynamic models (McCollom amp Shock1998)Seawater-altered and high-temperature recrystallized
diabases (protodykes) from the overlying sheeted dykecomplex stoped into the melt lenses could represent anindirect way for seawater to enter the system as they canremelt during their subsidence through the magmachamber A simple mass balance calculation suggeststhat this indirect contamination is not significant Assess-ments of the amount of recycled crust 1 in volumeby Nicolas et al (2000) based on the protodyke remnantsin the gabbro section or 20 by Coogan et al (2003)based on chlorine content in EPR basalts lead to a sea-waterrock ratio of the order of 104 to 103Thus following Nicolas et al (2003) we propose that
seawater has been introduced along microcracks down tothe walls of the magma chamber and has diffused alonggrain boundaries in the way described by Manning et al(2000) progressively invading the host gabbro Gabbroicdykes result from the injection of hydrous melt into thehost gabbros at falling temperatures as they drift awayfrom the magma chamber This water-saturated meltcould result from the extensive hydration of the crystal-lizing gabbros near the walls of the magma chamberwhen seawater penetrates through this wall (Fig 1)Similar plumbing systems driving seawater down to the
limit of the magma chamber and back to the surface havebeen already proposed in other environments such as theMid-Atlantic Ridge (Kelley amp Delaney 1987 Cooganet al 2001) the East Pacific Rise at Hess Deep (Gillis1995 Manning et al 1996a 1996b) the Tonga forearc(Banerjee amp Gillis 2001) and the Troodos ophiolite(Gillis amp Roberts 1999) The melting curve of wet basalthas a negative slope but is close to the dry solidus at lowpressure in the lower gabbros and the first melts areplagiogranitic (Spulber ampRutherford 1983) Such plagio-granites are ubiquitous in the highest gabbros and in theroot zone of sheeted dykes in Oman (Rothery 1983Juteau et al 1988 Nicolas amp Boudier 1991) in manyother ophiolites and in the oceanic crust where they havebeen interpreted as resulting either from wet anatexis ofhot gabbros or from the final product of crystallization ofa basaltic melt (Spulber amp Rutherford 1983) Plagio-granites are rare in the lower gabbros exceptionallyassociated with hydrous gabbro segregations inside thelayered gabbros Their constitutive minerals are also
remarkably absent along the VHTmicrocracks althoughthese gabbros were above the wet solidus A possibleexplanation has been proposed by McCollom amp Shock(1998) who wrote that at high temperature lsquoaqueousfluids may leave very little conspicuous petrological evid-ence for their presencersquo the reason being that thehigh-T conditions would be closer to batch meltingObviously more detailed studies on this specific problemare needed We conclude that the large degree of hydrousmelting locally required at the walls of the magma cham-ber to account for the generation of a gabbroic hydrousmelt may have erased the traces of the incipient plagio-granitic melt
CONCLUSIONS
Fostered by the recent discovery of high-temperatureseawater contamination in some gabbros of the Omanophiolite (Manning et al 2000) we have conducted astudy on 500 gabbro specimens from selected areas ofthe entire Samail ophiolite belt and on the hydrous gab-broic dykes that cross-cut them The highest-temperaturereactions (VHT) recorded in olivine and clinopyroxene ingabbros as orthopyroxene and pargasite coronas reach975C They are followed at lower HT conditions withreplacement of olivine by an assemblage of tremolitemagnetite and plagioclase surrounded by chlorite andby pargasite development in clinopyroxene followedfinally by the LT lizardite---olivine and saussurite---plagioclase greenschist-facies alteration With a fewexceptions the VHT alteration tends to be restricted tothe lower gabbros and to the vicinity of the Moho andthe HT alteration to the middle and upper gabbros Thepreservation of the vertical distribution of VHT---HTfacies indicates that progressive cooling during off-axisspreading did not obliterate this distribution and conse-quently that the VHT---HT hydrothermal alteration tookplace in a very restricted time interval at the limit of thecooling magma chamber In the deep and hot gabbrosthe main water channels may be sub-millimetre-sizedmicrocracks with a dominantly vertical attitude the ori-gin and dynamics of which have been investigated in aprevious paper (Nicolas et al 2003)Beyond these high-temperature alteration zones mas-
sively induced in the gabbros seawater may be respons-ible for the generation of clinopyroxene---pargasitegabbro dykes that cross-cut all gabbros and that areupsection clearly connected to the LT hydrothermalvein system Petrostructural evidence suggests that thesedykes were generated by hydration of the crystallizinggabbros near the internal wall of the LVZ---magmachamber The resultant melts were subsequently intrudedat various levels in the cooling lower and middle gabbrosPetrological observations of nearly all the gabbros ana-lysed in the present study located from the root zone of
BOSCH et al HYDROTHERMAL CIRCULATION IN OMAN OPHIOLITE
1205
the sheeted dyke complex down to the Moho emphasizethe imprint of VHT---HT hydrous alteration The VHThydrothermal event is characterized by temperatures ofequilibration around 900---1000C The temperatures atwhich the HT hydrous event occurred are estimated tobe in the range 550---900CStrontium and oxygen isotope compositions measured
on whole rocks and separated minerals significantlydepart from primary MORB values The Sr and O iso-tope data obtained for pargasite green hornblende andclinopyroxene record the participation of seawater at thetemperature of crystallization of the minerals Plagio-clases and whole rocks define a trend toward a compo-nent characterized by a high radiogenic 87Sr86Sr valueand a higher Sr content than normal seawater Seawateris clearly identified as the fluid responsible for the modi-fication of the Sr and O signatures for both massivegabbros and dykes and veins Nevertheless the high Srcontent of the extraneous component requires eitheropen-system exchange allowing penetration of a greaterquantity of seawater or penetration of seawater modifiedby interaction with the oceanic crust during its travel paththrough the crustal section The waterrock ratio calcu-lated on the basis of the Sr isotopes is between three andfive for the enclosing gabbros and for the least LT altereddykes The VHT---HT hydrothermal event affectingthese samples is related to penetrative alteration via therecharge and discharge hydrothermal system Thewaterrock ratio is 10 for dykes exhibiting evidenceof a LT hydrothermal alteration overprint and reaches22 for the epidosite dyke The depletion in d18O char-acterizes all the studied samples with the single exceptionof the micro-gabbroic dyke plagioclase This agreeswith the conclusions based on Sr isotopes suggestingthat these samples have undergone exchange with sea-water-derived fluids during a VHT---HTalteration hydro-thermal event
ACKNOWLEDGEMENTS
This work has benefited from financial support by theCentre National de la Recherche Scientifique (INSUInteerieur-Terre andDYETI programmes) and inOmanfrom the willingness of the Directorate of MineralsMinistry of Commerce and Industry and the hospitabil-ity of the people Technical help in the laboratory is alsoacknowledged including C Nevado for the thin sectionsE Ball for the illustrations B Galland for her assistancein the chemistry room and C Bassoulet for help duringthermal ionization mass spectrometric analyses of the Srresults from the leaching experiments Constructivereviews from R T Gregory C Lecuyer an anonymousreviewer and particularly from M Wilson greatlyhelped to improve an earlier version of the manuscript
REFERENCESAgrinier P Mevel C Bosch D amp Javoy M (1993) Metasomatic
hydrous fluids in amphibole peridotites from Zabargad Island (Red
Sea) Earth and Planetary Science Letters 120 187---205Albareede F A Michard A Minster J F amp Michard G (1981)
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Proceedings of the Ocean Drilling Program Scientific Results 106109
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1206
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(2001) Sr isotope geochemistry and hydrothermal alteration of the
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Kelemen P B Koga K amp Shimizu N (1997) Geochemistry of
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fracturing in mid-ocean ridge gabbros at temperatures greater than
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of the Samail ophiolite Oman Journal of Geophysical Research 86
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oceanic gabbros by seawater (Hess Deep Ocean Drilling Program
Leg 147) evidence from oxygen isotopes and elemental fluxes
Journal of Geophysical Research 101(B7) 15883---15897
Liou J G Kuniiyoshi S amp Ito K (1974) Experimental study of the
phase relations between greenschist and amphibolite in a basaltic
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MacLeod C J amp Rothery D A (1992) Ridge axial segmentation in
the Oman ophiolite evidence from along-strike variations in the
sheeted dyke complex Geological Society of America Special Papers 60
39---63
Manning C E MacLeod C J amp Weston P E (1996a) Fracture-
controlled metamorphism of Hess Deep gabbros site 984
constraints on the roots of mid-ocean ridge hydrothermal systems
at fast-spreading centers In GillisM C Allan KM ampMeyer P S
(eds) Proceedings of the Ocean Drilling Program Scientific Results 147
College Station TX Ocean Drilling Program pp 189---211Manning C E Weston P E amp Mahon K I (1996b) Rapid high-
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cracking front at fast-spreading ridges evidence from the East Pacific
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349 261---272McCollom T M amp Shock E L (1998) Fluid---rock interactions in
the lower oceanic crust thermodynamic models of hydrothermal
alteration Journal of Geophysical Research 103 547---575
McCulloch M T Gregory R T Wasserburg G J amp Taylor H P
(1981) Sm---Nd Rb---Sr and 18O16O isotopic systematics in an
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Geophysical Research 86(B4) 2721---2735Mevel C Gillis K M Allan J F amp Meyer P S (eds) (1996)
Proceedings of the Ocean Drilling Program Scientific Results 147 College
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Nehlig P Juteau T Bendel V amp Cotten J (1994) The root zone of
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1207
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Geochemical Society Special Publication 3 77---90
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oxygen isotope microprobe and field study Journal of Geophysical
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Wilcock W S D amp Delaney J R (1996) Mid-ocean ridge
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49---64
Witt G amp Seck H A (1989) Origin of amphibole in recrystallized
and porphyroclastic mantle xenoliths from Rhenish massif implica-
tions for the nature of mantle metasomatism Earth and Planetary
Science Letters 91 327---340
JOURNAL OF PETROLOGY VOLUME 45 NUMBER 6 JUNE 2004
1208
exchange with the surrounding rocks during its penetra-tion into the crustal section In an environment in whichfluid---rock interaction occurs at variable temperaturesthe d18O is greatly influenced by the temperature atwhich the exchange took place and by the waterrockratio It is evident from Figs 9b and 10b that none of thestudied gabbros have plagioclase and amphibole withMORB-like d18O and 87Sr86Sr values The separateminerals display a broad range of d18O values and mostexhibit extreme 18O depletion (Table 5) Hydrothermalexchange kinetics are similar in amphibole and pyroxeneand these two minerals are more resistant to oxygenexchange during water---rock interaction than coexistingplagioclase (Gregory et al 1989) The most probableexplanation of the low d18O values measured in bothamphiboles and pyroxenes is that they crystallized athigh temperature in the presence of a relatively low 18Oseawater-derived hydrothermal fluid [d18O from 01 to07 after Gregory amp Taylor (1981)] The anomalouslyhigh d18O value (thorn1009) measured for the plagioclase ofthe micro-gabbroic dyke (01 OA19d) can be interpretedas resulting from a superimposed overprint caused by alate-stage penetration of fluids at lower temperature Inthis sample the occurrence of such different 18O16Oratios between coexisting plagioclase and amphibole isnot consistent with equilibrium fractionation betweenthese two minerals at any possible temperature Thisobservation suggests that parts of the ophiolite sequencehave undergone a high-temperature hydrothermal eventprior to the later low-temperature event that producedthe strong 18O increase in coexisting plagioclaseThe depletion of the d18O values results from high-temperature hydrothermal alteration (Gregory amp Taylor1981 Stakes amp Taylor 1992) The oxygen isotopic datasupport the contention that most studied samples havebeen affected by a VHT---HT hydrothermal alteration byfluids derived from seawater Some of the analysed sam-ples presenting petrological evidence of crystallization ofLT secondary minerals have been overprinted by the latelow-temperature alteration thus swamping the earlierhigh-temperature alteration effects
Determination of waterrock ratio
To better evaluate the exchange between seawater andthe gabbroic rocks waterrock ratios (WR) have beenestimated for the whole rocks analysed during the courseof this study The different models used to constrain thisratio mainly differ by the calculation of the effective ratioor by the estimated weight ratio and by considering openor closed systems for water circulation (Albareede et al1981 McCulloch et al 1981) The WR ratios reportedin Table 5 have been calculated assuming a closed sys-tem Using this formulation these values are minimabecause the calculation always uses pristine fluid for the
initial fluid thus maximizing the value in the denomina-tor If the fluid was shifted to lower 87Sr concentrations byexchange with the rock on the downward path theinferred values would be much higher (Davis et al 2003)The calculated WR ratios for the samples from this
study range from 31 to 219 (Table 5) Two main groupsof samples can be recognized on the basis of their waterrock ratios The lowest ratios ranging from 31 to 47have been determined for massive gabbros or anatecticgabbros but also for dykes and veins devoid of late LTalteration Similar ratios have been previously calculatedfor example for the discharged hydrothermal solutions atthe 13N and 21N East Pacific Rise (Di Donato et al1990 Fouquet et al 1991) The gabbros from the Ibrasection in the Oman ophiolite gave effective WR ratiosof 05 33 and 42 (McCulloch et al 1981) in goodagreement with the values obtained in this study Theleast altered gabbro of the Ibra section yields a WR ratioof 05 in good agreement with the exceptionally fresh-ness of this rock characterized by typical mantle oxygenvalues for its minerals (McCulloch et al 1981) Samplescharacterized by waterrock effective ratios in the rangeof 3---5 can be related to penetrative alteration via thehydrothermal system Lecuyer amp Reynard (1996) havedemonstrated in oceanic gabbros from the Hess Deepaltered in similar HT conditions that WR mass ratios inthe range of 02---1 are sufficient to account for the mod-ified stable isotope signature of the gabbros Higherwaterrock values (WR 45) have been calculated forsamples affected by superimposed late hydrothermalalteration and for the epidosite samples (Table 5) Theyprobably acquired their isotopic characteristics throughinteraction with a larger volume of seawater duringgreenschist-facies metamorphism Similar or higherWR values have been published in previous studies(ie McCulloch et al 1981 Kawahata et al 2001)They correspond either to samples from the upper-crus-tal sequence (ie basalt sheeted dyke or plagiogranite) orto gabbros from the crustal section located in a particularenvironment such as for example the Wadi Fizh sectionthat is located close to a segment boundary along thepalaeo-spreading axis (Nicolas et al 2000)
Mechanism for seawater interaction withmagma
Nicolas et al (2003) have proposed that variable amountsof seawater can circulate at high temperature through-out the crustal section down to the walls of the magmachamber via a network of parallel microcracks a fractionof a millimetre wide Such microcracks and their relation-ship with the VHT---HT hydrous alteration in the sur-rounding gabbros have been described in detail byNicolas et al Via this network of microcracks largeamounts of seawater can have reacted with the magma
JOURNAL OF PETROLOGY VOLUME 45 NUMBER 6 JUNE 2004
1204
and induced variable changes of its Sr and O isotopiccomposition This regular network of parallel micro-cracks could constitute the recharge system and thehydrated gabbroic dykes the discharge system Physicalparameters and the geophysical models of Nicolas et al(2003) are in agreement with this scenario Such a pene-tration of seawater-derived hydrothermal fluids throughthe lower crust along grain boundaries and a microscopicfracture network at temperatures 4750C is consistentwith recent thermodynamic models (McCollom amp Shock1998)Seawater-altered and high-temperature recrystallized
diabases (protodykes) from the overlying sheeted dykecomplex stoped into the melt lenses could represent anindirect way for seawater to enter the system as they canremelt during their subsidence through the magmachamber A simple mass balance calculation suggeststhat this indirect contamination is not significant Assess-ments of the amount of recycled crust 1 in volumeby Nicolas et al (2000) based on the protodyke remnantsin the gabbro section or 20 by Coogan et al (2003)based on chlorine content in EPR basalts lead to a sea-waterrock ratio of the order of 104 to 103Thus following Nicolas et al (2003) we propose that
seawater has been introduced along microcracks down tothe walls of the magma chamber and has diffused alonggrain boundaries in the way described by Manning et al(2000) progressively invading the host gabbro Gabbroicdykes result from the injection of hydrous melt into thehost gabbros at falling temperatures as they drift awayfrom the magma chamber This water-saturated meltcould result from the extensive hydration of the crystal-lizing gabbros near the walls of the magma chamberwhen seawater penetrates through this wall (Fig 1)Similar plumbing systems driving seawater down to the
limit of the magma chamber and back to the surface havebeen already proposed in other environments such as theMid-Atlantic Ridge (Kelley amp Delaney 1987 Cooganet al 2001) the East Pacific Rise at Hess Deep (Gillis1995 Manning et al 1996a 1996b) the Tonga forearc(Banerjee amp Gillis 2001) and the Troodos ophiolite(Gillis amp Roberts 1999) The melting curve of wet basalthas a negative slope but is close to the dry solidus at lowpressure in the lower gabbros and the first melts areplagiogranitic (Spulber ampRutherford 1983) Such plagio-granites are ubiquitous in the highest gabbros and in theroot zone of sheeted dykes in Oman (Rothery 1983Juteau et al 1988 Nicolas amp Boudier 1991) in manyother ophiolites and in the oceanic crust where they havebeen interpreted as resulting either from wet anatexis ofhot gabbros or from the final product of crystallization ofa basaltic melt (Spulber amp Rutherford 1983) Plagio-granites are rare in the lower gabbros exceptionallyassociated with hydrous gabbro segregations inside thelayered gabbros Their constitutive minerals are also
remarkably absent along the VHTmicrocracks althoughthese gabbros were above the wet solidus A possibleexplanation has been proposed by McCollom amp Shock(1998) who wrote that at high temperature lsquoaqueousfluids may leave very little conspicuous petrological evid-ence for their presencersquo the reason being that thehigh-T conditions would be closer to batch meltingObviously more detailed studies on this specific problemare needed We conclude that the large degree of hydrousmelting locally required at the walls of the magma cham-ber to account for the generation of a gabbroic hydrousmelt may have erased the traces of the incipient plagio-granitic melt
CONCLUSIONS
Fostered by the recent discovery of high-temperatureseawater contamination in some gabbros of the Omanophiolite (Manning et al 2000) we have conducted astudy on 500 gabbro specimens from selected areas ofthe entire Samail ophiolite belt and on the hydrous gab-broic dykes that cross-cut them The highest-temperaturereactions (VHT) recorded in olivine and clinopyroxene ingabbros as orthopyroxene and pargasite coronas reach975C They are followed at lower HT conditions withreplacement of olivine by an assemblage of tremolitemagnetite and plagioclase surrounded by chlorite andby pargasite development in clinopyroxene followedfinally by the LT lizardite---olivine and saussurite---plagioclase greenschist-facies alteration With a fewexceptions the VHT alteration tends to be restricted tothe lower gabbros and to the vicinity of the Moho andthe HT alteration to the middle and upper gabbros Thepreservation of the vertical distribution of VHT---HTfacies indicates that progressive cooling during off-axisspreading did not obliterate this distribution and conse-quently that the VHT---HT hydrothermal alteration tookplace in a very restricted time interval at the limit of thecooling magma chamber In the deep and hot gabbrosthe main water channels may be sub-millimetre-sizedmicrocracks with a dominantly vertical attitude the ori-gin and dynamics of which have been investigated in aprevious paper (Nicolas et al 2003)Beyond these high-temperature alteration zones mas-
sively induced in the gabbros seawater may be respons-ible for the generation of clinopyroxene---pargasitegabbro dykes that cross-cut all gabbros and that areupsection clearly connected to the LT hydrothermalvein system Petrostructural evidence suggests that thesedykes were generated by hydration of the crystallizinggabbros near the internal wall of the LVZ---magmachamber The resultant melts were subsequently intrudedat various levels in the cooling lower and middle gabbrosPetrological observations of nearly all the gabbros ana-lysed in the present study located from the root zone of
BOSCH et al HYDROTHERMAL CIRCULATION IN OMAN OPHIOLITE
1205
the sheeted dyke complex down to the Moho emphasizethe imprint of VHT---HT hydrous alteration The VHThydrothermal event is characterized by temperatures ofequilibration around 900---1000C The temperatures atwhich the HT hydrous event occurred are estimated tobe in the range 550---900CStrontium and oxygen isotope compositions measured
on whole rocks and separated minerals significantlydepart from primary MORB values The Sr and O iso-tope data obtained for pargasite green hornblende andclinopyroxene record the participation of seawater at thetemperature of crystallization of the minerals Plagio-clases and whole rocks define a trend toward a compo-nent characterized by a high radiogenic 87Sr86Sr valueand a higher Sr content than normal seawater Seawateris clearly identified as the fluid responsible for the modi-fication of the Sr and O signatures for both massivegabbros and dykes and veins Nevertheless the high Srcontent of the extraneous component requires eitheropen-system exchange allowing penetration of a greaterquantity of seawater or penetration of seawater modifiedby interaction with the oceanic crust during its travel paththrough the crustal section The waterrock ratio calcu-lated on the basis of the Sr isotopes is between three andfive for the enclosing gabbros and for the least LT altereddykes The VHT---HT hydrothermal event affectingthese samples is related to penetrative alteration via therecharge and discharge hydrothermal system Thewaterrock ratio is 10 for dykes exhibiting evidenceof a LT hydrothermal alteration overprint and reaches22 for the epidosite dyke The depletion in d18O char-acterizes all the studied samples with the single exceptionof the micro-gabbroic dyke plagioclase This agreeswith the conclusions based on Sr isotopes suggestingthat these samples have undergone exchange with sea-water-derived fluids during a VHT---HTalteration hydro-thermal event
ACKNOWLEDGEMENTS
This work has benefited from financial support by theCentre National de la Recherche Scientifique (INSUInteerieur-Terre andDYETI programmes) and inOmanfrom the willingness of the Directorate of MineralsMinistry of Commerce and Industry and the hospitabil-ity of the people Technical help in the laboratory is alsoacknowledged including C Nevado for the thin sectionsE Ball for the illustrations B Galland for her assistancein the chemistry room and C Bassoulet for help duringthermal ionization mass spectrometric analyses of the Srresults from the leaching experiments Constructivereviews from R T Gregory C Lecuyer an anonymousreviewer and particularly from M Wilson greatlyhelped to improve an earlier version of the manuscript
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Bosch D (1991) Introduction drsquoeau de mer dans le diapir mantellique
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Chernosky J V Berman R G amp Bryndzia L T (1988) Stability
phase relations and thermodynamic properties of chlorite and
serpentine group minerals In Bayley S W (ed) Hydrous
Phyllosilicates Mineralogical Society of America Reviews in Mineralogy 19
295---346
Coogan L A Wilson R N Gillis K M amp MacLeod C J (2001)
Near-solidus evolution of oceanic gabbros insights from amphibole
geochemistry Geochimica et Cosmochimica Acta 65 4339---4357
Coogan L A Mitchell N C amp OrsquoHara M J (2003) Roof
assimilation at fast spreading ridges an investigation combining
geophysical geochemical and field evidence Journal of Geophysical
Research on line first httpwwwaguorgpubscurrentjb 108(B1)
1171
Davis A C Bickle M J amp Teagle D A H (2003) Imbalance in the
oceanic strontium budget Earth and Planetary Science Letters 211
173---187
Detrick R S Buhl P Vera E Mutter J Orcutt J Madsen J amp
Trocher T (1987) Multi-channel seismic imaging of a crustal
magma chamber along the East Pacific Rise Nature 326 35---41
De Villiers S (1999) Seawater strontium and SrCa variability in the
Atlantic and Pacific oceans Earth and Planetary Science Letters 171
623---634
Di Donato G Joron J L Treuil M amp Loubet M (1990)
Geochemistry of zero-age N-MORB from hole 648B ODP legs
106---109 MAR 22 In Detrick R S Honnorez J et al (eds)
Proceedings of the Ocean Drilling Program Scientific Results 106109
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