kuiper-binnenwerk 30-09-2003 15:03 pagina 123 chapter 5 · 40ar/39ar dating of tephra layers...

40Ar/39Ar DATING OF TEPHRA LAYERS INTERCALATED INASTRONOMICALLY TUNED MARINE, UPPER MIOCENE

SEDIMENTARY SEQUENCES IN THE WESTERNMEDITERRANEAN.

CHAPTER 5

Kuiper-binnenwerk 30-09-2003 15:03 Pagina 123

INTRODUCTION

Steenbrink et al. (1999) confirmed the astronomical theory of climate change for the Pliocene by 40Ar/39Ardating of volcanic ash layers distributed at several stratigraphic positions in the astronomically tunedsections in Ptolemais, resulting in an average period of 21.7 kyr for a cycle, equivalent to the expectedduration of the cycles based on astronomical theory. 40Ar/39Ar data of the volcanic deposits in the MelillaBasin will be used to confirm the astronomical theory of climate change for the late Miocene for the firsttime with a method (i.e., 40Ar/39Ar) completely independent from astronomical theory.

Once confirmed, the 40Ar/39Ar ages will be used for comparison with astronomical ages of the ash layers. Inchapter 3 a discrepancy between isotopic 40Ar/39Ar and astronomical ages for the eastern Mediterraneanwas presented, where 40Ar/39Ar ages tended to be systematically younger than the astronomicalcounterparts. In chapter 4 an astronomical time frame was established for late Miocene sediments in theMelilla Basin, Morocco. This basin is located near the source area of two volcanic complexes active duringthe late Miocene, resulting in a large number of very coarse grained volcanic deposits intercalated in theastronomically tuned sections. Therefore, the existence (or non-existence) of systematic differencesbetween the 40Ar/39Ar and astronomical tuning methods could be further explored in great detail, while thelarge crystal size (>1 mm) in some of the Melilla volcanic deposits enabled us to perform single crystal40Ar/39Ar dating, allowing the detection of contaminating (xenocrystic) grains. To supplement the data forthe western Mediterranean a few volcanic ash layers intercalated in the astronomically tuned Sorbas andNijar Basins (Sierro et al., 2001) have been dated as well, although the crystal sizes were much smaller(125-250 µm) and biotite was the main suitable K-rich mineral.

GEOLOGICAL BACKGROUND

The Betic-Rif Cordilleras of southern Spain and northern Morocco constitute the westernmost extension ofthe Mediterranean Alpine orogenic belt, which formed in response to the convergence between theEuropean and African plates during the Cenozoic. During the early to middle Miocene this convergencecaused thrusting and westward migration of the Internal zones over the External zones (Figure 5.1a).Ongoing convergence between Africa and Iberia caused deformation along NE-SW and NW-SE strike slipfaults and the formation of intramontane basins in SE Spain (Sanz de Galdeano and Vera, 1992). TheSorbas and Nijar Basins are two of those basins. The oldest Neogene sediments are conglomerates ofSerravallian age, which are overlain by turbiditic sandstones. A major unconformity separates the turbiditicsandstones from onlapping shallow marine calcarenites (Azagador member) of latest Tortonian age (~7Ma). The Azagador member changes upward into marls, clays and diatomites of early Messinian age (theAbad member). The Abad marls are deposited in the relatively deep parts (200-300 m, Troelstra et al.,1980) of the basins and they change laterally and vertically into platform carbonates and reefs. The Abadmarls can be divided in two distinct units: the Lower and the Upper Abad marls. The Lower Abad unitconsists of an alternation of indurated homogenous whitish marls and soft homogeneous grey marls rich inforaminifera. The Upper Abad is characterized by the intercalation of sapropels and indurated diatom-richlayers (Sierro et al., 2001). The Abad marls in the central part of the basin are overlain by massive gypsumdeposits (Yesares member), which is correlated to the Lower Evaporites of the Central Mediterranean(Krijgsman et al., 1999a).

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The extension, forming the Melilla-Nador Basin, started in the Serravallian (~14 - 10 Ma) and resulted in thedeposition of a folded, molassic clastic wedge (Guillemin and Houzay, 1982), followed by uplift of thePaleozoic metamorphic core due to NS-directed compression. Messinian extension provided accommodationspace for marl deposition in the basin and the development of a carbonate platform. The onset of marinesedimentation in the Melilla Basin is coeval to the deepening of the central Betic Corridor basins like Sorbasand Nijar (Van Assen et al., in press, chapter 4). This marine sedimentation phase ended with a lagoonal tolacustrine regressive sequence (Saint Martin et al., 1991). Sedimentary cyclicity in the Melilla Basin startswith bipartite cycles composed of indurated cherty laminites and homogeneous sediments with a change incolor from gray to brownish marls coeval to the change in color from the Lower to Upper (more brownish)Abad in the Sorbas and Nijar Basins. Therefore, a change in the bio-lithofacies characteristics in the MelillaBasin is coeval to the division in the Lower and Upper Abad (Van Assen et al., in press, chapter 4). Volcanichorizons suitable for 40Ar/39Ar dating are intercalated in both the cyclic sediments of the Sorbas, Nijar andMelilla Basins.

NEOGENE VOLCANISM IN THE WESTERN MEDITERRANEAN

Calc-alkaline, potassic and basaltic volcanism is scattered across the Alboran Sea and Betic-Rif systems(e.g., Hernandez and Bellon, 1985). The Neogene magmatic activity developed in the eastern Betics seemsto be closely related to the major strike slip faults. In the Guercif Basin magmatic activity seems also berelated to the faults along the border of the basin. The earliest Neogene igneous activity was a basaltic dyke

125

CHAP

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5

40Ar/39Ar DATA OF MELILLA, SORBAS AND NIJAR

RABAT

TANGER

SEVILLA

CADIZMALAGA

NIJAR

GIBRALTAR

TETOUAN

MELILLA

MEKNES Mou

loyaLEGEND:

I B E R I A N M E S E TA

N

FORTUNA

GUERCIFHAUTS

PLATEAUXMOROCCAN MESETA

FEZ

TAZA

VALENCIA

Internal Units

External Units

Marine Gateways

Major faults

Basement

GRANADAM

IDDLE

ATLAS

AT L A N T I C

O C E A N

M E D I T E R R A N E A N

S E A

B

C

A

× Jumilla

× Cancarix

× Calasparra

Mula ××

Murcia

× Fortuna

Zeneta×

Almeria×

Cabo de Gata

× Carboneras

Vera×

×Aguilas

×Cartagena

×Barqueros

×Mazzaron

0 50 km

Ncalc-alkaline series

lamproitic series

alkaline series

shoshonitic seriesNijar b

asin

Sorbas basin

SE SPAIN

7.7 Ma

2.8 - 2.6 Ma

7.0 - 5.7 Ma

8.2 - 6.8 Ma

7.0 - 5.7 Ma

15.1 - 7.3 Ma

Ras Tarf

Trois Fourches

× MelillaGourougou

Amjar

Sidi Maatoug

× Ain Zohra

Guilliz

Taourirt×

Chaffarines islands

× Oujda

ALGERIAMOROCCO

~12 Ma

9.6 - 4.7 Ma

8.0 - 4.9 Ma

< 5.9 Ma

< 5.9 Ma

< 5.9 Ma

13 - 7.9 Ma

5.2 - 1.5 Ma

~10 Ma N

0 50 km

SORBAS

FFiigguurree 55..11aa--cc:: GGeeoollooggiiccaall sseettttiinngg aanndd NNeeooggeennee

vvoollccaanniissmm iinn tthhee BBeettiicc--RRiiff CCoorrddiilllleerraass..

These figures show the geological setting, the

distribution of Neogene volcanism and the locations

of the studied sections in the Western Mediterranean.


swarm around 22 Ma located in the central and western internal zones of the Betics (Torres-Roldán et al.,1986). Furthermore, the oldest volcanic rocks are mainly calc-alkaline and are restricted to the Cabo deGata-Carboneras area in Spain (figure 5.1b). In the eastern Rif calc-alkaline series are more widespread(figure 5.1c). Dates obtained for this calc-alkaline volcanic suite range from 15-7 Ma in southern Spain,whereas the Rif-Tell (northern Africa) calc-alkaline mountains range from 13-8 Ma in age (e.g., Bellon et al.,1981; Hernandez and Bellon, 1985). In the Alboran Sea the Alboran Island is a calc-alkaline volcanic edificewith volcanism at 18-7 Ma (Aparico et al., 1991). A second suite of dominantly potassic-ultrapotassic rockswith a wide variety of compositions (shoshonitic to lamproitic) erupted in Spain between 8 and 5 Ma and inNorth Africa between 9 and 4 Ma. The lamproitic volcanic rocks are widely scattered in southern Spain, butdo not occur in northern Africa. The youngest volcanic rocks are alkaline basalts with an age of Pliocene toQuaternary in southern Spain, in North Africa the youngest volcanic rocks are Messinian to Quaternaryalkaline basalts.

Considering the ages of the different Neogene volcanic complexes in the western Mediterranean(figure 5.1b and c), and the relation between tephra thickness and distance from the source(Watkins et al., 1978), the volcanic tephras in the Messadit basin most probably originate fromthe Gourougou volcano located immediately south of the basin. The Trois Fourches complex, at~10 Ma immediately north of the basin is too old to be the source of the tephras studied here.The Guillez complex, approximately 100 km south of the Melilla Basin, is Messinian, but mightbe too far from the basin to account for the sometimes meters thick coarse grained (>1 mm)tephras.

The origin of the thin tephras intercalated in the Abad marls of the Sorbas and Nijar Basins insouthern Spain is less clear. They might be related to the local lamproitic volcanism in southeastSpain. However, Bellon et al. (1983) described the existence of some thin vitreous tuffs inseveral Messinian Basins (e.g., in the Sorbas Basin) and noticed that the mineralogy wascompletely different from the lamproitic volcanism. Therefore, the thin tephras intercalated inthe basin probably have another (andesitic / dacitic) source. They might for example originatefrom the Gourougou volcanic complex as well which is located ~250 km to the southwest.Chemical analyses could confirm this hypothesis, but this is beyond the scope of this study. Itmust, however be remarked that tephra Mes-4 in the Messadit section, which is by far thethickest tephra in this section, and the volcanic horizon a.1.2 from the Sorbas and Nijar Basinsoccur in exactly the same cycle suggesting that they originate from the same volcanic event.

ASTROCHRONOLOGICAL TIME CONTROL IN SORBAS, NIJAR AND MELILLA BASINS

In this study astronomical ages for the volcanic tephra layers intercalated in the sedimentarybasins are indispensable. Therefore, the astrochronological timeframe of the relevant sections issummarized here with a focus on potential uncertainties in astronomical ages (see also chapter2). The detailed astrochronological timeframe is described in detail in Sierro et al. (2001) andKrijgsman et al. (2001) for the Sorbas and Nijar Basins and in Van Assen et al. (in press) for theMelilla Basin. The astronomical ages assigned to the volcanic tephras are given in table 5.1 incombination with the 40Ar/39Ar data.

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THE ABAD MARLS

The cyclostratigraphic relations in the Abad marls (Sorbas and Nijar Basin) are discussed in detail by Sierroet al. (2001). The Lower Abad consists of bipartite cycles of indurated homogeneous whitish marls andsofter grey homogeneous marls. From cycle LA17 upwards a sapropelitic layer appears in the middle part ofthe homogeneous marls between two consecutive indurated layers. The Upper Abad is characterized by theintercalation of sapropels and indurated diatom rich layers in the homogeneous marls. The transitionbetween the Lower and Upper Abad marls is complicated by a hiatus near the margins or by sedimentinstability (several slumps) in the basin depocenter (Sierro et al., 2001). A second slump is located in theUpper Abad marls. However, the combination of several subsections, the presence of tephra interval a.1.3,the occurrence of the top acme of the G. scitula dextral group and the only continuous transition in theGafares section resulted in a reliable and continuous composite. The occurrence of the tephras and severalbiostratigraphic events in the several subsections were useful to confirm the cyclostratigraphic relations.

The Lower Abad marls could unambiguously be correlated to astronomically dated sections in Italy and onGavdos and Crete (Sierro et al., 2001; Hilgen et al., 1995; Krijgsman et al., 1997). Therefore all sedimentarycycles, bio-events and ash-layers of the Lower Abad can be directly related to the 65ºN summer insolationcurve of the La93 astronomical solution (Laskar et al., 1993a) with present day values for tidal dissipationand dynamical ellipticity. Although no sapropels occur in the cycles of the Lower Abad, the middle to upperpart of the homogeneous marls are rich in warm oligotrophic planktonic foraminiferal faunas, which are alsopresent in sapropels of the Upper Abad and other Neogene marine successions in the Mediterranean (Sierroet al., 1999). Sapropels in the Mediterranean Neogene are all linked to precession minima. Therefore, themiddle to upper part of the homogeneous marls is tuned to precession minima and summer insolationmaxima. The first sapropels in the Abad marls are without exception recorded in the middle to upper part ofthe homogeneous marls confirming this phase relation. The tuning of the lowermost 19 cycles seemsstraightforward. The homogeneous marl below the first indurated bed is characterized by a maximum inoligotrophic foraminifera and correlates well with the first prominent sapropel in other Messinian sections.Therefore this layer is correlated to the first high amplitude peak in insolation following an interval of low-amplitude fluctuations that corresponds to the interval at the base of the Abad marls where no distinctcyclicity occurs (Sierro et al., 2001). Upward tuning of each middle to upper part of the homogeneous marllevels shows also other characteristics supporting the proposed tuning, like an extra thick marl in cycle LA12corresponding to a longer precession cycle, the alternately thin-thick-thin marls of LA13, LA14 and LA15corresponding to lower-higher-lower amplitude in the insolation maxima and the relatively thin marls ofLA15 and LA16 corresponding to minimum amplitudes in the insolation minima. Therefore, an error in theastronomical ages of the tephras a.1.1 (just below the indurated bed of LA1) and a.1.2 (in thehomogeneous marl of LA17) due to incorrect tuning or “missing” cycles seems unlikely. This is supported bythe occurrence of several bio-events in the same cycles in sections throughout the Mediterranean, alsoindicating the synchroneity of these events (Sierro et al., 2001).

The correlation of the Upper Abad marls to the insolation curve also produces a series of characteristicpatterns observed in the sediments and the target curve. The extra-ordinary thick homogeneous marl (UA4)does fit with a precession cycle having a prolonged 29 kyr period. The thick homogeneous marl of UA17represents a double cycle where the upper sapropel corresponding to a very low amplitude insolation

127

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128

CHAPTER 5

C3

An

.1n

C3

An

.2n

C3

Bn

7.2

7.1

7.0

6.9

6.8

6.7

6.6

6.5

6.4

6.3

6.2

6.1

6.0

Summer 65oNLa93(1,1)

Age

(in

Ma)

minmaxAPTS

yy

IZ6

IZ2

IZ1

IZ5

IZ4

IZ3

0

4m

Izarorene (Morocco)

Iza-1

+ + + + + + + + + + Iza-3

+ + + + + + + + + + Iza-2

IF3

IF5

IF7

IF9

IF15

IF16

IF11

yyyyyyyyyyyyyy

IF1

0

4m

IF2

IF4

IF6

IF12

Ifounassene (Morocco)

Ifo-1aIfo-1b

Ifo-2aIfo-2b

+ + + + + + + + + + +

+ + + + + + + + + + +

+ + + + + + + + + + +

+ + + + + + + + + + +

Ifo-3

Ifo-5

Ifo-4

+ + + + + + + + + + +Ifo-6

ME24

ME19

ME17

ME15

ME13

ME11

ME9

ME7

ME5

ME3

ME1

ME25

ME26

ME28

ME31

ME35

yyyyyy

yyyyyyyyyyyyyyyyyyyyy

ME32

ME22

ME16

ME14

ME18

ME20

ME12

ME2

ME4

ME6

ME8

ME10

0

4m

Messâdit (Morocco)

+ + + + + + + + + + Mes-2

Mes-1

Mes-4

+ + + + + + + + + + Mes-3

+ + + + + + + + + + Mes-6

+ + + + + + + + + + Mes-5

+ + + + + + + + + + Mes-7

Mes-8

Mes-9

+ + + + + + + + + + Mes-10+ + + + + + + + + + Mes-11

+ + + + + + + + + + Mes-13+ + + + + + + + + +

Mes-14

Mes-12

+ + + + + + + + + + Mes-16+ + + + + + + + + + Mes-17+ + + + + + + + + + Mes-18

+ + + + + + + + + + Mes-19

Mes-15

Sorbas / Nijar basin (Spain)

composite

LA17

LA18

LA19

LA20

LA10

LA11

LA12

LA13

LA14

LA15LA16

LA9

LA1

LA2LA3LA4

LA5

LA6

LA7

LA8

UA1

UA8

UA7

UA6

UA5

UA4

UA2

UA3

UA15

UA14

UA13

UA12

UA11

UA10

UA9

UA21

UA20

UA19

UA18

UA17

UA16

UA31

UA32UA33

UA34

UA28

UA29UA30

UA24

UA25UA26

UA22

+ + + + + + + + + + A1.1

+ + + + + + + + + + A1.2

+ + + + + + + + + + A1.3

5

6

4

3

2

1

Diatomite/Laminite/Opal CT

Dark blue clay

Sapropel

Evaporites

Blue clayey marls

Tephras

Grey to brown marls

Halimeda-algae packstone

Pycnodonta levels

Lithology

yyyyyyyyFFiigguurree 55..22 SSttrraattiiggrraapphhiicc ccoolluummnn ooff tthhee AAbbaadd mmeemmbbeerr iinn tthhee SSoorrbbaass aanndd NNiijjaarr bbaassiinn aanndd tthhee MMeessssiinniiaann mmaarrllss iinn tthhee

MMeelliillllaa--NNaaddoorr bbaassiinn aanndd iittss ccoorrrreellaattiioonn ttoo tthhee AATTSS..

This figure shows the tuning of the sedimentary sections to the astronomical solutions of La93 (Laskar et al., 1993a). The locations

of ash layers are indicated. For details the reader is referred to Sierro et al. (2001) or Van Assen et al. (in press). The main

biostratigraphic marker-events registered within the studied sections correspond to 1) G. miotumida gr. FRO at 7.242 Ma (T/M

boundary), 2) G. nicolae FCO at 6.828 Ma, 3) G. nicolae LO at 6.722 Ma in the Moroccan sections (chapter 4) and at 6.713 in

Sorbas, 4) G. obesa FCO at 6.613 Ma, 5) N. acostaensis sinistral/dextral coiling change at 6.360 - 6.340 Ma (Krijgsman et al.,

1999a; Sierro et al., 2001) and 6) N. acostaensis first sinistral influx at 6.129 Ma in Morocco and 6.126 Ma in Sorbas.


maximum indeed lacks expression. Cycle UA31 is also interpreted as a double cycle in which the uppersapropel lacks expression. Further, the sapropel of cycle UA13 is extremely thin and devoid of warmoligotrophic fauna, but also cycles UA11 and UA9 are thinner than adjacent sapropels. These alternatingthin-thick-thin-thick patterns are reflected in the insolation curve by lower-higher-lower-higher insolationmaxima due to the combined effect of the eccentricity modulation of precession and precession-obliquityinterference. The sapropel thickness of UA4-UA8 also agrees with the pattern of the insolation curve apartfrom the expected reduction in amplitude in the insolation curve for cycle UA7, which is not observed. Thethickness of the diatomites in cycles UA26-UA33 correspond perfectly with the pattern of the insolationminima of the target curve where the ticker and more prominent diatomites correspond to the higheramplitude insolation minima.

THE MELILLA MARLS

Van Assen et al. (in press) discuss the cyclostratigraphic relations in the Neogene marls in the Melilla Basinin detail. The Melilla marls are characterized by mainly bipartite cycles of homogeneous marls and laminatedcherty layers or diatomites. The upper part of the section shows more sandy influxes. The astronomicaltuning of the Melilla marls is primarily based one five biostratigraphic marker-events, which occursimultaneously in the Mediterranean Neogene (Krijgsman et al., 1995; Sierro et al., 2001) and have beenidentified in the Melilla Basin (Van Assen et al., in press). The astronomical ages of these bio-events areused as calibration points to tune all sedimentary cycles, volcanic ash layers and other events to thesummer insolation target curve of La93(1.1). The phase relation of the sedimentary cycles with respect tothe target curve is based on the occurrence of high planktonic foraminifera contents within thehomogeneous intervals resembling the pattern as discussed above for the Abad marls (Sierro et al., 2001).Therefore, the homogenous marls in the Melilla Basin are correlated to a precession minimum or insolationmaximum. Additionally, the number of sedimentary cycles between the different bio-events is equivalent toother astronomically calibrated Mediterranean sections. Characteristic patterns are also recognized in theMelilla marls, like for examples the extra thick marl of Mec11 corresponds to UA4 of the Abad marls causedby an extra long (29 kyr) precession cycle. The extra thick cycles of MEc24 and IFc5 corresponding to theextra thick marls of cycle UA17 in the Abad marls are linked to a double minimum peak. The tuning of theupper part of the Melilla marls above the sinistral to dextral coiling change of N. acostaensis is lessstraightforward due to increased terrigeneous influx. However, the number of 10 sedimentary alternationsbetween the two recorded bio-events (figure 5.2) are in agreement with the ten cycles formed in otherastronomically tuned Mediterranean sections between these bio-events.

In chapter 2 we made a division in so-called category 1, 2 and 3 ash layers, where second category ashlayers were found in cyclic marine sequences that have been astronomically dated by applying anastronomically dated integrated stratigraphic framework, which was constructed with the help of othersections that are more suitable for astronomical tuning. Consistency in the number of basic sedimentarycycles in between clear-cut planktonic foraminiferal bio-events known to be synchronous in theMediterranean and the occurrence of characteristic sedimentary cycles indicate the reliability of the tuning.Above we discussed this category 2 tuning for the Sorbas, Nijar and Melilla Basins. Apart from the“correctness” of the tuning uncertainties in astronomical ages of volcanic ash layers might be due to 1)uncertainties in the applied astronomical solutions including the values for tidal dissipation and dynamical

129

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ellipticity, 2) uncertainties in the assumption of a constant sedimentation rate between two astronomicallytuned points to derive an astronomical ages for a layer intercalated between two such points and 3)uncertainties in the lag between the orbital forcing and sedimentary expression. Overall, the uncertainty inthe astronomical ages for the volcanic ash layers in Melilla and Sorbas/Nijar is estimated at ± 5 kyr providedthat the tuning is correct.

MATERIAL AND METHODS

In the Sorbas and Nijar Basin the three volcanic levels (a.1.1, a.1.2 and a.1.3) were collected at severalsites. Exact geographic locations of the sampled sections are given in figure 1 of Sierro et al., (2001). In theMelilla Basin the volcanic tephras were sampled in the Messadit, Ifounassene and Izarorene sections as isdescribed in Van Assen et al. (in press) and chapter 4. The bulk samples were crushed (depending on theirsolidity), washed and sieved. For the Melilla samples the 500-1000 µm fractions were used for standardmagnetic and heavy liquid separations for micas or sanidine. For the Sorbas and Nijar samples smaller sizefractions had to be used, but size fractions smaller than 125 µm were removed. Subsequently, all sampleswere handpicked. The samples were wrapped in Al-foil and loaded in a 5 mm ID quartz vial. Fish CanyonTuff (FC-2) sanidine and Taylor Creek Rhyolite (85G003) were wrapped in Cu-foil and loaded at the top andbottom positions and between each set of 3-5 samples. Samples were irradiated in several irradiationbatches (VU37, VU41 and VU42 for Melilla and VU32 and VU41 for Sorbas) for 7 hours in the Oregon StateUniversity TRIGA reactor in the cadmium shielded CLICIT facility for VU32 and VU37 and in the extendedtube CLICIT facility for VU41 and VU42. After irradiation samples and standards were loaded in 2 mmdiameter holes of a copper planchet and placed in an ultra-high vacuum extraction line. Samples andstandards were stepwise heated or directly fused and gas was analyzed with a Mass Analyzer Products LTD215-50 noble gas mass spectrometer (for details see chapter 1).

Multiple grain fractions of biotite samples have been measured by stepwise heating and in a few cases byfusion with a 24W continuous wave argon-ion laser. Multiple grain or single grain sanidine fractions ofstandards and samples have been preheated using a defocused laser beam with an output of 2W (samplesdid not glow and gas was pumped away) to remove undesirable atmospheric argon adsorbed to the crystalsurface. After the preheating step the samples and standards were analyzed by total fusion. Experimentswere replicated 5 to 10 times for the fusion experiments; most stepwise heating experiments have beenperformed in duplicate. Beam intensities were measured in a peak-jumping mode over the mass range 40-36 on a secondary electron multiplier. For data collection the mass spectrometer is operated with a modifiedversion of standard MAP software (i.e., valve control, laser control, variable integration times for differentisotopes). System blanks were measured every 3 steps. The total system blanks were in the range of 5.0 ×10-14 moles for mass 40, 4.0 × 10-15 moles for mass 39, 2.6 × 10-15 moles for mass 38, 2.6 × 10-14 moles formass 37 and 2.0 × 10-15 moles for mass 36. Mass discrimination (1.002-1.0100 per atomic mass unit) wasmonitored by frequent analysis of 40Ar/38Ar reference gas or 40Ar/36Ar air pipette aliquots (chapter 1). Theirradiation parameter J for each unknown was determined by interpolation using a 2nd order weightedpolynomial fitting between the individually measured standards (chapter 1).

Data reduction is performed using the in-house developed ArArCalc(v2.20c) software (Koppers, 2002).Blanks corrections are performed by subtracting the mean blank values of blank analyses before and after

130

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measurement of the unknown. Mass discrimination and interfering nuclear isotope corrections are describedin chapter 1. Weighted mean 40Ar/39Ar- (or F-) ratios and standard errors of the mean are given forcombined experiments from the same irradiation package. Weighted mean intercalibration factors () andstandard errors of the mean are calculated when experiments on the same ash layer, but from differentirradiation packages are combined. Ages have been calculated with standard age equations (equations 1.11and 1.12) relative to TCR of 28.34 Ma or FCT of 28.02 Ma (Renne et al., 1998) and with the decay constantsand decay constant errors of Steiger and Jäger (1977). Errors are reported at the 1σ level and includerespectively the analytical error in the unknown (I), the analytical error in the unknown and standard (II),the analytical errors and the uncertainties in 40Ar*/40K of the primary standard and intercalibration betweenprimary and secondary standards (III) and the error including decay constant uncertainties as well (IV).MSWD values are used to assess homogeneity of the data. For comparison ages are also calculated with aslightly modified version of the full external error calculation as in Min et al. (2000). These ages anduncertainties are calculated according to equation 1.14 with physical parameters and activity data asreported in table 1.4, column VII. This last approach should present the most realistic age and errorestimate (see also chapter 1 and 3).

RESULTS

Table 5.1 shows all the 40Ar/39Ar data obtained for volcanic ash layers in the Sorbas and Nijar Basins. Table5.2 shows the data for the ash layers in the Melilla Basin. Figure 5.3 visualizes the 40Ar/39Ar age estimatesfor the Sorbas / Nijar ash layers, while in figure 5.5 cumulative age probability distributions are shown forthe Melilla volcanic deposits.

SORBAS / NIJAR

All incremental heating experiments performed on biotite show reliable plateaus (figure 5.4) according thecriteria of e.g. Foland et al. (1986). The first steps and occasionally the last step are omitted from mostplateaus. Omitted steps show lower amounts of radiogenic 40Ar than the steps included in the plateaus.Although the fusion experiments on biotite produce MSWD’s <1, we do not consider these experiments inthe integrated ages, because the stepwise heating experiments showed the necessity of the removal ofsome steps. 40Ar/36Ar isochron intercepts are indistinguishable from 295.5 at the 1σ level, but due toclustering around the axis, uncertainties are sometimes large. The only exceptions are two experiments onash layer a.1.3 (VU32-C9 (fusion), VU32-C12), which show 40Ar/36Ar intercepts of respectively 307 ± 16 and310 ± 10, indicative of some excess argon. However, at a 95% confidence level they do not differ from theatmospheric ratio of 295.5. Further, we tried to separate pure sanidine from the same ash layers, butobtained and analyzed both sanidine/plagioclase mixtures (K/Ca 1-5) and sanidine separates (K/Ca >20)which appeared to be very heterogeneous and apart from VU41-B8 and VU41-B13 no reliable isochronscould be defined. The heterogeneity observed in the feldspar indicates that we might be dealing with areworked tuff, although biotite minerals from the same ash layer are reproducible. All analytical data onfeldspar are reported in table 5.1 and the “best” feldspar data (i.e., MSWD <1) also shown in figure 5.3.

For the biotite experiments all steps included in the different plateaus are combined to one integrated age.When all plateau steps are combined a.1.3 is 6.707 ± 0.010 (0.091) Ma, a.1.2 is 6.771 ± 0.009 (0.092) Ma

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132

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Ap

par

ent

40A

r/3

9A

r ag

e an

d e

rro

r (M

a)Id

enti

ty a

nd

# c

orr

esp

on

din

g t

ofi

gu

re 5

.3S

ecti

on

F un

kno

wn

σσσσF

NM

SW

D4

0A

r*

%

39A

r K%

K/C

aF S

tσσσσ

Fst

Ag

eI

IIII

IIV

Eq

uat

ion

1.1

4

Cyc

le U

A1

= a

.1.3

6.6

92

1 VU

32-C

9 (f

usio

n)G

afar

es I

I2.

0583

0.00

575

0.58

81.0

--

8.74

260.

0262

6.71

20.

019

0.02

70.

047

0.07

36.

710

± 0

.095

2 VU

32-C

8 (f

usio

n)G

afar

es I

2.04

630.

0089

50.

1264

.8-

-8.

7234

0.02

626.

688

0.02

90.

035

0.05

20.

076

6.68

6 ±

0.0

973

VU32

-C12

Gaf

ares

III

2.06

200.

0043

60.

7383

.779

.1-

8.81

560.

0264

6.66

90.

014

0.02

40.

045

0.07

26.

667

± 0

.093

4 VU

32-C

10Pe

rale

s2.

0517

0.00

756

0.19

61.2

88.7

-8.

7668

0.02

636.

673

0.02

40.

031

0.04

90.

074

6.67

0 ±

0.0

955

VU41

B28

Gaf

ares

I1.

9972

0.00

794

0.18

77.2

82.1

-8.

3615

0.01

256.

732

0.02

70.

029

0.04

80.

074

6.73

2 ±

0.0

956

VU41

B28

Gaf

ares

I2.

0109

0.00

723

0.16

69.3

75.7

-8.

3615

0.01

256.

778

0.02

40.

026

0.04

70.

074

6.77

8 ±

0.0

957

VU41

B26

Gaf

ares

III

2.01

160.

0195

30.

7277

.624

.0-

8.32

590.

0125

6.81

00.

066

0.06

70.

077

0.09

66.

810

± 0

.113

8 VU

41 B

26G

afar

es I

II1.

9860

0.00

854

0.34

76.2

51.2

-8.

3259

0.01

256.

723

0.02

90.

030

0.04

90.

075

6.72

3 ±

0.0

95

Cyc

le L

A1

7 =

a.1

.26

.78

910

VU

32-C

4 (f

usio

n)G

afar

es2.

0721

0.00

375

0.82

79.8

--

8.62

360.

0285

6.85

00.

012

0.02

60.

047

0.07

46.

848

± 0

.096

11 V

U32

-C4

Gaf

ares

2.04

510.

0041

70.

0980

.299

.1-

8.62

360.

0285

6.76

10.

014

0.02

60.

046

0.07

36.

759

± 0

.095

12 V

U32

-C6

(fus

ion)

Los

Mol

inos

2.04

600.

0056

70.

2775

.8-

-8.

6708

0.02

696.

728

0.01

80.

028

0.04

70.

073

6.72

5 ±

0.0

9513

VU

32-C

6Lo

s M

olin

os2.

0593

0.00

605

0.25

80.8

98.8

-8.

6708

0.02

696.

771

0.02

00.

029

0.04

80.

074

6.76

9 ±

0.0

9614

VU

41 B

45Pe

rale

s A

2.09

370.

0140

40.

3983

.381

.3-

8.71

530.

0131

6.77

10.

045

0.04

60.

060

0.08

36.

771

± 0

.102

15 V

U41

B45

Pera

les

A2.

0884

0.00

874

0.61

85.6

71.2

-8.

7153

0.01

396.

754

0.02

80.

030

0.04

90.

075

6.75

4 ±

0.0

9616

VU

41 B

44G

afar

es2.

0910

0.00

785

0.05

87.9

93.7

-8.

6960

0.01

306.

777

0.02

50.

027

0.04

70.

074

6.77

7 ±

0.0

9517

VU

41 B

44G

afar

es2.

0981

0.00

955

0.10

86.7

92.2

-8.

6960

0.01

306.

800

0.03

10.

032

0.05

10.

077

6.80

0 ±

0.0

9718

VU

41 B

32Lo

s M

olin

os2.

0252

0.01

504

0.30

81.8

83.5

-8.

4336

0.01

276.

768

0.05

00.

051

0.06

40.

086

6.76

8 ±

0.1

0419

VU

41 B

32Lo

s M

olin

os2.

0382

0.02

053

0.01

87.1

42.4

-8.

4336

0.01

276.

812

0.06

80.

069

0.07

90.

098

6.81

1 ±

0.1

1520

VU

41 B

32Lo

s M

olin

os2.

0334

0.01

054

0.07

69.0

95.4

-8.

4336

0.01

276.

796

0.03

50.

036

0.05

30.

078

6.79

5 ±

0.0

9821

VU

41 B

11 f

elds

par

Los

Mol

inos

1.95

220.

0041

50.

4199

.0-

3.0

8.19

940.

0123

6.71

10.

014

0.01

70.

042

0.07

16.

711

± 0

.092

V

U41

B12

fel

dspa

rPe

rale

s A

2.31

170.

0689

414

197

.1-

2.2

8.19

940.

0123

7.94

40.

236

0.23

70.

241

0.25

07.

944

± 0

.259

22 V

U41

B13

fel

dspa

rG

afar

es1.

9617

0.00

425

0.37

95.1

-4.

58.

1994

0.01

236.

743

0.01

40.

018

0.04

30.

071

6.74

3 ±

0.0

9223

VU

32-C

26 f

elds

par

Gaf

ares

2.25

680.

0026

60.

2994

.4-

22.3

9.48

080.

0265

6.78

70.

080

0.02

10.

044

0.07

26.

784

± 0

.094

Cyc

le L

A1

= a

.1.1

7.1

61

25 V

U32

-C5

(fus

ion)

Los

Mol

inos

SS

2.16

370.

0071

50.

5476

.3-

-8.

6471

0.02

777.

133

0.02

30.

033

0.05

20.

079

7.13

1 ±

0.1

0226

VU

32-C

5Lo

s M

olin

os S

S2.

1803

0.00

684

0.25

84.0

79.0

-8.

6471

0.02

777.

188

0.02

20.

032

0.05

20.

079

7.18

5 ±

0.1

0227

VU

41 B

29Lo

s M

olin

os2.

1451

0.00

945

0.26

81.4

90.0

-8.

3794

0.01

267.

215

0.03

10.

033

0.05

30.

081

7.21

4 ±

0.1

0328

VU

41 B

29Lo

s M

olin

os2.

1741

0.01

124

0.20

83.2

91.6

-8.

3794

0.01

267.

312

0.03

80.

039

0.05

70.

084

7.31

2 ±

0.1

0629

VU

41 B

29Lo

s M

olin

os2.

1608

0.00

604

0.34

83.0

84.7

-8.

3794

0.01

267.

267

0.02

00.

023

0.04

80.

078

7.26

7 ±

0.1

0030

VU

41 B

30Lo

s M

olin

os S

S2.

1644

0.01

434

0.02

86.7

67.7

-8.

3929

0.01

267.

268

0.04

80.

049

0.06

40.

089

7.26

7 ±

0.1

0931

VU

41 B

30Lo

s M

olin

os S

S2.

1753

0.01

913

0.66

84.2

82.8

-8.

3929

0.01

267.

304

0.06

40.

065

0.07

70.

099

7.30

4 ±

0.1

1832

VU

41 B

30Lo

s M

olin

os S

S2.

1619

0.00

474

0.04

86.3

76.0

-8.

3929

0.01

267.

259

0.01

60.

019

0.04

60.

077

7.25

9 ±

0.1

0033

VU

41 B

46Lo

s M

olin

os S

S2.

2554

0.00

545

0.56

78.8

90.0

-8.

7494

0.01

317.

265

0.01

70.

021

0.04

60.

077

7.26

4 ±

0.1

0034

VU

41 B

46Lo

s M

olin

os S

S2.

2470

0.00

554

0.19

87.0

86.0

-8.

7494

0.01

317.

238

0.01

80.

021

0.04

70.

077

7.23

7 ±

0.1

00 V

U41

B7

feld

spar

Los

Mol

inos

2.23

600.

0169

1268

93.2

-19

.98.

2080

0.01

237.

676

0.05

80.

059

0.07

40.

098

7.67

6 ±

0.1

1835

VU

41 B

8 fe

ldsp

arLo

s M

olin

os S

S2.

0709

0.00

465

0.09

91.0

-2.

78.

2037

0.01

237.

114

0.01

60.

019

0.04

50.

075

7.11

4 ±

0.0

97 V

U41

B9

feld

spar

Los

Mol

inos

SS

2.14

810.

0373

815

492

.6-

25.9

8.20

370.

0123

7.37

90.

128

0.12

90.

135

0.14

97.

379

± 0

.162

V

U32

C30

fel

dspa

rLo

s M

olin

os S

S2.

6887

0.05

526

116

85.3

-1.

69.

6542

0.02

617.

938

0.16

30.

164

0.17

00.

183

7.93

5 ±

0.1

95


and a.1.1 is 7.251 ± 0.008 (0.098) Ma. Uncertainties represent the analytical errors, between brackets fullerror estimates are given. Combined plateau ages are calculated by first establishing a weighted meanFash/FTC ratio of all steps included in a plateau, which is then inserted in equation 1.14. Combined plateauages of a.1.1 and a.1.2 show almost normal probability distributions, MSWD values lower than 1 andisochron intercepts indiscernible from the atmospheric ratio of 295.5 (296.2 ±. 3.9 for A.1.1, 294.9 ± 3.4 forA.1.2 for both normal and inverse isochrons). Ash layer a.1.3 has a slightly higher MSWD value (1.04), anisochron intercept very slightly deviating from the atmospheric ratio at the 1σ level (298.3 ± 2.4) and analmost normal probability distribution. The inverse isochron age for a.1.3 is 6.679 ± 0.027 (or 0.094 fullerror) Ma.

133

CHAP

TER

5


TTaabbllee 55..11:: 4400AA rr //3399AArr ddaattaa ooff SSoorrbbaass//NNiijjaarr tteepphhrraa wwiitthh ddiiffffeerreenntt eerrrroorr pprrooppaaggaattiioonn mmeetthhooddss ((pprreevviioouuss ppaaggee))..

Analytical data of Sorbas / Nijar ash layers. F ratios with standard error of the mean, number of analyses with between brackets

number of experiments excluded from dataset, radiogenic 40Ar* contents, 39Ar content included in plateau (for biotite), analytical

errors, ages calulated with standard age equations (equation 1.11/1.12) and “updated’ ages with full error estimates (equation

1.14) are reported. Reported errors represent analytical error of unknown (I), of unknown and standard (II), analytical errors and

uncertainties in 40Ar*/40K of primary standard and intercalibration factor(s) between primary and secondary standard(s) (III), and

the foregoing errors with decay constant uncertainties (IV). All experiments are stepwise heating experiments on biotite, unless

stated otherwise. For experiments on sanidine and/or plagioclase (mixtures) K/Ca ratios are mentioned. All experiments on

feldspar are multigrain fusion experiments. N represents the number of steps included in the plateau (or number of replicate

fusion experiments). Ages of VU32 experiments are calculated relative to TCR of 28.34 Ma and ages of VU42 experiments are

calculated relative to FCT of 28.02 Ma (Renne et al., 1998).

0 2 4 6 8 10 12 14 16 18 20 22 24 26 28 30 32 346.5

6.6

6.7

6.8

6.9

7.0

7.1

7.2

7.3

7.4

7.5

plag / san mix

plag

A

1-2 3-8

8,10

11,13-20

21,21

23 25 26-34 356.5

6.6

6.7

6.8

6.9

7.0

7.1

7.2

7.3

7.4

7.5

plag/san mix

plag

B

Number corresponding to table 5.1Number corresponding to table 5.1

Ag

e (M

a)

a.1.1a.1.1

a.1.2a.1.2

a.1.3a.1.3

biotitefusion

biotitestepwiseheating

sanK/Ca=22.3

biotitefusion

biotitestepwiseheatingbiotite

fusion

biotitestepwiseheating

FFiigguurree 55..33:: 4400AA rr //3399AArr aaggeess ooff vvoollccaanniicc aasshh llaayyeerrss iinn tthhee SSoorrbbaass aanndd NNiijjaarr BBaassiinnss..

The 40Ar/39Ar ages of the experiments performed on the Sorbas / Nijar volcanic layers are compared with the astronomical

ages (gray horizontal bars). Error bars represent analytical errors in samples and standards only (a) and a full error

propagation including uncertainties in absolute age of standards and activities according to equation 1.14 (b). The thickness

of the bar depicting the astronomical age represents the uncertainty in the astronomical age. The gray (black) markers

represent combined fusion experiments (incremental heating experiments). Experiments on plagioclase / sanidine are

indicated in the figure. The numbers on the X-axis correspond to the data reported in table 5.1.


Further, ash layer a.1.3 (cycle UA1) comprises 6 thinash layers spread over a range of 15 cm, where 3 ashlayers are coarser grained with Gafares I representingthe lower coarser grained ash and Gafares III the uppercoarser grained ash. In the same cycle in Perales only adouble ash was found. Although sampling, mineralseparation and 40Ar/39Ar dating has been performed onthe individual ash layers, the “accepted” 40Ar/39Ar agefor the a.1.3 is based on the combination ofexperiments, because with a cycle thickness of ~1.5 monly an uncertainty of 2 kyr will be introduced.

The feldspar experiments are also combined to assessthe behavior of the isochrons and to check the possibleoccurrence of excess argon. Isochrons for a.1.1 indeedshow intercepts significantly higher than theatmospheric ratio (normal 396 ± 48; inverse 470 ± 60),but uncertainties and MSWD values (68 and 71) arehigh. Isochron ages are respectively 7.16 ± 0.14 Ma(normal) and 7.17 ± 0.15 Ma (inverse) for a.1.1. Fora.1.2 the combination of experiments VU41-B11, VU-B13 and VU32-C26 produces reliable isochrons withintercepts of 334 ± 10 and 324 ± 10, MSWD’s of 1.10and 0.91 and ages of 6.72 ± 0.02 Ma and 6.73 ± 0.02

Ma for respectively normal and inverse isochrons. When VU41-B12 is included, isochrons are completelydisturbed.

MELILLA

Most experiments on the volcanic layers in the Melilla Basin concern single crystal sanidine fusions, butsingle crystal stepwise heating experiments have been performed in a few cases. Stepwise heating onfeldspar was not successful, because the major amount of gas was released in one or two steps due tounsuitability of the -argon ion continuous wave- laser for stepwise heating of transparent minerals. Analyseswith a 40Ar yield lower than 1 Volt (at 109 Ω with a relative gain of ~500) or with very low K/Ca ratios wereomitted from further interpretation and not included in table 5.2. For Ifo-6 the results of three stepwiseheating experiments on biotite from the same package are reported. MSWD values have been used toassess the heterogeneity of the samples. Almost all MSWD values (with a few exceptions) are lower than 1,

134

CHAPTER 5

6.67 ± 0.03 Ma

4.4

5.2

6.0

6.7

7.5

8.3

9.0

9.8

10.6

0 10 20 30 40 50 60 70 80 90 100

Ag

e (M

a)

4: VU32 C10 Weighted Plateau6.67 ± 0.03 Ma

Total Fusion6.67 ± 0.03 Ma

Normal Isochron6.63 ± 0.07 Ma

Inverse Isochron6.63 ± 0.07 Ma

MSWD 0.19

6.78 ± 0.03 Ma

5.0

6.0

7.1

8.1

9.2

10.2

11.3

12.3

13.4

0 10 20 30 40 50 60 70 80 90 100

Ag

e (M

a)

16: VU41 B44 Weighted Plateau6.78 ± 0.03 Ma

Total Fusion6.86 ± 0.03 Ma Normal Isochron6.77 ± 0.07 Ma

Inverse Isochron6.77 ± 0.07 MaMSWD 0.05

7.26 ± 0.02 Ma

5.3

6.0

6.7

7.5

8.2

8.9

9.6

10.3

11.0

0 10 20 30 40 50 60 70 80 90 100

Ag

e (M

a)

32: VU41 B30 Weighted Plateau7.26 ± 0.02 Ma

Total Fusion7.28 ± 0.02 Ma

Normal Isochron7.25 ± 0.07 Ma

Inverse Isochron7.25 ± 0.07 Ma

MSWD 0.04

Cumulative 39Ar Released (%)

FFiigguurree 55..44 EExxaammpplleess ooff ppllaatteeaauu aaggeess ffoorr tthhee SSoorrbbaass//NNiijjaarr

BBaassiinnss..

Plateaus are shown of three representative biotite samples of the

three volcanic layers in the Sorbas/Nijar Basin. All plateaus fulfill

the criteria as proposed by e.g. Foland et al. (1986).


135

CHAP

TER

5


Ap

par

ent

40A

r/3

9A

r ag

e an

d e

rro

r (M

a)Id

enti

tyF u

nkn

ow

nσσσσ

FN

MS

WD

40A

r*

%

39A

r K%

K/C

aF S

tσσσσ

Fst

Ag

eI

IIII

IIV

Eq

uat

ion

1.1

4

Ifo-

56

.27

0VU

42-A

211.

8456

0.00

138

(2)

0.79

98.7

-25

.78.

3749

0.01

766.

212

0.00

40.

014

0.03

80.

065

6.21

2 ±

0.0

85VU

37-B

221.

9708

0.00

341

-94

.8-

23.2

9.02

670.

0226

6.22

60.

011

0.01

90.

040

0.06

66.

223

± 0

.086

VU37

-B21

1.96

190.

0028

1-

94.4

-19

.78.

9909

0.02

346.

222

0.00

90.

018

0.04

00.

066

6.22

0 ±

0.0

86VU

37-B

191.

9433

0.00

153

0.55

96.7

-22

.48.

8949

0.02

316.

230

0.00

50.

017

0.03

90.

065

6.22

7 ±

0.0

86VU

37-B

181.

9367

0.00

231

-94

.8-

21.7

8.86

000.

0230

6.23

30.

007

0.01

80.

040

0.06

56.

231

± 0

.086

Ifo-

46

.28

1VU

42-A

201.

8611

0.00

148

(2)

1.02

98.8

-23

.98.

3704

0.01

766.

268

0.00

50.

014

0.03

90.

066

6.26

7 ±

0.0

85VU

37-B

141.

9230

0.00

222

(1)

0.07

98.3

-25

.28.

7620

0.02

286.

258

0.00

70.

018

0.04

00.

066

6.25

6 ±

0.0

86VU

37-B

151.

9223

0.00

202

0.04

99.0

-20

.48.

7620

0.02

286.

256

0.00

70.

018

0.04

00.

066

6.25

3 ±

0.0

86VU

37-B

171.

9322

0.00

212

0.67

98.1

-20

.78.

8353

0.02

306.

236

0.00

70.

017

0.04

00.

065

6.23

4 ±

0.0

86

Ifo-

36

.33

3VU

42-A

191.

8627

0.00

248

(2)

0.35

98.1

-21

.58.

3615

0.01

766.

280

0.00

80.

015

0.03

90.

066

6.28

0 ±

0.0

86VU

37-B

131.

9250

0.00

163

0.62

95.8

-16

.28.

7330

0.02

276.

285

0.00

50.

017

0.04

00.

066

6.28

3 ±

0.0

86VU

37-B

111.

9094

0.00

202

0.77

97.7

-19

.18.

6898

0.02

356.

265

0.00

60.

018

0.04

00.

066

6.26

3 ±

0.0

86VU

37-B

101.

9001

0.00

202

(1)

0.19

96.7

-19

.78.

6613

0.02

346.

255

0.00

60.

018

0.04

00.

066

6.25

3 ±

0.0

86

Ifo-

26

.43

2VU

42-A

161.

8997

0.00

139

(1)

1.05

99.6

-22

.48.

3526

0.01

846.

411

0.00

40.

015

0.04

00.

067

6.41

1 ±

0.0

87

Ifo-

16

.44

3VU

42-A

171.

9005

0.00

348

(2)

0.16

97.2

-22

.88.

3570

0.01

756.

411

0.01

10.

018

0.04

10.

068

6.41

0 ±

0.0

88

Mes

-18

6.2

60

VU42

-A15

1.84

510.

0017

7 (3

)0.

3399

.4-

23.1

8.35

260.

0184

6.22

70.

006

0.01

50.

039

0.06

56.

227

± 0

.085

Mes

-17

6.2

98

VU42

-A13

1.84

990.

0027

100.

3596

.7-

25.5

8.34

810.

0184

6.24

70.

009

0.01

60.

039

0.06

66.

246

± 0

.086

VU37

-B23

(ih

)1.

9729

0.00

652

0.19

95.1

96.8

29.1

9.06

290.

0227

6.20

70.

020

0.02

60.

044

0.06

86.

205

± 0

.087

VU37

-B25

1.98

870.

0063

20.

1195

.5-

21.3

9.12

030.

0228

6.21

80.

020

0.02

50.

043

0.06

86.

215

± 0

.087

VU37

-B25

(ih

)1.

9966

0.00

642

0.01

97.1

100

27.9

9.12

030.

0228

6.24

20.

020

0.02

50.

044

0.06

86.

240

± 0

.088

Mes

-16

6.3

13

VU42

-A12

1.85

500.

0028

100.

4397

.3-

23.7

8.34

810.

0184

6.26

40.

009

0.01

70.

040

0.06

66.

263

± 0

.086

Mes

-15

6.3

37

VU42

-A11

1.85

580.

0037

80.

0995

.6-

22.2

8.34

810.

0184

6.26

70.

012

0.01

80.

040

0.06

76.

266

± 0

.086

TTaabb

llee 55

..22::

4400 AA

rr//33

99 AArr

ddaattaa

ooff

MMeell

iillllaa

tteepphh

rraa ww

iitthh dd

iiffffee

rreenntt

eerrrroo

rr pprr

ooppaagg

aattiioo

nn mm

eetthhoodd

ss.. ((

ccoonnttii

nnuueedd

oonn nn

eexxtt

ppaaggee

ss))


136

CHAPTER 5

Ap

par

ent

40A

r/3

9A

r ag

e an

d e

rro

r (M

a)Id

enti

tyF u

nkn

ow

nσσσσ

FN

MS

WD

40A

r*

%

39A

r K%

K/C

aF S

tσσσσ

Fst

Ag

eI

IIII

IIV

Eq

uat

ion

1.1

4

Mes

-14

6.3

79

VU42

-A9

1.86

780.

0030

8 (2

)0.

6495

.2-

20.9

8.35

260.

0184

6.30

40.

010

0.01

70.

040

0.06

76.

304

± 0

.086

VU37

-B37

2.15

240.

0057

1 (1

)-

99.7

-52

.99.

6778

0.02

326.

342

0.01

70.

023

0.04

20.

068

6.33

9 ±

0.0

88VU

37-B

382.

1575

0.00

263

1.70

98.4

-13

.49.

7313

0.02

346.

322

0.00

70.

017

0.04

00.

066

6.31

9 ±

0.0

87VU

37-B

392.

1734

0.00

352

2.47

99.5

-10

.29.

7915

0.02

356.

329

0.01

00.

018

0.04

00.

067

6.32

7 ±

0.0

87

Mes

-12

6.4

50

VU42

-A8

1.90

350.

0030

9 (1

)0.

2595

.8-

23.1

8.35

260.

0184

6.42

40.

010

0.01

70.

041

0.06

86.

424

± 0

.088

VU37

B30

2.09

180.

0029

2 (1

)0.

3297

.1-

22.3

9.32

440.

0233

6.39

60.

009

0.01

80.

041

0.06

76.

394

± 0

.088

VU37

B29

2.08

950.

0039

1 (1

)-

97.3

-19

.09.

2807

0.02

326.

420

0.01

20.

020

0.04

20.

068

6.41

7 ±

0.0

89VU

37 B

272.

0628

0.00

163

1.30

98.4

-24

.39.

1891

0.02

306.

401

0.00

50.

017

0.04

00.

067

6.39

8 ±

0.0

88

Mes

-11

6.5

42

VU37

-C11

72.

1978

0.00

225

0.71

97.8

-1.

29.

6483

0.02

326.

421

0.00

60.

017

0.04

00.

068

6.42

1 ±

0.0

88

Mes

-10

6.5

52

VU42

-A7

1.92

950.

0043

6 (3

)0.

6395

.0-

20.3

8.35

700.

0192

6.50

80.

015

0.02

10.

043

0.07

06.

508

± 0

.090

Mes

-96

.58

2VU

42-A

51.

9297

0.00

329

(1)

0.30

97.1

-28

.58.

3660

0.01

926.

502

0.01

10.

018

0.04

20.

069

6.50

1 ±

0.0

89VU

37-B

3 (ih)

1.95

820.

0044

3 (1

)0.

1598

.497

.128

.28.

5445

0.02

486.

534

0.01

50.

024

0.04

40.

070

6.53

2 ±

0.0

91VU

37-B

3 (ih)

1.95

710.

0070

1 (1

)-

99.6

80.7

27.5

8.54

450.

0248

6.53

10.

023

0.03

00.

048

0.07

26.

528

± 0

.093

VU37

-B3

(ih)

1.96

130.

0056

20.

0494

.999

.927

.78.

5445

0.02

486.

545

0.01

90.

027

0.04

60.

071

6.54

2 ±

0.0

92VU

37-B

11.

9476

0.00

261

-99

.5-

24.7

8.50

770.

0255

6.52

70.

009

0.02

10.

043

0.06

96.

525

± 0

.091

VU37

-B2

1.95

440.

0018

30.

2199

.3-

22.8

8.52

150.

0247

6.53

90.

006

0.02

00.

042

0.06

96.

537

± 0

.090

VU37

-B5

1.96

530.

0042

1-

98.9

-25

.08.

5630

0.02

406.

544

0.01

40.

023

0.04

40.

070

6.54

2 ±

0.0

91

Mes

-86

.63

8VU

42-A

41.

9542

0.00

3210

0.19

96.7

-21

.58.

3704

0.02

016.

581

0.01

10.

019

0.04

20.

070

6.58

1 ±

0.0

91VU

37-B

562.

5400

0.00

96(2

)0.

4198

.0-

53.8

10.8

444

0.02

496.

678

0.02

50.

030

0.04

80.

074

6.67

6 ±

0.0

95VU

37-B

542.

5240

0.01

24(2

)1.

4198

.8-

95.8

10.7

269

0.02

366.

708

0.03

30.

036

0.05

20.

077

6.70

6 ±

0.0

97VU

37-B

532.

4897

0.00

67(1

)-

96.0

-40

.510

.697

90.

0235

6.63

50.

018

0.02

30.

044

0.07

16.

633

± 0

.092

Mes

-66

.71

8VU

37 B

452.

3772

0.01

092

0.34

98.3

-11

.210

.129

80.

0233

6.69

10.

031

0.03

40.

051

0.07

66.

688

± 0

.096

VU37

B46

2.34

300.

0287

(1)

-95

.6-

20.9

10.1

754

0.02

346.

565

0.08

00.

082

0.09

00.

105

6.56

3 ±

0.1

20VU

37 B

472.

4041

0.00

812

(1)

0.67

94.8

-19

.210

.241

20.

0225

6.69

30.

023

0.02

70.

047

0.07

36.

691

± 0

.094


137

CHAP

TER

5


Ap

par

ent

40A

r/3

9A

r ag

e an

d e

rro

r (M

a)Id

enti

tyF u

nkn

ow

nσσσσ

FN

MS

WD

40A

r*

%

39A

r K%

K/C

aF S

tσσσσ

Fst

Ag

eI

IIII

IIV

Eq

uat

ion

1.1

4

Mes

-46

.79

2VU

42-A

32.

0031

0.00

3910

0.68

98.0

-25

.68.

3749

0.02

096.

742

0.01

30.

021

0.04

40.

072

6.74

1 ±

0.0

93VU

41 B

41.

9647

0.00

259

(1)

0.43

99.0

-25

.78.

2339

0.01

406.

725

0.00

80.

014

0.04

10.

070

6.72

5 ±

0.0

92VU

37 B

562.

6048

0.01

19(1

)-

99.1

-14

.810

.844

40.

0249

6.84

80.

031

0.03

50.

052

0.07

76.

845

± 0

.099

VU37

-B57

2.58

020.

0051

2 (1

)0.

3099

.7-

26.0

10.9

268

0.02

516.

732

0.01

30.

020

0.04

30.

071

6.73

0 ±

0.0

93

Mes

-1>

6.8

6VU

37 B

52.

0709

0.00

222

2.97

99.4

-35

.18.

5630

0.02

406.

895

0.00

70.

021

0.04

40.

073

6.89

2 ±

0.0

95VU

37 B

62.

0733

0.00

153

0.36

99.5

-39

.58.

5769

0.02

406.

892

0.00

50.

020

0.04

40.

072

6.88

9 ±

0.09

5VU

37 B

72.

0766

0.00

202

0.00

98.9

-46

.78.

6002

0.02

326.

884

0.00

70.

020

0.04

40.

072

6.88

1 ±

0.0

95VU

37 B

92.

0938

0.00

321

-99

.7-

43.4

8.63

770.

0233

6.91

10.

011

0.02

10.

045

0.07

36.

908

± 0

.096

Iza-

16

.79

2VU

37 B

432.

3690

0.00

252

0.07

99.1

-24

.310

.046

30.

0231

6.72

30.

007

0.01

70.

042

0.07

06.

721

± 0

.092

VU37

B42

2.35

910.

0026

21.

5398

.5-

22.4

9.98

920.

0230

6.73

30.

007

0.01

70.

042

0.07

06.

731

± 0

.092

VU37

B41

2.34

450.

0024

2 (1

)0.

0797

.6-

19.2

9.92

660.

0228

6.73

40.

007

0.01

70.

042

0.07

06.

731

± 0

.092

VU37

C11

52.

3083

0.00

67(4

)0.

0394

.0-

24.2

9.92

660.

0239

6.63

00.

019

0.02

50.

045

0.07

16.

627

± 0

.093

Mes

-17

6.2

98

VU37

C12

1 (ih)

2.14

070.

0061

6 (5

)0.

3494

.989

.4-

9.89

560.

0237

6.16

90.

018

0.02

30.

042

0.06

66.

167

± 0

.086

Ifo-

6<

6.0

7VU

42-A

23 (

ih)

1.78

300.

0081

4 (4

)0.

4178

.380

.7-

8.38

390.

0176

5.99

60.

027

0.03

00.

046

0.06

85.

995

± 0

.086

VU42

-A23

(ih

)1.

7892

0.00

776

(3)

1.19

69.3

98.0

-8.

3839

0.01

766.

016

0.02

60.

029

0.04

50.

068

6.01

6 ±

0.0

86VU

42-A

23 (

ih)

1.79

960.

0060

6 (3

)0.

3279

.898

.4-

8.38

390.

0176

6.05

10.

020

0.02

40.

042

0.06

66.

051

± 0

.085

TTaabb

llee 55

..22::

4400 AA

rr//33

99 AArr

ddaattaa

ooff

MMeell

iillllaa

tteepphh

rraa ww

iitthh dd

iiffffee

rreenntt

eerrrroo

rr pprr

ooppaagg

aattiioo

nn mm

eetthhoodd

ss..

Anal

ytic

al d

ata

of M

elill

a as

h la

yers

. F

ratio

s w

ith s

tand

ard

erro

r of

the

mea

n, n

umbe

r of

ana

lyse

s w

ith b

etw

een

brac

kets

num

ber

of e

xper

imen

ts e

xclu

ded

from

dat

aset

,

radi

ogen

ic 4

0 Ar*

cont

ents

, 39

ArK

cont

ent

incl

uded

in p

late

au (

for

biot

ite),

K/C

a ra

tios,

ana

lytic

al e

rror

s, a

ges

calu

late

d w

ith s

tand

ard

age

equa

tions

(eq

uatio

n 1.

11)

and

“upd

ated

” ag

es w

ith fu

ll er

ror

estim

ates

(eq

uatio

n 1.

14)

are

repo

rted

(se

e di

scus

sion

in c

hapt

er 1

). R

epor

ted

erro

rs r

epre

sent

ana

lytic

al e

rror

of u

nkno

wn

(I),

of u

nkno

wn

and

stan

dard

(II

), a

naly

tical

err

ors

and

unce

rtai

ntie

s in

40 A

r*/4

0 K o

f pr

imar

y st

anda

rd a

nd in

terc

alib

ratio

n fa

ctor

(s)

betw

een

prim

ary

and

seco

ndar

y st

anda

rd(s

) (I

II),

and

the

fore

goin

g er

rors

with

dec

ay c

onst

ant

unce

rtai

ntie

s (I

V).

Ages

are

cal

cula

ted

with

Ste

iger

and

Jäg

er (

1977

) de

cay

cons

tant

s. A

ges

and

erro

rs a

ccor

ding

to

equa

tion

1.14

are

cal

cula

ted

with

par

amet

ers

as in

tab

le 1

.4, c

olum

n VI

I. (

1977

) M

ost ex

perim

ents

are

sin

gle

crys

tal f

usio

n ex

perim

ents

on

sani

dine

, a fe

w a

re in

crem

enta

l hea

ting

expe

rimen

ts o

n si

ngle

cry

stal

s of

san

idin

e. T

he e

xper

imen

ts o

f as

h la

yer

Ifo-

6 an

d m

iner

al s

plit

VU37

-C12

1 of

Mes

-17

are

incr

emen

tal h

eatin

g ex

perim

ents

on

biot

ite.

VU

37 e

xper

imen

ts a

re c

alcu

late

d re

lativ

e to

TCR

of

28.3

4 M

a an

d VU

41 a

nd V

U42

exp

erim

ents

are

cal

cula

ted

rela

tive

to F

CT

of 2

8.02

Ma

(Ren

ne e

t al

., 19

98).

The

stra

tigra

phic

sec

tions

are

indi

cate

d by

If

= I

foun

asse

ne, M

e =

Mes

sadi

t, I

z =

Iza

rore

ne. (ih)

= in

crem

enta

l hea

ting

expe

rimen

t.


indicating that the analytical uncertainty is overestimated. Cumulative probability distributions were used tovisualize the presence of contaminating crystals. On basis of inspection of the probability distributions someanalyses were removed from further interpretation (figure 5.5).

138

CHAPTER 5

> 6.87

6.4 6.5 6.6 6.7 6.8 6.9

7.0 7.16.6 6.7 6.8 6.9

6.0 6.1 6.2 6.3 6.4 6.5 6.0 6.1 6.2 6.3 6.4 6.5

6.0 6.1 6.2 6.3 6.4 6.5

6.0 6.1 6.2 6.3 6.4 6.5

6.2 6.3 6.4 6.5 6.6 6.7

6.4 6.5 6.6 6.7 6.8 6.9 7.0 6.4 6.5 6.6 6.7 6.8 6..9 7.0

6.4 6.5 6.6 6.7 6.8 6.9

6.4 6.5 6.6 6.7 6.8 6.9

6.6 6.7 6.8 6.9 7.0 7.1 6.6 6.7 6.8 6.9 7.0 7.1

6.6 6.7 6.8 6.9 7.0 7.1

6.6 6.7 6.8 6.9 7.0 7.1

6.0 6.1 6.2 6.3 6.4 6.5

6.2 6.3 6.4 6.5 6.6 6.7

6.0 6.1 6.2 6.3 6.4 6.5

6.4 6.5 6.6 6.7 6.8 6.9

6.3 6.4 6.5 6.6 6.7 6.8 6.3 6.4 6.5 6..6 6.7 6.8

Age (Ma)

Age (Ma)

Age (Ma) Age (Ma)

Mes-18n = 10

Mes-18n = 7

Mes-16n = 10

Mes-15n = 8

Mes-11n = 5

Mes-10n = 9

Mes-10n = 6

Mes-8n = 10

Mes-6n = 6

Mes-6n = 4

Mes-4n = 23

Mes-4n = 21

Mes-1n = 8

Iza-1n = 9

Iza-1n = 6

Mes-17n = 16

Mes-12n = 15

Mes-14n = 14

Mes-8n = 15

Mes-9n = 21

Mes-9n = 20

FFiigguurree 55..55.. EExxaammpplleess ooff pprroobbaabbiilliittyy ddiissttrriibbuuttiioonnss ffoorr MMeelliillllaa vvoollccaanniicc ddeeppoossiittss..

Probability distributions are shown for the Melilla tephras. In addition, the effect of removal of “outliers” on the age distributions is

shown as well. The area under the curve is proportional to the number of experiments. Therefore, the Y-axis does not display the

same scale in all figures. The Y-axes of Ifo-5, Ifo-4, Ifo-3, Mes-9 and Mes-4 is stretched 2.5 times, of Mes-17, Mes-14, Mes-12 2

times and of Ifo-1, Mes-1 and Iza-1 1.5 times relative to the Y-axes of the other figures.


The isochron intercepts of some ash layers (Mes-18, Mes-14 and Mes-11) deviate very slightly from the atmospheric40Ar/36Ar ratio at the 1σ level. However, isochron agesdiffer <10 kyr from the weighted mean ages and at the95% significance level the 40Ar/36Ar is indiscernible from295.5 for the three ash layers. Further, the crystalsanalyzed from ash layer Mes-11 appeared to beplagioclase/sanidine mixtures with K/Ca <1.2.Combination of the VU37 data of Mes-8 with thoseobtained in VU42 resulted in a non-normal probabilitydistribution for this tephra. Omitting the VU37 data issupported by the significant deviation (134 ± 51) from theatmospheric 40Ar/36Ar ratio. For the other volcanic ashlayers, it appeared that outliers are consistently older thanthe main age population indicating xenocrysticcontamination. Only for Mes-6 the outlying age appearedto be younger than the main population, but this analysisshowed a significantly lower amount of radiogenic 40Ar.Table 5.3 reports the combined weighted mean “tephra /standard” intercalibration factors (R) by weighting allindividual R’s with the inverse variance for each ash layer.

DISCUSSION

PREVIOUS RADIO-ISOTOPIC STUDIES IN THEMELILLA BASIN

Early chronological studies in the Melilla Basin focused onthe petrology and volcanology of the domes and lavaflows of the Trois Fourches and Gourougou volcaniccomplexes (Choubert et al., 1968; Hernandez and Bellon,1985). More recent studies (Cunningham et al., 1994,1997; Roger et al., 2000; Münch et al., 2001; Cornée etal., 2002) focused on the geochronology and development of the carbonate platform and its basinalequivalents. Several of the published 40Ar/39Ar ages are obtained for tephras intercalated in theastrochronologically tuned sections of Van Assen et al. (in press) and can be compared with the 40Ar/39Arages obtained here. In table 5.4 we summarize the relevant 40Ar/39Ar data from the original publications.The data of Cunningham et al. (1997) have been recalculated relative to FCT of 28.02 Ma. MWSD valueshave been estimated from data reported in the original publications. The 40Ar/39Ar ages for Me-13 and Me-5 of Roger et al. (2000) and Ta-2 of Cornée et al. (2002), show MSWD values <1. Ta-2 shows a narrow,symmetrical probability distribution. Therefore, the results for Ta-2 (Cornée et al., 2002) are considered as agood 40Ar/39Ar age estimate, where the reported uncertainty will increase when a full error estimateaccording to the modified method of Min et al. (2000) is given. The 40Ar/39Ar age estimates for Me-13 and

139

CHAP

TER

5


6.2 6.3 6.4 6.5 6.6

5.9 6.0 6.1 6.25.8

6.2 6.3 6.4 6.5 6.6 6.7 6.2 6.3 6.4 6.5 6.6 6.7

6.1 6.2 6.3 6.4 6.5 6.6 6.1 6.2 6.3 6.4 6.5 6.6

6.1 6.2 6.3 6.4 6.5 6.6 6.1 6.2 6.3 6.4 6.5 6.6

6.1 6.1 6.2 6.3 6.4 6.5 6.6

6.2 6.3 6.4 6.5 6.6 6.7 6.2 6.3 6.4 6.5 6.6 6.7

Age (Ma)

Age (Ma)

Ifo-5n = 14

Ifo-6n = 16

Ifo-1n = 10

Ifo-1n = 8

Ifo-3n = 18

Ifo-3n = 15

Ifo-4n = 17

Ifo-4n = 14

Ifo-5n = 16

Ifo-2n = 10

Ifo-2n = 9

FFiigguurree 55..55.. EExxaammpplleess ooff

pprroobbaabbii ll ii ttyy ddiissttrr iibbuutt iioonnss

ffoorr MMeelliillllaa vvoollccaanniicc ddeeppoo--

ssiittss ((ccoonnttiinnuueedd))..


140

CHAPTER 5

Ash

laye

rC

ycle

Min

eral

NM

SW

DR

σσσσR

Ast

ron

om

i-ca

l ag

e (M

a)σσσσ

astr

40A

r/3

9A

r ag

e (M

a)±

1se

Ap

par

ent

FCT

ag

e(M

a) ±

1σσσσ

a.1.

3U

A1Bi

otite

261.

044.

2028

0.00

646.

692

0.00

56.

71 ±

0.0

1 (0

.09)

27.9

6 ±

0.0

5a.

1.2

LA17

Biot

ite41

0.21

4.16

290.

0058

6.78

90.

005

6.77

± 0

.01

(0.0

9)28

.10

± 0

.04

a.1.

1LA

1Bi

otite

370.

573.

8867

0.00

447.

161

0.00

57.

25 ±

0.0

1 (0

.10)

27.6

8 ±

0.0

4

a.1.

2LA

17Sa

n/pl

ag m

ix16

1.34

4.17

280.

0047

6.78

90.

005

6.75

± 0

.01

(0.0

9)28

.16

± 0

.04

a.1.

1LA

1Sa

n/pl

ag m

ix5

0.08

3.96

140.

0093

7.16

10.

005

7.11

± 0

.02

(0.1

0)28

.20

± 0

.07

Ifo-

5If

-9Sa

nidi

ne14

(2)

0.18

4.54

000.

0057

6.27

00.

005

6.21

± 0

.01

(0.0

8)28

.29

± 0

.04

Ifo-

4If

-8Sa

nidi

ne14

(3)

0.44

4.49

800.

0059

6.28

10.

005

6.27

± 0

.01

(0.0

8)28

.08

± 0

.04

Ifo-

3If

-6Sa

nidi

ne15

(3)

0.14

4.48

860.

0067

6.33

30.

005

6.28

± 0

.01

(0.0

8)28

.25

± 0

.05

Ifo-

2If

-1Sa

nidi

ne9

(1)

0.32

4.39

450.

0051

6.43

20.

005

6.41

± 0

.01

(0.0

9)28

.10

± 0

.04

Ifo-

1If

-1Sa

nidi

ne8

(-)

0.13

4.39

730.

0087

6.44

30.

005

6.41

± 0

.01

(0.0

9)28

.16

± 0

.06

Mes

-18

Me-

28Sa

nidi

ne7

(3)

0.22

4.53

070.

0080

6.26

00.

005

6.22

± 0

.01

(0.0

8)28

.19

± 0

.05

Mes

-17

Top

Me-

26Sa

nidi

ne16

(-)

0.40

4.51

910.

0060

6.29

80.

005

6.24

± 0

.01

(0.0

8)28

.29

± 0

.04

Mes

-16

Base

Me-

26Sa

nidi

ne10

(-)

0.35

4.50

110.

0076

6.31

30.

005

6.26

± 0

.01

(0.0

8)28

.24

± 0

.05

Mes

-15

Me-

25Sa

nidi

ne8

(-)

0.08

4.49

840.

0097

6.33

70.

005

6.27

± 0

.01

(0.0

8)28

.33

± 0

.06

Mes

-14

Top

Me-

23Sa

nidi

ne14

(3)

0.68

4.46

160.

0050

6.37

90.

005

6.32

± 0

.01

(0.0

8)28

.29

± 0

.04

Mes

-12

Me-

19Sa

nidi

ne15

(3)

0.38

4.39

740.

0046

6.45

00.

005

6.41

± 0

.01

(0.0

9)28

.19

± 0

.04

Mes

-11

Top

Me-

14Pl

agio

clas

e5

(-)

0.31

4.34

190.

0068

6.54

20.

005

6.49

± 0

.01

(0.0

9)28

.23

± 0

.05

Mes

-10

Base

Me-

14Sa

nidi

ne6

(3)

0.52

4.33

100.

0107

6.55

20.

005

6.51

± 0

.02

(0.0

9)28

.21

± 0

.07

Mes

-9M

e-12

Sani

dine

20 (

3)0.

424.

3201

0.00

456.

582

0.00

56.

52 ±

0.0

1 (0

.09)

28.2

6 ±

0.0

4M

es-8

Me-

10Sa

nidi

ne10

(5)

0.15

4.28

260.

0081

6.63

80.

005

6.58

± 0

.01

(0.0

9)28

.26

± 0

.06

Mes

-6M

e-8

Sani

dine

4 (2

)0.

324.

2127

0.01

326.

718

0.00

56.

69 ±

0.0

2 (0

.09)

28.1

3 ±

0.0

9M

es-4

Me-

3Sa

nidi

ne21

(3)

0.45

4.18

800.

0045

6.79

20.

005

6.73

± 0

.01

(0.0

9)28

.28

± 0

.04

Iza-

1Iz

-3Sa

nidi

ne6

(5)

0.17

4.18

910.

0050

6.79

20.

005

6.73

± 0

.01

(0.0

9)28

.28

± 0

.04

Ifo-

6>

If-1

7Bi

otite

160.

734.

6779

0.01

12<

6.07

0.02

56.

03 ±

0.0

1 (0

.08)

28.2

2 ±

0.0

7M

es-1

<M

e-1

Sani

dine

8 (-

)0.

254.

0901

0.00

47>

6.86

0.04

6.89

± 0

.01

(0.0

9)27

.91

± 0

.04

Mes

-17

Top

Me-

26Bi

otite

60.

074.

5689

0.03

986.

298

0.00

56.

17 ±

0.0

5 (0

.10)

28.6

0 ±

0.2

5

TTaabb

llee 55

..33::

BBeess

tt 44

00 AArr//

3399 AA

rr aagg

ee eess

ttiimm

aatteess

ffoorr

SSoorr

bbaass

//NNiijj

aarr aa

nndd MM

eelliillll

aa ttee

pphhrraa

ss aann

dd ii

nnttee

rrccaall

iibbrraa

ttiioonn

wwiitt

hh FF

CCTT

..

In t

his

tabl

e da

ta p

rese

nted

in t

able

5.1

and

5.2

are

com

bine

d to

a s

ingl

e ag

e es

timat

e fo

r ea

ch a

sh la

yer.

In a

dditi

on, t

he c

ycle

in w

hich

the

ash

laye

r oc

curs

, the

num

ber

of a

naly

ses

incl

uded

and

MSW

D v

alue

s ar

e m

entio

ned.

The

inte

rcal

ibra

tion

fact

ors

R b

etw

een

ash

laye

rs a

nd the

FCT

sta

ndar

d ha

ve b

een

calc

ulat

ed a

nd e

stim

ates

for

the

astr

onom

ical

cal

ibra

ted

ages

for

FCT a

re g

iven

. The

R’s

of

the

VU

37 d

ata

wer

e ca

lcul

ated

rel

ativ

e to

TCR

and

hav

e be

en r

ecal

cula

te r

elat

ive

to F

CT w

ith

the

inte

rcal

ibra

tion

fact

or o

f 1.

0112

± 0

.001

0 be

twee

n TC

R a

nd F

CT (

Renn

e et

al.,

199

8).T

he a

naly

tical

sta

ndar

d er

rors

of th

e m

ean

are

repo

rted

for

the

40Ar

/39 A

r ag

es o

f th

e

ash

laye

rs w

ith -

bet

wee

n br

acke

ts -

the

ful

l err

or e

stim

ate

(eq.

1.1

4). 1σ

erro

rs a

ccor

ding

to

equa

tion

1.15

are

rep

orte

d fo

r th

e as

tron

omic

al c

alib

rate

d FC

T ag

es.


Me-5 of Roger et al. (2000) seem reliable, although probability distributions are asymmetrical (differentpopulations can not clearly be distinguished). Me-16 and If-4 of Roger et al. (2000) have MSWD values >1,indicating that either the assigned errors are too small to explain the observed scatter, or that the samplesare heterogeneous. Both, Me-16 and If-4 show a bimodal probability distribution (not shown). MSWD valuesfor the data published by Cunningham et al. (1997, table 5.4) are all >1. Additionally, none of ash layersshows a perfect normal probability distribution. Especially V1 and V2 display a multi modal distribution, V3shows a small age component around 6.3 Ma, while the dominant age is around 6.0 Ma and IR-1.0 is notcompletely symmetrical but represent the best approach to a normal distribution of the Cunningham et al.(1997) data. Nevertheless, the 40Ar/39Ar data of Cunningham et al. (1997), Roger et al. (2000) and Cornéeet al. (2002) do not contradict the astronomical ages where the age constraints cannot be specified in moredetail than older or younger than a certain astronomical age, because the volcanic layers are intercalated inthe sedimentary section at a position either below the first tuned or above the last tuned cycle (V1, IR-1.0,V3, Ta-2). The 40Ar/39Ar ages of V2, Me-5, Me-13 and Me-16 (Cunnigham, 1997; Roger et al., 2000) aresystematically younger than the astronomical ages, If-4 (Roger et al., 2000) is older. Further, most of the40Ar/39Ar ages of Cunningham et al. (1997), Roger et al. (2000) and Cornée et al. (2002) can not bedistinguished at the 1σ level (analytical uncertainties only) from the data reported here (table 5.4). Only thebimodal (with peaks at 6.38 and 6.55 Ma) Me-16 of Roger et al. (2000) deviates from the 40Ar/39Ar agereported here. The 40Ar/39Ar data in three different laboratories (New Mexico Geochronology ResearchCenter, Nice and the VUA) confirm the reproducibility of the 40Ar/39Ar ages.

Previous chronological studies in SE Spain mainly focussed on the lamproitic volcanism and on the Cabo deGata volcanic complex. The thin volcanic horizons intercalated in sediments described by Bellon et al.(1983) are dated as Messinian based on the biostratigraphic constraints in the basin. No previous radio-isotopic studies on the volcanic ash layers studied here are known.

SEDIMENTARY CYCLES IN THE MELILLA BASIN PRECESSION CONTROLLED?

Apart from the discussion about the accuracy of 40Ar/39Ar ages in an absolute way, 40Ar/39Ar ages provide arelative time frame, where only the analytical uncertainty has to be accounted for. When plotting the40Ar/39Ar data of the ash layers versus the cycle number in which they occur, an average period of 20.8 kyris derived for a cycle based on simple linear regression. In this way it is confirmed by a completelyindependent method that the observed cyclicity is indeed precession controlled. This was alreadyestablished independently for Pliocene cyclic continental successions (Steenbrink et al., 1999), but is nowalso confirmed for late Miocene marine successions.

DISCREPANCY BETWEEN 40Ar/39Ar AND ASTRONOMICAL AGES?

Figure 5.3 shows the 40Ar/39Ar data compared to the astronomical ages for incremental heating andcombined fusion experiments of ash layers from the Sorbas/Nijar Basin (table 5.1). The error bars in thefigure represent the combined analytical uncertainty of standards and samples (figure 5.3a). Mostgeochronological studies report analytical errors only. This is correct, when ages are only regarded asrelative ages or when comparing ages to check on reproducibility. When radio-isotopic data are comparedwith astronomical ages, i.e. completely independent methods, the full realistic error estimate must be

141

CHAP

TER

5



reported to consider a 40Ar/39Ar age as an absolute age. Therefore, also other uncertainties i.e. in absoluteages of standards, decay constants must be accounted for. Using a slightly modified error propagationmethod based on Min et al. (2000), the uncertainties in 40Ar/39Ar ages increase (figure 5.3b) and almost alldata are indistinguishable from their astronomical counterparts. However, the general picture is that theplateau ages of biotite are consistently older than the astronomical age for ash layer a.1.1. For ash layera.1.2. the biotite plateau and astronomical ages are the same and for ash layer a.1.3. data are morescattered. When all plateau steps are combined a.1.3 is 6.707 ± 0.010 (0.091) Ma, a.1.2 is 6.771 ± 0.009(0.092) Ma and a.1.1 is 7.251 ± 0.008 (0.098) Ma. Uncertainties are analytical errors, between brackets fullerror estimates were given. Combined plateau ages of a.1.1 and a.1.2 show almost normal probabilitydistributions, MSWD values lower than 1 and isochron intercepts indistinguishable from the atmosphericratio of 295.5 (296.2 ±. 3.9 for A.1.1, 294.9 ± 3.4 for A.1.2). Ash layer a.1.3 has a slightly higher MSWDvalue (1.04), an isochron intercept that very slightly deviates from the atmospheric ratio at the 1σ level(298.3 ± 2.4) and an almost normal probability distribution. The inverse isochron age for a.1.3 is 6.679 ±0.027 (or 0.094 full error) Ma. Overall, it can be concluded that the 40Ar/39Ar biotite data of Sorbas do notshow a systematic deviation towards younger ages relative to the astronomical ages (i.e., 6.71 ± 0.09 Maversus 6.69 Ma for a.1.3, 6.77 ± 0.09 Ma versus 6.77 Ma for a.1.2 and 7.25 ± 0.10 Ma versus 7.16 Ma fora.1.1). Due to the lack of high quality data on sanidine, we can not tell if there exist a difference betweensanidine and biotite 40Ar/39Ar ages, although biotite ages in Ptolemais and Faneromeni (chapter 3)appeared to be significantly older than the sanidine ages. The data reported on the sanidine / plagioclasemixtures for Sorbas / Nijar do show a tendency to be younger than the biotite (and astronomical) ages aswell. This might for example be due to recoil effects in biotite (e.g., Huneke and Smith, 1976).

In Melilla predominantly single crystals of sanidine have been dated. This enabled us to unequivocallydetect xenocrystic contamination. Figure 5.5 directly shows the influence of removal of contaminating data

142

CHAPTER 5

Ash layer40Ar/39Ar age,published (Ma) N MSWD Astronomical

ages (Ma)

40Ar/39Ar age,this study (Ma)

Cunningham et al, 1997V3 5.99 ± 0.10 12 3.20 <6.05 ± 0.02 6.03 ± 0.01

V2 6.72 ± 0.02 7 1.39

V2 6.76 ± 0.02 8 2.426.79 6.73 ± 0.01

V1 7.05 ± 0.14 10 8.48

V1 7.05 ± 0.14 7 3.29>6.83 6.89 ± 0.01

IR-1.0 6.90 ± 0.02 10 1.94 >6.86 -

Roger et al, 2000Me-16 6.46 ± 0.03 10 1.45 6.58 6.53 ± 0.01

Me-13 6.54 ± 0.04 10 0.35 6.63 6.58 ± 0.01

Me-5 6.73 ± 0.02 1 + 1 + 1 0.87 6.79 6.73 ± 0.01

If-4 6.29 ± 0.02 3 + 4 + 3 1.52 6.29 -

Cornée et al, 2002Ta-2 6.87 ± 0.02 4 + 4 + 3 0.66 >6.83 6.89 ± 0.01

TTaabbllee 55..44:: PPrreevviioouussllyy ppuubblliisshheedd ggeeoocchhrroonnoollooggiiccaall ddaattaa ffoorr tthhee MMeelliillllaa bbaassiinn..

The “accepted” 40Ar/39Ar ages of previous publications on the Melilla basin are reported relative to FCT is 28.02 Ma. Errors are

quoted at the 1σ level and represent analytical standard errors of the means.


on the age distributions. Therefore, the accepted 40Ar/39Ar ages for all tephras are normal distributions withMSWD <1 substantially lower than 1 (see table 5.3), indicating that samples are homogeneous andanalytical errors are not underestimated. A full error assessment has also been applied to the Melilla dataincreasing the (analytical) uncertainty about ~4 times. Looking at the analytical error only, all reliable40Ar/39Ar ages are systematically younger than their astronomical counterparts (even the biotite of Mes-6).With a full error assessment this discrepancy is not significant anymore. It must be remarked that thevariance of such a full error assessment is dominated for >75% by the uncertainty in the activity of thedecay of 40K to 40Ar and for ~14% by the uncertainty in the amount of radiogenic 40Ar in the primarystandard for all analyses. To obtain more accurate 40Ar/39Ar ages effort must be directed to decreasing theuncertainties in the values of these parameters (e.g., Begemann et al., 2001). Overall, it can be concluded40Ar/39Ar ages on sanidine indeed tend to be systematically younger than the astronomical counterparts inthe eastern (chapter 3) as well as in the western part (this chapter) of the Mediterranean, although with afull error assessment this discrepancy might disappear partly. Biotite ages show a less consistent picture.

IMPLICATIONS FOR AN ASTRONOMICALLY CALIBRATED 40Ar/39Ar STANDARD?

As stated before the uncertainties in the activity of the decay of 40K to 40Ar and in the amount of radiogenic40Ar in the primary standard dominate the total variance of 40Ar/39Ar ages. One way to diminish theinfluence of these two parameters is to use a primary standard, which is dated by completely independentmethods. Here we will use the astronomically dated ash layers as “primary” standard to establish an age forFCT. In that case the amount of radiogenic 40Ar in the primary standard does not occur in the age equationand the activity of the decay of 40K to 40Ar occurs only once (instead of twice) (Chapter 1). Table 5.3 showsthe obtained ages for FCT based on the ash layers in the western Mediterranean. The FCT agesintercalibrated to biotite ages show a different pattern than sanidine ages, but are not consistent. The FCTages obtained on the Sorbas/Nijar sanidine/plagioclase mixtures seem to support the data obtained forMelilla, but unfortunately, these data were not of the same excellent quality. The Melilla ash layers producea consistent age for FCT with only one ash layer (Ifo-4) significantly different at the 1σ level. This ash layerappeared to be the only multi-modal ash layer in Melilla (figure 5.5) with no indications for the “true” age,but with the youngest peak of 6.235 Ma producing an age of 28.22 Ma for FCT. Total variance is nowdominated by the analytical uncertainties (~70-90%) and the uncertainty in the astronomical age of thevolcanic ash layer (~10-30%). As shown in chapter 3 uncertainties in decay constants (or relatedparameters) hardly influence the data in this time interval and therefore the discussion about the “true”values for decay constants (e.g., Begemann et al., 2001) can be circumvented. The weighted mean age forFCT intercalibrated to all reliable sanidine data of Melilla (excluding Ifo-4 and plagioclase Mes-11) is 28.24 ±0.01 Ma (standard error of the mean, with a 10 kyr uncertainty in the astronomical age). The probabilitydistribution is normal and the MSWD is 0.51. This age for FCT is in agreement with the age of 28.21 ± 0.03Ma for the A1 ash on Crete, but is different from the age of 28.02 Ma of Renne et al. (1998) and 28.476 ±0.064 Ma zircon concordia age of Schmitz and Bowring (2001). However, Renne et al. (1998) report anuncertainty of 0.28 Ma, when all sources of error are included and therefore, the Renne et al. (1998) age isnot significantly different than ours, but ours is more accurate. The U/Pb age of Schmitz and Bowring(2001) might be affected by residence time in the magma chamber, producing an older age than the age oferuption.

143

CHAP

TER

5



CONCLUSIONS

Sanidine 40Ar/39Ar ages are indeed systematically younger than astronomical ages for volcanic ash layerfrom which the sanidine originates. This is most probably due to the uncertainties in decay constants andthe absolute age of the standards. Intercalibrating the 40Ar/39Ar data of FCT and 16 ash layers in Melillaresults in an age of 28.24 ± 0.01 Ma for FCT equivalent to 28.21 ± 0.03 Ma based on the A1 ash layer onCrete. The intercalibration with the Ptolemais data (chapter 3) is not consistent with these data. Xenocrysticcontamination observed (and accounted for) in Melilla could not be assessed in Ptolemais and Crete.However, possible occurrence of xenocrystic contamination in Ptolemais does not fully explain the observeddifference, because 40Ar/39Ar ages are too young and xenocrystic contamination tends to result in older40Ar/39Ar ages. This may imply possible uncertainties in the tuning in Ptolemais unrecognized so far.Additionally, to confirm an age of 28.24 Ma for FCT relative to an astronomically dated standard, we willextend our research to deep marine astronomically dated sections of middle Miocene age (category 1 ashlayers).

144

CHAPTER 5


kuiper-binnenwerk 30-09-2003 15:03 pagina 123 chapter 5 · 40ar/39ar dating of tephra layers...

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