u-series disequilibria in young (a.d. 1944) vesuvius rocks: preliminary implications for magma...

Ž .Journal of Volcanology and Geothermal Research 82 1998 97–111

ž /U-series disequilibria in young A.D. 1944 Vesuvius rocks:Preliminary implications for magma residence times and

volatile addition

Stuart Black a,), Ray Macdonald a, Benedetto DeVivo b, Christopher R.J. Kilburn c,Guiseppe Rolandi b

a EnÕironmental Science DiÕision, IEBS, Lancaster UniÕersity, Lancaster LA1 4YQ, UKb Dipartimento di Geofisica e Vulcanologia, UniÕersita di Napoli, Via Mezzocannone 8, 80134 Naples, Italy`

c Department of Geological Sciences, UniÕersity College London, Gower St., London WC1E 6BT, UK

Received 26 October 1996; accepted 1 January 1997

Abstract

The results of a preliminary U-series study of the timescale of magmatic processes at Vesuvius are presented.Phonotephrites of the 1944 eruption of Vesuvius show 0–15% 230Th–238U and 350–1150% 226Ra–230Th disequilibria.Apparent U–Th internal isochrons for a lava and a cumulate nodule suggest crystal residence times of 12 and 39 ka,respectively. A tephra sample shows isotopic heterogeneity, possibly related to mixing of younger crystal-laden melt andolder crystals giving apparent U–Th ages of 0.4 and 18 ka, respectively. Mineral 226Ra–230Th disequilibria on Ba-normal-ised internal isochron diagrams suggest Ra–Th ages of 1730–3300 years for the same rocks and phenocrysts. Minor226 230 Ž .Rar Th heterogeneity between minerals and groundmass or whole rock is evidence of open-system Ra–Th behaviour.This heterogeneity suggests that there have been recent, post-crystallisation changes in melt composition that affected 226Ramore than 230Th. Continued crystallisation in a Ra-enriched magma has subsequently resulted in Ra–Th disequilibriaprobably as a result of addition via a fluid-rich phase. Magma differentiation, residence time, transport, and pervasive gasaddition at Vesuvius apparently occur over geologically short periods. q 1998 Elsevier Science B.V. All rights reserved.

Keywords: U-series; magma residence times; volatile; crystallisation ages

1. Introduction

The Somma–Vesuvius composite volcano is situ-ated on top of thick Mesozoic and Cenozoic carbon-ate rocks of the Campanian–Lucanian carbonate

Ž .platform Ippolito et al., 1975 . The volcano has

) Corresponding author. Tel.: q44-1524-594209; fax: q44-1524-593985; e-mail: [email protected]

erupted silica-undersaturated, K-rich lavas and pyro-clastics for at least 25,000 years. The recent eruptive

Ž .history can be divided into three periods: 1 theearly historic period before the A.D. 79 ‘Pompei’

Ž .plinian eruption; 2 a middle period, covering A.D.Ž .79 to 1631; and 3 continuous activity from A.D.

1631 to 1944.The eruption of Vesuvius March 14–23, 1944,

represents the most recent activity of a volcano that

0377-0273r98r$19.00 q 1998 Elsevier Science B.V. All rights reserved.Ž .PII S0377-0273 97 00059-0

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Ž . Ž .Fig. 1. Map of the Vesuvius region after Belkin et al., 1993 . Scale 1:60,000, details of sample localities for those samples prefixed with V can be found in Belkin et al. 1993 .Samples MTQT comes from the Terzigno Quarry, 1.5 km southwest of Terzigno. 44CI comes from MTQT tephra deposit 5 m east of the locality of tephra sample.

( )S. Black et al.rJournal of Volcanology and Geothermal Research 82 1998 97–111 99

has erupted almost continuously through historicalŽ .times Fig. 1; Scandone et al., 1993 . The 1944

eruption is of particular interest in that not only wasit extensively observed but the 50 years of quies-cence that have followed the eruption suggests that itrepresents an end point for the largely effusive phaseof activity that started in 1631. This is important forunderstanding processes in the plumbing system ofVesuvius.

Although the eruptive chronology of the volcanois reasonably well established, the timescales overwhich magma formation and evolution occur arepoorly understood. Here we present the results of apreliminary study using 238 U-series disequilibriamethods to determine crystal residence times. Wehave made measurements of 238U–234 U–230 Th–226Raand 232 Th activity concentration ratios in three rocksand separated minerals from the 1944 eruption. Theresults point to residence times of the order of 104

years and strongly suggest open-system behaviour ofthe magma reservoirs.

2. 1944 eruption chronology and stratigraphy

The 1944 eruption started with the collapse ofconelets and moderate explosive activity on March13 and 14. Lava emission started approximately 8 hafter the collapse of the conelets and flows con-verged towards San Sebastiano and the Massa diSomma. This was followed by a period of lava firefountaining, starting at 17.00 h on March 21, andcontinuing with eight separate phases each lastingapproximately 45 min, until 18.00 h on March 22.Scoria falls were reported early on March 23, fallingon Terzigno and South Giuseppe until 13.00 h, whenthe fire fountains turned ashy with a sustained col-umn 6–7 km high. Large ash and pumice falls werereported around Terzigno, San Giuseppe Vesuvianaand Torre del Greco. Large explosions occurred onMarch 23, accompanied by a large earthquake. Thecrater formed by the eruption presently has a volume

6 3 Žof 25=10 m and a depth of 300 m Scandone et.al., 1993 .

3. Petrology

The 1944 eruptive rocks are strongly porphyritic,Ž .with 40–60% by volume phenocrysts. The domi-

nant phenocrysts are clinopyroxene, leucite and pla-gioclase; mean abundances are clinopyroxene 11%,leucite 36% and plagioclase 9%, but there is consid-erable variation in the relative proportions of the

Ž .phases Trigila and De Benedetti, 1993 . Olivine,biotite, titanomagnetite and apatite are less abundant

Ž .phenocrysts -1% .Compositional variation in whole rocks is signifi-

cant, e.g. MgO 3.3–10.4% and CaO 8.20–18.7%,and is largely due to mixing of phonotephritic meltŽ .SiO ;48%, MgO;3% with various proportions2

of accumulated phenocrysts, dominated by clinopy-Žroxene Villemant et al., 1993; Trigila and De

.Benedetti, 1993; Belkin et al., 1993 . However, theŽcalculated amounts of minerals accumulated Vil-

.lemant et al., 1993 in general match the modalabundances indicating that the latter are a reasonableapproximation of the fractionating assemblage.

An important constraint on the applicability of theU-series internal isochron method is that the mineralphases were in, or at least close to, chemical equilib-rium with the melt. We now assess this constraint forthe 1944 Vesuvius products, using the mineral chem-

Ž .ical data of Trigila and De Benedetti 1993 .Clinopyroxene phenocrysts in the 1944 rocks show

sector and oscillatory zonation. The compositionrange is Wo En Fs , with three main47 – 51 34 – 47 6 – 16

modes at En , En and En . Plagioclase phe-40 37 36

nocrysts are very strongly zoned, with corroded cores.Compositions range from An to An , but most62 90

analyses fall in two, smaller ranges, An and74 – 78

An . Olivine compositions vary from Fo to84 – 86 77Ž .Fo and are generally uniform Joron et al., 1987 .65

There are no data for leucite phenocrysts from theŽ1944 eruption. However, in similar, young 1714–

.1872 A.D. rocks from Vesuvius, leucite is close tostoichiometric and also shows little compositionalvariation.

Phenocrysts in 1944 rocks show, therefore, smallbut significant compositional variations, which Trig-

Ž .ila and De Benedetti 1993 and Villemant et al.Ž .1993 ascribe to imperfect re-equilibration follow-ing crystallisation over a range of low-pressure con-

Ž .ditions 2–4 km . The occasional presence of cor-Ž .roded green Fe-rich cores to clinopyroxene phe-

nocrysts also suggests that magma mixing was atleast locally operative.

The 1944 rocks provide, then, a good test of the

( )S. Black et al.rJournal of Volcanology and Geothermal Research 82 1998 97–111100

robustness of the U-series dating technique to rockswhere complete chemical equilibrium between crys-tal and melt has not been achieved.

4. U and Th decay series disequilibria

There are two naturally occurring alpha decayseries which may be used to study recent geological

Ž238 . Ž232 .processes—the uranium U and thorium Th .In each, a long-lived parent passes by a- or b-decayto a stable isotope of lead. The different geochemicalbehaviour of the parent and daughter products allowsthem to be fractionated, establishing radioactive dise-quilibrium. Because the half-lives of these daughtersare known, the time since fractionation can be as-sessed and the variety of half-lives permits the studyof processes operating on timescales ranging from

Ž .days to ;350 ka Fig. 2a and b .Uranium-series disequilibria studies utilise nu-

clide activities rather than mass concentrations. Theratios between the different nuclides in a decayseries are conventionally reported as activity ratios,denoted by parentheses, rather than atomic ratios.When a decay series is at secular equilibrium, allspecies of that series will be decaying at the samerate or activity A, where:

Asl N 1Ž .i i

ŽThus, nuclides with a high probability of decay l ,i.or decay constant will generally be present only inŽ .low concentrations N . All parent–daughter activityi

ratios within a given decay series at secular equilib-rium i.e. 238Ur230 Th ratios will be equal to one.Radioactive disequilibrium refers to any state wherethe activity ratio of any pair of nuclides from asingle decay series is not equal to one, for example238 230 Ž .Ur Th ratios/1 indicate recent -350 kafractionation.

Thorough reviews of the principles underlying238 U-series disequilibria are provided elsewhereŽAllegre, 1968; Capaldi et al., 1976; Condomines et`

.al., 1988; Gill et al., 1992; Macdougall, 1995 .Briefly, the 238 U–230 Th method is based on therestoration with time of equilibrium between theprogeny 230 Th and parent 238U after an event causes

Ž .Fig. 2. a U-Decay series radionuclides. Half-lives are in yearsŽ .unless otherwise stated. b Th-decay series radionuclides. Half-

lives are in years unless otherwise stated.

U:Th fractionation. U and Th isotope ratios of coge-netic minerals which have different initial degrees offractionation may form an isochron described by the

Ž .equation Allegre, 1968 :`230 Th 230 Th 238 U

yl t yl ts e q 1yeŽ .232 232 232ž / ž / ž /Th Th Tht 0 t

2Ž .230 Ž .where l is the decay constant of Th lnrhalf-life

Ž .where the half-life is 75.2 ka Meadows et al., 1980 ,and 232 Th is used as a reference isotope for normali-sation purposes. For this isotope system, secularequilibrium would be approached after around 350ka.

Absolute dating of volcanic rocks by 238 U-seriesdisequilibria methods is controversial because the


Table 1Modal analyses for the 1944 samples

Sample Gdm Lc Cpx Plag Ol Bi Op Mt Accesory Total

Ž .Tephra MTQT 67.1 13.2 12.3 1.8 — 2.0 — 3.0 -0.5 100.0Ž .Lava V138 44 35.8 10.8 8.9 — — 0.5 -0.5 -0.5 100.0

Ž .Cumulate 44CI 7.8 32.2 44.3 7.0 0.9 — — 4.0 2.0 100.0

Gdmsgroundmassrglass; Lcs leucite; Cpxsclinopyroxene; Plagsplagioclase; Olsolivine; Bisbiotite; Opsorthopyroxene; Mtsmagnetite. Acessory phrases are commonly apatite with occasional sphene.

required assumption of closed-system behaviour mayŽnot always be met Allegre and Condomines, 1976;`

Capaldi et al., 1982; Hemond and Condomines, 1985;ˆ.Pyle et al., 1988 . Ages defined by U–Th internal

isochrons may not necessarily represent eruption agesof the magma but times of crystal formation in themagma reservoir. If these ages are significant incomparison to the half-life of 230 Th then magma

Ž .residences times decay that occurs in the reservoirmay not be insignificant. At least one case,226Rar230 Th isotopic heterogeneity between miner-

Žals and groundmass in Mt. St. Helens rocks Volpe.and Hammond, 1991 clearly violates the closed

system assumption. Still, 238 U-series disequilibriumalso can be used to constrain the timescales and

Žquantify the extent of chemical fractionation Con-.domines et al., 1982, 1988; Gill et al., 1992 . In this

context, the results for Vesuvius will be described interms of understanding the magmatic processes.

5. Previous disequilibria measurements on Vesu-vius

The first U–Th disequilibria studies of VesuviusŽ . 230Oversby and Gast, 1968 reported Th excessŽ . 226 230 210 2267% with Rar Th ratiosG10 and Pbr Ra

Ž .disequilibria. More recently, Capaldi and Pece 1981Ž .and Capaldi et al. 1982 analysed mineral separates

from Vesuvius which did not satisfy the closedsystem criterion of thorium isotope homogeneity andform a coherent isochron. Whilst samples had similar

Ž .strontium isotope ratios Cortini and Hermes, 1981and an apparently equilibrium phenocryst assem-blage, in each case the whole-rock 230 Thr232 Th waslower than in each mineral. An isochron slope,formed with all their whole rock and mineral sam-

ples, defined an apparent age of 24,000 years. How-ever, their samples also had 226Rar230 Th)1, indi-cating a fractionation event -8000 years ago. The

Table 2Chemical analyses of samples from the 1944 eruption

Sample No.: V138 MTQT 44C1Analysis: A B B

Ž .SiO wt% 48.2 47.9 47.22

TiO 0.89 0.91 0.782

Al O 18.5 15.8 12.42 3

Fe O 2.88 8.11 7.212 3

FeO 4.90 N.A. N.A.MnO 0.14 0.14 0.12MgO 3.48 5.26 9.95CaO 8.46 10.9 15.6Na O 2.63 2.15 0.852

K O 7.85 6.04 3.492qH O 0.28 1.4 2.202yH O 0.10 N.A. N.A.2

P O 0.88 0.77 0.542 5

CO 0.01 N.A. N.A.2

Total 99.20 98.60 97.80

Ž .Zr ppm 197 204 128Rb 315 269 135Sr 1100 756 570

aBa 2390 2570 633bBa 2389 2561 621

Y 26 18 20Nb 31 22 7Pb 20 24 6

Trace elements Zr, Rb, Sr, Ba, and Y by EDXRF at Reston,unless otherwise stated. Pb values were determined by XRF atLancaster. A sground at Reston, Va, with steelraluminium, Nbvalue determined by EDXRF. BsXRF at Lancaster Universitywith all FE as Fe O .2 3a Ba EDXRF result.b Ba Dionex result.


U–Th internal isochron was considered to reflectthorium isotope inhomogeneity in the magma at thetime of eruption, whereas the favoured explanationfor the Ra disequilibria involved late-stage mixingwith fluids to transfer into the magma radium with a

Ž .‘crustal’ signature. Hemond and Condomines 1985ˆresampled and reanalysed the lava. They did not findany mineral–whole-rock disequilibrium, and no traceof a high 230 Th component in any mineral separate.They inferred that laboratory contamination by anunspecified component may have played a role in thevariation of the 230 Thr232 Th in the Capaldi mineralseparate data.

6. Samples and analytical procedures

We have determined U, Th concentrations andvarious U-series activity ratios including Ra, Pb andPo radionuclides, on a cumulate nodule and its hosttephra, together with a lava sample for internal U–Thand Ra–Th mineral isochrons from the 1944 erup-tion. The cumulate is composed of similar phases tothose in the lava, namely clinopyroxene, leucite,plagioclase, olivine and titanomagnetite; the cumu-

Ž .late also contains some interstitial glass Table 1 .The petrogenetic relationship of the nodule to thelavas and tephra has been uncertain—it might repre-sent cogenetic material from the magma reservoir, orit may be part of an earlier magmatic episode.

The lava sample, prefixed V, was a split of theŽ . Ž .sample analysed by Belkin et al. 1993 Table 2 ;

two additional samples were collected for this study.Sr, Nd, Pb, He and Be isotope data, phenocrystassemblages and locality details can be found in

Ž .Cortini and Van Calsteren 1985 , Belkin et al.Ž . Ž .1993 , Morris et al. 1993 and Graham et al.Ž .1993 .

Analytical methods are discussed in detail byŽ .Black 1994 . Briefly, U-series radionuclides were

measured by high resolution alpha spectrometry atLancaster University. Whole-rock sample sizesranged from 0.5 to 2.0 g whereas mineral massesrange from 0.8 to 5.1 g. Whole-rock and mineralseparates sample solutions were split into threealiquots: one for U–Th analyses, one for PorPbanalyses and another for Ba determinations. U–Thand Po were separated by conventional chemical

Žseparation techniques HF–HNO –HCl digestion3

followed by anion exchange separation and electro-.plating for U, Th and autoplating for Po . Radium

analyses were conducted on separate whole-rockaliquots by g-spectrometry, after sealing in an air-tight container for 21 days to allow 222 Rn to ingrowto equilibrium with 226Ra.

For internal isochrons, the phases separated froma single rock sample include leucite, pyroxene,olivine, biotite, glassrmatrix and a magnetic sepa-rate dominated by Fe–Ti oxides. Mineral and glassseparates were made using standard magnetic and

Table 3U-series and Ba results for the JF-1 and the A-THO standard

230 232 234 238238 232 Ž . Ž . Ž .Ur Th Thr Th Ur U U ppm Th ppm Ba ppm

U-series and Ba results for the JF-1 standardLancaster 0.866"0.012 0.864"0.013 0.32"0.01 1.16"0.03 1754"16Listed values for JF-1 N.A. N.A. 0.33 1.17 1750

U-series results for the A-THO standardŽ .Condomines Clermont-Ferrand TIMSra 0.907"0.009 1.029"0.014 2.24"0.5% 7.48"0.5%

Ž .O.U. TIMS 0.919"0.009 1.018"0.009 2.22"0.001 7.34"0.010Ž .Santa Cruz TIMS 0.919"0.002 1.015"0.004 1.007"0.003 2.27"0.003 7.49"0.009

Ž .Lancaster a 0.912"0.009 1.019"0.009 0.999"0.012 2.27"0.056 7.50"0.150

JF-1 standard is a Japan Geological Survey feldspar standard which was run with the mineral analyses. Errors quoted are 2s . Results for theŽ . Ž .A-THO standard run at Lancaster compared to results for the same standard, in Williams et al. 1992 mean of 3 analyses , Lancaster result

is the mean of 12 analyses. Errors quoted are 2s .


density techniques, followed by hand-picking to re-move impurities. Difficulties in obtaining absolutely

Ž .pure separates were imposed by glass melt inclu-sions and plagioclase in the leucite, fine intergrowthsof ulvospinel and other phases in the magnetic sepa-rate, and microphenocrysts in glass separates. Theestimated purity for leucite, pyroxene, biotite, olivineand glass was )95%. Other minerals present in thesamples, namely, apatite and plagioclase, occurred insuch small abundances that it was impossible toseparate them in sufficient abundance for a-spec-trometry.

For U–Th analyses a 232 U–228 Th yield monitorwas used with a decay and ingrowth correctionapplied to the daughter 224Ra nuclide. Typical yieldsranged from 87 to 100% for uranium and 85 to100% for thorium. Blank and background determina-tions were carried out frequently on low backgroundEGG ‘Ultraw ’ detectors, averaging -10 counts in10,000 under determinant peaks and -25 counts in10,000 under the 232 U–228Th peaks. Whole rocksamples were generally counted for between 2 and 6days, whereas mineral separates were counted forbetween 2 and 10 weeks.

The precision of the U–Th results can be assessedfrom analyses of the A-THO and JF-1 standards run

Ž .over a three year period Table 3 , which showreproducibility of the 238 Ur232 Th and 230 Thr232 Thratios to be better than 1.5%. Our U and Th massconcentration values also compare very well with theINAA determinations presented by Belkin et al.Ž . Ž1993 for splits of the same sample mean differ-

.ences of 3.4 and 2.8% for U and Th, respectively .Also the reproducibility of our analyses for separatealiquots of the same sample for both minerals andwhole rocks is better than 1.5%.

Barium analyses were also conducted on wholerock and mineral separates by Chelation liquid chro-matography on a CS12 column using a Dionex 4000i.Initial HF–HNO –HCl digestions were followed by3

uptake in 3% HNO and elution of Ba on a cation3

column after approximately 15 min. Whole-rock BaŽ .analyses by Dionex compare well "1.5% to

whole-rock XRF Ba analyses presented by Belkin etŽ . Ž .al. 1993 Table 2 .Polonium analyses were conducted on whole rocks

and some mineral separates following the methods ofŽ . Ž .Flynn 1968 , and are documented in Black 1994 .

Fig. 3. 210 Por230 Th versus 210 Pbr230 Th for the whole rock andmineral separates from the 1944 eruption. All the samples overlapwithin uncertainty, the 1:1 relationship indicating that 210 Po anal-yses can be used to determine 210 Pb analyses. 210 Po determinedby a-spectrometry and 210 Pb determined by g-spectrometry. Un-certainties are 2s .

Ž 2 . 210The very good correlation r s1.00 between PoŽ . 210 Ž .a and Pb g can be seen in Fig. 3 indicatinginternal consistency in our a- and g-spectrometrydata.

7. Results

Tables 4 and 5 list radionuclide concentration andactivity ratio data. For all samples 234 Ur238 Us1.00within analytical uncertainty. The rocks have238 Ur230 Th activity ratios between 0.982 and 1.145,

Ž .the majority showing relative U-excess Fig. 4 . Therelatively small Th–U fractionation shown by thelavas and tephra differs from determinations by Ca-

Ž .paldi et al. 1982 but is similar to that reported byŽ .Hemond and Condomines 1985 and to the moreˆ

Žextensive data on pre-1944 rocks Black, unpubl..data ; such small fractionation could be a characteris-

tic of the 1944 flows.Table 4 lists the concentration of Th, U and Ba in

mineral separates, matrix glasses and whole rocksfrom the Vesuvius samples. U and Th concentrationscan be used to evaluate mineralrmelt partition coef-ficients. The calculated partition coefficients


Table 4U–Th mass concentration data for internal isochron products erupted in 1944

Phase Whole rock Leucite Clinopyroxene Magnetic separate Glass Olivine Groundmass Biotite

Cumulate 44CIU 1.39 1.11 0.565 5.26 5.17 0.138Th 4.28 3.07 1.422 24.83 18.61 0.445Ba 633 3400 25 156 1801 13ThrU 3.09 2.77 2.52 4.72 3.60 3.22BarTh 148.0 1107 17.6 6.28 96.8 29.2

LaÕa V138U 6.28 0.826 0.759 0.931 7.11Th 19.44 2.093 1.88 1.901 19.64Ba 2390 2010 15 63 2651ThrU 3.10 2.53 2.47 2.04 2.76BarTh 122.9 960.0 7.98 33.1 135.0

Tephra MTQTU 6.25 1.19 0.946 1.075 11.547 1.333Th 19.37 1.11 2.030 2.126 34.369 0.795Ba 2570 3200 12 38 2816 269ThrU 3.10 0.933 2.146 1.978 2.98 0.596BarTh 132.6 2883 5.91 17.9 81.93 338.4

Table 5U–Th–Ra activity ratios of the 1944 whole rock and mineral separates

238 232 230 232 238 230 234 238 226 230 210 230a bSample Ur Th Thr Th Ur Th Ur U Rar Th Pbr Th

TephraWhole rock 0.990"0.021 1.009"0.024 0.982"0.023 0.999"0.016 8.00"0.13 8.10"0.35Leucite 3.283"0.068 0.962"0.070 3.091"0.081 0.998"0.021 69.48"8.63 70.10"6.17Biotite 5.143"0.244 1.594"0.041 3.036"0.078 1.007"0.019 N.A. 1.07"0.37Pyroxene 0.700"0.030 0.9001"0.031 0.699"0.035 1.000"0.024 N.A. 1.04"0.12Magnetic separate 1.550"0.021 0.970"0.021 1.449"0.036 0.985"0.029 N.A. 0.946"0.06Glass 1.030"0.010 0.955"0.009 0.976"0.019 0.991"0.016 N.A. 2.86"0.29

LaÕaWhole rock 0.990"0.021 0.901"0.024 1.099"0.024 1.003"0.015 8.46"0.10 8.85"0.40Leucite 1.210"0.031 0.911"0.022 1.329"0.036 1.007"0.019 55.01"6.27 53.37"4.49Pyroxene 0.806"0.032 0.865"0.031 0.932"0.039 1.000"0.024 N.A. 1.16"0.04Magnetic separate 0.666"0.010 0.856"0.014 0.778"0.023 0.985"0.029 N.A. 2.64"0.07Matrix 0.901"0.011 0.875"0.009 1.030"0.021 0.999"0.016 N.A. 11.09"0.42

CumulateWhole rock 0.994"0.031 0.868"0.023 1.145"0.037 1.017"0.021 11.61"0.22 11.70"0.20Leucite 1.110"0.016 0.904"0.018 1.228"0.023 0.998"0.021 72.74"4.45 73.10"6.90Pyroxene 0.821"0.014 0.815"0.016 1.007"0.021 1.007"0.019 N.A. 1.02"0.11Olivine 0.875"0.021 0.825"0.019 1.061"0.022 1.000"0.024 N.A. 1.02"0.19Magnetic separate 0.650"0.022 0.765"0.021 0.850"0.029 1.003"0.015 N.A. 1.21"0.10Glass 0.851"0.011 0.821"0.010 1.037"0.020 1.000"0.010 N.A. 9.40"0.33

a 226Ra determined by g-spectrometry using 214 Pb peak after sealing for 21 days.b 210 210 Ž210 . 226 210Pb via Po Po analyses by a-spectrometry . Ra and Pb values decay corrected back to the time of eruption.


Ž238 232 . Ž230 232 .Fig. 4. Ur Th versus Thr Th isochron diagram for the lava, tephra and cumulate samples from the 1944 eruption. Data fromŽ . Ž .Capaldi et al. 1982 are shown for comparison. Uncertainties are 2s for this study and 1s for Capaldi et al. 1982 .

D rD , are consistent with those given bymineral matrixŽ .Gill et al. 1992 . The apparent partition coefficients

for all the phases are greater than 0.02 and may beaffected by the presence of U-rich inclusionsŽ .apatite . D and D for clinopyroxene, leucite,U Th

olivine and biotite must, therefore be considered asŽmaximum values. D values for clinopyroxene -Ba

. Ž .0.013 and olivine -0.007 are low, but are higherŽ . Ž .for leucite 0-76-1.89 , magnetic separate -0.086Ž .and biotite 0.096 . The values for clinopyroxene,

olivine, leucite and biotite are in line with previouslyŽreported partition coefficients for Ba Baldridge et

.al., 1981; Gill et al., 1992 .Magnetite has been suggested as a possibility for

Žproducing fractionation between U and Th LaTour-.rette et al., 1991 . Our data show large variations in

Ž .D rD for the magnetic separate 0.67–1.31 . ThisTh U

variation, however, could be due to inclusions ofapatite or a variation in the oxidation state of theuranium. Clinopyroxene, olivine and biotite all havelow U and Th partition coefficients and D rD )1.Th U

However, the D for all fractions are higher, whereasU

D are comparable to previously reported valuesThŽ .Gill et al., 1992 , but not recent experimental dataŽ .LaTourrette and Burnett, 1992; Beattie, 1993 . Therelative U–Th fractionation which we observe inclinopyroxene and olivine is supported by experi-mental data.

The mineral separates have lower U and Th con-centrations than the glass and matrix. It is, however,likely that some U-rich impurities are included in theclinopyroxene, leucite and magnetic separates as il-lustrated by the order of decreasing UrTh ratios:Cumulate: pyroxene ) leucite ) whole rock )

olivine)glass)magnetic separate.Lava: magnetic separate ) pyroxene )

leucite)groundmass)whole rock.Tephra: biotite) leucite)magnetic separate

)pyroxene)glass)whole rock.Minerals exhibit greater 230 Th–238 U fractionation

than the whole rocks or groundmass. Leucite, onemagnetic separate, biotite and olivine are 238U-en-

Ž .riched 123–309, 145, 304 and 6%, respectivelyŽ .Table 5 . The magnetic separates in the lava and

Ž230 . Žcumulate are Th -enriched 22 and 15%, respec-.tively . The magnetic separate from the tephra con-

tains inclusions of glass and lesser amounts of opaqueminerals which it was impossible to separate. Largevariations are apparent from the lava and tephra; forexample, the leucite phenocrysts have variations in

Ž . Ž .Ba 3400–2010 ppm , Th 3.07–1.11 ppm ,226 230 Ž . 238 230 ŽRar Th 72.7–52.0 and Ur Th 3.09–

.1.23 indicating that there may be inclusions in thesephenocrysts or that they represent minerals of differ-ent magmatic events.

All the whole-rock samples are in secular equilib-


Fig. 5. 226Rar230 Th versus 238 Ur230 Th for lavas and tephra fromthe 1944 eruption. Field for 1944 lavas and tephras from Blackunpublished data. Uncertainties are 2s .

228 232 Ž228 232 .rium for the Ra– Th system Rar Ths1 .All show 226Rar230 Th disequilibrium, with excess226 Ž . 226Ra Fig. 5 . The tephra has 800% excess Raand the lava and cumulate have 846–1160% excess226Ra. The magnitude of Ra–Th disequilibrium inthese very young volcanics is greater than that ob-

Ž .served in axial MORB Goldstein et al., 1991 and inŽyoung rocks from Mount Etna and Stromboli Con-

.domines et al., 1982; Capaldi et al., 1983 and BaturŽ .Rubin et al., 1989 , but is of the same order as

ŽVesuvius rocks reported elsewhere Capaldi et al.,.1982 .

It is important to assess whether 226Ra mobilisa-tion during deuteric alteration andror weathering hasaltered magmatic 226Rar230 Th ratios. U and Ra aremobile in surficial, oxidising environments and al-tered volcanic rocks show 234 Ur238 U disequilibriumŽ .Gill et al., 1992 . None of our samples exhibits suchdisequilibrium, and we, therefore, conclude that the226Ra–230 Th disequilibrium results from magmaticprocesses.

8. Discussion: timescales of magma genesis

8.1. Isotope systematics

238 U decays to 206 Pb, and the intermediate iso-topes include 230 Th and 226Ra which have half-livesof 75,200 and 1600 years, respectively. At secularequilibrium samples have 226Rar230 Th and 238 Ur230

Th ratios of unity, and thus the observed ratios of226Rar230 Th and 238 Ur230 Th)1 reflect very recentincreases in those ratios. Radioactive equilibrium isrestored in five times the half-life of the parentisotope, so 226Rar230 Th ratios)1 reflect changes inthe last 8000 years, whereas 238Ur230 Th)1 reflectchanges in the last 350,000 years.

Fig. 6. 226Rar230 Th versus BarTh for the whole rock and mineral separates from the 1944 eruption. Fractional crystallisation will producesteep, positive trends, with radioactive decay moving samples vertically downwards as indicated by the arrow. The majority of minerals lieon a single mineral partitioning line, indicating that Ra and Ba have not fractionated from one another. Uncertainties are 2s . Inset is anexpanded portion of the lower part of the diagram.


Criteria for 230 Thr232 Th–238 Ur232 Th dating byinternal or mineral isochron methods are described

Želsewhere Condomines et al., 1976, 1988; Gill et.al., 1992 . Using Ba as a natural analogue for a

stable Ra isotope, a similar internal isochron diagram226 230 Ž .with Ra and Th activities dpmrg normalisedŽ . Žto Ba grg may be used for dating e.g. Volpe and

.Hammond, 1991; Schaefer et al., 1993 . This as-sumes that magmatic processes do not fractionate Raand Ba and that all minerals and the melt have thesame initial 226RarBa ratios. This condition is ap-parently met at Vesuvius; there is a positive correla-tion between 226Rar230 Th and BarTh element ratiosŽ . 226Fig. 6 , suggesting that Ra has partitioned simi-larly to Ba during phenocryst formation.

8.2. 230Th–238U and 226Ra–230Th disequilibrium

Fig. 7a shows an internal 230 Th–238U isochrondiagram for lava V138. The calculated age is 12q6

y5

ka. If this represents the age at which the phe-nocrysts crystallised, a crustal residence time of be-tween 7 and 18 ka is implied. The isotope systemat-ics require that the crystals in the magma had beenan essentially closed system for G7 ka. Ra–Th

Žrelationships reveal a more complex situation Fig.. q6107b . The phenocrysts define an isochron at 2630y490

years, much younger that the U–Th crystallisationage. Since we have precluded the possibility that Raand Ba were fractionated during crystallisation, theRa excess implies either that:

Ž .1 The phenocrysts and melt were not in chemi-cal equilibrium, i.e. had different initial 226RarBaratios. This might be consistent with compositionalzonation in pyroxene and plagioclase phenocrystsdiscussed earlier. Despite inferred 226Ra heterogene-ity in the mineral phases, the internal isochron forthe lava appears to be robust.

Ž .2 Ra was added to the magma after initialphenocryst formation.

Given the evidence from U–Th systematics ofcrystal–melt equilibria in this rock, we prefer thesecond explanation for Ra excess and suggest that aRa-rich volatile phase was added to the porphyriticmagma in the past 2.6 ka. The Ra–Th isochron thusdates the addition of the Ra-excess and subsequent

Ž . 238 232 230 232Fig. 7. a Ur Th– Thr Th internal isochrons for lavaV138. Uncertainties are 2s ; Some uncertainties are smaller thansymbols. The ages were measured by the slope of the isochron inall cases was calculates using a weighted regression method whichtakes into account the 2s counting errors associated with each

Ž . Ž . 226 . 230variable Williamson, 1968 . b Ra rBa– ThrBa internalisochron for lava sample V138. Uncertainties are 2s . 226Ra wasdetermined via 210 Po for the mineral separates.

continued crystallisation of the magma implying thatthe Ra-enrichment is associated with younger rims.

Ž . ŽThe melt groundmass shows Ra-excess i.e. plots.above the Ra–Th isochron due to continued Ra

addition beyond the time of Ra–Th crystallisationand represents the Ra-rich volatile component.

ŽThe calculated U–Th age for the cumulate Fig.. q98a is 39 ka, significantly older than the eruptiony8

age and the phenocryst crystallisation age for the


lava. The cumulate cannot be comagmatic with thelava and must represent an earlier crystallisationperiod, followed by effective isolation, in terms ofU–Th systematics, at the wall or roof of the magma

Ž .reservoir. An isochron on a Ra–Th plot Fig. 8b forthe cumulate indicates the presence of excess226Rar230 Th in the rock and a disequilibrium event-8000 years ago. The Ra–Th isochron age for thisrock is 3270q700 years, excluding the olivine andy540

pyroxene fractions that have 226Rar230 Th equal tounity. We suggest that a volatile or fluid phase hasvariably interacted with the cumulate phases, gener-

Ž . 238 232 230 232Fig. 8. a Ur Th– Thr Th internal isochrons for cumu-Ž . 226 230late sample 44CI. Uncertainties are 2s . b RarBa– ThrBa

internal isochron for cumulate sample 44CI. Uncertainties are 2s .226Ra was determined via 210 Po for the mineral separates.

Ž . 238 232 230 232Fig. 9. a Ur Th– Thr Th internal isochrons for tephraŽ . 226 230sample MTQT. Uncertainties are 2s . b RarBa– ThrBa

internal isochron for tephra sample MTQT. Uncertainties are 2s .226Ra was determined via 210 Po for the mineral separates.

ating considerable 226Rar230 Th heterogeneity. Con-Žtinued crystallisation of some phases leucite and

.those present in the magnetic separate has generatedRa–Th disequilibria together with a Ra excess in thegroundmass phase very similar to that generated inthe lava sample. This late stage fluid interaction withcrystal mushes is consistent with previous work onfluid inclusions of cumulate nodules from Vesuvius

Ž . Žby Belkin and DeVivo 1993 and elsewhere Wi-.dom et al., 1993 .

The data for the 1944 tephra define two lineararrays on a U–Th isochron plot, with least-squaresbest-fit slopes corresponding to ages of 18q2 andy2

q3 Ž .0.4 ka Fig. 9a . The older age corresponds to they3


biotite and pyroxene phenocrysts, whereas theyounger age is defined by leucite, magnetic separate,glass and whole rock. This division into twoisochrons is consistent with the lack of 226Rar230 Th

Ždisequilibrium in the pyroxene and biotite Table 5.and Fig. 9b thus they must be )8000 years old

similar to the situation observed in the cumulatenodule with the pyroxene and olivine phenocrysts.The other phases display excess 226Rar230 Th andmust be -8000 years old. The tephra apparentlycontains phenocrysts from two periods of crystallisa-tion, perhaps brought together by convective mixingof older crystals retained in the chamber withyounger, porphyritic melt. This is consistent withevidence from the Ra–Th for the same sample; theage defined by the phases with 226Rar230 Th disequi-libria is 1760q500 years, within error of the U–Thy450

age.The U-series data apparently define four crystalli-

sation episodes: 39, 18, 12 and 0.4 ka. Given that wehave analysed only three samples and that the errorsin each age are large, we might assume that crystalli-sation has been essentially continuous over the past40 ka. 226Rar230 Th data seem to require that the

Ž .magmatic system s represented by our specimensremained open, after crystallisation, to modificationby a volatile phase, and subsequent crystallisationbut closed as regards the U–Th systematics.

Ra data suggest that the magma chamber resi-dence times of minerals in the Vesuvius rocks arecommensurate with the half-life of 226Ra. Equilib-rium 226Rar230 Th ratios observed in some phe-nocrysts imply residence times greater than 8000years. The extreme 226Ra deficits for the magneticseparate in the tephra suggest that the residence timeis about 1800 years. These residence times, togetherwith U–Th isochrons are consistent with studiesincluding residence and settling times of 101–103

years calculated for crystals in convecting magmaŽ .chambers Martin and Nokes, 1988 , and with 3 ka

anorthoclase crystallisation in phonolite from MtŽ .Erebus Reagan et al., 1992 .

The data imply that recent Ra addition to themagma -8 kyr and perhaps -2 kyr ago. The dataalso suggest that many of the erupted products in the1944 eruption had crystallised over a long periodprior to eruption. Thus, conduit walls may have beenlined with ‘cumulates’ of many ages, some of which

Ž .leucites and melt may have subsequently becameimpregnated with oxidising Ra-rich fluids that didnot reset the U–Th in the minerals. This could haveoccurred either as discrete cumulate blocks or asdisaggregated minerals that were added to youngermagma, especially when more hydrous and subse-

Žquently erupted in a more explosive way e.g. the.tephra sample . The Vesuvius magma system must

involve an ‘open-hydrous’ system. The U–Th–Radata are consistent with this interpretation rather thana simple mixing of older crystals with younger meltŽas illustrated by the varying leucite compositionsand variations in initial 230 Thr232 Th ratios of the

.lava and tephra .

9. Conclusions

Ž .1 Significant U-excess in specimens from the1944 eruption of Vesuvius indicates that the relevantmagmas were generated less than 0.3 Ma ago.

Ž .2 Apparently robust U–Th internal isochronshave been obtained, despite evidence of composi-tional zonation in the phenocryst phases.

Ž . q9 q63 U–Th crystallisation ages of 39 and 12y8 y5

ka have been determined for a cumulate and lava,respectively. A tephra sample apparently recordsmixing of two phenocryst populations crystallising18q2 and 0.4q3 ka.y2 y3

Ž .4 Excess Ra was added to the parental magmaof the lava at ;2 ka, and to the other samples atsometime -8 ka. Ra–Th internal isochrons showages of 2630q610 years for the lava and 1760q500

y490 y450

years for the tephra indicating recent addition ofRa-rich fluids that continued leucite crystallisationand enriched the melt and whole rock compositions.The cumulate nodule also shows a much younger

Ž q700 .Ra–Th age 3270 years than its U–Th crystalli-y540

sation age would suggest.Ž .5 The preliminary data presented here confirm

that U-series disequilibria studied can play an impor-tant role in unravelling the nature and timescales ofprocesses in the Vesuvius magma chamber.

Acknowledgements

We thank Jim Gill and Wendy Bohrson for veryhelpful reviews of an early version of this manuscript.


We would also like to thank Mike Kelly for accessto his radiochemistry facility. This work was sup-ported by Lancaster University.

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u-series disequilibria in young (a.d. 1944) vesuvius rocks: preliminary implications for magma...

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