Oxygen isotope constraints on the petrogenesis of volcanic arc magmas from Martinique, Lesser Antilles

Earth and Planetary Science Letters, 95 (1989) 255-270 255 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands

[11

Oxygen isotope constraints on the petrogenesis of volcanic arc magmas from Martinique, Lesser Antilles *

J o n P. D a v i d s o n * a n d R u s s e l l S. H a r m o n * *

Department of Geological Sciences, Southern Methodist University, Dallas, TX 75275, U.S.A.

Received December 21, 1988; revised version accepted August 8, 1989

Volcanic rocks from the island of Martinique in the Lesser Antilles island arc are highly diverse isotopically. Values of 8180 (%0 SMOW) in lavas erupted over the last 5 Ma from five stratigraphically discrete and geochemically distinct volcanic centers are: Morne Jacob = +7.5 to +7.9%o; Pitons du Carbet = + 8.0 to +8.9%o; Diamant = +6.3 to +7.4%0; Piton Mt. Conil, +7.2 to +7.3%0, and Mt. Pelre = +6.5 to +7.4%o. 13 of the 27 samples have been corrected for variable amounts of secondary alteration and low-temperature hydration. The least 180-rich andesites on Martinique have 8180 values similar to those of the northern Lesser Antilles (8180 = +6.0 to +7.3%c) and other intra-oceanic arcs like the Marianas (8180 = +5.5 to +6.8%0). O-isotope ratios are compositionally dependent; correlations between 8180 and radiogenic isotope ratios (87Sr/86Sr, 143Nd/la4Nd, 2°6pb/2°4pb) and indices of fractionation, show that modification of isotopic compositions occurred during magmatic differentiation rather than at source.

A two-stage model is proposed for the origin and differentiation of magmas on Martinique. Primary basaltic melts, which are broadly similar in composition to primitive island-arc magmas elsewhere (e.g. Marianas, Aleutians, South Sandwich Islands), are generated from the subduction-modified asthenospheric mantle wedge. These magmas ascend into the arc crust and differentiate. On Martinique, crystal f~ctionation is accompanied by contamination with isotopically distinct crust, including a component of terrigenous sediments. Thus, subduction-zone processes at the Lesser Antilles and other island arcs are similar, involving only a small (albeit significant isotopically) volumetric contribution from subducted sediment.

1. Introduction

I t is w ide ly be l i eved tha t (1) s u b d u c t i o n

p rocesses recycle crus t to the man t l e , (2) c o m p o -

nen t s f r o m s u b d u c t e d s ed imen t s a n d h y d r o t h e r -

r ea l ly -a l t e red ocean i c l i t hosphe re are a d d e d to the

sub -a rc man t l e , a n d (3) this c h e m i c a l l y - m o d i f i e d

m a n t l e is the source for m o s t i s l and a rc m a g m a s .

Cha rac t e r i s t i c s c o n s i d e r e d d i s t inc t ive of a rc m a g -

m a s inc lude h igh l a rge- ion l i t hoph i l e e l e m e n t /

h igh- f ie ld s t r eng th e l e m e n t ( L I L E / H S F E ) a b u n -

d a n c e ra t ios (e.g. B a / N b ) as wel l as Sr-, N d - and

* This paper is dedicated to the memory of the late James Borthwick, whose analytical excellence in the field of stable isotopes will be greatly missed.

Present addresses: * Department of Earth and Space Sciences, University of

California, Los Angeles, CA 90024, USA. **NERC Isotope Geology Center, 64 Gray's Inn Road,

London WCIX 8NG, England, UK.

0012-821X/89/$03.50 © 1989 Elsevier Science Publishers B.V.

P b - i s o t o p e c o m p o s i t i o n s wh ich are d i sp l aced

s l ight ly f r o m those of M O R B . T h e i n v o l v e m e n t of

a c o m p o n e n t d e r i v e d f r o m s u b d u c t e d s e d i m e n t s in

m a n y arcs is s t rong ly i m p l i c a t e d by a 1°Be sig- n a t u r e a n d in the re l a t ive ly h igh 207 Pb /204 Pb ra t ios

wh ich cha rac t e r i z e arc lavas.

In the d i s cus s ion w h i c h fo l lows we focus on the

P l i o - P l e i s t o c e n e v o l c a n i c rocks on the i s l and of

M a r t i n i q u e (i.e. the " R e c e n t A r c " as d e f i n e d by

W e s t e r c a m p a n d T a z i e f f [1]), in the Lesser An t i l l e s

(Fig. 1). M a r t i n i q u e is an i m p o r t a n t site at wh ich

to e x a m i n e the r e l a t ive ro les o f source c o n t a m i n a -

t ion versus c rus ta l c o n t a m i n a t i o n because the

Lesse r An t i l l e s is an i n t r a - o c e a n i c arc, and on

M a r t i n i q u e an e x c e p t i o n a l l y wide r ange o f iso-

top ic a n d i n c o m p a t i b l e t race e l e m e n t c o m p o s i -

t ions is p r e s e n t in the e r u p t e d v o l c a n i c rocks [2].

S imi la r ly v a r i a b l e c o m p o s i t i o n s are o b s e r v e d at St.

Luc ia , o n l y 100 k m s o u t h o f M a r t i n i q u e , whe re

W h i t e et al. [3] h y p o t h e s i z e tha t la rge v o l u m e s of

s u b d u c t e d s e d i m e n t (ca. 1 5 - 3 0 % ) were i n v o l v e d in

256

(a) o ,'oo m i km M A R T I N I Q U E !

18 ° N $ - St. Kitts Nor th m ~

• ",~/ Atlantic " I N / " '. ° Ocean

16 ° N Statia ~ DSDP t 1 •

Dominica ~ Hole %43------ Martinique~

14 ° N ~ St. Lucia Mt. P

Caribbean ., ~ St. P i e r r e ~ ygc00. :. :\£fZ'~_~ ~

12 ° N e n

SOUTH ~ ~.j ~ ~ . ~ . x ~ :~) / '

Fort de France"~_. ~ -~

Rocher du ~Pit°nsduCarbet I Oiamant ~ # Marne Jacob i J

10 km ~ ] Intermediate and I I

Old Arc ( b )

Fig. l. Location map of (a) the Lesser Antilles island arc and (b) a simplified geological map of the island of Martinique. In (a) major geographic and physiographic features are identified; and the site of DSDP Hole 543 is indicated. In (b), the locations of the five volcanic centers studied from the Recent Arc are shown. The centers are listed in sequence from youngest (Mt. Pel6e) to oldest (Morne Jacob).

magma genesis. If such a simple sediment-mantle mixing model is tenable, then the central Lesser Antilles is unusual in that a much greater proportion of subducted sediment is being contributed to the sub-arc mantle here than the - 0.3-5% typically estimated for other arcs worldwide [4-7]. Alternatively, the proportion of sediment involved in magma genesis may be small in all arcs, with the isotopic variability observed at specific sites, such as Martinique (and probably St. Lucia), being the result of processes occurring within the arc crust which have overprinted the source characteristics. This is the central question addressed in this study•

Silicate materials that have experienced a cycle of crustal residence and weathering, or have been formed at low-temperatures in the presence of meteoric water or seawater, typically have high 6180 values, generally in excess of + 10%o, whereas oceanic basalts typically have ~]80 values between +5.5 and +6.5%0 ([8] and references therein)• Therefore, O-isotopes provide a possible means of assessing the extent to which ]80-rich

crustal materials (e.g. hydrothermally-altered oceanic lithosphere, pelagic sediments, altered volcaniclastic arc crust, and terrigenous sediments) may have played a role in arc magma genesis and evolution during transit and storage prior to eruption. The mantle source for arc basalts is probably similar to MORB in O-isotope composition (i.e. 6]SO = +5.7 + 0.3%0). Addition of a fluid component from the subducted slab will not significantly alter the oxygen budget of the source (which is already dominated by this element). However, the interaction of basaltic magma with crust during chemical differentiation has the potential to modify melt 1 8 0 / ] 6 0 ratios because crustal rocks typically are more ~80 rich than mantle melts. This study evaluates the extent to which it is possible to distinguish between crustal and source 180 enrichments in an intra-oceanic arc setting and to define the relative importance of the two processes. Caution is required in the analysis and interpretation of O-isotope ratios of volcanic rocks as they may be subject to modification by post-eruption subsolidus processes [52].

We present, therefore, additional analyses of structural water contents and H-isotope compositions, which enable us to assess the primary versus secondary nature of measured 81SO values. From these new data for the volcanic rocks of Martinique we show that there are multiple lines of petrologi- cal, geochemical, and isotopic evidence to support the hypothesis that magma-crust interactions have modified the compositions of ascending parental magmas.

2. Geological setting

The Lesser Antilles island arc (Fig. 1) has developed during the past 40 million years as a result of westward subduction of the Atlantic plate beneath the Caribbean plate. A double arc structure exists north of Dominica, where the active, young volcanic arc lies inboard of an older, Oligocene arc; south of Dominica, the older and younger arcs are superimposed (Fig. la).

Martinique (Fig. 1) is located in the central part of the Lesser Antilles island arc at a point where subduction of the Atlantic lithosphere beneath the Caribbean plate is roughly orthogonal. The Benioff zone beneath Martinique is located between 140 and 200 km depth [9]. Crustal thick- nesses, although variable in the region of Martinique and poorly constrained by seismic data, are probably in excess of 25 km [10]. The central Lesser Antilles is regarded as a mature arc; volcanism on Martinique has persisted for at least 35 Ma [11]. In contrast, the current volcanic arc to the north of Dominica records volcanism only since < 10 Ma, and gravity data suggest that the arc basement here is not very substantial. There- fore, the northern Lesser Antilles may be used as a reference example of an immature oceanic arc in which magmas have not had to ascend through a thick arc basement.

Martinique consists of three primary volcano- stratigraphic units (after [1], Fig. lb): (1) an "Old Arc" of hydrothermally altered and zeolitized volcanic rocks in the east and southeast, (2) an "Intermediate Arc" of submarine to subaerial volcanic deposits in central to southeast Martinique, and (3) a "Recent Arc" in the northwest and west of the island. By restricting our discussion to the younger volcanic centers of the Recent Arc we minimize (but do not completely

257

avoid) problems of weathering and alteration which may have modified significantly the O- and H-isotope characteristics in the older (especially submarine) rocks [16]. The Recent Arc consists of five discrete volcanic complexes spread over a distance of some 50 km (Fig. lb). In order of decreasing stratigraphic age, these are: (1) Morne Jacob, (2) Pitons du Carbet, (3) the Presqu'Ile des Trois Ilets peninsula, (4) Piton Mt. Conil, and (5) Mt. Pel6e. The lavas of Morne Jacob are dark, vitrophyric two-pyroxene andesites, which overlie the volcanics of the "Intermediate Arc" in the northwest of the island and form the sub-stratum upon which the Pitons du Carbet and Mt. Pel6e centers have developed. The Pitons du Carbet volcanic complex is formed by a series of deeply- dissected hornblende- and quartz-bearing dacitic domes, which are the erosional remains of much more extensive volcanic deposits. Overlap in K-Ar ages of between 3.1 and 0.8 Ma [11,12] for the Pitons du Carbet and Morne Jacob lavas suggests that at least some of the volcanic activity at these two centers was contemporaneous, despite their apparent stratigraphic superposition. The Pres- qtf'Ile des Trois Ilets peninsula in southwest Martinique consists of a number of small volcanic edifices, the most prominent of which is the andesitic center Rocher du Diamant (or Morne Lar- che r - -bu t hereafter simply referred to as "Di- amant"), which forms a single volcanic cone on the peninsula and a small, offshore island. The volcanic centers of Piton Mt. Conil and Mt. Pel6e form the northwestern portion of Martinique. Pi- ton Mt. Conil is comprised of hornblende-phyric andesites, ranging in age from 2.6 to 0.6 Ma [11,12]. These lavas were the immediate precursors to the Mt. Pel6e volcanics and are compositionally similar to the basal volcanics of the Mt. Pelde center [13]. Typically, the Mt. Pel6e lavas are two-pyroxene andesites. With the exception of the 1902 and 1929 summit domes, Mt Pel~e deposits consist almost entirely of pyroclastic material.

3. Geochemistry

Although spanning a limited range of bulk composition, with S i O 2 c o n t e n t s of only 55 to 63 wt.%, the volcanics of the Recent Arc exhibit large variations in trace element and radiogenic isotope composition [2]. However, compositional varia-

258

Mj=Morno Oa¢o b r~orne younqe, o Mt Pel

1.81-1 (Trend n o~Sm~,~l I "~ ' ~d " M ...... E a s / i o e r • o r n e Jacob

.~ " ["Field of Mt Pelee t ~ , ' , - . ~ / Carbet ~ . 0 ~ j (Includes data from ~ . " ~ - / ~ , ~ - . t ~ ~

- "~- F s~t , ~, R o o b o U _4L1:.~_/.=/// /.C-/m T /~

o.8 rx7/../J~.F// / ~ ~ , 0 . 6 t ~ P am~t M A R T I N I Q U E

55 6 0 65 SiO 2 (wt. %)

Fig. 2, K ; O - S i O 2 re la t ions for the vo lcan ic cen te r s o f the

Recent Arc on Martinique. Data from this study are indicated by symbols; the larger fields include analyses from [13,14,43]. Heavy dashed lines indicate the trend lines for Mt. Pelde (P), Pitons du Carbet (PC), and Marne Jacob (MJ) defined by [13]. The distinct K20-SiO 2 variations indicate that the lavas erupted at each center have followed independent differentiation paths, implying that the magmatic plumbing systems of five volcanic centers are independent and probably not inter- connected to a large magma reservoir of regional scale at depth. In this and subsequent figures filled and open symbols denote older and younger centers respectively (details on key).

tions are less extensive for individual centers. This is illustrated by the K20-SiO 2 relations shown in Fig. 2. Individual volcanos have erupted lavas which plot along approximately linear trends, but the slope of each trend differs, suggesting that different differentiation paths were followed. Within the five centers, magmatic evolution ap- pears to have been dominated by fractional crystallization processes involving pyroxenes and plagioclase _+ olivine, oxides, and amphibole [13,14], although some open-system behavior is required to account for the variable radiogenic isotope ratios observed at some centers (e.g. Pi- tons du Carbet, Table 1). Unfortunately, these data do not permit rigorous modelling as only restricted compositional ranges are present in the available outcrops. At Mr. Pel6e, where good exposures do ensure stratigraphic control and a range in S i O 2 c o n t e n t is present, major and trace element data are consistent with closed-system fractionation of a p lagioclase-hornblende-magne- t i te-pyroxene assemblage, as represented in rare cumulate blocks [2,13].

The basic question then arises as to the explanation for the differences in incompatible trace element ratios and radiogenic isotope compositions of the five young volcanic centers. Mixing,

between a mantle (or mantle-derived) end-member (parental melt) and a component with the chemical characteristics of ancient continental crust, in different proportions can explain the trends observed. Whether mixing occurs at source (i.e. through the addition of subducted sediments to the mantle wedge) or in the arc crust (i.e. contamination through magma-c rus t interaction) or both, has been much argued [2,3,15-20] and is the subject of this paper. Making a distinction between these possible models is particularly difficult; the effect of adding 1% contaminant to the mantle or 10% contaminant to a 10% partial melt of that mantle will be closely similar in terms of radiogenic isotope and incompatible element ratios in the resultant mixture. However, oxygen, unlike Sr, Nd and Pb, is a major element, and oxygen abundances between solid and melt are not significantly different. Thus, whereas sol id-melt S r /Nd , S r / P b and N d / P b ratios may be similar, S r /O, N d / O and P b / O ratios will be very different. Mixing lines between sediment + mantle and sediment + mantle-derived melts on Nd-Sr, Sr-Pb and Nd-Pb isotope plots cannot be distinguished. On plots of Sr-O, Nd-O or Pb-O isotopes a clear distinction can be made [21].

4. Sampling and analytical procedures

The rugged topography and thick rain forest cover on Martinique makes much of the island inaccessible and, therefore, stratigraphically-constrained sampling of the younger volcanic centers is not possible. Samples were collected from natural exposures, road cuttings, and quarries and selected for stable isotope analysis on the basis of geochemical character (to cover a representative range in compositions) and minimal macroscopic alteration. Although all samples are porphyritic, some are fine-grained, many contain groundmass glass, and a few exhibit minor visible alteration.

Table I presents new 180/a60 , D / H and H 2 0 + data for the five Pliocene to Recent volcanic centers of the Recent Arc on Martinique, together with the major element and radiogenic isotope data from Davidson [2]. Analytical measurements were made in the ISEM Stable Isotope laboratory at Southern Methodist University. Oxygen was liberated from 15-25 mg samples by reaction with C1F 3 at 650°C [22], and converted to CO 2 for

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260

isotopic analysis. For D / H - H 2 0 + analyses, structural water was liberated from 250-350 mg samples melted by induction heating in vacuum, after outgassing overnight at 120 o C. This water was converted to H 2 gas for isotopic analysis by reaction with uranium metal at 650 ° C and yields measured manometrically. Isotope ratios of 180/160 and D / H were measured on a MAT-251 mass spectrometer and are reported in Table 1 in the usual 6 notation as permil (%o) deviations from the SMOW standard [23]. Analytical preci- sion is on the order of _+0.1%o for 6180 values, +4%o for 6D values, and _+0.04 wt.% for H2 O+ contents. In the ISEM laboratory, reference sample NBS-28 had a 8180 value of +9.66 _+ 0.05%o during the course of this work.

5. Results

5.1. H20 +-6D-6180 relationships There are several lines of evidence to indicate

that vapor-saturated arc magmas contain on the order of a few weight percent dissolved water at depth, that partial degassing in the high-level conduits of a volcanic system to a water content of - 0.5 wt.% is necessary for the extrusion of a flow or dome, and that explosive eruption will result in near total water loss and a distinct lowering of D / H ratios [24]. Thus it would be expected that fresh volcanic rocks would contain small amounts of water (generally < 0.5 wt.%) and have variable 6D values if degassing has been the pr imary control on volatile content. Degassing should not affect O-isotope ratios. Also it is well known that glassy and fine-grained volcanic rocks are highly susceptible to secondary processes (such as de- uteric and hydrothermal alteration, hydration and weathering) which typically affect glassy silicic volcanic rocks more readily than their mafic coun- terparts [8,24-26]. The secondary uptake of water through low-temperature alteration, hydration or weathering at the Earth's surface will result in increased H 2 0 + and 6180 values, and 6D values which are determined by the latitude, temperature and elevation of the locality in question. Linked variation in all three parameters can be considered a diagnostic feature of secondary processes and, therefore, can be used as a means of distinguish- ing between primary degassing control on D / H ratios, and secondary alteration.

Structural water contents and D / H ratios for the Mt. Pel6e samples range from H2 O+ = 0.09 to 1.38% and 6D = - 9 9 to -40%o respectively; samples from the four other centers of the Recent Arc on Martinique fall within this range (Table 1). Of the samples analyzed, water contents for 12 are < 0.4%. Only 4 samples have high H2 O+ (> 0.8%: M8217, M8277, M8269, and 21898). There is a general tendency for (1) both 6D and 8180 values to increase with increasing H2 O+ contents, and (2) 8180 values to be higher in the older (Pitons du Carbet and Morne Jacob) lavas than in the younger (Diamant, Piton Mt. Conil and Mt. Pel6e) volcanic rocks (Table 1). Together, these features suggest that a majority of Martinique volcanics have been affected to some extent by post-eruption hydration and alteration processes, with elevated H2 O*, 6D and 8180 reflecting the secondary uptake of a D-rich fluid during alteration. Similar (but much larger) increases in lSO/160 ratios during low temperature hydration of siliceous glasses have been recorded from Plio-Pleistocene volcanics of the Turkana Basin in East Africa [25].

A further indication that the high water contents observed in a few of the Martinique samples most likely are a product of secondary processes is provided by Fig. 3, where water contents are plotted as a function of an index of magma differentiation (SiO2). On this diagram, which il-

LI L ' ' cpx÷ plag-dom noted' "~ / MARTINIQUE ~ ,4 froctionat~on | (Recent Arc) / % , , P /

F l ol- dominated 1.0| y0un ger -.-_,._~/ "? / , . - ' [ / /

08~" volcanics / .~U/ .,~ll" I ~ older -~ ' / / / - ' ' ' / I volcanies /

b(lsalti~- 0.4

F . . . . .

, o°!o,o,,,C:

SiO 2 (wt. %)

Fig. 3. Variat ion in H 2 0 + versus SIO 2 contents for the volcanic rocks of the Recent Arc on Martinique. The inset in the lower right of the diagram indicates the sense of H20+-SiO2 variation for various processes that might have affected the Martinique volcanic rocks [44]. The steep/vertical H20÷-SiO2 arrays suggest that the secondary uptake of water has affected rocks from both the older and younger volcanic centers.

lustrates H20+-SiO2 relationships for the five Martinique volcanic centers, the large field for Mt. Pelre encompasses nearly the entire range of observed variation for the Recent Arc. Model fractionation curves calculated by progressive removal of a cpx-plag-opx_+ol assemblage, with H20 treated as an incompatible species are shown, using a relatively primitive basaltic andesite from Statia in the northern Lesser Antilles [17] as a possible parental melt. There is no indication that the data reflect a fractionation trend. The samples with H20+< 0.4 wt.% fall in a restricted field, well below the limiting fractionation curve, as expected for andesitic magmas that had been variably devolatilized prior to crystallization and cool- ing. Variation in H2 O+ is independent of SiO 2 contents, and takes the form of broad sub-vertical

~" +9 o ¢/)

rr +8 o

MARTINIQUE (Recent Arc)

+1o[ I 0.4 wt % H20 +

+7

volcanics

II: 8180 = 0.51 H20++ 6.75

\ J

younger volcanics

0.5 1.0 1.5

H2 O+ (Wt. %)

Fig. 4. Whole-rock 8]80 values for Martinique Recent Arc rocks versus H20 + contents. The dark solid lines labelled I and 1I are the least squares regressions through the data from the older and younger centers respectively. For samples with > 0.4 wt.% HzO +, extrapolations have been made along lines of parallel slope assuming that the secondary uptake of water was accompanied by a systematic enrichment in 180 [27,28]. In this way "corrected" 8180 contents for 0.4 wt.% H20 + equivalent are obtained. These are listed in Table 1, in parentheses after the measured values.

261

arrays to higher H2 O+ contents, a feature that is diagnostic of hydration or alteration (Fig. 3, inset).

We have tried to correct for the secondary uptake of water, and its effect on whole-rock 1SO contents, by extrapolating measured 6180 values back to pre-alteration values based upon the as- sumption that systematic enrichment in 180 accompanied the uptake of water [27,28] from a primary post-eruption HzO + value of - 0.4 wt.%. Fig. 4 is a plot of measured 6180 versus H2 O÷ contents for the Recent Arc volcanic rocks. It is clear that the Martinique data form two distinct 6180-H2 O+ arrays: (I) a steep-sloped trend (6180 = 2.76 H20 ÷ + 7.06) defined by rocks of the two older centers, Pitons du Carbet and Morne Jacob, and (II) an array of shallow slope (6180 = 0.51 H 2 0 + + 6.75) described by the younger volcanic rocks of Diamant, Piton Mt. Conil and Mt. Pelre. We have corrected measured whole-rock 6180 values for secondary uptake of water using these two least squares regression relationships for the older and younger samples respectively. Such an ap- proach has been demonstrated by previous workers [27,28]. The corrected 8180 values for the 13 samples with high water contents (H20+ > 0.4 wt.%) are given in parentheses in Table 1, alongside the measured values. The difference between measured and corrected 81So values is generally small (< 0.5%0) and only exceeds 1%o for one sample, M8269. Corrected values are used throughout the following text and figures, but we stress that this is merely a refinement of the data, enabling us better to see through the "scatter" introduced by secondary alteration.

5.2. 6~80 variations

Ranges in whole-rock 6180 values (after adjust- ment for secondary water uptake) for the five volcanic centers of the Recent Arc on Martinique are: Morne Jacob = +7.5 to +7.9%o; Pitons du Carbet = + 8.0 to + 8.9%o; Diamant = + 6.3 to +7.4%o; Piton Mt. Conil, +7.2 to +7.3%0, and Mt. Pelre = + 6.5 to + 7.4%o (Table 1). The distribution of Martinique 6180 values is compared with O-isotope data from other intra-oceanic arcs in Fig. 5. Important features are: (1) the substantial 6180 variation present within this small portion of a single island relative to the northern Lesser Antilles over some 300 km arc length, and relative to most other island arcs, (2) the largely

262

OCEANIC ISLAND ARCS Banda Arc

n n n EL7 0 n n i i p 1

~ r ~ L 7 Volcano-lzu Arc

[ ~ ] ] 7 Marianas Arc

i i i

] ~ , Northern Lesser Antilles i , i r

j Martinique (Recent Arc) ~BB , , 'o'P'P'~IP'"'~'~'~I~I ~ ,

ras-~ I / I I I I i

+ 5 + 6 + 7 + 8 + 9 + 1 0 +11

8~80 (5'~ SMOW) Fig. 5. Comparison of whole-rock 8180 values for the volcanic rocks of the Recent Arc on Martinique with those for other intra-oceanic island arcs, including the northern Lesser Anti- lles, and mid-ocean-ridge basalts (MORB). J = Morne Jacob, C = Pitons du Carbet, D = Diamant, N = Piton Mt. Conil, and P = Mt. Pel~e. Data sources: MORB [45], northern Lesser Antilles Arc [48], Marianas Arc [29,46], Volcano-Izu, Arc [46,47], and Banda Arc [49] (which has also suffered crustal contamination).

distinct ~180 ranges for the five centers, particularly the older Morne Jacob and Pitons du Carbet centers relative to the younger Diamant, Piton Mt. Conil and Mt. Pel6e centers; (3) the distinct displacement of the Martinique 8180 distribution away from the field of mid ocean ridge basalts; and (4) the fact that the lowest 8180 values for Martinique andesites are comparable with the highest 61~O values observed for the northern Lesser Antilles [48], Marianas [29,46], and Volcano-Izu [46,47] arcs. It is notable that the Pitons du Carbet dacite lavas are significantly more toO-rich than the other samples analyzed. Petrographically, these rocks also are distinct in that all contain significant proportions of quartz, which, of the common rock-forming igneous minerals, is the phase that most strongly con- centrates tSO. In fact, dacite sample M8269 con-

tains up to 10% quartz, which is present as large, rounded and embayed grains that may be of xenocrystic origin, introduced into a precursor andesitic magma during differentiation in the arc crust.

Overall, the Recent Arc volcanic rocks of Martinique exhibit a correlated covariation of 180/160 with both elemental and radiogenic isotope compositions, such that the chemically more evolved rocks typically are enriched in 180 and 878r, and depleted in 143Nd (Table 1, Figs. 6 and 7). The 6180-SiO2 field for the younger centers overlaps the northern Lesser Antilles array, as well as that for the Marianas where magmatic differentiation through crystal fractionation is considered to be the primary process controlling 6180 variation [28]. The displacement of the older Morne Jacob and Pitons du Carbet lavas to even higher 61SO values at equivalent S i O 2 c o n t e n t s is evidence for open-system magmatic evolution involving the addition of an 180-rich crustal component during intracrustal magma differentiation.

6. Discussion

The laO systematics of the Recent Arc volcanic rocks on Martinique are consistent with modification of mantle-derived liquids by interaction with highJSO crustal material. Two questions need to be addressed: (1) what is the nature and origin of the crustal contaminant, and (2) is it possible to use other isotope and geochemical data to estab- lish the details of the contamination process?

Implicit in this discussion is the requirement that, if subducted sediments are not the sole agent responsible for the compositional diversity dis- played by the Martinique volcanic rocks, then it is necessary to identify a component within the crust which is a suitable contaminant. Turbidite sediments from Barbados, the Eocene Tufton Hall Formation of Grenada, and metasedimentary xenoliths from Dominica and St. Vincent (Thirl- wall and Davidson, unpublished data) do not have radiogenic isotopic compositions as extreme as the most contaminated Martinique samples. However, a number of factors do encourage us to consider that a sedimentary component with appropriate isotope-trace element characteristics (those of ancient crust or, more likely, terrigenous/detri tal sediments derived by erosion from such crust)

may exist in the arc crust. Included in Table 1 is the Martinique sample with the most extreme 87Sr/S6Sr (0.71017), 143Nd/a44Nd (0.51196), 2°6pb//z°4pb (19.92) ratios and 8180 value ( + 14%o); a garnet-bearing dacite (M8323; analyzed in [2]) from the volcanics of the Inter- mediate Arc on Gros Ilet (Fig. lb). This dacite outcrops about 10 km from the volcanics of the Diamant center, and was probably erupted less than 5 Ma earlier [30]. Mineral compositions indicate that the garnets are cognate (phenocrysts) and equilibrated at 850-680°C and approximately 3 kbar [31]. The peraluminous mineralogy of the Gros Ilet dacite is difficult to reconcile with evolution from a liquid originally in equilibrium with sub-arc mantle peridotite, unless substantial contamination by peraluminous, intracrustal sediment had occurred. We note that other occur- rences of garnet in calc-alkaline volcanic rocks (e.g. [32]) are from arcs developed on continental margins and detached continental fragments (e.g. Japan, New Zealand, England, California), where crustal contamination is likely to have occurred. If the Gros Ilet dacite is solely a result of intra-crustal melting, then it may be representative of the contaminant composition. If there is juvenile (i.e. mantle-derived) material mixed with it, then the contaminant must be even more extreme in isotopic composition than the dacite.

Plate tectonic reconstructions [33] show that the area now occupied by the Caribbean plate was probably a site of prolific sedimentation during early stages of rifting to form the Atlantic Ocean. Abundant cratonic sediments from the Guyana shield and interior of North America would have been deposited in the developing basin. Such sediments are characterized by a "terrigenous" trace e lement- i so tope signature, rather than the "pelagic" nature of sediments currently being subducted. As the Caribbean Plate moved eastward into this region, from its original location in the Pacific, a subduction zone developed on the lead- ing (eastern) edge of the plate. The resultant volcanic arc (now extinct) is thought to be represented by the Aves Ridge, a largely submerged bathymetric high to the west of the currently active Lesser Antilles. Therefore, the Lesser Anti- lles, along which volcanism began later, has been built on crust where the accretionary prism to the Aves Ridge (if it existed) had developed (G.

263

Wadge, personal communication), but which has become separated from the older Aves arc by opening of the back arc Grenada basin. An anal- ogy, therefore, may be recognized between the Aves Ridge arc-Lesser Antilles basement, and the present Lesser Antilles arc-Barbados Ridge sedimentary accumulation, respectively. We therefore speculate that the basement to the arc volcanic rocks on Martinique includes relatively old detri- tal sediments, and the garnet-beating dacite of Gros Ilet may represent an anatectic melt of this material.

The broad positive correlations between stable and radiogenic isotope ratios and bulk composition (Table 1, Fig. 6), indicate that changes in isotopic composition occurred during differentiation, i.e. within the arc crust. The inset cartoons on this figure indicate the sense of trends which would be expected as the result of different petrogenetic processes. If all the variation in Sr and Nd isotopes is inherited at source (by subduction zone contamination; vector s z c in Fig. 6, insets) then correlations between isotopic composition and indices of differention (e.g. SiO2) would not be expected since differentiation is a later, independent process. The scatter in the data is probably largely due to further fractionation during storage in high-level magma chambers prior to eruption, giving rise to increases in SiO2, which are not coupled to isotopic variations. That is, although the Recent Arc data as a whole show evidence for AFC, the compositions of differentiates at individual volcanoes is controlled in large part by simple fractional crystallization.

Fig. 7 shows ~180-143Nd/144Nd relationships for the Martinique Recent Arc volcanic centers. Different possible mixing trajectories are shown. The isotopic composition of a potential intracrustal contaminant is represented by the Gros Ilet dacite. The radiogenic isotope composition of subducted sediments has been estimated from data for piston core and DSDP (Hole 543) core analyses [34]. However, because there is a finite time required for surface sediments to reach the zone of magma genesis at 100-200 km depth through subduction, surface sediments sampled today cannot be considered representative of sediments in the subduction zone. At present, subduction rates in the Lesser Antilles, the youngest sediments which could have been subducted to a position beneath

264

the volcanic front are Miocene in age. Therefore, we consider only the deeper samples from DSDP Hole 543 [17,34], and not the piston core samples, as possible representatives of subducted sediment compositions. A notable feature of the sediments is their rather restricted Nd-isotope compositions (]43Nd/]44Nd = 0.51206-0.51185) compared with the large range in Sr-isotope variation (87Sr/86Sr =0.7138-0.7257 [17,34]), and is the reason for examining Nd-O rather than Sr-O isotope relations. In terms of a simple model in which the mantle is contaminated by subducted sediment, in order for mixing lines to pass through the data, Nd concentrations in the sediment can be only

+9 - ' ' ' ' a) MARTINIQUE ~ ( (Recent Arc) / 2 older

+ 8 /A~ / / ~ volcanics

0 ~E O0

o-~ / ~ [3 \ younger +7 Northern Lesser ~/ C) \ ~ v o I c a n i c s

- Antilles o

_m 6O _ closed-System r

+ 6 - ~ n ~ l s l a n d s

I ~ x ~ @ (b) o.513o

Antdles ~ . . . . . . . . . . ~ \

~- 0.5126 ~ \ \

z _ ?~% ,, , . ==~ \ / fractionation \ ,_ N-- / \

~r ! Subducfion-zone \ Oloer y o.5122r - contominafion \ volcanics"" \

m melting ~" ~ Gros Ilet dacite \~,

F ~ZC "~ractionation Field of a l l / ~ . . . . . a~

o sI le r .... :r22,',o~ ~a.,.i0ue0o,.

L L t J- & 50 55 60

Si0z ( w t . % )

Fig. 6. Plots o f (a) 31So versus SiO 2 and (b) 143Nd/ ]44Nd versus SiO 2 for the volcanic rocks of the Recent Arc of Martinique. Shown for reference are the fields for MORB [45], the northern Lesser Antilles [48] and the Marianas [29]. The bold line in (a) indicates the 3180-SiO2 trend expected for differentiation of a basalt with 46% SiO 2 and 3180 = +6%0 through closed system fractional crystallization given a bulk solid melt fractionation factor (a) of 0.99975. In (b) the dashed line indicates the field of 143Nd/]44Nd-SiO2 variation for all volcanic rocks on Martinique [2]. The insets indicate the sense of 31S0-si02 and 143Nd/144Nd-SiO2 variation for different processes that might have affected the volcanic rocks; m designates a mantle source (or melt) and s z e denotes subduction zone contamination.

+16

+14

o

,12

"~ + I0

o % + 8

+ 6

' I ' ' i , , I i ,

I E c.~0, ,,,,,o, % MARTIN QU i ~ ,~ ;o ........ ,, , , ~

L M 8 3 2 3 (GrosIlet ('younger a r c ' ) I - ~ / \ ~ dacite) 8~80 ~ ~ ~ ." , ~o

IAT .... o l d e r J43Ndp44Nd

volcanics

25 % ~ y o u n g e r volcanics

5% ""-----.........~ Northern Lesser Antilles

MORB Source =,~.V& ~ y MORE~ Subducted ~ Source Sediment

J I , , I , , I , ,0.51129 , , I , 05120 0.5125 0.5126 0.5152

143Nd/144Nd

Fig. 7. 3180 versus 143Nd/]44Nd for the volcanic rocks of the Recent Arc on Martinique. Shown on the diagram for reference are (1) the field of MORB (or MORB-source mantle), (2) the data field for the northern Lesser Antilles, and (3) the Gros llet garnet-bearing dacite (M8323). The curve labelled 1 il- lustrates mixing of subducted sediment ( 3 ] 8 0 = +20%~, 143Nd/144Nd = 0.5119, taken from [34]) into a MORB-source mantle (i.e. "source contamination"), whereas curve 2 il- lustrates the mixing of intracrustal sediment, as represented by the Gros Ilet dacite (which we propose may be an anatectic melt of terrigenous sediment from the arc crust), with a relatively primitive island-arc tholeiite OAT), represented by basalts from the northern Lesser Antilles. The principles of these models are shown schematically on the inset, after [21] (Ndm and Nd c refer to Nd contents in the mant le /mant le derived magma, and crust respectively).

2-7 times that of the mantle. This implies that either the unmodified sub-arc mantle is very enriched in Nd (in conflict with the low absolute HFSE and H R E E abundances in primitive Lesser Antilles magmas [2,15,17]) or the sediments are much less Nd-rich than those analyzed by White et al. [34]. Nevertheless, if we permit uncon- strained source and sediment compositions, then bulk mixing curves can be made to pass close to the data to satisfy either mixing model (Fig. 7). For the O-Nd isotope variations to be the result solely of s o u r c e contamination, two rather un- likely circumstances are required. First, the amount of source contamination would have to be different for each of the Martinique volcanic centers, implying distinct source-to-surface magmatic plumbing systems. Secondly, unusually large amounts of subducted sediment (e.g. - 2 0 - 2 5 % for Piton du Carbet and 10-15% for Morne Jacob) are required to explain their high O-isotope ratios, even if the sediment has 6~80 as high as + 20%0, as reported for DSDP pelagic sediments [17]. The

0.712

0.710

~ 0.708 % ;, 0.706

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a~ EL ~- 15.7

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I I i I

265

I I I

(b) DSDP Hole 5 4 3 Sediments

Fie ld of Northern : ! : I Lesser Ant i l les e~d: ' of Volcanic rocks Mart inique

' ~ ~ Volcanic rocks" DSDP Hole 5 4 5 Basolts

(c)

2OTpb

2o4pb

Mixing of relotively homogeneous sedimenis with heterogeneous MORB SOClr ce j

] 5 . 5 ~-~///////////////////.-~/////////////////L~--, I I 18.0 18.5 19.0 19.5 20.0

Zo6pb/2o4 Pb 206 pb/2O4 p b

@Mixing of sediments INFERRED into mantle wedge to ARC CRUST source mafic mogmas, I followed by contamina- y t i o n in the arc

Fig. 8. (a) 87Sr/86Sr versus 2°6pb/2°4pb. Measured 8180 values are shown next to appropriate data points. Shaded field is for all volcanic rocks from Martinique [2]. Curves 1 and 2 represent source and crustal contamination models, using similar end-members to those shown in Fig. 7. Tick marks indicate % contaminant added, together with calculated 8180 of that mixture. (b) 2°7pb/2°4pb versus Z°6pb/2°4pb for the volcanic rocks of the Recent Arc on Martinique. Error bars fit within symbols. Shown for reference are the fields for MORB (or MORB-source mantle), the volcanic rocks of the northern Lesser Antilles [17], the complete arc volcanic sequence on Martinique [2], and the sediments and basalts of DSDP Hole 543 [17,34]. (c) Three possible interpretations of the Pb-isotope data, illustrated schematically. See text for discussion.

re la t ions shown in Fig. 7 indica te that the i so topic var ia t ions shown by bo th the o lder and younger volcanic rocks on Mar t in ique can be expla ined in terms of a two-stage process in which the i sotopic s ignature of p r ima ry melts der ived f rom a subduc- t ion-modi f ied mant le source is overpr in ted by subsequent m a g m a - c r u s t in terac t ion ( A F C ) during in t ra-crus ta l d i f ferent ia t ion.

Sediment Pb- and Sr- isotope compos i t ions f rom D S D P Hole 543 are shown together with da t a for the volcanic rocks in Fig. 8a. The Mar t in ique samples descr ibe a mixing hype rbo l a asympto t i c to 2°6pb//z°4pb > 19.9. This is a higher ra t io than

any recorded so far f rom sediments o u t b o a r d of the Lesser Ant i l les arc. The s o u r c e - s e d i m e n t mixing hype rbo la d rawn in Fig. 8a passes close to the

field for the volcanic rocks of the nor the rn Lesser Anti l les . The da t a are cons is ten t with a two-s tage mode l involving source con tamina t ion , p roduc ing i so topic compos i t i ons s imilar to the tholei i t ic rocks of the nor the rn Lesser Anti l les , fo l lowed by A F C within the arc crust. Mix ing of a pa ren ta l maf ic mel t ( represen ted in Fig. 8a by a basa l t ic andes i te f rom the nor the rn Lesser Ant i l les) wi th a terr ige- nous sed iment ( represen ted b y the Gros I let ga rne t -bea r ing daci te) genera tes a curve which fits the Mar t i n ique d a t a array.

Pb- i so tope re la t ions for Mar t i n ique (Fig. 8b) cont ras t wi th Pb- i so tope da t a for m a n y o ther in- t ra -oceanic arcs, in which the volcanics form a b r o a d t rend f rom M O R B towards a b r o a d l y homogeneous local sed iment compos i t i on (e.g. Aleu-

266

tians [4]; South Sandwich Islands [6]). Although data from the northern islands lie between MORB and DSDP sediment compositions, the Martinique Pb-isotope data form a field which is sub-parallel to that of the sediments, with the DSDP sediments characterized by higher 2°7pb/Z°4pb ratios at a given 2°6pb/Z°4pb ratio than the volcanic rocks. Fig. 8c outlines schematically three alternative models for Martinique magma genesis, constrained by the Pb-isotope compositions of the potential reservoirs and the fact that mixing lines are straight on Pb-Pb isotope plots. In Model 1, a relatively homogeneous sediment is mixed with a heterogeneous mantle, and the converging mixing lines pass through the data points for the derived partial melts. This model cannot explain the more radiogenic isotopic ratios of the older lavas from Martinique [2]. Furthermore, there is no reason to suppose that the pre-subduction mantle beneath the Lesser Antilles was particularly heterogeneous. We note that in island arcs where Pb-isotope data are used to argue for a MORB source + subducted sediment mixture, a reasonably homogeneous mantle end-member is implied (e.g. the South Sandwich islands [6]). Model 2 shows the effect of mixing a homogeneous MORB-source mantle with the array of measured DSDP sediment Pb-isotope compositions. Mixing lines in this case do not pass through the entire data field for the Recent Arc volcanics of Martinique, let alone the less radiogenic lavas of the northern Lesser Antilles or the more radiogenic older rocks of Martinique. In fact, no sediments have been analyzed that contain sufficiently radiogenic Pb to explain the most radiogenic Martinique volcanics in terms of a subducted sediment-mantle mixing model. Model 3, the preferred explanation, envisages a simple source contamination process which adds sediments (represented by DSDP Hole 543 sediment data) to a MORB-source mantle (represented by Hole 543 basalts), to produce the Pb-isotope compositions of the northern Lesser Antilles arc and, perhaps, the least radiogenic of the Martinique volcanics. Subsequent crustal contamination gen- erates the array of Pb-isotope variation observed on Martinique. The only unsatisfactory aspect of this model is that the Pb-isotope composition of the crustal contaminant is merely inferred and does not correspond to any analyzed samples. Again, this problem can be alleviated if we accept

that the most isotopically evolved end of the Martinique array (the garnet-bearing dacite of Gros Ilet) is largely a peraluminous crustal melt, and therefore effectively represents the isotopic composition of the arc basement.

Many of the trace element characteristics of the Martinique volcanics are also inconsistent with one-stage mantle-sediment mixing models. The overall inadequacy of bulk man t l e+ sediment mixing to explain the relative trace element distribution of lavas from Martinique has been previ- ously demonstrated [17]. For instance, it is impos- sible to generate the range in B a / L a ratios in Martinique volcanic rocks (12-30) by mixing local sediments ( B a / L a = 4 - 1 2 ) with MORB-source mantle (Ba /La = 4). Furthermore, mixing of relatively high B a / L a sediment with low B a / L a mantle should give rise to correlations in which B a / L a increases with 878r/86Sr and decreases with a43Nd/144Nd. This is not observed for the Martinique volcanics. In fact a scattered negative correlation exists between B a / L a and 8VSr/S6Sr [17]. It has been noted that some lavas from the Lesser Antilles have a negative Ce anomaly [18,35] which might be inherited from subducted pelagic sediments (e.g. [5]). However, the DSDP sediments on average are characterized by a positive anomaly [34].

To summarize, although mixing between a MORB-source, sub-arc mantle and subducted sediment can account for some of the compositional features of the Martinique volcanics, too many important geochemical relationships remain unexplained by such a simple, single-stage model. This is not to say that subduction zone contamination as proposed by White and Dupr6 [15] does not occur, but rather that the effects of source enrichment may be variably overprinted and ex- tensively modified during differentiation, even in intra-oceanic arcs.

The large variations in the isotopic compositions of Recent Arc volcanic rocks on Martinique over small separations in time and space can be appreciated by reference to Fig. 9, in which O-, Sr, Nd-, and Pb-isotope compositions are projected onto a N W - S E section through the island. Studies of mantle samples (xenoliths and peridotite mas- sifs, e.g. [36,37]) show that fairly large differences in isotopic composition may exist over small distances, of the order of a few kilometers. However,

+ 10~- (a) 0-isotopes Age (Ma)

÷9~- I~lml I ~ 0 Pelee 0-05

-- <~ ~ G Diamant ~1 t ~ 1 1 Carbet 1-5 f f 19.6[- II •

~Adecob 2-7 ~ [ ~ 19.4I o (b)Pb-isotopes N 19.2 ~-C] ¢~,

f 0 (c)Nd-isotopes 0.5129 [] ~

~. 0.5125~- I~ ,

i • (d) Sr- isotopes 0.705

= 0.703[ @

! :i

Fig. 9. Isotopic compositions of (a) O, (b) Pb, (c) Nd and (d) Sr for the volcanic rocks of the Recent Arc on Martinique, projected onto a N W - S E section of the island, showing the steep isotopic gradients both in space and time. Asterisks mark sample localities (Mt. Pel~e samples were recovered from a continuous section at one locality).

when the mantle is melted these heterogeneities are homogenized to a large extent, so that mantle-derived volcanic rocks show less isotopic variation over comparable distances [36]. This seems reasonable in view of the elevated temperatures and associated rapid chemical diffusion associated with the actual process of melting. The high temperatures and associated large degrees of partial melting inferred in the subarc mantle wedge [38-40] will generate more homogeneous isotopic compositions. Other arc segments, such as the Aleutians, Marianas, South Sandwich Islands and even the northern Lesser Antilles, are remarkably homogeneous in isotopic composition over large distances along the arc. Consequently, it is difficult to conceive of such large isotopic gradients in space and time as observed in Martinique (Fig. 9) being preserved in the convecting mantle wedge.

267

Given that the volcanic pile that comprises the crust of the Lesser Antilles arc is most voluminous in the vicinity of Martinique and that the proportion of evolved volcanic rocks is greater than to the north or the south in the Lesser Antilles, it is more plausible that mantle-derived magmas have been contaminated during ascent. The strong isotopic gradients between volcanic centers are then generated within individual sub-volcanic plumbing systems through variable degrees of contamination. This interpretation is consistent with the observation that the volcanics from the older centers of the Recent Arc (Morne Jacob and Pi- tons du Carbet) are more contaminated than those of the younger centers (Diamant, Piton Mt. Conil, or Mt. Pel6e; Table 1). It is the magmas at these older centers which would have had the greatest opportunity to interact with the arc crust as magmatic conduits were being established and magmas moved slowly upwards toward high-level magma chambers. Magmas erupted from the younger centers, such as Mr. Pel6e, may have utilized the same deep plumbing systems that through time had become armored by cumulates and crystallized melts, thus preventing extensive interaction with the arc crust (cf. [41]).

7. A petrogenetic model

We propose a two-stage model for the Recent Arc in Martinique (and oceanic arcs in general) involving initial modification of the sub-arc mantle by components from the subducted slab and subsequent crustal contamination of the resultant basaltic melts (Fig. 10). Hydration of the sub-arc mantle wedge by a slab-derived fluid component provides both a trigger to partial melting (by lowering the solidus of the mantle wedge peridotite) and the chemical modification of the mantle wedge necessary to produce the characteristic island arc chemical signature of high L I L E / H F S E ratios and isotopic compositions that are displaced slightly from those of MORB. The contribution from subducted sediment, although important in terms of some geochemical characteristics such as high 2°7pb/2°4pb (and a ]°Be signature in some arcs), is restricted to probably < 3%. Primitive basaltic partial melts derived from the mantle wedge are considered to have chemical characteristics similar to the island-arc tholeiites of the

268

'% 143Nd 144Nd

ISource contamination I

~le 0

BalLa I

87Sr/86Sr

N o r t h e r n

Lesser An t i l l es

Contamination by \ terrlgenous =ediment \

500

~Lg.~la crustal L ~ ~/t_o c oo,om,°a,,on,///

~\ \ \ s,8o / / /

eTSr/e6Sr

MORB ~ IAV (eg. Northern Lesser Antilles )

- ~ Source enrichment (fluid from altered MORB and subducted sediment) ~ M8325

~:> Assimilation- fractiona[ crystallization

~ ] Mart inique Lovas

Fig. 10. A schematic model of magma genesis in the Lesser Antilles island arc, comparing the northern Lesser Antilles (source contamination) with Martinique in the central Lesser Antilles, where similar island-arc basaltic magmas are modified by intra-crustal contamination during transit through the thicker arc crust.

northern Lesser Antilles [42]. These parental mag, mas then rise into the arc crust, where they differentiate through fractional crystallization which on Martinique is accompanied by variable degrees of assimilation of hydrothermally-altered arc crust and terrigenous sediment intercalated in the crust. Crustal contamination leads to displacement of O-, Sr-, Nd- and Pb-isotope compositions away from those of island-arc tholeiites towards those of the crustal contaminant (Fig. 8) and produces calc-alkaline andesitic magmas. Later, high-level differentiation may be dominated by fractional

crystallization which disturbs the AFC relationships inherited from the earlier stage of deep- crustal evolution. Our main objections to an "extreme subduction contaminat ion" model [3,15] are based on the observations that: (1) correlations exist between isotopes of O, Sr, Nd, Pb and indices of differentiation, arguing that contamination occurs during differentiation (i.e. in the crust) rather than at source; (2) measured sediment compositions (from DSDP Hole 543) are inconsistent with those inferred for the crustal contaminant, as recently argued by Dupr6 and All6gre [51] from a multi-dimensional analysis of Pb-, Sr- and Nd-isotope systematics, and (3) high 180 enrichments indicate up to 25% contamination; if this occurred at source the incompatible trace element and Sr-Nd-Pb-isotopic compositions of the melts would be " swamped" by the sediment signature (i.e. they would look like sediments rather than volcanic arc magmas). Our model argues that the arc signature is the result of subduction zone processes, common to all arcs, and that the unusually large isotopic variations (e.g. 6180 values up to + 9%~ and 143Nd/a44Nd ratios as low as 0.5123) and trace element characteristics of many Martinique samples (e.g. lower K / R b and B a / L a than in many island arcs) are a function of an intra-crustal contamination process which over- prints the source signature. The basement to the Martinique volcanic arc is not exposed and, therefore, we can only infer the characteristics of this enigmatic contaminant . However, rare peraluminous volcanic rocks may be approximately anatectic melts and, therefore, provide a good (albeit indirect) representation of the basement composition.

Acknowledgements

The first author is very grateful to the late D. Westercamp of B R G M for his generous advice and assistance during fieldwork on Martinique. We thank the late J. Borthwick and B. Pratson for assistance with 180/160, D / H , and H2 O+ de- terminations in the Stable Isotope Laboratory at S.M.U. Reviews by B.M. Wilson, M. Thirlwall and K. Ferguson are greatly appreciated. We are grateful to W.M. White and an anonymous re- viewer for insightful comments and suggestions on behalf of the journal, which have greatly improved

the manuscript. The help of Susan Fast and Jean Sells with figure preparation, Hariett Arnoff and Paula Kunde with the tables, and C. Leblanc with manuscript preparation is also appreciated.

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Documents

Oxygen isotope constraints on the petrogenesis of volcanic arc magmas from Martinique, Lesser Antilles