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Geochimica et Cosmochimica Acta, Vol. 61, No. 1, pp. 161-169, 1997 Copyright © 1997 Elsevier Science Ltd Printed in the USA. All rights reserved

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PII S0016-7037(96) 00314-6

Oxygen isotopic composition of hydrous and anhydrous mantle peridotites

G1LLES CHAZOT,* DAVID LOWRY, MARTIN MENZIES, and DAVID MATTEY Department of Geology, Royal Holloway, University of London, Egham, Surrey TW20 0EX, UK

(Received September 28, 1995; accepted in revised form September 6, 1996)

Abst rac t - -Oxygen isotope ratios, determined using the laser fluorination technique, are reported for minerals from anhydrous and hydrous (i.e., amphibole-bearing) spinel lherzolites from Yemen, as well as from hydrous spinel lherzolites and amphibole megacrysts from Nunivak Island, Alaska. Oxygen isotopic compositions of olivine vary from 5.1-5.4%0 and of pyroxene from 5.5-6.0%e and no systematic difference exists between minerals in hydrous and anhydrous lherzolites. The oxygen isotopic composi- tion of the amphibole in the peridotites and of the amphibole megacrysts is also very homogeneous and varies from 6180 = 5.3-5.6%0. These results indicate that the metasomatic minerals in the lherzolites are in oxygen isotopic equilibrium with the peridotitic minerals. The only isotopic disequilibria are observed in minerals which have grown in melt-pockets formed by partial melting of amphibole. The homogeneity of the oxygen isotopic ratios of mantle minerals in this study indicate that the fluids circulating in the mantle and precipitating amphibole or mica had the same oxygen isotopic compositions as the mantle protolith or that the fluids had been buffered by the isotopic composition of the olivine, the most abundant mineral, during percolation through the peridotites. Copyright © 1997 Elsevier Science Ltd

1. INTRODUCTION

The high incompatible trace element contents of many basal- tic rocks and mantle xenoliths have provided evidence for metasomatic enrichment of the upper mantle. These enrich- ment processes are thought to involve circulation of silicate and nonsilicate melts in the mantle and their interaction with the peridotite protolith with concomitant modification of the composition of the primary minerals or crystallization of new metasomatic minerals. The chemical composition of hydrous and anhydrous mantle xenoliths are used to study the nature and composition of the fluids circulating in the mantle that are responsible for metasomatism. Stable iso- topes, and particularly oxygen, are important geochemical tracers of fluid-peridotite interaction in the mantle. The be- haviour and fractionation of oxygen isotopes at high temper- atures have been extensively studied from experimental works (e.g., Chiba et al., 1989; Clayton et al., 1989; Stolper and Epstein, 1991; Matthews et al., 1994; Rosenbaum et al., 1994) or from numerical calculations (e.g., Bottinga and Javoy, 1973, 1975; Richter and Hoernes, 1988; Clayton and Kieffer, 1991; Zheng, 1991, 1993a,b). Recently, laser fluo- rination techniques have been developed to analyse the oxy- gen isotopic composition of very small amounts of mineral grains or directly in situ in thin sections (Sharp, 1990; Elsen- heimer and Valley, 1992; Sharp, 1992; Mattey and Macpher- son, 1993). The combination of knowledge of mineral isoto- pic fractionation with the newly improved analytical tech- niques now allows greater constraints to be placed on the physicochemical processes operating in the upper mantle.

In this paper, we report the oxygen isotopic composition of coexisting mantle minerals in hydrous and anhydrous spinel

* Present address: D6partement de G6ologie, Universit6 Blaise Pascal, 5 rue Kessler, 63038 Clermont Ferrand cedex, France (g. chazot @ opgc.univ-bpclermont.fr).

lherzolite xenoliths from Ataq and Bir-Ali in Yemen and from Nunivak Island, Alaska. We will show that there is no contrast in oxygen isotopic composition of the metasomatic melts that clearly affected these parts of the upper mantle. A comparison of anhydrous and hydrous peridotites from these localities shows that their oxygen isotope compositions are uniform and that metasomatism had no effect on the oxygen isotopic composition of these mantle rocks.

2. SAMPLE DETAILS AND ANALYTICAL TECHNIQUES

2.1. Nunivak Samples

Nunivak is a small island located off the west coast of Alaska, in the Bering Sea. Mantle xenoliths are present in recent alkaline volca- nic rocks and are associated with many clinopyroxene and amphibole megacrysts. Metasomatism in some xenoliths has induced precipita- tion of amphibole, phlogopite, jadeitic clinopyroxene, and apatite. In a later event, the amphiboles experienced partial melting and were surrounded by a silicate glass containing small euhedral clinopyrox- ene, olivine, and spinel grains. The geochemistry of the Nunivak megacrysts and mantle xenoliths has been studied elsewhere (Fran- cis, 1976, Menzies and Murthy, 1980; Roden et al., 1984; Ben Oth- man et al., 1990). Herein we report oxygen isotope data for the samples from the Nd-Sr study of Menzies and Murthy (1980). 10051 is an amphibole-bearing spinel lherzolite with phlogopite veinlets and interstitial phlogopite (Roden et al., 1984). UM1 is an amphi- bole-bearing spinel lherzolite. 13002 is a coarse-grained pyroxenite composed predominantly of clinopyroxene and amphibole. 13003 and 13008 are large amphibole megacrysts.

161

2.2. Yemen Samples

Oceanic rifting in the Gulf of Aden began some 20 Ma ago and is associated with widespread volcanism represented by large strato- volcanoes (the Aden Volcanic Line, Cox et al., 1970) along the coast and by alkaline basaltic volcanic fields on the coast and inland. The mantle xenoliths were sampled from two Plio-Quaternary volca- nic fields on the south coast of Yemen: Bir All and Ataq. At Bir All, the xenoliths are anhydrous spinel lherzolites and some samples show evidence of limited melting of clinopyroxene and/or spinel replaced by clinopyroxene, spinel, and glass. In sample BA8, many

162 G. Chazot et al.

spinels are surrounded by plagioclase, indicating upwelling of the upper mantle during rifting along the Gulf of Aden. At Ataq, the xenoliths are amphibole-bearing spinel lherzolites or anhydous spi- nel lherzolites and sample JK5 contains occasional melt-pockets, with residual spinel and clinopyroxene surrounded by glass and new olivine, clinopyroxene, and spinel. In this sample, as in the amphibole-bearing lherzolites, clinopyroxene is enriched in the LREE and other incompatible elements (Chazot et al., 1996). The hydrous amphibole-beating lherzolites have recorded two different events during their evolution. The first one was a metasomatic event when a fluid reacted with enstatite to form new large grains of clinopyroxene, amphibole and, in some samples, apatite. All these metasomatic minerals are disseminated in the peridotite and the amphibole is always associated with large spinel grains. After this metasomatic event, amphibole underwent partial melting evident as large melt-pockets (up to 1 cm in size) containing small newly formed euhedral olivine, clinopyroxene, and spinel grains which have crystallized from a silicate melt now quenched to a glass be- tween these minerals. Mass-balance calculations, using the major element compositions of the different phases in the melt-pockets, showed that this partial melting occurred in a closed-system, or, was triggered by percolation of a metasomatic melt at very low fluid/ rock ratios (Chazot et al., 1996; Chazot et al., 1997), without inter- action with the host basalt. Lherzolite JK2 is crosscut by a vein of clinopyroxenite which also contains melt-pockets with amphibole.

2.3. Analytical Techniques

The xenoliths were coarsely crushed and olivine, orthopyroxene, clinopyroxene, spinel, amphibole, and mica were carefully hand- picked under the microscope from the 250-500 #m fraction. For JK2, the vein was separated from the wallrock pcridotite and will be referred as JK2 vein and considered as an individual sample. For samples JK2 and JK8, the melt-pockets were crushed separately. From the 125-250 #m fraction of these melt-pockets, new olivine and clinopyroxene were extracted, as well as the spinel grains associ- ated with the amphibole grains. Newly-formed spinels are often included in the olivine in these melt-pockets and are too small to be separated. The minerals were cleaned in acetone and deionised water before analysis and only grains devoid of small inclusions or alteration were selected for oxygen isotope measurements.

The laser-fluorination technique used to extract oxygen from the minerals is described elsewhere (Mattey and Macpherson, 1993). All the analyses were made on more than one grain in order to average single grain oxygen isotopic heterogeneity. The weight of the samples was between 1.1 and 1.5 mg except for spinel (which contains less oxygen) for which larger samples of 1.4-1.8 mg were required to obtain sufficient gas for analysis. The grains were laser- heated under 0.4 atmospheres of C1F3 for the clinopyroxenes and 0.2 atmospheres of C1F3 for the other phases until reaction was complete. Determination of the precise weight of the samples before analysis and gas pressure measurement after conversion into CO2 allowed the calculation of the oxygen yield for each sample. This is important as Mattey and Macpherson (1993) show isotopic frac- tionation associated with oxygen yield variations by laser-heating. Ionov et al. (1994) also reported correlation between low O2 yield and high 6 ]80 in laser heating fluorination of olivine. Oxygen yields greater than 98% were obtained for all olivine and other mineral analyses reported in this paper. The problem of isotopic fractionation associated with low yield is even more important for the spinel analyses, apparently due to coating of the sample with an impenetra- ble Fe-rich fluoride residue before the end of reaction.

After conversion into CO2 gas, isotope ratios for liberated 02 were measured on a VG Prism mass spectrometer. For all the spinels but one, only single analyses were performed. The other phases were analyzed between two and four times and the external sample reprod- uctibility was normally better than _+0.15%o. For each sample, the reported values (Table I ) represent the average of these analyses. Each tray of sixteen samples included two or three standards and of these, two were analyzed at the beginning and one at the end of each analytical session.

The primary standard was NBS-30 and data for unknown were normalized to NBS-30 = 5.10%o, but to monitor within-run and day

to day drift in this study we used an inhouse standard, a San Carlos olivine (Mattey and Macpherson, 1993; Mattey et al., 1994a) with a mean 6180 value of 4.86%o __ 0.18 (2s, n = 245). The magnitude of the daily corrections ranged from 0-0.3%0. For the olivine and clinopyroxene in the melt-pockets of sample JK2, the amount of mineral sample available was less than 0.45 mg. In order to avoid isotope fractionation effects associated with small samples (Mattey and Macpherson, 1993 ) which require a correction calculation, these were mixed with roughly the same amount of the San Carlos olivine standard and the oxygen isotopic compositions were calculated by mass balance from the measured value of the mixed sample.

As stated before, it is very difficult to obtain high yields of oxygen when analysing spinel with the laser. For the Yemen samples, we obtained yields between 44 and 100%, which correlated with the measured 6J80 (Table 1 ). Because of this analytical problem, we will consider in the paper only the spinel for which we obtained more than 97% oxygen during analysis.

All the results are reported in the Table 1 as permil (%0) deviations relative to Standard Mean Ocean Water (SMOW) using NBS-30 and San Carlos olivine at 5.1 and 4.9, respectively.

3. RESULTS

3.1. Peridotites

The results of the oxygen isotopic analyses of peridotite minerals are given in Table 1 and shown in Figs. 1 and 2. Ranges of 6 ~so values measured on the Yemen and Nunivak

peridotites (except in the melt-pockets) are very restricted. The 6 t so values range for olivines f rom 5.1-5.4%o (n = 13),

for c l inopyroxene from 5.5-5.8%o (n = 13 ) and for orthopy- roxene f rom 5.7-6.0%o (n = 13) (Fig. 1). In comparison, publ ished values f rom mant le peridotites range f rom 4 . 6 - 7.2%o for the olivine, f rom 4.8-6.7%o for the c l inopyroxene and from 5.3-6.5%0 for the or thopyroxene (Kyser et al., 1981; Harmon et al., 1986/87; Kempton et al., 1988). Frac-

t ionation between pyroxene and ol ivine are always positive, be tween 0.3 and 0.5%o for c l inopyroxene-ol ivine and be- tween 0.3 and 0.8%o for orthopyroxene-olivine. The fraction- ation between or thopyroxene and c l inopyroxene is also posi- tive for all the samples with the except ion of two.

An important point to stress f rom these data is the fact that there are no systematic differences of 6180 values between

olivine, cl inopyroxene, and or thopyroxene from hydrous and anhydrous lherzolites. The range of 6180 values measured

for these minerals is the same in the two kinds of lherzolites implying that the percolat ion of a metasomat ic mel t in the hydrous lherzolites did not change the oxygen isotopic com- posit ion of the peridotitic minerals.

The oxygen isotopic composi t ion of amphibole is very restricted and ranges f rom 5.3-5.5%0 (Fig. 2 ) . The mica in sample 10051 has a 6180 of 5.5%0, similar to the amphibole values in the other hydrous samples. This range of 6~80

values is very small when compared with data on mant le amphiboles f rom different localities (Boet tcher and O'Neil , 1980) , which range from 4.7-6.0%o. Whi le fractionation between hydrous minerals and olivine is small and posit ive for all the samples but one, fractionation between hydrous minerals and the two pyroxenes is large and negative. The fractionation between mica and or thopyroxene is lower than between amphibole and or thopyroxene in all other samples f rom Yemen or Nunivak.

Isotope composition of O in the mantle 163

Table 1. Oxygen isotopic composition of minerals from Yemen and Nunivak spinel lherzolites and standard deviation (2a). (n) are numbers of replicates. Only one replicate was analyzed for the spinels. 6'80 in brackets for spinel are analyses with low oxygen yield which are not used in the discussion.

Samples 618Ool (n) 618Oopx (n) 6tSOcpx (n) 618Oamp (n) 618Omica (n) 6'8Osp Yieldsp (%)

Anhydrous lherzolites

Bir Ali BA5 5.20 + 0.36 (2) 5.85 _+ 0.08 (2) 5.70 _+ 0.23 (2) (4.16) 86 BA7 5.12 _+ 0.01 (2) 5.90 _+ 0.04 (2) 5.60 _+ 0.05 (2) 4.75 100 BA8 5.22 _+ 0.05 (2) 5.84 _+ 0.23 (2) 5.53 + 0.04 (2) 4.54 97

Ataq JKI 5.20 _+ 0.15 (2) 5.68 _+ 0.13 (2) 5.71 _+ 0.01 (2) (4.02) 89 JK4 5.34 _+ 0.03 (2) 5.93 + 0.07 (2) 5.74 _+ 0.02 (2) 4.00 97 JK5 5.28 _+ 0.18 (2) 5.93 + 0.01 (1) 5.77 _+ 0.37 (2)

Hydrous lherzolites

Ataq JK2 5.21 (1) 5.72 _+ 0.37 (3) 5.69 + 0.41 (5) 5.35 _+ 0.02 (2) JK2vein 5.34 _+ 0.14 (2) 5.82 -4- 0.39 (3) 5.63 _+ 0.07 (2) 5.45 + 0.28 (2) JK3 5.21 _ 0.07 (2) 5.90 _+ 0.05 (2) 5.72 + 0.33 (3) 5.53 _+ 0.06 (2) JK7 5.37 _+ 0.04 (2) 5.83 _+ 0.13 (2) 5,77 + 0.17 (2) 5.35 _+ 0.35 (2) JK8 5.28 _+ 0.13 (2) 5.98 _+ 0.16 (2) 5.53 _+ 0.05 (2) 5.37 + 0.14 (2) JK2melt 5.98 (1) 7.01 (l) JK8melt 5.21 _+ 0.03 (2) 5.65 + 0.21 (2)

Nunivak UM1 5.29 _+ 0.04 (2) 5.92 _+ 0.13 (2) 5.62 _+ 0.36 (2) 5.34 + 0.02 (2) 10051 5.35 _+ 0.02 (2) 5.66 _+ 0.52 (2) 5.78 _+ 0.29 (2)

13002 5.57 _+ 0.07 (2) 13003 5.37 _+ 0.05 (2) 13008 5.47 -+-- 0.22 (2)

5.54 _ 0.02 (2)

(1.22) 69

3.99 99 (3.77) 83 (2.25) 44

3.82 100 (3.14) 61 (3.05) 55

3.2. Megacrysts and Pyroxenite, Nunivak

The amphibole 6 ~so values from two megacrysts and one pyroxenite range from 5.4%0 (amphibole megacryst) to 5.6%0 (amphibole in pyroxenite) (Fig. 2). These results are very similar to mica and amphibole values from Nunivak and Yemen hydrous lherzolites and are consistent with a common origin for these hydrous minerals, as proposed by Ben Othman et al. (1990) on the basis of lead isotopic data.

3.3. Melt-Pockets

The isotopic results for olivine and clinopyroxene in the melt-pockets are complex. In JK8 the 6180 values for the newly formed olivine (5.2%o) and clinopyroxene (5.7%o) are very similar to the values for the minerals in the lherzolite. In contrast, the values measured on these minerals in the melt- pockets from sample JK2 are much heavier, with 6 '80 = 6.0%0 for the olivine and 7.0%o for the clinopyroxene.

4. THERMOMETRY

The temperature-dependent isotope fractionation between several pairs of cogenetic minerals can be used to test i f the different minerals in a single rock are in equilibrium and to calculate the temperature of equilibration. For comparison, we determined the equilibration temperature for some of the lherzolites using the two-pyroxene thermometer of Wells (1977). The results range from 920-1080°C for the anhy- drous lherzolites and from 920-950°C for the hydrous sam- ples (Table 2).

We have also calculated temperatures using the 6180 val- ues obtained on the minerals and the fractionation factors made available by Chiba et al. (1989) and Zheng (1993a) for olivine, clinopyroxene, and magnetite-spinel (Table 2). All the calculated temperatures are shown on Fig. 3.

The range of temperatures calculated with the ol-cpx ther- mometer is quite large, from 670-1620°C. However, most of the temperatures occur between l l 0 0 and 1300°C and the values are consistent with experimentally determined fractionation factors (Chiba et al., 1989) and theoretically calculated factors (Zheng, 1993a). Minerals in the melt- pocket in sample JK2 give low temperatures, between 670 and 770°C. In contrast, the temperatures calculated for sam- ples BA8, JK2 vein, and JK8 are very high, ranging from 1470-1620°C. Using published fractionation factors for magnetite (Chiba et al., 1989), the calculated temperatures for JK4, JK3, and UM1 are within the 1100-1300°C range determined with the ol-cpx thermometer whereas BA7 and BA8 give temperatures far in excess, ranging from 1620- 2390°C. However, the use of the spinel fractionation factors of Zheng (1991) gives temperatures ranging from 9 6 0 - 1300°C, in good agreement with those calculated using the ol-cpx oxygen thermometer and the two-pyroxenes ther- mometer.

In all the samples where comparison was possible, the temperatures calculated with the oxygen thermometers are higher than, or equal to, the temperatures calculated with the two-pyroxene thermometer of Wells (1977) (Fig. 3). However, the large range of temperatures calculated with the oxygen thermometer is to be expected, given the uncer-

164 G. Chazot et al.

O ©

6.0

5.8

5.6

5.4

5.2

5.0

5.0

6.0

5.8

5.6

5.4

5.2

5.0

5.0

ZG /+O /

, , , , . . . . 0 . . . . ~ . . . . ,, . . . .

5.2 5.4 5.6 5.8 6.0

8180 ol (%°)

/

, , , , . . . . , . . . . , . . . . , . . . .

5.2 5.4 5.6 5.8 6.0

~180 ol (%0)

and clinopyroxene in the melt-pockets (JK2 and JK8) are very different. Temperatures calculated for JK8 melt are similar to the average temperatures calculated for other sam- ples, but temperatures calculated for JK2 melt are lower and may indicate re-equilibration at lower temperature in the melt-pockets (Table 2).

5. DISCUSSION

5.1. Interpretation of the Data

The oxygen isotopic composition of the upper mantle has been analyzed using the constituent minerals of peridotites from several localities around the world and related to the chemical processes involved during fluid-rock interactions. Investigating oxygen isotopic fractionation between the main minerals in the mantle (i.e., olivine, orthopyroxene, and cli- nopyroxene), Kyser et al. (1981, 1982) proposed the exis- tence of crossovers in mineral isotopic fractionation at high temperatures. These crossovers have little theoretical support and were questioned by Gregory and Taylor (1986a,b) who used ~-6 plots to show that disequilibrium arrays between olivine and clinopyroxene can be explained in terms of open- system metasomatism. They proposed an open-system model with oxygen exchange between peridotite and oxygen-bear- ing fluids of different isotopic composition. In this model, the olivine and spinel exchanged their oxygen with the perco- lating fluid at a faster rate than the pyroxenes and as a result had larger variations in oxygen isotopic composition. They also showed that ~80 heterogeneity was least in peridotites from oceanic lithosphere and greatest in continental litho- sphere (e.g., kimberlite-borne garnet peridotites associated

L,, + Yemen anhydrous II

I • Yemen hydrous o Nunivak hydrous

Fig. 1. Oxygen isotopic composition for olivine, orthopyroxene, and clinopyroxene from Yemen and Nunivak hydrous and anhydrous lherzolites. A line corresponding to A ~80 = 0 is shown for reference. For the cpx-ol diagram, two lines corresponding to fractionation at 1000 and 1200°C have been calculated using the fractionation factors of Chiba et al. (1989). The cross in the cpx-ol diagram represents the analytical uncertainty on the replicate analyses (_+0.15%o).

tainties in these thermometers at these high temperatures. Given the analytical error on the 6180 of the minerals and on the determination of the fractionation factors, the uncer- tainty on the calculated temperatures is certainly higher than _200°C at 1200°C. The range of values reported here is smaller than this and clearly suggests that there is oxygen isotopic equilibrium between the mineral phases in the Yemen and Nunivak lherzolites. The very high temperatures calculated using spinel in samples BA7 and BA8 with the fractionation factors of Chiba et al. (1989) may reflect differ- ences between the chemical composition of the spinel in the lherzolites and the magnetite used for determination of these fractionation factors.

Temperatures calculated from the newly formed olivine

6.2

6.0

5.8

5.6

5.4

~ 5.2

5.0

4.8

4.6

j ~ / ~ ~ 6.2

~,~ 6.0

5.8

5.6

o~/~ / ~ X ~ 5.4

5.0

4.8

4.6 4.6 4.8 5.0 5.2 5.4 5.6 5.8 6.0 6.2

glSO ol (%~)

t_ + Yemen anhydrous 1 • Yemen hydrous

I o Nunivak hydrous

Fig. 2. Oxygen isotopic composition for olivine and hydrous miner- als (amphibole and mica) from Yemen and Nunivak. The isotopic composition of the megacrysts from Nunivak is indicated by hori- zontal arrows, and the range of composition of mantle amphiboles reported by Boetcher and O'Neil (1980) is shown in a vertical box on the right of the diagram. The cross represents the analytical uncertainty on the replicate analyses (_+0.15%e).

Isotope composition of O in the mantle 165

Table 2. Calculated temperatures (°C) for mineral pairs in hydrous and anhydrous spinel lherzolites. Temperatures calculated with oxygen fractionation factors published by Chiba et al. (1989) and Zheng (1991, 1993a) can be compared with temperatures calculated with the two- pyroxenes thermometer of Wells (1977). v is for minerals in a vein and m is for minerals in melt-pockets.

Anhydrous lherzolites

BA5 BA7 BA8 JK1 JK4 JK5

Zheng (1991, 1993a) ol-cpx 1199 sp-ol sp-cpx

Chiba et al. (1989) ol-cpx 1093 sp-ol sp-cpx

Wells (1977) opx-cpx 1069

1211 1472 1180 1308 1317 1205 1020 1293 1254 1072

1107 1469 1071 1228 2388 1690 1125 1767 1618 1153

1200

1094

926 1076 917 940

Hydrous lherzolites

JK2 JK2v JK3 JK7 JK8 UM 1 10051 JK2m JK8m

Zheng (1991, 1993a) ol-cpx 1214 sp-ol sp-cpx

Chiba et al. (1989) ol-cpx 1111 sp-ol sp-cpx

Wells (1977) opx-cpx 945

1510 1181 1318 1559 1426 1277 771 1258 1048 987 1075 1059

1533 1072 1242 1622 1396 1188 672 1164 1192 1062 1157 1129

940 949 921

with eclogites). At the same time, Gregory and Criss (1986) addressed tectonic setting and the conditions necessary for generating high- 180 mantle and concluded that high tso peri- dotites are rather localized and rare.

These observations were used by Kempton et al. (1988) to explain the oxygen isotopic compositional variations in hydrous and anhydrous spinel lherzolites from the Eifel, Ger- many. Because of the large 6180 variation in olivines ( 1.2%o,

1900

1700

15oo

1300 [...,

1100

ra O0

I Zheng Chiba et al. i 0 ol-cpx • [] sp-ol • 0 sp-cpx •

o • • 8o ° 0 0

t ° o..a.o------~

900 ~ • • '. . . . . '. . . . . ! . . . . 900 950 1000 1050 1100

T (2 pyroxenes)

Fig. 3. Calculated temperatures for different mineral pairs. Fraction- ation factors published by Chiba et al. (1989) and Zheng (1991, 1993a) are used to compare ol-cpx, sp-ol and sp-cpx calculated temperatures for the Yemen and Nunivak lherzolites using oxygen isotopes with those calculated with the two-pyroxenes thermometer of Wells (1977).

Fig. 4) associated with the development of amphibole and mica, Kempton et al. (1988) suggested that the mantle under Germany had experienced several metasomatic episodes leading to LREE enrichment in the clinopyroxenes and im- portant oxygen isotopic modifications to the olivine and hy- drous minerals.

More recently, in a general study of the oxygen isotopic composition of olivine, clinopyroxene, and orthopyroxene in mantle peridotites using the laser fluorination technique, Mattey et al. (1994a) showed that the 6 t80 values for these minerals were homogeneous in spinel-, garnet-, and dia- mond-facies peridotites. The data we have obtained in this paper are well within the range of values published by Mattey et al. (1994a) and confirm that there is no systematic difference in the oxygen isotopic composition of co-existing minerals in hydrous and anhydrous spinel lherzolites from Yemen and Nunivak island (Fig. 4). Clearly hydration of the mantle has not modified the oxygen isotopic composition of olivine, orthopyroxene, and clinopyroxene in these localit- ies. This conclusion contrasts with the data from Eifel (Kempton et al., 1988 ) in which olivine and pyroxene show great variations in oxygen isotopic composition (Fig. 4). Clearly, melt infiltration in the Eifel peridotites had a very different effect compared to fluid infiltration in the Yemen and Nunivak mantle.

The 6180 values measured in amphiboles and mica should give us a direct insight into the oxygen isotopic composition of the metasomatic melts from which these minerals crystal- lized. Here again, the results are very homogeneous for all the hydrous phases aoalyzed in the two localities (Fig. 2).

166 G. Chazot et al.

7.0

~. 6.5

0 ~ 6.0 0 e~

~ 5.5

5.0

4.5 5.0

• Yemen and Nunivak

5.5 6.0 6.5 7.0

818Oolivin e (%o)

7.5

7.0

6.5

o

6.0

0 9 5.s

5.0

4.5 5.0 5.5 6.0 6.5 7.0 7.5

818Oolivin e (%o)

Fig. 4. 6 '80 clinopyroxene and orthopyroxene versus 6'80 olivine for Yemen and Nunivak hydrous and anhydrous spinel lherzolites compared to published data (Kyser et al., 1981; Harmon et al., 1986/ 87; Kempton et al., 1988). The field from Kempton et al. (1988) for Eifel lherzolites is shown as an indication of the large range in composition of minerals in hydrous and anhydrous lherzolites from this area. Data from Mattey et al. (1994a) represent laser-fluorination analyses of more than 100 spinel and gamet lherzolites.

This homogeneity contrasts with the large range in 6 1 8 0

values for hydrous mantle minerals reported in the literature. Examples for amphibole and mica are 4.65-6.79%o (Boettcher and O'Neil, 1980), 5.9-7.4%o (Ongley et al., 1987) and 7.0-8.1%o (Kempton et al., 1988). Furthermore, fractionation between amphibole and olivine ranges from 0 - 0.3%o and is in good agreement with the fractionation ex- pected from equilibrium fractionation factors calculated for these minerals (Bottinga and Javoy, 1975; Zheng, 1993b). This confirms, observations from the oxygen isotopic com- position of olivine and pyroxene, that the metasomatic melts from which these hydrous minerals crystallized were very close to equilibrium with the peridotite through which they percolated. Alternatively since these are a minor component

in the mantle, amphibole equilibrated with the peridotite minerals through exchange without significant impact on the oxygen isotopic composition of the other minerals.

In the case of the Nunivak samples, the similarity in oxy- gen isotopic composition between megacrysts and hydrous minerals in the peridotite matrix indicates that melts in veins and melts circulating in the matrix were in equilibrium with each other.

We analyzed several single grains of olivine from the same thin section (sample JK8) at increasing distance from a large melt-pocket containing amphibole (Table 3 ) to estab- lish whether fluid infiltration during amphibole crystalliza- tion modified the oxygen isotopic composition of the sur- rounding olivine grains or if re-equilibration of amphibole with olivine had a significant effect on the surrounding oliv- ine grains. The melt-pocket was ca 6 mm in diameter, two analyzed olivine grains were in direct contact with the melt- pocket, and three grains were at distances of 3, 9, and 14 mm from the melt-pocket. These individual grains weighed between 0.93 mg and 1.75 mg. The mean 6 '80 value mea- sured for JK8 olivine was 5.28%o and for the separated grains, the measured values were between 5.20%o and 5.30%o (Table 3), well within the range of reproducibility of the technique. The grains away from the melt-pocket (6 ' so from 5.24-5.30%0) may be slightly heavier than those in contact with it (6 '80 = 5.20 and 5.21%o), but the variations are not large enough, compared to the uncertainty of the measure- ment, to draw definite conclusions. These data do indicate that no large scale exchange occurred between olivine and melt or hydrous minerals (amphibole in JK8 ) with different oxygen isotopic composition during formation or melting of the amphibole in the hydrated lherzolites.

Few data have been published for spinel from peridotites (Fig. 5 ). Kyser et al. ( 1981 ) reported values for spinel from different localities with a 6 '80 range from 4-7%o. Other spinel grains analyzed by laser fluorination (Mattey et al., unpubl, data) display a smaller range of 6Jso, from 4 .2- 5.0. Our data extend the range towards low values and are split into two groups. Spinels in anhydrous lherzolites from Bir Ali have high 6 '80 values (>4.5%o) whereas spinels from Ataq and Nunivak have lower 6 ~80 values (-<4.0%o). These differences can be accounted for either by large varia- tions in fractionation factors between these two phases with the chemistry of the spinel (Zheng, 1991 ) or by incomplete equilibration of the spinel with the peridotite.

The 6 '80 data from the minerals in the melt-pockets are difficult to interpret. As previously noted, data obtained with electron and ion microprobes and mass balance calculations

Table 3. Isotopic composition of single grains from a thin section of lherzolite JK8.

Distance from Sample weight Sample melt-pocket (mm) (mg) 8'sO

JK8-3OL13 0 1.124 5.21 JK8-6OL1 0 1.157 5.20 JK8-9OL4 3 1.752 5.30 JK8-OL10 9 0.933 5.24 JK8-13OL8 14 1.243 5.30

Isotope composition of 0 in the mantle 167

8.0

7.0

6.0

.9

oo O 5.0

4.0

3.0 3.0 4.0 5.0 6.0 7.0 8.0

18Oolivine (%0)

Fig. 5. 6~O spinel (with oxygen yield >-97%) vs. ~'80 olivine for Yemen and Nunivak hydrous and anhydrous spinel lherzolites. Published data from Kyser et al. (1981) represent fifteen analyses. Data from Mattey et al. (unpubl.) represent twelve analyses obtained by the laser-fluorination technique. A line corresponding to A~O = 0 is shown for reference. Same symbols as in Fig. 1.

show that these melt-pockets have evolved in a closed-sys- tem, without infiltration of metasomatic fluid during melting of amphibole and crystallization of the secondary minerals olivine, clinopyroxene, and spinel (Chazot et al., 1996 and in press). The 6~80 values obtained in minerals from JK8 melt are consistent with these observations, as they are very similar to the data measured in minerals from the peridotitic matrix of the same sample. In contrast, 6~sO values measured in olivine and clinopyroxene in JK2 melt are higher than the values measured in the matrix and may suggest reequilibra- tion at lower temperature.

5.2. Implications for Metasomatic Fluids in the Mantle

Metasomatism in the Earth's mantle implies circulation of a fluid phase and reaction of this fluid with mantle miner- als. In the case of modal metasomatism, with crystallization of new minerals, the composition of these new minerals reflects the composition of the metasomatic fluids. The large range of oxygen isotopic compositions reported for hydrous minerals in peridotites (Boettcher and O'Neil, 1980; Kemp- ton et al., 1988) as well as the very large range of values reported for minerals in eclogites (Garlick et al., 1971; On- gley et al., 1987; Mattey et al., 1994b) are indicative of the diversity of metasomatic fluids and of their complex origin. The high ~51~O values reported by these authors strengthen the link between metasomatism and subduction processes and, for the eclogites, their direct origin from subducted slabs. (e.g., Gregory and Criss, 1986; Mattey et al., 1994b).

However, in other localities such as Yemen and Nunivak Island, the 6 J~O of mantle minerals and especially the hy- drous minerals are not that different from values measured in anhydrous mantle peridotites. In these cases, the metaso-

matic fluids are very close to equilibrium with the mantle itself and do not modify the oxygen isotopic composition of the peridotites through which they percolate. These metaso- matic fluids can originate in two different ways: ( 1 ) A sub- duction related origin similar to the fluids with high 6~O values recorded in hydrous minerals from other localities. In that case, their oxygen isotopic composition has been buffered by the oxygen isotopic composition of olivine dur- ing percolation through mantle peridotite. This would indi- cate a residence time longer than in the localities that have high 6~sO for the hydrous minerals or reactions with the mantle minerals at very low fluid/rock ratios. (2) Mantle- derived origin without any link to recent recycling of crustal material. Mattey et al. (1994a) has shown that the oxygen isotopic composition of most of the minerals from peridotites is very homogeneous. Segregation of carbonate or silicate melts or aqueous fluids fiom an isotopically homogeneous mantle peridotite will produce metasomatic fluids in oxygen isotopic equilibrium with the mantle minerals.

In the case of Nunivak Island, Deloule et al. ( 1991 ) mea- sured the 6D of hydrous minerals very similar to those ana- lyzed in this study and found very homogeneous values (be- tween -65 and - 92 ) , typical of a mantle origin and consis- tent with the homogeneous 6 ~O values we measured herein. One can conclude that the Nunivak samples are unaffected by recycled material from the subduction zone. Furthermore, the trace element composition of metasomatic minerals in the hydrous lherzolites from Yemen indicate that the melt responsible for metasomatism may have had a carbonated composition (Chazot et al., 1994). This observation is in good agreement with experimental data that demonstrated the transformation of enstatite in clinopyroxene and amphi- bole (Wallace and Green, 1988) during reaction of a carbon- ated fluid and a peridotite. These mineralogical reactions have been observed in the Yemen lherzolites. In a detailed geochemical study of Nunivak mantle xenoliths, Roden et al. (1984) concluded that metasomatic clinopyroxenes in their samples were not in equilibrium with a silicate basaltic melt, and that the metasomatic fluids were probably CO2 or HzO rich. Taking all these observations together indicates that the presence of several different kinds of metasomatic fluids are recorded in the oxygen isotopic composition of mantle minerals. In some cases, these fluids have a strong subduction signature leading to high 6~sO values as found in most eclogites (Mattey et al., 1994b). In other cases, such as Yemen and Nunivak, the metasomatic fluids had an oxygen isotopic composition (and hydrogen for Nunivak, Deloule et al., 1991) close to equilibrium with the mantle minerals and probably originated in the convecting upper mantle or were buffered by olivine during melt transfer through the mantle. It is important to stress that in these two last localities, the metasomatic fluids in equilibrium with the peridotitic mantle seem to be carbonated rather than silicate melts.

Clearly from these remarks, a general study of the oxygen isotopic composition of hydrous minerals in mantle xenoliths is needed for a better understanding of the origin, nature, and composition of the metasomatic fluids in the Earth's mantle.

168 G. Chazot et al.

6. CONCLUSIONS

The oxygen isotopic compositions of minerals in hydrous and anhydrous spinel lherzolites from Yemen and Nunivak Island, as well as in amphibole megacrysts, are homoge- neous. Olivine in the spinel lherzolites shows little variation and there is no systematic variation in the isotopic composi- tion of olivine and pyroxenes between hydrous and anhy- drous lherzolites.

Hydrous minerals (amphibole and mica) in Yemen and Nunivak mantle appear to be in isotopic equilibrium with the peridotitic minerals implying that melts percolating through the mantle originate from a typical mantle source or have been buffered by diffusive exchange with the most abundant mineral in the mantle during percolation (i.e., ol- ivine).

The results obtained in this study contrast with published data which report large variations in the oxygen isotopic composition of hydrous and anhydrous minerals in metaso- matized rocks.

Oxygen isotopic ratios of olivine and clinopyroxene euhe- dra in in situ melt-pockets suggest differing temperatures of equilibration.

Acknowledgments--We thank Z. Sharp, E. Young, R. Gregory, A. Matthews, and an anonymous reviewer for careful and very construc- tive reviews of this manuscript as well as D. Mittlefehldt for useful comments. This work has been supported by an E. C. Fellowship from the Human Capital and Mobility program.

Editorial handling: D. W. Mittlefehldt

REFERENCES

Ben Othman D., Tilton G.R., and Menzies M.A. (1990) Lead, neodymium, and stronium isotopic investigations of kaersutite and clinopyroxene from ultramafic nodules and their host basalts: the nature of the subcontinental mantle. Geochim. Cosmochim. Acta 54, 3449-3460.

Boettcher A. L. and O'Neil J. R. (1980) Stable isotope, chemical and petrographic studies of high-pressure amphiboles and micas: evidence for metasomatism in the mantle source regions of alkali basalts and kimberlites. Amer. J. Sci. 280-A, 594-621.

Bottinga Y. and Javoy M. (1973) Comments on oxygen isotope geothermometry. Earth Planet. Sci. Lett. 20, 250-265.

Bottinga Y. and Javoy M. (1975) Oxygen isotope partitioning among minerals in igneous and metamorphic rocks. Rev. Geophys. Space Phys. 13, 401-418.

Chazot G., Menzies M. A., Harte B., and Mattey D. (1994) Carbona- tite metasomatism and melting of the Arabian lithosphere: evi- dence from oxygen isotopes and trace element composition of spinel lherzolites. Mineral. Mag. 58A, 167-168.

Chazot G., Menzies M. A., and Harte B. (1996) Determination of partition coefficients between apatite, clinopyroxene, amphibole and melt in natural spinel lherzolites from Yemen: implications for wet melting of the lithospheric mantle. Geochim. Cosmochim. Acta 60, 423-437.

Chazot G., Menzies M. A., and Harte B. (1997) Silicate glasses in spinel lherzolites from Yemen: Origin and chemical composition. Chem. Geol. (in press).

Chiba H., Chacko T., Clayton R. N., and Goldsmith J. R. (1989) Oxygen isotope fractionation involving diopside, magnetite, and calcite: application to geothermometry. Geochim. Cosmochim. Acta 53, 2985-2995.

Clayton R. N. and Kieffer S. W. ( 1991 ) Oxygen isotopic thermome- ter calibrations. In Stable Isotope Geochemistry: A Tribute to Sam- uel Epstein (ed. H. P. Taylor et al.); Geochem. Soc. Spec. Publ. 3, 3-10.

Clayton R. N., Goldsmith J. R., and Mayeda T. K. (1989) Oxygen isotope fractionation in quartz, albite, anorthite, and calcite. Geo- chim. Cosmochim. Acta 53, 725-733.

Cox K. G., Gass I. G., and Mallick D. I. J. (1970) The peralkaline volcanic suite of Aden and Little Aden, South Arabia. J. Petrol. 11, 433-461.

Deloule E., Albar~de F., and Sheppard S. M. F. ( 1991 ) Hydrogen isotope heterogeneities in the mantle from ion probe analysis of amphiboles from ultramafic rocks. Earth Planet. Sci. Lett. 105, 543-553.

Elsenheimer D. and Valley J.W. (1992) In situ oxygen isotope analysis of feldspar and quartz by Nd:YAG laser microprobe. Chem. Geol. 101, 21-42.

Francis D. M. (1976) The origin of amphibole in lherzolite xenoliths from Nunivak Island, Alaska. J. Petrol. 17, 357-378.

Garlick G. D., MacGregor I. D., and Vogel D.E. (1971) Oxygen isotope ratios in eclogites from kimberlites. Science 172, 1025- 1027.

Gregory R. T. and Criss R. E. (1986) Isotopic exchange in open and closed systems. In Stable isotopes in high temperature geological processes (ed. J. W. Valley et al.); Rev. Mineral. 16, 91-127.

Gregory R. T. and Taylor H. P. (1986a) Possible non equilibrium oxygen isotope effects in mantle nodules, an alternative to the Kyser-O'Neil-Carmichael t80/~60 geothermometer. Contrib. Mineral. Petrol. 93, 114-119.

Gregory R. T. and Taylor H. P. (1986b) Non-equilibrium, metaso- matic ~80/~60 effects in upper mantle mineral assemblages. Con- trib. Mineral. Petrol. 93, 124-135.

Harmon R. S., Kempton P. D., Stosch H.-G., Hoefs J., Kovalenko V. I., and Eonov D. (1986/87) t80/~60 ratios in anhydrous spinel lherzolite xenoliths from the Shavaryn-Tsaram volcano, Mongolia. Earth Planet. Sci. Lett. 81, 193-202.

lonov D.A., Harmon R.S., France-Lanord C., Greenwood P.B., and Ashchepkov 1. V. (1994) Oxygen isotope composition of gar- net and spinel peridotites in the continental mantle: evidence from the Vitim xenolith suite, southern Siberia. Geochim. Cosmochim. Acta 58, 1463-1470.

Kempton P.D., Harmon R.S., Stosch H.-G., Hoefs J., and Hawkesworth C. J. (1988) Open-system oxygen isotope behav- iour and trace element enrichment in the sub-Eifel mantle. Earth Planet. Sci. Lett. 89, 273-287.

Kyser T. K., O'Neil J. R., and Carmichael I. S. E. ( 1981 ) Oxygen isotope thermometry of basic lavas and mantle nodules. Contrib. Mineral. Petrol. 77, 11-23.

Kyser T. K., O'Neil J. R., and Carmichael I. S. E. (1982) Genetic relations among basic lavas and ultramafic nodules: evidence from oxygen isotope compositions. Contrib. Mineral. Petrol. 81, 88- 102.

Mattey D. and Macpherson C. ( 1993 ) High-precision oxygen isotope microanalysis of ferromagnesian minerals by laser-fluorination. Chem. Geol. 105, 305-318.

Mattey D., Lowry D., and Macpherson C. (1994a) Oxygen isotope composition of mantle peridotite. Earth Planet. Sci. Lett. 128, 231-241.

Mattey D., Lowry D., Macpherson C. G., and Chazot G. (1994b) Oxygen isotope composition of mantle minerals by laser fluorina- tion analysis: homogeneity in peridotites, heterogeneity in eclog- ites. Mineral. Mag. 58A, 573-574.

Matthews A., Palin J. M., Epstein S., and Stolper E.M. (1994) Experimental ~80/t60 partitioning between crystalline albite, al- bitic glass, and CO2 gas. Geochim. Cosmochim. Acta 58, 5255- 5266.

Menzies M. A. and Murthy V. R. (1980) Nd and Sr isotope geo- chemistry of hydrous mantle nodules and their host alkali basalts: implications for local heterogeneities in metasomatically veined mantle. Earth Planet. Sci. Lett. 46, 323-334.

Ongley J. S., Basu A. R., and Kyser T. K. (1987) Oxygen isotopes in coexisting garnets, clinopyroxenes and phlogopites of Roberts Victor eclogites: implications for petrogenesis and mantle metaso- matism. Earth Planet. Sci. Lett. 83, 80-84.

Richter R. and Hoernes S. (1988) The application of the increment method in comparison with experimentally derived and calculated 0-isotope fractionations. Chem. Erde 48, 1 - 18.

Isotope composition of O in the mantle 169

Roden M. F., Frey F. A., and Francis D. M. (1984) An example of consequent mantle metasomatism in peridotite inclusions from Nunivak Island, Alaska. J. Petrol. 25, 546-577.

Rosenbaum J. M., Kyser T. K., and Walker D. (1994) High tempera- ture oxygen isotope fractionation in the enstatite-olivine-BaCO3 system. Geochim. Cosmochim. Acta 58, 2653-2660.

Sharp Z. D. (1990) A laser based microanalytical method for the in situ determination of oxygen isotope ratios of silicates and oxides. Geochim. Cosmochim. Acta 54, 1353-1357.

Sharp Z. D. (1992) In situ laser microprobe techniques for stable isotope analysis. Chem. Geol. 101, 3-19.

Sharp Z. D., Essene E. J., and Smyth J. R. (1992) Ultra-high temper- ature from oxygen isotope thermometry of a coesite-sanidine gros- pydite. Contrib. Mineral. Petrol. 112, 358-370.

Stolper E. and Epstein S. (1991) An experimental study of oxygen

isotope partitioning between silica glass and CO2 vapor. In Stable Isotope Geochemistry: A tribute to Samuel Epstein (ed. H.P. Taylor et al.); Geochem. Soc. Spec. Publ. 3, 35-51.

Wallace M. E. and Green D. H. (1988) An experimental determina- tion of primary carbonatite magma composition. Nature 335, 343- 346.

Wells P. R. A. (1977) Pyroxene geothermometry in simple and com- plex systems. Contrib. Mineral. Petrol 62, 129-139.

Zheng Y.-F. (1991) Calculation of oxygen isotope fractionation in metal oxides. Geochim. Cosmochim. Acta 55, 2299-2307.

Zheng Y.-F. (1993a) Calculation of oxygen fractionation in anhy- drous silicate minerals. Geochim. Cosmochim. Acta 57, 1079- 1091.

Zheng Y.-F. (1993b) Calculation of oxygen isotope fractionation in hydroxyl-bearing silicates. Earth Planet. Sci. Lett. 120, 247-263.