Sulphur isotope variations in the mantle from ion microprobe analyses of micro-sulphide inclusions

144

Earth and Planetary Science Letters, 92 (1989) 144-156 Elsevier Science Publishers B.V.. Amsterdam - Printed in The Netherlands

PI

Sulphur isotope variations in the mantle from ion microprobe of micro-sulphide inclusions Marc Chaussidon,

Francis Albarkde

C.R.P.G.-E.N.S.G.,

Received

September

analyses

and Simon M.F. Sheppard

B.P. 20, 54501 Vandoeuure I& Nancy (France)

29, 1988; revised version accepted

January

3, 1989

21 samples of sulphide trapped either as liquid globules or grains in various minerals (oli..ine, pyroxenes, ilmenite and garnet) or rocks (basalt glasses, peridotites, eclogites and kimberlites) of mantle origin, have been analysed for their sulphur isotope, and their Cu. Ni, Fe compositions by ion microprobe. The results show a wide range of S34S values between - 4.9 f 1 and + 8 k 1%. Sulphides with high nickel contents (up to 40% pentlandite), corresponding mostly to residual peridotites, have 634S values ranging from - 3.2% to + 3.6? 00 with a mode of + 3 f l%, compared to low Ni content sulphides, mostly contained in pyroxenites, OIB and kimberlites, ranging from - 3.6% to + 8%~ with a mode of +1*1%J. The 6%S of sulphides originating from within the mantle are variable. The sulphide globules with high Ni contents and iY3%3values close to + 3%~~are probably produced by lo-208 partial melting of a mantle source containing 300 ppm sulphur as an upper limit and having a 634S value of +0.5 k 0.5%. This difference in 634S values suggests a high-temperature S-isotope fractionation of = + 3% between liquid sulphide and the sulphur dissolved in the silicate liquid. The sulphur isotopes balance in the system upper mantle + oceanic crust + continental crust + seawater requires a mean 634S value of the primitive upper mantle of + 0.5%, slightly but significantly different from that of chondrites ( + 0.2 f 0.2%) [l].

1. Introduction Sulphur in magmatic rocks occurs as dissolved species, melt blebs in glasses and minerals, or as interstitial minerals [2]. The sulphur content of basic rocks ranges from several tens of ppm to = 2000 ppm, and mid-ocean ridge basalt (MORB) glasses are generally sulphur saturated at around 1000 _t 200 ppm [3-51. In contrast to this large range of sulphur contents, the isotope composition of primary sulphur in basic rocks has been assumed to be rather uniform at 0 + 3%0 [1,6] and very similar to that of meteorites, 0 + 0.5%0 [1,7]. However, recent studies of volcanic rocks (basalts, andesites and dacites) from the Japanese island arc [8] and the Mariana back arc basin [9] have shown that 634S values range between -0.2 and + 20.7%0. These rocks have probably assimilated a high 634S crustal component during the directly CRPG

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associated subduction processes. More undoubted proof of sulphur isotope heterogeneity within the mantle has been given by ion probe analyses of sulphide inclusions armoured in diamonds. The 634S values range from + 1 & 1%0, for a peridotitic diamond from Premier (South Africa), to + 9 + 1%0 for several eclogitic diamonds from Orapa (Bostwana) [lo]. The purpose of this study has been to use the recent technical developments in ion probe isotope analyses [11,12], to determine the 634S values of the small (< 100 pm) sulphide globules or inclusions present in mantle rocks, in order to constrain the geochemical evolution of sulphur in the mantle. In fact these samples are the only available witnesses of the sulphide immiscible melts suspected to occur at depth in the mantle (e.g. [13-161). They are therefore of prime importance for determining the 834S values of the mantle, and for setting limits on the flux of sulphur between the mantle and crustal reservoirs.

145

2. Samples description The samples selected represent a survey of the different kinds of occurrences of primary sulphides within mantle-derived rocks. Several sulphide textures, as already summarized (e.g. [16]), have been analysed: large (40-200 pm) globules within glasses or phenocrysts, small globules (< 40 pm) generally occurring as lines of numerous blebs within phenocrysts, and sulphide mineral grains of various sizes (20-200 pm) present either within fractured phenocrysts or at the intersection of several minerals. The globules are generally considered to represent sulphide melts which were immiscible with silicates. The host rocks (see Table 1) range from MORB and alkali basalt glasses to continental basalts, eclogites, peridotites and kimberlites. Host minerals are pyroxene, ilmenite, garnet and olivine. The sulphides, whether in globules or grains, are always pyrrhotite (FeS) plus various amounts of pentlandite (FeNiS) and chalcopyrite (CuFeS,). They indicate that the sulphides have crystallized from the high-temperature mono-sulphide solid solution (MSS, between S, Fe, Ni and Cu) [17]. However, in some cases

200

150

50

0 800

1000 1200 Temperature (“C)

1400

Fig. 1. P-T conditions for the host rocks or minerals of some of the sulphide assemblages studied, together with three model geotherms. Data are from 1221 (Hawaii lava lake), (231 (Kiglapait), [24] (Salt Lake Crater Hawaii), [25] (Koidu kimberlite), [26] (diamonds in general), J.P. Lorand, personal communication (Iherzolites from AriBge), [27] (peridotites from Ardtihe).

detailed electron microprobe analyses revealed the presence of small pyrite exsolutions which show that some grains or even some globules have reequilibrated at low temperatures, = 200°C (J.P. Lorand, personal communication). In addition to the grains (pyrrhotite + pentlandite f chalcopyrite), which occur as inclusions or between minerals, some pyrites, occurring as interstitial minerals, were also analysed. Such pyrites, however, cannot form as the major phase during the closed system evolution of a magmatic sulphur phase. Depending on their host rock or mineral, the majority of the sulphides can be considered to have been armoured from any secondary changes as no crack surrounds them. Thus they represent samples of the sulphide component of the mantle at different depths. The available or inferred P-T conditions of the host of the sulphides studied are plotted in Fig. 1, together with three model geotherms for ocean ridge, ocean plate and shield environments. Although a large range of mantle conditions has been covered, a gap exists between the deep samples (> 100 km), like globules or inclusions in kimberlite megacrysts or diamonds, and samples from shallower depths (< 50 km) like sulphides in peridotites or basalts. 3. Analytical techniques The S34S values as well as the chemical compositions of the sulphides were determined with a modified CAMECA IMS3F ion microprobe at the C.R.P.G. (Nancy), according to the procedures already described [11,12]. Samples, either as separated sulphides or pieces of rock containing sulphides, are embedded in epoxy, polished and coated with gold. The ion probe is operated with a lo-15 nA O- primary beam of 10 pm in diameter. Secondary ions are analysed over a 60 pm diameter area, in which the primary beam is always centered. When possible a raster of 60 pm was used to achieve a better reproducibility. Positive secondary ions are analysed without energy filtering, at a mass resolution of 3000 when hydrides are not observed, or 4000 when they are detected (by monitoring the ratios 33S/32S and 34S/32S). Large differences in instrumental mass fractionation A (with A = 634S measured - 634Srea,) similar to the ones already reported with the SHRIMP ion microprobe [18] are observed between pentlandite

146

(A = - 48.3%~) pyrrhotite (A = - 64.4%0) and chalcopyrite (A = -69.1%). In order to correct the measured value with a mean instrumental mass fractionation factor, the precise quantity of these three sulphides in the area analysed is determined before and after each isotope analysis by counting the Fe, Ni, Cu peaks. Using calibration curves of sulphide standards, an accuracy of +5% is obtained on the measurements of the pentlandite and chalcopyrite contents. The variation of the instrumental mass fractionation, due to the variation of the sulphide phases analysed, is generally < 10%0. Knowledge of this correction factor to * 5% results in a change of -=K0.5%0 in the corrected S34S value, and no bias of the final 634S value is observed between low Ni and high Ni points on the same samples (see Table 1). Finally since pyrrhotite can contain several percent of Ni, the major uncertainty is an over-estimation of the percentage of pentlandite in the analysed area. This uncertainty is in the order of a few percent in the estimated %Ni, and if over-estimated would decrease the corrected 634S values by less than l%o. The within-run statistics is generally = 0.8%0, and the reproducibility on both standard minerals and samples is mostly = 1%0. The reproducibility was worse for a few analyses (see Table 1). Note that samples which have been analysed more than once are repolished between analyses and that the nature of the exsolution texture implies that the analysed spot is not necessarily mineralogically identical each time. The S34S values are given in permil (W,) relative to the Canyon Diablo Troilite (CDT) standard ( 34S/32Sc.DT = 0.044994 [l]). Reliability of the S34S values is considered to be + 1%0 for most samples larger than 50 pm, but falling to + 3%0 for 10 pm ones (see Table 1). 4. Modifications

of

the

sulphide

liquids

during

to have remained closed throughout the history of the globule. However, in some cases the morphology of the globule is not clear and fractures cannot be excluded. Isotopic fractionations are conceivable since successive exsolutions of sulphur-bearing species, which can be separated later, take place during cooling of the sulphide liquids. For temperatures between 1000°C and 900-850” C, a sulphur-rich (= 98 wt.% S) liquid can coexist with a liquid sulphide having a composition like a Fe-Ni MSS [17]. During cooling of the MSS phase a Cu-rich liquid segregates around 850 o C. This liquid whose composition is close to the intermediate solid solution (ES) [17], will precipitate chalcopyrite on further cooling [15,19]. The remaining MSS yields pentlandite and pyrrhotite or Ni-pyrrhotite via subsolidus reactions around 600 o C. Sulphur fractionations which could result from these reactions or degassing processes are probably negligible. At 1000” C, the known fractionation factors are Z -0.4%0 A pyrrhotite-Hi,Sc,, [61 and AS,c,,_H,Sc,, = - 0.5%0 [20]. Similarly Apyrrhotite_cha,copyrite = + 0.1%0 at 850” C [21], and at 500 o C the biggest fractionation among sulphides (between pyrite and galena) is less than 2%0 [21]. The sulphide globules therefore undergo internal chemical and minor isotopic readjustments during cooling. The system, as a whole, can be considered to be generally chemically and isotopitally closed if pyrrhotite and smaller amounts of pentlandite and chalcopyrite are the major sulphide constituents. Moreover, the sulphur isotope analyses with the ion probe were generally performed on the mixed sulphide assemblage, because of the fine scale of the exsolution, with duplicates at different depths. In this way within run fractionations between different sulphides were minimized, and the analysis is representative of the whole globule.

cooling 5. Results

Sulphides of different shapes and chemical compositions have been studied. The question arises whether they are all isotopically perfectly representative of the sulphide liquids from which they originated. Sulphide globules, compared to sulphide grains, are assumed -to have been armoured by their host mineral or glass if no crack surrounds them. The sulphide system is then likely

The 634S values of 62 sulphides (38 globules, 19 grains, 3 inclusions not rounded, and 2 pyrite grains), are given in Table 1 together with their pentlandite and chalcopyrite contents. The 634S values of the globules range between - 3.6 and +8%0 (mean value + 1.5%0), and are indistinguishable from the sulphide grains. Note that

147 TABLE

1

Sulphur isotope compositions of sulphide globules and grains and of pyrite Sample

Sulphide type, shape and size

Content in the area analyzed (%) pentlandite

MORB glasses Mid-Atlantic Ridge (see [4,19,28] “): ALV 5261a globule, spherical ALV 5261b globule, spherical ALV 5261b b globule, spherical Rl-3 globule, spherical B21-10 globule, spherical B21-10 b globule, spherical

(500 pm) (500pm) (500 pm) (20 pm) (20 pm) (20 pm)

29 27.3 13.9 10 2.1 2.1

a3% CDT (*l(%o)

chalcopyrite

1 1 3.5 5 7 7

+3.8 + 3.1 + 3.4 +0.9*3 -1.3*3 + 3.5 f 1.5

Seamountand oceanic island glasses Rocard seamount (Tahiti), phonolites containing sulphides associated with magnetite (depth of collection 2400 m):
+ 2.2 + 2.8 + 4.2 +4 - 0.8 - 0.6 + 1.6 - 0.9 +1.6

(1
-3 -0.6 - 3.6 -1.6 -1.4

<1 (1 <1
+0.1 +1.1 - 2.3 -0.7 +8 + 7.8

5.8 5.3
+ 1.5 + 3.7 +3.7 + 2.2

3.1 3.1 5.1 1.5
+1.2&1.5 + 3.2+ 1.5 + 3.8 + 1.5 - 3.2+ 1.5 + 0.2 +3 +0.3 +1.4 - 0.2

3.6 1 1

+ 5.1 f 1.5 +3.6+1.5 + 3.6 + 1.5

148 TABLE Sample

1 (continued) Sulphide

Content in the area analyzed (S)

type, shape and size

Continental gabbros, eclogites and peridotites (continued) Bestiac (Lherz Ariege, France), spine1 lherzohte, massive peridotite: 79-61-l globule in olivine, spherical (60 pm) 79-61-2 grain (100X40 pm) 79-61-3 grain (80 X 50 pm) 79-61-4 grain (SO X 50 pm) Frey&in&de (At&e, France), vein of amphibole and garnet chnopyroxenite _ 7b-1841112-l grain (200 pm) _ 70-1841112-l ’ grain (200 pm) 70-1841112-2 grain (70 pm) 70-1841112-3 grain (100X40 pm) 70-1841112-4 grain (60 pm) Kimberlites Uintjesberg (South Africa): JJG2186(5) globule (20 am) in clinopyroxene megacryst Jagersfontein (South Africa): JF130b globule (30 pm) in pyroxene megacryst JF130b b globule (30 pm) in pyroxene megacryst JF132a-1 globule (30 pm) in pyroxene megacryst JF132a-2 globule (30 pm) in pyroxene megacryst Bulfontein (South Africa): B7 hexagonal grain Roberts Victor (South Africa): RV3-1 pyrite cubic grain (120 x 120 pm) RV3-2 globule in clinopyroxene RV3-3 grain RV3-4 globule(?) in clinopyroxene Koidu (West Africa) (see [34]): ilmenite megacryst KIL6-1 globule (50 pm) KIL6-2 elongated globule (500 pm) KIL6-2 b elongated globule (500 pm) garnet megacryst KGTll-1 globule (40 pm) KGTl l-2 globule (40 pm) KGTll-3 globule (40 nm) KGTll-4 globule (40 pm) KGTl l-5 globule (40 pm) eclogite nodule KEC86112a globule KEC86112a b globule KEC8664a-1 pyrite KEC8664a-1 b pyrite a Reference of work concerning b New analysis after repolishing.

the sulphides

measured

pentlandite

chalcopyrite

43.4 22.5 26.9 26.9 (see [33]): -Cl -Cl 2.2 (1 (1

Cl


+1.2 +7 +6 +5.6

xl (1 (1
+4.4 +3 + 4.5 i3.6 + 3.1

<1

il

-2.2*1.5

34.8 32 13.4 1.5

(1


+ 3.3 + 3.3 -4.9 - 2.9

4.4


-0.1


il

3.7 1.9 0

+ 2.5 +1.7 0 +4.8

1.8 19.3

+ 3.7 + 1.9 +1.8

9.6 3.3 2.5

1.3 2.6 <1

Cl

4.3 6.2 5 2.6 1.4


6.4 4.3

-

+ 2.9 -1.7 - 0.2 - 1.6 + 0.5

2.2


-

+ 1.6 +1.9 - 12.8 - 10.2

il (1
in this study.

the mean a-values presented here must be interpreted with caution since the number of different sample localities is still extremely limited for all types of environments represented here. It is the range of a-values which is of prime importance. In

general, 634S values determined on different globules and grains from the same sample are the same within the experimental uncertainties. Exceptions include grains and globules from the Lherz spine1 lherzolite

and

from

the Mg-

and

K-rich

diorite

149

from the Massif Central (Table 1). In both these examples the pentlandite contents of the grain and globules are also different. Of two apparently fresh pyrites from kimberlites, one has a mean 634S value of - 11 & l%o, which is in total contrast to the globule and grain values. Nevertheless, comparable values have been reported on pyrites from kimberlites using conventional mass spectrometric techniques [15]. The pentlandite contents of the globules and the grains are highly variable between = 0 and 43%. Low pentlandite contents are observed for both globules and grains, although more globules have high pentlandite contents. These high pentlandite contents seem to be restricted to high-temperature peridotites but are also found in part of the MORB and eclogite (metamorphic equivalents of MORB) samples. In contrast low pentlandite contents, which represent the majority of samples, are found in OIB (oceanic island basalts), continental basal& pyroxenites, and many kimberlites (Table 1). The S34S values are broadly related to the pentlandite contents (Fig. 2). A wide range of 634S values between -3.6 and t-8%0 is observed for the low pentlandite content samples, with a mode of + 1%0, contrary to + 3%0 for the high pentlandite ones (Fig. 2). This trend is con-

wt % pentlandite in the analyzed area Fig. 2. S34S values determined with the ion probe (this study) versus the percentage of pentlandite in the sulphide assemblage sampled by the ion probe, and histogram of the 13~~svalues of the globules. Note the variability of the pentlandite contents, which can reflect different equilibration conditions in the mantle and/or different histories of cooling of the MSS. The mode of the 634S values of the Ni-rich sulphide globules ( = +3%) is different from that of the Ni-poor globules ( = + 1%). The four pentlandite-rich grains could correspond to previous pentlandite-rich globules. In addition the 634S values obtained by conventional whole-rock analyses of comparable samples [15,35,36] (rocks containing Ni-rich sulphide grains and/or globules) are represented by the open squares in the histogram.

trary to the one observed for sulphide inclusions in diamonds, since in this latter case the supposed peridotitic diamond with a high Ni content has a 634S value close to O%o, whilst some of the eclogitic diamonds have very low Ni contents but have 634S values up to +9%0 [12]. Few 634S values of MSS sulphide grains in basic or mafic rocks have been previously measured by conventional mass spectrometry due to their small size. These data are plotted in Fig. 2, and include a whole rock analysis of a peridotite nodule probably containing pyrrhotite and Ni, Cu sulphides, from West Germany (Hirzstein near Giittingen) (S34S = + 3.3%0 [35]), whole rock analyses of two lherzolites and of a garnet peridotite xenolith from the Obnazhennaya kimberlite pipe (S34S = + 2.1, + 2.0 and + 0.6%0, respectively ]361), and analyses of pyrrhotite-pentlandite sulphide grains in a garnet megacryst from Premier mine (South Africa) and in an eclogite xenolith from Roberts Victor (South Africa) (634S = +0.2 and +2.1%0, respectively [15]). Our measurements are consistent with these data (Fig. 2). 6. Samples with low Ni contents About two thirds of the sulphide globules and grains analysed have low pentlandite contents. They are enclosed in OIB (Tahiti, Hawaii), continental gabbro and diorite (Kiglapait, Massif Central) and pyroxenites (Fig. 3 and Table 1). Kimberlite megacrysts usually, but not systematically, contain sulphides with low Ni contents. Kimberlites excluded, the host rocks represent the differentiated liquids (OIB), and cumulates and/or liquids (e.g. pyroxenites) produced during the partial melting of the mantle. These different rocks, kimberlites included, represent magmas extracted from a mantle different from the type I peridotites [37] and source of MORB. The low Ni contents of the sulphides can be attributed to the fact that they exsolved after extraction of their silicate host magma from an olivine-bearing source (type I peridotites). Subsequently they equilibrated, often in a low-pressure environment, with a differentiated, low-Ni magma. The Kiglapait sample, KI 3003 (olivine gabbro), corresponds to the upper zone of the intrusion after solidification of 85-90% by volume [23]. The chemistry and abundance of the sulphides in

150

Kiglapait rocks have been shown to be dependent on this percentage of crystallization [30], suggesting a late-stage equilibration with the silicate liquid. Similarly the sulphide globules from the Alae lava lake are thought to have exsolved during a late stage of the lava’s cooling [22]. They are contained in interstitial liquids, and not in the main magma body which remained sulphur undersaturated throughout the crystallization. Furthermore the Alae lava lake sulphides contain about 4% by weight oxygen, which suggests equilibration in a near-surface environment [22], since the solubility of oxygen in sulphide melts decreases with increase of pressure [38]. The sulphide blebs from the Tahiti phonolite are also associated with magnetite blebs, but intergrowth textures, similar to those of the Alae lake crater, are not observed, suggesting perhaps a greater depth of equilibration. The 634S values of the low-Ni sulphides are much more variable than those of the high-Ni sulphides (Fig. 2). They range from - 3.6 to + 8%0, with a mode of + 1 k 1% with no significant difference between globules and grains. All grains with 634S < 0% come from the Kiglapait or, for one sample, the Massif Central (Fig. 3). The sulphide globules from the Koidu kimberlite suite show a range of 634S values from -1.7 to +3.9%0, which represents a large part of the total variation so far observed in kimberlites (Fig. 3). If a mean value is taken for each sample, which is represented in Table 1 by 1-5 globules, the range is reduced from 0 to + 2.8%0. However, such an averaging process may not be justified if, by analogy with the 613C and 615N variations observed in single diamonds [39], the megacrysts had a complex growth history, trapping sulphide globules in a number of sites or times within the mantle. The variability of the pentlandite contents supports a polyphased trapping history. Their host silicates, ilmenite, garnet megacrysts or eclogite are supposed to have crystallized around = 47 kbar between 1000 and 1200 o C (Fig. 1) [25], and sulphur is supposed to control the redox state in which ilmenite is stable [34]. Similarly, the two pyroxene megacrysts from the Jagersfontein pipe have sulphide globules with different pentlandite contents and 634S values (Table 1). The variability of 634S values observed for the other low-Ni samples is also likely to reflect the

lo

dim

-5

0

Bo +lO

634s

(%o)+5

Fig. 3. Histograms of the IS’~S values of this study for different rock types, plus the values of sulphide inclusions in diamonds [lo]. Note that globules from MOR and 01 basalts tend to have positive 13~~s values and also exhibit smaller variations in comparison with the continental rocks.

complex history of these rocks. The pyroxenite layers are thought to represent cumulates of mantle-derived liquids [37,40]. The range of isotopic data is therefore interpreted in terms of mantle heterogeneity. Part of the variation may be generated within the mantle through exchange with migrating fluids. Different sulphur-bearing species (COS, H,S, SO,) have recently been detected in small quantities in deep mantle fluid inclusions [41], and sulphur isotope fractionation existing, for example, between SO, and H,S at 1000” C (+ 2.5% [l]), could modify the 634S values of the rock sulphur. 7. Samples with high Ni contents 7. I. Origin of the Ni-rich globules These samples include three globules from MORB glasses, all the grains and globules from

151

the high-temperature peridotites from Lherz, Ronda and Beni-Boussera, the globule from the Salt Lake Crater eclogite, and three globules in a pyroxene megacryst from the Jagersfontein kimberlite (Table 1). The REE patterns of these peridotites suggest that they are representative of the mantle depleted after one or several episodes of partial melting [24,40]. The sulphide globules from these latter samples have 634S values ranging from - 3.2 to + 3.6%~ with a mode of +3%0 (Table 1, Fig. 2). MORB globules with high Ni contents also have high 634S values (Table l), but MORB globules with lower Ni contents nevertheless exist (Fig. 3). Sample ALV 5261 (Table 1) is very unusual since it contains large (= 500 pm in diameter) sulphide globules [19]. The REE patterns of MORB glasses from the same area are relatively flat or even slightly light-REE enriched and are not typical of normal MORB [42]. Peach and Mathez [43] have measured the Au/Ir ratio in the large and small globules and the glasses from this site. This ratio was found to be the same in the glasses and the small globules but significantly higher in ALV 5261. It is therefore unclear whether globules from ALV 5261 represent sulphide liquids equilibrated with the surrounding glass or an exotic component incorporated during the latest stages of magma evolution. Despite the continuing debate on the value of the partition coefficient of Ni between olivine and sulphide melts (see for example [44]), the sulphide globules with a high pentlandite content are commonly assumed to have equilibrated in the mantle with olivine. For example, high Ni content sulphides from the Kilboume Hole xenoliths (New Mexico) were only found in lherzolites, whilst pyroxemte sulphides are poorer in Ni [45]. Sulphide inclusions in diamonds are also richer in Ni when contained in peridotitic type diamonds in comparison with eclogitic type diamonds [46]. In the case of the Kilboume Hole sulphides, the high Ni contents are likely to represent sulphides left in the residual peridotites after partial melting of the mantle. Our high-Ni sulphides contained in peridotites have certainly the same origin. 7.2. Constraints

on the S34S value of the depleted

mantle, from the Ni-rich

sulphides

The present data indicate that the mean S34S

values of the immiscible sulphides in the “depleted mantle” are close to + 3%0 (Fig. 2). These values are different from those of MORB glasses, which are generally close to 0.4 + 0.5%0 [5]. Four main hypotheses can explain this difference. (1) The mantle is isotopically heterogeneous and the present set of samples is inadequate to characterize the ordinary upper mantle. The present samples are indeed not fully representative of the depleted oceanic lithosphere (Pyrenees, Ronda) nor of its melts (ALV 5261). Furthermore, sampling is potentially biased by the 30 pm minimum size required for accurate isotopic analyses with the ion probe. However, the systematically high 634S values of the present high-Ni globules strongly suggest that immiscible sulphides produced during partial melting of the mantle have a mean 634S value of + 3 + 1%0 (Fig. 2). (2) Sulphur isotopic disequilibrium exists on a small scale in the mantle: the sulphide globules with 634S = + 3%;0predate the partial melting event and remained in disequilibrium relative to silicate melts with 634S values of +0.4 k 0.5%0. However, Hofmann and Hart [48] have shown that Sr isotopic heterogeneity in the mantle cannot last for much longer than lo9 years in a fluid-free mantle (sub-continental mantle) and 10’ years in a partially molten mantle (oceanic mantle) and this conclusion is likely to hold broadly for sulphur. In fact, the diffusion coefficient of sulphur in a tholeiite melt is only slightly lower (5.8 X lo-’ cm2/s at 1200°C [49]) than that of Sr (lo-’ to lop6 cm2/s at 1300 o C [48]). Disequilibrium melting of the mantle is therefore unlikely on a small scale and, in most cases, sulphide globules are expected to be in isotopic equilibrium with their surrounding melts. The 634S values of the sulphide globules should only be controlled by the unknown fractionation between immiscible sulphide and sulphur dissolved in the liquid. (3) The 634S value of the depleted mantle is + 3 f 1%0, and either sulphide melt segregation or SO, outgassing drives the sulphur isotope composition of MORB liquids down to 0 + 1%0. Indeed sulphides and vent fluids from the East Pacific Rise (EPR) have mean 634S values between + 1.6 and + 5%0 (e.g. [50-521) and between + 3.2 and + 7.4%0 [51,52] respectively, but this is usually interpreted as resulting from the addition of a seawater component to the basalt-derived

152

sulphur. Exsolution of immiscible sulphides from a MORB liquid upon cooling and differentiation is probably not a suitable mechanism to decrease the 634S value of MORB from + 3 to 0%~ Sakai et al.‘s [5] data show that sulphur isotope fractionations of up to + 7.5%~ may exist in MORB between either reduced (AS2-_me,t= - 0.5%0) or oxidized ( A so; - _me,t= +7%0) dissolved species. Reduced sulphur, which is the dominant species (= 90%) dissolved in MORB, is therefore slightly 34S depleted as compared with the whole-rock value. Unfortunately experimental evidence for S isotope fractionation between a sulphide liquid and reduced sulphur dissolved in a silicate liquid is lacking and the isotopic fractionation between sulphide and basaltic melts cannot be predicted. Decreasing the 634S value of MORB from + 3 to 0 by the withdrawal of a proportion (f) of sulphur as sulphide demands that Aln(1 -f) = - 3, where A is the S-isotope fractionation factor between the coexisting liquids. For an assumed maximum f of 0.2, an unacceptably high A value of +13.4%0 is computed. On the other hand SO, is thought to be the predominant sulphur species which was degassed from MORB. A ASO,_me,,= + 1%~at 900 o C was estimated from ISLESmeasurements of basalts and volcanic gases at Hawaii [53]. Again, the change of the mean 634S value of MORB from +3 to O%Oby fractional degassing of SO, would imply that = 95% sulphur has been lost as SO,. This figure is inconsistent with both the uniformly high sulphur content of MORB (3 900 ppm S) and the implied sulphur content of = 18,000 ppm S for the source magmas. Furthermore, this outgassing would correspond to a flux of = 4.8 X lOI g/y of sulphur into the atmosphere, assuming a flux of = 56.1015 g/y of MORB and a mean sulphur content of MORB of 900 ppm. This quantity is 4-20 times the commonly accepted rate of volcanic sulphur discharge into the atmosphere 1541. (4) The 634S value of the depleted mantle before melting is 0 f 1%0 and globule sulphur is some 3%~ heavier than silicate melt sulphur. As discussed previously, the isotopic fractionation involved cannot be established on the basis of available experimental data. We favour this last hypothesis which is not incompatible with the assumed percentage of partial melting in the mantle as well as with the sulphur contents and the S34S

0

100

200

300

Sulphur content of the mantle to partial melting (ppm)

400 prior

Fig. 4. Calculated trends of sulphur contents of silicate liquids and restites, and of 634S values of the silicate liquids, produced during partial melting of a mantle source with varying sulphur contents and with a 13~~s value of +0.5%. A sulphur saturation level of 1000 ppm is assumed for the silicate liquid. The sulphide globules are assumed to stay in the restite and to have a 13~~s value of +3%~. The shaded fields represent the range of sulphur contents of the mantle coherent with sulphur saturated liquids having 634S values of 0*0.5%0 and sulphide globules with a S34S value of + 3%0 being present in the restite.

of the evolved liquids. In fact these high 634S values of sulphides left in the source of MORB add new constraints to the sulphur content and 634S of the liquids and restites produced by partial melting as a function of the initial sulphur content of the upper mantle. Assuming a 1000 ppm sulphur saturation level in the liquid, a mantle 634S value of + 0.5%0 and a fractionation of + 3%0 between exsolved sulphides and dissolved sulphur, the 634S values of the liquids have been calculated and represented in Fig. 4. The percentage of partial melting consistent with liquids being sulphur saturated at = 1000 ppm S and having a 634S value of 0 f 0.5%0, e.g. MORB (dotted zones in Fig. 4) depends on the initial sulphur content of the source. The lower the assumed percentage of partial melting, the lower must be the sulphur content in the mantle source: a fraction of 10% liquid with S34S = 0 f 0.5%0 demands a low sulphur content (= 100 ppm S), whereas 20% melting suggests 200-300 ppm S. The range of calculated melt fractions and mantle source S contents increase if the fractionation between the

153

exsolved sulphides and the dissolved creases below + 3%0.

sulphur

de-

8. Present-day sulphur isotope budget of the mantle aud crust The data from this study show that the majority of the sulphides within the mantle have slightly positive values, probably due to fractionation during partial melting and subduction of altered oceanic crust. Javoy et al. [55] emphasized the importance of sedimentary material recycled into the mantle for the carbon balance in the mantle-crust system, whereas Chaussidon et al. [lo] accounted for 634S values as high as + 9% in eclogitic diamonds by subduction of altered oceanic crust. The subduction hypothesis is also consistent with the 634S value of the sulphide globule from the Salt Lake Crater olivine-eclogite (Table 1). The mean positive 634S values of the mantle suggested by the present data prompt us to re-evaluate the sulphur budget in the crust, seawater and upper mantle system. This budget is mainly dependent on the sulphur content assumed for the upper mantle, since this reservoir is the largest of the system (Table 2). A sulphur content of 350 ppm has been proposed for the depleted mantle [58]. This value agrees more or less with those that can be estimated from MORB glasses, whose sulphur contents range between 800 and 1500 ppm [4]. If these liquids have been formed by lo-20% of partial melting, and if sulphur is incompatible, the MORB source should have contained at least 80-300 ppm of sulphur. It

is, however, unlikely that MORB extraction from the mantle will exhaust sulphur from the residual peridotite. For instance, type I and type II peridotites from the Kilbourne Hole have the same range of sulphur contents between 20 and 400 ppm S, hosted in globules [45]. Furthermore, the high sulphur content of ridge basalts suggests that, in spite of the widely acknowledged residual character of the MORB source, it still contains enough sulphur to saturate the melts. The constant Ce/Pb and to a lesser extent Mo/Pr ratios, observed in MORB, has been interpreted as strong evidence against the presence of sulphides in the residue of oceanic basalt melts [60]. However, even with large partition coefficients between sulphide and silicate liquids and reasonable amounts of melt (5-20%) the noise on the Ce/Pb (25 * 5) and Mo/Pr (0.1-0.4) ratios still allows a substantial fraction of sulphur to be left in the restite. For the mass balance calculation, a value of 300 ppm sulphur is assumed for the depleted mantle (Table 2). Taking into account the mean S34S value of MORB (+0.4 + 0.5%) and the present set of data on high-Ni sulphides, two extreme values of S34S of -0.1 and +1.5%0 are assumed for the upper mantle. The estimates for the masses, sulphur contents and 634S values of the other reservoirs are given in Table 2. We assume that a mantle, termed primitive (pm), with an unknown S34S value was initially the source of the sulphur held by the depleted upper mantle (dm), the oceanic crust (oc), the continental crust (cc) and seawater (SW) and that no sulphur is lost from the system (no back flow

TABLE 2 Sulphur contents and isotope compositions of the mantle, crust and seawater Reservoir Oceanic crust Seawater Depleted mantle Continental crust

Total mass a

Sulphur content b

Sulphur mass

w

(ppm)

(S)

0.5 x 10 *s 1.4x 1o24 1 x102’ 2 x10*5

loo0 915 300 780

5 1.28 3 1.56

x10*’ x 10” x10*3 x lo**

a b ’ d

Data Data Data 634S

-

_

from [56]. from 15,571. from [47,57], and mean for MORB from [5]. of ordinary chondrites [1,7].

_

c

(6)

1.5 0.4 93.2 4.8

+1 +21 + 0.4 +7 Mean

Ordinary chondrites

Ps

% of total sulphur

_

+0.8 0.2 + 0.2 d

154

of material to the lower mantle). The mass balance of the whole system is given by: (ws,,

x 6?Sdm) + (ws,

+ (ws,, x 6-%,,) = 100 x 6%

x 634S,)

+ (ws,,

x 63‘S,,)

Pm

The calculated 634Sp, values range from +0.3%0 for S34S,, = -0.1%~ to +1X%0 for S34S,, = +1.5%. Therefore if we want the S34Sp, to be similar to the S34S values of chondrites (+0.2 If: is needed from our 0.2%) a S34S,, of -0.1% mass balance equation. This last value is inconsistent with the present data and the S34S values of MORB. This implies either the existence of an unknown reservoir with negative S34S values hidden somewhere in the mantle-crust system, or that isotopic fractionation has occurred between the upper and the lower mantle or even the core. The S34S values of the continental crust for which a34Spm = +0.2% (Table 2) range between +4%0 (for 634S,, = -0.1%) and -27% (for 634S,, = + 1.5%). Unless a large crustal reservoir with negative S34S, such as sedimentary sulphides in accretion prisms, has been overlooked, this estimate is lower than any S34S value suggested for the continental crust, (+ 7%, Table 2). Moreover the S34S values obtained for the altered oceanic crust at Hole 504B, by whole-rock or in-situ analyses, range between - 4.3 and + 8.1% with a mean of + 3 + 1.5% [61]. The whole budget implies therefore that either the chondritic S34S is not the value of the mantle or a back-flow drags material with negative S34S down to the lower mantle and/or core. Oxygen, carbon and nitrogen isotopes tend to favor an enstatite chondrite type material for the Earth [62]. In the case of sulphur, however, the S34S values of the different meteorite types are very similar [7]. Furthermore the large difference between the mean sulphur content of enstatite chondrites (4.29 wt.% [3]) and that of the upper mantle strongly suggests important movements of sulphur in the mantle, such as degassing and/or sinking of sulphides. The assumption of a chondritic S34S value for the mantle is uncertain. A S34S value of = +0.5% for the primitive mantle, distinct from the meteoritic value (0.2%) is in accordance with the present budget and is therefore quite acceptable.

9. Conclusion All the measurements from this study plus those on sulphide inclusions in diamonds [12] demonstrate that sulphur isotope heterogeneity exists within the mantle (- 4 to + 8%). The mean S34S value (+ 3 5 1%) of the high-Ni sulphide globules suggests that an isotopic fractionation occurs during partial melting of the mantle and that the upper limit of the sulphur content of their mantle source is 300 ppm. Sulphur isotope mass balance between the upper mantle, the oceanic crust, the continental crust and seawater, requires either an unknown reservoir such as sedimentary sulphides, the lower mantle and/or the core, with negative S34S values or that the S34S of the primitive mantle source was different from the S34S of chondrites. A S34S value of +0.5% is therefore proposed for the primitive mantle. Acknowledgements The authors are indebted to C.W. Devey, B. McDonough, W.L. Griffin, S.E. Haggerty, J.-P. Lorand, E.A. Mathez, J.M. Montel, S.A. Morse, N. Shimizu, B. Skinner, and C.B. Smith for discussions and for providing the invaluable samples of this study. The great ability of J.C. Demange in repairing and modifying the ion probe was greatly appreciated. We are very grateful to A.E. Fallick, an anonymous reviewer and especially S.R. Boyd for their thoughtful comments on the manuscript. This work was was supported by an INSU grant of the DBT program (No. 88-38-30). References H.G. Thode, J. Monster and H.B. Dunford, Sulfur isotope geochemistry, Geochim. Cosmochim. Acta 25, 159-174, 1961. W. Ricke, Ein Beitrag zur Geochemie des Schwefels, Geochim. Cosmochim. Acta 21, 35-80, 1960. A. Schneider, Sulfur: abundance in cosmos, meteorites, tektites and lunar material, in: Handbook of Geochemistry, 16-C K.H. Wedepohl, ed., Springer-Verlag, Berlin, 1978. E.A. Mathez, Sulfur sohtbility and magmatic sulfides in submarine basalt glass, J. Geophys. Res. 81, 4269-4276, 1976. H. Sakai, D.J. Des Marais, A. Ueda and J.G. Moore, Concentrations and isotope ratios of carbon, nitrogen and sulphur in ocean floor basal& Geochim. Cosmochim. Acta 48, 2433-2441, 1984.

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