Sulphur isotope composition of orogenic spinel lherzolite massifs from Ariege (North-Eastern Pyrenees, France): An ion microprobe study

0016.7037/90/$3.00 + .OO

Geochrmicu d Cwnochimrca Acra Vol. 54, pp. 2835-2846 Copyright 0 1990 Pergamon Press pk. Printed in U.S.A.

Sulphur isotope composition of erogenic spine1 lherzolite massifs from Ariege (North-Eastern Pyrenees, France): An ion microprobe study MARC CHAUSSIDON’and JEAN-PIERRELORAND* ‘Centre de Recherches Pttrographiques et Geochimiques (CRPG-CNRS), 5 rue Notre Dame des Pauvres, B.P. 20, 5450 I Vandoeuvre-Es-Nancy CCdex, France ‘Laboratoire de Mintralogie du MusCum National d’Histoire Naturelle, Unit6 associke au CNRS URA n”736, 61 Rue Buffon, F-75005 Paris, France (Received

December

29, 1989; accepted in revised form August 3, 1990)

Abstract-The erogenic spine1 lherzolite massifs from Ariege (Northeastern Pyrenees, France), which represent tectonically emplaced fragments of the sub-continental upper mantle, are composed mainly of variously depleted peridotites. These rocks are crosscut by two generations of pyroxenites. The first is made up of layered pyroxenites, which are interpreted either as crystal segregates from Triassic continental tholeiites or as subducted parts of the oceanic crust re-injected within the upper mantle. The second consists of amphibole-rich dikes separated from Cretaceous alkali basalts. Forty sulphide grains, occurring either as inclusions within silicates or as interstitial grains, were investigated by ion microprobe for their sulphur isotopic compositions. The 634S values range between -5.7 t 1.5 and t4.5 f 1.5%0 with an average of -0.7%0, being very similar to the value of 0 f 2%0 commonly assumed for the upper mantle. However, significant differences in 634S values are observed between the different rock types. Negative 634S values (average = -3.2%0 for the massive peridotites) are discovered for the first time in mantle peridotites. In contrast, amphibole pyroxenites have positive 634S values (mean = +3.3%0), while layered pyroxenites show a wide range of variations from -3.1 f 1.5 to $4.5 ? 1.5%0. Peridotites adjacent to layered pyroxenite display 634S values ranging from -5.2 + 1.5%0 to +2.8 + 1.5%0, which reflect partial re-equilibration with the tholeiitic melt. Comparison between sulphide inclusions in silicates and interstitial sulphide grains strongly suggests that serpentinization and pyrenean metamorphism had no significant effect on the 634S values. Likewise, these values are broadly independent of the degree of partial melting. The negative 634Svalues of the massive peridotites could represent an ancient depletion event in the upper mantle. By contrast, the positive 634S values observed in the layered pyroxenites and the amphibole-rich dikes indicate that the two parent magmas had in common a mantle source variously enriched in 34S. Therefore, the present study reveals two extreme reservoirs characterized by different 634S values in the upper mantle (= -3 and = +3%0, respectively). This range of variations can explain most 634S values found in MORB. continental tholeiites, and alkali basalts. INTRODUCTION

SULPHURISOTOPICVARIATIONSare

to measure 634S values in a whole section of sub-continental mantle composed of different rock types, each displaying its original structural relationships and containing a mantle-derived sulphide phase (LORAND1989a,b,c,d). The present data directly characterize possible sulphur isotope heterogeneities of the upper-mantle at different scales and between different rocks. Such 634S variations have already been proposed from studies of mantle-derived lavas and xenoliths (e.g., UEDA and

used in metallogeny to characterize sources of sulphur for sulphide mineralizations (OHMOTO, 1987). Mantle-derived rocks, comparatively, have been rarely investigated for their sulphur isotope compositions (see, e.g., reviews by KYSER, 1986, and HOEFS, 1987) mainly because their sulphur content is very low (down to a few tens of ppm; MITCHELL and KEAYS, I98 1; HARMON et al., 1987; LORAND, 1989a). Hence, 634S values are difficult to measure in such S-poor rocks. Moreover, conventional 634Sanalyses always require chemical extraction of S from 10 to 25 g of rock-powder, which implies that a whole-rock 634S value is always an average of several sulphide grains. Furthermore, several secondary or late processes such as alteration or contamination during emplacement, might change the original 634S values of the rock. The ion probe technique can overcome these difficulties by making in situ analyses possible, either of sulphide grains from rocks very poor in S or of magmatic sulphide inclusions enclosed in silicate minerals and therefore protected from secondary alteration or contamination (CHAUSSIDONet al., 1987, 1989). The aim of the present work is to obtain a representative data base for the sulphur isotope composition in mantle peridotites and associated pyroxenites. The ion probe was used COmmOnly

SAKAI, 1984; KYSER, 1986; HARMON et al.. 1987; CHAUSSIDONet al., 1989). The spine1 lherzolite massifs from Ariege (North-Eastern Pyrenees, France) are especially suitable for this study for several reasons. ( 1) These massifs have been thoroughly studied with respect to their petrography, major and trace element geochemistry, radiogenic and stable isotope geochemistry

(AVE LALLEMANT,1967; CONQUERS, 1977,1978; POLVEand ALL~GRE, 1980; JAVOY, 1980; LOUBET and ALL~GRE, 1982; FABRIES and CONQUERS, 1983;

CONQU~~R~and FABRIES, 1984; BODIN~ERet al., 1987a,b, 1988; VETIL et al., 1988; HAMELINand ALL~GRE, 1988; FABRIESet al., 1989). (2) They recorded almost all the petrogenetic processes recognized elsewhere in the upper mantle. (3) They display the whole range of compositions (major and trace elements as well as isotope ratios) of ultramafic xenoliths found in continental 2835

M. Chaussidon and J-P. Lorand

2836

lavas, but with structural relationships preserved at the outcrop scale. (4) Peridotites are very fresh (less than 30% serpentinized) and match most of the xenoliths in basalts as regards serpentinization degrees. This point is crucial because sulphur isotopes may re-equilibrate significantly at low temperatures with serpentinizing fluids. (5) The origin and the subsolidus history of sulphides are well constrained (LORAND 1987, 1989a,b,c,d). Since the sulphide phase recorded partial melting, mantle metasomatism, and metamorphic reactions in the crust, special attention was paid to the likely exchanges of S during these processes by analysing in particular all the sulphide occurrences (unfractured inclusions, fractured inclusions and interstitial grains). In addition, difficulties in ion probe analysis arise from the size (diameter down to 20 pm) and the mineralogy of the sulphide grains (up to 5 different major phases per sulphide grain). For each analysis, instrumental mass discrimination was calculated by taking into account both petrography and Ni, Fe, Cu contents of the analyzed sulphide. Summary of the Petrogenesis Peridotite Massifs

of the Eastern Pyrenean

The Eastern Pyrenean peridotite massifs outcrop within a small band of metamorphosed mid-Jurassic limestones along the North Pyrenean fault (Fig. I). They are considered to be fragments of the sub-continental upper-mantle emplaced tectonically within the continental crust during the Middle Cretaceous counter-clockwise motion of the Iberian plate with respect to the stable Europe (KORNPROBST and VIELZEUF,1984). Although the Etang de Lherz massif is one of the best known peridotite massifs, it is in fact only one of several ultramafic bodies found in the Eastern Pyrenees. Some are over 1 km in diameter while others are much smaller (e.g., Sem, 5 X 5 m). All of the massifs are composed of spine1 lherzolites with rare bands of spine1 websterites (cm size) which define a layering. The peridotites and the pyroxenites show a foliation plane parallel to the layering. The rocks studied here can be divided into four main types: (1) the massiveperidotites which range from depleted harzburgites to fertile spine1 Iherzolites, (2) the interlayered peridotites which correspond to small bands of lherzolites intercalated within pyroxenite layers, (3) the layered pyroxenites which correspond to layers of websterite or clinopyroxenite within the lherzolites and (4) the amphibole rich pyroxenites veins which crosscut all the three previous rock types. The complex pre-emplacement history of the Eastern Pyrenean peridotite massifs is summarized in Fig. 2. The earliest event is diapiric

uprise and partial melting which occurred in the garnet lherzolite stability field (-75 km depth). The layered pyroxenites are interpreted according to two different models. (I) In the first hypothesis, they would represent high-pressure, high-temperature crystal segregates separated from LREE-enriched tholeiitic magmas chemically akin to the Triassic continental tholeiites erupted along the Pyrenees (BODINIERet al., 1987a). These melts intruded the peridotites which formed the basis of the subcontinental lithosphere during the early Mesozoi’c. The thin peridotites interlayered within the pyroxenite bands are distinctly enriched in clinopyroxene, spinel, and Fe, Mn, Co, Al, Ca, Ti. SC,Hf, and HREEs, indicating melt infiltration from the surrounding pyroxenites (BODINIERet al., 1988). (2) The second hypothesis refers to the “marble cake mantle” model in which the layered pyroxenites would be either a basaltic or cumulative part of the oceanic crust re-injected by subduction into the mantle and stretched within Iherzolites, variously depleted after oceanic crust extraction, by solid-state mixing in the mantle (e.g., POLVEand ALL&GRE,1980; ALL~GREand TURCOTTE,1986; HAMEIJN and ALL&RE, 1988). After the accretion into the sub-continental lithosphere, both the peridotites and the layered pyroxenites cooled isobarically within the spine1 stability field down to temperatures of about 900°C for 1215kb pressure (40-45 km). Accessory Ti-pargasite (O-2% by volume) crystallized in almost all the peridotites prior to the end of this thermal relaxation (Fig. 2). Migration of alkali basalts in the lherzolites through vein conduits led to the flow crystallization of the amphibole-rich veins crosscutting the layered sequence (BODINIER et al., 1987b). Ages obtained for amphiboles from several pyroxenites suggest that these melts were emplaced about 100 Ma and are therefore probably related to the middle cretaceous alkali magmatism of the Pyrenees (BODINIERet al., 1987b; FABRIESet al., 1989). According to structural and petrological constraints, the amphibole pyroxenites veins were emplaced at depths of 30-40 km, i.e. in a cold thinned lithospheric uppermantle, a few km below the Moho (VETIL et al., 1988). Their emplacement was contemporaneous with a second episode of deformation and syn-kinematic recrystallization (D2, R2) related to the tectonic introduction of upper mantle slices into the lower crust (CONQUERSand FABRIES,1984) (Fig. 2). Both the lithospheric deformation and infiltration of alkali basalt liquids have produced significant changes in the chemistry and mineralogy of the peridotites (therefore increasing small scale heterogeneities in the upper-mantle). Infiltration of alkali basalts is responsible for minor modal metasomatism of the wall-rock peridotites (Ti-pargasite + ilmenite), which suggests that the alkali basaltic magmas infiltrated along silicate grains boundaries up to a few cm from the contact. Changes in the chemical composition of the peridotites are especially marked by LREE enrichments (BODINIERet al.. 1988). Later, the upper-mantle slices emplaced within small oceanic basins 90-95 Ma, during the high-temperature, low-pressure Pyrenean metamorphism (GOLDBERGet al., 1986) (Fig. 2). The most obvious

1

50

IL

km

FIG. I. Structural sketch map of the Pyrenees (modified from CHOUKROUNEand SEGURET, 1973). Dotted area indicates Hercynian and older materials (North Pyrenean Massifs and the Paleozoic Axial Zone). The studied peridotites massifs are located at the center of the black square. Oblique ruling indicates Mesozoic sediments of the North Pyrenean Zone and equivalents. The area shown in black is the North Pyrenean Metamorphic Zone. SPU = South Pyrenean Units; PAZ = Paleozoic Axial Zone; NPF = North Pyrenean Fault: NPFI’ = North Pyrenean Frontal Thrust.

Isotope composition

Eruptron of contrental thole~tes related to the triassic opening of Atfanbc ocean. [pyrenean ophites)

2837

of sulfur in the upper mantle

the

d,stenoon along Norfh Pyrenean opening of small oceamc baens Erupbon

of mrddle cretaceous

a/k&

Fault basafts

Pyrenean metamorphism

Teriiary

compressive

of the pyrenean

stages orogeny

FIG.2. Schematic history of the North-Eastern Pyrenees peridotites massifs from this study. The successive stages of deformation (Dl, D2) and of recrystallization (RI, R2) are only approximately delineated (vertical exageration). The temperatures indicated in italic are those of the peridotites.

changes are carbonate veins invading the periphery of the peridotite massifs, probably due to exchanges with fluids containing COZ, Cl, and perhaps S, as shown by the presence of scapolite (e.g., Caussou and Lherz, CONQU~RB, 1978). Subsequently, the massifs underwent slight serpentinization, but mostly at the periphery of the massifs and along fracture planes, thus preserving large areas nearly free of serpentine. EXPERIMENTAL Selection

of Samples

Ten samples representing all the different rock types present in the peridotites massifs from Ariege were selected for ion microprobe determination of sulphur isotopic compositions. In order to minimize crustal contaminations, they were picked up at distance from any contact with the country rocks and from the serpentinized fault planes which crosscut the largest massifs. A brief description (origin, size, type, mineralogy, alteration, sulphide mineralogy, reference of previous works .) of each sample, some of which are reference samples previously studied for petrogenetic purposes, is given in the Appendix 1. Description

and Petrogenesis

of the Sulphides

Sulphides are present in all of the peridotites and pyroxenites investigated in the Eastern Pyrenean peridotite massifs. Their mineralogy and petrology, as well as whole-rock distribution of S, have been thoroughly discussed in separate papers (LORAND, 1987, 1989a,b,c,d). Hence, only a brief summary of the main results is given below. The sulphides are more abundant in the pyroxenites (O. l-0.7% by volume) than in the peridotites (0.01-O. 1% by volume), due to different origins and conditions of formation. In the peridotites, the relative proportions of sulphides increase from the harzburgites to

the lherzolites (see Table 1). Sulphides, ranging in size from 30 pm to 1 mm, occur either as fractured or unfractured isolated inclusions trapped in silicates, or as interstitial grains among the silicates. The sulphide phase is composed of four main primary sulphides: pentlandite, pyrite, pyrrhotite, and chalcopyrite (in order of decreasing abundance). Pentlandite and pyrite form complex symplectic intergrowths resulting from low-temperature breakdown of Monosulphide Solid Solution (MSS). Sulphide inclusions and interstitial grains are also observed in both generations of pyroxenites but with a considerable increase in sulphide grain size. In the layered pyroxenites, the primary sulphide assemblages observed in unfractured inclusions are composed of nickeliferous pyrrhotite + N&rich pentlandite + chalcopyrite and minor pyrite, whereas the amphibole-rich pyroxenites contain mainly Ni-poor pyrrhotite, coexisting with minor amounts of pentlandite and chalcopyrite. Interstitial sulphide grains in peridotites and pyroxenites are generally composed of the same primary sulphide assemblage as the included sulphides. However, these grains behaved as an open system with respect to serpentinization-related hydrothermal fluids; hence, their primary sulphide assemblage is variously altered into secondary phases (e.g., troilite, mackinawite). The evolution of sulphides in response to serpentinization will be discussed later in conjunction with a3“S values. The sulphides in the peridotites, remote from the pyroxenites, represent the sulphide fraction which survived partial melting and was trapped as a Ni-rich, &-poor (2-4 wt%) immiscible liquid in the solid peridotite residue. A part of this liquid has been included in the silicates during the stage of plastic deformation DI-Rl (Fig. 2). The sulphides solidified as MSS + ISS (cupriferous intermediate solid solution) during the isobaric cooling stage between 1200 and 900°C. Afterwards, MSS and ISS decomposed at subsolidus temperature into the presently observed sulphides. The bulk Fe/Ni ratios recalculated from modal proportions and chemical compositions of the sulphides are rather homogeneous (0.6 < Fe/Ni < 0.8) which suggests a strong chemical control at subsolidus temperatures of the dominant silicate matrix on the sulphide microphases (LORAND,1987).

2838

M. Chaussidon and J-P. Lorand

In the two generations of pyroxenites, the sulphide phases separated as immiscible sulphide liquids from the basaltic melts injected in the peridotites (LORAND, 1989c,d). This interpretation is supported by the much higher Fe/Ni ratios (0.8- 1) of the presently observed sulphides and by the abundance of isolated sulphide inclusions in the silicates which segregated at liquidus temperatures from the parent basaltic meits. Once separated, the sulphides in the pyroxenites have cooled under the thermal regime imposed by the surrounding pyroxenites. Because of the high Fe/Ni ratios, the incipient solidification temperatures are probably around 1200- 1250°C (LORAND 1989c,d), although the accuracy of this value may depend on the presence of other fluid species, mainly O2 and/or COZ (LORAND1989d). Cu/Ni + Fe and Fe/Ni ratios in bulk-sulphide compositions vary significantly among the inclusions, probably due lo strain-induced remobilization of the sulphide liquids at tempemtures close to 900°C. The sulphide phase of both the peridotites and the pyroxenites was completely solidified during the lithospheric shearing deformation and synkinematic recrystallization (D2, R2) (Fig. 2). This latter event is responsible for the fracturation of some inclusions and for the local break up and mechanical dispersion of the largest interstitial sulphide grains into the fine-grained silicate matrix. Because some silicate grains recrystallized totally, it seems likely that an unknown number of sulphide inclusions were also expelled in the interstitial pores of the rocks. This could have increased the relative proportions of interstitial sulphide grains relative to inclusions. Analytical Conditions Sulphur isotopic ratios were obtained with a modified Cameca IMS3fion microprobe at the CRPG (Nancy, France). Sulphides were analysed on polished sections, which were coated with gold to minimize charging problems during the analyses (always less than 5 V after 45 min sputtering). The ion probe was operated with a O- primary beam at 10 kV accelerating voltage. The intensity of the Obeam was adjusted between 20 and 60 nA for a Econstant signal on S during aging of the duoplasmatron source between each cleaning. Under such conditions the diameter of the primary beam ranges between IO and 30 pm. Positive secondary ions were accelerated at +4.5 kV and analysed without energy filtering at a mass resolution (M/a) of 3500. Therefore possible interferences of “SH- ions with 34S- ions were resolved. Peaks were measured with an electron multiplier operating in counting mode. After 45 mn counting, the statistics on the ratio 34S/32Sis generally on the order of kO.5 to rrt1%OO for very small samples (-20 pm size). Measured isotopic ratios have to be corrected for a large instrumental mass fractionation A,,, (with A,., = d34SmeasUred - d3’S,,) which has been shown to be matrix dependent (DELOULEet al., 1986; ELDRIDGEet al., 1987; CHALISSI~XNand DEMANGE,1988). Values of Ainstare continuously checked between sample analyses against standard sulphides of various 634Svalues. Ain* depends mainly on fine settings of the machine and on aging of the duoplasmatron source (few permil shift). Average Ainuvalues during this study were -5O.OL for py~hotite, -32.4k for pentlandite, -44X& for chalcopyrite, and -39.9%” for pyrite. Variations of din., are not fuliy understood but there is for instance a strong positive correlation between A,.,, and the Ni/Fe ratio of the sulphide (Fig. 3). Therefore Ainrtis constant within ?0.5%0 for a given Ni/Fe ratio. Since we are dealing with polyphase sulphide grains, bulk Aina values depend upon the respective proportions of the four main sulphide phases in each probed area. For example, Ai,,=varies around -40 & IO?&for py~te-py~hotite-pentiandite assemblages. An uncertainty of +lO% on the proportion of these three sulphides wilf therefore yield an uncertainty of Ltl%o when determining the Ainst value. Modal proportions of sulphides have been calculated from ion microprobe determinations of Ni, Cu, and Fe contents of each analysed grain, coupled with Ni/Fe ratios of pentlandite previously measured by electron microprobe. Nickel, iron, and copper contents were obtained using a primary beam of O- with the same intensity as For the &MSanalyses, after closing of the energy slit in order to get the Fe signal measurable on the electron multiplier. A previous procedure (CHAUSSIDONet al., 1989) used a neutral primary beam for measurements of Ni, Cu, and Fe contents, but in some cases (e.g., small sulphide grains or charging samples) this procedure is not ad-

1

A i,,st= - 37.47 + 6.94 x Log,,,(NiiFe,I R=l.OO

-60

Log,,,WFesulphide1 FIG. 3. Example of variation of the instrumental mass fractionation A (A = 63?&,-d - S3”Si&of sulphur isotopes during ion microprobe m~surements of sulphides from the Fe-Ni-S system. The large range of variations of the Ni/Fe ratios found for some pentlandites from this study (between 0.7 1 and 1.33) creates a range of variations in A of =2%0. This was taken into account for the correction of the 6’4S measurements.

equate because the neutral beam does not sputter at exactly the same spot as the charged beam (see (4) in Table I for sample 79-60). However, if present in the sulphide assemblage, pyrite cannot be estimated by the present correction procedure. Pyrite contents have been estimated (I) from SEM image analysis and bulk sulphur contents, and (2) from detailed observation of the sample cratered at high magnifi~tion under the microscope. For very small sulphide intergrowths, this part of the correction is the most delicate since ion probe data represent a 2-5 pm depth-analyzed volume while electron probe or image electron analyses data correspond to the whole surface of the intergrowth. However, the error caused on the calculation of the bulk Ainn by the uncertainty of the pyrite content is moderate since pyrite has a Ainstvalue intermediate between pyrrhotite and pentlandite. Finally, all analyses for which the ion probe crater was observed to sweep across the nei~~u~ng silicate were systematically discarded since this may induce a defocussing of the secondary beam at high mass resolution and therefore a shift of the isotopic ratios. Sulphur isotopic measurements are given in 6?S notation versus the CDT standard (with 34S/32Scor= 0.044994 after THODEet al. 196 I). The reproducibihty of h”S measurements on homogeneous standards is generally on the order of +0.8L (+lu). Estimates of the relative percentages of each sulphide are valid within tlO% in the worst cases (small intergrowths between various sulphides). Therefore, the present d”S values are considered to be accurate within f 1.5%(t 1a), RESULTS

The 634Svalues of the Eastern Pyrenean peridotite massifs range between -5.7 i 1.5 and t4.5 rf; 1.5%0{Table I). The 634Svalue averaged on all ofthe 40 measurements is -0.7%0. However, this average must be taken cautiously since different rock types having different sulphur contents are considered. Taking into account the analytical uncertainty of the method (rtl.S%o) the value of -0.7%~ only indicates that the mean 634S value of the sub-continental upper mantle now represented by the erogenic-type spine1 lherzolite from Ariege is close to O%O,the mean value commonly assumed for the upper-mantle (e.g., THODE et al., 196 1; OHMOTO, 1987; KYSER, 1986).

2839

Isotope composition of sulfur in the upper mantle Table

1: Ion microprobe

5%

analyses of the sulphldes from onalysrd

Sample

Sulphur

Petrography

content Massive

several Eastern sulphidc

size

type

Pyr6nk

perldotites

and pyroxenites: ton

grorn

(ppm)

probe

sulphide

Fe/N1

area

IS%

assemblage

pentlandite

analysed

(-+1.5%CDT)

peridotites:

12.442

harzbqite

70.355

23

Cpx-poor Lhcrwlite

100

unhactud inclusion in olivme

5Ox30pm

0.75

intersititial grain

8OxSOpm

0.77

70Pn30Cp

-2.4

0.77

83Pn8Cp9Po

-3.7

75PnZSPy

-3.0

intersibtial 79.61

Ihermlite

Interlayered

260

gzam

-4.3f2

ZooX5Opm

Pn+Cp+Poll

unfrauured inclusion in olivme

1WW

PWPY22CP3(”

0.93

lmfrac.mred inclusion in Opx

7ox3ojlm

Pn2SP?+?‘8Cp42(‘)

0.93

42PnSPy33CpZOPo

-2.7

flacnued inclusion in Cpx

wx90pm

0.90

60Pn25Cpl5Po

-2 1

0.90

6SPn20PolOCpWy

-5.7

fractured . inclusion in olivine

1ooX2Opll

interstitizJ grain

5OFm

0.90

8OPnl scpspy

-3.8

Pn+Cp+Po

1.14

8OPnlOPolccp

-0.8

PVOTW

1.10

Id(*)

-1.2

5Ow

1.10

id(Z)

-1.1

interstitial grain interstitial grain

200pm

1.10 1.10

interstitial grain

8Wx2CQtm

1.10

perldotites:

79.60 c4)

Cpx-rich

500

lherrolite

fractured inclusion in olivine inter&&l

lwm

-1.4(‘)

grain

-5.2 -2.4

83Pnl6Cp WPn10Po

-0.8 -3.2 -1.4 +2.4 +2.8

PIl PO PO CP 50PnSOCp

anofysrd

Petrography

Sample

Sulphur

sulphide

s1z.e

type

ton probe

grain

content layered

.n.lysts

sulphlde

Fe/N1

assemblage

pentlandite

area snalysed (in %)

annlysrs

S”4S (*1.%%DT)

pyroxenites:

72.283

spine1 websterile

>I000

interstitial grain

lmmx4Qm

interstitial gmin

3COxZWpm

Pn+Py

0.75

Pll 90PnlOPy 8OPn2OPy

-I 8 -0.1 -3.9

Pn+Py+Cp

0.75

PY

-10.1 -10.1(3)

72.50

spinel+secondary garnet taring webstelite

3W

interstitial grain

ZOOx300~m

interstitial grain

l35x250km

interstitial grain

8COx3OOpm

Pn(+Py+Po”)

1.28

WPnlOTr WPnlOTr Pll 95Pn5Tr

-2.6 -3.8 +I .9 -1.7

mterstitial grain

2OOpm

Pn+Tr

1.19

76Pn24Tr

-1.5

intnstitial

5COpm

Pn+Cp

1 02

PIl 9lPn9Cp

+1.7 +3.0

0.89

95Pn5Po

-3.3

1.34

OOTr

Pn+Tr+Cp(+Mw)

Pn+Tr+Cp(+Py+PdI)

I 28

I .28

PO 82Pnl8Cp 8lPnl9Cp PII

+1.2 +2.3 +2.8 -4 -3.7w

80.166

spinel+sez.ondarygamec bearing webstmite

81.17

garnet clinopYmxenite

Amphibole-rich 70.257

340

interstitial

grain

amphitulega&and ohnne

II50

850

clinopynxenite

fractd inclusion in amphibole

lC@m

interstitial grain

3COxlWpm

measurements

Tr9OPnio(+Mw)

+I

Tr+Cp+Pn(+Mw)

1.40

I CQTr

+3.7

unhactwed inclusion in garnet (5)

<6Opm

Po(+Pn+Cp)

0.95

IoOTr

+3.1 +3 60)

interstitial gnin(6)

2Wx4OOpm

Tr(+Cp)

1CQTr

+4.4 +3.0(3)

mterstitial grain

70xl00&ml

Pd’

I CQPon

+4.5

interstitial grain

4Ox100~rn

PO

IOQPO

+3 6

Pn=pentlandite. Py=pyite. Po-pynhotite. Pon=wxndq pynholite. (I): quantimef image analyzer (B. R. G. M. GrlCans. France). W whole grain analysis. W: duplicate analysis afler qolisbing. c4): lour 8%

Pn@d'5

‘n

pyroxenitcs:

garnet-pvorolivineand amphibole baling alitgitc

70.184

410

of this sample were previously

repaied

Cp=chalwpyrite,

in CIlAUSSIDON

Tr=tmilite.

Mw=ma&inavite.

et a1.(1989). However

microscopic

revealed a problem in !he calibration of the Ni contents. and the previous 8% values arc lhcrcfore no more valuable. Tk been obtained after a “ew polishin of the sample and accordmg IO the analytical techniques described here. c5): named 70-1841112-4 grain in CHAUSSIDON e( al.(1989). @): from CILAUSSIDON et aL(l989).

exammation

of thus sample

present mea~u~~,,,~~ts have

_

M. Chaussidon and J-P. Lorand

2840

However, when considered in more detail, the mean 634S values show significant differences between the four rock types (Fig. 4). The massive peridotites are characterized by negative 634S values (from -5.7 to -0.8%0, average: -3.2%0), amphibole pyroxenites by positive values (from +l to +4.5%0, average: +3.3%0), and layered pyroxenites by intermediate values (from -4 to +3%0, average: -0.8%0). No significant difference in 634S values has been found within the massive peridotites between unfractured inclusions (= -3.3%0), fractured inclusions (= -3.9%0), and interstitial sulphides (- -2.3%0). The same is true for the interlayered peridotite sample (79-60) (fractured inclusion = - 1.3%0 and interstitial grains = - 1.1 o/00). However, this sample shows the largest range of 634S variations of all the samples studied (between -5.2 + 1.5 and +2.8 * 1.5%0for the interstitial grains) which overlaps that of layered pyroxenites. No definite relationship is observed between the mineralogy of the sulphide assemblage analyzed and its 634S value. Two analyses of chalcopyrite from a complex sulphide assemblage (sample 79-60) yield 634S values (+2.4 and +2.8 f 1.5%0) very different from those of adjacent pyrrhotite (- 1.4f 1.5%0) or pentlandite (-0.8 + 1.5%0).This is at variance with known sulphur isotopic fractionations between chalcopyrite and either pyrrhotite or pentlandite which predict lower 634Svalues in chalcopyrite than in the other sulphides (BACHINSKI, 1969; KAJIWARA and KROUSE, 197 I). Such isotopic shifts of a few

q q

massive peridotites

, errors ,

cl= pJmp!gJm 0 , , . I r I,

Interlayered

I +5

peridotites 0

cl f-J

r)nFgf-j

I,

I

I

I

I,

layered pyroxenites

I

I

I

+5

0 q q cl0 0000

I,

I 0

-5

m a I -5

amphibole

I I I 0

-5

rich pyroxenites

0 00

un Iqm a I,

I, 0

+5 R

FIG. 4. Variations of the @?Smeasurements from this study within each rock type and between the different rock types. The massive peridotites and the amphibole-rich pyroxenites are homogeneous in @“Swithin the analytical uncertainty. Mean 634Svalues increase from the massive peridotites (-3.2%0) to the layered pyroxenites (-O.S%o) and the amphibole-rich pyroxenites (+3.3%0).

permil for chalcopyrite are not fully understood but have also been observed in a Sudburry ore deposit (NALDRETT, 1981) and in the Stillwater complex (ZIENTEK and RIPLEY. 1988). In both cases the high 634S values of chalcopyrite were inferred to result indirectly from late crystallization of ISS from sulphide liquids. DISCUSSION Crustal Processes: Serpentinization Metamorphism

and Pyrenean

Because of the long and complex history (Fig. 2) of the Ariege peridotites, it is very likely that sulphides, and therefore 634S values, have been modified by secondary crustal processes. For instance, external sulphur could have been introduced by serpentinization and/or pyrenean metamorphism. One possible source of sulphur is the scapolite-bearing Mesozo’ic limestones. However, field observations, and petrographic, chemical, and isotopic constraints argue against this hypothesis. (1) Special care was taken to analyse samples collected remotely from the contact with the country rocks and serpentinized fault planes. Likewise, field, macroscopic or microscopic evidence of introduction of exotic sulphur in the studied samples, after the plastic deformation, is lacking (LORAND, 1989a,b). Pyrite is systematically of primary origin. It is present within the unfractured sulphide inclusions in silicates, which have been prevented from any contamination since the plastic deformation D 1-R 1. Furthermore, pyrite is intimately intergrown with pentlandite, suggesting that these two sulphides co-crystallized from the MSS. (2) The peridotites display a good correlation between their degree of depletion in incompatible elements and their wholerock S contents (LORAND, 1989a). Harzburgites have low S contents (e.g., 23 ppm for sample 72-442; Table 1) while fertile Iherzolites have S contents (e.g.. sample 79-61 with 260 ppm S) close to recent estimates proposed for the convecting asthenospheric upper-mantle (x250-300 ppm S; S. S. SUN pers. comm., 1989). Thus, the S contents of the peridotites have not significantly changed after partial melting. (3) If crustal fluids had infiltrated the peridotites, then they would have primarily contaminated the interstitial sulphide grains. Because the different sulphide occurrences (inclusions, interstitial grains) are not significantly different with regards to their 634S values, the peridotites are considered to have evolved as an isotopically closed system for S. (4) The three massive peridotites have similar 634S values in spite of different degrees of serpentinization (4% for sample 79-6 1, 8% for sample 70-355, 20% for sample 72-442, Appendix 1, Table 1, Fig. 4). Likewise, ABRAJANO and PASTERIS (1989) did not detect significant 634S variations in the sulphides from the serpentinized Zambales (Philippines) dunites. These two independant sets of data suggest only a restricted and localized remobilization of S during lizardite-producing, pseudomorphic serpentinization. However, one 634Svalue of - 10.1 %Ohas been found in a single pyrite grain from sample 72-283 (Table 1). Such a negative a3‘?j value is not yet understood but is significantly different from the only two values available so far on serpentinized lherzolite (+20.6%0) and

2841

Isotope composition of sulfur in the upper mantle pure

serpentinite

(+23.8%0)

(GRINENKO and UKHANOV,

1977). The possible effect of the lizardite-producing serpentinization on 634Svalues can be evaluated semi-quantitatively. Unlike the unfractured inclusions in the peridotites composed of a primary sulphide assemblage, the parageneses of the fractured inclusions and the interstitial grains vary as a function of the degree of serpentinization (LORAND, 1989b,c,d). In the least serpentinized samples (e.g., 79-60,79-6 l), primary pyrrhotite, which represents ~5 ~01%of the sulphide assemblage, has a composition of troilite (FeS). This change in pyrrhotite composition corresponds to a S loss of = 1.5 ppm, assuming 300 ppm S in the original whole rock. In the case of the most serpentinized samples (e.g., 70-355), the loss of S was more important since, in addition to the appearance of troilite, primary pyrite (-25 ~01% Fe& in the sulphide assemblage) is replaced by a secondary pyrrhotite (Fe&). This yielded a decrease of = 11 ppm S from an initial wholerock S content of ~300 ppm S. However, the loss of S could have been reduced by the transformation of pentlandite into mackinawite, which would have yielded a S gain. Assuming an isotopic fractionation factor between pyrite and H2S of t-2%0 at 2OO’C (OHMOTO and RYE, 1979) and a Rayleigh distillation model, a loss of 11 ppm S as H2S over 300 ppm would have only induced an increase of less than +O. 1%Oin the 634Svalue ofthe sulphide phase. Such an increase probably represents an upper limit for the serpentinization-related shift of the 634S values inasmuch as isotopic fractionation between pyrrhotite and H2S is much smaller than that of pyrite (Z +0.5%0 at 200°C; KAJIWARA and KROUSE, 1971; OHMOTO and RYE, 1979). These shifts are well within the analytical uncertainties of the ion probe technique and are therefore considered to have no significant influence on mantle-derived 634S values.

Mantle Metasomatism Several stages of metasomatism (e.g. BODINIERet al., 1988; FABRIESet al., 1989) affected the peridotites in the sub-continental lithosphere at elevated temperatures (>9OO”C) (Fig. 2). Asthenospheric 634Svalues may have significantly changed because a part of the sulphide component was still partially molten and S might have been present in metasomatic fluids (e.g. LORAND et al., 1989). The oldest metasomatic process is related to the introduction of the S-rich layered pyroxenites into the S-poor massive peridotites. As for the silicates (enrichment in spine1 and Cpx) there is some evidence of re-equilibration of the sulphide phases in the thin peridotites interlayered within the pyroxenite banded series. As a general rule the bulk Fe/Ni ratio of the sulphide phase increases while whole-rock S content considerably increases (e.g., up to 500 ppm S in sample 79-60, Table 1). This increase is attributed to infiltration of sulphide liquids into the lherzolites from the injected tholeiitic melts parent to the pyroxenite layers (LORAND, 1989a). The latter magmas are likely to have experienced very early S saturation since the silicate megacrysts in the pyroxenites contain numerous sulphide inclusions (LORAND, 1989~). The infiltration

of such S-saturated melts can account for the 634Svalues of sample 79-60 which are intermediate between 634Svalues of

massive

peridotites

and 634S values of layered pyroxenites shows that a 634S value of - 1.2%0 (similar to that of sample 79-60) may be obtained by adding ~650 ppm S (with a 634S = -0.8%0, similar to the mean value of layered pyroxenites) to an unmetasomatized lherzolite such as sample 79-6 1 (260 ppm S and mean 634S = -2.2%0). Taking into account the accuracy of the 634S measurements (?1.5%0) this value of 650 ppm drawn from sulphur isotopic ratios is of the same order of magnitude as the difference in S content between unmetasomatized samples (between 20 and ~300 ppm S) and the metasomatized ones (e.g., 500 ppm S for sample 79-60). The second metasomatic episode is related to the injection of the Cretaceous alkali basalts parent to the amphibole-rich veins. The peridotite wall rocks enclosing the amphibole pyroxenites re-equilibrated with the magma with regards to LREEs, Ti, and Fe (BODINIER et al., 1988). Sulphur transfer would be expected from the amphibole pyroxenites to the peridotites because of the differences in S contents between the two rock types (1000 vs. 20-300 ppm S; LORAND,

(Fig. 4). A simple mass balance calculation

1989a,d; Table 1). However, compared to unmetasomatized massive peridotites, neither the S content nor the 634Svalues have changed in sample 72-442, which comes from the immediate vicinity of a hornblendite dike (Appendix 1; Table 1). This lack of variation strongly suggests that asthenospheric 634S values of the massive peridotites were not significantly contaminated by the Cretaceous alkali basalts.

Comparison with 634S of Mantle-derived

Rocks

Peridotites The negative 634S values (mean -3.2 + 1.5%0) reported here are the first found for mantle peridotites. Actually, literature data are scarce but systematically positive. GRINENKO and UKHANOV ( 1977) reported 634S values ranging between +0.4 and +4.3%0, and SCHNEIDER (1970) reported a value of +3.3%0 for a basalt-hosted peridotite xenolith from West Germany. However, these previous values have been obtained by a whole-rock analysis technique which has the disadvantage of averaging 634S values of all the sulphides (interstitial grains or inclusions). Negative 634S values in mantle peridotites are of great importance for the budget of sulphur isotopes in the system upper-mantle + crust + seawater. Mean 634S values between 0 and +3%0 have been proposed for the whole system (e.g., AULT and KULP, 1959; THODE et al., 1961; HOLSER and KAPLAN, 1966). This budget is complicated because the sulphur isotopic composition of the upper mantle, which holds ~93% of the whole S of the system, is estimated only from mantle-derived magmas (634S values of 0 t 3%0; OHMOTO, 1987). Previous ion microprobe investigations of various sulphides from mantle origin failed to find any significant negative 634S reservoir (CHAUSSIDON et al., 1987, 1989). These authors thus suggested for the whole system a mean positive 634S value (+0.8%0) slightly different from that of meteorites (0.2 * 0.2%0; THODE et al., 1961). This discrepancy may be accounted for by assuming an unknown reservoir, with negative 634Svalues in the upper-mantle or the crust. The Eastern Pyrenean peridotites are probably proof of the existence of

such a reservoir.

2842

M. Chaussidon and J-P. Lorand

Because the origin of the layered pyroxenites, whether magmatic segregates from tholeiitic magmas or recycled oceanic crust, is still debated (see the summary of the petrogenesis of the peridotites massifs), our 6%Svalues of layered pyroxenites (ranging between -4 * 1.5 and C3 t 1.5%0,Table 1) have to be compared with whole-rock 634Sof both continental tholeiites and of fresh and altered oceanic crust (Fig. 5). Continental tholeiites range in 634Svalues between -3 and + 1.4%0(SCHNEIDER,1970), fresh glassy MORB between -0.6 and t-1.6%0 (KANEHIRA et al., 1973; GRINENKO and UKHANOVA, 1977; SAKAI et al., 1984), and sulphide globules from MORB between - I.5 +: I .5 and +3.8 + 1.5%0(CHAUSSIDON etal., 1989). Average 634Sof the altered oceanic crust is more difficult to estimate, mainly because most information on the oceanic crust at depth comes from only one hole (DSDP/ODP 5048) in the Pacific Ocean. There, whole-rock 634Svalues vary from - 1.8%0(with a large spread in the values) in the 600 m thick pillows to $3 -+ 1.2%~in the sulphurenriched transition zone (-300 m thick); they then fall to +0.6 t 1.4%0in the 500 m-thick sheeted dike complex (HUB-

s34s(%o) 3

-5

0

I

I

t5

t I

I----+--{

massive peridotites

Amphibole pyroxenitex

garnet ~rzburg;te from ~lf~“t~,“~‘~ t--_-i

peridotite8 3,

------_c---__-_!+I

layered pyroxenife: I+-continental

t

lholei~e~‘)

t._+_j M(3R(3t4’5.6)

i

sulphide globules from MkV?B(‘)

RERTEN, 1983; ALT et al., 1989). Even though whole-rock 634Svalues are close to O%O,a much greater scatter between -4.1 t 2.6 and +8.3 It 2.6%0 was revealed by in situ ion microprobe analyses of single pyrite and chalcopyrite grains from the transition zone and the sheeted dikes (ALT and CHAUSSON, 1990). On the other hand, GRINENKO et al. (1975) found a value of +6.5%0 for a coarse-grained massive gabbro from the Mid-Atlantic ridge. Finally, altered Pacific and Mid-Atlantic pillow basaits have 634S values between - 11.4 and + 12.7%0, with an extreme range of variation for epigenetic pyrite between -48.1 and +14.3%0 (SAKAI et al.. 1978; PUCCHELT and HUBBERTEN, 1979). To summarize, the effect of oceanic lithosphere subduction on the bulk 634Sof the upper mantle cannot be constrained because of the large uncertainties (I) on the mean 634Svalue of the altered oceanic crust and (2) on the proportion of sediments subducted, as well as the proportion and the 634Sof the S lost in arc ma~matism. Therefore, because the &34S values of continental tholeiites and subducted oceanic crust overlap widely (Fig. 5), the origin of the layered pyroxenites cannot be elucidated on the basis of the fis4Svalues alone. It should, however. be noted that the layered pyroxenites are lacking in high 634Svalues similar to the ones found in sulphides from inferred recycled materials such as eclogite nodules in kimberlites (between 0 ? 1 and $4.8 ?I 1%o;CHALJSSIDON et al., 1989) or inclusions in eclogitic diamonds (between +2.1 f 1 and C9.5 + 1%0; CHAUSSIDON et al., 1987).

The 6“‘s values of the amphibole-rich veins (+I t 1.5 to +4.5 + 1.5%00;Table 1) compare favourably with the cI’~S values ofcontinental alkali basalts (between +0.4 and +9.6%0: SCHNEIDER, 1970; HARMON et al., 1987). The same range

of ?j3’Svalues has also been reported for alkali basalt-related pyroxenite xenoliths in intraplate basalts (Salt Lake Crater, Hawaii, San Carlos, Arizona, and Dish Hill, California) (-0.2 I 1.5 to f5.1 -t 1.5%~;CHAUSSIDON et al., 1989). Thus, the 634Svalues of the amphibole-rich veins support the hypothesis

that these rocks are crystalline segregates from alkali basalts en route to the surface. -------_-a______

a~pbi~/erich

pyroxeffifes

I+]

(2.10)

ontinental alkali olivine basalts Continental alkali rich rockswJ\

I

,&+.___I


t SanCaries andDishHiNxenoiiths (1)

/8

I

FIG. 5. Comparison of the present ??S values with previous 6-S measurements of rocks of mantellic origin. Samples in italic correspond to previous ion microprobe measu~ments of 634Svalues on sulphide grains while others correspond to whole-rock analyses. The means of d34Svalues are indicated within the various fields by vertical bars. Data are from (1) CHALJSSIIXINet al. (1989);(2) SCHNEIDER (1970); (3) GRINENKOand UKHANOV(1977); (4) KANEHIRAet al. (1973): (5) GRINENKOet al. (1975); (6) SAKA!et al. (1984); (7) HUE BERTEN(I 983); (8) AI-Tet al., 1989: (9) ALT and CHAUSSIIXN (I 990); (I 0) HARMONet al. ( 1987).

Sources of 6% Variations in the Eastern Pytenean Peridotite Massifs

If all of the rock types present in the Eastern Pyrenean peridotite massifs are averaged then the resulting 634Svalue (= -0.7%0) is close to the supposed 634Svalue of the upper mantle. However, significant variations are observed between massive peridotites (mean 634Svalue = -3.2%0), layered pyroxenites (mean 634Svalue = -OX%), and amphibole-being pyroxenites (mean 634Svalue = i3.3%0). Inclusions in massive peridotites have more constant 634Svalues (in the range of -3.9 & 1.8%0)which, given the accuracy of the ion probe technique, compares well with the strong homogeneity of their whole-rock 6’*0 values (-5.63 +- 0.04aioo;JAVOY, 1980). The largest internal 634Svariations (broadly between -5 f 1.5 and +4 ?I 1.5%0) is shown by the layered pyroxenites (Table I). Similar variations around the “mantle values” are commonly reported for heavy radiogenic isotopes (Nd, Sr, and

2843

Isotope composition of sulfur in the upper mantle

Pb), showing heterogeneities at all scales in the upper mantle (e.g., MENZIES and MURTHY, 1976; ZINDLER et al., 1983; REISBERGand ZINDLER, 1986; ZINDLER and HART, 1986). Such variations have also been documented in the Eastern Pyrenean peridotite massifs (POLVE and ALL~GRE, 1980; HAMELIN and ALL~GRE, 1988; DOWNES et al., 1990; J. L. BODINIER, pers. comm., 1990). It is noteworthy that the present 634Svalues vary broadly in parallel with *‘Sr/“Sr ratios (Table 2). Quite obviously, the two generations of pyroxenites are characterized by higher “Sr/‘?& ratios and 634Svalues than the massive peridotites. Regarding Sr isotopes, this difference is interpreted as reflecting at least two different sources for the peridot&es and the pyroxenites (POLVEand ALL&GRE, 1980; DOWNESet al., 1990), high 87Sr/86Srratios originating from mantle sources variously enriched in incompatible trace elements. Source enrichments have also been suggested for both the Triassic continental tholeiites and the Cretaceous alkali basahs erupted throughout the Pyrenean range (ALIBERT, 1985; AZAMBREet al., 1990). At least two different metasomatic agents are postulated in the recent literature to be capable of mantle source enrichments, either recycled oceanic lithosphere or small degree partial melts originating from the lower mantle (e.g., MENZIES et al., 1987). By analogy with 34Scontents of meteorites, such melts are likely to have 634Svalues close to 0%. Small degrees of partial melting, indeed, would not significantly fractionate sulphur isotopes, because at high temperature (> 1OOO’C)no fractionation factors in excess of 1 or 2%0have been measured or calculated between various sulphur species (SAKAI, 1968; KAJIWARA and KROLJSE,1977; OHMOTO and RYE, 1979). Besides, no relationship has been found in the present work between 634Svalues and the depletion degree of the peridotites (Table l), unlike what would be expected for the residue of a Rayleigh isotopic fractionation process. Thus, owing to their similarities with the sulphur isotopic compositions of eclogite xenoliths (see above) the positive 634S values found in the pyroxenites are better interpreted by assuming introduction ofa recycled oceanic crust in the mantle sources ofthe parent magmas. The origin of the negative fi3“Svalues associated with low “Sr/*?Sr ratios in the massive peridotites (Table 2) cannot yeietbe fully elucidated. One inte~re~tion is that such negative values are the counter part of the positive &34Svalues of most crustal rocks and of mantle-derived magmas. The bulk 634S value of the system (upper mantle + crust) would be similar to that of meteorites (0 rt 0.2%0; THODE et al., 1961). However, further discussion is hampered by poor knowledge of bulk sulphur isotope fractionation mechanisms during extraction ofcrust from the upper-mantle. A second hypothesis calls for a few permil decrease of 634S values from 0%~ of parts of the upper-mantle by an early degassing of Sot, according to the positive isotopic fractionation factors between SOz and sulphide (SAKAI, 196X; OHMOTO and RYE, 1979). The negative SJ4S values of some pyroxenite layers which overlap the range of massive peridotites are still to be explained. Similar overlaps are known for 87Sr/86Sr ratios in the case of the thinnest websterite layers and for the margins of the thickest bands (Table 2). The low 87Sr/86Srvalues of these pyroxenites are explained by diffusion and re-equilibration processes with the massive peridotites (POLVE and

Table

?. : 834s

values

nnd

llrcrzolite msssifscompared Rock

S’Sr/@‘Sr to MORB

ratio of and OIB:

the

Eastern Pyrcoean

type

Massive

perldolites

Layered

pyroxenites

Amphibole.ricb

veins

Wall rock peridotites amphibole-rich vclns Thin web&rite within massive

of

layers peridotltes

-3,s to +3’pm * (average =&5!&)

0.7030 to 0.7035 2

+l%uto +4xh?* (nveruge =+33%0)

0.7028 f 0.003 *

-4 312960’

0.7027 trJo.7029 2

-O.l?& to -3.99&J*

0.7021 to 0.7023 1.2

ALL~GRE, 1980; ALLEGREand TURCOTTE, 1986). However, a S transfer from the massive peridotites to the layered pyroxenites is highly unlikely because the S contents of peridotites are 3 to 50 times lower than in the pyroxenites. Furthermore, unlike S which is a trace element concentrated by major silicates (Cpx, Opx), S is a major element in an accessory phase; exchanges of S are more likely to have occurred by mechanical transport of sulphide droplets than by diffusion of S in the solid state. Alternative explanations for the wide range of 634S values in the layered pyroxenites are (1) the existence of at least two different generations of layered pyroxenites which originated from two different sources or (2) a mixing between two sources: the first one similar in 634Sto the massive peridotites (-3.2 + 1.5%0)and the other one to that of the amphibole-bearing pyroxenite veins (+3.3 rt l.S%o). CONCLUSION The Eastern Pyrenean peridot&e massifs contain sulphide grains which range in 63JS values between -5.2 t 1.5 and +4.5 + l.S?&. The average of all the measurements (-0.7 + 1.5%0) is indistinguishable from the commonly assumed &34Svalue of the mantle (0 i 2%0).Mantle~e~v~ &34Svalues were not significantly disturbed by the crustal emplacement. Partial melting has no obvious effect on the 634Svalues of the variously depleted harzburgites and lherzolites, suggesting therefore a very minor isotopic fractionation between the different S-bearing phases during this process. &34Svalues increased by several permil in the peridotites closely interlayered with the pyroxenite layers, in response to sulphide melt infiltration from the pyroxenite. Irrespective of metasomatic contaminations in the subcontinental lithosphere, significant differences in 634S exist between the massive peridotites (mean 634S = -3.2%0), the layered pyroxenites (mean 634S = -0.8%~), and the amphibole-rich pyroxenites (mean 6’4S = +3.3%0). The positive 634Svalues are considered to result from variously enriched

M. Chaussidon and J-P. Lorand

2844

mantle sources trapped by the two generations of basaltic liquids which crosscut the Eastern Fyrenean lherzolite massifs. These massifs provide for the first time straightforward evidence of very small scale h3?3heterogeneities of a few permil in the sub-continental mantle. This range of 634Svalues should cause most of the 634S variations observed in most MORB, oceanic island basalts, as well as fresh continental tholeiites and alkali basalts free of late degassing or contamination by the crust. Acknowledgments-M. Chaussidon is grateful to F. Albar&de and S. M. F. Sheppard for many discussions on sulphur isotopes and to J. C. Demange, P. All&,and E. Deloule for help with the ion probe. J. P. Lorand thanks J. Fabriks, J. L. Bodinier, and C. Dupuy for a fruitful five years of collaborating work on the Eastern Pyrenean peridotites massifs. The manuscript has benefited from reviews by M. Guiraud. C. S. Eldridee. E. M. Riolev. and W. C. Shanks. This work was supported by the Centre National de la Recherche Scientifique and by INSU grants (programe DBT, thtme Dynamique globale) DBT no883862 to F. Albarede and DBT no893822 to J. P. Lorand. This is contribution CNRS-INSU no 177 and CRPG-CNRS n”847. Editorial handling: B. E. Taylor REFERENCES ABRAJANOT. A. and PASTERISJ. D. (1989) Zambales ophiolite, Philippines II. Sulfide petrology of the critical zone of the Acoje massif. Contrih. Mineral. Petrol. 103, 64-77. ALIBERTC. (1985) A Sr-Nd isotopic and REE study of late Triassic dolerites from the Pyrenees (France) and the Messejana dyke (Spain and Portugal). Earth Planet. Sci. Lett. 73, 81-90. ALL~GREC. J. and TURCOTTED. L. (1986) Implications of a two component marble-cake mantle. Nature 323, 123-126. ALT J. C. and CHAUSSIDONM. (I 990) Ion microprobe analyses of the sulfur isotope composition of sulfides in hydrothermally altered rocks DSDP/ODP hole 504B, Part B. ODP Proc., Leg 111 (eds. K. BECKERet al.). Vol. III, pp. 41-45. ALTJ. C., ANDERSON,T. F., and BONNELL. (1989) The geochemistry of sulfur in a 1.3 km section of hydrothermally altered oceanic crust, DSDP hole 504B. Geochim. Cosmochim. Acta 53, 101l1023. AIJI.T W. U. and KULP J. L. (1959) Isotopic geochemistry of sulfur. Geochitn. Cosmochim. Acta 16, 201-235. AVE LALLEMANDH. G. (1967) Structural and petrographic analysis of an “alpine type” peridotite. The lherzolite of French Pyrtnt!es. Leidse Geol. Mededel. 42, i -57. AZAMBREB., ROSSY M. and ALBAREDEF. (1990) Petrology and geochemistry of the alkalin magmatism from the Cretaceous North Pyrenean rift zone (France and Spain) and the origin of rift magmas. Contrib. Mineral. Petrol. (in press). BACHINSKID. J. (1969) Bond strength and sulfur isotopic fractionation in coexisting sulfides. Econ. Geol. 64, 56-65. BODINIERJ. L., GUIRAUD M., FABRIESJ., DOSTALJ., and DUPUY C. (1987a) Petrogenesis of layered pyroxenites from the Lherz, Freychinkde and Prades ultramalic bodies (AriPge, French Pyrt&es). Geochim. Cosmochim. Acta 51, 279-290. BODINIERJ. L., FABRIESJ., LORANDJ. P. DOSTALJ., and DUPLJY C. (1987b) Geochemistry of amphibole pyroxenite veins from the Lherz and Freychintde ultramafic bodies (Arige, French PyrknCes). Bull. Minwal. 110, 345-358. BODINIERJ. L., DUPUYC., and DOSTALJ. (1988) Geochemistry and petrogenesis of eastern pyrenean peridotites. Geochim. Cosmochim. Acca 52,2893-2907. CHAUSSIDONM. and DEMANGEJ.-C. (1988) Instrumental mass fractionation in ion microprobe studies of sulphur isotope ratios. In Secondary Ion Mass Spectrometry VI (eds. A. BENINGHOVEN et al.), pp. 937-940, J. Wiley & Sons. CHALJSSI~ON M.. ALBAR~DEF., and SHEPPARDS. M. F. (1987) Sulphur isotope heterogeneity in the mantle from ion microprobe

measurements of sulphide inclusions in diamonds. Nature 330, 242-244. CHAUSSID~NM., ALBAR~DEF., and SHEPPARDS. M. F. (1989) Sulphur isotope variations in the mantle from ion microprobe analyses of micro-sulphide inclusions. Earth Planet. Sci. Lett. 92, 144- 156. CHOUKROUNEP. and SEGURETM. (I 973) Carte structurale des Pw&I&S. Ed. ELF-ERAP, Boisseus, France. CONQI&R&F. (1977) PCtrologie des pyroxtnites lit&esdans les complexes ultramafiques de l’Ari2ge (France) et autres gisements de lherzolite B spinelle. Compositions min&alogiques et chimiques, Cvolution des conditions d’tquilibre des pyroxCnites. Bull. Sot. Fr. Min&al. Cristal. 100, 42-80. CONQUERSF. (1978) PCtrologie des complexes ultramafiques de Iherzolites i spinelle de I’Arisge (France). Th&e Dot. Etat, Paris. CONOU~R~~ F. and FABRIESJ. (1984) Chemical disequilibrium and itsihermal significance in sdinel peridotites from ;he Lherz and FreychinPde ultramalic bodies (AriZge, French PyrCnCes).In Kimber1ite.v II. The Mantle and Crust-Mantle Relationships (ed. J. KORNPROBST),pp. 3 19-33 1. Elsevier. DELOULEE., ALL~GREC. J.. and DOE B. (1986) Lead and sulfur isotope microstratigraphy in galena crystals from Mississippi Valley type deposits. Econ. Geol. 81, 1307- 1321. DOWNESH., BODINIERJ-L., THIRLWALLM. F.. LORANDJ-P., and FABRIESJ. (1990) Isotopic evidence for successive metasomatic events in peridotites massifs of the Eastern Pyrenees (abstr.). Proc. Intl. Ukshp. Orogenic Lherzolites and Mantle Processes: Montpellier. France, septemhre 1990 (in press). ELDRIDCEC. S., COMPSTONW., WILLIAMSI. S.. WALSHES. L., and BOTH R. A. (I 987) In situ microanalysis for 3“S/32Sratios using the ion microprobe SHRIMP. Intl. J. Mass Spectr. on Proc. 76, 65-83. FABRIESJ. and CONQUERSF. (1983)Les lherzolites g spinelle et les pyroxCnites B grenat associCes de Bestiac (Arigge, France). Bull. Minint:ral.106, 78 I-803. FABRIESJ., BODINIERJ. L., DUPUYC., LORANDJ. P., and BENKERROU C. (1989) Evidence for modal metasomatism in the erogenic spine1 lherzolite body from Caussou (Northeastern Pyr&nCes, France). J. Petrol. 30, 199-228. GOLDBERGJ. M., MALUSKIH., and LEYRELOUPA. F. (1986) Petrological and age relationship between emplacement of magamatic breccia. alkaline magmatism, and static metamorphism in the north pyrenean zone. Tectonoph.wlc.s 129, 275-290. GRINENKO L. N. and UKHANOVA. V. ( 1977) Sulfur levels and isotopic compositions in upper-mantle xenoliths from Obnazhennaya kimberlite pipe. Geochem. Intl. 14, 169- 17 I. GRINENKOV. A., DMITRIEVL. V., MIGDISOVA. A.. and SHARAS’KIN A. YA. (1975) Sulfur contents and isotope compositions for igneous and metamorphic rocks from mid-ocean ridges. Geochem. Intl 12, 132-137. HAMELINB. and ALL~GREJ. C. (1988) Lead isotope study oforogenic lherzolite massifs. Earth Planet. Sci. Lett. 91, 117- I3 I. HARMONR. S., HOEFSJ., and WEDEPOHLK. H. (1987) Stable isotope (0, H, S) relationships in tertiary basalts and their mantle xenoliths from the Northern Hessian depression, W-Germany. Contrih. Mineral. Petrol. 95, 350-369. HOEFSJ. (1987) Stable Isotope Geochemistry. Springer-Vedag. HOLSERW. T. and KAPLANI. R. (1966) Isotope geochemistry of sedimentary sulfates. Chem. Geol. 1, 93-l 35. HUBBERTEN H. W. (1983) Sulfur content and sulfur isotopes ofbasalts from the Costa Rica rift (Hole 504B. DSDP legs 69 and 70). In Init. Rept. DSDP (eds. J. HONNOREZet al.), Vol 69. pp. 629-635. US Govt. Printing Office. JAVOYM. (1980) ‘8O/‘6O and D/H ratios in high temperatures peridotites. In Orogenic Majic and C’ltramafc Associations. Colloq. Intl. CNRS 272.279-287. KAJIWARAY. and KROUSEH. R. (197 I) Sulfur isotope partitioning in metallic sulfide systems. Canudian J. Earth Sci. 8, 1397-1408. KANEHIRAK., YUI S., SAKAI H.. and SASAKIA. (1973) Sulphide globules and sulphur isotope ratios in the abyssal tholeiite from the Mid-Atlantic Ridge near 30”N latitude. Geochem J. 7, 89-96. KORNPROBST J. and VIELZEUFD. (1984) Transcurrent crustal thinning: a mechanism for the uplift of deep continental crust-upper mantle associations. In Kimherlitec II. The Mantle and Crust

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Isotope composition of sulfur in the upper mantle Mantle Relationships. (ed. J. KORNPROBST);Proc. 3rd Intl. Kimberlite Co& pp. 341-352. Elsevier. KYSER T. K. (1986): Stable isotopes variations in the mantle. In Stable Isotopes in High Temperature Geological Processes (eds. J. W. VALLEYet al.), Vol 16, Chap. 5, pp. 141-164. Mineral Sot. Amer. LORANDJ. P. (1987) Caractkres min&alogiques et chimiques g&raux des microphases du systkme Cu-Fe-N&S dans les roches du manteau supCrieur: exemples d’hCttrogCn&tCs en domaine subcontinental. Bull. Sot. G&o/ France (8) t III(4), 643-657. LORANDJ. P. (1989a) Abundance and distribution of Cu-Fe-Ni sulfides, sulfur, copper and platinum-group elements in orogenictype spine1 lherzolite massifs of Aribge (Northeastern PyrtnCes, France). Earth. Planet. Sci. Lett. 93, 50-64. LORANDJ. P. (1989b) Mineralogy and chemistry of Cu-Fe-Ni sulfides in erogenic-type spine1 peridotite bodies from Aricge (Northeastern Pyrenees, France). Contrib. Mineral. Petrol. 103, 335-343. LORANDJ. P. (1989~) Sulfide petrology of spine1 and garnet pyroxenite layers from mantle-derived spine1 lherzolite massifs of Arikge, northeastern Pyrenees, France. J. Petrol. 30, 987- 10 15. LORANDJ. P. (1989d) The Cu-Fe-Ni sulfide component of the amphibole-rich veins from the Lherz and Freychintde spine1 peridotite massifs (Northeastern Pyrenees, France): a comparison with mantle-derived megacrysts from alkali basalts. Lithos 23, 28 l-298. LORAND J. P., BODINIERJ. L., DUPUY C., and DOSTALJ. (1989) Abundance and distribution of gold in the erogenic-type spine1 peridotites from Arikge (Northeastern Pyrkntes, France). Geochim. Cosmochim. Acta 53, 3085-3090. LOUBETM. and ALL&RE C. J. (1982) Trace elements in erogenic lherzolites reveal the complex history of the upper-mantle. Nature 298, 809-8 14. MENZIESM. A. and MURTHYV. R. (1976) Sr isotopic composition of clinopyroxenes from some mediterranean alpine Iherzolites. Geochim. Cosmochim. Acta 40, 1571- I58 I. MENZIESM. A., ROGERSN., TINDLE,A., and HAWKESWORTH C. J. (1987)Metasomatic and enrichment processes in lithospheric peridotites, an effect of asthenosphere-lithosphere interaction. In Mantle Metasomatism (eds. M. A. MENZIESand C. J. HAWKESWORTH),pp. 3 13-36 1. Academic Press, London. MITCHELLR. H. and KEAYSR. R. (198 I) Abundance and distribution of gold, palladium and inidium in some spine1 and garnet Iherzolites: implications for the nature and origin of precious metalrich intergranular components in the upper-mantle. Geochim. Cosmochim. Acta 45,2425-2442. MONTICNYR., AZAMBREB., ROSSYM., and THUIZAT R. (1986) K-Ar study of cretaceous magmatism and metamorphism from the Pyrtntes-age and length of rotation of the Iberian Peninsula. Tectonophvsics 129, 257-273. NALDRETTA. J. (1981) Nickel sulphide deposits: Classification, composition and genesis. In Economic Geology Seventy-Fifth Anniversar_v Volume (ed. B. SKINNER), pp. 628-685. Economic Geology Publishing Company. OHMOTOH. (1987)Stable isotope geochemistry of ore deposits. In Stable Isotopes in High Temperature Geological Processes (eds. J. W. VALLEYet al.), Vol 16, Chap. 14, pp. 491-559. Mineral. Sot. Amer. OHMOTOH. and RYE R. 0. (1979) Isotopes of sulfur and carbon. In Geochemistry of Hydrothermal Ore Deposits, 2nd edn. (ed. H. L. BARNES),pp. 509-567. J. Wiley & Sons. POLVEM. and ALL~GREC. J. (1980) Orogenic lherzolite complexes studied by 87Rb-86Sr:A clue to understand the mantle convection processes? Earth Planet. Sci. Lett. 51, 7 l-93. PUCHELTH. and HUBBERTENH. W. (1979) Preliminary results of sulfur isotope investigations on deep sea drilling projects core from Legs 52 and 53. Init. Rep. DSDP. Vols. 51-53, pp. 1145-I 148. US Govt. Printing Office. REISBERGL. and ZINDLERA. (1986)Extreme isotopic variation in the upper mantle: Evidence from Ronda. Earth Planet. Sci. Lett. 81,29-45.

* All samples contain about 0.1 to 1% accessory Ti pargasite.

SAKAIH.

(1968)Isotopic properties of sulfur compounds in hydrothermal processes. Geochem. J. 2,29-49. SAKAIH., UEDA A., and FIELDC. W. (1978) &34Sand concentration of sulfide and sulfate sulfurs in some ocean-floor basalts and serpentinites. 4th Intl. Corzf Geochronology-Cosmochronology and Isotope Geology; USGS Open-File Rept. 78-701, pp. 372-314. SAKAI H., DES MARAISD. J., UEDA A., and MOORE J. G. (1984) Concentrations and isotope ratios of carbon, nitrogen and sulfur in ocean floor basalts. Geochim. Cosmochim. ilcta 48,2433-244 1. SCHNEIDER A. (I 970) The sulfur isotope composition ofbasaltic rocks. Contrib. Mineral. Petrol. 25, 95- I24. THODEH. G.. MONSTERJ.. and DUNFORDH. B. t I96 11Sulfur isotope geochemistry. Geockm. Cosmochim. Acta.‘ 25, 159- 174. UEDA A. and SAKAI H. (1984) Sulfur isotope study of quaternary volcanic rocks from the japanese island arc. Geochim. Cosmochim. Acta. 48, 1837-1848. VETIL J. Y., LORANDJ. P., and FABRIESJ. (1988) Conditions de mise en place des filons de pyroxtnites 5 amphibole du massif ultramafique de Lherz (Ari;ge, France). C. R. Acad. Sci. Paris t 307 (II), 587-593. ZIENTEKM. L. and RIPLEYE. M. (I 988) Sulfur isotopic studies of the Stillwater Complex and associated rocks, Montana. USGS Open-File Rept. 89-76. ZINDLERA. and HART S. R. (1986) Chemical geodynamics. Ann. Rev. Earth Planet. Sci. 14, 493-571. ZINDLERA., STAUDIGEL H., HARTS. R., ENDRESR., and GOLDSTEIN S. (1983) Nd and Sr isotopic study of a mafic layer from Ronda ultramafic complex. Nature 304, 226-230. APPENDIX

I: SAMPLE DESCRIPTIONS

Massive Peridotites” 72-442: Strongly depleted spine1 harzburgite (3% Cpx, 23 ppm S) from the top of the Lherz massif (700 m X 1.6 km), sampled at 20 cm from a hornblendite dike. This sample is 20% serpentinized and displays a porphyroclastic texture. A single Pentlandite (Pn) + Pyrite (Py) + Pyrrhotite (PO) + Chalcopyrite (Cp) composite sulphide assemblage enclosed in olivine was observed among the three polished sections investigated. (This sample was analysed by CONQUI?R&1978) 70-355: Refractory spine1 lherzolite (7% Cpx, 100 ppm S) from the Pit de G&al outcrop (x400 m). This sample has a slightly deformed coarse granular texture and is 8% serpentinized. The sulphides are present as three polyhedral isolated inclusions in olivine (composed of unaltered type 1 sulphide assemblage, Pn * Py +- Cp) and a few tens of interstitial grains (composed of type II sulphide assemblage, Fe rich Pn + Cp t Troilite (Tr) I Mackinawite (Mw), resulting from the low-temperature serpentinization-related decrease of the sulphur fugacity). (This sample is similar to sample GER 2 analysed by BODINIERet al., 1988, and LORAND, 1989a.) 79-61: Fertile spine1 lherzolite (15% Cpx, 260 ppm S) from the small outcrop of Bestiac (= 100 m). The texture is coarse-grained with an incipient secondary deformation (undulose extinction and elongation of the silicates). In this very fresh sample (4% serpentinized) numerous sulphide inclusions were observed in olivine and orthopyroxene. Intergrowths between Ni-rich Pn and Py are common and coexist with PO + Cp. The unaltered type 1 sulphide assemblage is predominant as interstitial grains. Average modal composition ofthe sulphide estimated from analyses of 40 individual sulphide grains is about Pn7,Po5 sPy, &p,. Mw and Tr are uncommon. (This sample was analysed by FABRIESand CONQUERS, 1983: BODINIERet al., 1988, and LORAND,1989a.) Interlayered Peridotite 79-60: 2 cm-thick metasomatized Cpx-rich lherzolite (25% Cpx, 500 ppm S), intercalated with websterite bands in the small outcrop of Bestiac (= 100 m). It displays a porphyroclastic texture and contains about 5% serpentine. A single isolated fractured sulphide inclusion has been observed in olivine, showing the altered Fe-rich Pn + Tr +- Mv sulphide assemblage. All the other sulphide grains are interstitial to the silicates, sometimes occurring as small flakes between cpx+opx+spinel kelyphitic aggregates. The interstitial sulphide grains

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M. Chaussidon and J-P. Lorand

are composed of the altered type II sulphide assemblage. Average modal compositions estimated from analyses of 40 grains is Pne,3Tr2,Py3Cp,. (This sample was analysed by FABRIESand CONQL&R~, 1983; BODINIERet al., 1988, and LORAND,1989a.) Layered Pyroxenites 72-283: Thin (4 cm), Mg-rich spine1 websterite layer (Op.u, Cpx, Sp, > 1000 ppm S) from the Lherz massif, intercalated within massive Iherzolites. This layer contains interstitial sulphide grains only. They are composed of the unaltered type I sulphide assemblage (N&rich Pn t PO + Py t Cp). The average modal composition estimated from analyses of 110 grains is Pn76P04Py8Cp12.(This sample is similar to sample 72-443 analysed by BODINIERet al., 1988.) 72-50: Recrystallized 20 cm-thick garnet-poor spine1 websterite layer (Opx, Cpx, Sp, Gt”, 390 ppm S) from the Freychinede massif. The investigated polished section was sampled at 8 cm from the layer margins. Two isolated sulphide inclusions have been found in secondary garnet. They display the unaltered type I sulphide assemblage. The interstitial sulphide phase is composed of large (mm size) sulphide aggregates composed of type II altered sulphide assemblages (Fe-rich Pn + Tr + PoII + Cp + Py). When present, the interstitial pyrite is replaced by secondary pyrrhotite. The average modal composition of the interstitial sulphides as estimated from the analyses of 120 grains is PnssTrzsPy,Cp,, (This sample belongs to the Freychinede “banded” series described by CONQL&R&1977. It was analyzed as 72-202 bv BODINIERet al.. 1987a. and as 72-50 bv LORAND.1989c.) 80-168: Transitional zone of a garnet-poor spine1 websterite (50% Opx, 48% Cpx, 2% Sp, 410 ppm S) from the intermediate part of a composite pyroxenite lens (80 cm-thick and 3 m-long), Freychinede massif. Large (2-4 cm) high-temperature, high-pressure megacrysts of Opx and Cpx displaying relic exsolution features survive in a mosdic recrystallized matrix. Opx megacrysts are subhedral and unfractured. They show a single set of very thin lamellae of Cpx + associated garnet exsolutions. Cpx are more brittle and fragmented into pieces of 5 mm immersed in a fine-grained matrix. They contain large lamellae ( IO- 100 pm wide) of Opx together with spine1 inclusions. The matrix is composed of abundant exsolved Opx, small fragments of Cpx megacrysts as well as vermicular spine1 surrounded by garnet coronas. The matrix resulted from tectonic granulation of the megatryst together with their exsolution products. The Cpx megacrysts contain numerous polyhedral sulphide inclusions ranging from 20 X 30 pm to 60 X 30 grn in size and composed of the unaltered type I sulphide assemblage (Ni-rich Pn + PO + Cp) coexisting with subordinate amounts of Py. Larger sulphide grains (up to 1 mm in diameter) are disseminated between the pyroxene megacrysts. Smaller aggregates are interspersed in the recrystallized

matrix or penetrate the megacrysts along exsolution planes. All the interstitial sulphide grains display the type I sulphide assemblage (Nirich Pn + PO t Py t Cp). However, pyrite is often replaced by a secondary pyrrhotite. The average modal composition estimated from analyses of 58 grains is Pn43PoZ7Py&p12. (This sample, belonging to the Frevchinede “banded” series. was described as 70-403 bv CONQUEER!& 1977 and analysed as 70-463 by JAVOY, 1980, as 70-385 by BODINIERet al., 1987a, and LORAND,1989c.) 81-17: Garnet clinopyroxenite (10% Opx, 45% Cpx, 44% Gt, 1% Sp, 340 ppm S) from the core of a thick (0.8 to 1 m) zoned layer in the Freychinede massif. This layer contains three polyhedral isolated sulphide inclusions in recrystallized Cpx and secondary garnet. Interstitial sulphides are common. Their average modal composition (estimated from analyses of 66 grains) is Pm2 sPo4,,5Py,Cp,, (type-II sulphide assemblage). (This sample is very similar to sample 70-357 belonging to the Freychinede “banded” series [CONQUEROR, 19771 and analysed by POLE and ALL~GRE,(1980), LOUBETand ALL~GRE, 1982, BODINIERet al., 1987, and LORAND,1989c.) Amphibole-pyroxenite Veins 70-257: 20 cm-thick, garnet-poor, amphibole ariegite (Amp t Cpx + 01 + (Opx) + (Gt) + Sp + (Plag) + (Ilm)) from one of the Lherz injection zones. This vein is sulphur-rich ( I 100 to 1200 ppm S) and contains both sulphide inclusions and interstitial grains. The sulphide inclusions are common in amphibole, spine], ilmenite, and garnet where they occur as ovoid, euhedral, or rounded grains ranging in diameter from 10 to 100 pm. The enclosed sulphides are composed of the type II unaltered sulphide assemblages except when fractured (Tr + Fe-rich Pn invaded by mackinawite blebs). The average grain size of interstitial sulphides is 200 pm. All the interstitial grains are composed of the type II altered sulphide assemblage. Their average modal composition (estimated from analyses of ~100 grains) is TrRdPnsCps. (This sample was analyzed as Lherz H51 by JAVOY, 1980, and is very similar to sample 73-2111,analysed by BODINIER et al., 1987b, and LORAND,1989d.) 70-184: 15 cm-thick, garnet-amphibole-olivine clinopyroxenite dike (Gt + Amph + Cpx + Plag + 01 + (Sp) + (TMagn) + (Ilm) + ((Hyp))) from Lherz. This rock contains 850 ppm S. Sulphide inclusions are common in garnet, amphibole, and Fe-Ti oxides where they are euhedrally shaped. When unfractured, these inclusions contain the typeI unaltered sulphide assemblage. The fractured inclusions and the interstitial grains are composed of predominant Tr associated with minor Pn and Cp. Several interstitial grains display unusual assemblages in which secondary PO replacing primary pyrite coexists with Mw + Cb + Cp intergrowths. (This sample is very similar to sample 73- 1B of CONQUERS, 1978.)