Lithos 154 (2012) 296–314

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In-situ geochemistry of sulfides in highly metasomatized mantle xenoliths fromKerguelen, southern Indian Ocean

Guillaume Delpech a,⁎, Jean-Pierre Lorand b, Michel Grégoire c, Jean-Yves Cottin d, Suzanne Y. O'Reilly e

a UMR-CNRS 8148 IDES, Université Paris Sud-XI, 91405 Orsay Cedex, Franceb LPGNantes, UMR-CNRS 6112, 2 Rue de la Houssinière, BP 92208, 44322 NANTES Cedex 3, Francec GET, Géosciences Environment Toulouse, UMR-CNRS 5563, Observatoire Midi-Pyrénées, 14, Avenue Edouard Belin, 31400, Toulouse, Franced “Transferts lithosphériques,” UMR-CNRS 6524 “Magmas et volcans,” 42, rue du Dr. Paul Michelon, 42023 Saint-Etienne Cedex, Francee Australian Research Council Centre of Excellence for Core to Crust Fluid Systems/GEMOC, Macquarie University, NSW 2109, Australia

⁎ Corresponding author. Tel.: +33 169154898; fax: +E-mail address: guillaume.delpech@u-psud.fr (G. De

0024-4937/$ – see front matter © 2012 Elsevier B.V. Alldoi:10.1016/j.lithos.2012.07.018

a b s t r a c t

a r t i c l e i n f o

Article history:Received 9 November 2011Accepted 27 July 2012Available online 11 August 2012

Keywords:MetasomatismCarbonate-rich meltCO2-rich fluidsBase metal sulfidesPGE fractionationsLA-ICP-MS

Mantle xenoliths from the Kerguelen Archipelago record a complex multistage history involving a highdegree (15 to 25%) partial melting that created a harzburgitic mantle completely stripped of Base MetalSulfides (BMS), followed by pervasive melt-rock reaction with alkaline melts above the Kerguelen mantleplume. Subsequent reaction of the highly refractory protolith with small volumes of carbonate-rich silicatemelts led to a re-enrichment in BMS (up to 0.05 wt.%). Two BMS precipitation mechanisms are suggested:immiscibility from the silicate-carbonatemelt and sulfidation reactions from aCO2-rich supercriticalfluid. In-situanalyses of chalcophile and siderophile elements (major and trace levels) in the BMS shed new light on theirorigin. The BMS phases that precipitated via immiscibility are metal-rich sulfide melts which progressivelyevolved toward Ni and Cu-rich end-members by cumulate fractionation of monosulfide solid solution (mss)during percolation inside the peridotites. Some cumulate mss have elevated and fractionated IPGE contents(200–900× C1-Chondrite abundances), indicating random digestion of preexisting Os, Ir, Ru-rich PGM by thepercolating sulfide melt. The BMS that precipitated by sulfidation reactions from a CO2-rich vapour phase aresubsolidus exsolution products from Cu-bearing but Ni-poorer mss. They have the highest concentrations ofPGEs and show selective enrichment in S, Pd, Pt and Os over Cu, S, Ir, Ru and Rh. Their PGE compositions confirmexperimental data, which demonstrate that S, Pd, Pt and Os can be efficiently transported in a CO2-richsupercritical fluid. Superchondritic (S/Se), (Os/Ir) and (Pd/Pt) in both bulk-rocks and individual sulfides areinferred to be the geochemical fingerprints of sulfide crystallisation from a CO2-rich vapour exsolved from ahighly evolved carbonate-rich metasomatic melt.

© 2012 Elsevier B.V. All rights reserved.

1. Introduction

Platinum-group elements (PGE, i.e., Os, Ir, Ru, Rh, Pt, Pd) and theircompanion chalcophile (Cu) and chalcogenide elements (S, Se, Te, Bi,As) have been shown to be powerful geochemical tracers of petroge-netic processes occurring in the upper mantle (Morgan, 1986; Pattouet al., 1996; Büchl et al., 2002; Rehkämper et al., 1999, van Acken et al.,2010, Lorand et al., 1999). The most refractory PGEs (Os, Ir, Ru and Rh)behave compatibly during partial melting and are concentrated intorefractory minerals such as monosulfide solid solutions (mss) (Alardet al., 2000; Ballhaus et al., 2001, 2006; Fleet et al., 1991; Li et al., 1996;Mungall et al., 2005) or in Os-Ir-Ru alloys/sulfides (Godel et al., 2007;Lorand et al., 2010; Luguet et al., 2003, 2007). Conversely, partialmelting depletes the upper mantle in elements that are stronglypartitioned into low-volume melting sulfide melts (Pd, Pt, Cu, S, Se, Te

33 169154891.lpech).

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and As; Alard et al., 2000; Bockrath et al., 2004; Ballhaus et al., 2006;Fleet et al., 1991; Lorand et al., 2008). Thus, partial melts shareenrichments in all of these elements compared to Os, Ir, Ru and Rh,which are retained in the mantle residue. However, interaction ofpercolating melts with the (depleted) lithospheric mantle can lead tocontrasting HSE signatures. Some studies have reported that melt-rockreactions at highmelt/rock ratio, betweenmigratingmelts and depletedmantle peridotites could result in further removal of HSE, leading tomantle rocks with very low HSE contents (Büchl et al., 2002; Lorandet al., 2003, 2004; Rehkämper et al., 1999). Other studies have shownthe opposite effect, leading to HSE enrichments in peridotites by thepercolating melt, which may also occur with sulfide addition duringperidotite-melt rock reactions at relatively low melt-rock ratios ( Alardet al., 2011, Ackermann et al., 2009; Liu et al., 2010; Luguet et al., 2003;Lorand et al., 2003, 2004). These metasomatic BMS commonly shareenrichments in Pd, Pt and their companion chalcophile elementscompared to Os, Ir, Ru and Rh. Even Os, which was long considered tobe inert in metasomatic fluids, has been shown to fractionate fromIr (Os/Ir>1) in volatile-rich aqueous fluids in the sub-arc mantle

297G. Delpech et al. / Lithos 154 (2012) 296–314

(McInnes et al., 1999; Lee, 2002) or in CO2-rich (carbonatitic) fluids inthe subcontinental or oceanic mantle domains (Alard et al., 2011;Lorand et al., 2004). Finally, in some cases, the metasomatic sulfide HSEsignatures may also fingerprint the random dissolution of pre-existingPGE-richminerals (PGM— PlatinumGroupMinerals) in the S-saturatedpercolating melt (Griffin et al., 2002, 2004; Lorand et al., 2010). SuchPGMs have been shown to control the HSE signature of residual mantlerocks if the partial melting degree continues beyond the point of sulfideexhaustion (~20%); they are found dispersed as rare micrometer-sizedgrains in the interstices of peridotites ( e.g., Os\Ir\Ru alloys, Luguetet al., 2007; Lorand et al., 2010).

Mantle xenoliths from the Kerguelen Islands offer the possibility totest the consequences of melting, melt-rock reaction and metasomaticprocesses on the PGE systematics in an oceanic plume environment.The complex history of Kerguelen peridotite xenoliths in terms ofmelting and metasomatism has already been unraveled using detailedmineralogy and lithophile major and trace element geochemistry byGrégoire et al. (1997, Grégoire et al., 2000a,b), Moine et al. (2001, 2004)and Delpech et al. (2004). In an earlier study, Lorand et al. (2004) usedthe bulk-rock S, Se, Cu and PGE systematics of a suite of mantlexenoliths from Kerguelen to assess the behaviour of PGEs during thecomplex history ofmelt-depletion andmetasomatism. In this suite, twopopulations of metasomatic sulfides were identified: 1) Cu-rich sulfidemelt droplets unmixed from a carbonatemelt, 2) Cu-poor Fe-Ni sulfidesprecipitated from a S- and Cl-bearing CO2 vapour phase. The aim ofthe present study is to further examine the distribution of PGEs andtheir chalcophile elements between the two different populations ofmetasomatic BMS that were added to the mantle residues. Mantlexenoliths from the Kerguelen lithospheric mantle are particularlysuitable for investigating such distribution since the original protolithwas completely stripped of residual BMS during the high degreemelting event that formed the lithosphericmantle (Lorand et al., 2004).

This study is based on five xenoliths previously analysed for bulk-rock PGE geochemistry (Lorand et al., 2004). In-situ geochemicaltechniques (Electron Microprobe-EMP and Laser-Ablation InductivelyCoupled PlasmaMass Spectrometry-LA-ICPMS)were coupledwith SEM(Scanning Electron Microscope) observations in the back-scatteredmode to examine the distribution of PGEs inside BMS and to detect PGE-rich microphases. Our results highlight the selective transport andprecipitation of some PGEs during metasomatism involving volatile-rich small melt fractions in the lithospheric mantle.

2. Petrogenetic characteristics of the peridotite xenoliths

2.1. General overview

Peridotite xenoliths from the Kerguelen Archipelago were broughtto the surface by plume-derived lavas, which occur in breccia pipescross-cutting the flood basalt sections. The peridotite suite consistsmainly of harzburgites and dunites and also contains compositexenoliths in which hornblendite or clinopyroxenite veins cross-cutting dunites (Delpech et al., 2004; Grégoire et al., 1997; Grégoireet al., 2000a,b; Moine et al., 2001, 2004).

Petrogenetic indicators such as mineral modes (clinopyroxeneb3 vol.%) and mineral compositions (high Mg#(olivine), highCr#(spinel), low Al, Na, Ti in clinopyroxene; Table 1) in the harzburgiticxenoliths point to formation in a high degree partial melting event (upto 25%melt removed), related to the formation of the Kerguelenmantlelithosphere near the South-East Indian ridge (SEIR), as described byGrégoire et al. (1997, 2000a). The high-degree partial melting (15–25%)left the protogranular harzburgites strongly depleted in S (b5 ppm),Se (b7 ppb), Cu (b5 ppm), Pd and Pt (Lorand et al., 2004; Table 1).Following this event, some residual harzburgites were subsequentlyaffected by intense melt percolation of CO2-bearing alkaline silicatemelts originating from the Kerguelen plume. Melt-rock reactions ledto different microstructures and mineralogy among harzburgites or

dunites depending on both the compositions of the metasomatic agentsand the melt/rock ratios involved (Grégoire et al., 2000a). Some residualharzburgites show evidence of incipient metasomatism by small meltfractions that only re-enriched the most incompatible lithophile traceelements in clinopyroxenes (e.g., “cryptic metasomatism”) withoutmicrostructuralmodification. In contrast, in the vicinity ofmelt pathways,protogranular harzburgites of residual origin were percolated by largevolumes of Si-undersaturated alkaline silicate melt which producedolivine and clinopyroxene at the expense of the orthopyxroxene, anddeveloped poikiloblastic or poikilitic microstructures. Clinopyroxene-and spinel-bearing dunites contain minerals with lower Mg# and haveclinopyroxenes enriched in basaltic elements (e.g., Al, Na, Ti); they are theend-products of intense melt-rock interactions at high melt/rock ratios(Grégoire et al., 1997, Grégoire et al., 2000a), as developed in themodel ofKelemen (1990). Most clinopyroxenes in poikiltic harzburgites anddunites show trace element compositions that indicate near equilibra-tion with the alkaline magmas erupted at the surface (Grégoire et al.,2000a). During this metasomatic event at high melt-rock ratios, thepoikilitic harzburgites and dunites were further depleted in most PGEsand especially (Os, Ir, Ru, Rh) and Cu compared to protogranularharzburgites , due to the percolation of S-undersatutared melts thatscavenged the PGEs left in the residual harzburgites (Fig. 1a, see Lorandet al., 2004 for details).

After the crystallisation of olivine and clinopyroxene, the residualhighly alkaline silicate melt became enriched in fluids (H2O, CO2) andprecipitated amphibole and phlogopite in some of the harzburgitesand dunites (Moine et al., 2001). Finally, the last derivative fractionsof these strongly modified silicate melts evolved towards volatile-rich(carbonate-rich) melts that ultimately unmixed into low volumesilica-rich and volatile-poor alkaline melt fractions (Grégoire et al.,2000b) and a carbonate-rich melt fraction, strongly enriched involatiles of the system C\O\H\S (Delpech et al., 2004; Moine et al.,2004), from which precipitated the BMS described in this study.

2.2. Studied samples

The five selected samples (one harzburgite, four dunites) com-prise a set of samples that display the mineralogical characteristics(phlogopite and/or carbonates), major and trace element fingerprintsof metasomatism by volatile-rich (carbonate-rich) melts (Grégoireet al., 2000a) and all contain BMS. Bulk-rock S, Se, Cu and PGE con-centrations are from Lorand et al. (2004). For easier reading, sampleIds were renamed in this study (e.g., GM453 in this study is GM92-453 of Lorand et al., 2004. Samples MM54 and 101 are samplesMM94-10 and MM94-54, GM214 is sample GM92-214 and sampleBOB640 is sample BOB93-640.1). These xenoliths are only weaklyserpentinised and there is optically no evidence of host-rock contami-nation such as basaltic melt veins cross-cutting the xenoliths. Thesecondary mineralogical assemblage described below thus predatestheir entrainment in the host lava or occurred shortly before it.

Sample GM453 is a Fe-enriched olivine-rich poikilitic harzburgite(Mg#=89.9, Table 1), which contains 2.5% modal clinopyroxene andlow modal orthopyroxene (11%). The bulk-rock (Table 1) displays astrong LREE enrichment [(La/Yb)N=5.3] associated with negative HFSEanomalies [(La/Nb)N=4.7]. This harzburgite contains carbonates (purecalcite) located interstitially into melt “pockets” along with formersilicate melt (now devitrified). The clinopyroxene often displays spongyrims that may contain secondary Cr-spinel inclusions and BMS. The fourdunites (GM214; BOB640; MM54 and MM101) are clinopyroxene- andphlogopite-bearing spinel dunites and are enriched in FeO (Mg#=88.9–86.2) compared to the refractory harzburgitic protolith (Mg#=91.1–91.7; Table 1). Phlogopite and carbonate-bearing pockets and/or veinletsin these samples are found disseminated in the interstices of the perido-titic matrix. All four dunites are enriched in LREE [(La/Yb)N=2.65–6.88]with concomitant depletions in HFSE (1.12b(La/Nb)Nb4.72) except forsampleMM101 [(La/Nb)N=0.79]. Moreover, the average clinopyroxene

Table 1Mineralogical and geochemical characteristics of the xenolith suite.

Sample B796-396 GM453 MM101 MM54 GM214 BOB640

Rock type H H D D D D

Location Lac Michèle Trièdre Vallée Ring Vallée Ring Pointe Espérance Capitole

group “Protolith” I I I II II

ModeOl 75.8 86 97 95.1 90.3 94Opx 21.8 11 − − 1.5 −Cpx 1 2.5 1.6 2.9 5.8 3.1Sp 1.2 0.5 1 1.6 2.5 0.6Phlog − − 0.4 0.4 − 0.8Amp − − − − − −Carb − + + + + +Mg#(Ol) 91.5 89.9 87.4 86.8 88.7 86.1Cr#(Sp) 47.7 46.3 46.6 31.6 25.0 45.7

Bulk-rockT(°C)⁎ 880–1010 1030–1040 965–985 980–1005 940 945–985Mg# 91.7 89.8 87.2 86.6 88.4 86.2(La/Yb)N 3.46 5.27 4.70 4.21 2.65 6.88(La/Nb)N 0.89 4.72 0.79 1.27 1.80 1.12S (ppm) 45 145 130⁎⁎ 130 120 170Se(ppb) b7 69 − 49 21 17Cu (ppm) 3.5 49.3 58.1 48.4 14.5 10.3Ni (ppm) 2620 3000 2959 2712 2702 2337S/Se >6428 2101 − 2653 5714 10000

PGE (ppb) after Lorand et al. (2004)Os 0.72 4.07 0.60 0.74 3.46–4.65 2.29Ir 0.64 3.78 0.67 0.50 1.20–1.78 1.25Ru 1.63 7.59 1.22 1.10 5.04–5.75 2.87Rh 0.19 1.41 0.22 0.15 0.31–0.36 0.70Pt 0.18 8.27 1.04 0.50 1.96 6.03Pd 0.13 4.76 1.20 0.73 6.86–7.86 13.22Au⁎⁎⁎ 0.38 0.75 3.50 0.70 1.00–8.50 0.26(Pd/Pt)N 1.33 1.06 2.12 2.68 6.43–7.36 4.03(Os/Ir)N 1.04 1.08 0.83 1.37 2.43–2.68 1.70

H; harzburgite, D; dunite. Modal compositions were calculated from least-square regressions from bulk-rock analyses by XRF and individual mineral analyses by EMP or by point-counting. Mg#=100*(Mg/Mg+Fet) on a molar basis. Trace element analyses are from Grégoire et al. (2000a) and chalcophile and siderophile elements from Lorand et al. (2004).All values are normalised to the CI-Chondrite values after McDonough and Sun (1995).

⁎ T°C were calculated by using the thermometer of Fabriès (1979) and Ballhaus et al. (1991) for the Ol-Sp pair.⁎⁎ S for MM101 in Lorand et al. (2004) is erroneous. We used the same value as for MM54.⁎⁎⁎ Au contents are unpublished data (Lorand and Delpech).

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trace element composition in each dunite displays high (La/Nb)N (11.9–22.7; Grégoire et al., 2000a).

The studied samples, throughwhich small fractions of the carbonate-rich melts, percolated, are all enriched in S (130–170 ppm), Cu (10.3–58 ppm), Se (17–69 ppb), Pd (0.73–13.22 ppb) and Pt (0.5–8.27 ppb)relative to the refractory harzburgitic protolith (e.g., BY96-396, Table 1).Sample BY96-396 (from Lorand et al., 2004) is considered in this study asa representative sample of the refractory harzburgitic protolith compo-sition since it has the lowest concentrations in terms of PGE and recordsevidence of both an extensive partial melting event and subsequentpercolation by large amounts of Si-undersaturated melt that furtherdepleted its PGE contents (see Lorand et al., 2004 for details). The bulk-rock chalcophile and siderophile element systematics of the studiedsamples led Lorand et al. (2004) to subdivide this set of five samples intotwo groups. The Group-I samples (harzburgite GM453 and dunitesMM54 and MM101) share a strong enrichment in Cu (48.4–58.1 ppm)but little enrichment in Pd over Pt (Pd/PtN b2.7), coupled with broadlychondritic S/Se (2100–2600) and Os/Ir ratios (Fig. 1). Dunites MM54and MM101 are extremely depleted in refractory PGE of the iridiumsub-group (IPGE; 0.0007–0.0015×CI-chondrites, Fig. 1), as a result ofmetasomatism at high melt/rock ratios (Fig. 1), a characteristic featureof their origin as poikiloblastic or poikilitic harzburgites formed in thevicinity of strongly alkalinemelt pathways (Table 1, see Fig. 1 of Lorandet al., 2004). The bulk-rock chalcophile/siderophile element systematicsof Group-I xenoliths was ascribed to the precipitation of Cu-rich sulfidemelt droplets unmixed by immiscibility from a carbonate melt (Lorand

et al., 2004). The Group-II samples (dunites GM214 and BOB640)display very moderate enrichments in Cu (10.3–14.5 ppm), but strongincreases in Os, Pd and S concentrations relative to Ir, Pt and Se (Fig. 1).These selective enrichments produced superchondritic S/Se ratios(5700–10000), Os/Ir (1.7-2.7), Pd/Ir (4.0-8.8) and Pd/Pt (4–7.4).Lorand et al. (2004) ascribed the chalcophile /siderophile element sys-tematics of Group-II samples to the precipitation of Cu-poor sulfides viasulfidation by a S- and Cl-bearing CO2 vapour phase.

3. Analytical methods

The BMS phases were studied in reflected light microscopy andanalysed for major elements with a Cameca® SX50 microprobe atGEMOC (Macquarie University, Australia). The analytical conditionsincluded an acceleration voltage of 20 kV, a beam current of 20 nAand a beam width of about 2 to 5 micrometers. The counting time foreach element was 20 s/peak and 40 s/background. Cu, Co and Niconcentrations were determined with pure metal standards, S and Fewith a natural pyrite and O with hematite. The relative standarddeviation (1SD) on the determination of S is 0.29%, 0.38% for Fe, 0.4%for Cu, 0.2% for Co and 0.5% for O. The LLD is 0.08 wt.% for S, 0.04 wt.%for Fe, 0.07 wt.% for Ni, 0.06 wt.% for Cu, 0.06 wt.% for Co and0.05 wt.% for O.

Platinum group minerals (PGM) were searched for on polished200 μm-thick standard-sized (40x25 mm) sections using a high-resolution scanning electron microscope (SEM, SupraTM 55VP Zeiss

0,1

1

10

1000100 10000S/Se

(Pd/

Pt)

N

Harzburgites

0 1 2 3

(Os/Ir)N

Gr. II

Gr. I

Dunites

CI

BY96-396"Protolith"

Gr. I

Gr. II

0 10 20 30 40 50 60Cu (ppm)

volatiletransport of S

BY96-396"Protolith"

Gr. II

Gr. I

0.1

0.01

0.001

Os Ir Ru Rh PdPt Au

Gr. I dunitesGr. II dunites

Partial melting residue (protogranular Hzb)High melt/rock ratio metasomatism (poikilitic Hzb)

Gr. I Hzb

0

1

2

3

4

5

6

7

(Pd/

Pt)

N

0

0,5

1

1,5

2

2,5

3

(Os/

Ir) N

0.0001

Effect of high melt/rock ratio metasomatism

453

54

101

640

214

BY96-396"Protolith"

396"Protolith"

a b

c d

volatiletransport of S

volatiletransport of S

immiscibleCu-rich sulphides

immiscibleCu-rich sulphides

immiscibleCu-rich sulphides

GM92-453

MM94-101

MM94-54

BOB93-640

GM92-214Gr.II

Gr.I

Bul

k-ro

ck/C

I-C

hond

rite

Fig. 1. a) PGE patterns of selected bulk-rock samples from Lorand et al. (2004), normalized to the CI-Chondrite values (McDonough and Sun, 1995). Yellow symbols, Group-Ixenoliths; green symbols, group-II xenoliths. The black thick line in the upper left panel has the composition of a partial melting residue after exhaustion of the sulfide component(Lorand et al., 2004). The black arrow pointing downwards shows the effect of metasomatism at high melt/rock ratio starting from a partial melting residue (thick black line), whichis to decrease the bulk PGE content of the metasomatised rock down to that of “protolith BY96-396” (see texte for details). The protolith here denotes the bulk rock PGE compositionof the most depleted rock prior to sulfide addition via metasomatism in group-I and -II xenoliths. b); (Pd/Pt)N vs. (S/Se), c) (Pd/Pt)N vs. (Os/Ir) N and d) (Os/Ir) N vs. Cu (ppm) inbulk-rock harzburgites and dunites from Lorand et al. (2004). The large PGE fractionations are related to different BMS precipitation mechanisms (Lorand et al., 2004). Group-Isamples display BMS precipitated following immiscibility from a carbonate-rich melt, Group-II samples show BMS precipitated from a CO2-rich vapour phase. N_CI–Chondritenormalisation fromMcDonough and Sun (1995). Black triangles; poikiloblastic harzburgites, black squares; protogranular harzburgites, black circles; poikilitic harzburgites (Lorandet al., 2004).

299G. Delpech et al. / Lithos 154 (2012) 296–314

FEG-SEM; Pierre andMarie Curie University, Paris VI) operating in thebackscattered mode and equipped with an energy dispersive Si(L)detector with a resolution of 129 eV full width at half maximum ofthe Fe peak. The SEM investigations were carried out with anacceleration voltage of 15 kV and a working distance of 8 mm. Noneof the thin sections were coated with Au for SEM observations.

The six PGEs (Os, Ir, Ru, Rh, Pd and Pt), as well as Au, As, Se, Te, andBi were determined by in-situ analyses of BMS, using the LA-ICPMSfacility at GEMOC, Macquarie University, (Australia). A 266 nm “in-house” custom-built Continuum Surelite I-20 Q-switched quadrupledfrequency Nd:YAG laser coupled with an Agilent 7500 ICP-MS wasused for the study. The analytical procedure used is similar to thatdescribed in Alard et al. (2000) and Lorand and Alard (2001a), andincludes a 5 Hz laser frequency for an energy of 0.5 mJ/pulse; theablation was carried out in a pure He atmosphere. The spot size wasadjusted to the size of the sulfide grains (40–80 μm). Selectedmeasured isotopes for major elements were 33S, 34S, 61Ni, 62Ni, 63Cuand 65Cu and for trace element analyses 75As, 82Se, 99Ru, 101Ru, 103Rh,105Pd, 106Pd, 108Pd, 126Te, 189Os, 192Os, 193Ir, 195Pt, 197Au and 209Bi.The production rate of the 63Cu40Ar and 65Cu40Ar argide interferenceson the measured 103Rh and 105Pd was measured by analysing a pieceof pure Cu metal (JMC Cu976) at different energies and frequencies atthe beginning and at the end of each analytical run. The 63Cu40Ar(103Rh) and 65Cu40Ar (105Pd) argide production rate was low (0.03%)and 103Rh and 105Pd were corrected for interferences according to thelinear relation between elemental concentration (103Rh or 105Pd) andinterference (63Cu40Ar and 65Cu40Ar) found by analyzing the pure Cumetal. The accuracy of the correction was verified by correcting 105Pdfor its 65Cu40Ar interference, and by comparing it to 106Pd, which is

free from any major interference, except for 66Zn40Ar, of negligiblevalues for mantle sulfides. For 103Rh, which is mono-isotopic, we notethat the 63Cu40Ar interference correctionmay not be fully accurate forsome Cu-rich sulfides (e.g., MM54). 99Ru and 101Ru may also beaffected by interferences involving cobalt (59Co40Ar) and nickel(61Ni40Ar) and were not corrected for interferences in this study. SinceCo is mono-isotopic (100%) and the natural abundance of 61Ni is 1.14%,possible interferences may be expected to be 100 times higher on 99Ruthan on 101Ru. As both 99Ru and 101Ru yielded similar concentrationswithin uncertainty in the PGE-A standard (101Ru/99Ru=1.001), a NiSbead with 71.5 wt.% Ni and 0.2 ppm Co, the contribution of 61Ni40Ar on101Ru was found negligible. Similarly, most analysed BMS yieldedsimilar 99Ru and 101Ru concentrations, irrespective of the Ni concentra-tions in sulfides, indicating negligible contribution from 59Co40Ar and61Ni40Ar interferences. Nonetheless, a few sulfides from differentsamples show higher 99Ru than 101Ru, which could indicate a con-tribution from the 59Co40Ar argide in some analyses. For such sulfides,the 101Ru concentrations are reported in Table 4 caption. Sulfur content,determined by EMP, was used as the internal standard (34S). The countswere converted into concentrations using the Glitter software package(Griffin et al., 2008) and by measuring PGE-A, a synthetic NiS beaddoped with PGEs, Au and some chalcophile elements (Se, Te, As, Bi) asexternal standard (Alard et al., 2000). Isotopes chosen for concentrationsare reported in the caption to Table 4. Typical detection limits for theconditions described above vary between 10 ppb (Ir) and 60 ppb (Ru)for all PGEs (see Table 4) and those for As, Se, Te and Bi between 20 ppb(Bi) and 1.86 ppm (Se). The Ni and Cu contents of BMS in Tables 3 and 4are not directly comparable since 1) the BMS observed in 2D are com-plex intergrowths of several sulfide types and 2) LA-ICP-MS analyses

300 G. Delpech et al. / Lithos 154 (2012) 296–314

result in non-modal sampling with depth compared to the observedBMS in 2D. More details on the analytical methods can be found athttp://www.gemoc.mq.edu.au/AnMethods/AnlyticalMeth.html.

4. Sulfide petrography and major element composition of BMS

The BMS assemblages identified in the five xenoliths vary sig-nificantly between Group-I and Group-II samples, in agreement withthe bulk-rock chalcophile and siderophile element contents and PGEsystematics (Table 2). Therefore, the BMS will be described hereafterby reference to these two groups rather than using the petrographicsub-types defined for the Kerguelen harzburgites and dunites (e.g.,Grégoire et al., 2000a). Minor mackinawite and violarite have beenidentified as secondary sulfides partially replacing the Pentlandite.Other secondary phases are valleriite replacing the chalcopyrite, andmagnetite replacing the pyrrhotite (Table 2). All the secondarysulfides and magnetite are preferentially developed in intergranularBMS grains, adjacent to serpentine veins or weathered cracks. Only thefreshest sulfides have been analysed for major- and trace- elements inthis study. Photomicrographs of the BMS are presented in Fig. 2 (highresolution copy supplied as supplement material Fig. ES1), chemicalmaps and SEM images of the BMS are given as supplementary material(Figs. ES2 and ES3 respectively). Representative major element com-positions of single BMS analyses are given in Table 3.

4.1. Group-I sulfides

Sulfides in Group-I xenoliths consist of monosulfide solid solution(mss), pyrrhotite, pentlandite, chalcopyrite and minor bornite(Table 2). Modal proportions of Fe-Ni-rich sulfides (mss, pyrrhotite,pentlandite) range from 60 to 66% whereas the modal proportions ofCu-rich sulfide (chalcopyrite) range from 34 to 40% (Table 2).

Harzburgite GM453 — no solitary BMS inclusions (i.e. inclusionsaway from fracture planes and from secondary fluid inclusions) wereobserved in the major silicates (olivine, pyroxenes) of the harzburgite.The BMSblebs range in size from20 μmto 200 μmacross and aremostlylocated inside the silicate (now devitrified) and carbonate-melt pockets(Fig. 2a,b,c and ES2a,c). The shape of the BMS blebs varies from roundedto contorted when in contact with the silicate and carbonate-melt, toangular when they adhere to the silicate matrix. Some triangular blebsalso seem to protrude into themelt veinlets separating two grains of thesilicatematrix (Fig. ES2b).Most BMS blebs in GM453, regardless of theirshape, consist of lamellar intergrowths of tiny pentlandite flames insidepyrrhotite. Large (blocky) pentlandite grains are rare and two dif-ferent patterns of pentlandite-pyrrhotite intergrowths were identified;a) “interlocking” lamellae cutting across rod-shaped fibrous pyrrhotiteaggregates (Fig. 2c); b) highly porous, very fine-grained and stronglycleaved pentlandite-pyrrhotite intergrowths (Fig. 2b and ES2a). Major

Table 2Paragenesis and characteristic features of the BMS assemblages in the mantle xenoliths.

Group Sample Interstitial BMS phases Suab

mss Po/Tr Pn Ni-Pn Ccp Ccp singlephase

%F

I GM453 +++ + + + 66I MM101 + + + + - 55I MM54 + + ++ + (Bo) 63II GM214 + + ++ (+) - 88II BOB640 + + (+) - 94

Po; Pyrrhotite, Pn; Pentlandite; NiQPn; Ni-rich Pentlandite, Ccp; Chalcopyrite, Tr; Troilitecarbonate minerals; Sil; silicate minerals. chalcopyrite is most commonly associated with othphase indicates the occurrence of ccp or bornite single phase not associated with other BMThe assemblage in column “oxidation”, together with the occurrence of Tr replacing pyrrhot1985). Numbers in brackets are propagated errors on sulfide modal abundance. See supple

element compositions of intergranular, strongly cleaved pentlandite-pyrrhotite intergrowths (Fig. 3, Table 3) plot inside the high-temperature limit (1000 °C) of the mss stability field in the Fe-Ni-Sternary diagram (Fig. 3), with Ni/Fe ratios ranging between Ni-rich(19.8 wt.% Ni) and Ni-poor extremes (5–11.0 wt.% Ni) and all haveuniformly low Cu contents (b0.05 wt.%). The pyrrhotites with Nicontents higher than 5 wt.% will be referred hereafter as mss. No pureanalysis of pentlandite could be obtained due to the very fine-grainedexsolution of pentlandite along the cleavage planes of the pyrrhotite–pentlandite intergrowths. Pentlandite–pyrrhotite intergrowths arecommonly associated with various amounts of chalcopyrite (Fig. 2b,cand ES2b). The chalcopyrite occurs inside the pentlandite–pyrrhotiteintergrowths as jagged blobs enclosing small mackinawite grains or asexternal granules occupying various positions (e.g., angular corners oftriangular sulfide bodies; Fig. 2b,c). It has a Cu/Fe ~1 (at.%) and it hasvariable but often lowNi contents from0.08-0.81 wt.%. The chalcopyritegrain margins are often convex toward the mss (Fig. 2b,c). Pure mono-mineralic chalcopyrite (chalcopyrite single phase) blebs were observedin poikilitic augite rims that have reacted with the silico-carbonatedmetasomatic melt (Fig. ES2c). These chalcopyrite grains display highlycontorted grain boundaries developing typical wetting features againstthe host augite (and may contain small Cr-Spinel inclusion; Fig. ES2c),they have low Ni contents (0.41 wt.%), no Co and slightly lower Fecontents (29.9 wt.%) compared to chalcopyrite blebs attached topyrrhotite-pentlandite intergrowths. Finally, Cu-rich sulfide films alsooccur inside the silicate matrix (Fig. 2a).

Dunites MM54 and MM101 — the dunites contain smaller BMS(average size=50 μm). Intergranular BMS blebs show some character-istic features reminiscent of BMS in the harzburgite GM453. Two types ofintergranular BMS are observed: 1) sulfide blebs consisting of pyrrhoti-te+pentlandite flames+chalcopyrite and 2) chalcopyrite+pentlanditetwo-phase grains. Solitary single sulfide inclusions occur in olivineneoblasts (Fig. 2e, less than 20 μm across). The olivine-hosted inclusionsdisplay BMS assemblages similar to the intergranular blebs and consist ofeither pyrrhotite–pentlandite–chalcopyrite assemblages or pentlandite-chalcopyrite two-phase droplets (Fig. 2e).

1) Pyrrhotite+pentlandite flames+chalcopyrite blebs are oftenrounded or ovoid and are common inside devitrified melt pocketsor in the K-feldspar+clinopyroxene+carbonate+apatite veinscross-cutting the dunite (Fig. 2d,g and ES2d,e). Petrographicobservations and X-ray maps indicate i) a higher pentlanditecontent in the dunitic BMS (pyrrhotite~10–30%; pentlandite~70–90%, pyrrhotite and pentlandite contents estimated from the X-raymaps for Ni using Adobe PhotoshopTM) than in theharzburgitewheremost grains are mss (Fig. 2b,c and ES2a), and ii) a higher contentof chalcopyrite, forming massive blebs attached to pentlandite-pyrrhotite assemblages (Fig. 2g, Table 2).

lphideundance

Sulphideoccurrence

Association Oxidation

e-Ni %Cu vein pocket Cb Sil

(6) 34 (4) ++ ++ ++ ++ + (Mag+Viol+Mack)(20) 45 (13) ++ ++ ++ ++ + (Mag+Val+Mack)(6) 37 (4) ++ ++ ++ ++ + (Mag+Val+Mack)(5) 12 (4) + ++ ++ + + (Mag+Val+Mack)(5) 6 (2) - ++ ++ - + (Mag+Val+Mack)

, Mag; Magnetite, Val; Valleriite, Mack; Mackinavite, Viol; Violarite, Bo; Bornite, Cb;er BMS to form three- or two-phase BMS blebs (see text for details). chalcopyrite singleS (e.g., mss, pyrrhotite, pentlandite).ite, indicate a typical paragenesis of incipient serpentinisation in mantle rocks (Lorand,mentary material ES5 for calculations.

MM94-54GM92-453

Ol

Cb

Ol

Cpx

Po-PnCcpCcp

Ol

PoPn

Ccp

Pn-Ccp

Cu-richsulf.

mss

Ccp

mss

MM94-101

Cb

Po-Pn

Cr-Sp

CcpPn

Ccp

Pn

Ol

Ol

Ol

Pn

Ccp

Ccp

0,5 mm 0,5 mm

a

b

c

e

f

g

i

j

k

GM92-214 BOB93-640

Cb

Ol

Ccp

Ol

Po

Pn

Po

Pn

Pn

OlPn

Po

Ccp

Pn

Ccp

50 µm 1 mm

30 µm 40 µm

50 µm 50 µm

m

n

o

Group I sulfides Group II sulfides

Ccp

h

PnCb

Ol

CpxCcpl

d

Fig. 2. Photomicrographs of BMS under the microscope of group-I and -II xenoliths. GM453: a) Interstitial mss (pyrrhotite+pentlandite) in pocket with devitrified glass and associatedwith metasomatic clinopyroxene. Note the sulfide inclusions in the cpx and the Cu-rich sulfide melt in the veins between the silicates. b) Interstitial pyrrhotite+pentlandite+chalcopyrite showing pentlandite exsolutions along the cleavage of the pyrrhotite and a chalcopyrite bleb with curved outward margins toward the mss. c) Quenched texture in a mss(pyrrhotite+pentlandite+chalcopyrite) showing an interlocking texture of the pyrrhotite+pentlandite assemblage. MM54 : d) Interstitial pentlandite+chalcopyrite associated withcarbonate in melt pocket inside the dunitic matrix. e) Unfractured pyrrhotite+pentlandite+chalcopyrite inclusion in an olivine grain. f) Bi-mineralic pentlandite-ccp bleb with cleavedpentlandite and massive chalcopyrite grains on the outer rim or inside the pentlandite core. Some chalcopyrite also exsolved inside the cleavage of the pentlandite. MM101: g) RoundedBMS showing a pyrrhotite-pentlandite intergrowth, a massive pentlandite core and massive chalcopyrite, in contact with a carbonate. h) Large bi-mineralic chalcopyrite+pentlanditegrain with dominant chalcopyrite in the interstice of the dunite. i) Bi-mineralic chalcopyrite-pentlandite grain with dominant chalcopyrite andwith Cu-rich films departing from the BMSin the interstices of the dunitic matrix. GM214 : j) Immiscible pentlandite+chalcopyrite grain in a former carbonate melt pocket (dolomite). Grey Fe-hydroxydes at the rim of thepentlandite+chalcopyrite result from low-T alteration (~20 μm sulfide globule). k) Unfractured enclosed pyrrhotite+pentlandite+chalcopyrite inclusion in a cpx grain (~17×10 μm).l) Bi-mineralic pn-ccp bleb with massive pentlandite grains with minor chalcopyrite on the rim, in contact with a carbonate. BOB640 : m) Abundant interstitial pyrrhotite+pentlandite(±chalcopyrite) sulfides inside the dunitic matrix. n) Unfractured pyrrhotite+pentlandite inclusion in an olivine grain (b20 μm sulfide globule). o) Interstitial large pyrrhotite+pentlandite+chalcopyrite sulfide away from serpentine veins. pentlandite occurs as flames inside the pyrrhotite. Note the blocky Cu-sulfide on the rims or inside of the dominantpyrrhotite+pentlandite.

301G. Delpech et al. / Lithos 154 (2012) 296–314

2) In addition to the pyrrhotite-pentlandite-chalcopyrite assemblages,both dunites contain abundant intergranular chalcopyrite+pentlandite two-phase grains (sometimes with minor pyrrhotite),which commonly have ovoid shapes (Fig. 2d,f,h,i and ES2e). Thepentlandite may appear blocky and highly fractured and is asso-ciated with massive chalcopyrite (Fig. ES2e). These bi-mineralicBMS blebsmay be found in apatite-rich veinlets (MM101, Fig. ES2d),where carbonate and apatite grains sometimes adhere to thesedroplets or may be included in either of the two sulfides (Fig. 2d,g).Other BMS may be associated with secondary Cr-rich spinel+clinopyroxene+apatite assemblages inside carbonate melt pockets(MM54, Fig. ES2e). Thin Cu-rich sulfide veins are observed branchingoff of these carbonate melt pockets, and fill olivine grain boundariesalong with interstitial glass (Fig. 2i). Cu-rich sulfide veins some-times grade in continuity into arrays of secondary fluid inclusionscutting across olivine grains. Bornite has occasionally been identifiedinside mono-mineralic chalcopyrite grains that are in contact withcarbonates.

The pyrrhotites in the pyrrhotite-pentlandite-chalcopyrite blebsare hexagonal-type pyrrhotite (Fig. 3, Hpo; Fe9S10; 38.1±0.1 wt.%S)with low oxygen (b0.8 wt.%) and low-Ni contents (Nib0.5 wt.%).They coexist with troilite-type FeS compositions in dunite MM101(Table 3), frequently producing pyrrhotite mixtures FeS-Fe11S12.Pentlandite in the group-I dunites displays two intervals of Ni/Feratios, depending on whether it coexists with pyrrhotite-pentlandite-

chalcopyrite three-phase grains or with pentlandite-chalcopyrite bi-mineralic grains. The pentlandite flames in pyrrhotite-pentlandite-chalcopyrite three-phase grains (Fig. 2 g, ES2d) are pentlandites(30–34 wt.% Ni) with Ni/Fe ratios in the range 0.8–1.0 and Co up to1 wt.%; they coexist with low-Ni pyrrhotite and chalcopyrite (Fig. 3).However, pentlandites in bi-mineralic pentlandite-chalcopyritegrains are often highly nickeliferous (up to 42 wt.% Ni, Fig. 3) andhave Ni/Fe ratios>1 (Table 3). Chalcopyrite coexisting with pyrrhotite-pentlandite-chalcopyrite or pentlandite-chalcopyrite blebs has Cu/Feratios~1 and is almost Ni-free (0.08 wt.%). The amount of oxygen isvariable in the cchalcopyrite (0–4.36 wt.%). High oxygen contentscorrespond to lower S and Cu contents, indicating incipient oxidation insome chalcopyrites.

4.2. Group-II sulfides

The Group-II dunites (GM214 and BOB640) contain abundantinterstitial sulfide grains (average grain size=80 μm, Fig. 2q), whichconsist of pyrrhotite, pentlandite and minor chalcopyrite (Table 2)and occur interstitially in the olivine matrix. The modal proportions ofFe\Ni-rich sulfides are much higher than in the Group-I xenoliths(88–94%) and the chalcopyrite modal proportions are low (6–12%).The rare chalcopyrite always occur within pyrrhotite or pentlanditegrains (Fig. 2n,l,o), in agreement with the low bulk-rock Cu contentsof these samples (Table 1). Some BMS are associated with fluidinclusion trails that crosscut the olivine neoblasts. The enclosed BMS

Table 3Major element data of sulfides in the harzburgite and dunites.

Group Sample Position Assemblage Phase core/rim Laser spot Fe Ni Co Cu S O Total Fe Ni Co Cu S Total M/S Ni/Fe

wt.% at.%I GM453 i mss mss c − 39.07 17.23 0.23 0.01 36.10 6.12 98.86 32.95 13.83 0.18 0.01 53.03 100.00 0.89 0.42I GM453 i mss+po mss r 453C1S2 43.92 19.55 0.49 0.07 35.03 0.24 99.33 35.41 14.99 0.38 0.05 49.18 100.00 1.03 0.42I GM453 i mss c 48.53 14.10 0.31 0.01 34.82 0.56 98.33 39.49 10.92 0.24 0.00 49.35 100.00 1.03 0.28I GM453 i Po c 57.48 4.36 0.11 0.05 37.00 0.36 99.39 45.54 3.29 0.08 0.03 51.06 100.00 0.96 0.07

GM453 i mss+ccp mss c 453C1S1 44.32 13.47 0.28 0.00 36.52 1.62 96.24 36.62 10.59 0.22 0.00 52.56 100.00 0.90 0.29I GM453 i ccp c 32.92 0.81 0.00 27.66 32.39 4.36 98.18 28.77 0.67 0.00 21.25 49.31 100.00 1.03 0.02I GM453 i mss+po Po c 453C4S1 55.13 4.33 0.11 0.07 38.26 1.17 99.06 43.74 3.27 0.08 0.05 52.87 100.00 0.89 0.07I GM453 i mss c 36.78 19.82 0.43 0.04 37.44 3.60 98.22 30.33 15.55 0.33 0.03 53.76 100.00 0.86 0.51I GM453 i mss c 44.44 13.63 0.24 0.00 38.70 1.79 98.83 35.54 10.37 0.18 0.00 53.91 100.00 0.86 0.29I GM453 e (cpx) ccp ccp r − 29.92 0.14 0.00 33.85 33.87 0.35 98.13 25.19 0.11 0.00 25.04 49.66 100.00 1.01 0.00I GM453 i mss+ccp mss c 453C7S1 44.54 14.16 0.23 0.00 34.99 2.78 96.73 37.37 11.31 0.18 0.00 51.14 100.00 0.96 0.30I GM453 i ccp r 30.88 0.47 0.00 33.08 33.24 1.00 98.69 26.11 0.38 0.00 24.57 48.94 100.00 1.04 0.01I MM101 i Po+Pn+Ccp Tr flame T101S1 62.88 0.32 0.00 0.03 35.95 0.36 99.54 49.98 0.24 0.00 0.02 49.76 100.00 1.01 0.00I MM101 i Ni-Tr c 59.16 3.42 0.02 0.05 34.40 1.55 98.59 48.34 2.66 0.01 0.03 48.95 100.00 1.04 0.05I MM101 i Ni-poor Pn c 39.99 25.84 0.41 0.05 33.49 0.44 100.22 32.43 19.93 0.31 0.03 47.29 100.00 1.11 0.61I MM101 i (vein) Pn+Ccp Ccp patch on rim U101S1 30.64 0.90 0.00 33.09 34.17 0.02 98.83 25.52 0.72 0.00 24.21 49.55 100.00 1.02 0.03I MM101 i (vein) Ni-Pn patch on rim 27.86 38.46 0.39 0.01 32.78 0.35 99.84 22.85 30.02 0.30 0.00 46.83 100.00 1.14 1.31I MM101 i (vein) Pn c 29.10 27.80 0.21 8.14 32.89 1.85 99.99 24.21 22.00 0.17 5.95 47.66 100.00 1.10 0.91I MM101 i Po+Pn+Ccp Ccp patch on rim PQ101C2S1 31.03 0.10 0.00 34.30 34.44 0.25 100.12 25.59 0.08 0.00 24.86 49.47 100.00 1.02 0.00I MM101 i Tr flame 63.16 0.99 0.00 0.04 36.01 0.42 100.62 49.79 0.74 0.00 0.03 49.44 100.00 1.02 0.01I MM101 i Pn c 38.28 27.75 0.47 0.06 33.46 0.28 100.29 31.01 21.39 0.36 0.04 47.20 100.00 1.12 0.69I MM101 i Ni-Tr c 60.78 3.37 0.02 0.05 35.74 0.20 100.16 48.13 2.54 0.02 0.03 49.29 100.00 1.03 0.05I MM54 i Po+Pn+Ccp Ccp c − 30.43 0.30 0.00 34.22 34.44 0.40 99.79 25.20 0.24 0.00 24.90 49.67 100.00 1.01 0.01I MM54 i Ni-Pn c − 32.87 33.46 0.62 0.19 32.80 0.25 100.20 26.81 25.97 0.48 0.14 46.60 100.00 1.15 0.97I MM54 i Hpo c − 61.45 0.45 0.00 0.08 37.94 0.35 100.27 48.00 0.33 0.00 0.05 51.61 100.00 0.94 0.01I MM54 i Po+Pn+Ccp Ni-Hpo c − 60.16 3.91 0.00 0.28 35.73 0.34 100.42 47.61 2.95 0.00 0.20 49.24 100.00 1.03 0.06I MM54 i Ni-poor Pn c − 46.17 19.08 0.33 0.06 34.49 0.23 100.36 37.01 14.55 0.25 0.04 48.15 100.00 1.08 0.39I MM54 i Pn+Ccp Ccp c 54C5S1 30.70 0.40 0.00 34.70 33.58 0.16 99.54 25.57 0.32 0.00 25.40 48.72 100.00 1.05 0.01I MM54 i Pn c 34.32 31.54 0.33 0.40 32.04 0.28 98.92 28.41 24.84 0.26 0.29 46.19 100.00 1.16 0.87I MM54 i Pn-ccp Ni-Pn c 54C6S1 25.81 39.76 0.47 0.30 31.98 0.28 98.60 21.50 31.52 0.37 0.22 46.39 100.00 1.16 1.47I MM54 i Ni-Pn c 25.68 39.53 0.47 0.25 32.29 0.23 98.45 21.36 31.29 0.37 0.18 46.79 100.00 1.14 1.46II BOB640 i Po+Pn Hpo c HBOBC1S1 61.54 0.36 0.00 0.02 38.17 0.40 100.49 47.94 0.27 0.00 0.01 51.78 100.00 0.93 0.01II BOB640 i Fe-Pn Patch Pn inside Po 35.48 29.61 0.77 0.03 33.22 0.78 99.89 29.02 23.04 0.60 0.02 47.32 100.00 1.11 0.79II BOB640 i Po+Pn+Ccp Hpo c IBOBC2S1 61.25 0.48 0.00 0.00 37.92 0.38 100.03 47.95 0.36 0.00 0.00 51.69 100.00 0.93 0.01II BOB640 e (ol) Po+Pn+Ccp Mpo c − 55.56 4.71 0.14 0.04 39.45 0.35 100.24 43.10 3.48 0.10 0.03 53.29 100.00 0.88 0.08II BOB640 i Po+Pn Hpo c KBOBC4S1 61.64 0.44 0.00 0.00 38.07 0.35 100.50 48.02 0.32 0.00 0.00 51.65 100.00 0.94 0.01II BOB640 i Pn r 32.87 32.13 0.85 0.17 33.10 0.40 99.52 26.93 25.05 0.66 0.13 47.24 100.00 1.12 0.93II BOB640 i Po+Pn+Ccp Hpo c MBOBC5S1 61.11 0.51 0.00 0.03 38.20 0.29 100.13 47.69 0.38 0.00 0.02 51.91 100.00 0.93 0.01II BOB640 i Pn r 32.41 31.50 0.97 0.81 32.28 0.67 98.63 26.96 24.93 0.76 0.59 46.76 100.00 1.14 0.92II BOB640 i Pn+Ccp Ccp Patch on sulf rim NBOBC6S1 30.57 0.34 0.00 34.21 34.41 0.24 99.76 25.29 0.27 0.00 24.87 49.57 100.00 1.02 0.01II BOB640 i Pn c 31.42 33.60 0.77 0.10 32.43 1.41 99.74 26.04 26.49 0.61 0.07 46.79 100.00 1.14 1.02II GM214 i Pn+Ccp Ccp Patch on rim F214C1S1 30.11 0.12 0.00 33.45 34.53 0.91 99.13 25.14 0.10 0.00 24.55 50.21 100.00 0.99 0.00II GM214 i Fe-Pn c 32.46 33.16 0.31 0.11 33.34 0.93 100.31 26.50 25.76 0.24 0.08 47.42 100.00 1.11 0.97II GM214 i Pn+Ccp Ccp c G214C1S1 30.87 0.25 0.00 33.47 34.45 0.32 99.36 25.61 0.19 0.00 24.41 49.79 100.00 1.01 0.01II GM214 i Fe-Pn c 38.36 26.12 0.32 0.12 33.49 0.56 98.97 31.46 20.38 0.25 0.09 47.83 100.00 1.09 0.65II GM214 i Pn+Po+Ccp Ni-Pn c H214C3S1 29.02 36.88 0.38 0.00 32.76 0.33 99.37 23.88 28.88 0.30 0.00 46.95 100.00 1.13 1.21II GM214 i Ni-Pn c 29.36 37.53 0.41 0.00 33.00 0.40 100.71 23.88 29.05 0.31 0.00 46.75 100.00 1.14 1.22II GM214 i Pn Pn Patch on rim − 27.38 29.66 0.57 7.41 32.18 1.07 98.31 23.07 23.78 0.45 5.49 47.21 100.00 1.12 1.03II GM214 i Ni-Pn c − 26.93 38.00 0.69 0.76 32.08 0.36 98.83 22.39 30.06 0.55 0.56 46.45 100.00 1.15 1.34

i; interstitial sulfide, e; enclosed sulfides, c; core of mineral, r; rim of mineral.Po; Pyrrhotite, Hpo; Hexagonal-type pyrrhotite, Mpo; Monoclinic-type pyrrhotite, Pn; Pentlandite; Ni-Pn; Ni-rich Pentlandite, Ccp; Chalcopyrite, Tr; Troilite.M/S=100*((Fe+Ni+Co+Cu)/S) at.%.

302G.D

elpechet

al./Lithos

154(2012)

296–314

S (wt.%)

Cu (wt.%) 30 102030

55S (wt.%)

GM453

Ccp

S (wt.%)

Cu (wt.%) 30 102030

55

CbnPn

Py

Tr

Vs

Vi Pm

Mi

Gs

MPoHPo

Ni+Co (wt.%)

S (wt.%)

Fe (wt.%)

S (wt.%)

Cu (wt.%)30 1020

55

S (wt.%)

Fe (wt.%) Ni+Co (wt.%)

Pn

Py

TrHPoMPo

Vs

Vi Pm

Mi

Gs

GM214

55

30

S (wt.%)

Cu (wt.%) 30 102030

55

S (wt.%)

Fe (wt.%) Ni+Co (wt.%)

Pn

Py

TrHPoMPo

Vs

Vi Pm

Mi

Gs

BOB640

S (wt.%)

Cu (wt.%) 30 102030

55

S

Fe

Ni+CoCu

55

30

40mss 1000°C mss 900°C

mss 1100°C

Gr II

Gr II

Gr I

Gr I

Gr I

Ni-Pn

Pn

Pn

mss30 40 50 6010 20

po mss

MM54

Ni+Co (wt.%)

S (wt.%)

Fe (wt.%)

Pn

Py

TrHPoMPo

Vs

Vi Pm

Mi

Gs30

Ni-PnPo Pn

Ccp

Ccp

Ccp

Ccp

55

55

70

30 40 50 6010 20

30 40 50 6010 20

30 40 50 6010 20

Cbn

Cbn

Cbn

Cbn

MM101

Fe (wt.%) Ni+Co (wt.%)

Pn

Py

TrHPoMPo

Vs

Vi Pm

Mi

Gs

55

30

Ni-PnPo Pn

30 40 50 6010 20

Interstitial po-pn-ccp Interstitial pn-ccp

IncludedInterstitial

Interstitial pn-ccp

pnccp

pn, poccp Mpo

Interstitial po-pn-ccp Interstitial pn-ccp

Interstitial ccp

Fig. 3. Sulfides compositions in the Fe-Ni-Cu system (wt.%). Mss compositions range at 900, 1000 and 1100 °C after Kullerud et al. (1969) and Craig and Scott (1974). Compositions forstoichiometric phases are shown in light grey. Ccp; chalcopyrite (CuFeS2), Cbn; cubanite (Fe2CuS3); Tr; troilite (FeS), Mpo; monoclinic pyrrhotite (Fe7S8), Hpo; hexagonal pyrrhotite(Fe9S10), Vi; violarite ((Ni,Fe)2 S4), Vs; vaesite (2(Fe,Cu)S2), Pm; polydymite (Ni3S4), Mi; millerite (NiS), Gs; godlevskite (Ni7S6), Pn; pentlandite ((Fe,Ni)9 S8), Py; pyrite (FeS2).

303G. Delpech et al. / Lithos 154 (2012) 296–314

in olivine grains have preserved homogeneous pyrrhotite-rich assem-blages where pentlandite and chalcopyrite occur as small grainslining the walls of the sulfide inclusions (Fig. 2k,n), as in BOB640where the pyrrhotite has a monoclinic-type composition (Fig. 3,Mpyrrhotite, Fe7S8) rich in Ni (4 wt.% Ni, Table 3). Intergranularsulfides in dunites GM214 and BOB640 have different characteristicsthat are detailed below.

The BMS in dunite GM214 display mineralogical characteristicsreminiscent of BMS in group-I dunites, such as small ovoid sulfides (20to 50 μm on average) found in (melt) pockets (Fig. 2j), whichsometimes display low dihedral angles against the olivine (Fig. ES2f).The dominant BMS is a highly fractured andmassive pentlandite, whichsometimes containsmicropatches of chalcopyrite distributed randomly(Fig. 2l and ES2f). Most pentlandite in the pn-ccp blebs is rich in Ni and

displays Ni/Fe ratios>1 (Fig. 3, Table 3) and the ccp associated to the pnhas Cu/Fe=1 and low Ni contents (0.07–0.27 wt.%). A micrometricPd\Te\As PGM (approximately 1 μm across) was found enclosed in amassive pentlandite grain (Fig. ES3a) and a gold micronugget wasobserved in the interstices of the peridotitic matrix (Fig. ES3b). Somerare BMS composed of pyrrhotite-pentlandite-chalcopyrite also coexistwith the abundant pentlandite-chalcopyrite blebs and most displayevidence of secondary alteration affecting mainly the pyrrhotite, whichis now replaced by magnetite+valleriite or has totally disappeared.No pyrrhotite was analysed in dunite GM214 given the evidence ofweathering.

The BMS in dunite BOB640 are preferentially located interstitiallyat triple junctions between olivine neoblasts (Fig. 2m) and frequentlycoexist with polycrystalline dolomite that shows rectilinear contacts

304 G. Delpech et al. / Lithos 154 (2012) 296–314

with the BMS, indicating textural equilibrium (Fig. ES2g), but werenot found in (melt) pockets (Fig. ES2g). The shape of BMS grains iscontrolled by the geometry of the interstitial pores (i.e., ovoid BMSblebs in ovoid interstitial pores; Fig. ES2g). In pockets where car-bonate is absent, the BMS occur as polygonal grains (up to 200 μmacross) with margins that curve inward but never with a dihedralangleb60° (Fig. 2o). The BMS invariably consist predominantlyof pyrrhotite containing abundant flame-like pentlandite (Fig. 2oand #ES2g). Modal proportions of pyrrhotite/pentlandite are highand range from 70/30 to 90/10 (estimated from the X-ray maps forNi using Adobe Photoshop™). The pyrrhotite associated with thepo-pn±ccp blebs is a hexagonal-type pyrrhotite (Fig. 3, Hpo, Fe9S10)with low Ni contents (0.33–0.68 wt.%). The pentlandite flames asso-ciated with the pyrrhotite have Ni/Fe~1 and very low Cu contents(Fig. 3, Table 3). In this sample are no bi-mineralic pn-ccp blebs and nopentlandite has Ni contents as high as in GM214 (Fig. 3). Secondaryalteration of the pyrrhotite is evidenced by its replacement bymagnetite and Fe-hydroxides or pyrrhotite cores, which have troilitecompositions (FeS, Fig. 3) coexisting with Fe-rich pentlandite flames.Chalcopyrite is observed as thin and discontinuous films commonlyseparating the BMS from the silicate matrix minerals (Fig. ES2g) or aspatches interlocked with the pentlandite (Fig. 2o). The chalcopyriteassociated with the pyrrhotite and pentlandite in the pyrrhotite-pentlandite–chalcopyrite blebs has Cu/Fe=1 (at.%) and low Nicontents (0.21–0.34 wt.%). A submicron-sized PGM microphase con-taining Pd-Pt-As asmajor elementswas found attached to the outer rimof a weathered pyrrhotite–pentlandite grain (Fig. ES3c).

6. In-situ analyses of chalcogenides and platinum-group elements

6.1. Se, Te, As and Bi

Concentrations of Se, As, Te and Bi in BMS vary by several orders ofmagnitude (Table 4) and co-variations of these elementswith Cu (ppm)and S (wt.%) are shown in Fig. 4. In Fig. 4a, mss-pyrrhotite and fine-grained pentlandite-pyrrhotite intergrowths in the harzburgite GM453and dunite BOB640 are poorer in Se thanmost pentlandite-chalcopyritebi-mineralic grains in other dunites (MM101,MM54, GM214). Sulfideswith differing S/Se ratios are foundwithin single samples, reflecting thediversity of the BMS mineralogy in each sample, especially in group-Idunite MM101 and harburgite GM453. The pyrrhotite-rich grains indunite BOB640 (Group-II) have low Se (27.9–82.8 ppm) and thehighest S/Se ratios (6000–12,000). The range of Se concentrationsmeasured in the BMS from Group-I dunites MM54 and MM101 wherepyrrhotite-pentlandite-chalcopyrite or pentlandite-chalcopyrite areabundant (78–150 ppm) is similar to the range of Se measured inother mantle-derived pentlandite-rich BMS assemblages (Lorand andAlard, 2010a; Lorand et al., 2008; Luguet et al., 2004). For example,Lorand and Alard (2010a) reported a range of Se concentrations inpentlandite from 73 to 261 ppm (mean 134±42 ppm) in Pyreneanperidotites. For dunites MM54 and BOB640, the measured in-situ S/Seratios in the BMS (S/Se=2991±628 and 8812±3861 respectively)match the bulk-rock (S/Se) reported by Lorand et al. (2004) reasonablywell (S/SeWR=2653 and 10000 respectively, Table 1), indicating that aBMS population representative of these samples has been analysed byLA-ICP-MS. In the harzburgite GM453, the bulk-rock S/Se appears low(S/SeWR=2101) compared to the in-situ data (S/Se=5293±1975), dueto the lack of analyses of the Cu-rich sulfide component (higher S/Se).

In Fig. 4b,c, the concentrations of Te (2.23–66.6 ppm) and Seleniumcorrelate positively with Cu in the BMS. Mss/pyrrhotite-rich assem-blages in BOB640 and GM453 have relatively low contents of both Se(b 100 ppm) and Te (b12 ppm). In contrast, bimineralic pentlandite-chalcopyrite assemblages in Group-I dunites show large variations inSe and Te with Cu except for two pyrrhotite-pentlandite-chalcopyritegrains in dunite MM101, which have a higher Se contents (#PQ101and #T101) at Cu content similar to mss/pyrrhotite-rich BMS. The

time-resolved LA-ICP-MS analyses show that some of the traceelements, especially for As, Se and Bi, are not distributed homoge-neously in some BMS. In dunite MM101, the time-resolved spectrashow the occurrence of several microphases rich in Te\Bi±Pt insideBMS #U101S1 (Table 4).

The As concentrations are also highly variable in the BMS andrange from 0.41to 224 ppm (Table 4). The As contents are higher inCu-poor BMS such as mss in harzburgite GM453 (224 ppm in#453C1S2, Table 4) than in most other analysed sulfides MM101and reflect the preferential partitioning of As into mss/pyrrhotitephases. However, As does not appear to be homogeneously distrib-uted in some cases as the LA-ICP-MS signal increases at some pointafter the beginning of the ablation. In most cases, the As signal is notcorrelated with any other chalcophile or siderophile elementsreported in this study. As contents may also be high in BMS blebsthat contain PGM microphases such as the Pt\Pd\As-rich PGMintercepted during ablation of one BMS in dunite BOB640 (#KBOBC4S1;Table 4). Some PGMmay occur on the outer rims of the BMS in the samerock such as the Pd\Pt\As-rich microphase that was observed on therim of a pyrrhotite-pentlandite BMS with the SEM. In dunite GM214,the SEM images show a rounded Pd\Te\As (\Pt) microphase wasobserved inside a Ni-rich pentlandite (Fig. #ES3a). Bismuth concentra-tions (0.14–6.7 ppm) do not show any clear correlation with either Cu,Te or As although some Te\Bi±Pt microphases were observed duringlaser ablation.

6.2. Platinum-group elements

Concentrations of platinum-group element are reported in Table 4and shown as CI-normalised PGE patterns in Fig. 5. Due to theirrelatively small size, LA-ICP-MS data of the BMS represent a mixtureof the different proportions of two or three BMS phases (pyrrhotite-pentlandite-chalcopyrite) in each bleb.

The fourmss intergrowths analysed inGM453 display highly variablecontents of Os (2.0–296 ppm), Ir (0.95–130 ppm), Ru (2.1–648 ppm)and Rh (2–67.3 ppm). Due to their high concentrations in Os-Ir-Ru-Rhrelative to Pd and Pt, the four mss-like phases exhibit negative-trendingCI-chondrite normalised PGE patterns (Fig. 5a) with (Pd/Ir)Nb1, andthey all have superchondritic Os/Ir and Ru/Ir ratios (1.73–2.10 and 1.23–3.65 respectively). Pd and Pt contents are somewhat less variable (e.g.,0.1bPdb7.76 ppm), and a selective enrichment in Pd relative to Pt(1.49b(Pd/Pt)Nb11.73) is observed in mss but not in the pentlandite-chalcopyrite assemblage (Table 4). The CI-chondrite normalised PGEpatterns of these mss-type sulfides are reminiscent of those of enclosedresidual sulfides in continental mantle xenoliths (Fig. 5b), but theenrichment in Pd is not. Gold contents vary from below LLD and up to1.26 ppm.

Of the four dunites, samples MM54 and MM101 have BMSwith the lowest PGE contents (Fig. 5c–f; 0.1–10×CI-Chondrite;McDonough and Sun, 1995). The CI-chondrite normalised PGEpatterns in Fig. 5c and d represent mixtures of the three-phase(pyrrhotite+pentlandite+chalcopyrite) or two-phase (pentlandite+chalcopyrite) BMS intergrowths (Table 4). In MM101, three BMS(#T101S1, #S101S1, #U101S1) have high contents of Os (4.53–28.2 ppm), Ir (2.53–20.9 ppm), Ru (9.17–45.8 ppm) compared to Pd, Ptand Au (Table 4, Fig. 7c) and display a negative-trending PGE patternfrom Os to Pt, reminiscent of mss found in the harzburgite (Fig. 5a).Among these three BMS, two are bi-mineralic Ni-rich pentlandite-chalcopyrite sulfides found in late metasomatic silicate veins cross-cutting the primarymineralogy (#S101S1, #U101S1, Table 4, #ES2d) andthey yielded similar PGE contents. Other three phase grains (pyrrhotite–pentlandite–chalcopyrite) or bi-mineralic pentlandite-chalcopyrite inboth dunites display variable but contents of Os, Ir and Pt (0.1×CI-Chondrite) lower than Ru and Rh, and are all slightly enriched in Pdrelative to Ir ((Pd/Ir)N=8–20), which reflects the larger amount ofchalcopyrite ablated in the BMS. Ni-rich pentlandite grains inMM54have

Table 4Highly siderophile and chalcophile trace element data of sulfides in the harzburgite and dunites.

Group Sample Position Analysis# Phase Os Ir Ru Rh Pt Pd Au Ni Cu As Se Te Bi

ppm ppm ppm ppm ppm ppm ppm wt.% wt.% ppm ppm ppm ppm

I GM453 I 453C1S1 mss+ccp 2.03 1.09 2.08b 0.21 0.05 a 0.10 0.03 a 10.27 0.77 113 51.6 2.29 0.29I GM453 I 453C1S2 mss+Po 2.06 0.95 1.87b 0.05 a 0.20 0.16 0.03 a 11.72 0.05 224 54.4 2.23 0.31I GM453 I 453C4S1 mss+Po 296 131 648 67.3 3.31 7.76 bdl 11.91 0.26 21.3 nd nd ndI GM453 I 453C7S1 mss+Ccp 17.6 9.15 52.10 6.69 1.14 7.30 0.13 26.49 2.94 8.80 87.2 4.29 0.91I GM453 I 453C6S1 Pn+Ccp 6.30 bdl 1.23 0.40 0.30 0.13 1.26 12.84 1.64 60.3 99.9 11.9 3.75I MM101 i O101C1S1 Ccp (+Pn) 0.45 bdl 2.04 bdl 2.75 4.58 0.56 9.15 16.30 bdl 247 66.6 1.64I MM101 I PQ101C2S1 Po+Pn+Ccp 0.11 0.06 4.62 b 0.28 0.67 1.02 bdl 22.92 0.43 bdl 146 6.21 1.32I MM101 I R101S1CA Ccp (+Po+Pn) 0.13 0.05 3.24 b 0.68 0.31 0.90 bdl 15.69 8.77 0.41 a 127 8.92 1.06I MM101 i (vein) S101S1 Pn+Ccp 5.91 3.07 9.54 0.42 5.84 2.50 0.98 10.41 10.17 2.73 164 44.9 2.35I MM101 I T101S1 Po+Pn+Ccp 28.2 20.9 45.8 1.49 4.28 1.92 0.28 19.98 0.10 1.80 168 27.2 2.31I MM101 i (vein) U101S1(⁎) Pn+Ccp 4.53 2.53 9.17 0.67 3.30⁎ 2.98 0.58 17.60 7.66 1.85 174 51.7⁎ 5.99⁎I MM54 I 54C2S1 Ni-Pn+Ccp 0.89 0.80 2.12 1.11 2.05 0.79 0.13 a 19.63 4.83 9.03 112 14.6 4.85I MM54 I 54C3S1 Pn+Ccp (altered) 0.70 0.85 5.51 2.77 1.06 0.82 bdl 16.64 7.42 1.19 90.3 10.8 2.18I MM54 I 54C4S1 Pn+Ccp+Po 0.90 bdl 3.62 2.54 1.46 0.35 bdl 10.82 6.77 2.22 91.2 8.11 0.86I MM54 I 54C7S1 Pn+Ccp 5.18 1.98 3.48 b 5.77 2.66 1.43 bdl 6.70 11.68 6.28 141 17.0 1.82I MM54 I 54C6S1 Ni-Pn+Ccp 2.09 0.98 7.94 2.02 2.41 1.19 0.04 a 18.16 7.70 1.82 135 13.5 2.33I MM54 I 54C5S1 Pn+ccp 1.21 bdl 4.6 b 1.20 1.54 0.60 bdl 7.75 11.41 5.27 134 30.3 0.14I MM54 I 54C1S1 Po (+Pn) 0.45 0.14 6.37 1.10 1.16 0.58 bdl 10.44 3.69 7.76 78.1 7.25 0.33II BOB640 I HBOBC1S1 HPo+Fe-Pn 4.70 5.00 16.8 3.53 13.2 39.6 13.3⁎ 16.06 0.28 nd 27.9 nd 6.70II BOB640 I IBOBC2S1 HPo+Pn (+Ccp) 8.28 3.06 16.7 b 3.3 7.23 28 bdl 17.71 0.07 nd 32.0 4.29 0.68II BOB640 I KBOBC4S1 (⁎) HPo+Pn 51.7 11.2 50.4 9.51 41.8⁎ 322⁎ 2.19⁎ 17.17 0.85 112⁎ 82.8 9.03 0.50II BOB640 I MBOBC5S1 HPo+Pn+ccp 47.0 8.41 41.9 10.3 12.8 42.9 0.19 19.28 0.34 nd nd nd 0.36II BOB640 I NBOBC6S1 Pn+ccp 1.83 0.57 12.5b 0.34 12.5 22.7 0.02 a 37.24 1.04 0.77 a 47.1 9.99 0.96II GM214 I F214C1S1 Pn+Ccp 28.9 5.36 41.4 bdl 28.7 22.2 bdl 21.43 5.81 nd nd 14.3 2.12II GM214 I G214C1S1 Pn+Ccp 9.95 2.75 32.0b 5.26 39.3 6.15 bdl 44.33 1.54 nd nd nd ndII GM214 I H214C3S1 Pn+Po+Ccp 17.5 1.10 40.1b 7.93 19.5 2.50 bdl 35.48 1.06 nd nd nd 5.47II GM214 I 214C3S1 Pn+Ccp 19.7 10.8 33.7 6.50 16.0 4.80 bdl 22.79 3.22 nd nd 8.52 0.94

Os Ir Ru Rh Pt Pd Au Ni Cu As Se Te Bi

Standard PGE-A NiS bead ppm ppm ppm ppm ppm ppm ppm ppm ppm ppm ppm ppm ppmAverage n=13 197 107 229 246 205 258 234 717,411 244 137 246 243 236Stdev 26 16 37 41 33 42 35 34,425 40 19 31 38 22%RSD 13 15 16 17 16 16 15 5 16 14 13 16 9Av 1σ 17 8 12 14 10 16 9 52,245 15 9 19 18 17LLD range 0.006–0.04 0.003–0.02 0.03–0.14 0.014–0.05 0.01–0.09 0.007–0.06 0.01–0.06 0.3–1.26 0.06–0.3 0.16–0.70 0.7–3.6 0.06–0.24 0.007-0.027

i; interstitial sulfide; bdl; below detection limit, nd; not determined. Phase in bracket denotes the minor phase in the BMS assemblage observed in 2D. Pd concentrations are the average 108Pd and 106Pd concentrations and 192Os was used forOs concentrations. Av 1σ , within run precision; Stdev, standard deviation; LLD: lower limit of detection (external precision).⁎ Element possibly affected by PGM microphase during laser ablation.a Element close to LLD.b 101Ru used in calculation, otherwise the average 101Ru and 99Ru was used.

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102 103 104 105 106

102 103 104 105 106

Fig. 4. a) Se (ppm) vs. S (wt.%) in the analysed BMS. Symbols as in Fig. 1. Lines withdifferent values of S/Se ratios are shown as dark grey lines. Filled black symbols are Sand Se contents of associated bulk-rock to each BMS, from Lorand et al. (2004) forcomparison with in-situ data. Group-II dunite GM214 does not appear since there areno Se data. Bulk-rock data for S and Se are theoretical S and Se contents (seesupplementary material for details). The grey thick line correspond to the S content(wt.%) of sample MM101 for which there is no bulk-rock Se data. The two differentmajor BMS assemblages (Fe\Ni-rich BMS as mss-pyrrhotite–pentlandite–chalcopyriteand Ni\Cu-ruch BMS as pentlandite-chalcopyrite) are circled with dashed lines. b) Sevs. Cu (ppm). c) Te vs. Cu (ppm). d) As vs. Cu (ppm) for the different BMS analysed inthis study.

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higher contents of Os, Ir and Ru (1.5–10×CI-Chondrite) coupled with aslight Pd enrichment (1b(Pd/Ir)Nb3) and a depletion in Pt. Goldconcentrations are systematically low or below detection limits anduncorrelated to the Cu abundances.

Taken as a whole, the Cu-rich BMS in dunites display (Pd/Pt)Nratios >1 that are proportional to the amount of chalcopyrite in thegrain ablated. However, the time-resolved LA-ICP-MS signals revealthat the concentrations of Pt in analysis #U101S1 (Table 4) werecontaminated by three micrometer-sized discrete Pt\Te\Bi mineralsinside the analysed BMS.

The BMS analysed in group-II dunites BOB640and GM214 are richerin PGE than the BMS in Group-I dunites (10–100×CI-Chondriteconcentrations; Fig. 5e,f) as suggested by the bulk-rock concentrationsin Table 1 and Fig. 1. Moreover, the Cu-poor sulfides in both dunitesshow normalised PGE patterns rather similar with lower abundancesof Ir and Pt compared to neighbouring elements (Fig. 5e,f), irrespectiveof the proportions of BMS phases in the pn+ccp or po+pn±ccpassemblages. Most sulfides from both dunites from Group-II yieldsuperchondritic ratios for Os/Ir, Pd/Ir and Pd/Pt (e.g., 1.7b(Os/Ir)Nb14.8); 1.23b(Pd/Ir) N≤33.1; 2.38b(Pd/Pt)Nb14.3). The LA-ICPMSdata thus confirm the superchondritic values for these ratios obtainedfor the bulk-rock analyses (Fig. 1, Table 1; Lorand et al., 2004). The CI-chondrite normalised PGE patterns of BMS in GM214 are characteristicof those of pentlandites found in mantle xenoliths (Alard et al., 2000;Luguet et al., 2003; Lorand et al., 2008, 2010; Table 4, Fig. 5e) whereasthe PGE patterns of BOB640 indicate amixture of pyrrhotite withminorpentlandite and chalcopyrite phases, in agreement with the mineral-ogical data (Figs. 2 and 5, Tables 3 and 4) and data literature on pureBMS analyses in ore deposits (e.g., Barnes et al., 2006, 2008; Godel et al.,2007; Holwell and McDonald, 2007). Concentrations of Os,Ir, Ru, Rh inthe pyrrhotites in BOB640are much more variable than in pentlanditesfrom GM214 (Fig. 5e,f). Concentrations of Pd and Pt in analysis#KBOBC4S1are only indicative since aPt-Pd-As-bearing PGMmicrophasewas detected in the time-resolved LA-ICPMS signal. Similarly to the Cu-rich sulfide samples in Group-I, Au concentrations are highly variable inBOB640 (0.02–13.3 ppm; Table 4) and always below the detection limitin the Ni-rich pentlandite from GM214. However, the occurrence of apure Aumicronugget in the peridotitic matrix of the later sample and thehighly variable contents in the bulk-rock replicate analyses (Table 1)indicate that Au may not reside in the BMS in this rock (Fig. ES2c).

7. Discussion

The bulk-rock systematics of PGE and chalcophile elementsdiscussed in Lorand et al. (2004) provide strong evidence that thehigh degree melting event experienced by the peridotites left behind aBMS-free harzburgitic protolith beneath the Kerguelen plateau. De-tailed petrographic observations, coupled with in-situ major and traceelement analyses, add supporting evidence to this interpretation of BMSin Kerguelen mantle xenoliths in terms of their metasomatic origin(Lorand et al., 2004). In particular, the present mineralogical andgeochemical study supports a progressive geochemical evolution of themetasomatic BMS assemblages from Group-I to Group-II samples, froma Cu\Ni-rich assemblage toward a Cu-poor, Fe-rich end-member. Thisevolution will be discussed first, before addressing the PGE fraction-ations/transfers associated with the volatile-rich metasomatic agents.

7.1. Cu\Ni rich sulfide melt immiscibility in Group-I xenoliths

In the Group-I xenoliths, the rounded or contorted shape of theBMS blebs attests to the crystallisation from a sulfide melt (Fig. 2b,d,g,h and ES2b,c,d). Petrographic observations indicate that the BMS havecrystallised together with metasomatic minerals (cpx, Cr-spinel,apatite, carbonate, Fe\Ti oxides) in interstitial reaction zones/meltpockets (silicate/carbonate melt) or in intergranular silicate meltveins (Fig. 2a,d,g and ES2c,d,e). Characteristic microtextures of liquid-

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Pt-Pd-As-(Au)microphase

Gr II

Fig. 5. Highly siderophile and chalcophile trace element patterns of individual sulfides plus Cu contents (ppm). Values are normalised to CI-Chondrite values (McDonough and Sun,1995). a) Sulfides in harzburgite GM453. b) interstitial mss in Group-I harzburgite GM453 and dunite MM101 (Table 4) compared to some enclosed mss in mantle xenolithsworldwide (Alard et al., 2000; Lorand & Alard, 2001). c–f) Sulfides in Group-I and II dunites. The grey line in a–c–d–e–f represents concentrations in the theoretical BMS. for eachsample calculated from the bulk-rock data from Lorand et al. (2004) and EMP data from this study, and recalculated to 100% sulfides (see Electronic supplement ES4 for details).Data close to the LLD are shown with an arrow pointing downwards. Broken lines between elements indicate data given as indicative when submicron PGM inclusions were found(e.g., #U101S1 in c) and #KBOBC4S1 in f); data with asterisks in Table 4). Grey fields in a-c-d-e-f are data from group-I or -II dunites for comparison.

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liquid immiscibility inside the carbonate melt pockets such as therounded shape developed at the contactwith carbonate and silicate glasssuggest that the Cu\Ni rich metasomatic sulfides were transported bythe carbonate-rich metasomatic silicate melt as a high-density sulfidemelt.

The coexistence of Fe\Ni-rich sulfides (low Ni-pyrrhotite+pentlandite flames+chalcopyrite) and Ni-Cu-rich sulfides (chalcopyrite+pentlandite two-phase droplets) included in olivine neoblasts or in theinterstitial pores of the rocks (Figs. 2 and ES2), both associated withcarbonates, suggest that both types of BMS were deposited duringa single metasomatic event involving immiscibility between a carbonate-rich melt/fluid and a precursor metal-rich sulfide melt. A similarassociation of included and interstitial BMS in garnet-bearingpyroxenitic xenoliths from Hawaii showing characteristic features ofimmiscibility between a sulfide and a silicatemeltwas also described bySen et al. (2010). In their study, both included and interstitial BMShave a rather uniform composition of mss, which they interpreted asa result of sulfide-silicate melt immiscibility at high pressure andtemperature (~1530±100 °C; 3.1±0.6 GPa), near the base of theoceanic lithosphere. The BMS described in this study are hosted inxenoliths that have much lower equilibration temperatures (~900–1000 °C at about 1.5 GPa, Table 1), indicating sulfide immiscibilityfrom a highly evolved carbonate melt at much lower depth in thelithosphere, and shortly before eruption.

Sulfidemelts inGroup-I dunites nowoccurring as lowNi-pyrrhotite+pentlandite flames+chalcopyrite assemblages or chalcopyrite+pentlandite two-phase droplets also underwent an in-situ differentiationfrom a precursor metal-rich sulfide melt. In Fig. 4b,c, the increase in Seand Te with the amount of Cu in the different BMS is consistent with apreferential partitioning of these elements into Cu\Ni rich sulfidemeltsduring differentiation by crystallization of mss as shown in other studies(Helmy et al., 2007; Rose-Weston et al., 2009; Lorand and Alard, 2010a,band ref. therein). The ratio between the Se concentrations measuredin-situ in the mss precursors (now three-phase pyrrhotite–pentlandite–chalcopyrite grains) and thosemeasured in pentlandite–chalcopyrite bi-mineralic blebs yield partition coefficients of ~0.6, in the range of thoseobtained between mss and Cu\Ni-rich sulfide melts (0.4–0.6; e.g.,Thériault and Barnes, 1998; Lorand and Alard, 2010a,b). It suggeststhat the two sulfide assemblages were in equilibriumwith respect to Seconcentrations. We interpret the Fe\Ni-rich sulfides (pyrrhotite-pentlandite-chalcopyrite three phase grains) as subsolidus reequilibra-tion products of former high temperature “mss precursors” while theNi\Cu-rich sulfide melts (now pentlandite-chalcopyrite bi-mineralicgrains) represent sulfide melts residual after mss crystallization. Asboth assemblages may occur separately in olivine neoblasts, the mssprecursor of pyrrhotite–pentlandite–chalcopyrite assemblages and themetal-rich BMS assemblages (Ni\Cu sulfides)must have been physicallyseparated within the mantle.

308 G. Delpech et al. / Lithos 154 (2012) 296–314

Moreover, the bulk compositions of sulfides in type I xenolithsperfectly account for such an evolution of sulfide compositions duringin-situ differentiation of a precursor metal-rich sulfide melt similar tothe bulk mss composition in harzburgite GM453. In Fig. 6a,b,c,d, thebulk sulfide composition of harzburgite GM453 is that of an mss at1000 °C and 725 °C, whereas the bulk sulfide compositions in dunitesare those of Ni\Cu-rich sulfide melts coexisting with mss at hightemperature. The coexistence of mss and Ni\Cu-rich sulfide melt isdetailed in phase diagrams of the Cu\Fe\Ni\S system (Fig. 6a,b,c,d).In these diagrams, an mss first separates at ca 1100 °C (Fig. 6a) andcoexists with a Cu-rich sulfide melt which can contain up to 18 wt.%Cu at 1000 °C (Fig. 6b). This Cu-rich sulfide melt solidifies as a cubicIntermediate Solid Solution (iss) at Tb870–880 °C (Dutrizac, 1976;Peregoedova and Ohnenstetter, 2002), a temperature somewhatlower than that of the solidus of a carbonate-bearing peridotitesolidus (~1000 °C at 1.5 GPa; Olafsson and Eggler, 1983). In theFe\Ni\Cu diagram at 550 °C (Fig. 6d), the positions of the bulk BMSassemblages in different domains indicate the different BMS assem-blages in equilibrium at subsolidus temperatures during differentiationof the original metal-rich sulfide melt in Group-I samples. Theircoupled Ni and Cu-enrichment (Fig. 6d) shows that Group-I dunitescrystallised Ni\Cu-rich BMS assemblages (Table 2 and 3) and evolved inthe domains Pentlandite solid solution-Intermediate solid solution-Monosulfide solid solution (MM54) or between the domains Pentlanditesolid solution-Intermediate solid solution-Monosulfide solid solutionand Pentlandite solid solution-Monosulfide solid solution-Bornite solidsolution (MM101) until chalcopyrite was stable. Mss and iss coexist inthe Cu\Fe\S system down to ca. 334 °C, when the tie lines betweenchalcopyrite andmss are established (Lusk and Bray, 2002). Mss such asin the harzburgite GM453 subsequently decomposed into twomss and apentlandite at Tb300 °C (Misra and Fleet, 1973). The very low Nicontents in chalcopyrite and the Cu contents of the coexisting mssindicate an almost complete subsolidus redistribution of Ni and Cubetween the chalcopyrite and the mss, which can only be achieved atvery low T, perhaps as low as 100 °C (Mc Queen, 1979; Fleet, 2006). Indunites, the mss then decomposed into low-Ni pyrrhotite+pentlanditeflames below 200 °C, according to the Fe\Ni\S phase diagrams (Misraand Fleet, 1973).

Finally, there is also evidence for a partial desulphidation event inthe Group-I xenoliths. In harzburgite GM453, the external distribu-tion of the chalcopyrite as granules with respect to the mss at angularcorners of triangular sulfide bodies or as distorted chalcopyritedroplets in these BMS (Fig. 2b,c), attest to the formation of a Cu-richsulfide liquid inside a solid mss that crystallised from a precursormetal-rich sulfide melt (Fig. 2b,c). The large grain-to-grain variationin the modal mss/ccp in the BMS (Figs. 2b,c and ES2a) and theoccurrence of Cu-rich sulfide films inside the silicate matrix (Fig. 2a)or Cu-rich sulfides in the spongy rims of clinopyroxenes (Fig. ES2c)suggest that a physical separation between both BMS occurred,allowing the Cu-rich melt to percolate in the interstitial pores of theperidotites (Fig. 2a). Textural features such as the low-wetting anglesof the Cu-rich sulfide melt against the silicate matrix in group-Idunites indicate that Cu-rich sulfide melts were mobile (Ballhauset al., 2001) and could leak away from crystallizing mss. The Ni-richnature of the original sulfide melt from which the mss crystallised inthe harzburgite GM453 is indicated by the occurrence of “rod-shaped” exsolution microtextures in some mss (Fig. 2c), similarmicrostructures have been produced experimentally at high degreesof supersaturation of pentlandite with respect to pyrrhotite (Durazzoand Taylor, 1982). Texturally similar metal-rich mss have also beenobserved in continental mantle harzburgites metasomatised byvolatile-rich small-melt fractions (e.g., Lorand and Conquéré, 1983;Lorand and Grégoire, 2006). Such “rod-shaped ” microstructurescould have been induced by an abrupt S loss from the mss (ordecrease in the sulfur fugacity) but in the present case only at atemperature range where mss and pentlandite can coexist in the Fe-

Ni-S system, i.e. at Tb600° (Craig and Kullerud, 1969; Naldrett et al,1967; Sinyakova et al., 1999). In harzburgite GM453, such adesulphidation step inside the mantle was prevented by the ol-opx-mss equilibrium, which buffered the fugacity of sulfur to valuescorresponding to the stability of the mss (Eggler and Lorand, 1993).Thus, as observed petrographically in this sample, only a Cu-rich meltformed under these P-T conditions.

The bulk sulfide compositions of Group-I dunites (MM54 and 101)plot inside the field of Ni\Cu-rich sulfide melts in the Fe\Ni\Ssystem and in the two-phase volumes in the Cu\Fe\S system at1000 °C (Fig. 6a,b). Peregoedova et al. (2004) showed experimentallythat interstitial S-poor and Ni\Cu-rich sulfides in mantle xenolithssuch as the Group-I dunites could correspond to partial melting of aformer S and Fe-rich mss in response to a decrease in the sulfurfugacity at T=1000 °C due to interaction with a fluid undersaturatedin S. A lower sulfur fugacity in Group-I xenoliths could have resultedfrom the preferential partitioning of sulfur into the H\C\O\S‐richfluids exsolved from the carbonate-silicate melt fraction undergoingcrystallisation in these xenoliths (Moine et al., 2004). Similar Ni-richsulfide compositions (45 wt.% Ni) have also been reported to coexistwith metal-rich mss in Montboissier xenoliths (Massif Central;France), also metasomatised by volatile-rich highly alkaline smallmelt fractions (Lorand and Conquéré, 1983).

7.2. Sulfidation reactions from a CO2-rich vapour phase in Group-IIxenoliths

The large difference in Cu content in the bulk-sulfide compositionsof the Group-II dunites compared to the Group-I samples (2–4 vs.12–14 wt.%; Table 5) indicate that the BMS from Group-II are derivedfrom a sulfide melt much poorer in Cu. The bulk composition of theBMS in dunite GM214 plots in the field of sulfide melt at 1000 °C inthe Fe\Ni\S systemwhereas the bulk sulfide composition calculatedfor BMS in BOB640 plots inside the stability field of mss at 1000 °C, inboth the Fe\Ni\S and the Cu\Fe\S systems (Fig. 6a,b,c). The sub-solidus history of Cu-poor BMS in dunite BOB640 may be described asfollows: at 1000 °C, the mss incorporated all of the available Cu fromthe Cu-poor sulfide melt. A homogeneous mss probably persisteddown to ca. 500–600 °C since the calculated bulk Cu content (1 wt.%)corresponds to the solubility of Cu in mss at this temperature range(e.g., Mc Queen, 1979). Chalcopyrite then exsolved along withpentlandite from mss inside the pyrrhotite–pentlandite intergrowthsas thin rims and sometimes as small blocky blebs at the edge of theBMS (Fig. 2o and ES2g). Flame-like pentlandite distributed onto thepyrrhotite walls at the edge of the sulfide, as observed in sampleBOB640 (e.g., Fig. ES2g) were interpreted by Wang et al. (2005) asproducts of low-temperature (Tb200 °C) homogeneous Ni nucleationproducts at. The very low Ni contents of the pyrrhotite (b0.5 wt.%)indicate that pentlandite probably continued to exsolve from thepyrrhotite down to lower temperatures.

On the basis of the bulk-rock chalcophile and siderophile elementsystematics in the same samples (Table 1), Lorand et al. (2004)concluded that the BMS in Group-II dunites formed by sulfidationreactions from a CO2-rich vapour phase. Several textural andmineralogical features of the BMS from the present study confirmthis conclusion and are detailed here: a) the occurrence of minutesulfide droplets associated with CO2-rich fluid inclusion trails cross-cutting the primary silicate mineralogy; b) the occurrence of anassemblage consisting of BMS and polycrystalline dolomite grains intextural equilibrium interstitially to the peridotitic matrix (but rarelyfound with silicate melt pockets as in group-I); c) the absence ofmicrotextural features of liquid-liquid immiscibility in most cases(Fig. 2l,o and #ES2f,g) such as in Group-I sulfides (except rarely inGM214, Fig. 2j).

Moreover, the high bulk S content of dunite BOB640 (170 ppm) andthe abundant Fe-Ni-rich sulfides (pyrrhotite-pentlandite-chalcopyrite

NiS2FeS2

mss

Liquid

Feat%

Feat%

Ni+Coat%

Ni+Coat%

Sat%

640

453 10154

214

S(L)+mss

S(L)+mss+NiS2

mss+NiS2

a

5050

50

1000°C

1100°C

(Ni,Fe)3±xS2

FeS2

214

54

101

453

640

Sat%

NiS2

mss

725°C

50 30 10

50

60

70

80

30

50

40

1000°C

mss

Swt.%

Fewt.%Cuwt.%

453

101 54640

214

Cu Fe

S

bnss,mss,pnss

10

20

30

40

20 3010 40

20

30

40

550°C

Cu (at.%)

mss,pnss

mss,pnss(Fe, Ni)

mss,pnss,bnss,(Fe, Ni)

mss,pnss,bnss,(Fe, Ni)

pnss,poss

mss,pnss,iss

bnss,iss,poss

Cu-pnss,iss, bnss

pnss,iss, bnss

mss,pnss, bnss

pnss,mss,iss,bnss

iss,bnss,mss

iss,bnss,mss, Vs

iss,bnss

Hz-iss,iss,bnss

Fe (at.%)

Ni (

at.%

)pn

ss

iss214

54101

453

640

b

c d

50

50

80

S(L)+mss+L

Liquid

Liquid

Haezelwoodite

Fig. 6. Plot of Kerguelen bulk-sulfide compositions in the central portion of the Cu-Fe-Ni-S tetrahedron with S, Fe, Ni and Cu in at. % from Table 5. a) and b) experimental phaseassemblages at 1,000 °C in the Fe\Ni\S system (a) and Fe\Cu\S system (b) are from Craig and Kullerud (1969) and Raghavan (2004). The dark grey field for mss at 1100 °C isgiven for reference. Mss; monosulfide solid solution, NiS2; vaesite; FeS2; pyrite, Bn; bornite, S(L) liquid sulfur. c) Experimental phase assemblages at 725 °C in the Fe\Ni\S systemafter Karup-Moller and Makovicky (1995), reporting only extensive solid solution of the high-temperature heazlewoodite Ni3S2. d) Bulk sulfide compositions of Kerguelen mantlexenoliths and stability fields of principal BMS assemblages (solid lines; field boundaries; grey fields: one-phase fields) in the pseudo-ternary system, Cu9S8\Fe9S8\Ni9S8 at 550 °C(from Mackovicky, 2002). Pnss; pentlandite solid solution, Mss; monosulfide solid solution, Bnss; bornite solid solution, Iss; intermediate solid solution, Vs; Vaesite.

309G. Delpech et al. / Lithos 154 (2012) 296–314

blebs) with low Se contents (Table 4) indicate that S was preferentiallytransported compared to Se in the fluid. In dunite BOB640, the averageS/Se ratio calculated from the in-situ analyses of the BMS is high (8812±3861), like the bulk-rock S/Se ratio (10000; Table 1, Lorand et al., 2004).Although the BMS display large grain-to-grain variations in S/Se ratios(4394–12838), their ratios all are high compared to sulfides in group-I

Table 5Bulk compositions of sulfide assemblages (wt.%) in Kerguelen mantle xenoliths.

Group I I

Rock type H D

Sample GM453 MM101

%BMS in rock 0.041 (0.003) 0.037 (0.005)%Fe-Ni BMS 66 (6) 55(20)%Cu BMS 34 (4) 45 (13)S 36 35Fe 42 35Ni 10 16Cu 12 14Total 100 100M/S* 0.99 1.03

Numbers in brackets are propagated errors on sulfide modal abundance. Error on the determmetal/sulfur ratio, i.e M/S, is calculated as 100×(Fe+Ni+Co+Cu/S) in at.%. See suppleme

dunites (2680±645). BMS precipitated by immiscibility in Group-Idunites have S/Se ratios comparable to those measured in most BMSfrommantle peridotites (3000±500;Morgan, 1986; Hattori et al., 2002;Lorand and Alard, 2010a and ref. therein) whereas S/Se ratios in BOB640pyrrhotites are at least two to three time higher. Such elevated S/Seratios in pyrrhotites are difficult to reconcile with the range of S/Se

I II II

D D D

MM54 GM214 BOB640

0.037 (0.003) 0.037 (0.002) 0.046 (0.004)63( 6) 88 (5) 94 (5)37 (4) 12 (4) 6 (2)35 34 3831 32 5522 30 612 4 1100 100 1001.02 1.08 0.93

inations of bulk compositions for S, Fe, Ni, Cu~4–10%, except Cu in MM101 (14%). Thentary material ES5 for calculations.

310 G. Delpech et al. / Lithos 154 (2012) 296–314

fractionations expected during magmatic processes such as crystalliza-tion (see Lorand et al., 2004) but are common geochemical fingerprintsof BMS precipitated from hydrothermal fluids (Thériault and Barnes,1998; Ripley et al., 1999; Luguet et al., 2004; Lorand and Alard, 2010band reference therein).

Finally, the bulk Ni content of BMS in BOB93-640 is 6 wt.% (Table 5),lower by at least a factor of twothan the composition of a hypotheticalsulfide melt in equilibrium with the olivines in dunite BOB640(NiO=0.31 wt.%; calculated using predicted partition coefficients deter-mined experimentally at magmatic temperatures; KdFe-Niol/sulfide=10–35;Brenan, 2003). Thus, the textural and petrographic characteristics ofthese BMS, as well as their geochemical fingerprints, favour an origin ofBMS in Group-II by crystallization from a CO2-rich fluid/vapour phase.Experimental work by Libaudé and Sabatier (1980) and Peregoedovaet al. (2006) has shown that nickeliferous mss may be produced duringreaction betweenmagnesian olivine (Fo90) and SO2 at 900 °C following aschematic reaction of the form:

Fe;NiO Olð Þ þ SO2→FeSþ 3=2 O2:

In the Kerguelen dunites, where H2S-rich fluid inclusions havebeen reported by Lorand et al. (2004), the schematic sulfidationreactions involving H2S is more likely to be:

Fe;NiO Olð Þ þ H2S→FeSþ H2O

The few BMS inclusions of monoclinic-type pyrrhotite (Fe7S8)hosted in olivine provide a record of the high sulfur fugacity in Group-II dunites. This high sulfur fugacity probably reflects the accumulationof S-bearing CO2-rich fluids in BOB640. The sharp contact betweenBMS and polycrystalline dolomites (Fig. ES2g) testifies to precipita-tion as a solid BMS, as expected from sulfidation reactions. However,some polyhedral BMS grains with convex-inward rims also suggestthat the sulfides were still fluid enough to be deformed against thewalls of olivine neoblasts (Fig. ES2f) while a few show liquid-liquidimmiscibility features only in dunite GM214 (Fig. 2j). Both observa-tions may be reconciled if the sulfidation reactions occurred at atemperature close to the solidus of a carbonate-bearing peridotite(~1000 °C at 1.5 GPa; Olafsson and Eggler, 1983). Such a mechanismof sulfide precipitation by sulfidation reactions involving olivine hasalso been invoked in mantle xenoliths from Montferrier (MassifCentral, France) in which BMS show similar geochemical fingerprintsto sulfides from Group-II dunites (high S/Se, (Os/Ir)N and (Pd/Pt)N>1; Alard et al., 2011).

7.3. PGE transfers during interaction between volatile-rich melts and astrongly PGE-depleted protolith

Dunites from both groups are the best rock type to discuss the PGEtransfers associated with the addition of metasomatic BMS sincethese rocks have lost more than 90% of their original mantle-derivedPGE budget (Lorand et al., 2004), during a high-degree partial meltingepisode and subsequent pervasive percolation of large volumes of S-undersaturated alkaline silicate melts into the residual harzburgites(high melt/rock ratio). Judging by the bulk-rock PGE concentrations inthese samples (Table 1, Fig.1a), theGroup-I dunites differ from themostdepleted poikiloblastic harzburgites (e.g., BY96-396 given for reference,Table 1, Fig. 1) only by having higher Pt, Pd and Au contents. Bulk-rockdata from Fig. 1 demonstrate that immiscible (Ni-) Cu-rich sulfidesassociated to carbonate-rich metasmatism in Group-I xenoliths crystal-lised from a sulfide melt enriched in Cu, Pd and Pt (Pd/Pt~1 or slightly>1) compared to Os and Ir (and little fractionation Os/Ir~1). In Fig. 7a,the bulk rock Pd/Ir ratios of Group-I xenoliths (~1, except Os/Ir inMM54 >1) are also in general much higher than that of partial meltingresidues or that of the protolith (BY96-396, Pd/Ir=0.2) and support apreferential addition of Pd (and Pt) compared to Os during carbonate-

rich metasomatism involving (Ni-Cu-rich) BMS precipitated by immis-cibility. However, the global enrichment in all PGE's with Cu remainslow to moderate for immiscible Cu-rich sulfides (group-I) compared togroup-II sulfides precipitated by sulfidation (Fig. 7b). This geochemicalsignature is rather typical of mantle derived-magmas in whichenrichments in PPGE's compared to IPGE's are observed (Rehkämperet al., 1999; Chazey and Neal, 2005; Bézos et al., 2005). We thereforeenvisage that the metasomatic medium that invaded the Group-Ixenoliths, a strongly evolved silicate-bearing carbonate melt throughrepeated melt-rock reactions with the depleted lithospheric mantlebeneath the Kerguelen Archipelago (Grégoire et al., 2000a), hadrelatively low contents of IPGE compared to Pd and Pt and was onlycapable of modifying the bulk-rock Pt and Pd budgets of these rocks.Interaction between basaltic melt enriched in PPGE and depletedperidotites has already been shown to produce such selectiveenrichments in Pt and Pd in metasomatised mantle rocks (e.g., Büchlet al., 2002, van Acken et al., 2010 among others). In Fig. 7c, theconcomittant enrichment in Pd and Pt in group-I BMS illustrated by thein-situ data testifies to the greater affinity of Pd and Pt for fractionatedCu-rich sulfide melts (pentlandite-chalcopyrite blebs). Upon cooling ofthese sulfide melts, crystallization of mss ins these rocks allowed Cu,Pd and Pt, alongwith the chalcogenides Se, Te, Bi (Figs. 4b,c and 7b,c,d),to preferentially concentrate in the fractionated Ni-Cu-rich sulfidemelt (iss). Further cooling of the Ni\Cu-rich sulfide melt producedpentlandite+chalcopyrite blebs. Pt-Te-Bi inclusions found during laserablation of some sulfide blebs in MM101 reflect the preferentialcomplexation of Pd and Pt with melts enriched in Te, Bi or As whichcrystallised PGM microphases during cooling (Helmy et al., 2007;Holwell and McDonald, 2010). Such an interpretation has already beenproposed for the origin of Pt\Pd\Te\Bi microphases associated withpentlandite-chalcopyrite-Bo assemblages inmantle peridotites (Lorandet al., 2008, 2010; Luguet et al., 2004) and in PGE ores (Gervilla andKojonen, 2002; Gervilla et al., 1997; Helmy et al., 2007; Holwell andMcDonald, 2010). The IPGE and Pd enrichments in some pentlanditeGroup-I dunites (MM101, Fig. 5c) indicate their origin from reactionsbetween crystallising mss (delivering Os, Ir, Ru and Rh) and crystal-lisation products from Ni\Cu rich sulfide melts (Hz-Iss solid solutionhosting the Pd; e.g., Godel et al., 2007; Barnes et al., 2006).

Compared to harzburgitic protoliths that are devoid of BMS (e.g.,BY96-396) and group-I dunites, there is an overall enrichment in PGEin Group-II dunites (Table 1, Fig. 1) as indicated by the more elevatedPGE contents of Cu-poor BMS (Figs. 7b and 5e,f, Table 4). This is incontrast to the PGE transfers associated with metasomatism by smallfractions of carbonate-rich melts in the Group-I xenoliths discussedabove, which show very moderate global enrichment in PGE (Fig. 7b)and suggest an efficient transport of all six PGEs by S-rich CO2 fluids inGroup-II dunites. Moreover, despite the difference in sulfide miner-alogy in both dunites (Ni-poor pyrrhotite+pentlandite in BOB640and Ni-pentlandite+chalcopyrite in GM214), the individual in-situanalyses of BMS (Fig. 5e,f) mimic the coupled Pd-Os enrichmentsrelative to Ir and Pt found in the bulk-rock PGE analyses (Fig. 1a). Thisis also demonstrated by the superchondritic Os/Ir and Pd/Pt in theBMS (Figs. 5e,f and 7a). The individual in-situ analyses of BMS indunites BOB640 and GM214 compare reasonably well with thetheoretical BMS calculated from bulk-rock PGE abundance and BMSmodal abundances (Electronic supplement ES4). Since we found noevidence of Ir-Pt alloys or any other Os-, Ir-, Ru-rich PGM in these rocksusing LA-ICP-MS or SEM techniques, the good similarity between thebulk-rock and the in-situ analyses confirm that: 1) these preferentialenrichments in Os and Pd are not an analytical artifact; 2) Os ispartitioned into the crystallographic sites of the BMS and notdistributed in discrete minerals. Unlike Os, which easily partitions intothe octahedral sites of the pyrrhotite (Ballhaus et al., 2006; Li et al.,1996;Mungall et al., 2005), Pd can only enter themonoclinic pyrrhotite,which has a distorted crystal structure suitable for accommodating thislow valence PGE (Ballhaus and Ryan, 1995; Ballhaus and Ulmer, 1995).

a b

c d

Fig. 7. a) (Os/Ir)N vs. (Pd/Ir)N, b) Cu vs. Σ(PGE) , c) Pt vs. Pd and d) Te vs. Pd in the two groups of BMS assemblages. Symbols as in Fig. 1. Drak grey field, field for partial meltingresidues and metasomatism at high melt/rock ratio (Lorand et al., 2004). CI-Chondrite values from McDonough and Sun (1995) are shown for reference only in panels a–c–d. Thetwo different mechanisms of sulfide precipitations (Lorand et al., 2004) are indicated by arrows. Sulfides precipitated by sulfidation reactions involving a volatile transport of S(Gr. II) are comparatively more enriched in most PGEs compared to immiscible Cu-rich sulfides precipitated by immiscibility (Gr. I).

311G. Delpech et al. / Lithos 154 (2012) 296–314

The occurrence of unfractured inclusions of monoclinic pyrrhotite inBOB640 suggests that the enhanced partitioning of Pd into thepyrrhotite was facilitated by the high fS2 caused by the accumulationof S-rich fluid in BOB640. Pd later partitioned at low-T during coolinginto the secondary pentlandite exsolutions observed inside thepyrrhotite, in agreement with the greater preference of Pd forpentlandite compared to Pt (Ballhaus and Sylvester, 2000; Luguetet al., 2004; Lorand et al., 2008, 2010 and ref. therein).

Since the sulfides precipitated by sulfidation reactions in group-IIdunites did not experience in-situ fractional crystallization, theirbulk-compositions provide direct information on the relative ele-mental mobility of PGE in the sulfur-bearing CO2 fluid that exsolvedfrom the carbonate-rich silicate melt in Kerguelen mantle xenoliths.The geochemical fingerprints of BMS from the Group-II dunites(Figs. 4, 5, 7) such as the high S/Se and superchondritic Os/Ir and Pd/Pt suggest the following qualitative order of elemental transfer;S>Pd>Pt>Os>Cu_Se_Ir_Ru_Rh. Based on sublimation experi-ments at 1000 °C, Fleet andWu (1993) suggested that both sulfur andchlorine are required for an appreciable transport of PGE in dry fluidsat magmatic temperatures. Au and Pt were shown to be highlysoluble in Cl-rich brines and Cl vapour at 800 °C (Hanley, 2005).However, as pointed out by Wood et al. (1993), base metals like Cuand Ni are hundreds of times more soluble in such brines. For Ni, theamount transferred as a dissolved species in the vapour cannot bedetermined with confidence as part of the Ni involved in thesulfidation reactions is hosted in olivine. The overall decrease of Cuconcentrations in Group-II BMS assemblages with respect to Group-IBMS is inconsistent with a transport by chlorine-rich vapours. Gold,which displays a strong preference for vapour relative to magmas

(DVapour/Melt=15±2.5; Simmons and Brown, 2006) should also be asenriched as Pd in the BMS assemblage of BOB640 and GM-92-214, ifchlorine-rich fluids were involved in the transport of PGE (e.g., Fleetand Wu, 1993). We observe that the measured BMS and thecalculated bulk-sulfide compositions for BOB640 are strongly deplet-ed in Au relative to Pd (Fig. 5f, Electronic supplement ES4) comparedto other group-I or -II xenoliths where Au is enriched over Pd. Aucommonly occurs in very low concentrations in BMS (pyrrhotite,pentlandite, chalcopyrite, e;g. b0.1 ppm) and is not hosted in anyparticular BMS phase (Barnes et al., 2006, 2008; Holwell andMcDonald, 2007), as also shown in this study (Table 4). The low Aucontents measured in BMS from both groups cannot balance the highAu/Pd observed in bulk rocks, suggesting Au does not reside in BMS.Therefore, the depletion in Au compared to Pd in BOB640 bulk-rockand BMS may be a primary feature of the S- and CO2-rich fluid, i.e.independent of the subsolidus exsolution/precipitation of discretegold microminerals. Peregoedova et al. (2006) showed experimen-tally that Os, Ir, Ru, Rh, Pd, Au and Cu could be transported in Cl-freedry sulfurous fluids, especially at the highest T (1100 °C). Theseauthors also show that vapour mass transfer of Au and Cu decreases ifthe fluid interacts with a base metal sulfide melt. If applied to dunitesfrom Group-II, these experiments could suggest that the S-rich CO2

fluid interacted with a (Cu)-Ni sulfide melt that retained Au and Cu,hence depleting these elements in the vapour compared toPd as seeninBOB640. Evidence of such an interaction may be found in the Au-rich bulk-rock compositions measured in the Group-I dunites and thehighly variable Au concentrations (1 to 8.5 ppb) of replicate bulk-rockanalyses of GM214 (Table 1). This peculiar sample provides evidencethat submicrometric Au particles were exsolved from the Ni-rich

312 G. Delpech et al. / Lithos 154 (2012) 296–314

sulfide melt (Fig. ES3b) and their heterogeneous distribution probablyaccounts for the poorly reproducible bulk-rock Au contents.

7.4. Evidence for assimilation of residual PGM micro-nuggets inmetasomatic BMS assemblages

Lorand et al. (2004) demonstrated that, in contrast to the dunites,the bulk-rock PGE composition of the harzburgite GM453 representsmixing between a metasomatic Cu-rich sulfide melt and a residualcomponent enriched in compatible PGEs (Os, Ir, Ru and Rh). The bulk-rock budget of these elements in GM453 is akin to that of the BMS-free residual harzburgites (thick black line in upper left panel of Fig. 1,see Lorand et al., 2004) and thus its elevated contents of Os, Ir, Ru andRh probably are inherited from the residual protolith produced byextensive partial melting beyond the point of exhaustion of sulfide.Beyond this point (F=25% in the spinel lherzolite stability field), thePGEs in residual mantle rocks are hosted in micrometric PGE-richminerals ((Ru, Os, Ir) di-sulfides/alloys) and platinum alloys/ sulfidesresiding mostly in the interstices of the peridotitic matrix (Lorandet al., 2010; Luguet et al., 2007). The interaction between formerinterstitial IPGE-rich PGMs and sulfide melt during metasomatism,resulting in the random capture and dissolution of these discreteminerals in the sulfide melt, has previously been proposed to explainmss compositions in xenocrystic olivine (mixing with former Pt-Ir-Osalloys) from the Udachnaya kimberlite pipe (Siberian Craton; Griffinet al., 2002) or in composite PGM-BMS blebs in Pyrenean orogeniclherzolites (Lorand et al., 2008, 2010). The absence of PGMs bothinside and outside the BMS in GM453, also supports the idea thatPGMs did not physically survive their entrapment in the sulfide melt.If such a process occurred, it is likely that PGE-rich micromineralsimparted their elevated budget in Os, Ir, Ru, Rh during interaction anddissolution in the percolating Cu-rich sulfide melt, which was poor inOs, Is and Ru and comparatively enriched in Pd, Pt and Au (i.e. basalticcomposition). The former presence of PGMs is attested by the highlyvariable Os, Ir, Ru and Rh contents of mss in GM453 (Table 4, Fig. 5a),contrary to the concentrations predicted by fractional/equilibriumcrystallization models involving mss. The strong enrichment in Osand Ru relative to Ir in some of the PGE patterns (Fig. 5a), which isnot observed in residual or cumulate mss compositions (Fig. 5a,b;Alard et al., 2000; Lorand and Alard, 2001), suggest the formeroccurrence of laurite/Ru-Os-Ir alloys in this rock. Such PGMs con-centrate much more Os and Ru than Ir, whereas Rh is betteraccommodated by Pt-alloys (Griffin et al., 2002). The mss accommo-dates weight percent levels of Os, Ir and Ru at magmatic temperatures(Bockrath et al., 2004; Li et al., 1996; Peregoedova et al., 2004);therefore, as shown experimentally by Andrews and Brenan (2002), itis unlikely to coexist with Os\Ir\Ru-rich PGMs for PGE concentra-tions at ppm levels as measured in Kerguelen BMS. The large inter-grain variation of mss PGE contents in GM453 implies their randomdistribution in the rock prior to metasomatism and therefore pointsto a random dissolution of such residual PGM into the percolatingsulfide melt. The same process probably also occurred in duniteMM101, which displays rare Os, Ir and Ru concentration peaks insome mss. This means that a few discrete PGE-rich minerals survivedthe extensive percolation of strongly alkaline melts within theserocks. The random incorporation of PGMmicrophases in metasomaticsulfides such as in harzburgite GM453 has important implications forthe interpretation of the 187Os/188Os isotopic system in individualsulfides since their 187Os/188Os will likely reflect that of the PGMmicrophase and not that of the metasomatic sulfide melt (e.g., Griffinet al., 2002; Lorand et al., 2010).

8. Conclusion

The diversity in the mineralogy of interstitial sulfides and the largeHSE fractionations observed in rocks affected by differing mechanisms

of sulfide precipitation (immiscibility vs. sulfdiation reactions) provesthat the HSEs are efficiently transported in highly evolved metasomaticmelts/fluids. The observations from this study are summarised below:

1) The large variations in the chalcophile and highly siderophileelements observed in metal-rich sulfides that precipitated afterimmiscibility from a carbonate-rich silicate melt (Group-I)document the in-situ differentiation during percolation of theCu-rich metasomatic sulfide melt in the studied mantle xenoliths.Se, Te, Pd, Pt are enriched in the Cu-rich residual sulfide meltfollowing crystallization of mss. The sulfides crystallised byimmiscibility from a carbonate-rich melt show about chondriticS/Se and Os/Ir ratios and slightly suprachondritic Pd/Pt.

2) Sulfidation reactions by a S- and CO2-enriched supercritical fluid,exsolved from the carbonate-rich melt fraction caused the crystal-lisation of Fe-Ni pyrrhotites at high fS2 with more elevatedconcentrations of PGE than in sulfides precipitated by immiscibility.Such sulfidation reactions imply an efficient and selective transportof chalcophile and siderophile elements with the following orderof preferential transport ; S>Pd>Pt>Os>Cu_Se_Ir_Ru_Rh.These sulfides have geochemical fingerprints with superchondriticS/Se, Os/Ir and Pd/Pt ratios.

3) Excess IPGE concentrations in the interstitial cumulate mssprecipitated after immiscibility from the carbonate-rich meltcannot be explained either by equilibrium or fractional crystal-lisation during differentiation. Instead, it provides evidence for thedissolution of rare interstitial residual IPGE-rich PGM in thepercolating metasomatic sulfide melt.

Supplementary data to this article can be found online at http://dx.doi.org/10.1016/j.lithos.2012.07.018.

Acknowledgements

We thank the French Polar Institute (IPEV, Brest, France) for theirsupport during fieldwork for the program “Cartoker.” Support for thisproject was provided from an ARC Discovery Grant (S.Y.O'R andothers), the GEMOC ARC National Key Centre, an ARC IREX Grant(S.Y.O'R and W.L. Griffin), a Macquarie University PostgraduateResearch Grant (G.D), and a Macquarie University InternationalPostgraduate Scholarship (G.D). The analytical data were obtainedusing instrumentation funded by DEST Systemic InfrastructureGrants, ARC LIEF, NCRIS, industry partners and Macquarie University.Bill Griffin, Norman Pearson and other members of the scientific teamat GEMOC are warmly acknowledged for their help and advice duringthe laboratory work. The authors thank A. Kerr for editorial work andC. Dale and an anonymous reviewer for detailed and constructivereviews that significantly improved the manuscript. This is contribu-tion 201 from the ARC Centre of Excellence for Core to Crust FluidSystems (http://www.ccfs.mq.edu.au) and 842 in the GEMOC KeyCentre (http://www.gemoc.mq.edu.au).

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