Earth and Planetary Science Letters 289 (2010) 298–310

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Earth and Planetary Science Letters

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Platinum-group element micronuggets and refertilization process in Lherz orogenicperidotite (northeastern Pyrenees, France)

Jean-Pierre Lorand a,⁎, Olivier Alard b, Ambre Luguet c

a Laboratoire de Minéralogie et Cosmochimie, Muséum National d'Histoire Naturelle and CNRS (UMR 7202), 61 Rue Buffon, 75005, Paris, Franceb Géosciences Montpellier, Université de Montpellier II and CNRS, Place Eugène Bataillon, 34095 Montpellier Cédex, Francec Steinmann Institut-Endogene Prozesse. Universität Bonn, Poppelsdorfer Schloss, 53115 Bonn, Germany

platinum-group minerals

⁎ Corresponding author.E-mail addresses: jplorand@mnhn.fr (J.-P. Lorand),

olivier.alard@gm.univ-montp2.fr (O. Alard), ambre.lugu

0012-821X/$ – see front matter © 2009 Elsevier B.V. Adoi:10.1016/j.epsl.2009.11.017

a b s t r a c t

a r t i c l e i n f o

Article history:Received 29 July 2009Received in revised form 19 October 2009Accepted 4 November 2009Available online 2 December 2009

Editor: R.W. Carlson

Keywords:highly siderophile elementsupper mantleorogenic peridotites

Highly siderophile elements (Platinum-group elements, Au and Re) are currently assumed to reside insidebase metal sulfides (BMS) in the convecting upper mantle. However, fertile lherzolites sampled by Pyreneanorogenic peridotite massifs are unexpectedly rich in 0.5–3 µm large micronuggets of platinum-groupminerals (PGM). Among those, sulfides from the laurite-erlichmanite series (Ru, Os(Ir)S(As)2), Pt–Ir–Osalloys and Pt–Pd–Te–Bi phases (moncheite–merenskyite) are predominant. Not only the BMS phases butalso the PGM micronuggets must be taken into account in calculation of the PGE budget of orogenic fertilelherzolites. Laurite is a good candidate for equilibrating the whole-rock budget of Os, Ir and Ru whileaccounting for supra-chondritic Ru/IrN. Textural relationships between PGMs and BMS highlightheterogeneous mixing between refractory PGMs (laurite/Pt–Ir–Os alloys) inherited from ancient refractorylithospheric mantle and late-magmatic metasomatic sulfides precipitated from tholeiitic melts. “Low-temperature” PGMs, especially Pt–Pd bismuthotellurides should be added to the list of mineral indicators oflithosphere refertilization process. Now disseminated within fertile lherzolites, “lithospheric“ PGMs likelyaccount for local preservation of ancient Os model ages (up to 2 Ga) detected in BMS by in-situ isotopicanalyses. These PGMs also question the reliability of orogenic lherzolites for estimating the PGE signature ofthe Primitive Silicate Earth.

et@uni-bonn.de (A. Luguet).

ll rights reserved.

© 2009 Elsevier B.V. All rights reserved.

1. Introduction

As highly siderophile elements, platinum-group elements (PGEs),Au and Re, are presumably concentrated in the metallic core of theEarth. It is commonly inferred that the late meteoritic influx that hitthe earth–moon system after accretion and core formation deliveredsupplementary PGE to the hypothetical Primitive Upper Mantle(PUM) of the Earth (Morgan, 1986; Lorand et al., 2008a and referencetherein). Updated estimates of the PGE composition of the PUMidentified reproducible (20–30%) deviations from the canonicalchondritic model in the light/heavy PGE ratios (i.e. Ru/Ir; Rh/Ir; Pd/Pt; Becker et al., 2006). However, PGE's are ultra-trace elements inmantle rocks (ng/g concentration level) and their budget in terms ofhost minerals is still debated. Pioneering works on separatedmineralsfrom fertile lherzolites (Morgan and Baedecker, 1983; Pattou et al.1996; Burton et al., 1999; Handler and Bennett, 1999) identifiedaccessory (b0.1 wt.%) base-metal sulfides (BMS i.e. Fe–Ni–Cu sul-fides) as the main PGE carriers. This conclusion received strongsupport from in-situ analyses using laser-ablation inductively coupled

plasma mass spectrometry (LA–ICP–MS) (Alard et al., 2000; Lorandand Alard, 2001; Luguet et al., 2001, 2004; Lorand et al., 2008b).

Fertile mantle peridotites that are rich in BMS are presumed to bedevoid or poor in discrete platinum-group minerals (PGM), owing tolow bulk PGE abundances in the upper mantle and the strong affinityof PGE for BMS at magmatic temperatures. However, Meisel et al.(2003) estimated non-trivial amount of Os (N30%) residing outsideBMS in UB-N, an orogenic lherzolite used as reference material. Massbalance calculations based on accessory BMS, often varying consid-erably in size and distribution may generate large (up to 200%) errors(Luguet et al., 2004; Lorand et al., 2008b), thus overlooking anycontribution of other PGE-rich trace minerals. Mostly of micrometricsize, PGMs are not straightforwardly detected in lherzolite sampleswith conventional analytical tools. Using synchrotron-XRF (SR-XRF)combined with microbeam technique, Kogiso et al. (2008) detectednine grains of Pt–Os–Ir, Pt and Au just from one thin section of spinellherzolite (1102-1A) from Horoman peridotite complex. Theydeduced that c.a. 10% of the Ir and Os budget is accounted for by Pt–Ir–Os alloys. However, their analytical configuration was too limitedto detect efficiently light PGEs (Ru, Rh, and Pd) and anions of semi-metals (S and heavy metalloid elements (Sb and Te)). Using SEM andcarefully polished thick (200 µm) sections, Lorand et al. (2008b)detected up to 12–14 PGM grains (Pt–Os–Ir alloys+Pt–Pd–Te–Bi

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phases) per standard-size polished thin section of FON-B 93, a fertilelherzolite from Fontête Rouge, near Lherz. This PGM assemblageaccounts for c.a. 90% of the whole-rock Pt budget, in agreement withprevious estimates performed on orogenic lherzolites (e.g. Luguetet al., 2004).

The actual abundance of micrometric PGM in fertile orogeniclherzolites, their contribution to the whole-rock PGE budget and theirpetrogenetic significance is poorly understood. To address the issue ina more quantitative way, we have performed an integrated, multi-technique study on 17 mantle peridotites collected mainly from theLherz orogenic massif (North Eastern Pyrenees, France), the type-locality of lherzolite and one of the best studied in terms ofpetrogenetic evolution. The PGMs were searched for using acombination of reflected light microscopy, high-resolution scanningelectron microscope (FEG-SEM) and LA-ICPMS analyses (14 samples).LA-ICP-MS is a powerful tool for detecting such minerals which createPGE concentration spikes in time-resolved spectra (Ballhaus andSylvester, 2000). We provide evidence that 1) PGM and BMS are notnecessarily co-genetic; 2) Ru–Os–Ir PGMs, identified here for the firsttime in fertile lherzolites, were inherited from refractory mantleperidotites; and 3) PGM fractionate some whole-rock PGE ratios thatare considered to be signature of the Primitive Silicate Earth.

2. Geological setting

The Lherz orogenic peridotite massif and its cluster of peridotiteoutcrops (Freychinède, Fontête Rouge) are three of the 40 or morebodies of mantle rocks spatially associated with high-grade meta-morphic rocks of the granulite facies in the North Pyrenean Zone. Itwas exhumed from the mantle c.a. 100 Ma ago by the movement ofthe Iberian plate relative to Europe, which resulted in crustal thinningassociated with successive opening and closing of elongated,asymmetrical pull-apart mesozoïc sedimentary basins (Fabriès et al.,1991; Lagabrielle and Bodinier, 2008 and ref. therein).

Pyrenean orogenic massifs consist mainly of weakly (b25%)serpentinized spinel lherzolites (10–16 wt.% cpx). Clinopyroxene-poor lherzolites (5–10 wt.% cpx) are much rarer and true harzburgites(cpx b5 wt.%) are known only in the Fontête Rouge and Lherz massifs(Fabriès et al., 1991). The Lherz massif displays the best exposure ofboudinaged lenses of highly refractory harzburgites, intermingled ona decametric scale with fertile lherzolites. The most fertile lherzolites(15% clinopyroxene) have long been interpreted as pristine mantleonly weakly affected by partial melting whereas harzburgites recordhigh (N20%) degrees of melt extraction (Bodinier et al., 1988; Lorand,1989a; Fabriès et al., 1991). Themelt extraction eventwas dated at c.a.2 Ga by the Re–Os chronometer (Reisberg and Lorand, 1995). Acomprehensive study of Os isotopic compositions, PGE abundancesand PGM distribution in the Lherz harzburgites recently confirmedthis model age while providing further evidence supporting theresidual origin of these rocks (Luguet et al., 2007).

However, from detailed investigations along two distinct, 7–8 m-long sections across harzburgite–lherzolite contacts, Le Roux et al.(2007, 2008, 2009) provided convergent structural and geochemicalarguments supporting a secondary origin for spinel lherzolites. Theconstant orientation of the foliation and lineation in harzburgitebodies, different from that in lherzolites and websterites, stronglysuggests that all harzburgites were once part of a single andcontinuous unit, deformed before the formation of the lherzolitesand websterites. This 2 Ga-old lithospheric (harzburgitic ?) protolithwas then infiltrated and reacted with ascending MORB-type basalticmelts of asthenospheric origin. A near-solidus refertilization reactioninvolving dissolution of olivine and precipitation of Cr- and Al-bearingminerals, mostly spinel and pyroxenes, gave rise to spinel lherzolites.The refertilization model adequately accounts for the geochemicalbehaviour of some elements such as Ti, Cr and Rare Earth Elements(REE) at odds with partial melting models (Le Roux et al., 2007).

Strong enrichment of large ion lithophile elements (LILE, includingREE) at melt infiltration fronts is indeed predicted by theoreticalmodelling of melt-consuming reactions combined with melt trans-port. The irregularly-shaped lherzolite–harzburgite contacts repre-sent therefore a convoluted melt-rock reaction front formed bycoalescence of porous melt infiltration channels. At the contact,lherzolites show larger andmore equant grains and olivine crystallinepreferred orientation (CPO) quite similar to harzburgites, but weaker.All structures and geochemical signatures were efficiently erasedwithin a fewmeters across the front. In lherzolites away from contacts(N20 m), olivine microstructures and CPO are consistent with thesteeply-dipping foliations and sub-horizontal lineations observedwithin the whole massif, and clinopyroxene displays the classic,N-MORB, REE pattern observed in orogenic lherzolites worldwide.

3. Petrographical and analytical notes

The seventeen samples selected for the present paper comprisetwo lherzolites from Freychinède and fifteen lherzolites and harzbur-gites from Lherz. The degree of serpentinisation ranges from 15 to 30%for the harzburgites to 5–20% for the lherzolites (Lorand, 1989a). AtLherz, two harzburgites (04Lh13; 04Lh11), one cpx-poor lherzolite(04Lh08; or cpx-rich harzburgites) and two fertile lherzolites(04Lh815; 04Lh15) are reference samples from the Le Roux et al.(2007) study of Site-2 section across one harzburgite-lherzolitecontact (hereafter referred to as Site-2 samples). Samples 04Lh37and 04Lh39 are lherzolites from Site-4 of the same study (hereafterreferred to as Site-4 samples). The other Lherz samples (cpx-richharzburgites, 82-4; 71-322; 12-1; 71-107; fertile lherzolites 71-321,71-326, 86-V2-5 and 71-324) and the two Freychinède lherzolites are“historic” samples, already analysed for whole-rock sulfur contentsand BMS mineralogy (Lorand, 1989a,b), Re-Os isotopic compositions(Reisberg and Lorand, 1995) and PGE systematics (Pattou et al., 1996;Lorand et al., 1999; Becker et al., 2006).

Site 2 and Site 4 samples were analysed for S using iodometrictitration of the SO2 produced by combustion of 500 mg powder aliquotat the National Museum of Natural History (see Gros et al., 2005 forfurther details). PGEs were separated at the MNHN from powderaliquots of 15 g each by aNiS-fire assay— Te coprecipitation procedureas described by Gros et al. (2002) and modified by Lorand et al.(2008b) to improve the recovery of PGE (i.e. secondNiS-fire assay stepand amount of Te increased to 28 ml). The analyses were performedusing a FISONS VG 353 PlasmaQuad PQplus inductively coupledplasma mass spectrometer (ICP-MS) (University of Montpellier II andNational Museum of Natural History, Paris). Sample 82-4, alreadyanalysed by Lorand et al. (1999) and Becker et al. (2006) wasreanalysed by isotope dilution-ICP-MS (Thermo-Finnigan Element 2magnetic-sector ICP-MS) at the Northern Centre for Isotopic andElemental Tracing (NCIET) at the University of Durham (UK). Sampledigestion was performed overnight at 300 °C and 100 bars in AntonPaar HPA-S pressure-sealed quartz vessels containing 7.5 ml reverseaqua-regia. Details on the separation procedure and analyticalconditions were given in Lorand et al. (2008b) and in Luguet et al.(2009).

Platinum group minerals (PGM) were located on carefullypolished 200 µm-thick standard-sized (40×25 mm) sections usingreflected light microscopy and high-resolution scanning electronmicroscope (Supra™ 55VP Zeiss FEG-SEM; Pierre and Marie CurieUniversity, Paris VI) equipped with an energy dispersive Si(L)detector with a resolution of 129 eV full width at half maximum atthe Fe peak. The SEM investigations were carried out at anacceleration voltage of 15 kV and a working distance of 8 mm toidentify platinum-group minerals qualitatively in the backscatteredmode. Manual scan of the thick sections in the backscattered modewere preferred over automated particle search procedures that may

Fig. 1. CI chondrite-normalized whole-rock PGE abundances for lherzolites (A) andharzburgites (B). B = Becker et al. (2006); Lo = Lorand et al. (1999). Normalizingvalues after McDonough and Sun (1995). See Table 2 for data.

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not detect particles on the nanometer scale in accessory BMS, despitestrong contrast in atomic numbers between host and included PGM.

Platinum-group element contents of base metal sulfides weredetermined along with a few other trace elements (Te, Bi, Pb) bylaser-ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) in fourteen samples from Lherz that displayed BMS grainslarge enough (N100 µm) to be probed. A GEOLAS excimer UV laseroperating at 193 nm, coupled with a Thermo-Finnigan Element 2magnetic-sector ICP-MS inductively coupled plasma mass spectrom-eter (ICP-MS) (University of Montpellier II) was used. Analyticalconditions were reported in Lorand et al. (2008b). The laser beamdiameter was set up at 51 µm, 5 Hz laser frequency and a beam energyof about 15 J/cm2. Accuracy was tested with in-house standards (NiSbeads doped with SARM 7 rock standard powder; SABS website 2005)analysed separately at Montpellier and at the “Museum Royal del'Afrique Centrale” at Tervuren (Belgium); both sets of analysesproduced very similar results (generally differing by no more than10%). Measured concentrations of Au and PGE fit the theoreticalconcentrations at 1 sigma level (Table 1). Concentrations of Os, Ir, Ruand Rh show the lowest reproducibility and thus, the largest relativestandard deviation (up to 70% for Ru and Ir). This results frommineralogical inhomogeneities in NiS beads (Gros et al., 2002)possibly linked to non-homegeneous mixing of powder particlesinside the NiS too.

4. Results

4.1. Whole-rock PGE and S abundances and BMS mineralogy

Lherzolites, cpx-rich harzburgites and harzburgites are character-ized by different whole-rock PGE systematics. The most fertilelherzolites show CI-chondrite normalized PGE abundance patternssimilar to those inferred for the Primitive Upper Mantle (Becker et al.,2006), i.e. positive anomalies of Ru, Rh and Pd relative to heavy PGEs(i.e. Ru/IrN and Pd/IrNN2; Rh/IrN=0.30.38: N=CI-chondrites-nor-malized; Fig. 1A). The two Site-2 harzburgites (04Lh13 and 04Lh11)display negatively trending Pd-depleted CI-chondrite normalizedpatterns (Pd/IrN=0.33–0.21; Pd/PtN=0.32–0.50), coupled withvery low S contents (38–51 ppm; Table 2), as is expected for solidmantle residues after 20–25% of partial melting (Lorand et al., 1999).Similar patterns were reported for BMS-free harzburgites collected inthe thickest harzburgite lenses from the south-eastern part of theLherz massif (Luguet et al., 2007). The three cpx-rich harzburgitesshare features with both the harzburgites (i.e. a strong Pt-depletionrelative to Ir; Pt/IrN=0.54–0.68) and the lherzolites (Pd enrichmentrelative to Pt; Pd/PtN=0.96–1.18; Fig. 1B), while displaying interme-diate S contents (56–80 ppm).

Regardless of the whole-rock S contents, BMS are 20–300 µmacross polyhedral (not spherical) blebs with convex-inward grain

Table 1Precision and accuracy of LAM-ICPMS analyses (1 sigma). MRAC=Musée Royal de l'Afrique CStandard (Annual Report. 2005).

Os(ppm)

Ir(ppm)

Ru(ppm)

SARM-7-10Montpellier 0.17 0.38 1.30

±0.10 ±0.26 ±0.90MRAC. Tervuren 0.16 0.31 1.30

±0.13 ±0.25 ±1.06Theoretical concentrations 0.12 0.28 0.98

SARM-7-1Montpellier 0.33 0.32 1.10

±0.11 ±0.11 ±0.48Theoretical concentrations 0.34 1.176

boundaries (Fig. 2). BMS blebs occur almost exclusively in intergran-ular pores, within the same microstructural sites as major mineralsinvolved in the refertilization process, i.e. opx, vermicular Al-spinel(which represents no more than 3% of host rock modal compositions)and cpx. A higher proportion of BMS was found to be adjacent toolivine in the harzburgites and the cpx-rich harzburgites, a likelyconsequence of the higher olivine modal abundances. The major

entrale, Tervuren, Brussels (courtesy of Jacques Navez). SABS= South African Bureau of

Rh(ppm)

Pt(ppm)

Pd(ppm)

Au(ppm)

0.75 7.49 3.95 0.66 N=12±0.30 ±0.98 ±1.55 ±0.190.67 7.23 4.07 0.74 N=20±0.42 ±1.54 ±1.33 ±0.220.56 7.24 2.96 0.6 SABS (2005)

0.60 8.76 3.89 0.69 N=12±0.22 ±2.86 ±0.39 ±0.230.61 8.64 3.55 0.552 SABS (2005)

Table 2Whole-rock PGE analyses. H = harzburgite; C+H = clinopyroxene-rich harzburgite; L = lherzolite; C+L = clinopyroxene-rich lherzolites.

CaO(wt.%)

Al2O3

(wt.%)S(ppm)

Os(ppb)

Ir(ppb)

Ru(ppb)

Rh(ppb)

Pt(ppb)

Pd(ppb)

04Lh11 1 Lherz (H) 0.38 0.51 38.5 5.14 4.21 7.51 1.23 6.07 1.6904Lh13 1 Lherz (H) 0.36 0.44 51 4 3.39 6.95 1.12 4.67 0.8471-322 3 Lherz (C+H) 0.68 1.52 56 n.a. 4.42 7.93 1.51 6.3 3.771-322 2 0.68 1.52 56 4.03 4.76 9.08 n.a. 7.06 3.682-4 3 Lherz (C+H) 0.81 1.14 80 n.a. 3.55 6.3 1.03 6.1 3.182-4 1 0.81 1.14 80 4.47 3.93 8.15 n.a. 4.6 3.0382-4 2 0.81 1.14 80 4.72 4.51 8 n.a. 4.86 2.6104Lh08 1 Lherz (L) 2.18 2.1 145 4.45 4.07 7.6 1.39 7.23 3.771-3211 Lherz (L) 2.87 3.1 220 4.53 3.62 5.8 1.2 7.2 5.671-324 1 Lherz (L) 2.23 2.96 170 3.92 3.37 6,6 1.27 7.8 6.971-3261 Lherz (L) 2.70 3.58 170 n.a. n.a. 7.55 n.a. 7.76 7.0404LH37b 1 Lherz (C+L) 3.73 4.13 415 4 3.87 7.06 1.39 7.68 6.7104LH39a 1 Lherz (L) 2.96 2.9 321 3.96 3.6 6.7 1.25 6.93 5.5504LH39b 1 Lherz (C+L) 5.39 5.59 348 3.13 2.7 6.3 1 5.57 5.4504Lh815 1 Lherz (L) 3.26 3.4 208 4.14 3.6 7.32 1.27 7.03 5.8304Lh15 1 Lherz (L) 3.29 3.96 198.5 3.6 3.55 6.79 1.35 7.01 6.2886-V2-5 2 Lherz (L) 3.0 3.6 250 5.85 3.02 6.18 0.953 5.77 4.9771 339 1 Freychinède (L) 2.5 3.23 240 3.41 3.15 6.4 1.25 6.76 6.4

1This study; 2Becker et al. (2006); 3Lorand et al. (1999). n.a.: not analysed.

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BMS are pentlandite-Pn (Fe/Niat.=1.15±0.3) and chalcopyrite-Cp (Cu/Feat.=0.96±0.04), representing 80–95% and 5–15% by volume,respectively. Bornite, pyrrhotite, pyrite and mackinawite are minorsulfides (Lorand, 1989b). Both Pn and Cp display nice wetting features(dihedral angles b60°) against olivine neoblasts. Some samples showBMS grains of two different sizes, i.e. large grains (generally located insp–opx–cpx clusters) surrounded by clouds of tens ofminute (b30 µm)

Fig. 2. Photomicrographs of base metal sulfides-BMS (A BSE image; B to D: plane polarizedangles with matrix silicates; B: two-phase intergranular BMS (pentlandite-Pn+chalcopyritecapacity of Cu–Ni-rich sulfide melts in the mantle; D: disaggregated BMS grains in a highly d

sulfide blebs, which result from mechanical dispersion of sulfide meltsby the strong plastic deformation and recrystallization of silicates.

4.2. Platinum-group minerals

Platinum-group minerals (PGM) were identified in all of the 17samples studied. Ninety five PGM grains ranging in size from

reflected light images). A: polyhedral pentlandite grain displaying very low dihedral-Cp); C: Polyhedral pentlandite–chalcopyrite intergrowth illustrating the high wettingeformed lherzolite; note the very small satellite BMS blebs all around the largest grains.

Fig. 3. Evolution of PGM assemblages as a function of whole-rock S contents. Note that laurite modal proportions decrease as whole-rock S concentrations (and thus BMS modalabundance) increase. The minerals showing Pt–Pd–Te–Bi combinations predominate in the PGM assemblages of lherzolites. The alloys of Pt–Ir–Os do not show clear trend, beingpresent in similar proportions from the lherzolites to the most depleted harzburgites. N = number of PGM grain per sample.

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b0.25×0.1 µm to 16 µm×1.4 µm were detected by SEM operating inthe BSE mode inside or in the immediate vicinity of BMS blebs.Quantitative analysis by electron microprobe was precluded by the verysmall grain size of PGMs and their location inside or at the vicinity ofBMS which generated continuous fluorescence on PGM analyses.According to EDS spectra (not shown), PGMs mainly consist in sulfidesfrom the laurite-erlichmanite series [Ru,Os(Ir)]S(As)2, Pt–Ir–Os rich alloysand Pt–Pd–Te–Bi phases (moncheite–merenskyite). The other PGMsidentified in Pyrenean peridotites are Pt arsenides/sulfarsenides(sperrylite), Cu–(Pt, Ir) sulfides (malanite), Pd–Ni–S (braggite), Pd–Cucompounds, Pt–Fe alloys and native gold. Additional PGM micronug-gets (56) were detected by in-situ LA-ICPMS analyses. When drillingthrough amicronugget, this latter generates concentration spikes in time-resolved spectra which allow qualitative assessment of which PGE ishosted in the micronuggets. In-situ analyses confirm the results of EDSspectra, namely the predominance of Pt–Ir–Os alloys and Pt–Pd–Te–Biphases over the other PGM species. Pt–Ir–Os alloys display variable Ir/Osratios. Native gold is not uncommon while native platinum is exception-ally scarce.

Fig. 4. Time-resolved LAM-ICP-MS spectra collected during three analyses of pentlandite. Theidentify a laurite nugget that contains trace amounts of Pt and Pd in A. B displays two spikes ofassociated with a Pt–Te–Bi phase. C corresponds to analysis of a pentlandite-chalcopyrite bl

Pyrenean peridotites display a continuum of PGM assemblageswhich vary sympathetically with whole-rock S contents, and thus BMSmodal abundances (Fig. 3). The BMS-poor residual harzburgites(04Lh11 and 04Lh13) show very similar PGM assemblages as theBMS-free harzburgites analysed by Luguet et al. (2007). Laurite coexistswith Pt–Ir–Os alloys and Cu–Pt-rich sulfides of the malanite–cupror-hodsite series; it can generate huge concentration spike in the time-resolved spectra for Os–Ir–Ru–As (Fig. 4A). Cpx-rich harzburgites, ofintermediate BMS content, differ from the harzburgites by occurrence ofa few Pt- and Pd-rich PGMs (bismutho-tellurides, sulfides and alloys;e.g. 71-322), coexisting with laurite and Pt–Ir–Os alloys. The relativeabundance of laurite drops in the lherzolites whereas that ofbismuthotellurides significantly increases along with the abundanceof BMS. The PGMassemblages in the Lherz lherzolites are dominated byPt–Ir–Os alloys and Pt–Te–Bi phases, in agreement with previousobservations on FONB-93 (Lorand et al., 2008b). The only PGMdetectedin some samples, especially poor in PGM but very rich in BMS (04Lh37)were Pt–Te–Bi phases. Platinum arsenides/sulfarsenides and discretenative gold were detected in PGM-rich samples.

huge spikes of Ru, Ir andOs concentrations, coupledwith aminor As concentration spikePt, Ir andOs concentrations corresponding to Pt–Ir–(Os)micronuggets; the second one iseb containing a mix of Pt–Ir–(Os) alloy and Pt–Te–Bi phase.

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Although PGM are systematically associated with BMS grains, thetotal number of PGM detected in a single sample does not correlatewith whole-rock S contents. Sample 71-322, a Lherz cpx-harzburgiteis as rich in PGM as the lherzolites in spite of markedly lower Scontents (Fig. 3). Conversely, S-rich lherzolites may be very PGM-poor(04Lh39; 321 ppm S; 1 PGM). In addition to real random distributionand strong random sectioning effect onmicrometer-sizedmineral, thebetween-sample variation in PGM modal abundance seems tocorrelate with the intensity of plastic deformation. The PGM-richsamples (71-322, 04LH15, LH 815 and 71-339) are sub-myloniticsamples more extensively deformed and recrystallized, and richer inminute (b50 µm) sulfide blebs than the other samples. The highabundance of PGM in those samples is confirmed by a largernumber of concentration spikes in time-resolved laser ablationspectra. Each of the 8 BMS grains analysed in-situ in Lh15 generatedPGE concentration spikes. Some grains display evidence of at leastthree PGMs located inside the analysed BMS grain (Fig. 4B).

Although very scarce in fertile lherzolites, laurite was detected inBSE images of 71-339 and in three LAM-ICPMS signal vs. timediagrams of Lherz lherzolites (04 LH08; 04Lh815; 86-V2-5. In 71-339,laurite and Pt–Ir–Os alloys preferentially occur within small-sized(b80 µm across) satellite sulfide grains around larger BMS blebs.Laurite crystals (3×2 µm in maximum dimensions) occur as a combi-nation of cubic and octahedral morphology, often Os-enriched (OsS2)towards grain rims. It has been observed as i) cubes included in Pncores (Fig. 5A), ii) as external granules sharing one face with matrixsilicates or Al–Cr-spinel (Fig. 5A, C) or iii) as discrete, euhedral grainssharing just one face with BMS, which may be Pn or Cp (Fig. 5D). Thesequence from Fig. 5D to A likely corresponds to different stage ofentrapment of laurite by BMS blebs. It is worth noting that very fewlaurite were detected in LAM-ICPMS time-resolved spectra. This is inagreement with the preferred location of laurite nearby very smallBMS grains which are too small to be analysed with laser ablationtechniques.

Unlike laurite, in-situ analyses detected numerous concentrationsspikes corresponding to Pt–Ir–Os alloys and Pt–Te–Bi phases. Pt–Ir–Osalloys tend to be acicular (up to 10×0.1 µm). These single-phase grainsare concentrated preferentially with small-sized pentlandite blebs(average diameter of 17 grains: 29±22 µm). Pt–Ir–Os alloys arecommonly associated with laurite inside the same BMS (Fig. 5A), asinclusions in homogeneous Pn cores (Fig. 5E) or as external needlesadhering to Pn (Fig. 5F). Pt–Ir–Os alloy inclusions in Cp–Pn intergrowthare very scarce. Overall, PGE spikes are randomly distributed in time-resolved spectra. In the mylonitized samples, spikes of Pt–Ir–Os alloysoccur preferentially as the S signal is decreasing rapidly. This observa-tion is consistentwith theBSE images. Both suggests that Pt–Ir–Osalloysare adhering to, but are not fully enclosed by BMS.

Pd–Te–Bi phases are intimately associated with Cp or Cp–Pnintergrowths. Pt-rich tellurides were commonly found to occur inpentlandite-chalcopyrite intergrowths as thin (b1 µm thick), cleavedtrigonal sub-equant platelets (Fig. 5H). Aside their marked preferencefor Cu-rich sulfides, Pt–Te–Bi minerals may be attached to euhedrallaurite, occupying embayments along with hydrous silicates (Fig. 5B).Native gold contorted micronuggets inside cracks or altered contactbetween Cp and Pn. In sample 71-339, a complex, four-phase PGMassemblage displays a Pt–Fe alloy and a Pt–bismuthotelluride rodprotruding from an euhedral crystal of laurite+Pt–arsenide (Fig. 5G).

Fig. 5. Back scattered electron (BSE) scanning electron microscope images of platinum grinclusion (La) in the core of a minute pentlandite bleb, coexisting (but not in contact) with awith a Pt–Te–Bi phase and hydrous silicate (dark grey); C: laurite granule adhering to thewalof an external laurite granule in contact with pentlandite; low-brightness spongy materialF: thin Pt–Ir–Os lamella adhering to a pentlandite bleb. G: three-phase PGM grain (euhedchalcopyrite intergrowth (Pn-Cp); H: euhedral lamella of Pt–Te–Bi phases included in pentlTe–Bi phases. Scale bar=1 µm, unless otherwise stated.

Such four-phase PGM composite inclusions share one face with Al–Crspinel.

5. Discussion

5.1. Significance of PGM composition and mineralogy in the petrogenetichistory of the Pyrenees Orogenic Peridotite Massifs

The Lherz and Freychinède peridotites show the widest range of«primary» PGM combinations yet reported in fertile mantle peridotites,with some minerals (sulfides of the laurite-erlichmanite series) reportedfor the first time in such rocks. Our results shed some new light on theorigin of PGMs as a whole in orogenic lherzolites. However beforediscussing their origin, it is necessary to recall some basic features ofmantle-derived BMS. Phases like monoclinic pyrrhotite, pentlandite andchalcopyrite are low-temperature minerals, not stable under P–Tconditions of the upper mantle. Paragenetic succession of BMS has beenextensivelydiscussed forpentlandite-rich(80–95%Pn;5–15%Cp;Table3)assemblage such as those studied here (Lorand et al., 2008b). Theseassemblage ultimately derived from metal-rich (i.e. metal/sulfur atomicratio N1) Cu–Ni sulfidemelt (Alard et al., 2000, 2005; Ballhaus et al., 2001;Luguet et al., 2003; Lorandet al., 2008a,b).At1.5 Gpa, theaveragepressureof equilibration of Pyrenean peridotites within the lithospheric continen-tal mantle (Fabriès et al., 1991), this assemblage should be molten at c.a.1150±50 °C (Bockrath et al., 2004a,b). On cooling down from 1150 to1000 °C, this composition is expected to precipitate almost equalproportions of a Ni-rich monosulfide solid solution (22–25 wt.% Ni)withmetal/sulfur (M/S) ratioof0.9, andaNi-rich sulfidemeltmoremetal-rich than the Mss, which will subsequently crystallizing the hightemperature form of heazlewoodite (Hz) at 860 °C. On further cooling,the sulfide melt is also enriched in Cu until Iss (Intermediate solidsolution) crystallizes at 880-840 °C (Peregoedova and Ohnenstetter,2002). A reaction betweenHz-Iss andMss at Tb600 °C producedmassivepentlandite,while chalcopyrite is stable below557 °C (Fleet, 2006 and ref.therein).Note that a slightlydifferent coolingpathwasdeducedby Lorand(1989b) for some pyrite-rich Pyrenean lherzolites that exsolved pent-landite at lower temperature (b300 °C). As pyrite is either absent orpresent in very minor proportion, the 600 °C figure is considered to bemore likely for the Lherz and Freychinède samples studied here. In bothpaths, some readjustment of Ni and Cu distribution between Cp and Pnoccurred until closure temperature below 100 °C is reached (Lorand,1989b).

The solubility of Os, Ir and Ru and Rh in (Fe–Ni)–S monosulfide(solid or molten) is several thousands of ppm to several percents (e.g.Alard et al., 2000; Brenan and Andrews, 2001; Brenan, 2002; Bockrathet al., 2004a,b) whereas Pt and Pd tend to go to the coexisting Cu-Nisulfide melt (Dmss/sulfide meltb0.3; Li et al., 1996; Peregoedova et al.,2004; Mungall et al., 2005; Ballhaus et al., 2006). Pentlandite, themajor low-temperature BMS (N80 vol.%) in Lherz lherzolites candissolve ten percent levels of Ru, Rh and Pd, as shown by experimentalworks (Makovicky et al., 1986) and natural occurrence of rutheniumand/or rhodium equivalent of low-temperature pentlandite havebeen reported in PGE ores (Cabri 1992; Augé, 1988; Zacarini et al.,2005). Even at 25 °C, the Ru, Ir and Os content of pentlandites fromPGE ores is 2–4× the Ru–Ir–Os concentration range measured withinLherz pentlandites (4–55 ppm; Table 3) (Cabri, 1992; Ballhaus and

oup minerals in Lherz and Freychinède peridotites (PGM; white). A: euhedral lauritePt–Ir–Os alloy; B: zoned euhedral laurite crystals partly entrapped by pentlandite, alongl of a chalcopyrite grain which has exsolved a Pt–Pd–Te needle; D: octahedral sectioningis secondary magnetite. E regularly faceted pentlandite showing a Pt–Ir–(Os) needle;ral laurite-La+sperrylite-PtAs2+Pt–Pd–Te–Bi phase) partly enclosed in pentlandite-andite+chalcopyrite intergrowths. Note the fracture planes that cut across the Pt–Pd–

Table 3Contribution of BMS to the whole-rock PGE budget of Pyrenean orogenic peridotites(concentration in ppm).

Os Ir Ru Rh Pt Pd N

82-4 (0.024 wt.% sulfides)calculated BMS 18.7 15 32 3.8 19 12.2 5Pn96Cp4 ±6.0 ±4.3 ±8.0 ±1.35 ±4.0 ±3Measured BMS 8.63* 5.44 10.5 2.11 2.75 32.3

±1.8 ±5 ±2 ±1.5 ±6

71-322 (0.016-0.011 wt.% sulfides)calculated BMS 26.7 30.3 53.3 10.07 45.3 23.7 5Pn95Cp5 ±18 ±20 ±25 ±5 ±12 ±11.7Measured BMS 2.00 2.57 5.36 1.62 1.26 28.40

±0.9 ±1.65 4.38 ±1 ±1.29 ±8.56

04Lh08 (0.044 wt.% sulfides)Calculated BMS 10 9.25 17.3 3.15 16.43 8.5 8Pn84Cp+(Bo)16Measured BMS 7.30 4.06 5.88 1.90 0.87 12.82

±2.1 2.20 ±3.18 ±0.85 ±0.57 ±2.20

71-321(0,067-0.093 wt.% sulfides)calculated BMS 6.0 4.82 10.66 1.6 9.6 7.5 6Pn95Cp5(3)Measured BMS 7.84 4.53 5.00 1.5 1.64 18.2

±3.02 ±2.86 ±2.60 ±0.9 ±0.75 ±7.6

71-326 (0.055-0.065 wt.% sulfides)Calculated 6.7 5.99 12.58 2.10 12.93 11.73 10

Measured 6.9 3.70 4.7 1.50 0.34 10.7±3.39 ±0.90 ±2.23 ±0.42 ±0.33 ±5.15

71-324 (0.055 wt.% sulfides)Calculated 7.13 6.13 12.1 2.31 14.181 12.54 5Pn85Cp15Measured 7.74 6.58 6.34 2.0 0.36 12.99

±2.62 ±3.27 ±4.37 ±0.54 ±0.25

04Lh15(0.06 wt.% sulfides)Calculated 6.5 5.9 11.32 2.25 11.69 10.5 6Pn85Cp-Bo15Measured 5.28 4.13 6.17 1.90 0.24 22.041

±2.84 ±2.70 ±3.70 ±3.07 ±0.99 ±0.08

04Lh815 (0.06 wt.% sulfides)Calculated 6.9 6.0 12.2 2.12 11.8 9.72 11Measured 6.5 5.39 6.09 1.35 0.68 16.45

±3 ±2.39 ±2.28 ±0.6 ±0.37 ±5.41

71-339 (0.071 wt.% sulfides)Calculated 4.9 4.25 9.5 1.76 9.56 8.7 6Pn84Cp16Measured 8.32 3.95 2.94(6) 1.46 0.26 11.1/16.0

±3.95 ±2.39 ±1.74 ±1.12 ±0.22 1.4/±4.9

Calculated sulfide compositions are based on whole-rock analyses of PGE (Table 2)normalized to 100 wt.% sulfides. Pn = pentlandite; Cp = chalcopyrite; Bo = bornite.Sulfide modal composition, necessary to convert whole-rock S contents into sulfidemodal abundances were estimated from systematic traversing of polished thin section(Lorand, 1989a); where available, Cp/Pn ratios were calculated from whole-rock Cu/Sratios, assuming negligible amount of Cu in silicates (Lorand, 1989a; Lorand et al., 1999;Le Roux, unpublished data). Measured sulfides are themean PGE contents measured in-situ with LA-ICPMS. PGM's-bearing BMS grains (i.e. exhibiting well-characterizedspikes of noble metals) were not used. Propagated errors are given as one standarddeviation. N = number of in-situ analyses taken into account.

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Sylvester, 2000; Godel et al., 2007; Godel and Barnes, 2008 and ref.therein).

Of course, owing to their low PGE content, BMS residing in the uppermantle are expected to be undersaturated with respect to PGM (Table 3).The sulfide-PGM assemblages of mantle samples actually reflect acomplex series of events. Basically, three episodes can be identified atwhich the sulfides and the PGE phases could have formed (1) during thepartialmelting andmelt depletion eventswhich eliminated the BMS from

the lithospheric protolith of (2) the refertilization process that addedelements like Se, As, Sb, Te, and Bi (chalcogenes and semimetals) whichwere able to scavenge PGE from BMS and (3) crystallization of the basemetal sulfides during decompression and emplacement of the peridotitebodies within the crust, as temperature decrease tends to expel PGEformerly in solution in BMS.

5.2. BMS, Pt–Pd–Te–Bi phases and the refertilization model

Of likely subsolidus origin are Pt–Pd–Te–Bi phases (and Pt arsenidestoo) that are systematically enclosed in BMS. Those PGMs and the BMSphase are co-genetic, as suggested by the coupled increase in themodalabundances of bothminerals (Fig. 3): semi-metal anions of Te, Bi, andAswere delivered (like S), by the tholeiitic melt involved in therefertilization process that affected the harsburgitic protolith.

Textural (i.e. location in the interstitial pores of the rocks, withinthe same microstructural sites as major minerals involved in therefertilization process) and compositional features lead us to inter-preted BMS in Lherz and Freychinède lherzolites as crystallizationproducts from an exotic component, for instance the tholeiitic meltthat refertilized the harzburgitic protolith in the Le Roux et al. (2007,2008, 2009) model. Our interpretation of late-magmatic/metasomaticorigin is supported by 1) the shape of BMS blebs and their mineralassemblage (pentlandite–chalcopyrite intergrowths, occasional born-ite) are typical of Cu–Ni sulfide melt already identified in abyssal andcontinental mantle peridotites that reacted with basaltic melts and 2)Lherz pentlandites display only Pd-enriched (i.e. Type-II; Fig. 6)chondrite-normalized PGE patterns characterizing metasomatic sul-fides in oceanic peridotites refertilized by basaltic melts (Luguet et al.,2003, 2004; Alard et al., 2005). Of specific interest in that discussion isthe origin of BMS in the cpx-rich harzburgites and the harzburgites.Melting models suggest that these refractory rocks should becharacterized by Cu-poor Mss inclusions (or reequilibration productsof this mineral, i.e. Pn–Po intergrowths) displaying a gradualdepletion in Pt and Pd (Pd/IrNb0.2; Pt/IrNb0.5) (Ballhaus et al.,2006). Such enclosed Mss are commonly preserved by peridotitexenoliths in alkaline lavas (e.g. Alard et al., 2000; Lorand and Alard,2001). However, at Lherz, BMS in harzburgites are in no way differentfrom BMS in lherzolites. They are composed of intergranular Pn-Cpblebs displaying nice wetting features of metal-rich sulfide melts andconstant Cu/S ratios (0.1–0.2), with no evidence of trapped Mss. Therefertilization model that implies a unique source for the BMS (i.e. thetholeiitic melt) accounts for such constant mineralogical featuresfrom the lherzolites to the S-depleted harzburgites. The REE variationin harzburgitic Cpx indicates that small melt fractions residual afterthe refertilization reaction migrated in the harzburgite protolith (LeRoux et al., 2007, 2009), thus precipitating Pd-rich sulfides in formerlyPd-depleted peridotites. These late-magmatic BMS may account forwhole-rock Pd/PtN≥1 much better than partial melting models (c.f.Luguet et al., 2003) because Pd is more incompatible than Pt.

Platinum does not enter the octahedral site of pentlandite (norchalcopyrite), the two main sulfides in the samples under discussion.All Pyrenean pentlandites analysed in-situ are strongly depleted in Pt(by factors of up to 100) relative to the other PGEs (Fig. 6). This Ptdeficit is also noted in sulfides from PGE deposits (see Ballhaus andRyan, 1995). It reflects exsolution of Pt–Fe alloys when mssrecrystallizes to Po and Pn and when Iss recrystallizes to Cp. Platinumwas also scavenged from BMS to form PtAs2, PtAsS and Pt tellurideswhich are rather stable once formed. At Lherz, like in sample FON B-93from Fontête Rouge (Lorand et al. 2008b), the preferential occurrenceof bismuthotellurides in isolated Cp or Cp–Pn intergrowths isconsistent with the strong affinity of these highly incompatibleelements for Cu-rich sulfide melts (Yi et al., 2000). Semi-metals formsoft ligands that stabilize Pt and Pd in the sulfide melt, thus furtherdecreasing their Dmss/sulfide melt partition coefficients (Ballhaus andSylvester, 2000). Sulfide melt and telluride melt are fully miscible

Fig. 6. CI chondrite-normalized PGE abundances in PGM-free lherzolitic BMS. Analyses showing positive Ru anomalies in bold. Normalizing values after McDonough and Sun (1995).

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above 1150 °C (Helmy et al., 2007). For c.a. 70,000 ppmof Te+Pt+Pd,the telluridemelt is saturatedwith respect to Pt-telluride at TN1015 °Cwhile Pd–Ni–telluride saturation is shifted down to subsolidustemperatures (b700 °C; Gervilla and Kojonen, 2002). However,saturation in Pt–Te–(Bi) phases in Pyrenean peridotites is expectedto have occurred at much lower temperature because mantle-derivedBMS at Lherz are several orders of magnitude poorer in Pt+Pd+Te(b100 ppm; Table 3). For such low contents of Pt+Pd+Te, Pt–Pdbismuthotellurides likely precipitated at sub-solidus temperature ofthe BMS assemblage from the very last Cu-sulfidemelt fractionswhichshould be the most Pt–Pd–Te-enriched. Note that the Te contentrecorded in the sulfides is that expected for equilibration temperatureof ca. 200 °C (C. Ballhaus, personal communication).

Although moderately compatible into Cu-rich sulfides (chalcopy-rite, cubanite; Ballhaus and Sylvester, 2000; Lorand and Alard, 2001;Luguet et al., 2004), Pd can enter pentlandite at all temperatures Thus,there is very little room for pentlandite to exsolve Pd-rich minerals.Alternatively, palladium bismutho-tellurides associated with pent-landite in Pyrenean peridotites may be direct precipitation productsfrom vapor at Tb700 °C (Gervilla and Kojonen, 2002). Likewise, not allPt bismutho-tellurides can be interpreted as direct crystallizationproducts from Cu-rich sulfide melts. For instance, Fig. 5B and 5Gprovide evidence that some Pt-tellurides nucleated directly ontolaurite or Pt–Ir–Os alloys, sometimes associated with hydrous silicatesand Pt-arsenides. Such assemblages point to local accumulation ofvolatile elements (Te, Bi, As) by the refertilization process. Separateevidence is provided by lithophile trace element patterns anddisseminated Ti-pargasite in Lherz and Freychinède lherzolites(Fabriès et al., 1991; Le Roux et al., 2007).

5.3. Laurite and Pt–Ir–Os alloys: refractory PGMs tracking the 2 Ga-oldharzburgitic protolith inside Pyrenean lherzolites

Unlike Pt–Pd–Te–Bi phases, laurite and BMS are not cogeneticsince their modal abundance are negatively correlated (cf. Fig. 3).Euhedrally shaped laurite is usually interpreted as direct crystalliza-

tion product from S-bearing, sulfide-undersaturated silicate melts(Augé, 1988; Brenan and Andrews, 2001; Andrews and Brenan, 2002;Bockrath et al., 2004b). Such melts may form in a mantle pieceundergoing increasing degree of partial melting once the ultimateBMS fractions (compositionally close to Fe–Ni monosulfides-Mss) aredissolved into the silicate melt. At Lherz, Luguet et al. (2007)demonstrated that the whole-rock PGE budget of the 2 Ga-old S-free harzburgites is effectively hosted in discrete PGM of the laurite-erlichmanite series, Pt–Ir–Os rich alloys and complex Cu–Pt sulfides.Thus, laurite crystals now disseminated inside lherzolites areinterpreted as relicts from this lithospheric protolith. Our interpreta-tion is supported by BMS-laurite textural relationships, namelyintermediate stages of entrapment by pentlandite, from externallaurite granules simply attached to pentlandite by one crystal face tofully enclosed cubes. Likewise, our interpretation does account prettywell for the laurite crystals that are pasted on Cp or Pn–Cpintergrowths (e.g. Fig. 5C) because Ru (like Os and Ir) is not solublein Cp and Iss-phases nor within their high-temperature precursor (i.e.Cu-rich sulfide melt, Li et al, 1996; Lorand and Alard, 2001).“Lithospheric” PGMs were mechanically collected by droplets ofmagmatic sulfide melt during rejuvenation reactions that refertilizedthe harzburgitic protolith. A preferential distribution within the grainboundary network of the lithospheric protolith (Luguet et al., 2007)likely helped “lithospheric” PGMs to be captured by lherzolitic BMS.Plastic deformations coeval to the refertilization event (Le Roux et al.,2008) also played a role, as suggested by abundant PGMs in the highlydeformed peridotites. At 1100 °C, the BMS were molten to partiallymolten, which likely enhanced their mechanical dispersion amongsilicate neoblasts, generating the nice wetting figures of Fig. 2.

The great similarity between Pt–Ir–Os alloys and laurite as regardtheir textural relationships and their associations with BMS grainsargue for a similar origin (cf. Lorand et al, 2008b). Both coexist in BMS-free harzburgites (Luguet et al., 2007) and Pt–Ir–Os alloys are high-temperature phases (N1100 °C) in the Fe–Pt–Ir–S system (Fleet andStone, 1991; Cabri et al., 1996). Ruthenium-poor, Ir+Os-rich alloyand laurite coexist stably at 1100 °C in the phase equilibrium data of

Fig. 7. Ru vs. Os, Os vs. Ir and Rh vs. Os diagrams for Lherz and Freychinède lherzoliticBMS. The population displaying suprachondritic Ru/Ir ratios is Os- and Rh- enrichedrelative to chondritic ratios; these features are ascribed to laurite crystals dissolvedinside BMS. CR=chondritic ratio (McDonough and Sun, 1995).

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Andrews and Brenan (2002). Pt–Ir–Os alloy modal abundances varyirrespective of whole-rock S contents (cf. Fig. 3). One may speculatethat lithospheric Pt–Ir–Os alloys, once trapped inside lherzolites, werenot eliminated by the refertilization process because Pt is poorlysoluble within Mss. Note that, although experimental data relevant toconditions of the upper mantle precludes definite answer, Peregoe-dova et al. (2004) reported Mss+Cu–Ni sulfide melt saturated in Pt–Ir alloys at 1000 °C for bulk Pt contents of similar order of magnitudeas the bulk concentrations estimated for Lherz/Freychinède sulfides(Table 3). Platinum systematically forms its own alloy phases inmetal-rich (b50 at.% S) BMS assemblages of the Cu–Fe–Ni–S systemover a wide range of temperature (Makovicky, 2002).

The decrease of laurite modal abundance with increase of BMSmodal abundances in Fig. 3 most probably indicate dissolution oflithospheric crystals inside metasomatic sulfide melts. This is notsurprising because 1) laurite and sulfide melts can coexist over a veryrestricted range of sulfur fugacity (Andrews and Brenan, 2002;Bockrath et al., 2004b; see also Fig. 5 in Luguet et al., 2007) b)Theoretical BMS phase calculated fromwhole-rock PGE and S analyses(Table 3) are low in Os, Ir, Ru and Rh compared with the solubility ofthe corresponding elements in pentlandite. Another evidence forlaurite dissolution inside the BMS is provided by positive Ruanomalies in the chondrite-normalized PGE patterns of somepentlandite (Fig. 6) with Os/Ir, Ru/Os and Rh/Ir≫CI-chondritic ratios(Fig. 7). Nugget effects in those pentlandite grains are ruled out by thelack of coupled peaks in the signal of Ru+Os+Ir in time-resolvedspectra. Each lherzolite analysed in the present study shows such Ru-rich BMS which are most abundant in 71-339, the richest in laurite.The simplest explanation is that Ru, Os and Ir were not able torehomogeneise their concentrations. Pyrenean lherzolites werequickly uplifted as small-sized bodies (b1 km3) and cooled at 600 °Cwithin crustal country rocks within less than 1 Ma (Fabriès et al.,1998). It is worth noting that FON-B 93 is devoid of laurite, although itis as rich in PGMs as the richest samples at Lherz and Frechinède. Wemay speculate that laurite totally dissolved inside the BMS owing tothe longer period of annealing of silicate assemblages at mantletemperatures that generated the secondary protogranular microtex-ture of Fontête Rouge lherzolites (Fabriès et al., 1991).

A set of peculiar conditions allowed the laurite to be locallypreserved. Epitaxial growth of sulfide melts on pre-existing lauritescan account for much of the laurite-BMS microtextures in Fig. 5. Asimilar explanation also holds true for those Pt–Ir–Os alloys which arenot enclosed, just attached on small-sized Pn blebs. Plastic deforma-tions in Pyrenean peridotites ended at rather low temperature(750 °C, 0.7–1.0 Gpa) during solid-state emplacement at Mohodepth (Fabriès et al., 1991, 1998). Late stage deformations disag-gregated the largest BMS blebs into ten-times smaller grains, thusincreasing surface/volume ratio and creating opportunity for thesmallest BMS blebs to intercept “lithospheric” PGMs at subsolidustemperatures (b900 °C). This is the likely reason why laurite wasfound to be attached to the smallest BMS blebs. Sharing only one ortwo crystalline faces with BMS grain, those grains suffered limiteddiffusion of Ru, Os and Ir with the BMS.

5.4. Implications for the PGE budget of Pyrenean peridotites

Not only the BMS phases but also PGM micronuggets must betaken into account to calculate the PGE budget of orogenic fertilelherzolites. Laurite is a good candidate for fully equilibrating thewhole-rock budget of Ru, coupledwith Pt–Ir–Os alloys for Os and Ir. Inaddition to an overall Pt deficit (75–99%) that reflects the abundanceof Pt-rich PGMs, the measured BMS compositions in the cpx-richharzburgites and lherzolites that are rich in PGMs and poor in BMS donot balance the whole-rock PGE budget for Os, Ru, Ir and Rh (Table 3).Of course, the unaccounted fraction decreases from 71-322 to 82-4and 04Lh08 as BMS modal abundance increases. Sample 04Lh08, as

rich in BMS as UB-N (145 vs. 141-138 ppm S; Lorand et al., 2008a)displays unaccounted fractions of Os and Ir not residing inside BMS ofsimilar extent as this rock standard (30±20%Osand40±20% Ir;Meiselet al., 2003). By contrast, as suggestedbyFONB-93 (Lorandet al., 2008b),fertile lherzolites, which are the most-enriched in BMS, do not displaysuch adeficit. Calculated andmeasured contents of Os, Ir andRh agree at1 sigma level; the rather large uncertainties on measured compositionspreclude a precise estimate of the contribution of PGMs.

Unlike Os and Ir, the concentrations of Ru measured in BMS offertile lherzolites display rather a constant deficit of ca. 50% comparedto calculated concentrations. This obviously tracks laurite, although

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this sulfide, preferentially adhering to the smallest BMS blebs, has notbeen detected in all the analysed samples. A simple calculation basedon sample 71 339 (4±2 perfectly cubic laurite crystals, eachrepresenting arbitrarily 8 µm3 by volume per 30 µm-thick standardsizedpolished thin section) allows the volumeof laurite tobe estimated at1±0.3×10−9%, which transposes into 2±1×10−9 g/g (density=6.4).EDS spectra indicate higher Ru contents than Os and much lower Irabundances relative to Ru and Os in lherzolitic laurites. AssumingRu content of 40±10 wt.%, laurite may account for 0.5–1.5 ppb Ru, i.e.up to 25% of the whole-rock budget of Ru of a lherzolite with a PUM-likePGE composition (7.1 ppb Ru). That contribution is obviously under-estimated compared to mass balance calculations of Table 3 because 1)volumeestimateofmicrominerals is prone to considerable error, 2)not allof the laurite crystals were intercepted by the thin sections of 71 339and 3) in-situ LAM-ICPMS analyses provide evidence for one lauritepopulation already dissolved into the BMS phase.

6. Conclusions

Pyrenean lherzolites clearly contain two generations of PGMs(a) refractory minerals (laurite-erlichmanite series, Pt–Ir–Os alloys)inherited from ancient highly depleted harzburgitic protolith and(b) “low-temperature” bismuthotellurides and arsenides/sulfarsenides,intimately associated with Pd-rich metasomatic BMS, indicators oflithosphere refertilization process, in agreement with Le Roux et al.(2007, 2008, 2009) model. Such platinum-group minerals have beenidentified in lherzolites from several orogenic locations, all displayingunambiguous petrological evidence of magma refertilization processesof formerly depleted lithosphere (e.g. Corsican plagioclase lherzolites(Ohnenstetter, 1992), Ligurides ophiolites (Luguet et al 2004),Horoman(Kogiso et al., 2008). Conversely, “Lithospheric” PGM micronuggetslikely carry the unradiogenic Os isotopic compositions of the litho-spheric protolith. Now disseminated within fertile lherzolites, theyaccount for local preservation of ancient Os model ages (up to 2 Ga)detected in BMS by in-situ isotopic analyses (Alard et al., 2005, 2008).

With palladium being hosted by metasomatic BMS, and Ir byrefractory alloy+laurite, refertilization process can generate devia-tion of whole-rock Pd/Ir ratio from chondritic values, depending onthe relative proportions of these two phases. Therefore, the Pd/Ir ratioof orogenic lherzolites should not be used as face values for PUMestimates (e.g. Becker et al., 2006). Similar conclusion also hold truefor whole-rock Ru/Ir ratios as both elements may be partitioned intomicronuggets (laurite and alloys) inherited from the harzburgiticprotolith. Integrated studies coupling identification of Os–Ir–Rucarriers, in-situ analyses of Os isotopic compositions, and Ru/Ir andOs/Ir ratios should be able to decipher whether the positive deviationof Ru/Ir (2.1±0.15) from chondritic ratio is a primordial feature of thePUM or result from refertilization process of ancient lithosphere, asdocumented in the Lherz and Freychinède lherzolites.

Acknowledgements

The authors acknowledge M. Marot (MNHN) for the polished thinsections, O. Boudouma for the FEG-SEM study (Paris VI university)and O. Brugier who operated the LAM-ICPMS facility at Montpellier.Jacques Navez (Musée Royal de l'Afrique Centrale) is warmly thankedfor the analysis of SARM7 NiS beads. We thank Chris Ballhaus andJames Brenan who provided helpful comments on the manuscript,and Richard W. Carlson for editorial advices.

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