Platinum-Group Element Micronuggets and Refertilization Process in Lherz Orogenic Peridotite (Northeastern Pyrenees, France)

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 orogenic peridotite (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, France b Géosciences Montpellier, Université de Montpellier II and CNRS, Place Eugène Bataillon, 34095 Montpellier Cédex, France c Steinmann Institut-Endogene Prozesse. Universität Bonn, Poppelsdorfer Schloss, 53115 Bonn, Germany article info abstract

Article history: Highly siderophile elements (Platinum-group elements, Au and Re) are currently assumed to reside inside Received 29 July 2009 base metal sulﬁdes (BMS) in the convecting upper mantle. However, fertile lherzolites sampled by Pyrenean Received in revised form 19 October 2009 orogenic peridotite massifs are unexpectedly rich in 0.5–3 µm large micronuggets of platinum-group Accepted 4 November 2009 minerals (PGM). Among those, sulﬁdes from the laurite-erlichmanite series (Ru, Os(Ir)S(As) ), Pt–Ir–Os Available online 2 December 2009 2 alloys and Pt–Pd–Te–Bi phases (moncheite–merenskyite) are predominant. Not only the BMS phases but Editor: R.W. Carlson also the PGM micronuggets must be taken into account in calculation of the PGE budget of orogenic fertile lherzolites. Laurite is a good candidate for equilibrating the whole-rock budget of Os, Ir and Ru while

Keywords: accounting for supra-chondritic Ru/IrN. Textural relationships between PGMs and BMS highlight highly siderophile elements heterogeneous mixing between refractory PGMs (laurite/Pt–Ir–Os alloys) inherited from ancient refractory upper mantle lithospheric mantle and late-magmatic metasomatic sulﬁdes precipitated from tholeiitic melts. “Low- orogenic peridotites temperature” PGMs, especially Pt–Pd bismuthotellurides should be added to the list of mineral indicators of platinum-group minerals lithosphere refertilization process. Now disseminated within fertile lherzolites, “lithospheric“ PGMs likely account for local preservation of ancient Os model ages (up to 2 Ga) detected in BMS by in-situ isotopic analyses. These PGMs also question the reliability of orogenic lherzolites for estimating the PGE signature of the Primitive Silicate Earth. © 2009 Elsevier B.V. All rights reserved.

1. Introduction plasma mass spectrometry (LA–ICP–MS) (Alard et al., 2000; Lorand and Alard, 2001; Luguet et al., 2001, 2004; Lorand et al., 2008b). As highly siderophile elements, platinum-group elements (PGEs), Fertile mantle peridotites that are rich in BMS are presumed to be Au and Re, are presumably concentrated in the metallic core of the devoid or poor in discrete platinum-group minerals (PGM), owing to Earth. It is commonly inferred that the late meteoritic influx that hit low bulk PGE abundances in the upper mantle and the strong affinity the earth–moon system after accretion and core formation delivered of PGE for BMS at magmatic temperatures. However, Meisel et al. supplementary PGE to the hypothetical Primitive Upper Mantle (2003) estimated non-trivial amount of Os (N30%) residing outside (PUM) of the Earth (Morgan, 1986; Lorand et al., 2008a and reference BMS in UB-N, an orogenic lherzolite used as reference material. Mass therein). Updated estimates of the PGE composition of the PUM balance calculations based on accessory BMS, often varying consid- identified reproducible (20–30%) deviations from the canonical erably in size and distribution may generate large (up to 200%) errors chondritic model in the light/heavy PGE ratios (i.e. Ru/Ir; Rh/Ir; Pd/ (Luguet et al., 2004; Lorand et al., 2008b), thus overlooking any Pt; Becker et al., 2006). However, PGE's are ultra-trace elements in contribution of other PGE-rich trace minerals. Mostly of micrometric mantle rocks (ng/g concentration level) and their budget in terms of size, PGMs are not straightforwardly detected in lherzolite samples host minerals is still debated. Pioneering works on separated minerals with conventional analytical tools. Using synchrotron-XRF (SR-XRF) from fertile lherzolites (Morgan and Baedecker, 1983; Pattou et al. combined with microbeam technique, Kogiso et al. (2008) detected 1996; Burton et al., 1999; Handler and Bennett, 1999) identified nine grains of Pt–Os–Ir, Pt and Au just from one thin section of spinel accessory (b0.1 wt.%) base-metal sulfides (BMS i.e. Fe–Ni–Cu sul- lherzolite (1102-1A) from Horoman peridotite complex. They fides) as the main PGE carriers. This conclusion received strong deduced that c.a. 10% of the Ir and Os budget is accounted for by Pt– support from in-situ analyses using laser-ablation inductively coupled Ir–Os alloys. However, their analytical configuration was too limited to detect efficiently light PGEs (Ru, Rh, and Pd) and anions of semimetals (S and heavy metalloid elements (Sb and Te)). Using SEM and ⁎ Corresponding author. E-mail addresses: [email protected] (J.-P. Lorand), carefully polished thick (200 µm) sections, Lorand et al. (2008b) [email protected] (O. Alard), [email protected] (A. Luguet). detected up to 12–14 PGM grains (Pt–Os–Ir alloys+Pt–Pd–Te–Bi

0012-821X/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.epsl.2009.11.017 J.-P. Lorand et al. / Earth and Planetary Science Letters 289 (2010) 298–310 299 phases) per standard-size polished thin section of FON-B 93, a fertile Strong enrichment of large ion lithophile elements (LILE, including lherzolite from Fontête Rouge, near Lherz. This PGM assemblage REE) at melt infiltration fronts is indeed predicted by theoretical accounts for c.a. 90% of the whole-rock Pt budget, in agreement with modelling of melt-consuming reactions combined with melt trans- previous estimates performed on orogenic lherzolites (e.g. Luguet port. The irregularly-shaped lherzolite–harzburgite contacts repre- et al., 2004). sent therefore a convoluted melt-rock reaction front formed by The actual abundance of micrometric PGM in fertile orogenic coalescence of porous melt infiltration channels. At the contact, lherzolites, their contribution to the whole-rock PGE budget and their lherzolites show larger and more equant grains and olivine crystalline petrogenetic significance is poorly understood. To address the issue in preferred orientation (CPO) quite similar to harzburgites, but weaker. a more quantitative way, we have performed an integrated, multi- All structures and geochemical signatures were efficiently erased technique study on 17 mantle peridotites collected mainly from the within a few meters across the front. In lherzolites away from contacts Lherz orogenic massif (North Eastern Pyrenees, France), the type- (N20 m), olivine microstructures and CPO are consistent with the locality of lherzolite and one of the best studied in terms of steeply-dipping foliations and sub-horizontal lineations observed petrogenetic evolution. The PGMs were searched for using a within the whole massif, and clinopyroxene displays the classic, combination of reflected light microscopy, high-resolution scanning N-MORB,REEpatternobservedinorogeniclherzolitesworldwide. electron microscope (FEG-SEM) and LA-ICPMS analyses (14 samples). LA-ICP-MS is a powerful tool for detecting such minerals which create PGE concentration spikes in time-resolved spectra (Ballhaus and 3. Petrographical and analytical notes Sylvester, 2000). We provide evidence that 1) PGM and BMS are not necessarily co-genetic; 2) Ru–Os–Ir PGMs, identified here for the first The seventeen samples selected for the present paper comprise time in fertile lherzolites, were inherited from refractory mantle two lherzolites from Freychinède and fifteen lherzolites and harzbur- peridotites; and 3) PGM fractionate some whole-rock PGE ratios that gites from Lherz. The degree of serpentinisation ranges from 15 to 30% are considered to be signature of the Primitive Silicate Earth. for the harzburgites to 5–20% for the lherzolites (Lorand, 1989a). At Lherz, two harzburgites (04Lh13; 04Lh11), one cpx-poor lherzolite 2. Geological setting (04Lh08; or cpx-rich harzburgites) and two fertile lherzolites (04Lh815; 04Lh15) are reference samples from the Le Roux et al. The Lherz orogenic peridotite massif and its cluster of peridotite (2007) study of Site-2 section across one harzburgite-lherzolite outcrops (Freychinède, Fontête Rouge) are three of the 40 or more contact (hereafter referred to as Site-2 samples). Samples 04Lh37 bodies of mantle rocks spatially associated with high-grade meta- and 04Lh39 are lherzolites from Site-4 of the same study (hereafter morphic rocks of the granulite facies in the North Pyrenean Zone. It referred to as Site-4 samples). The other Lherz samples (cpx-rich was exhumed from the mantle c.a. 100 Ma ago by the movement of harzburgites, 82-4; 71-322; 12-1; 71-107; fertile lherzolites 71-321, the Iberian plate relative to Europe, which resulted in crustal thinning 71-326, 86-V2-5 and 71-324) and the two Freychinède lherzolites are associated with successive opening and closing of elongated, “historic” samples, already analysed for whole-rock sulfur contents asymmetrical pull-apart mesozoïc sedimentary basins (Fabriès et al., and BMS mineralogy (Lorand, 1989a,b), Re-Os isotopic compositions 1991; Lagabrielle and Bodinier, 2008 and ref. therein). (Reisberg and Lorand, 1995) and PGE systematics (Pattou et al., 1996; Pyrenean orogenic massifs consist mainly of weakly (b25%) Lorand et al., 1999; Becker et al., 2006). serpentinized spinel lherzolites (10–16 wt.% cpx). Clinopyroxene- Site 2 and Site 4 samples were analysed for S using iodometric poor lherzolites (5–10 wt.% cpx) are much rarer and true harzburgites titration of the SO2 produced by combustion of 500 mg powder aliquot (cpx b5 wt.%) are known only in the Fontête Rouge and Lherz massifs at the National Museum of Natural History (see Gros et al., 2005 for (Fabriès et al., 1991). The Lherz massif displays the best exposure of further details). PGEs were separated at the MNHN from powder boudinaged lenses of highly refractory harzburgites, intermingled on aliquots of 15 g each by a NiS-fire assay — Te coprecipitation procedure a decametric scale with fertile lherzolites. The most fertile lherzolites as described by Gros et al. (2002) and modified by Lorand et al. (15% clinopyroxene) have long been interpreted as pristine mantle (2008b) to improve the recovery of PGE (i.e. second NiS-fire assay step only weakly affected by partial melting whereas harzburgites record and amount of Te increased to 28 ml). The analyses were performed high (N20%) degrees of melt extraction (Bodinier et al., 1988; Lorand, using a FISONS VG 353 PlasmaQuad PQplus inductively coupled 1989a; Fabriès et al., 1991). The melt extraction event was dated at c.a. plasma mass spectrometer (ICP-MS) (University of Montpellier II and 2 Ga by the Re–Os chronometer (Reisberg and Lorand, 1995). A National Museum of Natural History, Paris). Sample 82-4, already comprehensive study of Os isotopic compositions, PGE abundances analysed by Lorand et al. (1999) and Becker et al. (2006) was and PGM distribution in the Lherz harzburgites recently confirmed reanalysed by isotope dilution-ICP-MS (Thermo-Finnigan Element 2 this model age while providing further evidence supporting the magnetic-sector ICP-MS) at the Northern Centre for Isotopic and residual origin of these rocks (Luguet et al., 2007). Elemental Tracing (NCIET) at the University of Durham (UK). Sample However, from detailed investigations along two distinct, 7–8m- digestion was performed overnight at 300 °C and 100 bars in Anton long sections across harzburgite–lherzolite contacts, Le Roux et al. Paar HPA-S pressure-sealed quartz vessels containing 7.5 ml reverse (2007, 2008, 2009) provided convergent structural and geochemical aqua-regia. Details on the separation procedure and analytical arguments supporting a secondary origin for spinel lherzolites. The conditions were given in Lorand et al. (2008b) and in Luguet et al. constant orientation of the foliation and lineation in harzburgite (2009). bodies, different from that in lherzolites and websterites, strongly Platinum group minerals (PGM) were located on carefully suggests that all harzburgites were once part of a single and polished 200 µm-thick standard-sized (40×25 mm) sections using continuous unit, deformed before the formation of the lherzolites reflected light microscopy and high-resolution scanning electron and websterites. This 2 Ga-old lithospheric (harzburgitic ?) protolith microscope (Supra™ 55VP Zeiss FEG-SEM; Pierre and Marie Curie was then infiltrated and reacted with ascending MORB-type basaltic University, Paris VI) equipped with an energy dispersive Si(L) melts of asthenospheric origin. A near-solidus refertilization reaction detector with a resolution of 129 eV full width at half maximum at involving dissolution of olivine and precipitation of Cr- and Al-bearing the Fe peak. The SEM investigations were carried out at an minerals, mostly spinel and pyroxenes, gave rise to spinel lherzolites. acceleration voltage of 15 kV and a working distance of 8 mm to The refertilization model adequately accounts for the geochemical identify platinum-group minerals qualitatively in the backscattered behaviour of some elements such as Ti, Cr and Rare Earth Elements mode. Manual scan of the thick sections in the backscattered mode (REE) at odds with partial melting models (Le Roux et al., 2007). were preferred over automated particle search procedures that may 300 J.-P. Lorand et al. / Earth and Planetary Science Letters 289 (2010) 298–310 not detect particles on the nanometer scale in accessory BMS, despite strong contrast in atomic numbers between host and included PGM. Platinum-group element contents of base metal sulfides were determined along with a few other trace elements (Te, Bi, Pb) by laser-ablation inductively coupled plasma mass spectrometry (LA- ICP-MS) in fourteen samples from Lherz that displayed BMS grains large enough (N100 µm) to be probed. A GEOLAS excimer UV laser operating at 193 nm, coupled with a Thermo-Finnigan Element 2 magnetic-sector ICP-MS inductively coupled plasma mass spectrometer (ICP-MS) (University of Montpellier II) was used. Analytical conditions were reported in Lorand et al. (2008b). The laser beam diameter was set up at 51 µm, 5 Hz laser frequency and a beam energy of about 15 J/cm2. Accuracy was tested with in-house standards (NiS beads doped with SARM 7 rock standard powder; SABS website 2005) analysed separately at Montpellier and at the “Museum Royal de l'Afrique Centrale” at Tervuren (Belgium); both sets of analyses produced very similar results (generally differing by no more than 10%). Measured concentrations of Au and PGE fit the theoretical concentrations at 1 sigma level (Table 1). Concentrations of Os, Ir, Ru and Rh show the lowest reproducibility and thus, the largest relative standard deviation (up to 70% for Ru and Ir). This results from mineralogical inhomogeneities in NiS beads (Gros et al., 2002) possibly linked to non-homegeneous mixing of powder particles inside the NiS too.

4. Results

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

Lherzolites, cpx-rich harzburgites and harzburgites are characterized by different whole-rock PGE systematics. The most fertile lherzolites show CI-chondrite normalized PGE abundance patterns similar 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/IrN N2; Rh/IrN =0.30.38: N=CI-chondrites-normalized; Fig. 1A). The two Site-2 harzburgites (04Lh13 and 04Lh11) display negatively trending Pd-depleted CI-chondrite normalized patterns (Pd/Ir =0.33–0.21; Pd/Pt =0.32–0.50), coupled with N N Fig. 1. CI chondrite-normalized whole-rock PGE abundances for lherzolites (A) and very low S contents (38–51 ppm; Table 2), as is expected for solid harzburgites (B). B = Becker et al. (2006);Lo=Lorand et al. (1999). Normalizing mantle residues after 20–25% of partial melting (Lorand et al., 1999). values after McDonough and Sun (1995). See Table 2 for data. Similar patterns were reported for BMS-free harzburgites collected in the thickest harzburgite lenses from the south-eastern part of the Lherz massif (Luguet et al., 2007). The three cpx-rich harzburgites boundaries (Fig. 2). BMS blebs occur almost exclusively in intergran- share features with both the harzburgites (i.e. a strong Pt-depletion ular pores, within the same microstructural sites as major minerals relative to Ir; Pt/IrN =0.54–0.68) and the lherzolites (Pd enrichment involved in the refertilization process, i.e. opx, vermicular Al-spinel relative to Pt; Pd/PtN =0.96–1.18; Fig. 1B), while displaying interme- (which represents no more than 3% of host rock modal compositions) diate S contents (56–80 ppm). and cpx. A higher proportion of BMS was found to be adjacent to Regardless of the whole-rock S contents, BMS are 20–300 µm olivine in the harzburgites and the cpx-rich harzburgites, a likely across polyhedral (not spherical) blebs with convex-inward grain consequence of the higher olivine modal abundances. The major

Table 1 Precision and accuracy of LAM-ICPMS analyses (1 sigma). MRAC=Musée Royal de l'Afrique Centrale, Tervuren, Brussels (courtesy of Jacques Navez). SABS = South African Bureau of Standard (Annual Report. 2005).

Os Ir Ru Rh Pt Pd Au (ppm) (ppm) (ppm) (ppm) (ppm) (ppm) (ppm)

SARM-7-10 Montpellier 0.17 0.38 1.30 0.75 7.49 3.95 0.66 N=12 ±0.10 ±0.26 ±0.90 ±0.30 ±0.98 ±1.55 ±0.19 MRAC. Tervuren 0.16 0.31 1.30 0.67 7.23 4.07 0.74 N=20 ±0.13 ±0.25 ±1.06 ±0.42 ±1.54 ±1.33 ±0.22 Theoretical concentrations 0.12 0.28 0.98 0.56 7.24 2.96 0.6 SABS (2005)

SARM-7-1 Montpellier 0.33 0.32 1.10 0.60 8.76 3.89 0.69 N=12 ±0.11 ±0.11 ±0.48 ±0.22 ±2.86 ±0.39 ±0.23 Theoretical concentrations 0.34 1.176 0.61 8.64 3.55 0.552 SABS (2005) J.-P. Lorand et al. / Earth and Planetary Science Letters 289 (2010) 298–310 301

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

CaO Al2O3 S Os Ir Ru Rh Pt Pd (wt.%) (wt.%) (ppm) (ppb) (ppb) (ppb) (ppb) (ppb) (ppb)

04Lh11 1 Lherz (H) 0.38 0.51 38.5 5.14 4.21 7.51 1.23 6.07 1.69 04Lh13 1 Lherz (H) 0.36 0.44 51 4 3.39 6.95 1.12 4.67 0.84 71-322 3 Lherz (C+H) 0.68 1.52 56 n.a. 4.42 7.93 1.51 6.3 3.7 71-322 2 0.68 1.52 56 4.03 4.76 9.08 n.a. 7.06 3.6 82-4 3 Lherz (C+H) 0.81 1.14 80 n.a. 3.55 6.3 1.03 6.1 3.1 82-4 1 0.81 1.14 80 4.47 3.93 8.15 n.a. 4.6 3.03 82-4 2 0.81 1.14 80 4.72 4.51 8 n.a. 4.86 2.61 04Lh08 1 Lherz (L) 2.18 2.1 145 4.45 4.07 7.6 1.39 7.23 3.7 71-3211 Lherz (L) 2.87 3.1 220 4.53 3.62 5.8 1.2 7.2 5.6 71-324 1 Lherz (L) 2.23 2.96 170 3.92 3.37 6,6 1.27 7.8 6.9 71-3261 Lherz (L) 2.70 3.58 170 n.a. n.a. 7.55 n.a. 7.76 7.04 04LH37b 1 Lherz (C+L) 3.73 4.13 415 4 3.87 7.06 1.39 7.68 6.71 04LH39a 1 Lherz (L) 2.96 2.9 321 3.96 3.6 6.7 1.25 6.93 5.55 04LH39b 1 Lherz (C+L) 5.39 5.59 348 3.13 2.7 6.3 1 5.57 5.45 04Lh815 1 Lherz (L) 3.26 3.4 208 4.14 3.6 7.32 1.27 7.03 5.83 04Lh15 1 Lherz (L) 3.29 3.96 198.5 3.6 3.55 6.79 1.35 7.01 6.28 86-V2-5 2 Lherz (L) 3.0 3.6 250 5.85 3.02 6.18 0.953 5.77 4.97 71 339 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.

BMS are pentlandite-Pn (Fe/Niat. =1.15±0.3) and chalcopyrite-Cp (Cu/ sulfide blebs, which result from mechanical dispersion of sulfide melts Feat. =0.96±0.04), representing 80–95% and 5–15% by volume, by the strong plastic deformation and recrystallization of silicates. respectively. Bornite, pyrrhotite, pyrite and mackinawite are minor sulfides (Lorand, 1989b). Both Pn and Cp display nice wetting features 4.2. Platinum-group minerals (dihedral angles b60°) against olivine neoblasts. Some samples show BMS grains of two different sizes, i.e. large grains (generally located in Platinum-group minerals (PGM) were identified in all of the 17 sp–opx–cpx clusters) surrounded by clouds of tens of minute (b30 µm) samples studied. Ninety five PGM grains ranging in size from

Fig. 2. Photomicrographs of base metal sulfides-BMS (A BSE image; B to D: plane polarized reflected light images). A: polyhedral pentlandite grain displaying very low dihedral angles with matrix silicates; B: two-phase intergranular BMS (pentlandite-Pn+chalcopyrite-Cp); C: Polyhedral pentlandite–chalcopyrite intergrowth illustrating the high wetting capacity of Cu–Ni-rich sulfide melts in the mantle; D: disaggregated BMS grains in a highly deformed lherzolite; note the very small satellite BMS blebs all around the largest grains. 302 J.-P. Lorand et al. / Earth and Planetary Science Letters 289 (2010) 298–310

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 modal abundance) 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, being present in similar proportions from the lherzolites to the most depleted harzburgites. N = number of PGM grain per sample.

b0.25×0.1 µm to 16 µm×1.4 µm were detected by SEM operating in Pyrenean peridotites display a continuum of PGM assemblages the BSE mode inside or in the immediate vicinity of BMS blebs. which vary sympathetically with whole-rock S contents, and thus BMS Quantitative analysis by electron microprobe was precluded by the very modal abundances (Fig. 3). The BMS-poor residual harzburgites small grain size of PGMs and their location inside or at the vicinity of (04Lh11 and 04Lh13) show very similar PGM assemblages as the BMS which generated continuous fluorescence on PGM analyses. BMS-free harzburgites analysed by Luguet et al. (2007). Laurite coexists According to EDS spectra (not shown), PGMs mainly consist in sulfides with Pt–Ir–Os alloys and Cu–Pt-rich sulfides of the malanite–cupror- from the laurite-erlichmanite series [Ru,Os(Ir)]S(As)2,Pt–Ir–Os rich alloys hodsite series; it can generate huge concentration spike in the time- and Pt–Pd–Te–Bi phases (moncheite–merenskyite). The other PGMs resolved spectra for Os–Ir–Ru–As (Fig. 4A). Cpx-rich harzburgites, of identified in Pyrenean peridotites are Pt arsenides/sulfarsenides intermediate BMS content, differ from the harzburgites by occurrence of (sperrylite), Cu–(Pt, Ir) sulfides (malanite), Pd–Ni–S(braggite),Pd–Cu a few Pt- and Pd-rich PGMs (bismutho-tellurides, sulfides and alloys; compounds, Pt–Fe alloys and native gold. Additional PGM micronug- e.g. 71-322), coexisting with laurite and Pt–Ir–Os alloys. The relative gets (56) were detected by in-situ LA-ICPMS analyses. When drilling abundance of laurite drops in the lherzolites whereas that of through a micronugget, this latter generates concentration spikes in time- bismuthotellurides significantly increases along with the abundance resolved spectra which allow qualitative assessment of which PGE is of BMS. The PGM assemblages in the Lherz lherzolites are dominated by hosted in the micronuggets. In-situ analyses confirm the results of EDS Pt–Ir–Os alloys and Pt–Te–Bi phases, in agreement with previous spectra, namely the predominance of Pt–Ir–Os alloys and Pt–Pd–Te–Bi observations on FON B-93 (Lorand et al., 2008b). The only PGM detected phases over the other PGM species. Pt–Ir–Os alloys display variable Ir/Os in some samples, especially poor in PGM but very rich in BMS (04Lh37) ratios. Native gold is not uncommon while native platinum is exception- were Pt–Te–Bi phases. Platinum arsenides/sulfarsenides and discrete ally scarce. native gold were detected in PGM-rich samples.

Fig. 4. Time-resolved LAM-ICP-MS spectra collected during three analyses of pentlandite. The huge spikes of Ru, Ir and Os concentrations, coupled with a minor As concentration spike identify a laurite nugget that contains trace amounts of Pt and Pd in A. B displays two spikes of Pt, Ir and Os concentrations corresponding to Pt–Ir–(Os) micronuggets; the second one is associated with a Pt–Te–Bi phase. C corresponds to analysis of a pentlandite-chalcopyrite bleb containing a mix of Pt–Ir–(Os) alloy and Pt–Te–Bi phase. J.-P. Lorand et al. / Earth and Planetary Science Letters 289 (2010) 298–310 303 304 J.-P. Lorand et al. / Earth and Planetary Science Letters 289 (2010) 298–310 J.-P. Lorand et al. / Earth and Planetary Science Letters 289 (2010) 298–310 305

Although PGM are systematically associated with BMS grains, the Such four-phase PGM composite inclusions share one face with Al–Cr total number of PGM detected in a single sample does not correlate spinel. with whole-rock S contents. Sample 71-322, a Lherz cpx-harzburgite is as rich in PGM as the lherzolites in spite of markedly lower S contents (Fig. 3). Conversely, S-rich lherzolites may be very PGM-poor 5. Discussion (04Lh39; 321 ppm S; 1 PGM). In addition to real random distribution and strong random sectioning effect on micrometer-sized mineral, the 5.1. Significance of PGM composition and mineralogy in the petrogenetic between-samplevariationinPGMmodalabundanceseemsto history of the Pyrenees Orogenic Peridotite Massifs correlate with the intensity of plastic deformation. The PGM-rich samples (71-322, 04LH15, LH 815 and 71-339) are sub-mylonitic The Lherz and Freychinède peridotites show the widest range of samples more extensively deformed and recrystallized, and richer in «primary» PGM combinations yet reported in fertile mantle peridotites, minute (b50 µm) sulfide blebs than the other samples. The high with some minerals (sulfides of the laurite-erlichmanite series) reported abundance of PGM in those samples is confirmed by a larger for the first time in such rocks. Our results shed some new light on the number of concentration spikes in time-resolved laser ablation origin of PGMs as a whole in orogenic lherzolites. However before spectra. Each of the 8 BMS grains analysed in-situ in Lh15 generated discussing their origin, it is necessary to recall some basic features of PGE concentration spikes. Some grains display evidence of at least mantle-derived BMS. Phases like monoclinic pyrrhotite, pentlandite and three PGMs located inside the analysed BMS grain (Fig. 4B). chalcopyrite are low-temperature minerals, not stable under P–T Although very scarce in fertile lherzolites, laurite was detected in conditions of the upper mantle. Paragenetic succession of BMS has been BSE images of 71-339 and in three LAM-ICPMS signal vs. time extensively discussed for pentlandite-rich (80–95% Pn; 5–15% Cp; Table 3) diagrams of Lherz lherzolites (04 LH08; 04Lh815; 86-V2-5. In 71-339, assemblage such as those studied here (Lorand et al., 2008b). These laurite and Pt–Ir–Os alloys preferentially occur within small-sized assemblage ultimately derived from metal-rich (i.e. metal/sulfur atomic (b80 µm across) satellite sulfide grains around larger BMS blebs. ratio N1) Cu–Ni sulfide melt (Alard et al., 2000, 2005; Ballhaus et al., 2001; Laurite crystals (3×2 µm in maximum dimensions) occur as a combi- Luguet et al., 2003; Lorand et al., 2008a,b). At 1.5 Gpa, the average pressure nation of cubic and octahedral morphology, often Os-enriched (OsS2) of equilibration of Pyrenean peridotites within the lithospheric continen- towards grain rims. It has been observed as i) cubes included in Pn tal mantle (Fabriès et al., 1991), this assemblage should be molten at c.a. cores (Fig. 5A), ii) as external granules sharing one face with matrix 1150±50 °C (Bockrath et al., 2004a,b). On cooling down from 1150 to silicates or Al–Cr-spinel (Fig. 5A, C) or iii) as discrete, euhedral grains 1000 °C, this composition is expected to precipitate almost equal sharing just one face with BMS, which may be Pn or Cp (Fig. 5D). The proportions of a Ni-rich monosulfide solid solution (22–25 wt.% Ni) sequence from Fig. 5D to A likely corresponds to different stage of with metal/sulfur (M/S) ratio of 0.9, and a Ni-rich sulfide melt more metal- entrapment of laurite by BMS blebs. It is worth noting that very few rich than the Mss, which will subsequently crystallizing the high laurite were detected in LAM-ICPMS time-resolved spectra. This is in temperature form of heazlewoodite (Hz) at 860 °C. On further cooling, agreement with the preferred location of laurite nearby very small the sulfide melt is also enriched in Cu until Iss (Intermediate solid BMS grains which are too small to be analysed with laser ablation solution) crystallizes at 880-840 °C (Peregoedova and Ohnenstetter, techniques. 2002). A reaction between Hz-Iss and Mss at Tb600 °C produced massive Unlike laurite, in-situ analyses detected numerous concentrations pentlandite, while chalcopyrite is stable below 557 °C (Fleet, 2006 and ref. spikes corresponding to Pt–Ir–Os alloys and Pt–Te–Bi phases. Pt–Ir–Os therein). Note that a slightly different cooling path was deduced by Lorand alloys tend to be acicular (up to 10×0.1 µm). These single-phase grains (1989b) for some pyrite-rich Pyrenean lherzolites that exsolved pent- are concentrated preferentially with small-sized pentlandite blebs landite at lower temperature (b300 °C). As pyrite is either absent or (average diameter of 17 grains: 29±22 µm). Pt–Ir–Os alloys are present in very minor proportion, the 600 °C figure is considered to be commonly associated with laurite inside the same BMS (Fig. 5A), as more likely for the Lherz and Freychinède samples studied here. In both inclusions in homogeneous Pn cores (Fig. 5E) or as external needles paths, some readjustment of Ni and Cu distribution between Cp and Pn adhering to Pn (Fig. 5F). Pt–Ir–Os alloy inclusions in Cp–Pn intergrowth occurred until closure temperature below 100 °C is reached (Lorand, are very scarce. Overall, PGE spikes are randomly distributed in time- 1989b). resolved spectra. In the mylonitized samples, spikes of Pt–Ir–Os alloys The solubility of Os, Ir and Ru and Rh in (Fe–Ni)–S monosulfide occur preferentially as the S signal is decreasing rapidly. This observa- (solid or molten) is several thousands of ppm to several percents (e.g. tion is consistent with the BSE images. Both suggests that Pt–Ir–Os alloys Alard et al., 2000; Brenan and Andrews, 2001; Brenan, 2002; Bockrath are adhering to, but are not fully enclosed by BMS. et al., 2004a,b) whereas Pt and Pd tend to go to the coexisting Cu-Ni Pd–Te–Bi phases are intimately associated with Cp or Cp–Pn sulfide melt (Dmss/sulfide melt b0.3; Li et al., 1996; Peregoedova et al., intergrowths. Pt-rich tellurides were commonly found to occur in 2004; Mungall et al., 2005; Ballhaus et al., 2006). Pentlandite, the pentlandite-chalcopyrite intergrowths as thin (b1 µm thick), cleaved major low-temperature BMS (N80 vol.%) in Lherz lherzolites can trigonal sub-equant platelets (Fig. 5H). Aside their marked preference dissolve ten percent levels of Ru, Rh and Pd, as shown by experimental for Cu-rich sulfides, Pt–Te–Bi minerals may be attached to euhedral works (Makovicky et al., 1986) and natural occurrence of ruthenium laurite, occupying embayments along with hydrous silicates (Fig. 5B). and/or rhodium equivalent of low-temperature pentlandite have Native gold contorted micronuggets inside cracks or altered contact been reported in PGE ores (Cabri 1992; Augé, 1988; Zacarini et al., between Cp and Pn. In sample 71-339, a complex, four-phase PGM 2005). Even at 25 °C, the Ru, Ir and Os content of pentlandites from assemblage displays a Pt–Fe alloy and a Pt–bismuthotelluride rod PGE ores is 2–4× the Ru–Ir–Os concentration range measured within protruding from an euhedral crystal of laurite+Pt–arsenide (Fig. 5G). Lherz pentlandites (4–55 ppm; Table 3)(Cabri, 1992; Ballhaus and

Fig. 5. Back scattered electron (BSE) scanning electron microscope images of platinum group minerals in Lherz and Freychinède peridotites (PGM; white). A: euhedral laurite inclusion (La) in the core of a minute pentlandite bleb, coexisting (but not in contact) with a Pt–Ir–Os alloy; B: zoned euhedral laurite crystals partly entrapped by pentlandite, along with a Pt–Te–Bi phase and hydrous silicate (dark grey); C: laurite granule adhering to the wall of a chalcopyrite grain which has exsolved a Pt–Pd–Te needle; D: octahedral sectioning of an external laurite granule in contact with pentlandite; low-brightness spongy material is secondary magnetite. E regularly faceted pentlandite showing a Pt–Ir–(Os) needle;

F: thin Pt–Ir–Os lamella adhering to a pentlandite bleb. G: three-phase PGM grain (euhedral laurite-La+sperrylite-PtAs2 +Pt–Pd–Te–Bi phase) partly enclosed in pentlandite- chalcopyrite intergrowth (Pn-Cp); H: euhedral lamella of Pt–Te–Bi phases included in pentlandite +chalcopyrite intergrowths. Note the fracture planes that cut across the Pt–Pd– Te–Bi phases. Scale bar=1 µm, unless otherwise stated. 306 J.-P. Lorand et al. / Earth and Planetary Science Letters 289 (2010) 298–310

Table 3 the lithospheric protolith of (2) the refertilization process that added Contribution of BMS to the whole-rock PGE budget of Pyrenean orogenic peridotites elements like Se, As, Sb, Te, and Bi (chalcogenes and semimetals) which (concentration in ppm). were able to scavenge PGE from BMS and (3) crystallization of the base Os Ir Ru Rh Pt Pd N metal sulﬁdes during decompression and emplacement of the peridotite 82-4 (0.024 wt.% sulﬁdes) bodies within the crust, as temperature decrease tends to expel PGE calculated BMS 18.7 15 32 3.8 19 12.2 5 formerly in solution in BMS. Pn96Cp4 ±6.0 ±4.3 ±8.0 ±1.35 ±4.0 ±3 Measured BMS 8.63* 5.44 10.5 2.11 2.75 32.3 5.2. BMS, Pt–Pd–Te–Bi phases and the refertilization model ±1.8 ±5 ±2 ±1.5 ±6

71-322 (0.016-0.011 wt.% sulfides) Of likely subsolidus origin are Pt–Pd–Te–Bi phases (and Pt arsenides calculated BMS 26.7 30.3 53.3 10.07 45.3 23.7 5 too) that are systematically enclosed in BMS. Those PGMs and the BMS Pn95Cp5 ±18 ±20 ±25 ±5 ±12 ±11.7 phase are co-genetic, as suggested by the coupled increase in the modal Measured BMS 2.00 2.57 5.36 1.62 1.26 28.40 abundances of both minerals (Fig. 3): semi-metal anions of Te, Bi, and As ±0.9 ±1.65 4.38 ±1 ±1.29 ±8.56 were delivered (like S), by the tholeiitic melt involved in the 04Lh08 (0.044 wt.% sulfides) refertilization process that affected the harsburgitic protolith. Calculated BMS 10 9.25 17.3 3.15 16.43 8.5 8 Textural (i.e. location in the interstitial pores of the rocks, within Pn84Cp +(Bo)16 the same microstructural sites as major minerals involved in the Measured 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 refertilization process) and compositional features lead us to interpreted BMS in Lherz and Freychinède lherzolites as crystallization 71-321(0,067-0.093 wt.% sulfides) products from an exotic component, for instance the tholeiitic melt calculated BMS 6.0 4.82 10.66 1.6 9.6 7.5 6 that refertilized the harzburgitic protolith in the Le Roux et al. (2007, Pn95Cp5(3) 2008, 2009) model. Our interpretation of late-magmatic/metasomatic 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 origin is supported by 1) the shape of BMS blebs and their mineral assemblage (pentlandite–chalcopyrite intergrowths, occasional born- 71-326 (0.055-0.065 wt.% sulfides) ite) are typical of Cu–Ni sulfide melt already identified in abyssal and Calculated 6.7 5.99 12.58 2.10 12.93 11.73 10 continental mantle peridotites that reacted with basaltic melts and 2)

Measured 6.9 3.70 4.7 1.50 0.34 10.7 Lherz pentlandites display only Pd-enriched (i.e. Type-II; Fig. 6) ±3.39 ±0.90 ±2.23 ±0.42 ±0.33 ±5.15 chondrite-normalized PGE patterns characterizing metasomatic sulfides in oceanic peridotites refertilized by basaltic melts (Luguet et al., 71-324 (0.055 wt.% sulfides) 2003, 2004; Alard et al., 2005). Of specific interest in that discussion is Calculated 7.13 6.13 12.1 2.31 14.181 12.54 5 the origin of BMS in the cpx-rich harzburgites and the harzburgites. Pn85Cp15 Measured 7.74 6.58 6.34 2.0 0.36 12.99 Melting models suggest that these refractory rocks should be ±2.62 ±3.27 ±4.37 ±0.54 ±0.25 characterized by Cu-poor Mss inclusions (or reequilibration products of this mineral, i.e. Pn–Po intergrowths) displaying a gradual 04Lh15(0.06 wt.% sulfides) depletion in Pt and Pd (Pd/IrN b0.2; Pt/IrN b0.5) (Ballhaus et al., Calculated 6.5 5.9 11.32 2.25 11.69 10.5 6 Pn85Cp-Bo15 2006). Such enclosed Mss are commonly preserved by peridotite Measured 5.28 4.13 6.17 1.90 0.24 22.041 xenoliths in alkaline lavas (e.g. Alard et al., 2000; Lorand and Alard, ±2.84 ±2.70 ±3.70 ±3.07 ±0.99 ±0.08 2001). However, at Lherz, BMS in harzburgites are in no way different from BMS in lherzolites. They are composed of intergranular Pn-Cp fi 04Lh815 (0.06 wt.% sul des) blebs displaying nice wetting features of metal-rich sulfide melts and Calculated 6.9 6.0 12.2 2.12 11.8 9.72 11 – Measured 6.5 5.39 6.09 1.35 0.68 16.45 constant Cu/S ratios (0.1 0.2), with no evidence of trapped Mss. The ±3 ±2.39 ±2.28 ±0.6 ±0.37 ±5.41 refertilization model that implies a unique source for the BMS (i.e. the tholeiitic melt) accounts for such constant mineralogical features fi 71-339 (0.071 wt.% sul des) from the lherzolites to the S-depleted harzburgites. The REE variation Calculated 4.9 4.25 9.5 1.76 9.56 8.7 6 Pn84Cp16 in harzburgitic Cpx indicates that small melt fractions residual after Measured 8.32 3.95 2.94(6) 1.46 0.26 11.1/16.0 the refertilization reaction migrated in the harzburgite protolith (Le ±3.95 ±2.39 ±1.74 ±1.12 ±0.22 1.4/±4.9 Roux et al., 2007, 2009), thus precipitating Pd-rich sulfides in formerly Calculated sulfide compositions are based on whole-rock analyses of PGE (Table 2) Pd-depleted peridotites. These late-magmatic BMS may account for normalized to 100 wt.% sulfides. Pn = pentlandite; Cp = chalcopyrite; Bo = bornite. whole-rock Pd/PtN ≥1 much better than partial melting models (c.f. Sulfide modal composition, necessary to convert whole-rock S contents into sulfide Luguet et al., 2003) because Pd is more incompatible than Pt. modal abundances were estimated from systematic traversing of polished thin section Platinum does not enter the octahedral site of pentlandite (nor (Lorand, 1989a); where available, Cp/Pn ratios were calculated from whole-rock Cu/S fi ratios, assuming negligible amount of Cu in silicates (Lorand, 1989a; Lorand et al., 1999; chalcopyrite), the two main sul des in the samples under discussion. Le Roux, unpublished data). Measured sulfides are the mean PGE contents measured in- All Pyrenean pentlandites analysed in-situ are strongly depleted in Pt situ with LA-ICPMS. PGM's-bearing BMS grains (i.e. exhibiting well-characterized (by factors of up to 100) relative to the other PGEs (Fig. 6). This Pt spikes of noble metals) were not used. Propagated errors are given as one standard deficit is also noted in sulfides from PGE deposits (see Ballhaus and deviation. N = number of in-situ analyses taken into account. Ryan, 1995). It reflects exsolution of Pt–Fe alloys when mss recrystallizes to Po and Pn and when Iss recrystallizes to Cp. Platinum

was also scavenged from BMS to form PtAs2, PtAsS and Pt tellurides Sylvester, 2000; Godel et al., 2007; Godel and Barnes, 2008 and ref. which are rather stable once formed. At Lherz, like in sample FON B-93 therein). from Fontête Rouge (Lorand et al. 2008b), the preferential occurrence Of course, owing to their low PGE content, BMS residing in the upper of bismuthotellurides in isolated Cp or Cp–Pn intergrowths is mantle are expected to be undersaturated with respect to PGM (Table 3). consistent with the strong affinity of these highly incompatible The sulfide-PGM assemblages of mantle samples actually reflect a elements for Cu-rich sulfide melts (Yi et al., 2000). Semi-metals form complex series of events. Basically, three episodes can be identified at soft ligands that stabilize Pt and Pd in the sulfide melt, thus further which the sulfides and the PGE phases could have formed (1) during the decreasing their Dmss/sulfide melt partition coefficients (Ballhaus and partial melting and melt depletion events which eliminated the BMS from Sylvester, 2000). Sulfide melt and telluride melt are fully miscible J.-P. Lorand et al. / Earth and Planetary Science Letters 289 (2010) 298–310 307

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). above 1150 °C (Helmy et al., 2007). For c.a. 70,000 ppm of Te+Pt+Pd, tion product from S-bearing, sulfide-undersaturated silicate melts the telluride melt is saturated with respect to Pt-telluride at TN1015 °C (Augé, 1988; Brenan and Andrews, 2001; Andrews and Brenan, 2002; while Pd–Ni–telluride saturation is shifted down to subsolidus Bockrath et al., 2004b). Such melts may form in a mantle piece temperatures (b700 °C; Gervilla and Kojonen, 2002). However, undergoing increasing degree of partial melting once the ultimate saturation in Pt–Te–(Bi) phases in Pyrenean peridotites is expected BMS fractions (compositionally close to Fe–Ni monosulfides-Mss) are to have occurred at much lower temperature because mantle-derived dissolved into the silicate melt. At Lherz, Luguet et al. (2007) BMS at Lherz are several orders of magnitude poorer in Pt+Pd+Te demonstrated that the whole-rock PGE budget of the 2 Ga-old S- (b100 ppm; Table 3). For such low contents of Pt+Pd+Te, Pt–Pd free harzburgites is effectively hosted in discrete PGM of the laurite- bismuthotellurides likely precipitated at sub-solidus temperature of erlichmanite series, Pt–Ir–Os rich alloys and complex Cu–Pt sulfides. the BMS assemblage from the very last Cu-sulfide melt fractions which Thus, laurite crystals now disseminated inside lherzolites are should be the most Pt–Pd–Te-enriched. Note that the Te content interpreted as relicts from this lithospheric protolith. Our interpreta- recorded in the sulfides is that expected for equilibration temperature tion is supported by BMS-laurite textural relationships, namely of ca. 200 °C (C. Ballhaus, personal communication). intermediate stages of entrapment by pentlandite, from external Although moderately compatible into Cu-rich sulfides (chalcopy- laurite granules simply attached to pentlandite by one crystal face to rite, cubanite; Ballhaus and Sylvester, 2000; Lorand and Alard, 2001; fully enclosed cubes. Likewise, our interpretation does account pretty Luguet et al., 2004), Pd can enter pentlandite at all temperatures Thus, well for the laurite crystals that are pasted on Cp or Pn–Cp there is very little room for pentlandite to exsolve Pd-rich minerals. intergrowths (e.g. Fig. 5C) because Ru (like Os and Ir) is not soluble Alternatively, palladium bismutho-tellurides associated with pent- in Cp and Iss-phases nor within their high-temperature precursor (i.e. landite in Pyrenean peridotites may be direct precipitation products Cu-rich sulfide melt, Li et al, 1996; Lorand and Alard, 2001). from vapor at Tb700 °C (Gervilla and Kojonen, 2002). Likewise, not all “Lithospheric” PGMs were mechanically collected by droplets of Pt bismutho-tellurides can be interpreted as direct crystallization magmatic sulfide melt during rejuvenation reactions that refertilized products from Cu-rich sulfide melts. For instance, Fig. 5B and 5G the harzburgitic protolith. A preferential distribution within the grain provide evidence that some Pt-tellurides nucleated directly onto boundary network of the lithospheric protolith (Luguet et al., 2007) laurite or Pt–Ir–Os alloys, sometimes associated with hydrous silicates likely helped “lithospheric” PGMs to be captured by lherzolitic BMS. and Pt-arsenides. Such assemblages point to local accumulation of Plastic deformations coeval to the refertilization event (Le Roux et al., volatile elements (Te, Bi, As) by the refertilization process. Separate 2008) also played a role, as suggested by abundant PGMs in the highly evidence is provided by lithophile trace element patterns and deformed peridotites. At 1100 °C, the BMS were molten to partially disseminated Ti-pargasite in Lherz and Freychinède lherzolites molten, which likely enhanced their mechanical dispersion among (Fabriès et al., 1991; Le Roux et al., 2007). silicate neoblasts, generating the nice wetting figures of Fig. 2. The great similarity between Pt–Ir–Os alloys and laurite as regard 5.3. Laurite and Pt–Ir–Os alloys: refractory PGMs tracking the 2 Ga-old their textural relationships and their associations with BMS grains harzburgitic protolith inside Pyrenean lherzolites argue 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- Unlike Pt–Pd–Te–Bi phases, laurite and BMS are not cogenetic temperature phases (N1100 °C) in the Fe–Pt–Ir–S system (Fleet and since their modal abundance are negatively correlated (cf. Fig. 3). Stone, 1991; Cabri et al., 1996). Ruthenium-poor, Ir+Os-rich alloy Euhedrally shaped laurite is usually interpreted as direct crystalliza- and laurite coexist stably at 1100 °C in the phase equilibrium data of 308 J.-P. Lorand et al. / Earth and Planetary Science Letters 289 (2010) 298–310

Andrews and Brenan (2002).Pt–Ir–Os alloy modal abundances vary irrespective of whole-rock S contents (cf. Fig. 3). One may speculate that lithospheric Pt–Ir–Os alloys, once trapped inside lherzolites, were not eliminated by the refertilization process because Pt is poorly soluble within Mss. Note that, although experimental data relevant to conditions 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 magnitude as the bulk concentrations estimated for Lherz/Freychinède sulfides (Table 3). Platinum systematically forms its own alloy phases in metal-rich (b50 at.% S) BMS assemblages of the Cu–Fe–Ni–S system over a wide range of temperature (Makovicky, 2002). The decrease of laurite modal abundance with increase of BMS modal abundances in Fig. 3 most probably indicate dissolution of lithospheric crystals inside metasomatic sulfide melts. This is not surprising because 1) laurite and sulfide melts can coexist over a very restricted 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 from whole-rock PGE and S analyses (Table 3) are low in Os, Ir, Ru and Rh compared with the solubility of the corresponding elements in pentlandite. Another evidence for laurite dissolution inside the BMS is provided by positive Ru anomalies in the chondrite-normalized PGE patterns of some pentlandite (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 the lack of coupled peaks in the signal of Ru+Os+Ir in time-resolved spectra. 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 to rehomogeneise their concentrations. Pyrenean lherzolites were quickly uplifted as small-sized bodies (b1km3) and cooled at 600 °C within 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 it is as rich in PGMs as the richest samples at Lherz and Frechinède. We may speculate that laurite totally dissolved inside the BMS owing to the longer period of annealing of silicate assemblages at mantle temperatures 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 locally preserved. Epitaxial growth of sulfide melts on pre-existing laurites can account for much of the laurite-BMS microtextures in Fig. 5.A similar explanation also holds true for those Pt–Ir–Os alloys which are not enclosed, just attached on small-sized Pn blebs. Plastic deformations in Pyrenean peridotites ended at rather low temperature (750 °C, 0.7–1.0 Gpa) during solid-state emplacement at Moho depth (Fabriès et al., 1991, 1998). Late stage deformations disaggregated the largest BMS blebs into ten-times smaller grains, thus increasing surface/volume ratio and creating opportunity for the smallest BMS blebs to intercept “lithospheric” PGMs at subsolidus temperatures (b900 °C). This is the likely reason why laurite was Fig. 7. Ru vs. Os, Os vs. Ir and Rh vs. Os diagrams for Lherz and Freychinède lherzolitic found to be attached to the smallest BMS blebs. Sharing only one or BMS. The population displaying suprachondritic Ru/Ir ratios is Os- and Rh- enriched relative to chondritic ratios; these features are ascribed to laurite crystals dissolved two crystalline faces with BMS grain, those grains suffered limited inside BMS. CR=chondritic ratio (McDonough and Sun, 1995). diffusion of Ru, Os and Ir with the BMS.

5.4. Implications for the PGE budget of Pyrenean peridotites rich in BMS as UB-N (145 vs. 141-138 ppm S; Lorand et al., 2008a) Not only the BMS phases but also PGM micronuggets must be displays unaccounted fractions of Os and Ir not residing inside BMS of taken into account to calculate the PGE budget of orogenic fertile similar extent as this rock standard (30±20% Os and 40±20% Ir; Meisel lherzolites. Laurite is a good candidate for fully equilibrating the et al., 2003). By contrast, as suggested by FONB-93 (Lorand et al., 2008b), whole-rock budget of Ru, coupled with Pt–Ir–Os alloys for Os and Ir. In fertile lherzolites, which are the most-enriched in BMS, do not display addition to an overall Pt deficit (75–99%) that reflects the abundance such a deficit. Calculated and measured contents of Os, Ir and Rh agree at of Pt-rich PGMs, the measured BMS compositions in the cpx-rich 1 sigma level; the rather large uncertainties on measured compositions harzburgites and lherzolites that are rich in PGMs and poor in BMS do preclude a precise estimate of the contribution of PGMs. not balance the whole-rock PGE budget for Os, Ru, Ir and Rh (Table 3). Unlike Os and Ir, the concentrations of Ru measured in BMS of Of course, the unaccounted fraction decreases from 71-322 to 82-4 fertile lherzolites display rather a constant deficit of ca. 50% compared and 04Lh08 as BMS modal abundance increases. Sample 04Lh08, as to calculated concentrations. This obviously tracks laurite, although J.-P. Lorand et al. / Earth and Planetary Science Letters 289 (2010) 298–310 309 this sulfide, preferentially adhering to the smallest BMS blebs, has not Alard, O., Le Roux, V., Bodinier, J.-L., Lorand, J.-P., O'Reilly, S.Y., Griffin, W.L., 2008. How primitive is the “primitive” mantle? XVIIIè V.M. Goldschmidt Conf, JUL, 2008 been detected in all the analysed samples. A simple calculation based Vancouver, Canada: Geochim. Cosmochim. Acta, vol. 72 Issue: 12, p. A13-A13. on sample 71 339 (4 ±2 perfectly cubic laurite crystals, each Andrews, D.R., Brenan, J.M., 2002. Phase equilibrium constraints on the magmatic origin representing arbitrarily 8 µm3 by volume per 30 µm-thick standard of laurite+Ru–Os–Ir alloy. Can. Mineral. 40, 1705–1716. Augé, T., 1988. Platinum group minerals in the Tiebaghi and Vourinos ophiolitic sized polished thin section) allows the volume of laurite to be estimated at complexes: genetic implications. Can. Mineral. 33, 1023–1045. −9 −9 1±0.3×10 %, which transposes into 2±1×10 g/g (density=6.4). Ballhaus, C., Ryan, C.G., 1995. Platinum-group elements in the Merensky reef. I. PGE in EDS spectra indicate higher Ru contents than Os and much lower Ir solid solution in base metal sulfides and the down-temperature equilibration – abundances relative to Ru and Os in lherzolitic laurites. Assuming history of Merensky ores. Contrib. Mineral. Petrol. 122, 241 251. Ballhaus, C., Sylvester, P., 2000. Noble metal enrichment processes in the Merensky Ru content of 40±10 wt.%, laurite may account for 0.5–1.5 ppb Ru, i.e. Reef, Bushveld Complex. J. Petrol. 41, 545–561. up to 25% of the whole-rock budget of Ru of a lherzolite with a PUM-like Ballhaus, C., Bockrath, C., Wohlgemuth-Ueberwasser, C., Laurenz, V., Berndt, J., 2006. PGE composition (7.1 ppb Ru). That contribution is obviously under- Fractionation of the noble metals by physical processes. Contrib. Mineral. Petrol. 152, 667–684. estimated compared to mass balance calculations of Table 3 because 1) Ballhaus, C., Tredoux, M., Späth, A., 2001. Phase relations in the Fe–Ni–Cu–PGE–S volume estimate of microminerals is prone to considerable error, 2) not all system at magmatic temperature and application to massive sulfide ores of the of the laurite crystals were intercepted by the thin sections of 71 339 Sudbury Igneous Complex. J. Petrol. 42, 1921–1926. Becker, H., Horan, M.F., Walker, R.J., Gao, S., Lorand, J.-P., Rudnick, R.L., 2006. Highly and 3) in-situ LAM-ICPMS analyses provide evidence for one laurite siderophile element composition of the Earth's primitive upper mantle: constraints population already dissolved into the BMS phase. from new data on peridotite massifs and xenoliths. Geochim. Cosmochim. Acta 70, 4528–4550. Bockrath, C., Ballhaus, C., Holzheid, A., 2004a. Fractionation of the Platinum-group 6. Conclusions elements during mantle melting. Science 305, 1951–1953.

Bockrath, C., Ballhaus, C., Holzheid, A., 2004b. Stabilities of Laurite RuS2 and Pyrenean lherzolites clearly contain two generations of PGMs monosulfide liquid solutions at magmatic temperatures. Chem. Geol. 208, – – 247–264. (a) refractory minerals (laurite-erlichmanite series, Pt Ir Os alloys) Bodinier, J.-L., Dupuy, C., Dostal, J., 1988. Geochemistry of Eastern Pyrenean peridotites. inherited from ancient highly depleted harzburgitic protolith and Geochim. Cosmochim. Acta 52, 2893–2907. (b) “low-temperature” bismuthotellurides and arsenides/sulfarsenides, Brenan, J.M., 2002. Re–Os fractionation in magmatic sulfide melt by monosulfide solid – intimately associated with Pd-rich metasomatic BMS, indicators of solution. Earth Planet. Sci. Lett. 199, 257 268. Brenan, J.M., Andrews, D.R., 2001. High-temperature stability of laurite and Ru–Os–Ir lithosphere refertilization process, in agreement with Le Roux et al. alloy and their role in PGE fractionation in mafic magmas. Can. Mineral. 39, 341–360. (2007, 2008, 2009) model. Such platinum-group minerals have been Burton, K.W., Schiano, P., Birck, J.-L., Allègre, C.J., 1999. Osmium isotope disequilibrium – identified in lherzolites from several orogenic locations, all displaying between mantle minerals in a spinel lherzolite. Earth Planet. Sci. Lett. 172, 311–322. unambiguous petrological evidence of magma refertilization processes Cabri, L.J., 1992. The distribution of trace precious metals in minerals and mineral of formerly depleted lithosphere (e.g. Corsican plagioclase lherzolites products. The 23rd Hallimond Lecture (1991): Min. Mag., vol. 56, pp. 289–308. (Ohnenstetter, 1992), Ligurides ophiolites (Luguet et al 2004), Horoman Cabri, L.J., Harris, D.C., Weiser, T.W., 1996. Mineralogy and distribution of platinum- – “ ” group mineral (PGM) placer deposits of the world. Explor. Mining. Geol. 5, 73 167. (Kogiso et al., 2008). Conversely, Lithospheric PGM micronuggets Fabriès, J., Lorand, J.-P., Bodinier, J.-L., Dupuy, C., 1991. Evolution of the upper mantle likely carry the unradiogenic Os isotopic compositions of the litho- beneath the Pyrenees: evidence from orogenic spinel lherzolite massifs. In: spheric protolith. Now disseminated within fertile lherzolites, they Menzies, M.A., Dupuy, C., Nicolas, A.J. (Eds.), J. Petrology, Special Lherzolites Issue, pp. 55–76. account for local preservation of ancient Os model ages (up to 2 Ga) Fabriès, J., Lorand, J.-P., Bodinier, J.L., 1998. Petrology of some Central and Western detected in BMS by in-situ isotopic analyses (Alard et al., 2005, 2008). Pyrenean lherzolite massifs. Tectonophysics 292, 145–167. With palladium being hosted by metasomatic BMS, and Ir by Fleet, M.E., Stone, W.E., 1991. Partitioning of platinumgroup elements in the Fe–Ni–S system and their fractionation in nature. Geochim. Cosmochim. 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Determination of total sulfur contents in PUM or result from refertilization process of ancient lithosphere, as international rock standard SY-2 and other mafic and ultramafic rocks using an documented in the Lherz and Freychinède lherzolites. improved scheme of combustion/iodometric titration. Geoanalysis Geostand. Newsl. 29, 123–130. Handler, R.M., Bennett, V.C., 1999. Behavior of Platinum-group elements in the Acknowledgements subcontinental mantle of eastern Australia during variable metasomatism and melt depletion. Geochim. Cosmochim. Acta 63, 3597–3618. Helmy, H., Ballhaus, C., Berndt, J., Bockrath, C., Wohlgemuth-Ueberwasser, C., 2007. The authors acknowledge M. Marot (MNHN) for the polished thin Formation of Pt, Pd and Ni tellurides: experiments in sulfide–telluride systems. sections, O. Boudouma for the FEG-SEM study (Paris VI university) Contrib. Mineral. Petrol. 153, 493–524. and O. Brugier who operated the LAM-ICPMS facility at Montpellier. 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