Platinum-group element abundances in the upper mantle: new constraints from in situ and whole-rock analyses of Massif Central xenoliths (France)

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Abstract

Fourteen peridotite xenoliths collected in the Massif Central neogene volcanic province (France) have been analyzed for platinum-group elements (PGE), Au, Cu, S, and Se. Their total PGE contents range between 3 and 30 ppb and their PGE relative abundances from 0.01 to 0.001 × CI-chondrites, respectively. Positive correlations between total PGE contents and Se suggest that all of the PGE are hosted mainly in base metal sulfides (monosulfide solid solution [Mss], pentlandite, and Cu-rich sulfides [chalcopyrite/isocubanite]). Laser ablation microprobe-inductively coupled plasma mass spectrometry analyses support this conclusion while suggesting that, as observed in experiments on the Cu-Fe-Ni-S system, the Mss preferentially accommodate refractory PGEs (Os, Ir, Ru, and Rh) and Cu-rich sulfides concentrate Pd and Au. Poikiloblastic peridotites pervasively percolated by large silicate melt fractions at high temperature (1200°C) display the lowest Se (<2.3 ppb) and the lowest PGE contents (0.001 × CI-chondrites). In these rocks, the total PGE budget inherited from the primitive mantle was reduced by 80%, probably because intergranular sulfides were completely removed by the silicate melt. In contrast, protogranular peridotites metasomatized by small fractions of volatile-rich melts are enriched in Pt, Pd, and Au and display suprachondritic Pd/Ir ratios (1.9). The palladium-group PGE (PPGE) enrichment is consistent with precipitation of Cu-Ni-rich sulfides from the metasomatic melts. In spite of strong light rare earth element (LREE) enrichments (Ce/YbN < 10), the three harzburgites analyzed still display chondrite-normalized PGE patterns typical of partial melting residues, i.e., depleted in Pd and Pt relative to Ir and Ru. Likewise, coarse-granular lherzolites, a common rock type in Massif Central xenoliths, display Pd/Ir, Ru/Ir, Rh/Ir, and Pt/Ir within the 15% uncertainty range of chondritic meteorites. These rocks do not contradict the late-veneer hypothesis that ascribes the PGE budget of the Earth to a late-accreting chondritic component; however, speculations about this component from the Pd/Ir and Pt/Ir ratios of basalt-borne xenoliths may be premature.

Introduction

It is now well known that compared with chondritic meteorites, the Earth’s mantle is depleted in siderophile elements due to separation of the metallic core (e.g., O’Neill, 1991, and references therein). However, highly siderophile elements (Re, Au, and platinum group elements [PGE]) are several orders of magnitude more abundant than expected from core-mantle equilibrium partitioning models at low pressure (Borissov et al., 1994) and occur in roughly chondritic relative abundances. These particular geochemical features are usually interpreted in terms of the late-veneer hypothesis, i.e., addition of an unfractionated chondritic component to the silicate earth after core formation Chou 1978, Morgan 1986, O’Neill 1991, Rehkämper et al 1997. Recent studies of fertile lherzolites from various localities in the world supported the hypothesis of a primitive upper mantle reservoir characterized by a chondriticlike evolution of the Re/Os ratio Meisel et al 1996, Meisel et al 2001. Preliminary investigations on the 186Os-188Os systematics of abyssal peridotites yield a similar conclusion for the Pt/Os ratio (Brandon et al., 2000). Nevertheless, the late-veneer hypothesis has recently been challenged by alternative models such as high-pressure core-mantle partitioning (Righter and Drake, 1997) or limited exchanges of core-forming materials with the silicate earth throughout geological times (cf. Snow and Schmidt, 1998). This latter interpretation was based on the discovery of abyssal peridotites enriched in the less refractory PGE (Ru, Rh, Pt, and Pd) relative to Ir and Os and to chondritic ratios.

Geochemical information on the primitive upper mantle is usually drawn from the study of fertile lherzolites stored in the subcontinental lithospheric mantle and then sampled as basalt-hosted xenoliths or as kilometer-sized, tectonically emplaced orogenic lherzolites (e.g., McDonough and Sun, 1995). Data accumulated over the past 5 years indicate that orogenic lherzolites show PGE distribution patterns closely similar to oceanic peridotites, i.e., superchondritic Ru/Ir, Rh/Ir, and Pd/Ir ratios (Pattou et al 1996, Lorand et al 1999, Lorand et al 2000, and references therein). Despite the fact that mantle xenoliths provide important information on the mantle, high-quality PGE data remain scarce and produced conflicting results. PGE relative abundances have been shown to vary from basically chondritic, in agreement with the late-veneer hypothesis Mitchell and Keays 1981, Wilson et al 1996, Rehkämper et al 1997, Handler and Bennett 1999, to distinctly suprachondritic for Pd/Ir (e.g., Morgan, 1986) and occasionally Rh/Ir (Handler and Bennett, 1999). Interpretations of such deviations are ambiguous, given the poor external reproducibility that characterizes PGE analyses of mantle xenoliths in general (cf. Handler and Bennett, 1999). Recent studies of mantle sulfides by laser ablation microprobe-inductively coupled plasma mass spectrometry (LAM-ICP-MS) (Alard et al., 2000) have reemphasized the strong affinity of PGEs for sulfide microphases as proposed by numerous authors (e.g., Morgan and Baedecker 1983, Hart and Ravizza 1996, Pattou et al 1996, Burton et al 1999). However, these studies have also revealed the different partitioning of iridium-group (PGE) (IPGE) (i.e., Os, Ir, Ru; Barnes et al., 1985) and PPGE (Pt, Pd; Barnes et al., 1985) between monosulfide solid solution (Mss) and Cu-Ni-rich phases (Alard et al., 2000).

The present article provides whole-rock PGE analyses for 14 peridotite xenoliths collected in representative localities of the Neogene volcanic province of the Massif Central, France. The main sulfide phases were analyzed by LAM-ICP-MS, and whole-rock abundances of S and Se have been determined to monitor the abundance of sulfides in the studied samples. These analyses enable a critical assessment of the part played by mantle sulfides in concentrating PGE other than Os. These xenoliths were also analyzed for incompatible trace elements to evaluate the role of metasomatic processes in the continental lithosphere in altering the PGE budget.

Section snippets

Texture and petrogenesis of Massif Central xenoliths

More than 100 occurrences of mantle xenoliths are known in the Massif Central and the related area of Neogene volcanic activity (Causses and Bas-Languedoc). These xenoliths (hereafter referred to as MC xenoliths) have been extensively studied over the past 20 years with regard to microstructures (Coisy and Nicolas, 1978) and trace element geochemistry Downes and Dupuy 1987, Alard et al 1996, Xu et al 1998, Lenoir et al 2000. In a comprehensive study on more than 30 xenolith localities scattered

Location and main petrogenetic features of the samples analyzed

The 14 xenoliths analyzed (Table 1) are spinel peridotite xenoliths devoid of oxidation features. They were collected in 6 localities from the Massif Central, one locality in the late Miocene volcanism of the Causses (Sauclière), and one locality (Plan de Célessou) in the Lodévois-Escandorgue area of quaternary volcanism (Fig. 1). The Massif Central localities (sensu stricto) are Montboissier (Livradois Miocene volcanism) in the Northern Massif Central and, south of latitude 45°30′ N and

Analytical techniques

PGE and gold were analyzed with a VG 353 Plasmaquad ICP-MS (University of Montpellier II) with external calibration standards. The main difficulty of PGE analyses in mantle rocks comes from the fact that PGE-bearing phases may be heterogeneously distributed on a small scale, causing strong nugget effects (e.g., Handler and Bennett, 1999). Analyzing large (>10 g) powder aliquots may generally reduces such effects (Pattou et al. 1996). In the present study, PGEs and Au were analyzed in 15-g

Whole-rock data

Absolute PGE concentrations in the 14 MC xenoliths analyzed range between 0.53 and 4.29 ppb for Ir, 1.2 and 7.6 ppb for Ru, 0.13 and 1.21 ppb for Rh, 0.8 and 7.3 ppb for Pt, 0.4 and 6.3 ppb for Pd, and 0.2 and 1.5 ppb for Au (Table 3). This is the same compositional range reported by other studies of basalt-hosted spinel peridotite suites (e.g., Morgan 1986, Rehkämper et al 1997, Handler and Bennett 1999). The lherzolites display considerable sample-to-sample variations in PGE abundances that

PGE hosts in Massif Central xenoliths

The LAM-ICP-MS data support the assumption that PGE in the upper mantle are strongly concentrated in base metal sulfides. However, they reveal a clear antithetic relationship in PGE affinity—i.e., a much greater affinity of IPGE + Rh for Mss structures and a stronger preference of Pd and Au for Cu-rich sulfides. This relationship was already inferred from studies of PGE-Ni-Cu-rich ore deposits (e.g., Barnes et al 1997, Thériault and Barnes 1999) as well as from xenolith suites from worldwide

Conclusions

Whole-rock analyses and LAM-ICP-MS data demonstrate that base metal sulfides account for most of the PGE budget in Massif Central xenoliths. Mss accommodates the refractory PGE (Os, Ir, Ru, and Rh). Pd is located preferentially in volumetrically minor Cu-Ni-rich sulfides, whereas Pt exhibits a more complex partitioning behavior between Mss, Cu-rich sulfides, and probably discrete Pt alloys of likely subsolidus origin. The heterogeneous distribution on the hand-sample scale of the Pd and Pt host

Acknowledgements

Financial support for whole-rock PGE analyses was provided by grants from CNRS-INSU (Intérieur de la Terre). The development of sulfide analysis by LAM-ICP-MS was supported by a Macquarie University Postgraduate Research grant (O.A. and S. Y.O’Reilly) and is part of O.A.’s Ph.D. thesis, financed in part by a ARC grant to S. Y. O’Reilly. Liliane Savoyant (University of Montpellier) is gratefully acknowledged for operating the Montpellier ICP-MS; we thank N. J. Pearson, A. Sharma, and C. Lawson

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