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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| Institution: | 1. Laboratoire de Minéralogie, Muséum National d’Histoire Naturelle, Unité Associée au CNRS, 61 Rue Buffon, 75005 Paris, France;2. Key Centre for the Geochemical Evolution and Metallogeny of Continents (GEMOC), Department of Earth and Planetary Sciences, Macquarie University, Sydney, NSW 2109, Australia;3. Laboratoire de Tectonophysique UMR5568, Université de Montpellier II, Place E. Bataillon, 34095 Montpellier, Cedex 5, France;1. Department of Earth Sciences, Utrecht University, P.O. Box 80.021, 3508 TA Utrecht, The Netherlands;2. Physics of Geological Processes (PGP), University of Oslo, P.O. Box 1048, Blindern, N-0316 Oslo, Norway;3. Department of Earth, Ocean and Atmospheric Sciences and Mineral Deposit Research Unit (MDRU), The University of British Columbia, Vancouver, BC V6T 1Z4, Canada;4. Geosciences Department, University of Bremen, Klagenfurter Strasse, 28359 Bremen, Germany;5. Institut des Sciences de la Terre (ISTerre), UJF-CNRS, 38041 Grenoble Cedex 9, France;1. Department of Geology, University of Maryland, College Park, MD 20742, USA;2. Laboratoire de Géologie de Lyon, Ecole Normale Supérieure de Lyon and Université Claude Bernard Lyon 1, CNRS UMR 5276, 46 Allée d’Italie, 69007 Lyon, France;1. Freie Universität Berlin, Institut für Geologische Wissenschaften, Malteserstrasse 74-100, 12249 Berlin, Germany;2. Universität Bayreuth, Bayerisches Geoinstitut, 95440 Bayreuth, Germany;1. State Key Laboratory of Geological Processes and Mineral Resources, School of Earth Sciences, China University of Geosciences, Wuhan 430074, China;2. Institut de Physique du Globe de Paris, Université de Paris, CNRS UMR 7154, 75238 Paris Cedex 05, France;3. RC Centre of Excellence for Core to Crust Fluid Systems, Dept. of Earth and Planetary Sciences, Macquarie University, North Ryde, New South Wales 2109, Australia |
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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. |
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