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

Fourteen peridotite xenoliths collected in the Massif Central neogene volca nic 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 X CI-chondrites, respecti vely. 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 [chaleopyrite/isocubanite ]). Laser ablation microprobe-inductively coupled plasma mass spectrometry analyses support this conclusion while suggesting that, as observed in expe riments on the Cu-Fe-Ni-S system, the Mss preferentially accommodate refrac tory PGEs (Os, Ir, Ru, and Rh) and Cu-rich sulfides concentrate Pd and Au. Poikiloblastic peridotites pervasively percolated by large silicate melt fr actions at high temperature (1200 degreesC) display the lowest Se (<2.3 ppb ) and the lowest PGE contents (0.001 X CI-chondrites). In these rocks, the total PGE budget inherited from the primitive mantle was reduced by 80%, pr obably because intergranular sulfides were completely removed by the silica te melt. In contrast, protogranular peridotites metasomatized by small frac tions of volatile-rich melts are enriched in Pt, Pd, and Au and display sup rachondritic Pd/Ir ratios (1.9). The palladium-group PGE (PPGE) enrichment is consistent with precipitation of Cu-Ni-rich sulfides from the metasomati c melts. In spite of strong light rare earth element (LREE) enrichments (Ce /Yb-N < 10), the three harzburgites analyzed still display chondrite-normal ized PGE patterns typical of partial melting residues, i.e., depleted in Pd and Pt relative to Ir and Ru. Likewise, coarse-granular lherzolites, a com mon 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 roc ks do not contradict the late-veneer hypothesis that ascribes the PGE budge t of the Earth to a late-accreting chondritic component; however, speculati ons about this component from the Pd/Ir and Pt/Ir ratios of basalt-borne xe noliths may be premature. Copyright (C) 2001 Elsevier Science Ltd.