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RESEARCH PRODUCT

Are highly siderophile elements (PGE, Re and Au) fractionated in the upper mantle of the earth? New results on peridotites from Zabargad

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Abstract Seven peridotite samples from Zabargad Island (Red Sea) were analyzed for highly siderophile elements (HSE), including five platinum-group elements (PGE: Os, Ir, Ru, Rh, Pd) and Re and Au. Petrography and chemical composition of the samples had been published earlier [Kurat, G., Palme, h., Embey-Isztin, A., Touret, J., Ntaflos, T., Spettel, B., Brandstatter, F., Palme, C., Dreibus, G., Prinz, m., 1993. Petrology and geochemistry of peridotites and associated vein rocks of Zabargad Island, Red Sea, Egypt. Mineralogy and Petrology 48, 309–341]. Five samples with chemical compositions typical of upper mantle rocks, from fertile to increasingly depleted mantle (CaO: 3.39 to 0.21%), have Ir concentrations from 1.98 to 3.28 ppb, within the range in spinel and garnet-lherzolites from worldwide occurrences. There is, in Zabargad and in other localities, no systematic dependence of the contents of Ir and other PGE on the degree of fertility of the host rock (amount of cpx) nor on the geologic setting, xenolith or massive peridotite. Two samples, an orthopyroxenite (7.9 ppb Ir) and a plagioclase wehrlite (29.08 ppb) have significantly higher oncentrations of all HSE, except Re, reflecting the mobility of these elements in a metasomatized mantle. The more or less unfractionated PGE-patterns and the comparatively low Re contents of the enriched rocks indicate that the PGE were apparently transported with a single mobile phase, presumably sulfides, and that Re is chemically decoupled from the other HSE. However, because of the approximately constant level of PGE in upper mantle rocks, it is unlikely that such phases are the main carriers of PGE in these rocks. Although the CI-normalized patterns of the HSE in the Zabargad samples are to a first order chondritic a detailed inspection reveals deviations from chondritic ratios within about a factor of two, significantly beyond analytical uncertainties. Six of the seven samples show enrichments in Pd (factor of two)and some enrichments in Ru (30%)and Rh (30%)relative to the CI-concentrations of Ir, Os. Rhenium contents are variable and decoupled from the PGE abundances. If the HSEs were added to the Earth's mantle with a late chondritic veneer ratios among these elements should better agree with chondritic ratios. Variations within the groups of chondritic meteorites are too small to explain the observed “anomalies’’. The fractionated patterns of PGE in the Zabargad rocks either indicate the presence of additional, perhaps local, components of HSEs in the mantle, or they reflect processing of the PGE within the upper mantle, or, alternatively, the late veneer hypothesis is invalid. It is important in future work to better define the lateral and/or vertical extent of such anomalies in the Earth's mantle.