Non-chondritic platinum-group element ratios in oceanic mantle lithosphere: petrogenetic signature of melt percolation?

The concentrations of the platinum-group elements (PGE) Ir, Ru, Pt and Pd w ere determined in 11 abyssal peridotites from ODP Sites 895 and 920, as wel l in six ultramafic rocks from the Horoman peridotite body, Japan, which is generally thought to represent former asthenospheric mantle. Individual oc eanic peridotites from ODP drill cores are characterized by variable absolu te and relative PGE abundances, but the average PGE concentrations of both ODP suites are very similar. This indicates that the distribution of the no ble metals in the mantle is characterized by small-scale heterogeneity and large-scale homogeneity. The mean Ru/Ir and Pt/Ir ratios of all ODP peridot ites are within 15% and 3%, respectively, of CI-chondritic values. These re sults are consistent with models that advocate that a late veneer of chondr itic material provided the present PGE budget of the silicate Earth. The da ta are not reconcilable with the addition of a significant amount of differ entiated outer core material to the upper mantle. Furthermore, the results of petrogenetic model calculations indicate that the addition of sulfides d erived from percolating magmas may be responsible for the variable and gene rally suprachondritic Pd/Ir ratios observed in abyssal peridotites. Ultrama fic rocks from the Horoman peridotite have PGE signatures distinct from aby ssal peridotites: Pt/Ir and Pd/Ir are correlated with lithophile element co ncentrations such that the most fertile Iherzolites are characterized by no n-primitive PGE ratios. This indicates that processes more complex than sim ple in-situ melt extraction are required to produce the geochemical systema tics, if the Horoman peridotite formed from asthenospheric mantle with chon dritic relative PGE abundances. In this case, the PGE results can be explai ned by melt depletion accompanied or followed by mixing of depleted residue s with sulfides, with or without the addition of basaltic melt. (C) 1999 El sevier Science B.V. All rights reserved.