NONCHONDRITIC PT/PD RATIOS IN ARC MANTLE XENOLITHS: EVIDENCE FOR PLATINUM ENRICHMENT IN DEPLETED ISLAND-ARC MANTLE SOURCES

Abstract

Mantle-derived spinel harzburgites from the Kamchatka arc have fractionated Pd- group element patterns, and Pt is clearly enriched relative to Pd and Rh. This fractionation is also consistent with chondritic Ir-group platinum-group element distribution and super- chondritic Pt/Pd, chondrite-normalized (Pt/Os)N and (Pt/Ir)N found in harzburgite xenoliths from the Tubaf seamount in the Lihir island group of the Tabar-Lihir-Tanga-Feni island arc in Papua New Guinea. The nonchondritic Pt/Pd ratios and Pt enrichment in the island-arc mantle are best explained by extraction of melt in a back-arc volcanic-arc setting, which left refractory Pt-Fe alloys in the residual mantle. Pt-Fe alloys selectively concentrate Pt relative to Pd and, overall, increase the Pt/Pd in a subarc mantle wedge. Partial melting of this Pt-enriched mantle wedge above the subduction zone, due to hydrous flux from the subducting lithosphere, is potentially capable of producing Pt-rich primitive melts parental to the Alaskan-type ultramafic-mafic complexes.