Nonchondritic Pt/Pd ratios in arc mantle xenoliths: Evidence for platinum enrichment in depleted island-arc mantle sources

Mantle-derived spinet harzburgites from the Kamchatka arc have fractionated Pd-group element patterns, and Pt is clearly enriched relative to I'd and Rh. This fractionation is also consistent with chondritic Ir-group-platinum -group element distribution and superchondritic Pt/Pd, chondrite-normalized (Pt/Os)(N) and (Pt/Ir)(N) found in harzburgite xenoliths from the Tubaf se amount in the Lihir island group of the Tabar-Lihir-Tanga-Feni island arc i n Papua New Guinea. The nonchondritic Pt/Pd ratios and Pt enrichment in the island-are mantle are best explained by extraction of melt in a back-are v olcanic-arc setting, which left refractory Pt-Fe alloys in the residual man tle. Pt-Fe alloys selectively concentrate Pt relative to Pd and, overall, i ncrease the Pt/Pd in a subarc mantle wedge. Partial melting of this Pt-enri ched mantle wedge above the subduction zone, due to hydrous flux from the s ubducting lithosphere, is potentially capable of producing Pt-rich primitiv e melts parental to the Alaskan-type ultramafic-mafic complexes.