FRACTIONATION OF PLATINUM-GROUP ELEMENTS IN UPPER-MANTLE - EVIDENCE FROM PERIDOTITE XENOLITHS FROM WANGQING

The peridotite xenoliths from Wangqing display a downward-concave PGE pattern with Pt at maximum due to Pt-Pd fractionation, which is differ ent from the flat or negatively sloped patterns commonly observed for worldwide peridotite xenoliths and massifs. Taking into account the di fference in partition coefficients of Pt and Pd between alloy and sulf ide melt, it is suggested that some of the Wangqing peridotites must h ave been equilibrated with a melt in which alloy and sulfide coexisted . Segregation of Pt-rich alloy from Pd-sulfide melts due to density co ntrast and extraction of residual melts without sulfide saturation res ulted in the fractionation of Pt from Pd.