NON-CHONDRITIC PLATINUM-GROUP ELEMENT RATIOS IN THE EARTHS MANTLE

THE Earth's upper mantle has an overabundance of highly siderophile el ements (platinum-group elements, gold and rhenium), relative to what w ould be expected from equilibrium with the metallic core(1-3), This ex cess is now widely believed to have been introduced as a ''late veneer '' by meteorite bombardment during early Earth history, but after sepa ration of the core(4), Here we report high-precision analyses of plati num-group elements (PGEs) from fertile upper-mantle lherzolites, which show variations in relative abundances that exceed those in the chond ritic meteorites that are thought to have furnished the late veneer(5) , In particular, the Pd/Ir ratio, at about 1.76, is significantly grea ter than that of known chondritic meteorites(6), Our results, combined with previous indications of both non-chondritic and nearly chondriti c PGE ratios in the few other fertile mantle lherzolites that have bee n analysed(2,3,7-11), strongly suggest that the mantle is heterogeneou s in its PGE content on scales of similar to 100 kilometres, If the ob served PGE ratios can be reconciled with a plausible meteoritic source , the observed heterogeneity may reflect an original spatial variation in the late veneer; otherwise, an additional fractionation mechanism seems to be required.