Non-chondritic platinum-group element ratios in the Earth's mantle

THE Earth's upper mantle has an overabundance of highly siderophile elements (platinum-group elements, gold and rhenium), relative to what would be expected from equilibrium with the metallic core(1-3), This excess is now widely believed to have been introduced as a ''late veneer'' by meteorite bombardment during early Earth history, but after separation of the core(4), Here we report high-precision analyses of platinum-group elements (PGEs) from fertile upper-mantle lherzolites, which show variations in relative abundances that exceed those in the chondritic meteorites that are thought to have furnished the late veneer(5), In particular, the Pd/Ir ratio, at about 1.76, is significantly greater than that of known chondritic meteorites(6), Our results, combined with previous indications of both non-chondritic and nearly chondritic PGE ratios in the few other fertile mantle lherzolites that have been analysed(2,3,7-11), strongly suggest that the mantle is heterogeneous in its PGE content on scales of similar to 100 kilometres, If the observed 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.