Abstract
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 core1–3. 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 core4. Here we report high-precision analyses of platinum-group elements (PGEs) from fertile upper-mantle Iherzolites, which show variations in relative abundances that exceed those in the chondritic meteorites that are thought to have furnished the late veneer5. In particular, the Pd/Ir ratio, at about 1.76, is significantly greater than that of known chondritic meteorites6. Our results, combined with previous indications of both non-chondritic and nearly chondritic PGE ratios in the few other fertile mantle Iherzolites that have been analysed2,3,7–11, strongly suggest that the mantle is heterogeneous in its PGE content on scales of ∼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.
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Pattou, L., Lorand, J. & Gros, M. Non-chondritic platinum-group element ratios in the Earth's mantle. Nature 379, 712–715 (1996). https://doi.org/10.1038/379712a0
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DOI: https://doi.org/10.1038/379712a0
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