Constraints on Earth accretion deduced from noble metals in the oceanic mantle

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If the Earth's mantle were in equilibrium with its core, the mantle would contain three orders of magnitude less of the noble metals (platinum- group elements Pt, Os, Ir, Ru, Pd and Rh, plus Au and Re) than are observed. An explanation put forward to account for this disparity has been that the last 1% of the Earth's accretion occurred after the iron-rich core had separated from the mantle. Recent debate has accordingly centred on which meteorite class or classes made up this 'late veneer' of accretion. Here we present analyses of noble-metal concentrations in oceanic peridotites (plutonic rocks which are thought to represent samples of the Earth's upper mantle). We find that the average oceanic-mantle Os/Ir ratio is indistinguishable from that in the CI-type carbonaceous chondrites, but that Ru/Ir, Pt/Ir, Rh/Ir and Pd/Ir ratios are about 40% higher. A late veneer composed of strictly CI-type carbonaceous chondritic composition is therefore not compatible with these observations. The data also allows us to rule out other carbonaceous chondrites, enstatite chondrites and ordinary, chondrites as significant late veneer components. We propose that mixing of differentiated outer-core material back into the mantle core separation could account for the observed noble-metal ratios and abundances in the mantle without any late accretionary vencer.

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