The osmium isotopic composition of the Earth's primitive upper mantle

THE elevated abundances of highly siderophile elements in the Earth's mantle, relative to what would be predicted from metal–silicate equilibrium, have often been cited as evidence for the accretion to the Earth of a 'late veneer' of chondritic material following core formation1. As rhenium and its decay-product osmium are both highly siderophile, the evolution of the Re–Os isotope system in a terrestrial reservoir provides a robust, time-averaged constraint on the siderophile abundances of the reservoir; thus, the broadly chondritic evolution of Os isotopes in the oceanic upper mantle provides strong support for the late accretion model2,3. But the Re–Os composition of the late veneer is still poorly defined, because the mantle has differentiated into 187Os-enriched and -depleted reservoirs4–7. Here we report a value for the Os isotopic composition of the modern 'primitive upper mantle' (PUM), a hypothetical undifferentiated upper-mantle reservoir. From suites of variably melt-depleted mantle xenoliths from three continents, we derive a minimum 187Os/188Os ratio for PUM of 0.1290 ± 0.0009, by using a correlation between 187Os/188Os and geochemical indices of 'fertility' to extrapolate to the Os isotope ratio of undepleted mantle. Comparing this value to the 187Os/188Os ratios measured in different classes of chrondritic meteorite, we infer that the late veneer had siderophile element abundances similar to those of enstatite or ordinary chondrites (187Os/188Os = 0.1286 ± 0.0010), rather than carbonaceous chondrites (0.1258 ± 0.0005).