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

Abstract

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 model^2,3. But the Re–Os composition of the late veneer is still poorly defined, because the mantle has differentiated into ¹⁸⁷Os-enriched and -depleted reservoirs^4–7. 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).