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 formation(1). 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 Os-187-enriched 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 Os-187/Os-188 ratio for PUM of 0.1290+/-0.0009, by using a correlation between Os-187/Os-188 and geochemical indices of 'fertility' to extrapolate to the Os isotope ratio of undepleted mantle. Comparing this value to the Os-187/Os-188 ratios measured in different classes of chondritic meteorite, we infer that the late veneer had siderophile element abundances similar to those of enstatite or ordinary chondrites (Os-187/Os-188 = 0.128+/-0.0010), rather than carbonaceous chondrites (0.1258 +/- 0.0005).