Core formation and metal-silicate fractionation of osmium and iridium from gold

The abundances of the highly siderophile elements as well as their relative proportions¹ in the mantle deviate from those predicted by equilibrium partitioning between metal and silicate during the formation of the Earths core. This discrepancy is generally explained by invoking the addition of a late veneer of extraterrestrial material to the mantle after core formation was complete². Recently reported partition coefficients for gold, platinum and palladium^3-5 could result in mantle abundances consistent with equilibrium partitioning. However, whether these results can be extrapolated to all highly siderophile elements, and thereby preclude the need for a late veneer, remains to be verified. Here we use high-temperature experiments to determine the metal-silicate partition coefficients for osmium, iridium and gold. On the basis of our estimates, equilibrium partitioning during core formation can explain the observed concentration of gold in the mantle, but not that of osmium and iridium. We conclude that not all highly siderophile elements were affected by core formation in the same way, and that the abundances of elements such as osmium and iridium require the addition of a late veneer.