Partitioning of palladium at high pressures and temperatures during core formation

An early equilibration of the Earth's mantle with the metals that later formed the core may explain the concentrations of iron-loving (siderophile) elements in the mantle. However, a subset of these elements, the highly siderophile elements including palladium, are present in greater concentrations than expected. Moreover, their relative ratios are similar to those found in chondrites, that is, primitive solar-system materials(1-4). On the basis of very high partition coefficients for these elements derived from experiments at low pressures and temperatures (for example, a coefficient for palladium > 10(4)), it has been argued that the high mantle concentrations of the highly siderophile elements and also of volatile elements originated from the addition of chondritic material after core formation as a 'late veneer'(1-4). Here we present experiments at higher pressures and temperatures that approximate the conditions of early Earth, and find much lower partition coefficients for palladium, about 480, consistent with an equilibration scenario. This obviates the need for a late veneer to explain the terrestrial-mantle palladium content, and calls into question traditional explanations for the origin of the Earth's volatile elements.