David. C. Rubie

Evidence for a late chondritic veneer in the Earth's mantle from high-pressure partitioning of palladium and platinum

The high-pressure solubility in silicate liquids of moderately siderophile ‘iron-loving’ elements (such as nickel and cobalt) has been used to suggest that, in the early Earth, an equilibrium between core-forming metals and the silicate mantle was established at the bottom of a magma ocean. But observed concentrations of the highly siderophile elements—such as the platinum-group elements platinum, palladium, rhenium, iridium, ruthenium and osmium—in the Earths upper mantle can be explained by such a model only if their metal–silicate partition coefficients at high pressure are orders of magnitude lower than those determined experimentally at one atmosphere (refs 3,4,5,6,7,8). Here we present an experimental determination of the solubility of palladium and platinum in silicate melts as a function of pressure to 16 GPa (corresponding to about 500 km depth in the Earth). We find that both the palladium and platinum metal–silicate partition coefficients, derived from solubility, do not decrease with pressure—that is, palladium and platinum retain a strong preference for the metal phase even at high pressures. Consequently the observed abundances of palladium and platinum in the upper mantle seem to be best explained by a ‘late veneer’ addition of chondritic material to the upper mantle following the cessation of core formation.