Solubility of palladium in silicate melts: Implications for core formation in the Earth

Abstract

Palladium solubilities in silicate melts of anorthite-diopside-eutectic composition were determined at a wide range of oxygen fugacities, from pure O 2 to f o2 slightly below the iron-wüstite buffer and at temperatures ranging from 1343 to 1472°C. Experiments were performed by heating palladiumloops with silicates inside a gas controlled furnace. Palladium concentrations were determined by neutron activation analysis. Repeated analyses of the glasses after removal of the outer layers and several reversed experiments with initially high Pd in the glass showed that equilibrium was attained in the experiments. At 1350°C concentrations of Pd in silicate melts range from 428 ppm to 1.2 ppm with decreasing palladium content at decreasing oxygen fugacities. The dependence of log Pd on log f o 2 indicates a change in valence of the dominant palladium species in the silicate melt. The data can be explained by the presence of complexes containing Pd 2+ and Pd 0. Alternatively, a good fit is obtained by assuming mixtures of Pd 2+, Pd 1+and Pd 0 in the melt with increasing contributions of the lower valence species at increasingly reducing conditions. Solubilities increase with temperature at fixed oxygen fugacities independent of the absolute fugacity. This is an unexpected result. From the solubility data, metal/silicate partition coefficients were calculated using known activity coefficients of Pd in Fe-metal. Extrapolations were made to higher temperatures and lower oxygen fugacities. A palladium metal/silicate partition coefficient of 1.6 · 10 7 is inferred for 1623 K and IW-2. Extrapolation to 3500 K leads to a partition coefficient of 3.8 · 10 3. From earlier data on Ir solubilités, a metal/silicate partition coefficient of 2 · 10 8 was estimated for the same conditions. The high absolute metal/silicate partition coefficients for Pd and Ir and the large difference between the two partition coefficients are not compatible with a global core/mantle equilibrium as a source of the highly siderophile elements in the Earth mantle. The data favour models invoking the accretion of a late chondritic veneer after core formation without further metal segregation.