Experiments on crystal/liquid partitioning of Ru, Rh and Pd for magnetite and hematite solid solutions crystallized from silicate melt

Abstract

Experiments to characterize crystal/melt partitioning of Ru, Rh and Pd between Fe-oxides (magnetite and hematite solid solutions) and silicate melt are reported for oxygen fugacities imposed by CO 2 decomposition. Oxides were equilibrated near 1275°C at 1 atm with silica-saturated melts in the compositional system “FeO”CaOAl 2 O 3 SiO 2± MgO±Cr 2 O 3± TiO 2 . Ru and Rh are strongly compatible while Pd is slightly incompatible. Partition coefficients (elemental weight ratios) for Ru are variable but range from 100 to > 4000 depending, in a poorly defined way, on the oxide crystal composition. We report enhanced compatibility for Ru compared to previous work for spinels in an Fe-free system. Rh compatibility is also enhanced compared to the Fe-free system, but to a lesser degree. However, Rh partition coefficients (grand average is 250±120) are more uniform than those measured for Ru. Pd is slightly incompatible in magnetite and hematite (grand average is 0.7 ± 0.3) in contrast to the Fe-free system where Pd is highly incompatible. The differences in compatibility among Ru, Rh and Pd suggest that spinels could play a role in determining platinum-group element (PGE) fractionation trends, in particular, for rocks crystallizing at high oxygen fugacity where it is most likely that the dissolved PGE's are present as oxidized species. We also propose a reaction mechanism to explain the common observation of platinum-group minerals (PGM's) included within spinel phases. Finally, our data suggest that Ru, at least, should be concentrated in spinels crystallizing in Earth's atmosphere, either within micrometeroids, or from atmospheric crystallization of impact ejecta.