Element Redistribution in Fe-Ni-O Alloys by a Thermal Gradient: Implications for Siderophile Element Partitioning During Core Formation and Crystallization

Abstract:

Experimentally determined partition coefficients for Fe-rich metallic systems are essential to constrain models of the formation and evolution of planetary cores. Solid metal-liquid metal partition coefficients (D^Sol/Liq) for many siderophile elements depend upon the light-element content of the liquid phase. This dependence can be described by an empirically determined interaction parameter (β). Oxygen is a potential light element in the cores of large planetary bodies. Direct measurements of the partition coefficients between solid and liquid alloys in the Fe-O system, however, are restricted to liquids with low O contents (<2.2 wt% O at 15 GPa; Langlade et al. LPSC, 2008). Measurements to derive β-values from Soret diffusion experiments allow us to extend our investigation to the larger compositional space present at higher temperature (Brenan & Bennett, EPSL, 2010). We performed experiments at 15 GPa in a 1500-tonne multi-anvil press, using starting materials that comprised Fe, Ni and FeO powders mixed with ~200 ppm each of the platinum group metals, W, Mn, Re and Au. Run-products were analysed by electron microprobe (Fe, Ni, O) and LA-ICP-MS (PGMs, W, Mn, Re, Au). Temperature along the sample was estimated from the thickness of a spinel layer formed at the interface between the Al₂O₃ capsule and MgO sleeve. Initial results show siderophile elements typically display O avoidance behavior and are concentrated toward the cold, Fe-rich portion of the sample. In one experiment however, that contains W and Re in weight percent concentrations, Mn and Re are concentrated toward the O-rich portion of the sample. Au concentrations remain approximately constant along the sample length. Results are used to predict the evolution of element ratios in the outer-core during inner-core crystallization. β-values may also be used to estimate changes to D^Met/Sil that arise from O dissolved in core-forming metal.