Experimental constraints on the thermodynamic modelling of siderophile element distribution during core-segregation and basalt petrogenesis

Abstract

In order to effectively model the partitioning behaviour of siderophile elements during processes ranging from planetary differentiation in the early solar system to contemporary examples of igneous petrogenesis, we require an accurate determination of the physiochemical variables that control their distribution between phases. An X-ray absorption near edge structure (XANES) spectroscopic study of silicate glasses, synthesized at high pressure and quenched isobarically, has been conducted in order to assess the effect of pressure on the coordination environment of divalent Ni, Co, Fe, trivalent Fe and hexavalent W. The principle Ni coordination state is shown to transition from predominantly tetrahedral symmetry at low pressures to predominantly octahedral symmetry by 3 to 4 GPa, while the coordination environments of divalent Co and Fe remain essentially unchanged. The effects of this transition are clearly mirrored in experimentally determined distribution coefficients for Ni and Co between Ir alloy and liquid melt of an identical composition over the same pressure interval, indicating that the coordination environments present in high pressure and temperature melts are preserved on quenching to glass. Considering core-segregation models primarily reliant on linear regression of siderophile element partition coefficients to determine conditions that reconcile current mantle abundances, these results provide a structural basis for non-linear, pressure dependent Ni distribution in the upper mantle and exclude its abundance and near chondritic Ni/Co ratio as evidence for simple single-stage equilibration at the base of a magma ocean. Fe K-edge XANES spectra of trivalent Fe bearing glasses indicate an increase in average coordination state between 0.55 and 4 GPa while W L{u2083}-edge spectra of hexavalent W bearing glasses indicate a tetrahedral environment over all pressures investigated. The recognition of differential changes in the coordination of siderophile melt species as a function of pressure provides an essential constraint when modelling a wide range of igneous processes. While an experimental approach allows us to isolate the role of individual parameters to more effectively constrain predictive models, developing a thermodynamic framework for the interpretation of experimental results is essential to rationalise and quantify such effects. To assess partitioning relations between ternary (Mg,Fe,Ni) olivine, silicate melt and solid fcc Fe-Ni-Co alloys at controlled oxygen fugacity, the results of 1 bar isothermal experiments were used to develop thermodynamically consistent predictive models for the partitioning of Ni and Fe between metal and olivine, and Ni and Mg between olivine and melt, and to assess activity composition relations in ternary metal alloys by comparing activity-composition models from the metallurgical literature. Modelling of activity coefficients in Mg-Fe-Ni and Mg-Fe-Co olivines using regular solution formalism indicates both Ni and Co display non-ideal interaction with Fe. We report updated values for the interaction parameters W(Fe-Ni) = 20 kJ/mol and W(Fe-Co) =12 kJ/mol.