Metal-Silicate Partitioning of Si, O, and Mg at High Pressures and High Temperatures: Implications to the Compositional Evolution of Core-Forming Metallic Melts

Abstract

High-pressure and high-temperature experiments were conducted to investigate the partitioning behaviors of Si, O, and Mg between molten Fe-alloys and silicate melts in the Fe-Si-O-Mg system under conditions of 2–72 GPa and 2000–5500 K, using both laser-heated diamond anvil cells and a multi-anvil press. Combing our new experimental results with previously published data, we evaluated the effects of pressure, temperature, and metallic compositions on the partitioning behaviors of Si, O, and Mg. A set of internally consistent interaction parameters between Si, O, and Mg were obtained by the simultaneous fitting of distribution coefficients for all three elements in the Fe-Si-O-Mg system. The composition-dependent distribution coefficients were applied in calculating the compositional evolution of metallic melts during multi-stage core formation. Our results suggest that the core-forming metallic melts would contain more Si and O than previously estimated due to the attractive interactions of light elements in the metal. Compared to the geophysically constrained core composition, these findings imply the exsolution of light elements, likely in the form of SiO₂, from the outer core upon cooling.