Partitioning of nickel and cobalt between metal and silicate melts: Expanding the oxy-barometer to reducing conditions

Moderately siderophile elements (MSEs) are potential tracers of the thermodynamic conditions prevailing during planetary core formation because their metal-silicate partition coefficients (D-met/sil) vary as a function of P, T, and oxygen fugacity (fO(2)). Those properties result in the production of planetary mantles with unique MSE depletion signatures. Among the MSEs, Ni and Co are reliable barometers in magma oceans because their D-met/sil values are strongly correlated with pressure, decreasing by almost 3 orders of magnitude between 1 bar and 100 GPa. Current pressure-dependent expressions of D-met/sil were calibrated based on experiments performed under relatively oxidizing conditions, mostly at fO(2) slightly below the iron-wustite Fe-FeO buffer (IW), which is relevant to the mantles of Earth and Mars. However, planets and asteroids formed under a wide range of redox conditions, from Mercury, the most reduced (similar to IW - 5.5), to the most oxidized angrite parent body (IW - 1.5 to IW + 1). In this study, we performed and analyzed 38 metal-silicate partitioning experiments over a wide range of pressures (1 bar to 26 GPa) and oxygen fugacities (IW - 6.4 to IW - 1.9) to expand the available Ni and Co D-met/sil values to reducing conditions. We then parameterized 255 Ni and 194 Co D-met/sil values as a function of T (1573-5700 K), P (1 bar to 100 GPa), and fO(2) (IW - 6.4 to IW + 0.2). We also modeled the evolution of Ni and Co D-met/sil values along the liquidus of a chondritic mantle at various P and fO(2) conditions to investigate the thermodynamic conditions of various planetary bodies' magma oceans. The P and fO(2) conditions we obtained for Earth, Mars, the Moon, and Vesta are consistent with previous studies using similar methods, and the pressure during core formation is strongly correlated to planetary size. Finally, we also applied our model to several achondrite parent bodies; our results indicate a wide variety of objects, from the asteroid-sized, oxidized angrite parent body to the planet-sized, highly reduced aubrite parent body.