195 / 2023-04-15 18:29:49

Ni and Co metal/silicate partitioning: tracing pressure and oxygen fugacity conditions of planetary differentiation

planetary science,meteorites,asteroids,magma ocean,core formation

高压物理与材料科学 > 高压地球科学-I

Moderately siderophile elements (MSE) are potential tracers of the thermodynamic conditions prevailing during core formation because their metal/silicate partition coefficients (D^met/sil) vary as a function of P, T, and oxygen fugacity (fO₂). The intrinsic conditions of differentiation therefore lead to planetary mantles with unique MSE depletion signatures. Among the MSE, Ni and Co are excellent and reliable magma ocean barometers because their D^met/sil are strongly correlated to pressure, decreasing over almost 3 orders of magnitude between 1 bar and 100 GPa. Current pressure-dependent expressions of D^met/sil were calibrated on experiments performed under relatively oxidizing conditions, mostly with fO₂ slightly below the IW buffer, corresponding to the redox conditions of the terrestrial and Martian mantle. However, other planets, planetary embryos, and differentiated planetesimals formed under a wide range of redox conditions going from the most reduced Mercury (fO₂ ~ IW-3 to IW-7) to the most oxidized angrite parent body (fO₂ ~ IW+1). In this study, we performed and analyzed 38 experiments with equilibrated metal and silicate melts over a wide range of pressures (1 bar to 26 GPa) and oxygen fugacities (IW-6.4 to IW-1.9) to expand the Ni and Co D^met/sil database to more reducing conditions. Adding previously published data, we then parameterize 350 Ni and Co D^met/sil as a function of T, P and fO_2. This parametrization accurately predicts the evolution of Ni and Co D^met/sil between 1 bar and 80 GPa, IW to IW-7, and 1550 K to 4450 K.  Using our parameterization, we model Ni and Co D^met/sil along the liquidus of a chondritic mantle at various P and fO₂, to build an oxy-barometer. Finally, we apply this tool to investigate the thermodynamic equilibrium of various planetary bodies’ magma ocean. The P and fO₂ obtained for Earth, Mars, Moon and Vesta are strongly correlated to these planetary sizes and bulk silicate FeO contents, respectively. The P and fO₂ obtained for other achondrites suggest a wide variety of core formation conditions, from the small and oxidized angrite parent body, to a planet-sized and highly reduced aubrite parent body.