High P-T diffusive transport properties of liquid iron alloys and peridotite melt

Abstract:

Diffusive transport properties of peridotite melt and molten iron alloys at high pressures and temperatures are important for understanding large-scale geodynamic processes and thermochemical evolution of planetary interiors, such as the time and length scales of metal-silicate equilibration during core formation and chemical exchange across core-mantle boundaries during cooling. In order to determine the pressure and temperature dependence of diffusion in these geologically relevant liquids, we have conducted experiments of Si, O, and Cr diffusion in liquid iron over the P-T range of 1.5—18 GPa and 1850—2450 K, and Si, O, Mg, Ca tracer and Ni, Co chemical diffusion in peridotite liquids over the P-T range of 4—24 GPa and 2248—2623 K in a multi-anvil apparatus. Our results show a very small pressure dependence (< 1 cm³/mol) of alloying element diffusion (Si, O, Cr) in liquid iron over the P-T range of the study and remarkably good consistency with first principles calculations (Pozzo et al. 2013. Phys. Rev. B 87, 014110; Ichikawa and Tsuchiya. 2015. Phys. Earth Planet. Inter., in press) when extrapolated to outer core conditions. Diffusion rates of Si, O, Ca, Mg, Ni, Co in silicate melt decrease with increasing pressure to a minimum at approximately 10 GPa. An anomalous pressure effect is observed above ~10 GPa such that diffusivities rapidly increase with increasing pressure for all elements to a maximum at ~12 GPa, consistent with previous work on peridotite viscosity to 13 GPa (Liebske et al. 2005. Earth Planet. Sci. Lett. 240, 589). Above ~12 GPa, diffusivities decrease with increasing pressure to 24 GPa. Peridotite melt viscosities calculated using the Eyring relation, oxygen self-diffusion rates, and average jump distance (λ) of 5.4 nm yield ~0.01 Pa s at 24 GPa and 2423 – 2623 K.