Experimental determination of oxygen diffusion in liquid iron at high pressure

Abstract

Oxygen diffusion experiments in liquid iron have been performed at 3–18 GPa and 1975–2643 K using a multi-anvil apparatus. Diffusion couples consisted of a pure iron rod and a sintered disk of Fe0.85O0.15 placed end-to-end in a vertical orientation. Images and chemical spot analyses were acquired along the full length of the quenched sample on lines perpendicular to the diffusion interface. Exsolution features that formed during quenching consist mostly of spherical oxide blobs of at least two size populations, as well as feathery dendritic textures in more oxygen-rich regions near the top of the samples. Diffusion during heating (i.e. prior to reaching the peak annealing temperature, Tf) is treated numerically to refine Arrhenian parameters from simultaneous least-squares fits to several concentration profiles obtained from experiments at constant pressure and variable Tf. Diffusion coefficients range from ∼6×10−9 to ∼2×10−8 m2s−1 over the P–T range of the study, with activation enthalpies of less than 100 kJ mol−1. We find a very weak effect of pressure on oxygen diffusion with an activation volume of 0.1±0.1 cm3mol−1, in agreement with computational studies performed above 100 GPa. Arrhenian extrapolation of diffusion coefficients for oxygen to P–T conditions of the Earth's outer core yields faster average diffusion rates (∼3×10−8 m2s−1) than for Si or Fe in silicon-rich liquid iron alloys or pure liquid iron (∼5×10−9 m2s−1) reported previously. Oxygen diffusion data are used to constrain the maximum size of descending liquid metal droplets in a magma ocean that is required for chemical equilibration to be achieved. Our results indicate that if the Earth's core composition is representative of equilibrium chemical exchange with a silicate magma ocean, then it could only have been accomplished by large-scale break-up of impactor cores to liquid iron droplet sizes no larger than a few tens of centimeters.