Diffusive Transport and Structural Properties of Liquid Iron Alloys at High Pressure
Abstract
Diffusive transport properties of liquid iron alloys at high pressures (P) and temperatures (T) place important kinetic constraints on processes related to the origin and evolution of planetary cores. Earth's core composition is largely controlled by the extent of chemical equilibration achieved between liquid metal bodies and a silicate magma ocean during core formation, which can be estimated using chemical diffusion data. In order to estimate the time and length scales of metal-silicate chemical equilibration, we have measured chemical diffusion rates of Si, O and Cr in liquid iron over the P-T range of 1-18 GPa and 1873-2643 K using a multi-anvil apparatus. We have also performed first-principles molecular dynamic simulations of comparable binary liquid compositions, in addition to pure liquid Fe, over a much wider P-T range (1 bar-330 GPa, 2200-5500 K) in order to both validate the simulation results with experimental data at conditions accessible in the laboratory and to extend our dataset to conditions of the Earth's core. Over the entire P-T range studied using both methods, diffusion coefficients are described consistently and well using an exponential function of the homologous temperature relation. Si, Cr and Fe diffusivities of approximately 5 × 10-9 m2 s-1 are constant along the melting curve from ambient to core pressures, while oxygen diffusion is 2-3 times faster. Our results indicate that in order for the composition of the Earth's core to represent chemical equilibrium, impactor cores must have broken up into liquid droplet sizes no larger than a few tens of cm. Structural properties, analyzed using partial radial distribution functions from the molecular dynamics simulations, reveal a pressure-induced structural change in liquid Fe0.96O0.04 at densities of 8 g cm-3, in agreement with previous experimental studies. For densities above 8 g cm-3, the liquid is essentially close packed with a local CsCl-like (B2) packing of Fe around O under conditions of the Earth's core.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2017
- Bibcode:
- 2017AGUFMMR24A..02P
- Keywords:
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- 3909 Elasticity and anelasticity;
- MINERAL PHYSICS;
- 3924 High-pressure behavior;
- MINERAL PHYSICS;
- 5120 Plasticity;
- diffusion;
- and creep;
- PHYSICAL PROPERTIES OF ROCKS