High P-T experiments and first principles calculations of Si, O, and Cr diffusion in liquid iron

Abstract:

The mobility of alloying elements in liquid iron has important kinetic implications for timescales and processes occurring in metallic cores and core-mantle boundaries of differentiated bodies. According to current models of the Earth’s core-mantle segregation, substantial amounts of light elements (Si and/or O), as well as Cr, should have partitioned from a magma ocean into metallic Fe-Ni cores of planetesimals during accretion. In contrast to these predictions, however, the Si, O, and Cr contents of iron meteorites, which are derived from the metallic cores of early-formed planetesimals, are surprisingly low (e.g. < 1 ppm). The partitioning of Si, O, and Cr into liquid iron has been shown experimentally to increase with temperature so that that the alloy component of a planetesimal core should decrease during cooling. Ongoing metal-silicate interaction at the CMB of larger bodies, such as the Earth, and potential diffusive profiles of light elements in the outmost region of the Earth’s outer core have been used to model and interpret deviations from reference model seismic wave speeds in these respective regions. We are conducting a series of high P-T experiments and first principles calculations to constrain the diffusivity of Si, O, and Cr in liquid iron in order to understand the kinetics of chemical transport and equilibration during core formation and processes occurring at CMBs.

Experimental diffusion couples comprised of highly polished cylindrical disks of 99.97% Fe and metallic Fe alloy (Fe₆Si, Fe₁₂Si, Fe₉O, Fe₉₂Cr, Fe₈₅Si₁₄Cr) were contained in an MgO capsule and annealed within the P-T range 1642–2573 K and 3–18 GPa in a multi-anvil apparatus. A series of experiments are conducted at each pressure using variable heating rates, final temperatures (T_f), and time duration at T_f. To extend our dataset to P-T conditions of the Earth’s core-mantle boundary, we have begun first principles molecular dynamics (FP-MD) calculations. Fe supercells are overheated to induce melting, compressed along several isotherms (2000–5000K) and allowed for changes in chemistry (Si, O, Cr) and alloy concentration. Diffusion coefficients are computed from the atomic trajectories in the simulation cell via the Einstein relation.