Chemical evolution of planetesimal cores during the early history of the solar system

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The Earth formed by the accretion of numerous small 10-100 km-sized planetesimals together with a smaller number of larger Moon- to Mars-sized embryos. According to current models of Earth's core formation, the bodies that accreted early to the Earth should have undergone core-mantle differentiation under highly reducing conditions, such that significant amounts of silicon, chromium and vanadium partitioned into their metallic Fe-Ni cores. Iron meteorites are derived from the metallic cores of early-formed planetesimals, and, in contrast to the predictions, the concentrations of Si and Cr in these meteorites are extremely low (e.g. <1 ppm). The aim of this proposal is to test the following hypothesis: Molten planetesimal cores originally contained high concentrations of the weakly siderophile elements Si and Cr but during cooling these elements became increasingly lithophile and partitioned back into the overlying molten mantle (magma ocean). This hypothesis will be tested by first determining the chemical diffusivities of Si and Cr in molten iron both experimentally (at 1-25 Gigapascals) and theoretically (using molecular dynamics simulations over a larger pressure range). Second we will develop models of chemical interaction at planetesimal core-mantle boundaries for realistic cooling histories, assuming that both the liquid core and overlying magma ocean are in a state of convection. The rate of chemical evolution will be controlled by diffusion through boundary layers for which the diffusion results will be required.