Heterogeneous Earth Accretion and Incomplete Metal-Silicate Reequilibration at High Pressure During Core Formation

Details Heterogeneous Earth Accretion and Incomplete Metal-Silicate Reequilibration at High Pressure During Core Formation Heterogeneous Earth Accretion and Incomplete Metal-Silicate Reequilibration at High Pressure During Core Formation

Dec 2007

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2007agufm.u11a0015r&link_type=abstract

American Geophysical Union, Fall Meeting 2007, abstract #U11A-0015

Other

1009 Geochemical Modeling (3610, 8410), 1015 Composition Of The Core, 1027 Composition Of The Planets, 3610 Geochemical Modeling (1009, 8410)

Scientific paper

We present a new model of core formation, based on the partitioning of siderophile elements, that involves accreting the Earth through a series of collisions with smaller bodies that had already differentiated at low pressure. Each impact results in a magma ocean in which the core of the impactor reequilibrates with silicate liquid at high pressure before merging with the Earth's protocore. The oxygen contents of the chondritic compositions of the proto-Earth and impactors can be varied. The compositions of coexisting metal and silicate are determined through mass balance combined with partitioning equations for Ni, FeO, Si and other siderophile elements. The oxygen fugacity is fixed by the partitioning of FeO and is a function of P, T and bulk oxygen content. An important constraint for core formation is that core-mantle partition coefficients for Ni and Co must both converge to values of 23-28. Based on a recent study of the partitioning of Ni and Co over a wide P-T range (Kegler et al., EPSL, submitted) together with other published data, this constraint is not satisfied by a single- stage core formation model at any conditions because the partition coefficients converge at values that are much too low. In the present multi-stage model, the correct values can be reached if only part of each impactor core reequilibrates with silicate liquid in the magma ocean (as proposed by previous models based on Hf-W isotope studies). Physically, this would mean that impactor cores fail to emulsify completely as they sink through the magma ocean. Incorporating other elements (e.g. V and Cr) in the model requires, in addition, that the bulk composition of the impactors changes during accretion from reduced (FeO-poor) to oxidised FeO-rich). Then, with the resulting increase in fO2, incomplete reequilibration of the cores during the final 20-30% of Earth accretion is required to satisfy the Ni-Co constraint. In addition, this model enables the concentrations of O and Si in the core to be estimated.

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