Constraints on Core Composition from Shock-Wave Data

Physics

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Scientific paper

Seismic data demonstrate that the density of the liquid core is some 8-10% less than pure iron. Equations of state of Fe-Si, C, FeS2, FeS, KFeS2 and FeO, over the pressure interval 133-364 GPa and a range of possible core temperatures (3500-5000 K), can be used to place constraints on the cosmochemically plausible light element constituents of the core (Si, C, S, K and O). The seismically derived density profile allows from 14 to 20% Si (by mass) in the outer core. The inclusion of Si, or possibly C (up to 11%), in the core is possible if the Earth accreted inhomogeneously within a region of the solar nebulae in which a C: O (atomic) ratio of about 1 existed, compared with a C: O ratio of 0. 6 for the present solar photosphere. In contrast, homogeneous accretion permits Si, but not C, to enter the core by means of reduction of silicates to metallic Fe-Si core material during the late stages of the accumulation of the Earth. The data from the equation of state for the iron sulphides allow up to 9-13% S in the core. This composition would provide the entire Earth with a S: Si ratio in the range 0.14-0.3, comparable with meteoritic and cosmic abundances. Shock-wave data for KFeS2 give little evidence for an electronic phase change from 4s to 3d orbitals, which has been suggested to occur in K, and allow the Earth to store a cosmic abundance of K in the metallic core.

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