SHOCK WAVE DATA AND THE COMPOSITION OF THE EARTH'S CORE

Abstract

The pressures existing in the outer liquid core of the earth are 130 to 328 GPa. Seismic data demonstrate that the density of the liquid core is some 8 to 10% less than pure iron. Equations of state of Fe-Si, Fe3C, C, FeS2, FeS, KFeS2 and FeO, over the appropriate pressure interval, and a range of possible core temperatures (3500 to 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 wt. % Si in the outer core. The inclusion of Si or, possibly C (up to 11 wt. %), in the core is possible if, the earth accreted inhomogeneously within a region of the solar nebulae in which a C/0 (atomic) ratio of ≃ 1 existed, as compared to C/0 ∼ 0.6 for the present solar photosphere. In contrast, homogeneous accretion permits Si but not C, to enter the core via reduction of silicates to metallic Fe-Si core material during the late stages of the accumulation of the earth. The equation of state data for the iron sulfides allow up to 8 to 12% S in the core. This composition would provide the entire earth, with a S/Si ratio in the range 0.14 to 0.3 comparable to meteoritic and cosmic abundances. Shock-wave data for KFeS2 give little evidence for an electronic phase change from 4-s to 3-d orbitals which has been suggested to occur in K and allow the earth to store a cosmic abundance of K in the metallic core.