Siderophile Elements: Systematics and Significance

Abstract

Siderophile elements preferentially partition into metal phases during cosmochemical and geochemical processes. The siderophile elements include: the highly siderophile elements (HSE; Re, Os, Ir, Ru, Pt, Rh, Au, Pd), which have low-pressure metal-silicate distribution coefficients (D values) of more than 10,000; the moderately siderophile elements (MSE; including Mo, W, Fe, Co, Ni, P, Cu, Ga, Ge, As, Ag, Sb, Sn, Tl, Bi), with D values that are typically more than 10, but less than 10,000; and the slightly siderophile elements (SSE; Mn, Cr and V). During condensation of elements at the formation of the solar nebula, siderophile elements sequestered into refractory metallic alloys, or into iron sulfide. Early differentiation of Earth led to siderophile elements being extracted into a metallic iron core, and corresponding depletion of these elements in Earth's silicate mantle and crust. Application of low-pressure metal-silicate partition coefficients shows that the predicted abundances of some siderophile elements, especially the HSE, are higher than expected from metal-silicate partitioning in the bulk silicate Earth. The overabundance of siderophile elements in Earth's mantle has been attributed to “late accretion” of materials with generally chondritic bulk composition after core formation ceased, to account for the generally flat, chondrite-relative abundances of the HSE. Experimental partitioning studies have found that some siderophile elements have decreasing metal-silicate partitioning coefficients at high-pressures. Thus, the possibility of high-pressure core-mantle equilibration to account for some of the excess siderophile element—and specifically HSE—abundances in the bulk silicate Earth cannot be ruled out.