The Composition and Early Evolution of Earth

Abstract

A weighted average composition is suggestd as the "Best Bulk Silicate Earth" (BBSE). BBSE, thought to represent Earth's primitive mantle (modern mantle + crust), is a concept based on analyses of oceanic theoleiites and continental flood basalts, oceanic and continental alkali basalts, ultramafic ophiolites, Archean komatiites, and spinel- and garnet-bearing xenoliths from basalts and kimberlites. BBSE shows the combined effects of core formation and volatility, presumably including condensation and sublimation in the solar nebula and volatile loss during the accretion and early evolution of Earth. The abundances of volatile lithophile elements show a volatility trend (VT) when plotted against condensation temperature. The slope of VT suggests that Earth accreted matter that had condensed over a wide range of temperatures, consistent with planet-formation scenarios of G. W. Wetherill (1988, Mercury, pp. 670-691, Univ. of Arizona Press, Tucson; 1990, Annu. Rev. Earth Planet. Sci. 18, 205-256). Using VT, BBSE, and long-standing seismic constraints, we infer the composition of the entire Earth and its core. The bulk Earth's VT closely resembles the average composition of H-chondrites (H3-H6). The 10 most abundant elements in the core nominally include ∼85.55 mass% Fe, 5.18% O, 4.88% Ni, 2.69% S, 0.45% Cr, 0.41% Mn, 0.35% P, 0.22% Co, 0.07% Cl, and possibly 0.02% K. The core may contain most of Earth's heavy alkali and halogen elements, in addition to chalcophiles and siderophiles; alternatively, depletions of alkali halide components in BBSE may have resulted from impact blow-off of an early saline ocean. It is unclear whether the core contains a significant fraction of Earth's K and Pb. Possible partitioning of substantial quantities of O, Rb, and Cs into the core and the partitioning behavior of fourth-period transition metals suggests that metal-silicate equilibration occurred at characteristic pressures of several hundred kilobars or more; if correct, this implies core formation during or after accretion of Earth rather than during the planetesimal stage. The inferred partitioning between core and primitive mantle reflects periodic elemental properties, such as covalent radius; elements that possess covalent radii similar to those of iron are most strongly partitioned into the core. The periodic nature of inferred core/mantle partioning conflicts with models involving simple addition of siderophile elements and volatiles in a late chondritic veneer; however, this aspect is in accord with conceptual models involving an approach toward equilibrium partitioning during core formation. In one model, equilibrium melting and segregation of metal occurs continuously over a range of interior pressures and temperatures as fresh material accretes and mixes into Earth. BBSE may reflect retention of a small amount of the last dregs of metal whose composition was controlled by this "dynamic equilibrium."