Composition of the core and implications for origin of the earth.

Abstract

The density of the core is about 8 percent smaller than that of pure iron under similar P, T conditions, implying the presence of a substantial amount of light element(s). Sulphur is popularly considered to be the principal light element in the core. If so, the earth accreted about 44 percent of the primordial complement of this element. To accrete sulphur as efficiently as this, and at the same time, to account for the observed depletions by much larger factors of Na, K, Mn, Rb, F, Cs, Zn and Cl, the earth must have accreted in an environment where hydrogen was depleted relative to sulphur by a factor of about 100, compared to the solar nebula. The conditions required are restrictive and encourage evaluation of other light elements as possible components of the core, for example, oxygen. Experimental observations show that the solubility of FeO in liquid iron increases rapidly between 1, 500 and 2, 000°C. Thermodynamic extrapolation of solubility data implies solubility of 40mol.% FeO at 3, 000°C and complete miscibility of liquid Fe and FeO above 3, 500°C. Calculations show that solubility is greatly increased by high pressures and that at 2, 500°C, the liquid metal phase in equilibrium with the probable mineral assemblage in the lower mantle (FeO/(FeO + MgO) = 0.12) would contain more than 50mol.% FeO at a pressure of 300kb, and would be about 10 percent less dense than pure iron. These results imply that FeO is probably a major constituent of the earth's core. Solubility of FeO in metal may be accompanied by a large increase in oxygen fugacity of the core-mantle system relative to the situation where solution of FeO in molten iron does not occur. This effect is enhanced by the pressure-induced partial disproportionation of Fe2+ into Fe3+ + Fe0 in the lower mantle. Accordingly, the distribution of siderophile elements between mantle and core and the occurrence of oxidized species such as H2O, CO2 and Fe3+ in the mantle could result from attainment of local chemical equilibrium at high pressures between (Fe-FeO) metal and silicate phases in the more oxidizing environment prevailing during core segregation. Models of greater complexity, involving heterogeneous accretion of the earth and chemical disequilibrium between metal and silicate phases may not be necessary. Thus, if FeO indeed enters the core as suggested, rather simple models whereby the earth formed by homogeneous accretion from a mixture consisting of 10% of low-temp. oxidized primordial condensate similar to C1 chondrites (20% H2O) and 90% of devolatilized, reduced material (mainly metallic iron and magnesium silicates) can provide a satisfactory explanation of the earth's bulk composition.