Core formation in the shergottite parent body and comparison with the Earth

Abstract

The mantle of the shergottite parent body (SPB) is depleted relative to the bulk SPB in siderophile and chalcophile elements; these elements are inferred to reside in the SPB's core. Our chemical model of these depletions rests on a physically plausible process of segregation of partially molten metal from partially molten silicates as the SPB grows and is heated above silicate and metallic solidi during accretion. Metallic and silicate phases equilibrate at low pressures as new material is accreted to the SPB surface. Later movement of the metallic phases to the planet's center is so rapid that high‐pressure equilibration is insignificant. Partitioning of siderophile and chalcophile elements among solid and liquid metal and silicate determines their abundances in the SPB mantle. Using partition coefficients and the SPB mantle composition determined in earlier studies, we model the abundances of Ag, Au, Co, Ga, Mo, Ni, P, Re, S, and W with free parameters being oxygen fugacity, proportion of solid metal formed, proportion of metallic liquid formed, and proportion of silicate that is molten. Abundances of all elements except Au, Mo, and Re may be reproduced adequately with oxygen fugacity near one log‐unit below the iron‐wüstite buffer, proportion of molten silicate near 0.3, and a core composed of subequal parts of solid and liquid metal constituting approximately 30% of the mass of the SPB. The abundances of Au and Re may be reproduced using different (but still permissible) partition coefficients by invoking a small “chondritic veneer” mixed into the mantle after core formation or by assuming that small fractions of solid and liquid metal are trapped in the mantle during core formation. Molybdenum cannot be modeled adequately; problems may include inaccurate partition coefficients, inaccurate estimation of the abundance of Mo in the SPB mantle and the possibility that the mantle source of the SNC meteorites is not representative of the whole SPB mantle. In contrast to the SPB, there is at present no hypothesis capable of quantitatively reproducing elemental abundances in the earth's mantle. The contrast suggests that these two terrestrial planets may have accreted or differentiated differently, but the cause of such differences is unclear.