Partitioning behavior of moderately siderophile elements in Ni-rich systems: Implications for the earth and moon.

Abstract

Several scenarios set forth to explain the siderophile element abundance patterns in the mantles of the Earth and the Moon involve the segregation of Ni-rich metal to the cores of those bodies under oxidizing conditions. To test these models, the partition coefficients of Ni, Co, Mo, W, P, and Ga between basaltic liquid, Ni-rich metal, and Ni-rich sulfide were experimentally determined under a wide range of oxygen fugacities. The partition coefficients are then used in mass balance calculations to test these scenarios involving oxidizing conditions during the formation of both the Earth's and the Moon's cores. The results show that the siderophile element pattern in the Earth's mantle is consistent with a late stage segregation of a small fraction of metal that consists of approximately 70% Ni at low degrees of partial melding of the silicates. This result is not consistent with the early Earth being substantially molten as the result of a giant impact to form the Moon. The results for the Moon show that the siderophile element pattern in the lunar mantle is consistent with the segregation of a small Ni-rich core at high degrees of melting of the silicates if some elements are initially depleted by some other process--presumably volatility during a giant impact. The high degree of partial melting of silicates is consistent with the postulated magma ocean on the Moon.