Moderately and slightly siderophile element constraints on the depth and extent of melting in early Mars

Abstract

The thermal history of Mars during accretion and differentiation is important for understanding some fundamental aspects of its evolution such as crust formation, mantle geochemistry, chronology, volatile loss and interior degassing, and atmospheric development. In light of data from new Martian meteorites and exploration rovers, we have made a new estimate of Martian mantle siderophile element depletions. New high pressure and temperature metal-silicate experimental partitioning data and expressions are also available. Using these new constraints, we consider the conditions under which the Martian mantle may have equilibrated with metallic liquid. The resulting conditions that best satisfy six siderophile elements-Ni, Co, W, Mo, P, and Ga-and are consistent with the solidus and liquidus of the Martian mantle phase diagram are a pressure of 14±3GPa and temperature of 2100±200K. The Martian mantle depletions of Cr and V are also consistent with metal-silicate equilibration in this pressure and temperature range if deep mantle silicate phases are also taken into account. The results are not consistent with either metal-silicate equilibrium at the surface or at the current-day Martian core-mantle boundary. Recent measurements and modeling have concluded that deep (∼17GPa or 1350km) mantle melting is required to explain isotopic data for Martian meteorites and the nature of differentiation into core, mantle, and crust. This is in general agreement with our estimates of the conditions of Martian core formation based on siderophile elements that result in an intermediate depth magma ocean scenario for metal-silicate equilibrium. © The Meteoritical Society, 2011.