Mineralogy of the Martian interior up to core‐mantle boundary pressures

Abstract

In order to determine the mineralogy of the Martian interior along a high‐temperature areotherm, multianvil experiments have been performed with a model Martian mantle composition up to 23.5 GPa. The Dreibus and Wänke [1985] Martian mantle composition yields an upper mantle that consists of olivine + clinopyroxene + orthopyroxene + garnet at pressures up to 9 GPa. Above 9 GPa, orthopyroxene is no longer present. The transition zone is marked by the appearance of γ spinel at 13.5 GPa. Up to 15 GPa, clinopyroxene and majorite coexists with β phase and/or γ spinel. By 17 GPa, clinopyroxene is entirely replaced by majorite and the modal abundance of γ spinel increases at the expense of β phase. The dominant assemblage throughout most of the transition zone is γ spinel + majorite. Two experiments completed in the perovskite stability field indicate that the lower mantle consists of Mg‐Fe silicate‐perovskite, magnesiowüstite, and majorite. CaSiO 3‐perovskite is not present in these experiments. Both the presence of a Martian lower mantle, i.e., an Mg‐Fe silicate‐perovskite bearing zone, and the phase assemblage stable in the Martian lower mantle are very sensitive to the temperature profile of the interior. A low‐temperature profile may stabilize stishovite in the lower mantle or it may lead to the absence of the lower mantle because of the higher transition pressure required for forming perovskite at lower temperatures. Regardless of the temperature profile assumed, the Martian upper mantle and transition zone will account for a larger proportion of the planet's interior than is the case for the Earth's interior because of the smaller size of Mars.