Magma ocean fractional crystallization and cumulate overturn in terrestrial planets: Implications for Mars

Crystallization of a magma ocean on a large terrestrial planet that is significantly melted by the energy of accretion may lead to an unstable cumulate density stratification, which may overturn to a stable configuration. Overturn of the initially unstable stratification may produce an early basaltic crust and differentiated mantle reservoirs. Such a stable compositional stratification can have important implications for the planet's subsequent evolution by delaying or suppressing thermal convection and by influencing the distribution of radiogenic heat sources. We use simple models for fractional crystallization of a martian magma ocean, and calculate the densities of the resulting cumulates. While the simple models presented do not include all relevant physical processes, they are able to describe to first order a number of aspects of martian evolution. The models describe the creation of magma source regions that differentiated early in the history of Mars, and present the possibility of an early, brief magnetic field initiated by cold overturned cumulates falling to the core-mantle boundary. In a model that includes the density inversion at about 7.5 GPa, where olivine and pyroxene float in the remaining magma ocean liquids while garnet sinks, cumulate overturn sequesters alumina in the deep martian interior. The ages and compositions of source regions are consistent with SNC meteorite data.