Magma Ocean Cumulate Overturn and Its Implications for the Thermo-chemical Evolution of Mars

Early in the history of terrestrial planets, the fractional crystallization of primordial magma oceans may have led to the formation of large scale chemical heterogeneities. These may have been preserved over the entire planetary evolution as suggested for Mars by the isotopic analysis of the so-called SNC meteorites. The fractional crystallization of a magma ocean leads to a chemical stratification characterized by a progressive enrichment in heavy elements from the core-mantle boundary to the surface. This results in an unstable configuration that causes the overturn of the mantle and the subsequent formation of a stable chemical layering. Assuming scaling parameters appropriate for Mars, we first performed simulations of 2D thermo-chemical convection in Cartesian geometry with the numerical code YACC. We ran a large set of simulations spanning a wide parameter space, by varying systematically the buoyancy ratio B, which measures the relative importance of chemical to thermal buoyancy, in order to understand the basic physics governing the magma ocean cumulate overturn and its consequence on mantle dynamics. Moreover, we derived scaling laws that relate the time over which chemical heterogeneities can be preserved (mixing time) and the critical yield stress (maximal yield stress that allows the lithosphere to undergo brittle failure) to the buoyancy ratio. We have found that the mixing time increases exponentially with B, while the critical yield stress shows a linear dependence. We investigated then Mars early thermo-chemical evolution using the code GAIA in a 2D cylindrical geometry and assuming a detailed magma ocean crystallization sequence as obtained from geochemical modeling. A stagnant lid forms rapidly because of the strong temperature dependence of the viscosity. This immobile layer at the top of the mantle prevents the uppermost dense cumulates to sink, even when allowing for a plastic yielding mechanism. The convection pattern below this dense stagnant lid is dominated by small-scale structures caused by perturbations in the chemical component. Therefore, large-scale volcanic features observed over Mars surface cannot be reproduced. Assuming that the stagnant lid will break, the inefficient heat transport due to the stable density gradient and the entire amount of heat sources above the core-mantle-boundary (CMB) lead to a strong increase of the temperature to values that exceed the liquidus. We conclude that a fractionated global and deep magma ocean is difficult to reconcile with observations. Other scenarios like shallow or hemispherical magma ocean or even another freezing mechanism, which would reduce the strength of chemical gradient need to be considered.