Magma ocean solidification: consequences for mantle dynamics and evolution

Abstract:

Global-scale melting expected to follow the largest accretionary impacts suggests that the mantle of terrestrial
bodies may have been processed through one or multiple magma oceans before reaching a solid state. Several studies
have discussed how the fractional crystallization of a magma ocean can establish a primordial compositional
stratification that may represent a suitable initial condition to model the thermal evolution of the interior as
well as the formation, mixing, and sampling through partial melting of geochemical reservoirs. Fractional
crystallization is expected to produce a compositional stratification characterized by a progressive enrichment in
heavy incompatible elements from the core-mantle boundary to the surface. Such configuration is gravitationally
unstable, it may cause the overturn of the mantle and the formation of a stable chemical layering. This scenario is
successful in explaining the generation of the Moon's mare basalts as a consequence of the convective instability
of overturned ilmenite-bearing cumulates and the preservation of Mars' geochemical reservoirs as required by the
isotopic characteristics of the SNC meteorites. However, it also has a few consequences that can be difficult to
reconcile with the subsequent evolution of the interior such as the rapid formation of a stagnant lid preventing
the uppermost dense cumulates from sinking into the mantle or the difficulty for thermal convection to start
because of the strength of the stable compositional gradient established after the overturn. Using numerical
simulations of thermo-chemical convection, we illustrate the above issues and their consequences for the thermal
evolution of a Mars-like planet. We also present recent results showing that, under a variety of conditions,
solid-state convection can start mixing the mantle well before the overlying magma ocean has completely solidified,
thereby partly or completely erasing the effects of its fractional crystallization.