Onset of solid-state mantle convection and mixing during magma ocean solidification

The energy sources involved in the early stages of the formation of terrestrial bodies can induce partial or even complete melting of the mantle, leading to the emergence of magma oceans. The fractional crystallization of a magma ocean can cause the formation of a compositional layering that can play a fundamental role for the subsequent long-term dynamics of the interior and for the evolution of geochemical reservoirs. In order to assess to what extent primordial compositional heterogeneities generated by magma ocean solidification can be preserved, we investigate the solidification of a whole-mantle Martian magma ocean, and in particular the conditions that allow solid-state convection to start mixing the mantle before solidification is completed. To this end, we performed 2-D numerical simulations in a cylindrical geometry. We treat the liquid magma ocean in a parameterized way while we self-consistently solve the conservation equations of thermochemical convection in the growing solid cumulates accounting for pressure-, temperature-, and, where it applies, melt-dependent viscosity. By testing the effects of different cooling rates and convective vigor, we show that for a lifetime of the liquid magma ocean of 1Myr or longer, the onset of solid-state convection prior to complete mantle crystallization is likely and that a significant part of the compositional heterogeneities generated by fractionation can be erased by efficient mantle mixing. We discuss the consequences of our findings in relation to the formation and evolution of compositional reservoirs on Mars and on the other terrestrial bodies of the solar system. Plain Language Summary Early in their history, terrestrial planets likely had a molten rocky mantle because of the vast amount of heat released during their formation. They may have been covered by deep oceans of molten rock that are expected to solidify within a few thousand years. The solidification of a magma ocean results in a chemically-stratified mantle with light material at depth and dense, iron-rich material near the top. According to previous models, this gravitationally unstable configuration causes a global overturn, leading to a stably-stratified configuration, which may strongly prevent further creeping flow of the rocky interior, the so-called mantle convection. However, in the case of Mars, its large volcanic provinces along with the evidence for recent volcanism hint at a long-lived dynamic interior, a scenario that is difficult to reconcile with that of a stably stratified mantle. The solidification of a magma ocean can also be accompanied by the growth of a thick atmosphere of water and carbon dioxide, which hinders the cooling of the molten mantle maintaining a high surface temperature through greenhouse effect over a few million years. In this work, we show that, for a slowly solidifying magma ocean, convection can occur in the already solidified mantle and start mixing it while the overlying magma ocean is still solidifying. In contrast to the traditional scenario, no highly unstable configuration is reached at the end of the magma ocean stage. Rather, the planet is left with a partly homogeneous mantle, which can sustain long-term convection in agreement with observations.