From a magma ocean to a solid mantle: implications for the thermo-chemical evolution of Mars

Several studies suggest that Mars went through an episode of Magma Ocean (MO) early in its history. When the MO crystallises, solid mantle appears. The crystallisation of this MO starts at the Core-Mantle Boundary (CMB) and continues upwards to the surface of the planet. Assuming that this process occurs by fractional crystallisation, the solid cumulates that form are progressively enriched in incompatible elements, including iron, and an unstable density stratification is developed. This stratification is thought to have resulted in a planetary-scale mantle overturn after MO crystallisation, potentially explaining the early magnetic field, crustal dichotomy and chemical heterogeneities present on martian mantle.

However, previous studies on the thermo-chemical evolution of Mars consider only fractional crystallisation of the MO, and lack the possibility of re-melting/re-freezing of material at the mantle-MO interface, before the MO is fully crystallised.

In this study we investigate the effect of re-melting/re-freezing of material at the mantle-MO interface during MO crystallisation, on the dynamics and composition of the solid mantle. We use a numerical method with the convection code StagYY. The solid mantle is represented by a 2D spherical annulus geometry, and the MO by a 0D object at top of the mantle. The boundary condition applied to the solid domain allows the parameterisation of fractional crystallisation/re-melting of material at the mantle-MO interface. We model the growth of the solid mantle from the CMB up to the surface of the planet, and we account for core cooling and the presence of an atmosphere.

We show that by taking re-melting/re-freezing of material into account, the onset of convection can start earlier in Mars history. These results bring implications for the density stratification and overturn, and to the existence of isotopically distinct reservoirs on the mantle. Moreover, our results show that the mode of convection is preferentially degree-1, which can potentially explain the crustal dichotomy.