Timing of Cumulate Compaction and Cumulate Overturn

Magma ocean (MO) crystallization can generate gravitationally unstable Fe-Mg chemical stratification in terrestrial planets and can lead to subsequent solid-state mantle overturn. While only an ideal limiting case, fractional crystallization followed by mantle overturn can explain important geochemical features of both the Moon and Mars. However, recent models of MO cumulate dynamics show that thermally and/or compositionally driven convection in the cumulate can also occur prior to complete MO solidification. The timing of convection in the cumulate is crucial as it controls the degree of mantle mixing at the onset of mantle dynamics.

Early convection in the cumulate occurs if cumulate viscosity is low enough or the time of solidification is long enough, for the convective timescale to be shorter than the crystallization timescale. We argue that the amount of retained melt in the cumulate is the main control on cumulate viscosity. Using a numerical compaction model, we relate the timescale of MO solidification with the amount of retained melt in the cumulate. We then investigate the links between the time of MO solidification (i.e., MO cooling rate), the degree of differentiation governed by the amount of interstitial melt and the timing of early mixing by cumulate convection.

The model shows that the lower the interstitial melt fraction is, i.e.., the closer to fractional crystallization the mantle is, the later the onset of cumulate convection is. Efficient chemical differentiation in the MO implies a viscous strengthening of the cumulate that preserves the new formed geochemical reservoir from mantle mixing. The formation of a strong compositional layering does not favor its subsequent early mixing by mantle convection. Highly differentiated body such as the Moon, are less likely to experience early convection of the cumulate.