The Consequences Of Fractional Crystallization Of The Basal Magma Ocean On Present-Day Mantle Structure

Terrestrial planets evolve through stages of large-scale melting, or magma oceans, due to the energy release during accretion and differentiation. Previous work shows that a FeO-enriched molten layer or basal magma ocean (BMO) is stabilized at the core-mantle boundary for a few billion years. The BMO itself is expected to freeze by fractional crystallization (FC) because it cools very slowly, FC always yield a highly iron-enriched BMO and last stage cumulates. Other crystallization mode could be dominated and has not yet been systemically explored. To explore the fate of the BMO cumulates in the convecting mantle, we explore 2D geodynamic models with a moving-boundary approach. Flow in the mantle is explicitly solved, but the thermal evolution and related crystallization of the BMO are parameterized. The composition of the crystallizing cumulates is self-consistently calculated in the FeO-MgO-SiO2 ternary system according to Boukaré et al. (2015). In some cases, we also consider the effects of Al2O3 on the cumulate density profile. We then investigate the entrainment and mixing of BMO cumulates by solid-state mantle convection over billions of years as a function of BMO initial composition and volume, BMO crystallization timescales, distribution of internal heat sources, and mantle rheological parameters (Ra# and activation energy). For all our model cases, we find that most of the cumulates (first ~90% by mass) are efficiently entrained and mixed through the mantle. However, the final ~9% of the cumulates are too dense to be entrained - either fully or partially - and rather remain at the base of the mantle as a strongly FeO-enriched solid layer. We highlight that this inevitable outcome of BMO fractional crystallization is inconsistent with the geophysical constraints. Our results suggest that a BMO was either very small initially, or did not crystallize by FC. An alternative mode of crystallization may be driven by efficient reaction between a highly-enriched last-stage BMO with the overlying mantle due to chemical disequilibrium.