Protracted Crystallization of a Dense Basal Magma Ocean

The presence of silicate melt in a thin layer at the base of Earth's mantle was proposed by Williams and Garnero (1996) to explain the ultralow-velocity zone. Recent ab initio calculations by Stixrude and Karki (2005) suggest that an Fe-enriched silicate melt will be gravitationally stable at the bottom of the mantle. Using energy balances and regional-scale numerical models of convection in the lowermost mantle, we investigate the conditions necessary for the survival of such a melt layer in a cooling earth and its implications for core-mantle thermal coupling. The lifetime of a basal melt layer τ_c is controlled by the balance between heat transfer across the solid mantle and heat capacity of the core times the temperature decrement Δ T_c required to entirely freeze the melt layer. Δ T_c depends on the phase diagram and can also change depending upon whether the layer undergoes bulk or fractional crystallization. In contrast to the short lifetime of the surface magma ocean (~ 10 \mathrm{Myr}), τ_c is plausibly of order several billion years, with longer lifetimes permitted if the layer undergoes fractional crystallization. Fluctuations of heat flow by mantle convection induce fluctuations in the rate of crystallization which in turn buffers core-mantle heat transfer due to the diffusive adjustment time of the boundary layer at the base of the solid mantle. This could help the dynamo to survive periods of low heat flow across the mantle. Fractional crystallization of the basal melt layer (and subsequent Fe enrichment) could lead to a correlated chemical evolution in the crystallizing solids which, after some time, may become too dense to be entrained by solid state convection in the overlying mantle. This denser material tends to accumulate at the base of up-welling mantle currents and is underlain by material that is naturally buffered to the solidus temperature throughout the lifetime of the melt layer, perhaps providing a simple way to explain the presence of partially molten material at the base of Earth's mantle at the present time.