Small-Scale Overturn of High-Ti Cumulates Promoted by the Long Lifetime of the Lunar Magma Ocean

Overview & Synopsis

The fractional crystallization of the lunar magma ocean (LMO) results in a gravitationally unstable layering, with dense Fe- and Ti-oxides overlying lighter mafic cumulates. Due to their high density, these are prone to overturn via Rayleigh-Taylor instability. However, this instability competes with the downward growth of the cold and stiff stagnant lid, which tends to trap the cumulates preventing their overturn. The fate of high-Ti cumulates (HTC) plays a major role in several aspects of the Moon's history, including mare volcanism, early magnetism, and the presence of a partially molten layer at the core-mantle boundary (CMB). To assess the extent of the overturn of HTC, we use 2D simulations of thermo-chemical mantle convection in the presence of a solidifying LMO. The long lifetime of the magma ocean caused by the insulating effect of the plagioclase crust delays the growth of the stagnant lid and promotes the onset of solid-state convection during magma ocean solidification. Both phenomena favor a high degree of mobilization of the HTC. Independent of the rheology of the mafic and HTC, the overturn is always characterized by small-scale instabilities and is completed within ∼300-600 Myr. High-Ti material accumulates at the CMB where it undergoes partial melting until present-day, in agreement with the existence of a deep, partially molten layer inferred from geodetic data. Part of the overturned cumulates is entrained by mantle flow and can participate in secondary melting until ∼1 Ga, in agreement with the age of high-Ti mare basalts.