Lunar Cumulate Mantle Overturn: A Model Constrained by Ilmenite Rheology

Lunar cumulate mantle overturn has been proposed to explain the abundances of TiO2 and heat-producing elements (U, Th, and K) in the source region of lunar basalts. Ilmenite-bearing cumulates (IBCs) that were formed near the end of lunar magma ocean solidification are the driving force for overturn. IBCs are enriched with dense TiO2 and FeO contents and have lower viscosity and solidus than those of the underlying lunar cumulate mantle. We investigate the effects of temperature- and ilmenite-dependent mantle rheology on the dynamic process of lunar cumulate mantle overturn and conditions for long-wavelength downwellings of an IBC layer in a 3-D spherical geometry. Our results show that the ilmenite-induced rheological weakening is necessary to decouple the IBC layer from the top stagnant lid and facilitate overturn. Models with IBC viscosity derived from the experimental scaling can only produce short-wavelength downwellings (spherical harmonic degree >3) and show an overturn timescale more than 100Ma. A viscosity of the IBC layer at least 10(-4) lower than that of the ambient mantle can produce the long-wavelength (spherical harmonic degree 3) downwellings in similar to 10Ma and even a hemispheric downwelling. Such low IBC viscosity requires additional weakening mechanisms, such as remelting or/and water enrichment. During the overturn, the cold downwellings displace upward the materials from hot lower mantle and produce partial melting in upper mantle, which may serve as a viable mechanism for early lunar magmatisms. The settling of cold downwellings on the core-mantle boundary stimulates a transient high heat flux, which may contribute to generating an early lunar dynamo event.