Overturn of ilmenite-bearing cumulates in a rheologically weak lunar mantle

Kurzfassung

The accretion of the debris resulting from the giant collision that formed the Earth‐Moon system caused the formation of a deep lunar magma ocean. The initial composition profile of the Moon results from the solidification of this magma ocean. One of the late solidification products is a thin layer containing ilmenite, which is highly enriched in titanium and forms over a depth of about 40–80 km. This layer is much denser than the underlying mantle, it tends thus to overturn and sink toward the core. The overturn hypothesis explains several features of the evolution of the Moon. However, the strong temperature dependence of the viscosity makes the part of the mantle where this titanium‐rich layer solidifies very stiff and its overturn very difficult. In this work, we used numerical simulations to infer which deformation properties the mantle needs to have for the overturn to be possible. We found that the overturn requires a significantly weaker and less temperature dependent viscosity than usually thought. The required low viscosity and its weak temperature dependence can be explained by the presence of water and residual melt in the Moon and/or by the effects of nonlinear deformation dependent both on temperature and stress.