The participation of ilmenite-bearing cumulates in lunar mantle overturn

Abstract

The ilmenite-bearing cumulates (IBC) formed from the solidification of the lunar magma ocean are thought to have significantly affected the long-term evolution of the lunar interior and surface. Their high density is considered to trigger Rayleigh–Taylor instabilities which allow them to sink into the solidified cumulates below and drive a large-scale overturn in the lunar mantle. Knowledge of how the IBC participate in the overturn is important for studying the early lunar dynamo, chemistry of surface volcanism, and the existence of present-day partial melt at the lunar core–mantle boundary. Despite early efforts to study this process as Rayleigh–Taylor instabilities, no dynamical models have quantified the degree of IBC sinking systematically. We have performed quantitative 2-D geodynamical simulations to measure the extent to which IBC participate in the overturn after their solidification, and tested the effect of a range of physical and chemical parameters. Our results show that IBC overturn most likely happened when the magma ocean had not yet fully solidified, with the residual melt decoupling the crust and IBC, resulting in 50–70% IBC sinking. Participation of the last dregs of remaining magma ocean melt is unlikely, leaving its high concentrations of radiogenic elements close to the surface. Our simulations further indicate that foundered IBC can stay relatively stable at the core–mantle boundary until the present day, at temperatures consistent with the presence of a partially molten zone in the deep mantle as inferred from geophysical data. 30–50% of the primary IBC remain at shallow depths throughout lunar history, enabling their assimilation by rising magma to form high-Ti basalts.