Weakness of ilmenite revealed by new rheological measurements with implications for lunar cumulate mantle overturn

Introduction: Several lines of evidence suggest there was once a deep lunar magma ocean [1]. Experimental work [2] and studies of terrestrial layered intrusions [3] indicate that as the magma ocean crystallized the composition of the liquid evolved from a Mg-rich picritic liquid to an Fe-rich felsic liquid saturated in plagioclase, clinopyroxene, and ilmenite [4]. Because the intermediate and late stage pyroxene and ilmenite (FeTiO3) minerals were more Fe-rich than the olivine cumulates that preceded them they were denser than the early cumulates, which led to a gravitationally unstable density stratification. It has been argued that the gravitational instability was relieved by solid-state cumulate overturn [5,6], where early ultramafic cumulates rose to the top displacing the denser layer of ilmenite-bearing cumulates that crystallized late at the base of lunar crust.