Vestiges of a lunar ilmenite layer following mantle overturn revealed by gravity data

The lunar crust and mantle formed through the crystallization of a magma ocean, culminating in a solid cumulate mantle with a layer of dense ilmenite-bearing cumulates rich in incompatible elements forming above less dense cumulates. This gravitationally unstable configuration probably resulted in a global mantle overturn, with ilmenite-bearing cumulates sinking into the interior. However, despite abundant geochemical evidence, there has been a lack of physical evidence on the nature of the overturn. Here we combine gravity inversions together with geodynamic models to shed light on this critical stage of lunar evolution. We show that the observed polygonal pattern of linear gravity anomalies that surround the nearside mare region is consistent with the signature of the ilmenite-bearing cumulates that remained after the global mantle overturn at the locations of past sheet-like downwellings. This interpretation is supported by the compelling similarity between the observed pattern, magnitude and dimensions of the gravity anomalies and those predicted by geodynamic models of the ilmenite-bearing cumulate remnants. These features provide physical evidence for the nature of the global mantle overturn, constrain the overturn to have occurred before the Serenitatis and Humorum basin-forming impacts and support a deep Ti-rich mantle source for the high-Ti basalts.