Chloé Michaut, Jerome A. Neufeld
Journal of Fluid Mechanics
The discovery of primitive lunar anorthosites by the Apollo 11 mission was surprising. Their global distribution and purity suggested flotation of anorthite crystals over a liquid magma ocean. Flotation theory elegantly explains early formation of the Highlands and the Procellarum KREEP terrane composition, interpreted as the residual liquid from crystallization, enriched in incompatible elements. Since then, many lunar meteorites have been analyzed and the lunar surface has been studied remotely. Lunar anorthosites appear more heterogenous in composition than suggested by ferroan anorthosite samples only, contradicting the classical flotation scenario where the liquid ocean is the common source of all anorthosites. The ∼300 Ma age range for ferroan anorthosites also appears difficult to reconcile with a mostly liquid ocean. A reevaluation of the magma ocean solidification history thus seems necessary. In the low lunar gravity, efficient crystal-liquid separation may be difficult to achieve. Here, we investigate the case where a slushy crystal suspension persists throughout magma ocean solidification and the crust instead forms by extraction of melts enriched in an anorthite component. Crustal magmatism then produced the lunar anorthosites, explaining their observed diversity. The crust formation timescale is several hundreds of million years, matching the observed age range of lunar anorthosites.