Constraining the Protoliths of the Apollo Granulite Suite

Abstract The Apollo granulite suite represents the metamorphosed products of impact-contaminated polymict and monomict lunar breccias. We combine bulk and mineral major and trace element systematics with noble gas isotopes to constrain the highland lithologies that contributed to the feldspathic granulite suite protoliths. Ferroan anorthosites dominate the protolith of the ferroan granulite subtypes, whereas a KREEP-poor Mg-rich lithology dominates the protolith of the magnesian granulite. This magnesian lithology, while compositionally similar to Apollo Mg-suite rocks in major elements, is comparably poor in incompatible trace elements. Similar magnesian lithologies have been identified from granulites sampled by lunar meteorites and at the Chang’e 5 landing site. This adds to the body of evidence that a KREEP-poor Mg-suite lithology represents an important rock type within the lunar crust that was not sampled in a pristine form by the Apollo missions. Granulites have a range of noble gas systematics with contributions from solar wind and cosmogenic sources. Samples with a strong solar contribution indicate that they were formed from regolith-rich protoliths with components that had spent significant time at the lunar surface. Solar-wind-poor samples either indicate a protolith with contribution from regolith with limited exposure to the lunar surface or were sourced at depth where such regolith components are absent. There is no correlation between ferroan/magnesian subtypes and near-surface exposure duration. This indicates that granulites were formed from a range of protoliths and highlights the importance of the granulites for expanding the range of lunar highland lithologies, helping to place important constraints for lunar differentiation and crust building.