“Ground truth” occurrence of Pink Spinel Anorthosite (PSA) as clasts in lunar meteorite Northwest Africa (NWA) 15500: Chemical evidence for a genetic relationship with lunar highlands Mg-suite and formation by magma–wallrock interactions

Abstract

Pink spinel anorthosite (PSA), a distinctive plagioclase and spinel-rich lithology (spinel >20%) observed on the lunar surface by the Moon Mineralogy Mapper (M³) imaging spectrometer, has sparked considerable interest in understanding magmatic processes on the Moon that cannot be explained by the well-established lunar magma ocean paradigm. Competing ideas on the PSA-forming mechanisms have invoked either (1) impact melting of troctolitic source rocks on the lunar surface or (2) magma–wallrock interactions between anorthositic crust and Mg-suite parental melts, but have been difficult to evaluate given the lack of ground truth samples. Here, we investigate the textures and mineral compositions of seven PSA clasts in lunar meteorite Northwest Africa (NWA) 15500, and the bulk trace element compositions of a PSA clast separate and NWA 15500 host lithologies A and B. Our findings suggest derivation of PSA from an incompatible-element-poor source and are consistent with PSA representing an Mg-suite lithology genetically related to pink spinel troctolites that reflects increased degrees of crustal assimilation during magma–wallrock interactions, and a sourcing of PSA far from the Procellarum KREEP Terrane. Excavation of PSA material was followed by multiple, subsequent localized impact events, resulting in the formation of Lithologies A and B.