Evolution of the lunar crust recorded in the meteoritic feldspathic regolith breccias Northwest Africa 10291 and 11182: insights into the heterogeneity and petrogenesis of crustal lithologies using petrology and mineral chemistry

• Main Author: Boyle, Shannon
• Format: Text
• Language: unknown
• Published: No Publisher Supplied 2019
• Subjects: South Pole; Aitken; South pole
• Online Access: https://dx.doi.org/10.7282/t3-ngqe-8276; https://rucore.libraries.rutgers.edu/rutgers-lib/61694/

The geology of the Moon represents nearly a continuous geological record from its formation during the giant impact that resulted in the Earth-Moon system, to its state today. Therefore, it is a prime location for understanding one path of planetary evolution. Lunar meteorites are on average samples of rock and regolith from random areas on the lunar surface. As such, they represent our best available tools to study the crustal evolution of the Moon because they provide data on lunar petrology, geochemistry, and chronology, as well as data on the variety of existing lunar lithologies different from Apollo and Luna mission sample return sites. We investigated two lunar feldspathic regolith breccias found in 2017, Northwest Africa (NWA) 10291 and NWA 11182, to understand their petrogenetic origin and, more broadly, the evolution of lithologies present in unsampled areas of the Moon and place constraints on lunar crustal evolution in these areas. Both meteorites experienced terrestrial weathering, NWA 10291 less so than NWA 11182. NWA 10291 contains a variety of lithic clasts from the lunar highlands ranging from anorthosites to norites and gabbronorites, to pyroxenites. The matrix is mostly dominated by pyroxene grains that mostly have a very low Ti-basalt signature. In addition, NWA 10291 contains mineral grain fragments in the matrix of zircon, silica, and Na- and K-rich plagioclase. The bulk composition of other meteorites paired with NWA 10291, however, show low Th and low FeO content, indicating that this meteorite may have originated from the lunar farside feldspathic highland terrane (FHT), within the vicinity of South Pole Aitken basin. In contrast, most clasts in NWA 11182 are granulites, impact melt breccias, or recrystallized impact melt. They show a large compositional variation in Mg# ranging from hyperferroan (Mg# ~40) to highly magnesian (Mg# 80); however, a bimodal distribution between ferroan anorthosites (FAN) and magnesium-anorthosites (MAN) was identified using the Lilliefors test at the 95 and 99% confidence level. Judging from the lack of KREEP and presence of both FAN and MAN lithologies, NWA 11182 most likely originated from a region far from the Procellarum KREEP Terrane, between the lunar farside highland terrane (more Mg-rich) and the nearside highland terrane (more ferroan). Future bulk composition measurements could be used to further constrain the source regions of both meteorites.