Complexity of the NWA 773 Clan: New Evidence from Lunar Olivine Gabbro NWA 6950

Abstract

NWA 6950 is a type of cumulate gabbro meteorite that displays features indicating a lunar origin. Specifically, the Fe/Mn values of olivines and pyroxenes in the meteorite suggest a lunar origin, as does the presence of Fe-Ni metal. The meteorite has also undergone intense shock metamorphism, which is evidenced by the presence of ringwoodite, tuite, and xieite (a type of chromite with a CaTi₂O₄ structure) within the shock melt veins (SMVs). The texture, mineral modal abundances, and bulk compositions (measured from the SMVs) of NWA 6950 are similar to those of the NWA 773 clan, as are the concentrations and patterns of rare-earth-elements in olivine, pyroxene, plagioclase, and phosphate. In-situ U-Pb dating of baddeleyite and phosphate in NWA 6950 has determined its crystallization age to be 3 133 ± 11 and 3 129 ± 23 Ma, which is consistent with age data provided by Shaulis et al. (2017). Further, the chronology of the NWA 773 clan appears to be at least bimodal when considering the age of NWA 3333 (3 038 ± 20 Ma; Merle et al., 2020). The tight range of ages for the NWA 773 clan at approximately 3.1 Ga coincides with a change in the eruption flux and style on the Moon. This suggests that lunar volcanism may have shifted from extrusive-dominated to intrusive-dominated at approximately 3.1 Ga, resulting in the widespread distribution of gabbro lithologies on the Moon.

Keywords

lunar rocks / geochronology / geochemistry / meteorites / shock metamorphism / magmatism / planetary geology

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