Petrology and shock history of hybrid lunar feldspathic–troctolitic breccia Northwest Africa 11515

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Y. Li, P. J. A. McCausland, R. L. Flemming, G. R. Osinski

MAPS, Version of Record online: 09 January 2025

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“Lunar impact breccia meteorites contain clasts from unknown lunar regions, including areas not studied by past missions. These meteorites offer a unique opportunity to expand our knowledge of the Moon’s crustal and mantle composition and processes. The recently classified lunar meteorite Northwest Africa (NWA) 11515 is a moderately shocked feldspathic breccia with anorthite plagioclase and mafic minerals. In this work, we report the shock history of lithic clasts using 2-D micro-X-ray diffraction, detailed mineralogy from micro-X-ray fluorescence, and electron probe microanalysis. NWA 11515 shows moderately shocked anorthite and highly shocked olivine and pyroxene. The plagioclase composition is invariant (An96.4 ± 0.7, n = 52), with variable mafic clasts overlapping Mg- and FAN-suite lithologies (Mg# 84.5 to 45.6 for olivine; Mg# 85.6 to 32.2 for pyroxene), similar to KREEP-depleted troctolites in Allan Hills A81005. Spinel-group oxides vary from aluminous spinel to chromite and ulvöspinel. We also observed slow-cooled augite Ca-poor pyroxene exsolution clasts and fast-quenched fine-grained anorthite–olivine co-crystallized clasts (<5 μm), indicating different cooling histories. Combining petrological observations with published geochemical data, we show NWA 11515 has the mixed lithology of ferroan anorthosites with KREEP-poor magnesian rock fragments. With shock analysis, the materials are likely from a crater with minimum size of 7 km. Finally, we examined the published geochemical data for other lunar meteorites and hypothesize that other typical feldspathic breccias could contain magnesian clasts, suggesting the subdivision of typical feldspathic breccia into magnesian clast-hosting breccia and ferroan feldspathic breccia. This implies that non-KREEP magnesian magmatism might be more widespread in the post-LMO era on lunar highlands.”

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