Abstract
Lunar meteorites are the fragments of rocks that fell on Earth because of the impacts of asteroids on the Moon. Such rocks preserve information about the composition, evolutionary process, and shock history of the lunar surface. NWA 13120 is a recently discovered lunar breccia meteorite having features of strong shock, which is composed of lithic and mineral clasts in a matrix of very fine-grained (<10 μm) and recrystallized olivine-plagioclase with a poikilitic-like texture. As the most abundant lithic clasts, the crystalline impact melt (CIM) clasts can be divided into four types according to their texture and mineral composition: (1) anorthosites or troctolitic anorthosite with a poikilitic-like texture, but the mineral content is different from that of the matrix; (2) anorthosites containing basaltic fragments and rich in vesicles; (3) troctolitic anorthosite containing metamorphic olivine mineral fragments; (4) troctolitic anorthosite containing troctolite fragments. Based on the petrology and mineralogy, NWA 13120 is a lunar meteorite that was derived from the ferrous anorthosite suite (FANs) of the lunar highlands, while its texture suggests it is a crystalline impact melt breccia. In addition, we infer that the parent rock of NWA 13120 is a lunar regolith breccia enriched in glass fragments. During the shock process, at pressures of more than 20 GPa, all plagioclase fragments were transformed into maskelynites, and olivine fragments occurred metamorphism. The post-shock temperature led to the partial melting of the basaltic fragments. Subsequently, all glass with diverse components in the parent rock were devitrified and recrystallized, forming the common olivine-plagioclase poikilitic-like texture and different CIM clasts. Meanwhile, the devitrification of maskelynite formed the accumulation of a large number of plagioclase microcrystals. Therefore, NWA 13120 is a meteorite of great significance for understanding the local shock metamorphism of lunar rocks on the lunar surface.
Highlights
About 382 kg of lunar samples were obtained during the Apollo, Luna and Chang’E missions, these samples were collected only in the small areas of the near side of the Moon [1,2]
The breccia meteorite in this study, NWA 13120 has similar petrological, mineralogical, and geochemical characteristics with the lunar samples and meteorites, indicating that NWA 13120 originated from the Moon: (1) The recrystallized matrix and melted texture indicate that NWA 13120 has undergone a strong shock melting process
The bulk composition shows high content of Al2O3 (30.7 to 31.9 wt%) and poor in mafic minerals ((MgO + FeO)/Al2O3 = 0.13 to 0.15), which is consistent with the typical anorthitic lunar samples [4]. (4) The Fe/Mn ratio of NWA 13120 is 70.1, which is within the range of other anorthosite lunar rocks (60 to 80) [6,21,22]
Summary
About 382 kg of lunar samples were obtained during the Apollo, Luna and Chang’E missions, these samples were collected only in the small areas of the near side of the Moon [1,2]. More than 470 lunar meteorites have been collected (Meteoritical Society: https://meteoritical.org/, date accessed: 12 August 2021), the number is still very small. The breccias are formed from a variety of rocks, minerals, and glassy materials, and they are widely distributed on the surface of the Moon, of which anorthosite breccia meteorites account for about two-thirds of all recovered lunar meteorites (Meteoritical Society: https://meteoritical.org/, date accessed: 12 August 2021). Because of the complexity of lunar breccia composition (materials from different lunar regions, multi-stage shock sputtered materials, and complex rock fragments), it is often difficult to accurately determine the initial location of the samples. By understanding the composition of the breccia meteorites, we can constrain their source region and metamorphism (e.g., [7,8,9,10]). The shock process has altered the overall texture of the lunar meteorites, which record important information about the shock events, and provides evidence about the metamorphism of the lunar rocks after the shock (e.g., [12,13,14])
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