Lunar meteorite Northwest Africa 11962: A regolith breccia containing records of titanium‐rich lunar volcanism and the high alkali suite

Abstract

Northwest Africa (NWA) 11962 is a lunar regolith breccia composed of a wide range of different clasts. The lunar origin of this meteorite is confirmed by oxygen isotope analysis and the Fe/Mn ratio in pyroxene and olivine. In the present study, the clasts and the matrix of NWA 11962 are characterized by optical and electron microscopy along with electron microprobe analyses and micro‐Raman spectroscopy. The meteorite has a glassy impact melt matrix, which accounts for 35% of the surface area in the two thin sections examined, and which contains a very large variety of different lithic clasts, monomineralic clasts, and glass fragments. The presence of volcanic and impact‐related glass spherules led to the classification of this meteorite as a regolith breccia. Lithic clasts include numerous fragments of quartz monzogabbro and lunar felsite, which are quite rare lithologies in the lunar alkali suite. However, the most abundant components in the breccia are gabbroic clasts. The mineral chemistry of the pyroxenes in the gabbroic clasts and the chemistry of various types of glass fragments in the breccia indicate an origin of the regolith from an area with low‐Ti to high‐Ti mare basalt volcanism. In addition to the peculiar petrographic characteristics of NWA 11962, the possible pairing relationships with other lunar meteorites are discussed.

Geochemistry of lunar meteorite Northwest Africa 11962 and its potential source region/crater in the Procellarum KREEP terrane

“…Table 1 further shows the INAA results of paired lunar meteorites NWA 4472/4485 (from Korotev et al., 2009), which are very similar to NWA 11962 for many elements. This is particularly striking for Fe, Th, and the REEs, suggesting a launch pairing relation (see Bechtold et al., 2021), that is, it suggests that these meteorites were launched from the same impact event on the lunar surface. The REE pattern in Fig.…”

“…Major differences between these two lunar meteorites are mainly in the elements that are most likely to be affected by terrestrial weathering, for example, Ba and Br (see Table 1). Other differences between NWA 11962 and NWA 4472/4485 are the abundant glasses and mafic minerals in NWA 11962, which are derived from low‐Ti to high‐Ti magmas (Bechtold et al., 2021). Figure 1 also shows the mean REE pattern of Apollo 15 group C impact melt breccias, with similarities to NWA 11962.…”

“…Thus, the high Sc content possibly reflects a high pyroxene content of the regolith breccia or relatively abundant occurrence of other mineral phases known to contain appreciable amounts of Sc, for example, zircon or ilmenite. Both minerals were identified as important accessory minerals in thin sections of NWA 11962 (Bechtold et al., 2021). The abundance of these accessory minerals may be the result of a relatively higher contribution of mare basalt or alkali suite rocks.…”

“…(2021) based on the investigation of two thin sections, NWA 11962 consists mainly of large gabbroic clasts and smaller clasts of the lunar alkali suite and high‐Mg suite. The mafic silicates in the gabbroic clasts are not Mg‐rich and the plagioclase composition ranges from An 83 to An 87 (table 1 in Bechtold et al., 2021); therefore, they are most likely derived from a mare basalt source rock. A source rock of the Mg‐Suite would by indicated by Mg‐rich mafic silicates and plagioclase >An 90 (Shearer et al., 2015).…”

“…Details on the lunar origin, classification, and a petrographic description of the meteorite can be found in Bechtold et al. (2021). In this study, we discuss potential source regions with the aim to identify young lunar impact craters as possible source craters that are located within these regions.…”