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We present petrology and mineralogy for two lunar granulitic breccia meteorites that were detected in Northwest Africa (NWA), the samples NWA 15062 and NWA 15063. The fragments primarily consist of plagioclase and olivine mineral clasts, with minor amounts of anorthosite clasts and one troctolite clast. The anorthosite clasts are dominated by plagioclase/maskelynite, with minor olivine and pyroxene. A troctolite clast, composed of olivine and maskelynite, occurs in NWA 15063. The olivine clasts display mosaic extinction and usually have a homogeneous Mg‐rich composition. However, all olivine mineral clasts exhibit two distinct ranges of their major element composition (Mg#: 85–88 and 77–78, respectively). Large individual plagioclase clasts show heterogeneous compositions (Ab content: 2.5–4.8) and have different Raman peak positions in different domains. The matrix of the meteorites appears semitransparent and is composed of olivine and pyroxene aggregates associated with maskelynite, constituting a granoblastic texture. Pyroxenes of the matrix are dominantly enstatites, associated with a few augites. Both meteorite samples exhibit shock‐induced melt veins ranging from 50 to 200 μm width. These melt veins traverse the entire samples and contain rare, very fine‐grained (2–3 μm) Mg‐rich olivine clasts (Mg# = 90–93) and mafic silicate glass. Some Cr‐spinel grains exhibit slight compositional zonation, characterized by a magnesium‐rich core (Mg# = 56, Cr# = 23) and Cr‐rich rims (Mg# = 50, Cr# = 28), with decomposition at the edges. The significantly differing Mg# contents of the mafic silicate minerals in the matrix, lithic clasts, and mineral clasts of the two meteorites indicate a diverse origin of the clasts. Based on their petrology, mineral chemistry, and bulk composition, NWA 15062 and NWA 15063 are classified as anorthositic troctolitic granulitic polymict breccia. Textural evidence suggests that the parent rocks of NWA 15062 and NWA 15063 were affected by high pressure of up to 30 GPa during impact‐induced shock metamorphism, causing crystal structure deformation in olivine and the transformation of plagioclase to maskelynite. During cooling from peak temperatures of 1600–1700°C, the coarse‐grained maskelynite mineral clasts were partially devitrified, and the granoblastic texture of the matrix was developed. Mg‐rich anorthosite was formed before this shock event. Cr‐spinel was formed in a troctolitic melt, which was probably differentiated after the crystallization of anorthite and magnesium‐rich olivine. However, the possibility of the formation of the Mg‐rich melt through interaction with the lunar anorthositic crust cannot be ruled out. The meteorite NWA 15062/15063 strongly resembles the textural, chemical, and mineralogical characteristics of the NWA 5744 meteorite group. Therefore, we interpret the two samples as a new member of the NWA 5744 meteorite group.
We present petrology and mineralogy for two lunar granulitic breccia meteorites that were detected in Northwest Africa (NWA), the samples NWA 15062 and NWA 15063. The fragments primarily consist of plagioclase and olivine mineral clasts, with minor amounts of anorthosite clasts and one troctolite clast. The anorthosite clasts are dominated by plagioclase/maskelynite, with minor olivine and pyroxene. A troctolite clast, composed of olivine and maskelynite, occurs in NWA 15063. The olivine clasts display mosaic extinction and usually have a homogeneous Mg‐rich composition. However, all olivine mineral clasts exhibit two distinct ranges of their major element composition (Mg#: 85–88 and 77–78, respectively). Large individual plagioclase clasts show heterogeneous compositions (Ab content: 2.5–4.8) and have different Raman peak positions in different domains. The matrix of the meteorites appears semitransparent and is composed of olivine and pyroxene aggregates associated with maskelynite, constituting a granoblastic texture. Pyroxenes of the matrix are dominantly enstatites, associated with a few augites. Both meteorite samples exhibit shock‐induced melt veins ranging from 50 to 200 μm width. These melt veins traverse the entire samples and contain rare, very fine‐grained (2–3 μm) Mg‐rich olivine clasts (Mg# = 90–93) and mafic silicate glass. Some Cr‐spinel grains exhibit slight compositional zonation, characterized by a magnesium‐rich core (Mg# = 56, Cr# = 23) and Cr‐rich rims (Mg# = 50, Cr# = 28), with decomposition at the edges. The significantly differing Mg# contents of the mafic silicate minerals in the matrix, lithic clasts, and mineral clasts of the two meteorites indicate a diverse origin of the clasts. Based on their petrology, mineral chemistry, and bulk composition, NWA 15062 and NWA 15063 are classified as anorthositic troctolitic granulitic polymict breccia. Textural evidence suggests that the parent rocks of NWA 15062 and NWA 15063 were affected by high pressure of up to 30 GPa during impact‐induced shock metamorphism, causing crystal structure deformation in olivine and the transformation of plagioclase to maskelynite. During cooling from peak temperatures of 1600–1700°C, the coarse‐grained maskelynite mineral clasts were partially devitrified, and the granoblastic texture of the matrix was developed. Mg‐rich anorthosite was formed before this shock event. Cr‐spinel was formed in a troctolitic melt, which was probably differentiated after the crystallization of anorthite and magnesium‐rich olivine. However, the possibility of the formation of the Mg‐rich melt through interaction with the lunar anorthositic crust cannot be ruled out. The meteorite NWA 15062/15063 strongly resembles the textural, chemical, and mineralogical characteristics of the NWA 5744 meteorite group. Therefore, we interpret the two samples as a new member of the NWA 5744 meteorite group.
The enstatite chondrite class is known to have complex thermal histories, often interpreted to include impact melting and shock metamorphism. Highly equilibrated (type 6) EH group enstatite chondrites are rare and thought to have formed through collisional heating. We studied two EH6 chondrites, NWA 7976 and NWA 12945, for their textural, chemical, and mineralogical characteristics. The samples we studied contain subhedral to anhedral grains of enstatite and plagioclase, suggesting solid‐state recrystallization. They show low degrees of shock and no evidence of shock melting. Additionally, the ubiquitous occurrence of daubréelite exsolution lamellae in troilite and the Ni content of schreibersite suggest slow cooling at greater burial depths in the parent body, rather than rapid cooling as a result of an impact event. Based on the characteristics and scarcity of type 6 EH chondrites, and the ubiquitous shock effects and melt rocks in the enstatite chondrite class, we conclude that the unshocked NWA 7976 and NWA 12945 were formed by heat derived from impact melt sheets, analogous to contact metamorphism.
The Apollo granulite suite represents the metamorphosed products of impact-contaminated polymict and monomict lunar breccias. We combine bulk and mineral major and trace element systematics with noble gas isotopes to constrain the highland lithologies that contributed to the feldspathic granulite suite protoliths. Ferroan anorthosites dominate the protolith of the ferroan granulite subtypes, whereas a KREEP-poor Mg-rich lithology dominates the protolith of the magnesian granulite. This magnesian lithology, while compositionally similar to Apollo Mg-suite rocks in major elements, is comparably poor in incompatible trace elements. Similar magnesian lithologies have been identified from granulites sampled by lunar meteorites and at the Chang’e 5 landing site. This adds to the body of evidence that a KREEP-poor Mg-suite lithology represents an important rock type within the lunar crust that was not sampled in a pristine form by the Apollo missions. Granulites have a range of noble gas systematics with contributions from solar wind and cosmogenic sources. Samples with a strong solar contribution indicate that they were formed from regolith-rich protoliths with components that had spent significant time at the lunar surface. Solar-wind-poor samples either indicate a protolith with contribution from regolith with limited exposure to the lunar surface or were sourced at depth where such regolith components are absent. There is no correlation between ferroan/magnesian subtypes and near-surface exposure duration. This indicates that granulites were formed from a range of protoliths and highlights the importance of the granulites for expanding the range of lunar highland lithologies, helping to place important constraints for lunar differentiation and crust building.
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