Petrogenesis of lunar granulitic breccia meteorites Northwest Africa 15062 and 15063

Abstract

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.