The geological context of Lunar meteorites

Calzada-Diaz, Abigail (2017) The geological context of Lunar meteorites. Doctoral thesis, Birkbeck, University of London.

Abstract

Meteorites are ejected from random areas of the lunar surface and therefore provide geological information about the Moon far from areas sampled during the Apollo and Luna programs. However, unlike those samples, the exact launch locations and geological setting of lunar meteorites are unknown, limiting greatly the knowledge we can obtain from them. During this project the launch location of lunar meteorites were studied using both laboratory analytical techniques and remote sensing instruments. This approach enabled me to provide meteorites with a geological context, increasing the geological knowledge it is possible to obtain from them. The lunar feldspathic meteorites Miller Range (MIL) 90036 and MIL 090070 bulk-rock compositions and mineral chemistries were studied. MIL 090036 is a feldspathic immature regolith breccia (26.72 wt.% Al2O3, 5.27 wt.% FeO) that exhibits a Th-enrichment (1.89 ppm Th). This could be related to a high alkali component that has been observed in impact melt breccia clasts in this meteorite, but also it could be related to a KREEP component that, although it was not observed in the particular sample studied, it can be present in the bulk meteorite. Most of the provenance results show that the regolith from the surroundings of the Procellarum KREEP Terrane (PKT) in the nearside are the most similar to the composition of this meteorite and therefore the likely source location for MIL 090036. MIL 090070 is a feldspathic immature regolith breccia with bulk-rock compositions of 30.72 wt.% Al2O3, 3.77 wt.% FeO and a low Th abundance (0.44 ppm). The geochemistry of MIL 090070 may be formed by a mixture of ferroan and magnesian anorthositic rocks, although other possible explanation could be that they were all essentially ferroan-derived and that this rock types are more related to high-magnesium suite that previously thought. MIL 090070 was probably launched from the farside of the Moon as suggested by its compositional differences when compared to Apollo 16 regolith breccias, including the lack of KREEP. The present work has studied the possible launch locations of 37 lunar meteorites (67 individual stones) using bulk-rock FeO, TiO2 and Th compositions and the 2-degree LP-GRS dataset from Prettyman et al. 2006. My method compares the composition of both the analytical and the remote sensing measurements. The outcome is a shapefile that allows for a correct visualization in ArcGIS™. Results obtained suggest that differences in KREEP components between the PKT and other areas of the Moon are not the only indicator of a heterogeneous mantle. This study also indicates that ferroan anorthosites are widely distributed with patches occurring within the central farside highlands. The large number of feldspathic meteorites that contain both magnesian and ferroan anorthosites compared to those that contain only one or the other, could indicate that these rock types are genetically related and that the wide Mg# distribution in FAN and MAN could be produced by serial magmatism. If this is correct, some of the ferroan anorthositic lithologies observed within lunar samples would not be the direct product of the magma ocean crystallization.

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