Northwest Africa 482: A crystalline impact-melt breccia from the lunar highlands

摘要

Northwest Africa 482 (NWA 482) is a crystalline impact-melt breccia from the Moon with highlands affinities. The recrystallized matrix and the clast population are both highly anorthositic. Clasts are all related to the ferroan anorthosite suite, and include isolated plagioclase crystals and lithic anorthosites, troctolites, and spinel troctolites. Potassium-, rare-earth-element-, and phosphorus-bearing (KREEP) and mare lithologies are both absent, constraining the source area of this meteorite to a highland terrain with little to no KREEP component, most likely on the far side of the Moon. Glass is present in shock veins cutting through the sample and in several large melt pockets, indicating a second impact event. There are two separate events recorded in the ~(40)Ar-~(39)Ar system: one at ～3750 Ma, which completely reset the K-Ar system, and one at approx.< 2400 Ma, which caused only partial degassing. These events could represent, respectively, crystallization of the impact-melt breccia and later formation of the glass, or the formation of the glass and a later thermal event. The terrestrial age of the meteorite is 8.6 ± 1.3 Ka. This age corresponds well with the modest amount of weathering in the rock, in the form of secondary phyllosilicates and carbonates. Based on terrestrial age and location, lithology, and chemistry, NWA 482 is unique among known lunar meteorites.