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

Abstract

Abstract-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 40Ar-39Ar system: one at -3750 Ma, which completely reset the K-Ar system, and one at :::;2400 Ma, which caused only partial degassing. These events could represent, respectively, crystallization ofthe 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 ofweathering 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.

Osmium isotope and highly siderophile element systematics of lunar impact melt breccias: Implications for the late accretion history of the Moon and Earth

“…This is due to the lack of a KREEP component in the NWA482 melt and to the fact that it represents a feldspathic melt that was produced by an impact into a much different crustal terrain than the Apollo 14 and 17 melts described above (Korotev et al, 2006). The impact event that generated the impact melt breccia occurred at $3.75 Ga (Daubar et al, 2002). Following that impact event, the rock was shocked in another, less intense impact event.…”

“…The complete stone had a mass of about 1 kg and represents 3% of the total mass of known lunar meteorites. It is a very fine-grained crystalline impact melt breccia with highlands affinities (Grossman and Zipfel, 2001;Daubar et al, 2002). The matrix and the clast population are both highly feldspathic (Fig.…”

Constraining the source regions of lunar meteorites using orbital geochemical data

“…Mare basalts, KREEP, and Mg‐suite lithologies are absent (Daubar et al. ; Korotev et al. ; Warren et al.…”

Regolith history of lunar meteorites

“…The sample was crushed to a grain size <750 µm to obtain average bulk material. We measured two splits of 22.31 mg and 22.80 mg. Daubar et al (2002) concluded that NWA 482 is a crystalline impact melt breccia from the highlands of the Moon, and furthermore, that its short terrestrial age, combined with the unique lithology, preclude any pairing with other NWA lunar meteorites. The chemical composition was measured by .…”