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Northwest Africa 482: A crystalline impact‐melt breccia from the lunar highlands
Author(s) -
Daubar Ingrid J.,
Kring David A.,
Swindle Timothy D.,
Jull A. J. Timothy
Publication year - 2002
Publication title -
meteoritics and planetary science
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.09
H-Index - 100
eISSN - 1945-5100
pISSN - 1086-9379
DOI - 10.1111/j.1945-5100.2002.tb01164.x
Subject(s) - breccia , geology , geochemistry , plagioclase , geology of the moon , meteorite , pyroxene , impact crater , anorthosite , population , basalt , partial melting , olivine , astrobiology , paleontology , quartz , physics , demography , sociology
— 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 ≲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.