An oriented lunar meteorite weighing 1,015 g was originally found by a Berber nomad in late 2000 during a desert search, probably in Algeria. It was later sold in Morocco to a group of American meteorite collectors. Although much of the fusion crust on this meteorite has been replaced by desert varnish, the interior shows little sign of terrestrial weathering. Northwest Africa 482 is a polymict, crystalline impact-melt breccia, consisting of scattered white clasts of anorthosite within a fine-grained matrix of anorthitic plagioclase (An96.3), olivine, pigeonite, and augite, with rare troilite, FeNi-metal (Ni-rich), ilmenite, and whitlockite. Besides anorthosite clasts, other clast types that are present include troctolite, anorthositic troctolite, troctolitic anorthosite, and pleonaste spinel. No KREEP, regolith, Mg-suite, or mare components have been observed (Daubar et al., 2002), and thus a lunar far side origin is considered more favorable.
The rock was melted and brecciated, and clasts were introduced during a severe shock event, possibly occurring ~3.75 b.y. ago. Isotopic composition and elemental abundance studies indicate that the impactor component in the NWA482 impact melt breccia had a bulk composition comparable to an EH chondrite (Puchtel et al., 2008). Thereafter, the rock experienced a moderate shock event, possibly occurring ~2.4 b.y. ago, which resulted in shock melting and the formation of the black, vesiculated, shock-melt veins and melt pockets in the form of quenched glass. Finally, this rock was ejected from the Moon in a separate impact event, likely responsible for some of the shock veins. This lunaite has a relatively high porosity of ~13% (Warren et al., 2005). A more comprehensive treatment of this special meteorite is maintained by the owners at LunarRock.com.
Northwest Africa 482 is similar to sample #65015 that was returned by Apollo 16 from the lunar highlands, which is thought to be a sample of ancient lunar highlands, probably 4.4–4.5 b.y. old. However, in contrast to the Apollo samples, NWA 482 is derived from a KREEP-deficient terrane, more consistent with a location on the far side of the Moon. However, since an impact-melt breccia may be formed at a significant depth beneath the large-ion-lithophile contaminated regolith, its low-Th, low-Fe signature by itself is not an adequate determinant for either a nearside or a farside origin for this type of lunar meteorite (R. Korotev).
Noble gas studies indicate that NWA 482 records the longest residence in the lunar regolith observed to date—2.07 (±0.42) b.y—which occurred at a depth of >2.8 m (>500 g/cm² [shielding depth] divided by 1.8 g/cm³ [ave. regolith density]) (Lorenzetti et al., 2005). Northwest Africa 482 has a cosmic-ray exposure age of 280 t.y., and it arrived on Earth 8.6 (±1.3) t.y. ago (Nishiizumi, 2003). According to Nishiizumi and Caffee (2010), this relatively long Moon–Earth transit time would be consistent with a launch from a depth of >5.6 m. They determined that shallower launches from depths of <1–4.7 m correspond to the shortest transit times of <0.1 m.y. Through studies of cosmogenic nuclides, its pre-atmospheric diameter was calculated to be ~11–14 cm, while the presence of 26Al is indicative of relatively low ablation, removing 1–1.5 cm from its surface.
The specimen of NWA 482 pictured above is a 0.29 g partial slice displaying intricate microbrecciation. The top photo below shows the slice from which the above specimen was removed, while the bottom photo below shows a close-up of the large shock vein within the slice.
Photos courtesy of Anne Black—Impactika Meteorites
The photos below show the complete oriented main mass exhibiting flow lines in the thin remnant crust or weathering rind. On the right is what may be the most beautiful photo taken of this awesome meteorite, showing a large cut face. A sizable portion of this rare meteorite has been donated for scientific study.
Photos courtesy of Jim Strope—www.darksideofthemoon.com
Tom Phillips has provided an exquisite look at the intricacies of this lunar impact melt breccia utilizing a thin section, which he photographed in cross polarized light at 400× using a 1/4-wave retardation filter.
