NORTHWEST AFRICA 2058


H-melt rock
(H chondrite in MetBull DB)
standby for nwa 2058 photo
Found 2001
no coordinates recorded

Five fusion-crusted stones weighing together 80.1 g were found in the desert of northwest Africa. Classification was completed at Northern Arizona University (T. Bunch and J. Wittke) and NWA 2058 was determined to be an H chondrite melt rock which is composed of over 90% melt phase and has been severely shocked to at least stage S6. The meteorite is terrestrially weathered to a grade of W2/3.

This meteorite has experienced extreme metamorphic forces, probably during a highly energetic cosmic impact. This energy has caused crushing and ductile deformation to produce milonite with its characteristic foliation (planar alignment) and lineation (much longer in one dimension than in the other two), as well as shear deformation, melting, and flowing to produce pseudotachylite. Pseudotachylite is commonly found in faults associated with impact craters such as the Sudbury impact crater in Canada and the Vredefort impact structure in South Africa. Frictional melting of rock caused by slippage of the crater walls, followed by rapid cooling, produced the glassy textures in terrestrial pseudotachylite, and perhaps by a similar process in NWA 2058.

The matrix of NWA 2058 consists of fine-grained, recrystallized olivine containing relict grains which exhibit strain-induced undulatory extinction. Unmelted grains or clasts of the local rock are incorporated within the glassy matrix. The meteorite contains FeNi–FeS droplets that have been melted and quenched.

With the possiblity of large uncertainies not accounted for, an 40Ar–39Ar analysis for NWA 2058 resulted in a degassing age of 509 (±105) m.y. (Swindle et al., 2006; Kring and Swindle, 2008; Swindle et al., 2009). This gas retention age is the same within error as that measured for L chondrites, which is thought to have suffered impact disruption 470 m.y. ago; the two collisional events might be related. Alternatively, the widespread CRE age that is found for H chondrites of 5–10 m.y. could reflect resetting during a more recent disruption event. The specimen of NWA 2058 shown above is a 0.19 g part slice.