Maskelynite in Basaltic Meteorites: an Indicator of Shock Pressure during Impact Ejection from Parent Bodies

Introduction: Basaltic rocks available for laboratory study include samples from the Earth, Moon, Mars and at least seven asteroids – the parent bodies of eu-crites (probably 4 Vesta), angrites, mesosiderites, and Shock Effects in Retrieved Lunar Rocks: None of the shock effects in Apollo samples were produced during launch of these rocks off the lunar surface. Consequently , most Apollo mare basalts are weakly shocked or unshocked [6]. Only 3 out of 201 basalts in Apollo 11, 12, 15 and 17 soil contain maskelynite [7-9] (which forms at shock pressures 29 GPa [10]). It appears that few small lunar mare basalts (~1%) were shocked on the surface of the Moon to 29 GPa. Makelynite is also absent in 54 larger Apollo 17 high-Ti mare basalt samples (average size 364 g) [9]. and NWA 2977. Maskelynite-rich samples constitute ~29% (5/17) of lunar basaltic meteorites. Martian Basaltic Meteorites: As of this writing, there appear to be ~75 separate martian meteorites classified as basaltic shergottites, basaltic breccias, gabbros, diabases, olivine-phyric shergottites and au-gite basalt. Maskelynite has been reported in all but five of these samples. This indicates that ~93% of martian basalts contain maskelynite and must have been shocked to at least 29 GPa. (The martian basaltic meteorites for which maskelynite has not been reported are NWA 4222, NWA 4797, NWA 6342, Dofar 378, and NWA 7034 (and paired specimens). The first four samples contain plagioclase glass probably formed by large impacts prior to ejection from Mars [e.g., 11].) Eucrites: Eucrites are likely to come from 4 Vesta [12,13], the second largest asteroid (mean diameter 525.4±2 km). Without taking possible pairings into account, there are currently 858 eucrites in the Mete