Shock Veins in Terrestrial and Extraterrestrial Materials: Examples from the Steen River Impact Structure and NWA 8159 Martian Augite Basalt

English Abstract:

It is well documented that many meteorites contain shock veins where high temperature and high pressure minerals have been discovered. It is widely accepted that these veins are analogous to terrestrial micro-pseudotachylites, reported from only a few impact structures including Manicouagan, Ries and Vredefort. The importance of shock veins is twofold: 1) the minerals formed by crystallization from impact melt or solid-state transformation can give us a better understanding of the pressure-temperature conditions during shock, and 2) comparison between shock veins in terrestrial impact structures, where stratigraphic position within the crater is known, and shock veins in meteorites, can help provide context to impact ejection mechanisms. However, if shock metamorphism recorded in the meteorite occurred in a larger impact event prior to ejection, then comparison allows us to draw conclusions regarding the pre-ejection location on their parent body. The high pressure mineral inventory of thin shock veins from the central uplift of the 25-km diameter Steen River impact structure (SRIS) in NW Alberta and nearly identical features in a newly recognized type of martian meteorite – NWA 8159 augite basalt – have been characterized using SEM, EMPA, Raman and TEM. In the SRIS, shear-induced shock veins cut across the metamorphic fabric of a plagioclase-amphibole-biotite-quartz gneiss. Large mm-size grains of ferro-pargasite have undergone a nearly isochemical phase transformation to almandine garnet with 36% majorite component. This amphibole-garnet transformation is observed only in those grains in direct contact with the shock vein and in small fragments entrained within the vein. Likewise, plagioclase-maskelynite transformation is only observed near to the shock veins. The opaque, fine-grained, granular matrix of the shock vein contains euhedral almandine garnet with 11% majorite component and other minor phases such as pyroxene and stishovite. The high-pressure mineral inventory of shock veins from the SRIS is similar to, but distinct from, veins in NWA 8159 where Na-rich almandine-grossular garnets (38% majorite) have crystallized with clinopyroxene (jadeite-ferroan augite s.s.) and stishovite. Shock pressure and temperature estimates for both rocks are within the range ~15‒23 GPa.