SHOCK-INDUCED MELTING AND VAPORIZATION OF SHATTER CONE SURFACES - EVIDENCE FROM THE SUDBURY IMPACT STRUCTURE

Analytical scanning electron microscopy has been used to investigate t he surface textures and compositions of newly exposed shatter cones fr om the 1.85 Ga Sudbury impact structure, Canada. Unusual surface micro structures are observed at the micron scale, including silicate melt s mears, melt fibres and melt splats. Silicate and Ni-rich spherules up to 5 mu m in diameter adorn earlier-formed surface features, and we in terpret these to be condensates formed due to shock-induced vaporizati on of the shatter cone surfaces. The development of striations on the shatter cones is attributed to shock-related fracture and slip. Format ion of melts and spherules indicates that the highest ranks of shock m etamorphism (Stages IV and V) were realized, but only on a very locali zed scale. Shatter cone surfaces are, therefore, likely sites for the development of high-pressure polymorphs and, if the chemistry is appro priate, fullerenes. As such, they may be equivalent to ''Type A'' pseu dotachylytes and shock veins in meteorites.