KALKKOP CRATER, CAPE PROVINCE, SOUTH-AFRICA - CONFIRMATION OF IMPACT ORIGIN USING OSMIUM ISOTOPE SYSTEMATICS

The Kalkkop structure, a circular depression with a diameter of 640 m. is situated in the Eastern Cape Province, South Africa. In 1992, a bo re hole was drilled in the center of the crater to a depth of 15 1.8 m . To assess a possible impact origin of this structure, the abundances and isotopic ratios of osmium and rhenium were measured in breccias a nd in sandstones and shales derived from the basement. The basement ro cks show rhenium and osmium abundances and isotopic compositions of os mium and neodymium (Os-187/Os-188 = 0.61 to 0.92 and epsilon(Nd) = -6 to 7) that are typical for continental crust. One of the breccia sampl es shows a significantly elevated osmium content and a much lower Os-1 87/Os-188 ratio of about 0.215 (Os-187/Os-186 ratio = 1.79). This rati o is much closer to meteoritic compositions than to crust, indicating the presence of about 0.05% of an extraterrestrial component in the br eccia. This is the first time that osmium isotope systematics have bee n used to confirm the impact origin of a crater structure, and demonst rates their sensitivity for impact studies.