Kalkkop Crater, Cape Province, South Africa: Confirmation of impact origin using osmium isotope systematics

Abstract

The Kalkkop structure, a circular depression with a diameter of 640 m, is situated in the Eastern Cape Province, South Africa. In 1992, a bore hole was drilled in the center of the crater to a depth of 151.8 m. To assess a possible impact origin of this structure, the abundances and isotopic ratios of osmium and rhenium were measured in breccias and in sandstones and shales derived from the basement. The basement rocks show rhenium and osmium abundances and isotopic compositions of osmium and neodymium ( 187Os 188Os = 0.61 to 0.92 and ϵ Nd = −6 to−7) that are typical for continental crust. One of the breccia samples shows a significantly elevated osmium content and a much lower ( 187Os 188Os ratio of about 0.215 (( 187Os 188Os ratio = 1.79 ). This ratio is much closer to meteoritic compositions than to crust, indicating the presence of about 0.05% of an extraterrestrial component in the breccia. This is the first time that osmium isotope systematics have been used to confirm the impact origin of a crater structure, and demonstrates their sensitivity for impact studies.