Re–Os isotope systematics as a diagnostic tool for the study of impact craters and distal ejecta

The Re–Os isotopic system is based on the β-decay of 187Re to 187Os (half-life=42.3±1.3 Ga). During partial melting of mantle rocks, Os remains in the residue but Re is enriched in the melt. Thus, crustal rocks have high Re and low Os concentrations and the crustal 187Os/188Os ratio increases rapidly with time. The present-day 187Os/188Os ratio of mantle rocks is about 0.13. Meteorites also have low 187Os/188Os ratios of about 0.11–0.18. Osmium is much more abundant in meteorites than Re, leading to only small changes in the meteoritic 187Os/188Os ratio with time. ntinental crust has 187Os/188Os ratios of about 0.67–1.61, which are distinctly different from the meteoritic values. This allows the use of Re–Os isotope systematics for the study of impact craters and ejecta. Impact melts, breccias, and different materials in ejecta consist of terrestrial target rocks, in some cases mixed with a very small (<1%) admixture of recondensed projectile material, the so-called meteoritic component. Up to the 1990s, this component has been identified in the form of significantly enhanced abundances of some siderophile elements. Because of the high Os abundances in meteorites, the admixture of only a small meteoritic component to crustal target material will drastically change the Os isotope characteristics of the resulting breccias or impact melt rocks. cent development of negative thermal ionization mass spectrometry allowed the determination of abundances and isotopic ratios of Os and Re at low abundance levels and using relatively small amounts of material. We review the results of Re–Os isotope studies of material from various impact craters, e.g., Bosumtwi (Ghana), Kalkkop, Saltpan, and Vredefort (South Africa), Chicxulub (Mexico), Manson (U.S.A.), Sudbury (Canada), and at the K–T boundary. Re–Os isotope systematics allow the determination and quantification of the meteoritic component in impact-derived materials (in comparison to target rocks) and may help to understand the mixing between the bolide and target rocks. An interesting application of this method is the confirmation of an impact origin for unusual sedimentary layers of possible impact origin or structures of doubtful geological origin (which may be of importance for eroded structures). The study of Os isotopes may become a tool of similar diagnostic power as the study of shock metamorphism in confirming impact structures.