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 beta-decay of Re-187 to Os-1 87 (half-life = 42.3 +/- 1.3 Ga). During partial melting of mantle roc ks, Os remains in the residue but Re is enriched in the melt. Thus, cr ustal rocks have high Re and low Os concentrations and the crustal Os- 187/Os-188 ratio increases rapidly with time. The present-day Os-187/O s-188 ratio of mantle rocks is about 0.13. Meteorites also have low Os -187/Os-188 ratios of about 0.11-0.18, Osmium is much more abundant in meteorites than Re, leading to only small changes in the meteoritic O s-187/Os-188 ratio with time. Old continental crust has Os-187/Os-188 ratios of about 0.67-1.61, which are distinctly different from the met eoritic values. This allows the use of Re-Os isotope systematics for t he study of impact craters and ejecta. Impact melts, breccias, and dif ferent materials in ejecta consist of terrestrial target rocks, in som e cases mixed with a very small (<1%) admixture of recondensed project ile material, the so-called meteoritic component. Up to the 1990s, thi s component has been identified in the form of significantly enhanced abundances of some siderophile elements. Because of the high Os abunda nces in meteorites, the admixture of only a small meteoritic component to crustal target material will drastically change the Os isotope cha racteristics of the resulting breccias or impact melt rocks. The recen t development of negative thermal ionization mass spectrometry allowed the determination of abundances and isotopic ratios of Os and Re at l ow 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 Vredefor t (South Africa), Chicxulub ( Mexico), Manson (U.S.A.), Sudbury (Canad a), and at the K-T boundary. Re-Os isotope systematics allow the deter mination and quantification of the meteoritic component in impact-deri ved materials (in comparison to target rocks) and may help to understa nd the mixing between the bolide and target rocks. An interesting appl ication of this method is the confirmation of an impact origin for unu sual sedimentary layers of possible impact origin or structures of dou btful geological origin (which may be of importance for eroded structu res). The study of Os isotopes may become a tool of similar diagnostic power as the study of shock metamorphism in confirming impact structu res. (C) 1997 Elsevier Science B.V.