Comparison of the osmium and chromium isotopic methods for the detection of meteoritic components in impactites: Examples from the Morokweng and Vredefort impact structures, South Africa

Breccias and melt rocks found at possible meteorite impact structures on Earth may contain a minor extraterrestrial component. In the absence of evidence of shockmetamorphic effects in such rocks, the unambiguous detection of an extraterrestrial component can be of diagnostic value regarding the impact origin of a geologic structure. Previously the concentrations of siderophile elements, mainly the platinumgroup elements (PGEs), or osmium isotopic studies have been used to detect such a meteoritic component. The Cr isotopic method has shown great potential for identifying the type of impactor. Here we use a combination of trace element (PGE) and Os and Cr isotopic data for rocks from the two large impact structures, Vredefort and Morokweng, both in South Africa, to compare the two isotopic methods directly for the first time. The percentage of a meteoritic component in Vredefort granophyre was too low to yield a Cr isotopic signal, but there was a distinct Os isotope effect. For Morokweng, both methods yielded positive results, the Cr isotope data suggesting that the projectile had the composition of an ordinary chondrite, probably an L chondrite. Comparison of the two methods thus confirms that the Os isotopic method is a Koeberl, C., Peucker-Ehrenbrink, B., Reimold, W.U., Shukolyukov, A., and Lugmair, G.W., 2002, Comparison of the osmium and chromium isotopic methods for the detection of meteoritic components in impactites: Examples from the Morokweng and Vredefort impact structures, South Africa, in Koeberl, C., and MacLeod, K.G., eds., Catastrophic Events and Mass Extinctions: Impacts and Beyond: Boulder, Colorado, Geological Society of America Special Paper 356, p. 607–617. *E-mails: Koeberl, [email protected]; Peucker-Ehrenbrink, [email protected]; Reimold, [email protected]; Shukolyukov, [email protected]; Lugmair, [email protected] C. Koeberl et al. 608 more sensitive tool for detecting an extraterrestrial component, whereas the Cr isotopic method, where applicable, can provide additional information regarding the nature of the impactor.