Evidence for a meteoritic component in impact melt rock from the Chicxulub structure

The Chicxulub structure in Yucatan, Mexico, has recently been recognized as a >200-kmdiameter multi-ring impact crater of K-T boundary age. Crystalline impact melt rocks and breccias from within the crater, which have compositions similar to those of normal continental crustal rocks and which show shock metamorphic effects, have been studied for trace element and Re-0s isotope compositions. Re-0s isotope systematics allow the sensitive and selective determination of an extraterrestrial component in impact-derived rocks. A melt rock sample shows elevated iridium concentrations. an osmium concentration of 25 ppb, and a low 1870s/‘s80s ratio of 0.113, which are incompatible with derivation from the continental crust. Even though the ‘*‘OS/‘~~OS ratio is slightly lower than the range so far measured in meteorites, a mantle origin seems unlikely for mass balance reasons and because the cratering event is unlikely to have excavated mantle material. The data support the hypothesis of a heterogeneously distributed meteoritic component in the Chicxulub melt rock. A sample of impact glass from the Haitian K-T boundary at Beloc yielded about 0.1 ppb osmium and an 1870s/‘R80s ratio of 0.25 1, indicating the presence of a small meteoritic component in the impact ejecta as well.