EVIDENCE FOR A METEORITIC COMPONENT IN IMPACT MELT ROCK FROM THE CHICXULUB STRUCTURE

The Chicxulub structure in Yucatan, Mexico, has recently been recogniz ed as a >200-km-diameter multi-ring impact crater of K-T boundary age. Crystalline impact melt rocks and breccias from within the crater, wh ich have compositions similar to those of normal continental crustal r ocks and which show shock metamorphic effects, have been studied for t race element and Re-Os isotope compositions. Re-Os isotope systematics allow the sensitive and selective determination of an extraterrestria l component in impact-derived rocks. A melt rock sample shows elevated iridium concentrations, an osmium concentration of 25 ppb, and a low Os-187/Os-188 ratio of 0.113, which are incompatible with derivation f rom the continental crust. Even though the Os-187/Os-188 ratio is slig htly lower than the range so far measured in meteorites, a mantle orig in seems unlikely for mass balance reasons and because the cratering e vent is unlikely to have excavated mantle material. The data support t he 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 Os-187/Os-188 ratio of 0.251, indicating the presence of a small meteoritic componen t in the impact ejecta as well.