OXYGEN ISOTOPE CONSTRAINTS ON THE ORIGIN OF IMPACT GLASSES FROM THE CRETACEOUS-TERTIARY BOUNDARY

Laser-extraction oxygen isotope and major element analyses of individual glass spherules from Haitian Cretaceous-Tertiary boundary sediments demonstrate that the glasses fall on a mixing line between an isotopically heavy (delta-O-18 = 14 per mil) high-calcium composition and an isotopically light (delta-O-18 = 6 per mil) high-silicon composition. This trend can be explained by melting of heterogeneous source rocks during the impact of an asteroid (or comet) approximately 65 million years ago. The data indicate that the glasses are a mixture of carbonate and silicate rocks and exclude derivation of the glasses either by volcanic processes or as mixtures of sulfate-rich evaporate and silicate rocks.