GEOCHEMICAL CONSTRAINTS ON SOURCE REGION OF CRETACEOUS TERTIARY IMPACT GLASSES

THE 50-cm-thick Cretaceous/Tertiary boundary layer at Beloc in Haiti contains spherules of silicic black glass, 1-8 mm in diameter, which have been attributed to impact fusion of continental crust 1. Calcic yellow glass (up to 30 wt% CaO) forms a coating on and streaks within some black glass spherules, and has been attributed to melting of carbonate-rich sediment 1, or organic-rich or pyritic limestone 2. Here we present new trace-element and stable and radiogenic isotope data which show that the silicic glass is derived from continental crust of andesitic composition, whereas the high-Ca glass formed by melting of evaporite-rich sediment. This is confirmed by melting experiments with evaporite and andesite at 1,200-1,400-degrees-C, which approximately reproduce the high-Ca glass. The temperature-dependent variation of sulphur content in synthetic high-Ca glasses indicates a formation temperature of 1,300-degrees-C for the Haiti glasses. The geology of the impact site inferred from the geochemistry of the Haiti glasses matches the lithologies found in the 180-km Chicxulub structure, which occurs in Cretaceous evaporite deposits in Mexico. The high sulphur content of the calcic glasses suggests that the impact may have generated significant emissions of sulphur dioxide to the atmosphere, causing short-term global cooling.