CARBON CONTENT AND ISOTOPIC COMPOSITION OF K T IMPACT GLASSES FROM HAITI/

A microscope study of dark brown impact-produced glasses from the K/T boundary beds of Beloc (Haiti) has revealed the presence of bubbles (5 0-300 mu m in size). A simple experiment to release any enclosed gases proved positive in some cases and demonstrated pressure in the bubble s equal to or greater than atmospheric. Stepped combustion analyses co mbined with static mass spectrometry was used to verify that the gas b ubbles contained CO2; the content and stable carbon isotopic compositi on of this gas was determined along with background measurements of th e glass itself. Results indicated that the brown Si-rich glasses were heterogeneous in carbon content ranging from 0.007-2.62 wt% carbon, bu t that sharp releases of gas were afforded at relatively low temperatu res with a carbon isotopic composition up to -0.8 parts per thousand, similar to that of marine carbonate. The glasses had been cleaned and acid-etched to remove contamination, ensuring that any carbon released was indigenous to the glass. The Ca-rich yellow glasses were analysed in the same way and released 0.265-0.343 wt% carbon with a delta(13)C of -29 parts per thousand, which is quite different from the Si-rich glasses. The proposed source crater for these impact glasses is the Ch icxulub crater, which has a pre-impact stratigraphy containing carbona te. Vaporisation of the carbonate during the impact would have release d large quantities of CO2 into the atmosphere, which could have been i ncorporated into the glasses as bubbles. Alternatively, the bubbles co uld have been formed by degassing of the glasses themselves during coo ling. This would account for the marine carbonate carbon isotopic sign ature identified during stepped heating of the high-Si glass. We belie ve this provides a further link between the impact glasses of Haiti an d the Chicxulub impact crater although our results for the high Ca gla sses proved inconclusive. Copyright (C) 1998 Elsevier Science Ltd.