Carbon content and isotopic composition of K/T impact glasses from Haiti

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A microscope study of dark brown impact-produced glasses from the K/T boundary beds of Beloc (Haiti) has revealed the presence of bubbles (50-300 μm in size). A simple experiment to release any enclosed gases proved positive in some cases and demonstrated pressure in the bubbles equal to or greater than atmospheric. Stepped combustion analyses combined with static mass spectrometry was used to verify that the gas bubbles contained CO2; the content and stable carbon isotopic composition of this gas was determined along with background measurements of the glass itself. Results indicated that the brown Si-rich glasses were heterogeneous in carbon content ranging from 0.007-2.62 wt% carbon, but that sharp releases of gas were afforded at relatively low temperatures with a carbon isotopic composition up to -0.8‰, 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 δ13C of - 29‰, which is quite different from the Si-rich glasses. The proposed source crater for these impact glasses is the Chicxulub crater, which has a pre-impact stratigraphy containing carbonate. Vaporisation of the carbonate during the impact would have released large quantities of CO2 into the atmosphere, which could have been incorporated into the glasses as bubbles. Alternatively, the bubbles could have been formed by degassing of the glasses themselves during cooling. This would account for the marine carbonate carbon isotopic signature identified during stepped heating of the high-Si glass. We believe this provides a further link between the impact glasses of Haiti and the Chicxulub impact crater although our results for the high Ca glasses proved inconclusive. Copyright © 1998 Elsevier Science Ltd.

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Geochimica et Cosmochimica Acta