Sulfur and boron isotope study of high-Ca impact glasses from the K/T boundary: Constraints on source rocks

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A boron and sulfur isotope study has been conducted by ion microprobe on 20 high-Ca yellow impact glass particles from the K/T boundary deposit in Haiti. The yellow glasses are S-rich with up to 1.24 wt% SO3 and show a large range of δ34S values (+1.5 ± 2‰ to +13.2 ± 4‰) and δ11B values (-9.8 ± 2‰ to +11.0 ±2‰). In addition to the previously recognized andesite (or continental crust), anhydrite, and carbonate components, continental sedimentary rocks containing clays and diagenetic minerals (e.g., loess or sandstones) are indicated by the high B contents and low δ11B values of some yellow glasses. This component could correspond to the Todos Santos red-bed formation, recognized by drilling in the Yucatan and likely to be present at Chicxulub. Mass balance mixing calculations (based on SiO 2, CaO, B contents, and δ11B values) show that all the isotopic characteristics (δ11B, δ34S, and δ18O) of the yellow glasses can be explained by mixing during impact of very variable proportions of andesite (31 to 75%, mean 49%), sulfate (5 to 47%, mean 30%), red sandstone (3 to 26%, mean 14%), and carbonate (0 to 26%, mean 7%). The major element compositions modeled for the yellow glasses, taking into account the large volatilization of SO2 and CO 2, match closely the natural composition of the yellow glasses. The present observations suggest that the amount of SO2 released in the atmosphere represents ε 18.0 wt% of the impacted terranes, whereas only ε 2.6 wt% of the impacted terranes is released as CO2 to the atmosphere.

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Special Paper of the Geological Society of America