Special Paper 356: Catastrophic events and mass extinctions: impacts and beyond

Abstract

In this work we report the presence of coesite in suevite from the Chicxulub impact structure. We studied polished thin sections from sample Y6N14, which represents the fallback suevite breccia from the depth interval 1208-1211 m of the Yucatan-6 well. Micro-Raman spectroscopy was selected as the most adequate technique to search for high-pressure silica polymorphs. More than 60 shocked quartz grains were subjected to Raman study. Spectral data were compared with synthetic coesite as well as with published data for quartz and coesite. In four cases characteristic spectra of coesite displaying Raman shifts of ∼521, 271, 178, and 119 cm1 were obtained in shocked quartz grains. In one case the coesite-bearing quartz grains form an aplite-like aggregate. The three other grains were found in one fragment of partially melted crystalline basement rock. Optical microscopy reveals that coesite-bearing quartz grains contain comparable petrographic shock-metamorphic features, such as mosaicism, brownish domains, low birefringence, planar fractures, and/or planar deformation features. The coesite occurs as shapeless polycrystalline aggregates up to 50 lm or as fine, well-rounded, brownish crystals <10 μm in size, with positive relief relative to quartz, situated either along some planar deformation features, or arranged in short curved chains. We infer that the quartz grains where coesite was identified were subjected to shock pressures between 30 and 35 GPa.