Evidence for shock‐induced anhydrite recrystallization and decomposition at the UNAM‐7 drill core from the Chicxulub impact structure

Abstract

Drill core UNAM‐7, obtained 126 km from the center of the Chicxulub impact structure, outside the crater rim, contains a sequence of 126.2 m suevitic, silicate melt‐rich breccia on top of a silicate melt‐poor breccia with anhydrite megablocks. Total reflection X‐ray fluorescence analysis of altered silicate melt particles of the suevitic breccia shows high concentrations of Br, Sr, Cl, and Cu, which may indicate hydrothermal reaction with sea water. Scanning electron microscopy and energy‐dispersive spectrometry reveal recrystallization of silicate components during annealing by superheated impact melt. At anhydrite clasts, recrystallization is represented by a sequence of comparatively large columnar, euhedral to subhedral anhydrite grains and smaller, polygonal to interlobate grains that progressively annealed deformation features. The presence of voids in anhydrite grains indicates SOx gas release during anhydrite decomposition. The silicate melt‐poor breccia contains carbonate and sulfate particles cemented in a microcrystalline matrix. The matrix is dominated by anhydrite, dolomite, and calcite, with minor celestine and feldspars. Calcite‐dominated inclusions in silicate melt with flow textures between recrystallized anhydrite and silicate melt suggest a former liquid state of these components. Vesicular and spherulitic calcite particles may indicate quenching of carbonate melts in the atmosphere at high cooling rates, and partial decomposition during decompression at postshock conditions. Dolomite particles with a recrystallization sequence of interlobate, polygonal, subhedral to euhedral microstructures may have been formed at a low cooling rate. We conclude that UNAM‐7 provides evidence for solid‐state recrystallization or melting and dissociation of sulfates during the Chicxulub impact event. The lack of anhydrite in the K‐Pg ejecta deposits and rare presence of anhydrite in crater suevites may indicate that sulfates were completely dissociated at high temperature (T > 1465 °C)—whereas ejecta deposited near the outer crater rim experienced postshock conditions that were less effective at dissociation.

Organic matter from the Chicxulub crater exacerbated the K–Pg impact winter

“…These results support the hypothesis that thermally mature organic carbon within the target rock was a significant source for PAHs at the K-Pg boundary. Target rock organics at Chicxulub were pyrolyzed and vaporized on impact, at temperatures that exceeded 1,400°C (78). The PAHs were redeposited at the impact site as well as globally in ocean sediments containing fallout.…”

Petrographic and chemical studies of the Cretaceous-Paleogene boundary sequence at El Guayal, Tabasco, Mexico: Implications for ejecta plume evolution from the Chicxulub impact crater

“…At the UNAM-7 borehole near the outer crater rim (Fig. 1), a >400-m-thick impact-breccia sequence was deposited (Urrutia-Fucugauchi et al, 1996Salge et al, 2019). Here, a polymict suevitic breccia that is rich in altered silicate melt particles from crystalline target lithologies overlies an evaporite-rich megabreccia.…”

“…Chicxulub impactites and Cretaceous-Paleogene ejecta deposits, however, reveal that sedimentary ejecta display great variability, indicating various degrees of shock metamorphism, followed in part by thermal overprinting during ejection and atmospheric transport. Salge et al (2019) described impactinduced recrystallization and decomposition for anhydrite clasts at the UNAM-7 drill core. The lack of anhydrite in the crater suevites from the drill cores Yaxcopoil-1 (e.g., Dressler et al, 2004;Kring et al, 2004) (Gulick et al, 2017a), as well as in the Cretaceous-Paleogene ejecta deposits, may indicate that sulfates were completely dissociated at high temperature (T > 1465 °C), whereas ejecta deposited close to the outer crater rim experienced postshock conditions that were less effective at dissociation (Salge et al, 2019).…”

“…SO x gases, as well the emission of soot from impact-driven firestorms, may have caused a short, but probably severe cooling interval, whereas CO 2 gas may have led to subsequent long-term global warming (e.g., Kruge et al, 1994;Pierazzo et al, 1998;Harvey et al, 2008;Schulte et al, 2010;Brugger et al, 2017;Vellekoop et al, 2016;Artemieva et al, 2017;Tabor et al, 2020). The response of sedimentary target rocks to high shock pressures, however, is still not well understood (e.g., Langenhorst and Deutsch, 2012;Hamann et al, 2018;Salge et al, 2019;Hörz et al, 2020). For carbonates, vaporization (e.g., Kieffer and Simonds, 1980;Boslough et al, 1982;O'Keefe and Ahrens, 1989;Pierazzo and Artemieva, 2012;Hörz et al, 2015;Bell, 2016), melting (e.g., Graup, 1999;Osinski and Spray, 2001;Ivanov and Deutsch, 2002;Osinski et al, 2008;Walton et al, 2019), and calcite crystallization by the back-reaction of the decomposition products CaO and CO 2 (Agrinier et al, 2001;Yancey and Guillemette, 2008) are sedimentary target lithologies.…”

“…The goal was to provide insights into impact and alteration processes. Melting, aggregation, and thermal alteration processes during the initial stage of the Chicxulub impact Salge et al, 2019). event and during prolonged ejecta transport will be discussed.…”