The ejecta blanket of the Chicxulub impact crater, Yucatán, Mexico

Impactites from the Chicxulub impact crater (D~180 km, ~65 Ma) on the Yucatan peninsula, Mexico, were studied petrographically and chemically to investigate the compositional evolution of ejecta during transit through the atmosphere. The target lithology is a ~3 km thick carbonate-anhydrite-dominated sedimentary sequence on top of a crystalline silicate basement. Three different sites were studied including impactites from the UNAM-5 and UNAM-7 boreholes near the outer crater rim and the K-P site of El Guayal (~520 km SW of the crater centre). At the UNAM-5, a 172 m thick suevitic breccia was encountered between 332.0 and 504.0 m depths. At UNAM-7, the sequence of impactites consists of a suevitic breccia (222.2 to 348.4 m) on top of a polymict silicate melt-poor breccia. The latter is intercalated with evaporite megablocks and represents an analogue to the Bunte breccia of the Nördlinger Ries crater. At El Guayal, an impact-related sequence was emplaced between Upper Cretaceous limestones and earliest Palaeocene marls. It consists of (1) a basal ~40 m thick monomict carbonate megabreccia, (2) followed by an ~10 m thick suevitic unit, where an upper ~2.3 m subunit contains at its basis abundant accretionary lapilli, and (3) on top a several cm thick clay layer. In the clay of El Guayal, the presence of Chicxulub impact ejecta together with the PGE-enriched impactor component links the K-P boundary with the Chicxulub impact. The cooling history of the impact ejecta is recorded by the texture of the carbonate melts. In the polymict silicate melt-poor breccia of UNAM-7, carbonate melts were deposited mainly in liquid state, whereas at El Guayal carbonate melts were rapidly quenched. The cogenetic association of carbonate melts with variably textured calcite crystals displays temperature variations during transport through the atmosphere. The carbonates in the polymict silicate melt-poor breccia of UNAM-7 show complete melting and/ or recrystallisation. A vesiclebearing crystallisation sequence at an anhydrite core coated with silicate melt indicates coherent melting and decomposition of anhydrite at T~1465°C. These observations indicate a pronounced thermal alteration and transport in the ejecta curtain with higher temperatures than the Bunte breccia from the Nördlinger Ries crater. At El Guayal, larger variations of thermal alteration are recorded by recrystallisation at the rim of solid calcite particles and quenched carbonate melt particles. Intermixing of hot silicate melt with calcite resulted in annealing of the different components. Degassing voids associated with microcrystalline calcite crystals in silicate melt indicate a complex annealing reaction including calcite recrystallisation and decomposition. In the polymict silicate melt-poor breccia of UNAM-7, calcite decomposition and subsequent back-reaction is indicated by degassing vesicles associated with intercalations of microcrystalline calcite into plastically deformed anhydrite particles. There, a significant amount of the calcite matrix formed by similar processes and not solely by crystallisation from a carbonate melt as indicated by the preservation of vesicular shapes of carbonate melt particles. In the overlying suevitic breccia, silicate melt particles were deposited in liquid state and alteration due to reaction with seawater caused enrichment in Br, Cl, Sr and SO3. This hydrothermal alteration suggests that the calcite matrix at UNAM-7 formed by an exothermic reaction of decomposed calcite and anhydrite (CaO) with water and subsequent back-reaction with CO2. This reaction and impact melts provided the thermal energy for the crystallisation of euhedral anhydrite in the matrix from sulphate-rich fluids at T>350°C. The following five stages can be distinguished for the deposition and alteration of the Chicxulub ejecta blanket: (1) Impact-induced seismic activity resulted to the collapse of the Yucatan platform and proximal deposition of local material near the shelf region. In the highpressure zone between impactor and sedimentary target, jetting accelerated decomposition products (CaO, CO2, SOX) in the atmosphere. A hot vapour flow was initiated by target water explosively vaporised and a prolonged energy release during fast-back-reaction of CaO with CO2 to calcite. (2) Carbonate melts from intermediate target lithologies were excavated with anhydrite and carbonate megablocks and initiated a lateral extending ejecta curtain. Thereby, calcite reformation induced a transport at elevated temperatures. Additional steam generation changed an initially ballistic transport to a flow-like transport. (3) After some 10’s of min, the turbulent expanding ejecta plume partially collapsed separating the falling back suevite from impactor material that had been lifted into the stratosphere. Combination of hot silicate melt with carbonates initiated a gas-driven basal flow (T~550–1100°C). In an upper moderately tempered (T~100°C), turbulent ash cloud steam condensed and accretion of ash-sized material formed accretionary lapilli. (4) The impactor component was deposited with the finest ejecta in the K-P clay of El Guayal presumably for weeks to years. (5) The prolonged transport of ejecta in the hot ejecta plume induced the alteration processes observed in the ejecta deposits. It can be concluded that aggregation reduced the amount of ash from the atmosphere and possibly the duration of global darkening. A certain amount of CO2 has backreacted to calcite, whereas SOX gases were completely liberated into the atmosphere. These observations including the abundant presence of carbonate melts support that the amount of CO2 released to the atmosphere during the Chicxulub impact was overestimated previously.