Stable Isotope Record of Post-impact Fluid Activity in the Chicxulub Crater as Exposed by the Yaxcopoil-1 Borehole

Introduction: Cavities of secondary calcite and veins of secondary anhydrite and quartz in the Yuca-tan-6 borehole drilled into the Chicxulub structure suggested the impact generated a hydrothermal system [1, 2]. Based on work at other craters, mostly Siljan and Puchezh-Katunki [3, 4], we estimate this system could be several kilometers deep in the center of the crater and may extend to the rim, albeit at potentially shallower depths and cooler temperatures. Calculations of conductive cooling suggest the system may have been long-lived, up to 10 5 to 10 6 years [5, 6]. The first continuous core through impactites within the crater was recently recovered by the Chicxulub Scientific Drilling Project, providing a new opportunity to evaluate post-impact hydrothermal activity between the peak ring and rim of the crater. Yaxcopoil-1: The Yax-1 borehole recovered continuous core from a depth of 404 m to 1511 m, penetrating Tertiary cover, ~100 m of impactites, and one or more megablocks of Cretaceous target sediments. The borehole is located ~60-65 km from the center of the Chicxulub impact crater and ~20 beyond the limit of the estimated ~50 km radius of the transient crater, on nearly the same radial line as the Yucatan-6 borehole. The impactites are diverse, consisting of melt-rich polymict breccias and a coherent green melt unit that was brecciated after solidification [7]. The units have been affected by fluid-dominated alteration processes , producing a mineralogical measure of post-impact activity [8]. To complement the mineralogical evaluation of the fluid activity, a series of stable isotope analyses of whole rocks, carbonate fractions, sili-cate fractions, and isolated impact melts were made to determine the source of the water in the system and, at this stage of the study, a qualitative assessment of system temperatures. Oxygen Isotope Compositions: We analyzed a series of samples from within the impactite sequence, which ranged from a depth of 794 to 895 m [7, 8]. We also analyzed two samples from a depth of 1398 m, which represents a block of target carbonate and a dolomitic dike that crosscuts the block [9]. Oxygen isotope compositions as a function of depth are shown in Fig. 1. Many of the impactites had sili-cate and carbonate components, so oxygen isotopes were measured in both. The carbonate in the samples is in the form of clasts of target material, matrix, and