• Composition of impact melt particles and the effects of post-impact alteration in suevitic rocks at the Yaxcopoil-1 drill core, Chicxulub crater, Mexico

      Hecht, L.; Wittmann, A.; Schmitt, R.-T.; Stöffler, D. (The Meteoritical Society, 2004-01-01)
      Petrographical and chemical analysis of melt particles and alteration minerals of the about 100 m-thick suevitic sequence at the Chicxulub Yax-1 drill core was performed. The aim of this study is to determine the composition of the impact melt, the variation between different types of melt particles, and the effects of post-impact hydrothermal alteration. We demonstrate that the compositional variation between melt particles of the suevitic rocks is the result of both incomplete homogenization of the target lithologies during impact and subsequent post-impact hydrothermal alteration. Most melt particles are andesitic in composition. Clinopyroxene-rich melt particles possess lower SiO2 and higher CaO contents. These are interpreted by mixing of melts from the silicate basement with overlying carbonate rocks. Multi-stage post-impact hydrothermal alteration involved significant mass transfer of most major elements and caused further compositional heterogeneity between melt particles. Following backwash of seawater into the crater, palagonitization of glassy melt particles likely caused depletion of SiO2, Al2O3, CaO, Na2O, and enrichment of K2O and FeOtot during an early alteration stage. Since glass is very susceptible to fluid-rock interaction, the state of primary crystallization of the melt particles had a significant influence on the intensity of the postimpact hydrothermal mass transfer and was more pronounced in glassy melt particles than in wellcrystallized particles. In contrast to other occurrences of Chicxulub impactites, the Yax-1 suevitic rocks show strong potassium metasomatism with hydrothermal K-feldspar formation and whole rock K20 enrichment, especially in the lower unit of the suevitic sequence. A late stage of hydrothermal alteration is characterized by precipitation of silica, analcime, and Na-bearing Mg-rich smectite, among other minerals. This indicates a general evolution from a silica-undersaturated fluid at relatively high potassium activities at an early stage toward a silica-oversaturated fluid at relatively high sodium activities at later stages in the course of fluid rock interaction.