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

Abstract

Abstract-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 SiO 2 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 SiO 2 , Al 2 O 3 , CaO, Na 2 O, and enrichment of K 2 O and FeO tot 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 K 2 0 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.

Geochemistry of drill core samples from Yaxcopoil‐1, Chicxulub impact crater, Mexico

“…8c, 11b) and, therefore, clearly indicate a calcite-dolomite mixture with a dominant calcite component. However, these samples show abundant secondary calcite (formed during post-impact hydrothermal alteration; Hecht et al 2004), which may have disturbed the primary calcite/dolomite ratios. In most of the other Yax-1 suevite samples, the MgO contents are clearly higher than in the Y6 suevite (Figs.…”

“…Where possible, four to six typical samples from each suevite subunit were chosen for chemical analysis. The same samples were used for petrographic studies (see accompanying articles by Stöffler et al 2004;Hecht et al 2004). The stratigraphic location of the suevite samples is shown in Fig.…”

Impact‐related dike breccia lithologies in the ICDP drill core Yaxcopoil‐1, Chicxulub impact structure, Mexico

“…The sample types are explained in the text; "Ø" refers to the electron beam diameters for the respective measurements; FeO is total iron. Kettrup et al (2000), and Hecht et al (2004).…”

“…1) of Yax-1. They are equivalent to "type 3 and 4 melt rock particles" according to Hecht et al (2004). "Type 3" and "type 4" melt rock particles show variable shapes from subangular to elongated and lumpy.…”

“…The shaded areas MP 1-2 and MP 3-4 represent melt rock particles that are similar in appearance to SD 1 and SD 2, respectively. MP 1-2 encompasses the data for 20 particles of "type 1 and 2 melt rock particles" of Hecht et al (2004). They represent altered melt rock particles of samples from the upper three sub-units of the "impact breccias and melt rocks" (Fig.…”