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

Abstract

Abstract-Petrographic descriptions of three dike breccia lithologies from drill core Yaxcopoil-1 (Yax-1) are presented. They occur within allochthonous units of displaced sedimentary megablocks of the Chicxulub impact structure. The suevitic dike breccias are the uppermost dike lithology. They contain melt rock particles and melt injections into the dike groundmass. Shock features occur ubiquitously and indicate a strong thermal annealing. Flow textures suggest a highly energetic emplacement process, possibly during the excavation stage as a ground-surge related deposit. The impact melt rock dikes are present in a strongly brecciated megablock interval as flow textured, anastomozing veinlets of impact melt rock that were altered to clay minerals. The melt impregnated a dolomitic host rock, indicating a low viscosity and, thus, high initial temperatures. Brecciation of the impact melt rock dikes occurred while they were still below the glass transition temperature, suggesting that dynamic conditions prevailed shortly after the emplacement process. Major element data indicates that the impact melt rock dikes differ in composition from the homogenized impact melt rock of Chicxulub. This could point to an emplacement during the late compression or early excavation stages of cratering. The clastic polymict dike breccias are coeval with pervasive brittle fracturing of the host rocks. They bear clasts including some crystalline basement and possible melt rock particles in a fine-grained dolomite matrix with turbulent flow textures. Fabric and texture indicate a granular flow at ambient pressures. Such conditions could be envisaged for the excavation phase while the transient cavity grew and fractures opened.

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

“…Enrichments of the K 2 O content in impactites due to post-impact hydrothermal alteration have been reported from many impact craters in the past (e.g., French et al 1997;Reimold et al 1994). Hydrothermal alteration was already recognized in the Y6 and C1 drill cores (e.g., Schuraytz et al 1994;Kettrup et al 2000;Claeys et al 2003), but is much more dominant in the Yax-1 drill core, which partially displays a strong potassium metasomatism, especially in the lower suevite and the suevitic and impact melt rock dikes Wittmann et al 2004). …”

“…Several suevitic dikes were observed in the depth interval of 909-917 m. An approximately 8 cm-thick suevitic dike occurs at the depth of ~909 m and an ~80 cm-thick dike at 915.99-916.79 m (Dressler, personal communication;Wittmann et al 2003aWittmann et al , 2004. Both analyzed suevitic dike samples come from the latter dike.…”

“…An ~33 cm-thick impact melt rock dike was observed at the depth of 1348 m (Dressler, personal communication;Wittmann et al 2003aWittmann et al , 2004. Sample Yax-1_1347.96 m is from the center of this dike.…”

“…Within the matrix, some micritic carbonate clasts were observed. Iron sulfides (mostly pyrite) occur ubiquitously in the suevitic dikes, their host rocks, and the impact melt rock dike Wittmann et al 2004). Clastic breccia dikes within the megablock unit show variable abundances in clasts and thicknesses (Wittmann et al 2003b.…”

“…The TiO 2 , Al 2 O 3 , Fe 2 O 3 , Rb, Zr, and possibly MgO contents are enriched, while the SiO 2 , CaO, and possibly Na 2 O contents are depleted, compared with the composition of the Yax-1 and Y6 suevite. As in the suevitic dike, the relatively high SO 3 content of the impact melt rock dike is caused by the occurrence of iron sulfides Wittmann et al 2004). The chemical composition of this sample does not show any similarity to the Y6 impact melt rock (Table 3) either.…”