Browsing Meteoritics & Planetary Science, Volume 39, Number 6 (2004) by Authors
Geochemistry of drill core samples from Yaxcopoil-1, Chicxulub impact crater, MexicoSchmitt, R. T.; Wittmann, A.; Stöffler, D. (The Meteoritical Society, 2004-01-01)The chemical composition of suevites, displaced Cretaceous target rocks, and impactgenerated dikes within these rocks from the Yaxcopoil-1 (Yax-1) drill core, Chicxulub impact crater, Mexico, is reported and compared with the data from the Yucatán 6 (Y6) samples. Within the six suevite subunits of Yax-1, four units with different chemical compositions can be distinguished: a) upper/lower sorted and upper suevite (depth of 795-846 m); b) middle suevite (depth of 846-861 m); degrees C) brecciated impact melt rock (depth of 861-885 m); and d) lower suevite (depth of 885-895 m). The suevite sequence (a), (b), and (d) display an increase of the CaO content and a decrease of the silicate basement component from top to bottom. In contrast, the suevite of Y6 shows an inverse trend. The different distances of the Yax-1 and Y6 drilling sites from the crater center (~60, and ~47 km, respectively) lead to different suevite sequences. Within the Cretaceous rocks of Yax-1, a suevitic dike (depth of ~916 m) does not display chemical differences when compared with the suevite, while an impact melt rock dike (depth of ~1348 m) is significantly enriched in immobile elements. A clastic breccia dike (depth of ~1316 m) is dominated by material derived locally from the host rock, while the silicate-rich component is similar to that found in the suevite. Significant enrichments of the K2O content were observed in the Yax-1 suevite and the impact-generated dikes. All impactites of Yax-1 and Y6 are mixtures of a crystalline basement and a carbonate component from the sedimentary cover. An anhydrite component in the impactites is missing (Yax-1) or negligible (Y6).
Impact-related dike breccia lithologies in the ICDP drill core Yaxcopoil-1, Chicxulub impact structure, MexicoWittmann, A.; Kenkmann, T.; Schmitt, R. T.; Hecht, L.; Stöffler, D. (The Meteoritical Society, 2004-01-01)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.
Platinum group elements in impactites of the ICDP Chicxulub drill core Yaxcopoil-1: Are there traces of projectile?Tagle, R.; Erzinger, J.; Hecht, L.; Schmitt, R. T.; Stöffler, D.; Claeys, P. (The Meteoritical Society, 2004-01-01)This study presents results of platinum group element (PGE) analyses of impactites from the Yaxcopoil-1 (Yax-1) and Yucatán 6 drill cores of the 180 km-diameter Chicxulub crater. These are the main elements used for projectile identification. They were determined by nickel sulfide fire assay combined with inductively coupled plasma mass spectrometry. The concentration of PGE in the samples are low. The concentration patterns of the suevite samples resemble the pattern of the continental crust. We conclude that any meteoritic fraction in these samples is below 0.05%. A synand post-impact modification of the PGE pattern from meteoritic toward a continental crust pattern is very unlikely. The globally distributed fallout at the Cretaceous-Tertiary (K/T) boundary, however, has high PGE concentrations. Therefore, the lack of a significant meteoritic PGE signature in the crater is not an argument for a PGE-poor impactor. Taking the results of three-dimensional numerical simulations of the Chicxulub event into account, the following conclusions are drawn: 1) The main fraction of the impactor was ejected into and beyond the stratosphere, distributed globally, and deposited in the K/T boundary clay; and 2) the low amount of projectile contamination in the Yax-1 lithologies may reflect an oblique impact. However, the role of volatiles in the mixing process between projectile and target is not well-understood and may also have played a fundamental role.