Geochemical and petrographic characteristics of impactites and Cretaceous target rocks from the Yaxcopoil‐1 borehole, Chicxulub impact structure, Mexico: Implications for target composition

Abstract

All tables for this article are available online at http://meteoritics.org.Abstract-We present major and trace element data as well as petrographic observations for impactites (suevitic groundmass, bulk suevite, and melt rock particles) and target lithologies, including Cretaceous anhydrite, dolomite, argillaceous limestone, and oil shale, from the Yaxcopoil-1 borehole, Chixculub impact structure. The suevitic groundmass and bulk suevite have similar compositions, largely representing mixtures of carbonate and silicate components. The latter are dominated by melt rock particles. Trace element data indicate that dolomitic rocks represented a significant target component that became incorporated into the suevites; in contrast, major elements indicate a strong calcitic component in the impactites. The siliceous end-member requires a mafic component in order to explain the low SiO 2 content. Multicomponent mixing of various target rocks, the high alteration state, and dilution by carbonate complicate the determination of primary melt particle compositions. However, two overlapping compositional groups can be discerned-a high-Ba, low-Ta group and a high-Fe, high-Zn, and high-Hf group. Cretaceous dolomitic rocks, argillaceous limestone, and shale are typically enriched in U, As, Br, and Sb, whereas anhydrite contains high Sr contents. The oil shale samples have abundances that are similar to the North American Shale Composite (NASC), but with a comparatively high U content. Clastic sedimentary rocks are characterized by relatively high Th, Hf, Zr, As, and Sb abundances. Petrographic observations indicate that the Cretaceous rocks in the Yaxcopoil-1 drill core likely register a multistage deformation history that spans the period from preto post-impact. Contrary to previous studies that claimed evidence for the presence of impact melt breccia injection veins, we have found no evidence in our samples from a depth of 1347-1348 m for the presence of melt breccia. We favor that clastic veinlets occur in a sheared and altered zone that underwent intense diagenetic overprint prior to the impact event.

Trace element concentrations in the Mexico‐Belize ejecta layer: A link between the Chicxulub impact and the global Cretaceous‐Paleogene boundary

“…Although elevated values of As and Sb are not ubiquitous at the K/T boundary (Ebihara and Miura 1996), the anomalies are distributed to such an extent that they are most likely linked to a global dispersal event. Major and trace element data obtained from the target rocks at the Chicxulub impact site show that the Cretaceous dolomite, argillaceous limestone, and shale are in fact enriched in As, Sb, U, and Br (Tuchscherer et al 2005). Thus the elevated concentrations of these elements are likely derived from the target rock, and represent a genetic link between the Chicxulub impact, the proximal ejecta layer exposed in Mexico/Belize, and the global K/T boundary layer.…”

Hydrothermal alteration in the Bosumtwi impact structure: Evidence from 2M1‐muscovite, alteration veins, and fracture fillings

“…In contrast, the samples from the Yaxcopoil-1 drill core from the Chicxulub impact structure in Mexico experienced a greater extent of hydrothermal alteration, but mostly at lower temperatures that did not lead to extensive chlorite or illite formation (Newsom et al 2006). The nature of the hydrothermal episodes in these craters can be explained by three controlling factors: 1) the target lithology, 2) drilling location and distance to heat source from the impact, and 3) proximity to flooding by sea water in the case of Yaxcopoil-1 (Tuchscherer et al 2005). The Bosumtwi drill cores and the Manson drill core were obtained from the vicinity of the central uplifts, while the Yaxcopoil-1 core was located outside the transient cavity away from other heat sources.…”

Mobile element analysis by secondary ion mass spectrometry (SIMS) of impactite matrix samples from the Yaxcopoil-1 drill core in the Chicxulub impact structure

“…Petrographic evidence for hydrothermal alteration has been documented in the Yax-1 impactites by several studies Hecht et al 2004;Lüders and Rickers 2004;Zürcher and Kring 2004;Rowe et al 2004;Tuchscherer et al 2004Tuchscherer et al , 2005Goto et al 2004). This alteration has been divided into a sequence with several different stages by Zürcher and Kring (2004).…”

“…Initial studies of the mineralogy and geochemistry of the impactites in the drill core identified the presence of alteration phases due to postimpact hydrothermal processes (Tuchscherer et al 2004(Tuchscherer et al , 2005Hecht et al 2004;Ames et al 2004;Zürcher and Kring 2004). Variations in the concentrations of mobile elements in the core have been documented with bulk analyses of the impactite breccias by Tuchscherer et al (2004Tuchscherer et al ( , 2005, and by bulk analyses of the carbonate-rich sediments overlying the impact breccias by Rowe et al (2004).…”

“…Variations in the concentrations of mobile elements in the core have been documented with bulk analyses of the impactite breccias by Tuchscherer et al (2004Tuchscherer et al ( , 2005, and by bulk analyses of the carbonate-rich sediments overlying the impact breccias by Rowe et al (2004). We have analyzed altered and hydrated material having the textural characteristics of clays in thin section with electron microprobe, SIMS, and X-ray diffraction (XRD) to see if there is trace element evidence for fluid mobile element transport.…”