Chicxulub impactites: Geochemical clues to the precursor rocks

Abstract

Abstract— The 65 Ma Chicxulub impact structure, Mexico, with a diameter of ∼180 km is the focus of geoscientific research because of its link to the mass extinction event at the Cretaceous‐Tertiary (K/T) boundary. Chicxulub, now buried beneath thick post‐impact sediments, is probably one of the best‐preserved terrestrial impact structures known. Because of its inaccessibility, only limited samples on the impact lithologies from a few drill cores are available. We report major element and Sr‐, Nd‐, O‐, and C‐isotopic data for Chicxulub impact‐melt lithologies and basement clasts in impact breccias of drill cores C‐1 and Y‐6, and for melt particles in the Chicxulub ejecta horizon at the K/T boundary in Beloc, Haiti. The melt lithologies with SiO2 ranging from 58 to ∼63 wt% show significant variations in the content of Al, Ca, and the alkalies. In the melt matrix samples, δ13C of the calcite is about −3%o. The δ18O values for the siliceous melt matrices of Y‐6 samples range from 9.9 to 12.4%o. Melt lithologies and the black Haitian glass have rather uniform 87Sr/86Sr ratios (0.7079 to 0.7094); only one lithic fragment displays 87Sr/86Sr of 0.7141. The Sr model ages TSrUR for most lithologies range from 830 to 1833 Ma; unrealistic negative model ages point to an open Rb‐Sr system with loss of Rb in a hydrothermal process. The 143Nd/144Nd ratios for all samples, except one basement clast with 143Nd/144Nd of 0.5121, cluster at 0.5123 to 0.5124. In an ɛNd‐ɛSr diagram, impactites plot in a field delimited by ɛNd of −2 to −6, and ɛSr of 55 to 69. This field is not defined by the basement lithologies described to occur as lithic clasts in impact breccias and Cretaceous sediments. At least one additional intermediate to mafic precursor component is required to explain the data.