Chicxulub impactites: Geochemical clues to the precursor rocks

The 65 Ma Chicxulub impact structure, Mexico, with a diameter of similar to 180 km is the focus of geoscientific research because of its link to the m ass extinction event at the Cretaceous-Tertiary (WT) boundary. Chicxulub, n ow buried beneath thick post-impact sediments, is probably one of the best- preserved terrestrial impact structures known. Because of its inaccessibili ty, only limited samples on the impact lithologies from a few drill cores a re available. We report major element and Sr-, Nd-, O-, and C-isotopic data for Chicxulub impact-melt lithologies and basement clasts in impact brecci as of drill cores C-l and Y-6, and for melt particles in the Chicxulub ejec ta horizon at the K/T boundary in Beloc, Haiti. The melt lithologies with S iO2 ranging from 58 to similar to 63 wt% show significant variations in the content of Al, Ca, and the alkalies. In the melt matrix samples, delta C-1 3 of the calcite is about -3 parts per thousand. The delta O-18 values for the siliceous melt matrices of Y-6 samples range from 9.9 to 12.4 parts per thousand. Melt lithologies and the black Haitian glass have rather uniform Sr-87/Sr-86 ratios (0.7079 to 0.7094); only one lithic fragment displays S r-87/Sr-86 of 0.7141. The Sr model ages T-UR(Sr) for most lithologies range from 830 to 1833 Ma; unrealistic negative model ages point to an open Rb-S r system with loss of Rb in a hydrothermal process. The Nd-143/Nd-144 ratio s for all samples, except one basement clast with Nd-143/Nd-144 of 0.5121, cluster at 0.5123 to 0.5124. In an epsilon (Nd)-epsilon (Sr) diagram, impac tites plot in a field delimited by epsilon (Nd) of -2 to -6, and epsilon (S r) of 55 to 69. This field is not defined by the basement lithologies descr ibed to occur as lithic clasts in impact breccias and Cretaceous sediments. At least one additional intermediate to mafic precursor component is requi red to explain the data.