Isotopic signatures of black tektites from the K-T boundary on Haiti: Implications for the age and type of source material

Abstract— U-Th-Pb, Rb-Sr, and Sm-Nd isotopic signatures of corroded, but unaltered, black glassy tektites from Cretaceous-Tertiary (K-T) boundary rock on Haiti are not consistent with their derivation from an impact on MOR-derived oceanic crust or continental regions involving middle Proterozoic or older crustal material. Two single-grain and two batches of these tektites yielded present-day ?_Nd = ?3.0 to ?3.4, ?_Sr = +55 to 56, ²⁰⁶Pb/²⁰⁴Pb = 18.97; ²⁰⁷Pb/²⁰⁴Pb = 15.74; ²⁰⁸Pb/²⁰⁴Pb = 38.91 values, and Pb, Rb, Sr, Sm, and Nd concentrations of ～6, ～45, ～535, ～4.7, and ～22 ppm, respectively. Initial ?_Nd and ?_Sr values for the tektites are different from time-integrated Nd-Sr isotopic signatures for almost all oceanic crustal types. Age-corrected Pb isotopic values are similar to those for pelagic sediments with distinctly higher ²⁰⁷Pb/²⁰⁴Pb values compared to MORB. However, these results do not exclude the possibility of an oceanic impact site, if the tektites were derived from fine-grained sediments that typically overlie such regions, although other mineralogic and chemical evidence from K-T boundary debris suggests otherwise. Moreover, the Nd average crustal residence age of ～ 1080 Ma (T_DM) for the black tektites eliminates impact sites on continental crustal regions involving middle Proterozoic or older rocks, or sedimentary rocks largely derived from them. Previously reported major and trace element data from the black tektites suggest that the source material was possibly sedimentary with a composition similar to average shale or graywacke. If this is the case, then the Nd isotopic data suggest that the source rocks were not older than Silurian (T_CHUR = 400 Ma) in age, and were composed largely of young (< 1080 Ma) crustal material. Of the suspected K-T boundary impact sites, both the Manson (Iowa) and Chicxulub (Yucatan) structures occur in suitable lithologies to yield the Haitian black tektites, although neither structure has as yet proven to be the tektite source.