Carbon isotope study of impact diamonds in Chicxulub ejecta at Cretaceous-Tertiary boundary sites in Mexico and the Western Interior of the United States

Diamonds have been found in the ejecta of the Chicxulub Crater in northeastern Mexico and in the Western Interior of the United States, They are located in both the ejecta claystone and the fireball layer that form the impact doublet at the Cretaceous-Tertiary boundary and range in size from 5 nm to 30 mu m. Stepped heating experiments consistently reveal carbonaceous components in the acid-resistant residues, and cubic diamond was the only form observed using transmission electron microscopy, The carbon isotopic profiles for combustion of the diamond indicate multiple components with a range of compositions from -11 to -30 parts per thousand (delta(13)C). The isotopic compositions are representative of a mixture of carbon sources in which a meteoritic component is at most very minor along a mixing trend with carbon from target lithologies. The question remains whether diamond is also present within crater lithologies at Chicxulub in the same way as at other craters such as Popigai in Russia and Ries in Germany.