Fractionation of ruthenium from iridium at the Cretaceous-Tertiary boundary

New data on Ru/Ir abundance ratios are presented for nonmarine (Hell Creek, Montana; Frenchman River, Saskatchewan) and marine Cretaceous-Tertiary boundary sites (Brazos River, Texas; Beloc, Haiti; DSDP 577 and DSDP 596). The Ru/Ir ratio varies from 0.5 to 1 within 4000 km of Chicxulub and increases to 2–3 at paleodistances (65 Ma) of up to 12,000 km from the impact site. For CI chondrites, Ru/Ir= 1.5. A ballistic model of ejecta cloud cooling and expansion, which employs the available vapor-pressure versus temperature data for Ru and Ir, predicts qualitatively similar global variation in the Ru/Ir ratio but by only a factor of 1.5. We infer that several other factors, such as remobilization of PGE during diagenesis, preferential oxidation of Ru, condensation kinetics and atmospheric chemical and circulation processes, may account for the observed larger Ru/Ir variation.