Preliminary results from U-Pb dating and geochemical characterization of GRIND-ECT carbonate samples from Namibia

Abstract

Establishing geochronological controls on ancient sedimentary successions is critical for evaluating cause and effect, as well as rates of change, in ancient environments. Towards this goal, U-Pb dating of carbonate phases by in situ LA-ICP-MS offers a rapid and reasonably precise method for evaluating not only depositional ages but also the ages of post-depositional processes affecting carbonate rock. However, this technique is still in its first decade of application to carbonate rocks. Determining the best practices for applying this technique and evaluating the meaning, reliability, and robustness of the data it produces remain areas of interest. In this study, we present the first results from U-Pb dating and geochemical characterization of samples from recent drillcores from Namibia, drilled by International Continental Scientific Drilling Program project GRIND-ECT. GRIND-ECT aims to capture, in drillcore, the Ediacaran-Cambrian transition in key successions worldwide. The Namibian portion of GRIND-ECT targets the Nama Group of southern Namibia, well-known for its abundant fossils and detailed geochemical and geochronological records from outcrop, with the goal of elucidating changes in the Earth system during the diversification of early animals. With both abundant interbedded ashes amenable to U-Pb zircon dating and abundant carbonate, these drillcores offer a natural experiment for comparing results from LA-ICP-MS carbonate dating and other geochronological techniques. Our results show that a significant fraction of samples yield geologically meaningful ages when dated using LA-ICP-MS. These ages are consistent with depositional or early post-depositional ages. Microbially-influenced facies are often the most successful. Theses ages can be further refined using higher-precision techniques. Efforts to date vein calcites, which are often very low in U, have not yet yielded robust ages. Integration of Sr and trace and rare earth element data indicate the promise and the limitations of using these proxies as tests for “datability.”