Carbonate boundstone as a geochronometer

ended

Recent analytical developments are reinvigorating the study of carbonate as a geochronometer in deep time (>1 Ma) using the U-Pb system. Boundstone, a type of carbonate rock made by the trapping and binding of carbonate sediment by microbial mats, is often sampled for use in U-Pb dating of carbonates. It has even been used as a reference material in recent studies. However, we lack a process-based understanding as to why boundstone works so well for geochronology, and which geological processes create the material we date. This work, supervised by Dr. Axel Gerdes at Goethe University Frankfurt, investigates these questions. This project tests a hypothesis, supported by early results, that microbially-induced carbonate precipitation under reducing conditions creates geochronologically viable early cements. This hypothesis will be tested by petrographically, geochemically, and geochronologically characterizing boundstones from Earth’s last 1 Ga. To test the accuracy and robustness of the boundstone chronometer, I then will compare boundstone dates with radiometric ages derived from zircon and black shales in two stratigraphic succesions: the Neoproterozoic of Oman and the Ordovician of Anticosti Island. First, boundstone samples will be characterized petrographically. High-throughput laser ablation inductively coupled mass spectrometry (LA-ICP-MS) will be used to geochemically and geochronologically characterize samples, a subset of which will advance to high-precision dating by isotope dilution (ID).