U-Pb Geochronology in the John Day Formation, Oregon
In this study, we sought to determine the age of plant and vertebrate fossils in the Turtle Cove Member of the John Day Formation at John Day Fossil Beds National Monument. Also, we worked towards concluding paleoclimate change and possible relationships between this change, as well as the evolution of fossil organisms during the deposition of sediment. We hypothesized that U-Pb zircon geochronology will be able to provide more precise and accurate ages for the fossil assemblages than those of the previous Ar-Ar dating. Due to the greater stability of zircons and the greater accuracy of U decay constants, U-Pb dating can offer a precision and accuracy of 0.05%. After collecting samples, zircons were separated via density, magnetic, and optical properties, from the Blue Basin and AB Tuffs for U-Pb dating. After being extracted, they will be mounted in epoxy, imaged by cathodoluminescence on a scanning electron microscope before being analyzed for their isotope ratios using laser ablation inductively coupled plasma mass spectrometer (LAICPMS) and thermal ionization mass spectrometer (TIMS). The determined ages will then allow us to place the rock sequence into context and allow comparisons to other rocks in the formation. Also, they can lead to comparisons to the global climate at the time as well as with fossils and minerals of similar age.
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