In situ ion microprobe U-Pb dating and REE abundances of a Carboniferous conodont

We report here in situ ion microprobe U-Fb dating of a conodont micro-fossil using an ion microprobe method. Thirteen spots on the single fragment of the Carboniferous conodont (size: approximately 800 mum x 100 mum) yield a U-238/Pb-206 isochron age of 323+/-36 Ma and a Tera-Wasserburg concordia intercept age of 332+/-44 Ma in a three-dimensional U-238/Pb-206-Pb-207/Pb-206-Pb-204/Pb-206 diagram. These ages are consistent with the depositional and early diagenetic ages of the fossil in its host Mississippian sedimentary sequence within experimental error. The success of the method depends on the chemical fractionation of U from Pb within a hundred-mum length scale and the consequent variations in Pb isotopic compositions due to radioactive decay. Shale-normalized rare earth element (REE) abundances of two spots on the same sample show flat patterns from light REE to middle REE and decreases from middle REE to heavy REE with negative anomalies of both Ce and Eu. The REE characteristics are significantly different from those of Devonian conodonts reported by other workers, suggesting discrepant redox states and/or formation environments.