Abstract:
We report here direct ion microprobe dating of fossil tooth (dentine) of a Permian fresh-water shark, Orthacanthus senckenbergianus using the SHRIMP instrument recently installed at Hiroshima University. Fifteen spots on the small sample (approximately 2 mmx1 mm) indicate a 238U/206Pb isochron age of 266+/-18 Ma and a Tera-Wasserburg concordia intercept age of 266+/-24 Ma in a three-dimensional 238U/206Pb-207Pb/206Pb-204Pb/206Pb diagram. These Permian ages are consistent with a 235U/207Pb age of 453+/-170 Ma and a 232Th-208Pb age of 235+/-310 Ma, suggesting indistinguishable depositional and early diagenetic ages of the fossil in its sedimentary sequences. The success of the method depends on the chemical fractionation of uranium from lead in a specimen a few hundred microns in size and the consequent variations in lead isotopic compositions due to radioactive decay.