In situ ion microprobe Th-Pb dating of Silurian conodonts

We report in situ ion microprobe U-Th-Pb dating of an index fossil conodont derived from Langkawi Island, northern Malaysia. Fifteen spots on five fra gments of an early Silurian conodont yield Th-232-Pb-208* isochron age of 4 29 +/- 50 Ma (2 sigma, MSWD = 0.95). The U-238-Pb-206* isochron age of 436 +/- 270 Ma (2 sigma, MSWD = 0.47) has a large uncertainty, since the U-238 abundance is significantly small, but the age is comparable to that of Th-2 32-Pb-208*. These ages are consistent with the depositional and early diage netic ages of the fossil in a Llandoverian sedimentary sequence (430-439 Ma ). The success of the method depends on the heterogeneity of diagenetically incorpolated Th in a hundred,um size and the consequent variations in Pb i sotopic compositions due to radioactive decay. Shale-normalized rare earth element (REE) abundance shows a concave pattern with middle REE enrichment. There is not a significant anomaly of Ce and Eu. These characteristics are different from those of a Carboniferous conodont, probably due to a differ ent formation environment as suggested by other workers.