In situ ion microprobe Th-Pb dating of Silurian conodonts

Abstract

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 fragments of an early Silurian conodont yield 232Th-208Pb* isochron age of 429 ± 50 Ma (2σ, MSWD = 0.95). The 238U-206Pb* isochron age of 436 ± 270 Ma (2σ, MSWD = 0.47) has a large uncertainty, since the 238U abundance is significantly small, but the age is comparable to that of 232Th-208Pb*. These ages are consistent with the depositional and early diagenetic 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 μm size and the consequent variations in Pb isotopic 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 different formation environment as suggested by other workers.