U-Pb dating of a speleothem of Quaternary age

We demonstrate that U-Pb dating is a promising method for secondary carbona te materials of Quaternary age and older by obtaining a Pb-206*/U-238 age f or a speleothem with high U (>10 mu g g(-1)) and very low Pb (<10 ng g(-1)) that is supported by an independent Th-230 age. Thermal ionisation mass-sp ectrometry was used to determine the U and Pb isotopic ratios and concentra tions for subsamples of a stalactite from Winnats Head Cave, Peak District, UK. We obtained Pb-206/Pb-204 ratios up to 50, and determined a Pb-206*/U- 238 age of 248 +/- 10 ka, which is within error of the Pb-207*/U-235 age of 333 +/- 79 ka and a-spectrometric U-Th age of similar to 255 ka. For sampl es of Tertiary and Quaternary age, the initial state of U-series disequilib rium is an important consideration and, as with most radiometric dating tec hniques, the mineral must have remained closed to U, Th. Pb, and all interm ediate daughters. We show that dense calcite speleothems are ideal in this respect and that no loss of Rn has occurred. Unlike U-series disequilibrium methods. U-Pb dating has no upper limit and, hence. materials of Quaternar y age older than 0.6 Ma can be analysed to investigate landscape developmen t, paleoclimate, hominid evolution or hydrogeochemistry in carbonate terrai ns. Copyright (C) 1998 Elsevier Science Ltd.