232Th carbonates by multi-collector ICP- MS

We describe a new method for the measurement of U/P b ratios by isotope dilution multi-collector induct ively coupled plasma mass spectrometry (MC-ICP-MS) for th e dating of geologically young clean carbonates, pa rticularly 15 speleothems. The method is intended for materials c ontaining little or no initial 232Th. We illustrate and validate the method with four examples ranging from 0.57 Ma to 20 Ma ol d. The new method is capable of applying the 235U-207 and 238U-234U206Pb chronometers, common Pb and quantifiable residua l 234U/238U disequilibrium permitting. These provide an alter native to the more widely used 238U-206Pb chronometer, which can be highly inaccurate for samples a few million years old, owing to uncertainties in the excess initial 234U (hence, excess radiogenic 206Pb) commonly observed in speleothems. 20