Accurate measurements of Th–U isotope ratios for carbonate geochronology using MC-ICP-MS

Abstract

We present an advanced method to produce accurate and reproducible U and Th isotope data for carbonate geochronology using very small sample sizes. We analysed speleothem, marine calcite and coral samples by MC-ICP-MS and compared the data and its precision to U and Th data obtained by TIMS using the same samples. MC-ICP-MS needs between 0.05 and 0.34 ng of Th compared with 0.3–1.4 ng Th for TIMS analyses to obtain comparable precision. No separation of Th and U is necessary because of U and Th recoveries of circa 100% and 75–100%, respectively. Th and U analyses are carried out for a range from masses 228 to 238 in two separate runs, with mass ²³⁰Th on the Daly detector for measuring Th isotope ratios and ²³⁴U on the Daly detector for measuring U isotope ratios. Silicate rock standard “Table Mountain Latite” (TML) ²³⁵U/²³⁸U ratios are used to correct for instrumental mass bias. For calibrating the gain of the Daly and Faraday detectors (Daly–Faraday gain), a 1 ppt Aldrich U solution spiked with ²³³U–²³⁶U double spike is measured with mass ²³⁵U in the Daly detector. We demonstrate the accuracy and precision of our analytical data obtained for standard solution UCSC-Th-A, the TML and eleven spiked TML samples over the past 10 months. Literature data obtained by both TIMS and MC-ICP-MS lie well within the error of our UCSC-Th-A ²³⁰Th/²³²Th value of 0.00000585 ± 5 (2 sd, n = 29). Our (²³⁴U/²³⁸U) and (²³⁰Th/²³⁸U) activity ratios for the TML compare well with results of other MC-ICP-MS studies. Our external precision of ²³⁰Th/²³²Th and ²³⁴U/²³⁸U ratios for the unspiked TML is slightly worse than other MC-ICP-MS studies, but reflects either significantly smaller sample sizes or much shorter time for measurement.