Accurate U–Th radioactive disequilibrium analyses of carbonate rock samples using commercially available U and Th reagents and multi-collector ICP-MS

This study aims at developing a new analytical procedure for U–Th radioactive disequilibrium of carbonate rock samples. The procedure utilizes commercially available U and Th reagents as spikes for isotope dilution analysis with a multiple-collector inductively coupled mass spectrometer (MC-ICP-MS). Uranium and thorium in a carbonate sample were purified by Fe-coprecipitation, anion exchange resin, and U/TEVA resin. Isotopic compositions of purified spiked and unspiked U and Th were measured respectively for abundance and isotopic analyses. The correction of mass fractionation and the Daly/Faraday gain factor for Th measurements is carried out with a U standard solution. Accuracy of our method was confirmed by analyses of a coral sample that had been dated with thermal ionization mass spectrometry (TIMS). Our results indicate that U–Th disequilibrium studies are possible using easily available U and Th reagents as spikes.