High-Precision Determination of the 238U/235U Isotope Ratio in Rocks by Multicollector Inductively Coupled Plasma Mass Spectrometry

Abstract

The paper describes the high-precision U-isotope analysis of rocks. The analysis is based on multicollector inductively coupled plasma mass spectrometry (MC−ICP−MS) and includes, as an essential part, the three-step ion-exchange chromatography separation of uranium from chemically complex solutions. In the analysis, the 235U and 238U mass-bias of the MC−ICP−MS is corrected against the reference ratio of 233U and 236U in a double-spike added. The doublespiking precedes sample chemical digestion. This approach is useful as it corrects isotope data for a possible U-isotope fractionation that could have occurred in the chemical preparation of samples. The double-spike 236U/233U is 1.03183 ± 0.00005. The extraction procedure ensures uranium yield of more than 80%. The accuracy of the analysis was estimated by the study of 238U/235U in an international standard sample IRMM-3184 as well as in monitor samples. The uncertainty was estimated at ±0.008% (2SD), which is comparable with the accuracy of the results previously published by foreign researches. The analysis was elaborated for studying natural 238U/235U ratio variations. It was used by the authors for the high-precision determination of the U-isotope composition in rocks (granites, shales, volcanic rocks) and U-bearing minerals.