Precise determination of U isotopic compositions in low concentration carbonate samples by MC-ICP-MS

We developed a fast and simple analytical procedure for precise determination of U isotopic compositions in low concentration natural samples. The main advantage of the new method is that it requires only 12 ng U and can obtain all U isotopic ratios without using spike. Five carbonate reference materials (JCp-1, RKM-4, RKM-5, GBW04412 and GBW04413) and 3 international standards with different matrices (IAPSO, IRMM-3184 and CRM-U010) were analyzed for (234U/238U) and 238U/235U ratios by MC-ICPMS. Using our method, the results for these standards are in close agreement with the certified values, 1.144±0.004, 0.966±0.004 and 0.990±0.003 for (234U/238U) and 137.72±0.13, 137.64±0.15 and 98.63±0.04 for 238U/235U, in IAPSO, IRMM-3184 and CRM-U010, respectively. The long-term reproducibility of (234U/238U) and 238U/235U is 0.970±0.002 and 137.56±0.09; 1.144±0.004 and 137.72±0.13, respectively, for in-house U solution and IAPSO. The new (234U/238U) results for carbonates show much better precision than previous studies and also reflect their age variability. The obtained 238U/235U ratios, representing the first measurements in these carbonate specimens, are rather constant. The method described here requires only 12 ng of U for analysis and can be completed in 5.2 min. The approach provides a fast method to measure (234U/238U) and 238U/235U ratios in sample matrices commonly encountered in studies of chemical weathering, oceanography and paleoclimatology.