High-precision barium isotope measurements by MC-ICP-MS

We present a high precision method to measure Ba isotopes by multiple-collector inductively coupled plasma-mass spectrometry (MC-ICP-MS). Barium is separated from matrices by using a cation exchange resin (AG50W-X12, 200-400 mesh). Instrumental mass bias of Ba isotopes was corrected by a sample-standard bracketing method using SRM3104a as the bracketing standard. Potential effects of different matrices from resin and samples, and acid molarity and concentration mismatch were rigorously evaluated in this study. The precision and accuracy of this method were tested by the measurement of a synthetic solution made by mixing SRM3104a Ba with other matrix elements. The average delta Ba-137/134 of the synthetic solution is -0.005 +/- 0.047 parts per thousand (2SD, n = 36) relative to SRM3104a. The robustness of this method was further assessed by replicated analyses of 8 reference materials, including igneous rocks with mafic to felsic compositions. The delta Ba-137/134 of basalt standards BCR-2, BHVO-2, and JB-2 is 0.050 +/- 0.039 parts per thousand (2SD, n = 13), 0.047 +/- 0.028 parts per thousand (2SD, n = 22), and 0.085 +/- 0.035 parts per thousand (2SD, n = 19), respectively; diabase standard W-2 is 0.035 +/- 0.022 parts per thousand (2SD, n = 11); andesite standard AGV-1 is 0.047 +/- 0.040 parts per thousand (2SD, n = 11) and JA-2 is 0.038 +/- 0.048 parts per thousand (2SD, n = 17); rhyolite standard RGM-1 is 0.142 +/- 0.030 parts per thousand (2SD, n = 15); and granodiorite standard GSP-2 is 0.013 +/- 0.046 parts per thousand (2SD, n = 15). Two late Mesozoic basalts from China have a delta Ba-137/134 of 0.132 +/- 0.020 parts per thousand (2SD, n = 7) and 0.001 +/- 0.034 parts per thousand (2SD, n = 7), respectively. Based on repeated analyses of the synthetic standard and a carbonate standard IAEA-CO-9, the long-term external precision of our method is better than +/- 0.05 parts per thousand, much smaller than the variation of delta Ba-137/134 in these reference standards and samples (up to 0.27 parts per thousand). Therefore, the Ba isotopic composition can be used as a novel tracer to study geochemical processes.