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

Abstract

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 δ^137/134Ba of the synthetic solution is −0.005 ± 0.047‰ (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 δ^137/134Ba of basalt standards BCR-2, BHVO-2, and JB-2 is 0.050 ± 0.039‰ (2SD, n = 13), 0.047 ± 0.028‰ (2SD, n = 22), and 0.085 ± 0.035‰ (2SD, n = 19), respectively; diabase standard W-2 is 0.035 ± 0.022‰ (2SD, n = 11); andesite standard AGV-1 is 0.047 ± 0.040‰ (2SD, n = 11) and JA-2 is 0.038 ± 0.048‰ (2SD, n = 17); rhyolite standard RGM-1 is 0.142 ± 0.030‰ (2SD, n = 15); and granodiorite standard GSP-2 is 0.013 ± 0.046‰ (2SD, n = 15). Two late Mesozoic basalts from China have a δ^137/134Ba of −0.132 ± 0.020‰ (2SD, n = 7) and 0.001 ± 0.034‰ (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‰, much smaller than the variation of δ^137/134Ba in these reference standards and samples (up to 0.27‰). Therefore, the Ba isotopic composition can be used as a novel tracer to study geochemical processes.