Determining Ba isotopes for barite using Na2CO3 exchange reaction and double-spike method on MC-ICP-MS
This study presents a practical method of high precision Ba isotope measurement for barite using Na2CO3 exchange reaction and 135Ba-136Ba double-spike method by MC-ICP-MS. The effects of matrices and conditions of exchange reaction were rigorously examined. The precision and accuracy of this method were tested by measurement of a synthetic barite standard made by reacting Ba(NO3)2 with Na2SO4. The δ137/134Ba of the synthetic barite is 0.06 ± 0.03‰ (2SD, n=16) relative to SRM3104a, which is well consistent with the recommended value (0.07± 0.04‰, 2SD, n=769). The robustness of this method was further assessed by replicated analyses of six barite reference materials, which cover a large range of BaSO4 contents from 18.87 wt.% to nearly 100 wt.%. The δ137/134Ba of pure barite international standard NBS127 is -0.21±0.03‰ (2SD, n=20); δ137/134Ba of natural standards GBW07811, 07812, 07814, 07815 and 07816 are -0.06±0.05‰ (2SD, n=12), 0.24±0.05‰ (2SD, n=48), -0.02±0.02‰ (2SD, n=10), -0.05±0.04‰ (2SD, n=18), -0.01±0.05‰ (2SD, n=20), respectively. Based on repeated analyses of these standards, the long-term external precision of δ137/134Ba is better than 0.05‰ (2SD), much smaller than the variation in these barite standards (up to 0.45‰), revealing the large Ba isotope fractionation in natural barites.