Analysis of stable isotope ratios of Ba by double-spike standard-sample bracketing using multiple-collector inductively coupled plasma mass spectrometry

A precise analytical method for determining stable Ba isotope ratios was developed by applying a Ba-130-Ba-135 double-spike corrected standard-sample bracketing method with multiple-collector inductively coupled plasma mass spectrometry (MC-ICP-MS). Data were expressed as per mil deviation from a Ba standard in the delta Ba-137/134 notation. Careful examinations of the temporal instabilities of the instrument and of the Xe-130 and Xe-134 interference permitted accurate analysis of delta Ba-137/134. The isotopic ratios of the Ba-130-Ba-135 double-spike used in this study were Ba-134/Ba-130 = 0.076528, Ba-135/Ba-130 = 1.060129, and Ba-137/Ba-130 = 0.209145. These were determined iteratively by measurements of delta Ba-137/134 in the IAEA-CO-9 standard with different spike-sample mixing ratios in order to achieve an invariant delta Ba-137/134 value of IAEA-CO-9. The reproducibility of delta Ba-137/134 of IAEA-CO-9 was +/- 0.032 parts per thousand (2SD, n = 42), which was about 5 times better than that reported by a previous study. The delta Ba-137/134 values of JB-2, JA-2, and BHVO-2 igneous rock standard reference materials were 0.086 +/- 0.016 parts per thousand (2SD, n = 6), 0.016 +/- 0.034 parts per thousand (2SD, n = 7), and 0.058 +/- 0.019 parts per thousand (2SD, n = 5), respectively. A significant difference was observed between the JB-2 and JA-2 results, whereas no significant difference was observed for BHVO-2. These results show that the improved precision opens up a possibility to use delta Ba-137/134 as a tracer in igneous processes.