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

Abstract

A precise analytical method for determining stable Ba isotope ratios was developed by applying a ¹³⁰Ba-¹³⁵Ba 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 δ^137/134Ba notation. Careful examinations of the temporal instabilities of the instrument and of the ¹³⁰Xe and ¹³⁴Xe interference permitted accurate analysis of δ^137/134Ba. The isotopic ratios of the ¹³⁰Ba-¹³⁵Ba double-spike used in this study were ¹³⁴Ba/¹³⁰Ba = 0.076528, ¹³⁵Ba/¹³⁰Ba = 1.060129, and ¹³⁷Ba/¹³⁰Ba = 0.209145. These were determined iteratively by measurements of δ^137/134Ba in the IAEA-CO-9 standard with different spike-sample mixing ratios in order to achieve an invariant δ^137/134Ba value of IAEA-CO-9. The reproducibility of δ^137/134Ba of IAEA-CO-9 was ±0.032‰ (2SD, n = 42), which was about 5 times better than that reported by a previous study. The δ^137/134Ba values of JB-2, JA-2, and BHVO-2 igneous rock standard reference materials were 0.086 ± 0.016‰ (2SD, n = 6), 0.016 ± 0.034‰ (2SD, n = 7), and 0.058 ± 0.019‰ (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 δ^137/134Ba as a tracer in igneous processes.