Potential tourmaline reference materials for microbeam B and Sr isotopic analyses

Abstract

Tourmaline, a borosilicate, is widespread in many kinds of rocks. Due to its high boron content, tourmaline is quite suitable for in situ B isotope analysis. Tourmaline contains moderate Sr but very low Rb and recent studies show that this mineral has the potential for in situ Sr isotope analysis. Here, we present three new natural tourmaline reference materials GIGT, XJ-1 and XJ-3 for in situ B and Sr isotope measurement by LA-MC-ICP-MS. LA-MC-ICP-MS analyses for these three tourmalines show homogeneous B isotopic compositions with an average δ¹¹B of −12.63 ± 0.51‰ (2SD, n = 74) for GIGT, −11.90 ± 0.64‰ (2SD, n = 78) for XJ-1 and −11.91 ± 0.64‰ (2SD, n = 73) for XJ-3, which agree well with results determined by solution MC-ICP-MS (−12.65 ± 0.12, −11.90 ± 0.24, −11.89 ± 0.19). XJ-1 has relatively high Sr content (∼160 μg g⁻¹), but extremely low Rb/Sr (<0.0005). A total of 176 LA-MC-ICP-MS analyses of XJ-1 yield an average ⁸⁷Sr/⁸⁶Sr of 0.70827 ± 0.00021 (2SD), which is in good agreement with the result measured by solution MC-ICP-MS (0.708249 ± 0.000020, 2SD). The homogeneous B isotopes and Sr isotopes suggest that all three tourmalines presented in this study can serve as primary reference materials for in situ B isotope analysis and XJ-1 is suitable to be a monitor reference material for in situ Sr isotope analysis by LA-MC-ICP-MS.