Tourmaline Megacryst Reference Materials for High Precision In Situ Boron Isotope and Elemental Measurement by LA-MC-ICP-MS

Abstract

Tourmaline serves as a vital recorder of geological processes. However, suitable tourmaline reference materials (RMs) for elemental and stable isotopic composition in situ measurements are still limited. In this study, three tourmaline megacrysts (MD-B66, IM-B232 and BR-DG68) were characterised as potential RMs for B isotope measurements by LA-MC-ICP-MS and mass fraction determinations by LA-ICP-MS. Over 251 measurements with LA-MC-ICP-MS on ten randomly selected fragments from MD-B66 consistently yielded B isotope ratios of -7.74 ± 0.25‰ (2s), establishing MD-B66 as a suitably homogeneous primary reference material for high-precision, in situ microbeam B isotope measurements. Notably, the (long-term) intermediate precision of 0.25‰ (2s) for in situ B isotope measurements obtained using this reference material is comparable to that reported from solution MC-ICP-MS methods in the literature. Two other tourmaline megacrysts, with intermediate precision ranging from ± 0.48‰ to ± 0.61‰ (2s) for δ¹¹B measurement, can be employed as secondary RMs for quality control. The mean δ¹¹B values determined by solution MC-ICP-MS for MD-B66 (-7.71 ± 0.32‰, n = 12), IM-B232 (-13.17 ± 0.62‰, n = 8) and BR-DG68 (-13.85 ± 0.32‰, n = 12), with expanded uncertainties at the 95% confidence level, are consistent with those determined by LA-MC-ICP-MS. Among these three new RMs, BR-DG68 displays relatively homogeneous major and trace element mass fractions. Characterisation using both in situ and wet chemical techniques demonstrated the suitability of BR-DG68 as the first tourmaline reference material for elemental measurement by LA-ICP-MS, which would permit matrix-matched and therefore more accurate elemental measurement in tourmalines. Unlike electron probe microanalysis with B and Li contents calculated based on stoichiometric assumptions, direct and accurate measurements of the two low atomic number elements, along with other major and trace elements can be achieved by LA-ICP-MS with the aid of the newly developed tourmaline reference material BR-DG68. Overall, current and future studies in geochemistry may benefit from these newly proposed tourmaline RMs, which should lead to significantly improved precision and accuracy for in situ B isotope and elemental measurement.