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Abstract
Six tourmaline samples were investigated as potential reference materials (RMs) for boron isotope measurement by secondary ion mass spectrometry (SIMS). The tourmaline samples are chemically homogeneous and cover a compositional range of tourmaline supergroup minerals (primarily Fe, Mg and Li end‐members). Additionally, they have homogeneous boron delta values with intermediate precision values during SIMS analyses of less than 0.6‰ (2s). These samples were compared with four established tourmaline RMs, that is, schorl IAEA‐B‐4 and three Harvard tourmalines (schorl HS#112566, dravite HS#108796 and elbaite HS#98144). They were re‐evaluated for their major element and boron delta values using the same measurement procedure as the new tourmaline samples investigated. A discrepancy of about 1.5‰ in δ11B was found between the previously published reference values for established RMs and the values determined in this study. Significant instrumental mass fractionation (IMF) of up to 8‰ in δ11B was observed for schorl–dravite–elbaite solid solutions during SIMS analysis. Using the new reference values determined in this study, the IMF of the ten tourmaline samples can be modelled by a linear combination of the chemical parameters FeO + MnO, SiO2 and F. The new tourmaline RMs, together with the four established RMs, extend the boron isotope analysis of tourmaline towards the Mg‐ and Al‐rich compositional range. Consequently, the in situ boron isotope ratio of many natural tourmalines can now be determined with an uncertainty of less than 0.8‰ (2s).
Highlights
Authors Katharina Marger, Matthieu Harlaux, Andrea Rielli, Lukas P
A recent breakthrough achievement in the secondary ion mass spectrometry (SIMS) technique – the Hyperion radio-frequency oxygen plasma source (Hyperion RF source) – provides even smaller spot sizes, < 5 lm (Liu et al 2018). Such small analytical volumes serve to decipher geological processes at higher spatial resolution. For accurate analyses both SIMS and LA-MC-ICP-MS require a set of reference materials (RMs), which are chemically homogeneous at the microscopic scale, span the major element compositional range of the minerals to be analysed, and have well-defined homogeneous boron isotope compositions
Chemical compositions of all tourmaline RMs were measured by electron probe microanalyser (EPMA) to determine their species and to evaluate their major and minor element chemical homogeneity
Summary
Authors Katharina Marger, Matthieu Harlaux, Andrea Rielli, Lukas P. The three Harvard tourmalines and the IAEA-B-4 schorl served as RMs during SIMS measurements, and the correction was made with the published reference values (Leeman and Tonarini 2001, Gonfiantini et al 2003).
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