Boron isotope analysis of silicate glass with very low boron concentrations by secondary ion mass spectrometry

Author Posting. © The Author(s), 2014. This is the author's version of the work. It is posted here by permission of International Association of Geoanalysts for personal use, not for redistribution. The definitive version was published in Geostandards and Geoanalytical Research 39 (2015): 31-46, doi:10.1111/j.1751-908X.2014.00289.x.Here we present an improved method for the determination of the boron isotopic composition of volcanic glasses with boron concentrations of as low as 0.4–2.5 μg g−1, as is typical for mid-ocean ridge basalt glasses. The analyses were completed by secondary ion mass spectrometry using a Cameca 1280 large-radius ion microprobe. Transmission and stability of the instrument and analytical protocol were optimised, which led to an improvement of precision and reduction in surface contamination and analysis time compared with earlier studies. Accuracy, reproducibility (0.4–2.3‰, 2 RSD), measurement repeatability (2 RSE = 2.5–4.0‰ for a single spot with [B] = 1 μg g−1), matrix effects (≪ 0.5‰ among komatiitic, dacitic and rhyolitic glass), machine drift (no internal drift; long-term drift: ~ 0.1‰ hr−1), contamination (~ 3–8 ng g−1) and machine background (0.093 s−1) were quantified and their influence on samples with low B concentrations was determined. The newly developed set-up was capable of determining the B isotopic composition of basaltic glass with 1 μg g−1 B with a precision and accuracy of ± 1.5‰ (2 RSE) by completing 4–5 consecutive spot analyses with a spatial resolution of 30 μm × 30 μm. Samples with slightly higher concentrations (≥ 2.5 μg g−1) could be analysed with a precision of better than ± 2‰ (internal 2 RSE) with a single spot analysis, which took 32 min.This study was financially supported by the NSF ocean sciences program (OCE grant 1232996 to Dorsey Wanless and HRM).2015-06-1