Improved Precision and Reference Materials for Stable Carbon Isotope Measurement in Basaltic Glasses using Secondary Ion Mass Spectrometry

We introduce three new synthetic basalt reference materials and a new high-precision set-up for stable carbon isotope measurement in basaltic glasses using a large-geometry secondary ion mass spectrometry (SIMS) instrument. The new reference materials, characterised for carbon mass fraction and isotope composition, show homogeneity for in situ analysis for the reported set-up. Their bulk hydrogen mass fraction and isotope ratios are reported. Our SIMS protocol uses multi-collection, cycling between concurrent measurements of ¹²C and ¹³C on electron multipliers, and either ³⁰Si or ¹⁸O, as a reference mass, on a 10¹¹ Ω resistor Faraday cup. This set-up achieves high measurement repeatability for δ¹³C down to ± 0.35‰ 1RSE at 1706 ⁺⁸⁹/_-88 μg g^-1 CO₂, with ± 1.00‰ 1RSE or better between 163 ^+5.1/_-5.2 and 267 ^+8.9/_-8.9 μg g^-1 CO₂, using a 10 nA primary beam current and a 40 μm analytical pit over a 100 cycle analysis. Carbon blanks were characterised by measuring carbon-free olivines, allowing for blank corrections on δ¹³C measurements. After blank and instrument mass fractionation corrections, we measure δ¹³C in glasses down to 26.16 ^+0.85/_-0.86 μg g^-1 CO₂ with a final measurement standard sample deviation of ± 2.97‰ 1s. We report in situ measurements on an ocean floor basaltic glass and a set of synthetic basaltic glasses to demonstrate our approach. Reference materials and the SIMS set-up improve the accuracy and precision of δ¹³C measurements in natural basaltic glasses across a wide range of geologically relevant carbon contents.