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.
