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 analysis in basaltic glasses using large-geometry secondary ion mass spectrometry (SIMS) instrument. The new reference materials, characterised for carbon concentration and isotope composition by step-heating gas extraction and manometry followed by isotope ratio mass spectrometry, show homogeneity for in situ analysis for the reported set-up. Their bulk hydrogen concentration and isotope ratios are reported. Our SIMS protocol uses multi-collection, cycling between concurrent measurements of (^12)C and (^13)C on electron multipliers, and either (^30)Si or (^18)O, as a reference mass, on a 10^11 Ω resistor Faraday cup. This set-up achieves high internal precision for δ_ ^13 C down to ± 0.35 ‰ 1RSE at 1706_(-88)^(+89)  μg g^(-1) CO_2, with ± 1.00 ‰ 1RSE or better between 163_(-5.2)^(+5.1) and 267_(-8.9)^(+8.9)  μg g^(-1) CO_2, 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 δ_ ^13 C measurements. After blank and instrument mass fractionation corrections, we measure δ_ ^13 C in glasses down to 〖26.16〗_(-0.86)^(+0.85)  μg g^(-1) CO_2 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 used here can improve the accuracy and precision of δ_ ^13 C measurements in natural basaltic glasses applicable across a wide range of geologically relevant carbon contents