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 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 measurement repeatability for δ 13 C down to ± 0.35‰ 1RSE at 1706 +89 / ‐88 μg g ‐1 CO 2 , with ± 1.00‰ 1RSE or better between 163 +5.1 / ‐5.2 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 +0.85 / ‐0.86 μg g ‐1 CO 2 with a final measurement standard sample deviation of ± 2.97‰ 1 s . 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 δ 13 C measurements in natural basaltic glasses across a wide range of geologically relevant carbon contents.