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

Joshua Shea , Ery Hughes, Robert Balzar, Ilya Bindeman, Jon Blundy, Richard Brooker, Roman Botcharnikov, Pierre Cartigny, EIMF -, Glenn Gaetani, Geoff Kilgour, John Maclennan, Brian Monteleone, David Axford Neave , Oliver Shorttle 

We introduce three new 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 hydrous basaltic 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. Additionally, their hydrogen concentration and hydrogen 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. The analysis involves rastering over an area of 20 〖μm〗^2 for 100 cycles, resulting in a 40 μm-wide analytical pit. 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 precision of ± 1.00 ‰ 1RSE or better between 163_(-5.2)^(+5.1) and 267_(-8.9)^(+8.9)  μg g^(-1) CO_2, depending on set-up sensitivity. Precision reported here is improved by a factor of three at comparable concentrations to that previously reported elsewhere. Carbon blanks were characterised by measuring carbon-free olivines, allowing for accurate blank corrections on δ_ ^13 C measurements. After correcting for blank signals and instrument mass fractionation, we measure δ_ ^13 C in glasses with low CO_2 concentrations 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 from the East Pacific Rise and a set of synthetic basaltic glasses are presented to demonstrate our approach. The reference materials and SIMS set-up can be used to significantly improve the accuracy and precision of del13C measurements in natural basaltic glasses and are applicable across a wide range of geologically relevant carbon contents.