Development of sulfur isotope microanalysis of calcite using secondary ion mass spectrometry: methodology and matrix effects†

Sulfur isotope microanalysis of calcite using secondary ion mass spectrometry (SIMS) enables high spatial resolution investigations of microstructures and biological influences – areas where conventional bulk analytical methods fall short. This technique relies heavily on matrix-matched reference materials. However, the development of SIMS applications in calcite is currently limited by the scarcity of such standards; only one calcite standard is available, and it lacks systematic assessment of isotopic homogeneity. In this study, a biogenic calcite sample (Jas-outer-Mariana) underwent 295 SIMS sulfur isotope analyses and was found to exhibit homogeneous sulfur isotopic compositions, with an external reproducibility of <0.41‰ (1σ), making it a promising SIMS reference material. Matrix effects were evaluated using various carbonate samples, including abiotic aragonite (Vs001/1-A), low-organic-matter biogenic calcites (G-Mariana and B-Indian), and a high-organic-matter biogenic calcite (C-Mariana). The results reveal substantial matrix effects on the secondary ion yields of sulfur and its isotopes, particularly between low- and high-organic-matter calcites, likely due to the presence of organic matter and organic sulfur. This study highlights both the potential and the limitations of in situ sulfur isotope analysis in calcite by SIMS and emphasizes the need for caution when interpreting data from high-organic-matter samples.