Tracing sulfur sources in the crust via SIMS measurements of sulfur isotopes in apatite

We present a refined approach for acquiring sulfur (S) isotope compositions (S-33/S-32, S-34/S-32) in apatite by secondary ion mass spectrometry (SIMS), including the characterisation of new reference materials. In order to test the method, we analyzed potential apatite reference samples for their S isotope ratios via three different bulk methods. The investigated apatite samples contain S concentrations between similar to 160 mu/g and 3100 mu/g and their S-34/S-32(delta S-34) ratios deviate by more than 25%0 from the Vienna-Canyon Diablo Tmilite (VCDT) standard. We identified four candidates as new primary reference materials for routine SIMS S isotope measurements of apatite. Based on ICP-MS, EA-IRMS, and fluorination analyses, recommended S isotope values are +12.27 +/- 0.22 (2 sigma) parts per thousand delta S-34 for SAP1, +14.02 +/- 0.22 (2 sigma) parts per thousand delta S-34 for Bigl, -1.06 +/- 0.80 (2 sigma) parts per thousand delta S-34 for Durango-A, and -1.39 +/- 0.48 (2 sigma) parts per thousand for Durango-B. By selecting one of those four primary standards for SIMS analysis, the S isotope values of the other reference materials and additional tested apatite specimens can be reproduced to within 1 parts per thousand. Under optimized SIMS conditions, single spot uncertainty for delta(34)Sthat combines the within-spot precision and the repeatability of measurements of the primary apatite reference material during an analytical session is +/- 0.4 parts per thousand (95% CI). We also show that in apatite with S > 1000 mu g/g, SIMS analysis permits the detection of mass-independent S isotope signatures (i.e., Delta S-33) that are larger than similar to 1.0 parts per thousand if an average of multiple grains is used, and larger than similar to 1.5 parts per thousand for a single analytical point. Furthermore, our study shows that apatite can record S isotope signatures from extremely diverse environments, making this near-ubiquitous mineral a key candidate for tracing S source reservoirs and to track the pathway of magmatic-hydrothermal fluids in a wide range of geological settings.

Automated summary

The study introduces a refined method for acquiring sulfur isotope compositions in apatite via SIMS, identifying four new primary reference materials. Under optimized conditions, SIMS analysis can reproduce S isotope values within 1‰ for other reference materials and apatite samples. Additionally, the study demonstrates the ability of apatite to record diverse sulfur isotope signatures, making it a key candidate for tracing sulfur source reservoirs and tracking magmatic-hydrothermal fluid pathways in various geological settings.