A study on a natural pyrite sample as a potential reference material for simultaneous measurement of sulfur and iron isotopes using fs-LA-MC-ICP-MSs

Abstract

Pyrite has been extensively utilized for tracing geological processes through its iron and sulfur isotope micro-analysis. However, the lack of matrix-matched reference materials with characterized Fe and S isotopic compositions poses a challenge. In this study, we examined a natural pyrite sample (IGGPy-1) to evaluate its potential as a matrix-matched reference material for in situ micro-analysis of iron and sulfur isotopes. Electron probe microanalysis (EPMA) confirmed the homogeneity of major elements without any growth zoning in pyrite grains. Random spot isotopic analyses conducted using laser ablation multi-collector inductively coupled plasma mass spectrometry (LA-MC-ICP-MS) yielded variances in δ⁵⁶Fe and δ³⁴S of 0.07‰ (2S, N = 105) and 0.13‰ (2S, N = 105), respectively. Based on the results and statistical analysis, it was determined that the IGGPy-1 sample was isotopically homogeneous and had the potential to be used as a reference material. The δ³⁴S_VCDT value, measured using elemental analyzer-isotope ratio mass spectrometry (EA-IRMS), was 17.09 ± 0.30‰ (2S, N = 6). Additionally, δ⁵⁶Fe_IRMM-014 and δ⁵⁷Fe_IRMM-014 values, measured using solution-nebulizing multi-collector inductively coupled plasma mass spectrometry (SN-MC-ICP-MS), were −1.31 ± 0.06‰ (2S, N = 21) and −1.94 ± 0.12‰ (2S, N = 21), respectively. These values are recommended as the isotopic reference values for IGGPy-1. Since the samples with complex core-rim textures resulting from multiple stages in the formation of many sulfide deposits face a significant risk of mismatch in the Fe and S isotopic data obtained from different analytical domains, we presented the technique of simultaneous in situ measurements of Fe–S isotopes in pyrite using a single laser spot by coupling a femtosecond laser ablation system with two sets of MC-ICP-MS. Furthermore, our results clearly demonstrated the significant impact of position effects on the accuracy of Fe and S isotopic data. Therefore, it is crucial to identify appropriate domains within the ablation chamber for placing the “standard” and samples for mitigating these effects.