Two new barite reference materials for SIMS sulfur isotope analysis: evaluation of the crystallographic orientation effect and homogeneity†

Barite serves as a crucial archive for reconstructing sulfur cycling evolution throughout geologic history. Microscale space variations in sulfur isotope compositions (δ³⁴S) of barite provide valuable insights into the barite precipitation process. While secondary ion mass spectrometry (SIMS) microanalysis has emerged as a powerful tool for such investigations, the availability of high quality reference materials remains a fundamental requirement for accurate measurements. Although several barite reference materials have been developed for SIMS analysis, the potential influence of crystallographic orientation on sulfur isotope measurements has not been systematically evaluated. Moreover, the development of more in situ barite reference materials with diverse sulfur isotopic compositions would greatly facilitate cross-laboratory data comparison. In this study, we conducted a comprehensive investigation of the crystallographic orientation effect by combining electron backscatter diffraction (EBSD) and SIMS analyses. Our results demonstrate that crystallographic orientation does not produce significant analytical bias in SIMS barite sulfur isotope measurements at current levels of analytical precision. Furthermore, we present two new well-characterized potential reference materials NJU-Ba-1 and NJU-Ba-2 barite specifically developed for microbeam sulfur isotope analysis. Detailed characterization of texture and major element composition confirms the absence of internal zoning in ground fragments from both barite specimens. These two reference materials establish an extended δ³⁴S_V-CDT calibration range for microanalytical studies, with certified values of 6.18 ± 0.34‰ (2SD, N = 17) and 14.16 ± 0.26‰ (2SD, N = 9) for NJU-Ba-1 and NJU-Ba-2 respectively, as determined by gas-source isotope ratio mass spectrometry (GS-IRMS). Extensive SIMS analyses revealed exceptional homogeneity at both inter- and intra-unit scales, with δ³⁴S variations of 0.36‰ (2SD, N = 328) for NJU-Ba-1 and 0.45‰ (2SD, N = 343) for NJU-Ba-2. These performance characteristics indicate that NJU-Ba-1 and NJU-Ba-2 are promising candidates as reference materials for high-precision microanalytical studies of barite sulfur isotopes.