Barite, Anhydrite and Gypsum Reference Materials for In Situ Oxygen and Sulfur Isotope Ratio Measurements

Secondary ion mass spectrometry was used to test the δ¹⁸O and δ³⁴S nanogram-scale homogeneity of a suite of candidate sulfate minerals, ultimately selecting three barite, two anhydrite, and two gypsum samples from the Royal Ontario Museum that have repeatabilities for their SIMS measurements of better than ±0.39‰ and ±0.37‰ (1s) for oxygen and sulfur isotope ratios, respectively. Metrological splits of each of the seven materials were sent to multiple gas source isotope ratio mass spectrometry laboratories in order to establish their absolute ¹⁸O/¹⁶O and ³⁴S/³²S ratios. The inter-laboratory results of GS-IRMS analyses yielded reasonably narrow ranges in δ¹⁸O_VSMOW, whereas larger variations in δ³⁴S_VCDT values were found between the results from the gas source laboratories. All samples have good reproducibility within laboratories of GS-IRMS 10³δ¹⁸O values of between ±0.24‰ and ±0.44‰ (1s). The reproducibility within laboratories of GS-IRMS 10³δ³⁴S values range from ±0.07‰ to ±0.99‰ (1s). Here we also discuss some of the current analytical limitations affecting these isotope-mineral systems. A total of 256 metrological splits have been prepared from each of these seven materials; these aliquots will be made available to the global geochemical community.