Grain‐scale stable carbon and oxygen isotopic variations of the international reference calcite, IAEA‐603

Rationale The new international reference material IAEA‐603 (calcite) for stable carbon and oxygen isotopes ( δ 13 C and δ 18 O values) was released in 2016 to replace the previous reference material, NBS19 (exhausted). We examined the grain‐scale isotopic variations in IAEA‐603 for application to microscale isotopic analysis of carbonate samples. Methods Individual grains of IAEA‐603 were analyzed with an IsoPrime100 isotope ratio mass spectrometer with a customized continuous‐flow gas preparation system (MICAL3c). The individual grains of IAEA‐603 were observed by optical and scanning electron microscopy, and their observational characteristics (grain color and size) were compared with their stable isotope compositions. Results Translucent grains (main component of IAEA‐603; grain weight, 4–132 μg) had homogeneous isotopic ratios, comparable with the grain‐scale isotopic homogeneity of NBS 19. Their average δ 13 C and δ 18 O values were the same as the recommended values determined by the IAEA. Opaque (whitish) grains (1–2 per 100 grains; grain weight, 8–63 μg) were significantly more depleted in 13 C and 18 O than the translucent grains. Conclusions Low‐abundance opaque grains (1–2 grains out of 100 grains) have lower δ 13 C and δ 18 O values, suggesting that these grains should be eliminated when using IAEA‐603 for single‐grain (microscale) isotope analysis.