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 C and δ 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 C and δ 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 C and O than the translucent grains.

Conclusions: Low-abundance opaque grains (1-2 grains out of 100 grains) have lower δ C and δ O values, suggesting that these grains should be eliminated when using IAEA-603 for single-grain (microscale) isotope analysis.