Grain-scale heterogeneities in the stable carbon and oxygen isotopic compositions of the international standard calcite materials (NBS 19, NBS 18, IAEA-CO-1, and IAEA-CO-8)

摘要

We determined grain-scale heterogeneities (from 6 to 88 mu g) in the stable carbon and oxygen isotopic compositions (delta C-13 and delta O-18) of the international standard calcite materials (NBS 19, NBS 18, IAEA-CO-1, and IAEA-CO-8) using a continuous-flow isotope ratio mass spectrometry (CF-IRMS) system that realizes a simultaneous determination of the delta C-13 and the delta O-18 values with standard deviations (S.D.) of less than 0.05 parts per thousand for CO2 gas. Based on the S.D. of the delta C-13 and delta O-18 values determined for CO2 gases evolved from the different grains of the same calcite material, we found that NBS 19, IAEA-CO-1, and IEAE-CO-8 were homogeneous for delta C-13 (less than 0.10 parts per thousand S.D.), and that only NBS 19 was homogeneous for delta O-18 (less than 0.14 parts per thousand S.D.). On the level of single grains, we found that both IAEA-CO-1 and IAEA-CO-8 were heterogeneous for delta O-18 (1.46 parts per thousand and 0.76 parts per thousand S.D., respectively), and that NBS 18 was heterogeneous for both delta C-13 and delta O-18 (0.34 parts per thousand and 0.54 parts per thousand S.D., respectively). Closer inspection of NBS 18 grains revealed that the highly deviated isotopic compositions were limited to the colored grains. By excluding such colored grains, we could also obtain the homogeneous delta C-13 and delta O-18 values (less than 0.18 parts per thousand and less than 0.16 parts per thousand S.D., respectively) for NBS 18. We conclude that NBS 19, IAEA-CO-1, or pure grains in NBS 18 are suitable to be used as the standard reference material for delta C-13, and that either NBS 19 or pure grains in NBS 18 are suitable to be used as the reference material for delta O-18 during the grain-scale isotopic analyses of calcite. Copyright (C) 2008 John Wiley & Sons, Ltd.