RESUMO
RATIONALE: Measurements of oxygen-17 excess ((17)Oexcess) in carbonates have become of great importance. However, to compare results obtained by different laboratories, it is necessary to normalize them to international standards. With this purpose in mind, we measured accurate and high precision δ(17)O and (17)Oexcess values for NBS19 and NBS18, two international standards, for which δ(18)O values are already widely used as the references for carbonates. METHODS: The measurements are based on isotopic exchange at steady state between O2 and CO2 over hot platinum sponge at 750°C. Dual-inlet isotope ratio mass spectrometry (IRMS) measurements of the δ(17)O and δ(18)O values of this O2 allow δ(17)O values of CO2 to be obtained with very high precision (0.01 to 0.03) and, correspondingly, accurate (17)Oexcess values with a precision of 5 per meg. RESULTS: We measured, for the first time, the δ(17)O values and (17)Oexcess of CO2 liberated from NBS19 at 25°C (39.196 ± 0.026; 20.276 ± 0.015 and -227 ± 4 per meg, respectively) and NBS18 (17.591 ± 0.041; 9.253 ± 0.018 and 3 ± 5 per meg, respectively). The values are given versus VSMOW. CONCLUSIONS: Accurate values of δ(17)O and (17)Oexcess of the international standards NBS19-CO2 and NBS18-CO2 are now available. The new values should be used for normalization of measured oxygen isotope ratios of carbonates to allow meaningful comparison of results among different laboratories.
