RATIONALE
Oxygen triple isotope compositions give key information for understanding physical processes during isotopic fractionation between the geo‐, hydro‐, bio‐, and atmosphere. For detailed discussion of these topics, it is necessary to determine precise ¹⁷O‐excess values of terrestrial silicate/oxide minerals with respect to Vienna Standard Mean Ocean Water (VSMOW).
METHODS
Water was fluorinated in an electrically heated Ni‐metal tube into which water and BrF₅ were loaded for the quantitative extraction of oxygen. Silicate/oxide minerals were fluorinated by heating with a CO₂ laser in an atmosphere of BrF₅. The extracted oxygen was purified and isotope ratios of the oxygen triple isotope compositions were determined using a Finnigan MAT253 isotope ratio mass spectrometer.
RESULTS
The oxygen triple isotope compositions of meteoric water and terrestrial silicate/oxide minerals fall on statistically distinguishable fractionation lines, defined as [ln(δ¹⁷O + 1) = λln(δ¹⁸O + 1) + Δ], where λ and Δ correspond to the slope and intercept, respectively. The fractionation line for meteoric water has λ = 0.5285 ± 0.0005 and Δ = 0.03 ± 0.02‰ and for terrestrial silicate/oxide minerals has λ = 0.5270 ± 0.0005 and Δ = –0.070 ± 0.005‰, at the 95% confidence limit.
CONCLUSIONS
All the analyzed terrestrial silicate/oxide minerals including internationally accepted reference materials (NBS‐28, UWG‐2, and San Carlos olivine) have a negative ¹⁷O‐excess with respect to VSMOW. We propose that it is necessary to specify if the determined δ¹⁷O values of terrestrial and extraterrestrial samples are expressed as the difference from VSMOW or the terrestrial silicate mineral‐corrected value.