Determination of ¹⁷O-excess of terrestrial silicate/oxide minerals with respect to Vienna Standard Mean Ocean Water (VSMOW)

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 CO ₂ 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(δ ¹⁸O + 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.