Two-point normalization for reducing inter-laboratory discrepancies in δ17O, δ18O, and Δ′17O of reference silicates

Abstract

The δ17O and δ18O values of a number of terrestrial minerals and rocks have been determined using laser fluorination method worldwide. For the comprehensive and congruous interpretation of oxygen isotope data, the δ-values should be normalized by the two-point method (i.e., the VSMOW-SLAP scale) to eliminate inter-laboratory bias. In this study, the δ17O and δ18O values of VSMOW and SLAP were measured to calibrate our laboratory working standard O2 gas. The O2 gas liberated from the water samples was purified using the preparation line normally employed for solid samples, and analyzed by the same mass spectrometer. From the analyses of VSMOW and SLAP, the oxygen isotope compositions of the international silicate standards (UWG2 garnet, NBS28 quartz, and San Carlos olivine) were normalized to the VSMOW-SLAP scale (two-point calibration), and then the Δ′17O values were determined. Using the δ-values obtained in this way, the inter-laboratory discrepancy of the δ17O and δ18O results of the silicate standards could be reduced. The VSMOW-SLAP scaling for δ17O and δ18O analysis of silicates provides the most effective way to obtain accurate and precise data. In reporting the Δ′17O values, it is important to make the choice of the reference fractionation line into account because the Δ′17O value is quite variable owing to the slope and y-intercept of the linear relation of the δ-values. The reference fractionation line obtained from the measurement of the low- and high-δ18O reference silicates would help to compare ∆′17O values. We confirmed that the ∆′17O results of the international silicate standards based on the two-point silicate reference line were consistent with the results from other laboratories.

High-precision Δ′¹⁷O measurements of geothermal H₂O and MORB on the VSMOW-SLAP scale: evidence for active oxygen exchange between the lithosphere and hydrosphere

“…The δ 18 O and ∆′ 17 O values of JFB were previously determined to be in the range from +5.33‰ to +5.593‰ and from -64 × 10 −6 to -1 × 10 −6 , respectively (Ahn et al, 2012;Kim et al, 2019;Kusakabe and Matsuhisa, 2008). The revised values on the VSMOW-SLAP scale were reported recently to be +5.692‰ and -43 × 10 −6 , respectively (Kim et al, 2020). Additionally, NBS28, which is a quartz isotope standard distributed by IAEA, was routinely measured to verify the accuracy of our measurement.…”

“…Additionally, the normalization ranges can be easily changed depending on the approximate oxygen isotopic compositions in the samples. Although δ 18 O value of silicate is typically out of the VSMOW-SLAP range (approximately +5‰ versus VSMOW in δ 18 O) in general, we can accurately determine the oxygen isotope ratios of silicate using the in-house H 2 O standards by interpolation rather than extrapolation (Kim et al, 2020;Pack et al, 2016;Sharp et al, 2016;Wostbrock et al, 2020). In addition to the three in-house H 2 O standards, we analyzed Greenland ice sheet precipitation (GISP), which is a H 2 O isotope standard distributed by IAEA.…”

“…Stable isotope ratios of H 2 O have provided useful information on the origin of terrestrial H 2 O in the global hydrologic cycle. Along with traditional 2 H/ 1 H and 18 O/ 16 O ratios, recent advances in high-precision measurements of the ∆′ 17 O [=ln(δ 17 O+1) -0.528•ln(δ 18 O+1); detailed in Subsection "Definitions"] of H 2 O (Barkan and Luz, 2005;Luz and Barkan, 2010;Steig et al, 2014) and silicates (Kim et al, 2020;Pack et al, 2016;Sharp et al, 2016;Tanaka and Nakamura, 2013;Wostbrock et al, 2020) have enabled researchers to clarify various geological and geochemical processes such as evolution of the terrestrial hydrosphere and lithosphere throughout the geologic time scale (Herwartz et al, 2021;Pack and Herwartz, 2014;Sengupta et al, 2020;Sengupta and Pack, 2018;Tanaka and Nakamura, 2013). Many recent studies tion of temperature (Matsuhisa et al, 1978;Sharp et al, 2016).…”

“…The fluorination technique using BrF 5 (Clayton and Mayeda, 1963) has been used to quantitatively convert oxygen in silicates to O 2 and to determine the accurate and precise oxygen isotope ratios of silicates using isotope-ratio mass spectrometry (IRMS). Moreover, the 17 O/ 16 O and 18 O/ 16 O ratios of isotopic reference silicates such as NBS28, UWG-2, and San Carlos olivine have been measured relative to Vienna standard mean ocean water (VSMOW) (Kim et al, 2020;Pack et al, 2016;Sharp et al, 2016;Tanaka and Nakamura, 2013;Wostbrock et al, 2020) to normalize the oxygen isotope ratios of silicates on the VSMOW scale, which has been used to normalize the oxygen isotope ratios of H 2 O in general. However, significant disagreement has been reported on the determined ∆′ 17 O values of isotopic reference silicates (Pack et al, 2016;Sharp et al, 2016).…”

“…However, significant disagreement has been reported on the determined ∆′ 17 O values of isotopic reference silicates (Pack et al, 2016;Sharp et al, 2016). To resolve this discrepancy, two-point normalization based on VSMOW and standard light Antarctic precipitation (SLAP) indices has been adopted in recent studies (Kim et al, 2020;Sharp et al, 2016;Wostbrock et al, 2020) following the procedure previously used to normalize oxygen and hydrogen isotope ratios of H 2 O in general. Pack et al (2016) determined the ∆′ 17 O value of San Carlos olivine at two different laboratories based on VSMOW and SLAP.…”