An internally consistent triple oxygen isotope calibration of standards for silicates, carbonates and air relative to VSMOW2 and SLAP2

Abstract

High precision triple oxygen isotope measurements are becoming a more common analysis in laboratories. There is a lack of calibrated standards to use for triple oxygen isotope measurements and this has led to data being presented on different scales rather than to the traditional VSMOW2-SLAP2 scale. Here we present triple oxygen isotope values of standard carbonates, CO2 liberated from carbonates, silicates and air calibrated to the VSMOW2-SLAP2 scale. We analyzed VSMOW2 and SLAP2 to calibrate our reference gas. Our measured δ18O value of SLAP2 is −55.55‰, indistinguishable from the accepted value of −55.5‰. Our Δ′17O value of SLAP2 (λ = 0.528) is not zero, but rather −0.015‰, corresponding to a δ17O value of −29.741‰. The Δ′17O values of carbonate standards NBS19, IAEA603 and NBS18 are −0.102, −0.100 and − 0.048‰, respectively (±0.010). For CO2 of calcite liberated by phosphoric acid digestion at 25 °C, the θACID value at 25 °C is 0.5230 ± 0.0003. These results can be used to correct triple oxygen isotope measurements of CO2 released by phosphoric acid digestion in other laboratories. We present triple oxygen isotope values for UW Garnet-2, NBS-28, San Carlos Olivine (NM-SCO), and our in-house quartz standard (NM-Q). Aliquots of NM-Q and NM-SCO are available from the Center for Stable Isotopes (CSI), New Mexico for interlaboratory comparison. Our δ17O and δ18O values for air are 12.178‰ (±0.066) and 24.046‰ (±0.117), respectively, with a corresponding Δ′17O value of −0.441 ± 0.012‰. With the availability of common standards, all laboratories making δ17O-δ18O measurements can calibrate their reference gas relative to the VSMOW2-SLAP2 scale. Laboratories making triple oxygen isotope measurements on CO2 released from carbonates using phosphoric acid digestion can correct to the bulk carbonate value.