Standardizing the reporting of Δʹ17O data from high precision oxygen triple-isotope ratio measurements of silicate rocks and minerals

Abstract

Accurate and precise Δʹ17O data are important for quantifying and understanding processes responsible for variations of oxygen triple-isotope ratios in nature. Without universally agreed numerical values of λRL and γRL in the definition Δʹ17O=ln(1+δ17O) – λRLln(1+δ18O) – γRL however, as is currently the situation for measurements on rocks and minerals, reported Δʹ17O data may not be directly comparable. Furthermore, the accuracy of Δʹ17O measurements is linked to calibration (at the ppm level) of individual laboratory’s ‘working standard’ O2 relative to the VSMOW water reference or to the VSMOW-SLAP scale, whereas few laboratories have the facility to make high precision δ17O and δ18O measurements on silicates and waters. Both points may be addressed by defining a reference line from measurements of a pair of silicates which differ substantially in oxygen isotopic composition (comparable to the difference between VSMOW and SLAP), rather than by assigning λRL and γRL values arbitrarily. The resulting reference line has no physical significance. However, because Δʹ17O values reported relative to it are independent of the calibration of the working standard O2, this greatly facilitates Δʹ17O reproducibility and comparability. We propose silicate standards for this purpose and report comparative measurements – from two institutions – of the respective δ17O and δ18O values relative to VSMOW, together with Δʹ17O data relative to various assigned reference lines. We also report Δʹ17O measurements of UWG-2 garnet, San Carlos olivine and NBS 28 quartz relative to our reference line. The proposed standards are: (1) KRS (Khitostrov Rock Standard, δ18O = –25.20 ‰), which consists of sintered, fine-grained constituents (primarily garnet) of a very low-δ18O amphibolite-grade Palaeoproterozoic rock from Khitostrov, Karelia, Russia; (2) SKFS (Stevns Klint Flint Standard, δ18O=33.93 ‰), consisting of thermally dehydrated grains of a high-δ18O nodule of flint sampled from just below the exposed Cretaceous–Palaeogene boundary at Stevns Klint, southeast Zealand Island, Denmark. Both silicates fluorinate readily, using standard laser-assisted procedures, and the associated low blanks allow for overnight pre-treatment. Small quantities of these materials may be supplied, on request, for standardizing Δʹ17O measurements of silicate rocks and minerals according to the proposed procedure.