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

Miller, Martin F.; Pack, Andreas; Bindeman, Ilya N. and Greenwood, Richard C. (2020). Standardizing the reporting of Δʹ¹⁷O data from high precision oxygen triple isotope ratio measurements of silicate rocks and minerals. Chemical Geology, 532, article no. 119332.

DOI: https://doi.org/10.1016/j.chemgeo.2019.119332

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 λ_RL and γ_RL in the definition Δʹ¹⁷O = ln(1 + δ¹⁷O) – λ_RLln(1 + δ¹⁸O) – γ_RL 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 λ_RL and γ_RL 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 δ1¹⁷O 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.

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