A novel method for high-precision triple oxygen isotope analysis of diverse Earth materials using high temperature conversion–methanation–fluorination and isotope ratio mass spectrometry

Abstract

Triple oxygen isotope geochemistry is a growing field that investigates mass dependent and mass independent fractionation in 17O/16O relative to 18O/16O imparted by geochemical, physical, and biological processes. Many applications require 10 ppm-level precision in measurement of the Δ′17O parameter. Such analytical precisions are rarely realized for some materials (e.g. sulfates), and no method presently exists for many organic compounds. We describe a new analytical method that combines the versatility of high-temperature conversion (HTC, also known as Temperature Conversion Elemental Analysis, TC/EA) with the high-precision of the methanation-fluorination method described by Passey et al. (2014). In this method, materials including organics, phosphates, sulfates, nitrates, carbonates, and silicates undergo high-temperature conversion of their oxygen to CO in a glassy carbon reactor at temperatures >1400 °C. This CO is then reacted with hydrogen at 560 °C over an iron catalyst to transfer CO oxygen to oxygen in H2O (a methanation reaction). This H2O is subsequently fluorinated by CoF3 to yield O2, which is then analyzed by mass spectrometry to determine δ17O and δ18O. Sample oxygen conversion is quantitative (or nearly so) for most compatible materials, resulting in excellent reproducibility in Δ′17O (<10 ppm 1σ) and no clear evidence of fractionation affecting Δ′17O. We report the first (to our knowledge) high-precision Δ′17O measurements for organics, including benzoic acid (IAEA-601, IAEA-602), cellulose (IAEA-CH-3), keratin (USGS KHS), and honey (USGS 83). We present results for silver phosphates (NBS 120c, USGS 80, USGS 81, B2207), carbonates (IAEA-603, IAEA-C1, NBS 18), sulfates (IAEA-SO-5, IAEA-SO-6), a silicate (NBS 28), and waters (VSMOW2, SLAP2). We also present results from several in-house reference materials, including a silver nitrate and a phosphate that was equilibrated with isotopically exotic waters. Analysis of carbonates, sulfates, and silicates requires accelerants (e.g., graphite and AgCl or KF) to achieve high yields and ppm-level precision in Δ′17O. Waters can be analyzed using the same method during the same analytical sessions as solid materials, making determination of water-mineral fractionation factors straightforward and enabling values to be placed directly on the VSMOW-SLAP scale by concurrent analyses with those reference materials. We find that the method is especially well-suited for organics, sulfates, phosphates, and nitrates, however traditional fluorination methods may be better suited for many silicate minerals.