Oxygen Isotope Analysis for Silicate Minerals Using a High-Temperature Conversion/Elemental Analyzer-Isotope Ratio Mass Spectrometer

Rationale

Oxygen isotope composition of silicates is an important indicator of the formation environment and/or subsequent geochemical processes of minerals. Compared to carbonate minerals and organic matter, silicate is a less common target for oxygen isotope measurements because of the need for specialized fluorination instruments to break strong Si–O bonds.

Methods

We introduce a simple method using a high-temperature conversion elemental analyzer-isotope ratio mass spectrometer (TC/EA-IRMS), which does not require specialized instrumentation. Silicate powder with various fluorine compounds and ratios of fluorine/oxygen were decomposed at 1450°C, and then, mass spectral characteristics, oxygen yields, and δ¹⁸O values were compared.

Results

NaF and KF were the most reactive fluorine sources, followed by polytetrafluoroethylene, LiF, CaF₂, BaF₂, and AlF_3, with decreasing reactivity. The F/O ratio affected the tailing of the CO peaks in the mass spectrum. Higher F/O ratios show a more rapid regression to background, resulting in higher reproducibility of oxygen yield and δ¹⁸O. In addition to simply adding fluorides to the sample, homogenization treatment also improved the reactivity. Activated carbon catalyzed by nickel is a better carbon source than graphite in terms of reactivity. The quartz and smectite with NaF added at an F/O ratio of 6 provided 90.0%–99.3% of oxygen yields, δ¹⁸O values that deviated less than 1.42‰ from literature values, and 0.18‰–0.30‰ of precision.

Conclusions

Although further investigation is required to verify its accuracy, the potential of the TC/EA-IRMS method for silicates is demonstrated. Because the hydrogen isotope composition can also be examined using the same instrument, it is expected that this method can be applied to a wider range of earth materials, including hydrous silicates such as clay minerals.