Introduction

Oxygen is the most ubiquitous element in Earth’s crust. It exists in various gases (e.g., atmospheric O2 and water vapor), liquids (e.g., water, oxygen-containing ions, and dissolved organic matter), and solid forms (e.g., organic matter and minerals). Three oxygen isotopes, 16O, 17O, and18O, are involved in other elements such as carbon, nitrogen, and sulfur. The oxygen isotope composition as the per mil (‰) difference of the sample form the standard was defined using delta(δ ) notation 1:
\begin{equation} \delta^{18}O\ =\ \frac{R_{\text{sample}}}{R_{\text{standard}}}-1\nonumber \\ \end{equation}
where R is the ratio of the heavy isotope (18O) to the light isotope (16O) of the sample material relative to the isotope ratio of an international standard, Vienna Standard Mean Ocean Water (VSMOW). Among the oxygen-bearing terrestrial materials, silicate minerals consist of 92% of the earth’s crust 2, and thus, oxygen isotope compositions of silicates are highly important in diverse geochemical fields, such as meteorology 3, paleoclimatology 4, 5, and ore geology 6, to trace reaction processes and origins. Oxygen isotope analysis of silicates has been conducted for many years; however, it is not yet a popular target compared to carbonate and organic matter. This is due to the difficulty in extracting oxygen from silicates, in which strong covalent bonds exist between Si-O exist. Several methods have been developed to extract oxygen from silicates. These include the traditional Ni-bomb fluorination using BrF5 7 and CO2 laser fluorination 8. The latter often involves mixing the sample with fluoride compounds such as LiF 9 or BaF2 4, 10. Applications of oxygen isotope measurements to a range of silicate minerals may be limited by the need for specific equipment that these fluorination procedures require, which is separate from the isotope ratio mass spectrometer (IRMS). In contrast, a thermal decomposition method using a high temperature conversion elemental analyzer (TC/EA) has been proposed11, 12. They mixed samples with fluoride compounds, KF or polytetrafluoroethylene (PTFE), and then fluorinated at high temperatures (1450–1500˚C). This TC/EA-IRMS method is expected to be a simple and safe method that does not use hazardous gases such as BrF5, requires no specialized external apparatus, and requires less time for sample fluorination than conventional methods. However, despite their potential, there are not yet many applications for silicate minerals, and verification of pyrolysis methods is required. Recently, a high temperature conversion-methanation-fluorination method for high-precision triple oxygen isotope analysis was proposed that decomposes various earth materials, including silicates 13. A dual-inlet system was used to enable highly precise analysis through multiple gas generation/purification traps. By repeating the measurements for both purified sample gases and references, dual-inlet (offline) systems generally offer higher precision than continuous-flow (online) IRMS; nevertheless, they necessitate larger sample volumes and longer pretreatment durations. For the high-throughput measurement of a large number of samples required in geoscience, it is necessary to establish the measurement method using a continuous-flow system and a simpler instrument configuration. In addition, it is necessary to increase the oxygen yield from 80% 11,13 to close to 100%. In this study, pyrolysis and oxygen isotope ratio measurements were performed using a TC/EA-IRMS in a continuous-flow system with a simple instrument configuration. Silicate minerals were mixed with fluorides using various recipes and thermally decomposed to produce CO gas. Their reactivity, oxygen yields, and δ18O values obtained were compared. In addition to quartz, which is a typical silicate mineral, we also investigated its applicability to clay minerals, which are hydrous silicate minerals that cover the majority of the Earth’s surface, to explore the potential for the continuous measurement of oxygen and hydrogen isotopes using the same instrument.