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.