Pressure measurement and detection of small H 2 O amounts in high‐pressure H 2 O–CO 2 fluid up to 141 MPa using Fermi diad splits and bandwidths of CO 2

Dependence of residual pressures of fluid inclusions on their size and host mineral species provides valuable information related to the depth provenance and P – T – t path of the rocks. Although Raman‐based barometry is an effective method for ascertaining the internal pressure of H 2 O–CO 2 fluid inclusions, few studies have elucidated Raman spectral features of CO 2 in a system of high‐pressure H 2 O–CO 2 . New experiments using a high‐pressure optical cell in this binary system with compositions of 100, 75 ± 2, and 60 ± 2 mol% CO 2 were conducted for this study to verify the availability of Raman CO 2 barometers for use in assessing the temperature and pressure conditions of approximately 22°C and 17.3–141.4 MPa. Our results demonstrate that the existence of H 2 O does not affect the relation between Fermi diad splits (Δ, cm −1 ) and total pressure of pure CO 2 . These results suggest that the Δ–total pressure relation obtained from pure CO 2 is also applicable to H 2 O–CO 2 systems, even at high pressure. However, unlike Δ, because the peak positions of the Fermi diad in the system of H 2 O–CO 2 shift to a higher wavenumber than those of pure CO 2 at given pressure higher than 30 MPa, the peak positions are not very suitable for the pressure scale in an H 2 O–CO 2 system. Additionally, we confirmed the availability of bandwidths of CO 2 as an indicator of compositions that can identify the presence of very small amounts of H 2 O (at least 0.3 mol% H 2 O), even at room temperature.