Pressure measurement and detection of small H2O amounts in high‐pressure H2O–CO2 fluid up to 141 MPa using Fermi diad splits and bandwidths of CO2

Yuuki Hagiwara, Junji Torimoto, Junji Yamamoto

doi:10.1002/jrs.5865

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

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

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