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

Abstract

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 H2O–CO2 fluid inclusions, few studies have elucidated Raman spectral features of CO2 in a system of high‐pressure H2O–CO2. New experiments using a high‐pressure optical cell in this binary system with compositions of 100, 75 ± 2, and 60 ± 2 mol% CO2 were conducted for this study to verify the availability of Raman CO2 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 H2O does not affect the relation between Fermi diad splits (Δ, cm−1) and total pressure of pure CO2. These results suggest that the Δ–total pressure relation obtained from pure CO2 is also applicable to H2O–CO2 systems, even at high pressure. However, unlike Δ, because the peak positions of the Fermi diad in the system of H2O–CO2 shift to a higher wavenumber than those of pure CO2 at given pressure higher than 30 MPa, the peak positions are not very suitable for the pressure scale in an H2O–CO2 system. Additionally, we confirmed the availability of bandwidths of CO2 as an indicator of compositions that can identify the presence of very small amounts of H2O (at least 0.3 mol% H2O), even at room temperature.

Temperature dependence of a Raman CO2 densimeter from 23°C to 200°C and 7.2 to 248.7 MPa: Evaluation of density underestimation by laser heating

“…(b) Comparison of published Δ– ρ relations. All densimeters shown are within the density region in which calibration measurements were made and effective at near room temperature [ 28–31,35,37,42,43 ] [Colour figure can be viewed at wileyonlinelibrary.com]…”

“…[27][28][29] Raman-based CO 2 densimetry uses the relation between CO 2 fluid density (ρ) and Fermi diad splitting ðΔ ¼ ν þ F:D: -ν -F:D: Þ. [18,25,[27][28][29][30][31][32][33][34][35][36][37][38][39][40][41][42] Although Raman CO 2 densimeter is useful, some caution is necessary for application of this method to density measurements of natural fluid inclusions: (1) effect of intermolecular interactions between CO 2 and trace molecules other than CO 2 on the spectral properties of CO 2 , [28,[32][33][34][35][36][37][43][44][45][46] (2) discrepancy in the Δ-ρ relations derived in earlier studies, [27,37,47] and (3) apparent density reduction because of the local temperature rise caused by excitation lasers. [48] Regarding the first problem, this method has been improved vigorously to quantify the fluid pressure in inclusions with various P-V-T-x properties.…”

“…This correction term can be found by optimizing the residual sum of squares between the density estimated by any Δ-ρ relation from the Δ values of several standard fluid inclusions and the known density to the minimum. [37] Alternatively, in the region where the Δ-ρ relation can be approximated linearly, it is also effective to derive an original Δ-ρ relation applicable to the Raman system to be used, using two standard fluid inclusions with known density. [27] For the third one, because (∂Δ/∂T) ρ is always negative, at least in the density range investigated by earlier studies, [35,36] the density estimated from the measured Δ might be underestimated if the excitation laser causes unintended heating.…”

The perfection of Raman spectroscopic gas densimeters

“…A common type of distribution curve is not presented in literature (e.g., Yuan and Mayanovic [ 32 ] ). A Gaussian curve was used by Fukura et al, [ 17 ] Wang et al, [ 43 ] Shang et al, [ 44 ] Lamadrid et al, [ 2 ] and Hagiwara et al [ 34 ] ; a Lorentzian curve was used by Kawakami et al [ 47 ] and Yamamoto and Kagi [ 48 ] ; a combined Gaussian–Lorentzian curve was used by Lin et al, [ 18 ] Fall et al, [ 41 ] Wang et al, [ 40 ] Le et al, [ 37 ] and Sublett et al [ 4 ] ; a pseudo‐Voigt curve was used by Remigi et al [ 22 ] Some studies do not mention the method to reproduce shape properties and peak positions of Raman bands, [ 1,20,49,50 ] and none of the publications give information about the peak position estimation of atomic emission lines. The spectra of atomic emission “lines” are always defined by multiple pixels of the detector, comparable with a Raman band.…”

Interlaboratory Application of Raman CO2 Densimeter Equations: Experimental Procedure and Statistical Analysis Using Bootstrapped Confidence Intervals

“…Each densimeter equation is represented by a polynomial function, where the degree of the best fitting curve has been chosen based on the calculation of the root mean square error. 19,20 Although equations well fit descriptive statistics of data, published Raman densimeters calculate two distinct classes of density values for each Δ value measured in spectra, having a standard deviation of 0.09 ± 0.02 g/cm 3 on average (Fig. 1b).…”