Determination of methane content in NaCl–H2O fluid inclusions by Raman spectroscopy. Calibration and application to the external part of the Central Alps (Switzerland)

• Quantification of CH4,aq by Raman spectroscopy in a high-pressure optical cell
• Calibration on > 1000 data points in the ranges 30–1000 bar, 60–180 °C, 0–4 m NaCl
• Application to aqueous fluid inclusions: determination of PT trapping conditions

The reconstruction of the entrapment conditions of geological fluids requires determining the volumetric and composition properties of the inclusions containing these fluids. In some cases, the analytical data necessary for PVTX determination cannot be obtained from microthermometry. The quantification of the dissolved gases in aqueous fluid inclusions by Raman spectroscopy, following proper calibration of the instrument and methodology, can provide alternative data. In the present study, the intensity of the Raman signal of the symmetric stretching vibrational mode of methane was calibrated in order to (1) determine CH4 concentration in pure and saline water and (2) quantify the molar fractions of H2O and CH4 in the gas phase. High-pressure optical cell (HPOC), i.e. a pressurization system connected to a silica microcapillary heated on a customized heating–cooling stage, reveals to be much more convenient and accurate than synthetic fluid inclusions. Moreover, a wide range of pressure, temperature, and salinity can be covered by this methodology. Over 1000 measurements were produced to define the calibration curves, covering the ranges in temperature, pressure and salinity of 60–180 °C, 30–1000 bar, and 0–4 mol.kg− 1 NaCl, respectively, which corresponds to a CH4 molality up to 0.6 mol.kg− 1. The CH4 solubility vs. CH4/H2O Raman peak area ratio was fitted by a second-order polynomial curve (R2 = 0.996). In the gas phase, the molar fraction of H2O vs. Raman peak area ratio is fitted by a straight line (R2 = 0.990). The calibration was applied to a set of natural fluid inclusions trapped within late quartz Alpine fissure of the external part of the Central Alps (Switzerland). The determination of CH4 concentration in the studied fluids provided valuable insight on conditions of trapping and on the pressure regimes prevailing in this low-grade metamorphic setting.

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