Raman spectroscopic characterization on the OH stretching bands in NaCl–Na2CO3–Na2SO4–CO2–H2O systems: Implications for the measurement of chloride concentrations in fluid inclusions

• We improve the Raman spectroscopic measurements on the chloride concentration in FIs.
• The effects of SO42 −, CO32 −, and CO2 have been investigated.
• This method can be applied to CH4- ad CO2-bearing fluid inclusions.

Raman spectroscopy has been demonstrated to be an in situ and non-destructive method for the qualitative and quantitative analyses of fluid inclusions, especially for those fluid inclusions with high CO2 densities and/or small size (i.e., < 5 μm). Here we addressed this issue by studying the Raman spectra of fluids of the NaCl–H2O, Na2SO4–H2O, Na2CO3–H2O, CO2–H2O, NaCl–Na2SO4–H2O and NaCl–Na2CO3–H2O systems. Their characteristics in fused silica capillary capsules (FSCCs) or a high-pressure optical cell (HPOC) were collected at ~ 22 °C. The Raman OH stretching bands were fitted using two Gaussian sub-bands near 3220 cm− 1 and 3450 cm− 1. It is found that the peak height and width of the two Gaussian sub-bands can be used to construct the calibration curve for the calculation of chloride concentrations in aqueous NaCl solutions. Our results indicate that (1) this method can be applied to chloride-dominated fluids because the presence of other anions, such as CO32 − and SO42 −, will result in an overestimation of the calculated chloride concentrations; (2) the presence of CO2 does not have an obvious influence on the Raman OH stretching bands and, thus, this method can be applied to CO2-bearing fluid inclusions; and (3) each Raman spectroscopic system should be calibrated before the application, and reference standards can be easily prepared in fused silica capillary capsules.