Determining Salinity of Fluid Inclusions by Cryogenic Raman Spectroscopy

Chlorides are the most common and important solutes in liquid phase of natural fluid inclusions, but they are difficult to study by Laser Raman Microscopy (LRM) at room temperature, due to their strong ionic bonds, which are not Raman active under experimental conditions. This problem seriously hinders the utilizing of LRM in a fluid inclusion study, neither for their specific components in both vapor and liquid phases, nor for their salinity. In combination with laser Raman Spectrometry and hot and cold operation boards, Raman spectra of ice, sodium chloride hydrate and calcium chloride hydrate, which were formed by frozen NaCl-H₂O and CaCl₂-H₂O solutions with different concentrations at low temperature (-185℃) were acquired. According to systematic analysis and comparison of Raman spectra of sodium and calcium chloride hydrates with different concentrations, an attempt was made to establish a method for salinity determination by cryogenic laser Raman Spectroscopy. Results show that the area ratios of Raman bands at about 3425 cm^-1 of sodium chloride hydrates to which at about 3120 cm^-1 of ice were in good correlation with prepared concentrations of NaCl-H₂O solutions (r²=0.9995). Similarly, area ratios of Raman bands at about 3431 cm^-1 of calcium chloride hydrates to that at about 3120 cm^-1 of ice were in good correlation with prepared concentrations of CaCl₂-H₂O solutions, too (r²=0.9458). Reliability of this method was examined by synthetic NaCl-H₂O and CaCl₂-H₂O inclusions, which were prepared by healing microfracture of quartz crystals under high pressure and temperature conditions when the crystals were dipped in solution of specific concentration. Inspection results show that when this method is applied to > 0.5 mol/L NaCl-H₂O fluid inclusions, the accuracy is better than 20%. Meanwhile, when applied to > 0.5 mol/L CaCl₂-H₂O fluid inclusions, the best accuracy might reach to 5%. The results meet semi-quantitative to quantitative analysis requirements for fluid inclusions. The interior pressure of fluid inclusions might be less important in salinity determination of fluid inclusions by cryogenic Raman Spectroscopy. The Raman bands of ice were somewhat different to those acquired by most other researchers (at about 3120 cm^-1 in this study), which is most likely due to different polymorphs formed under the experimental condition. This method allows knowledge of the main fluid system to be attained as well as the salinity of fluid inclusions semi-quantitatively to quantitatively, with improved accuracy over traditional techniques.