Determining Fluid Compositions in the H2O-NaCl-CaCl2 System with Cryogenic Raman Spectroscopy: Application to Natural Fluid Inclusions

Previous cryogenic Raman spectroscopic analysis of H2O-NaCl-CaCl2 solutions has identified the Raman peaks of various hydrates of NaCl and CaCl2, and established a linear relationship between Raman band intensity of the hydrates and the composition of the solution (NaCl/(NaCl+CaCl2) molar ratio, or XNaCl) using synthetic fluids, which created the opportunity to quantitatively determine the solute composition of aqueous fluid inclusions with cryogenic Raman spectroscopy. This paper aims to test the feasibility of this newly established method with natural fluid inclusions. Twenty-five fluid inclusions in quartz from various occurrences which show a high degree of freezing during the cooling processes were carefully chosen for cryogenic Raman analysis. XNaCl was calculated using their spectra and an equation established in a previous study. These inclusions were then analyzed with the thermal decrepitation-SEM-EDS method. The XNaCl values estimated from the two methods show a 1:1 correlation, indicating that the new, non-destructive cryogenic Raman spectroscopic analysis method can indeed be used for fluid inclusion compositional study.