The Study of Fluid Inclusion Salinity in Minerals by Raman Spectroscopy Revisited

Raman spectroscopy is an efficient non-contact and non-destructive method and has been widely employed in many research fields, including fluid inclusion studies. The Raman spectra of OH vibrations can be used to determine the salinity of solutions and consequently the salinity of natural fluid inclusions in minerals. However, when analyzing a poorly resolved spectrum with a low signal-to-noise ratio, the problem of the ambiguity of its conventional deconvolution using peak fitting and the determination of weak changes in the parameters of its elementary components arises. The aim of the study was to develop and test the central moment (kurtosis) method for processing Raman spectra by the example of a series of quartz samples with fluid inclusions and to determine their salinity using the H2O vibrational modes avoiding the conventional peak fitting. The calibration curves for salinity determination were constructed using 26 model solutions of chemically pure NaCl with a mass fraction of 1 to 26%. To simulate the birefringence effect, a quartz plate was used to cover the cuvettes with solutions. A quantitative assessment of spectral shape variations was performed according to the values of their integral parameters (skewness and kurtosis). The regression coefficient for linear approximation of skewness and kurtosis based calibration curves was 0.97 and 0.98, respectively. The convergence with the microthermometric data for the two-phase fluid inclusions in quartz from a number of magnesite deposits of the Southern Ural was satisfactory.