Raman spectroscopy of pure H2O and NaCl-H2O containing synthetic fluid inclusions in quartz-a study of polarization effects

The O-H stretching region of the Raman spectrum of aqueous solutions (H2O-NaCl mixtures) is a set of poorly defined broad bands between 2,800cm(-1) and 3,800cm-1. The morphology of this spectral region is systematically characterised by deconvolution, using three Gaussian-Lorentzian contributions. This method allows a purely geometrical description of the spectra of pure H2O as well as saline solutions at room temperatures. A pure H2O reference solution has peak positions at 3,223cm(-1) (Peak1), 3,433cm(-1) (Peak2) and 3,617cm(-1) (Peak3). The morphology of the Raman bands is influenced by the presence of NaCl in the aqueous solution. The variation in Peak1 values is correlated with the salinity of the aqueous solutions according to the equation: Delta nu(1) = 3222.8 + 1.69 sal where sal is the salinity (in mass%) and Delta nu(1) is the relative wavenumber (cm(-1)) of Peak1. The contours of the Raman spectra of aqueous solutions in fluid inclusions in quartz are influenced by the birefringence of the host crystal, the orientation of the quartz-fluid interface, and the depth of the inclusion. The variation in Peak1 due to birefringence is 3,220cm(-1) to 3,239cm(-1) using a polarised laser beam. Reflection polarisation caused by the orientation of the quartz-fluid interface results in a variability of 14cm(-1), independently from the orientation of the indicatrix. The effect of birefringence is reduced with increasing depth. The Raman spectra of aqueous solutions in natural fluid inclusions can be used to calculate the salinity (in mass% NaCl) if the host quartz can be orientated with its optical axis parallel to the polarisation plane of the laser. Raman equipment that does not offer this possibility still can be used to analyse the salinity: multiple spectra must be acquired over at least 90 rotation of the sample; the lowest value of the deconvolved Gaussian-Lorentzian contribution of Peak1 is a measure of the salinity.