Determinations of water, hydrates and pH in fluid inclusions by micro-Raman spectrometry

Abstract

This paper demonstrates four new possibilities in the analysis of fluid inclusions by micro-Raman spectroscopy. 1) Very low water contents, around 1 mol%, can be detected in fluid inclusions with no visible water coating the wall of the cavity, using a molecular symmetric stretching vibration at 3636 cm −1 . With increasing water content, the Raman spectrum broadens and shifts towards lower wavenumbers, indicating H 2 O-H 2 O interactions which prevent any quantification if X(H 2 O)>0.1. 2) The Raman spectrum of H 2 S clathrate hydrate shows two bands at 2595 and 2605 cm −1 which are assigned to the stretching vibrations of the H 2 S molecule; it is compared with the spectrum of vapour, liquid and aqueous H 2 S. 3) The Raman spectrum of the LiCl.5H 2 O hydrate exhibits a stretching vibration band central around 3400 cm −1 , with an unusual large half-peak width (300 cm −1 ) compared to other stochiometric salt hydrates at −160 o C. The succession of phase melting above −160 o C in a natural fluid inclusion was identified owing to the Raman spectrum of salt hydrates including LiCl.5H 2 O. 4) Very acidic solutions (pH between 1 and −0.5) were documented from the identification of HSO 4 − and SO 4 −− by Raman spectroscopy and the calibration of their intensity as a function of pH