In situ cryogenic Raman spectroscopic studies on the synthetic fluid inclusions in the systems H2O and NaCl-H2O

Abstract

Salt-hydrates have diagnostic cryogenic Raman spectra, which can reflect the composition of the parent solutions. As analogue to the natural fluid inclusions, the synthetic inclusions can be used to validate numerous assumptions related to fluid inclusion research. They can also be used to test the feasibility of application of laser Raman spectroscopy to individual fluid inclusion analysis. Using the technique proposed by Sterner and Bodnar(1984), synthetic inclusions of the systems H2O and NaCl-H2O (with NaCl as 5.12 wt%, 9.06 wt%, 16.6 wt% and 25 wt%) were formed under the pressures from 50Mpa to 100Mpa and at the temperatures from 500°C? to 600°C. In situ cryogenic Raman spectra were collected at about −180°C by combined use of freezing-heating stage and Laser Raman Microscopy. It is shown that hydrohalite (NaCl·2H2O), the salt-hydrate of NaCl in the fluid inclusions has the specific Raman spectrum and can be used as the standard to verify the existence of NaCl in the aqueous inclusions. The crystalline ice other than amorphous ice (glasses) formed from the aqueous phase whthin the synthetic inclusions during the initial freezing, but hydrohalite did not form. Subsequent warming of these inclusions induced a phase change, typically between approximately −40 and −22°C, that represents the hydrohalite crystallization event but not a eutectic melting event. So, for fluid inclusions in the system NaCl-H2O, interpretation of phase behavior below the eutectic temperature (−20.8?) should be made with caution. The ratios of the relative intensity and the area of Raman spectra between 3423 cm−1 peak of hydrohalite and 3098 cm−1 peak of ice show positive correlations to the salinities in aqueous inclusions, which can be used to determine the salinity of NaCl-H2O system inclusions.