Raman spectra of fluid and crystal mixtures in the systems H ₂O, H₂O-NaCl and H₂O-MgCl ₂ at low temperatures: Applications to fluid-inclusion research

A combination of Raman spectrometry and microthermometry has been applied to synthetic fluid inclusions filled with pure H2O, a NaCl brine and a MgCl2 brine, in order to analyze spectra between -190° and +100°C. The combined technique allows: (1) the determination of the types of dissolved salts from the presence of salt hydrates at low temperatures, and (2) an accurate estimate of true temperatures of melting, even of phases that are difficult to observe within fluid inclusions. Raman spectra of water, brines, ice, glass and salt hydrates were analyzed by combined Gaussian-Lorentzian fitting of components. These fits illustrate the presence of singularities in the water spectra, around -35°C in a NaCl brine and around -30°C in a MgCl2 brine. During freezing experiments, inclusions may contain different configurations of phases at the same temperature. Rapid freezing of a MgCl2 brine inhibits the formation of a MgCl2 hydrate, and in such inclusions, ice and supersaturated brine are present down to -190°C. The phase MgCl2•12H2O forms only during slow cooling. Temperatures of phase changes, including eutectic point and final melting, were accurately determined by changes in measured Raman spectra of fluid inclusions. The variable freezing behavior of the same fluid inclusion, depending on cooling rates and cycling procedures, indicates the care with which natural fluid inclusions should be treated to obtain true salinities.