RAMAN SPECTRA OF FLUID AND CRYSTAL MIXTURES IN THE SYSTEMS H2O, H2O NaCl AND H2O MgCl2 AT LOW TEMPERATURES: APPLICATIONS TO FLUID-INCLUSION RESEARCH

Abstract

A combination of Raman spectrometry and microthermometry has been applied to synthetic fluid inclusions filled with pure H 2 O, a NaCl brine and a MgCl 2 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 MgCl 2 brine. During freezing experiments, inclusions may contain different configurations of phases at the same temperature. Rapid freezing of a MgCl 2 brine inhibits the formation of a MgCl 2 hydrate, and in such inclusions, ice and supersaturated brine are present down to -190 C. The phase MgCl 2 •12H 2 O 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.

Unexpected phase assemblages in inclusions with ternary H2O-salt fluids at low temperatures

“…Furthermore, the temperature of phase transitions (i.e. eutectic and melting points) was determined by changes in Raman spectra in a microthermometric heating experiment [9,22]. A LINKAM THMSG 600 stage was combined with the Raman spectrometer to perform heating and freezing experiments.…”

“…Raman spectroscopy was primarily applied to species yielding wavenumbers ranging from 2800 to 4000 cm −1 , where the variety of salt hydrates, ice, and aqueous liquid solutions can be characterized by specific spectra. The phases in this study were identified with standard Raman spectra as illustrated in Bakker [22], including water, brine, ice, hydrohalite, MgCl 2 hydrates, and metastable phases, such as super-cooled water and a glassy brine, and, and in Baumgartner and Bakker [25], including ice, hydrohalite, CaCl 2 hydrates, and MgCl 2 hydrates.…”

“…The nature of a phase assemblage in fluid inclusions is difficult to identify by optical means; assemblages may represent both stable and metastable conditions. Furthermore, the presence of a specific phase, whether stable or metastable, can often not be determined by optical microscope observations [9,22]. In this study, phases (liquid, ice, and salt hydrates) in fluid inclusions are identified by a combination of microthermometry and Raman spectroscopy according to the method described by Bakker [22].…”

“…Furthermore, the presence of a specific phase, whether stable or metastable, can often not be determined by optical microscope observations [9,22]. In this study, phases (liquid, ice, and salt hydrates) in fluid inclusions are identified by a combination of microthermometry and Raman spectroscopy according to the method described by Bakker [22]. In addition, phase changes are accurately characterized by changes in Raman spectra at low temperatures, see also [9,22,23].…”

“…In this study, phases (liquid, ice, and salt hydrates) in fluid inclusions are identified by a combination of microthermometry and Raman spectroscopy according to the method described by Bakker [22]. In addition, phase changes are accurately characterized by changes in Raman spectra at low temperatures, see also [9,22,23]. Fluid inclusions with varying compositions are synthesized in natural quartz in the ternary systems H 2 O-NaCl-CaCl 2 , H 2 O-NaCl-MgCl 2 , and H 2 O-CaCl 2 -MgCl 2 , to study the formation of stable and metastable phase assemblages during microthermometric experiments.…”