Cryogenic Raman spectroscopic studies in the system NaCl-CaCl2-H2O and implications for low-temperature phase behavior in aqueous fluid inclusions

The character of Raman spectra in the O-H stretching region collected at low temperatures (-180 degrees C) from fine-grained aggregates of ice and salt hydrate (hydrohalite or antarcticite or both) in the system H2O-NaCl-CaCl2 reflects the composition of the parent solution and can be used to estimate the ratio of NaCl to CaCl2 in the system. Comparison of these spectra with spectra collected from moderate-salinity natural fluid inclusions from the Oregon 3 granitic pegmatite, Colorado, allows non-destructive testing, and confirmation that these inclusions contain CaCl2, as inferred from microthermometry. in both synthetic solutions in the H2O-NaCl-CaCl2 system and in natural fluid inclusions. Raman spectra collected after initial freezing indicate the presence of ice, but not salt hydrates. Upon warming, a phase change is observed at temperatures of between -70 and -50 degrees C that appears as a darkening of the inclusion and that, during routine microthermometry, could be interpreted as a eutectic melting event. Raman spectra collected aft er this event indicate that it represents the crystallization of salt hydrates (hydrohalite and antarcticite), which we suggest occurs from an interstitial, hypersaline liquid. Many of the low eutectic temperatures reported in the literature may thus not represent first melting (stable or metastable), and inferences made about the composition of fluid inclusions from such measurements may be erroneous.