Cryogenic Raman spectroscopic studies in the system NaCl-CaCl (sub 2) -H (sub 2) O and implications for low-temperature phase behavior in aqueous fluid inclusions

Low-T ( approximately 180 degrees C) Raman spectra in the O-H stretching region, from fine-grained aggregates of ice and salt hydrate (hydrohalite and/or antarcticite) in the system H (sub 2) O-NaCl-CaCl (sub 2) , depend on the parent solution composition, and provide an estimate of the NaCl to CaCl (sub 2) ratio. Comparing these spectra with those from moderate-salinity natural fluid inclusions from the Oregon 3 granite pegmatite, Colorado, confirms the presence of CaCl (sub 2) , as inferred from microthermometry. In both synthetic solutions and natural fluid inclusions, Raman spectra collected after initial freezing indicate the presence of ice, but not salt hydrates. Upon warming, a phase change between -70 and -50 degrees C could be interpreted as a eutectic melting event, and spectra collected after this event show that crystallization of salt hydrates (hydrohalite and antarcticite) has occurred, probably from an interstitial, hypersaline liquid. Hence, many of the low 'eutectic' T reported in the literature may not record first melting (stable or metastable), and inferences made about fluid inclusion compositions from such measurements may be erroneous.