Application of low-temperature microthermometric data for interpreting multicomponent fluid inclusion compositions

Fluid inclusions are commonly the best available source of information on the compositions of fluids in past geologic environments. Microanalytical data, predominantly from LA-ICPMS, allow assessment of the relative abundances of chemical elements in fluid inclusions. Such data show that geologic fluids commonly contain appreciable concentrations of multiple salts in addition to NaCl, particularly KCl, CaCl2, and FeCl2 as major components. Quantification of absolute salt concentrations generally requires an internal standard concentration, which is typically derived from microthermometric measurements interpreted according to the vapor-saturated liquidus relations of simpler systems such as H2O-NaCl or H2O-NaCl-CaCl2. Here, we review and reassess compositional information obtainable from microthermometric measurements in multicomponent chloride-dominated aqueous systems. To do so, we investigate the systematics of vapor-saturated liquidus phase equilibria in complex multicomponent electrolyte solutions through thermodynamic modeling based on Pitzer's equations. We focus on low- to intermediate-salinity chloride-dominated inclusions, in which ice is the liquidus phase, and on the temperature range from subsolidus conditions to < 25 °C. On the basis of measured ice and hydrohalite melting temperatures, fluids with predominantly monovalent (Na ± K) chlorides, or mixtures of monovalent and divalent cation chlorides can be identified and a robust value of mNa as a proportion of total cations be calculated. We show that microthermometric measurements alone do not allow unequivocal determination of the identity of salts that are present in addition to NaCl. In combination with microanalytical determination of cation ratios, however, robust compositional results for multi-salt aqueous fluid inclusions can be obtained using microthermometric measurements interpreted with generic H2O-(Na,K)Cl-ΣXn+Cln phase stability relations.