Salinity of multivolatile fluid inclusions determined from clathrate hydrate stability

Measurements of the final dissociation temperature of gas-clathrate hydrates (Tm_CLA) are routinely used to determine the salinity of fluid inclusions which contain a volatile component in addition to water. Traditionally, experimental data are used to quantitatively relate Tm_CLA to the inclusion electrolyte concentration. Because of limitations in the experimental database, however, this method has hitherto not been applicable to the multivolatile fluid inclusions that are common in crustal rocks.A general solution to this problem is provided by statistical thermodynamics predictions of multivolatile clathrate stability. Published theoretical models explicitly account for the effect of aqueous NaCl in depressing the stability of clathrates composed of any mixture of CO₂, N₂, H₂S, CH₄ and higher-order hydrocarbons. Analysis of phase relations in complex clathrate systems shows that such theoretical predictions yield model salinities if the following fluid inclusion data are available: (1) the identity of the phase assemblage at Tm_CLA, (2) the relative concentrations of the volatile species, and (3) either the homogenization temperature of the volatile fluid fraction (bubble point or dew point, either stable or metastable), or an independent estimate of internal pressure at Tm_CLA. Additional data on fluid inclusion cation ratios can be incorporated in the calculations to recast equivalent weight percent aqueous NaCl in terms of effective electrolyte concentrations.New experimental data on mixed N₂-CO₂ clathrates, obtained from synthetic fluid inclusions, provide a test of both the model predictions and of the analytical procedure proposed for natural fluid inclusions. While the accuracy of the predictions varies between volatile compositions, the uncertainties in the salinities derived from the statistical thermodynamics method are generally of the order acceptable for geochemical applications. Applications to multivolatile, multi-electrolyte fluid inclusions from gold-quartz deposits illustrate the practical operations involved in determining salinity.