SALINITY OF MULTIVOLATILE FLUID INCLUSIONS DETERMINED FROM CLATHRATE HYDRATE STABILITY

Measurements of the final dissociation temperature of gas-clathrate hy drates (Tm(CLA)) are routinely used to determine the salinity of fluid inclusions which contain a volatile component in addition to water. T raditionally, experimental data are used to quantitatively relate Tm(C LA) to the inclusion electrolyte concentration. Because of limitations in the experimental database, however, this method has hitherto not b een applicable to the multivolatile fluid inclusions that are common i n crustal rocks. A general solution to this problem is provided by sta tistical thermodynamics predictions of multivolatile clathrate stabili ty. 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 theor etical predictions yield model salinities if the following fluid inclu sion data are available: (1) the identity of the phase assemblage at T m(CLA), (2) the relative concentrations of the volatile species, and ( 3) either the homogenization temperature of the volatile fluid fractio n (bubble point or dew point, either stable or metastable), or an inde pendent estimate of internal pressure at Tm(CLA). Additional data on f luid inclusion cation ratios can be incorporated in the calculations t o recast equivalent weight percent aqueous NaCl in terms of effective electrolyte concentrations. New experimental data on mixed N2-CO2 clat hrates, obtained from synthetic fluid inclusions, provide a test of bo th 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 de rived from the statistical thermodynamics method are generally of the order acceptable for geochemical applications. Applications to multivo latile, multi-electrolyte fluid inclusions from gold-quartz deposits i llustrate the practical operations involved in determining salinity.