STABILITY OF CO2 CLATHRATE HYDRATE+CO2 LIQUID+CO2 VAPOUR+AQUEOUS KCL-NACL SOLUTIONS - EXPERIMENTAL-DETERMINATION AND APPLICATION TO SALINITY ESTIMATES OF FLUID INCLUSIONS

The dissociation temperature of CO2 clathrate hydrate (Tm(cia) in the presence of CO2 liquid + CO2 vapour + aqueous KCl-NaCl solutions has been determined by microthermometry of fluid inclusions synthesized in quartz. The reliability of the experimental method as a means to examine clathrate equilibria in this four-phase (Q2) assemblage has been confirmed by reproducing independent results in the literature on the NaCl-CO2-H2O system. Regression fitting of the new experimental measurements and interpolation with published NaCl-CO2-H2O data allows the stability of CO2 clathrate in the Q2 assemblage of the KCl-NaCl-CO2-H2O quaternary system to be described as follows: wt% (KCl + NaCl) = 15.5296 + 4.2947X(KCl) + 0.5061(X(KCl)2 - 1.0709Tm(CLA) 0.4751X(KCl)Tm(CLA) - 0.048(Tm(CLA)2 (1) where wt% (KCl + NaCl) denotes the total salt concentration in the aqueous liquid (with respect to the salt-H2O subsystem), Tm(CLA) is in degrees-C, and X(KCl) denotes the relative weight fraction of salts: wt% KCl/(wt% KCl + wt% NaCl). This equation is valid over the range 0-19 wt% (KCl + NaCl), and 0-1 X(KCl). When coupled with fluid inclusion KCl/NaCl ratios analyzed by methods other than microthermometry. Eqn. (1) yields compositions in terms of individual electrolyte concentrations. In natural fluid inclusions, depending on their bulk compositions and densities, clathrate may dissociate as part of the Q2 assemblage (for which Th(CO2) > Tm(CLA)), or in either of two three-phase assemblages (for which Th(CO2) < Tm(CLA)). The new experimental data on the Q2 assemblage, and those in the literature, may be applied to natural three-phase assemblages as well. This is enabled by the following functions which relate Tm(CLA), and Th(CO2) measured in the metastable absence of clathrate, to equivalent wt% NaCl in the salt-H2O subsystem of the aqueous liquid. For inclusions in which CO2 homogenizes via a bubble-point transition (Th(CO2bub) < Tm(CLA): eq. wt% NaCl = 15.6151 - 0.03627x + 0.00164x2 - 0.9491y - 0.00287xy - 0.02464y2 - 0.00107xy2 0.00222y3. (2) For inclusions in which CO2 homogenizes via a dew-point transition (Th(CO2dew) < Tm(CLA)): eq. wt% NaCl - 15.5131 + 0.065705x - 0.00778x2 - 1.05135y + 0.02687xy - 0.04717y2 + 0.00138xy2 - 0.00411y3 (3) where x denotes Th(CO2) between -20 and +10-degrees-C, y denotes Tm(CLA), and eq. NaCl concentration is between 0 and 21 wt%. Equations (2) and (3) thus permit salinities to be estimated for the large class of natural KCl-NaCl-CO2-H2O fluid inclusions in which Th(CO2) is higher than the clathrate nucleation temperature.