Other
Scientific paper
Jan 1992
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1992gecoa..56..273d&link_type=abstract
Geochimica et Cosmochimica Acta, vol. 56, Issue 1, pp.273-280
Other
21
Scientific paper
The dissociation temperature of CO 2 clathrate hydrate ( Tm cla ) in the presence of CO 2 liquid + CO 2 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 (Q 2 ) assemblage has been confirmed by reproducing independent results in the literature on the NaCl-CO 2 -H 2 O system. Regression fitting of the new experimental measurements and interpolation with published NaCl-CO 2 -H 2 O data allows the stability of CO 2 clathrate in the Q 2 assemblage of the KCl-NaCl-CO 2 -H 2 O quaternary system to be described as follows: wt% ( KCl + NaCl ) = 15.5296 + 4.2947 X KCL + 0.5061 ( X KCL ) 2 -1.0709 Tm CLA - 0.4751 Tm CLA - 0.048( Tm CLA ) 2 (1) where wt% (KC1 + NaCl) denotes the total salt concentration in the aqueous liquid (with respect to the salt-H 2 O subsystem), Tm CLA is in °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% (KC1 + 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 Q 2 assemblage (for which Th CO 2 > Tm CLA ), or in either of two three-phase assemblages (for which Th CO 2 < Tm CLA ). The new experimental data on the Q 2 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 CO 2 measured in the metastable absence of clathrate, to equivalent wt% NaCl in the salt-H 2 O subsystem of the aqueous liquid. For inclusions in which CO 2 homogenizes via a bubblepoint transition ( Th CO 2 < Tm CLA ): eq. wt% NaCl = 15.6151 - 0.03627 x + 0.00164 x 2 - 0.9491 y - 0.00287 xy - 0.02464 y 2 - 0.00107 xy 2 - 0.00222 y 3 . (2) For inclusions in which CO 2 homogenizes via a dew-point transition ( Th CO 2 dew < Tm CLA ): eq. wt% NaCl = 15.5131 + 0.065705 x - 0.00778 x 2 - 1.05135 y + 0.02687 xy - 0.04717 y 2 + 0.00138 xy 2 - 0.00411 y 3 (3) where x denotes Th CO 2 between -20 and + 10°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-CO 2 -H 2 O fluid inclusions in which Th CO 2 is higher than the clathrate nucleation temperature.
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