Isotope salt effect

Because HDO or H2 0 molecules cannot be expected to have the same solubility in the brine of the main solvent species H2 0 (hereafter simply denoted H2O), their differential behavior must be accounted for by introduction of the appropriate activity coefficient ratio (or isotope salt effect cf Horita et ah, 1993a,b) F ... 

Table 11.14 Calibration of isotope salt effect in single-salt solntions (eq. 11.161), after Horita et al. (1993a).

The same problems encountered in mineral-water isotopic thermometry could be present in T estimates based on isotopic equilibrium between brine and dissolved species, such as SO ". Isotope geo thermometers based on mineral couples or gaseous couples (e.g., CH4-CO2), however, are not affected by isotopic salt effect in brines (Horita et al., 1993b). 

As already treated in the hydrogen section, isotope fractionations, the hydration of ions may play a significant role in hydrothermal solutions and volcanic vapors (Driesner and Seward 2000). Such isotope salt effects may change the oxygen isotope fractionation between water and other phases by several permil. 

Figure 9. Experimental determined isotope salt effects (103 InF) reported by Horita et al. (1993a) for (A) D/H and (B) fractionation at 100° C plotted against molality of the salt solution. The data were...

Table 4. Isotope salt effects determined by vapor-liquid equilibration.

Figure 11. (A) Hydrogen isotope salt effects in NaCl solntions to 450°C. Plotted points represent data obtained from liqnid-vapor eqnilibration, H2-water eqnilibration, and mineral-water exchange experiments. (B) Oxygen isotope salt effects in NaCl solntions to 400°C. Plotted points represent data from liqnid-vapor eqnilibration, C02-water eqnilibration, and mineral-water exchange experiments. Solid enrves are based on the liqnid-vapor eqnilibration experiments of Horita et al. (1995a).

Combined effects. Because temperature, pressure and solution composition all affect the physical properties of water, these three variables can act in concert to influence fractionation factors (Horita et al., in press Hu and Clayton, in press). Increases in pressure and NaCl concentration both work to decrease the fluid s affinity for deuterium. The isotopic effect is most pronounced at low pressures and NaCl concentrations, which relates to the fact that the largest changes in the density of the fluid occurs over this region of pressure-composition space. It appears that the fractionation factor and the density of aqueous NaCl solutions are closely related to each other. With additional systematic experiments, empirical equations can be designed that relate the isotope salt effects and the density of aqueous solutions. 

Isotope salt effect F OL A-aqueous soln OL A - pure water where A is a reference phase... 

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