# H_{2}O activity in concentrated KCl and KCl-NaCl solutions at high temperatures and pressures measured by the brucite-periclase equilibrium

DOI: 10.1007/s004100050279

- Cite this article as:
- Aranovich, L. & Newton, R. Contrib Mineral Petrol (1997) 127: 261. doi:10.1007/s004100050279

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## Abstract

H_{2}O activities in supercritical fluids in the system KCl-H_{2}O-(MgO) were measured at pressures of 1, 2, 4, 7, 10 and 15 kbar by numerous reversals of vapor compositions in equilibrium with brucite and periclase. Measurements spanned the range 550–900 °C. A change of state of solute KCl occurs as pressures increase above 2 kbar, by which H_{2}O activity becomes very low and, at pressures of 4 kbar and above, nearly coincident with the square of the mole fraction (*x*_{H2O}). The effect undoubtedly results primarily from ionic dissociation as H_{2}O density (ρ_{H2O}) approaches 1 gm/cm^{3}, and is more pronounced than in the NaCl-H_{2}O system at the same *P-T-X* conditions. Six values of solute KCl activity were yielded by terminal points of the isobaric brucite-periclase *T-x*_{H2O} curves where sylvite saturation occurs. The H_{2}O mole fraction of the isobaric invariant assemblage brucite-periclase-sylvite-fluid is near 0.52 at all pressures, and the corresponding temperatures span only 100 °C between 1 and 15 kbar. This remarkable convergence of the isobaric equilibrium curves reflects the great influence of pressure on lowering of both KCl and H_{2}O activities. The H_{2}O and KCl activities can be expressed by the formulas: *a*_{H2O} = γ_{H2O}[*x*_{H2O}+(1 + (1 + α)*x*_{KCl})], and *a*_{KCL} = γ_{KCl}[(1 + α)*x*_{KCl}/(*x*_{H2O} +(1 + α)*x*_{KCl})]^{(1 + α)}, where α is a degree of dissociation parameter which increases from zero at the lowest pressures to near one at high pressures and the γ's are activity coefficients based on an empirical regular solution parameter W: ln γ_{i} = (1 − x_{i})^{2}W. Least squares fitting of our H_{2}O and KCl activity data evaluates the parameters: α = exp(4.166 −2.709/ρ_{H2O}) − 212.1*P/T*, and W = (−589.6 − 23.10*P*) /*T*, with ρ_{H2O} in gm/cm^{3}, *P* in kbar and *T* in K. The standard deviation from the measured activities is only ± 0.014. The equations define isobaric liquidus curves, which are in perfect agreement with previous DTA liquidus measurements at 0.5–2 kbar, but which depart progressively from their extrapolation to higher pressures because of the pressure-induced dissociation effect. The great similarity of the NaCl-H_{2}O and KCl-H_{2}O systems suggests that H_{2}O activities in the ternary NaCl-KCl-H_{2}O system can be described with reasonable accuracy by assuming proportionality between the binary systems. This assumption was verified by a few reconnaissance measurements at 10 kbar of the brucite-periclase equilibrium with a Na/(Na + K) ratio of 0.5 and of the saturation temperature for Na/(Na + K) of 0.35 and 0.50. At that pressure the brucite-periclase curves reach a lowest *x*_{H2O} of 0.45 and a temperature of 587 °C before salt saturation occurs, values considerably lower than in either binary. This double-salt eutectic effect may have a significant application to natural polyionic hypersaline solutions in the deep crust and upper mantle in that higher solute concentrations and very low H_{2}O activities may be realized in complex solutions before salt saturation occurs. Concentrated salt solutions seem, from this standpoint, and also because of high mechanical mobility and alkali-exchanging potential, feasible as metasomatic fluids for a variety of deep-crust and upper mantle processes.