H2O ACTIVITY IN CONCENTRATED KCL AND KCL-NACL SOLUTIONS AT HIGH-TEMPERATURES AND PRESSURES MEASURED BY THE BRUCITE-PERICLASE EQUILIBRIUM

H2O activities in supercritical fluids in the system KCl-H2O-(MgO) wer e measured at pressures of 1, 2, 4, 7, 10 and 15 kbar by numerous reve rsals of vapor compositions in equilibrium with brucite and periclase. Measurements spanned the range 550-900 degrees C. A change of state o f solute KCl occurs as pressures increase above 2 kbar, by which H2O a ctivity becomes very low and, at pressures of 4 kbar and above, nearly coincident with the square of the mole fraction (xH(2)O). The effect undoubtedly results primarily from ionic dissociation as H2O density ( rho H2O) approaches 1 gm/cm(3), and is more pronounced than in the NaC l-H2O system at the same P-T-X conditions. Six values of solute KCl ac tivity were yielded by terminal points of the isobaric brucite-pericla se T-x(H2O) curves where sylvite saturation occurs. The H2O mole fract ion of the isobaric invariant assemblage brucite-periclase-sylvite-flu id is near 0.52 at all pressures, and the corresponding temperatures s pan only 100 degrees C between 1 and 15 kbar. This remarkable converge nce of the isobaric equilibrium curves reflects the great influence of pressure on lowering of both KCl and H2O activities. The H2O and KCl activities can be expressed by the formulas: (H2O)=gamma(H2O)[x(H2O)/( x(H2O)+(1+alpha)x(KCl))], and alpha)x(KCl)/(x(H2O)+(1+alpha)x(KCl))](( 1+alpha)), where alpha is a degree of dissociation parameter which inc reases from zero at the lowest pressures to near one at high pressures and the gamma's are activity coefficients based on an empirical regul ar solution parameter W: In gamma(i)=(1-x(i))W-2. Least squares fittin g of our H2O and KCl activity data evaluates the parameters: alpha=exp (4.166 -2.709/rho(H2O))-212.1P/T, and W=(-589.6-23.10P)/T, with rho(H2 O) 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 li quidus curves, which are in perfect agreement with previous DTA liquid us measurements at 0.5-2 kbar, but which depart progressively from the ir extrapolation to higher pressures because of the pressure-induced d issociation effect. The great similarity of the NaCl-H2O and KCl-H2O s ystems suggests that H2O activities in the ternary NaCl-KCl-H2O system can be described with reasonable accuracy by assuming proportionality between the binary systems. This assumption was verified by a few rec onnaissance measurements at 10 kbar of the brucite-periclase equilibri um 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 cu rves reach a lowest xH(2)O of 0.45 and a temperature of 587 degrees C before salt saturation occurs, values considerably lower than in eithe r binary. This double-salt eutectic effect may have a significant appl ication to natural polyionic hypersaline solutions in the deep crust a nd upper mantle in that higher solute concentrations and very low H2O activities may be realized in complex solutions before salt saturation occurs. Concentrated salt solutions seem, from this standpoint, and a lso because of high mechanical mobility and alkali-exchanging potentia l, feasible as metasomatic fluids for a variety of deep-crust and uppe r mantle processes.