We present experimental results constraining water activity in H2O-N2 fluids containing 40–90 mol% water at pressures of 6–13 kbar and temperatures of 680–840 °C. In the experiments, the displacement of the brucite dehydration equilibrium was used as a gauge of water activity. The experiments were performed in a conventional piston-cylinder apparatus, with NaCl pressure medium and silver azide, AgN3, as a source of nitrogen. Reversals of the dehydration reaction were used to bracket the equilibrium fluid compositions within 3 mol% H2O. Water activities were computed from the equilibrium brucite dehydration conditions in pure H2O as determined by Aranovich and Newton (1996) using thermodynamic data of Holland and Powell (1998). The experimentally derived activities were fit to a van Laar-type equation that reproduces our compositional data with a standard error of 1.6 mol% H2O:

\[R\mathit{T}ln{\gamma}_{1}\ =\ (\mathit{X}_{2})^{2}\mathit{W}{\{}\mathit{V}_{1}^{0}(\mathit{V}_{2}^{0})^{2}/[(\mathit{V}_{1}^{0}\ +\ \mathit{V}_{2}^{0})\ (\mathit{X}_{1}\mathit{V}_{1}^{0}\ +\ \mathit{X}_{2}\mathit{V}_{2}^{0}]{\}}\]

where γ1 is the activity coefficient of H2O, Xi is the mole fraction of end-member i (1 = H2O and 2 = N2), V0i is the molar volume of the pure end-member at the pressure (P) and temperature (T) of interest, and W is analogous to a regular solution parameter. The parameter W was fit as a function of pressure and temperature by the expression W = (A − BT)[1 − exp(−20P)] + C·P0.3T, with A = 40005 J, B = 51.735 J/K, C = 14.848 J/(K·kbar0.3), P in kbar and T in K. With these expressions, activity-concentration relations in H2O-N2 fluids can be reconstructed in a broad P-T-X range using any equation of state (EOS) for pure H2O and N2. The activity-concentration relations are similar to the semi-empirical EOS of Duan et al. (2000) and the theoretical EOS of Churakov and Gottschalk (2002a), although the former somewhat underestimates activities within the experimental pressure range whereas the latter appears to overestimate activities of the components at pressure above 20 kbar.

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