• journal_title：American Mineralogist
• Contributor：Andreas Haefner ; Leonid Y. Aranovich ; James A.D. Connolly ; Peter Ulmer
• Publisher：Mineralogical Society of America
• Date：2002-
• Format：text/html
• Language：en
• journal_abbrev：American Mineralogist
• issn：0003-004X
• volume：87
• issue：7
• firstpage：822
• section：Articles

d="p-1">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 d="xref-ref-4-1" class="xref-bibr">Aranovich and Newton (1996) using thermodynamic data of d="xref-ref-21-1" class="xref-bibr">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:

d="p-2"> disp-formula" id="disp-formula-1">d/tex-math-1.gif" />

d="p-3">where γ₁ is the activity coefficient of H₂O, X_i is the mole fraction of end-member i (1 = H₂O and 2 = N₂), V ⁰_i 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·P^0.3T, with A = 40005 J, B = 51.735 J/K, C = 14.848 J/(K·kbar^0.3), 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 d="xref-ref-17-1" class="xref-bibr">Duan et al. (2000) and the theoretical EOS of d="xref-ref-12-1" class="xref-bibr">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.