THERMODYNAMIC MIXING PROPERTIES OF MUSCOVITE-PARAGONITE CRYSTALLINE SOLUTIONS AT HIGH TEMPERATURES AND PRESSURES, AND THEIR GEOLOGICAL APPLICATIONS

Abstract

Reversed Na-K exchange data between mica and a 2 molal aqueous (Na,K)Cl fluid (Flux & Chatterjee, 1986) have been employed to model the thermodynamic mixing behaviour of muscovite-paragonite crystalline solutions on the basis of the Redlich-Kister equation. For these binary micas, Gexm may be expressed as Gexm=(1−Xms)Xms[A+B(1−2Xms)+C(1−2Xms)2] where A=11222+1.389 T+0.2359 P, B=?1134+6.806 T?0.0840 P, and C=?7305+9.043 T, with T in K, P in b, Gexm, A, B, and C in joules/mol. Gmex is well constrained between 450 and 620°C, and may be extrapolated beyond that range with caution. The calculated solvi are skewed toward the paragonite end member. In the range up to 15 kb, the critical temperature, Tc and the critical composition, Xc may be expressed as a function of P by the relations: Tc(in K )=977.68+1.53∗10−2P+2.45∗10−7P2 and Xc(in X ms )=0.2624−2.34∗10−6 P with P indicated in bars. Calculated phase relations of muscovite-paragonite crystalline solutions have been depicted in terms of the system KAlSi3O8-NaAlSi3O8-Al2O3-SiO2-H2O. These data may be applied to appropriate assemblages involving mica, alkali feldspar, an Al2 polymorph, and quartz to estimate P, T and aH2O conditions of their equilibration. In principle, the muscovite limb of the solvus may be used to obtain geothermometric data for coexisting muscovite-paragonite pairs, provided the equilibrium pressure is independently known. However, such application must be restricted for the present to micas on the ideal muscovite-paragonite join. Mica-alkali feldspar-Al2SiO5-quartz or mica-plagioclase-Al2SiO5-quartz assemblages may be used to deduce aH2O in the coexisting fluid, if P, and T of equilibrium are independently known. Examples of such geological applications are given.