Phase Equilibria Involving Nonideal Solutions and Outlines of Geothermometry and Geobarometry

Abstract

Given the thermodynamic data for pure substances (mineral end-members or pure fluid species) and the equations of state for the solutions, the chemical potentials of the components in any phase, solid or fluid, are readily computed at any pressure, temperature, and composition. The knowledge of the chemical potentials sets the stage for the computation of phase diagrams comprising nonideal solutions. A phase diagram predicted from thermodynamic data may be compared with mineral assemblages observed in nature to gain insight into the conditions of the formation of rocks and mineral deposits. The computed phase diagram may also be utilized to plan new phase equilibria experiments. Alternatively, knowledge of the chemical potentials permits recovery of intensive parameters (T, P, a _i, etc.) from compositional data on the coexisting minerals of rocks, providing the observed compositions reflect the condition of equilibrium. The extraction of the temperature of equilibration is geothermometry; geobarometry is the calculation of the pressure of equilibrium for any assemblage. Because the T and P of solid-solid reactions are independent of the activity or fugacity of fluids prevalent during equilibration, solid-solid equilibria form the backbone of geothermometry and geobarometry. Once temperature and pressure of equilibration of an assemblage are known, a solid-fluid equilibrium may be used to evaluate a _i (or f _i) of the fluid i present during equilibration.