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Abstract

Mineral assemblages are only stable in specific pressure and temperature ranges in the Earth. At fixed external variables such as, for example, pressure, temperature or the composition of an infiltrating fluid phase, the compositions of minerals in a given assemblage are fixed as well. However, changes in, say, pressure (P) and/or temperature (T) lead to compositional variations in solid solutions between specific end-member components or compositions. This interdependence between the chemistry of mineral solid solutions and P and T forms the basis of most pressure and temperature estimations (i.e., geothermobarometry). Assuming that the compositions of at least some of the minerals preserved in a rock reflect equilibrium, the thermodynamic equilibrium condition forumla can be used to calculate the PT conditions which the rock experienced at some stage during its tectono-metamorphic history. ΔG0 is the standard state Gibbs energy change of a reaction and depends on pressure and temperature, R is the universal gas constant, and K, the equilibrium constant, is a function of the activities of the components in a solid solution which, in turn, are related to mineral compositions. In order to apply the fundamental equation, the thermodynamic properties of the mineral end-member components and the appropriate form of the activity–composition (ax) relationships of the solid solution must be known. For most minerals, however, the activities of the components in a solid solution are a complex function of pressure, temperature and mineral composition.

There are several possibilities to decipher quantitatively the conditions of formation of rocks through thermodynamic calculations.

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