METAMORPHISM OF NONHYDROSTATICALLY STRESSED ROCKS

The condition for metamorphic phase equilibrium within a fluid-saturated porous rock subject to a macroscopic nonhydrostatic stress depends upon the reaction site and mechanism. In most cases the local equilibrium condition depends explicitly on the microscopic state of stress within the solid phases; this precludes finding an approximate condition that depends only on the macroscopic thermodynamic parameters. If the reaction occurs by means of dissolution of the solid reactants followed by precipitation of the solid reaction products at the fluid-solid interface, the macroscopic condition is DELTA(r)G(P(f), T) almost-equal-to 0 where DELTA(r)G(P(f), T) is the change in the Gibbs free energy of reaction at the pore fluid pressure P(f) and temperature T. Alternative approximate equilibrium conditions that depend upon the mean solid stress or other macroscopic parameters in addition to P(f) and T are difficult to justify.