EQUILIBRIUM....OR NOT? CLUES FROM REACTION MECHANISMS IN METAMORPHIC ROCKS

A traditional approach in metamorphic petrology has been to assume that metamorphic reactions proceed under near equilibrium conditions, so that the mineral assemblages, modes, and compositions can be used to decipher the metamorphic conditions when the minerals grew. This requires that the reaction mechanisms that allow minerals to grow and dissolve operate under local equilibrium conditions so that minerals in the immediate vicinity of each other are at or near equilibrium, although spatially separated products and reactants are out of equilibrium because the products must be more stable than the reactants. This condition imposes constraints on the chemical potential gradients that control the grain-boundary material transport required between reactants and products when a mineral grows surrounded by a matrix that is not of the same composition. If local equilibrium is maintained, the whole-rock reaction is accomplished by local reactions that are controlled by the growing mineral’s composition and the Gibbs-Duhem relations of the phases in local equilibrium that surround it. This causes minerals consumed in the local reactions to be depleted around a growing porphyroblast. When the minerals consumed are disseminated in the matrix around a growing porphyroblast, mantles or halos depleted in the reactant mineral develop around the porphyroblast. If a reactant minerals is irregularly distributed (e.g. as earlier formed porphyroblasts) the reactant crystals nearest the growing porphyroblast will be replaced, while those farther away will be unaffected until the crystals close to the product porphyroblast have been consumed.

When these features are present , it indicates that the reaction proceeded under local equilibrium conditions. When these features are absent, it suggests that local equilibrium was not maintained and the reaction proceeded by disequilibrium mechanisms. Irreversible thermodynamics has been combined with nucleation kinetics to investigate how reaction mechanisms develop when an isograd is crossed. The results of these calculations show that features in some rocks indicate reactions proceeded under local equilibrium conditions while reactions in other rocks appear to have been dominated by disequilibrium, suggesting both processes play a role in metamorphism.