PETROLOGIC CONSEQUENCES OF VARIATIONS IN METAMORPHIC REACTION AFFINITY

The extent to which kinetic barriers to nucleation and growth delay the onset of metamorphic reaction is related to reaction affinity, defined as the Gibbs free energy difference between the thermodynamically stable, but not-yet-crystallized, products and the metastable reactants. Mineral reactions which release large quantities of H₂O, such as chlorite-consuming reactions, have a higher entropy/volume change, and therefore a higher reaction affinity per unit of T/P overstep, than those which release little or no H₂O, such as chlorite-free reactions. The former are expected to be overstepped in T or P less than the latter, as observed in a number of natural contact metamorphic settings. Some implications of the above include: (1) metamorphic reaction intervals are expected to be discrete rather than continuous, including in broad multivariant domains across which smooth, continuous reaction is predicted; (2) reaction intervals may not correspond in a simple way to reaction boundaries and domains in an equilibrium phase diagram, and can involve metastable reactions; (3) overstepping can lead to a ‘cascade effect’, in which several stable and metastable reactions involving the same reactant phases proceed simultaneously; (4) fluid generation, and possibly fluid presence in general, is expected to be episodic rather than continuous, corresponding to discrete intervals of reaction; (5) thermobarometry based on combined use of phase diagram sections and mineral modes/compositions on the one hand, and classical thermobarometry methods on the other, may not agree even if the same thermodynamic data are used. The extent to which these findings apply to regional metamorphism depends on several factors. Nucleation-kinetic modeling based on classical nucleation theory suggests that, contrary to common supposition, the faster heating rate of contact metamorphism compared to regional metamorphism has little effect on predicted overstepping. A more important factor may be the enhanced deformation of regional metamorphism, which may favour the development of low-interfacial-energy sites for nucleation.