REACTION AFFINITY DURING REGIONAL METAMORPHISM: CHALLENGES TO INTERPRETATIONS OF METAMORPHISM

Numerical models of diffusion-controlled nucleation and growth of garnet crystals, which successfully replicate diverse textures in 13 porphyroblastic rocks from 7 localities, quantify the scale of departures from equilibrium during crystallization. Here we use the term “reaction affinity” to describe the departure from equilibrium quantified from chemical-potential gradients arising from the supersaturation of Al in the intergranular fluid, following the notion that Al is the rate-limiting component of each reaction. The reaction affinity is calculated at the site and time of each nucleation event providing both the reaction affinity estimates of each nucleation event and a mean reaction affinity for the whole rock throughout the crystallization interval. For the first crystals to nucleate, the reaction affinities range from 0.7 to 5.8 kJ·mol^-1 of 12-oxygen garnet (4-62 °C thermal overstepping), and the maximum whole-rock reaction affinities range from 4.7 to 16.0 kJ·mol^-1 (50-125 °C thermal overstepping). The results demonstrate that impediments to crystallization significantly delay nucleation and retard reaction, with the consequence that perceptible modes (~1 volume %) of the product crystals may not be observed until well beyond the equilibrium conditions of the reaction, and nucleation of new crystals extends throughout nearly the entire crystallization interval. This potential for protracted reaction during prograde metamorphism, with reactions continuing to temperatures well beyond the equilibrium conditions, suggests the likelihood of overstepping of multiple—possibly competing—reactions that can progress simultaneously. Therefore, isograds and ranges of stability for metamorphic assemblages along a metamorphic field gradient may be significantly offset from the positions predicted from calculations based on equilibrium assumptions, which poses a substantial challenge to accurate interpretation of metamorphic history.