LOCAL CONTROL OF REACTION AFFINITY IN ROCKS WITH PORPHYROBLASTS

The affinity of a reaction is an important control on the nucleation of new minerals when a reaction is overstepped during metamorphism. In rocks with a uniform distribution of reactant minerals, thermodynamic calculations based on the overall bulk composition of the rock may provide a reliable estimate of the affinity as a reaction is overstepped. However, rocks with porphyroblasts are not compositionally uniform, and reaction affinity is dependent upon the scale of equilibration. If the reaction is overstepped so that the scale of equilibration is large, all minerals in a rock, including porphyroblasts, will be at or near equilibrium, so the reaction affinity will be determined by the metastable assemblage involving all the reactant phases in a rock. However, if the scale of equilibration is smaller than the distance between different types of porphyroblasts, the reaction affinity will be locally buffered by several meta-stable mineral assemblages, resulting in regions with different reaction affinities in the same rock. For example, a pelite with Grt and St porphyroblasts that is progressing from the St zone to the Sil zone has several different possibilities for the spatial control of super-saturation of Sil as the reaction is overstepped. When metamorphic conditions allow the domains of equilibrium to be large, including both Grt and St porphyroblasts, the supersaturation of Sil will be controlled by overstepping of the stable reaction involving all the minerals in the rock. However, if domains of equilibrium do not enclose both Grt & St porphyroblasts, the activity of Sil will depend on the affinities of three meta-stable reactions which locally buffer the activity of Sil: one involving Grt, one involving St and a third involving neither porphyroblast . For rocks undergoing isobaric heating, the activity Sil rises most rapidly in domains away from both St and Grt porphyroblasts, closely followed by domains in local equilibrium with Grt. The activity of Sil near St is lowest in this situation. The resulting distribution of Sil supersaturation suggests that most Sil nuclei should form in the matrix away from either St or Grt, followed by nucleation near Grt and then near St. This is what is commonly observed in natural rocks that have undergone isobaric heating.