Controls on nucleation in metamorphic rocks

Most rocks undergoing metamorphism are composed of a matrix of minerals that provide a variety of nucleation sites for new minerals. At low grades, nucleation and growth of new minerals is generally controlled by variations in reaction affinity related to local changes in composition inherited from the protolith. At higher grades of metamorphism, nucleation is controlled by compositional heterogeneity due to the distribution of porphyroblasts produced by lower grade reactions. In this case, nucleation of new minerals are suppressed or enhanced by reaction affinities that are buffered by local reactions between pre-existing porphyroblasts and surrounding matrix minerals. A common example of this phenomenon is observed when garnet-staurolite schists are heated so that sillimanite becomes stable. In this situation, sillimanite commonly nucleates in biotite in local environments where the where sillimanite is most supersaturated. The primary control on sillimanite supersaturation in these types of rocks are local equilibrium reactions between preexisting porphyroblasts and matrix. Local equilibrium with staurolite buffers the activity of sillimanite to a low level, so that biotites near staurolite are not typically sites of sillimanite nucleation. Local equilibrium with garnet buffers the activity of sillimanite at an intermediate level. The most favorable sites for sillimanite nucleation are in matrix biotites that are isolated from both garnet and staurolite because the reaction affinity for sillimanite is highest in chlorite+biotite+muscovite+quartz assemblages that are not buffered by local equilibrium with staurolite or garnet.