LOCAL REACTION AFFINITIES AND THEIR EFFECT ON NUCLEATION PATTERNS IN METAMORPHIC ROCKS

Rocks undergoing metamorphism are always heterogeneous at some scale. If the scale of equilibration is smaller than the scale of heterogeneity, nucleation of a new mineral can be strongly influenced by reaction affinities that develop in local domains of rock. For example, a pelitic shist with garnet and staurolite porphyroblasts set in a fine matrix of chlorite, muscovite, biotite, and quartz that crosses the first sillimanite isograd has one stable reaction and three metastable reactions that can form sillimanite nuclei. In situations where the scale of equilibration is small, areas with the largest amounts of overstepping for sillimanite nucleation are regions where the matrix assemblage is far from staurolite and garnet porphyroblasts, so that the matrix in these areas is not in equilibrium with either mineral. If the matrix assemblage is equilibrated with garnet but not staurolite, it is overstepped about 2 degrees C less than the matrix assemblage alone. However, assemblages equilibrated with staurolite porphyroblasts are overstepped by about 20 degrees C less than staurolite-free assemblages, making sillimanite nucleation near staurolite less likely. This explains why sillimanite segregations in regional contact metamorphic environments tend to form in areas away from pre-existing staurolite porphyroblasts. Another type of heterogeneity that will affect nucleation patterns is the local variation in modes of the matrix phases because of features such as pseudomorphs. In this situation, small volumes of equilibration lead to heterogeneities that behave as distinctly different bulk compositions, causing mineral forming reactions to be crossed at different temperatures in different regions of a rock. This leads to the largest amounts of overstepping in regions that cross a reaction first, resulting in clusters of nuclei in these areas of rock. An example of this type of behavior is observed as clusters of many small 2nd generation staurolites within muscovite + chlorite psedomorphs after large 1st generation staurolites. These examples show that realistic models of nucleation and growth of metamorphic minerals need to account for many possible overstepping reactions and mechanisms in order to effectively simulate processes in real rocks.