MODELING HETEROGENEOUS METAMORPHIC REACTION MECHANISMS AND THEIR RELATIONSHIP TO TEXTURES AND P-T-X-T PATHS

The textures produced by metamorphic reaction mechanisms record valuable information about the Pressure-Temperature-Composition-time (P-T-X-t) history of a rock. Important factors that control how a particular P-T-X-t path will affect the reaction mechanisms that develop in a rock are: 1) the nucleation of new minerals in a rock as reactions are overstepped, 2) the growth of a new mineral once a stable nucleus has formed, 3) the transport of constituents to and from the reaction site and 4) consumption of early formed crystals as new reactions develop. Typically, rocks cross more than one reaction during metamorphism, resulting in complex nucleation and growth mechanisms among different assemblages in multiple heterogeneous local environments within a single hand specimen.

These processes are modeled in 3 dimensions with an approach that calculates nucleation rates using all reactions that are overstepped in the local environment and relates the growth or consumption of crystals that have previously nucleated to surface kinetics and material transport through the region around the reaction site. The position, nucleation time and growth history of each crystal is recorded and the texture evolution of the rock is sequentially drawn for each time step. The spatial and size distributions of the entire crystal population, detailed information about each crystal (such as its time of nucleation, growth rate and compositional zoning pattern), and “movies” showing the texture development can be extracted from these data.

The method is illustrated using calculated thermal fields around mid-crustal plutons. Important concepts that are revealed by these combined models are: 1) metastable reactions probably play an important role during nucleation and early growth of many metamorphic minerals, 2) reaction overstepping due to rapid heating and cooling may result in significant growth of “prograde” minerals during cooling, and 3) domains of equilibrium progressively enlarge as reactions proceed, allowing late stages of growth to establish equilibrium throughout much of a thinsection. This can result in the incorrect interpretation that nucleation and growth was due to stable reactions, when in fact these processes were controlled by metastable reactions in local domains that were not in equilibrium with each other.