Geochemistry of recrystallized rocks

Abstract:

Experiments were conducted to evaluate mechanisms operating along grain boundaries that control compositional and textural properties during recrystallization of rocks. Grain boundaries act as important geochemical reservoirs because elements that are incompatible in the dominant matrix minerals of a rock are partitioned into grain boundaries. Compositional variations in recrystallized minerals adjacent to grain boundaries in recrystallized rocks result from processes that are similar to those that modify compositions in mineral/melt and mineral/fluid systems. In fluid-free systems the transport of elements in polycrystalline materials must occur by diffusion along pathways composed of grain boundaries. From a purely geometrical standpoint, the only available pathway for coexisting minerals to ‘communicate’ with one another in response to changes in P–T is by grain-boundary diffusion. Because grain-boundary diffusion is commonly 10⁴ to 10⁶ times higher than diffusion through the lattices of most rock forming minerals, rapid transport of components through low volume grain boundaries may govern the kinetics of textural and geochemical equilibration. Minerals adjacent to grain boundaries may incorporate elements from the grain boundary region. The extent to which minerals adjacent to grain boundaries equilibrate is largely dependent upon the mechanism by which those minerals incorporate impurities from the grain-boundary region; lattice diffusion from a grain-boundary source may produce limited equilibration whereas chemically induced grain-boundary migration may produce extensive equilibration. The presence of a free fluid phase significantly affects the transport along grain boundaries, and in many cases will promote equilibration through dissolution and precipitation of minerals adjacent to grain boundaries.