PHYSICOCHEMICAL CONTROLS ON REACTION KINETICS AND EQUILIBRATION: SOMETIMES TURNING UP THE HEAT DOESN'T MATTER MUCH…

Rates of diffusion through an intergranular medium — which often govern the kinetics of metamorphic reaction and chemical equilibration — increase exponentially with temperature, yet in nature, thermal effects are subsidiary to the profound influence of physical and chemical features of the intergranular medium: its degree of fluid saturation, its volume fraction (which depends on precursor grain size), and the composition of any intergranular fluid.

Diffusion coefficients for Al at 600 °C in fluid-saturated, hydrous-but-fluid-undersaturated, and nearly anhydrous systems range over nearly 7 orders of magnitude (from 10^-18.8 to 10^-25.4m²·sec^-1). Thermal effects are smaller: for example, in fluid-saturated systems, even a large temperature increase from 400 °C to 900 °C raises the diffusion coefficient by only 4.5 orders of magnitude.

Diffusional fluxes in fluid-saturated systems scale in direct proportion to the volume fraction of interconnected porosity, so reaction rates are strongly influenced by the grain size of the intergranular medium during reaction. The volume fraction of grain edges is proportional to the squared inverse of grain diameter, so at constant temperature, reaction rates in a 10-µm matrix are faster by 2 orders of magnitude than they are in a 100-µm matrix, and length scales of chemical equilibration are an order of magnitude longer. Consequently, precursor grain size is a key determinant of reaction kinetics, and slowing of reaction due to grain coarsening can counterbalance or overwhelm the effects of thermal acceleration during prograde metamorphism.

Diffusional fluxes in fluid-saturated systems are also directly proportional to the solubility of the diffusing component in the intergranular fluid. In a revealing occurrence at Harpswell Neck, Maine, low solubility for Mn, Fe, and Mg in early CO₂-rich fluids yielded disequilibrium distributions in garnet while higher solubility allowed equilibration of Ca and Y; in later H₂O-rich fluids, higher solubility for all elements expanded length scales of diffusion to permit rock-wide equilibration. Analogous effects on solubility, transport, and equilibration should be expected from contrasts in the abundance and speciation of dissolved cations due to variations in pH, chlorinity, and oxidation state of the intergranular fluid.