REACTION-INFILTRATION FEEDBACK IN METAMORPHISM: THEORY AND EXAMPLES

Timescales of compaction during metamorphism predict that metamorphic permeability is often short-lived, and coincides with locally high fluid pressure. These conditions should generally prohibit the pervasive infiltration of externally derived fluids. However, chemical, isotopic and mineralogical alteration of metamorphic rocks is commonly cited as evidence for the percolation of large quantities of externally derived fluids. If these two observations are to be reconciled, then additional processes must operate to preserve permeability over metamorphic timescales. One such process is the generation of porosity and permeability by infiltration-driven mineral reactions. Most metamorphic reactions that produce porosity also evolve volatile species which locally elevate fluid pressure. Mineral reaction thereby influences fluid flow by modifying both permeability and fluid pressure simultaneously, often with contradictory effects: porosity increases will tend to accelerate and focus fluid flow while fluid production will retard and defocus flow. In addition, the efficiency of these processes relative to the rate of fluid infiltration and compaction will determine if mineral reactions have any meaningful influence on hydrodynamics.

Two reaction-specific dimensionless parameters, the pore generation potential and the over pressure potential, allow these effects to be evaluated. The parameter space can be divided into three fields: (1) in which mineral reaction does not promote incursion of externally-derived fluids, (2) in which fluid is drawn and focused into discrete reaction zones, and (3) in which homogeneous fluid infiltration is promoted. Examples drawn from the Horsethief Creek contact metamorphic aureole (southeast British Columbia) and the Mineral Hill wollastonite skarn (southwestern B.C.) illustrate environments in which infiltration-driven alteration extends over 100s of metres to kilometers. In both these examples, the mineral reactions at or near the leading edge of the infiltration fronts have pore generation potential that is favourable for the preservation of permeability. Differences in the over pressure potential between these systems, however, may account for differences in the geometry of the alteration and reaction fronts.