MODELING METAMORPHIC FLUID-FLOW WITH REACTION-COMPACTION-PERMEABILITYFEEDBACKS

Existing models of metasomatic now do not allow for the effect that re action has on the now patterns. Instead, it is assumed that the volati les produced are negligible in volume compared to those infiltrated an d that reaction does not modify permeability. This is clearly unlikely to be true for infiltration-driven decarbonation reactions. The rates of porosity creation by reaction and porosity loss by creep have been calculated for a representative volume of calcite-quartz-wollastonite marble, and it is found that, even for a weak calcite matrix, the rat e of porosity generation by reaction is likely to outstrip the collaps e of porosity, as long as the system is out of equilibrium. We have ap plied a self-consistent ID finite-difference model to the reaction of calcite + quartz to wollastonite in a 10 m thick marble, in response t o influx of H2O rich fluid, with fixed boundary conditions. The model allows us to evaluate the effect of reaction on the porosity structure and fluid pressure variation across the layer, from which local Darcy fluxes can be evaluated. The progress of reaction that we model is co nstrained by hydrological considerations, with the requisite parameter s recalculated as reaction progresses, assuming creep compaction of ro ck under the stress difference between lithostatic and fluid pressures . We find that the volume of fluid released by decarbonation, driven b y influx of H2O, is sufficient to create a back-now, so that further a dvancement of the reaction front is only possible as a result of diffu sion of water against the Darcy flux. The effect of creep driven by di fferences between fluid pressure and lithostatic pressure is to reduce the permeability of the layer and especially reduce the secondary por osity developed in the zone at and behind the advancing reaction front . We predict that in a 3D situation, the porous zone of reacted marble becomes a conduit for layer-parallel now, and the secondary porosity is infilled by calc-silicate minerals due to silica metasomatism.