Reaction enhanced permeability during retrogressive metamorphism

Retrogressive metamorphism is in most cases associated with influx of external fluids into igneous or metamorphic rocks with low to negligible initial permeabilities. Thus fluid migration is associated with some process that creates the necessary channels for migration. One such mechanism is associated with the volume changes resulting from volatilization reactions. Petrographic studies of a partly serpentinized troctolite from the Duluth complex in Minnesota reveal how the increase in solid volume associated with hydration of olivine phenocrystals generates an extensive network of microfractures in the surrounding plagioclase-dominated matrix. This microfracture network allows hydrous fluids to penetrate further into the rock. A 2-D spring-network model illustrates how expanding polygons (or non-spherical grains) in an elastically deforming matrix cause dilatant microfracturing and porosity generation even if the overall stresses are compressive. The model emphasises how fluid infiltration may be promoted by stress heterogeneities set up by metamorphic reactions.