The Mechanics of Metamorphic Fluid Expulsion

Metamorphic devolatilization generates fluid and grain-scale porosity. Evidence for high fluid pressure indicates the devolatilization occurs under poorly-drained conditions. Under such conditions fluid expulsion is limited by the capacity of the reacted rocks to resist compaction or the rate at which deformation modifies the permeability of the overlying rocks. In the former case the compaction time scale must be greater than the metamorphic time scale and flow patterns are dictated by details of permeability. The alternative is that compaction processes are fast relative to metamorphism. In this case flow is compaction-driven and accomplished by waves of fluid-filled porosity.