The role of the fluid phase during regional metamorphism and deformation

Evidence from rock microstructures, mass transfer and isotopic exchange indicates that substantial quantities of aqueous fluids are involved in low‐ and medium‐grade regional metamorphism. Similar conclusions are drawn from many retrograde environments, whereas high‐grade metamorphic fluids may be melt dominated. The mobile fluids play essential roles in metamorphic reactions, mass transport and deformation processes. These processes are linked by the mechanical consequences of metamorphic fluid pressures (P_f) generally being greater than or equal to the minimum principal compressive stress. Under such conditions metamorphic porosity comprises grain boundary tubules and bubbles together with continuously generated (and healed) microfractures. Deformation results in significant interconnected porosity and hence enhanced permeability. Lithologically and structurally controlled permeability variations may cause effective fluid channelling. Simple Rayleigh‐Darcy modelling of a uniformly permeable, crustal slab shows that convective instability of metamorphic fluid is expected at the permeabilities suggested for the high P_f metamorphic conditions. Complex, large‐scale convective cells operating in overpressured, but capped systems may provide a satisfactory explanation for the large fluid/rock ratios and extensive mass transport demonstrated for many low‐ and medium‐grade metamorphic environments. Such large‐scale fluid circulation may have important consequences for heat transfer in and the thermal evolution of metamorphic belts.