Fluid trapping in mid-crustal reservoirs by H₂O–CO₂ mixtures

Abstract

A THIN reservoir of free aqueous or carbonic fluids at mid-crustal levels, perhaps in the form of water sills¹, has been suggested as the source of fault-transmitted metasomatizing fluids^2–4, as the cause of observed deep crustal electrical conductivity and anomalous seismic reflectivity⁵, and as a lubricated potential detachment zone for thin-skin tectonics⁶. A problem with this hypothesis is that estimated crustal permeabilities are too high to permit long-term retention of such fluids^7,8. Most mechanisms suggested for achieving the required permeability reduction rely on maintaining unusually low porosity ^2,6,9,10. Because porosity creation, by tectonic deformation, fluid release or infiltration, is a ubiquitous process, permeability reductions achieved by these mechanisms are hard to sustain for long periods of time. A sealing (permeability reduction) mechanism that can operate in the presence of significant porosity is required. Here I propose that the required mechanism is capillary sealing by immiscible CO₂-H₂O mixtures derived from rising volatiles.