THE PERMEABILITY OF THE CONTINENTAL CRUST: IMPLICATIONS FOR THE COUPLING OF FLUID RESERVOIRS

The variation in permeability (k, in m²) with depth (z, in km) in the tectonically active continental crust can be estimated as log k ~ -14 - 3.2 log z on the basis of (1) hydrologic models invoking geothermal data as constraints and (2) the progress of metamorphic reactions driven by fluid flow (RoG 37, 127; Geology 27, 1,107). Although this log fit shows permeability decaying by about an order of magnitude below the brittle-ductile transition, the data below ~12.5 kilometers are actually fitted just as well by a constant permeability of log k ~ -18.3 m². The absence of a permeability discontinuity at the transition implies that fluids from the lower crust can readily be transmitted to the upper crust. The permeability-depth curve thus provides a quantitative basis for estimating the communication between several major reservoirs of volatiles: the mantle; the lower crust, where fluids are derived internally from hydrous mineral phases; the upper crust, dominated by meteoric groundwater; and the world ocean. The results have tantalizing, albeit speculative, implications for tectonism, metamorphic outgassing, and the evolution of the hydrosphere over geologic time. For instance, W.W. Rubey's long-accepted hypothesis that volcanic outgassing can account for the origin of sea water (GSA Bull. 62, 1,111) has recently been challenged by findings that the rate of subduction of sea water is much larger than the volcanic outgassing rate. A first-order calculation based on our permeability-depth data suggests a similarly large capacity for diffuse degassing through tectonically active continental crust, and weakens this mass-balance argument against a terrestrial source.