Permeability of the oceanic crust based on experimental studies of basalt permeability at elevated pressures

Abstract

Permeabilities have been measured as a function of confining pressure at constant temperature for basalt dredge samples from the Juan de Fuca and Tonga-Kermadec regions. Permeabilities at 40 MPa are 1.1 × 10 −19 m 2 for the Juan de Fuca basalt and 7.4 × 10 −19 m 2 for the higher porosity Tonga-Kermadec basalt. Permeabilities decrease significantly with increasing confining pressure. The measured values of permeability are considerably lower than the estimated 10 −14 to 10 −15 m 2 permeabilities of the upper few hundred meters of basement rocks of the oceanic crust, thereby supporting oceanic crustal convection models in which seawater convection occurs largely through macrocracks at relatively shallow depths. Based on our laboratory measurements it is estimated that permeabilities of layer 2C and layer 3 are generally less than 1.0 × 10 −19 m 2. Permeabilities of layers 2A and 2B in young oceanic crust are expected to be anisotropic due to the preferred orientation of fractures. An anisotropic upper crustal model is presented in which layer-2 permeabilities are low parallel to spreading directions.