Electrical properties of mid‐ocean ridge basalt and implications for the structure of the upper oceanic crust in Hole 504B

Abstract

The electrical resistivity, porosity, and cation exchange capacity (CEC) of mid‐ocean ridge basalt (MORB) samples from Deep Sea Drilling Project hole 504B have been measured in the laboratory. The presence of chlorites, zeolites and particularly smectites as alteration products of MORB is reflected by high values of CEC, with high and uniform CEC values in the massive units of layers 2A and 2B, and even higher values in the pillows. The porosity and the “intrinsic” formation factor are related, in the massive units, by an inverse power law similar to Archie's formula, with m = 1.0 and a = 10.0. Such a low m value equates to current conduction in cracks and microcracks present at mineral scale throughout the rock. During leg 111 of the Ocean Drilling Program, the Joides Resolution D.V. returned in the equatorial Pacific to deepen hole 504B and to perform a series of downhole experiments. A continuous electrical resistivity profile permitted to discriminate the large‐scale seismic layers of the upper oceanic crust and to isolate individual lithologic units. In the extrusive part of the crust, the massive flows (10‐m‐thick or more) are found to constitute permeability barriers and, subsequently, to constrain fluid circulation. Within layer 2A, the massive flows of unit 2D are associated with the underpressured aquifer located underneath, within Unit 3. In layer 2B, unit 27 is the boundary between low‐temperature, seawater alteration facies of basalt, and higher‐temperature alteration phases. This relationship between morphology, hydrological regime, and therefore alteration of the basaltic basement is proposed to be related to the accretion process of the upper oceanic crust. The porosity estimate derived from in situ measurements of electrical resistivity is reduced when accounting for surface conduction, with high values computed in layer 2A only. A permeability profile computed on the basis of in situ resistivity measurements reproduces those obtained in situ from packer experiments, and therefore provides a key to the low‐permeability high‐“apparent”‐porosity paradox obtained in the past.