The physical properties of oceanic basement rocks from deep drilling on the Mid-Atlantic Ridge

Abstract

The physical properties of compressional and shear velocity, electrical resistivity, bulk and grain density, porosity and water content, and thermal conductivity have been measured on a large collection of basalt samples and a few gabbros and serpentinized peridotites recovered by drilling on the mid-Atlantic ridge near 37°N. Samples were recovered to 582 m into basaltic basement in one hole, and rocks that may be representative of the lower crust were recovered from another hole. The mean compressional velocity of the basalts of 5.94 km s−1 agrees well with previous fresh unweathered sea floor basalts. It, however, can be reconciled with the much lower average upper crustal layer 2 seismic refraction velocities generally observed only by the presence of extensive large-scale fracturing and voids in the crust. The laboratory velocities of the gabbros are consistent with upper layer 3, or oceanic layer, refraction velocities. Lower layer 3 refraction velocities are consistent with a composition of a mix of gabbro and unserpentinized peridotite. Poisson's ratio for little-weathered laboratory samples of basalt is found to decrease systematically with decreasing velocity. The mean electrical resistivity of the basalts is 220 ohm m at 25°C. A close dependence of resistivity on porosity and a moderate increase in resistivity with pressure suggest conduction primarily through pore fluid. The basalt resistivity decreases rapidly with increasing temperature, the mean becoming about 20 ohm m at 100°C and about 5 ohm m at 150°C. The mean basalt bulk density is 2.795 g cm−3, the mean porosity is 7.8%, and the mean thermal conductivity at 21°C is 3.97 mcal cm−1 s−1 °C−1.