Electrical properties of slow-spreading ridge gabbros from ODP Site 735, Southwest Indian Ridge

Abstract

ODP hole 735B (ODP Legs 118 and 176) samples a block of igneous crust which was accreted at the ultraslow-spreading Southwest Indian Ridge, and was uplifted to seafloor by progressive unroofing along a north-dipping low-angle detachment fault. Physical properties of a set of gabbroic samples from ODP Hole 735B have been measured in the laboratory, with a particular emphasis on the analysis of electrical properties. The electrical formation factor (F) and surface conductivity (Cs) are calculated using the model of Revil and Glover [Geophys. Res. Lett., 25 (1998) 691], from measurements at room pressure, and different salinities of the saturating fluid. The acoustic compressional velocities are in the same range as those previously measured on ODP Leg 118 samples [Proc. ODP, Sci. Results, 118 (1991) 227]. The porosity (φ) is low (<1%) in most, fresh samples. The analysis of the porosity structure, characterised by the electrical tortuosity (τ) and the electrical cementation factor (m), reveals that the cored gabbro section is segmented in two parts. The upper part (approximately the upper half) has a nearly constant τ of 15, independent of the degree of alteration of the sample, indicating that the porous network is controlled by primary microstructures, such as grain boundaries, and by extension plastic foliations. Modifications with time and alteration are restricted to increases of φ and m, i.e. a higher variability of the channel thicknesses. In the lower part of the hole, fresh rocks predominate, and the porosity structure is different, with variable τ (3–10), very low m (1.4±SEM) and low φ (0.8±SEM%). This indicates a simpler porous network compared to that at shallower depths, probably dominated by well aligned cracks. The change in porosity structure downhole may be explained several ways, and may integrate the whole history of the crustal section cored at ODP site 735 since accretion, including plastic deformation related to unroofing of the lower crust gabbros along a low-angle detachment fault, cooling, and alteration due to hydrothermal flow at various temperatures. The electrical conduction via electronic processes in metallic minerals is not taken into account in our analysis of the electrical properties; we propose a new constitutive law for electrical conduction, that would account for the conductive mineral matrix.