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

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 up lifted to seafloor by progressive unroofing along a north-dipping low-angle detachment fault. Physical properties of a set of gabbroic samples from OD P Hole 735B have been measured in the laboratory, with a particular emphasi s on the analysis of electrical properties, The electrical formation factor (F) and surface conductivity (C-S) 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 c ompressional velocities are in the same range as those previously measured on ODP Leg 118 samples [Proc. ODP. Sci. Results, 118(1991)227]. The porosit y (phi) is low (<1%) in most, fresh samples. The analysis of the porosity s tructure, characterised by the electrical tortuosity (7) 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 c onstant <tau> of 15. independent of the degree of alteration of the sample, indicating that the porous network is controlled by primary microstructure s, such as grain boundaries, and by extension plastic foliations. Modificat ions with time and alteration are restricted to increases of phi 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, wi th variable tau (3-10), very low m (1.4 +/- SEM) and low phi (0.8 +/- SEM%) . This indicates a simpler porous network compared to that at shallower dep ths, probably dominated by well aligned cracks. The change in porosity stru cture downhole may be explained several ways, and may integrate the whole h istory of the crustal section cored at ODP site 735 since accretion, includ ing plastic deformation related to unroofing of the lower crust gabbros alo ng a low-angle detachment fault, cooling, and alteration due to hydrotherma l Row at various temperatures. The electrical conduction via electronic pro cesses in metallic minerals is not taken into account in our analysis of th e electrical properties: we propose a new constitutive law for electrical c onduction, that would account fur the conductive mineral matrix. (C) 2001 E lsevier Science B.V. All rights reserved.