Résumé
The resistivity, porosity, and cation exchange capacity of 36 basaltic samples recovered in Hole 504B during four DSDP and ODP legs have been measured in the laboratory at room temperature and atmospheric pressure. The presence of chlorites and particularly smectites as alteration products of basalt phases is reflected by high values of cation exchange capacity (CEC). Whereas the massive units of Layers 2A and 2B are defined by high and uniform CEC values, the more fractured and altered pillows are characterized in the entire hole by even higher values of CEC and a large variability. The lowest CEC values, measured for the massive units of Layer 2C, are due to changes of basalt alteration fades with depth and the related decreasing abundance of smectites with increasing depth in the oceanic crust.The porosity and the apparent formation factor (computed from resistivity measurements made with a fluid salinity similar to that of seawater) are related by an inverse power law similar to Archie's formula, with m close to 1.0 and a as large as 9.1. A review of the literature shows that such a low m value equates to current conduction in cracks and microcracks present throughout the rock. The presence of these microstructures reflects the extensional regime under which the rock formed at the ridge axis, and they were conserved by precipitation of clay minerals attributable to intense hydrothermal circulation. The comparison of these results to similar studies of mid-ocean ridge basalt physical properties indicates that m tends to increase to values close to 2.0 with age. Such values are, in fact, similar to those found for sedimentary rocks and probably reflect an increased tortuosity of the conducting pore space with increasing age. An inverse relationship also relates CEC and apparent formation factor, indicating that surface conduction of clay minerals plays an important role during downhole electrical experiments. This provides a plausible key to the paradox of low permeability and high apparent porosity obtained from comparing the in-situ experiments conducted in Hole 504B.