Resistivity, porosity, grain density and CEC for samples of ODP Hole 118-735B

The resistivity, porosity, and cation exchange capacity of 29 gabbroic samples recovered in Hole 735B during Leg 118 of the Ocean Drilling Program (ODP) were measured in the laboratory at room temperature and atmospheric pressure. The samples of lithologic Units IV and VI containing iron and titanium oxides have the highest electrical conductivity and the largest grain density sampled in Hole 735B. The fresh samples are the most resistive of the data set, and the tectonized samples (either due to plastic or brittle deformation) have intermediate values.
Cation exchange capacity and electrical resistivity measurements performed at four different fluid salinities indicate that surface conduction mechanisms can be ignored for in-situ measurements of electrical resistivity at Site 735. The main conduction mechanisms for electrical current consequently are reduced to (1) electrolytic conduction in the pore space and (2) electronic conduction in metallic grains of the matrix in the presence of Fe-Ti oxides. In the last case, matrix conduction might become the dominant conduction mechanism, which explains the low resistivity values recorded in situ with the dual laterolog in lithologic Unit IV and in the lower part of the hole. The conduction equation is then written as follows:
Co = [(Cw/FF) + Coxides],
and the low m value obtained when relating porosity to formation factor with
FF = (8.04) phi**-1.08,
which suggests that the conducting pore space of gabbros is similar to that of mid-ocean ridge basalts (MORB), that is, mainly constituted of cracks and microcracks, as elsewhere observed in thin sections.