Electrical conductivity and streaming potential coefficient in a moderately alkaline lava series

Abstract

Coupled hydraulic and electrical problems are being investigated with renewed interest in geophysical applications. In this study, electrical conductivity and streaming potential coefficient (SPC) have been measured in the laboratory for crushed oceanite, hawaite, and trachyte samples from an almost complete moderately alkaline lava series from the Mururoa atoll (French Polynesia). For pH varying from 7 to 9.5 and ionic strength varying from 0.1 to 100 mmol/l, despite the closely related mineralogy of the rocks, contrasted results are obtained for the surface conductivity as defined by Revil and Glover: 0.10±0.07 mS/m, 8.3±0.2 mS/m and 1.05±0.16 mS/m for oceanite, hawaite, and trachyte, respectively, and for the inferred ζ potential: −6.6±0.3 mV, −26.3±0.5 mV, and −14.6±0.3 mV, respectively. The higher values obtained with hawaite suggest that electrical properties are controlled by secondary and accessory minerals such as clay minerals and zeolites resulting from low-temperature seawater alteration, rather than by the magmatic differentiation. Furthermore, a synthesis of results obtained with 20 rock samples indicates that signatures of the alteration process may emerge from the study of the ζ potential as a function of surface conductivity. The results with the lava series can be used to derive estimates of the scaling of basalt resistivity and associated SPC as a function of permeability. For permeability values smaller than 10−15 m2, the value of the SPC can be dramatically affected by the value of the surface conductivity. At least in the brittle crust, a better knowledge of the contributions of clay minerals and zeolites to surface conduction is therefore needed, both for the interpretation of electrical conductivity profiles and for the estimation of electrical variations associated with groundwater flow, as could be produced for example during volcanic or tectonic cycles.