Electrical properties of zeolitized volcaniclastic materials

Abstract

The interpretation of electromagnetic anomalies associated with volcanic activity requires a good understanding of two rock properties, the electrical conductivity and the streaming potential coupling coefficient. We measured these properties on 22 consolidated tuff samples containing clays and zeolites typically found in hydrothermal systems or in other areas of high water‐rock interactions associated with active volcanic areas. These rocks exhibit unusually high surface conductivity and electrical cementation exponent (2.5–3.3). These features are explained by the highly complex texture of these rocks due to hydration/dissolution processes of the volcanic glass and the formation of clay minerals and zeolites as alteration products. At a pore fluid conductivity of 1.0 ± 0.2 S m−1, the streaming potential coupling coefficient ranges from −3.55 to −10.7 mV MPa−1. The zeta potential (a key electrochemical property of the pore water/mineral interface) determined from these measurements at T0 = 20°C and pH ∼6–7 and corrected for surface conductivity is ∼−28 ± 8 mV at 0.1 M NaCl. Two clay‐free samples exhibit a zeta potential ∼−16 ± 5 mV, a value associated with zeolites (clinoptilolite). The temperature dependence of the ζ potential is rather strong with ζ(T) = ζ(T0) [1 + νζ (T − T0)], where νζ ≈ 4.2 × 10−2 C−1 in the temperature range 20–60°C. These data are applied to the understanding of large‐scale self‐potential anomalies located at the ground surface of Miyake‐Jima volcano.