High‐pressure and high‐temperature measurements of electrical conductivity in basaltic rocks from Mount Etna, Sicily, Italy

Abstract

We have investigated the electrical properties of Etnean rocks by in situ complex impedance spectroscopy using a multianvil apparatus. From these measurements we determined the electrical conductivity of a lava flow sample, whose basaltic composition can be considered close to that of the parent magma and also that of a mafic nodule representative of the high‐density cumulates interpreted as responsible for the main high‐velocity anomaly observed beneath the volcano. The electrical conductivities of the two samples were measured at pressures of 0.9 and 1.5 GPa and temperatures from 400 to 1100°C at frequencies from 0.1 to 105 Hz. To investigate the electrical properties of the Etnean products as a function of partial melting, a few experiments were performed in a piston‐cylinder apparatus prior to the electrical measurements. The obtained data were approximated using an equivalent circuit fitting technique. For the lava flow sample, electrical conductivity displays Arrhenian behavior over the entire investigated temperature range, with an activation energy of ∼0.8 eV. Within the uncertainties of our measurements, we do not observe any effect of pressure on conductivity between 0.9 and 1.5 GPa. On the contrary, experiments performed on the series of partially molten samples indicate that conductivity increases with increasing quantity of glass. While conductivities of samples with minor amounts of glass are comparable to that of the lava flow starting material, with increased melting, conductivity can increase by as much as a factor of 3. The mafic nodule was observed to have a conductivity higher than the lava flow sample (e.g., at 800°C and 0.9 GPa, a factor of 4 higher). However, scatter in the data is somewhat higher in comparison to the lava flow sample, most likely because of the coarse texture of the samples and a consequence to their chemical and structural heterogeneity. Using our results, we illustrate the effects that a layer of hot magma surrounded by a cooler wall rock has on apparent resistivities determined by one‐dimensional forward calculations. Our modeling demonstrates that both size and depth of magmatic intrusions strongly influence apparent resistivity and that these parameters can be extracted from field data if the electrical properties of the rocks below the surface are well understood.