Electrical conductivity of HCl-bearing aqueous fluids to 700 ºC and 1 GPa

Abstract

Subsurface magmatic–hydrothermal systems are often associated with elevated electrical conductivities in the Earthʼs crust. To facilitate the interpretation of these data and to allow distinguishing between the effects of silicate melts and fluids, the electrical conductivity of aqueous fluids in the system H2O–HCl was measured in an externally heated diamond anvil cell. Data were collected to 700 °C and 1 GPa, for HCl concentrations equivalent to 0.01, 0.1, and 1 mol/l at ambient conditions. The data, therefore, more than double the pressure range of previous measurements and extend them to geologically realistic HCl concentrations. The conductivities $$\sigma$$ σ (in S/m) are well reproduced by a numerical model log $$\sigma$$ σ = −2.032 + 205.8 T−1 + 0.895 log c + 3.888 log $$\rho$$ ρ + log$$\Lambda_{0}$$ Λ 0 (T,$$\rho$$ ρ ), where T is the temperature in K, c is the HCl concentration in wt. %, and $$\rho$$ ρ is the density of pure water at the corresponding pressure and temperature conditions. $$\Lambda_{0}$$ Λ 0 (T,$$\rho$$ ρ ) is the limiting molar conductivity (in S cm2 mol−1) at infinite dilution, $$\Lambda_{0}$$ Λ 0 (T,$$\rho$$ ρ ) = 2550.14 − 505.10$$\rho$$ ρ − 429,437 T−1. A regression fit of more than 800 data points to this model yielded R2 = 0.95. Conductivities increase with pressure and fluid densities due to an enhanced dissociation of HCl. However, at constant pressures, conductivities decrease with temperature because of reduced dissociation. This effect is particularly strong at shallow crustal pressures of 100–200 MPa and can reduce conductivities by two orders of magnitude. We, therefore, suggest that the low conductivities sometimes observed at shallow depths below the volcanic centers in magmatic–hydrothermal systems may simply reflect elevated temperatures. The strong negative temperature effect on fluid conductivities may offer a possibility for the remote sensing of temperature variations in such systems and may allow distinguishing the effects of magma intrusions from changes in hydrothermal circulation. The generally very high conductivities of HCl–NaCl–H2O fluids at deep crustal pressures (500 MPa–1 GPa) imply that electrical conductors in the deep crust, as in the Altiplano magmatic province and elsewhere, may at least partially be due to hydrothermal activity.

Electrical conductivity of siderite and its implication for high conductivity anomaly in the slab-mantle wedge interface

“…It may reflect the decomposition reaction of the sample that occurred in high-temperature regions and produced highly conductive materials, resulting in the abrupt increase of conductivity. The analogous spectra were also observed in some highly conductive materials such as silicate/carbonatite Frontiers in Earth Science frontiersin.org melts (Sifré et al, 2015;Guo et al, 2017;Guo et al, 2018;Yoshino et al, 2018;Guo et al, 2021), NaCl-bearing (Guo and Keppler, 2019;Klumbach and Keppler, 2020) or water-bearing fluids from the dehydration of hydrous minerals (Manthilake et al, 2016;Pommier et al, 2019), and metallic minerals (Dai et al, 2019). In the second cooling cycle shown in Figure 5C, all of the impedance spectra exhibit similar vertical lines, implying the highly conductive phases in the sample dominate the conductivity.…”

Hydrothermal and Magmatic System of a Volcanic Island Inferred From Magnetotellurics, Seismicity, Self‐potential, and Thermal Image: An Example of Miyakejima (Japan)

“…On volcanoes and geothermal areas, the presence of steam, or supercritical water in porous media is known to increase electrical resistivity compared to a liquid water phase (e.g., Árnason et al, 2010;Gasperikova et al, 2015;Hersir et al, 2020;Piña-Varas et al, 2014;Soyer et al, 2018;Yamaya et al, 2013). Such field observations are also attested by numerous laboratory measurements (e.g., Klumbach & Keppler, 2020;Nono et al, 2020;Quist & Marshall, 1968;Roberts et al, 2001).…”