Electrical Conductivity of NaCl‐Bearing Aqueous Fluids to 900 °C and 5 GPa

Abstract

The electrical conductivity of aqueous fluids containing 0.01, 0.1, and 1 M NaCl was measured in a piston‐cylinder apparatus up to 900 °C and 5 GPa. The conductivity generally increases with NaCl concentration, while the pressure and temperature effects are more complex. At 1–2 GPa, the effect of temperature on conductivity is small, while at higher pressures, conductivity increases with temperature. Pressure also enhances conductivity, but only above 300–400 °C. This effect may be due to enhanced ion dissociation in response to an increasing dielectric constant. However, at lower temperatures, conductivity decreases with pressure, probably due to an increase in fluid viscosity. The measured conductivities of NaCl‐H2O fluids up to 900 °C and 5 GPa are reproduced (R2 = 0.953) by a numerical model with log σ = −0.919 − 872.5/T + 7.61 log ρ + 0.852 log c + log Λ0, where σ is the conductivity in S/m, T is temperature in K, c is NaCl concentration in wt%, ρ is the density of pure water (in g/cm3) at given pressure and temperature, and Λ0 is the molar conductivity of NaCl in water at infinite dilution (in S·cm2·mol−1), Λ0 = 1,573 − 1,212 ρ + 537,062/T – 208,122,721/T2. We use our data to model the elevated electrical conductivity in the mantle wedge above subducted slabs. We find that due to the strong conductivity enhancement, in most cases less than one volume percent of aqueous fluid with moderate salinity is sufficient to explain the observed conductivity anomalies.

Electrical Conductivity of Clinopyroxene‐NaCl‐H2O System at High Temperatures and Pressures: Implications for High‐Conductivity Anomalies in the Deep Crust and Subduction Zone

“…The impedance spectra were composed of an approximate semicircle in the frequency region 10 6 –10 3 Hz and an additional tail at 10 3 –10 −1 Hz. The characteristics of the impedance spectra are similar to those in relevant studies (H. H. Guo & Keppler, 2019; X. Z. Guo et al, 2015).…”

“…The electrical conductivities of the saline fluids with a NaCl concentration of 1 mol/L were higher than those of the Cpx‐NaCl‐H 2 O, Ab‐NaCl‐H 2 O, and Pl‐NaCl‐H 2 O systems. The conductivities of the saline fluids with NaCl concentration of 0.01 mol/L were close to those of the Cpx‐NaCl‐H 2 O system with a salinity of 10 wt% and a fluid fraction of 10 vol% (H. H. Guo & Keppler, 2019). The slope of the linear relationship between the logarithmic electrical conductivities of the Cpx‐NaCl‐H 2 O system and temperature was close to that for the Ab‐NaCl‐H 2 O system, but the slope for the Pl‐NaCl‐H 2 O system was smaller at a low salinity and fluid fraction.…”

“…When the NaCl concentration is sufficiently large (> 25 wt%), temperature has no influence on the electrical conductivity of saline fluids. The influence of temperature on the electrical conductivity of the Cpx‐NaCl‐H 2 O system is similar to that of the plagioclase‐NaCl‐H 2 O, albite‐NaCl‐H 2 O, and NaCl‐bearing fluids (H. H. Guo & Keppler, 2019; X. Z. Guo et al, 2015; Li et al, 2018).…”

“…Comparisons of electrical conductivity of Cpx‐NaCl‐H 2 O systems measured at 1 GPa in this study with those of other relevant brine‐bearing systems in previous studies. The red dashed lines represent the electrical conductivity of saline fluids with different molarities at 5.0 GPa (Guo & Keppler, 2019), and the green and blue dashed lines represent the conductivities of Ab‐NaCl‐H 2 O (Guo et al, 2015) and Pl‐NaCl‐H 2 O (Li et al, 2018) systems at 1.2 GPa, respectively.…”

“…Z. Guo et al, 2015; Li et al, 2018; Shimojuku et al, 2012). It has been proposed that NaCl‐bearing aqueous fluids may affect the high‐conductivity anomalies under the deep crust and subduction zone significantly (H. H. Guo & Keppler, 2019).…”