Electrical Conductivity of NaCl‐Bearing Aqueous Fluids to 900 °C and 5 GPa. Issue 2 (23rd February 2019)

• Record Type: Journal Article
• Authors: Guo, Haihao; Keppler, Hans
• Is Part Of: Journal of geophysical research. Volume 124:Issue 2(2019)
• Page Start: 1397
• Page End: 1411
• Publication Date: 2019-02-23
• Subjects: electrical conductivity -- aqueous fluid -- NaCl -- subduction zones -- fluid fractions; Geomagnetism -- Periodicals; Geochemistry -- Periodicals; Geophysics -- Periodicals; Earth sciences -- Periodicals; 551.1
• DOI: 10.1029/2018JB016658
• Languages: English
• ISSNs: 2169-9313

Abstract

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‐H2 O fluids up to 900 °C and 5 GPa are reproduced ( R 2 = 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/cm 3 ) at given pressure and temperature, and Λ0 is the molar conductivity of NaCl in water at infinite dilution (in S·cm 2 ·mol −1 ), Λ0 = 1, 573 − 1, 212 ρ + 537, 062/ T – 208, 122, 721/ T 2 . 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. Key Points: A new cell for electrical conductivity measurements of fluids at high pressures was designed and tested Electrical conductivities of NaCl‐bearing aqueous fluids were for the first time measured to 5 GPa and 900 ˚C The elevated electrical conductivity below volcanic arcs can in many cases be explained by less than 1 % of moderately saline fluids