Electrical Conductivity of KCl‐H2O Fluids in the Crust and Lithospheric Mantle

Abstract

AbstractEvidence from diamond inclusions suggests that KCl‐bearing fluids may be abundant in the subcratonic mantle and these fluids may also cause local anomalies of high electrical conductivity. We therefore measured the electrical conductivity of aqueous fluids containing 6.96, 0.74 and 0.075 wt% KCl using a hydrothermal diamond anvil cell to 2.5 GPa and 675°C, and with a piston‐cylinder apparatus to 5 GPa and 900°C. We found that below 550°C, increasing pressure generally decreases solution conductivity, while at higher temperatures the effect is opposite. However, at high pressures, the conductivity of KCl in H2O is smaller than for NaCl, possibly due to a hydration shell collapse. The experimental data are described by two numerical models. The first model (R2 = 0.999), is preferable for crustal conditions: log σ = −2.03 + 25.0 × T−1 + 0.923 × log c + 0.990 × log ρ + log Λ0, where σ is the conductivity in S/m, T is temperature in K, c is KCl concentration in wt%, ρ is the density of pure water (in g/cm3) at given pressure and temperature. Λ0 is the limiting molar conductivity of KCl (in S·cm2·mol−1): Λ0 = 1,377 – 1,082 × ρ + 6,883 × 102 × T−1 – 2,471 × 105 × T−2. The second model (R2 = 0.986), is applicable to the lithospheric mantle: log σ = −1.52 – 357 × T−1 + 0.865 × log c + 1.72 × log ρ + log Λ0, with the same equation for Λ0. The latter model shows that already traces of KCl‐bearing aqueous fluid may account for high conductivity anomalies in the subcratonic mantle.