Electrical conductivity and dissociation of fluids in crust and mantle
Final Report Abstract
The electrical conductivity of fluids in the systems NaCl-H2O and KCl-H2O was measured to 5 GPa and 900 ˚C. The original target of the proposal, to reach 3 GPa in this type of measurement was therefore exceeded. This was possible by the development of a new experimental technique, which allows measuring the conductivity of a fluid in the pore space of diamond powder inside a piston cylinder apparatus. This method extends the measurements in an externally heated diamond cell, which were used up to 2 GPa, to pressures up to 5 GPa. Numerical models were developed in order to describe the conductivities in both the NaCl-H2O and KCl-H2O systems. The data were directly applied to constrain the volume fraction and plausible salinity range of fluids in the mantle wedge above subduction zones and in the deep cratonic mantle. Most of these anomalies can be explained by the presence of far less than 1 vol. % of fluid with moderate salinity. High pressure enhances the conductivity of NaCl-bearing fluids much more than for KCl, which may be attributed to a collapse of hydration shells. The HCl-H2O system, which is mostly relevant for subvolcanic systems, was studied to 1 GPa. The data show that at typical shallow crustal pressures, electrical conductivity decreases strongly with temperature, due to the reduced dissociation of NaCl as the dielectric constant of the fluid decreases. Therefore, some electrically resistive zones in the centers of hydrothermal systems that were traditionally attributed to solidified magma bodies may in fact be areas of hightemperature fluid circulation. Since the response of the electrical conductivity of shallow aqueous fluids is opposite to that of silicate melts, it may be possible to use this effect in order to distinguish new magma intrusions from changes in hydrothermal activity in volcanically active zones, such as the Campi Flegrei or the Yellowsone caldera. The data generated here could therefore be directly usable for forecasting volcanic hazards.
Publications
- (2019) Electrical conductivity of NaCl-bearing aqueous fluids to 900 ˚C and 5 GPa. J Geophys Res-Solid Earth 124: 1397-1411
Guo H, Keppler, H
(See online at https://doi.org/10.1029/2018JB016658) - (2020) Electrical conductivity of HCl-bearing aqueous fluids to 700 ˚C and 1 GPa. Contrib Mineral Petrol 175: Article Number 114
Klumbach S, Keppler H
(See online at https://doi.org/10.1007/s00410-020-01754-5) - (2021) Electrical conductivity in texturally equilibrated fluid-bearing forsterite aggregates at 800 ˚C and 1 GPa: Implications for the high electrical conductivity anomalies in mantle wedges. J Geophys Res-Solid Earth 126: Article Number: e2020JB021343
Huang YS, Guo H, Nakatani T, Uesugi K, Nakamura M, Keppler H
(See online at https://doi.org/10.1029/2020JB021343)
