The Electrical Conductivity of Subducted Continental Crust

Case Western Reserve University, Cleveland OH

Investigators

Abstract

Subduction zones are regions where crust and lithosphere are recycled into the Earth’s mantle. While oceanic crust dominates in subducting plates, continental crust can also be subducted. It has been inferred that subducted continental crust may explain the high electrical conductivity observed in some subduction zones at depth greater than ~300 km. This high conductivity has previously been attributed to the release of hydrous fluids. Here, the researchers investigate an alternative possibility: that the high conductivity is due to the presence of liebermannite. This mineral becomes stable at the extreme pressure prevailing in deep subducted plates. It may account for a significant fraction of subducted continental crust. Materials with similar structures often display superionic conductivity. This type of conductivity - where charge carriers are ions and vacancies in the solid - does not require the presence of fluids. Here, the team carries out experiments at the extreme conditions prevailing in the mantle. They use an apparatus capable of generating pressures in excess of 200,000 atm. They quantify the effect of temperature, pressure, and mineral composition on electrical conductivity. They gradually unveil whether liebermannite can explain the high conductivities observed in subduction zones. Outcomes of this project may find applications beyond Earth Sciences in Materials Science. The award supports a principal investigator from a group underrepresented in science. It involves high-school students notably from underrepresented groups. It also promotes educational outreach toward the public. Here, the team measures the electrical conductivity of (K,Na)AlSi3O8 libermannite. They investigate the conductivity of the pure K-endmember, as well as that of K-Na solid solutions. They use the impedance spectroscopy technique in a multi-anvil high-pressure-temperature device. To complement the conductivity measurements, they perform K self-diffusion and Na-K interdiffusion measurements at similar conditions of pressure and temperature. This provides further insight on the conduction mechanism inferred from the conductivity measurements. The results provide the basis for calculating liebermannite conductivity as a function of pressure, temperature, and composition over a wide range of mantle depths. The study also improves estimates of the electrical conductivity of continental crust. It establishes whether “dry” continental crust may explain regions of high conductivity at depths greater than 300 km. The team also investigate the crystal orientation dependence of the electrical conductivity in liebermannite. The goal is to evaluate whether subducted continental crust may have significant electrical anisotropy. The data are used, along with previously published data on other minerals, to calculate the electrical conductivity of continental crust in subduction zone. It provides an improved basis for the identification of subducted continental crust by magnetotelluric measurements. This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.