Electrical Conductivity of Hydrous SiO2: Implications for the Superionic State and High Conductivity Anomalies Beneath Subduction Zones

Abstract

Electrical conductivity (EC) is a key physical property of minerals and rocks that constrains the composition and structure of Earth's deep interior. Theoretical studies predict that the CaCl₂-type hydrous Al-bearing SiO₂ phase, present in subducted crustal materials, becomes superionic—where protons are no longer bonded to specific oxygen atoms but instead become mobile within the SiO₂ lattice—under high-pressure and high-temperature conditions of the lower mantle. The enhancement of the EC upon such superionic transition has not been experimentally verified yet. Here, we measured the EC of Al-bearing SiO₂ containing 1,750 ppm H₂O at pressures up to 82 GPa and temperatures up to 2610 K by employing a recently developed technique designed for measuring transparent materials. Results demonstrate a sudden increase in EC to approximately 10 S/m at temperatures of 1,100–2,200 K, depending on pressure. This is several to 10 times higher than the conductivity of the surrounding shallow to mid-lower mantle and is consistent with a transition to the superionic state. If hydrous SiO₂ is substantially weaker than other coexisting phases and thus forms an interconnected film in subducted mid-oceanic ridge basalt (MORB) crust, the EC of the bulk MORB materials is significantly enhanced by superionic SiO₂ to ∼1,800 km depth, which may explain the high EC anomalies observed at subduction zones underneath northeastern China. The observed EC anomalies can be matched by the EC of subducted MORB materials containing Al-bearing SiO₂ with a water content of approximately 0.2 wt%, providing insights into deep H₂O circulation and mantle distribution.