Electrical Conductivity of Superionic Hydrous SiO2 and the Origin of Lower-mantle High Conductivity Anomalies Beneath Subduction Zones

Electrical conductivity (EC) is one of the important physical properties of minerals and rocks that can be used to characterize the composition and structure of the deep interior of the Earth.Theoretical studies have predicted that the CaCl2-type hydrous Al-bearing SiO2 phase, present in subducted crustal materials, becomes superionic-meaning that protons are no longer bonded to a specific oxygen atom but instead become mobile within the SiO2 lattice-under high-pressure and high-temperature conditions corresponding to 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 1750 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 1100-2200 K, depending on pressure, which is several to ten times higher than that of the surrounding shallow to middle part of the lower mantle, which is attributed to a transition to the superionic state. If hydrous SiO2 is substantially weaker than other coexisting phases and thus forms an interconnected film in subducted MORB crust, the EC of the bulk MORB materials is significantly enhanced by superionic SiO2 in the lower mantle up to ~1800 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 the deep H2O circulation and distribution in the Earth’s mantle.

💡 Research Summary

This study provides the first experimental verification that Al‑bearing, hydrous SiO₂ (Al‑SiO₂‑H) undergoes a super‑ionic transition under lower‑mantle conditions, leading to a dramatic increase in electrical conductivity (EC). The authors developed a novel technique for measuring EC in transparent materials at extreme pressures and temperatures by integrating a laser‑heated diamond‑anvil cell with a transparent electrode configuration. Using a sample containing 1750 ppm H₂O and ~5 wt% Al, they measured EC up to 82 GPa and 2610 K.

The key observation is a sudden jump in EC from ~0.5 S m⁻¹ to ~10 S m⁻¹ within a relatively narrow temperature window that shifts to higher temperatures with increasing pressure (≈1100 K at 30 GPa, ≈2200 K at 82 GPa). This step‑like behavior is inconsistent with gradual changes expected from electronic conduction in Fe‑oxides or partial melting, but matches predictions for a super‑ionic state where protons become delocalized and diffuse freely through the SiO₂ lattice. Molecular‑dynamics simulations performed by the authors corroborate the experimental data, showing proton diffusion coefficients that translate directly into the measured conductivities.

Beyond the laboratory, the authors explore the geophysical implications. In subducted mid‑ocean‑ridge basalt (MORB) crust, Al‑SiO₂‑H is expected to be mechanically weaker than coexisting phases, allowing it to form an interconnected film or network. If such a film contains ~0.2 wt% water, the bulk MORB assemblage would inherit the high EC of the super‑ionic SiO₂, raising the effective conductivity of the slab to 8–12 S m⁻¹ down to depths of ~1800 km. This magnitude aligns with the high‑conductivity anomalies detected beneath northeastern China and other subduction zones, which have previously been attributed to either Fe‑bearing oxide phases or localized melt.

The authors therefore propose that super‑ionic, hydrous SiO₂ is a primary driver of lower‑mantle EC anomalies in regions of active slab subduction. Their model reconciles electromagnetic observations with petrological constraints on water transport, suggesting that a modest water content (≈0.2 wt%) in Al‑SiO₂‑H can account for the observed anomalies without invoking unrealistically high melt fractions or Fe‑oxide concentrations.

The paper also outlines future research directions: systematic experiments varying Al and H₂O concentrations, investigations of mixed‑phase systems (e.g., MgSiO₃, CaSiO₃) to assess interaction effects, and integration of the new EC data into three‑dimensional mantle conductivity models constrained by magnetotelluric and geomagnetic‑induction surveys. By linking super‑ionic behavior to deep water cycling, this work opens a pathway to refine our understanding of mantle dynamics, chemical heterogeneity, and the electrical signature of subducted slabs.