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

TL;DR

This study experimentally demonstrates that hydrous SiO2 becomes superionic at high pressures and temperatures, significantly increasing electrical conductivity, which explains high conductivity anomalies observed in Earth's lower mantle beneath subduction zones.

Contribution

First experimental verification of superionic transition in hydrous SiO2 at mantle conditions, linking it to observed electrical conductivity anomalies in subduction zones.

Findings

EC of hydrous SiO2 increases to ~10 S/m at high P-T conditions

Superionic transition enhances bulk mantle conductivity significantly

Results match observed anomalies in subduction zone electrical conductivity

Abstract

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.