Hydrogen site-dependent physical properties of hydrous magnesium silicates: implications for water storage and transport in the mantle transition zone

TL;DR

This study combines computational methods to reveal hydrogen behavior in hydrous magnesium silicates under mantle transition zone conditions, showing implications for water storage, electrical conductivity, and planetary magnetic processes.

Contribution

It introduces a pressure-induced hydrogen substitution mechanism and identifies double superionicity in hydrous silicates, advancing understanding of deep Earth water cycling and planetary dynamics.

Findings

Hydrogen migrates from Mg2+ to Si4+ sites near 410 km depth.

Water content in the MTZ estimated at approximately 1.6 wt%.

Double superionicity observed at temperatures above 2000 K.

Abstract

The Earth's mantle transition zone (MTZ) is widely recognized as a major water reservoir, exerting significant influence on the planet's water budget and deep cycling processes. Here, we employ crystal structure prediction and first-principles calculations to identify a series of stable hydrous magnesium silicate phases under transition zone conditions. Our results reveal a pressure-induced hydrogen substitution mechanism in wadsleyite, where H+ preferentially migrates from Mg2+ sites to Si4+ sites near 410 km depth. This transformation leads to a substantial decrease in electrical conductivity, consistent with geophysical observations. We estimate the water content in the MTZ to be approximately 1.6 wt%, aligning with seismic and conductivity constraints. Furthermore, using machine learning-enhanced molecular dynamics, we discover double superionicity in hydrous wadsleyite and ringwoodite at temperatures exceeding 2000 K, wherein both H+ and Mg2+ exhibit high ionic mobility. This dual-ion superionic state has potentially profound implications for mass transport, electrical conductivity, and magnetic dynamo generation in rocky super-Earth exoplanets.