Hydrogen site-dependent physical properties of hydrous magnesium silicates in the mantle transition zone

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 show a series of stable hydrous magnesium silicate phases under transition zone conditions, identified using crystal structure prediction and first-principles calculations. 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, indicating that conductivity-based estimates may significantly underestimate MTZ water content if substitution effect is not taken into account. 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 important implications for mass transport, electrical conductivity, and magnetic dynamo generation in rocky super-Earth exoplanets.