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<p>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.</p>

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

  • 期刊论文

  • 出版模式：

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  • 发表日期：

    2026年5月4日

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  • Zifan Wang,
  • Yu He,
  • Ho-kwang Mao,
  • Duck Young Kim

摘要

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.