Hydrogen site-dependent physical properties of hydrous magnesium silicates: implications for water storage and transport 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 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.

💡 Research Summary

The paper investigates how water is stored and transported in the Earth’s mantle transition zone (MTZ, 410–660 km depth) by focusing on the site‑dependent behavior of hydrogen in hydrous magnesium silicates. Using crystal‑structure prediction (USPEX) combined with density‑functional theory (DFT) calculations, the authors identify a series of stable hydrous phases of wadsleyite (Mg₂SiO₄) and ringwoodite under MTZ pressure–temperature conditions (13–24 GPa, 1200–2000 K).

A key finding is a pressure‑driven hydrogen substitution mechanism in wadsleyite. At pressures around 13 GPa (≈410 km), H⁺ ions that initially occupy Mg²⁺ octahedral sites become energetically unfavorable and migrate to Si⁴⁺ tetrahedral sites, forming Si–OH bonds. This migration shortens the H–O bond (from ~1.60 Å to ~1.45 Å) and reduces the electronic density of states near the Fermi level, leading to a marked drop in electrical conductivity—by two orders of magnitude—consistent with geophysical conductivity profiles that show a pronounced low‑conductivity layer near the 410 km discontinuity.

Thermodynamic analysis based on Gibbs free energies yields an estimated water content of ~1.6 wt % for the whole MTZ. This value aligns with independent constraints from seismic velocities and magnetotelluric observations, which have long suggested that the MTZ can host up to a few weight percent of water.

Beyond static properties, the study employs machine‑learning‑enhanced molecular dynamics (ML‑MD) to explore high‑temperature behavior. At temperatures exceeding 2000 K, both H⁺ and Mg²⁺ become highly mobile, entering a “double superionic” state. While superionic hydrogen in silicates is well documented, the simultaneous superionic diffusion of Mg²⁺ is unprecedented. Diffusion coefficients for H⁺ rise to ~10⁻⁶ m² s⁻¹, and Mg²⁺, which is normally immobile in the solid, reaches comparable values, indicating that the lattice essentially melts for both species while the oxygen framework remains crystalline. This dual‑ion superionic phase dramatically enhances bulk electrical conductivity and reduces viscosity, implying that mass, charge, and heat can be transported much more efficiently through the MTZ than previously thought.

The authors discuss the geodynamic implications of these findings. The migration of hydrogen from Mg²⁺ to Si⁴⁺ sites provides a mechanistic explanation for the observed conductivity drop at the 410 km discontinuity and suggests that water released from subducted slabs may become “locked” in Si‑OH groups at greater depths, stabilizing a substantial water reservoir. The double superionic state could facilitate rapid upward transport of water‑rich material, influencing melt generation, volcanic outgassing, and the long‑term water cycle between the deep mantle and the surface. Moreover, the enhanced electrical currents associated with superionic H⁺ and Mg²⁺ may contribute to localized magnetic field generation, offering a possible link between deep mantle dynamics and the geomagnetic field.

Finally, the paper extrapolates these mechanisms to rocky super‑Earth exoplanets, where interior pressures and temperatures exceed those of Earth’s mantle. Under such extreme conditions, the double superionic phase is expected to be even more pervasive, implying that super‑Earths could store significantly larger water inventories and exhibit markedly different electrical and thermal conductivity profiles. This could affect their mantle convection patterns, surface tectonics, and the likelihood of sustaining a magnetic dynamo.

In summary, the study (1) reveals a pressure‑induced hydrogen site transition in wadsleyite that explains conductivity anomalies, (2) quantifies MTZ water content at ~1.6 wt %, (3) discovers a novel double superionic state of H⁺ and Mg²⁺ at high temperature, and (4) outlines the profound implications of these phenomena for Earth’s deep water cycle, mantle dynamics, and the interior physics of massive terrestrial exoplanets.