(Press-News.org) The cycling of water within Earth's interior regulates plate tectonics, volcanism, ocean volume, and climate stability, making it central to the planet's long-term evolution and habitability and a key scientific question.
While subducting slabs are known to transport water into the mantle, scientists have long assumed that most hydrous minerals dehydrate at high temperatures, releasing fluids as they descend. Whether water can survive the extreme conditions of the deep lower mantle, however, has remained an open question.
To fill this knowledge gap, a research team from the Institute of Geochemistry of the Chinese Academy of Sciences (IGCAS), together with collaborators, used complementary molecular dynamics simulations, combining ab initio and deep-learning potential methods. Their findings, published in Science Advances on January 28, reveal that under deep lower mantle and core–mantle boundary (CMB) conditions, water and the key hydrous mineral δ-AlOOH enter a superionic state—which combines features of a solid crystal lattice with liquid-like mobile ions—thereby fundamentally altering their stability and dehydration behavior.
In particular, the simulations showed that liquid water is thermodynamically unstable in the deep lower mantle—freezing into superionic ice phases, where hydrogen ions diffuse rapidly through an oxygen lattice.
Meanwhile, the hydrous mineral δ-AlOOH undergoes a remarkable "double superionic transition" at the CMB—under pressures of approximately 140 GPa and temperatures near 3800 K. This transition is marked by highly diffusive hydrogen and aluminum ions, whose enhanced mobility contributes substantial entropy. This stabilizes the crystal structure and raises its melting temperature to levels comparable to those at the CMB.
Additionally, the researchers conducted free-energy calculations and found that the dehydration of δ-AlOOH is both energetically and kinetically unfavorable under deep lower mantle conditions. Because water exists as superionic ice rather than free fluid, the conventional dehydration mechanism is effectively suppressed. As a result, water from Earth's early stages or carried into the mantle by subduction may be preserved over geological timescales, accumulating as a long-term water reservoir near the base of the mantle.
By uncovering how the physical state of water controls dehydration in Earth's deep interior, the study provides a new framework for understanding deep water cycling, the nature of lowermost mantle structures, and the long-term storage of ancient water and hydrogen within the planet.
END
Study reveals how superionic state enables long-term water storage in Earth's interior
2026-01-28
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