...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...

...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...