Absence of dehydration due to superionic transition at Earth’s core-mantle boundary

TL;DR

This study shows that water can remain stable in Earth's deep mantle due to a superionic transition, forming a long-term reservoir at the core-mantle boundary.

Contribution

The discovery of a doubly superionic transition in δ-AlOOH and its role in stabilizing water in the deep mantle is novel.

Findings

δ-AlOOH undergoes a superionic transition with highly diffusive hydrogen and aluminum ions.

Water tends to freeze under deep mantle conditions, making dehydration energetically unfavorable.

Superionic water may accumulate as a long-term reservoir at the base of Earth's mantle.

Abstract

The properties and stability of hydrous phases are crucial to unraveling the mysteries of the deep water cycle. Under deep lower mantle conditions, water and hydrous phases transition into a superionic state. However, superionic effect on their stability and dehydration behavior remains unclear. Using ab initio and deep learning potential molecular dynamics simulations, we discovered a doubly superionic transition in δ-AlOOH, characterized by the highly diffusive behavior of both hydrogen and aluminum ions within the oxygen sublattice. These highly diffusive elements contribute external entropy to the system, stabilizing the structure at 140 GPa and 3800 K. Our free-energy calculations reveal that water tends to freeze under deep lower mantle conditions, so dehydration becomes energetically and kinetically unfavorable even under core-mantle boundary (CMB) conditions. This implies that superionic water may accumulate in the deep lower mantle over geologic time, forming a long-term reservoir at the base of the mantle.

Dehydration becomes unfavorable due to superionic transition, resulting in water reservoir in the deep lower mantle.

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