Role of hydrogen and proton transportation in Earth’s deep mantle

Qingyang Hu,Ho-Kwang Mao

doi:10.1063/5.0069643

Abstract

Hydrogen (H) is the most abundant element in the known universe, and on the Earth’s surface it bonds with oxygen to form water, which is a distinguishing feature of this planet. In the Earth’s deep mantle, H is stored hydroxyl (OH−) in hydrous or nominally anhydrous minerals. Despite its ubiquity on the surface, the abundance of H in the Earth’s deep interior is uncertain. Estimates of the total H budget in the Earth’s interior have ranged from less than one hydrosphere, which assumes an H-depleted interior, to hundreds of hydrospheres, which assumes that H is siderophile (iron-loving) in the core. This discrepancy raises the questions of how H is stored and transported in the Earth’s deep interior, the answers to which will constrain its behavior in the deep lower mantle, which is defined as the layer between 1700 km depth and the core–mantle boundary.

Highlights

Mantle boundary (Fig. 1), which is consistent with a recent kinetic experiment involving the heating of FeOOH and the prediction of FeOOH0.75 as a stable stoichiometry.5 The regression of P–nH indicates that cross-boundary diffusion is more intense at relatively low pressures
H may be liberated from hydroxyl bonding and diffuse freely in the host crystalline lattice, entering an exotic superionic phase
If such solid–solid diffusion were to dominate mantle convection, chemical equilibrium by diffusion would be negligible beyond a few meters, even over the entire history of the Earth

Summary

Introduction

Mantle boundary (Fig. 1), which is consistent with a recent kinetic experiment involving the heating of FeOOH and the prediction of FeOOH0.75 as a stable stoichiometry.5 The regression of P–nH indicates that cross-boundary diffusion is more intense at relatively low pressures. Grotthuss-type diffusion is dominant for H-incorporated silicate in the asthenosphere, in regions under relatively high-temperature and low-pressure conditions.1 With increasing depth, H may be liberated from hydroxyl bonding and diffuse freely in the host crystalline lattice, entering an exotic superionic phase.2,3 The concept of a superionic phase is borrowed from the electric battery industry, and the existence of such a phase in ice is widely recognized.

Results

Conclusion