Biogeochemical Cycles & Geobiology

Ultrahigh-Pressure Magnesium Hydrosilicates As Reservoirs Of Water In The Early Earth

By Keith Cowing
February 3, 2022
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Ultrahigh-Pressure Magnesium Hydrosilicates As Reservoirs Of Water In The Early Earth
Stability and structures of Mg2SiO5H2 phases. (a) Enthalpy of formation of α-, β-Mg2SiO5H2,relative to the enthalpy of MgSiO3 + MgO + H2O assemblage, as a function of the pressure. Note that zero point energy (ZPE) is not included in these enthalpies. (b) Crystal structure of α-Mg2SiO5H2 in the [010] direction. (c) Crystal structure of β-Mg2SiO5H2 in the [010] direction. The Mg, Si, O, and H atoms are shown as orange, blue, red, and white spheres, respectively.

The origin of water on the Earth is a long-standing mystery, requiring a comprehensive search for hydrous compounds, stable at conditions of the deep Earth and made of Earth-abundant elements.

Previous studies usually focused on the current range of pressure-temperature conditions in the Earth’s mantle and ignored a possible difference in the past, such as the stage of the core-mantle separation. Here, using ab initio evolutionary structure prediction, we find that only two magnesium hydrosilicate phases are stable at megabar pressures, α-Mg2SiO5H2 and β-Mg2SiO5H2, stable at 262-338 GPa and >338 GPa,respectively (all these pressures now lie within the Earth’s iron core). Both are superionic conductors with quasi-one-dimensional proton diffusion at relevant conditions.

In the first 30 million years of Earth’s history, before the Earth’s core was formed, these must have existed in the Earth, hosting much of Earth’s water. As dense iron alloys segregated to form the Earth’s core, Mg2SiO5H2 phases decomposed and released water. Thus, now-extinct Mg2SiO5H2 phases have likely contributed in a major way to the evolution of our planet.

Han-Fei Li, Artem R. Oganov, Haixu Cui, Xiang-Feng Zhou, Xiao Dong, Hui-Tian Wang

Subjects: Geophysics (physics.geo-ph); Earth and Planetary Astrophysics (astro-ph.EP); Materials Science (cond-mat.mtrl-sci)
Journal reference: Phys. Rev. Lett. 128, 035703 (2022)
DOI: 10.1103/PhysRevLett.128.035703
Cite as: arXiv:2202.00752 [physics.geo-ph] (or arXiv:2202.00752v1 [physics.geo-ph] for this version)
Submission history
From: Xiao Dong
[v1] Sun, 30 Jan 2022 16:12:08 UTC (1,528 KB)

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