Ultrahigh-Pressure Magnesium Hydrosilicates as Reservoirs of Water in Early Earth

TL;DR

This study predicts stable magnesium hydrosilicate phases at extreme pressures relevant to early Earth's conditions, suggesting they stored and released water during Earth's formation, impacting its water inventory.

Contribution

It identifies two stable magnesium hydrosilicate phases at megabar pressures and proposes their role in Earth's early water reservoir and planetary evolution.

Findings

Two stable Mg hydrosilicate phases at 262-338 GPa.

These phases are superionic conductors with proton diffusion.

They likely contributed to Earth's water during early history.

Abstract

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, $\alpha$-Mg$_2$SiO$_5$H$_2$ and $\beta$-Mg$_2$SiO$_5$H$_2$, 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, Mg$_2$SiO$_5$H$_2$ phases decomposed and released water. Thus, now-extinct Mg$_2$SiO$_5$H$_2$ phases have likely contributed in a major way to the evolution of our planet.