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

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, $\alpha \text{−}{\mathrm{Mg}}_2{\mathrm{SiO}}_5{\mathrm{H}}_2$ and $\beta \text{−}{\mathrm{Mg}}_2{\mathrm{SiO}}_5{\mathrm{H}}_2$, stable at 262–338 GPa and $>338\mathrm{GPa}$, 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, ${\mathrm{Mg}}_2{\mathrm{SiO}}_5{\mathrm{H}}_2$ phases decomposed and released water. Thus, now-extinct ${\mathrm{Mg}}_2{\mathrm{SiO}}_5{\mathrm{H}}_2$ phases have likely contributed in a major way to the evolution of our planet.

Figure 1

Stability and structures of ${\mathrm{Mg}}_2{\mathrm{SiO}}_5{\mathrm{H}}_2$ phases. (a) Enthalpy of formation of $\alpha$-, $\beta \text{−}{\mathrm{Mg}}_2{\mathrm{SiO}}_5{\mathrm{H}}_2$, relative to the enthalpy of ${\mathrm{MgSiO}}_3+\mathrm{MgO}+{\mathrm{H}}_2\mathrm{O}$ assemblage, as a function of pressure. Note that zero point energy (ZPE) is not included in these enthalpies. (b) Crystal structure of $\alpha \text{−}{\mathrm{Mg}}_2{\mathrm{SiO}}_5{\mathrm{H}}_2$ in the [010] direction. (c) Crystal structure of $\beta \text{−}{\mathrm{Mg}}_2{\mathrm{SiO}}_5{\mathrm{H}}_2$ in the [010] direction. The Mg, Si, O, and H atoms are shown as orange, blue, red, and white spheres, respectively.

Figure 2

Proton dynamics in ${\mathrm{Mg}}_2{\mathrm{SiO}}_5{\mathrm{H}}_2$. (a,b) Mean square displacements of protons in $\alpha \text{−}{\mathrm{Mg}}_2{\mathrm{SiO}}_5{\mathrm{H}}_2$ at 300 GPa and (a) 1000 K, (b) 4000 K. (c)–(f) Projection of the atomic trajectories along the (c),(d) [010] and (e),(f) [100] direction of $\alpha \text{−}{\mathrm{Mg}}_2{\mathrm{SiO}}_5{\mathrm{H}}_2$ from the last 3 ps run representing (c),(e) the normal (1000 K) and (d),(f) the superionic (4000 K) states.

Figure 3

Pressure–temperature stability field of $\alpha \text{−}{\mathrm{Mg}}_2{\mathrm{SiO}}_5{\mathrm{H}}_2$. The high-temperature dissociation boundary of $\alpha \text{−}{\mathrm{Mg}}_2{\mathrm{SiO}}_5{\mathrm{H}}_2$ is shown by the solid line. Note that with ZPE correction, the formation pressure at zero temperature shifts to 247 GPa. Normal and superionic states are marked by triangles and circles, respectively; their boundary at around 2300 K is indicated by a blue solid line. For comparison, the temperature of the superionic transition of ${\mathrm{H}}_2\mathrm{O}$ is shown by a dash-dotted line, based on the previous work [27].

Figure 4

Water storage model in (a) early stages of core formation, (b) after the core-mantle separation.