${\text{FeSiO}}_4{\text{H}}_2$ stabilized at subducting slab conditions: A geologically viable water carrier into the Earth's lower mantle

Hydrous minerals hold the key to unlocking the enduring mystery of the water cycle deep inside the Earth. Tremendous efforts have been devoted to identifying geologically viable minerals meeting stringent pressure-temperature-density stability requirements for descent into deep Earth, and such pursuits remain active. Here, we identify two hydrous iron silicates, $\alpha$- and $\beta \text{−}{\mathrm{FeSiO}}_4{\mathrm{H}}_2$, formed by a reaction of Earth-abundant ${\mathrm{FeSiO}}_3$ and ${\mathrm{H}}_2\mathrm{O}$ and stabilized at the pressure-temperature conditions in cold subducting slabs. These phases have a sufficiently high density for a stable descent into the Earth's lower mantle, and then decompose to release water after reaching equilibrium with the mantle geotherm. Moreover, $\mathrm{Mg}\left(\mathrm{Fe}\right){\mathrm{SiO}}_4{\mathrm{H}}_2$ solutions are found to be more stable than the pure substances and can serve as effective carriers to transport substantial amounts of water to lower-mantle regions via the cold subduction zones. These findings establish a viable and robust material basis for the deep-Earth water cycle, with major implications for elucidation of many prominent geological processes.

Figure 1

The crystal structure of (a) $\alpha \text{−}{\mathrm{FeSiO}}_4{\mathrm{H}}_2$ and (b) $\beta \text{−}{\mathrm{FeSiO}}_4{\mathrm{H}}_2$. The Fe, Si, O, and H atoms are represented by brown, blue, red, and pink spheres, respectively.

Figure 2

(a) Calculated enthalpy as a function of pressure of $\beta \text{−}{\mathrm{FeSiO}}_4{\mathrm{H}}_2$, $\text{Fe}+{\mathrm{Fe}}_2{\mathrm{O}}_3+3{\mathrm{SiO}}_2+3{\mathrm{H}}_2\mathrm{O}$, $\text{FeO}+{\mathrm{SiO}}_2+{\mathrm{H}}_2\mathrm{O}$, ${\mathrm{FeSiO}}_3+{\mathrm{H}}_2\mathrm{O}$, and ${\mathrm{Fe}}_2{\mathrm{SiO}}_4+{\mathrm{H}}_2\mathrm{O}\text{-FeO}$ measured relative to $\alpha \text{−}{\mathrm{FeSiO}}_4{\mathrm{H}}_2$. Negative relative enthalpy indicates that ${\mathrm{FeSiO}}_4{\mathrm{H}}_2$ is stable in the pressure ranges of 18–61 GPa, which spans the pressures in the upper mantle (UM), mantle transition zone (MTZ), and lower mantle (LM), as indicated by the distinct color-shaded zones. (b) The pressure-temperature phase diagram of ${\mathrm{FeSiO}}_4{\mathrm{H}}_2$. The boundary between $\alpha$- and $\beta \text{−}{\mathrm{FeSiO}}_4{\mathrm{H}}_2$ phases is shown by the gray solid line. Purple and orange dashed lines represent the dissociation boundary where ${\mathrm{FeSiO}}_4{\mathrm{H}}_2$ decomposes into $\text{FeO}+{\mathrm{SiO}}_2+{\mathrm{H}}_2\mathrm{O}$ and ${\mathrm{FeSiO}}_3+{\mathrm{H}}_2\mathrm{O}$, respectively. The gray zone represents the geothermal conditions of the subducting slab, while the blue line indicates the geothermal conditions of the mantle. The pressure boundary of the mantle transition zone (MTZ) and the lower mantle (LM) is about 25 GPa, as indicated at the bottom of the panel.

Figure 3

(a) Gibbs free energies of ${\mathrm{Fe}}_x{\mathrm{Mg}}_{1-x}{\mathrm{SiO}}_4{\mathrm{H}}_2$ relative to those of pure substances ${\mathrm{MgSiO}}_4{\mathrm{H}}_2$ and ${\mathrm{MgSiO}}_4{\mathrm{H}}_2$ at 35 GPa and 0, 500, and 1000 K. (b) Density of hydrous minerals AlOOH, ${\mathrm{MgSiO}}_4{\mathrm{H}}_2$, ${\mathrm{FeSiO}}_4{\mathrm{H}}_2$, $\left({\mathrm{Fe}}_{0.45}{\mathrm{Mg}}_{0.55}\right){\mathrm{SiO}}_4{\mathrm{H}}_2$, and $\left({\mathrm{Fe}}_{0.7}{\mathrm{Mg}}_{0.3}\right){\mathrm{SiO}}_4{\mathrm{H}}_2$ compared with that of Earth's mantle according to the preliminary reference Earth model [65].