Aluminous hydrous magnesium silicate as a lower-mantle hydrogen reservoir: a role as an agent for material transport

Abstract

The potential for storage of a large quantity of water/hydrogen in the lower mantle has important implications for the dynamics and evolution of the Earth. A dense hydrous magnesium silicate called phase D is a potential candidate for such a hydrogen reservoir. Its MgO–SiO2–H2O form has been believed to be stable at lower-mantle pressures but only in low-temperature regimes such as subducting slabs because of decomposition below mantle geotherm. Meanwhile, the presence of Al was reported to be a key to enhancing the thermal stability of phase D; however, the detailed Al-incorporation effect on its stability remains unclear. Here we report on Al-bearing phase D (Al-phase D) synthesized from a bridgmanite composition, with Al content expected in bridgmanite formed from a representative mantle composition, under over-saturation of water. We find that the incorporation of Al, despite smaller amounts, into phase D increases its hydrogen content and moreover extends its stability field not only to higher temperatures but also presumably to higher pressures. This leads to that Al-phase D can be one of the most potential reservoirs for a large quantity of hydrogen in the lower mantle. Further, Al-phase D formed by reaction between bridgmanite and water could play an important role in material transport in the lower mantle.

State-of-the-art hydrogen generation techniques and storage methods: A critical review

“…These materials display a systematic inorganic/organic framework that supports a steady porous structure with pores in the range of 0.2-2.0 nm [235]. Microporous materials mainly include activated carbon, carbon nanotubes, metal-organic frameworks, silicates, and zeolites and exhibit type-I gas adsorption isotherm [236][237][238][239][240]. In such type of materials, the potential field of attraction among the wall of the pore and the adsorbate molecules overlap and enhance the force of attraction that acts on the adsorbate, thereby enhancing the hydrogen adsorption.…”

Calculated Elasticity of Al-Bearing Phase D

“…Recent studies indicate that aluminum substitution into DHMSs increases the thermodynamic stability of these phases [7,[12][13][14][15][16][17][18], and that Al-bearing DHMSs may host more water than their magnesium endmember counterparts [18,19]. Additionally, Al-bearing phase D is a likely precursor to the solid solution formed by phase H [MgAlO 2 (OH) 2 ] and δ-(Al,Fe)OOH-a solid solution with P-T stability that extends to the core-mantle boundary [13,14,20,21].…”

“…In Mg-endmember phase D, the SiO 6 and MgO 6 octahedra occur in two separate layers stacked along c-axis, leading these sites to be referred to as the S-site and M-site, respectively (Figure 1a). In Al-bearing phase D, the aluminum occupies both the S-and M-sites [18] (Figure 1b). Aluminum substitutes into phase D via a Tschermak Si 4+ + Mg 2+ ←→ 2Al 3+ substitution, and experiments report a range of compositions, including the near Al-endmember composition referred to as 'super-aluminous' phase D [19].…”