Stability of a hydrous δ-phase, AlOOH-MgSiO2(OH)2, and a mechanism for water transport into the base of lower mantle

- Hydrogen is stored in hydrous δ-phase AlOOH-MgSiO2(OH)2 in the lower mantle.
- Hydrous δ-phase coexists with perovskite and post-perovskite in the lower mantle.
- Hydrous δ-phase has a high bulk modulus due to the O-H-O bonding symmetrization.
- Hydrous δ-phase in the slabs transports hydrogen to the lower mantle and core.

The global water cycle in the Earth is one of the most important issues in geodynamics, because water can affect the physical and rheological properties of the mantle. However, it is still a matter of debate whether water can be transported into the lower mantle and core. Here we report a new reaction between aluminous perovskite and water to form alumina-depleted perovskite and hydrous δ-phase AlOOH-MgSiO2(OH)2 along the mantle geotherm in the lower mantle. Chemical analysis of the coexisting phases showed that the perovskite and post-perovskite phases were depleted in Al2O3, whereas hydrous δ-phase contains at least 44 mol% of MgSiO2(OH)2 component at 68 GPa and 2010 K, and 23 mol% of this component at 128 GPa and 2190 K. The present experiments revealed that hydrous δ-phase AlOOH-MgSiO2(OH)2 can coexist with alumina-depleted MgSiO3 perovskite or post-perovskite under the lower mantle conditions along the slab geotherm. Thus this hydrous phase in the slabs can transport water into the base of the lower mantle.