Stability of dense hydrous magnesium silicate phases and water storage capacity in the transition zone and lower mantle

We evaluated the water storage capacity in the mantle based on the phase relations of silicate systems containing water. We conducted high pressure and temperature synthesis experiments on some silicate systems, such as the superhydrous phase B (=phase C) and phase G (=phase D and F) compositions in the MgO-SiO₂-H₂O system and the CMAS pyrolite-2 wt.% H₂O system up to the uppermost part of the lower mantle. Superhydrous phase B is stable at temperatures below 1200°C at 18 GPa and below 1300°C at 20 GPa. The following reaction sequences were observed with increasing temperature from 1200 to 1600°C in the pressure range from 18 to 20 GPa: superhydrous phase B → phase B + liquid → wadsleyite + periclase + liquid → anhydrous phase B + periclase + liquid. Phase G is stable at temperatures below 1000°C at 18 GPa, below 1100°C at 20-22 GPa, and below 1200°C at 25 GPa. Wadsleyite + stishovite + liquid are stable above 1000°C at 18 GPa, ringwoodite (or phase E) + stishovite + liquid above 1100°C at 22 GPa, ilmenite + stishovite + liquid above 1100°C at 23.5 GPa, and perovskite + stishovite + liquid above 1200°C at 25 GPa. Superhydrous phase B is stable at 1200°C at 18.5 and 25 GPa in CMAS pyrolite-2 wt.% H₂O composition, whereas it decomposes at around 1400°C at 25 GPa in the composition. There may be a layered structure in the mantle in its water storage potential (solubility of water per unit mass); i.e. the upper and lower mantles have relatively small water storage potentials, whereas the transition zone has a larger potential. The water content in the transition zone might be large, because the primordial water trapped in the lower mantle and the recycled water transported by the slab subduction might have been released upwards and stored in the transition zone during geological time.