The phase relations in the MgO-SiO₂-H₂O system are investigated at 23 GPa and 2000 K using multi anvil experimental techniques combined with mass balance calculations. It is found that the water content in hydrous melt is strongly correlated to the coexisting solid phases. When the melt coexists with ringwoodite or bridgmanite/akimotoite, the water content in melt is about 20 – 25 wt.% under the experimental pressure and temperature conditions. In contrast, when melt coexists with only periclase or only stishovite, the water content in melt increases with increasing the bulk MgO-content or SiO₂-content in the system, respectively. It reaches more than 89 wt.% in the MgO-free SiO₂-H₂O binary system. Therefore, hydrous melt with a composition close to pure H₂O (H₂O-rich fluid) can be stabilized under SiO₂-rich conditions near the 660-km boundary, which explains the occurrence of natural ice-VII originated from the bottom of mantle transition zone or topmost lower mantle.

Hydrous melt; MgO-SiO2-H2O; phase relation; lower mantle