Elevated melting temperature and superionic transition of H₂O ice

Subduction of oceanic lithosphere effectively transports substantial amounts of water deep into the Earth, altering the properties of the surrounding mantle minerals and facilitating chemical interactions between the subducting lithosphere and mantle. Ice-VII is considered the stable phase of H₂O released from minerals within the cold subducting slabs by progressive dehydration. This study investigates the melting boundary of high-pressure ices and the transformation of ice-VII to superionic (SI) states under the specific pressure-temperature (P-T) conditions of subducting slabs. We conducted X-ray diffraction measurements on H₂O phases up to 42 GPa and 1400 K using a newly developed externally-heated diamond anvil cell system with precise P-T control. The experiments confirm the elevated melting temperature of high-pressure ice starting at 9.5 GPa, likely due to the appearance of SI body-centered cubic ice phase. The phase diagram of H₂O, particularly the elevated melting temperatures and transition to SI state, provides crucial insights into the role of water in cold subducting slabs and its possible association with deep-seated seismic activity and mantle dynamics.