Source: Science Daily
The existence of water in deep Earth is considered to play an important role in geodynamics, because water drastically changes the physical properties of mantle rock, such as melting temperature, electric conductivity, and rheological properties. Water is transported into deep Earth by the hydrous minerals in the subducting cold plates. Hydrous minerals, such as serpentine, mica and clay minerals, contain water in the form of hydroxyl (-OH) in the crystal structure. Most of the hydrous minerals decompose into anhydrous minerals and water when they are transported into deep Earth, at 40-100 km depth, due to the high temperature and pressure conditions.