To determine the p-t stability of hydrous phases in ultramafic compositions in subduction zones, and the conditions at which h2o is released from subducting slabs in dehydration reactions; to obtain thermodynamic data for hydrous phases through phase-equilibrium experiments and in situ volume measurements; and to constrain the equation of state of h2o at high pressures. Reversed phase-equilibrium experiments will be conducted on reactions in the systems mgo-sio2-h2o and mgo-al2o3-sio2-h2o; thermodynamic data for hydrous phases wil be extracted from p-t postions of solid-solid reactions; positions of dehydration reactions will constrain the equation of state of h2o; volumes of phases will be measured in situ using synchrotron radiation