Melting relations of hydrous pyrolite in CaO-MgO-Al₂O₃-SiO₂-H₂O system at the transition zone pressures

Phase relations and melt compositions in CaO-MgO-Al₂O₃-SiO₂,-pyrolite under hydrous (+2% of H₂O) and anhydrous conditions have been determined at 13-20 GPa and 1600-2220°C. Liquidus and solidus temperatures for the hydrous system are about 50-100°C and 180-240°C lower than those for the dry system, respectively. Majorite is a liquidus phase of the hydrous pyrolite from 13 to 20 GPa. Olivine is a liquidus phase at 13 GPa and both periclase and majorite are the liquidus phases at 20 GPa in the dry pyrolite. We observed expansion of the stability field of anhydrous phase B in hydrous experiments. Compositions of partial melts at 13-20 GPa are generally similar in dry and hydrous systems, but hydrous melts contain more SiO₂ at 13-17 GPa. The melts formed by low degree of melting have Al₂O₃-depleted and CaO-rich compositions. Trends of hydrous melt compositions are generally consistent with those of aluminum-depleted komatiite magmas.