Thermodynamic calculation on the phase transformation and water release of subducted sediment from 10 to 35 kbar

Thermodynamic calculation results of a clastic sediment composition (K2O–Na2O–CaO–MgO–FeO–Fe2O3–Al2O3–TiO2–SiO2–H2O) at 10 to 35 kbar and 300–900 °C show that H2O release of subducted sediment in subduction zone is mainly controlled by the metamorphic dehydrations of hydrous minerals: chlorite, muscovite, carpholite, paragonite, lawsonite and chloritoid. At lower (≤70 km) and deeper (≥70 km) depths in subduction zone, the dehydration temperature of sediment varies from 350–800 to 420–600 °C, respectively. Above 780 °C, hydrous minerals are broken down entirely in the system except for muscovite. The phase transition and dehydration of sediment may be responsible for the H2O source of the shallow level fluid in young (hot) subduction zone and the deeper fluids in elder (cold) subduction zone.