Phase relations of peridotites under H2O-saturated conditions and ability of subducting plates for transportation of H2O

Phase relations of peridotites under H2O-saturated conditions up to 28 GPa and 2600 °C have been clarified based on the high-pressure experimental results in the MgO(–FeO)–SiO2–H2O, MgO–SiO2–Al2O3–H2O, CaO–MgO–Al2O3–SiO2–H2O and natural systems. Based on the phase assemblages deduced and the chemical compositions of the phases, the maximum H2O contents for two bulk rock compositions (lherzolite and harzburgite) have been calculated using mass balance. Then the potential ability of subducting plates for transportation of H2O is discussed by simple models combining the phase relations and the thermal structures of subduction zones as a function of the age of subducting plate a (15–130 Ma), the subduction velocity u0 (2.25–18 cm/year) and angle α (30° and 60°), and the mantle potential temperature Tp (1300 and 1350 °C). The results confirm the importance of “choke point” (a cusp around 6.2 GPa and 550 °C at which the stability field of major hydrous phases is minimized in terms of temperature), although the choke point is exceeded by the Mg-sursassite-bearing assemblage that has the maximum H2O content of 0.4–0.7 wt.% around 5–7 GPa and 550–700 °C. If the geotherm of the coldest part across the subducting plate passes below the choke point, maximum 4.6 wt.% H2O can subduct, whereas H2O less than 0.4–0.7 wt.% can subduct above the choke point. The critical conditions are clarified as follows: if α=30° and Tp=1300 °C, a∼15 Ma with u0=9 cm/year, or a∼30 Ma with u0=2.25 cm/year is the critical condition. If α=60° and Tp=1300 °C, or α=30° and Tp=1350 °C, the critical conditions slightly shift to an older age or a greater velocity (e.g., a∼30 Ma with u0=3.5 cm/year). The slab thermal parameter (au0sinα) is useful to predict roughly the critical condition. However, within the younger plate, the spatial extent of a cold region with major hydrous phases is thinner, hence less H2O is subducted, even with the same slab thermal parameter. This indicates that the age of the plate and the subduction velocity act differently on the H2O-transportation ability of subducting plates.