Host: The Japanese Association of Mineralogists, Petrologists and Economic Geologists
Water plays an important role in slab dynamics near the 660 km discontinuity. We have determined the phase boundary of the post-garnet transformation by high pressure and temperature in situ X-ray diffraction experiment and the conventional quenching experiment using the Kawai anvil apparatus. We observed that the phase boundary in wet MORB is lower by about 2 GPa than that in the dry MORB, and MORB and peridotite has an equal density at 24.6 GPa and 1200 °C under the wet condition. Change of the post-garnet transformation boundary in wet conditions indicates that along the geotherm of the cold subducting slab, basalt is denser than periodotite and there is no density crossover between basalt and surrounding peridotite. The bottom of the transition zone and the uppermost lower mantle are supposed to be a zone of dehydration of the down going slabs due to decomposition of hydrous ringwoodite and/or superhydrous phase B in peridotite. Thus, hydrated basaltic crust, in which water for hydration can be readily provided from the peridotite layer below, can penetrate effectively into the lower mantle. The former oceanic crust is buoyant and can be trapped at the depth of 660 km discontinuity under relatively dry conditions. Whereas, under the wet conditions the subducting basaltic crust continuously sinks into the lower mantle without separation from the underlying peridotite along the lower temperature geotherm corresponding to the cold subduction
