We have conducted in situ X-ray diffraction study for determination of the phase boundary of hydrous peridotite and basalt compositions. The slope of the decomposition boundary of ringwoodite under the dry condition is very small around -0.4~-1.3 MPa/K, whereas that for the hydrous peridotite becomes about –2 MPa/K. The garnet-perovskite transformation boundary in hydrous basalt is by about 1.5 GPa lower than that in the dry basalt. Thus, there are no density crossover between hydrous peridotite and basalt components in the slabs. Thus, the basaltic layer can easily penetrate into the lower mantle under the wet conditions.
