Isothermal compressibility of hydrous ringwoodite and its relation to the mantle discontinuities

High pressure X-ray diffraction experiments of hydrous ringwoodite, contain ing 2.8 +/- 0.2 wt% H2O, have been carried out by using X-rays from a synch rotron radiation source with a diamond anvil cell under hydrostatic pressur e up to 6 GPa. The isothermal bulk modulus (K-0) was determined to be 148 /- 1 GPa with the pressure derivative (K-0') fixed to 5 using the Birch - M urnaghan equation of state. This value is almost 80% of that of anhydrous r ingwoodite. The relations of the bulk modulus and zero pressure volumes (V- 0) with H2O content are estimated within the solid solution range. Combinin g the present results with recent seismological data, the nature of the 520 km and 660 km seismic discontinuities is discussed.