Phase transformations in serpentine, the major hydrous mineral in the upper part of the subducting slabs, have been studied at pressures to 26 GPa and at temperatures between 500 and 1400 ¿C. While serpentine completely dehydrates at temperatures higher than 600--800 ¿C and at pressures to 12 GPa, phase assemblages including some dense hydrous magnesium silicates (DHMS) such as phases A, E, superhydrous B and D were formed at higher pressures. Phase D was found to possess a stability field of particularly wide pressure and temperature regime, suggesting that this phase may play an important role in transportation of water into the lower mantle via subduction of cold slabs. ¿ 1998 American Geophysical Union