We performed laser-heated diamond anvil cell experiments on bulk compositions in the
systems MgO–SiO 2–H 2 O (MSH) and MgO–Al 2 O 3–SiO 2–H 2 O (MASH) that constrain
the stability of hydrous phases in Earth's lower mantle. Phase identification by synchrotron
powder diffraction reveals a consistent set of stability relations for the high-pressure, dense
hydrous silicate phases D and H. In the MSH system phase D is stable to~ 50 GPa,
independent of temperature from~ 1300 to 1700 K. Phase H becomes stable between 35 …

Laser-heated diamond anvil cell experiments were performed at pressures from~ 30 to 125
GPa on bulk compositions in the system MgO-Al2O3-SiO2-H2O (MASH) to constrain the
stability of hydrous phases in Earth's lower mantle. Phase identification in run products by
synchrotron powder diffraction reveals a consistent set of stability relations for the high-
pressure, dense hydrous silicate phases D and H. Experiments show that aluminous phase
D is stable to~ 55 GPa. Aluminous phase H becomes stable at~ 40 GPa and remains stable …