Stabilities and equations of state of dense hydrous magnesium silicates

The thermodynamic stability of high-pressure phases in the ternary System MgO-SiO2-H2O is reviewed with special emphasis on hydrated phases including the "alphabet phases". On the basis of recent experimental data, a stability diagram for these phases along a geotherm appropriate for a subducting slab is constructed. This suggests that at realistic water contents, the breakdown of antigorite will produce olivine plus phase A in the upper mantle, but that the ability of wadsleyite to accommodate wt.% levels of water means that phases E and D are unlikely to occur in the upper portion of the transition zone. For the lower part of the transition zone in which ringwoodite becomes stable, the coexisting hydrous phase would be superhydrous phase B, provided ringwoodite is not able to accommodate significant amounts of water. At the top of the lower mantle, phase D is stable but only at temperatures below 1300 degreesC. As the maximum solubility of H2O in magnesium silicate perovskite is also quite low stabs may dehydrate somewhat as they enter the lower mantle. The available thermodynamic data for these phases is also reviewed. From the most recent measurements of equations of state, it is concluded that the elasticity of the phases in the MgO-SiO2-H2O ternary system is primarily dependent upon the density and not upon the water content. It is, therefore, concluded that the presence of even significant amounts of hydrated phases in subducting slabs could not be unequivocally identified from seismological observations, (C) 2001 Elsevier Science B.V. All rights reserved.