Phase relations in the system MgO-SiO2 with low volatile contents of H2O, F2, and Cl2 were experimentally investigated at 100--234 kbar pressures and 850¿--2050 ¿C temperatures using a split-sphere anvil apparatus (USSA-2000). Superphase B, Mg10Si3O144, was observed at 160--234 kbar, fluorous phase B, Mg12Si4O19F2, at 178 kbar, and hydrous phase F, Mg4Si7O164, also at 178 kbar. At lower pressures, the volatiles were present in phase A, Mg7Si2O86, phase C, Mg(7-x)Si(2+x)O(9+x)4, and phase E, Mg(2-x)Si(1+x)O(4+x-y)2y. With the exception of the fluorous phase B, all these phases can coexist with stishovite at pressures above 130 kbar. In a purely hydrous system, the dehydration temperatures of the assemblage superphase B+stishovite increase from 1200 ¿C at 160 kbar to 1400 ¿C at 234 kbar. Dehydration produces beta phase, Mg2SiO4, possibly containing up to 7 wt% of structural H2O. About 3 wt% of H2O in beta phase expands the stability of the assemblage beta phase + stishovite at the expense of ilmenite by 30 kbar. In more complex compositions, the volatile solidus is below 1350 ¿C, thus at the temperatures which are much lower than the expected mantle temperatures at the relevant depths. Consequently, most of the mantle is in a partially molten state. The volatile-bearing phases can only be present in the colder subducted slabs but not in the rest of the transition zone (400--670 km). Hydrous beta phase and stishovite are the most likely constituents of the slabs. Incongruent melting of the hydrous beta phase coexisting with stishovite, producing garnet and melt, could be the cause of deep-focus earthquakes. Evidence for a mineralogically and chemically stratified upper mantle is reviewed and its structure is discussed in the context of the present results. ¿ American Geophysical Union 1993