EFFECT OF WATER ON MELTING PHASE-RELATIONS AND MELT COMPOSITION IN THE SYSTEM MG2SIO4-MGSIO3-H2O UP TO 15 GPA

Melting experiments in the system Mg2SiO4-MgSiO3-H2O up to pressures of 15 GPa show that the presence of H2O drastically changes melting temperature, liquidus phases and the coexisting melt composition. The system Mg2SiO4-H2O shows incongruent melting characterized by a high Mg/Si ratio of the eutectic composition exceeding 2.0 above 12 GPa, whereas it exhibits congruent melting up to at least 7.7 GPa. In the MgSiO3-H2O system, incongruent melting is observed up to 3 GPa, which switches to congruent melting above 5.5 GPa, and persists to at least 12 GPa. The variation of the eutectic composition in the system Mg2SiO4-MgSiO3-H2O is consistent with the variations of the melting temperatures in the systems Mg2SiO4-H2O and MgSiO3-H2O. The melting temperature (solidus) in the system Mg2SiO4-H2O decreases from 1800-degrees-C at atmospheric pressure to less than 1200-degrees-C at 10 GPa, while in the system MgSiO3-H2O it is reduced to a minimum value of 1250-degrees-C at approximately 5.5 GPa and then increases with pressure. The composition of the initial melt in this system drastically changes at around 5.5 GPa, the Mg/Si ratio being about 3 at 15 GPa. Variations of the ternary phase diagrams (MgO-SiO2-H2O) with pressure are compiled and compared with previous work on the relevant systems. A 'modified spinel-like' hydrous phase is synthesized at 15.5 GPa. The maximum amount of H2O accommodated in this phase is estimated to be 3.3% wt. from observed chemical composition and crystal-chemical considerations, implying that a significant amount of H2O can be accommodated in the mantle transition zone. The overall effect of H2O on ultrabasic magma genesis in the mantle is discussed. A possibility is presented for the formation of komatiite magma involving the effect of H2O in the mantle of the early Earth: Mg-rich komatiite magma was possibly generated at a depth of only 200 km at temperatures much lower than that having been assumed previously.