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摘要
We determined the melting phase relations and conditions of the dense hydrous magnesium silicate phase D (nominally MgSi2O4(OH)(2)) on composition in MgO-SiO2-H2O (MSH), MgO-Al2O3-SiO2-H2O (MASH), and FeO-MgO-Al2O3-SiO2-H2O (FMASH), and on a mixture of phase D + olivine + enstatite (MSH) at 22-32 GPa and 1000-1800 degrees C. Contrasting to previous studies, we performed H2O-undersaturated experiments. Bulk compositions were synthetic mixtures of brucite + silica or brucite + olivine + enstatite on the silica-rich side of the tie-line perovskite-H2O. At 22-24 GPa, the maximum thermal stability of phase D is between 1350 and 1400 degrees C in MSH and FMASH, but 1600 degrees C at 24 GPa in the Fe-free, Al-bearing bulk composition (MASH). Apparently, addition of Al2O3 increases the stability field of phase D by 200 degrees C, an effect that is counter balanced by addition of FeO. At 32 GPa, the stability of phase D (MSH and FMASH) is between 1350 and 1400 degrees C. At 22 GPa, phase D melts to a Mg-rich melt coexisting with MgSi-ilmenite + stishovite, whereas at 24-32 GPa melt coexists with perovskite and stishovite. Even melts from bulk compositions in the silica-rich part of the MSH system (molar bulk Mg/Si < 0.5) are magnesian-rich (Mg/Si molar ratio of 2-5) and are distinct from aqueous fluids and hydrous melts at lower pressures. The temperature stabilities determined in this study indicate that slabs that thermally relax when stagnating on top of the 660-km discontinuity or penetrating into the lower mantle will have their last dense hydrous magnesium silicate phase, i.e., phase D, melting and producing a magnesian and hydrous melt that will rise through the transition zone. Such a melt could be responsible for observed low velocity zones, and may be neutrally buoyant at the 410-km discontinuity and will affect the structure and dynamics of the mantle.
- 出版日期2014-11-15
