New Al-Bearing Hydrous Mg-Silicate in the Deep Upper Mantle

Abstract:

Subducting slabs are supplying water into the deep mantle, so the stabilities of hydrous minerals under high pressure and high temperature are important issue to clarify the water transportation mechanism into deeper part of the Earth’s mantle. Here we reported a new Al-bearing hydrous Mg-silicate, which was synthesized at 10 GPa and 1000ºC in the system MgO-Al₂O₃-SiO₂-H₂O (phase A (Mg₇Si₂O₈(OH)₆) + Al₂O₃ + H₂O system). The composition of this new phase was determined to be Mg₁₁Al₂Si₄O₁₆(OH)₁₂, and it contains about 12 wt% water. Powder X-ray diffraction and electron diffraction showed that this new phase has a hexagonal structure, with a = 5.1972(2) Å, c = 22.991(4) Å, and V = 537.8(2) ų, and the possible space group is P‾6c2, P6₃cm, or P6₃mcm. The calculated density is 2.761 g/cm³, which was determined by assuming that the formula unit per cell (Z) is 1. This crystal structure is quite unique among mantle minerals in having an extraordinarily long c axis. We confirmed the stability P-T region of this new phase, and found that its stability region is very similar to that of phase A which is the high pressure hydrous phase in serpentine (Mg₆Si₄O₁₀(OH)₈) composition. We further confirmed that this new phase was stable in the chlorite (Mg₅Al₂Si₃O₁₀(OH)₈) composition at 10 GPa and 1000ºC. The present results indicate that this new phase will be stable in Al-rich subducting slab instead of phase A and transport water into the deep upper mantle or even into the upper part of the transition zone.