[S-MP38] Physics and Chemistry of Minerals

[SMP38-15] Stability and substitution mechanism of Al-bearing superhydrous phase B at the mantle transition zone and the upper most lower mantle

*Sho Kakizawa¹, Toru Inoue^1,2,3, Hideto Nakano¹, Naoya Sakamoto⁴, Hisayoshi Yurimoto^4,5,6 (1.Geodynamics Reseach Center, Ehime University, 2.Department of Earth and Planetary Systems Science, Hiroshima, 3.Hiroshima Institute of Plate Convergence Region Research, 4.Creative Research Institute, 5.Faculty of Science, Hokkaido University, 6.Institute of Space and Astronautical Science, JAXA)

Keywords:superhydrous phase B, dense hydrous magnesium silicates, hydrous mineral, stability, substitution mechanism

Water is the most abundant and important volatile component in the deep mantle. Water is transported into the Earth’s deep interior by hydrous minerals in a cold subducted slab. However, all Mg-Si endmembers of dense hydrous magnesium silicates break down at temperatures close to the typical mantle geotherm. Recent studies suggest that incorporation of Al increases the high-temperature stability of these hydrous minerals. Al-bearing phase D breaks down at apploximatly1600 °C and 24 GPa, which is approximately 200 °C higher than that of Mg -end member phase D (Ghosh and Schmidt, 2014). Moreover, Al -end member phase D (Al-phase D, Al₂SiO₄(OH)₂) is stable at a temperature of over 2000 °C and pressure of 26 GPa (Pamato et al., 2015). In the case of phase H, it can form a solid solution with δ-AlOOH (Nishi et al., 2014) and this solid solution is stable even along a typical mantle geotherm (Ohira et all., 2014). On the other hand, phase A cannot contain Al. Instead, Al is included in 23 Å phase (Mg₁₁Al₂Si₄O₁₆(OH)₁₂), which has stability region similar to phase A (Cai et al., 2015). However, the effect of Al on the stability of superhydrous phase B is not known. To clarify the stability of Al-bearing superhydrous phase B in the mantle transition zone and the uppermost lower mantle under a typical mantle geotherm, we conducted high-pressure and high-temperature experiments using a multi-anvil apparatus Here, we report the stability of Al-bearing superhydrous phase B and the tendency of its chemical composition, and we will also discuss a mechanism of water transport to the lower mantle.

The high pressure and high temperature experiments were conducted using a multi anvil high pressure apparatus (ORANGE-2000) installed at the Geodynamics Research Center, Ehime University, Japan. The starting materials for these experiments were powdered mixtures of MgO, Al₂O₃, SiO₂, Mg(OH)₂, and Al(OH)₃ with 6 different compositions. The experiments were performed at 20 and 24 GPa at 1400 and 1600 °C. Water content of the selected samples was quantified using a SIMS installed at Hokkaido University, Japan.

Superhydrous phase B contained significant amounts of Al₂O₃, from15-33 wt%, and Al-bearing superhydrous phase B remained stable, even at 1600 °C and pressures of approximately 20-24 GPa. Moreover, two types of superhydrous phase Bs with different chemical compositions coexisted at 20-24 GPa and 1600 °C. The Al₂O₃ and H₂O contents increased, and the MgO and SiO₂ contents decreased as the pressure and temperature increased. The maximum H₂O content of Al-bearing superhydrous phase B is 11.1(3) wt%, which is ~1.9 times larger than that of the Mg-endmember. The substitution mechanism of Al and H can be described as 2Mg²⁺ + Si⁴⁺ to 2Al³⁺ + 2H⁺ + Vc (cation vacancy). The crystal structures of the two coexisting superhydrous phase Bs are expected to be slightly different from each other. The present results indicate that Al-bearing superhydrous phase B can be stable in a subducted slab with a high Al content at temperatures typical of the mantle transition zone and lower mantle. Thus, water can be transported to the lower mantle by Al-bearing superhydrous phase B in a subducting slab, even at the typical mantle geotherm.