5:15 PM - 6:30 PM
[SCG57-P10] Iron-titanium oxyhydroxides as a water transporter into the Earth’s mantle transition zone
Keywords:deep upper mantle, mantle transition zone, water, titanium, hydrous phase, basaltic crust
We synthesized single phase in the system FeOOH-TiO2 at pressures of 8-16 GPa and temperatures of 900–1600°C which corresponds to conditions of the deep upper mantle and the mantle transition zone (Nishihara and Matsukage, 2016). Seven different compositions in the FeOOH-TiO2 system having molar ratios of x = Ti/(Fe + Ti) = 0, 0.125, 0.25, 0.375, 0.5, 0.75 were used as starting materials. High-pressure and high-temperature experiments were carried out using Kawai-type multi-anvil apparatus (Orange-1000 at Ehime University, SPI-1000 and SAKURA at Tokyo Institute of Technology). In this system, we identified two stable iron-titanium oxyhydroxide phases whose estimated composition is expressed by (FeH)1 – xTixO2 . One is the Fe-rich solid solution (x < 0.23) with e-FeOOH type crystal structure (e-phase, orthorhombic, P21nm) that was described by the previous studies (e.g., Suzuki 2010), and the other is the more Ti-rich solid solution (x > 0.35) with a-PbO2 type structure (a-phase, orthorhombic, Pbcn). The a-phase is stable up to 1500ºC for a composition of x = 0.5 and at least to 1600ºC for x = 0.75. Our result means that this phase is stable at average mantle temperature in the Earth’s mantle transition zone.
We also found that the hydrous phase with a-PbO2 type structure was stable in basalt + H2O system at wide pressure range at deep upper mantle and mantle transition zone (8-17 GPa), and it dehydrate at pressure of ~17 GPa. Above 17 GPa, CaTi perovskite was formed as a Ti-bearing phase. After dehydration of FeTi oxyhydroxide, Al-bearing phase D, which is one of major water carriers in deep mantle, was stable (Liu et al., this meeting). Therefore our findings suggest that water transport in the Earth’s deep interior by basaltic crust is probably much more efficient than had been previously thought.