Iron-titanium oxyhydroxides which transport water into the deep upper mantle and mantle transition zone

Abstract:

We experimentally discovered a new hydrous phase in the system FeOOH-TiO₂ at pressures of 10-16 GPa and temperatures of 1000–1600°C which corresponds to conditions of the deep upper mantle and the Earth’s mantle transition zone. 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 that were prepared by mixing reagent grade a-FeOOH (goethite) and TiO₂ (anatase) powders 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 and SPI-1000 at Tokyo Institute of Technology). In this system, we identified two stable iron-titanium oxyhydroxide phases whose estimated composition is expressed by (FeH)_{1 – x}Ti_xO₂ . 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. The Iron-titanium-rich hydrous phases was possible to stable in basalt + H₂O system (e.g., Hashimoto and Matsukage 2013). Therefore our findings suggest that water transport in the Earth’s deep interior is probably much more efficient than had been previously thought.