We experimentally explored phase relations in the system FeOOH-TiO2 at pressures of 16 GPa and temperatures of 1000–1600 °C, which corresponds to conditions of the Earth’s mantle transition zone. Analyses of the recovered samples revealed that, in the studied conditions, there are 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 ɛ-FeOOH type crystal structure (orthorhombic, P21nm), and the other is the more Ti-rich solid solution (x > 0.35) with α-PbO2 type structure (orthorhombic, Pbcn). The ɛ-FeOOH phase is stable up to ~1100 °C irrespective of chemical composition, whereas the α-PbO2 type phase is stable up to 1500 °C for a composition of x = 0.5 and at least to 1600 °C for x = 0.75, and thus the α-PbO2 type phase is stable at average mantle temperature in the Earth’s mantle transition zone. Iron-titanium-rich phases found previously in basalt + H2O system are estimated to be the ɛ-FeOOH and α-PbO2 type phases where the phase with iron-rich composition found at relatively low temperature (<1100 °C) is ɛ-FeOOH phase and the phase with titanium-rich composition is α-PbO2 type phase. The α-PbO2 type and ɛ-FeOOH phases may be stable in the subducted basaltic crust at pressures in the mantle transition zone under water-rich conditions.

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