Phase Relation and Equation of State of Iron-Titanium Oxyhydroxides With -PbO₂ Type Crystal Structure at Deep Mantle Conditions

Phase relation and equation of state of iron-titanium oxyhydroxide with alpha-PbO2-type crystal structure (hydrous alpha-phase) was investigated at pressure from middle upper mantle to mantle transition zone. The experiments were performed using multi-anvil apparatus with in situ synchrotron X-ray diffraction. The metal-diamond sample container was used to maintain a closed system with respect to water while allowing X-ray transmittance. Starting materials were mixture of reagent grade goethite and anatase with Fe:Ti = 1:1 and 1:3. The hydrous alpha-phase was synthesized at 11-14 GPa and 800-900 degrees C. The X-ray diffraction data were obtained over a wide range of pressures from 5 to 22 GPa and temperatures from room temperature to 930 degrees C. Using pressure-volume-temperature data of the hydrous alpha-phase with Fe:Ti = 1:1 collected at room temperature to 800 degrees C and at 8.9-20.8 GPa, we determined the isothermal bulk modulus (K-T0 = 183 (1) GPa) and the thermal expansivity (alpha(0) = 3.29 (1) x 10(-5)K(-1)) at ambient condition. The stability field of the hydrous alpha-phase and phase relation of FeOOH-TiO2 system at 900 degrees C was well constrained. It was found that the hydrous alpha-phase decomposes into baddeleyite-type TiO2 + epsilon-FeOOH at pressure of approximately 20-21 GPa, and into ilmenite + rutile at 5-6 GPa. This stability field equivalent to depth of 180-600 km in the subduction zone. Our results suggest that the hydrous alpha-phase is an important water reservoir in the middle upper mantle to mantle transition zone.