9:45 AM - 10:00 AM
[SMP26-04] Stability of α-PbO2 type FeTi-oxyhydroxide at mantle transition zoon
Keywords:FeTi-oxyhydroxide, H2O transportation, hydrous phase, mantle transition zone
Recently, FeTi-oxyhydroxides were discovered in quenched samples from high-temperature and high-pressure experiments of hydrous basalts at deep mantle conditions (Matsukage et al. 2017, Liu et al. 2019). XRD analysis and FE-EPMA surface chemical analysis of the quenched samples revealed two types of crystal structures in this iron-titanium hydroxide: ε-FeOOH-type and α-PbO2-type. α-PbO2-type is known as the high-pressure phase of rutile, and it has been reported that the α-PbO2 type TiO2 phase can contain up to more than 50 mol% FeOOH components (Nishihara and Matsukage 2016). It has also been suggested that the phase is stable up to a temperature of about 1600°C, depending on the Ti# (= Ti/(Ti + Fe) atomic ratio).
In this study, we investigated the crystal structure of FeOOH-TiO2 oxyhydroxide and its stability field by in-situ X-ray diffraction experiments to elucidate the water transport process in the deep Earth. The experiments were performed using a multi-anvil apparatus (SPEED-1500) installed at SPring-8 (BL04B1). Pressure was generated by eight 26mm WC anvils with TEL 5mm. A Cr-doped semi-sintered MgO octahedron with 10 mm edge length was as a pressure medium. A LaCrO3 sleeve was used as the heater, Mo as the electrode, MgO as the insulator and pressure medium, and W3Re - W25Re thermocouples were used for temperature measurement. Pressures up to 22 GPa and temperatures up to about 900°C were generated. Au was used as the pressure marker. Since FeTi oxyhydroxides are hydrous phases, water will dissipate from the system if the sample is not welded in a metal capsule. In order to follow the synthesis process and the subsequent phase change by in-situ observation using synchrotron X-rays while keeping water in the capsule, we used a sample capsule made of a diamond single crystal disk and an Au tube in this experiment. The starting material was a mixture of reagent FeOOH and TiO2 with Fe:Ti = 1:1. The α-PbO2 type high-pressure phase was first synthesized at 12 GPa and 800°C. The sharp X-ray diffraction pattern confirms that the ideal single phase was synthesized. Thereafter, X-ray diffraction data were obtained over a wide range of temperatures and pressures from 12 GPa to 22 GPa and from room temperature to 900°C while gradually increasing the pressure to observe the crystal structure and stability. As a result, it was found that the α-PbO2 phase decomposes into Baddeleyite and ε-FeOOH at a pressure of about 21 GPa (depth of about 600 km). In addition, the synthesis of α-PbO2 phase after the decomposition of Baddeleyite + ε-FeOOH (reverse reaction experiment) was also carried out. The stability field of the α-PbO2-type FeTi oxyhydroxide with Ti#=0.5 was well constrained by this study. In the future, it will be necessary to perform similar experiments with various Ti# clarify the correlation between the high-pressure phases in the ε-FeOOH-TiO system.
In this study, we investigated the crystal structure of FeOOH-TiO2 oxyhydroxide and its stability field by in-situ X-ray diffraction experiments to elucidate the water transport process in the deep Earth. The experiments were performed using a multi-anvil apparatus (SPEED-1500) installed at SPring-8 (BL04B1). Pressure was generated by eight 26mm WC anvils with TEL 5mm. A Cr-doped semi-sintered MgO octahedron with 10 mm edge length was as a pressure medium. A LaCrO3 sleeve was used as the heater, Mo as the electrode, MgO as the insulator and pressure medium, and W3Re - W25Re thermocouples were used for temperature measurement. Pressures up to 22 GPa and temperatures up to about 900°C were generated. Au was used as the pressure marker. Since FeTi oxyhydroxides are hydrous phases, water will dissipate from the system if the sample is not welded in a metal capsule. In order to follow the synthesis process and the subsequent phase change by in-situ observation using synchrotron X-rays while keeping water in the capsule, we used a sample capsule made of a diamond single crystal disk and an Au tube in this experiment. The starting material was a mixture of reagent FeOOH and TiO2 with Fe:Ti = 1:1. The α-PbO2 type high-pressure phase was first synthesized at 12 GPa and 800°C. The sharp X-ray diffraction pattern confirms that the ideal single phase was synthesized. Thereafter, X-ray diffraction data were obtained over a wide range of temperatures and pressures from 12 GPa to 22 GPa and from room temperature to 900°C while gradually increasing the pressure to observe the crystal structure and stability. As a result, it was found that the α-PbO2 phase decomposes into Baddeleyite and ε-FeOOH at a pressure of about 21 GPa (depth of about 600 km). In addition, the synthesis of α-PbO2 phase after the decomposition of Baddeleyite + ε-FeOOH (reverse reaction experiment) was also carried out. The stability field of the α-PbO2-type FeTi oxyhydroxide with Ti#=0.5 was well constrained by this study. In the future, it will be necessary to perform similar experiments with various Ti# clarify the correlation between the high-pressure phases in the ε-FeOOH-TiO system.