[S-IT17] Mass and energy transport in the crust and mantle: from properties to processes

[SIT17-10] Role of a Mixed-Valence Hydrous Iron-Rich Oxide (HH1 phase) on the sound velocity anomalies in the hydrated lower mantle

Lu Liu¹, *Eiji Ohtani², Li Zhang¹, Takayuki Ishii³, Daijo IKUTA³, Hiroshi Fukui^4,5, Alfred Q.R. Baron^4,5 (1.Center for High Pressure Science and Technology Advanced Research, Shanghai, China, 2.Department of Earth and Planetary Materials Science, Graduate School of Science, Tohoku University, 3.Institute for Planetary Materials, Okayama University, 4.RIKEN SPring-8 Center, 5.Japan Synchrotron Radiation Research Institute)

Keywords:Hydrous HH1 phase, Sound velocity, Inelastic x-ray scattering, High pressure, lower mantle, Seismic scatterer

Peridotite-water reactions create DHMS phases at the top of the lower mantle (1). Various hydrous phases such the HH1 phase, an iron-rich hydrous hexagonal phase Fe_16.72O₁₈H_x (x~4.5) (2) have been discovered recently under the lower mantle conditions. The HH1 phase, which is likely to be the same phase as hydrous η-phase (3), is stable in the middle lower mantle. The phase is synthesized in the Fe-O-H system, Fe-Al-O-H system, Fe-Mg-O-H system, and Fe-Mg-Si-O-H system, and it is stable at the pressure corresponding to 61 GPa to at least 82 GPa and 1800-2400 K. The reactions of ferropericlase and olivine with water at the middle lower mantle produce iron-depleted ferropericlase, iron depleted bridgemanite together with the HH1-phase (2). We measured the sound velocity of the HH1 phase at high pressure by using the inelastic x-ray scattering method combined with the diamond anvil cell. Our sound velocity measurement revealed that the hydrous peridotite including the HH1 phase exhibits higher v_S and a lower v_P/v_S ratio compared to anhydrous peridotite. Thus, the shear wave velocity v_s and density changes associated with hydration and formation of hydrous phase HH1 in peridotite lower mantle may create seismic scatterers with a positive v_s anomaly observed in the middle lower mantle (4, 5).

References: (1) Ohtani and Ishii (2024) Prog. Earth Planet. Sci. 11, 65. (2) Liu, L. et al. (2022) JGR: Solid Earth, 127(5), 1–11. (3) Chen, H. et al. (2020) EPSL, 299, 116551. (4) Kaneshima, S. (2013) Phys. Earth Planet. Inter., 222, 35–46. (5) Liu, L. et al. (2025) Geophys. Res. Lett., 51, e2024GL112963.