[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

劉 璐¹、*大谷 栄治²、張 莉¹、石井 貴之³、生田 大穣³、福井 宏之^4,5、バロン アルフレッド^4,5 (1.北京高圧科学研究中心上海、2.東北大学大学院理学研究科地学専攻、3.岡山大学惑星物質研究所、4.理化学研究所放射光科学研究センター、5.高輝度光科学研究センター)

キーワード：含水HH1相、音速、Ｘ線非弾性散乱、高圧、下部マントル、地震波散乱体

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.