[S-IT21] Interaction and Coevolution of the Core and Mantle in the Earth and Planets

[SIT21-15] Ultralow-velocity zones possibly explained by light element-enriched iron compounds

*Liang Yuan¹, Eiji Ohtani¹, Xiang Wu², Shengxuan Huang³, Daijo Ikuta¹, Tatsuya Sakamaki¹, Seiji Kamada¹, Hiroshi Fukui^4,5, Satoshi Tsutsui⁶, Hiroshi Uchiyama⁶, Daisuke Ishikawa^5,6, Naohisa Hirao⁶, Alfred Q. R. Baron⁵ (1.Department of Earth and Planetary Materials Science, Graduate School of Science, Tohoku University, Sendai 980-8578, Japan, 2.State Key Laboratory of Geological Processes and Mineral Resources, China University of Geosciences, Wuhan 430074, China, 3.Key Laboratory of Orogenic Belts and Crustal Evolution, MOE, Peking University and School of Earth and Space Sciences, Peking University, Beijing 100871, China, 4.Center for Novel Material Science under Multi-Extreme Conditions, Graduate School of Material Science, University of Hyogo, Hyogo 678-1297, Japan, 5.Materials Dynamics Laboratory, RIKEN SPring-8 Center, Hyogo 679-5148, Japan, 6.Japan Synchrotron Radiation Research Institute (JASRI), SPring-8, Hyogo, 679-5198, Japan)

Keywords:Core–mantle boundary, Ultralow-velocity zones, Light elements, Iron compounds

The core–mantle boundary (CMB) is the most fundamental chemical discontinuity in the Earth. Recent experiments showed that, when water meets iron at the CMB, hydrogen-bearing iron peroxide FeO₂Hx can be produced. The results indicate that water can interact with iron metal very differently than it does on the surface of the earth. Seismic waves speeds in this hydrogen bearing iron peroxide are much slower compared with lower-mantle silicate solid phases, and therefore it may help explain the seismic anomalies at the CMB.

Given the steep geothermal gradient across the CMB, the hydrogen-bearing iron peroxide FeO₂Hx likely undergoes high-temperature decomposition into anhydrous iron oxides (e.g., Fe₂O₃) and fluids. We measured the sound velocity of Fe₂O₃ post-perovskite (ppv) through inelastic X-ray scattering up to 132 GPa and 1,800 K, relevant to the lowermost mantle. Combined with first-principles investigations, we are able to show that Fe₂O₃ ppv has very low sound velocities and strong anisotropy with respect to lower-mantle silicates. Therefore, both hydrogen-bearing iron peroxide FeO₂Hx and post-perovskite Fe₂O₃ are candidate phases for ultralow-velocity zones at the CMB.

Funding acknowledgment: This work was supported by the JSPS Japanese–German Graduate Externship.