1703 / 2019-08-27 12:16:24

Sound velocities of δ-(Al0.95Fe0.05)OOH to core-mantle boundary pressures: Implications for seismic anomalies in the deep mantle

Deep water; sound velocity; Brillouin light scattering; high pressure

8、地球和空间物理 > 专题6：地球与行星内部物理化学属性与过程

Water plays an important role in the evolution and state of Earth. The presence of water (hydroxyl or hydrogen) would significantly affect the physical and chemical properties of Earth and planetary materials, such as melting, viscosity, diffusion rate, electrical conductivity, phase transition, and elasticity [Ni et al., 2017]. δ-AlOOH is plausibly one of the most important hydrous phases and it likely carries water into the deep mantle through deep subduction. Knowledge of the physical properties of δ-AlOOH under the deep mantle conditions are thus beneficial to decipher the circulation of deep water and abnormal features in global seismic observations [Mashino et al., 2016]. Furthermore, previous studies demonstrated that the presence of Fe2+ and/or Fe3+ could greatly influence sound velocities, elasticity, phase stability, and transport properties of major mantle minerals [Fu et al., 2018; Liu et al., 2015; Okuda et al., 2019]. To date, sound velocities of iron-bearing δ-AlOOH are scarcely investigated under conditions relevant to the deep mantle. Therefore, we systematically investigated the compressibility and sound velocities of δ-(Al0.95Fe0.05)OOH at high pressure up to 135 GPa across hydrogen bond symmetrization and spin-pairing transition of iron, using diamond-anvil cell techniques coupled with Brillouin light scattering (BLS) and X-ray diffraction (XRD) spectroscopies. We found that the incorporation of 5 mol% Fe in δ-AlOOH decreases the shear velocity by ~5% compared to that of pure δ-AlOOH . Interestingly, the shear wave velocity of δ-(Al0.95Fe0.0)OOH is ~10% lower than iron-bearing bridgmanite Mg0.96Fe0.05Si0.99O3 under CMB pressure conditions [Fu et al., 2018], likely contributing to seismic anomalies observed in the large low-shear-velocity province (LLSVP) and/or ultralow-velocity zones (ULVZs) at the bottom of the lower mantle.