δ-(Al,Fe)OOH is considered to be one of the most important hydrous phases on Earth, remaining stable under the extreme conditions throughout the mantle. The behavior of δ-(Al,Fe)OOH at high pressure is essential to understanding the deep water cycle. δ-(Al0.956Fe0.044)OOH crystals synthesized at 21 ​GPa and 1473 ​K were investigated by high-pressure Brillouin light scattering spectroscopy and synchrotron X-ray diffraction up to 135.4 ​GPa in diamond anvil cells. The incorporation of 5 ​mol% FeOOH increases the unit-cell volume of δ-AlOOH by ~1% and decreases the shear-wave velocity (VS) by ~5% at 20–135 ​GPa. In particular, the compressional (VP) and shear (VS) wave velocities of δ-(Al0.956Fe0.044)OOH are 7%–16% and 10%–24% greater than all the major minerals in the mantle transition zone including wadsleyite, ringwoodite, and majorite. The distinctly high sound velocities of δ-(Al0.956Fe0.044)OOH at 20–25 ​GPa may contribute to the seismic anomalies observed at ~560–680 ​km depths in the cold and stagnant slab beneath Izu-Bonin and/or Korea. Furthermore, the VS of δ-(Al0.956Fe0.044)OOH is about 10% and 4%–12% lower than iron-bearing bridgmanite Mg0.96Fe0.05Si0.99O3 and ferropericlase (Mg0.92Fe0.08)O, respectively, under the lowermost mantle conditions, which might partially contribute to the large low-shear-velocity provinces and ultralow velocity zones at the bottom of the lower mantle.

Xiaowan Su, Chaoshuai Zhao, Chaojia Lv, Yukai Zhuang, Nilesh Salke, Liangxu Xu, Hu Tang, Huiyang Gou, Xiaohui Yu, Qiang Sun, Jin Liu, The effect of iron on the sound velocities of δ-AlOOH up to 135 GPa, Geoscience Frontiers, Vol 12, Issue 2, 2021, Pages 937-946, ISSN 1674-9871, abstract