The effect of iron on the sound velocities of δ-AlOOH up to 135 ​GPa

Abstract

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.

Keywords

• Deep water cycle
• Fe-bearing delta phase
• Seismic anomalies
• High pressure