The influence of δ-(Al,Fe)OOH on seismic heterogeneities in Earth's lower mantle.

Scientific Reports
Itaru OhiraEiji Ohtani

Abstract

The high-pressure phases of oxyhydroxides (δ-AlOOH, ε-FeOOH, and their solid solution), candidate components of subducted slabs, have wide stability fields, thus potentially influencing volatile circulation and dynamics in the Earth's lower mantle. Here, we report the elastic wave velocities of δ-(Al,Fe)OOH (Fe/(Al + Fe) = 0.13, δ-Fe13) to 79 GPa, determined by nuclear resonant inelastic X-ray scattering. At pressures below 20 GPa, a softening of the phonon spectra is observed. With increasing pressure up to the Fe3+ spin crossover (~ 45 GPa), the Debye sound velocity (vD) increases. At higher pressures, the low spin δ-Fe13 is characterized by a pressure-invariant vD. Using the equation of state for the same sample, the shear-, compressional-, and bulk-velocities (vS, vP, and vΦ) are calculated and extrapolated to deep mantle conditions. The obtained velocity data show that δ-(Al,Fe)OOH may cause low-vΦ and low-vP anomalies in the shallow lower mantle. At deeper depths, we find that this hydrous phase reproduces the anti-correlation between vS and vΦ reported for the large low seismic velocity provinces, thus serving as a potential seismic signature of hydrous circulation in the lower mantle.

References

Aug 3, 2001·Nature·G R Helffrich, B J Wood
Dec 2, 2015·Scientific Reports·Jing YangNaotaka Tomioka
May 16, 2017·Nature Communications·Paula KoelemeijerJeroen Ritsema
Nov 17, 2017·Nature·Harriet C P LauDavid Al-Attar

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