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

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 (v_D) increases. At higher pressures, the low spin δ-Fe13 is characterized by a pressure-invariant v_D. Using the equation of state for the same sample, the shear-, compressional-, and bulk-velocities (v_S, v_P, and v_Φ) are calculated and extrapolated to deep mantle conditions. The obtained velocity data show that δ-(Al,Fe)OOH may cause low-v_Φ and low-v_P anomalies in the shallow lower mantle. At deeper depths, we find that this hydrous phase reproduces the anti-correlation between v_S and v_Φ reported for the large low seismic velocity provinces, thus serving as a potential seismic signature of hydrous circulation in the lower mantle.