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.
Highlights
The circulation and distribution of “water” in the forms of hydrogen, hydroxyl, and molecular H2O in the Earth’s interior are important factors affecting the evolution and dynamics of the Earth’s interior[1,2,3,4]
The spin transition of F e3+ in the octahedral site is observed in the Fe-bearing NAL phase[37]
A computational study by ref.[38] shows that the spin transition pressure of F e3+ in the octahedral site of the NAL phase is ~ 40 GPa, and it remains mostly invariant to temperature and the width moderately increases with temperature
Summary
The circulation and distribution of “water” in the forms of hydrogen, hydroxyl, and molecular H2O in the Earth’s interior are important factors affecting the evolution and dynamics of the Earth’s interior[1,2,3,4]. The chemical analyses of the run products at 117 GPa and 2050 K in this system showed that the composition of the synthesized δ-phase was (Mg0.03(2)Si0.07(3)Al0.81(4)Fe0.09(3))OOH18 Such a Al-rich composition is similar to the hydrous phase (Mg0.11Al0.63Si0.2Fe0.03)OOH, formed in a natural basalt system at 25–26 GPa and 1273–1473 K17 and closer to AlOOH than FeOOH and M gSiO4H2. A theoretical study proposed a negative correlation of pressure (P)–shear modulus (μ) for low-spin ε-FeOOH, which causes a decrease of vS/vΦ with increasing pressure[26]. These studies point to the possibility that the solid solution may in part be responsible for regionally seismic heterogeneities observed in the lower m antle[27,28]. By combining our results from NRIXS with the equation of state[22], we discuss the potential relationships between δ-(Al,Fe)OOH and seismic anomalies in the lower mantle
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