Itaru Ohira

Knowledge of the structural behavior of silicate melts and/or glasses at high pressures provides fundamental information for discussing the nature and properties of silicate magmas in the Earth’s interior. The behavior of Si-O structures under high-pressure conditions has been widely studied, while the effect of cation atoms on the high-pressure structural behavior of silicate melts or glasses has not been well investigated. In this study, we investigated the structures of MgSiO3 and CaSiO3 glasses up to 5.4 GPa by in situ X-ray pair distribution function measurements to understand the effect of different cations (Mg2+ and Ca2+) on high-pressure structural behavior of silicate glasses. We found that the structural behavior of MgSiO3 and CaSiO3 glasses are different at high pressures. The structure of MgSiO3 glass changes by shrinking of Si-O-Si angle with increasing pressures, which is consistent with previous studies for SiO2 and MgSiO3 glasses. On the other hand, CaSiO3 glass shows almost no change in Si-Si distance at high pressures, while the intensities of two peaks at ~3.0 and ~3.5 Å change with increasing pressure. The structural change in CaSiO3 glass at high pressure is interpreted as the change in the fraction of the edge-shared and corner-shared CaO6-SiO4 structures. The different high-pressure structural behavior observed in MgSiO3 and CaSiO3 glasses may be the origin of differences in properties, such as viscosity between MgSiO3 and CaSiO3 melts at high pressures. This signifies the importance of different structural behaviors due to different cations in investigations of the nature and properties of silicate magmas in Earth’s interior.

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