Itaru Ohira

<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.

Structure of an Al-containing silicate glass (60 mol. % Al2O3-40 mol. % SiO2, A40S) is investigated up to 131 GPa, a pressure close to that of the Earth's core-mantle boundary, by using our recently developed double stage large volume cell. The first peak (r1) of the pair distribution function, which corresponds to T-O distance (T = Al, Si), rapidly increases below 16 GPa, indicating an increase of average coordination number (CN) of T-O from ~4 to 6. The r1 linearly decreases in the pressure range of 25-110 GPa, but it displays a slope change and becomes nearly constant above 110 GPa. The slope change may imply a structural change in the A40S glass around 110 GPa, and may be explained by the change in Al-O distance associated with the Al-O CN increase from 6 to >6 as predicted by molecular dynamics simulations (Ghosh and Karki, 2018). Our observations suggest an important role for aluminum in densification of aluminosilicate at the deep lower mantle, which might imply a dense aluminosilicate magma with negative buoyancy.

Extensive experimental studies on the structure and density of silicate glasses as laboratory analogs of natural silicate melts have attempted to address the nature of dense silicate melts that may be present at the base of the mantle. Previous ultrahigh-pressure experiments, however, have been performed on simple systems such as SiO2 or MgSiO3, and experiments in more complex system have been conducted under relatively low-pressure conditions below 60 GPa. The effect of other metal cations on structural changes that occur in dense silicate glasses under ultrahigh pressures has been poorly understood. Here, we used a Brillouin scattering spectroscopic method up to pressures of 196.9 GPa to conduct in situ high-pressure acoustic wave velocity measurements of SiO2-Al2O3 glasses in order to understand the effect of Al2O3 on pressure-induced structural changes in the glasses as analogs of aluminosilicate melts. From 10 to 40 GPa, the transverse acoustic wave velocity (VS) of Al2O3-rich glass (SiO2 + 20.5 mol% Al2O3) was greater than that of Al2O3-poor glass (SiO2 + 3.9 mol% Al2O3). This result suggests that SiO2-Al2O3 glasses with higher proportions of Al ions with large oxygen coordination numbers (5 and 6) become elastically stiffer up to 40 GPa, depending on the Al2O3 content, but then soften above 40 GPa. At pressures from 40 to ~100 GPa, the increase in VS with increasing pressure became less steep than below 40 GPa. Above ~100 GPa, there were abrupt increases in the P-VS gradients (dVS/dP) at 130 GPa in Al2O3-poor glass and at 116 GPa in Al2O3-rich glass. These changes resemble previous experimental results on SiO2 glass and MgSiO3 glass. Given that changes of dVS/dP have commonly been related to changes in the Si-O coordination states in the glasses, our results, therefore, may indicate a drastic structural transformation in SiO2-Al2O3 glasses above 116 GPa, possibly associated with an average Si-O coordination number change to higher than 6. Compared to previous acoustic wave velocity data on SiO2 and MgSiO3 glasses, Al2O3 appears to promote a lowering of the pressure at which the abrupt increase of dVS/dP is observed. This suggests that the Al2O3 in silicate melts may help to stabilize those melts gravitationally in the lower mantle.

The global water cycle in the Earth is one of the most important issues in geodynamics, because water can affect the physical and rheological properties of the mantle. However, it is still a matter of debate whether water can be transported into the lower mantle and core. Here we report a new reaction between aluminous perovskite and water to form alumina-depleted perovskite and hydrous δ-phase AlOOH-MgSiO2(OH)2 along the mantle geotherm in the lower mantle. Chemical analysis of the coexisting phases showed that the perovskite and post-perovskite phases were depleted in Al2O3, whereas hydrous δ-phase contains at least 44 mol% of MgSiO2(OH)2 component at 68 GPa and 2010 K, and 23 mol% of this component at 128 GPa and 2190 K. The present experiments revealed that hydrous δ-phase AlOOH-MgSiO2(OH)2 can coexist with alumina-depleted MgSiO3 perovskite or post-perovskite under the lower mantle conditions along the slab geotherm. Thus this hydrous phase in the slabs can transport water into the base of the lower mantle. © 2014 Elsevier B.V.