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.

The (plagioclase) lherzolitic shergottite Northwest Africa (NWA) 7397 consists of poikilitic and non-poikilitic lithologies. Coarse-grained low-Ca pyroxene oikocrysts enclose olivine and chromite grains in the poikilitic lithology. The major constituents of the non-poikilitic lithology are olivine, Ca-pyroxene, and plagioclase. Minor amounts of chromite, ilmenite, alkali feldspar, Ca-phosphate, and iron-sulfide are included in the non-poikilitic lithology. Most plagioclase grains in the non-poikilitic lithology have become maskelynite. A melt pocket occurs in the non-poikilitic lithology. Plagioclase in contact with the melt pocket has dissociated into zagamiite + stishovite. Apatite and merrillite entrained in the melt pocket have transformed into tuite. Olivine in contact with the melt pocket has dissociated into bridgmanite (almost vitrified) + ferroan-periclase. Alteration products, iron oxides and hydroxides, also occur in the dissociated olivine although it is not clear when the aqueous alteration occurred. The dissociation reactions of olivine and plagioclase into the high-pressure polymorphs (bridgmanite, ferroan-periclase, zagamiite, and stishovite) are found from lherzolitic shergottites for the first time. The estimated peak shock-pressure and -temperature conditions recorded in melt pockets of NWA 7397 are similar to 23 GPa and 2,000 degrees C at least, respectively, based on the high-pressure mineral assemblages.

<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.

The roles of water in the mantle transition zone, lower mantle, and the core-mantle boundary are investigated. The evidence for a wet mantle transition zone has been suggested based on hydrous mineral inclusions in diamond. Seismic wave velocity and electrical conductivity profiles together with mineral physics data are consistent with existence of stagnant slabs in a wet mantle transition zone. The transition zone may contain continental crustal components in these stagnant slabs. Dense hydrous magmas may exist at the base of the upper mantle. Fluids or volatile-rich magmas may also exist at the top of the lower mantle due to the large contrast in water contents between the mineral assemblages in the mantle transition zone and the lower mantle, and the crossing of the convective descent of the cold hydrated materials. Dense magmas are not likely to be formed at the top of the lower mantle and hydrous magmas generated in this region move upwards and metasomatize the overlying mantle transition zone. Water can be transported deeper into the lower mantle by gravitational collapse of the stagnant slabs, which supply water into the lower mantle, including the core-mantle boundary. Hydrous δ-H solid solution may be the most important hydrous phase in lower mantle, and existence of this phase reduces the aluminum content in coexisting bridgmanite and post-perovskite, and thus modifies the physical properties of the lower mantle. Hydrous δ-H solid solution can accumulate at the base of the lower mantle. The iron-water reaction at the core-mantle boundary can also create pyrite-type FeOOH which can be a potential candidate material for the ultralow velocity zone (ULVZ).

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.