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Hydrogen may be recycled into the Earth's lower mantle by subduction and stabilized in solid solutions between phase H (MgSiO4H2), δ-AlOOH, ε-FeOOH, and SiO2 post-stishovite. In high-pressure oxyhydroxide phases, hydrogen is incorporated following the typical (OHO) sequence, adopting the asymmetric configuration O-H⋯O that evolves into a symmetric disordered state upon compression. Moreover, these iron-bearing aluminum oxyhydroxides [δ-(Al,Fe)OOH] present a structural phase transition from P21nm to Pnnm as pressure increases. Here, the single-crystal elasticity of the P21nm phase of δ-(Al0.97Fe0.03)OOH has been experimentally determined across the P21nm→Pnnm transition up to 7.94(2) GPa by simultaneous single-crystal X-ray diffraction (XRD) and Brillouin scattering at high pressures. The transition appears to be continuous, and it can be described with a second-, fourth-, and sixth-order terms Landau potential. Our results reveal an enhanced unit-cell volume compressibility, which is linked to an increase of the b- and a-axes linear compressibility in the P21nm phase of δ-(Al0.97Fe0.03)OOH prior to the transition. In addition, we observed the presence of elastic softening in the P21nm phase that mostly impacts the elastic stiffness coefficients c12, c22, and c23. The observed elastic anomalies cause a significant change in the pressure dependence of the adiabatic bulk modulus (KS). These results provide a better understanding of the relation between elasticity, P21nm→Pnnm structural phase transition, and hydrogen dynamics in δ-(Al0.97Fe0.03)OOH, which may be applied to other O-H⋯O-bettring phases. [ABSTRACT FROM AUTHOR]

The role of stishovite in transporting water in subducting slabs has been a subject of debate for several decades. Here we investigated stishovite's water solubility and its potential role in water transportation as a function of temperature from 1300 to 2100°C at 22 GPa. Under water‐saturated conditions, high‐quality, Al‐free stishovite single crystals were synthesized with multi‐anvil press, and their water contents were measured using infrared spectroscopy. The H2O solubility in stishovite increases from 128(20) to 521(47) wt. ppm with increasing temperature from 1300 to 1700°C and decreased to 145(26) wt. ppm at 2100°C. The maximum H2O content was about seven times larger than earlier high‐temperature multi‐anvil studies but significantly lower than recent laser‐heated diamond anvil cell and low‐temperature multi‐anvil studies. We suggest that Al‐free stishovite may not be a significant H2O carrier in subducting slabs, at least at the topmost lower mantle corresponding to our experimental conditions. Plain Language Summary: Stishovite is a high‐pressure phase of silicon dioxide (SiO2). It was recently reported that stishovite could contain a substantial amount of water based on diamond anvil cell experiments and, consequently, transport water into the lower mantle through subducting slabs. However, the water contents in their samples lack precise constrain due to the limited size of the recovered crystals, making them inaccessible for measurements using reliable FTIR spectroscopy or secondary‐ion mass spectroscopy. To settle this debate, we produced high‐quality single crystals of stishovite under water‐saturated conditions at high pressures and temperatures and measured their water contents using infrared spectroscopy. We discovered that stishovite contained only several hundred ppm of water under conditions at the top of the lower mantle. Therefore, it is likely that stishovite does not contain much water in the upper region of the lower mantle and does not transport water into the deep lower mantle. Key Points: H2O content in Al‐free stishovite increases with temperature up to 1700°C and decreases beyond this temperatureThe maximum H2O content in Al‐free stishovite at 22 GPa is 521 (47) wt. ppmAl‐free stishovite cannot be the major H2O carrier at least up to the uppermost lower mantle conditions [ABSTRACT FROM AUTHOR]

Stishovite, a rutile-structured polymorph of SiO2, is a main component of subducted basaltic lithologies in the lower mantle. At mid lower-mantle depths, a second-order ferroelastic transition to orthorhombic CaCl2-type (post-stishovite) structure occurs, causing extensive elastic shear softening. Previous studies showed that Al incorporation can decrease the transition pressure, while it is still debated whether H has a similar effect. Here we report the equations of state, structural evolution, and phase transformation of Si0.948Al0.052O1.983H0.018 (Al5) stishovite and Si0.886Al0.114O1.980H0.074 (Al11) post-stishovite samples using diamond-anvil cells in combination with synchrotron X-ray diffraction and Raman spectroscopy. The Al5 sample transformed to the orthorhombic polymorph upon compression to 16 GPa, displaying a drop of ~12% in its bulk modulus across the transformation. The Al11 sample did not undergo any phase transition in the pressure range investigated. Single-crystal structural refinements and Raman spectroscopy measurements on the Al5 sample show that the soft optic mode B1g is decoupled from the tetragonal-to-orthorhombic structural transformation and shows a plateau in the stability field of post-stishovite, between 20 and 30 GPa. This observation indicates that the transformation is not pseudo-proper ferroelastic as in SiO2 stishovite and that existing Landau expansions are likely not applicable to H-rich Al-bearing silica samples. Using the equation of state parameters of orthorhombic Al5 and Al11 and literature data on SiO2 post-stishovite we then discuss the possibility of non-ideal mixing along the SiO2-AlOOH join. [ABSTRACT FROM AUTHOR]

Fe3O4 magnetite is an important mineral commonly found in various geological settings, including the planet Mars, whose thermoelastic properties at high pressure and temperature are still poorly constrained. We performed X-ray difraction measurements on natural magnetite using resistive-heated diamond-anvil cells up to 16 GPa and 1100 K. We fitted a thermal equation of state (EoS) to the collected data resulting in K0 = 182(1) GPa, K′0 = 4, θD = 660 K, γ = 1.8(1), and q = 2.7. Moreover, it was possible to explore the structural evolution of magnetite in detail using single-crystal measurements. Over the studied pressure and temperature range, we found no evidence of a transformation from an inverse to a normal spinel structure. The EoS parameters obtained in this study will be implemented into currently available databases for self-consistent thermodynamic modeling. In particular, our results are used to model and compare the sound wave velocities of a magnetite-bearing and magnetite-free martian upper mantle assemblage. We observe that the incorporation of magnetite reduces the sound wave velocities; however, the magnitude of the effect is below the current seismic detection limit of the InSight mission on Mars at the low abundance of magnetite expected in the martian mantle. [ABSTRACT FROM AUTHOR]

Orthorhombic CaFe2O4-structured (Cf) Na-rich aluminous silicate (space group Pbnm) is a major mineral of metabasaltic rocks at lower mantle conditions and can, therefore, significantly affect the physical properties of subducted oceanic crusts. We attempted to synthesize single crystals of Cf-type phases in the systems NaAlSiO4, NaAlSiO4-MgAl2O4, NaAlSiO4-MgAl2O4-Fe3O4, and NaAlSiO4- MgAl2O4-Fe3O4-H2O at 23-26 GPa and 1100-2200 °C. Under dry conditions, single crystals of Cf-type phase up to 100-150 ^m in size were recovered from 23 GPa and 2000-2200 °C. Single-crystal X-ray diffraction and composition analyses suggest that the synthesized Cf-type phases have a few percent of vacancies in the eightfold-coordinated site with Na, Mg, and Fe2+ and partially disordered Al and Si in the octahedral sites. Iron-bearing Cf-type phases have 32-34% Fe3+ that is hosted both in the octahedral sites and in the eightfold-coordinated site. In NaAlSiO4-MgAl2O4-Fe3O4-H2O system, no formation of Cf-type phase was observed at 24 GPa and 1100-2000 °C due to the formation of hydrous Na-rich melt and Al-rich oxides or hydroxides, suggesting the possible absence of Cf-type phase in the hydrous basaltic crust. The single-crystal syntheses of Cf-type phases will be useful for investigating their physical properties, potentially improving models of lower mantle structure and dynamics. [ABSTRACT FROM AUTHOR]

Three single crystals of CaTi[sub 2]O4 (CT)-type, CaCaFe2O4 (CF)-type, and new low-density CaFe2O4 (LD-CF) related MgAl2O4 were synthesized at 27 GPa and 2500 °C and also CT-type MgAl2O4 at 45 GPa and 1727 °C using conventional and advanced multi-anvil technologies, respectively. The structures of CT-type and LD-CF related MgAl2O4 were analyzed by single-crystal X-ray diffraction. The lattice parameters of the CT-type phases synthesized at 27 and 45 GPa were a = 2.7903(4), b = 9.2132(10), and c = 9.3968(12) Å, and a = 2.7982(6), b = 9.2532(15), and c = 9.4461(16) Å, respectively, (Z = 4, space group: Cmcm) at ambient conditions. This phase has an AlO6 octahedral site and an MgO8 bicapped trigonal prism with two longer cation-oxygen bonds. The LD-CF related phase has a novel structure with orthorhombic symmetry (space group: Pnma), and lattice parameters of a = 9.207(2), b = 3.0118(6), and c = 9.739(2) Å (Z = 4). The structural framework comprises tunnel-shaped spaces constructed by edge- and corner-sharing of AlO6 and a 4+1 AlO5 trigonal bipyramid, in which MgO5 trigonal bipyramids are accommodated. The CF-type MgAl2O4 also has the same space group of Pnma but a slightly different atomic arrangement, with Mg and Al coordination numbers of 8 and 6, respectively. The LD-CF related phase has the lowest density of 3.50 g/cm³ among MgAl2O4 polymorphs, despite its high-pressure synthesis from the spinel-type phase (3.58 g/cm³), indicating that the LD-CF related phase formed via back-transformation from a high-pressure phase during the recovery. Combined with the previously determined phase relations, the phase transition between CF-and CT-type MgAl2O4 is expected to have a steep Clapeyron slope. Therefore, CT-type phase may be stable in basaltic- and continental-crust compositions at higher temperatures than the average mantle geotherm in the wide pressure range of the lower mantle. The LD-CF related phase could be found in shocked meteorites and used for estimating shock conditions. [ABSTRACT FROM AUTHOR]

The combination of seismic observations and mineral physics data represents a unique tool to understand the structure and evolution of the deep Earth's interior. However, to date, elasticity data on both compressional (vP) and shear (vS) wave velocities of MgSiO3 bridgmanite are limited to shallow mantle conditions, hampering the resolution of mineral physics models. Here, we report the first single‐crystal measurements of vP and vS of MgSiO3 bridgmanite up to ∼79 GPa using high‐pressure Brillouin scattering and single‐crystal X‐ray diffraction in a diamond anvil cell. At shallow lower mantle pressures, the elastic anisotropy of MgSiO3 bridgmanite was found to be similar, albeit smaller than that of Fe,Al‐bearing bridgmanite of Kurnosov et al. (2017) but differed significantly from that proposed in the recent study of Fu et al. (2019). Using the elastic stiffness coefficients of bridgmanite obtained in this study at different pressures, we calculate the pressure dependence of the adiabatic bulk modulus, KS0 = 257.1(6) GPa, K'S0 = 3.71(4), and of the shear modulus, G0 = 175.6(2) GPa, G'0 = 1.86(1). These elastic parameters are included in a self‐consistent thermodynamic model to calculate seismic wave velocities along a lower mantle adiabat for a primitive upper mantle bulk composition in the FeO‐CaO‐MgO‐SiO2 system, which is currently the most complex system for which sufficient data exist. This preliminary model provides a good match to the vS and vP of 1D seismic models, implying that the composition of the lower mantle may be closer to pyrolite, rather than being more bridgmanite rich. Key Points: We measured both compressional and shear wave velocities of MgSiO3 bridgmanite up to mid‐lower mantle pressure for the first timeThe effect of chemistry on the elastic moduli and elastic anisotropy of bridgmanite at lower mantle pressures is discussedWe compare the seismic velocities for a simplified primitive upper mantle composition with 1D seismic models of Earth's lower mantle [ABSTRACT FROM AUTHOR]