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Thermoelastic properties of mantle candidate minerals are essential to our understanding of geophysical phenomena, geochemistry, and geodynamic evolutions of the silicate Earth. However, the lower-mantle mineralogy remains much debated due to the lack of single-crystal elastic moduli (Cij) and aggregate sound velocities of (Al,Fe)-bearing bridgmanite, the most abundant mineral of the planet, at the lower mantle pressure-temperature (P-T) conditions. Here we report single-crystal Cij of (Al,Fe)-bearing bridgmanite, Mg0.88Fe0.1Al0.14Si0.90O3 (Fe10-Al14-Bgm) with Fe3+/ΣFe = ~0.65, up to ~82 GPa using X-ray diffraction (XRD), Brillouin light scattering (BLS), and impulsive stimulated light scattering (ISLS) measurements in diamond-anvil cells (DACs). Two crystal platelets with orientations of (–0.50, 0.05, –0.86) and (0.65, –0.59, 0.48), that are sensitive to deriving all nine Cij, are used for compressional and shear wave velocity (νP and νS) measurements as a function of azimuthal angles over 200° at each experimental pressure. Our results show that all Cij of singe-crystal Fe10-Al14-Bgm increase monotonically with pressure with small uncertainties of 1–2% (±1σ), except C55 and C23, which have uncertainties of 3–4%. Using the third-order Eulerian finite-strain equations to model the elasticity data yields the aggregate adiabatic bulk and shear moduli and respective pressure derivatives at the reference pressure of 25 GPa: KS = 326 ± 4 GPa, µ = 211 ± 2 GPa, KS′ = 3.32 ± 0.04, and µ′ = 1.66 ± 0.02 GPa. The high-pressure aggregate νS and νP of Fe10-Al14-Bgm are 2.6–3.5% and 3.1–4.7% lower than those of MgSiO3 bridgmanite end-member, respectively. These data are used with literature reports on bridgmanite with different Fe and Al contents to quantitatively evaluate pressure and compositional effects on their elastic properties. Comparing with one-dimensional seismic profiles, our modeled velocity profiles of major lower-mantle mineral assemblages at relevant P-T suggest that the lower mantle could likely consist of about 89 vol% (Al,Fe)-bearing bridgmanite. After considering uncertainties, our best-fit model is still indistinguishable from pyrolitic or chondritic models.

Acoustic compressional and shear wave velocities (VP, VS) of anhydrous (AHRG) and hydrous rhyolitic glasses (HRG) containing 3.28 wt% (HRG-3) and 5.90 wt% (HRG-6) total water concentration (H2Ot) have been measured using Brillouin light scattering (BLS) spectroscopy up to 3 GPa in a diamond-anvil cell at ambient temperature. In addition, Fourier-transform infrared (FTIR) spectroscopy was used to measure the speciation of H2O in the glasses up to 3 GPa. At ambient pressure, HRG-3 contains 1.58 (6) wt% hydroxyl groups (OH–) and 1.70 (7) wt% molecular water (H2Om) while HRG-6 contains 1.67 (10) wt% OH– and 4.23 (17) wt% H2Om where the numbers in parentheses are ±1σ. With increasing pressure, very little H2Om, if any, converts to OH– within uncertainties in hydrous rhyolitic glasses such that HRG-6 contains much more H2Om than HRG-3 at all experimental pressures. We observe a nonlinear relationship between high-pressure sound velocities and H2Ot, which is attributed to the distinct effects of each water species on acoustic velocities and elastic moduli of hydrous glasses. Near ambient pressure, depolymerization due to OH– reduces VS and G more than VP and KS. VP and KS in both anhydrous and hydrous glasses decrease with increasing pressure up to ~1–2 GPa before increasing with pressure. Above ~1–2 GPa, VP and KS in both hydrous glasses converge with those in AHRG. In particular, VP in HRG-6 crosses over and becomes higher than VP in AHRG. HRG-6 displays lower VS and G than HRG-3 near ambient pressure, but VS and G in these glasses converge above ~2 GPa. Our results show that hydrous rhyolitic glasses with ~2–4 wt% H2Om can be as incompressible as their anhydrous counterpart above ~1.5 GPa. The nonlinear effects of hydration on high-pressure acoustic velocities and elastic moduli of rhyolitic glasses observed here may provide some insight into the behavior of hydrous silicate melts in felsic magma chambers at depth.

By using neutron Laue diffraction, strong hydrogen bonding was observed in the framework structure of phase E of dense hydrous magnesium silicate. Such bonding plays a crucial role in stabilizing large amounts of hydrogen in the crystal structures of minerals under high-pressure and high-temperature conditions inside deep Earth., A large amount of hydrogen circulates inside the Earth, which affects the long-term evolution of the planet. The majority of this hydrogen is stored in deep Earth within the crystal structures of dense minerals that are thermodynamically stable at high pressures and temperatures. To understand the reason for their stability under such extreme conditions, the chemical bonding geometry and cation exchange mechanism for including hydrogen were analyzed in a representative structure of such minerals (i.e. phase E of dense hydrous magnesium silicate) by using time-of-flight single-crystal neutron Laue diffraction. Phase E has a layered structure belonging to the space group R 3 m and a very large hydrogen capacity (up to 18% H2O weight fraction). It is stable at pressures of 13–18 GPa and temperatures of up to at least 1573 K. Deuterated high-quality crystals with the chemical formula Mg2.28Si1.32D2.15O6 were synthesized under the relevant high-pressure and high-temperature conditions. The nuclear density distribution obtained by neutron diffraction indicated that the O—D dipoles were directed towards neighboring O2− ions to form strong interlayer hydrogen bonds. This bonding plays a crucial role in stabilizing hydrogen within the mineral structure under such high-pressure and high-temperature conditions. It is considered that cation exchange occurs among Mg2+, D+ and Si4+ within this structure, making the hydrogen capacity flexible.

During the decompression of plastically deformed glasses at room temperature, some aspects of irreversible densification may be preserved. This densification has been primarily attributed to topological changes in glass networks. The changes in short-range structures like cation coordination numbers are often assumed to be relaxed upon decompression. Here the NMR results for aluminosilicate glass upon permanent densification up to 24 GPa reveal noticeable changes in the Al coordination number under pressure conditions as low as ∼6 GPa. A drastic increase in the highly coordinated Al fraction is evident over only a relatively narrow pressure range of up to ∼12 GPa, above which the coordination change becomes negligible up to 24 GPa. In contrast, Si coordination environments do not change, highlighting preferential coordination transformation during deformation. The observed trend in the coordination environment shows a remarkable similarity to the pressure-induced changes in the residual glass density, yielding a predictive relationship between the irreversible densification and the detailed structures under extreme compression. The results open a way to access the nature of plastic deformation in complex glasses at room temperature.

In this study, we investigated the single-crystal elasticity of (Al,Fe)-bearing bridgmanite (Bgm) with chemical compositions of Mg 0.95 Fe 0.033 2 + Fe 0.027 3 + Al0.04Si0.96O3 (Fe6-Al4-Bgm) and Mg 0.89 Fe 0.024 2 + Fe 0.096 3 + Al0.11Si0.89O3 (Fe12-Al11-Bgm) using combined experimental results from Brillouin light scattering (BLS), impulsive stimulated light scattering (ISLS), and X-ray diffraction (XRD) measurements in diamond anvil cells at 25 and 35 GPa. Based on experimentally measured compressional and shear wave velocities ( V P , V S ) as a function of azimuthal angles within selected crystal platelets that are sensitive to derivation of nine elastic constants for each composition, we reliably derived the full elastic constants of Fe6-Al4-Bgm and Fe12-Al11-Bgm at the two experimental pressures. Our results show that the combined Fe and Al substitution results in a reduction of both V S and V P in Fe12-Al11-Bgm up to 2.6(±0.5)% and 1.5(±0.3)%, respectively, compared with those in Fe6-Al4-Bgm at the experimental pressures. In particular, we observed strong combined Fe and Al effects on V S splitting anisotropy of (Al,Fe)-bearing Bgm at the two experimental pressures: Fe6-Al4-Bgm exhibits the highest V S splitting anisotropy of ∼8.23-9.0% along the [001] direction, while the direction shifts to the midway between [100] and [001] directions for Fe12-Al11-Bgm with V S splitting anisotropy of ∼7.68-11.06%. Combining the single-crystal elasticity data of Fe6-Al4-Bgm and Fe12-Al11-Bgm with the crystallographic preferred orientation (CPO) results of deformed Bgm at relevant lower-mantle pressure-temperature (P-T) conditions from literature, we modeled the seismic V S radial anisotropy of deformed (Al,Fe)-bearing Bgm near a subducting slab at conditions relevant to the topmost lower mantle. Taking into account the Fe and Al contents in (Al,Fe)-bearing Bgm with depth in the Earth's topmost lower mantle, the results of our model show that the deformation of Fe6-Al4-Bgm and Fe12-Al11-Bgm crystals would produce ∼0.9% and ∼0.8% V S radial anisotropy at depths of ∼670 and ∼920 km, respectively. These findings provide mineral physics explanations to the distinct seismically-detected V S radial anisotropies at the topmost lower mantle near subducted slabs, especially in the Tonga-Kermadec subduction region.

In situ femtosecond x-ray diffraction measurements and ab initio molecular dynamics simulations were performed to study the liquid structure of tantalum shock released from several hundred gigapascals (GPa) on the nanosecond timescale. The results show that the internal negative pressure applied to the liquid tantalum reached $\ensuremath{-}5.6\text{ }(0.8)\text{ }\text{ }\mathrm{GPa}$, suggesting the existence of a liquid-gas mixing state due to cavitation. This is the first direct evidence to prove the classical nucleation theory which predicts that liquids with high surface tension can support GPa regime tensile stress.

Earth’s mantle composition is essential to our understanding of its physics and dynamics. Here we report single-crystal elasticity (Cij) of (Al,Fe)-bearing bridgmanite, Mg0.88Fe0.1Al0.14Si0.90O3 with Fe3+/∑Fe=~0.65, up to ~82 GPa measured in diamond anvil cells. Together with heat capacity measurements on bridgmanite and ferropericlase, we develop a fully internally-consistent thermoelastic model to simultaneously evaluate lower-mantle mineralogy and geotherm via comparisons of P-wave, S-wave velocities, and density (VP, VS, and ρ) with one-dimensional seismic profiles. Our best-fit model demonstrates the lower mantle consists of ~89 vol% (Al,Fe)-bearing bridgmanite, ~4 vol% ferropericlase, and ~7 vol% CaSiO3 perovskite. A chemically layered mantle with pyrolitic upper mantle and bridgmanite-predominant lower mantle would display ~3.2(±1.5)%, ~5.2(±1.5)%, and ~5.0(±1.0)% jumps in VP, VS, and ρ, respectively, across the 660-km discontinuity, which are well consistent with seismic reflection observations. The lower mantle could have become bridgmanite-predominant via accumulations of ancient silica-rich materials, which helps explain current deep-Earth seismic and geochemical signatures.

A dense magnesium iron silicate polymorph with a structure intermediate between olivine, ringwoodite, and wadsleyite was theoretically predicted about four decades ago. As this group of minerals constitute the major component of shocked meteorites, constraining their transitional forms and behaviour is of potential importance for understanding impact events on their parent bodies. Here we use high-resolution transmission electron microscopy techniques and single-crystal X-ray diffraction analyses to identify naturally occurring examples of this mineral – recently named poirierite – in shocked chondritic meteorites. We observe nanoscale lamellar poirierite topotactically intergrown within wadsleyite, and additionally within ringwoodite as recently reported. Our results confirm the intermediate structure of poirierite and suggest it might be a relay point in the shear transformations between its polymorphs. We propose that poirierite formed during rapid decompression at relatively low temperature in retrograde shock metamorphism of the meteorites. Naturally occurring poirierite is identified in three shocked meteorites where it probably formed as a metastable phase during rapid phase transitions between olivine and its high-pressure polymorphs, according to high-resolution crystallographic analyses

Iron is the most abundant transition metal in the Earth's interior, yet considerable uncertainties remain as to why mantle-derived rocks have diverse iron isotopic compositions. In particular, the isotopic fractionation behavior of iron in the lower-mantle minerals bridgmanite and ferropericlase are largely unexplored. The reason is that it is challenging to study isotopic fractionation at the high pressures relevant to the deep mantle. Here we report in situ measurements of the mean force constants of iron bonds in these minerals pressurized in diamond anvil cells using the technique of nuclear resonant inelastic X-ray scattering (NRIXS). We find that the transition from high- to low-spin iron in ferropericlase ((Mg0.75Fe0.25)O) at approximately 60 GPa drastically stiffens its iron bonds in the low-spin state. The mean force constant of iron bonds in both Fe-bearing and (Fe,Al)-bearing bridgmanite exhibits softening by 21% at approximately 40–60 GPa, which seems to be associated with changes in the iron local environment during the transition from low to high quadrupole splitting states. These results indicate that in the lower mantle, low-spin ferropericlase is enriched in heavy iron isotopes relative to bridgmanite and metallic iron by +0.15‰ and +0.12‰, respectively. Based on these results, we investigate whether terrestrial magma ocean crystallization could have fractionated iron isotopes. We conclude that this process cannot be responsible for the heavy iron isotope enrichment measured in terrestrial basalts.

Preferential iron partitioning into melt during melting and crystallization of lower mantle minerals – bridgmanite and ferropericlase – can play a critical role in our understanding of the origin of the early Earth and its evolution to form chemically and seismically distinct regions in the present lowermost mantle. Of particular interest is the consequence of iron spin crossover in ferropericlase on the physical and chemical properties of the molten materials under relevant pressure–temperature (P–T) conditions of the lowermost mantle. However, the spin crossover in liquid (Mg, Fe)O and its effects on melting curves, iron partitioning, melt density – and thus the evolution of an early basal magma ocean – remain poorly studied. Here we conducted high P–T melting experiments on ferropericlase with a starting composition of (Mg0.86Fe0.14)O using synchrotron X-ray diffraction up to ∼120 GPa and ∼5400 K in laser-heated diamond anvil cells, together with chemical analyses on quenched samples using focused ion beam and energy dispersive spectroscopy technique. An ideal solid solution model could be satisfactorily used to fit the experimental data of the liquidus and solidus of (Mg, Fe)O for pure high-spin (HS, below ∼83 GPa), and low-spin (LS, above ∼120 GPa) states, respectively. The experimental solidus and liquidus at 99 GPa and ∼4000–5200 K strongly deviate from ideal solid solution behavior for pure HS and LS states alone, but can be qualitatively explained using a thermodynamics model for a mixture of HS and LS states across the spin crossover. We found that LS (Mg, Fe)O exhibits ∼6–8% lower solidus and liquidus temperature than its HS counterpart. Furthermore, our results show that iron preferentially partitions into melt within the spin crossover to generate iron-rich LS melt. Such iron-rich LS (Mg, Fe)O is ∼27(±5)% denser than materials expected for lowermost mantle and could potentially persist as residual melt in the lowermost mantle at the late stage of magma ocean crystallization. Modeled results indicate that the existence of the dense, iron-rich LS (Mg, Fe)O melt in the lowermost mantle could provide plausible explanations for characteristic seismological signatures of ultra-low velocity zones (ULVZs).

Quasielastic neutron scattering is applied to analyse the atomic scale hydrogen transport processes occurring within the crystal lattices of hydrous minerals. Two types of transport processes were observed in Mg(OH)2, which has a prototypical layered hydrogen lattice structure., It is demonstrated that quasielastic neutron scattering is a novel and effective method to analyse atomic scale hydrogen transport processes occurring within a mineral crystal lattice. The method was previously characterized as sensitive for analysing the transport frequency and distance of highly diffusive hydrogen atoms or water molecules in condensed matter. Here are shown the results of its application to analyse the transport of much slower hydrogen atoms which are bonded into a crystal lattice as hydroxyls. Two types of hydrogen transport process were observed in brucite, Mg(OH)2: a jump within a single two-dimensional layer of the hydrogen lattice and a jump into the next nearest layer of it. These transport processes observed within the prototypical structure of brucite have direct implications for hydrogen transport phenomena occurring within various types of oxides and minerals having layered structures.

A single-crystal neutron diffraction study was performed on hydrogen incorporation in ringwoodite, which is the most important host mineral of water in the Earth’s deep mantle. Its hydrogen incorporation mechanism, bonding geometry and occupancy at the relevant hydrogen site were unambiguously revealed., Ringwoodite [(Mg,Fe2+)2SiO4 spinel] has been considered as one of the most important host minerals of water in the Earth’s deep mantle. Its extensive hydration was observed in high-pressure synthesis experiments and also by its natural occurrence. Water can dissolve into ringwoodite as structurally bound hydrogen cations by substituting other cations, although the hydrogen site and its occupancy remain unclear. In this study, neutron time-of-flight single-crystal Laue diffraction analysis was carried out for synthetic hydrous ringwoodite. Hydrogen cations were found only in the sites in MgO6 octahedra in the ringwoodite structure, which compensated the reduced occupancies of both magnesium and silicon cations. The refined cation occupancies suggest that the most plausible hydration mechanism is that three hydrogen cations simultaneously occupy an MgO6 octahedron, whereas four such hydrogenated octahedra surround a vacant SiO4 tetrahedron.

Complex seismic, thermal, and chemical features have been reported in Earth's lowermost mantle. In particular, possible iron enrichments in the large low shear-wave velocity provinces (LLSVPs) could influence thermal transport properties of the constituting minerals in this region, altering the lower mantle dynamics and heat flux across core-mantle boundary (CMB). Thermal conductivity of bridgmanite is expected to partially control the thermal evolution and dynamics of Earth's lower mantle. Importantly, the pressure-induced lattice distortion and iron spin and valence states in bridgmanite could affect its lattice thermal conductivity, but these effects remain largely unknown. Here we precisely measured the lattice thermal conductivity of Fe-bearing bridgmanite to 120 gigapascals using optical pump-probe spectroscopy. The conductivity of Fe-bearing bridgmanite increases monotonically with pressure, but drops significantly around 45 gigapascals due to pressure-induced lattice distortion on iron sites. Our findings indicate that lattice thermal conductivity at lowermost mantle conditions is twice smaller than previously thought. The decrease in the thermal conductivity of bridgmanite in mid-lower mantle and below would promote mantle flow against a potential viscosity barrier, facilitating slabs crossing over the 1000-km depth. Modeling of our results applied to LLSVPs shows that variations in iron and bridgmanite fractions induce a significant thermal conductivity decrease, which would enhance internal convective flow. Our CMB heat flux modeling indicates that, while heat flux variations are dominated by thermal effects, variations in thermal conductivity also play a significant role. The CMB heat flux map we obtained is substantially different from those assumed so far, which may influence our understanding of the geodynamo.

In this study, we performed synchrotron X-ray diffraction (XRD) and Mossbauer spectroscopy (SMS) measurements on two single-crystal bridgmanite samples [ Mg 0.94 Fe 0.04 2 + Fe 0.02 3 + Al 0.01 Si 0.99 O 3 ( Bm 6 ) and Mg 0.89 Fe 0.024 2 + Fe 0.096 3 + Al 0.11 Si 0.89 O 3 ( Al-Bm 11 ) ] to investigate the combined effect of Fe and Al on the hyperfine parameters, lattice parameters, and equation of state (EoS) of bridgmanite up to 130 GPa. Our SMS results show that Fe 2+ and Fe 3+ in Bm6 and Al-Bm11 are predominantly located in the large pseudo-dodecahedral sites (A-site) at lower-mantle pressures. The observed drastic increase in the hyperfine quadrupole splitting (QS) between 13 and 32 GPa can be associated with an enhanced local distortion of the A-site Fe 2+ in Bm6. In contrast to Bm6, the enhanced lattice distortion and the presence of extremely high QS values of Fe 2+ are not observed in Al-Bm11 at high pressures. Our results here support the notion that the occurrence of the extremely high QS component of approximately 4 mm/s in bridgmanite is due to the lattice distortion in the high-spin (HS) A-site Fe 2+ , instead of the occurrence of the intermediate-spin state. Both A-site Fe 2+ and Fe 3+ in Bm6 and Al-Bm11 remain in the HS state at lower-mantle pressures. Together with XRD results, we present the first experimental evidence that the enhanced lattice distortion of A-site Fe 2+ does not cause any detectable variation in the EoS parameters, but is associated with anomalous variations in the bond length, tilting angle, and shear strain in the octahedra of Bm6. Analysis of the obtained EoS parameters of bridgmanite at lower-mantle pressures indicates that the substitution of Fe in bridgmanite will cause an enhanced density and a reduced bulk sound velocity ( V Φ ), whereas the Al and Fe substitution has a reduced effect on density and a negligible effect on V Φ . These experimental results provide new insight into the correlation between lattice, hyperfine, and EoS parameters of bridgmanite in the Earth’s lower mantle.

Spin and valence states of iron in single-crystal bridgmanite (Mg0.89Fe0.12Al0.11Si0.89O3) are investigated using X-ray emission and Mossbauer spectroscopies with laser annealing up to 115 GPa. The results show that Fe predominantly substitutes for Mg2+ in the pseudo-dodecahedral A site, in which 80% of the iron is Fe3+ that enters the lattice via the charge-coupled substitution with Al3+ in the octahedral B site. The total spin momentum and hyperfine parameters indicate that these ions remain in the high-spin state with Fe2+ having extremely high quadrupole splitting due to lattice distortion. (Al,Fe)-bearing bridgmanite is expected to contain mostly high-spin, A-site Fe3+, together with a smaller amount of A-site Fe2+, that remains stable throughout the region. Even though the spin transition of B-site Fe3+ in bridgmanite was reported to cause changes in its elasticity at high pressures, (Fe,Al)-bearing bridgmanite with predominantly A-site Fe will not exhibit elastic anomalies associated with the spin transition.

We discuss extended X-ray absorption fine structure (EXAFS) measurements on light element compounds under high pressure. An absorption edge in light elements, ≤2 keV in energy, is measured by X-ray Raman scattering using 20 keV X-rays. To enhance the intensity, a large-culet, sintered polycrystalline diamond anvil cell is used, so that a scattering volume is maximized. For a pressure monitor, diffraction lines from NaCl powders compressed together with the sample are measured through an X-ray Raman spectrometer. With this set-up, we have measured EXAFS features at the oxygen K-edge in MgSiO3 glass up to 30 GPa. As the pressure increases, the oscillation pattern drastically changes, associated with the densification of the glass.

Iron may critically influence the physical properties and thermochemical structures of Earth’s lower mantle. Its effects on thermal conductivity, with possible consequences on heat transfer and mantle dynamics, however, remain largely unknown. We measured the lattice thermal conductivity of lower-mantle ferropericlase to 120 GPa using the ultrafast optical pump-probe technique in a diamond anvil cell. The thermal conductivity of ferropericlase with 56% iron significantly drops by a factor of 1.8 across the spin transition around 53 GPa, while that with 8–10% iron increases monotonically with pressure, causing an enhanced iron substitution effect in the low-spin state. Combined with bridgmanite data, modeling of our results provides a self-consistent radial profile of lower-mantle thermal conductivity, which is dominated by pressure, temperature, and iron effects, and shows a twofold increase from top to bottom of the lower mantle. Such increase in thermal conductivity may delay the cooling of the core, while its decrease with iron content may enhance the dynamics of large low shear-wave velocity provinces. Our findings further show that, if hot and strongly enriched in iron, the seismic ultralow velocity zones have exceptionally low conductivity, thus delaying their cooling.

Hydrogen site positions and occupancy in the crystal structure of dense hydrous magnesium silicate (DHMS) phase E were determined for the first time by pulsed neutron powder diffraction. A fully deuterated pure phase E powder sample, which had space group $$R\overline{3} m$$ and lattice parameters of a = 2.97065(8) A and c = 13.9033(4) A, was synthesized at 15 GPa and 1100 °C. Through quantitative evaluation of refined structure parameters obtained with sufficient spatial resolution and very high signal-to-background ratio, we conclude that the O–D dipoles in the refined phase E structure are tilted by 24° from the direction normal to the layers of edge-shared MgO6 octahedra (octahedral layers). The tilted dipole structure of phase E is in remarkable contrast to that of brucite, Mg(OH)2, which has dipoles exactly normal to the octahedral layer. This contrast exists because the O–Si–O bonding unique in the phase E structure connects two adjacent octahedral layers and thereby reduces the interlayer O···O distance. This shrinkage of the interlayer distance induces the tilting of the O–D dipole and also generates unique O–D···O hydrogen bonding connecting all the layers in the phase E structure.

Lattice thermal conductivity of ferropericlase and radiative thermal conductivity of iron bearing magnesium silicate perovskite (bridgmanite) – the major mineral of Earth’s lower mantle– have been measured at room temperature up to 30 and 46 GPa, respectively, using time-domain thermoreflectance and optical spectroscopy techniques in diamond anvil cells. The results provide new constraints for the pressure dependencies of the thermal conductivities of Fe bearing minerals. The lattice thermal conductivity of ferropericlase Mg0.9Fe0.1O is 5.7(6) W/(m * K) at ambient conditions, which is almost 10 times smaller than that of pure MgO; however, it increases with pressure much faster (6.1(7)%/GPa vs 3.6(1)%/GPa). The radiative conductivity of a Mg0.94Fe0.06SiO3 bridgmanite single crystal agrees with previously determined values for powder samples at ambient pressure; it is almost pressure-independent in the investigated pressure range. Our results confirm the reduced radiative conductivity scenario for the Earth’s lower mantle, while the assessment of the heat flow through the core-mantle boundary still requires in situ measurements at the relevant pressure–temperature conditions.

The presence of water in the Earth’s deep mantle is an issue of increasing interest in the field of highpressure mineralogy. An important task for further advancing research in the field is to create homogeneous single crystals of candidate deep-mantle water-bearing minerals of 1 mm or larger in size, which is required for applying them for the time-of-flight (TOF) single-crystal Laue diffraction method with a third-generation neutron instrument. In this study, we perform several experiments to demonstrate an improved methodology for growing hydrous crystals of such large sizes at relevant transition zone and lower-mantle conditions via very slow cooling over a maximum period of 1 day. Successfully synthesized crystals using this methodology include dense hydrous magnesium silicate (DHMS) phase E, hydrous wadsleyite, hydrous ringwoodite, and bridgmanite (silicate perovskite). It is also demonstrated that these hydrous crystals can be grown from deuterium enriched starting materials in addition to those having a natural hydrogen isotope ratio. Magnitudes of chemical and crystallographic heterogeneities of the product crystals were characterized by comprehensive analysis of X-ray precession photography, single-crystal X-ray diffraction (SCXRD), field-emission scanning electron microscope (FE-SEM), electron probe microanalyzer (EPMA), secondary ion mass spectroscopy (SIMS), powder X-ray diffraction (PXRD), and TOF neutron powder diffraction (TOF-NPD). The product crystals were confirmed to be inclusion free and crystallographically homogeneous. Compositional and isotopic differences of major elements and hydrogen isotope abundances were lower than 1 and 3%, respectively, among intracrystals and intercrystals within each recovered sample capsule. Phase E crystals up to 600 μm in the largest dimension were grown at a constant temperature of 1100 °C kept for 3 h. Using a lattice parameter-to-temperature relation of phase E, the thermal gradient in the sample capsules for the phase E synthesis has been evaluated to be 20 °C/mm. Hydrous wadsleyite crystals up to 1100 μm in the largest dimension were grown at 1390 °C with a temperature reduction of 70 °C during heating for 10 h. Hydrous ringwoodite crystals up to 1000 μm in the largest dimension were grown at around 1400 °C with a temperature reduction of 110 °C during heating for 12 h. Bridgmanite crystals up to 600 μm in the largest dimension were grown at 1700 °C with a temperature reduction of 30 °C during heating for 12 h. A TOF single-crystal diffraction instrument has been successfully used for analyzing one of the hydrous wadsleyite crystals, which demonstrated that single crystals appropriate for their expected usage are created using the method proposed in the present study.

High-pressure polymorphism of olivine (α-phase of Mg2SiO4) is of particular interest for geophysicists aiming to understand the structure and dynamics of the Earth’s interior because of olivine’s prominent abundance in the upper mantle. Therefore, natural and synthetic olivine polymorphs have been actively studied in the past half century. Here, we report a new high-pressure polymorph, the ε*-phase, which was discovered in a heavily shocked meteorite. It occurs as nanoscale lamellae and has a topotaxial relationship with the host ringwoodite (γ-phase of Mg2SiO4). Olivine in the host rock entrapped in a shock-induced melt vein initially transformed into polycrystalline ringwoodite through a nucleation and growth mechanism. The ringwoodite grains then coherently converted into the ε*-phase by shear transformation during subsequent pressure release. This intermediate metastable phase can be formed by all Mg2SiO4 polymorphs via a shear transformation mechanism. Here, we propose high-pressure transformations of olivine that are enhanced by diffusionless processes, not only in shocked meteorites but also in thick and cold lithosphere subducting into the deep Earth.

The volume diffusion coefficient of water in ice VI was determined in the pressure-temperature range of 1.3–1.9 GPa and 300–320 K by in situ isotope tracer diffusion experiments. We determined the activation energy of the volume diffusion to be 61.9 ± 9.5 kJ/mol. The viscosity of polycrystalline ice VI under diffusion creep was estimated from the diffusion coefficients based on the theory of the diffusion creep. From a compilation of viscosity values in the current diffusion creep regime and the viscosity previously determined by plastic deformation experiments of ice VI in a high stress regime (Durham et al. 1996), here we provide the relationships between viscosity, stress, and the average grain size of polycrystalline ice VI. The most plausible deformation mechanism of the layers of ice VI underneath the internal oceans in large icy bodies was inferred from the viscosity-stress-average grain size relationship. We also discuss the critical thickness of the ice VI layer, which determines the onset of thermal convection.

Hydrogen site positions and occupancies in the crystal structure of synthetic hydrous ringwoodite have been determined for the first time by high-resolution neutron powder diffraction conducted at the pulsed neutron source at Japan Proton Accelerator Research Complex (J-PARC). It is demonstrated that hydrogen exchanges not only with Mg or Fe but also with Si cations to form hydroxyl and hydrogen bonding, both within the relevant octahedra or tetrahedra created by their surrounding oxygen anions. The hydrated octahedra shrink, whereas the hydrated tetrahedra expand. The occurrence of simultaneous hydration of octahedra and tetraheda is thus unambiguously confirmed, whereby the latter plays an especially unique role in reducing seismic velocity and for enhancing the electrical conductivity of ringwoodite at high pressures and temperatures. These results provide distinctive implications for discussing the physical properties of hydrous ringwoodite occurring in the mantle transition zone and evaluating the actual water content in the zone.

A dynamic high-pressure experiment was performed on magnesium hydride to measure the Hugoniot equation-of-state point using laser-driven shock waves. By measuring the mean shock velocity of MgH2 and the time-resolved shock velocity of the quartz reference, the Hugoniot state of MgH2 at 150 GPa was determined. The reliability of the data is ensured by using another reference material, polystyrene, which is next to the sample and is compressed by the same. This work validates the experimental methodology to obtain further high-pressure data of MgH2 with high accuracy.

A new design of opposed anvil cell for time-of-flight neutron powder diffraction was prepared for use at advanced pulsed sources. A couple of single-crystal sapphire sphere anvils and a gasket of fully hardened Ti–Zr null alloy were combined to compress 35 mm3 of sample volume to 1 GPa and 11 mm3 to 2 GPa of pressures, respectively. A very high-quality powder diffraction pattern was obtained at Japan Proton Accelerator Research Complex for a controversial high pressure phase of methane hydrate. The counting statistics, resolution, absolute accuracy and d-value range of the pattern were all improved to be best suitable for precise structure refinement. The sample is optically accessible to be measured by Raman and fluorescence spectroscopy during and after compression. The current cell will be an alternative choice to study hydrogenous materials of complex structures that are stable at the described pressure regime.

Proton diffusion in brucite was investigated by conducting hydrogen-deuterium (H-D) exchange experiments using multi-anvil high-pressure apparatuses at pressures from 3 to 15 GPa and temperatures in the range of 750–1050 K. The diffusion couple was composed of a natural proton-dominated brucite single crystal surrounded by synthesized D-doped brucite polycrystals. Micro-Raman spectroscopy was used to determine the diffusion profiles of the samples. The D/H diffusion profile across the boundary between single crystal and polycrystalline D-doped brucite showed an asymmetric pattern characterized by faster diffusion in aggregates. The D/H interdiffusion rate determined from the analysis of the single crystal side indicates that the interdiffusion rate increases with increasing H/D ratio. The H-D interdiffusion rate in the direction perpendicular to the c -axis is about 0.5 orders of magnitude higher than that in the direction parallel to the c -axis. At 3 GPa, the H-D interdiffusion coefficients [ D (m2/s)] along and perpendicular to the c -axis of brucite at compositions of C ODnorm = 0.2 in the single-crystal region were determined to be 3.30(±1.77) ×10−11exp[−48.2(±5.8) (kJ/mol)/R T ] and 1.43(±1.33) ×10−9exp[−67.5(±23.2) (kJ/mol)/R T ], respectively. The H-D interdiffusion rate perpendicular to and along the c -axis increased about one order of magnitude by compression from 3 to 10 GPa, but the pressure enhancement became weaker above 10 GPa. From 10 to 15 GPa, there is almost no pressure dependence of proton diffusion for both directions. As pressure increases up to 10 GPa, enhancement of the proton migration is strongly correlated with the activation of the atomic interaction and decrease of O···O′ distance induced by compression. The positive pressure effect on the proton diffusion in brucite suggests that proton diffusion in higher-pressure hydrous phase becomes faster because of the shorter O···O′ distance.

The self-diffusion of ice VII in the pressure range of 5.5-17 GPa and temperature range of 400-425 K was studied using micro Raman spectroscopy and a diamond anvil cell. The diffusion was monitored by observing the distribution of isotope tracers: D2O and H2 (18)O. The diffusion coefficient of hydrogen reached a maximum value around 10 GPa. It was two orders of magnitude greater at 10 GPa than at 6 GPa. Hydrogen diffusion was much faster than oxygen diffusion, which indicates that protonic diffusion is the dominant mechanism for the diffusion of hydrogen in ice VII. This mechanism is in remarkable contrast to the self-diffusion in ice Ih that is dominated by an interstitial mechanism for the whole water molecule. An anomaly around 10 GPa in ice VII indicates that the rate-determining process for the proton diffusion changes from the diffusion of ionic defects to the diffusion of rotational defects, which was suggested by proton conductivity measurements and molecular dynamics simulations.

Evidence from seismological and mineralogical studies increasingly indicates that water from the oceans has been transported to the deep earth to form water-bearing dense mantle minerals. Wadsleyite [(Mg, Fe2+)2SiO4] has been identified as one of the most important host minerals incorporating this type of water, which is capable of storing the entire mass of the oceans as a hidden reservoir. To understand the effects of such water on the physical properties and chemical evolution of Earth’s interior, it is essential to determine where in the crystal structure the hydration occurs and which chemical bonds are altered and weakened after hydration. Here, we conduct a neutron time-of-flight single-crystal Laue diffraction study on hydrous wadsleyite. Single crystals were grown under pressure to a size suitable for the experiment and with physical qualities representative of wet, deep mantle conditions. The results of this neutron single crystal diffraction study unambiguously demonstrate the method of hydrogen incorporation into the wadsleyite, which is qualitatively different from that of its denser polymorph, ringwoodite, in the wet mantle. The difference is a vital clue towards understanding why these dense mantle minerals show distinctly different softening behaviours after hydration.

Microscopic dynamics of compressed molecular hydrogen is important for understanding its transport properties at high density as well as for improvement of hydrogen storage technology. To observe it at very high density, nuclear magnetic relaxation times of molecular hydrogen were measured at room temperature using a diamond anvil cell. Collision dynamics of the compressed H2 gas was determined at pressures to 1.8 GPa, and the results are in good agreement with the standard kinetic theory assuming hard-sphere molecules. Collision and diffusion dynamics of dense H2 in a H2O framework (filled-ice Ic hydrogen’s hydrate) were determined at pressures to 4.6 GPa, and it was demonstrated that active translational diffusion of H2 through the leaky H2O framework occurs throughout the observed pressure range.

type structure named “post-perovskite” (PPv) at 120 GPa and 2300 K by using a laser-heated diamond anvil cell (LHDAC). Therefore, it has been widely accepted that the ubiquitous presence of the PPv phase at the bottom of the lower mantle is responsible for for-mation of the D″ layer. Many seismic anomalies such as velocity jumps and anisotropy observed in the D″ layer (e.g., Wysession et al. 1998; Panning and Romanowicz 2004) can be explained by the physical properties of the PPv phase (Iitaka et al. 2004; Murakami et al. 2007a, 2007b; Merkel et al. 2007; Yamazaki et al. 2006a). In this context, the Clapeyron slope, d

ab ST rac T Electronic states of iron in Earth’s mantle minerals including ferropericlase, silicate perovskite, and post-perovskite have been previously investigated at high pressures and/or temperatures using various experimental techniques, including X-ray emission and Mossbauer spectroscopies. Although such methods have been used to infer changes in the electronic spin and valence states of iron in lower mantle minerals, they do not directly probe the 3d electronic states quantitatively. Here we use 1s2p resonant X-ray emission spectroscopy (RXES) at the Fe K pre-edge to directly probe and assess the 3d electronic states and the crystal-field splittings of Fe 2+ in the lower-mantle ferropericlase [(Mg0.75,Fe0.25) O] at pressures up to 90 GPa. The pre-edge features from X-ray absorption spectroscopy in the partial fluorescence yield (PFY-XAS) and RXES results explicitly show three excited states for high-spin Fe 2+ (a lower-energy 4 T1g state, a 4 T2g state, and a higher-energy 4 T1g state) and a single 2 Eg state for low-spin Fe 2+ , attributed to the (t2g) 0 (eg) 3 excited configuration. This latter feature begins to appear at 48 GPa and grows with pressure, while the peaks related to high-spin Fe 2+ vanish above 80 GPa. The observed pre-edge features are consistent with purely quadrupolar transitions resulting from the centrosymmetric character of the Fe 2+ site. The K pre-edge RXES spectra at the incident energy of

Micrometer- to nanometer-scale structures of the cut surfaces of single- and polycrystalline diamonds by a pulsed ultraviolet laser have been thoroughly investigated by scanning and transmission electron microscopy. Within the laser-cut grooves, the processed diamond surfaces are extensively covered with laser-modified debris which consists of complex layered units of graphite with various crystallinities. The units consist of 1) highly oriented graphite, 2) corrugated graphite, and 3) nanocrystalline graphite, which are sequentially located from the surface of the underlying diamond substrate to the center of the grooves. Detailed textural examinations revealed that the highly oriented graphite unit is a product of the initial graphitization of diamond by a solid-state diffusion process, whereas the latter two units are deposition products from the liquid and/or vapor phases of carbon in the later stage. The present study demonstrates that the laser-cutting of diamonds proceeds in a two-step process: 1) extensive graphitization of laser-scanning path and 2) subsequent sublimation of the pre-formed graphite. These processes are basically identical among the three different types of diamonds (single crystal type Ib, single crystal type IIa and nano-polycrystalline aggregate) tested in this study.

We have recently carried out a neutron powder diffraction experiment at high pressure using nano-polycrystalline diamond as opposed anvils. Although spurious diffraction peaks from these anvils are rather large, a few peaks from Pb sample with 0.7 mm3 volume were clearly observed. Future potential and problem of the current scheme toward neutron diffraction at the highest pressure regime have been demonstrated.

Laser beam micromachining was applied to super-hard nano-polycrystalline diamond (NPD) synthesized by the direct conversion of graphite at high pressure and high temperature. Three types of pulsed lasers were tested: nanosecond near-infrared, nanosecond near-ultraviolet, and femtosecond near-infrared lasers. The latter two were also applied for synthetic single crystal of diamond to compare the results with those of the NPD. It was demonstrated that the nanosecond near-infrared laser was the most efficient device for rough shaping of the NPD, while the ultraviolet and femtosecond lasers give satisfactory results for precise surface finishing of it. The properties of the laser-processed surfaces were analyzed by scanning and transmission electron microscopy, laser scanning microscopy, and micro Raman spectroscopy. These analyses demonstrated that the three types of lasers play different and complementary roles, and that their combination is the best suitable solution for micromachining of the hardest diamond into any desired shapes.

We have conducted laser processing of ultrahard nano-polycrystalline and single crystalline diamonds (NPD, SCD, respectively) using nano-pulsed near-ultraviolet laser, and the machining properties were compared through microstructural examinations by SEM, TEM and Raman spectroscopy. The cut depth of the laser-cut grooves was observed to be deeper for the NPD than for the SCD. This is probably due to the lower thermal conductivity feature of NPD, which provides higher absorption efficiency of the laser energy and decreases the laser ablation threshold. TEM cross-section observation showed that the processed grooves in the both types of diamonds are covered with identical laser-modified layers (~ 1 µm thick) composed of roughly oriented nanocrystalline graphite. A marked difference was observed between the laser-processed surfaces of NPD and SCD: in the former the diamond–graphite interface is almost linear and undamaged, whereas in the latter the boundary is slightly folded and significantly distorted. These textural features suggest that different laser-machining processes are involved between NPD and SCD in the microscopic scale. Our results demonstrate that pulsed laser can be used even more effectively for the fabrication of nano-polycrystalline diamond than the case for single crystal diamond.

In order to analyse the dynamics of molecules at high pressures, we are applying high-resolution nuclear magnetic resonance (NMR) spectroscopy for samples at gigapascals of pressures in a diamond anvil cell. Here we report some results of its application to various phases of hydrogen hydrates. These hydrates are stable only at high pressures and have never been analyzed in situ by NMR. The observed 1H-NMR spectra of filled-ice hydrogen hydrates at pressures 1 to 4 GPa gave anomalously narrow resonances of the H2 guests encapsulated into hydrogen-bonded H2O frameworks. Observed effects of pressure on NMR relaxation times of these H2 guests indicate that molecular rotation and translational diffusion contribute together to their spin relaxation. We determined the two motional correlation times of the H2 guest molecules as a function of pressure. From the diffusion correlation time, liquid-like fast diffusion of the H2 guests within the hydrate, of the order of 10-8 cm2/s, has been deduced. For hydrogen clathrate hydrate stable at much lower pressure, such diffusion is even faster, which was separately confirmed by pulsed-gradient field NMR method using a sapphire gas-pressure cell.