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The interplay between crystal–melt and grain boundary interfaces in partially melted polycrystalline aggregates controls many physical properties of mantle rocks. To understand this process at the fundamental level requires improved knowledge about the interfacial structures and energetics. Here, we report the results of first-principles molecular dynamics simulations of two grain boundaries of (0l1)/[100] type for tilt angles of 30.4° and 49.6° and the corresponding solid–liquid interfaces in Mg₂SiO₄forsterite at the conditions of the upper mantle. Our analysis of the simulated position time series shows that structural distortions at the solid–liquid interfacial region are stronger than intergranular interfacial distortions. The calculated formation enthalpy of the solid–solid interfaces increases nearly linearly from 1.0 to 1.4 J/m²for the 30.4° tilt and from 0.8 to 1.0 J/m²for the 49.6° tilt with pressure from 0 to 16 GPa at 1500 K, being consistent with the experimental data. The solid–liquid interfacial enthalpy takes comparable values in the range 0.9 to 1.5 J/m²over similar pressure interval. The dihedral angle of the forsterite–melt system estimated using these interfacial enthalpies takes values in the range of 67° to 146°, showing a decreasing trend with pressure. The predicted dihedral angle is found to be generally larger than the measured data for silicate systems, probably caused by compositional differences between the simulation and the measurements.

 

Hydrogen solubility was determined in olivine and orthopyroxene under water‐saturated conditions atP = 3–5 GPa andT = 1373–1573 K. For olivine, polycrystalline samples were prepared from San Carlos olivine, and for orthopyroxene synthetic samples were prepared from oxide mixture containing 1.5–5 wt% of Al₂O₃. Olivine and orthopyroxene were placed next to each other and annealed under various pressure and temperature conditions for 3–5 hr. Hydrogen content was measured across each sample by FTIR spectroscopy. Under the water‐saturated conditions, the hydrogen solubility in olivine increases with pressure and temperature similar to previous results. Hydrogen solubility in Al₂O₃‐bearing orthopyroxene also increases with temperature and pressure for a fixed Al₂O₃content. Based on these observations we calculated the partition coefficients of hydrogen between orthopyroxene and olivine assuming the fugacity dependence of hydrogen solubility in olivine and Al₂O₃‐bearing orthopyroxene reported by previous studies. We find that the partition coefficient depends weakly on temperature but strongly on pressure and water fugacity. Our results are extended to an open system where Al₂O₃content in orthopyroxene changes with pressure and temperature. At relatively low pressures and low water fugacity (in the lithosphere (shallower than ∼50 km)), the partition coefficient is high and a majority of hydrogen is present in orthopyroxene. Consequently, the influence of water on the bulk physical properties is small. In contrast, at higher pressures and higher water fugacity (in the asthenosphere), the partition coefficient is smaller and a substantial amount of hydrogen is present in olivine. Consequently, hydrogen has a strong effect on the bulk properties of the asthenosphere reducing viscosity and increasing electrical conductivity.

 

Transient creep may play an important role in short‐term (small strain) time‐dependent deformation such as the post‐glacial rebound and the post‐seismic deformation. Among various mechanisms of transient creep, I focus on inter‐granular transient dislocation creep caused by plastic anisotropy in each grain. The essence of this process is the evolution of strain accommodation across grain‐boundaries from elastic to plastic (viscous) accommodation. Extending the previous studies, I show that evolution of strain accommodation leads to two asymptotic behaviors: at small strain, strain accommodation across grain‐boundaries is by elastic deformation and the strength is controlled largely by that of the soft slip system. In contrast, when strain exceeds a threshold value, strain accommodation occurs mostly by plastic deformation and the strength is largely controlled by that of the hard slip system. The model predicts that the threshold strain is on the order of elastic strain (depending also on the degree of plastic anisotropy) and the strength contrast between transient and steady‐state creep depends on the degree of plastic anisotropy of the crystal. Consequently, transient creep plays an important role in these short‐term time‐dependent processes. Since the soft and the hard slip systems have different properties, applications of rheological properties inferred from short‐term time‐dependent deformation to long‐term geodynamic processes needs to be made with a great care.

 

Water (H2O) as one of the most abundant fluids present in Earth plays crucial role in the generation and transport of magmas in the interior. Though hydrous silicate melts have been studied extensively, the experimental data are confined to relatively low pressures and the computational results are still rare. Moreover, these studies imply large differences in the way water influences the physical properties of silicate magmas, such as density and electrical conductivity. Here, we investigate the equation of state, speciation, and transport properties of water dissolved in Mg1-xFexSiO3 and Mg2(1-x)Fe2xSiO4 melts (for x = 0 and 0.25) as well as in its bulk (pure) fluid state over the entire mantle pressure regime at 2000 to 4000 K using first-principles molecular dynamics. The simulation results allow us to constrain the partial molar volume of the water component in melts along with the molar volume of pure water. The predicted volume of silicate melt+water solution is negative at low pressures and becomes zero above 15 GPa. Consequently, the hydrous component tends to lower the melt density to similar extent over much of the mantle pressure regime irrespective of composition. Our results also show that hydrogen diffuses fast in silicate melts and enhances the melt electrical conductivity in a way that differs from electrical conduction in the bulk water. The speciation of the water component varies considerably from the bulk water structure as well. Water is dissolved in melts mostly as hydroxyls at low pressure and as -O-H-O-, -O-H-O-H- and other extended species with increasing pressure. On the other hand, the pure water behaves as a molecular fluid below 15 GPa, gradually becoming a dissociated fluid with further compression. On the basis of modeled density and conductivity results, we suggest that partial melts containing a few percent of water may be gravitationally trapped both above and below the upper mantle-transition region. Moreover, such hydrous melts can give rise to detectable electrical conductance by means of electromagnetic sounding observations. 

The effect of pressure on grain‐growth kinetics of olivine was investigated up to 10 GPa at 1773 K under relatively water‐poor conditions. The results are interpreted using a relationto obtain the activation volume = 5.0 ± 1.1 cm³/mol forn = 2 or = 5.2 ± 1.1 cm³/mol forn = 3. The small activation volume means that grain‐growth kinetics in pure olivine aggregates is fast even in the dry deep upper mantle, implying that grain‐size is controlled by the pinning by other phases or by dynamic recrystallization except for the early stage after the phase transformation from wadsleyite in upwelling materials. The present results are applied to seismic wave attenuation that is likely controlled by grain‐boundary processes. The inferred peak in attenuation just below the oceanic lithosphere‐asthenosphere boundary from the NoMelt array is difficult to be explained by the pressure effects assuming the absorption band behavior because such a model requires a much larger activation volume than determined in this work and it also fails to explain high attenuation in the deep asthenosphere. This suggests that either melt accumulation or other processes such as elastically accommodated grain‐boundary sliding (EAGBS) is responsible for the peak in attenuation. The present results are also applied to EAGBS. We suggest that the deep upper mantle is likely to be relaxed by EAGBS, which implies that the shear velocity of the deep upper mantle can be several percent smaller than that inferred from single crystal elasticity.

 

High pressure and temperature experiments were carried out on the oxide mixtures corresponding to the bridgmanite stoichiometry under the hydrous shallow lower mantle conditions (24–25 GPa and 1673–1873 K with 5–10 wt. % of water in the starting material). Oxide mixtures investigated correspond to MgSiO₃, (Mg, Fe)SiO₃, (Mg, Al, Si)O₃, and (Mg, Fe, Al, Si)O₃. Melting was observed in all runs. Partitioning of various elements, including Mg, Fe, Si, and H is investigated. Melting under hydrous lower mantle conditions leads to increased (Mg + Fe)O/SiO₂in the melt compared to the residual solids. The residual solids often contain a large amount of stishovite, and the melt contains higher (Mg,Fe)O/SiO₂ratio than the initial material. (Mg + Fe)O‐rich hydrous melt could explain the low‐velocity anomalies observed in the shallow lower mantle and a large amount of stishovite in the residual solid may be responsible for the scattering of seismic waves in the mid‐lower mantle and may explain the “stishovite paradox. Since stishovite‐rich materials are formed only when silica‐rich source rock (MORB) is melted (not a typical peridotitic rock [bulk silicate Earth]), seismic scattering in the lower mantle provides a clue on the circulation of subducted MORB materials. To estimate hydrogen content, we use a new method of estimating the water content of unquenchable melts, and also propose a new interpretation of the significance of superhydrous phase B inclusions in bridgmanite. The results provide revised values of water partitioning between solid minerals and hydrous melts that are substantially higher than previous estimates.

 

Trace amounts of hydrogen in olivine can significantly increase its conductivity. However, the conduction mechanism in hydrous olivine is still unclear, which hinders the in‐depth understanding of the high conductivity structures of the asthenosphere. We investigate the proton conduction mechanism in hydrous olivine usingab initiocalculations. Several models were examined using climbing image nudged elastic band andab initiomolecular dynamics methods. We found that hydrogen trapped at the Mg (or Fe) defect is more mobile than hydrogen trapped at the Si defect. At high temperature, we observed the ionization of hydrogen from cation defects leading to high and anisotropic proton conductivity along the [100] direction. The highly anisotropic conductivity caused by thermal ionized hydrogen at high temperature explains the experimental observations on olivine single crystals.