Tetsuo Irifune

Dislocation-accommodated grain boundary sliding as the major deformation mechanism of olivine in the Earth’s upper mantle

Grain size–sensitive creep controls the flow in the middle and deep upper mantle. Understanding the deformation mechanisms of olivine is important for addressing the dynamic processes in Earth’s upper mantle. It has been thought that dislocation creep is the dominant mechanism because of extrapolated laboratory data on the plasticity of olivine at pressures below 0.5 GPa. However, we found that dislocation-accommodated grain boundary sliding (DisGBS), rather than dislocation creep, dominates the deformation of olivine under middle and deep upper mantle conditions. We used a deformation-DIA apparatus combined with synchrotron in situ x-ray observations to study the plasticity of olivine aggregates at pressures up to 6.7 GPa (that is, ~200-km depth) and at temperatures between 1273 and 1473 K, which is equivalent to the conditions in the middle region of the upper mantle. The creep strength of olivine deforming by DisGBS is apparently less sensitive to pressure because of the competing pressure-hardening effect of the activation volume and pressure-softening effect of water fugacity. The estimated viscosity of olivine controlled by DisGBS is independent of depth and ranges from 1019.6 to 1020.7 Pa·s throughout the asthenospheric upper mantle with a representative water content (50 to 1000 parts per million H/Si), which is consistent with geophysical viscosity profiles. Because DisGBS is a grain size–sensitive creep mechanism, the evolution of the grain size of olivine is an important process controlling the dynamics of the upper mantle.

Crystal chemistry of dense hydrous magnesium silicates: The structure of phase H, MgSiH2O4, synthesized at 45 GPa and 1000 °C

Abstract The crystal structure of the dense hydrous magnesium silicate phase H, MgSiH2O4, synthesized at 45 GPa and 1000 °C, was investigated by single-crystal X-ray diffraction. Although showing a deterioration process under the X-ray beam, the compound was found to be orthorhombic, space group Pnnm (CaCl2- type structure), with lattice parameters a = 4.733(2), b = 4.3250(10), c = 2.8420(10) Å, V = 58.18(3) Å3, and Z = 1. The structure was refined to R1 = 0.0387 using 53 observed reflections [2s(I) level]. Magnesium and silicon were found to be disordered at the same octahedral site (with a mean bond distance of 1.957 Å). Hydrogen was not located in the difference Fourier maps, but it is very likely disordered at a half-occupied 4g position. The centrosymmetric nature of the structure of phase H is examined in relation to that reported for pure d-AlOOH at ambient conditions (non-centrosymmetric, P21nm), and the possibility that these two compounds can form a solid solution at least at high pressure is discussed.

High-pressure generation using double stage micro-paired diamond anvils shaped by focused ion beam

Micron-sized diamond anvils with a 3 μm culet were successfully processed using a focused ion beam (FIB) system and the generation of high pressures was confirmed using the double stage diamond anvil cell technique. The difficulty of aligning two second-stage micro-anvils was solved via the paired micro-anvil method. Micro-manufacturing using a FIB system enables us to control anvil shape, process any materials, including nano-polycrystalline diamond and single crystal diamond, and assemble the sample exactly in a very small space between the second-stage anvils. This method is highly reproducible. High pressures over 300 GPa were achieved, and the pressure distribution around the micro-anvil culet was evaluated by using a well-focused synchrotron micro-X-ray beam.

Phase relations and formation of chromium-rich phases in the system Mg4Si4O12–Mg3Cr2Si3O12 at 10–24 GPa and 1,600 °C

Phase relations in the system Mg4Si4O12–Mg3Cr2Si3O12 were studied at 10–24 GPa and 1,600xa0°C using a high-pressure Kawai-type multi-anvil apparatus. We investigated the full range of starting compositions for the knorringite–majorite system to derive a P–X phase diagram and synthesize garnets of a wide compositional range. Samples synthesized in the pressure range 10–14 GPa contain knorringite–majorite garnet and Cr-bearing pyroxene. With increasing Cr content in the starting materials, an association of knorringite–majorite garnet and eskolaite is formed. Garnets contain a significant portion of majorite (>10xa0mol%) even for a pure Mg3Cr2Si3O12 starting composition. Knorringite–majorite garnets were obtained in the pressure range from 10 to 20 GPa. With increasing pressure, the phase assemblages include Cr-bearing MgSiO3 akimotoite and MgSiO3 bridgmanite, as well as MgCr2O4 with calcium titanate structure, and stishovite. Single-crystal X-ray diffraction shows that the incorporation of Cr into the structure of garnet, as well as MgSiO3 akimotoite, and bridgmanite results in an increase in their unit cell parameters. Results of the experimental high-pressure investigation of the pseudo-binary system Mg4Si4O12–Mg3Cr2Si3O12 (SiO2–MgO–Cr2O3) may be applied to the origin of high chromium phases (mostly garnet) found as inclusions in peridotitic diamonds and formed in bulk rock compositions with high Cr/Al ratios in relation to the primitive mantle.

Raman spectra of phase a at various pressures and temperatures

Abstract Variations of the Raman spectra of phase A (Mg 7 Si 2 O 14 H 6 ) were investigated up to about 400 kbar at room temperature and in the range 80–853 K at atmospheric pressure. Phase A appears to undergo a reversible phase transition around 180 kbar at room temperature, and it becomes amorphous above 813 K at atmospheric pressure. The Raman frequencies of the two strong OH bands of phase A decrease linearly with increasing pressure, but increase non-linearly with increasing temperature. The frequencies of the other Raman bands of phase A increase non-linearly with increasing pressure but decrease both linearly and non-linearly with increasing temperature within the experimental uncertainties and the range investigated. The trends of the pressure and temperature effects on the Raman frequencies of phase A parallel those observed in the hydrous β-phase, but nonlinear behaviour was not observed in the latter.

Fracture-induced amorphization of polycrystalline SiO2 stishovite: a potential platform for toughening in ceramics.

Silicon dioxide has eight stable crystalline phases at conditions of the Earths rocky parts. Many metastable phases including amorphous phases have been known, which indicates the presence of large kinetic barriers. As a consequence, some crystalline silica phases transform to amorphous phases by bypassing the liquid via two different pathways. Here we show a new pathway, a fracture-induced amorphization of stishovite that is a high-pressure polymorph. The amorphization accompanies a huge volume expansion of ~100% and occurs in a thin layer whose thickness from the fracture surface is several tens of nanometers. Amorphous silica materials that look like strings or worms were observed on the fracture surfaces. The amount of amorphous silica near the fracture surfaces is positively correlated with indentation fracture toughness. This result indicates that the fracture-induced amorphization causes toughening of stishovite polycrystals. The fracture-induced solid-state amorphization may provide a potential platform for toughening in ceramics.

Pressure-induced nano-crystallization of silicate garnets from glass

Transparent ceramics are important for scientific and industrial applications because of the superior optical and mechanical properties. It has been suggested that optical transparency and mechanical strength are substantially enhanced if transparent ceramics with nano-crystals are available. However, synthesis of the highly transparent nano-crystalline ceramics has been difficult using conventional sintering techniques at relatively low pressures. Here we show direct conversion from bulk glass starting material in mutianvil high-pressure apparatus leads to pore-free nano-polycrystalline silicate garnet at pressures above ∼10u2009GPa in a limited temperature range around 1,400u2009°C. The synthesized nano-polycrystalline garnet is optically as transparent as the single crystal for almost the entire visible light range and harder than the single crystal by ∼30%. The ultrahigh-pressure conversion technique should provide novel functional ceramics having various crystal structures, including those of high-pressure phases, as well as ideal specimens for some mineral physics applications.

Elastic wave velocity of polycrystalline Mj 80 Py 20 garnet to 21 GPa and 2,000 K

AbstractnThe elastic wave velocities of polycrystalline Mj80Py20 garnet along the majorite–pyrope system have been measured at pressures up to 21xa0GPa and temperatures up to 2,000xa0K using ultrasonic interferometry in conjunction with in situ X-ray diffraction techniques in a Kawai-type multi-anvil apparatus. The elastic moduli of Mj80Py20 garnet and their pressure and temperature derivatives are determined by a two-dimensional linear fitting of the present experimental data, yielding: KSxa0=xa0161.5 (7) GPa, ∂KS/∂Pxa0=xa04.42 (4), ∂KS/∂Txa0=xa0−0.0154 (2)xa0GPa/K, Gxa0=xa086.2 (2)xa0GPa, ∂G/∂Pxa0=xa01.28 (1), ∂G/∂Txa0=xa0−0.0096 (5)xa0GPa/K. The present results together with those of the studies on the majorite–pyrope solid solutions suggest the pressure and temperature derivatives of elastic moduli are insensitive to the majorite content in the majorite–pyrope system. The velocity gradients of the majoritic garnets in the majorite–pyrope system are 3xa0~xa06 times lower than those required to account for the high seismic velocity gradients observed in the mantle transition zone.

Short-range order of compressed amorphous GeSe2

The structure of amorphous GeSe2 (a-GeSe2) has been studied by means of a combination of two-edges X-ray absorption spectroscopy (XAS) and angle-dispersive X-ray diffraction under pressures up to about 30u2009GPa. Multiple-edge XAS data-analysis of a-GeSe2 at ambient conditions allowed us to reconstruct and compare the first-neighbor distribution function with previous results obtained by neutron diffraction with isotopic substitution. GeSe2 is found to remain amorphous up to the highest pressures attained, and a reversible 1.5u2009eV red-shift of the Ge K-edge energy indicating metallization, occurs between 10u2009GPa and 15u2009GPa. Two compression stages are identified by XAS structure refinement. First, a decrease of the first-neighbor distances up to about 10u2009GPa, in the same pressure region of a previously observed breakdown of the intermediate-range order. Second, an increase of the Ge-Se distances, bond disorder, and of the coordination number. This stage is related to a reversible non-isostructural transition involving a gradual conversion from tetra- to octa-hedral geometry which is not yet fully completed at 30u2009GPa.

Chromium solubility in anhydrous Phase B

The crystal structure and chemical composition of a crystal of (Mg14−xCrx)(Si5−xCrx)O24 (xxa0≈xa00.30) anhydrous Phase B (Anh-B) synthesized in the model system MgCr2O4–Mg2SiO4 at 12xa0GPa and 1600xa0°C have been investigated. The compound was found to be orthorhombic, space group Pmcb, with lattice parameters axa0=xa05.900(1), bxa0=xa014.218(2), cxa0=xa010.029(2)xa0Å, Vxa0=xa0841.3(2)xa0Å3 and Zxa0=xa02. The structure was refined to R1xa0=xa00.065 using 1492 independent reflections. Chromium was found to substitute for both Mg at the M3 site (with a mean bond distance of 2.145xa0Å) and Si at the octahedral Si1 site (mean bond distance: 1.856xa0Å), according to the reaction Mg2+xa0+xa0Si4+xa0=xa02Cr3+. Such substitutions cause a reduction in the volume of the M3 site and an increase in the volume of the Si-dominant octahedron with respect to the values typically observed for pure Anh-B and Fe2+-bearing Anh-B. Taking into account that Cr3+ is not expected to be Jahn–Teller active, it appears that both the Cr3+–for–Mg and Cr3+–for–Si substitutions in the Anh-B structure decrease the distortion of the octahedra. Electron microprobe analysis gave the Mg13.66(8)Si4.70(6)Cr0.62(4)O24 stoichiometry for the studied phase. The successful synthesis of this phase provides new information for the possible mineral assemblages occurring in the Earth’s deep upper mantle and shed new light on the so-called X discontinuity that has been observed at 275–345xa0km depth in several subcontinental and subduction zone environments.