Hisako Hirai

H2OCO2 fluids supplied in alpine-type mantle peridotites: electron petrology of relic fluid inclusions in olivines

Fluids supplied in alpine-type mantle peridotites and trapped as fluid inclusions in olivines have been fixed by low-temperature reactions, and theirCO2/H2O ratios can be deduced from the minerals in the inclusions. Relic fluid inclusions were commonly observed by the optical microscope in olivines from almost all examined solid intrusive ultramafic complexes (Papua, Oman, Troodos and eleven alpine-type complexes of Japan). Such complexes were emplaced into the crust in a solid state. Electron microscopic studies of olivines from three complexes, Higashiakaishi, Horoman and Iwanai-dake, showed that relic fluid inclusions in these olivines have distinctive mineral parageneses: serpentine + magnesite + talc, serpentine + magnesite + brucite, and serpentine + brucite, respectively, depending on theCO2/(H2O+CO2) ratio of the trapped fluid. It is deduced that the fluids had been supplied to peridotites, at least partly, but almost wholly in some case, when the peridotites were still hot, probably at the upper mantle for the following reasons: (1) the curved surfaces along which the inclusions are distributed are cut by post-emplacement serpentine veins; (2) for the Higashiakaishi dunite, the relic fluid inclusions are exclusively found in porphyroclast olivines and are totally absent in matrix olivines recrystallized during the Sanbagawa metamorphism. Recent models on the derivation of ophiolitic or some alpine-type peridotites favor the island-arc or fore-arc settings. Dehydration of the descending oceanic slab may supply H2OCO2 vapor to the overlying mantle wedge. Fluid inclusions trapped in such mantle wedge may abound in H2O component. H2O-bearing fluid inclusions may, therefore, be important H2O containers in the upper mantle, especially near the edge of the mantle wedge above downgoing oceanic slabs.

Phase changes of CO2 hydrate under high pressure and low temperature

High pressure and low temperature experiments with CO(2) hydrate were performed using diamond anvil cells and a helium-refrigeration cryostat in the pressure and temperature range of 0.2-3.0 GPa and 280-80 K, respectively. In situ x-ray diffractometry revealed that the phase boundary between CO(2) hydrate and water+CO(2) extended below the 280 K reported previously, toward a higher pressure and low temperature region. The results also showed the existence of a new high pressure phase above approximately 0.6 GPa and below 1.0 GPa at which the hydrate decomposed to dry ice and ice VI. In addition, in the lower temperature region of structure I, a small and abrupt lattice expansion was observed at approximately 210 K with decreasing temperature under fixed pressures. The expansion was accompanied by a release of water content from the sI structure as ice Ih, which indicates an increased cage occupancy. A similar lattice expansion was also described in another clathrate, SiO(2) clathrate, under high pressure. Such expansion with increasing cage occupancy might be a common manner to stabilize the clathrate structures under high pressure and low temperature.

Origin of iridescence in garnet: An optical interference study

The optical interference phenomenon of an iridescent grandite garnet was examined by using white light and laser light on the analogy of the X-ray diffraction method. A kinematical interpretation was also made for the observed angle and intensity of interference light. The regularly stratified layers which cause iridescence are deduced to be periodic twins oriented parallel to the growth layers in each sector with a periodicity of about 1,000 Å.

Structural changes of filled ice Ic structure for hydrogen hydrate under high pressure

High-pressure experiments of hydrogen hydrate, filled ice Ic structure, were performed using a diamond-anvil cell in the pressure range of 0.1-80.3 GPa at room temperature. In situ x-ray diffractometry (XRD) revealed that structural changes took place at approximately 35-40 and 55-60 GPa, and that the high-pressure phase of hydrogen hydrate survived up to at least 80.3 GPa. Raman spectroscopy showed that the changes in vibrational mode for the hydrogen molecules in hydrogen hydrate occurred at around 40 and 60 GPa, and these results were consistent with those of the XRD. At about 40 GPa, the intermolecular distance of host water molecules consisting the framework attained the critical distance of symmetrization of the hydrogen bond for water molecules, which suggested that symmetrization of the hydrogen bond occurred at around 40 GPa. The symmetrization might introduce some structural change in the filled ice Ic structure. In addition, the existence of the high-pressure phase above 55-60 GPa implies that a denser structure than that of filled ice Ic may exist in hydrogen hydrate.

Structural changes of filled ice Ic hydrogen hydrate under low temperatures and high pressures from 5 to 50 GPa.

Low-temperature and high-pressure experiments were performed on the filled ice Ic structure of hydrogen hydrate at previously unexplored conditions of 5-50 GPa and 30-300 K using diamond anvil cells and a helium-refrigeration cryostat. In situ x-ray diffractometry revealed that the cubic filled ice Ic structure transformed to tetragonal at low temperatures and high pressures; the axis ratio of the tetragonal phase changed depending on the pressure and temperature. These results were consistent with theoretical predictions performed via first principle calculations. The tetragonal phase was determined to be stable above 20 GPa at 300 K, above 15 GPa at 200 K, and above 10 GPa at 100 K. Further changes in the lattice parameters were observed from about 45-50 GPa throughout the temperature region examined, which suggests the transformation to another high-pressure phase above 50 GPa. In our previous x-ray study that was performed up to 80 GPa at room temperature, a similar transformation was observed above 50 GPa. In this study, the observed change in the lattice parameters corresponds to the beginning of that transformation. The reasons for the transformation to the tetragonal structure are briefly discussed: the tetragonal structure might be induced due to changes in the vibrational or rotational modes of the hydrogen molecules under low temperature and high pressure.

Stabilizing of methane hydrate and transition to a new high-pressure structure at 40 GPa

Abstract High-pressure experiments of methane hydrate with a composition of full-occupancy of structure I were performed in a pressure range from 0.2 to 86 GPa. X-ray diffractometry and Raman spectroscopy revealed that methane hydrate transformed from a known high-pressure structure, filled-ice-Ih structure, to a new high-pressure structure at approximately 40 GPa. The reason for the outstanding retention of the filled-ice-Ih structure up to 40 GPa was examined, because the filled-ice-Ih structures for other gas hydrates decompose below 6.5 GPa. In the Raman spectra, new intramolecular vibration modes softer than the original ones appeared at 14 to 17 GPa, indicating that additional intermolecular interaction arose around the methane molecules. The additional interaction might be induced by symmetrization of the hydrogen bonds forming the framework. The symmetrization of the framework and the subsequent additional interactions between the methane molecules and the framework water molecules and also between the methane molecules are likely the cause of the excellent stabilization. The new high-pressure structure survived at least to 86 GPa.

Compositional variation of calcic amphiboles in Mineoka metabasites, central Japan, and its bearing on the actinolite-hornblende miscibility relationship

Abstract Calcic amphiboles from the metabasites of the medium-pressure type from Mineoka, central Japan, are chemically continuous from Al-poor actinolite to Al-rich hornblende even in a single grain. Calcic amphibole with the intermediate composition between actinolite and hornblende was confirmed to be a single phase by analytical electron microscope and X-ray diffraction studies. Calcic amphiboles from Mineoka metabasites, transitional between greenschist and epidote amphibolite facies, are characterized by the continuous deficiency of Ca, which is due to the increase of crossite and cummingtonite components from actinolite to hornblende. In metabasites of medium-pressure type, however, calcic amphiboles with low Ca deficiency have the compositional gap between actinolite and hornblende. It is most probable that the miscibility gap in calcic amphiboles of metabasites of medium-pressure type tends to be narrowed with increase of crossite and cummingtonite components.

Shock Compaction of an Acicular Iron-Alloy Powder to Produce a Nanocrystalline Magnet

A shock-compaction technique of an acicular magnetic iron-alloy powder was developed to produce a nanocrystalline magnet preserving its magnetic properties and acicular features. Although a precompacted disk of the powder had undesirable initial conditions which were a large porosity of 50% and a wide distribution of pore size, use of a double-plate flyer consisting of copper and aluminum made it possible to produce a good compact having a density 98% of the theoretical value, 6.2 GPa Vickers microhardness, and maximum energy product of 15.1 kJ/m3. One-dimensional wave-propagation analysis implies effective compression at the initial stage suppressing the total thermal energy and the localization of heat in the vicinity of large pores. The method is discussed for improving the magnetic properties.

High pressure X-ray diffraction and Raman spectroscopic studies of the phase change of D2O ice VII at approximately 11 GPa

High pressure experiments were performed on D2O ice VII using a diamond anvil cell in a pressure range of 2.0–60 GPa at room temperature. In situ X-ray diffractometry revealed that the structure changed from cubic to a low symmetry phase at approximately 11 GPa, based on the observed splitting of the cubic structures diffraction lines. Heating treatments were added for the samples to reduce the effect of non-hydrostatic stress. After heating, splitting diffraction lines became sharp and the splitting was clearly retained. Although symmetry and structure of the transformed phase have not been determined, change in volumes vs. pressure was calculated, assuming that the low-symmetry phase had a tetragonal structure. The bulk modulus calculated for the low-symmetry phase was slightly larger than that for the cubic structure. In Raman spectroscopy, the squared vibrational frequencies of ν1 (A1g), as a function of pressure, showed a clear change in the slope at 11–13 GPa. The full width at half maxima of the O-D modes decreased with increasing pressure, reaching a minimum at approximately 11 GPa, and increased again above 11 GPa. These results evidently support the existence of phase change at approximately 11 GPa for D2O ice VII.

Phase changes of filled ice Ih methane hydrate under low temperature and high pressure

Low-temperature and high-pressure experiments were performed with filled ice Ih structure of methane hydrate under 2.0-77.0 GPa and 30-300 K using diamond anvil cells and a helium-refrigeration cryostat. In situ X-ray diffractometry revealed distinct changes in the compressibility of the axial ratios of the host framework with pressure. Raman spectroscopy showed a split in the C-H vibration modes of the guest methane molecules, which was previously explained by the orientational ordering of the guest molecules. The pressure and temperature conditions at the split of the vibration modes agreed well with those of the compressibility change. The results indicate the following: (i) the orientational ordering of the guest methane molecules from an orientationally disordered state occurred at high pressures and low temperatures; and (ii) this guest ordering led to anisotropic contraction in the host framework. Such guest orientational ordering and subsequent anisotropic contraction of the host framework were similar to that reported previously for filled ice Ic hydrogen hydrate. Since phases with different guest-ordering manners were regarded as different phases, existing regions of the guest disordered-phase and the guest ordered-phase were roughly estimated by the X-ray study. In addition, above the pressure of the guest-ordered phase, another high-pressure phase developed in the low-temperature region. The deuterated-water host samples were also examined, and the influence of isotopic effects on guest ordering and phase transformation was observed.