Kiyoshi Fujino

Dislocation microstructures in naturally deformed silicate garnets

Abstract Dislocation microstructures of naturally deformed silicate garnets and olivines in garnet-peridotites and silicate garnets in eclogites from four localities have been observed with a transmission electron microscope (TEM) to clarify the dislocation characteristics of silicate garnets. We have obtained the following results: (1) dislocation densities of garnets in all the garnet-peridotites (ρ = 105−107 cm−2) are always nearly an order of magnitude lower than those of co-existing olivines; (2) dislocation densities of garnets in eclogites (ρ = 105−108 cm−2) which are embedded in garnet-peridotites are almost an order of magnitude higher than those of garnets in the surrounding garnet-peridotites; (3) the dominant Burgers vector, b, of mainly edge dislocations in garnet is 〈100〉 for specimens with dislocation density ρ = 105−106 cm−2, while b = 1 2 〈111〉 for specimens with ρ = 107−108 cm−2. Result (1) indicates that the observed dislocations in garnets were formed by plastic deformation under the same stresses as for co-existing olivines, and that there is a similar relationship between applied stress and dislocation density for garnets as for olivines. Result (2) suggests that the stress concentration occurred around eclogites embedded in garnet-peridotites, and the resulting differential stress in garnets in eclogites was further elevated by the interlocking of neighboring hard garnet grains. Finally, result (3) indicates that the dominant Burgers vector of mainly edge dislocations in garnet changes from 〈100〉 to 1 2 〈111〉 with increasing applied differential stress.

Stability of majorite (Mg, Fe)SiO3 at high pressures and 1800°C

Abstract Phase relations in the system MgSiO 3 -FeSiO 3 were studied experimentally up to a pressure of 24 GPa at 1800°C. Alumina-free tetragonal garnet exists in the pressure range from 16 to 23.5 GPa. The compositional range of the tetragonal garnet phase extends from pure MgSiO 3 to (Mg 0.76 Fe 0.24 )SiO 3 at 18.2 GPa and 1800°C. The sequence of the phase transitions in Mg-rich part of this system at 1800°C is: clinopyroxene→17GPatetragonal garnet→22.5GPailmenite→23GPaMg-perovskite. The lattice constants of tetragonal garnet increase with increasing iron content from a = 11.517 ± 0.002Aandc = 11.435 ± 0.003Afor MgSiO 3 toa = 11.543 ± 0.002A and c = 11.476 ± 0.003A for (Mg 0.76 Fe 0.24 )SiO 3 . Cubic garnet was not observed in the pressure and temperature range studied in this work. The alumina-free tetragonal garnet observed in this study may exist in the harzbergite layer in the descending slab.

High-pressure phase transitions in FeCr2O4 and structure analysis of new post-spinel FeCr2O4 and Fe2Cr2O5 phases with meteoritical and petrological implications

Abstract We determined phase relations in FeCr2O4 at 12-28 GPa and 800-1600 °C using a multi-anvil apparatus. At 12-16 GPa, FeCr2O4 spinel (chromite) first dissociates into two phases: a new Fe2Cr2O5 phase + Cr2O3 with the corundum structure. At 17-18 GPa, the two phases combine into CaFe2O4- type and CaTi2O4-type FeCr2O4 below and above 1300 °C, respectively. Structure refinements using synchrotron X-ray powder diffraction data confirmed the CaTi2O4-structured FeCr2O4 (Cmcm), and indicated that the Fe2Cr2O5 phase is isostructural to a modified ludwigite-type Mg2Al2O5 (Pbam). In situ high-pressure high-temperature X-ray diffraction experiments showed that CaFe2O4-type FeCr2O4 is unquenchable and is converted into another FeCr2O4 phase on decompression. Structural analysis based on synchrotron X-ray powder diffraction data with transmission electron microscopic observation clarified that the recovered FeCr2O4 phase has a new structure related to CaFe2O4-type. The high-pressure phase relations in FeCr2O4 reveal that natural FeCr2O4-rich phases of CaFe2O4- and CaTi2O4-type structures found in the shocked Suizhou meteorite were formed above about 18 GPa at temperature below and above 1300 °C, respectively. The phase relations also suggest that the natural chromitites in the Luobusa ophiolite previously interpreted as formed in the deep-mantle were formed at pressure below 12-16 GPa.

High-pressure high-temperature transitions in MgCr2O4 and crystal structures of new Mg2Cr2O5 and post-spinel MgCr2O4 phases with implications for ultrahigh-pressure chromitites in ophiolites

Abstract We determined phase relations in MgCr2O4 at 12-28 GPa and 1000-1600 °C using a multi-anvil apparatus. At 12-15 GPa, spinel-type MgCr2O4 (magnesiochromite) first decomposes into a mixture of new Mg2Cr2O5 phase + corundum-type Cr2O3 at 1100-1600 °C, but it dissociates first into MgO periclase + corundum-type Cr2O3 at l000 °C. At about 17-19 GPa, the mixture of Mg2Cr2O5 phase + corundum-type Cr2O3 transforms to a single MgCr2O4 phase. Structure refinements using synchrotron X-ray powder diffraction data indicated that the high-pressure MgCr2O4 phase has a CaTi2O4-type structure (Cmcm), and that the basic structure of the Mg2Cr2O5 phase is the same as that of recently found modified ludwigite-type Mg2Al2O5 and Fe2Cr2O5 (Pbam). The phase relations in this study may suggest that natural chromitites in the Luobusa ophiolite regarded as the deep-mantle origin were derived from the mantle shallower than the depths corresponding to pressure of 12-15 GPa because of absence of the assemblage of (Mg,Fe)2Cr2O5 + Cr2O3 in the chromitites.

Pressure-induced spin transition in FeCO3-siderite studied by X-ray diffraction measurements

We have collected synchrotron X-ray diffraction patterns of FeCO3-siderite after or in-situ laser heating at high pressures to 66 GPa. Diffraction peaks of FeCO3 in all diffraction patterns obtained can be indexed as a trigonal cell. However, calculated cell volumes show an abrupt decrease (about 6.5%) between 47 and 50 GPa at room temperature. This abrupt change of the cell volume on FeCO3 is possibly due to a pressure-induced spin transition of ferrous Fe (HS: high-spin → LS: low-spin). Because cell parameters obtained at high temperature and at pressures above 50 GPa suggest HS state rather than LS state, the Clapeyron slope of the HS-to-LS transition of FeCO3 should be positive.

Pressure and temperature dependence of cation distribution in Mg-Mn olivine

Synthetic (Mg0.51, Mn0.49)2SiO4 olivine samples are heat-treated at three different pressures; 0, 8 and 12 GPa, all at the same temperature (∼500° C). X-ray structure analyses on these single crystals are made in order to see the pressure effect on cation distribution. The intersite distribution coefficient of Mg and Mn in M1 and M2 sites, KD = (Mn/Mg)M1/(Mn/Mg)M2, of these samples are 0.192 (0 GPa), 0.246 (8 GPa) and 0.281 (12 GPa), indicating cationic disordering with pressure. The small differences of cell dimensions between these samples are determined by powder X-ray diffraction. Cell dimensions b and c decrease, whereas a increases with pressure of equilibration. Cell volume decreases with pressure as a result of a large contraction of the b cell dimension. The effect of pressure on the free energy of the cation exchange reaction is evaluated by the observed relation between the cell volume and the site occupancy numbers. The magnitude of the pressure effect on cation distribution is only a fifth of that predicted from the observed change in volume combined with thermodynamic theory. This phenomenon is attributed to nonideality in this solid solution, and nonideal parameters are required to describe cation distribution determined in the present and previous experiments. We use a five-parameter equation to specify the cationic equilibrium on the basic of thermodynamic theory. It includes one energy parameter of ideal mixing, two parameters for nonideal effects, one volume parameter, and one thermal parameter originated from the lattice vibrational energy. The present data combined with some of the existing data are used to determine the five parameters, and the cation distribution in Mg-Mn olivine is described as a function of temperature, pressure, and composition. The basic framework of describing the cationic behavior in olivine-type mineral is worked out, although the result is preliminary: each of the determined parameters is not accurate enough to enable us to make a reliable prediction.

Electron microscopy studies of akermanites (Ca1−xSrx)2CoSi2O7 with modulated structure

Single crystals of akermanite (Ca1−xSrx)2Co-Si2O7 solid solution were grown in nitrogen by the floating zone method using a lamp-image furnace. The grown crystals were 6 mm in diameter by 50 mm in length. Microprobe analyses indicate uniform strontium content x except in the initially crystallized part. Synthetic crystals with x from 0.0 to 0.3 give, at room temperature, satellite reflections and circular diffuse scatterings in the electron diffraction pattern, which are related to an incommensurate phase and microdomains, respectively. With increasing Sr content the wavelength of a modulation increases and the intensity of satellites decreases, but the intensity of circular diffuse scattering increases up to x=0.15 and then decreases until eventually the satellites and the circular diffuse scatterings disappear at x = 0.3. The circular diffuse scattering is explained by the cluster model for the transition state, proposed by De Ridder et al. (1976).

Highly conductive iron-rich (Mg,Fe)O magnesiowüstite and its stability in the Earth's lower mantle

It has been repeatedly suggested that an iron-rich oxide might accumulate on the Earths core-mantle boundary by various processes. Recent studies showed that FeO with a rock salt (B1)-type structure undergoes pressure- and temperature-induced metallization at the Earths lower mantle conditions. This implies similar metallization or decomposition of the lower mantle phase, (Mg,Fe)O, under high pressure-temperature conditions. We performed simultaneous X-ray diffraction and electrical conductivity measurements on (Mg0.20Fe0.80)O and (Mg0.05Fe0.95)O magnesiowustites up to 140u2009GPa and 2100u2009K, and we examined recovered samples by means of an analytical transmission electron microscope. The experiments revealed very high electrical conductivity of the magnesiowustite samples and their minimal temperature dependence above 85u2009GPa and 1300u2009K, yet the samples remained insulators. We also found decomposition of (Mg0.05Fe0.95)O into almost pure FeO and iron-rich (Mg,Fe)O due to metallization of the FeO component, while such a reaction was not observed in (Mg0.20Fe0.80)O. The observed high electrical conductivity and decomposition of iron-rich (Mg,Fe)O magnesiowustite could enhance the heterogeneities in the electrical and thermal conductivity at the Earths core-mantle boundary region.

Preferred orientation of antiphase boundaries in pigeonite as a cooling ratemeter

Antiphase domains (APDs) of pigeonite lamellae in natural and heated augite crystals from the Hakonetoge andesite have been examined by a transmission electron microscope (TEM). Antiphase boundaries (APBs) of the pigeonite lamellae in natural specimens have a sigmoidal shape cutting the c axis in (010) sections. APBs in specimens heated at temperatures above the high-low inversion and then quenched are nearly parallel to the c axis with almost straight boundaries. These observations indicate that the preferred orientation of APBs in (010) sections depends on cooling rate; at fast colling rates the APBs are nearly parallel to the c axis, whereas at slower cooling rates they are inclined to the c axis.The cooling rate of the natural augite specimen from Hakonetoge is estimated to be about 0.01 °C/h from the experimentally determined time-temperature-transformation (TTT) diagram for the APB orientations.APD sizes are large in specimens quenched from above the inversion temperature; they are at a minimum after cooling rates of around 1–0.1 °C/h, and then become larger with slower cooling rates.

Synchrotron X-ray diffraction study for crystal structure of solid carbon dioxide CO2-V

Synchrotron X-ray diffraction study for solid CO2 at 40?100 GPa and around 2,000 K has shown that the diffraction pattern of the high pressure phase CO2-V is consistently interpreted in terms of a tetragonal uni_t cell (Z = 4, a = 3.584 ?, c = 5.908 ? at 50GPa). A ?-cristobalite structure (space group I2d) gives a good account of our data qualitatively. Isothermal molar volume (300K) of the CO2-V in the present study is smaller than that indexed as a tridymite structure proposed by previous studies at any pressures.