Nobumasa Funamori

Thermoelastic properties of MgSiO3 perovskite determined by in situ X ray observations up to 30 GPa and 2000 K

In situ X ray experiments on MgSiO3 perovskite at pressures of 21–29 GPa and temperatures of 300–2000 K were carried out using an MA8-type high-pressure apparatus, employing sintered diamond anvils, combined with synchrotron radiation. The thermal expansion at 25 GPa up to 2000 K was determined from interpolation of the P-V-T data obtained in the present study. The 95% confidence level was estimated by taking all possible errors into account, including statistical error and systematic error caused by uncertainty of pressure scales, etc. The thermal expansivity at 25 GPa is expressed as αT,25 = a25 + b25T − c25T2 with the best-fit parameters of a25 = 2.11 × 10−5 K−1, b25 = 1.80 × 10−9 K−2, and c25 = 1.93 K. The equation of state of MgSiO3 perovskite has been determined using our new data combined with the lower-pressure data of Wang et al. [1994] and Utsumi et al. [1995]. The optimal set of parameters of the third-order Birch-Murnaghan equation of state, which is expressed as P = (3/2)KT,0[(VT,0/V)7/3 − (VT,0/V)5/3]{1 − (3/4)(4 − KT,0′)[(VT,0/V)2/3 − 1]}, where KT,0 = K300,0 + (∂KT,0/∂T)P(T − 300), KT,0′ = K300,0′, VT,0 = V0exp ∫300TαT,0dT, and αT,0 = a0 + b0T − c0T−2, is K300,0 = 261 GPa, K300,0′ = 4, a0 = 1.982 × 10−5 K−1, b0 = 0.818 × 10−8 K−2, c0 = 0.474 K, and (∂KT,0/∂T)P = −0.0280 GPa/K. The reliability of the result is discussed in detail.

High pressure and high temperature in situ X‐ray observation of MgSiO3 Perovskite under lower mantle conditions

High pressure and high temperature in situ X-ray observations of MgSiO3 perovskite were carried out using a newly developed Drickamer-type apparatus combined with synchrotron radiation. Variations of the unit cell parameters of the perovskite were observed as a function of temperature up to about 1900 K at 36 GPa. The orthorhombic distortion changed very little and no indication of transformations to higher symmetry phases was observed up to 1900 K. This result indicates that the orthorhombic perovskite is the stable phase of MgSiO3 in the most part of the lower mantle. The thermal expansivity of the perovskite is determined to be larger than 1.7(2) × 10−5 /K at 36 GPa and room temperature. This value will give an important constraint to the chemical composition of the lower mantle.

High‐pressure transformations in MgAl2O4

X ray diffraction and transmission electron microscopy on laser-heated diamond cell samples show that with increasing pressure MgAl2O4 spinel transforms first to Al2O3 corundum + MgO periclase, then to the CaFe2O4-structured phase, and finally to a new phase having the CaTi2O4 structure above ∼40 GPa. The CaFe2O4 and the CaTi2O4 structures are closely related and have almost the same densities and bulk moduli. Transformation from the CaFe2O4 to the CaTi2O4 phase would be expected to take place in oceanic crust that is subducted deep into the lower mantle.

Garnet-perovskite transformation under conditions of the Earth's lower mantle: an analytical transmission electron microscopy study

Natural pyrope garnets with three different chemical compositions have been transformed at 30–60 GPa in the laser-heated diamond-anvil cell (DAC). Recovered samples were examined by analytical transmission electron microscopy (ATEM). For all three pyropes, the dominant post-garnet phase was Mg-rich orthorhombic silicate perovskite. The Al content of Mg-perovskite increased significantly with increasing pressure and temperature, and its chemical composition became close to that of the starting material. Mg-perovskite with Al2O3 content less than 25 mol% was quenched as a single phase of orthorhombic perovskite at ambient conditions, whereas Mg-perovskite with Al2O3 content of 25–28 mol% transformed into the alternating lamellae of orthorhombic perovskite and lithium niobate phases. With further increasing the Al2O3 content, Mg-perovskite converted into a single phase of the lithium niobate structure with polysynthetic twinning on {1012} at ambient conditions. The high Al content may also induce the stabilization of Fe3+ in the perovskite structure accompanied by metallic iron. Two Al-rich phases, corundum and a new Al-rich phase (the NAL phase) were recognized with Mg-perovskite mostly at lower pressures. The NAL phase is close to M3Al4Si1.5O12 (M=Mg, Fe, Ca, Na, K), i.e., the middle of A2O3 and AB2O4 type compounds, and is accompanied by stishovite. The electron diffraction patterns are consistent with the space group P63/m or P63 with a=8.85 (2) A, c=2.78 (2) A, V=188 (1) A3. The existence of large cations such as Ca, K, and Na seems to stabilize the NAL phase relative to the corundum phase, but excess Ca and Na contents are likely to induce the formations of Ca-rich cubic perovskite and the calcium ferrite phase, respectively.

High-pressure and high-temperature in situ x-ray Diffraction study of iron to above 30 Gpa using MA8-type apparatus

In situ x-ray diffraction experiments on iron at pressures of 22–32 GPa and temperatures of 300–1500 K were carried out using an MA8-type high-pressure apparatus, employing sintered diamond anvils, combined with synchrotron radiation. No phases other than e(hcp) and γ(fcc) were observed in this pressure and temperature range. It became clear that non-hydrostaticity or chemical reaction with hydrogen causes serious problems in high-pressure and high-temperature x-ray diffraction study on iron. The average thermal expansivity of e phase between 300 K and 1000 K and the zero-pressure bulk modulus of γ phase at 1400 K were determined by assuming that no chemical reaction occurred in the present study. The thermal expansivities of e iron at 22 GPa and 32 GPa are determined to be 3.88 × 10−5 K−1 and 3.16 × 10−5 K−1, respectively. In this case, the volume at 22–32 GPa and room temperature is about 1% larger than the literature value. The bulk modulus of γ iron is determined to be 120 GPa, when its pressure derivative is fixed at 5.

LATTICE STRAINS IN CRYSTALS UNDER UNIAXIAL STRESS FIELD

An expression for the lattice strains in a polycrystalline specimen under uniaxial stress field has been extended for all crystal systems. Apparent Miller indices (HKL) are introduced from Miller indices (hkl) and lattice parameters. The lattice strain e(l1l2l3) of the direction l1l2l3, normal to the plane HKL, can be uniquely expressed for all crystal systems as follows: e(l1l2l3)={αβ(l1l2l3)+(1−α)[1/(3KV)]}σ p+α(−(t/3)(1−3 cos2 ψ){(1/2)[3/E(l1l2l3) −β(l1l2l3)]})+(1−α){−(t/3)(1−3 cos 2 ψ)[1/(2GV) ]}, where β(l1l2l3) and E(l1l2l3) denote the linear compressibility and the Young modulus, respectively. Bulk modulus KV and shear modulus GV are values for isostrain model. Variable ψ is the angle between loading axis and the normal of the plane HKL. The first term is the strain caused by the hydrostatic stress component σp. The second and third term, strains caused by the differential stress t, correspond to the isostress and the isostrain model, respectively. The parameter α takes a value between 0 (isostrain...

Thermal expansivity of MgSiO3 perovskite under high pressures up to 20 GPa

Volume measurement of MgSiO3 perovskite was made in the temperature range from 25 to 500°C as a function of pressure up to 20 GPa by in situ x-ray diffraction, using a DIA apparatus combined with synchrotron radiation. The measured thermal expansivity ranges from 1.8 to 2.5*10−5 /K and decreases only slightly with pressure. Our present results are in good agreement with the previous lower pressure (up to 11 GPa) data of Wang et al. [1994] and the higher pressure (36 GPa) data of Funamori and Yagi [1993], but contrast with the results using diamond anvil cell by Mao et al. [1991], particularly in a low pressure range.

Deviatoric stress measurement under uniaxial compression by a powder x‐ray diffraction method

The complete stress field in a polycrystalline sample compressed in a modified Drickamer‐type apparatus was determined from x‐ray diffraction data. The incident x rays, from a synchrotron source, were perpendicular to the compression axis, and the diffracted energy‐dispersive signals were simultaneously determined for two directions relative to the compression axis. The two sets of d values measured by this system were analyzed by making use of a new equation derived by Singh, and the uniaxial stress component σ1−σ3 and the parameter α, which describes the stress and strain conditions across the grain boundaries of the sample, were obtained. This method was applied to NaCl and the results give the important information on the stress state and the pressure determination method under direct compression of a solid sample.

Mineral assemblages of basalt in the lower mantle

High-pressure X-ray diffraction and analytical transmission electron microscopy of laser-heated diamond cell samples show temperature-dependent phase assemblages in mid-ocean ridge basalt (MORB) at lower mantle pressures. The assemblage observed below ∼2250 K consists of only stishovite plus an orthorhombic perovskite phase (space group Pbnm or Cmcm) having a composition close to that of MORB but slightly depleted in SiO2. With increasing temperature, the assemblage transforms to orthorhombic and cubic perovskites plus stishovite and then at ∼3250 K to orthorhombic and cubic perovskites plus stishovite and the CaFe2O4 phase. All of these assemblages could be present in slabs that have penetrated deep into the mantle and can therefore contribute to the seismological heterogeneity of the lower mantle.

Amorphization of Serpentine at High Pressure and High Temperature

Pressure-induced amorphization of serpentine was observed at temperatures of 200° to 300°C and pressures of 14 to 27 gigapascals with a combination of a multianvil apparatus and synchrotron radiation. High-pressure phases then crystallized rapidly when the temperature was increased to 400°C. These results suggest that amorphization of serpentine is an unlikely mechanism for generating deep-focus earthquakes, as the temperatures of subducting slabs are significantly higher than those of the rapid crystallization regime.