Multi-anvil Pressure Apparatus Research Articles

Crystal-Chemical Properties of Synthetic Almandine-Pyrope Solid Solution by X-Ray Single-Crystal Diffraction and Raman Spectroscopy

Crystal-chemical properties of synthetic Almandine-Pyrope (Alm-Pyr) solid solutions were investigated by X-ray single-crystal diffraction and Raman spectroscopy. Garnet solid solution with different compositions were synthesized from powder at 4.0 GPa and annealed at 1200 °C for 48 h by a multi-anvil pressure apparatus. Garnet crystals with different sizes (about 60–1000 μm) were obtained from synthesis. The results of X-ray single-crystal diffraction show that the unit cell constants decrease with increasing Pyr contents in the synthetic Alm-Pyr crystals due to the smaller ionic radius of Mg2+ in eightfold coordination than that of Fe2+. The data exhibit obviously positive deviations from ideal mixing volumes across the Alm-Pyr join which may be caused by the distortion of the SiO4 tetrahedron. Moreover, the significant decrease in the average M-O bond length and volume of the [MgO8]/[FeO8] dodecahedron with increasing Pyr contents are the most important factors to the decrease in the Alm-Pyr crystal unit cell constant and volume. On the other hand, selected bond distances (average <M-O>, <Al-O>, and <D-O> distances) have a linear correlation with the unit-cell parameter, but the <Si-O> distance has nonlinear correlation. With increasing the unit-cell parameter, the average <M-O> distance increases significantly, followed by the average <D-O> and <Al-O> distances. While the <Si-O> distance changes negligibly further confirming the conclusion that the significant decrease of the average M-O bond length of the [MgO8]/[FeO8] dodecahedron with increasing Pyr contents are the most important factors to the decrease in the Alm-Pyr crystal unit cell volume. In the Raman spectra collected for the Alm-Pyr solid solutions, Raman vibration mode assignments indicate that the Raman vibrational spectra change along the Alm-Pyr binary solution. The mode frequencies of Si-O stretching, Si-O bending, and the rotation of the SiO4-tetrahedron (R(SiO4)) decrease linearly, while the translational modes of the SiO4-tetrahedron (T(SiO4)) increase with increasing Alm contents.

3D velocity distribution of P- and S-waves in a biotite gneiss, measured in oil as the pressure medium: Comparison with velocity measurements in a multi-anvil pressure apparatus and with texture-based calculated data

Ultrasonic measurements of the 3D velocity distribution of P- and S-waves were performed on a spherical sample of a biotite gneiss from the Outokumpu scientific drill hole. Measurements were done at room temperature and pressures up to 400 and 70MPa, respectively, in a pressure vessel with oil as a pressure medium. A modified transducer/sample assembly and the installation of a new mechanical system allowed simultaneous measurements of P- and S-wave velocities in 132 independent directions of the sphere on a net in steps of 15°. Proper signals for P- and S-waves could be recorded by coating the sample surface with a high-viscosity shear wave gel and by temporal point contacting of the transmitter and receiver transducers with the sample surface during the measurements. The 3D seismic measurements revealed a strong foliation-related directional dependence (anisotropy) of P- and S-wave velocities, which is confirmed by measurements in a multi-anvil apparatus on a cube-shaped specimen of the same rock. Both experimental approaches show a marked pressure sensitivity of P- and S-wave velocities and velocity anisotropies. With increasing pressure, P- and S-wave velocities increase non-linearly due to progressive closure of micro-cracks. The reverse is true for velocity anisotropy. 3D velocity calculations based on neutron diffraction measurements of crystallographic preferred orientation (CPO) of major minerals show that the intrinsic bulk anisotropy is basically caused by the CPO of biotite constituting about 23vol.% of the rock. Including the shape of biotite grains and oriented low-aspect ratio microcracks into the modelling increases bulk anisotropy. An important finding from this study is that the measurements on the sample sphere and on the sample cube displayed distinct differences, particularly in shear wave velocities. It is assumed that the differences are due to the different geometries of the samples and the configuration of the transducer–sample assembly. On the spherical sample with a point source and point receiver the first break at the velocity arrival time is suggested to define group velocities in most cases, whereas the receivers in the multi-anvil apparatus recorded the flat part of the wavefront, that is, phase velocities.

PRECISE STRUCTURE ANALYSES OF ADVANCED MATERIALS UNDER HIGH-PRESSURE AND HIGH-TEMPERATURE

Precise structure analyses of advanced materials under pressure were performed using each advantage of X-ray absorption fine structure (XAFS) and diffraction methods. Measurements were performed in-situ under pressure and temperature using a large-volume multi-anvil pressure apparatus and synchrotron radiation. XAFS spectra are useful for phase study under high temperature and high pressure. The XAFS Debye Waller factor provides anharmonic effective pair potential with a pressure-dependent and temperature-independent shape. The phonon energies and anharmonicity are affected largely by the change in local structure and bonding character.

High-pressing preparation of rock glasses and their elastic properties

Melt glasses for seven types of rock ranging from acid to basic were prepared under 1.0 GPa on a multi-anvil pressure apparatus, YJ-3000 ton press. Densities and elastic properties of the melt glasses were compared with those described in previous studies. It was found that the glasses melted under 1.0 GPa were consistent in density with both naturally-occurring glassy rocks and artificially prepared glasses melted at ambient pressure. The densities of glasses are negatively correlated with the SiO2 contents and positively correlated with the (NgO+FeO) contents. The compressive velocity (Vp) of glass tends to increase with decreasing SiO2 contents and increasing (MgO+FeO) contents. The shear velocity (Vs) of glass tends to increase slightly with increasing SiO2 contents, which has little connection with the (MgO+FeO) contents. It was calculated from densities and velocities that the elastic moduli of glasses are negatively correlated with the SiO2 contents and positively correlated with the (MgO+FeO) contents.

Experimental measurement of the electrical conductivity of pyroxenite at high temperature and high pressure under different oxygen fugacities

Electrical conductivities of pyroxenite were measured between frequencies of 10−1 and 106 Hz in a multi-anvil pressure apparatus using different solid buffers (Ni+NiO, Fe+Fe3O4, Fe+FeO and Mo+MoO2) to stabilize the partial pressure of oxygen. The temperature ranged from 1073 to 1423 K (800 to 1200 °C) and the pressure from 1.0 to 4.0 GPa. We observe that: (1) the electrical conductivity (σ) of pyroxenite depends on frequency; (2) σ tends to increase with rising temperature (T), and Log σ and 1/T obey a linear Arrhenius relationship; (3) under control of the buffer Fe+Fe3O4, σ tends to decrease with rising pressure, nevertheless the activation enthalpy tends to increase. For the first time we have obtained values for the activation energy and activation bulk volume of the main charge carriers, which are (1.60±0.07) eV and (0.05±0.03) cm3/mol, respectively; (4) for a given pressure and temperature, σ tends to rise with increased oxygen fugacity, whereas the activation enthalpy and preexponential factor tend to decrease; and (5) the behaviour of the electrical conductivity at high temperature and high pressure can be reasonably interpreted by assuming that small polarons provide the dominant conduction mechanism in the pyroxenite samples.

Temperature dependence of elastic P- and S-wave velocities in porous Mt. Unzen dacite

Laboratory measurements of elastic properties of volcanic rocks are crucial for the modelling of volcano seismic activity. Compared to the large database reported in the literature for sedimentary, igneous and metamorphic rocks, the data set for volcanic rocks is limited and mostly restricted to basalts. Data for more silica-rich rocks are sparse. In particular, velocity data for silica-rich volcanic rocks measured at elevated temperature are lacking. We measured the elastic P- and S-wave velocities and the velocity anisotropy of porous dacitic rocks from Unzen Volcano, Japan, exhibiting an open porosity of 3.3 to 24.3 vol.%. The measurements were done at temperatures of up to 600 °C and confining pressures of 100 MPa, corresponding to depths of ∼ 3000–4000 m. Samples with even higher porosities failed at the required pressures. The measurements were carried out in a cubic multi-anvil pressure apparatus, using the pulse transmission technique. In contrast to low-porosity magmatic and metamorphic rocks, the seismic velocities of the investigated volcanic rocks increased and the velocity anisotropies decreased with increasing temperature, due to further sample compaction. There is a close relationship between velocity, density and porosity. The higher the density (and the lower the porosity) the higher are the P- and S-wave velocities. These results can contribute to a better understanding of the propagation of seismic energy through the volcanic edifice.

Compressional elastic wave velocities of serpentinized olivine-bearing pyroxeniteup to 960°C at 1.0 GPa

Compressional wave velocities (Vp)were measured in situ in serpentinized olivine-bearing pyroxenite at 1.0 GPa fromroom temperature to 960°C in a multi-anvil pressure apparatus. The velocityshowed a remarkable decrease from temperature more than 500̂C which was from6.2 to 5.0 km s−1. The decline in velocity is attributed to the changes of rockfabric, including the following aspects: composition, size and shape of grain,crack, grain boundary and dislocation changes resulting from dehydrationat high temperature. The fine-granular aggregates of minerals formedthrough dehydration were chemically analysed, and they were found to becomposed of forsterite and enstatite. Dehydration of serpentine has asignificant effect on the low-velocity zone and earthquake generation.

Elastic and electrical properties of serpentinite dehydration at high temperatureand high pressure

P-wave velocities (VP) andelectrical conductivities (σ)in serpentinite, collected from Ailaoshan orogenic belt, Yunnan province,China, were measured at 2.0–3.5 GPa and high temperature with the pulsetransmission–reflection combined method and impedance spectroscopyby the means of a multi-anvil pressure apparatus, the YJ-3000 press.VP decreasedand σincreased markedly at temperature higher than about 560°C. It is arguedthat the onset of the dehydration of serpentine is the main cause forVP decreasingand σincreasing. Combining the present experimental data with the results of previousexperiments, we were led to speculate that the formation of the low-velocity layerbetween the subducted slab and mantle wedge is closely related to thedehydration of serpentine.

Compressional elastic wave velocities of serpentinized pyroxenite at high pressures and high temperatures and its geological significance

1) Ultrasonic laboratory measurements of ultrasonic elastic vP carried out on serpentinized pyroxenite at 1.0 GPa from room temperature to 960 °C in a multi-anvil pressure apparatus, and four quenched products were gotten. The velocity was from 6.4 km/s to 5.0 km/s at temperatures more than 500 °C, with a remarkable decrease of 22%. At the temperature from 660 °C to 800 °C, the vP abnormally increased.

A technique for thermoelectric power measurements at high pressure in an octahedral multianvil press

We report a method for measuring the thermoelectric power of samples compressed in an octahedral multianvil pressure apparatus. Results are presented for the thermoelectric power at 300 K of doped Sb1.5Bi0.5Te3 up to 9 GPa and Nd0.45Ce2.55Pt3Sb4 up to 4.5 GPa.

The effect of hydration and dehydration reactions on wave velocities in basalts

Compressional and shear wave velocities were measured on zeolite-bearing and zeolite-free basalts at pressures up to 600 MPa and at temperatures up to 900°C. Measurements were done under dry and wet conditions in a multi-anvil pressure apparatus and in an internally heated gas apparatus at different effective pressures ( P eff): (1) with atmospheric pore pressure ( P eff= P conf), and (2) at pore pressure equal to confining pressure ( P eff=0), respectively. Temperature-induced abrupt velocity variations were found to be caused by hydration and dehydration processes. At conditions of zero effective pressure, the formation of zeolite gives rise to an increase of wave velocities at around 200°C in the zeolite-free basalt due to filling of cracks and fissures by the newly formed minerals and the coeval enhancement of grain-boundary contacts. Dehydration reactions in some zeolite-bearing tholeiitic basalts at initially high effective pressures cause a significant abrupt and irreversible drop of P- and S-wave velocities at temperatures around 350°C in the initially dry rock. The declines of velocity are attributed to spontaneous lowering of effective pressure due to internally created pore pressure that induces the opening of pore spaces and reconstitution of pore geometry.