Takeshi Sanehira

Development of the Multi-anvil Assembly 6-6 for DIA and D-DIA type high-pressure apparatuses

A newly designed anvil assembly, Multi-anvil Assembly 6-6 (MA6-6), was developed. This assembly consists of six small anvils with an anvil guide, and can be compressed by DIA-type and deformation-DIA (D-DIA) apparatuses. The use of this anvil assembly simplifies the process of replacing anvils with those having different truncated edge length and/or hardness. As a consequence, the time needed for anvil replacement is significantly shortened. This is a benefit to experiments at synchrotron facilities because anvil replacement has to be carried out within the limited beamtime. Using a combination of an MA6-6 and DIA-type apparatus, pressure above 12 GPa was generated. A deformation experiment of polycrystalline MgO was performed using an MA6-6 with D-DIA at 4 GPa and room temperature. Two-dimensional X-ray diffraction patterns and X-ray radiographic images were collected from the deforming sample using monochromatic X-rays. Quantitative deformation experiments can be carried out using this experimental setup.

In situ determination of the spinel-post-spinel transition in Fe3O4 at high pressure and temperature by synchrotron X-ray diffraction

Abstract The position of the spinel-post-spinel phase transition in Fe3O4 has been determined in pressuretemperature space by in situ measurements using a multi-anvil press combined with white synchrotron radiation. Pressure measurement using the equation of state for MgO permitted pressure changes to be monitored at high temperature. The phase boundary was determined by the first appearance of diffraction peaks of the high-pressure polymorph (h-Fe3O4) during pressure increase and the disappearance of these peaks on pressure decrease along several isotherms. We intersected the phase boundary over the temperature interval of 700-1400 °C. The boundary is linear and nearly isobaric, with a slightly positive slope. Post-experiment investigation by TEM confirms that the reverse reaction from h-Fe3O4 to magnetite during decompression leads to the formation of microtwins on the (311) plane in the newly formed magnetite. Observations made during the phase transition suggest that the transition has a pseudomartensitic character, explaining in part why magnetite persists at conditions well within the stability field of h-Fe3O4, even at high temperatures. This study emphasizes the utility of studying phase transitions in situ at simultaneously high temperatures and pressures since the reaction kinetics may not be favorable at room temperature.

In situ investigation of high-pressure melting behavior in the Fe-S system using synchrotron X-ray radiography

We studied the high-pressure melting behavior of Fe-S mixtures containing 9 wt% sulfur using the synchrotron X-ray radiographic method in a large volume press. By opening two graphite windows along the X-ray path and using boron nitride capsules, we were able to observe segregation of sulfur-rich liquid and iron-rich solid upon initial melting and determine the eutectic temperature of the Fe-S system at pressures between 13 and 16 GPa. Our results are in excellent agreement with the existing data from quench experiments, and are further supported by in situ X-ray diffraction measurements. The liquidus temperatures of this composition, determined on the basis of the disappearance of the solid phase on the radiographs, are also consistent with literature values. By observing radiographic changes with time, we monitored the approach to equilibrium and obtained preliminary data on the kinetics of the melting process.

Density profiles of pyrolite and MORB compositions across the 660 km seismic discontinuity

Precise density changes across 660 km seismic discontinuity have been determined for pyrolite and mid-oceanic ridge basalt (MORB) compositions at 1873 K at high pressures using a combination of in situ X-ray diffraction on unit cell measurements and chemical composition analyses on the recovered products, using a multiple sample chamber cell assembly. Density jump for the post-spinel transformation in pyrolite was determined to be ∼9%, which is comparable to that of PREM. The density of MORB is found to be lower than that of pyrolite at the depths of ∼590–680 km (using Andersons equation of state for Au), however, it becomes denser than the pyrolite below ∼680 km. The density cross-over region at ∼590–680 km may contribute to the stagnation of subducted slabs near the 660 km discontinuity, as suggested by seismological observations as well as earlier experimental studies based on quench experiments and extrapolated in situ X-ray measurements.

Interfacial tension measurement of Ni-S liquid using high-pressure X-ray micro-tomography

High-pressure, high-temperature X-ray tomography experiments have been carried out using a large volume toroidal cell, which is optimized for interfacial tension measurements. A wide anvil gap, which corresponds to a field of view in the radiography imaging, was successively maintained to high pressures and temperatures using a composite plastic gasket. Obtained interfacial tensions of Ni-S liquid against Na, K-disilicate melt, were 414 and 336 mN/m at 1253 and 1293 K, respectively. Three-dimensional tomo-graphy images revealed that the sample had an irregular shape at the early stage of melting, suggesting either non-equilibrium in sample texture and force balance or partial melting of surrounding silicate. This information cannot always be obtained from two-dimensional radiographic imaging techniques. Therefore, a three-dimensional tomography measurement is appropriate for the precise interfacial measurements.

The thermal equation of state of FeTiO3 ilmenite based on in situ X-ray diffraction at high pressures and temperatures

Abstract We present in situ measurements of the unit-cell volume of a natural terrestrial ilmenite (Jagersfontein mine, South Africa) and a synthetic reduced ilmenite (FeTiO3) at simultaneous high pressure and high temperature up to 16 GPa and 1273 K. Unit-cell volumes were determined using energy-dispersive synchrotron X-ray diffraction in a multi-anvil press. Mössbauer analyses show that the synthetic sample contained insignificant amounts of Fe3+ both before and after the experiment. Results were fit to Birch- Murnaghan thermal equations of state, which reproduce the experimental data to within 0.5 and 0.7 GPa for the synthetic and natural samples, respectively. At ambient conditions, the unit-cell volume of the natural sample [V0 = 314.75 ± 0.23 (1σ) Å3] is significantly smaller than that of the synthetic sample [V0 = 319.12 ± 0.26 Å3]. The difference can be attributed to the presence of impurities and Fe3+ in the natural sample. The 1 bar isothermal bulk moduli KT0 for the reduced ilmenite is slightly larger than for the natural ilmenite (181 ± 7 and 165 ± 6 GPa, respectively), with pressure derivatives K0′ = 3 ± 1. Our results, combined with literature data, suggest that the unit-cell volume of reduced ilmenite is significantly larger than that of oxidized ilmenite, whereas their thermoelastic parameters are similar. Our data provide more appropriate input parameters for thermo-chemical models of lunar interior evolution, in which reduced ilmenite plays a critical role.

A combination of a Drickamer anvil apparatus and monochromatic X-rays for stress and strain measurements under high pressure

A modified Drickamer anvil apparatus has been developed to combine with monochromatic synchrotron radiation for high-pressure X-ray diffraction and radiography in the GSECARS bending-magnet station, 13-BM-D, at the Advanced Photon Source, Argonne, USA. Using this experimental set-up, deformation experiments can be carried out at pressures in excess of 30 GPa at high temperatures. Differential stresses and total axial strains of polycrystalline platinum and Mg(2)SiO(4) ringwoodite have been measured up to 32 GPa at room temperature using tungsten carbide anvils. The total axial strain of the platinum increases with pressure and reaches about 55% at the highest pressure. A test run using a composite sintered diamond anvil system was performed. The use of X-ray-tranparent anvils enables the entire Debye rings to be observed up to 10 degrees 2theta. With high-energy photons (65-70 keV), this allows a coverage in Q (= 2pi sintheta/lambda) to about 3 A(-1), thus making it possible to evaluate hydrostatic pressure and differential stress in crystalline minerals using diffraction. This, coupled with the ability to determine axial strain, allows deformation studies to be performed to pressures above 30 GPa.