Noriyoshi Tsujino

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.

Mantle dynamics inferred from the crystallographic preferred orientation of bridgmanite

Seismic shear wave anisotropy is observed in Earth’s uppermost lower mantle around several subducted slabs. The anisotropy caused by the deformation-induced crystallographic preferred orientation (CPO) of bridgmanite (perovskite-structured (Mg,Fe)SiO3) is the most plausible explanation for these seismic observations. However, the rheological properties of bridgmanite are largely unknown. Uniaxial deformation experiments have been carried out to determine the deformation texture of bridgmanite, but the dominant slip system (the slip direction and plane) has not been determined. Here we report the CPO pattern and dominant slip system of bridgmanite under conditions that correspond to the uppermost lower mantle (25 gigapascals and 1,873 kelvin) obtained through simple shear deformation experiments using the Kawai-type deformation-DIA apparatus. The fabrics obtained are characterized by [100] perpendicular to the shear plane and [001] parallel to the shear direction, implying that the dominant slip system of bridgmanite is [001](100). The observed seismic shear- wave anisotropies near several subducted slabs (Tonga–Kermadec, Kurile, Peru and Java) can be explained in terms of the CPO of bridgmanite as induced by mantle flow parallel to the direction of subduction.

Isothermal compression of face-centered cubic iron

Abstract Isothermal compression curves of face-centered cubic iron (γ-Fe) were determined at high temperatures (1273 and 1073 K) up to 27 GPa by in situ X-ray diffraction experiments using synchrotron radiation and the Kawai-type multi-anvil apparatus. Fits of the third-order Birch-Murnaghan equation of state to pressure-volume data yielded V0 = 48.997 ± 0.040 Å3, KT0 = 108.3 ± 2.4 GPa, and K′T = 5.8 ± 0.2 for 1273 K, and V0 = 48.600 ± 0.098 Å3, KT0 = 88.9 ± 5.1 GPa, and K′T = 8.9 ± 0.7 for 1073 K, where V0, KT0, and K′T are unit-cell volume, bulk modulus and its pressure derivative, respectively, at ambient pressure. The relatively large values of K′T are attributable to successive electronic spin state transitions from mixed-spin at lower pressures to low-spin at higher pressures. When discussing the constituents of Earth’s (or other planets’) solid inner core in terms of density and equations of state, one must carefully consider the influence of the electronic spin state.

P-V-T relations of γ-Ca3(PO4)2 tuite determined by in situ X-ray diffraction in a large-volume high-pressure apparatus

Abstract Tuite, γ-Ca3(PO4)2, is regarded as an important phosphate mineral in the deep mantle playing a crucial role as a host for rare earth elements, large ion lithophile elements, and phosphorus. In this study we report the thermoelastic properties of synthetic γ-Ca3(PO4)2 at simultaneously high pressures and temperatures of up to 35.4 GPa and 1300 K, respectively, as determined by means of in situ energydispersive X‑ray diffraction in a large-volume multi-anvil apparatus. The pressure-volume-temperature data obtained for γ-Ca3(PO4)2 were fitted by the high-temperature Birch-Murnaghan equation of state to yield V2 = 447.4(4) Å3, KT0 = 100.8(18) GPa, K′T0 = 5.74(13), (∂KT/∂T)P = -0.020(1) GPa/K, and αT = 3.26(18) × 10-5 + 1.76(24) × 10-8 T. In addition, fitting the present data to the Mie-Grüneisen-Debye equation of state gives γ0 = 1.35(6), Θ0 = 944(136) K, and q = 0.37(29). Based on the thermoelastic properties obtained in our study, the density profiles of γ-Ca3(PO4)2 tuite along typical cold and hot slab geotherms were calculated and are compared with those of the coexisting silicate minerals in subducting mid-ocean ridge basalt.

Effect of cation substitution on bridgmanite elasticity: A key to interpret seismic anomalies in the lower mantle

Seismological observations show that, in some regions of the lower mantle, an increase in bulk sound velocity, interestingly, occurs in the same volume where there is a decrease in shear velocity. We show that this anti-correlated behavior occurs on cation substitution in bridgmanite by making single crystal elasticity measurements of MgSiO3 and (Mg,Fe,Al)(Si,Al)O3 using inelastic x-ray scattering in the ambient conditions. Cation substitution of ferrous iron and aluminum may explain large low shear velocity provinces in the lower mantle.

Stress measurement under high pressure using Kawai-type multi-anvil apparatus combined with synchrotron radiation.

A system for stress measurement under high pressure has been developed at beamline BL04B1, SPring-8, Japan. A Kawai-type multi-anvil apparatus, SPEED-1500, was used to pressurize polycrystalline KCl to 9.9 GPa in a mechanically anisotropic cell assembly with the KCl sample sandwiched between dense Al(2)O(3) pistons. The variation of deviatoric stress was determined from the lattice distortion measured using two-dimensional X-ray diffraction with monochromatic synchrotron X-rays. The low-pressure B1 phase transformed to the high-pressure polymorph B2 during compression. The deviatoric stress increased with increasing pressure in both the B1 and B2 phases except for the two-phase-coexisting region at a pressure of 2-3 GPa. This new system provides one of the technical foundations for conducting precise rheological measurements at conditions of the Earths lower mantle.

Generation of pressures over 40 GPa using Kawai-type multi-anvil press with tungsten carbide anvils

We have generated over 40 GPa pressures, namely, 43 and 44 GPa, at ambient temperature and 2000 K, respectively, using Kawai-type multi-anvil presses (KMAP) with tungsten carbide anvils for the first time. These high-pressure generations were achieved by combining the following pressure-generation techniques: (1) precisely aligned guide block systems, (2) high hardness of tungsten carbide, (3) tapering of second-stage anvil faces, (4) materials with high bulk modulus in a high-pressure cell, and (5) high heating efficiency.

Semiconductor diamond heater in the Kawai multianvil apparatus: an innovation to generate the lower mantle geotherm

Semiconductor diamond is considered the best heater material to generate ultra-high temperatures in a Kawai cell. In two pioneering studies, a mixture of graphite and amorphous boron (or boron carbide, B4C) was converted to semiconductor diamond in the diamond stability field and was confirmed to generate 2000°C and 3500°C, respectively. Following these works, we synthesized a homemade boron-doped graphite block with fine machinability. With this technical breakthrough, we developed a semiconductor diamond heater in a smaller Kawai-type cell assembly. Here, we report the procedure for making machinable boron-doped graphite, and the performance of the material as a heater in a Kawai cell at 15 GPa using tungsten carbide anvils and at ∼50 GPa using sintered diamond anvils. Furthermore, we present a finite element simulation of the temperature distribution generated by a semiconductor diamond heater, which is much more homogeneous than that generated by a metal heater.

Pressure generation to 65 GPa in a Kawai-type multi-anvil apparatus with tungsten carbide anvils

ABSTRACT We have expanded the pressure ranges at room and high temperatures generated in a Kawai-type multi-anvil apparatus (KMA) using tungsten carbide (WC) anvils with a high hardness of Hv = 2700 and a Young’s modulus of 660 GPa. At room temperature, a pressure of 64 GPa, which is the highest pressure generated with KMA using WC anvils in the world, was achieved using 1°-tapered anvils with a 1.5-mm truncation. Pressures of 48–50 GPa were generated at high temperatures of 1600–2000 K, which are also higher than previously achieved. Tapered anvils make wide anvil gaps enabling efficient X-ray diffraction. The present pressure generation technique can be used for studying the upper part of the Earth’s lower mantle down to 1200 km depth without sintered diamond anvils.

Elastic wave velocity anomalies of anorthite in a subducting plate: In situ experiments

Abstract To understand the origin of observed low velocities in the crustal portion of subducting plates, we performed in situ measurements of elastic wave velocities of anorthite at temperatures up to 1373 K at pressure of ~1 GPa and up to 773 K at 2.0-7.0 GPa. A fine-grained polycrystalline anorthite, which was synthesized using a gas pressure apparatus, was used for the measurements. The high-pressure experiments were performed using the multi-anvil apparatus installed on beamline BL04B1 at SPring-8. The elastic wave velocity was measured by the ultrasonic pulse method with synchrotron X‑ray radiographic imaging and X‑ray diffraction techniques. At ~1.0 GPa, elastic wave velocities exhibited a sharp temperature-induced kink at ~500 K. Below 500 K, the elastic wave velocities decrease with increasing temperature. In contrast, above 500 K, the elastic wave velocities show an increasing trend in the range of 500-900 K, and then revert back to a decreasing trend at above 900 K. We also found a pressure-induced velocity anomaly of anorthite. At 300-373 K, νP is constant up to 4 GPa, but decrease above 4 GPa with increasing pressure, while νS decreases monotonously with increasing pressure. These elastic anomalies are considered to be attributable to the tilting behavior of the corner-sharing TO4 (T = Al, Si) tetrahedra in three-dimensional frameworks of anorthite. Our results suggest the presence of plagioclase feldspar has the potential to causes low-velocity anomaly in the subducting oceanic crust when it survives as a metastable phase in the slab at higher pressure and lower temperature conditions.