Tetsu Watanuki

In situ X‐ray observations of phase assemblages in peridotite and basalt compositions at lower mantle conditions: Implications for density of subducted oceanic plate

[1]xa0Phase relationships in KLB-1 peridotite, normal-type mid-oceanic ridge basalt (NMORB) and K-rich basalt compositions were investigated experimentally from 28 to 143 GPa and 2050 to 3000 K. A laser-heated diamond anvil cell with the synchrotron X-ray diffraction was developed to perform in situ observations of samples at simultaneous high pressures and temperatures. We confirmed that the peridotite composition crystallized into an assemblage of Mg perovskite + Ca perovskite + magnesiowustite at pressures below 97 GPa and that NMORB and K-rich basalt compositions both crystallized as Mg perovskite + Ca perovskite + stishovite + an aluminous phase with a CaFe2O4-type structure at pressures below 68 GPa. The orthorhombic Mg perovskite is stable in natural rock compositions at pressures less than 97 GPa, corresponding to about 2200 km depth. Phase transition of silica phase from stishovite to CaCl2-type phase was observed at 78 GPa in NMORB composition. Moreover, we observed that NMORB crystallized in to an assemblage of CaIrO3-type (Mg,Fe)SiO3 + Ca perovskite + α-PbO2-type silica + an aluminous phase with a calcium CaTi2O4-type structure at 143 GPa, corresponding to the core-mantle boundary depth. We measured the unit cell volumes of minerals in the NMORB composition at high P-T and thereby estimated the density of subducted MORB to a depth of 1800 km in the lower mantle. In this depth regime, MORB is denser than model mantle densities, e.g., PREM and AK135. The estimated density of MORB may contribute to an understanding of dynamics of the subducted slab in the deep lower mantle.

Construction of laser-heated diamond anvil cell system for in situ x-ray diffraction study at SPring-8

A double-sided laser heated diamond anvil cell (DAC) system was constructed at a high brilliance, undulator beamline (BL10XU) at SPring-8 a third generation synchrotron radiation facility, for performing in situ angle-dispersive x-ray diffraction experiments under high temperature and high pressure. The design of this system puts emphasis on reliable data collection for the structural analysis. With this system, the adjustment of the optical systems for both x-ray and laser beams can be done easily, and high quality diffraction data can be obtained typically within several minutes. A system for temperature measurement of ten points in a sample area at the same time was also developed. The performance of the laser heated DAC system was tested by observing phase transitions of natural olivine.

Laser heated diamond anvil apparatus at the Photon Factory and SPring-8: Problems and improvements

Two high pressure high temperature in situ x-ray diffraction systems constructed at the Photon Factory and at SPring-8 are described. Both systems consist of a laser heated diamond anvil cell, CO2 or YAG laser heating systems, and an angle dispersive powder x-ray diffraction system with an imaging plate detector. CO2 laser heating has an advantage in that the sample can be heated stably and uniformly due to stable absorption of the sample. Use of type-IIa diamond, however, caused plastic deformation of the anvils. YAG laser heating, compared to CO2 laser heating, can be used to much higher pressures and temperatures, although a good control system is required for stable heating. Various problems encountered during construction are discussed.

Equation of state of CeCu2Ge2 at cryogenic temperature

Abstract The isothermal equation of state of CeCu2Ge2 at 10xa0K has been studied up to 20xa0GPa in a diamond-anvil cell using angle-dispersive X-ray diffraction technique with a synchrotron source and an imaging plate detector. At 15.0±1.6xa0GPa, an anomalous contraction of the volume was observed while the crystal structure remained unchanged. Relevance of the result with the enhancement of the superconducting transition temperature reported in this vicinity is discussed.

The compressibility of hexagonal Al-rich NAL phase: similarities and differences with calcium ferrite-type (CF) phase with implications for the lower mantle

Abstract We have determined the equation of state of a hexagonal Al-rich type NAL phase [K0.07Na0.81Ca0.12]Σ1.01[Mg1.62Fe0.38]Σ2.00[Al4.98Fe0.10Ti0.05Si0.88]Σ6.01O12, an aluminum host mineral that is likely to occur in subducted oceanic crusts (mid-oceanic ridge basaltic (MORB)) in the Earth’s lower mantle. Angle-dispersive X-ray powder diffraction experiments were conducted up to 36xa0GPa at room temperature at the BL04-B2 at Spring-8. Fitting a third-order Birch–Murnaghan equation of state to the data gives a bulk modulus of K 0 =214 (±2) xa0GPa with K 0 ′=3 (±0.1) , a volume V0=184.55(6)xa0A3 and a zero-pressure density of ρ0=4xa0g/cm3. When K0′ is fixed at 4, the value of K 0 =202.3 (±0.9) xa0GPa. These values are comparable to those for similar anhydrous hexagonal aluminous phases reported previously by Ono et al. [Phys. Chem. Minerals 29 (2002a) 527]. The density of NAL phase is lower than that of other minerals (excluding the CF phase) co-existing in the subducted oceanic crust, such as Mg-perovskite, Ca-perovskite and stishovite. We find that the elastic properties of the hexagonal Al-rich phase do not change despite substantial variations in Si, Al, Fe, Mg, Na and K content in the NAL chemical composition.