Kei Hirose

Bio: Kei Hirose is an academic researcher from Tokyo Institute of Technology. The author has contributed to research in topics: Mantle (geology) & Post-perovskite. The author has an hindex of 64, co-authored 261 publications receiving 15730 citations. Previous affiliations of Kei Hirose include Japan Agency for Marine-Earth Science and Technology & German Aerospace Center.

Post-Perovskite Phase Transition in MgSiO3

Abstract: In situ x-ray diffraction measurements of MgSiO3 were performed at high pressure and temperature similar to the conditions at Earth9s core-mantle boundary. Results demonstrate that MgSiO3 perovskite transforms to a new high-pressure form with stacked SiO6-octahedral sheet structure above 125 gigapascals and 2500 kelvin (2700-kilometer depth near the base of the mantle) with an increase in density of 1.0 to 1.2%. The origin of the D″ seismic discontinuity may be attributed to this post-perovskite phase transition. The new phase may have large elastic anisotropy and develop preferred orientation with platy crystal shape in the shear flow that can cause strong seismic anisotropy below the D″ discontinuity.

The fate of subducted basaltic crust in the Earth's lower mantle

Abstract: The subduction of oceanic lithosphere into the Earth's deep interior is thought to drive convection and create chemical heterogeneity in the mantle. The oceanic lithosphere as a whole, however, might not subduct uniformly: the fate of basaltic crust may differ from that of the underlying peridotite layer because of differences in chemistry, density and melting temperature. It has been suggested that subducted basaltic crust may in fact become buoyant at the mantle's 660-km discontinuity, remaining buoyant to depths of at least 800 km, and therefore might be gravitationally trapped at this boundary to form a garnetite layer1, 2. Here we report the phase relations and melting temperatures of natural mid-ocean ridge basalt at pressures up to 64 GPa (corresponding to ∼1,500 km depth). We find that the former basaltic crust is no longer buoyant when it transforms to a perovskitite lithology at about 720 km depth, and that this transition boundary has a positive pressure–temperature slope, in contrast to the negative slope of the transition boundary in peridotite. We therefore predict that basaltic crust with perovskitite lithology would gravitationally sink into the deep mantle. Our melting data suggest that, at the base of the lower mantle, the former basaltic crust would be partially molten if temperatures there were to exceed 4,000 K.

Experimentally determined postspinel transformation boundary in Mg2SiO4 using MgO as an internal pressure standard and its geophysical implications

Abstract: [1] We have determined the postspinel transformation boundary in Mg2SiO4 by combining quench technique with in situ pressure measurements, using multiple internal pressure standards including Au, MgO, and Pt. The experimentally determined boundary is in general agreement with previous in situ measurements in which the Au scale of Anderson et al. [1989] was used to calculate pressure: Using this pressure scale, it occurs at significantly lower pressures compared to that corresponding to the 660-km seismic discontinuity. In this study, we also report new experimental data on the transformation boundary determined using MgO as an internal standard. The results show that the transition boundary is located at pressures close to the 660-km discontinuity using the MgO pressure scale of Speziale et al. [2001] and can be represented by a linear equation, P(GPa) = 25.12 − 0.0013T(°C). The Clapeyron slope for the postspinel transition boundary is precisely determined and is significantly less negative than previous estimates. Our results, based on the MgO pressure scale, support the conventional hypothesis that the postspinel transformation is responsible for the observed 660-km seismic discontinuity.

First-principles calculations of the ferroelastic transition between rutile-type and CaCl2-type SiO2 at high pressures

Abstract: The tetragonal to orthorhombic ferroelastic phase transition between rutile- and ${\text{CaCl}}_{2}$-type ${\text{SiO}}_{2}$ at high pressures is studied using first-principles calculations and the Landau free-energy expansion. The phase transition is systematically investigated in terms of characteristic phonon modes with ${\text{B}}_{1g}$ and ${\text{A}}_{g}$ symmetries, shear moduli, transverse-acoustic mode, rotation angle of the ${\text{SiO}}_{6}$ octahedra, spontaneous symmetry-breaking and volume strains, and enthalpy. The results show that these physical behaviors at the transition are well described using the Landau free-energy expansion parametrized by the first-principles calculations.

Crystal structure prediction using ab initio evolutionary techniques: principles and applications.

Abstract: We have developed an efficient and reliable methodology for crystal structure prediction, merging ab initio total-energy calculations and a specifically devised evolutionary algorithm. This method allows one to predict the most stable crystal structure and a number of low-energy metastable structures for a given compound at any P-T conditions without requiring any experimental input. Extremely high (nearly 100%) success rate has been observed in a few tens of tests done so far, including ionic, covalent, metallic, and molecular structures with up to 40 atoms in the unit cell. We have been able to resolve some important problems in high-pressure crystallography and report a number of new high-pressure crystal structures (stable phases: epsilon-oxygen, new phase of sulphur, new metastable phases of carbon, sulphur and nitrogen, stable and metastable phases of CaCO3). Physical reasons for the success of this methodology are discussed.