HR: 08:45h
AN: MR21A-04    [Abstracts]
TI: Theoretical and experimental discovery of a CaIrO$_{3}$-type MgSiO$_{3}$ phase in Earth's D" layer
AU: * Ono, S
EM: sono@jamstec.go.jp
AF: Institute for Research on Earth Evolution, Japan Agency for Marine-Earth Science and Technology, 2-15 Natsushima-cho, Yokosuka-shi, Kanagawa-ken, 237-0061 Japan
AU: Oganov, A R
EM: a.oganov@mat.ethz.ch
AF: Laboratory of Crystallography, Department of Materials, ETH Honggerberg, Wolfgang-Pauli-Strasse 10, Zurich, CH-8093 Switzerland
AB: Recent seismic studies have shown that the region above the core-mantle boundary (CMB), called the D" layer, contains strong seismic anomalies, such as seismic discontinuity, anisotropy, and anticorrelation between the shear and bulk sound velocities. For a long time, mineralogists and petrologists believed that the Earth's lower mantle is composed mainly of Fe- and Al-bearing MgSiO$_{3}$ perovskite. However, a high-pressure experimental study predicted a phase transition of MgSiO$_{3}$ perovskite using an analogy of the phase transition sequence of iron oxide, Fe$_{2}$O$_{3}$. A new high-pressure phase of Fe$_{2}$O$_{3}$ with an orthorhombic (Cmcm) CaIrO$_{3}$-type structure was stable above 60 GPa [1]. Recently, this idea was confirmed simultaneously by recent theoretical and experimental studies [e.g., 2]. We explored this idea using {\it ab initio} simulations and high-pressure X-ray diffraction experiments using a laser-heated diamond anvil cell. Theoretical calculations were based on density functional theory within the local density approximation (LDA) and the generalised gradient approximation (GGA). High-pressure experiments were performed using an angle-dispersive X-ray diffraction technique at the synchrotron beam line BL10XU, SPring-8 in Japan. The transition depth from the orthorhombic MgSiO$_{3}$ perovskite to CaIrO$_{3}$-type phase matched that of the observed seismic discontinuity at D" layer. A Clapeyron slope of this transition, calculated to be 8-10 MPa/K, agrees with both seismic observations and experimental results. The elastic properties of the CaIrO$_{3}$-type phase explain most of the previously puzzling properties of the D" layer: its seismic anisotropy, strongly undulating shear-wave discontinuity at its top, and possibly the anticorrelation between shear and bulk sound velocities. This new phase is therefore likely to be a major Earth-forming mineral, and its discovery will change our understanding of the deep Earth's interior. We will also discuss our latest, quite unexpected, results on this phase. [1] Ono et al, J. Phys. Chem. Solid 65 (2004)1527-1530. [2] Oganov and Ono, Nature 430 (2004) 445-448.
DE: 8124 Earth's interior--composition and state (old 8105)
DE: 3630 Experimental mineralogy and petrology
DE: 3924 High-pressure behavior
DE: 3954 X ray, neutron, and electron spectroscopy and diffraction
DE: 1212 Earth's interior--composition and state (8105)
SC: Mineral and Rock Physics [MR]
MN: 2004 AGU Fall Meeting