HR: 08:00h
AN: MR21A-01    [Abstracts]
TI: Discovery of MgSiO$_{3}$ Post-Perovskite Phase Transition
AU: * Hirose, K
EM: kei@geo.titech.ac.jp
AF: Dept. Earth & Planetary Sciences, Tokyo Institute of Technology, 2-12-1 Ookayama, Meguro, Tokyo, 152-851 Japan
AU: Murakami, M
EM: mmurakam@geo.titech.ac.jp
AF: Dept. Earth & Planetary Sciences, Tokyo Institute of Technology, 2-12-1 Ookayama, Meguro, Tokyo, 152-851 Japan
AU: Kawamura, K
EM: kats@geo.titech.ac.jp
AF: Dept. Earth & Planetary Sciences, Tokyo Institute of Technology, 2-12-1 Ookayama, Meguro, Tokyo, 152-851 Japan
AU: Sata, N
EM: sata@jamstec.go.jp
AF: Institute for Research on Earth Evolution, Japan Agency for Marine-Earth Science and Technology, 2-15 Natsushima-cho, Yokosuka, 237-0061 Japan
AU: Ohishi, Y
EM: ohishi@spring8.or.jp
AF: Japan Synchrotron Radiation Research Institute, 1-1 Koto, Mikaduki, 679-5198 Japan
AU: Tateno, S
EM: stateno@geo.titech.ac.jp
AF: Dept. Earth & Planetary Sciences, Tokyo Institute of Technology, 2-12-1 Ookayama, Meguro, Tokyo, 152-851 Japan
AU: Takafuji, N
EM: takafuji@fox25.hucc.hokudai.ac.jp
AF: Div. Earth & Planeraty Sciences, Hokkaido University, N10 W8, Sapporo, 060-0810 Japan
AB: Recent progress in X-ray diffraction (XRD) measurement in situ at high-pressure and -temperature in a laser-heated diamond-anvil cell (LHDAC) enables us to do a search for new high-pressure phases to the core-mantle boundary condition. MgSiO$_{3}$ perovskite is a principal mineral at least in the upper part of the lower mantle, but its stability and possible phase transition at greater depths have long been controversial. We first found the phase transition of MgSiO$_{3}$ perovskite in an experiment with multi-component natural mantle composition, in which XRD pattern significantly changed at 115 GPa and 2550 K. Later, we observed similar change in the XRD pattern of pure MgSiO$_{3}$ composition above 127 GPa and 2500 K. Based on the fifteen new diffraction peaks from pure MgSiO$_{3}$, we determined the crystal structure of post-perovskite phase by the molecular dynamics (MD) calculations (Murakami et al., Science, 2004). The new phase has SiO$_{6}$ octahedral sheet stacking-structure with orthorhombic symmetry (space group: Cmcm) and is isostructural with UFeS$_{3}$ and CaIrO$_{3}$. Stability of MgSiO$_{3}$ post-perovskite phase was confirmed later by theory. First-principles calculations further demonstrate that it can cause the DO_L discontinuity, S-wave anisotropy, and anti-correlation between S-wave and bulk-sound velocities in the lowermost mantle. Our group has been working on the post-perovskite phase transition in a variety of bulk chemical compositions. It occurs in both MgGeO$_{3}$ (Hirose et al., Am. Mine., 2004) and MnGeO$_{3}$ around 60 GPa at 1600 K, indicating that they are low-pressure analogues to MgSiO$_{3}$. In contrast, CdGeO$_{3}$ perovskite becomes less distorted with increasing pressure, and the perovskite structure is stable at least to 110 GPa and 2000 K. The Al-bearing MgSiO$_{3}$-rich post-perovskite phase also appears in a MORB composition above 108 GPa and 2200 K. The buoyancy relationships between the mantle and former basaltic crust can be complex at the post-perovskite phase transition in both the compositions. It has important implications for the fate of basaltic crust component in the deep interior.
DE: 3630 Experimental mineralogy and petrology
SC: Mineral and Rock Physics [MR]
MN: 2004 AGU Fall Meeting