HR: 16:00h
AN: MR44A-01    [Abstracts]
TI: Influence of water on the phase transitions of olivine and its polymorphs in the Earth's mantle
AU: * Litasov, K D
EM: klitasov@ganko.tohoku.ac.jp
AF: Institute of Mineralogy, Petrology and Economic Geology, Faculty of Science, Tohoku University, Aoba-ku, Sendai, 980-8578 Japan
AU: Ohtani, E
EM: ohtani@mail.tains.tohoku.ac.jp
AF: Institute of Mineralogy, Petrology and Economic Geology, Faculty of Science, Tohoku University, Aoba-ku, Sendai, 980-8578 Japan
AB: Seismic observations indicate many unusual variations of the depth of 410-, 520-, and 660-km seismic discontinuities, which are usually attributed to the phase changes in (Mg,Fe)₂SiO₄ from olivine to wadsleyite, wadsleyite to ringwoodite and ringwoodite to Mg-perovskite + ferropericlase (post-spinel transformation), respectively. Studies of topography of the discontinuities indicate elevations of 410-km and depression of 660-km in the subduction zones and opposite effect in the hot spots/mantle plumes. Until recently these variations were considered to be consistent with experimentlally observed Clapeyron slopes of the olivine to wadsleyite and post-spinel transformations. Newest data for Mg₂SiO₄ and peridotite (pyrolite) systems obtained by in situ X-ray diffraction studies (Katsura et al., 2003; Fei et al., 2004; Litasov et al., 2005) indicate that the post-spinel transformation boundary has a gentle negative Clapeyron slope, which varies between -0.4 and -1.3 MPa/K. These studies create significant complexities to explain the topography of the 650-km discontinuity and indicate that the post-spinel phase transition may account for less than half of the variations of a depth of the 650-km discontinuity. In present study using in situ X-ray diffraction measurements we observed shift of the post-spinel transformation boundary in hydrous pyrolite by 0.6 GPa at 1473 K, which may account for the missing half of 30-40 km depressions of the 660 km discontinuity in the hot subduction zones. Recent quench multianvil experiments (Smyth and Frost, 2002; Chen et al., 2002; Litasov and Ohtani, 2003) showed a clear shift of the olivine-wadsleyite boundary to lower pressures by 1-2 GPa if we add water. The observed shifts of the olivine-wadsleyite and post-spinel transition boundaries are consistent with the different water solubility in wadsleyite and olivine and ringwoodite and Mg-perovskite, respectively. Wadsleyite may accommodate 5 to 40 times more water relative to olivine at about 1400 K and ringwoodite contains much more water (up to 2.6 wt.%) than Mg-perovskite and ferropericlase (less than 100 ppm). Litasov and Ohtani (2003) and Kawamoto (2004) reported a significant shift of wadsleyite to ringwoodite phase boundary to the higher pressure by addition of water to the peridotite system. These observations may be consistent with irregular distribution or splitting of the discontinuity at 520 km. However, causes of the significant displacement of the wadsleyite/ringwoodite phase boundary are not yet clarified.
DE: 1038 Mantle processes (3621)
DE: 3613 Subduction zone processes (1031, 3060, 8170, 8413)
DE: 3954 X-ray, neutron, and electron spectroscopy and diffraction
DE: 8124 Earth's interior: composition and state (1212, 7207, 7208, 8105)
SC: Mineral and Rock Physics [MR]
MN: Fall Meeting 2005