HR: 0830h
AN: V11D-0519    [PDF]
TI: Influence of water on the compressional mechanism of $\beta$ and $\gamma$ spinels
AU: * Yamamoto, T
EM: tyama@cms.mtl.kyoto-u.ac.jp
AF: Fukui Institute for Fundamental Chemistry, Kyoto University, 34-4 Takano-nichihiraki-cho, Sakyo-ku, Kyoto, 606-8103 Japan
AU: Yuen, D A
EM: davey@krissy.geo.umn.edu
AF: Department of Geology and Geophysics and Minnesota Supercomputing Institute, University of Minnesota, 310 Pillsbury Drive SE, Minneapolis, MN 55455-0219 United States
AU: Tanaka, I
EM: tanaka@cms.mtl.kyoto-u.ac.jp
AF: Department of Materials Science and Engineering, Kyoto University, Yoshida-honmachi, Sakyo-ku, Kyoto, 606-8501 Japan
AU: Adachi, H
EM: adachi@cms.mtl.kyoto-u.ac.jp
AF: Department of Materials Science and Engineering, Kyoto University, Yoshida-honmachi, Sakyo-ku, Kyoto, 606-8501 Japan
AB: It is widely accepted that the atmosphere and the oceans of the Earth are formed by degassing of the Earth's mantle. Most of the water may have been lost or it may still be stored in the Earth's mantle. If considerable amounts of water are present in the Earth's mantle, such water plays a key role in the geodynamics of the Earth's interior, because it affects the melting temperature and the transport properties of minerals as well as their elastic properties. Recent high pressure experiments suggested that main components of the transition zone of the Earth_fs mantle, the wadsleyite and the ringwoodite, can store significant amount of water [1, 2]. More recently, the lower mantle minerals, consisting of Mg-perovskite, magnesiowustite and Ca-perovskite, can potentially store considerable amounts of water [3]. However, the effects of water solution on their physical properties and the substitution mechanisms of water in these materials have not yet been fully understood. Then the first principles calculations within the density functional theory are performed here to investigate change in compressional mechanism of hydrous wadsleyite ($\beta$-Mg$_{2}$SiO$_{4}$) and ringwoodite ($\gamma$-Mg$_{2}$SiO$_{4}$), and substitution mechanisms of water in these minerals. Here two types of substitution mechanisms have been examined for wadsleyite changing the positions of substituted water molecule. Observed cell parameters and bulk moduli of the anhydrous and hydrous wadsleyites are well reproduced by the present calculations. Bulk moduli of these two calculated hydrous wadsleyite become significantly lower than that of water-free one, which strongly supports the high pressure experiment. For ringwoodite, we also examined two types of substitution models corresponding to the experimental stoichiometries, in which Mg$^{2+}$ and Si$^{4+}$ ions are replaced by two and four H$^{+}$ in the first and second models, respectively. Resultant reduction of the bulk moduli due to water incorporations reproduces the experimental ones as well as the case for wadsleyite. The influence of water incorporations on the transition pressure from $\beta$ to $\gamma$ spinels are also examined here using the free energies here calculated. Present calculations show that this transition pressure become larger by a few GPa when these minerals contain water. [1] Inoue et al., Geophys. Res. Lett. 22, 117-120 (1995). [2] Yusa et al., Geophys. Res. Lett. 27, 413-416 (2000). [3] Murakami et al., Science 295, 1885-1887 (2002).
DE: 3900 MINERAL PHYSICS
DE: 3904 Defects
DE: 3909 Elasticity and anelasticity
DE: 3919 Equations of state
DE: 3924 High-pressure behavior
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2003 Fall Meeting