高圧地球科学　　下部マントルの流動特性

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  • High Pressure Earth Science. Rheological Properties of the Lower Mantle.
  • カブ マントル ノ リュウドウ トクセイ

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The earth's lower mantle is mainly composed of (Mg, Fe) SiO3 perovskite and (Mg, Fe) O magneSiOwustite. It is essential to determine the rheological properties of MgSiO3 perovskite and periclase for understanding the rheology of the lower mantle. High pressure and temperature experiments were carried out under lower mantle conditions to determine their rheological properties. The grain growth rates of perovskite and periclase were determined to be G10. 6 [m] = 1×10-57. 4t [sec] exp (-320. 8 [kJ/mol] /RT) and G10. 8 [m] =1×10-62. 3t [sec] exp (-247. 0[kJ/mol]/RT), respectively, where G is grain size at time t, R is the gas constant and T is the absolute temperature. The lattice diffuSion coefficient (D1) and grain boundary diffuSion coefficient (Dgb) of silicon in MgSiO3 perovskite were determined at 25GPa and 1673-2073 K to be D1 [m2/sec] = 3. 76×10-10exp (-338 [kJ/mol] /RT) and δDgb [m3/sec] =1. 02×10-16exp (-303 [kJ/mol]/RT), respectively, where δ is the width of the grain boundary. The grain size of perovskite in the lower mantle is estimated to be 1-10 mm, which suggests diffusion creep (Nabarro-Herring creep) as a dominant deformation mechanism in the greater part of the lower mantle. The present results indicate that the subducting slab is much softer than the surrounding lower mantle due to the slow grain growth rate.

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