| Research Abstract |
At the 670-km discontinuity, spinel dissociates to a fine-grained mixture of perovskite plus magnesiowustite. The fine-grained dissociation product may exhibit superplastic behavior ; we have thus accounted for the observed absence of seismicity below -700 km depth. In order to investigate the rheological properties of the dissociation product, grain growth of MnO + MnTiO_3 (an analogue of Pv + Mw) was studied at 0.9-1.6GPa and 1000-1320ﾟC. Even after annealing for 256 hours, grain size was less than 10 mum. Such a slow growth rate causes at least several tens of kilometers superplastic zone in the lower mantle slab. Further the dissociation product of Mn_2TiO_4 was deformed in a piston-cylinder high-pressure apparatus. Long and narrow grains of MnO and MnTiO_3, which are typical in superplastic texture, were observed in the deformed sample.
Based on the creep and anelasticity data of mantle rocks at high pressure and temperature, we have derived a relationship between viscosity and seismic Q. This relation was used to estimate mantle viscosity from seismic Q structure. For the upper mantle beneath northeastern Japan, viscosity is low (-10^<20>Pa・s) in the back-arc side of volcanoes, and increases to -10^<23>Pa・s toward the tip of the mantle wedge. Viscosity of the descending slab is greater than 10^<23>Pa・s. Beneath the Pacific Ocean, upper mantle viscosity from the bottom of the lithosphere to 300 km depth is in the range of 3x10^<18>-3x10^<22>Pa・s. Viscosity at the lithosphere-asthenosphere boundary is in the range of 10^<20>-10^<21>Pa・s, and lithospheric viscosity is found to be larger than 10^<21>Pa・s. For the asthenosphere beneath the Iceland Plateau, Qs=7 (0-5 Ma) and 50 (5-10 Ma) yield 10^<16> and 10^<18>Pa・s, respectively. These viscosity values are consistent with the thermal states of the two regions, where one may expect 3-7 vol.% partial melt in 0-5 Ma and about 90 % of the solidus temperature in 5-10 Ma.
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