[S-IT21] 固体地球科学と材料科学の融合が切り拓く新展開

[SIT21-08] 高圧実験に基づくブリッジマナイトの変形機構図

*辻野 典秀¹、山崎 大輔¹ (1.岡山大学・惑星物質研究所)

キーワード：変形機構図、ブリッジマナイト、高圧実験、レオロジー

Geophysical observations have revealed that the lower mantle has the largest viscosity in the Earth’s mantle. The lower mantle viscosity would play a key role in the global mantle dynamics. In order to understand the lower mantle viscosity, it is important to determine viscosity of bridgmanite which is regarded as the most dominant mineral of the lower mantle. Nevertheless, experimental data on viscosity of bridgmanite is quite limited due to experimental difficulty. Therefore, we conducted in-situ stress-strain measurements of bridgmanite by uniaxial deformation experiments at the lower mantle pressure conditions by newly developed deformation apparatuses with the Kawai type cell. Experimental conditions covered temperatures of 1473-1673 K and pressures of 23-27 GPa. The obtained stress exponent and activation enthalpy suggest that the dominant deformation mechanism of bridgmanite is climb-controlled dislocation creep. Bridgmanite is found to have the largest creep strength among mantle constituent minerals at same strain-rate even under nominally dry conditions. Based on this results and diffusion coefficient of Si in bridgmanite reported by Yamazaki et al. (2000), the deformation mechanisms map of bridgmanite was constructed. Combining deformation mechanism map of bridgmanite with geophysical observations indicates that grain-size of bridgmanite and stress conditions at top of the lower mantle would be > 8 mm and < 3×10⁵ Pa, respectively. The expected grain size of bridgmanite with 3-8 mm is much larger than that of bridgmanite in multi-phase system after the phase transition from ringwoodite across the 660 km discontinuity during convection of the mantle even followed by the grain growth for 1 billion years (e.g., Yamazaki et al., 1996). This fact suggests that large amount of the lower mantle materials would not have experienced the dissociation during the mantle convection and would have been isolated from the mixing by mantle stirring after crystallization of the magma ocean.