[S-IT38_30PO1] Rheology and Transport Phenomena of Geomaterials

[SIT38-P06] Grain growth experiment on pyrolite materialunder lower mantle conditions

*Masahiro IMAMURA¹, Tomoaki KUBO¹, Takumi KATO¹ (1.Kyushu Univ.)

Keywords:lower mantle, multi-anvil, pyrolite, grain growth

Grain size is a key parameter for understanding viscosity of Earth's mantle. Grain growth rate is one of important factors controlling the grain size. Especially, it is indispensable to examine grain growth kinetics in multiple phases because the grain growth rate of major phase drastically changes with the proportion of secondary phases (e.g., Hiraga et al., 2010). In the lower mantle, Mg-perovskite is major phase, and ferro-periclase, Ca-perovskite, and majoritic garnet are present as secondary phases (e.g., Irifune, 1994; Nishiyama and Yagi, 2003). The previous grain growth experiment (Yamazaki et al., 1996) in the two-phase system of MgSiO₃ perovskite and MgO periclase using Mg₂SiO₄ forsterite as a starting material suggests that the grain growth rate is too slow to explain the lower mantle viscosity constrained by geophysical observations. This inconsistency may arise from effects of the eutectoid transformation prior to the grain growth process (e.g., Solomatov et al., 2002). It is also necessary to examine effects of the chemical composition that affects the proportion of secondary phases and diffusivity. Here, we report preliminary results of the grain growth experiment on pyrolite material under lower mantle conditions. High-pressure and temperature experiments were conducted using a Kawai-type multi-anvil apparatus (QDES) installed at Kyushu University. Starting material is a powder with pyrolite composition that was used in the previous phase equilibrium study (e.g., Irifune, 1994). We conducted annealing experiments at 25-28 GPa and 1600-1800℃ for 6-600 min. Chemical compositions, microstructures and grain sizes of recovered samples were examined using a FE-SEM with an energy-dispersive analytical system. Four phases of Mg-perovskite, Ca-perovskite, ferro-periclase and majoritic garnet were present in recovered samples annealed at 25 GPa and 1600-1800℃. The normalized grain size distribution in the recovered samples showed Gaussian-like shape and the largest grain size is smaller than three times of the mean grain size, suggesting that normal grain growth occurred. The grain growth rate is faster than that of the previous study (Yamazaki et al., 1996). Preliminary analysis of the kinetic data of Mg-perovskite obtained showed the smaller grain growth exponent of 4.3 than that reported in the previous study. On the other hand, three phases of Mg-perovskite, Ca-perovskite and ferro-periclase were present at higher pressure of 28 GPa and 1800℃, in which the volume fraction of Mg-perovskite increased compared to the four-phases experiment. While the microstructure and the grain size distribution in the three-phase assemblage was similar to those of the four-phase assemblage, the grain size was larger probably due to the smaller proportion of the secondary phases. Our preliminary results provide some insights into the grain-size evolution in the lower mantle and suggest that further quantitative grain growth data with possible lower mantle conditions are needed.