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• W4322005801 abstract "The absence of seismic anisotropy in most regions of the lower mantle suggests that diffusion creep may be the dominant mechanism in the lower mantle. Because the diffusion-creep rate is inversely proportional to the 2~3 power of grain size, knowledge of the grain-growth kinetics is crucial for studying lower-mantle dynamics. For these reasons, this study determined the grain-growth kinetics of bridgmanite at a pressure of 27 GPa using advanced multi-anvil techniques.We first measured the grain sizes of bridgmanite in an olivine bulk composition with various annealing durations at 2200 K. The results were fitted to an equation dn &#8211; d0n = kt, where d and d0 are the final and initial grain sizes, respectively, n is the grain-size exponent, t is the annealing duration, and k is the growth-rate constant. This fitting yielded n = 5.2 &#177; 0.3, which is much smaller than given by a previous study [Yamazaki et al., 1996], n = 10.6 &#177; 1.1. This discrepancy may be because Yamazaki et al.&#8217;s [1996] olivine starting material may have contained adhesive water, which enhanced grain growth at the beginning of annealing. We then conducted runs at various temperatures, yielding the activation energy of 260 &#177; 20 kJ/mol. These results suggest that the bridgmanite grain sizes over 0.1 &#8211; 1 Gyr should have grain sizes of 150-230 &#956;m, which is one order of magnitude larger than Yamazaki et al.&#8217;s [2006] estimation. Consequently, the lower mantle should be much harder than previously considered.Furthermore, we measured the grain-growth kinetics as a function of the fraction of coexisting ferropericlase. Although the grain-growth kinetics is almost independent of the ferropericlase fraction down to 20 vol.%, it rapidly increases with decreasing ferropericlase fraction at lower fractions. Over 0.1~4.5 Gyr, the bridgmanite grain sizes in pure-bridgmanite rock should be 2 ~ 3 orders of magnitude larger than those coexisting with 20 vol.% of ferropericlase. These results suggest that pure-bridgmanite rock has 4 ~ 9 orders of magnitude lower flow rates than pyrolite if the diffusion creep is dominant. Since the diffusion creep rate in pure-bridgmanite rock is so low, the dislocation creep should dominate in pure-bridgmanite rock. We estimated that the pure-bridgmanite rock should have 1 ~ 2.5 orders of magnitude more viscous than pyrolite if the stress condition is 0.1~0.5 MPa in the lower mantle. This variation may interpret the viscosity variation in the lower mantle inferred from the geoid analysis [Rudolph et al., 2015], subduction speed [van der Meer et al., 2018], and plume morphology [French & Romaniwicz, 2016]." @default.
• W4322005801 created "2023-02-26" @default.
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• W4322005801 date "2023-05-15" @default.
• W4322005801 modified "2023-09-29" @default.
• W4322005801 title "Grain growth kinetics of bridgmanite under topmost lower-mantle" @default.
• W4322005801 doi "https://doi.org/10.5194/egusphere-egu23-7732" @default.
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