EGU23-7732, updated on 25 Feb 2023
EGU General Assembly 2023
© Author(s) 2023. This work is distributed under
the Creative Commons Attribution 4.0 License.

Grain growth kinetics of bridgmanite under topmost lower-mantle

Hongzhan Fei1, Ulrich Faul2, Maxim Ballmer3, Nicolas Walte4, and Tomoo Katsura1
Hongzhan Fei et al.
  • 1Bayerisches Geoinstitut, University of Bayreuth, Bayreuth, Germany (
  • 2Earth Atmospheric and Planetary Sciences, Massachusetts Institute of Technology, Cambridge, MA, USA
  • 3Department of Earth Sciences, University College London, London, UK
  • 4Heinz Maier-Leibnitz Zentrum (MLZ), Technische Universität München, Garching, Germany

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 dnd0n = 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 ± 0.3, which is much smaller than given by a previous study [Yamazaki et al., 1996], n = 10.6 ± 1.1. This discrepancy may be because Yamazaki et al.’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 ± 20 kJ/mol. These results suggest that the bridgmanite grain sizes over 0.1 – 1 Gyr should have grain sizes of 150-230 μm, which is one order of magnitude larger than Yamazaki et al.’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].

How to cite: Fei, H., Faul, U., Ballmer, M., Walte, N., and Katsura, T.: Grain growth kinetics of bridgmanite under topmost lower-mantle, EGU General Assembly 2023, Vienna, Austria, 24–28 Apr 2023, EGU23-7732,, 2023.