Grain growth exponent of bridgmanite with ferropericalse


 The pioneering work by Yamazaki et al., 2005] investigated the grain growth kinetic of the bridgmanite(Brg)+ferropericlase(fPc) assemblage. One of the astonishing results of their study is that its grain size exponent was extremely high, i.e., 10~11, which concludes that the grain size of bridgmanite does not exceed 20 μm in the whole of Earth's history.  We re-investigated the grain growth kinetics of the Brg+fPc assemblage at a pressure of 27 GPa. The left figure shows an example of the determination of grain growth kinetics from presynthesized Brg+fPc. We found that the grain growth kinetics in the initial stages largely depend on starting material. Especially, the use of olivine powder produces huge grain-size exponents (lower left figure), probably due to adhesive water. We obtained a more robust exponent of 5~6 for bridgmanite grain growth. The grain size of bridgmanite can reach 100 μm in the Earth's history (lower right figure).



Left: BSE images (left) and grain size distributions (right) of the samples annealed at the same pressure and temperature conditions (27 GPa, 2200 K), but different run durations (1.5 to 1000 min)

A comparison of grain sizes with different types of starting materials (olivine powder, olivine single crystal, pre-synthesized post-spinel, and Fe-free forsterite single crystal) with forsterite powder from Yamazaki et al. [1996]. Ol: San Carlos olivine. Fo: Fe-free forsterite.

Grain sizes of bridgmanite at the topmost of the lower mantle for different temperatures as a function of geological time. The grain sizes based on Yamazaki et al. [1996] are also shown for comparison.

Fei, H., Faul, U., Katsura, T., The grain growth kinetics of bridgmanite at the topmost lower mantle, Earth and Planetary Science Letters 561, 116820, 2021. 10.1016/j.epsl.2021.116820