The Effect of Pressure on Grain‐Growth Kinetics in Olivine Aggregates With Some Geophysical Applications.

The effect of pressure on grain‐growth kinetics of olivine was investigated up to 10 GPa at 1773 K under relatively water‐poor conditions. The results are interpreted using a relation dn−d0n=k0texp(−(Ek∗+PVk∗)/RT) to obtain the activation volume Vk∗ = 5.0 ± 1.1 cm3/mol for n = 2 or Vk∗ = 5.2 ± 1.1 cm3/mol for n = 3. The small activation volume means that grain‐growth kinetics in pure olivine aggregates is fast even in the dry deep upper mantle, implying that grain‐size is controlled by the pinning by other phases or by dynamic recrystallization except for the early stage after the phase transformation from wadsleyite in upwelling materials. The present results are applied to seismic wave attenuation that is likely controlled by grain‐boundary processes. The inferred peak in attenuation just below the oceanic lithosphere‐asthenosphere boundary from the NoMelt array is difficult to be explained by the pressure effects assuming the absorption band behavior because such a model requires a much larger activation volume than determined in this work and it also fails to explain high attenuation in the deep asthenosphere. This suggests that either melt accumulation or other processes such as elastically accommodated grain‐boundary sliding (EAGBS) is responsible for the peak in attenuation. The present results are also applied to EAGBS. We suggest that the deep upper mantle is likely to be relaxed by EAGBS, which implies that the shear velocity of the deep upper mantle can be several percent smaller than that inferred from single crystal elasticity. Plain Language Summary: Processes involving grain‐boundaries play an important role in Earth's interior. They include seismic wave attenuation and resultant reduction in seismic wave velocities. These properties are controlled by thermally activated processes, and hence they are expected to be sensitive to pressure. However, direct experimental studies in seismic attenuation are difficult under high‐pressure conditions. In this study, we performed experiments of grain‐growth in olivine (the dominant mineral of the Earth's upper mantle) at various pressures and applied the experimental data to discuss how the propagation of seismic waves in the Earth's mantle is affected by grain boundary‐related processes. Key Points: The activation volume of grain‐growth in olivine aggregates was experimentally determinedA small pressure effect was observed, indicating that grain‐growth is fast even in the deep upper mantle unless pinning is effectiveThe anelasticity in the deep upper mantle is evaluated under the assumption that it has a similar activation volume as grain‐growth [ABSTRACT FROM AUTHOR]