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

Abstract

AbstractThe 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 to obtain the activation volume = 5.0 ± 1.1 cm3/mol for n = 2 or = 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.