Effect of Impurities on Polycrystal Anelasticity

Abstract

AbstractRock anelasticity causes seismic low velocity and high attenuation if, and only if, attenuation spectrum has a large “high‐frequency peak” superposed on the monotonic background. As a result, it is critical to define the conditions for peak existence. It is known that a large and broad high‐frequency peak exists at near solidus temperatures because grain boundary sliding is enhanced by the significant increase in grain boundary disorder called pre‐melting. The current study experimentally investigated the high‐frequency peak caused by impurities using organic polycrystals as an analog to mantle rock. A method for removing impurities was developed and anelasticity, viscosity, and grain growth rate of polycrystalline borneol with and without impurities were measured. The results showed that impurities can also significantly disorder the grain boundaries and cause a large and broad high‐frequency peak. This impurity‐induced peak disappeared, when the impurities were removed. On the basis of these results, this study draws a conclusion that the existence or non‐existence of the high‐frequency peak is strongly coupled with the grain boundary structure. This study also shows that the underlying mechanism of the peak is diffusionally accommodated grain boundary sliding, which is different from the prediction of existing models, and that further refinement of the models is needed. I further confirmed that the effects of impurities were minor in the published data which demonstrated the mechanical effects of premelting.