Effect of Melt on Polycrystal Anelasticity

Abstract This study provides the quantitative assessment of the effect of melt on the polycrystal anelasticity by grain boundary sliding, which is necessary to evaluate the effect of melt on seismic wave velocity and attenuation. We measured elasticity, anelasticity, and viscosity of rock analog samples by changing temperature continuously from subsolidus to supersolidus. Our previous studies have shown that when homologous temperature exceeds 0.9, anelastic relaxation by grain boundary sliding is significantly enhanced, probably due to the increase in grain boundary disorder called pre‐melting. In this study, we found that when homologous temperature exceeds 1, grain boundary sliding is further enhanced by melt and that the viscoelastic property at supersolidus temperatures shows the combined total effect of disordered grain boundaries and melt. The relaxation strength due to melt depends on melt fraction, while that due to disordered grain boundaries does not. When melt fraction is very small (≪1%), the melt effect is negligibly small and the effect of disordered grain boundaries is dominant. Based on these experimental results, we present a new viscoelasticity model seamlessly applicable for a broad temperature range from subsolidus to supersolidus temperatures and for a broad frequency range from seismic waves to geodetic deformation. The new model covers an upper mantle region where a moderate amount of melt exists (e.g., near subduction zones). We further compare our data with those of rock samples and infer that a common physical mechanism, subsolidus and supersolidus enhancements on grain boundary sliding, may be dominant in these two data sets.