Grain-size Sensitivity of Viscoelastic Relaxation in Dry, Melt-free Polycrystalline Olivine: An Update

For the purpose of better understanding seismic attenuation in the upper mantle, dynamic shear modulus and strain-energy dissipation of polycrystalline olivine (usually dry and melt-free) have been measured by mechanical testing with low-frequency forced oscillation (0.001 1 Hz) at elevated temperatures ( 1300 C). Although significant grain-size sensitivity of the results has been previously observed in our laboratory and fitted to an extended Burgers model for extrapolation to upper mantle conditions, there are some experimental limitations in these works. Firstly, coarse-grained samples have not been extensively tested and were mostly derived from San Carlos olivine. Secondly, Ni70Fe30 discs routinely inserted into the specimen assembly at either end of the cylindrical specimen to prevent reaction at high temperature with the alumina pistons introduces uncertainty concerning additional interfacial compliance. Thirdly, the previous work on grain-size sensitivity employed a control specimen of polycrystalline (LucaloxTM) alumina with somewhat uncertain viscoelastic behavior. Accordingly, in this study, we newly prepared a suite of genuinely melt-free specimens ranging in grain size from a few to a few tens m by hot pressing sol-gel precursor material. We removed the two NiFe discs from the specimen assembly for forced-oscillation testing, resulting in no more than minimal reaction with the adjacent alumina, and employed a single-crystal sapphire control specimen, plausibly treated as elastic. Preliminary indications of stronger grain-size sensitivity of anelastic relaxation in the new forced-oscillation experiments, to be embedded within an extended Burgers model, will enhance our understanding of relaxation mechanisms in olivine aggregates and allow more robust interpretation of seismic attenuation in the upper mantle.