The Influence of Water on Seismic Wave Attenuation in the Upper Mantle

Trace amounts of water, present as protons structurally bound in olivine crystal defects, are inferred to significantly enhance the low-strain solid-state viscoelastic relaxation responsible for attenuation and dispersion of seismic waves in the upper mantle. This inferrence is supported by recent observation of water weakening at moderate compressive strains in synthetic, water-undersaturated aggregates (Faul et al., in preparation). In these fine-grained olivine polycrystals of Fo₉₀ composition, doped with 0.02wt% TiO₂, “water” is incorporated in the remarkably stable Ti-clinohumite defect. Such synthetic olivine specimens reproduce the infrared spectra of natural mantle olivines (Berry et al., 2005), and present the advantage of being melt-free and of low dislocation density. The water contents in such synthetic polycrystalline olivine aggregates, which can be quantitatively measured by Fourier Transform Infrared Spectroscopy (FTIR), range up to 90 ppm, and are thus representative of water-undersaturated conditions in the upper mantle. We will report here the outcome of torsional-oscillation tests,in which attenuation and shear modulus were measured at seismic frequencies (mHz-Hz) and various temperatures up to 1300C on Pt-encapsulated, Ti-doped olivine specimens, enclosed within a mild-steel jacket.