[1] In order to extend the range of conditions that can be obtained in experiments, we have measured the viscoelastic properties of polycrystalline organic borneol, as an analogue to mantle rock. Using a custom fabricated apparatus, the Young's modulus E and attenuation Q E −1 were measured accurately over a broad frequency range (10 −4 ≤ f (Hz) ≤ 2.15) and at low strain amplitude (10 −5 -10 −6 ). Creep experiments were performed with the same apparatus to measure the steady state viscosity. Anelasticity and viscosity were measured at high homologous temperatures (T = 22-48°C; T/T m = 0.61-0.67) and various grain sizes (3-22 mm), the growth of which was controlled by annealing. Using the measured viscosities h and the unrelaxed modulus E U determined from ultrasonic experiments, the frequency of the entire data set was normalized by the Maxwell frequency f M = E U /h, resulting in E and Q −1 master curves. The Q −1 data from previous studies on olivine-dominated samples also collapse onto the same curve when scaled by f M, , demonstrating the universality of anelasticity for polycrystalline materials. The similitude by the Maxwell frequency scaling indicates that the dominant mechanism for the anelasticity observed in this study and in previous studies is diffusionally accommodated grain boundary sliding. A generalized formulation for this similitude is provided to extrapolate the experimental data to velocity and attenuation of seismic shear waves.Citation: McCarthy, C., Y. Takei, and T. Hiraga (2011), Experimental study of attenuation and dispersion over a broad frequency range: 2. The universal scaling of polycrystalline materials,

Seismic attenuation measurements provide a powerful tool for sampling mantle properties.Laboratory experiments provide calibrations at seismic frequencies and mantle temperatures for dry meltfree rocks, but require $10 2 210 3 extrapolations in grain size to mantle conditions; also, the effects of water and melt are not well understood. At the same time, body wave attenuation measured from dense broadband arrays provides reliable estimates of shear wave attenuation (Q 21 S ), affording an opportunity for calibration. We reanalyze seismic data sets that sample arc and back-arc mantle in Central America, the Marianas, and the Lau Basin, confirming very high attenuation (Q S $ 25-80) at 1 Hz and depths of 50-100 km. At each of these sites, independent petrological studies constrain the temperature and water content where basaltic magmas last equilibrated with the mantle, 1300-1450 C. The Q S measurements correlate inversely with the petrologically inferred temperatures, as expected. However, dry attenuation models predict Q S too high by a factor of 1.5-5. Modifying models to include effects of H 2 O and rheologydependent grain size shows that the effects of water-enhanced dissipation and water-enhanced grain growth nearly cancel, so H 2 O effects are modest. Therefore, high H 2 O in the arc source region cannot explain the low Q S , nor in the back arc where lavas show modest water content. Most likely, the high attenuation reflects the presence of melt, and some models of melt effects come close to reproducing observations. Overall, body wave Q S can be reconciled with petrologic and laboratory inferences of mantle conditions if melt has a strong influence beneath arcs and back arcs.

One contribution of 11 to a theme issue 'Microdynamics of ice'. Using a new biaxial friction apparatus, we conducted experiments of ice-on-rock friction in order to better understand basal sliding of glaciers and ice streams. A series of velocity-stepping and slidehold-slide tests were conducted to measure friction and healing at temperatures between −20°C and melting. Experimental conditions in this study are comparable to subglacial temperatures, sliding rates and effective pressures of Antarctic ice streams and other glaciers, with load-point velocities ranging from 0.5 to 100 µm s −1 and normal stress σ n = 100 kPa. In this range of conditions, temperature dependences of both steady-state friction and frictional healing are considerable. The friction increases linearly with decreasing temperature (temperature weakening) from μ = 0.52 at −20°C to μ = 0.02 at melting. Frictional healing increases and velocity dependence shifts from velocity-strengthening to velocityweakening behaviour with decreasing temperature. Our results indicate that the strength and stability of glaciers and ice streams may change considerably over the range of temperatures typically found at the ice-bed interface. Subject Areas: glaciology KeywordsThis article is part of the themed issue 'Microdynamics of ice'.