MR022-0006
Experimental investigation of the nonlinear viscoelasticity of olivine single crystals

Wednesday, 16 December 2020
Poster
Ri Cao, University of Oxford, Oxford, United Kingdom, Lars N Hansen, University of Minnesota - Twin Cities, Department of Earth and Environmental Sciences, Minneapolis, United States, Christopher A. Thom, University of Oxford, Department of Earth Science, Oxford, United Kingdom and David Wallis, University of Cambridge, Department of Earth Science, Cambridge, United Kingdom
Abstract:
The viscoelasticity of upper-mantle rocks controls short-term geodynamic scenarios, such as post-seismic relaxation, tidal dissipation, glacial isostatic adjustment, and seismic wave propagation. Most previous investigations of viscoelasticity in geological materials have focused on grain-boundary processes in polycrystalline olivine at small strain amplitudes, which is manifested as strain-amplitude-independent viscoelastic behavior. However, oscillatory deformation at larger strain amplitudes may induce intracrystalline mechanism such as dislocation multiplication, unpinning, and long-range interaction with other defects, resulting in strain-amplitude-dependent viscoelasticity (i.e., nonlinear viscoelasticity). The transition to nonlinear viscoelasticity in olivine has been observed in a few previous studies, but the details of the nonlinear behavior and the responsible microphysical mechanisms remain largely unexplored.

To characterize the nonlinear viscoelasticity of olivine, we performed forced-oscillation experiments on single crystals of San Carlos olivine at 1150 to 1300 °C using a uniaxial deformation apparatus. Oscillatory stresses with amplitudes of 0.1–2.6 MPa were applied on top of stresses ranging from 56–108 MPa. Consequently, oscillatory stresses were superimposed on creep at a constant strain rate. Applied mean stresses correspond to dislocation densities ranging from 6×1011 to 1.5×1012 m-2. Our results reveal that an increase in the mean stress causes an increase in attenuation, demonstrating that attenuation increases with increasing dislocation density. The magnitude of attenuation ranges from 10-1 to 100, and the modulus defect ranges from 0.03 to 0.75. Amplitude-dependent behavior is observed above strain amplitudes of ~10-5 and is characterized by a peak in attenuation at amplitudes of 2×10-5. The shape of hysteresis loops is inconsistent with models for attenuation based on dislocation-breakaway from point defects (e.g., the Granato-Lucke model). Instead, our data are consistent with models based on the interactions of dislocations with the long-range stress fields of other defects. These observations indicate that models of the viscoelasticity of olivine at strains >10-5 should be based on long-range interactions of dislocations.

<< Previous Abstract | Next Abstract >>