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Abstract Dislocation‐based dissipation mechanisms potentially control the viscoelastic response of Earth's upper mantle across a variety of geodynamic contexts, including glacial isostatic adjustment, postseismic creep, and seismic‐wave attenuation. However, there is no consensus on which dislocation‐based, microphysical process controls the viscoelastic behavior of the upper mantle. Although both intergranular (plastic anisotropy) and intragranular (backstress) mechanisms have been proposed, there is currently insufficient laboratory data to discriminate between those mechanisms. Here, we present the results of forced‐oscillation experiments in a deformation‐DIA apparatus at confining pressures of 3–7 GPa and temperatures of 298–1370 K. Our experiments tested the viscoelastic response of polycrystalline olivine—the main constituent of the upper mantle—at stress amplitudes from 70 to 2,800 MPa. Mechanical data are complemented by microstructural analyses of grain size, crystallographic preferred orientation, and dislocation density. We observe amplitude‐ and frequency‐dependent attenuation and modulus relaxation and find that numerical solutions of the backstress model match our results well. Therefore, we argue that interactions among dislocations, rather than intergranular processes (e.g., plastic anisotropy or grain boundary sliding), control the viscoelastic behavior of polycrystalline olivine in our experiments. In addition, we present a linearized version of the constitutive equations of the backstress model and extrapolate it to conditions typical of seismic‐wave propagation in the upper mantle. Our extrapolation demonstrates that the backstress model can explain the magnitude of seismic‐wave attenuation in the upper mantle, although some modification is required to explain the weak frequency dependence of attenuation observed in nature and in previous experimental work.
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A cryogenic forced oscillation apparatus to measure anelasticity of ice
We have developed a new cryogenic uni-axial forced oscillation apparatus to measure the anelastic behavior of ice by adapting the design of a previous high-precision apparatus for use in low-temperature (<0 °C) conditions. With this new apparatus, Young’s modulus and attenuation can be measured over a broad frequency range from 10−4 to 10 Hz. We have performed calibration tests with standard materials (steel spring, stainless steel, and acrylic samples) under various conditions to assess the apparatus properties and correct the effects on the obtained raw data. Young’s modulus and attenuation for an acrylic sample after all of the data corrections show good agreement with previously published values, demonstrating the validity of the data corrections and reliability of the obtained data. We further obtained a preliminary dataset of Young’s modulus and attenuation for an ice polycrystalline sample under small median stress and small stress amplitude. The anelastic response was not strain amplitude-dependent, that is, the response is linear. Moreover, the attenuation data are consistent with the data measured for other polycrystalline materials under similarly small stress conditions in terms of the Maxwell frequency scaling, which is known as a scaling law applicable to linear anelasticity induced by the diffusionally accommodated grain boundary sliding mechanism. Although there is still room for improving the control of testing conditions, we show that the new forced oscillation apparatus is capable of systematic studies on the anelastic properties of ice, the subject of future studies.
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- Award ID(s):
- 1855423
- PAR ID:
- 10592740
- Publisher / Repository:
- American Institute of Physics
- Date Published:
- Journal Name:
- Review of Scientific Instruments
- Volume:
- 95
- Issue:
- 7
- ISSN:
- 0034-6748
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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