Dislocation interactions during low-temperature plasticity of olivine and their impact on the evolution of lithospheric strength.

Wallis, D. and Hansen, L. N. and Kumamoto, K. M. and Thom, C. A. and Plümper, O. and Ohl, M. and Durham, W. B. and Goldsby, D. L. and Armstrong, D. E. J. and Meyers, C. D. and Goddard, R. and Warren,, J. M. and Breithaupt, T. and Drury, M. and Wilkinson, A. J. (2020) Dislocation interactions during low-temperature plasticity of olivine and their impact on the evolution of lithospheric strength. Earth and Planetary Science Letters, 543. ISSN 0012 821X DOI https://doi.org/10.1016/j.epsl.2020.116349

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The strength of the lithosphere is typically modelled based on constitutive equations for steady-state flow. However, models of lithospheric flexure reveal differences in lithospheric strength that are difficult to reconcile based on such flow laws. Recent rheological data from low-temperature deformation experiments on olivine suggest that this discrepancy may be largely explained by strain hardening. Details of the mechanical data, specifically the effects of temperature-independent back stresses stored in the samples, indicate that strain hardening in olivine occurs primarily due to long-range elastic interactions between dislocations. These interpretations provided the basis for a new flow law that incorporates hardening by development of back stress. Here, we test this hypothesis by examining the microstructures of olivine samples deformed plastically at room temperature either in a deformation-DIA apparatus at differential stresses of ≤ 4.3 GPa or in a nanoindenter at applied contact stresses of ≥ 10.2 GPa. High-angular resolution electron backscatter diffraction maps reveal the presence of geometrically necessary dislocations with densities commonly above 10 14 m-2 and intragranular heterogeneities in residual stress on the order of 1 GPa in both sets of samples. Scanning transmission electron micrographs reveal straight dislocations aligned along slip bands and interacting with dislocations of other types that act as obstacles. The stress heterogeneities and accumulations of dislocations along their slip planes are consistent with strain hardening resulting from long-range back-stresses acting between dislocations. These results corroborate the mechanical data in supporting the form of the new flow law for low-temperature plasticity and provide new microstructural criteria for identifying the operation of this deformation mechanism in natural samples. Furthermore, similarities in the structure and stress fields of slip bands formed in single crystals deformed at low temperatures and those formed at high temperatures suggest that similar hardening processes occur in both regimes, providing a new constraint for models of transient creep at high temperatures.

Item Type: Article
Uncontrolled Keywords: NILAREP; IA76
Subjects: 05 - Petrology - Igneous, Metamorphic and Volcanic Studies
Divisions: 05 - Petrology - Igneous, Metamorphic and Volcanic Studies
07 - Gold Open Access
Journal or Publication Title: Earth and Planetary Science Letters
Volume: 543
Identification Number: https://doi.org/10.1016/j.epsl.2020.116349
Depositing User: Sarah Humbert
Date Deposited: 02 Jun 2020 01:56
Last Modified: 02 Jun 2020 01:56
URI: http://eprints.esc.cam.ac.uk/id/eprint/4756

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