Evolution of ice crystal microstructure during creep experiments

Hamann, Ilka; Weikusat, Christian; Azuma, Nobuhiko; Kipfstuhl, Sepp

doi:10.3189/002214307783258341

Cited by 35 publications

(40 citation statements)

References 47 publications

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“…In the high‐porosity/low‐density firn in the uppermost 40 m depth, straight subgrain boundaries (Figure 5) are the most frequent intracrystalline features. The straight subgrain boundaries probably originate from some sort of grain subdivision process, like intracrystalline shear, bend or twist [ Bons and Jessell , 1999; Hamann et al , 2007; Faria et al , 2009, also Is Antarctica like a birthday cake?, preprint 33/2006, Max Planck Institute for Mathematics in the Sciences, Leipzig, Germany, 2006, available at http://www.mis.mpg.de/preprints/2006/preprint2006_33.pdf]. Note the wedge‐shaped grain in the grain at the lower left side showing multiple straight subgrain boundaries.…”

Section: Resultsmentioning

confidence: 99%

Evidence of dynamic recrystallization in polar firn

et al. 2009

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[1] Microstructural analyses have been performed on polar firn from the European Project for Ice Coring in Antarctica drilling site in Dronning Maud Land, Antarctica. The results derived from images of the firn structure in microscopic resolution indicate that dynamic recrystallization is active in firn at all depths, and it dominates the evolution of the microstructure when the firn density exceeds a critical value of 730 kg/m 3 (overburden snow load $0.2 MPa). At the firn-ice transition (density $820 kg/m 3 ) the microstructure is characterized by many small grains and bulged or irregularly shaped grain boundaries. More than half of all grains show subgrain boundaries. Thus, strain-induced boundary migration is an essential feature to describe the irregular grain structure. In agreement with previous studies, significant grain growth has been observed with depth for the largest grains in the samples. However, our microscopic analysis reveals that the grain growth with depth in fact vanishes if all grains larger than 65 mm in diameter are taken into account. This result reflects the fact that the growth of the largest grains is counteracted by grain size reduction by shrinking and subdivision of old grains, as well as production of new grains. Consequently, previous conclusions that grain growth in polar firn is essentially analogous to normal grain growth in metallic and ceramic sinters and that the stored strain energy is small in comparison with grain boundary energy can no longer be supported. Additionally, our observations show that the incipience of dynamic recrystallization in polar ice sheets is not as sensitive to temperature as supposed so far. A discussion of the change of the mean grain size due to the measuring technique is imperative.

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Section: Resultsmentioning

confidence: 99%

Evidence of dynamic recrystallization in polar firn

et al. 2009

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“…Significant amounts of the plastic strain at steady state are 226 being accomplished by the motion of dislocations. Consequently, steady-state flow in 227 these regimes is consistent with a subgrain structure, which has been recently 228 characterized both by careful experimental ice-rheology study (Hamann et al, 2007) data for the coarsest-grain-sized material, which exhibit significant experimental scatter, 240 the data for the experiments take the same form as the high-temperature-background 241 -master curve‖; (2) the attenuation of the deforming ice aggregates in our experiments is 242 approximately an order of magnitude greater at a given frequency than that of the 243 materials in the master curve. (Note that, although there is no theoretically justified 244 reason to do so, normalizing instead with a GBS or dislocation viscosity also yields a 245 curve significantly offset from the master curve of the other studies.…”

Section: Previous Studies: Diffusionally Accommodated Gbs 153mentioning

confidence: 88%

Tidal dissipation in creeping ice and the thermal evolution of Europa

McCarthy

Cooper

2016

Earth and Planetary Science Letters

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10The thermal and mechanical evolution of Europa and comparable icy satellites-the 11 physics behind creating and sustaining a subsurface water ocean-depends almost 12 entirely on the mechanical dissipation of tidal energy in ice to produce heat, the 13 mechanism(s) of which remain poorly understood. In deformation experiments, we 14 combine steady-state creep and low-frequency, small-strain periodic loading, similar 15 conditions in which tectonics and tidal flexing are occurring simultaneously. The data 16reveal that the relevant, power-law attenuation in ice (i) is non-linear, depending on strain 17 amplitude, (ii) is independent of grain size, and (iii) exceeds in absorption the prediction 18of the Maxwell solid model by an order of magnitude. The Maxwell solid model is 19 widely used to model the dynamics of planetary ice shells, so this discrepancy is 20important. The prevalent understanding of damping in the geophysical context is that it is 21 controlled by chemical diffusion on grain boundaries, which renders attenuation strongly 22 dependent on grain size. In sharp contrast, our results indicate instead the importance of 23 intracrystalline dislocations and their spatial interactions as the critical structural variable 24 affecting dissipation. These dislocation structures are controlled by stress and realized by 25 accumulated plastic strain. Thus, tectonics and attenuation are coupled, which, beyond 26 the icy satellite/subsurface ocean problem, has implications also for understanding the 27 attenuation of seismic waves in deforming regions of the Earth's upper mantle. 28McCarthy & Cooper: Dissipation in creeping ice 2 steady-state rheology (e.g., Ojakangas and Stevenson, 1989). However, the disparity 36 between predicted behavior using Maxwell's model and observed behavior from recent 37 satellite-based measurements (e.g., geysers and tectonics) demonstrate the need to refine 38 such models to include the spectrum of mechanical response from elastic to anelastic to 39 viscous (Shoji et al., 2013). To account for significant heat generated by mechanical 40 dissipation of tidal forces, a transient, anelastic response is required. Since microstructure 41 influences transient/anelastic properties and steady-state behavior establishes and sustains 42 microstructure, the overlap of transient and steady-state behavior is important for tidal 43 processes. Further, the ability to extrapolate laboratory data to planetary dynamics 44 requires understanding of the physics of dissipation and how it scales with frequency, 45 grain size, and temperature, as well as with stress and accumulated strain. 46On Europa, the global stress field within the icy shell includes a diurnal component with 47

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“…Given the extensive grain boundary migration that occurs at all levels in ice cores (Montagnat and Duval, 2004;Weikusat et al, 2009a), it is likely that many SGBs may have been extended by growth behind migrating boundaries. This is conceivable in particular with the common observation that only a part of SGBs crosses grains completely, while many others fade out towards the centre of a grain (Weikusat et al, 2009b;Hamann et al, 2007), such as SGB 9 in Fig. 3 and SGB 11 in Fig.…”

Section: Subgrain Boundaries Without Connection To Host Grain Crystalmentioning

confidence: 98%

EBSD analysis of subgrain boundaries and dislocation slip systems in Antarctic and Greenland ice

et al. 2017

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Abstract. Ice has a very high plastic anisotropy with easy dislocation glide on basal planes, while glide on non-basal planes is much harder. Basal glide involves dislocations with the Burgers vector b = a , while glide on non-basal planes can involve dislocations with b = a , b = [c], and b = c + a . During the natural ductile flow of polar ice sheets, most of the deformation is expected to occur by basal slip accommodated by other processes, including non-basal slip and grain boundary processes. However, the importance of different accommodating processes is controversial. The recent application of micro-diffraction analysis methods to ice, such as X-ray Laue diffraction and electron backscattered diffraction (EBSD), has demonstrated that subgrain boundaries indicative of non-basal slip are present in naturally deformed ice, although so far the available data sets are limited. In this study we present an analysis of a large number of subgrain boundaries in ice core samples from one depth level from two deep ice cores from Antarctica (EPICA-DML deep ice core at 656 m of depth) and Greenland (NEEM deep ice core at 719 m of depth).EBSD provides information for the characterization of subgrain boundary types and on the dislocations that are likely to be present along the boundary. EBSD analyses, in combination with light microscopy measurements, are presented and interpreted in terms of the dislocation slip systems. The most common subgrain boundaries are indicative of basal a slip with an almost equal occurrence of subgrain boundaries indicative of prism [c] or c + a slip on prism and/or pyramidal planes. A few subgrain boundaries are indicative of prism a slip or slip of a screw dislocations on the basal plane. In addition to these classical polygonization processes that involve the recovery of dislocations into boundaries, alternative mechanisms are discussed for the formation of subgrain boundaries that are not related to the crystallography of the host grain.The finding that subgrain boundaries indicative of nonbasal slip are as frequent as those indicating basal slip is surprising. Our evidence of frequent non-basal slip in naturally deformed polar ice core samples has important implications for discussions on ice about plasticity descriptions, rate-controlling processes which accommodate basal glide, and anisotropic ice flow descriptions of large ice masses with the wider perspective of sea level evolution.

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Evolution of ice crystal microstructure during creep experiments

Cited by 35 publications

References 47 publications

Evidence of dynamic recrystallization in polar firn

Evidence of dynamic recrystallization in polar firn

Tidal dissipation in creeping ice and the thermal evolution of Europa

EBSD analysis of subgrain boundaries and dislocation slip systems in Antarctic and Greenland ice

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