Crystallographic preferred orientation (CPO) development governs strain weakening in ice: insights from high-temperature deformation experiments

© The Author(s), 2021. This article is distributed under the terms of the Creative Commons Attribution License. The definitive version was published in Fan, S., Cross, A. J., Prior, D. J., Goldsby, D. L., Hager, T. F., Negrini, M., & Qi, C. Crystallographic preferred orientation (CPO) development governs strain weakening in ice: insights from high-temperature deformation experiments. Journal of Geophysical Research: Solid Earth, 126(12), (2021): e2021JB023173, https://doi.org/10.1029/2021JB023173.Strain weakening leads to the formation of high-strain shear zones and strongly influences terrestrial ice discharge. In glacial flow models, strain weakening is assumed to arise from the alignment of weak basal planes—the development of a crystallographic preferred orientation, CPO—during flow. However, in experiments, ice strain weakening also coincides with grain size reduction, which has been invoked as a weakening mechanism in other minerals. To interrogate the relative contributions of CPO development and grain size reduction toward ice strain weakening, we deformed initially isotropic polycrystalline ice samples to progressively higher strains between −4 and −30°C. Microstructural measurements were subsequently combined with flow laws to separately model the mechanical response expected to arise from CPO development and grain size reduction. Magnitudes of strain weakening predicted by the constitutive flow laws were then compared with the experimental measurements. Flow laws that only consider grain size do not predict weakening with strain despite grain size reduction. In contrast, flow laws solely considering CPO effects can reproduce the measured strain weakening. Thus, it is reasonable to assume that strain weakening in ice is dominated by CPO development, at least under high temperature (Th ≥ 0.9) and high stress (>1 MPa), like those in our experiments. We speculate that at high homologous temperatures (Th ≥ 0.9), CPO development will also govern the strain weakening behavior of other viscously anisotropic minerals, like olivine and quartz. Overall, we emphasize that geodynamic and glaciological models should incorporate CPOs to account for strain weakening, especially at high homologous temperatures.This work was supported by a NASA fund (grant no. NNX15AM69G) to David L. Goldsby and two Marsden Funds of the Royal Society of New Zealand (grant nos. UOO1116, UOO052) to David J. Prior. Sheng Fan was supported by the University of Otago doctoral scholarship, the Antarctica New Zealand doctoral scholarship, a research grant from New Zealand Ministry of Business, Innovation and Employment through the Antarctic Science Platform (ANTA1801) (grant no. ASP-023-03), and a New Zealand Antarctic Research Institute (NZARI) Early Career Researcher Seed Grant (grant no. NZARI 2020-1-5)

Crystallographic orientation mapping of lizardite serpentinite by Raman spectroscopy

The serpentine mineral lizardite displays strong Raman anisotropy in the OH-stretching region, resulting in significant wavenumber shifts (up to ca. 14.5 cm−1) that depend on the orientation of the impinging excitation laser relative to the crystallographic axes. We quantified the relationship between crystallographic orientation and Raman wavenumber using well-characterised samples of Monte Fico lizardite by applying Raman spectroscopy and electron backscatter diffraction (EBSD) mapping on thin sections of polycrystalline samples and grain mounts of selected single crystals, as well as by a spindle stage Raman study of an oriented cylinder drilled from a single crystal. We demonstrate that the main band in the OH-stretching region undergoes a systematic shift that depends on the inclination of the c-axis of the lizardite crystal. The data are used to derive an empirical relationship between the position of this main band and the c-axis inclination of a measured lizardite crystal: y=14.5cos 4 (0.013x+0.02)+(3670±1), where y is the inclination of the c-axis with respect to the normal vector (in degrees), and x is the main band position (wavenumber in cm −1) in the OH-stretching region. This new method provides a simple and cost-effective technique for measuring and quantifying the crystallographic orientation of lizardite-bearing serpentinite fault rocks, which can be difficult to achieve using EBSD alone. In addition to the samples used to determine the above empirical relationship, we demonstrate the applicability of the technique by mapping the orientations of lizardite in a more complex sample of deformed serpentinite from Elba Island, Italy.publishedVersio

Crystallographic preferred orientations of ice deformed in direct-shear experiments at low temperatures

Synthetic polycrystalline ice was sheared at temperatures of-5,-20 and-30 °C, to different shear strains, up to γ = 2.6, equivalent to a maximum stretch of 2.94 (final line length is 2.94 times the original length). Cryo-electron backscatter diffraction (EBSD) analysis shows that basal intracrystalline slip planes become preferentially oriented parallel to the shear plane in all experiments, with a primary cluster of crystal c axes (the c axis is perpendicular to the basal plane) perpendicular to the shear plane. In all except the two highest-strain experiments at-30 °C, a secondary cluster of c axes is observed, at an angle to the primary cluster. With increasing strain, the primary c-axis cluster strengthens. With increasing temperature, both clusters strengthen. In the-5 °C experiments, the angle between the two clusters reduces with strain. The c-axis clusters are elongated perpendicular to the shear direction. This elongation increases with increasing shear strain and with decreasing temperature. Highly curved grain boundaries are more prevalent in samples sheared at higher temperatures. At each temperature, the proportion of curved boundaries decreases with increasing shear strain. Subgrains are observed in all samples. Microstructural interpretations and comparisons of the data from experimentally sheared samples with numerical models suggest that the observed crystallographic orientation patterns result from a balance of the rates of lattice rotation (during dislocation creep) and growth of grains by strain-induced grain boundary migration (GBM). GBM is faster at higher temperatures and becomes less important as shear strain increases. These observations and interpretations provide a hypothesis to be tested in further experiments and using numerical models, with the ultimate goal of aiding the interpretation of crystallographic preferred orientations in naturally deformed ice

Translation, Cross-Cultural Adaptation, and Validation of the Portuguese Version of the Rotterdam Elderly Pain Observation Scale

INTRODUCTION: This study reports on the translation, cultural adaptation, and validation of a Portuguese version of the Rotterdam Elderly Pain Observation Scale (REPOS), a Dutch scale to assess pain in patients who cannot communicate, with or without dementia. METHODS: This is a multicenter study in pain and neurological units involving Brazil (clinical phase) and the Netherlands (training phase). We performed a retrospective cross-sectional, 2-staged analysis, translating and culturally adapting the REPOS to a Portuguese version (REPOS-P) and evaluating its psychometric properties. Eight health professionals were trained to observe patients with low back pain. REPOS consists of 10 behavioral items scored as present or absent after a 2-min observation. The REPOS score of ≥3 in combination with the Numerical Rating Scale (NRS) of ≥4 indicated pain. The Content Validity Index (CVI) in all items and instructions showed CVI values at their maximum. According to the higher correlation coefficient found between NRS and REPOS-P, it may be suggested that there was an adequate convergent validity. RESULTS: The REPOS-P was administered to 80 patients with a mean age of 60 years (SD 11.5). Cronbach's alpha coefficient showed a moderate internal consistency of REPOS-P (α = 0.62), which is compatible with the original study of REPOS. All health professionals reached high levels of interrater agreement within a median of 10 weeks of training, assuring reproducibility. Cohen's kappa was 0.96 (SD 0.03), and the intraclass correlation coefficient was 0.98 (SD 0.02), showing high reliability of REPOS-P scores between the trainer (researcher) and the trainees (healthcare professionals). The Pearson correlation coefficient was 0.95 (95% confidence interval 0.94–0.97), showing a significant correlation between the total scores of REPOS-P and NRS. CONCLUSION: The REPOS-P was a valuable scale for assessing elderly patients with low back pain by different healthcare professionals. Short application time, ease of use, clear instructions, and the brief training required for application were essential characteristics of REPOS-P

Microstructure and crystallographic preferred orientations of an azimuthally oriented ice core from a lateral shear margin: Priestley Glacier, Antarctica

A 58 m long azimuthally oriented ice core has been collected from the floating lateral sinistral shear margin of the lower Priestley Glacier, Terra Nova Bay, Antarctica. The crystallographic preferred orientations (CPO) and microstructures are described in order to correlate the geometry of anisotropy with constrained large-scale kinematics. Cryogenic Electron Backscatter Diffraction analysis shows a very strong fabric (c-axis primary eigenvalue ∼0.9) with c-axes aligned horizontally sub-perpendicular to flow, rotating nearly 40° clockwise (looking down) to the pole to shear throughout the core. The c-axis maximum is sub-perpendicular to vertical layers, with the pole to layering always clockwise of the c-axes. Priestley ice microstructures are defined by largely sub-polygonal grains and constant mean grain sizes with depth. Grain long axis shape preferred orientations (SPO) are almost always 1–20° clockwise of the c-axis maximum. A minor proportion of “oddly” oriented grains that are distinct from the main c-axis maximum, are present in some samples. These have horizontal c-axes rotated clockwise from the primary c-axis maximum and may define a weaker secondary maximum up to 30° clockwise of the primary maximum. Intragranular misorientations are measured along the core, and although the statistics are weak, this could suggest recrystallization by subgrain rotation to occur. These microstructures suggest subgrain rotation (SGR) and recrystallization by grain boundary migration recrystallization (GBM) are active in the Priestley Glacier shear margin. Vorticity analysis based on intragranular distortion indicates a vertical axis of rotation in the shear margin. The variability in c-axis maximum orientation with depth indicates the structural heterogeneity of the Priestley Glacier shear margin occurs at the meter to tens of meters scale. We suggest that CPO rotations could relate to rigid rotation of blocks of ice within the glacial shear margin. Rotation either post-dates CPO and SPO development or is occurring faster than CPO evolution can respond to a change in kinematics

Strength and rheological evolution of the lower continental crust: an experimental study of the deformation behavior of feldspar and quartz at high pressure and temperature

Models representing the strength of the lithosphere (i.e. strength-depth profiles) are mainly based on the rheological properties of the minerals quartz and feldspar that, to a large extent, are determined in laboratory-based rock deformation studies. The study of the deformation behavior of quartz and feldspar is of fundamental importance for the understanding and modeling of plate tectonics and orogenic processes in general. However, the rheology of the crust is intensely debated because field observations sometimes do not support models based on geophysical data. Experiments may resolve some of the discrepancies. The aim of this study is to reproduce in the laboratory, by means of high pressure and high temperature deformation experiments, the microstructures and reactions observed in quartz and feldspar in nature in order to quantify and explore deformation processes in the lower continental crust. The deformation experiments were performed on natural single crystals and powders of quartz and feldspar with a Griggs-type solid medium deformation apparatus. High confining pressures (Pc= 0.75-1.5 GPa) and temperatures (700 °C-1000 °C) were chosen in order to simulate lower crustal conditions. As in many laboratory deformation experiments, the use of very high temperatures was necessary to promote deformation of quartz and feldspar at the strain rates that can be achieved using the Griggs apparatus (10-6 to 10-7 in this work). Two main crustal processes were studied: (1) The effect of deformation on the TitaniQ (titanium in quartz) geothermobarometer and; (2) the deformation behavior and rheology of melt bearing feldspar single crystals and gouges. (1) The geothermobarometer TitaniQ is based on Si-Ti substitution in quartz, which is both pressure and temperature dependent. Because the microstructural evolution of quartz under varying pressure and temperature conditions is relatively well characterized, the correlation between quartz microstructures and Ti content represents a promising method to directly estimate the pressure and temperature conditions of deformation. However, the effect that different mechanisms of recrystallization have on Ti incorporation is not fully understood, and thus it is unknown whether the TitaniQ geothermobarometer can be applied to deformed rocks of the lower crust. Our high pressure and temperature deformation experiment on quartz single crystals (Chapter 3) demonstrate that in a fluid-present and Ti-saturated environment, the Si—Ti substitution in quartz is not likely to occur during deformation, regardless of the recrystallization mechanism involved in the deformation process. In our experiments, neither quartz grains that deformed by subgrain rotation recrystallization nor those showing grain boundary migration features incorporate equilibrium contents of Ti. These results suggest that the application of the TitaniQ geothermobarometer to deformed rocks under prograde metamorphic conditions is not as straightforward as previously thought. (2) Previous experimental studies suggest that fluid-bearing feldspar deforms by viscous processes at temperatures > 700 °C. However, field observations indicate that under lower crustal conditions feldspar can deform by brittle processes, even at high temperature. Microstructural observations and mechanical data from high-temperature and high-pressure deformation experiments on K-feldspar single crystals (Chapter 4) demonstrate that deformation is initially accommodated by brittle fractures, which cause grain size reduction and formation of gouges. Dilatation along the fractures leads to a local decrease of the confining pressure and promotes melting in cracked regions. However, the presence of melt in the system does not significantly influence the strength of the rock as melt is present only in small fractions and, during deformation, it remains isolated without forming an interconnected network. Preliminary microstructural observations of the fine grained fault gouges formed along brittle fractures suggests the activity of dissolution precipitation creep as the dominant deformation mechanism. In order to better quantify and study the processes occurring in the gouges, shear experiments were performed on layers of fine grained aggregates of feldspar (Chapter 5). Microstructural observations supported by mechanical data indicate that the composition of the starting material has only a minor influence on the deformation behavior of water bearing feldspar gouges. Furthermore, no systematic differences were observed in melt bearing and melt free aggregates. The presence of regions of gouge showing pervasive compositional changes, the nucleation and growth of new euhedral grains with a different chemical composition, grains with lobate and indented grain boundaries, and overgrowth structures all suggest that in wet feldspar gouges at high pressure and temperature deformation is mainly accommodated by dissolution precipitation processes