Quantifying geometrically necessary dislocations in quartz using HR-EBSD: Application to chessboard subgrain boundaries

This study presents the first use of high-angular resolution electron backscatter diffraction (HR-EBSD) to quantitatively characterise geometrically necessary dislocations in quartz subgrain structures. HR-EBSD exploits cross-correlation of diffraction patterns to measure intragranular misorientations with precision on the order of 0.01° with well-constrained misorientation axes. We investigate the dislocation structures of chessboard subgrains in quartz within samples from the Greater Himalayan Sequence, Nepal. Our results demonstrate that chessboard subgrains are formed primarily from two sets of subgrain boundaries. One set consists primarily of {m}[c] edge dislocations, the other consists primarily of dislocations with Burgers vectors. Apparent densities of geometrically necessary dislocations vary from > 10^13 m−2 within some subgrain boundaries to < 10^12 m−2 within subgrain interiors. The results suggest that at pressures above approximately 10 kbar, chessboard subgrains may form within the α-quartz stability field. Most importantly, this study demonstrates the potential of HR-EBSD as an improved method for analysis of intragranular microstructures in quartz that are used as indicators of deformation conditions.D. Wallis and L.N. Hansen acknowledge support from the Natural Environment Research Council Grant NE/M000966/1. A.J. Parsons acknowledges support from the Natural Environment Research Council (training grant NE/J50001X/1)

Infinite Multiple Membership Relational Modeling for Complex Networks

Learning latent structure in complex networks has become an important problem fueled by many types of networked data originating from practically all fields of science. In this paper, we propose a new non-parametric Bayesian multiple-membership latent feature model for networks. Contrary to existing multiple-membership models that scale quadratically in the number of vertices the proposed model scales linearly in the number of links admitting multiple-membership analysis in large scale networks. We demonstrate a connection between the single membership relational model and multiple membership models and show on "real" size benchmark network data that accounting for multiple memberships improves the learning of latent structure as measured by link prediction while explicitly accounting for multiple membership result in a more compact representation of the latent structure of networks.Comment: 8 pages, 4 figure

Bridging the translational gap:adenosine as a modulator of neuropathic pain in preclinical models and humans

Objectives: This review aims to analyse the published data on preclinical and human experimental and clinical adenosine modulation for pain management. We summarise the translatability of the adenosine pathway for further drug development and aim to reveal subgroups of pain patients that could benefit from targeting the pathway. Content: Chronic pain patients suffer from inadequate treatment options and drug development is generally impaired by the low translatability of preclinical pain models. Therefore, validating the predictability of drug targets is of high importance. Modulation of the endogenous neurotransmitter adenosine gained significant traction in the early 2000s but the drug development efforts were later abandoned. With the emergence of new drug modalities, there is a renewed interest in adenosine modulation in pain management. In both preclinical, human experimental and clinical research, enhancing adenosine signalling through the adenosine receptors, has shown therapeutic promise. A special focus has been on the A 1 and A 3 receptors both of which have shown great promise and predictive validity in neuropathic pain conditions. Summary: Adenosine modulation shows predictive validity across preclinical, human experimental and clinical investigations. The most compelling evidence is in the field of neuropathic pain, where adenosine has been found to alleviate hyperexcitability and has the potential to be disease-modifying. Outlook: Adenosine modulation show therapeutic potential in neuropathic pain if selective and safe drugs can be developed. New drug modalities such as RNA therapeutics and cell therapies may provide new options.</p

Low-Frequency Measurements of Seismic Moduli and Attenuation in Antigorite Serpentinite

Laboratory measurements of seismic moduli and attenuation in antigorite serpentinite at a confining pressure of 200 MPa and temperatures up to 550 °C provide new results relevant to the interpretation of geophysical data in subduction zones. A polycrystalline antigorite specimen was tested via forced oscillations at small strain amplitudes and seismic frequencies (millihertz to hertz). The shear modulus has a temperature sensitivity, ∂G/∂T, averaging −0.017 GPa/K. Increasing temperature above 500 °C results in more intensive shear attenuation ( urn:x-wiley:grl:media:grl58579:grl58579-math-0001) and associated modulus dispersion, with urn:x-wiley:grl:media:grl58579:grl58579-math-0002 increasing monotonically with increasing oscillation period and temperature. This “background” relaxation is adequately captured by a Burgers model for viscoelasticity and possibly results from intergranular mechanisms. Attenuation is higher in antigorite ( urn:x-wiley:grl:media:grl58579:grl58579-math-0003 at 550 °C and 0.01 Hz) than in olivine ( urn:x-wiley:grl:media:grl58579:grl58579-math-0004 below 800 °C), but such contrast does not appear to be strong enough to allow robust identification of antigorite from seismic models of attenuation only

Testing for difference between two groups of functional neuroimaging experiments

We describe a meta-analytic method that tests for the di#erence between two groups of functional neuroimaging experiments. We use kernel density estimation in three-dimensional brain space to convert points representing focal brain activations into a voxel-based representation. We find the maximum in the subtraction between two probability densities and compare its value against a resampling distribution obtained by permuting the labels of the two groups. The method is applied on data from thermal pain studies where &quot;hot pain&quot; and &quot;cold pain&quot; form the two groups

Insight into the microphysics of antigorite deformation from spherical nanoindentation.

The mechanical behaviour of antigorite strongly influences the strength and deformation of the subduction interface. Although there is microstructural evidence elucidating the nature of brittle deformation at low pressures, there is often conflicting evidence regarding the potential for plastic deformation in the ductile regime at higher pressures. Here, we present a series of spherical nanoindentation experiments on aggregates of natural antigorite. These experiments effectively investigate the single-crystal mechanical behaviour because the volume of deformed material is significantly smaller than the grain size. Individual indents reveal elastic loading followed by yield and strain hardening. The magnitude of the yield stress is a function of crystal orientation, with lower values associated with indents parallel to the basal plane. Unloading paths reveal more strain recovery than expected for purely elastic unloading. The magnitude of inelastic strain recovery is highest for indents parallel to the basal plane. We also imposed indents with cyclical loading paths, and observed strain energy dissipation during unloading-loading cycles conducted up to a fixed maximum indentation load and depth. The magnitude of this dissipated strain energy was highest for indents parallel to the basal plane. Subsequent scanning electron microscopy revealed surface impressions accommodated by shear cracks and a general lack of dislocation-induced lattice misorientation. Based on these observations, we suggest that antigorite deformation at high pressures is dominated by sliding on shear cracks. We develop a microphysical model that is able to quantitatively explain Young's modulus and dissipated strain energy data during cyclic loading experiments, based on either frictional or cohesive sliding of an array of cracks contained in the basal plane. This article is part of a discussion meeting issue 'Serpentinite in the earth system'.This work was supported by the Natural Environment Research Councilthrough grant no. NE/M016471/1 to L.N.H. and N.B., and by the European Research Councilunder the European Union’s Horizon 2020 research and innovation programme (project RockDEaF, grant agreement no. 804685)

The role of grain-environment heterogeneity in normal grain growth: a stochastic approach

The size distribution of grains is a fundamental characteristic of polycrystalline solids. In the absence of deformation, the grain-size distribution is controlled by normal grain growth. The canonical model of normal grain growth, developed by Hillert, predicts a grain-size distribution that bears a systematic discrepancy with observed distributions. To address this, we propose a change to the Hillert model that accounts for the influence of heterogeneity in the local environment of grains. In our model, each grain evolves in response to its own local environment of neighbouring grains, rather than to the global population of grains. The local environment of each grain evolves according to an Ornstein-Uhlenbeck stochastic process. Our results are consistent with accepted grain-growth kinetics. Crucially, our model indicates that the size of relatively large grains evolves as a random walk due to the inherent variability in their local environments. This leads to a broader grain-size distribution than the Hillert model and indicates that heterogeneity has a critical influence on the evolution of microstructure.Comment: 24 pages, 8 figures, to be published in Acta Materiali

Dislocation interactions in olivine control postseismic creep of the upper mantle.

Changes in stress applied to mantle rocks, such as those imposed by earthquakes, commonly induce a period of transient creep, which is often modelled based on stress transfer among slip systems due to grain interactions. However, recent experiments have demonstrated that the accumulation of stresses among dislocations is the dominant cause of strain hardening in olivine at temperatures ≤600 °C, raising the question of whether the same process contributes to transient creep at higher temperatures. Here, we demonstrate that olivine samples deformed at 25 °C or 1150-1250 °C both preserve stress heterogeneities of ~1 GPa that are imparted by dislocations and have correlation lengths of ~1 μm. The similar stress distributions formed at these different temperatures indicate that accumulation of stresses among dislocations also provides a contribution to transient creep at high temperatures. The results motivate a new generation of models that capture these intragranular processes and may refine predictions of evolving mantle viscosity over the earthquake cycle

Dislocation theory of steady and transient creep of crystalline solids: predictions for olivine

Significance Many important deformation processes take place at strain rates that are too slow to be investigated experimentally. For example, strain rates in Earth’s mantle are typically ten orders of magnitude slower than in the laboratory. To bridge this gap, empirical relationships are extrapolated with large epistemic uncertainties. We propose a model for deformation derived from the microphysics of deformation. In application to olivine, the main mineral of Earth’s upper mantle, this model explains the scaling relationships observed under a range of laboratory conditions. In extrapolation to Earth’s mantle, the model predicts a transition in the dominant microphysical processes, leading to predictions distinct from previous studies. For instance, following abrupt stress changes, it predicts rapid transient deformation. Abstract In applications critical to the geological, materials, and engineering sciences, deformation occurs at strain rates too small to be accessible experimentally. Instead, extrapolations of empirical relationships are used, leading to epistemic uncertainties in predictions. To address these problems, we construct a theory of the fundamental processes affecting dislocations: storage and recovery. We then validate our theory for olivine deformation. This model explains the empirical relationships among strain rate, applied stress, and dislocation density in disparate laboratory regimes. It predicts the previously unexplained dependence of dislocation density on applied stress in olivine. The predictions of our model for Earth conditions differ from extrapolated empirical relationships. For example, it predicts rapid, transient deformation in the upper mantle, consistent with recent measurements of postseismic creep