Patrick Trimby

Orientation mapping of nanostructured materials using transmission Kikuchi diffraction in the scanning electron microscope.

In this study, the new technique of transmission Kikuchi diffraction (TKD) in the scanning electron microscope (SEM) has been applied for the first time to enable orientation mapping of bulk, nanostructured metals. The results show how the improved spatial resolution of SEM-TKD, compared to conventional EBSD, enables reliable mapping of truly nanostructured metals and alloys, with mean grain sizes in the 40-200 nm range. The spatial resolution of the technique is significantly below 10nm, and contrasting examples are shown from both dense (Ni) and lighter (Al-alloy) materials. Despite the burden of preparing thin, electron-transparent samples, orientation mapping using SEM-TKD is likely to become invaluable for routine characterisation of nanocrystalline and, potentially, highly deformed microstructures.

Crystal-plastic deformation of zircon: a defect in the assumption of chemical robustness

Orientation contrast imaging and quantitative electron backscatter diffraction analysis of a zircon collected from an Indian Ocean gabbro reveal intragrain crystallographic misorientations (up to 14°) and low-angle orientation boundaries concentrated in the zircon tips. These features represent the formation and migration of dislocations and provide the first evidence of crystal-plastic deformation of zircon under crustal conditions. Panchromatic and wavelength cathodoluminescence (CL), combined with quantitative rare earth element (REE) ion microprobe analyses, demonstrate modification of zircon REE chemistry within the areas of crystal plasticity. These data indicate that the enhanced diffusion of REEs into the zircon is spatially linked to the presence of dislocations that behave as high-diffusivity pathways, increasing bulk diffusion rates and effective diffusion distances in the zircon by several orders of magnitude. In addition, discrete ∼2 μm zones of reduced panchromatic CL correspond exactly to the position of low-angle orientation boundaries and demonstrate a defect dependence on CL signal at high dislocation densities. The presence of deformation-related crystal-plastic microstructures in zircon, and their role in modifying elemental diffusion, questions the commonly made assumption of chemical robustness and has fundamental implications for the interpretation of zircon trace-element and isotopic data.

Grain boundary hierarchy development in a quartz mylonite

Abstract Orientation contrast imaging using forescatter detectors and backscattered electron diffraction techniques in the scanning electron microscope have been used to investigate the boundary hierarchy characteristics of a quartz mylonite shear zone from Torridon, NW Scotland. The boundary hierarchy is defined as the relationship between the boundary misorientations, their enclosed domain size and their frequency distribution. By measuring the misorientation across every grain and subgrain boundary, the characteristics of the boundary hierarchy can be found. Two microstructural domains were studied: one partially recrystallized low strain domain with large relict grains and one fully recrystallized high strain mylonitic domain. Our results indicate that the processes of recovery, subgrain rotation and grain boundary migration recrystallization each produce identifiable boundary hierarchy signatures. In the relict quartz grains we have identified the processes of recovery and subgrain rotation; in the recrystallized quartz a cyclic steady state exists between these and other processes. Coupling these data with traditional microstructural observations allows a more rigorous investigation into the development of a high strain, fine grained mylonite from a coarse grained, undeformed protolith. We suggest that this type of detailed crystallographic microstructural analysis can greatly further our understanding of microstructural development in shear zones and may have implications for the effective use of (sub)grain size palaeopiezometers.

Deformation-induced trace element redistribution in zircon revealed using atom probe tomography.

Trace elements diffuse negligible distances through the pristine crystal lattice in minerals: this is a fundamental assumption when using them to decipher geological processes. For example, the reliable use of the mineral zircon (ZrSiO4) as a U-Th-Pb geochronometer and trace element monitor requires minimal radiogenic isotope and trace element mobility. Here, using atom probe tomography, we document the effects of crystal–plastic deformation on atomic-scale elemental distributions in zircon revealing sub-micrometre-scale mechanisms of trace element mobility. Dislocations that move through the lattice accumulate U and other trace elements. Pipe diffusion along dislocation arrays connected to a chemical or structural sink results in continuous removal of selected elements (for example, Pb), even after deformation has ceased. However, in disconnected dislocations, trace elements remain locked. Our findings have important implications for the use of zircon as a geochronometer, and highlight the importance of deformation on trace element redistribution in minerals and engineering materials.

The weighted Burgers vector: a new quantity for constraining dislocation densities and types using electron backscatter diffraction on 2D sections through crystalline materials

The Weighted Burgers Vector (WBV) is defined here as the sum, over all types of dislocations, of [(density of intersections of dislocation lines with a map) × (Burgers vector)]. Here we show that it can be calculated, for any crystal system, solely from orientation gradients in a map view, unlike the full dislocation density tensor, which requires gradients in the third dimension. No assumption is made about gradients in the third dimension and they may be non‐zero. The only assumption involved is that elastic strains are small so the lattice distortion is entirely due to dislocations. Orientation gradients can be estimated from gridded orientation measurements obtained by EBSD mapping, so the WBV can be calculated as a vector field on an EBSD map. The magnitude of the WBV gives a lower bound on the magnitude of the dislocation density tensor when that magnitude is defined in a coordinate invariant way. The direction of the WBV can constrain the types of Burgers vectors of geometrically necessary dislocations present in the microstructure, most clearly when it is broken down in terms of lattice vectors. The WBV has three advantages over other measures of local lattice distortion: it is a vector and hence carries more information than a scalar quantity, it has an explicit mathematical link to the individual Burgers vectors of dislocations and, since it is derived via tensor calculus, it is not dependent on the map coordinate system. If a sub‐grain wall is included in the WBV calculation, the magnitude of the WBV becomes dependent on the step size but its direction still carries information on the Burgers vectors in the wall. The net Burgers vector content of dislocations intersecting an area of a map can be simply calculated by an integration round the edge of that area, a method which is fast and complements point‐by‐point WBV calculations.

Microstructural imaging techniques: a comparison between light and scanning electron microscopy

Abstract Both light microscopy and scanning electron microscopy are commonly used in microstructural studies of geological specimens. This contribution uses polished thin sections to analyse the same areas in two specimens using both microscopy techniques. The images from a quartz mylonite and a granodiorite are compared to quantitative crystallographic data obtained using electron backscatter diffraction (EBSD). The results indicate that light microscopy images pick out all high angle (>10° misorientation) boundaries but only some low angle and twin boundaries, therefore making it suitable for imaging more general microstructural trends. Orientation contrast imaging, using forescatter detectors in the SEM, images all types of crystallographic boundary: this can obscure the general microstructural trend but makes it more suitable for detailed microstructural analyses. Future studies are advised, wherever possible, to utilise the imaging advantages from a combination of both techniques.

The First Occurrence in the Fossil Record of an Aquatic Avian Twig-Nest with Phoenicopteriformes Eggs: Evolutionary Implications

Background We describe the first occurrence in the fossil record of an aquatic avian twig-nest with five eggs in situ (Early Miocene Tudela Formation, Ebro Basin, Spain). Extensive outcrops of this formation reveal autochthonous avian osteological and oological fossils that represent a single taxon identified as a basal phoenicopterid. Although the eggshell structure is definitively phoenicopterid, the characteristics of both the nest and the eggs are similar to those of modern grebes. These observations allow us to address the origin of the disparities between the sister taxa Podicipedidae and Phoenicopteridae crown clades, and traces the evolution of the nesting and reproductive environments for phoenicopteriforms. Methodology/Principal Findings Multi-disciplinary analyses performed on fossilized vegetation and eggshells from the eggs in the nest and its embedding sediments indicate that this new phoenicopterid thrived under a semi-arid climate in an oligohaline (seasonally mesohaline) shallow endorheic lacustine environment. High-end microcharacterizations including SEM, TEM, and EBSD techniques were pivotal to identifying these phoenicopterid eggshells. Anatomical comparisons of the fossil bones with those of Phoenicopteriformes and Podicipediformes crown clades and extinct palaelodids confirm that this avian fossil assemblage belongs to a new and basal phoenicopterid. Conclusions/Significance Although the Podicipediformes-Phoenicopteriformes sister group relationship is now well supported, flamingos and grebes exhibit feeding, reproductive, and nesting strategies that diverge significantly. Our multi-disciplinary study is the first to reveal that the phoenicopteriform reproductive behaviour, nesting ecology and nest characteristics derived from grebe-like type strategies to reach the extremely specialized conditions observed in modern flamingo crown groups. Furthermore, our study enables us to map ecological and reproductive characters on the Phoenicopteriformes evolutionary lineage. Our results demonstrate that the nesting paleoenvironments of flamingos were closely linked to the unique ecology of this locality, which is a direct result of special climatic (high evaporitic regime) and geological (fault system) conditions.

Redox-freezing and nucleation of diamond via magnetite formation in the Earth's mantle.

Diamonds and their inclusions are unique probes into the deep Earth, tracking the deep carbon cycle to >800 km. Understanding the mechanisms of carbon mobilization and freezing is a prerequisite for quantifying the fluxes of carbon in the deep Earth. Here we show direct evidence for the formation of diamond by redox reactions involving FeNi sulfides. Transmission Kikuchi Diffraction identifies an arrested redox reaction from pyrrhotite to magnetite included in diamond. The magnetite corona shows coherent epitaxy with relict pyrrhotite and diamond, indicating that diamond nucleated on magnetite. Furthermore, structures inherited from h-Fe3O4 define a phase transformation at depths of 320–330 km, the base of the Kaapvaal lithosphere. The oxidation of pyrrhotite to magnetite is an important trigger of diamond precipitation in the upper mantle, explaining the presence of these phases in diamonds.

An automated method of quantifying ferrite microstructures using electron backscatter diffraction (EBSD) data

The identification and quantification of the different ferrite microconstituents in steels has long been a major challenge for metallurgists. Manual point counting from images obtained by optical and scanning electron microscopy (SEM) is commonly used for this purpose. While classification systems exist, the complexity of steel microstructures means that identifying and quantifying these phases is still a great challenge. Moreover, point counting is extremely tedious, time consuming, and subject to operator bias. This paper presents a new automated identification and quantification technique for the characterisation of complex ferrite microstructures by electron backscatter diffraction (EBSD). This technique takes advantage of the fact that different classes of ferrite exhibit preferential grain boundary misorientations, aspect ratios and mean misorientation, all of which can be detected using current EBSD software. These characteristics are set as criteria for identification and linked to grain size to determine the area fractions. The results of this method were evaluated by comparing the new automated technique with point counting results. The technique could easily be applied to a range of other steel microstructures.

Structural analysis of reactionary dentin formed in response to polymicrobial invasion

In response to microbial invasion of dentin odontoblasts secrete an altered calcified matrix termed reactionary dentin (Rd). 3D reconstruction of focused-ion-beam scanning electron microscopy (FIB-SEM) image slices revealed helical tubular structures in Rd that contrasted with regular cylindrical tubules characteristic of dentin from healthy teeth and affected so-called physiological dentin (Pd) lying exterior to Rd. This helical structure in Rd provided effective constriction of tubule lumen diameter that formed a barrier to bacterial advance towards the dental pulp. SEM of resin cast preparations revealed altered extension of odontoblast processes through Rd. The distribution of key mineral elements was studied by combination of 3D reconstruction of focused-ion-beam based X-ray microanalysis (FIB-EDS), laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) and diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS). There was a marked redistribution of calcium and phosphorous in Rd together with an increase of diffusely deposited magnesium compatible with the mineral deposition phase of synthesis of this altered matrix. Changes in tubule structure and mineral content characteristic of Rd are consistent with reduced hardness and lower elastic modulus reported for this matrix. Findings provide insight into the unique structure of Rd synthesised as a primary response to infection.