G. M. Pennock

Current issues and new developments in deformation mechanisms, rheology and tectonics

Abstract We present a selective overview of current issues and outstanding problems in the field of deformation mechanisms, rheology and tectonics. A large part of present-day research activities can be grouped into four broad themes. First, the effect of fluids on deformation is the subject of many field and laboratory studies. Fundamental aspects of grain boundary structure and the diffusive properties of fluid-filled grain contacts are currently being investigated, applying modern techniques of light photomicrography, electrical conductivity measurement and Fourier Transform Infrared (FTIR) microanalysis. Second, the interpretation of microstructures and textures is a topic of continuous attention. An improved understanding of the evolution of recrystallization microstructures, boundary misorientations and crystallographic preferred orientations has resulted from the systematic application of new, quantitative analysis and modelling techniques. Third, investigation of the rheology of crust and mantle minerals remains an essential scientific goal. There is a focus on improving the accuracy of flow laws, in order to extrapolate these to nature. Aspects of strain and phase changes are now being taken into account. Fourth, crust and lithosphere tectonics form a subject of research focused on large-scale problems, where the use of analogue models has been particularly successful. However, there still exists a major lack of understanding regarding the microphysical basis of crust- and lithosphere-scale localization of deformation.

Cryogenic EBSD on ice: preserving a stable surface in a low pressure SEM

Naturally deformed ice contains subgrains with characteristic geometries that have recently been identified in etched surfaces using high‐resolution light microscopy (LM). The probable slip systems responsible for these subgrain boundary types can be determined using electron backscattered diffraction (EBSD), providing the etch features imaged with reflected LM can be retained during EBSD data acquisition in a scanning electron microscope (SEM). Retention of the etch features requires that the ice surface is stable. Depending on the pressure and temperature, sublimation of ice can occur. The equilibrium temperature for a low pressure SEM operating at 1 × 10−6 hPa is about −112°C and operating at higher temperatures causes sublimation. Although charging of uncoated ice samples is reduced by sublimation, important information contained in the etch features are removed as the surface sublimes. We developed a method for collecting EBSD data on stable ice surfaces in a low pressure SEM. We found that operating at temperatures of <–112°C reduced sublimation so that the original etch surface features were retained. Charging, which occurred at low pressures (<1.5 × 10−6 to 2.8 × 10−5 hPa) was reduced by defocusing the beam. At very low pressures (<1.5 × 10−6 hPa) the spatial resolution with a defocused beam at 10 kV was about 3 μm in the x‐direction at −150°C and 0.5 μm at −120°C, because at higher temperature charging was less and only a small defocus was needed to compensate it. Angular resolution was better than 0.7° after orientation averaging. Excellent agreement was obtained between LM etch features and EBSD mapped microstructures. First results are shown, which indicate subgrain boundary types comprised of basal (tilt and twist) and nonbasal dislocations (tilt boundaries).

Nanodiamonds and wildfire evidence in the Usselo horizon postdate the Allerød-Younger Dryas boundary

The controversial Younger Dryas impact hypothesis suggests that at the onset of the Younger Dryas an extraterrestrial impact over North America caused a global catastrophe. The main evidence for this impact—after the other markers proved to be neither reproducible nor consistent with an impact—is the alleged occurrence of several nanodiamond polymorphs, including the proposed presence of lonsdaleite, a shock polymorph of diamond. We examined the Usselo soil horizon at Geldrop-Aalsterhut (The Netherlands), which formed during the Allerød/Early Younger Dryas and would have captured such impact material. Our accelerator mass spectrometry radiocarbon dates of 14 individual charcoal particles are internally consistent and show that wildfires occurred well after the proposed impact. In addition we present evidence for the occurrence of cubic diamond in glass-like carbon. No lonsdaleite was found. The relation of the cubic nanodiamonds to glass-like carbon, which is produced during wildfires, suggests that these nanodiamonds might have formed after, rather than at the onset of, the Younger Dryas. Our analysis thus provides no support for the Younger Dryas impact hypothesis.

Electron backscattered diffraction as a tool to quantify subgrains in deformed calcite

In this work, we investigated processing methods to obtain subgrain sizes from electron backscattered diffraction data using samples of experimentally deformed calcite (CaCO3) polycrystals. The domain boundary hierarchy method, based on area measurements of domains enclosed by boundaries larger than a given misorientation angle, was applied to these calcite samples and was found to be limited by: (i) topological problems; (ii) undersampling of large grains; and (iii) artefacts caused by nonindexing. We tested two alternative methods that may reduce the problems: (i) the measured linear intercept hierarchy method, based on measurements of linear intercept between boundaries having larger misorientations than a given minimum angle; and (ii) the calculated linear intercept hierarchy method, based on the total length of boundaries having misorientations larger than a given minimum angle. The measured linear intercept hierarchy method was found to produce results more representative for the microstructure than the calculated linear intercept hierarchy method, because the calculated linear intercept hierarchy method has a significant uncertainty related to the grid‐based nature of the measurements. Preliminary results on calcite suggest that the measured linear intercept hierarchy method is related, in a complex way, to deformation conditions such as stress, strain and temperature as well as to the characteristics of subgrain rotation and grain boundary migration processes.

Subgrain Rotation Recrystallization in Minerals

Subgrain rotation is a common mechanism of continuous dynamic recrystallization in minerals and some metals. The mechanism involves new grain boundary formation by progressive rotation of subgrains or subgrain boundary migration in regions with an orientation gradient. This paper reviews the status of our current knowledge of rotation recrystallization in minerals. In minerals a misorientation angle (θ) of 10˚ is often taken as the transition from subgrain boundary to grain boundary but recent studies on olivine indicate a much higher transition angle between 15-25˚. In contrast to a high transition angle, the onset of subgrain boundary mobility may occur at much lower angles between 3-10˚. In consequence, rotation recrystallization in minerals often involves an initial stage of subgrain rotation followed by subgrain growth once medium angle boundaries have formed. Current models assume that all subgrain boundaries increase in misorientation with strain. However, recent studies show that many different types of subgrain boundary develop in minerals. The formation of new high angle grain boundaries is only likely along some types of geometrically necessary boundary (GNB). The mineral halite (NaCl) is often quoted as the classic example of rotation recrystallization yet recent electron backscattered diffraction (EBSD) studies show that only limited grain sub-division occurs in NaCl polycrystals. This grain sub-division occurs on the scale of large subgrains that divide the old grain into a few domains and not by the rotation of the smaller equiaxed subgrains, as envisaged in current models. The small scale, equiaxed, mainly low angle network of subgrain boundaries that develop in many minerals may be incidental boundaries, as found in metals, or could be smaller length-scale GNBs. As minerals have high plastic anisotropy and a limited number of slip systems GNBs may dominate over incidental subgrain boundaries formed by trapping of statistically stored dislocations. New and extended models for rotation recrystallization are needed that consider i) incidental subgrain boundaries as well as different types of GNB, ii) the potential high mobility of medium angle (3-15˚) subgrain boundaries and iii) a link between the development of subgrain misorientation and texture development.

Low-angle subgrain misorientations in deformed NaCl.

The development of subgrain boundary misorientations with strain in NaCl polycrystals has been investigated. At low strains, a power law relationship exists between strain and average misorientations. The accuracy of this relationship is assessed in terms of material and electron backscattered diffraction (EBSD) processing parameters and is found to hold for a material of constant grain size deformed in compression, providing EBSD mapping and processing conditions were similar. Average misorientations are strongly influenced by grain orientation, suggesting that the misorientation–strain relationship may also be texture dependent in materials with high plastic anisotropy. A slight grain size dependency of the average misorientations was observed.

Misorientation distributions in hot deformed NaCl using electron backscattered diffraction.

EBSD orientation mapping has been used to derive subgrain boundary misorientation distributions in a series of hot deformed and etched NaCl samples. The main objective of this study has been to examine the influence of data processing, noise caused by angular resolution limits and step size on the subgrain misorientation distributions in hot deformed NaCl. Processing of non‐indexed EBSD patterns increased the average misorientations in etched NaCl. Noise contributed significantly to low angle misorientation peaks for step sizes less than the minimum subgrain size. Orientation data collected using a step size larger than the average subgrain size cumulated misorientations across individual subgrains and effectively measured an orientation gradient between steps. Orientation gradient distributions were not influenced by noise. Average misorientation values calculated from large step data correlated well with average misorientation from small step size data. Average misorientations showed a power law relationship with strain. Three types of substructures were identified using scanning electron microscopy and EBSD mapping, equiaxed subgrains, long subgrain boundaries and a core‐mantle subgrain arrangement.

Grain boundary populations in wet and dry NaCl

Abstract The presence of water causes significant weakening of polycrystalline NaCl because fluid assisted recrystallisation and diffusion creep occurs along grain boundaries. In metals and ceramics, special low energy boundaries are often not wetted by fluids and this affects properties. The aim of the present paper is to use EBSD to determine whether special boundaries exist in NaCl and whether these are wetted. Wet and dry polycrystalline NaCl were examined, deformed by dislocation creep and statically and dynamically recrystallised. Coincident site lattice (CSL) boundaries in recovered material occurred with frequencies expected from the texture, whereas boundary populations of recrystallised material were significantly different from those expected from the texture, and may have been influenced by special boundaries, approximately parallel to {100} planes. Coincident site lattice boundaries became discontinuous with increasing strain in dry NaCl because of subgrain rotation. Both CSL and special boundaries migrated like general boundaries at low temperatures and are therefore likely to have been wetted.

Investigation of Subgrain Rotation Recrystallization in Dry Polycrystalline NaCl

NaCl is plastically anisotropic and forms a well developed substructure during deformation at 0.3-0.5Tm. EBSD was used to assess subgrain misorientations up to 0.5 true strain in dry NaCl. Equiaxed subgrains were ubiquitous but misorientations along segments of subgrain boundaries differed. Three types of subgrain boundary were identified: boundaries that surrounded equiaxed subgrains, boundaries that partly surrounded mantle subgrains, and extended subgrain boundaries, longer than the equiaxed subgrains. All of these subgrain features were recognised at low strains, <0.15. Misorientations of the majority of equiaxed subgrains were generally <2° at 0.5 strain, although segments could reach higher misorientations along kink-like boundaries. Mantle subgrains along grain boundaries tended to develop higher misorientations than in core subgrains. Long subgrain boundaries reached very high misorientations along segments of their length by 0.5 strain. Small new grains formed at triple points and more rarely within grains. Microstructures in NaCl are similar to those found in aluminium. Therefore, the dominant mechanism of high angle subgrain development at 0.5 strain and at 0.4Tm is probably an orientation splitting mechanism rather than equiaxed subgrain rotation.

Cosmic impact or natural fires at the Allerød–Younger Dryas boundary: A matter of dating and calibration

Wittke et al. (1) present evidence of a major cosmic impact at the onset of the Younger Dryas (YD) episode, including some markers found in the top of the well-known Usselo marker horizon (UH). This finding is contrary to our extensive radiocarbon dating effort from this horizon (2), which shows that the UH at Aalsterhut postdates the onset of the YD. Furthermore, Wittke et al. (1) misinterpret the origin of the UH: this horizon is a well-defined paleosoil that formed during the Allerod and the early YD in the top part of coversand. This coversand was deposited before the Allerod, during cold and dry conditions, and is part of the European Sandbelt. Wildfires were common and occurred throughout this period, rather than synchronously with the onset of the YD (3).