Richard Spiess

The application of electron backscatter diffraction and orientation contrast imaging in the SEM to textural problems in rocks

Abstract In a scanning electron microscope (SEM) an electron beam sets up an omni-directional source of scattered electrons within a specimen. Diffraction of these electrons will occur simultaneously on all lattice planes in the sample and the backscattered electrons (BSE), which escape from the specimen, will form a diffraction pattern that can be imaged on a phosphor screen. This is the basis of electron backscatter diffraction (EBSD). Similar diffraction effects cause individual grains of different orientations to give different total BSE. SEM images that exploit this effect will show orientation contrast (OC). EBSD and OC imaging are SEM-based crystallographic tools. EBSD enables measurement of the crystallographic orientation of individual rock-forming minerals as small as 1 μm, and the calculation of misorientation axes and angles between any two data points. OC images enable mapping of all misorientation boundaries in a specimen and thus provide a location map for EBSD analyses. EBSD coupled to OC imaging in the SEM enables complete specimen microtextures and mesotextures to be determined. EBSD and OC imaging can be applied to any mineral at a range of scales and enable us to expand the microstructural approach, so successful in studies of quartz rocks, for example, to the full range of rock-forming minerals. Automated EBSD analysis of rocks remains problematic, although continuing technical developments are enabling progress in this area. EBSD and OC are important new tools for petrologists and petrographers. Present and future applications of EBSD and OC imaging include phase identification, studying deformation mechanisms, constraining dislocation slip systems, empirical quantification of microstructures, studying metamorphic processes, studying magmatic processes, and constraining geochemical microsampling. In all these cases, quantitative crystallographic orientation data enable more rigorous testing of models to explain observed microstructures.

Some garnet microstructures: an illustration of the potential of orientation maps and misorientation analysis in microstructural studies

The microstructures of two contrasting garnet grains are mapped using automated electron backscatter diffraction. In both cases there is a very strong crystallographic preferred orientation, with measurements clustered round a single dominant orientation. Each garnet grain is divided into domains with similar orientations, limited by boundaries with misorientations of 2° or more. In both samples most of misorientation angles measured across orientation domain boundaries are significantly lower than those measured between random pairs of orientation domains. One sample is a deformed garnet that shows considerable distortion within the domains. Lines of orientation measurements within domains and across domain boundaries show small circle dispersions around rational crystallographic axes. The domain boundaries are likely to be subgrain boundaries formed by dislocation creep and recovery. The second sample is a porphyroblast in which the domains have no internal distortion and the orientation domain boundaries have random misorientation axes. These boundaries probably formed by coalescence of originally separate garnets. We suggest that misorientations across these boundaries were reduced by physical relative rotations driven by boundary energy. The data illustrate the potential of orientation maps and misorientation analysis in microstructural studies of any crystalline material.

Coseismic recrystallization during shallow earthquake slip

Solidified frictional melts, or pseudotachylytes, remain the only unambiguous indicator of seismic slip in the geological record. However, pseudotachylytes form at >5 km depth, and there are many rock types in which they do not form at all. We performed low- to high-velocity rock friction experiments designed to impose realistic coseismic slip pulses on calcite fault gouges, and report that localized dynamic recrystallization may be an easy-to-recognize microstructural indicator of seismic slip in shallow, otherwise brittle fault zones. Calcite gouges with starting grain size −1 , and total displacements between 1 and 4 m. At coseismic slip velocities ≥0.1 m s −1 , the gouges were cut by reflective principal slip surfaces lined by polygonal grains 2 + CaO. The recrystallized calcite aggregates resemble those found along the principal slip surface of the Garam thrust, South Korea, exhumed from

Dynamic weakening along incipient low-angle normal faults in pelagic limestones (Southern Apennines, Italy)

Slip along low-angle normal faults is a mechanical paradox requiring activation of strain weakening mechanisms. Microstructures present in the slip zones of incipient low-angle normal faults cutting carbonates in the Southern Apennines of Italy show that slip was promoted by two weakening mechanisms producing a reduction of the friction coefficient: (1) high pore fluid pressures; (2) dynamic weakening related to thermal decomposition indicated by decarbonation microstructures and concomitant localized dynamic calcite recrystallization. Furthermore, as a consequence of thermal decomposition, nanoparticles occur as infilling of injection veins, suggesting that powder lubrication processes are active along the slip surface during seismic slip. Supplementary materials: A geological sketch of the study area, detailed field photographs of the studied faults and detailed micrographs are available at http://www.geolsoc.org.uk/SUP18806.

From the upper to the lower continental crust exposed in Calabria

Outcrops of the pre-Mesozoic basement, representative of the whole Hercynian continental crust are exposed in Calabria. This is the result of Tertiary geological evolution that brought to the surface different crustal levels. This geological field trip aims to provide a general picture of the continental crust that hopefully may represent a reference frame for geochemical, rheological and geophysical models. The itinerary develops in central and southern parts of Calabria, namely in the Serre massif and in the promontories of Capo Vaticano and Monte Sant’Elia. In three days it is possible to examine compositional and structural features across an entire crust section. Thus rocks affected by very low-grade to granulite facies metamorFrom the upper to the lower continental crust exposed in Calabria A. Caggianelli G. Prosser V. Festa A. Langone R. Spiess geogical feld tips 2013 51.2) DOI: 10.3301/GFT.2013.02 in fo rm a io n 5 phism and distinctive features of granitoids emplaced at different structural levels will be examined. The effects of the intense thermal perturbation produced by granitoid emplacement are visible both in the upper and in the lower crust, in a sharp metamorphic aureole and in a migmatitic border zone, respectively. Finally, some cases of Paleozoic rocks with strongly partitioned deformation, produced by Tertiary tectonics in the brittle and the ductile domains, can be observed.