Exploring Dislocation Structures with Conventional EBSD

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The interior of the Earth is not frozen but continuously turns over. The motion of rigid plates at the Earth’s surface is driven by the thermal convection of solid rocks in the planet interior. Plate tectonics likely began early in Earth’s history. The continuous deformation leaves a record within rocks that can be sampled at the surface. Such a record is also observed in minerals and rocks deformed experimentally at conditions reproducing natural deformations. The record includes the size and shape of the minerals forming the rocks, called microstructure, as well as specific defects within individual crystals. These defects contain critical information about the permanent deformation – called plastic deformation – that the mineral experienced. A key ingredient of crystal plastic deformation, first recognized and extensively studied in metals, are line defects called dislocations. These small linear defects can be imaged by electron microscopy. Yet, because of their small size, it is challenging to observe them together with their host crystal, usually much larger. Here, the team develops a new method to image dislocations over areas that are representative of the microstructure of natural rocks. The method, based on scanning electron microscopy, can be routinely applied to experimentally and naturally deformed rocks. It provides a new time- and cost-effective way to study processes that shape the Earth. The researchers first benchmark the method on crystals which have been experimentally deformed and extensively studied by a range of imaging techniques. They then apply the new imaging technique to naturally deformed rocks, gradually unveiling their deformation history. The project supports an early-career female scientist and the training of undergraduate students at SUNY College at New Paltz (NY). Its outcomes provide the scientific community with a blueprint for improving microstructural studies of Earth materials.Most of the Earth's crust and upper mantle deform by dislocation creep. Grain-internal structures of experimentally and naturally deformed samples are usually examined either by oxidative decoration of dislocations or TEM imaging. The former is not a routine analysis method and cannot resolve the full geometry of dislocations. While the latter comprehensively characterizes dislocations, the investigated volume is only a few microns, a fraction of the grain size even of fine-grained experimental samples. Up to now, electron backscatter diffraction (EBSD) mapping has primarily been used to determine grain sizes and lattice preferred orientation; but the speed and quality of EBSD indexing has substantially improved over the last decade. Automated EBSD mapping allows routine imaging of relatively large areas (up to thin section scale). High-resolution EBSD mapping has been shown to be able to image dislocation structures in olivine and quartz. HR-EBSD, however, requires substantial additional resources in comparison to conventional EBSD. Here, the team investigate whether conventional EBSD mapping can provide accurate enough indexing to characterize dislocation type/slip systems, for both distributed dislocations and sub-grain boundaries. To test the method, the team map single crystals previously deformed experimentally, for which the dislocation structures have been comprehensively evaluated by oxidative decoration and TEM. The method will then applied to natural samples with different fabric types, which allows indexing their dislocation microstructures in a cost-effective way.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.