Evaluation of Nanostructures in Oriented Thin Films of Tectonically Deformed Rocks: Integrating SEM-EBSD and TEM studies in Structural Geology
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Date:
23/05/2024
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Time:
4:00 pm
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A lot of Structural Geology research in the present decade involves data collated from macroscopic to microscopic scales using field, petrographic and SEM-EBSD studies. However, deformation that is manifested in tectonically deformed rocks in the field, the optical microscope as well as the SEM, in-fact initiates at the atomistic scale. Therefore, to gain in-depth knowledge of deformation of any mineral, it is necessary to evaluate nanostructures under TEM in thin films. Following materials scientists, geoscientists, in recent years, have been using SEM-based Focused Ion Beam (FIB) technique for extracting thin films (lamella) from a mineral of interest hosted in a deformed rock. Whilst this has helped in observing nanostructures and deciphering dislocation density and deformation mechanism/s, the application to relate the same to larger-scale kinematics has been rather restricted. This is because most FIB lamella excavations are plane normal (PN), i.e., the thin film is extracted perpendicular to the plane of observation. In Structural Geology, kinematic studies in the field as well as rock thin sections are done in a plane parallel to the stretching lineation and perpendicular to the foliation (i.e., XZ section of the strain ellipsoid). Hence, PN FIB lamella excavated from an XZ thin section is not parallel to the kinematic reference frame. In order to relate nanostructures to kinematics, it is necessary to excavate thin film parallel to the XZ thin section. In this lecture, I will focus on this aspect and discuss in detail a protocol that has been developed to excavate “In-Plane” (IP) FIB lamella from the kinematic reference frame. SEM-EBSD and TEM data from magnetite hosted in a mylonitized schist and a BIF sample will be presented to highlight the nanostructures observed in PN and IP thin films, respectively. In addition, details will be presented to keep track of the geographic reference frame while excavating IP FIB lamella from an oriented rock thin section; this helps relate the nanostructures and kinematics in the oriented thin film with the kinematics in the oriented thin section. It is envisaged that the study of nanostructures on these lines will form the basis of a lot of Structural Geology research in the coming decades.