A new method to track the evolution of rock microstructures in shear deformation (torsion) experiments.

Rock microstructures evolve during creep deformation and have a fundamental control on creep mechanisms and rheology. Understanding of creep deformation would improve significantly if we could track the evolution of individual grains and grain boundaries during experiments. No creep deformation experiments of polycrystalline materials relevant to the crust or upper mantle have succeeded in tracking the microstructural evolution of individual grains. This proposal aims to develop and test a new, relatively easy and inexpensive method to track the evolution of specific rock microstructures (e.g. individual grains) during laboratory creep experiments to high shear-strain at elevated pressure (P) and temperature (T); a method applicable to a wide range of rock types. In the case for support we demonstrate that a single cylindrical sample can be twisted incrementally in torsion and the microstructures on the outer surface of the cylinder imaged between each strain increment. The proposal requests funding for a stage for a scanning electron microscope (SEM) that will enable 1) microstructural mapping of the outer surface of cylindrical samples without distortions and 2) accurate registration of data from different strain increments. A pilot study, shearing Carrara marble, will track the orientation and internal structure of individual grains, migration of grain boundaries, and the orientation and evolution of recrystallized grains and should give new insights into polycrystal deformation and recrystallization.