NANOSCALE INVESTIGATION OF GARNET GRAIN BOUNDARIES IN METAMORPHIC ROCKS

Grain boundaries, which are the interfaces between mineral grains, are important for understanding metamorphic processes including intergranular element mobility, the addition of atoms to growing mineral surfaces, and mechanisms and length-scales of chemical equilibration. We examine the role of grain boundaries in mediating the equilibration and growth of garnet in the upper crust. We compare and contrast the nanoscale structure of garnet grain boundaries with surrounding matrix mineral phases, including quartz and chlorite, in a sample from the Nelson Aureole, British Columbia (Pattison & Tinkham, J Met Geol, 2009). Textural evidence suggests that this rock just reached the conditions of initial garnet stability (530ºC and 4.0 kbar), so it is ideal for studying the prograde relationship between a growing garnet porphyroblast and the surrounding matrix; it was not subsequently affected by higher temperature or pressure, and shows limited macro-scale evidence for late deformation.

Grain boundaries surrounding a euhedral garnet crystal were prepared for Transmission Electron Microscopy (TEM) analysis using a Focused Ion Beam. Electron diffraction, imaging, and elemental quantification in the vicinity of the grain boundary was performed on a JEOL 2100 TEM. Imaging of the grain boundaries reveals minimal dislocations and deformation in the garnet crystal, but a complex array of deformation in the matrix phases. This could potentially be the result of a rigid porphyroblast exerting stress on the surrounding phases as it grows into the matrix. Diffraction confirms that the porphyroblast is a single crystal with lattice continuity. Diffraction of a garnet-chlorite grain boundary reveals coincident lattice sites between the two phases, implying an epitaxial relationship between mineral grains that reduces the free energy of the grain boundary. Chemical analysis reveals <100 nm compositionally distinct phases at the garnet-chlorite boundary. These results highlight how grain boundary processes are important for understanding mineral growth and equilibration in metamorphic systems.