Chemical heterogeneities along grain boundaries in garnet occur across a wide range of metamorphic conditions, yet the processes underlying their development remain poorly understood. Here we integrate electron backscattered diffraction (EBSD) and atom probe tomography (APT) to evaluate the mechanisms driving nanoscale trace element mobility to deformation microstructures in a granulite-facies garnet. This approach shows that low-angle boundaries can be enriched in Ca, Ti, P, Cu, K, Na, Cl, and H. Based on the correlation between EBSD and APT data, we propose that solute ions (Ca, Ti, P, and Cu) were segregated to the interface during the migration of dislocation associated with ductile deformation of the grain. In contrast, elements such as K, Na, Cl, and H are interpreted to reflect diffusion along the low-angle boundary from an externally derived fluid source. These results provide the missing link between chemical heterogeneity and deformation-related microstructures in garnet. Our approach shows that a combination of microstructural and nanoscale geochemical analyses can provide unprecedented insights into mechanisms of element transfer within minerals.
The geometry and composition of deformation-related low-angle boundaries in naturally-10 deformed olivine were characterized by electron backscattered diffraction (EBSD) and atom probe 11 tomography (APT). EBSD data show the presence of discrete low-angle tilt boundaries, which formed 12 by sub-grain rotation recrystallisation associated with the (100)[001] slip system during fluid-13 catalysed metamorphism and deformation. APT analyses of these interfaces show the preferential 14 segregation of olivine-derived trace elements (Ca, Al, Ti, P, Mn, Na and Co) to the low-angle 15 boundaries. Boundaries with < 2° show marked enrichment associated with the presence of multiple, 16 non-parallel dislocation types. However, at larger disorientation angles (> 2°), the interfaces become 17 more ordered and linear enrichment of trace elements coincides with the orientation of dislocations 18 inferred from the EBSD data. These boundaries show a systematic increase of trace element 19 concentration with disorientation angle. Olivine-derived trace elements segregated to the low-angle 20 boundaries are interpreted to be captured and travel with dislocation as they migrate to the sub-grain 21 boundary interfaces. However, the presence of exotic trace elements Cl and H, also enriched in the 22 low-angle boundaries, likely reflect the contribution of an external fluid source during the fluid-23
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