ESTABLISHING THE LINK BETWEEN GEOCHEMISTRY AND MICROSTRUCTURE WITHIN DEFORMED MINERAL GRAINS
A zircon grain from a lower crustal xenolith from the Udachnaya kimberlite, Siberia, records a subgrain microstructure associated with crystal-plastic deformation. Localized Ti depletion from 30 ppm to 2.6 ppm (corresponding to an apparent Ti-in-zircon temperature range of over ~200°C) and enrichment of U, Th and REE within the grain are spatially related to low-angle (<5°) boundaries. Diffusion modeling illustrates that volume diffusion is an unlikely explanation for these observations and the data are best explained by enhanced trace element diffusion along fast-diffusion pathways associated with crystal-plastic deformation.
Detrital pyrite grains in ore from the Witwatersrand gold province record bedding-perpendicular shortening that is manifest by brittle (Hertzian) fracture and limited crystal-localized crystal-plastic deformation around grain-grain contacts. This deformation overprints a more widespread, but still heterogeneous, crystal-plastic deformation that we identify via orientation mapping and which we interpret to have taken place prior to erosion and resedimentation of the pyrite grains. Quantitative microstructural and geochemical data from one of these early-deformed detrital pyrites shows localised trace element (As, Co, Ni) enrichment coincident with specific geometries of low-angle boundaries. This observation indicates that microstructures associated with different slip systems may have different diffusivities for element migration within complexly deformed mineral grains.
The results of these two studies highlight complex relationships between deformation and geochemistry at the intragrain scale that may be utilised to address specific geological problems.