HOW DOES GARNET CRYSTALLIZE DURING METAMORPHISM?

Microstructural data on the orientation and distribution of minerals can be utilized to better understand the processes controlling mineral crystallization during metamorphism and the extent to which equilibrium versus kinetic factors control the evolution of metamorphic rocks. Here, we present microstructural (EBSD, TEM, and X-Ray CT) data on three samples from two terranes: the Nelson Aureole, British Columbia and the Mosher’s Island Formation, Nova Scotia, the latter which was metamorphosed during the formation of the Appalachians. The orientations of twenty-two garnet crystals were analyzed to investigate the crystallographic relationships between garnet and both muscovite and chlorite (which define the foliation in these rocks). All twenty-two garnet crystals have one of three crystal directions parallel to the foliation of the rock. Comparison of atomic-scale crystal structure models of garnet, muscovite, and chlorite in those orientations reveal that the crystal structure of garnet and muscovite (specifically the spacing and orientation of Al and Si) is similar enough that garnet could crystallize by templating on the crystal structure of muscovite. This highlights the fact that garnet crystallizes via epitaxial nucleation on pre-existing muscovite crystals.

Interpretation of X-Ray CT data on the 3D size, location, and shape of garnet crystals in a sample of the Mosher’s Island Formation reveals that the average garnet radius in 126 µm and the crystal size distribution (CSD) is skewed to the right. 3D statistical analysis of the garnet crystals show that they are ordered relative to a random distribution up to the mean nearest neighbor distance between garnet crystals. This is interpreted such that the rate limiting step for garnet crystallization was likely the intergranular diffusion of Al through the matrix.

Combining EBSD and X-Ray CT data reveals that there is a complex interplay between template providing mineral distribution, epitaxy, and competition for nutrient elements (Al) during mineral crystallization.