Garnet growth patterns and processes: An analysis of the three-dimensional spatial distribution and compositional zoning patterns in metamorphic garnet from northwest Connecticut and Harpswell Neck, Maine

The application of serial sectioning and imaging with a flatbed scanner yields the three-dimensional size and spatial distribution of garnet porphyroblasts in metamorphic rocks from northwest Connecticut and Harpswell Neck, Maine. Two garnet-zone and one staurolite-bearing schist from the Everett Formation, northwest Connecticut and one sample each from the garnet-zone, staurolite-zone and andalusite-zone rocks from Harpswell Neck, Maine were examined. Statistical measures of garnet spatial distributions, with the pair correlation function (PCF), indicate that garnet crystals are weakly to strongly clustered at length scales between 2 and 10 mm for the Connecticut samples and 0.5 to 6 mm for the Maine samples. Covariance measures between garnet size and nearest-neighbor distance, using the mark covariance function (MCF), show a weak correlation between crystal size and nearest-neighbor distance for length scales of $\approx 500mm$ or less that is inconsistent with the interface-controlled growth simulations. Radius-rate plots calculated from two-dimensional Mn compositional maps indicate that garnet crystals in all samples grew by approximately equal radial increments consistent with an interface-controlled growth model. The statistical and compositional data suggest that diffusional gradients, if present, were small and did not greatly influence nucleation and growth patterns and thus, garnet growth appears to be interface-controlled.; Three-dimensional compositional zoning patterns in garnet-zone garnets from Harpswell Neck, Maine, United States, suggest complex patterns of nucleation and growth. Compositional zoning is often asymmetric, and core compositions do not always coincide with the geometric center of the garnet. In detail, garnet zoning patterns show multiple, isolated high-MnO areas that we interpret as sites of garnet nucleation. Computer-generated, three-dimensional animations of these patchwork MnO zoning patterns reveal that many garnets contain multiple nuclei that grew and coalesced to form a single crystal. New nuclei often appeared next to preexisting porphyroblasts and became incorporated into the growing crystal. Contrary to the widely held idea that garnets grow radially outward from a single nucleus, we propose that many of the garnets examined in this study grew by the coalescence of multiple nuclei.