Abstract

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. If it is assumed that garnet growth is close to equilibrium and the Mn zoning is not significantly modified by diffusion, then the MnO contours or isosurfaces can serve as a proxy for patterns of garnet nucleation and growth. 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.

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