HIGH-RESOLUTION IMAGING OF GARNET GROWTH ZONING IN METAPELITES: CAN IT INFORM US ABOUT GROWTH PROCESSES?

During the formative stages of his career, the senior author was much influenced by the pioneering work of Lincoln Hollister on garnet zoning, and so is pleased to participate in this session honoring Prof. Hollister.

From the 1960s onward, there has been steady progress in understanding the distribution of garnet end-member components in metamorphic garnets. One early and unsuccessful strategy that preceded EPMA techniques was an attempt by J.B. Thompson to physically abrade large garnets from Vermont and perform chemical analysis on the resulting sequential powders. The onset of the EPMA era in the 1960s allowed researchers such as Hollister and others to perform quantitative analytical rim-to-rim traverses (one-dimensional). The discovery of the typical Mn “bell curve” in metapelite garnets, and the Rayleigh fractionation model for Mn matrix diffusion, was a result. In the 1970s, the 1-dimensional model of garnet zoning was extended to two dimensions with the first areal maps of zoning, based on manual contouring of analytical points. In the 1990s, electron microprobes became capable of creating 2-dimensional X-ray intensity maps (analog chemical maps) that revealed unexpected complexity that confounded simple concentric models for garnet growth. Images and data presented in this talk illustrate a variety of intriguing and puzzling features of longer- and shorter-scale garnet zoning that reveal details of how material is added to growing garnet crystals in metamorphic rocks during solid-state crystal growth. Examples will include amphibolite facies garnets from New England, HT and UHT garnets from New England and elsewhere, and UHP pyrope-rich garnets from the Dora Maira massif, Italian Alps. The preservation of original growth features is typically blurred or obliterated by post-growth diffusive modification of zoning for most components except for Ca (in some but not all cases) and for HFSEs like Y, Ti, Sc and Cr, which generally occur at trace levels and are difficult to image. But in ideal cases where original zoning is preserved by some components, the deviations from simple concentric growth, and modifications due to causes other than post-growth diffusion, can be spectacular, highly informative, and generally difficult to reconcile with simple models of crystal growth in metamorphic systems.