The 3-dimensional growth history of two garnet zone samples (Grt + Chl + Bt + Ms + Pl + Qtz + Ilm) from southwestern Maine was examined by serial sectioning and 3-D reconstructions of compositional zoning from backscatter images and X-ray maps.

Mn, Fe, Mg and Ca zoning is broadly concentric. The concentration of Mn in garnet cores generally correlates with size (d = 50 to 750 microns), indicating progressive nucleation. In detail, all elements show irregular, patchy zoning in the cores. Assuming constancy of Mn on the rims of all garnets in a rock volume plus no subsequent diffusional modification, Mn concentration can be used as a "time line" for garnet growth. Examination of the evolution of individual garnets reveals that multiple nuclei formed simultaneously in the core regions and nuclei expanded by growth in amoeba-shape forms along preexisting mineral grain boundaries (primarily quartz and plagioclase), dissolving the interior grains until the grains were either gone or encapsulated, at which time dissolution ceased. Amoeba-shaped garnets coalesced as they grew and, simultaneously, new nuclei appeared in the nearby matrix. The net result was a single garnet porphyroblast that formed by the growth and coalescence of multiple nuclei.

Radius-rate plots, constructed by counting pixels in 2-D images, reveal that crystals grew at the same radial rate, regardless of size. The observation of continuous nucleation in the vicinity of preexisting crystals plus the radius-rate plots rule out diffusion over length scales on the order of the garnet radius or greater as the rate-limiting step and are consistent with either diffusion over shorter length scales or interface control as rate-limiting to garnet growth. The strong clustering of garnet nuclei requires clustering of favorable nucleation sites, which may have been caused by favorable orientation of garnet precursor minerals (i.e. chlorite + quartz).

Keywords: kinetics, nucleation, growth, garnet