Controls on scales of chemical equilibration during metamorphism : insights from garnet zoning patterns
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Garnets in metapelites from Harpswell Neck, Maine display atypical compositional zoning patterns that reveal the effects of unusual fluid compositions on rates of intergranular diffusion and scales of chemical equilibration during garnet growth. At Harpswell Neck, pelitic rocks metamorphosed to 525-575 °C and 0.2-0.3 GPa contain chlorite + biotite + muscovite + garnet + quartz + plagioclase + ilmenite ± graphite ± pyrrhotite ± pyrite ± chalcopyrite. Garnet crystals display a wide variety of unusual patterns of compositional zoning. Mn is commonly concentrated in multiple isolated patches in the interior of garnet; this effect is superimposed on the more common tendency for Mn to partition into garnet cores during early stages of growth. Fe and Mg exhibit patterns of compositional zoning inverse to those of Mn. Ca concentration is slightly elevated or flat in garnet interiors and increases in concentric annuli near garnet rims. Y concentrations are highest in garnet cores and in concentric annuli near garnet rims. Relative rates of intergranular diffusion can be interpreted from these garnet compositional zoning patterns. Isolated, patchy zoning of Mn, Fe, and Mg is interpreted as overprint zoning, indicating very slow rates of intergranular diffusion for these elements. In contrast, Ca and Y display patterns of higher core concentrations and concentric rings, which indicate that these elements achieved rock-wide chemical equilibrium. The observed combination of zoning patterns and inferred length scales of diffusion are not compatible with prior models in which temperature is the dominant factor controlling relative rates of intergranular diffusion. The composition of intergranular fluid affects the solubility of elements during metamorphism, thereby exerting a fundamental control on rates of intergranular diffusion and scales of chemical equilibration. To explain observed garnet compositional patterns, the intergranular fluid would need to produce high solubility for Ca and Y and low solubility of Mn, Fe, and Mg. Limited data on solubility suggest that a fluid poor in Cl⁻ and F⁻, but rich in SO₄²⁻, is likely to have these effects; such a fluid might be generated by de-sulfidation of pyrite to pyrrhotite, or oxidation of pyrite and pyrrhotite during metamorphism, or both. Direct analysis of fluid inclusions within garnet using laser ablation inductively coupled mass spectrometry could provide a further test of this hypothesis. In contrast to prior studies in which thermal effects dominate, this work provides evidence that variations in the composition of intergranular fluids can be important determinants of the scales of chemical equilibrium during metamorphism. The patchy zoning patterns observed in garnet from Harpswell Neck are caused by fluids of unusual composition: these fluids yield low solubility and thus slow intergranular diffusion for Mn, Fe, and Mg, but high solubility and thus rapid intergranular diffusion for Ca and Y.