A thermodynamic model of mineral segregations in the lower sillimanite zone near Rangeley, Maine

A local equilibrium, irreversible thermodynamic approach is used to model metamorphic segregations in pelites from the lower sillimanite zone near Rangeley, Maine. Relative phenomenological coefficients for the model are derived from a reaction involving sillimanite and another reaction involving staurolite using a least-squares technique to give the best fit to the observed data. The mean values of the relative coefficients in an SiO₂ fixed (= inert marker?) reference frame from five rocks in the middle of the lower sillimanite zone are: Al_1.5 = 5.9, FeO = 2.6, MgO = 1.8, KO_0.5 = 1.0, NaO_0.5 = 0.9, TiO₂ = 0.4, and CaO = 0.2. The phenomenological coefficients are used with Gibbs-Duhem relations and conservation equations to give the sequence of mantles, mineral modes in each mantle, chemical potential profiles, and size of segregations that are expected to form around reacting sillimanite, staurolite, and garnet. The model predicts that: sillimanite growth will produce a segregation with a muscovite-free, biotite-rich mantle surrounding a sillimanite- and quartz-rich core; staurolite will be replaced by a muscovite-rich pseudomorph; and matrix garnets should be rimmed by thin biotite-free mantles. Comparison of the computed textures with those observed in the rocks shows a good match except for the case of garnet, where new garnet growth was not sufficient to produce well-defined mantles in the relatively coarse-grained matrix.