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Experimental investigation of the kinetics of the reaction wollastonite + calcite + anorthite = grossular + CO₂

Institut für Mineralogie, Petrologie und Geochemie der Universität Tübingen Wilhelmstrasse 56, D-72074 Tübingen, Germany

Mechanisms and kinetics of the reaction wollastonite + calcite + anorthite grossular + carbon dioxide were studied in powder experiments in the presence of a H₂O-CO₂ fluid with 8 to 13 mole percent CO₂ at 665° to 750°C and 400 MPa. The reaction was studied in terms of overall reaction rates, nucleation rates, and crystal growth rates. Grossular was the only solid reaction product. The crystals were grown around minute andradite-rich cores, which enriched the grossular with traces of iron. At a given temperature, the conversion to grossular + CO₂ increased linearly with time at least up to 25 percent. Reaction rates increased in the temperature range from 710° to 750°C by a factor of 22. At the constant temperature of 730°C the reaction rate increased by a factor of 2.4 when overstepping of the equilibrium temperature was increased from 55° to 95°C due to a decrease in the CO₂-concentration in the fluid.

Nucleation and growth rates were measured at 730°C as a function of time. There was a sharp nucleation rate maximum during the second day of the experiments followed by a steady state nucleation rate at a much lower level during the rest of the experiment. Crystal growth rates permanently decreased either with respect to diameter, surface area, or volume. In experiments with varied surface area for either the reactants or for the grossular, no consistent relationship between surface area and reaction rate was found and thus no single rate-limiting step could be identified.

For the rate-controlling process of the overall reaction a model similar to competitive, diffusion-controlled growth of garnet porphyroblasts is suggested. In this model each grossular crystal is surrounded by a diffusion halo where nutrients deplete with time. Once the diffusion halos impinge throughout the reactant mixture, the diffusion gradients are leveled. Consequently, the nucleation rate drops down to a steady state level and crystal growth rates decrease. Nucleation never totally ceases because the nucleation barrier is overcome by enrichment of andradite component in the garnet cores. Analog enrichment of trace elements during nucleation might be common in metamorphic crystallization. The results achieved will serve to better understand garnet formation in porphyroblastic rocks.

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