Ostwald ripening as a possible mechanism for zircon overgrowth formation during anatexis: Theoretical constraints, a numerical model, and its application to pelitic migmatites of the Tickalara Metamorphics, northwestern Australia

A fundamental dichotomy exists between the low solubility of zircon in pera luminous melt predicted by experimental and geochemical studies and the lar ge volume proportions of zircon overgrowths formed during high-temperature metamorphism and anatexis that are revealed by cathodoluminescence imaging. We investigate the potential of Ostwald ripening as a possible mechanism f or overgrowth formation by presenting a numerical solution to the continuit y equation governing open system, diffusion rate-limited Ostwald ripening i n a zircon-saturated melt. Application of the model to a typical (log-norma l) initial zircon crystal size distribution (CSD) suggests that despite unc ertainties associated with the interfacial free energy of zircon, significa nt grain coarsening is possible via this mechanism under geological conditi ons and time scales relevant to high-grade metamorphism. Primary influences on the rate at which Ostwald ripening proceeds are (i) the temperature of the system, (ii) the duration of the time interval for which the system is above its solidus, and (iii) the nature of the initial (premelting) zircon CSD. To test the viability of the model, we examine zircon CSDs from three high-grade pelitic migmatites of the Tickalara Metamorphics (northwestern A ustralia), assuming that zircon crystals hosted by melanosome biotite were permanently occluded from the melt (and therefore approximate the premeltin g CSD). The model predicts that within 1 to 2 Ma, these biotite-hosted zirc on CSDs will evolve into the observed leucosome-hosted zircon CSDs via melt -present Ostwald ripening, under geological conditions applicable to peak m etamorphism. Although we have not conclusively demonstrated that Ostwald ripening contri buted to changes in zircon CSDs during anatexis of the Tickalara metapelite s, our results suggest that Ostwald ripening is a viable mechanism for zirc on volume transfer in a zircon-saturated melt and capable of playing a sign ificant role in overgrowth formation in rocks where the total volume of zir con overgrowths substantially exceeds the concentration of zircon dissolvab le in the coexisting melt. Copyright (C) 2001 Elsevier Science Ltd.