ON THE TRAIL OF METAMORPHIC ZIRCON GROWTH (Invited Presentation)

Zircon is unusually well-suited for investigating metamorphic processes because it is readily analyzed for U-Pb ages, it harbors diverse mineral inclusions, and its chemistry can be linked to metamorphic parageneses and P-T paths. Here, we review how metamorphic chemistry and inclusion assemblages have been used to link the age of a zircon domain to metamorphic P-T conditions, but also emphasize unsolved questions of how metamorphic zircon grows and potential future approaches.

Mass balance considerations alone suggest that most metamorphic zircon should form during the retrograde/exhumational phase of metamorphism through breakdown of Zr-bearing minerals or crystallization from in situ melts. Yet domain-specific ages and inclusion assemblages from ultrahigh-pressure zircons show that metamorphic zircon domains do form during prograde metamorphism at temperatures as low as 500-600 °C. Insofar as prograde zircon must grow at the expense of older grains, what catalyzes redistribution of zircon in a metamorphic rock?

Dissolution of metamict grains and precipitation of new zircon can be effective at sub-greenschist conditions (<250 °C), but annealing above that temperature should block this process. A new consideration of Ostwald ripening (surface energy-driven dissolution of small zircon grains to produce larger ones) shows theoretically that zircon should not coarsen at sub-anatectic conditions, but can readily coarsen in the presence of partial melts. Observationally, trace elements, inclusion assemblages and oxygen isotopes in zircon suggest that dehydration reactions may also catalyze zircon growth. Using a new HREE mass balance model for garnet, zircon and whole-rock, we show that, indeed, prograde zircon preferentially grows at dehydration reactions, but for reasons that remain obscure.

Future research should include (a) identifying natural systems that constrain crystal-chemical controls on trace element uptake in zircon and garnet for understanding how rare-earth budgets and patterns change during metamorphism, and (b) identifying underlying principles that govern the dissolution and reprecipitation of zircon during metamorphism.