METAMORPHISM ALONG MIGRATING GRAIN BOUNDARIES AND THE EQUILIBRIUM ORIGIN OF SOME MINERAL INCLUSIONS: A TRIBUTE TO THE MEMORY OF BOB TRACY (Invited Presentation)
To continue investigating the mechanisms of metamorphic reactions, we combine detailed analysis of rocks metamorphosed in a variety of geologic settings with carefully-designed experiments to investigate the driving forces, grain-scale processes, and consequences of reactions in a simple system with clearly defined solubility and diffusivity relationships: Ti-in-quartz. Hydrostatic recrystallization experiments conducted on synthetic quartz aggregates reveal local migration of boundaries between neighboring grains with out-of-equilibrium Ti contents. A consequence of this process is to produce a two-phase material – rutilated quartz – in the wake of a migrated boundary. This de-alloying process is driven by compositional instability between adjacent grains and is described by the mechanism of chemically-induced grain boundary migration. This presentation will describe recent work to elucidate the significance of driving forces that arise from trace-level compositional differences between adjacent grains in an aggregate, the consequences of these driving forces for recrystallization and microstructure development, and the importance of this process for the equilibrium formation of mineral inclusions.