METAMORPHISM ALONG MIGRATING GRAIN BOUNDARIES AND THE EQUILIBRIUM ORIGIN OF SOME MINERAL INCLUSIONS: A TRIBUTE TO THE MEMORY OF BOB TRACY (Invited Presentation)

Robert J. (“Bob”) Tracy was truly a renaissance-man petrologist whose wide-ranging expertise touched on many facets of the origin and evolution of solid earth materials. His encyclopedic knowledge of all-things petrology is reflected by the breadth of research topics investigated over the course of his career. A common thread to his work was seeking to understand not only the driving forces for metamorphic reactions but also the grain-scale mechanisms by which rocks adjust their mineralogy and compositions to respond to changing environmental conditions. This expertise was impressed upon us during our years of interacting with Bob and undoubtedly contributed to shaping our scientific perspective.

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.