Rates and Mechanisms of Intracrystalline and Intergranular Diffusion

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Metamorphic processes that take place deep in the earth's crust or in its upper mantle cannot be observed directly, and they span time scales much longer than human lifetimes, so knowledge of them must come from study of the rocks they produce. To reconstruct geologic histories -- to learn, for example, how long it takes for a mountain belt to form, how ancient it is, how rapidly it arose or was eroded -- we must be able to read the record of those processes that is encoded in minerals that formed at great depth and have been brought to the surface by tectonic processes. The central goal of this research is to learn better how to read such records in garnet, a mineral with a remarkable ability to encode details of its history in its chemical composition. Garnet, during growth, commonly develops differences in the concentrations of constituent elements from the cores of crystals out to their rims, and these variations are modified by diffusion, the movement of atoms through the solid crystal structure. Diffusional modifications occur to varying degrees, reflecting the various lengths of time that the crystals spent at different temperatures and pressures during their post-growth histories. The key to transforming these diffusional modifications into detailed information on geologic processes is quantitative knowledge of the rates and mechanisms of diffusion of different elements through the garnet structure.Prior work has produced robust measurements of diffusion rates for elements that are abundant in garnet, but to glean information on high-temperature processes, such data are needed for trace elements, those with very low abundance. Thus the first objective of this research is to determine diffusion rates of trace elements by analyzing and modeling the evolution of concentration profiles from core to rim in garnet crystals with special, well-constrained histories. A second objective is to develop new theories to explain, in a fundamental way, the atomic-scale mechanisms of diffusion, by sophisticated computer modeling (molecular-dynamics simulation) of the motions of atoms during diffusional transport. A third objective is to quantify rates of a different type of diffusion, namely intergranular diffusion -- meaning diffusion along crystal boundaries rather than within the crystals themselves. The rate of intergranular diffusion of aluminum is a principal control on rates of metamorphic reactions and chemical exchange among minerals as they grow. It can be quantified in special circumstances, in which a crystal is 'caught in the act' of reacting with its surroundings to produce new minerals, by analysis and modeling of the compositional changes that build up at crystal's outer periphery as it reacts.New data on trace-element diffusion in garnet should add rigor to applications as diverse as high-temperature heating/cooling histories and timescales of high-temperature thermal and metasomatic events, Sm-Nd and Lu-Hf garnet geochronology, understanding of metamorphic equilibration between garnet and accessory minerals, and interpretation of rare-earth patterns of mantle minerals and melts. Molecular-dynamics simulation should answer the puzzling question of why garnet and other minerals fail to obey elastic-strain theories for diffusion, and should thereby reveal so-far-unrecognized factors governing the kinetics of diffusion in the solid state. New estimates for intergranular diffusivity of Al should extend previous results to higher temperatures, significantly improving our ability to treat quantitatively the length scales and time scales of metamorphic reactions, and the range and degree of chemical equilibration during metamorphism.