Constraining the rates and timescales of garnet growth and associated dehydration during metamorphism
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This study incorporates high precision zoned garnet samarium-neodymium geochronology and thermodynamic analysis of garnet forming dehydration reactions to determine the amount of water release during both subduction and mountain building. Garnet grows during rock dehydration, providing both a temporal and geodynamic record of not only its growth, but of associated dehydration. Laboratory experiments and geodynamic models have been used to predict amounts of dehydration during metamorphism based on equilibrium assumptions. If equilibrium is not maintained, or if aspects of the geodynamic modeling are incorrect, these model-based predictions will prove inaccurate. Field-based evidence is necessary to test such model predictions and to elucidate both the timing and duration of dehydration and the role of kinetics during metamorphism. Localities that have undergone dehydration and associated fluid flow provide natural laboratories in which to study these geologic processes. This study focuses on two geologic settings: regional orogenesis (Townshend Dam, Vermont) and subduction zone metamorphism (Sifnos, Greece). Regional metamorphism of the pelitic schists of Townshend Dam occurred during the Acadian orogeny peaking at ~381 Ma. Garnet growth lasted for 4.2 ± 2.4 million years. Thermodynamic forward modeling from this study has shown that an early stage of burial of the rocks without significant heating first occurred, followed then by a period of intense heating at depth, during which, roughly 2 vol.% water was lost from the rock. In contrast, metamorphism, and thus dehydration, during subduction of a continental margin in Sifnos, Greece was found to have occurred in as brief a timespan as tens to hundreds of thousands of years, releasing 2-3 vol.% water during a period of intense heating at ~75 km depth between ~47-44 million years ago. This short time interval represents a discrete pulse of dehydration and heating within the context of the process of subduction, which probably occurred over timescales of 10 to 20 million years in this location. This is the first study to provide a field-based constraint on the magnitude, timing, and rate of dehydration during subduction, a process that causes intermediate-depth earthquakes, mantle melting and volcanism, and large scale changes to the global water cycle.