EAR-PF: Deciphering the Timescales of Metamorphism via Size Sorted Garnet Geochronology

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Dr. Allie Nagurney has been granted an NSF EAR Postdoctoral Fellowship to investigate the timescales and nature of metamorphism via size sorted garnet geochronology. This work will take place at the University of South Carolina under the mentorship of Dr. Besim Dragovic. Mountain-building events thicken and heat the upper most layer of the Earth (the lithosphere), causing physical and chemical changes in crustal rocks, called metamorphism. Establishing the duration of metamorphic processes during mountain building events is crucial to characterizing the thermal evolution of the lithosphere. Traditional models suggest that metamorphism happens over time scales of more than 10 million years and is driven by slow cooling of overthickened crust. However, more recent studies find that metamorphism is likely driven by increased, small-scale heat production and may happen in geologically short (5-10 million year) pulses during mountain-building events. Garnet is a mineral that occurs in metamorphic rocks and is useful for determining time scales of metamorphism because it progressively crystallizes (nucleates and grows) as metamorphic processes progress. This project will couple textural analysis and size sorted garnet geochronology to determine the duration of garnet crystallization in multiple samples across the Western Blue Ridge Terrane, North Carolina. During her tenure as an NSF postdoctoral fellow, Dr. Nagurney will mentor undergraduate researchers, teach an Appalachian geology seminar course, and develop educational resources about the methods explored in this project for NAGT's Teach the Earth portal. The proposed research will utilize textural and compositional analyses of garnet crystals to apply a methodology of size sorted garnet geochronology to constrain the nature and duration of metamorphic processes during orogenesis. Metamorphic rocks from the Western Blue Ridge terrane, North Carolina (part of the Appalachian Taconic Orogeny) will be studied via X-Ray Computed Tomography (CT Scanning) to determine the size and location of garnet crystals, the distribution of garnet crystal sizes, and analyzed to interpret the mechanisms of garnet crystallization in each sample. Representative garnets from each crystal size class will be characterized for major and trace element compositions via electron microprobe and LA-ICP-MS, and dated using the Sm-Nd isotopic system via MC-ICP-MS. The objectives of this study are to understand: i) if garnet crystallization is pulsed or protracted in a sample, ii) if the mechanisms controlling garnet crystallization vary across a terrane, and iii) if metamorphism is pulsed or protracted across a terrane during orogenesis.This project is jointly funded by the EAR Postdoctoral Fellowship program, the Established Program to Stimulate Competitive Research (EPSCoR), and the EAR Petrology and Geochemistry program.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.