Collaborative Research: Developing a Practical and Quantitative Method for Measurement of Metamorphic Porphyroblast Crystallization Kinetics and Strain Rate

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"This award is funded under the American Recovery and Reinvestment Act of 2009 (Public Law 111-5)." This project, which is a collaboration between researchers at Western Washington University, the University of Alabama at Tuscaloosa, and Boston University, is focused on improving our understanding of what happens when rocks are heated, pressurized, and deformed deep within the Earth's crust, a process (metamorphism) that is occurring every day under our feet. The researchers strive to understand the rates at which rocks are deformed and at which new crystals grow during metamorphism. These basic parameters of metamorphism are very poorly known. The researchers will develop and test a new method for determining these rates on rocks from Townshend Dam, VT, which were metamorphosed about 380 million years ago during a collision between North America and a micro-continent to the east. The study will estimate the rates of metamorphism and deformation during this collision (and by extension, other similar collision events), but more importantly, the project will develop a new method by which these and related aspects of metamorphism can be studied in other areas by other researchers in the future.The investigation will focus on garnet, a commonly formed metamorphic mineral that tends to record details of its environment as it grows. The research relies on recent advances in Sm-Nd radiometric dating of tiny volumes of garnet. The investigators will date well-characterized parts of garnet crystals in a rock to link the garnet chemistry to the time of garnet growth. The crux of the method is that it will allow numerous indirect dates to be easily obtained by reference to the arduously obtained time-chemistry link. The specific questions being studied are: What are metamorphic nucleation rates and how do they vary with time? What are growth rates for individual crystals and how do they vary over the course of a crystal's growth? How fast are rocks deformed during continental collisions, and is that deformation continuous or episodic? How fast are rocks heated and loaded during collisions? Finally, the study will provide the first data set of its kind, against which theoretical models of nucleation and growth can be tested.