Collaborative Research: Dating Subduction Zone Metamorphism: Testing the Effect of Pressure-temperature Path on Lawsonite Lu-Hf Geochronology

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Subduction zones are places on the Earth where dense ocean crust descends (or subducts) beneath more buoyant continental or oceanic crust. Active subduction zones are responsible for most of Earth's large earthquakes and explosive volcanic eruptions. Ancient subduction zones now exposed at the Earth's surface provide important information on the processes, both physical and chemical, occurring within active subduction zones. Determining the timing and rates of past events in these ancient subduction zones, however, has proven extremely difficult. This proposed research will develop a new technique to determine ages on lawsonite, an important mineral that forms during metamorphism associated with subduction zones. Dating lawsonite, therefore, provides geologists the ability to determine the timing and rates of ancient subduction processes.This research will develop and refine the technique of dating lawsonite by the Lu/Hf method. Lawsonite forms at distinct pressure and temperature conditions during the metamorphic history of a rock. Depending on the metamorphic conditions, lawsonite may grow before or after other high-Lu minerals such as garnet and epidote, which may effect the ability to date lawsonite by the Lu/Hf method. This proposal will test the technique in the Franciscan Complex of California and the Sivrihisar Massif of Turkey. The Franciscan Complex contains Lawsonite-bearing rocks that are of similar metamorphic age but that preserve different metamorphic histories. These rocks also contain both garnet and epidote, making the Franciscan Complex an ideal place to examine lawsonite growth with respect to these minerals. The Sivrihisar Massif contains garnet and lawsonite-bearing rocks that formed under a large range of pressures and temperatures and is therefore ideally suited to compare the relative closure temperatures of Lu and Hf in garnet and lawsonite. The dating will be combined with quantitative thermodynamic models to directly relate the ages to the metamorphic history of a given sample. The results of this study can be applied to other lawsonite-bearing localities and will further develop this powerful tool for dating subduction zone metamorphism.