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

俯冲带是地球上致密的海洋地壳下降（或俯冲）到浮力更大的大陆或海洋地壳之下的地方。 活跃的俯冲带是地球大部分大地震和火山爆发的原因。 现在暴露在地球表面的古代俯冲带提供了关于活跃俯冲带内发生的物理和化学过程的重要信息。 然而，确定这些古老俯冲带过去事件的时间和速率已被证明是极其困难的。 这项拟议中的研究将开发一种新的技术，以确定年龄的硬硅钙石，一个重要的矿物，形成在变质作用与俯冲带。 因此，硬毛石定年为地质学家提供了确定古俯冲过程的时间和速率的能力。本研究将发展和完善Lu/Hf法测定硬毛石年龄的技术。 在岩石的变质历史中，碳铝石在不同的压力和温度条件下形成。 根据变质条件的不同，硬毛钠石可能在其他高Lu矿物如石榴石和绿帘石之前或之后生长，这可能影响Lu/Hf法测定硬毛钠石年龄的能力。 该提案将在加州的方济会建筑群和土耳其的Sivrihisar Massif测试该技术。 方济各杂岩包含具有相似变质年龄但保留不同变质历史的含劳森岩。这些岩石还含有石榴石和绿帘石，使方济各会综合体成为研究硬柱石生长与这些矿物的理想场所。 Sivrihisar地块含有石榴石和硬铝石的岩石，形成于大范围的压力和温度下，因此非常适合比较石榴石和硬铝石中Lu和Hf的相对闭合温度。 定年将与定量热力学模型相结合，直接将年龄与给定样品的变质历史联系起来。 本研究结果可应用于其他含硬铝石的地区，并将进一步发展这一强有力的俯冲带变质定年工具。

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.