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

合作研究：俯冲带变质作用测年：测试压力-温度路径对硬钠石 Lu-Hf 地质年代学的影响

• 批准号: 1119237
• 负责人: Jeffrey Vervoort
• 金额: $ 4.85万
• 依托单位: Washington State University
• 依托单位国家: 美国
• 项目类别: Standard Grant
• 财政年份: 2011
• 资助国家: 美国
• 起止时间: 2011-08-01 至 2014-07-31
• 项目状态: 已结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=1119237&HistoricalAwards=false
• 关键词: Collaborative; Research; Dating; Subduction; Zone

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.

俯冲带是地球上的一些地方，在那里致密的海洋地壳下降(或俯冲)到更浮力的大陆或海洋地壳之下。活动俯冲带是地球上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动俯冲带内发生的物理和化学过程的重要信息。然而，事实证明，在这些古老的俯冲带中确定过去事件的时间和速度是极其困难的。这项拟议的研究将开发一种新的技术来确定劳钠铝石的年龄，这是一种重要的矿物，在与俯冲带有关的变质过程中形成。因此，测年为地质学家提供了确定古代俯冲过程的时间和速率的能力。这项研究将发展和完善利用Lu/Hf方法测年的技术。在岩石的变质历史过程中，在不同的压力和温度条件下形成了硬钠铝石。根据变质条件的不同，在石榴石和绿帘石等高Lu矿物之前或之后可能生长，这可能会影响用Lu/HF方法测定Lawsonite的年龄。这项提议将在加利福尼亚州的方济各会建筑群和土耳其的Sivrihisar Massif测试这项技术。方济各会杂岩含有含劳钠铝石的岩石，这些岩石具有相似的变质时代，但保存了不同的变质历史。这些岩石还含有石榴石和绿帘石，这使得方济各会杂岩成为研究与这些矿物相关的劳钠铝石生长的理想场所。Sivrihisar地块含有在很大的压力和温度范围内形成的含石榴石和硬钠铝石的岩石，因此非常适合于比较石榴石和硬钠铝石中Lu和Hf的相对闭合温度。测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。这一研究成果可推广到其他含劳钠铝石地区，并将进一步发展这一强有力的俯冲带变质作用测年工具。