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.

俯冲带是地球上致密的海洋地壳下降（或俯冲）到更具浮力的大陆或海洋地壳下方的地方。 活跃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有关活跃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，确定这些古代俯冲带过去事件的发生时间和发生率极其困难。 这项拟议的研究将开发一种新技术来确定硬铝石的年龄，硬铝石是一种在俯冲带相关变质作用过程中形成的重要矿物。 因此，对硬钠石进行测年，使地质学家能够确定古代俯冲过程的时间和速率。本研究将开发和完善通过 Lu/Hf 法对硬钠石进行测年的技术。 在岩石变质历史过程中，硬钠石在不同的压力和温度条件下形成。 根据变质条件，硬锂石可能在其他高钇矿物（例如石榴石和绿帘石）之前或之后生长，这可能会影响通过铍/铪法测定硬石榴石的年代。 该提案将在加利福尼亚州方济各会综合体和土耳其西夫里希萨尔地块测试该技术。 方济会杂岩包含含硬铝石的岩石，这些岩石具有相似的变质年龄，但保留了不同的变质历史。这些岩石还含有石榴石和绿帘石，使方济各会杂岩成为检查这些矿物的硬长石生长的理想场所。 Sivrihisar 地块包含含石榴石和硬铝石榴石的岩石，这些岩石是在大范围的压力和温度下形成的，因此非常适合比较石榴石和硬铝石榴石中 Lu 和 Hf 的相对闭合温度。 测年将与定量热力学模型相结合，将年龄与给定样品的变质历史直接联系起来。 这项研究的结果可以应用于其他含有硬钠长石的地区，并将进一步开发这种用于测定俯冲带变质作用的强大工具。