EAR-PF: Deciphering the Timescales of Metamorphism via Size Sorted Garnet Geochronology

EAR-PF：通过尺寸排序石榴石地质年代学破译变质作用的时间尺度

• 批准号: 2052848
• 负责人: Alexandra Nagurney
• 金额: $ 17.4万
• 依托单位: Nagurney, Alexandra Bobiak
• 依托单位国家: 美国
• 项目类别: Fellowship Award
• 财政年份: 2021
• 资助国家: 美国
• 起止时间: 2021-07-01 至 2023-02-28
• 项目状态: 已结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=2052848&HistoricalAwards=false
• 关键词: EAR; PF; Deciphering; Timescales; Metamorphism

Dr. Allie Nagurney has been granted an NSF EAR Postdoctoral Fellowship to investigate the timescales and nature of metamorphism via size sorted garnet geochronology. This work will take place at the University of South Carolina under the mentorship of Dr. Besim Dragovic. Mountain-building events thicken and heat the upper most layer of the Earth (the lithosphere), causing physical and chemical changes in crustal rocks, called metamorphism. Establishing the duration of metamorphic processes during mountain building events is crucial to characterizing the thermal evolution of the lithosphere. Traditional models suggest that metamorphism happens over time scales of more than 10 million years and is driven by slow cooling of overthickened crust. However, more recent studies find that metamorphism is likely driven by increased, small-scale heat production and may happen in geologically short (5-10 million year) pulses during mountain-building events. Garnet is a mineral that occurs in metamorphic rocks and is useful for determining time scales of metamorphism because it progressively crystallizes (nucleates and grows) as metamorphic processes progress. This project will couple textural analysis and size sorted garnet geochronology to determine the duration of garnet crystallization in multiple samples across the Western Blue Ridge Terrane, North Carolina. During her tenure as an NSF postdoctoral fellow, Dr. Nagurney will mentor undergraduate researchers, teach an Appalachian geology seminar course, and develop educational resources about the methods explored in this project for NAGT's Teach the Earth portal. The proposed research will utilize textural and compositional analyses of garnet crystals to apply a methodology of size sorted garnet geochronology to constrain the nature and duration of metamorphic processes during orogenesis. Metamorphic rocks from the Western Blue Ridge terrane, North Carolina (part of the Appalachian Taconic Orogeny) will be studied via X-Ray Computed Tomography (CT Scanning) to determine the size and location of garnet crystals, the distribution of garnet crystal sizes, and analyzed to interpret the mechanisms of garnet crystallization in each sample. Representative garnets from each crystal size class will be characterized for major and trace element compositions via electron microprobe and LA-ICP-MS, and dated using the Sm-Nd isotopic system via MC-ICP-MS. The objectives of this study are to understand: i) if garnet crystallization is pulsed or protracted in a sample, ii) if the mechanisms controlling garnet crystallization vary across a terrane, and iii) if metamorphism is pulsed or protracted across a terrane during orogenesis.This project is jointly funded by the EAR Postdoctoral Fellowship program, the Established Program to Stimulate Competitive Research (EPSCoR), and the EAR Petrology and Geochemistry program.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.

Allie Nagurney博士获得了NSF EAR博士后奖学金，通过石榴石年代学研究变质作用的时间尺度和性质。这项工作将在贝西姆·德拉戈维奇博士的指导下在南卡罗来纳大学进行。造山事件使地球最上层(岩石圈)增厚并加热，导致地壳岩石的物理和化学变化，称为变质作用。确定造山事件期间变质过程的持续时间对于表征岩石圈的热演化至关重要。传统的模型表明，变质作用发生在1000万年以上的时间尺度上，是由过厚的地壳缓慢冷却所驱动的。然而，最近的研究发现，变质作用可能是由增加的小规模热生产驱动的，并可能在造山事件期间以地质上的短脉冲(500万-1000万年)发生。石榴石是一种出现在变质岩中的矿物，对确定变质作用的时间尺度很有用，因为随着变质过程的进行，它会逐渐结晶(成核并生长)。该项目将结合结构分析和大小分类石榴石年代学，以确定北卡罗来纳州西部蓝岭地体多个样品中石榴石结晶的持续时间。在担任NSF博士后期间，Nagurney博士将指导本科生研究人员，教授阿巴拉契亚地质学研讨会课程，并为NAGT的Teach the Earth门户网站开发关于该项目中探索的方法的教育资源。这项拟议的研究将利用石榴石晶体的结构和成分分析，应用大小排序的石榴石年代学方法，以限制造山作用期间变质过程的性质和持续时间。对北卡罗来纳州西部蓝岭地体(阿巴拉契亚塔肯造山运动的一部分)的变质岩进行了X射线计算机断层扫描(CT)研究，以确定石榴石晶体的大小和位置、石榴石晶体的尺寸分布，并对每个样品进行分析，以解释石榴石结晶的机制。每种晶体大小的代表性石榴石将通过电子探针和LA-ICPMS表征主量元素和微量元素组成，并通过MC-ICPMS使用Sm-ND同位素系统测定年龄。这项研究的目的是了解：i)石榴石结晶是否在样品中脉动或延长，ii)控制石榴石结晶的机制是否在地体中变化，以及iii)变质作用是否在造山过程中脉动或延长跨越地体。该项目由EAR博士后奖学金计划、既定的激励竞争研究计划(EPSCoR)以及EAR岩石学和地球化学计划共同资助。该奖项反映了NSF的法定使命，并通过使用基金会的智力优势和更广泛的影响审查标准进行评估，被认为值得支持。