Addressing Current Challenges in (U-Th)/He Thermochronology and Running Summer Student Workshops

应对当前 (U-Th)/He 热年代学挑战并举办暑期学生研讨会

• 批准号: 0910577
• 负责人: Peter Reiners
• 金额: $ 28.01万
• 依托单位: University of Arizona
• 依托单位国家: 美国
• 项目类别: Standard Grant
• 财政年份: 2009
• 资助国家: 美国
• 起止时间: 2009-09-01 至 2011-08-31
• 项目状态: 已结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=0910577&HistoricalAwards=false
• 关键词: Addressing; Current; Challenges; Th; He

"This award is funded under the American Recovery and Reinvestment Act of 2009 (Public Law 111-5)."Intellectual Merit: Expanded use of (U-Th)/He and 4He/3He thermochronology in tectonic and geomorphic studies over the last decade have elucidated several first-order challenges facing their practical application and interpretation. Quantitative understanding of the phenomena behind these challenges is required to confidently integrate He dating with other approaches, develop realistic diffusion models, and robustly interpret thermal histories in general. This proposal focuses on addressing two primary challenges in zircon and apatite He dating. The first challenge is development of a robust kinetic model for He diffusion in zircon that incorporates anisotropy and the effects of radiation damage. Zircon He ages show two distinct types of correlations between age and parent nuclide concentrations, depending on the specific thermal histories of individual samples. This leads to a hypothesis whereby initially high and anisotropic diffusivity becomes progressively lower and more isotropic with increasing damage, followed by a rapid decrease in diffusivity after damage exceeds a critical threshold. Evaluation of this behavior may provide constraints on distinct parts of sample thermal histories. Kinetic calibration will be done through a series of crystallographically controlled He diffusion experiments and age analyses on zircons with varying degrees of radiation damage and thermal histories. The second challenge is to understand the origin of anomalous and highly dispersed apatite He ages in some samples. Detailed examination of detrital and bedrock samples documents formation of secondary phases in grain boundaries around at least some fraction of apatite crystals; in many cases these phases have U-Th concentrations comparable to or higher than the apatite. Such samples are likely to be affected by extracrystalline U-Th-bearing secondary phases in distinct ways, depending on the timing of secondary phase growth and whether some proportion of the phases are analyzed with the apatite. The secondary-phase hypothesis will be tested through a series of analyses to characterize in situ U-Th distributions in and around apatite in detrital and Laramide basement bedrock samples through SEM, ion-imaging, and induced-track imaging of thin sections. Methods to ameliorate or avoid the problem will be tested using grain abrasion and measurement of elements associated with the secondary phases on problematic samples. A final component of this study involves funding for continued support of two-week summer student workshops to be held each year for undergraduate and graduate students, allowing them to learn analytical and interpretational aspects of low-temperature thermochronology as applied to samples from their own research projects.Broader Impacts: This work will fund the research of two promising students who are members of underrepresented groups: PhD student Kendra Murray, and undergraduate Guleed Ali, and will provide partial funding for establishing FT analytical facilities for Research Scientist Dr. Stuart Thomson. The student workshops will provide direct training and research experience to a broad spectrum of students in the tectonics and geomorphology communities.

“该奖项由 2009 年美国复苏和再投资法案（公法 111-5）资助。” 智力优势：过去十年在构造和地貌研究中广泛使用 (U-Th)/He 和 4He/3He 热年代学阐明了其实际应用和解释面临的几个首要挑战。需要对这些挑战背后的现象进行定量理解，才能自信地将氦测年与其他方法相结合，开发现实的扩散模型，并稳健地解释一般的热历史。该提案的重点是解决锆石和磷灰石 He 定年中的两个主要挑战。第一个挑战是开发一个稳健的氦在锆石中扩散的动力学模型，其中考虑了各向异性和辐射损伤的影响。锆石 He 年龄显示出年龄与母体核素浓度之间两种不同类型的相关性，具体取决于单个样品的特定热历史。这导致了一个假设，即随着损伤的增加，最初较高的各向异性扩散率逐渐降低且各向同性增强，随后在损伤超过临界阈值后扩散率迅速下降。对这种行为的评估可以为样本热历史的不同部分提供约束。动力学校准将通过一系列晶体学控制的氦扩散实验和对具有不同程度的辐射损伤和热历史的锆石的年龄分析来完成。第二个挑战是了解一些样品中异常且高度分散的磷灰石 He 年龄的起源。对碎屑和基岩样品的详细检查记录了至少部分磷灰石晶体周围晶界中二次相的形成；在许多情况下，这些相的 U-Th 浓度与磷灰石相当或更高。此类样品可能会以不同的方式受到晶外含 U-Th 的二次相的影响，具体取决于二次相生长的时间以及是否用磷灰石分析某些相的比例。第二相假说将通过一系列分析进行检验，通过 SEM、离子成像和薄片诱导径迹成像来表征碎屑和 Laramide 基底基岩样品中磷灰石内部和周围的原位 U-Th 分布。将使用晶粒磨损和测量与有问题的样品上的第二相相关的元素来测试改善或避免该问题的方法。这项研究的最后一个组成部分是资助每年为本科生和研究生举办为期两周的夏季学生研讨会，使他们能够学习低温热年代学的分析和解释方面的知识，并将其应用于自己研究的样本更广泛的影响：这项工作将资助两名有前途的学生的研究，他们是代表性不足的群体的成员：博士生 Kendra Murray 和本科生 Guleed Ali，并将为研究科学家 Stuart 博士建立 FT 分析设施提供部分资金汤姆森.学生研讨会将为构造学和地貌学界的广大学生提供直接的培训和研究经验。