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

“该奖项是根据2009年美国复苏和再投资法案(公法111-5)资助的。”学术价值：在过去十年中，(U-Th)/He和4He/3He热年代学在构造和地貌研究中的广泛应用阐明了它们在实际应用和解释方面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的理解，才能自信地将He测年与其他方法结合起来，开发现实的扩散模型，并有力地解释总体上的热历史。这项建议的重点是解决锆石和磷灰石He测年方面的两个主要挑战。第一个挑战是开发一个包含各向异性和辐射损伤影响的锆石中氦扩散的稳健动力学模型。年龄和母体核素浓度之间显示出两种不同类型的锆石年龄相关性，这取决于个别样品的特定热历史。这导致了一种假设，即最初高的各向异性扩散率随着损伤的增加而逐渐降低并变得更加各向同性，随后在损伤超过临界阈值后扩散率迅速下降。对这一行为的评估可以对样品热历史的不同部分提供约束。动力学校准将通过对具有不同程度的辐射损伤和热历史的锆石进行一系列结晶控制的氦扩散实验和年龄分析来完成。第二个挑战是了解一些样品中异常和高度分散的磷灰石年龄的来源。对碎屑和基岩样品的详细检查证明，在至少部分磷灰石晶体周围的晶界上形成了第二相；在许多情况下，这些相的U-Th浓度与磷灰石相当或高于磷灰石。这类样品可能会受到晶外含U-Th的第二相的不同影响，这取决于第二相生长的时间，以及是否用磷灰石分析了一定比例的相。第二相假说将通过一系列分析来验证，通过扫描电子显微镜、离子成像和薄片的诱导径迹成像来表征碎屑基岩和拉拉酰胺基岩样品中磷灰石及其周围的U-Th分布。改善或避免问题的方法将通过颗粒磨损和对有问题的样品上与第二相相关的元素的测量来测试。这项研究的最后一个组成部分是继续资助每年为本科生和研究生举办的为期两周的暑期学生研讨会，让他们学习应用于自己研究项目样本的低温热年代学的分析和解释方面。广泛影响：这项工作将资助两名有前途的学生的研究，他们是代表不足的群体的成员：博士生肯德拉·默里和本科生古利德·阿里，并将为研究科学家斯图尔特·汤姆森博士建立英国《金融时报》的分析设施提供部分资金。学生工作坊将为构造学和地貌学领域的广泛学生提供直接的培训和研究经验。

"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.