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

“这项奖励是根据2009年美国复苏和再投资法案（公法111-5）资助的。”知识价值：在过去的十年中，(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中面临的几个一级挑战。需要对这些挑战背后的现象进行定量理解，以自信地将He测年与其他方法相结合，开发现实的扩散模型，并在一般情况下可靠地解释热历史。该建议侧重于解决锆石和磷灰石He定年中的两个主要挑战。第一个挑战是建立一个包含各向异性和辐射损伤影响的锆石中He扩散的鲁棒动力学模型。锆石年龄在年龄与母核素浓度之间表现出两种不同类型的相关性，这取决于单个样品的特定热史。这导致了一种假设，即随着损伤的增加，最初高且各向异性的扩散系数逐渐降低且更各向同性，随后在损伤超过临界阈值后扩散系数迅速下降。对这种行为的评价可能会对样品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He扩散实验和年龄分析来完成，这些锆石具有不同程度的辐射损伤和热历史。第二个挑战是了解一些样品中异常和高度分散的磷灰石He年龄的来源。对碎屑和基岩样品的详细检查表明，在至少一部分磷灰石晶体周围的晶界中形成了次生相；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。这些样品很可能受到晶外含u - th二次相的不同影响，这取决于二次相生长的时间和是否有一定比例的相与磷灰石一起分析。第二相假说将通过一系列分析来验证，通过扫描电镜、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。改善或避免这个问题的方法将在有问题的样品上使用颗粒磨损和与二次相相关的元素的测量进行测试。这项研究的最后一个组成部分是为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提供资金支持，使他们能够学习低温热年代学的分析和解释方面，并应用于他们自己的研究项目中的样本。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 Thomson博士建立FT分析设施提供部分资金。学生工作坊将为构造和地貌学领域的广大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.