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。“智力优点：在过去的十年中，（U-Th）/He和4 He/3 He热年代学在构造和地貌研究中的广泛应用阐明了其实际应用和解释所面临的几个一阶挑战。这些挑战背后的现象的定量理解是需要自信地整合他定年与其他方法，开发现实的扩散模型，并在一般情况下稳健地解释热历史。该提案侧重于解决锆石和磷灰石He定年中的两个主要挑战。第一个挑战是开发一个强大的动力学模型，他在锆石中的扩散，结合各向异性和辐射损伤的影响。锆石He年龄显示两种不同类型的年龄和母体核素浓度之间的相关性，这取决于个别样品的特定热历史。这导致了一个假设，即最初的高和各向异性的扩散率变得逐渐降低，更各向同性的损伤增加，随后由一个快速下降后，损害超过一个临界阈值的扩散率。这种行为的评估可以提供对样品热历史的不同部分的约束。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的锆石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年龄的起源。碎屑和基岩样品的详细检查文件形成的第二相在晶界周围至少有一部分磷灰石晶体，在许多情况下，这些阶段的铀-钍浓度相当于或高于磷灰石。这样的样品可能会受到影响的晶外U-Th轴承第二相在不同的方式，这取决于第二相生长的时间，以及是否有一定比例的阶段与磷灰石进行了分析。第二阶段的假设将通过一系列的分析来测试，以表征原位U-Th分布在磷灰石和周围的碎屑和Laramide基底基岩样品，通过扫描电镜，离子成像，并诱导跟踪成像的薄片。改善或避免该问题的方法将使用颗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。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，使他们能够学习低温热年代学的分析和解释方面，并将其应用于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。学生研讨会将提供直接的培训和研究经验，以广大学生在构造和地貌社区。

"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.