Postdoctoral Fellowship: EAR-PF: The effects of grain-scale deformation on helium diffusion and thermochronometric ages of accessory minerals

地球的热结构被许多过程改变，这些过程在岩石中表现为独特的地球化学和同位素指纹。热年代学利用矿物中的这些同位素记录来推断热历史，（U-Th）/He系统就是其中的一种工具。这种技术的不确定性相对较低，而且在许多研究中，这种方法已经产生了可重复的、地质上合理的年龄。然而，有时数据是“过度分散的”，这意味着日期的差异大于量化的不确定性，这可能使数据集难以解释。这可能有几个原因，但一个未充分研究的原因是晶体缺陷。PI Ross的目标是通过对矿物中的这些缺陷如何影响氦扩散进行跨学科研究，来研究这种颗粒尺度过程作为过度分散的潜在贡献者。在地质学界，仔细描述热年代学样品的特征以研究更广泛的地质问题变得越来越重要，包括更好地估计侵蚀，限制板块构造，探索其他行星，以及为我们的未来开发能源和矿产资源。PI Ross将通过与两个完善的科罗拉多博尔德大学研究培训项目合作，指导一名本科生，这些项目旨在招募和留住代表性不足的学生。PI Ross还将编排一个舞蹈，帮助非地质学家了解如何在单个晶体内的原子尺度上容纳复杂的变形，加强艺术与科学之间的联系。虽然热年代学界已经研究了许多影响氦扩散的因素，如晶粒尺寸，辐射损伤和各向异性，仍然有扩散动力学方面没有得到很好的理解，表现为许多（U-Th）/He数据集的过度分散。由于变形的缺陷可能会大大影响氦在矿物中的扩散动力学，因此有助于（U-Th）/He年龄分散在一个低估的方式。该项目通过扫描和透射电子显微镜对晶粒内缺陷进行详细的微观表征，He扩散实验（连续斜坡加热和4 He/3 He），并通过常规（U-Th）/He分析和激光消融He映射和斑点分析进行定年。通过完成各种分析，对相同的晶粒产生不同类型的见解，可以在单晶粒变形，扩散动力学和年龄之间进行直接比较和量化。变形在这些过程中发挥的作用，在亚晶粒尺度的研究不足，尽管这些计时器在构造复杂地区的变形岩石中的频繁应用。这个项目的更广泛的影响是双重的：PI罗斯将指导代表性不足的本科生，并创建一个“错位之舞”。该EAR-PF项目得到了美国国家科学基金会（NSF）地质构造计划的全力支持。该奖项反映了NSF的法定使命，并通过使用基金会的知识价值和更广泛的影响审查标准进行评估，被认为值得支持。

The Earth’s thermal structure is altered by many processes which show up in rocks as distinct geochemical and isotopic fingerprints. Thermochronology uses these isotopic records in minerals to infer thermal histories and the (U–Th)/He system is one of these tools. This technique’s uncertainties are relatively low and the method has yielded reproducible, geologically reasonable ages in many studies. Sometimes, however, the data are “overdispersed”, meaning that the spread in dates is greater than the quantified uncertainty, which can make datasets challenging to interpret. There could be several reasons for this, but one understudied reason is crystal defects. PI Ross aims to investigate this grain-scale process as a potential contributor to overdispersion by carrying out a cross-disciplinary study of how these defects within minerals affect helium diffusion. It has become increasingly important in the geology community to carefully characterize thermochronology samples to study a wider set of geological problems including better estimating erosion, constraining plate tectonics, exploring other planets, and developing energy and mineral resources for our future. PI Ross will mentor an undergraduate student by working with two well-established University of Colorado Boulder research training programs aimed at recruiting and retaining underrepresented students. PI Ross will also choreograph a dance that helps non-geologists understand how complicated deformation is accommodated on the atomic scale within a single crystal, bolstering the connection between the arts and sciences. Although the thermochronology community has studied many of the factors that affect Hediffusion such as grain size, radiation damage, and anisotropy, there are still aspects of diffusion kinetics that are not well understood, as manifested by the overdispersion of many (U-Th)/He datasets. Defects due to deformation may substantially affect helium diffusion kinetics in minerals and therefore contribute to (U-Th)/He age dispersion in an underappreciated way. This project integrates detailed microscopic characterization via scanning and transmission electron microscopy of intragrain defects, He diffusion experiments (Continuous Ramped Heating and 4He/3He), and dating via conventional (U-Th)/He analyses and Laser-Ablation He mapping and spot analyses. By completing a variety of analyses yielding different types of insights on the same grains, a direct comparison and quantification can be made between single-grain deformation, diffusion kinetics, and age. The role that deformation plays in these processes at the sub-grain scale is understudied, despite frequent applications of these chronometers in deformed rocks from tectonically complex regions. The broader impacts of this project are two-fold: PI Ross will mentor underrepresented undergraduate students and create a “Dance of Dislocations”. This EAR-PF project has been fully supported by the EAR Tectonics 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.