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

地球的热结构受到许多过程的改变，这些过程在岩石中表现为独特的地球化学和同位素指纹。热年代学使用这些矿物中的同位素记录来推断热历史，(U-Th)/He系统是这些工具之一。该技术的不确定性相对较低，并且在许多研究中得出了可重复的、地质上合理的年龄。然而，有时数据“过度分散”，这意味着日期的分散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，但一个未被充分研究的原因是晶体缺陷。PI Ross的目标是通过开展一项跨学科研究，研究矿物中的这些缺陷如何影响氦扩散，来研究这种粒度过程作为过度分散的潜在因素。为了研究更广泛的地质问题，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，仔细表征热年代学样本已变得越来越重要。PI Ross将指导一名本科生，与科罗拉多大学博尔德分校的两个成熟的研究培训项目合作，旨在招募和留住代表性不足的学生。PI Ross还将编舞一段舞蹈，帮助非地质学家理解在单晶的原子尺度上复杂的变形是如何发生的，从而加强艺术与科学之间的联系。尽管热年代学界已经研究了影响氦扩散的许多因素，如晶粒尺寸、辐射损伤和各向异性，但扩散动力学的一些方面仍未得到很好的理解，如许多(U-Th)/He数据集的过度分散。由于变形引起的缺陷可能会严重影响矿物中的氦扩散动力学，从而导致(U-Th)/He年龄分散，但未被充分认识。该项目通过扫描和透射电子显微镜对颗粒内部缺陷进行详细的微观表征，进行He扩散实验（连续斜坡加热和4He/3He），并通过常规(U-Th)/He分析和激光烧蚀He绘图和斑点分析进行测年。通过完成各种分析，对同一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。变形在亚粒尺度上的作用还没有得到充分的研究，尽管这些计时器经常应用于构造复杂地区的变形岩石。这个项目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，并创造一个“错位之舞”。这个EAR- pf项目得到了EAR Tectonics项目的全力支持。该奖项反映了美国国家科学基金会的法定使命，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.