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不同的地球化学和同位素指纹。热年代学使用矿物中的这些同位素记录来推断热历史，(U-Th)/He系统就是其中之一。这项技术的不确定度相对较低，在许多研究中，该方法已经获得了可重复性的、地质上合理的年龄。然而，有时数据是“过度分散的”，这意味着日期上的差异大于量化的不确定性，这可能会使数据集难以解释。这可能有几个原因，但一个未被研究的原因是晶体缺陷。皮罗斯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，将这种颗粒尺度的过程作为过度分散的潜在贡献者进行研究。在地质界，仔细研究热年代学样品以研究更广泛的地质问题已变得越来越重要，包括更好地估计侵蚀、约束板块构造、探索其他行星以及为我们的未来开发能源和矿产资源。皮罗斯将与科罗拉多大学博尔德分校的两个著名的研究培训项目合作，指导一名本科生，该项目旨在招收和留住代表性不足的学生。皮罗斯还将编排一种舞蹈，帮助非地质学家了解单个晶体内原子尺度上的复杂变形是如何适应的，从而加强艺术和科学之间的联系。尽管热年代学研究了许多影响氦扩散的因素，如颗粒大小、辐射损伤和各向异性，但扩散动力学的一些方面仍然没有被很好地理解，表现在许多(U-Th)/He数据集的过度分散。变形引起的缺陷可能对氦在矿物中的扩散动力学有很大影响，因此对(U-Th)/He年龄分散的贡献被低估了。该项目通过扫描和透射电子显微镜对晶内缺陷进行了详细的微观表征，通过He扩散实验(连续斜坡加热和4He/3He)，以及通过常规(U-Th)/He分析和激光烧蚀He图谱和光斑分析进行了测年。通过完成对同一颗粒产生不同类型洞察的各种分析，可以在单晶变形、扩散动力学和年龄之间进行直接比较和量化。尽管这些计时仪经常应用于来自构造复杂地区的变形岩石，但在亚颗粒尺度上，变形在这些过程中所起的作用还没有得到充分的研究。这个项目的更广泛的影响是双重的：Pi Ross将指导未被充分代表的本科生，并创造一场“错位之舞”。这个EAR-PF项目得到了EAR构造计划的全力支持。这一奖项反映了NSF的法定使命，并通过使用基金会的智力优势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.