Thermochronology at the nanoscale: delineating the challenges and opportunities in Pb isotope analysis of zircon by atom probe tomography

该博士后奖学金授予泰勒布鲁姆博士，他将在威斯康星州麦迪逊大学和德国杜塞尔多夫的Max-Plank-Institut fur Eisenforschung GmbH（MPIE）工作。拟议的工作将分析来自美国Beartooth山脉的几个3.3-4.0亿年的锆石，以研究形成小型微量元素丰富的集群的地质控制（热，时间和晶体学），以及如何使用它们来推断有关古老的热历史的独特信息，脱离上下文的锆石。最近的技术进步使原子探针层析成像（APT）应用于地质材料，它提供了一种独特的手段来研究矿物结构和化学（同位素敏感性）在三维和近原子尺度。这项工作扩大了最近的原子探针分析锆石，矿物最广泛依赖于地质事件的约会，并期待提高我们的理解如何集群的微量元素在纳米尺度上与矿物结构和温度的变化在地质时期。重要的是，它将有助于描绘APT在地质年代学中的应用和局限性，在地球历史中可以利用它的时间段，以及对模拟星团形成年龄很重要的地质变量。这项工作也将有助于更好地了解放射性衰变过程中产生的纳米级缺陷结构及其在地质时期的特征;这些结晶域的稳定性和行为与核废料形式的更有效工程高度相关。与威斯康星州-麦迪逊地质博物馆大学合作，这项工作还将包括关于地质时间尺度、我们对早期地球的理解和矿物锆石的宣传和教育。纳米级微量元素富集簇的形成被认为代表了辐射损伤积累、结构退火和微量元素迁移的原子尺度过程之间的复杂相互作用。因此，这些星团的化学成分，以及它们的相关系和形成年龄，有可能记录地球历史早期以前未被认识的高温事件。从星团中提取信息的能力取决于对其形成和稳定性的透彻理解，以及年龄测定中的仪器和地质限制。该项目结合了二次离子质谱（西姆斯）、电子背散射衍射（EBSD）、高光谱拉曼光谱、透射电子显微镜（TEM）和APT，从颗粒尺度到原子尺度表征锆石的结构和化学。通过将这套表征工具应用于来自美国Beartooth山脉的太古代锆石颗粒和实验室退火锆石，该提案旨在解决：（1）构造和热作用对富微量元素团簇形成的控制;（2）测定~（207）Pb/~（206）Pb比值的地质和仪器限制（以及集群的建模形成年龄），以及（3）该奖项反映了NSF的法定使命，并通过使用基金会的智力价值进行评估，被认为值得支持和更广泛的影响审查标准。

This postdoctoral fellowship is awarded to Dr. Tyler Blum to work at the University of Wisconsin Madison and Max-Plank-Institut fur Eisenforschung GmbH (MPIE) in Dusseldorf, Germany. The proposed work will analyze several 3.3-4.0 billion-year-old zircons from the Beartooth Mountains, USA, in order to study the geological controls (thermal, temporal, and crystallographic) on the formation of small, trace element-rich clusters, and how they can be used to deduce unique information about the thermal histories for old, out of context zircons. Recent technological advances have enabled the application of atom probe tomography (APT) to geological materials, where it offers a unique means to study mineral structure and chemistry (with isotope sensitivity) in three dimensions and at the near atomic scale. This work expands upon recent atom probe analysis of zircon, the mineral most widely relied upon for dating geologic events and looks to improve our understanding of how clustering of trace elements at the nanometer scale relates to changes in mineral structure and temperature over geologic time. Importantly, it will help to delineate the applications and limitations of APT in geochronology, the time periods in Earth's history where it can be utilized, and the geological variables important to modeling cluster formation ages. This work will also contribute to a greater understanding of the nanometer-scale defect structures produced during radioactive decay and their characteristics over geologic time; the stability and behavior of these crystallographic domains is highly relevant to more effective engineering of nuclear waste forms. In collaboration with the University of Wisconsin -Madison Geology Museum, this work will also include outreach and education on the geological time scale, our understanding of the early earth, and the mineral zircon. The formation of nanoscale trace element-rich clusters is thought to represent a complex interaction between the atomic-scale processes of radiation damage accumulation, structural annealing, and trace element migration. As such, there is potential for the chemistry of these clusters, as well as their phase relations and formation ages, to record previously unrecognized high-temperature episodes early in Earth's history. The ability to extract information from clusters hinges on a thorough understanding of their formation and stability, as well as the instrumental and geological limitations in age determination. This project combines secondary ion mass spectrometry (SIMS), electron backscatter diffraction (EBSD), hyperspectral Raman spectroscopy, transmission electron microscopy (TEM) and APT to characterize zircon structure and chemistry from the grain scale to the atomic scale. By applying this set of characterization tools to both Archean zircon grains from the Beartooth Mountains, USA, and laboratory annealed zircon, this proposal seeks to resolve: (1) the structural and thermal controls on formation of trace element-rich clusters, (2) the geological and instrumental limitations for determination of 207Pb/206Pb ratios (and modeled formation ages for clusters), and (3) where APT analysis of zircon can be applied within the geological record to extract novel information about zircon thermal histories.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.