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Tyler Blum博士，他在威斯康星大学麦迪逊分校和德国杜塞尔多夫的Max-Plank-Institut Fur Eisenforschung GmbH(MPIE)工作。这项拟议的工作将分析来自美国熊牙山脉的几个33-40亿年前的锆石，以研究地质控制(热、时间和晶体学)对小的、微量元素丰富的星系团的形成的控制，以及如何利用它们来推断古老的、断章取义的锆石的热历史的独特信息。最近的技术进步使原子探针层析成像(APT)能够应用于地质材料，它提供了一种在三维和近原子尺度上研究矿物结构和化学(具有同位素敏感性)的独特手段。这项工作扩展了最近对锆石的原子探针分析，锆石是最广泛依赖的矿物，用于测定地质事件的年龄，并希望提高我们对纳米尺度上的微量元素聚集如何与矿物结构和温度随地质时间的变化的关系的理解。重要的是，它将有助于描述APT在地质年代学中的应用和局限性，地球历史上可以利用它的时间段，以及对模拟星系团形成年龄至关重要的地质变量。这项工作还将有助于更好地理解放射性衰变过程中产生的纳米级缺陷结构及其随地质时间的特征；这些结晶域的稳定性和行为与更有效地设计核废料形式高度相关。与威斯康星大学麦迪逊地质博物馆合作，这项工作还将包括关于地质时间尺度的宣传和教育，我们对早期地球的理解，以及矿物锆石。纳米级痕量元素富集团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等原子尺度过程之间的复杂相互作用。因此，这些星系团的化学成分，以及它们的相关系和形成年龄，有可能记录下地球历史早期以前未被发现的高温事件。从星系团中提取信息的能力取决于对其形成和稳定性以及在确定年龄方面的仪器和地质限制的透彻了解。该项目结合二次离子质谱仪(SIMS)、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表征了从颗粒尺度到原子尺度的锆石结构和化学组成。通过将这套表征工具应用于来自美国熊牙山的太古宙锆石颗粒和实验室退火的锆石，这项建议试图解决：(1)构造和热控对微量元素富集团形成的控制，(2)确定207Pb/206Pb比值的地质和仪器限制(以及团簇的模拟形成年龄)，以及(3)可以在地质记录中应用对锆石的APT分析，以提取有关锆石热历史的新信息。这项裁决反映了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。

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.