EAGER: Collaborative Research: Developing new laser ablation (U-Th)/(He-Pb) hematite double dating techniques to date ancient oxidation

EAGER：合作研究：开发新的激光烧蚀 (U-Th)/(He-Pb) 赤铁矿双重测年技术来测定古代氧化的年代

• 批准号: 2203532
• 负责人: Rebecca Flowers
• 金额: $ 8.02万
• 依托单位: University of Colorado at Boulder
• 依托单位国家: 美国
• 项目类别: Standard Grant
• 财政年份: 2021
• 资助国家: 美国
• 起止时间: 2021-12-15 至 2023-11-30
• 项目状态: 已结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=2203532&HistoricalAwards=false
• 关键词: EAGER; Collaborative; Research; Developing; new

This project is focused on improving methods that Earth scientists can use to determine the formation age of the mineral hematite. Hematite is a wide-spread mineral in rocks and soils and often forms due to oxidation on Earth’s surface as well as on other planets — notably Mars. Hematite can incorporate uranium when it forms. Given that uranium undergoes radioactive decay it is possible to conduct both uranium-lead dating (U-Pb) and uranium-thorium-helium dating ((U-Th)/He) on hematite crystals. Investigators will apply methods that can make these measurements simultaneously at a small scale on hematite crystals by blasting them with a laser and measuring the isotopes through mass spectrometry. An additional property of hematite is that it records the magnetic field at the time it crystallizes. This ability to record the ancient magnetic field enables Earth scientists to reconstruct the past position of Earth’s continents and can also be used to gain insight into the timing of hematite formation. They will make magnetic measurements of hematite including measurements that use new capabilities to make magnetic maps at the microscopic scale using an instrument called a quantum diamond microscope. The study will focus on hematite within ancient sedimentary rocks known as iron formations. In the United States, iron formations in the Lake Superior region are the major source of domestic iron production. By applying these dating methods to iron formation, they will constrain the timing of hematite formation in these units, where the timing of oxidation (that is, hematite formation) is debated. If this project is successful, these combined methods of measuring the age of hematite will enable a multitude of future studies. For example, it would be possible to pursue more advanced studies on hematite formation within iron formation, to study the processes and timescales of deep soil leaching (also known as laterization), and determine the timing of ancient surface exposure and chemical alteration in far greater detail using these combined methods. The project will support the research of an early-career scientist, advance development of new analytical capabilities at both University of Carlifornia Berkeley and University of Colorado Boulder, and support both undergraduate and graduate student research for first-generation students.These investigators seek to develop simultaneous in situ laser ablation (U-Th)/He and U-Pb dating of hematite, which is termed LA-(U-Th)/(He-Pb). Although bulk (U-Th)/He and in situ (U-Pb) methods have been used previously on hematite, the coupled laser-ablation technique has never been applied. This method can provide a powerful tool for assessing the timing of oxidation and weathering in a wide range of hematite-bearing environments. Researchers will investigate the timing of hematite crystallization in Lake Superior region iron formation through this method development in conjunction with paleomagnetic data, which can provide complementary chronologic insight. They will focus on a carefully selected set of samples that will enable method development and give new insights into the origin of iron formations. All samples will be characterized prior to geochronologic measurements via electron backscatter diffraction and electron microprobe to understand the chemical heterogeneity of the samples and the distribution of crystallites within hematite aggregates. This characterization will permit targeting of individual crystallites through LA-(U-Th)/(He-Pb) and provide context relative to potential polycrystalline diffusion behavior. This study has two main sample targets: (1) large, high-purity hematite samples from iron formation that will be used for method development; (2) typical iron formation from the Menominee Group. Menominee Group samples will be analyzed through both LA-(U-Th)/(He-Pb) and paleomagnetism. Paleomagnetic analyses will be conducted at both the centimeter and micrometer scale to constrain hematite formation relative to folding and through comparison to Laurentia’s apparent polar wander path. The well-constrained regional history of deposition, tectonism, and near-surface weathering provides testable hypotheses for the timing of hematite formation in these samples. Successful radiometric dating of these materials will provide confidence in the utility of the LA-(U-Th)/(He-Pb) method in natural samples beyond museum-quality hematite specimens.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.

该项目的重点是改进地球科学家用来确定赤铁矿形成年龄的方法。赤铁矿是一种广泛存在于岩石和土壤中的矿物，通常是由于地球表面以及其他行星上的氧化而形成的。赤铁矿在形成时可以掺入铀，鉴于铀会发生放射性衰变，因此可以进行铀-铅测年（U-Pb）和铀-铅测年。赤铁矿晶体上的铀-钍-氦测年（(U-Th)/He）研究人员将采用可以在小范围内同时对赤铁矿晶体进行这些测量的方法，方法是用激光对赤铁矿晶体进行爆破并通过质谱法测量同位素。赤铁矿的另一个特性是它可以记录结晶时的磁场，这种记录古代磁场的能力使地球科学家能够重建地球大陆过去的位置。他们将用于深入了解赤铁矿形成的时间，包括使用量子钻石显微镜的仪器在微观尺度上绘制磁性图的测量。该研究将重点关注古代赤铁矿。在美国，苏必利尔湖地区的铁矿层是国内铁矿生产的主要来源，通过将这些测年方法应用于铁矿层，它们将限制这些单位中赤铁矿形成的时间。的时间安排如果该项目成功，这些测量赤铁矿年龄的组合方法将有助于未来的大量研究，例如，可以对赤铁矿的形成进行更深入的研究。铁的形成，研究深层土壤淋滤（也称为红土化）的过程和时间尺度，并使用这些组合方法更详细地确定古代地表暴露和化学蚀变的时间。职业科学家，推进加州大学伯克利分校和科罗拉多大学博尔德分校新分析能力的开发，并支持第一代学生的本科生和研究生研究。这些研究人员寻求开发同时原位激光烧蚀（U-Th）/赤铁矿的 He 和 U-Pb 定年法被称为 LA-(U-Th)/(He-Pb)，尽管之前已对赤铁矿使用了本体 (U-Th)/He 和原位 (U-Pb) 方法。 ，耦合的激光烧蚀技术从未被应用过，这种方法可以为评估各种含赤铁矿的环境中的氧化和风化时间提供有力的工具，研究人员将通过这种方法研究苏必利尔湖地区铁形成的赤铁矿结晶的时间。方法开发与古地磁数据相结合，可以提供补充的年代学见解。他们将重点关注一组精心挑选的样品，这些样品将有助于方法开发并为铁形成的起源提供新的见解。通过电子背散射衍射和电子微探针进行地质年代学测量，以了解样品的化学异质性和赤铁矿聚集体中微晶的分布，这种表征将允许通过 LA-(U-Th)/(He-Pb) 定位单个微晶并提供。与潜在的多晶扩散行为相关的背景。本研究有两个主要样本目标：（1）来自铁矿层的大型高纯度赤铁矿样本，用于方法开发；（2）典型的铁矿样本。梅诺米尼群铁的形成将通过 LA-(U-Th)/(He-Pb) 和古地磁分析进行厘米和微米尺度的分析，以限制与折叠相关的赤铁矿形成。通过与劳伦西亚明显的极地漂移路径进行比较，严格限制的沉积、构造作用和近地表风化的区域历史为这些地区赤铁矿形成的时间提供了可检验的假设。这些材料的成功放射性测年将为 LA-(U-Th)/(He-Pb) 方法在博物馆级赤铁矿样本之外的自然样本中的实用性提供信心。该奖项反映了 NSF 的法定使命，并被视为值得通过使用基金会的智力优点和更广泛的影响审查标准进行评估来支持。