Neoarchean to Early Proterozoic evolution of Earth's core: Paleomagnetic tests using dikes and sills of the Zimbabwe craton

地核的新太古代到早元古代演化：利用津巴布韦克拉通的岩墙和岩台​​进行的古地磁测试

• 批准号: 1045651
• 负责人: John Tarduno
• 金额: $ 26万
• 依托单位: University of Rochester
• 依托单位国家: 美国
• 项目类别: Continuing Grant
• 财政年份: 2011
• 资助国家: 美国
• 起止时间: 2011-02-15 至 2015-01-31
• 项目状态: 已结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=1045651&HistoricalAwards=false
• 关键词: Neoarchean; Early; Proterozoic; evolution; Earth

The history of Earth's magnetic field (paleomagnetism) recorded by magnetic minerals when rocks form provides a way to probe conditions in Earth's core in the past. Earth's magnetic field also shields the atmosphere from erosion by energetic particles streaming from the Sun (the solar wind), and thus may have played an important role in the evolution of the atmosphere. We will test two recent hypotheses concerning the development of Earth's core and atmosphere by sampling a magnificent set of igneous rocks (dikes and sills) preserved in Zimbabwe. The first hypothesis suggests that the onset of growth of Earth's solid inner commenced more than 2 billion years ago. By sampling the dikes and sills and investigating their paleomagnetic signature, we will test whether they record evidence for initial growth of Earth's inner core. Earth's atmosphere was somehow transformed about 2.3 billion years ago, from mildly reducing to oxidizing conditions. The second hypothesis suggests that this change was aided by removal of hydrogen from the atmosphere by the solar wind. We will test this hypothesis by gauging the past intensity of Earth's magnetic field (and hence its atmospheric shielding capacity) through paleomagnetic analyses. Our work could lead to a transformative change in how we relate deep Earth processes and evolution of the atmosphere. The research will be integrated with educational efforts, involving graduate and undergraduate students who will receive training in the field and laboratory. We will also undertake outreach activities to communicate our results to the local Rochester community and to the wider public.Two recent hypotheses relate the nature of the geomagnetic field to fundamental aspects of core and atmosphere evolution. In the first hypothesis, inner core growth is postulated to occur prior to 2 billion years ago, as recorded by a lower quadrupole family contribution to Archean geomagnetic secular variation. This hypothesis in turn favors a small Phanerozoic core-mantle boundary heat flow. The second hypothesis relies on new paleointensity data and solar wind estimates for the Archean. Intense solar wind from the rapidly rotating young Sun is envisioned as stripping H from Earth's atmosphere, contributing to the transformation from mildly reducing to oxidizing conditions, potentially contributing to the ∼2.3 billion-year-old Great Oxidation Event. We will examine these ideas through paleomagnetic and paleointensity studies of a magnificent record of mafic dikes and sills exposed on the Zimbabwe craton. To test prior inferences on the nature of the paleosecular variation and its potential relationship to inner core growth, we will collect paleomagnetic directional data from these units, following a major U-Pb regional dating effort by our collaborators. We focus of three time windows spanning the Great Oxidation Event: 1.89-1.88, 2.51-2.41 and 2.58 billion-years ago. To examine the hypothesis of H-loss from the atmosphere, we will conduct paleointensity analyses using single silicate minerals; these values combined with estimates of solar winds will allow us to calculate magnetopause standoff distances that are needed to evaluate atmospheric effects. Our study will address fundamental issues of broad interest to the scientific community interested in deep Earth processes and evolution of the atmosphere. The paleomagnetic approach we outline is one of the few probes we have to gauge inner core growth, the onset of which is an essential element of thermal models for Earth. Moreover, determining whether external forcing (i.e. solar wind) had a role in atmosphere evolution could lead to transformative changes in how we view long-term Earth history. A key part of our work is the integration of research and educational efforts, including graduate and undergraduate education. The work will contribute to at least one Ph.D. thesis and will involve several undergraduates, who will receive training in the field and laboratory. We will also undertake a small number of K-12 activities, integrating our graduate and undergraduate teaching efforts with the Rochester community, as well as outreach efforts to disseminate the results of our study through the media and museums.

当岩石形成时，磁性矿物记录的地球磁场（古地磁）的历史提供了一种探测过去地核条件的方法。地球的磁场也保护大气层免受来自太阳的高能粒子流（太阳风）的侵蚀，因此可能在大气层的演化中发挥了重要作用。我们将通过对保存在津巴布韦的一组壮观的火成岩（岩脉和岩床）进行取样，来检验最近关于地核和大气发展的两个假设。第一种假设认为，地球固体内部的增长开始于20多亿年前。通过对岩脉和岩床进行采样并研究它们的古地磁特征，我们将测试它们是否记录了地球内核初始生长的证据。地球的大气层在大约23亿年前发生了某种变化，从轻度还原到氧化。第二种假设认为，这种变化是由于太阳风从大气中移除氢而促成的。我们将通过古地磁分析来测量地球磁场过去的强度（以及它的大气屏蔽能力）来检验这一假设。我们的工作可能会导致我们如何将地球深部过程与大气层演变联系起来的变革。这项研究将与教育工作相结合，涉及研究生和本科生，他们将在实地和实验室接受培训。我们还将开展外展活动，将我们的研究结果传达给当地罗切斯特社区和更广泛的公众。最近的两个假说将地磁场的性质与地核和大气演化的基本方面联系起来。在第一个假设中，内核的增长被假定发生在20亿年前，记录了较低的四极家庭的贡献太古代地磁长期变化。这一假设反过来又有利于一个小的中生代核幔边界热流。第二个假设依赖于新的古强度数据和太阳风估计的太古代。来自快速旋转的年轻太阳的强烈太阳风被设想为从地球大气中剥离H，有助于从轻度还原到氧化条件的转变，可能导致23亿年前的大氧化事件。我们将通过对津巴布韦克拉通上暴露的镁铁质岩脉和岩床的宏伟记录进行古地磁和古强度研究来检验这些观点。为了测试先前的推断的性质的paleosecular变化及其潜在的关系，内核的增长，我们将收集古地磁方向数据，从这些单位，以下主要的U-Pb区域测年的努力，我们的合作者。我们聚焦于大氧化事件的三个时间窗口：1.89-1.88亿年前，2.51-2.41亿年前和25.8亿年前。为了检验大气中H损失的假设，我们将使用单一硅酸盐矿物进行古强度分析;这些值与太阳风的估计相结合，将使我们能够计算评估大气影响所需的磁层顶距离。我们的研究将解决对地球深部过程和大气层演变感兴趣的科学界广泛感兴趣的基本问题。我们概述的古地磁方法是我们测量内核生长的少数探测器之一，内核生长的开始是地球热模型的基本要素。此外，确定外部强迫（即太阳风）是否在大气演化中发挥作用可能会导致我们如何看待地球长期历史的变革。我们工作的一个关键部分是研究和教育工作的整合，包括研究生和本科教育。该工作将为至少一名博士学位做出贡献论文，并将涉及几个本科生，谁将在现场和实验室接受培训。我们还将开展少量的K-12活动，将我们的研究生和本科教学工作与罗切斯特社区相结合，以及通过媒体和博物馆传播我们研究成果的外联工作。