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.

当岩石形式时，磁性矿物记录的地球磁场（古磁性）的历史为过去的探测地球核心的条件提供了一种方法。地球的磁场还通过从太阳（太阳风）中流出的能量颗粒（太阳风）屏蔽了气氛，因此在大气的演变中可能发挥了重要作用。我们将通过对津巴布韦保存的一组宏伟的火成岩（堤防和窗台）进行采样，以测试有关地球核心和大气发展的两个最近的假设。第一个假设表明，地球固体内在的增长开始了20亿年前。通过对堤防和窗台进行采样并研究其古磁性签名，我们将测试它们是否记录了地球内核最初生长的证据。从轻度减少到氧化条件，地球的大气在某种程度上转变为23亿年。第二个假设表明，通过太阳风从大气中清除氢，这一变化得到了帮助。我们将通过衡量地球磁场的过去强度（以及其大气屏蔽能力）通过古磁分析来检验这一假设。我们的工作可能会导致我们如何将深层过程和大气进化的方式发生变化。这项研究将与教育工作融合，涉及将在现场和实验室接受培训的毕业生和本科生。我们还将开展外展活动，将我们的结果传达给当地的罗切斯特社区，并向更广泛的公众传达。两个最近的假设将地磁领域的性质与核心和气氛进化的基本方面联系起来。在第一个假设中，假设内部核心生长发生在20亿年前，这是由较低的四极杆家族对大帝地磁世俗变异的贡献所记录的。该假设反过来有利于一个小的phanerogoic核心掩体边界热流。第二个假设依赖于天堂的新的古意义数据和太阳风估计。从地球大气中剥离了H的迅速旋转的太阳，从地球大气中剥离了H，这有助于从轻度减少到氧化条件的转变，有可能导致23亿; 23亿年前的大氧化事件。我们将通过古磁性和古意志研究来研究这些思想，该研究对津巴布韦克拉顿（Zimbabwe Craton）上的镁铁质堤防和窗台的宏伟记录进行了研究。为了测试对古生物变异性质及其与内核增长的潜在关系的先前推断，我们将根据我们的合作者进行了主要的U-PB区域约会工作，从这些单元中收集古磁定向数据。我们将三个时间的窗户重点放在了大氧化事件中：1.89-1.88、2.51-2.41和25.8亿年前。为了检查大气中H损失的假设，我们将使用单个硅酸盐矿物进行古显着分析。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效应所需的磁笼僵局。我们的研究将解决对深层过程和大气进化感兴趣的科学界广泛关注的基本问题。我们概述的古磁方法是我们必须衡量内部核心生长的少数探针之一，其开始是地球热模型的重要元素。此外，确定外部强迫（即太阳风）是否在大气进化中发挥作用会导致我们对长期地球历史的变革性变化。我们工作的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。这项工作将至少有助于一位博士学位。论文并将涉及几名本科生，他们将在现场和实验室接受培训。我们还将进行少量的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融为一体，并通过媒体和博物馆进行宣传工作，以传播我们的研究结果。