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

当岩石形成时，磁性矿物记录的地球磁场(古地磁)的历史为探测过去地球核心的条件提供了一种方法。地球磁场还保护大气层不受太阳(太阳风)喷出的高能粒子的侵蚀，因此可能在大气的演化中发挥了重要作用。我们将通过取样一组保存在津巴布韦的壮丽的火成岩(岩脉和岩床)来检验最近关于地核和大气发展的两个假说。第一种假设表明，地球固体内部的生长始于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最初增长的证据。地球大气层在大约23亿年前以某种方式发生了变化，从温和的还原状态转变为氧化状态。第二种假设认为，这种变化是由于太阳风将氢从大气中移除。我们将通过古地磁分析测量过去地球磁场的强度(以及它的大气屏蔽能力)来验证这一假设。我们的工作可能会导致我们将地球深处的过程与大气演化联系起来的方式发生革命性的变化。这项研究将与教育工作相结合，涉及将接受实地和实验室培训的研究生和本科生。我们还将开展外联活动，向当地罗切斯特社区和更广泛的公众传达我们的结果。最近的两个假设将地磁场的性质与核心和大气演化的基本方面联系起来。在第一个假设中，内核的增长被认为发生在20亿年前，正如一个较低的四极杆家族对太古代地磁长期变化的贡献所记录的那样。这一假设反过来又有利于显生界的小热流。第二种假设依赖于新的古强度数据和太古宙的太阳风估计。来自快速旋转的年轻太阳的强烈太阳风被认为是从地球大气中剥离氢，有助于从温和的还原条件转变为氧化条件，并有可能导致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想法。为了测试关于古分子变化的性质及其与内核生长的潜在关系的先前推断，我们将在我们的合作者进行重大的U-Pb区域测年工作后，从这些单元收集古地磁方向数据。我们关注了大氧化事件的三个时间窗口：1.89-1.88，2.51-2.41和25.8亿年前。为了检验大气中氢损失的假说，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，将使我们能够计算出评估大气影响所需的磁层顶距离。我们的研究将解决科学界对地球深部过程和大气演化感兴趣的基本问题。我们概述的古地磁方法是我们测量内核增长的为数不多的探测器之一，它的开始是地球热学模型的基本要素。此外，确定外力(即太阳风)是否在大气演化中发挥作用，可能会导致我们看待地球长期历史的方式发生革命性变化。我们工作的一个关键部分是将研究和教育努力结合起来，包括研究生和本科教育。这项工作将为至少一篇博士论文做出贡献，并将涉及几名本科生，他们将接受现场和实验室方面的培训。我们还将开展少量的K-12活动，将我们的研究生和本科生教学工作与罗切斯特社区结合起来，以及通过媒体和博物馆传播我们的研究结果的外展努力。

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.