A paleomagnetic and geochronological re-investigation of the ~1.1 Ga Coldwell complex: Implications for the reversal asymmetry in Keweenawan rocks

对 ~1.1 Ga Coldwell 杂岩的古地磁和地质年代学重新研究：对 Keweenawan 岩石反转不对称性的影响

• 批准号: 1045406
• 负责人: Aleksey Smirnov
• 金额: $ 16.62万
• 依托单位: Michigan Technological University
• 依托单位国家: 美国
• 项目类别: Standard Grant
• 财政年份: 2011
• 资助国家: 美国
• 起止时间: 2011-05-01 至 2014-04-30
• 项目状态: 已结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=1045406&HistoricalAwards=false
• 关键词: paleomagnetic; geochronological; re; investigation; 1.1

The conceptual model for Earth's magnetic field is that of a dipole (i.e. bar magnet) positioned at Earth's center and aligned with the rotational axis of the Earth. This allows us to predict the direction of the magnetic field at any location on Earth?s surface using the fundamental equations of a dipole field. The geomagnetic field periodically reverses (i.e. a magnetic compass which points north will now point south and vice versa) and these reversals are symmetrical (i.e. the normal and reversed field directions are exactly anti-parallel). The above is the fundamental assumption used to reconstruct continents to their past positions using the ancient magnetic field recorded in rocks (fossil magnetism). Our research seeks to shed light on one of the long- standing puzzles in Earth science - the presence of asymmetrical reversals in ~1.1 billion year old rocks in North America and elsewhere which may indicate that our basic understanding of the behavior of Earth's magnetic field is incomplete. The rocks of the Coldwell complex in Canada, recording more than one asymmetrical reversal, provide a unique opportunity to study this unusual field behavior. If our investigation confirms a significant field asymmetry at ~1.1 billion years, this would represent a breakthrough in our understanding the mechanism that generates Earth's magnetic field and would have important implications in how we use the magnetic field records to decipher the geological history of our planet. The project will involve undergraduate students at Michigan Tech both in the field and in the laboratory analyses of the samples thus training the next generation of scientists. This research will also become a part of a Ph.D. thesis. In order to increase the general public awareness of Earth science, the results will be disseminated through a series of science exploration sessions.Data on the long-term behavior and configuration of the geomagnetic field during the Precambrian are crucial in understanding the nature of Earth's early geodynamo. One of the intriguing problems related to the Precambrian field is the apparent reversal asymmetry which is manifested in rocks of Keweenawan age around the Lake Superior basin and elsewhere. Current thinking suggests two possible causes for the asymmetry: a significant non-dipole field at ~1.1 Ga, or a fast or unaccounted plate motion during that time period. A recent re-study of the Mamainse Point volcanics has suggested that the apparent asymmetry is exclusively caused by rapid plate motion. However, the robustness of that study may be in question due to inadequate statistical treatment and the lack of geochronological control. The alkaline Coldwell complex is the only other Keweenawan unit that records multiple reversals, which makes it a key location to investigate the asymmetry problem. Prior analyses suggest that the complex was emplaced within a 1 to 2 million year period at ~1108 Ma and cooled rapidly. Our re-interpretation of the available data shows that, if taken at face value, these data would imply an unrealistically fast plate motion, and hence may support the non-dipole field as the cause of the asymmetry. However, the existing age determinations for the intrusive centers of the complex are not sufficiently precise to exclude plate motion as the reason of the observed asymmetry. In our project we will address these questions by: (1) Reinvestigation of the paleomagnetism of the Coldwell complex using modern demagnetization and statistical analysis techniques to obtain a high-quality paleodirectional dataset; (2) A detailed geochronological study using the chemical abrasion thermal ionization mass spectrometry method to obtain the precise U-Pb ages for all three intrusive centers of the Coldwell complex.

地球磁场的概念模型是位于地球中心并与地球旋转​​轴对齐的偶极子（即条形磁铁）。这使我们能够使用偶极场的基本方程来预测地球表面任何位置的磁场方向。地磁场周期性地反转（即指向北方的磁罗盘现在将指向南方，反之亦然），并且这些反转是对称的（即正常和反转的磁场方向完全反平行）。上述是利用岩石中记录的古代磁场（化石磁力）将大陆重建到过去位置的基本假设。我们的研究旨在阐明地球科学中长期存在的难题之一——北美和其他地方约 11 亿年前的岩石中存在不对称反转，这可能表明我们对地球磁场行为的基本理解是不完整的。加拿大科德威尔综合体的岩石记录了不止一次不对称逆转，为研究这种不寻常的场行为提供了独特的机会。如果我们的调查证实大约 11 亿年存在显着的磁场不对称性，这将代表我们在理解地球磁场产生机制方面的突破，并对我们如何利用磁场记录破译地球的地质历史产生重要影响。该项目将让密歇根理工大学的本科生参与现场和实验室样品分析，从而培训下一代科学家。这项研究也将成为博士学位的一部分。论文。为了提高公众对地球科学的认识，研究结果将通过一系列科学探索会议进行传播。有关前寒武纪地磁场的长期行为和配置的数据对于了解地球早期地球发电机的性质至关重要。与前寒武纪地场相关的有趣问题之一是明显的反转不对称性，这在苏必利尔湖盆地周围和其他地方的基威纳湾时代的岩石中表现出来。目前的想法提出了造成不对称的两个可能原因：~1.1 Ga 处的显着非偶极场，或者该时间段内快速或无法解释的板块运动。最近对马曼斯点火山的重新研究表明，明显的不对称完全是由快速板块运动引起的。然而，由于统计处理不充分和缺乏地质年代学控制，该研究的稳健性可能受到质疑。碱性科德韦尔复合体是唯一一个记录了多次反转的基韦纳旺单元，这使其成为研究不对称问题的关键地点。先前的分析表明，该复合体在约 1108 Ma 的温度下在 1 到 2 百万年的时间内就位并迅速冷却。我们对现有数据的重新解释表明，如果从表面上看，这些数据将意味着不切实际的快速板块运动，因此可能支持非偶极场作为不对称的原因。然而，现有的复合体侵入中心的年龄测定不够精确，无法排除板块运动作为观察到的不对称性的原因。在我们的项目中，我们将通过以下方式解决这些问题：（1）利用现代退磁和统计分析技术重新研究科尔德韦尔杂岩体的古地磁，以获得高质量的古方向数据集； (2) 使用化学磨损热电离质谱法进行详细的地质年代学研究，以获得 Coldwell 杂岩体所有三个侵入中心的精确 U-Pb 年龄。