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）。这使我们能够预测地球上任何位置的磁场方向。用偶极子场的基本方程求解S曲面。地磁场周期性地反转（即一个指向北方的磁罗盘现在指向南方，反之亦然），这些反转是对称的（即正常和反向的磁场方向完全反平行）。以上是利用岩石中记录的古代磁场（化石磁）来重建大陆过去位置的基本假设。我们的研究旨在揭示地球科学中一个长期存在的难题——在北美和其他地方约11亿年前的岩石中不对称倒转的存在，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Coldwell复合体的岩石记录了不止一次的不对称反转，为研究这种不寻常的磁场行为提供了一个独特的机会。如果我们的研究证实了大约11亿年的磁场不对称，这将代表着我们对地球磁场产生机制的理解的一个突破，并将对我们如何利用磁场记录来破译地球的地质历史具有重要意义。该项目将包括密歇根理工大学的本科生在现场和实验室对样本进行分析，从而培养下一代科学家。这项研究也将成为博士论文的一部分。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。关于前寒武纪地磁场的长期行为和结构的数据对于理解地球早期地球动力学的性质至关重要。与前寒武纪野外有关的一个有趣的问题是明显的反转不对称性，这种不对称性在苏必利尔湖盆地周围和其他地方的Keweenawan时代的岩石中表现出来。目前的想法提出了两种可能的不对称原因：在~1.1 Ga有一个显著的非偶极子场，或者在这段时间内快速或未解释的板块运动。最近对马曼斯火山的重新研究表明，这种明显的不对称完全是由板块的快速运动造成的。然而，该研究的稳健性可能由于统计处理不充分和缺乏地质年代学控制而受到质疑。碱性Coldwell复合体是Keweenawan唯一记录多次逆转的单元，这使得它成为研究不对称问题的关键位置。先前的分析表明，该复合体是在约1108 Ma的100万至200万年期间形成的，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，这些数据将意味着一个不切实际的快速板块运动，因此可能支持非偶极子场作为不对称的原因。然而，现有的对杂岩侵入中心的年龄测定还不够精确，不能排除板块运动是观测到的不对称的原因。在本项目中，我们将通过以下方法来解决这些问题：(1)利用现代消磁和统计分析技术重新研究Coldwell复合体的古地磁，获得高质量的古定向数据集；(2)利用化学磨损热电离质谱法进行了详细的年代学研究，获得了Coldwell杂岩三个侵入中心的精确U-Pb年龄。