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

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。的表面使用偶极子场的基本方程。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）。以上是利用岩石中记录的古代磁场（化石磁性）重建大陆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11亿年前岩石中存在不对称逆转，这可能表明我们对地球磁场行为的基本理解是不完整的。加拿大科尔德韦尔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。如果我们的调查证实了大约11亿年的显著场不对称性，这将代表着我们在理解产生地球磁场的机制方面的突破，并将对我们如何使用磁场记录来破译地球的地质历史产生重要影响。该项目将涉及密歇根理工大学的本科生在现场和实验室分析的样本，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，将通过一系列科学探索会议传播研究结果。有关前寒武纪地磁场长期行为和结构的数据对于了解地球早期地球发电机的性质至关重要。与前寒武纪领域相关的一个有趣的问题是明显的反转不对称性，这在苏必利尔湖上级盆地和其他地方的基韦纳万年龄的岩石中表现出来。目前的想法提出了两个可能的原因，为不对称：一个显着的非偶极子场在~1.1 Ga，或快速或不明板块运动在这段时间内。最近对马曼斯角火山岩的重新研究表明，这种明显的不对称性完全是由快速的板块运动造成的。然而，由于统计处理不当和缺乏地质年代学控制，该研究的稳健性可能受到质疑。碱性的科尔德韦尔复合体是唯一一个记录了多次反转的基韦纳万单元，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，该杂岩体在1108 Ma左右的100万至200万年内侵位，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，现有的年龄测定的侵入中心的复杂的是不够精确，以排除板块运动的原因所观察到的不对称性。本项目将通过以下几个方面来解决这些问题：（1）利用现代退磁技术和统计分析技术重新研究Coldwell杂岩的古地磁学，获得高质量的古方向数据集;（2）利用化学磨蚀热电离质谱法进行详细的年代学研究，获得Coldwell杂岩三个侵入中心的精确U-Pb年龄。

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.