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自转轴对齐的偶极(即条形磁铁)。这使得我们可以利用偶极场的基本方程来预测地球上任何位置的磁场方向？？S表面。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)，这些反转是对称的(即，法向和反向场方向正好是反平行的)。以上是使用岩石中记录的古代磁场(化石磁性)重建大陆到其过去位置所用的基本假设。我们的研究试图解开地球科学中一个长期存在的谜团--北美和其他地方11亿年前的岩石中存在不对称的反转，这可能表明我们对地球磁场行为的基本理解是不完整的。加拿大科德威尔杂岩的岩石记录了不止一次不对称的反转，为研究这种不同寻常的野外行为提供了独特的机会。如果我们的研究证实了约11亿年的显著磁场不对称，这将代表着我们在理解地球磁场产生机制方面的突破，并将对我们如何利用磁场记录来破译地球的地质历史产生重要影响。该项目将让密歇根理工学院的本科生参与对样本的实地和实验室分析，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，这些成果将通过一系列科学探索会议进行传播。前寒武纪期间地磁场的长期行为和构造数据对于了解地球早期地球发电机的性质至关重要。与前寒武纪油田有关的一个耐人寻味的问题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Keweenawan时代的岩石中表现出来。目前的想法提出了两种不对称的可能原因：在~1.1Ga的显著非偶极子磁场，或者在这段时间内快速或未被解释的板块运动。最近对马门塞点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，由于统计处理不充分和缺乏地质年代学控制，这项研究的稳健性可能受到质疑。碱性科尔德威尔杂岩是唯一另一个记录了多次反转的Keweenawan单位，这使得它成为研究不对称性问题的关键位置。先前的分析表明，该杂岩侵位于约1108 Ma的100-200万年内，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子场作为不对称的原因。然而，现有的杂岩侵入中心的年龄测定不够精确，不能排除板块运动是观察到的不对称的原因。在我们的项目中，我们将通过以下方式解决这些问题：(1)使用现代退磁和统计分析技术重新调查Coldwell杂岩的古地磁，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方法进行详细的地质年代学研究，以获得Coldwell杂岩所有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.