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

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.

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