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The Origin of Ancient Magnetic Activity on Small Planetary Bodies: a Nanopaleomagnetic Study

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The Origin of Ancient Magnetic Activity on Small Planetary Bodies: a Nanopaleomagnetic Study The Origin of Ancient Magnetic Activity on Small Planetary Bodies: A Nanopaleomagnetic Study James Francis Joseph Bryson Department of Earth Sciences University of Cambridge This dissertation is submitted for the degree of Doctor of Philosophy Selwyn College October 2014 To my family and teachers Declaration I hereby declare that except where specific reference is made to the work of others, the contents of this dissertation are original and have not been submitted in whole or in part for consideration for any other degree or qualification in this, or any other University. This dissertation is the result of my own work and includes nothing which is the outcome of work done in collaboration, except where specifically indicated in the text. This dissertation contains fewer than 225 pages of text, appendices, illustrations, captions and bibliography. James Francis Joseph Bryson October 2014 Acknowledgements First and foremost, I would like to acknowledge my supervisors, Richard Harrison and Simon Redfern. Without Richard’s hard work, dedication, supervision and direction this project would not have been possible, and I feel privileged to have worked with him. Simon should be thanked for his guidance, hours of entertainment and awful jokes. I would like to acknowledge all of my collaborators, in particular Nathan Church, Claire Nichols, Roberts Blukis, Julia Herrero-Albillos, Florian Kronast, Takeshi Kasama and Francis Nimmo. Each has played an invaluable role in acquiring and understanding the data in this thesis and I would not have reached this point without their expertise and help. Martin Walker must be thanked for his assistance and calming influence. I would like to also thank Ioan Lascu for proof-reading this thesis and general advice. I would like to thank the Natural Environment Research Council, the European Research Council, the Helmholtz-Zentrum Berlin and Selwyn College for their generous funding. I would also like to thank Dan Pemberton and the staffat the Sedgwick Museum of Earth Sciences and Sara Russell, Deborah Cassey and the staffat the Natural History Museum for assistance and loaning me samples. Finally, I would like to thank all of my friends. They have motivated me when I needed to work, kept me calm when I needed to relax and kept me grounded when I needed to be realistic. It is thanks to them that my time at Cambridge has been such an enjoyable and unforgettable experience. Last and in no way least, special thanks to Sarah Humbert; partly because I know she will read this, but also for her friendship and support. ‘Let’s think the unthinkable, let’s do the undoable. Let us prepare to grapple with the ineffable itself, and see if we may not effit after all.’ -Douglas Adams, Dirk Gently’s Holistic Detective Agency Publications Arising from this Thesis Peer-reviewed Papers Bryson J. F. J., Kasama, T., Dunin-Borkowski, R. E., and Harrison, R. J. (2013). Fer- rimagnetic/ferroelastic domain interactions in magnetite below the Verwey transition: Part II. Micromagnetic and image simulations. Phase Transitions, 86,88–102. Bryson, J. F. J., Church, N. S., Kasama, T., and Harrison, R. J. (2014). Nanomagnetic inter-growths in Fe–Ni meteoritic metal: The potential for time-resolved records of plan- etesimal dynamo fields. Earth and Planetary Science Letters, 388, 237–248. Bryson, J. F. J., Herrero-Albillos, J., Kronast, F., Ghidini, M., Redfern, S. A. T., van der Laan, G., and Harrison, R. J. (2014). Nanopaleomagnetism of meteoritic Fe–Ni studied using X-ray photoemission electron microscopy. Earth and Planetary Science Letters, 396,125–133. Bryson, J. F. J., Nichols C. I. O., Herrero-Albillos, J., Kronast, F., Kasama T., Ali- madadi H., van der Laan, G., Nimmo F. and Harrison, R. J. (2015). Long-lived mag- netism from solidification-driven convection on the pallasite parent body. Nature, 517, 472-475. Computer Scripts Bryson, J. F. J. and Harrison, R. J. (2011). ATHLETICS, https://wserv4.esc.cam.ac.uk/nanopaleomag/?page_id=273 Abstract The magnetic remanence stored within meteorites represents a window into the struc- tural, dynamic and thermochemical conditions that prevailed within small planetary bodies (<1000 km radius) during the early solar system. Paleomagnetic measurements indicate that many meteorites recorded a vestige of ancient planetary magnetic fields, indicating that numerous small bodies contained, at one time, molten metallic cores that convected to generate dynamo activity. However, the temporal evolution of these fields has remained elusive, thus the thermochemical conditions that governed this activity are uncertain. Also, the nm- and µm-scale magnetic and structural properties of the reman- ence carriers are largely unstudied, thus their capabilities as paleomagnetic recorders are unknown. In this thesis, I apply state-of-the-art magnetic transmission electron microscopy techniques (electron holography), image simulations and atomic-scale Monte Carlo sim- ulations to investigate the fundamental properties of the ‘cloudy zone’ (CZ), a nm-scale intergrowth unique to meteoritic metal. The CZ is found to capture an extremely stable chemical transformation remanent magnetisation, which, due to its formation history, corresponds to the temporal evolution of the field experienced by this intergrowth. To further investigate this remanence, I apply cutting-edge synchrotron X-ray mag- netic microscopy techniques (X-ray photoemission electron microscopy, XPEEM) and image simulations to the CZ in the Tazewell IIICD iron meteorite. The observed mag- netisation reveals that the domain pattern of the older CZ is a direct reflection of the planetary field; this is not the case for the younger CZ, where the domain pattern is adversely influenced by the underlying nanostructure. These properties were exploited to infer time-resolved records of planetary magnetic activity from XPEEM images of the Imilac and Esquel pallasite meteorites, and Bishop Canyon and Steinbach IVA iron meteorites. The field inferred from the pallasites was unidirectional, with an intens- ity that decayed in a step-wise fashion over millions of years from ~100 µTto0µT. Through planetary cooling and dynamo models, this behaviour is interpreted as due to compositional convection resulting from bottom-up inner core solidification on the pallasite parent body. The decay events are interpreted as a dipolar-multipolar field transition, and the shut down of dynamo activity associated with the completion of core solidification. The field inferred from IVA meteorites was intense (>100 µT) and reversed orientation once in the brief recording period studied ( 30 kyr). The IVA par- Ø ent body was likely an exposed planetary core (proposed to be the extant asteroid (16) Psyche), and thus will have solidified from the top-down. Dynamo generation models demonstrate that this process involved the proposed ‘Fe-snow’ solidification mechanism, vi and that the field was generated by compositional convection resulting either from ‘Fe- snow’ itself, or from inner core solidification in the compositionally stratified core liquid produced by ‘Fe-snow’. Prior to this study, the magnetic remanence within meteorites had only been sugges- ted to relate to planetary magnetic activity generated by thermal convection, which was inefficient and would have been short-lived. On the other hand, the compositionally- driven dynamo activity identified in this thesis was efficient, and would have been per- mitted by the ancient thermochemical structure predicted for nearly all small bodies. Hence, across their period of core solidification, the vast majority of small bodies (re- gardless of the direction of core solidification) would have contributed to an ancient and widespread epoch of long-lasting, stable and intense magnetic activity. Contents Contents vii List of Figures xii List of Tables xvi 1 Introduction 1 1.1 Planetary Formation . 1 1.2 Planetary Dynamos . 6 1.3 Extraterrestrial Paleomagnetism . 8 1.4 Open Questions . 10 1.4.1 Planetary evolution . 10 1.4.2 Meteoritic magnetism . 12 1.5 Aims and Thesis Outline . 14 2 Magnetic Microscopy 18 2.1 Introduction . 18 2.2 Transmission Electron Microscopy Techniques: Off-axis Electron Holo- graphyandLorentzMicroscopy . 20 2.2.1 Experimentation and method . 20 2.2.1.1 Sample preparation . 20 2.2.1.2 Off-axis electron holography . 20 2.2.1.3 Lorentz microscopy . 22 2.2.2 Theory of image simulation . 23 2.3 X-ray methods: X-ray Magnetic Circular Dichroism, X-ray Photoemission Electron Microscopy and Scanning Transmission X-ray Microscopy . 26 2.3.1 Experimentation and method . 26 2.3.1.1 X-ray magnetic circular dichroism . 27 Contents viii 2.3.1.2 X-ray photoemission electron microscopy . 27 2.3.1.3 Scanning transmission X-ray microscopy . 29 2.3.2 Theory of image simulation . 30 2.4 Scanning Methods: Stray Field and Magnetic Force Microscopy . 31 2.4.1 Experimentation and method . 31 2.4.1.1 Magnetic force microscopy . 31 2.4.1.2 Stray field . 32 2.4.2 Theory of image simulation . 32 2.5 ATHLETICS ................................. 34 2.6 Case Study: Magnetic TEM of Magnetite Below the Verwey Transition . 36 2.6.1 Scientific background . 36 2.6.2 Results and discussion . 38 2.6.2.1 Lamellar twins . 38 2.6.2.2 In-plane and out-of-plane domain walls . 41 2.6.2.3 Twin-induced closure domains . 45 2.7 Summary and Conclusions . 48 3 Structural and Magnetic Fundamentals of the Cloudy Zone 50 3.1 Introduction and Scientific Background . 50 3.1.1 Cloudy zone formation . 52 3.1.2 Open questions regarding the cloudy zone . 55 3.2 Magnetic Properties of the Cloudy Zone . 57 3.2.1 Methods . 57 3.2.1.1 Experimental procedures and methods . 57 3.2.1.2 Image simulations . 59 3.2.1.3 Micromagnetic simulations . 60 3.2.2 Results . 61 3.2.2.1 Electron diffraction, EDS maps and STEM images . 61 3.2.2.2 Electron holography . 62 3.2.2.3 Image simulations .
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