Mineral Magnetism: Some Surprises And Future Challenges

Abstract:

The magnetism of minerals has played a major role in revolutionizing the Earth Sciences through establishment of the plate tectonic paradigm. In particular, recognition that Earth’s magnetic field can undergo polarity reversals unlocked understanding of the global history of seafloor spreading and paleomagnetic analysis enabled determination of apparent polar wander paths that define global plate motions. Paleomagnetic analysis also provides an understanding of Earth’s magnetic field and the deep-Earth dynamo processes that generate the field. This is all possible because nanoparticulate magnetic rock-forming minerals (e.g., magnetite, hematite, their titanium-substituted counterparts, maghemite, pyrrhotite, greigite, goethite) occur widely in nature within the ideal single domain (SD) size range. SD materials have homogeneous magnetizations that can remain stable for periods exceeding the age of the Earth. This is why paleomagnetism works. Despite the fact that mineral magnetism has been exploited paleomagnetically for more than 100 years, and despite its application to an increasing array of research questions, and a burgeoning literature, I consider mineral magnetism to be a middle-aged rather than a mature subject. Much fundamental information remains unknown and a concerted plan is needed to turn many unknowns into knowns. Such work will provide an even firmer and broader foundation for applying our science. The Bullard Lecture is a wonderful forum for celebrating past achievements. I prefer to look forward and will propose a roadmap for future work. In this lecture, I will explore some curiosities that I hope will surprise even hardened veterans of mineral magnetism, I will seek to convince you of what we need to do to bring mineral magnetism to maturity, and I will explore future challenges that I hope will motivate the next generation of researchers to achieve this objective.