High-quality paleointensity data are essential for improving our understanding of the geomagnetic field; however, it is challenging to find materials that reliably record full vector magnetization going back in time. Here, we examine a new candidate material for paleointensity studies: clinkers, which are rocks that have been baked, metamorphosed, or melted by underlying coal seam fires. Previous studies conducted on clinkers suggest that they may be high-fidelity magnetic field recorders. However, due to the inhomogeneity of clinker deposits and limited scope of previous studies, it is unknown under what conditions these conclusions hold true. To better assess this, we quantified the variation of magnetic properties within clinker deposits collected from the Powder River Basin, Montana, as a function of lithology, oxidation state, distance from the coal seam, and location. Our results indicate that the clinker products contain three main magnetic minerals: magnetite, hematite, and the rare ε-Fe2O3. Clinker lithology was found to be the primary control on magnetic mineralogy, where strongly baked sediment and porcellanite are dominated by varying proportions of hematite, ε-Fe2O3, and magnetite, and paralavas are dominated by low-Ti magnetite. All clinker materials are thermally stable and likely experienced temperatures in excess of the magnetite Curie temperature. Grain size analysis indicates that the magnetic particles in all clinker materials are amenable to high-quality paleointensity study. In total, our study confirms that clinkers should be reliable full vector paleomagnetic recorders.
Bibliographical noteFunding Information:
The authors thank the Institute for Rock Magnetism (IRM), the UK National Environmental Research Council, and the University of Liverpool for support of this work. Sample analysis was funded by an IRM visiting fellowship awarded to the first author and the UMN Undergraduate Research Opportunity Program awarded to the third author. Field work was funded by the University of Liverpool's Early Career and Returners fund awarded to Courtney J. Sprain. Courtney J. Sprain was additionally funded by the Natural Environment Research Council (NERC standard grant, NE/P00170X/1) and Richard K. Bono was supported by the Leverhulme Trust (Research Leadership Award, RL‐2016‐080; Early Career Fellowship, ECF‐2020‐617). The authors further thank Yael Engbers and Daniele Thallner at the University of Liverpool for help with field work. The authors also thank Andrew Biggin for his support on this project. The authors would also like to thank the entire IRM staff for their overwhelming support and kindness and especially Bruce Moskowitz who pointed us in the direction of ε‐FeO. The authors additionally thank Jérôme Gattacceca and Lisa Tauxe for their constructive reviews. 2 3
© 2021. The Authors.
- clinker deposits
- rock magnetism