MagNetZ with Courtney Sprain (2020/08/12) - Magnetic recording of rocks baked by coal seam fires
Channel: Magnetics Information Consortium (MagIC)
Date: 2020-08-22

Landscapes on Fire: Do rocks baked by coal seam fires reliably record the geomagnetic field?

The video is also available for download at: https://earthref.org/ERDA/2443/
and can be referenced with the doi: 10.7288/V4/ERDA/2443

Author: Courtney Sprain (University of Florida)
Author Website: https://geology.ufl.edu/people/faculty/courtney-sprain/

Abstract:
High quality paleointensity data is essential for improving our understanding of the long-term evolution of the geodynamo as well as short-term paleosecular variation; however, it is challenging to find materials that reliably record Earth’s full-vector magnetic field going back in time. This study examines 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, MT, as a function of lithology, oxidation state, distance from the coal seam, and location. Our results indicate that the clinker products are dominated by three main magnetic minerals: magnetite, hematite, and the rare ε-Fe2O3. No major differences in magnetic properties were observed as a function of location, distance from the coal seam, or primary lithology. Clinker material, instead, is the primary control on magnetic mineralogy, where baked sediment and porcellanite are dominated by varying proportions of hematite, ε-Fe2O3, and magnetite, and paralavas (melted sediment) are dominated by low-Ti magnetite. All clinker materials are thermally stable and likely experienced temperatures in excess of the magnetite Curie T, potentially as far away as 8 m from the coal seam. 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 paleomagnetic recorders and that so long as collection is within the clinker deposit, material amenable to full-vector paleomagnetic study should be obtained.  These conclusion are corroborated by preliminary demagnetization and paleointensity results, which will also be presented.