GEOCHEMICAL CHARACTERIZATION OF CLINKER PROFILES IN POWDER RIVER BASIN, MONTANA

Understanding the evolution of Earth’s magnetic field has important implications for a range of Earth processes e.g. the evolution of the deep interior, deflection of solar wind, and evolution of life. In order to enhance our understanding of this behavior, we need a reliable characterization of recent (0-5 Myr) magnetic data. Clinkers in the Powder River Basin have young ages (<5 Myr) and requisite magnetic properties to improve our recent magnetic record in this way. These rocks are defined as a suite of pyro-metamorphic rocks that have been baked, welded, and/or melted by the natural burning of underlying coal beds. To ensure precision of these magnetic records, we must constrain the thermal history of the Powder River Basin clinkers to confirm that they are reliable full-vector magnetic recorders. This cannot be done with magnetic analyses or thermal models alone. One of the primary magnetic recorders in these samples is the high-temperature version of hematite, ε-Fe₂O₃. In laboratory experiments, this mineral phase forms at temperatures in excess of 900 ºC, but its formation temperature in natural samples is poorly constrained. In this project, we attempt to address two questions: 1) the thermal history of clinker deposits during and after spontaneous combustion of an underlying coal seam, and 2) the maximum temperature relative to distance from the ignited seam and correlated mineral formation. To assess geochemical changes, we used X-Ray fluorescence and inductively coupled plasma mass spectrometry. We also used X-Ray diffraction to determine mineral phases and to identify high-T minerals present in our samples and how they occur as a function of distance from the coal seam fire within a clinker profile. Here, we present evidence of high-T mineral phases which form at temperatures well surpassing the Curie temperatures of the most common magnetic recorders in clinker deposits that occur throughout a clinker profile. This work demonstrates that clinkers achieve very high temperatures for a sustained period after burning and that they should record thermal remanent magnetizations, making them excellent candidates for constraining the full-vector Quaternary magnetic history