Impaktstrukturen: Einfluss von post-Schocktemperaturbedingungen auf die Magneto-Mineralogie

• Antragstellerin: Professorin Dr. Agnes Kontny
• Fachliche Zuordnung: Paläontologie ; Mineralogie, Petrologie und Geochemie ; Physik des Erdkörpers
• Förderung: Förderung von 2020 bis 2024
• Projektkennung: Deutsche Forschungsgemeinschaft (DFG) - Projektnummer 432762445

• Erstellungsjahr: 2024

Zusammenfassung der Projektergebnisse

Large scale meteorite impacts indicate catastrophic geological processes, releasing substantial energy that vaporize, melt and metamorphose the target rocks. These impacts often exhibit magnetic anomalies due to contrasts between the total magnetization in the shocked target, newly formed impactites, and the regional background magnetization. Magnetite, the dominant magnetic mineral in Earth's crust, is also found in shocked rocks within impact craters. However, during impact events, magnetite can lose up to 90% of its induced magnetization due to fractures and defects in the crystal lattice. These lattice imperfections enhance magnetic domain wall-pinning, leading to a decrease in the apparent domain state of magnetite, from its original multi- domain state toward pseudo-single domain or even single domain state-like behavior. In our project, we studied magnetite from the shocked granitoid basement of the Chicxulub and Nördlinger Ries impact structures to test if the findings from laboratory experiments also occur in natural systems. Compared to its amount of pure, stoichiometric magnetite, the shocked basement shows conspicuously low magnetic susceptibility, which is in line with the experimental studies. However, this finding is masked by the presence of hematite, an oxidation product of magnetite commonly found in crustal rocks. Laboratory heating experiments result in hematite-to-magnetite transformation above 560°C. These results are similar for annealing shock-induced lattice-defects in magnetite and the hematite-to-magnetite transformation. To distinguish between the two effects, we successfully employed high-resolution first order reversal curve (FORC) diagrams at both room temperature and elevated temperatures to distinguish the two mechanisms. While annealing partially restores pre-shock magnetic behavior and increases the apparent bulk-sample domain state, hematite-to-magnetite transformation generates new fine-grained magnetite that significantly overprints the original signal, ultimately decreasing the average bulk-sample domain state. Understanding this transformation process is crucial for interpreting paleomagnetic data, especially considering that magnetite oxidation is widespread in surface rocks. Additionally, our project revealed that only high temperatures (>540°C) can anneal shocked magnetite. Although natural annealing can occur in contact with impact melt, the hydrothermal fluid temperatures at Chicxulub and Nördlinger Ries were insufficient for annealing. Consequently, the hydrothermal system does not significantly impact the magnetic anomalies observed over the two studied impact craters. Instead, demagnetized basement rocks and newly formed impact materials play a crucial role in these anomalies.

Projektbezogene Publikationen (Auswahl)

• Peak-ring magnetism: Rock and mineral magnetic properties of the Chicxulub impact crater. GeoKarlsruhe 2021, DGGV Annual Meeting, Karlsruhe.
  Mendes, B. D. L., Kontny, A., Gaus, K., Kuipers, B. W. M. & Dekkers, M. J.
  
• Peak-ring magnetism of the Chicxulub Impact Crater derived from rock and mineral magnetic properties of the IODP-ICDP drill core M0077A. 17th Castle Meeting on Paleo, Rock and Environmental Magnetism, Trakošćan, Croatia.
  Mendes, B. D. L., Kontny, A., Gaus, K., Kuipers, B. W. M. & Dekkers, M. J.
  
• Peak-ring magnetism: Rock and mineral magnetic properties of the Chicxulub impact crater. Geological Society of America Bulletin.
  Mendes, Bruno Daniel Leite; Kontny, Agnes; Poelchau, Michael; Fischer, Lennart A.; Gaus, Ksenia; Dudzisz, Katarzyna; Kuipers, Bonny W.M. & Dekkers, Mark J.
  
• Post-Shock Temperature Effects in Magnetism: The Chicxulub Impact Crater – Natural Case Study. Visiting Fellow Talk. 1/2/2023, Institute for Rock Magnetism, Minneapolis, USA.
  Mendes, B. D. L.
  
• Use of magnetic fabrics and X-ray diffraction to reveal low strains in experimentally deformed Maggia gneiss. International Journal of Earth Sciences, 112(3), 867-879.
  Kumar, Sachin; Agarwal, Amar; Rae, Auriol S. P.; Kontny, Agnes & Poelchau, Michael H.
  
• Changes in thermomagnetic and X-ray diffraction properties of Asby dolerite with distance from the Lockne impact structure, Sweden. Physics of the Earth and Planetary Interiors, 348, 107145.
  Phukon, Pradyut; Agarwal, Amar; Mendes, Bruno Daniel Leite; Kontny, Agnes & Joshi, Gaurav
  
• Impact structures: How post-impact thermal conditions influence magnetic behavior. Dissertation at AGW / KIT, 116 pp.
  Mendes, B. D. L.
  
• Restoration and Transformation: The Response of Shocked and Oxidized Magnetite to Temperature. Journal of Geophysical Research: Solid Earth, 129(2).
  Mendes, Bruno Daniel Leite & Kontny, Agnes
  
• Ries magnetic mineralogy: Exploring impact and post‐impact evolution of crater magnetism. Meteoritics & Planetary Science, 59(7), 1577-1609.
  Mendes, Bruno Daniel Leite; Kontny, Agnes; Dudzisz, Katarzyna & Wilke, Franziska D. H.
  
• Stress-induced changes in magnetite: insights from a numerical analysis of the Verwey transition. Geophysical Journal International, 238(2), 794-805.
  Fuchs, Helena; Kontny, Agnes & Schilling, Frank R.