Magnetic properties and microfabrics of shocked minerals and their influence on the magnetic anomalies in the Ries, Vredefort and Manicouagan impact structures
Final Report Abstract
Impact-cratering subjects target rocks to extremely high stresses, strain rates and temperatures that permanently modify the minerals within them. These shock effects bear heavily on the magnetic properties of ferrimagnetic minerals within impactites, which in turn leads to distinct magnetic anomalies of the impact structures. Our project combined competencies in rock genesis (petrography) with rock physics (magnetism) to decipher how the extreme conditions imprint the target rocks with a focus on Fe-Ti-oxides. Three impact structures were studied: Ries (Germany), Vredefort (South Africa) and Manicouagan (Canada). 1) The Ries impact structure contains well-preserved ejecta deposits consisting of melt-free lithic breccia overlain by suevite. We found a distinct difference in magnetic properties and shock histories of the polymict breccia clasts depending on rock type. A three-stage model explains the thermal and structural formation of impact ejecta. In the basement clasts, crystallographic relationships of ilmenite grains with foam structure indicate a back-transformation from shock pressures >16 GPa. Fe-Ti-oxides help indicate local stress, temperature and oxygen fugacity conditions. 2) The Vredefort impact structure has two circular, strongly negative, aeromagnetic anomalies: one within crystalline basement rocks and the other in collar metasedimentary rocks. In the basement rocks, we found two populations of magnetite within ilmenite—one preceding impact and another that grew as a result of impact. We concluded that impact related fracturing increased the remanence carrying capacity of the pre-existing magnetite while the newly formed magnetite further increased the remanent magnetization of the host rock. Together they contributed to the strong, negative anomalies near the impact structure’s center. Ground-based profiles across the sediments using a fluxgate magnetometer mounted on a mountain bicycle allowed us to pinpoint the rocks responsible for the extremely negative anomalies. The magnetic signal resides in 10-100 m thick banded iron formations (BIFs) at the base of the supergroup as the main producer of the anomalies. Both, the strongly negative magnetic anomalies in the collar region and in the crater’s center were created during crater formation. The rocks originally resided at temperatures higher than the Curie temperature of magnetite (580°C) until the impact rapidly brought them close to the surface where magnetite cooled to acquire a thermal remanence in the 2 Ga field. 3) Our study of cores penetrating the basement at Manicouagan shows that the most magnetic rocks contain hydrothermally altered garnets rimmed by magnetite or with magnetite filling fractures in the garnets. We argue that alteration from hot fluids produced the magnetite, which together with thick differentiated impact melts, produce the prominent positive anomaly.
Publications
-
Ilmenite and magnetite microfabrics in shocked gneisses from the Vredefort impact structure, South Africa. Contributions to Mineralogy and Petrology, 177(9).
Dellefant, Fabian; Trepmann, Claudia A.; Gilder, Stuart A.; Sleptsova, Iuliia V.; Kaliwoda, Melanie & Weiss, Benjamin P.
-
Emplacement of shocked basement clasts during crater excavation in the Ries impact structure. International Journal of Earth Sciences, 113(4), 951-971.
Dellefant, Fabian; Seybold, Lina; Trepmann, Claudia A.; Gilder, Stuart A.; Sleptsova, Iuliia V.; Hölzl, Stefan & Kaliwoda, Melanie
-
Ilmenite phase transformations in suevite from the Ries impact structure (Germany) record evolution in pressure, temperature, and oxygen fugacity conditions. American Mineralogist, 109(6), 1005-1023.
Dellefant, Fabian; Trepmann, Claudia A.; Schmahl, Wolfang W.; Gilder, Stuart A.; Sleptsova, Iuliia V. & Kaliwoda, Melanie
-
Thermal and Structural History of Impact Ejecta Deposits, Ries Impact Structure, Germany. Journal of Geophysical Research: Solid Earth, 129(1).
Sleptsova, Iuliia V.; Gilder, Stuart A.; Dellefant, Fabian; Trepmann, Claudia A.; Ahanin, Nastaran & Pohl, Jean
-
Origin of the intensely negative magnetic anomalies in Witwatersrand strata, Vredefort impact structure, South Africa. Geophysical Journal International, 243(1).
Sleptsova, Iuliia V.; Gilder, Stuart A.; Le Goff, Maxime; Dellefant, Fabian; Trepmann, Claudia A.; Lhuillier, Florian & Webb, Susan J.
