THURSDAY 16 FEBRUARY, 4 PM CET (WEBSEMINAR)
Jean-Guillaume Feignon
Title : Characterization of the impactites from the Chicxulub impact structure peak ring (Yucatán Peninsula, Mexico)
Abstract : The Chicxulub (200 km-diameter, 66.05 Ma, Yucatán Peninsula, Mexico), is the only impact structure on Earth with a well-preserved peak ring. It was drilled in 2016 by the joint IODP-ICDP Expedition 364 which recovered a continuous 829 m core. This provided a unique opportunity to investigate the nature, properties, and composition of the peak ring rocks, in order to better understand how they emplaced, and finally refine the Chicxulub impact event scenario, as well as the Yucatán basement geology. During this seminar, I will introduce you the main results obtained during my PhD at the University of Vienna (Austria).
The drill core was divided into four main lithological units, from top to bottom: (1) post-impact Paleogene, carbonate-rich sedimentary rocks, (2) an impact melt-bearing polymict impact breccia (defined as suevite), (3) impact melt rock, and (4) a shocked granitic crystalline basement unit, crosscut by intercalations of pre-impact volcanic dikes (dolerite, felsite, dacite) and intercalations of impact melt rock-bearing units. My work focused on the investigation of shocked quartz grains within the granite unit of the drill core, revealing a high level of shock (high abundance of shocked-induced planar microstructures in quartz grains, but also in other minerals), with estimated shock pressures between 16 and 18 GPa. Detailed petrographic and geochemical (major, trace elements, and Sr-Nd isotopic compositions) investigations were performed on granite samples, indicating that they are coarse-grained, highly shocked and altered, rich in K and probably emplaced in a volcanic arc context during the Carboniferous. The granite further experienced a fluid metasomatic event ~50 Myr after its formation (possibly related to the first stages of Pangea breakup). Then, the granite was further altered by post-impact hydrothermal alteration.
Measurements of highly siderophile element (Ir, Os, Pt, Re) contents and Re-Os isotopic compositions in the Chicxulub peak ring impact melt rocks did not reveal any unambiguous or detectable meteoritic component, excepted in one sample having a possible, very low (~0.01-0.05%) chondritic component. This may be explained by several factors, including the presence of a significant mafic component within impact melt rocks, the steeply-inclined trajectory and high velocity of the Chicxulub impactor, and also post-impact hydrothermal alteration.