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Volatilization and overprint processes through the Chicxulub impact structure: isotopic evidence from the Expedition 364 core


Thomas Déhais
AMGC, Vrije Universiteit Brussel

ABSTRACT: The Chicxulub impact structure in Mexico has been drilled by the joint IODP-ICDP Expedition 364 in 2016. A unrivalled continuous sequence of the upper peak ring has been retrieved, giving access from bottom to top to felsic basement rocks, impact melt, suevite, and post-impact sediments [1]. The suevite unit consist in a melt-bearing polymict impact breccia with a clastic matrix. It is the result of the fallback of ejecta material from the vapour-plume emitted by the impact and their wash-back by
tsunamis in the crater. The timing of the impact-plume formation and the deposition of ejecta (from proximal to distal) after an hypervelocity impact are still not well constrained. It is crucial to understand and disentangle the different processes occurring within the plume such as volatilization, condensation and hydrothermal overprint.

For this project, isotopic analyses of Fe, Cu, and Zn have been realised by MultiCollector –Inductively Coupled Plasma – Mass Spectrometry (MC-ICP-MS) on selected samples from the Expedition 364 core. All lithologies have been analysed with a focus on the transitional (core section 40R-1) and suevite (core section 40R-2 to 95R-3) units as they might have endured more the thermodynamics conditions of the vapour-plume. Isotopic data have been coupled with major and trace elemental analyses, and petrographic observations.

The isotopes of Fe, Cu and Zn show different behaviours while exposed to the vapour-plume thermodynamic conditions. This is due to the difference of volatility orders between Fe, Cu, and Zn [2]. Therefore, this innovative technique that has never been applied on ejecta material, allow to distinguish the different processes that affected the lithologies within the Chicxulub impact structure. Further isotopic analyses on selected new samples along the Exp 364 core will allow a complete overview and will help to confirm first interpretations.

[1] Morgan J. V., et al., 2016. The formation of peak rings in large impacts craters. Science, 354 (6314), 878-882.

[2] Allègre C., et al., 2001. Chemical composition of the Earth and the volatility control on planetary genetics. Earth and Planetary Science Letters 185, 49-69.