¹Jennifer R. Davis, ¹Carolyn A. Crow, ²Timmons M. Erickson, ¹Jeffrey Benowitz
Earth and Planetary Science Letters, 698, 120443
Open Source Link to Article [DOI: 10.1016/j.epsl.2026.120443]

¹Department of Earth Science, University of Colorado Boulder, 2200 Colorado Ave, Boulder, CO 80309, USA
²Amentum-JETS II, NASA Johnson Space Center, Houston, TX 77058, USA
Copyright Elsevier

Dating meteorite impact structures is confounded by shock-temperature heterogeneity and potential long-lived (>10⁶ years) post-event hydrothermal systems. Meteorite-impact airfall ejecta may provide independent material for corroborating an impact age or establishing a primary impact event age. However, dating ejecta deposits may be complicated by the presence of detrital and/or unreset target-rock material. Post-depositional heating and fluid flow may further disturb isotopic systems. To further our ability to date ejecta material, here we present the results of a test-of-concept, coordinated microstructural (using electron backscatter diffraction [EBSD]; N_zircon=217) and in-situ U-Pb geochronology (via secondary ion mass spectrometry [SIMS]; N_spots=116 on 92 zircons) survey of ZrSiO₄ from Chicxulub ejecta in the Raton Basin of southern Colorado and northern New Mexico (USA). Seventy-three ZrSiO₄ grains were identified as Chicxulub-derived based on the presence of mechanical microtwins, reidite, granular recrystallization, and/or FRIGN (former reidite in granular neoblastic) zircon microtextures, of which 42 were selected for geochronology. U-Pb spots (N = 12) on only partially granular grains were generally older (>230 Ma) than spots on fully granular grains (N = 38). ∼75% of the Chicxulub grains that experienced ≥90% Pb loss contained FRIGN zircon domains. Our results suggest that a microstructural survey of zircons and targeted in-situ geochronology of fully granular zircons with FRIGN domains is a viable method for determining the age of an impact event from distal ejecta materials. Furthermore, a compilation of paired ejecta-crater (N = 11) dates demonstrates overlap, and such dual dating should be considered best practice for dating meteorite impact events when both materials are present.