¹Jin-Young Lee, ²Jeongmin Kim, ¹Sei-Sun Hong
Meteoritics & Planetary Science (in Press) Open Source Link to Article [DOI: 10.1111/maps.70216]

¹Quaternary Geological Research Center, Korea Institute of Geoscience and Mineral Resources, Daejeon, Korea
²Research Center of Earth and Environmental Sciences, Korea Basic Science Institute, Cheongju, Korea
Published by arrangement with John Wiley & Sons

We present the first micrometer-scale, phase-resolved compositional characterization of impact melt glass from the Hapcheon crater (~1.4 km apparent diameter), the first confirmed impact structure on the Korean Peninsula, integrating EPMA-WDS (5 μm spot), confocal Raman spectroscopy (<1 μm spot), XRD, and 40Ar/39Ar step-heating geochronology. EPMA of 37 points resolves three compositionally distinct groups separated by a clear compositional gap: silica-glass cores (CORE; n = 23; 98.4 wt% SiO2, K2O 0.023 wt%), feldspathic–mafic mantling glass (MANTLE; n = 6; ~55 wt% SiO2, K2O 2.24 wt%), and feldspathic glass clasts (UNCERTAIN; n = 8; ~61 wt% SiO2, K2O 3.10 wt%), demonstrating phase-segregated melting of the quartz and feldspathic–mafic fractions of the Cretaceous Dongmyeong target rock rather than whole-rock homogenization. Confocal Raman spectroscopy confirms coesite in 74% of 34 spectra and identifies diaplectic quartz glass, documenting micrometer-scale shock heterogeneity; a 30-point colocation data set confirms coesite within the ultralow-K2O glass phase. The ultralow K2O causes systematic 40Ar/39Ar age overestimation through inherited 40Ar; all seven plateau ages (1.8–5.6 Ma) exceed the independent 10Be burial age of 1.33 Ma. Step-heating Ca/K systematics confirm multiphase assemblages, and inverse-isochron 40Ar/36Ar intercepts independently corroborate inherited 40Ar. The youngest compositionally anchored age brackets the impact at 1.33–3.07 Ma (Plio-Pleistocene).