Characterization of coesite‐bearing impact melt glass from the Hapcheon crater, Korea: Trimodal phase architecture, shock metamorphism, and implications for ⁴⁰ Ar / ³⁹ Ar geochronology

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 40 Ar/ 39 Ar 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% SiO 2 , K 2 O 0.023 wt%), feldspathic–mafic mantling glass (MANTLE; n = 6; ~55 wt% SiO 2 , K 2 O 2.24 wt%), and feldspathic glass clasts (UNCERTAIN; n = 8; ~61 wt% SiO 2 , K 2 O 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‐K 2 O glass phase. The ultralow K 2 O causes systematic 40 Ar/ 39 Ar age overestimation through inherited 40 Ar; all seven plateau ages (1.8–5.6 Ma) exceed the independent 10 Be burial age of 1.33 Ma. Step‐heating Ca/K systematics confirm multiphase assemblages, and inverse‐isochron 40 Ar/ 36 Ar intercepts independently corroborate inherited 40 Ar. The youngest compositionally anchored age brackets the impact at 1.33–3.07 Ma (Plio‐Pleistocene).