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

Abstract

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).