• Shock-induced incipient melting of pyroxene in Lonar impact-melt bombs. • Raman spectra reveal structural disorder linked to localized shock intensity. • Microtextures show fracturing, vesiculation, and partial melting of pyroxene. • Thermal modelling constrains projectile size, velocity, and impact obliquity. • Pyroxene is a sensitive recorder of shock heterogeneity in basaltic targets. We present microstructural and spectroscopic evidence of shock-induced incipient melting of pyroxene in impact-melt bombs from the Lonar crater, India, formed by oblique impact on Deccan basalts. Although previous studies suggested that pyroxene remained largely in the solid state during basaltic target melting, our observations reveal pervasive fracturing, vesiculation, granular textures, and selective pyroxene-grain melting. Back-scattered electron imaging highlights marginal recrystallization, granularization, and irregular Fe-enrichment/Mg-depletion patterns, indicative of localized high-temperature effects. Micro-Raman spectroscopy documents systematic shifts and broadening of characteristic pyroxene peaks, with dendritic and vesiculated domains showing pronounced structural disorder. These trends correspond to crystallization at elevated temperatures consistent with incipient melting under ∼ 55 GPa shock pressure and ≥ 1606 K. Using heat conduction modelling of pyroxene melting timescales, we constrain the shock pulse duration and refine estimates of the impactor’s size and velocity. Our calculations suggest the Lonar crater was formed by a ∼ 193–219 m diameter chondritic projectile, impacting at an angle of 30 − 45° with an impact velocity of ∼ 5.28–5.48 km/s, generating a shock wave of sufficient intensity and duration to induce melting and structural transformation in pyroxene. These findings reconcile microstructural and thermal observations with impact dynamics and underscore the importance of pyroxene behavior in the generation and evolution of basaltic impact melts. This study provides new insights into shock metamorphism of pyroxene and its contribution to melt production in basaltic targets, with broader implications for interpreting impact processes on the Moon, Mars, and basaltic asteroids.
Tiwari et al. (Mon,) studied this question.