Chemical and Structural Evidence for Melt-Induced Amorphization of Alkali Feldspar in Lunar Meteorite DEW 12007: Insight into Shock Amorphization Mechanisms

Abstract

Feldspar minerals, including alkali feldspar and plagioclase, commonly undergo shock-induced amorphization, making them key indicators for reconstructing the impact history and thermal evolution of planetary materials. This study focuses on alkali feldspar in the lunar meteorite DEW 12007 to elucidate the mechanisms of amorphization using Raman spectroscopy and electron probe microanalysis. Our findings reveal a strong correlation between crystallinity and chemical composition, with Na enrichment and K depletion observed in amorphous regions compared to the crystalline counterparts. These compositional shifts can be explained by the chemical behavior of sanidine-albite system, supporting impact melt-induced partial amorphization as the dominant mechanism. The crystalline counterparts near amorphous boundaries exhibit increased K content, reflecting the compositional redistribution during partial melting and quenching. To assess the utility of these findings, feldspar from the well-documented L6 ordinary chondrites were analyzed. Feldspars from the ordinary chondrites showed opposite trends, exhibiting the amorphous phases enriched in K and depleted in Na relative to the crystalline counterparts. These differences are consistent with the chemical behavior of sanidine-albite system. This ability has a potential to differentiate whether maskelynite was formed via solid-state transformation or melt-quenching processes, providing critical insights into the impact history and thermal evolution of the rocky crusts of the Moon and other solar system bodies. Extending this framework to other meteorites, returned samples, and planetary bodies enhances our understanding of impact processes, offering a robust approach for unraveling the complex interplay between shock metamorphism and planetary evolution.