Partial Melting-Induced Chemical Evolution in Shocked Crystalline and Amorphous Plagioclase From the Lunar Meteorite Mount DeWitt 12007

Abstract

Determining the formation mechanism of maskelynite is essential to understanding the shocked environments of meteorites on their parent bodies. Maskelynite has been accepted as a diaplectic glass for several decades, but there have been suggestions that it is a normal glass quenched from a dense melt. Morphological characteristics have been generally investigated to identify the formation mechanism of maskelynite, but the chemical difference between maskelynite and plagioclase has not been fully understood. In this study, we investigated the morphological, atomic structural, and chemical characteristics of maskelynite in the lunar meteorite DEW 12007 to constrain the formation mechanism of maskelynite via chemical analysis. The morphological characteristics showed that plagioclase was partially converted into maskelynite though partial melting. Two-dimensional Raman mapping confirmed the disordered and crystalline structures of the maskelynite and plagioclase regions. Quantitative chemical analyses revealed that the maskelynite regions were more albite-rich than the plagioclase regions, likely due to the partial melting of plagioclase. Under shocked conditions, the partial melting of plagioclase induced chemical variation between normal glass-type maskelynite and plagioclase, as this variation occurred upon partial melting at ambient pressure. The morphological and structural changes correspond well with the chemical variations, indicating that amorphization induced such variations as a result of the partial melting of plagioclase. The chemical differences between maskelynite and plagioclase in other meteorites also could be understood to be the results of partial melting. Thus, the chemical differences between maskelynite and plagioclase in partially amorphized grains could elucidate the formation mechanism of maskelynite in many meteorites.