Evidence for impact events can be found in the mineralogy and appearance of lunar samples and meteorites that originate from other bodies in the solar system. This shock metamorphism is ubiquitous in meteorites from asteroids, which, as airless bodies, are more susceptible to impact effects than planets that have protective atmospheres. Understanding the intensity of shock effects in meteorites is important for elucidating the geologic processes that dominated the early solar system as shock metamorphism not only provides a record of large-scale impact events but can also obscure the records of how planets and asteroids originally formed. While several methodologies have been used for classifying the degree of shock metamorphism in meteorites, a more detailed classification scheme is needed to better understand weakly-shocked samples.
Recently, Rei Kanemaru of The Graduate University for Advanced Studies, SOKENDAI (Japan), and colleagues combined optical and spectroscopic observations of sixteen weakly shocked eucrites, differentiated meteorites thought to form via volcanic processes and originate from the asteroid Vesta, to create a classification scheme to describe the shock metamorphic effects they observed. These samples contain the minerals plagioclase and pyroxene, but most of the study focused on the plagioclase components. The study used Raman spectroscopy to examine the distortion of the crystals, which quantifies the degree of shock damage. They also used color cathodoluminescence to investigate how the minerals luminesce when exposed to a beam of electrons. In this study, it was found that in mildly shocked samples, plagioclase emitted strongly at yellow wavelengths, while in more severely shocked samples, it emitted strongly at blue wavelengths.
The classification scheme that Kanemaru and colleagues defined consists of five levels ranging from shock degree A, described as relatively unshocked, to shock degree E, described as highly shocked, based on their combination of optical and spectroscopic analyses. The implementation of a universal shock classification scheme is important for future sample studies because it provides a consistent basis for comparing meteorites and understanding large-scale planetary processes. For example, when Kanemaru and colleagues applied their shock classification scheme to eucrites and integrated their observations to what they already knew about Vesta’s thermal history, they concluded that the shock metamorphism present in the samples was the result of impact events that occurred after Vesta had cooled significantly after its formation. READ MORE