AFM, FE-SEM and optical imaging of a shocked L/LL chondrite: implications for martensite formation and wave propagation

Introduction: Martensite is a structure of metallic alloys due to a diffusionless transformation. In steels, the martensitic transformation is usually associated to fast cooling rates but it is indeed a shearing process involving very little atoms displacements. Faster the process, finer are the layers of martensite, while the total amount only depends on the available energy. Martensite formation is very sensitive to me- tallic composition, impurities [1], and initial pressure [2]. In meteorites, the cooling rates are typically provided by the size of cloudy zones of the metallic phases [3] or by the nickel spatial distribution in the taenite- kamacite interface [4], while martensite is usually as- sociated to shock metamorphism from S4 to S6 [5], [6]. We analysed a thin section of an ordinary chondrite (L/LL4 S3, determined by petrographic indicators [7]), showing a barred chondrule (figure 1) and many me- tallic zones with martensite (figure 2). The rather pronounced heterogeneity of the shock pressure is clearly visible in the spatial distribution of the martensite over the whole section. We show images obtained by optical, field emission scanning electron microscopy (Fe-SEM) and atomic force microscopy (AFM) in order to understand the wave propagation and the martensite formation. The AFM images, in particular, have been collected with a spatial resolution of 2 nm. Such an approach allow to identify martensite as surface topography and/or lateral friction changes.