Shock metamorphism in the CM chondrites revealed through combined chondrule and calcite analysis

To study the shock and aqueous alteration histories of CM chondrites, we analyzed the microstructure of chondrules and calcite grains using SEM-EDS and SEM-EBSD in a selection of CMs spanning a wide range of grades, including one thermally metamorphosed CM. Using EDS maps, we identified calcite grains, and in these maps we measured the long and short axes of chondrules and the orientations of those axes. For calcite grains, we collected EBSD maps. We measured the twin densities in the calcite grains, as twin density is causally related to the degree of deformation. Our samples universally show evidence for chondrule strain and petrofabric development, with a weak correlation between the degree of chondrule strain and the degree of aqueous alteration. Most chondrule populations have two preferred orientations for their long axes, with the direction of chondrule extension lying in between. We speculate that this may be due to a primary predeformational fabric, likely depositional, which was later modified by shock-related deformation. Most calcites in calcite-bearing samples are not deformed based on their twin densities, with Murchison being the only exception. We infer that most shock metamorphism preceded most aqueous alteration, though there is evidence for an overlap between the time periods of these processes, necessary to explain the minority of shock-deformed calcites. We find that PCA 02010, a strongly thermally metamorphosed CM, is less shocked than SCO 06043 and Jbilet Winselwan, both of which are less thermally metamorphosed that PCA 02010. Therefore, it is unlikely there is a simple and direct relationship between thermal metamorphism and shock metamorphism in the CMs.