Three-dimensional petrography of metal phases in equilibrated L chondrites-Effects of shock loading and dynamic compaction

To examine the role of impacts in the evolution of asteroids as seen through their chondritic offspring, we have performed a quantitative three dimensional (3D) study of metal grains in a suite of increasingly shocked L chondrites with synchrotron X-ray microtomography (XMT). Our data allow rigorous quantification of size-number distributions and collective morphology of Fe(Ni) metal phases in chondritic meteorites. At the resolution of our XMT measurements (8.4-17.9 μm/voxel), the number of metal particles increase with higher degrees of petrographically identified shock loading, indicating a coalescing of Fe-Ni metal at or below this scale. Our results demonstrate that collective degrees of metal grain preferred orientation increase with greater degrees of impact-related compaction and shock loading. Ductile metal grains in L chondrites begin to show foliation at peak shock pressures < 5 GPa, pressures great enough to compact and indurate loosely bound chondritic material, and our results constitute evidence for multiple generations of impact events acting on the L parent body or bodies.