Coexisting polymorphs in shocked rocks investigated by EBSD and TEM

Parallel to the study of accessory minerals as potential shock indicators, the main rock forming minerals quartz and olivine, representative of terrestrial impact cratering and planetary collisions recorded in meteorites, respectively, are still intensively used for constraining shock metamorphism. Here, we present investigations on shocked quartz from a terrestrial impact structure and shocked olivine from a chondrite meteorite, where at least two high pressure polymorphs coexist, with the aim of constraining the formation process and, thus, the shock conditions.

In the L6 ordinary chondrite Alfianello, three polymorphs with olivine composition were observed, namely olivine itself, wadsleyite, and ringwoodite. The occurrence of fine-grained aggregates of wadsleyite and ringwoodite, investigated by transmission electron microscopy (TEM) and 3D electron diffraction, and the presence of lamellar ringwoodite in olivine support the coexistence of different shock-induced processes in the same sample. In the case of the wadsleyite-ringwoodite aggregates, the random mutual crystallographic orientation and the complementary Fe/Mg ratio, as well as the occurrence in clasts within impact melt pockets or in the vicinity of shock veins lend support to formation by fractional crystallization from an impact melt with olivine composition during the shock pulse. On the other hand, the lamellar ringwoodite, oriented along notable crystallographic planes of olivine, suggests formation by solid-state transformation from olivine at the shock front.

The pseudotachylitic breccia from the Vredefort impact structure, one of the oldest and largest impact structures preserved on Earth, contains up to three silica polymorphs: quartz, stishovite, and coesite. In the investigated clasts in pseudotachylitic veins, quartz and coesite occur together, forming a fine-grained aggregate. The frequency of coesite occurrence, mostly limited to the center of the clasts, seems to be correlated with the position of the clast within the vein, with a maximum towards the center of the vein. Electron backscatter diffraction (EBSD) enabled the identification of the crystallographic relationship between coesite and quartz, allowing the discrimination of the most likely formation process for coesite between crystallization from the impact melt under high-pressure conditions during the shock pulse and solid-state transformation from a likely already strongly deformed quartz or from diaplectic glass.

This work remarks the potential of modern analytical techniques to help scientists reconstructing complex deformation mechanisms, constraining the local formation conditions. These data can be used for better modelling shock metamorphic processes, improving our understanding of the global implications of planetary collisions.