Abstract Shock-melt veins in ordinary chondrites record ultrafast, high-pressure mineral transformation reactions. However, resolving the nano- to microscale mineral assemblages that form and quench during these events remains challenging. Here we demonstrate that near-axis transmission Kikuchi diffraction (NA-TKD), combined with scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDS), can reliably resolve crystal structures and Fe–Mg zoning in sub-micron, high-pressure olivine phases across 10–15 μm fields of view, providing new insights into shock transformation mechanisms. We apply this approach to shock-melt veins in the Catherwood L6 chondrite, where host olivine along the shock-vein margin transforms into dense, randomly oriented clusters of ringwoodite crystallites. These textures indicate rapid solid-state transformation by homogeneous intracrystalline nucleation and interface-controlled growth under strongly overstepped conditions. Olivine fragments entrained within the melt preserve similar ringwoodite-dominated cores but develop Fe-rich reaction zones and Mg-rich wadsleyite rims at melt-wetted grain boundaries, accompanied by interstitial majoritic garnet. These features record brief melt infiltration, partial dissolution, and melt-assisted recrystallization during shock events. Together, the observed microstructures define a two-stage, but spatially heterogeneous transformation sequence: initial solid-state ringwoodite formation followed by localized melt-mediated overprinting and wadsleyite crystallization. This demonstrates that pressure–temperature conditions vary substantially across a single shock-melt vein, allowing multiple transformation mechanisms to operate sequentially or simultaneously within the same system. By enabling phase discrimination, orientation mapping, and coupled chemical–structural analysis at ∼10–30 nm spatial resolution, NA-TKD combined with EDS provides nanoscale crystallographic mapping within the SEM. This approach allows shock transformation sequences to be reconstructed across micrometer-scale fields of view that are difficult to access using SEM and TEM alone, providing a powerful framework for interpreting high-pressure reaction pathways in planetary materials.
Abbott et al. (Wed,) studied this question.