EBSD Analysis of Iron‐Nickel Metal in H Chondrites: 2. Formation of Metal With the M‐Shaped Ni Profile

Abstract Iron‐nickel metals, primarily taenite and kamacite, are major components in most meteorites. Taenite exhibiting the M‐shaped Ni profile has traditionally been interpreted as a product of slow cooling and is widely used to estimate the thermal histories of planetary bodies. However, our Electron Backscatter Diffraction analyses of H chondrites reveal that metal grains with M‐shaped Ni profiles consist of a low‐Ni martensite core surrounded by a high‐Ni tetrataenite rim. The presence of martensite, which forms via rapid quenching of taenite, is difficult to reconcile with its formation by slow cooling. Integrating these microstructural observations with the thermal history of H chondrites, we propose that these metal assemblages most likely formed during impact‐related reheating events. In this scenario, impact‐induced heating facilitates the nucleation and growth of high‐nickel tetrataenite along the margins of pre‐existing kamacite monocrystals, followed by the formation of lower‐nickel taenite in the core. This process results in a metallic assemblage characterized by the M‐shaped nickel profile. During subsequent rapid cooling, the taenite core transforms either martensitically into martensite or via spinodal decomposition into duplex plessite. When martensite forms, it inherits the Ni composition of the precursor taenite core, preserving the M‐shaped profile. These results suggest that, at least for the samples investigated here, M‐shaped Ni profiles may record impact‐related thermal processes. The formation of these assemblages requires shock metamorphism of at least stage S3.