Microstructures of metal grains in ordinary chondrites: Implications for their thermal histories

This paper reports one of the first attempts to investigate by analytical transmission electron microscopy (ATEM) the microstructures and compositions of Fe-Ni metal grains in ordinary chondrites. Three ordinary chondrites, Saint Severin (LL6), Agen (H5), and Tsarev (L6) were selected because they display contrasting microstructures, which reflects different thermal histories. In Saint Severin, the microstructure of the Ni-rich metal grains is due to slow cooling. It consists of a two-phase assemblage with a honeycomb structure resulting from spinodal decomposition similar to the cloudy zone of iron meteorites. Microanalyses show that the Ni-rich phase is tetrataenite (Ni = 47 wt%) and the Ni-poor phase, with a composition of similar to 25% Ni, is either martensite or taenite, these two occurring adjacent to each other. The observation that the Ni-poor phase is partly Sec resolves the disagreement between previous transmission electron microscopy (TEM) and Mossbauer studies on iron meteorites and ordinary chondrite metal. The Ni content of the honeycomb phase is much higher than in mesosiderites, confirming that mesosiderites cooled much more slowly. The high-Ni tetrataenite rim in contact with the cloudy zone displays high-Ni compositional variability on a very fine scale, which suggests that the corresponding area was destabilized and partially decomposed at low temperature. Both Agen and Tsarev display evidence of reheating and subsequent fast cooling obviously related to shock events. Their metallic particles mostly consist of martensite, the microstructure of which depends on local Ni content. Microstructures are controlled by both the temperature at which martensite forms and that at which it possibly decomposes. In high-Ni zones (>15 wt%), martensitic transformation started at low temperature (<300 degrees C). Because no further recovery occurred, these zones contain a high density of lattice defects. In low-Ni zones (<15 wt%), martensite grains formed at higher temperature and their lattice defects recovered. These martensite grains present a lath texture with numerous tiny precipitates of Ni-rich taenite (Ni = 50 wt%) at lath boundaries. Nickel composition profiles across precipitate-matrix interfaces show that the growth of these precipitates was controlled by preferential diffusion of Ni along lattice defects. The cooling rates deduced from Ni concentration profiles and precipitate sizes are within the range 1-10 degrees C/year for Tsarev and 10-100 degrees C/year for Agen.