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

This paper reports one of the first attempts to investigate by analytical t ransmission electron microscopy (ATEM) the microstructures and compositions of Fe-Ni metal grains in ordinary chondrites. Three ordinary chondrites, S aint Severin (LL6), Agen (H5), and Tsarev (L6) were selected because they d isplay contrasting microstructures, which reflects different thermal histor ies. 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 struct ure resulting from spinodal decomposition similar to the cloudy zone of iro n 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 disagre ement 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 cont act with the cloudy zone displays high-Ni compositional variability on a ve ry 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 cool ing obviously related to shock events. Their metallic particles mostly cons ist 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). B ecause no further recovery occurred, these zones contain a high density of lattice defects. In low-Ni zones (<15 wt%), martensite grains formed at hig her temperature and their lattice defects recovered. These martensite grain s present a lath texture with numerous tiny precipitates of Ni-rich taenite (Ni = 50 wt%) at lath boundaries. Nickel composition profiles across preci pitate-matrix interfaces show that the growth of these precipitates was con trolled by preferential diffusion of Ni along lattice defects. The cooling rates deduced from Ni concentration profiles and precipitate sizes are with in the range 1-10 degrees C/year for Tsarev and 10-100 degrees C/year for A gen.