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

Abstract

Abstract-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 -25% Ni, is either martensite or taenite, these two occurring adjacent to each other. The observation that the Ni-poor phase is partly fcc 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 "C). Because no further recovery occurred, these zones contain a high density of lattice defects. In low-Ni zones (<I 5 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 1-1 0 "C/year for Tsarev and 10-1 00 "C/year for Agen.

A study of ordinary chondrites by Mössbauer spectroscopy with high‐velocity resolution

“…The doublet corresponded to residual silicates in extracted metal (∼5.2%), sextets corresponded to ferromagnetic phases in metal with different values of the effective magnetic field on the 57 Fe nucleus and singlet corresponded to paramagnetic phase in metal. Metallographic data of chondrite Tsarev L5 demonstrated that its matter was strong shock affected in the space (Migdisova et al 1982;Leroux et al 2000). Microphotograph of typical metal grain is shown in Fig.…”

AFM and SNOM characterization of ordinary chondrites: A contribution to solving the problem of asteroid reddening

“…In this context, the choice of a particular metal as reference is not critical. Since martensite is more easily formed in high-Ni areas [24], we show this element as a reference for the typical featureless optical behavior of metals.…”

Complex Thermal Histories of L Melt Breccias NWA 5964 and NWA 6580

“…Metals and sulfides have been shown to contain vital information about shock processing and cooling conditions (Smith and Goldstein, 1977;Bennett and McSween, 1996). A variety of studies have been conducted in attempts to experimentally recreate the variety of compositions and textures seen in the metal and sulfide structures in ordinary chondrites (Scott, 1982;Reisener and Goldstein, 2003b;Leroux et al, 2000;Bennett and McSween, 1996;Wilkening, 1978). Onorato et al (1978) propose that metal grains within a single chondrite may record various stages of cooling, which Stöffler et al (1991) describe as two stages of cooling.…”

“…The closure temperature is the temperature in which the diffusion of Ni through the solid metal is stopped. The transformations that occur depend on multiple parameters: 1) initial composition of the metal (somewhat different for the H, L, and LL chondrite groups, Figure 2B), 2) nucleation sites for further transformations, and 3) cooling rate (Leroux et al, 2000;and references therein). A typical bulk nickel content in metal grains for H or L chondrites is about 10-15%, so following the phase diagram in Figure 2, the typical slow cooling trend is for a metal to begin in the taenite field, then around 700˚C kamacite will begin to exsolve forming kamacite + taenite assemblages (Reisener and Goldstein, 2003a).…”

“…One value was extrapolated from the Schmitt (2000) Reisener and Goldstein (2003a). Part A shows the diagram with points marked for metals experimentally heated and allowed to cool under equilibrium conditions to a closure temperature of 200 ˚C (Leroux et al, 2000). Part B…”