Magnetic microstructures of metal grains in equilibrated ordinary chondrites and implications for paleomagnetism of meteorites
Résumé
Meteorites are a primary source of information about past magnetic field in the solar system. Yet, the small-scale magnetic properties of FeNi metals, which are the magnetic carriers of most meteorites, are poorly known. We study here the magnetic microstructures of FeNi metals in two equilibrated chondrites. Two types of tetrataenite-bearing microstructures are revealed: (1) Zoned taenite particles that consist of a “cloudy zone” (20–250 nm large tetrataenite precipitates embedded in Ni-poor matrix) and a 1–10 μm thick tetrataenite rim. (2) Zoneless plessite particles that consist of large tetrataenite grains (> 1 μm) embedded in a kamacite matrix. Magneto-optical imaging of saturation remanence shows that, the submicron-sized tetrataenite islands in cloudy zone carry a much stronger remanence than the μm-sized tetrataenite crystals in the tetrataenite rims and plessite. Micron-scale mapping of coercivity of remanence (Bcr) shows that the center part of the cloudy zone has finer tetrataenite grains (20 nm) and higher Bcr values (~ 1 T) than the outer part (250 nm and 400 mT, respectively). These results suggest that the micron-sized tetrataenite is in a multi domain state, whereas the submicron-sized tetrataenite in the cloudy zone are in a single domain-like state and may be regarded as a potentially good paleomagnetic recorder in meteorites. The stability of the remanent magnetization in ordinary chondrites is a function of the amount of the cloudy zones of the zoned taenite grains rather than the bulk amount of tetrataenite.
The Fe–Ni phase diagram indicates that precursor of tetrataenite is paramagnetic when the metamorphic temperature was above 350–400 °C. Therefore, the remanent magnetization of tetrataenite cannot be an evidence of early magnetic activity on the parent body (e.g., dynamo activity) during the first 10 to 50 Myr after the peak of metamorphism, assuming 900 °C peak temperature and 50–100 °C/Myr cooling rate. Our TEM observations show that tetrataenite has a homogeneous crystallographic orientation in an individual zoned taenite grain. In low or null field, tetrataenite may acquire a spontaneous magnetization whose direction is controlled by this crystallographic orientation, which varies from grain to grain. The small-scale heterogeneity of remanence observed in equilibrated chondrites may imply that no significant magnetic field (e.g., dynamo field) was present during cooling below 350–400 °C.