Microscopic Magnetic Study of Ordinary Chondrite Using Magneto-optical Imaging

Fe-Ni alloys, which are kamacite (<10 wt% Ni), taenite (30 ~ 40 wt % Ni), and tetrataenite (ordered Fe 50 Ni 50), are the dominant magnetic minerals in the metamorphic (type 5-6) ordinary chondrites [1], and the carrier of the remanent magnetization is high coercivity tetrataenite [1,2,3]. Metallographic studies reported that there are two types of Fe-Ni grains in the metamor-phic ordinary chondrites: zoned taenite + kamacite and zoneless plessite that consists of tetrataenite micron-sized plates and kamacite matrix [4]. In the high-Ni outer region of zoned taenite grains, tetrataenite appear as clear-etching 1-3 µm-thick rim and dark-etching " cloudy-zone (CZ) " consists of submicron inter-growth of tetrataenite, taenite, and martensite [4, 5]. Despite various shape and size, relationships between tetrataenite morphologies and their remanent magnetizations are still unknown due to the small grain size of metallic grains (< 1 mm). Recently, we succeeded to observe magnetizations of rock samples by magneto-optical imaging (MOI) microscopy [6]. This technique utilizes Faraday rotation of garnet film and can observe vertical component of surface magnetic field in 10 µm resolution under reflected light microscope. Using this new technique, we observed Fe-Ni grains in Ausson (L5, S2) ordinary chondrite [7] and characterized the carrier of the remanent magnetization. Sample is 6 mm in thickness and magnetized by 3T upward field. Magnetic images and petrographic settings of metallic grains were simultaneously observed by reflected light micros-copy. After acquisition of magnetic images, the metal particles were etched with Nital (1 vol% HNO3, balance ethanol) for 30 sec. Microstructure of metallic grains were observed by reflected light microscopy, scanning electron microscope (SEM) and field-emission SEM (FE-SEM) after etching. Magnetic and optical microscopy reviled that zoned taenite + kamacite grains are magnetized. We dmagnetized zoneless ples-site grains were not found in the present observation. To compare with etched-grain images, magnetic signatures do not correspond to clear-etching tetrataenite rim but accord with dark-etching CZ. Some large grains contain non-magnetic core that consists of tae-nite or martensite. FE-SEM observations revealed that magnetic part of CZ consists mainly of <100nm spherical or fiber-shaped particles. To compare with previous SEM study of CZ [4], these particles are probably tetrataenite. Therefore, these results suggest that such fine tetrataenite particles are the dominant magnetic carrier in metamorphic ordinary chondrites.