Temperature dependence of coercivity for chondrites: Allende, Allan Hills-769, and Nuevo Mercurio

Temperature dependence of the hysteresis parameters (saturation magnetization J_S, saturation remanent magnetization J_R (SIRM), coercive force H_C, remanent coercive force H_ and initial susceptibility X_i) was examined from 30℃ to 750℃ for Allende (CV3), ALH-769 (L6) and Nuevo Mercurio (H5) chondrites. The NRMs of these chondrites were thermally demagnetized for determination of the NRM blocking (T_B) temperature. The hysteresis parameters were compared with the T_B in order to identify which parameter is the most sensitive to the high coercivity grains. The J_S-T and X_i-T curves showed almost the same transition temperature (Θ_, Θ_), but the Θ_ andΘ_ were not coincident with the T_B for these chondrites. On the other hand, the main transitions of theΘ_, Θ_ andΘ_ in the J_R-T, H_C-T and H_-T curves were coincident with the T_B. Consequently the temperature dependency of the J_R, H_C and H_ is useful for the determination of the high coercivity grains which carry the stable NRM. The reliable NRMs are carried by tetrataenite in ALH-769,and Nuevo Mercurio, although their main magnetic mineral is kamacite

Characteristics of natural remanent magnetization of Nova Petropolis iron meteorite (II)

Nova Petropolis (medium octahedrite, IIIA) was studied magnetically focusing on the acquisition mechanism of natural remanent magnetization (NRM). Thermomagnetic properties, hysteresis properties, NRM stabilities and the study of fine magnetic structures by north seeking bacteria (NSB) were examined. The sample consists of polycrystalline kamacite, taenite and cloudy taenite, while tetrataenite is absent. The NRM directions scattered widely among subsamples, but each subsample was extremely stable against AF demagnetization up to 100mT. The S pole alignments observed by NSB were not parallel completely with the crystallographic axis of kamacite. In order to understand the stable NRM of Nova Petropolis, we must consider the magnetic domains that were reconfigured and stabilized due to deformation by hyper-velocity shocks

Magnetic analysis of Antactic ordinary chondrites and achondrites on the basis of a magnetic binary system model

Magnetic hysteresis cycles of 9 ordinary chondrites and 10 achondrites, collected mainly from Antarctica, are analyzed on the basis of a newly proposed model of a non-interactive magnetic binary system, by taking into account their thermomagnetic characteristics for identifying ferromagnetic phases involved. All the chondrites and achondrites examined consist of a high-coercivity (a) component and a low-coercivity (b) component. In ordinary chondrites, the (a) component is often identified to be tetrataenite (tetragonal-ordered crystal of FeNi) having an apparent coercive force H^_C≳10^3 Oe (TT-type phase), while in other chondrites the (a) component comprises fine grains of shape-anisotropic single-domain structure (A. SD-type phase) having H^_C=(2∿5)×10^2 Oe. The (b) component consists of mostly multi-domain grains of kamacite and/or taenite of H^_C≲20 Oe. In most achondrites, the (a) component comprises A. SD-type phase of H^_C=(2∿5)×10^2 Oe except a special case consisting of a small amount of tetrataenite. The structure of the (b) component in achondrites is the same as in ordinary chondrites. An anomalous hump-shape rise in saturation magnetization (I_s) between 330℃ and Curie point of taenite (540∿590℃) during the initial heating process is found in diogenites and a eucrite. The thermomagnetic hump is considered here to be due to an increase of A. SD-type phase of taenite associated with a relatively smaller increase of the (b) component multi-domain phase. However, a possible metallographical interpretation of the hump phenomenon has not yet been obtained

Collection of Yamato meteorites by the 35th Japanese Antarctic Research Expedition

The 35th Japanese Antarctic Research Expedition found 9 ordinary chondrites and 7 fragments around Camp 1 (latitude 71°32′49″S, longitude 35°24′07″E) located 5km westward from Massif C in the Yamato Mountains. In this area meteorites have not been collected previously. Meteorite surveys were also carried out along the route of the Camp migrations and around the Camp 3,although no meteorites were found. The chondrites examined microscopically may be classified into either H4 or H6 categories. The biggest chondrite (Yamato-9403) which is classified as H6 was examined magnetically

Magnetic fabric analysis of deformed rocks in the Riiser-Larsen Main Shear Zone, East Antarctica

Anisotropy of magnetic susceptibility (AMS) in deformed rocks in the Riiser-Larsen Main Shear Zone (RLMSZ) was analyzed in order to demonstrate the changes in rock magnetic properties due to deformation. Sixty-nine samples were collected at six sites from sheared gneisses and sheared dolerites. Experimental results of stepwise acquisition of isothermal remanence, demagnetization of a composite IRM and thermomagnetic measurement indicate the presence of Ti-poor titanomagnetite. Pyrrhotite also occurs characteristically in specimens with mylonitic textures. Magnetic foliations of AMS for the mylonite at three sites show good agreement with mylonitic foliation at each site. The mylonites showed enhancement of anisotropy degree from protoliths, indicating overprinting of the original magnetic fabrics. Their maximum susceptibility axes are well defined within each site, and dip about 50-60° northward. The magnetic lineation probably indicates the maximum stretching direction in the RLMSZ

The magnetic anomaly and NRM directions of Odessa octahedrite

The magnetic anomaly of the Odessa octahedrite (3.7 kg, a single crystal) was measured at 1, 5, 10, and 40 cm from the surface using a uniaxial fluxgate magnetometer in a μ-metal magnetic shield environment. The NRM directions of cubic subsamples were measured to take into consideration the role of the octahedron crystallography during AF demagnetization. These results indicated that the observed anomalies are strongly controlled by the shape anisotorpy of the sample rather than the crystallography of the octahedrite, although weak complicated anomalies which may be related to structure appeared above 1 cm from the surface. The natural remanent magnetization of cubic subsamples was controlled by the octahedral γ(111) planes, as demagnetized directions occurred along these planes. Namely, the preferable direction of NRM is the intersection among the great circles of γ(111) crystallographic planes which are consistent with the result obtained by A. Brecher and L. Albright (J. Geomagn. Geoelectr., 29, 379, 1977). The origin of the stable NRM component was investigated using the results of microscopic analysis, thermomagnetic curve and hysteresis parameters. Consequently, we concluded that small amounts of taenite grains and lamellae carry the stable component which is distributed along the octahedral γ(111) planes

Magnetic properties of high petrologic grade L-LL chondrites: Tenham,Tuxtuac,Willard and Forrest(b)

Temperature dependence of the hysteresis parameters (saturation magnetization, Js; saturation remanent magnetization, Jr; coercivity, Hc; and initial susceptibility, Xi) were examined from room temperature to 780℃ for Tenham (L6), Tuxtuac (LL5), Willard (L6) and Forrest (b) (L6) chondrites. Their main magnetic minerals were determined from the analyses of transition temperatures (Θ_, Θ_, Θ_). The NRM characteristics of these chondrites were measured and are discussed on the basis of magnetic properties. The main magnetic minerals of the Tenham chondrite are taenite (γ-(Fe, Ni)) with 54 at % Ni and kamacite (α-(Fe, Ni)) with 7 at % Ni, and the main NRM carrier is taenite. Tenham has a stable NRM. Tuxtuac mainly contains tetrataenite and taenite (the ratio tetrataenite : taenite is 15 : 85,T. NAGATA et al., Mem. Natl Inst. Polar Res., Spec. Issue, 41,364,1986) and they both contribute to NRM. The NRM of Tuxtuac is rather unstable magnetically. The main magnetic minerals of the Willard chondrite are the same as those of Tenham, i. e. taenite with 54 at % Ni and kamacite with 7 at % Ni. The main NRM carrier is taenite. Willard has an unstable NRM. Forrest is suggested to have experienced some alteration, on the basis of the analysis of the Js-T curve, and may have acquired a secondary NRM of probable terrestrial origin

Magnetic properties and natural remanent magnetization of carbonaceous chondrites containing pyrrhotite

Magnetic properties, NRM characteristics and magnetic minerals of four carbonaceous chondrites, the Allende, the Leoville, Y-74662 and Y-81020,are examined. These C-chondrites contain ferrimagnetic pyrrhotite grains in addition to magnetite, kamacite and/or taenite as magnetic minerals possessing NRM. The low temperature NRM component which is possessed by ferrimagnetic pyrrhotite at temperatures below 300℃, indicates that the corresponding paleointensity (F_p) is around 1 Oe in order of magnitude. The high temperature NRM component possessed by magnetite and/or taenite is magnetic at temperatures below about 600℃, giving rise to F_p≲0.1 Oe. The kamacite magnetization contributes very little at temperatures below 770℃

Development of Autonomous Unmanned Aerial Vehicles for Geoscience in Antarctica and its potentiality

第3回極域科学シンポジウム/第32回極域地学シンポジウム 11月29日（木） 統計数理研究所 ３階セミナー

Magnetic properties of the mixtures of Fe-Ni alloys simulated to Y-74354, Y-74362 and Y-74190 chondrites

The magnetic properties of the mixtures of Fe-Ni alloys simulated to Y-74354,Y-74362 and Y-74190 chondrites have been investigated by magnetic measurements and the Mossbauer effect. It was explained from the martensitic transformation that the intensity of RM decreased after cooling at 77K in zero magnetic field and was enhanced by cooling in geomagnetic field. The phase changes were observed by the Mossbauer effect. The thermo-magnetic properties were also discussed