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

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 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

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

Paleomagnetic Investigation of McMurdo Sound Region, Southern Victoria Land, Antarctica

A total of 602 rock samples for paleomagnetism were collected from the McMurdo Sound region, southern Victoria Land, Antarctica. These collected samples, ranging from the Cambro-Ordovician to the Cenozoic age, were examined using several tests of rock magnetism to check the evidence of stability of natural remanent magnetization (NRM) and were then studied paleomagnetically as follows : The samples of the Cenozoic volcanic rocks have stable NRM. The position of Antarctica has been almost the same from the late Pliocene age up to the present. The position of the virtual geomagnetic pole (VGP) (11 normals and 3 reverses) from the Pliocene age to the present is within a polar cap area of about 30 colatitude. The sequence of lava eruption was determined by synthetic evidence from paleomagnetic, geological and geochronological data. The almost samples of Ferrar dolerite of Jurassic age have stable NRM. All these samples were magnetized to the normal polarity, and the calculated VGP positions were 45.3°S, 152.0°W for the Wright Valley, 47.0°S, 133.2°W for the Allan Hills and 68.6°S, 139.5°W for Mt. Fleming. These VGP positions are in reasonablagreement with other Jurassic VGPs from Antarctica. The most samples of Beacon Supergroup have stable NRM of parallel direction to that of Ferrar dolerite. These samples were remagnetized in the Jurassic age and the primary magnetization disappeared. However part of the samples has stable depositional remanent magnetization of the Permo-Triassic age. The direction of NRM is parallel to that of Ferrar dolerite. East Antarctica had no shift from the Permo-Triassic to the Jurassic and Australia must have been linked to Antarctica at least up to the Jurassic age. The basement complex of Cambro-Ordovician age of Wright Valley has a stable component of NRM. The directions of NRM distributed on a meridian gathered gradually to low in latitude by thermal demagnetization up to 500℃, however distributed in high latitude dispersed by that demagnetization. These characteristics are related to the Curie points of the samples. These rocks were heated to 500℃ in the Jurassic age by a hidden Ferrar dolerite body. Consequently primary magnetization was remagnetized for the samples which have a Curie point lower than 500℃ in the Jurassic age by a hidden Ferrar dolerite body, but survived for the samples included in magnetite grains. The obtained VGP positions of the Cambro-Ordovician age consistent with previous date of the same age in East Antarctica. The differences in the declination of the Cambro-Ordovician age from East Antarctica are consistent with an angular rifting of 15-20° having occurred subsequently with in East Antarctica, probably along the line of the Amery Ice Shelf and Lambert Glacier. From these viewpoints, following synthetic results were obtained : The reconstruction model of Gondwanaland by SMITH and HALLAM (1970) is available ; the southern continents and India were continuous large land mass until Jurassic age, and the break-up had begun after intrusion of dolerite in Gondwanaland at that time ; newly apparent polar-wander path for Antarctica was determined

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

Magnetic contaminations of small iron meteorites, Odessa and Gibeon

Small samples of Odessa iron meteorites and Gibeon iron meteorites were studied to assess any magnetic contaminations acquired after the meteorites reached the surface of the earth. Odessa showed a stable component of natural remanent magnetization (NRM) present during AF demagnetization up to 50mT. The NRM intensity decreased from one side to the other side with the exception of an interior sample. These variations in NRM might be acquired by artificial magnetic contamination, which is supported by the REM (the ratio of NRM to saturation isothermal remanent magnetization (SIRM)) value proposed by WASILEWSKI and DICKINSON (1998). The Gibeon\u27s subsample directions with stronger NRM intensities made one cluster and those weaker intensities clustered at a different site. According to the REM values, the more intense NRM\u27s might be the result of a magnetic contamination overprint. From these experimental subsamples analyses, from small iron meteorites, we find that the possibility of magnetic contamination must be considered during NRM analyses of meteorite magnetism

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℃

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