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Volume 41, Issue 1
Displaying 1-12 of 12 articles from this issue
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T. Nishitani, Y. Otofuji1989 Volume 41 Issue 1 Pages 1-2
Published: 1989
Released on J-STAGE: August 11, 2011
JOURNAL FREE ACCESSDownload PDF (122K) -
A ReviewNaoji SUGIURA1989 Volume 41 Issue 1 Pages 3-17
Published: 1989
Released on J-STAGE: September 14, 2010
JOURNAL FREE ACCESSRecent studies on the measurements of magnetization at high temperatures are reviewed. Use of a second gradient magnetometer is the main advance in instrumentation. Direct observation of how remanent magnetization is acquired and/or lost by changing temperature provides strong evidence about the origin of thermo-remanent magnetization. According to such observation, Neel's single domain theory seems to be correct. Neel's multi domain theory (which is originally a two domain theory, but is frequently applied to grains with many domains), however, cannot explain the behavior of magnetization at high temperatures. The behavior of magnetization in pseudo single domain (psd) grains is strongly dependent on the grain size. It seems that the heterogeneity in the magnetic grains and the screening effect are the main effects which characterize the psd moment.
It should be emphasized that at high temperatures, there are still many interesting phenomena which have not been adequately described and/or explained. Some of these interesting phenomena are shown as a guide for future studies.View full abstractDownload PDF (1126K) -
Tadashi NISHITANI, Masaru KONO1989 Volume 41 Issue 1 Pages 19-38
Published: 1989
Released on J-STAGE: September 14, 2010
JOURNAL FREE ACCESSChanges in remanent magnetization and hysteresis parameters associated with low-temperature oxidation have been investigated using x=0.3, 0.5 and 0.7 synthetic titanomagnetite samples and natural submarine basalts. The intensity of remanence and the median destructive field (MDF) of TRM decreases with increasing oxidation. As oxidation proceeds, the intensity of CRM increases and its MDF decreases, except for x=0.7 titanomagnetite samples, which show maximum intensity and MDF around 0.6 of oxidation state (z). The intensity and stability of CRM become almost comparable to those of TRM in a high oxidation state. Saturation magnetization (Js) decreases with increasing oxidation and in a fully oxidized state Js is about 60% of initial values. The saturation remanence of titanomagnetite samples is almost constant until z-0.6 and tend to decrease as z increases further. The coercive force and coercivity of remanence decrease for x=0.3 and 0.5 samples monotonically with increasing z, whereas those for x=0.7 samples show maximum values for oxidation states of about 0.6. The correlation is not good between oxidation state z and the NRM intensity of submarine basalts, while the NRM becomes more stable with increasing low-temperature oxidation. Changes exhibited by DSDP samples against AF demagnetization show almost the same tendency for MDF of CRM for the x=0.7. However, other parameters are not always consistent with those of synthetic samples.View full abstractDownload PDF (1662K) -
Koichiro FUJIMOTO, Eiichi KIKAWA1989 Volume 41 Issue 1 Pages 39-64
Published: 1989
Released on J-STAGE: September 14, 2010
JOURNAL FREE ACCESSInfluences of hydrothermal alteration on titanomagnetites and magnetic properties of two pyroxene andesite lavas in the Noya geothermal area were investigated. The chemical compositions of titanomagnetites easily change in response to changes in environmental conditions. Most x-values lie between 0.25 and 0.35 for titanomagnetites without an indication of low temperature oxidation or hydrothermal alteration. Ti-migration due to high temperature hydrothermal alteration (>ca. 150°C) results in a decrease of the x-value down to 0.0, whereas loss of Fe due to low temperature oxidation characterized by maghemitization results in an increase of x-value up to 0.5. The x-values show a gradual decrease with increasing alteration temperature (maximum at up to about 300°C). Hydrothermally altered titanomagnetites characteristically contain small dots, blebs and lamellae composed of rutile and/or sphene. On the other hand, altered titanomagnetites at low temperature (<ca. 150°C) have nearly the same characteristics as low temperature oxidized titanomagnetite.
The main carrier of magnetization is relatively Ti-poor titanomagnetites. Magnetic susceptibility and the intensity of magnetization have lower values in intensely altered rocks because of the decomposition of magnetite into hematite and/or pyrite. Curie temperatures increase from 520 to 580°C with increasing alteration temperature.View full abstractDownload PDF (7284K) -
Yozo HAMANO1989 Volume 41 Issue 1 Pages 65-75
Published: 1989
Released on J-STAGE: September 14, 2010
JOURNAL FREE ACCESSLattice parameter measurements were made at high temperatures of up to 700°C on synthetic titanomagnetite samples with x values of 0.0, 0.3, 0.7, and 1.0. A high temperature X-ray camera was used for the measurement, where heating was made in vacuum. The variation of the lattice constants of the titanomagnetite series (x=0.0-1.0) can be approximated as d[A]=(8.395+8.5×10-5T[°C]+7.5×10-8T2)+(0.14-6.0×10-5T)x, which can be used to identify the chemical change of titanomagnetites and titanomaghemites during the heating of synthetic and natural samples.View full abstractDownload PDF (836K) -
M. FUNAKI, H. SAKAI, T. MATSUNAGA1989 Volume 41 Issue 1 Pages 77-87
Published: 1989
Released on J-STAGE: September 14, 2010
JOURNAL FREE ACCESSMagnetotactic bacteria (north seeking bacteria) have been used to identify the magnetic S pole of iron-nickel grains selected from St. Séverin LL6 chondrite. The results indicate that the bacteria are sensitive magnetic sensors which can be used to detect not only the S pole in the grains but also the directions of lines of magnetic force radiated from the grains. The magnetic coercive force and the stability of natural remanent magnetization can also be measured with the bacteria by applying a steady magnetic field. These methods can in principle be applied to terrestrial rocks having relatively strong natural remanent magnetization. Thus, the magnetotactic bacteria can give useful information for rock magnetism and paleomagnetism as a bio-magnetometer.
Combining the method of south seeking bacteria and Bitter pattern analyses using colloidal magnetite particles, complex magnetization structures on the surface of Fe-Ni grains from the St. Séverin meteorite have been revealed, which is important for an understanding the chondrite magnetism.View full abstractDownload PDF (2997K) -
Evidence for Secondary MagnetizationHayao MORINAGA, Hiroo INOKUCHI, Haruo DOMEN, Katsumi YASKAWA, Michihir ...1989 Volume 41 Issue 1 Pages 89-99
Published: 1989
Released on J-STAGE: September 14, 2010
JOURNAL FREE ACCESSThe Akiyoshi limestone group is an accreted reef complex originated on oceanic basaltic seamounts. The direction value of the natural remanent magnetization (NRM) of the Akiyoshi limestone is not adequate to deduce where the limestone was deposited, because the NRM seems to be secondary (a remagnetized value). The mean in situ direction, namely the remagnetized direction, of the limestone is 49.90° in declination and 61.1° in inclination (Fisher statistic parameters: k=90.0 and α95=4.1°) for fifteen sites with the precision parameter k>10.0. This mean direction is almost the same as the in situ directions of welded tuffs from four formations distributed around the Akiyoshi Plateau, which have been already reported by previous study. These welded tuffs have fission track ages between 85 and 94Ma. The remagnetized direction is also rather similar to the well-known paleomagnetic direction for Southwest Japan before about 15Ma. These correspon-dences suggest that the Akiyoshi limestone and the welded tuffs were remagnetized by some event after about 90Ma and before 15Ma. Results of thermal demagnetization and observation in thin sections of the limestone suggest that dominant magnetic particle is magnetite, and that the limestone may not have suffered high temperature (>500°C) alteration. This implies the presence of a low temperature chemical event.View full abstractDownload PDF (995K) -
Pitambar GAUTAM1989 Volume 41 Issue 1 Pages 101-117
Published: 1989
Released on J-STAGE: September 14, 2010
JOURNAL FREE ACCESSDetailed paleomagnetic investigation was carried out on 33 trachyte and trachyandesite rock samples collected at 5 sites from the Aulis volcanics in the Tansen area, the Lesser Himalaya, Nepal. Analysis of magnetic behavior by stepwise alternating field (up to 53mT) and then thermal demagnetization shows complex multi-component remanence composed of at least four components as follows: 1) Component A (D=001°, I=47°)—of recent field origin, 2) Component B (D=339°, I=32°)—interpreted as acquired during early Miocene, 3) Component C (D=330°, I=-49°)—probably acquired during Late Cretaceous, and 4) Component D, poorly defined, with southerly declination and downward inclination of about 55°—probably acquired prior the acquisition of component C. All these components show in situ grouping suggesting their secondary nature.
Thermomagnetic analysis showed double-Curie point curves indicating two major magnetic phases with Curie temperatures around 565°C and 360°C. These phases are interpreted as of Ti-poor magnetite and titanomaghemite, respectively, which were formed due to low-temperature oxidation of original titanomagnetite. Analysis of unblocking temperature spectra suggests that the 360°C phase contributes to the component B magnetization.View full abstractDownload PDF (1329K) -
Yo-ichiro OTOFUJI, Kazuhiko KATSURAGI, Hiroo INOKUCHI, Katsumi YASKAWA ...1989 Volume 41 Issue 1 Pages 119-135
Published: 1989
Released on J-STAGE: September 14, 2010
JOURNAL FREE ACCESSSixty-eight samples are collected from 24 sites on either side of the wing of the synclinal structure of the Choson and P'yongan Supergroups. Almost all specimens show a stable behavior with respect to the alternating field demagnetization and thermal demagnetization. The sedimentary sequences are normally magnetized and their directions of natural remanent magnetization (NRM) (D=-2.7°, I=58.1°, α95=5.2°) are along the present geocentric axial dipole field in situ. The fold test is negative at 99% probability level. This evidence indicates that the original NRM acquired during the formation of the sedimentary strata has been overcome by the new NRM which was acquired quite recently, probably in the Brunhes epoch. The new NRM is carried by hematite and pyrrhotite. These minerals have been produced in the sedimentary strata due to metamorphism which had occurred before the Brunhes epoch. We conclude that the remagnetization of the strata is not associated with the production of the magnetic carrier. Remagnetization may be attributed to the alignment of the fine grains of hematite and pyrrhotite within calcium carbonate rich water in the interstices among quartz or calcite grains.View full abstractDownload PDF (3112K) -
Haruaki ITO, Kanichi MOMOSE, Katsuyasu TOKIEDA, Yukio NOTSU1989 Volume 41 Issue 1 Pages 137-145
Published: 1989
Released on J-STAGE: September 14, 2010
JOURNAL FREE ACCESSA boring core of 41m in length and 3.2cm in diameter has been obtained from the Takase granite dated to between 51Ma and 41Ma by the K-Ar dating method. We preliminarily measured the remanent magnetization and examined magnetic minerals in the pilot samples of the core. Then, a section of the core from 32m to 41m in depth was cut into the form of cylinders 1.8cm high and 1.8cm in diameter. From the remanent magnetization of about 350 cylindrical samples obtained, an apparent magnetic polarity change was found to take place at short intervals at approximately 35.8m to 37.4m in depth. The interval between the normal polarity and reversed polarity states is about 1.6m in length, however, it is unknown whether it provides a true interval perpendicular to isothermal planes. On the other hand, the variation of remanent intensity during this polarity transition had no connection with a change in inclination of the remanent magnetization, but it was significantly concordant with the variation of intensity of the saturation magnetization.View full abstractDownload PDF (584K) -
Takaharu SATO, Kazuo KOBAYASHI1989 Volume 41 Issue 1 Pages 147-159
Published: 1989
Released on J-STAGE: September 14, 2010
JOURNAL FREE ACCESSClose correlation of the variations in saturation isothermal remanent magnetization (SIRM) is found between two deep-sea sediment cores with different sedimentation rates, which were sampled 500km apart from each other. The clear correlation of the variations of the natural remanent magnetization (NRM) to SIRM in the relative inter-core time scale obtained from the SIRM correlation, and the rock magnetic diagnosis for the sediments, indicate that the variations probably reflect secular variation of the geomagnetic field intensity in almost the whole length of the Quaternary period. Long period secular variation of declination ranging from 0.02My to the order of 0.1My is revealed from a comparison of the relative declination between the cores in the early Brunhes epoch. The width of magnetizing zone, represented by half fixing depths, is estimated by comparing deconvoluted variations in declination and NRM/SIRM.View full abstractDownload PDF (987K) -
Hajimu KINOSHITA, Naoko MATSUDA1989 Volume 41 Issue 1 Pages 161-173
Published: 1989
Released on J-STAGE: September 14, 2010
JOURNAL FREE ACCESSDecay of a magnetic anomaly pattern to the north of the Nankai Trough, off Shikoku, Japan is discussed based upon studies of the seismic structure of the oceanic crust of the region. A simple subduction model of the lithosphere with a temperature gradient in the crust to reduce the resultant amplitude of the total magnetic field anomaly on the sea surface is not sufficient to explain the large decay rate of the total force anomaly versus landward distance from the Trough axis. Assumptions are made that some kind of destruction of the topmost part of the magnetic basement is causing the rapid decay. Effects of the geometrical disordering of the magnetic layer to reduce the resultant field intensity is studied by model experiments. Several models were tested to conclude that the destruction of the topmost part (ca. 1km) of the oceanic basement, the magnetic source of the anomaly field, may affect the decay rate in two ways; i. e., randomization of natural remanent magnetization as well as the subduction of the rest of the nondisturbed magnetic basement. Subduction of the Philippine Sea plate along the Nankai Trough area and strong friction may enhance the destruction of the magnetism. Some of the present-day destruction and subsequent dismembering of the part of the oceanic basement seem to be occurring at intermediate depths between the crust and subducting lithosphere.View full abstractDownload PDF (1194K)
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