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Volume 43, Issue 5
Displaying 1-13 of 13 articles from this issue
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J. Shaw, D. Tarling1991 Volume 43 Issue 5 Pages 323-324
Published: 1991
Released on J-STAGE: August 11, 2011
JOURNAL FREE ACCESSDownload PDF (443K) -
Minisamples versus Standard SizeM. J. AITKEN, L. J. PESONEN, M. LEINO1991 Volume 43 Issue 5 Pages 325-331
Published: 1991
Released on J-STAGE: September 14, 2010
JOURNAL FREE ACCESSPalaeointensities using 3×3mm mini-cylinders and a cryogenic magnetometer have been compared with those using 23×25mm standard cylinders and a spinner magnetometer. When samples satisfy the reliability criteria of both approaches there is good agreement but the proportion of samples rejected is higher in the case of minisamples. One possible reason is that the minisamples are more vulnerable to physico-chemical alteration during laboratory heating, because of their much higher surface to volume ratio.View full abstractDownload PDF (664K) -
D. WALTON1991 Volume 43 Issue 5 Pages 333-339
Published: 1991
Released on J-STAGE: September 14, 2010
JOURNAL FREE ACCESSA technique has been developed which largely eliminates the errors resulting from mineral alteration during heating. It has been used to measure the intensity of the earth's magnetic field from eight sites in the Eastern Mediterranean. The results show that the secular variation in intensity appears to be largely periodic with a single strong component in the power spectrum with a period of 1430 years. There appear to be no contributions from fluctuations with shorter periods. The scatter in the results is sufficiently large to permit limited archaeomagnetic dating with a precision of between one and about two hundred years between 900 BC and AD 400.View full abstractDownload PDF (800K) -
Graham J. SHERWOOD1991 Volume 43 Issue 5 Pages 341-349
Published: 1991
Released on J-STAGE: September 14, 2010
JOURNAL FREE ACCESSHoffman et al.'s multi-specimen variation of the Thellier palaeointensity method has been applied to a number of historically dated lavas from Etna (Sicily), Vesuvius (Italy), and the Canary Islands. The majority of experiments resulted in palaeointensity estimates which were in good agreement with values obtained by other palaeointensity methods and geomagnetic field models. A lava which produced an overestimate of the actual field value using the Rolph and Shaw method also failed to give a reasonable intensity with the Hoffman method. A Miocene lava from New Zealand which had given a very high field intensity using the Rolph and Shaw method gave a lower value with Hoffman, but it is uncertain whether this value is a better estimate of the actual field intensity. Detailed rock magnetic studies were performed on every sub-specimen of six Miocene lavas from the Vogelsberg (Germany): bulk susceptibility and IRM acquisition curves were measured, and the magnetic minerals were observed using reflected light microscopy. Changes in susceptibility were often observed, but they do not correspond with changes in TRM capacity as observed on the Arai plots. Very similar susceptibility versus heating temperature plots were observed from sister samples which had been used in modified Thellier experiments, suggesting that the majority of thermal alteration occurs on the first heating to a particular temperature. The IRM and optical observations showed no obvious trends which could be related to thermal alteration.View full abstractDownload PDF (940K) -
C. RADHAKRISHNAMURTY, S. D. LIKHITE, P. W. SAHASRABUDHE1991 Volume 43 Issue 5 Pages 351-355
Published: 1991
Released on J-STAGE: September 14, 2010
JOURNAL FREE ACCESSA prior knowledge of the domain states and composition of the magnetic grains in basalts could be helpful in planning palaeointensity investigations and this could be obtained by simple techniques. The behaviour of basalt samples containing different types of magnetic grains on heating and cooling is presented and the feasibility of estimating palaeointensity from such samples is discussed.View full abstractDownload PDF (557K) -
M. E. EVANS1991 Volume 43 Issue 5 Pages 357-361
Published: 1991
Released on J-STAGE: September 14, 2010
JOURNAL FREE ACCESSArchaeomagnetic measurements on a kiln thought to be operating at the time of the Pompeii disaster, 24th August 79 A. D., indicates a normal field direction with an archaeointensity value of 61 microtesla. The low error and precise date makes this an important reference point to which other less accurately dated results can be compared.View full abstractDownload PDF (483K) -
C. TANG, J. Y. ZHENG, D. J. LI, S. F. WEI, Q. Y. WEI1991 Volume 43 Issue 5 Pages 363-368
Published: 1991
Released on J-STAGE: September 14, 2010
JOURNAL FREE ACCESSThe Thellier method has been used to determine the total intensity of the geomagnetic field between 500 to 300 B. C. (Spring-Autumn dynasty) and 1763 to 1800 A. D. (Qing dynasty). The intensity of the field increased from -55μT to -82μT during the last 500 years B. C. followed by a decrease to 63μT sometime between 960 and 1279 A. D. It subsequently peaked at -71μT between 1279 and 1368 A. D. and has since been decreasing.View full abstractDownload PDF (450K) -
S. P. BURLATSKAYA, Z. A. CHELIDZE1991 Volume 43 Issue 5 Pages 369-373
Published: 1991
Released on J-STAGE: September 14, 2010
JOURNAL FREE ACCESS155 samples from 170 collected at 36 archaeological sites dating between the 15th and 1st century B. C. defines the variation in declination (±20°), inclination (±10°) and ancient intensity (±15-20μT) in Georgia. These new observations are consistent with earlier observations and, when combined with them, these errors are reduced to ±5°, ±4°, and ±5-6μT. The virtual dipole path shows an anticlockwise rotation during this period.View full abstractDownload PDF (351K) -
H. TANAKA, M. KONO1991 Volume 43 Issue 5 Pages 375-388
Published: 1991
Released on J-STAGE: September 14, 2010
JOURNAL FREE ACCESSPalaeointensity determinations, using Coe's version of the Thellier method, for 7 flows on Hawaii are used to illustrate some of the problems in palaeointensity determinations by this method. It is shown that the linearity of NRM-TRM is not an adequate criterion for acceptance, but that it is very powerful when combined with the PTRM test. Examples are given of the effect of chemical changes during heating, viscous remanence and the use of low field susceptibility measurements. The importance of multiple determinations from different samples from the same site is also demonstrated. The within site dispersion is some 10% although it is hoped to reduce this to 3%.View full abstractDownload PDF (1148K) -
J. BOCHNÍCEK, P. HEJDA, V. KROPÁCEK1991 Volume 43 Issue 5 Pages 389-393
Published: 1991
Released on J-STAGE: September 14, 2010
JOURNAL FREE ACCESSTRM is generated at the contacts of igneous and sedimentary rocks; it provides information about the paleointensity of the magnetic field at the time the igneous rock was formed. Using a plane model of the intrusion, the process of cooling inside and outside a lava formation is studied by numerical methods depending on the formation's thickness. The effect of crystallization heat on cooling is taken into account.View full abstractDownload PDF (445K) -
Palaeointensity Results from the Deccan Traps (India) and the Disko Lavas (Greenland)J. SHAW, G. J. SHERWOOD, A. E. MUSSETT, T. C. ROLPH, K. V. SUBBARAO, P ...1991 Volume 43 Issue 5 Pages 395-408
Published: 1991
Released on J-STAGE: September 14, 2010
JOURNAL FREE ACCESSThe lavas of the Deccan Traps (India) and Disko Island (West Greenland) were both erupted close to the Cretaceous-Tertiary (K-T) boundary. Ar-Ar dating has provided plateau dates of 67.1±1.1Ma and 67.4±0.4Ma for samples respectively near the base and near the top of the Deccan succession, and the uppermost flow of the Disco succession has a preliminary age of 64.7±1.4Ma. Palaeointensities have been obtained from orientated samples from 47 Indian and 38 Greenlandic sites, using the SHAW (1974) technique with the ROLPH and SHAW (1985) correction for laboratory alteration. The intensity values show considerable scatter, but the mean virtual dipole moment calculated for the Deccan is 4.2±0.3×1022Am2 and for Disko is 5.8±0.6×1022Am2 compared to the present value of about 8×1022Am2. Studies of the variation of low-field susceptibility between -196°C and 700°C shows that the proportion of samples containing multidomain magnetite is somewhat higher than in some younger Lavas, suggesting hydrothermal alteration, which could account for some of the scatter in the palaeointensity values.View full abstractDownload PDF (1208K) -
Poorna C. PAL1991 Volume 43 Issue 5 Pages 409-428
Published: 1991
Released on J-STAGE: September 14, 2010
JOURNAL FREE ACCESSThe correlation of long-term trends in the geomagnetic field's strength and polarity stability (PAL and ROBERTS, 1988) is re-examined using all the available palaeointensity data. The Phanerozoic VDMs (virtual dipole moments) decrease systematically with age, whereas the Precambrian estimates are comparable to the present field. This is ascribed here to a steady low-temperature oxidation of titanomagnetites which reduces the NRM (natural remanent magnetization) intensities in the initial stages. When this Phanerozoic decrease is modeled by an exponential trend, the palaeointensity data from igneous rocks and baked sediments are indeed found to be substantially weaker during the middle-late Cenozoic and middle Mesozoic intervals of frequent reversals than during the middle Cretaceous normal (N) and Permian reversed (R) superchrons of fixed polarity state. Assuming that polarity reversals are endemic to the geomagnetic dynamo behaviour, the examination of available insights from the kinematic geodynamo theory too favours a stronger field during a fixed polarity superchron than when reversals are frequent. The observational record is also taken to favour BRAGINSKII'S (1976) model-Z geodynamo, over BUSSE'S (1975) weak-field model, suggesting that a high reversal frequency implies a stronger toroidal field in the core than when reversals are infrequent. The long-term modulations in reversal frequency may therefore involve complimentary changes in the α-and ω-effects that perhaps reflect changes in core-mantle coupling.View full abstractDownload PDF (2801K) -
Masaru KONO, Masayuki HOSHI, Koji YAMAGUCHI, Yosuke NISHI1991 Volume 43 Issue 5 Pages 429-443
Published: 1991
Released on J-STAGE: September 14, 2010
JOURNAL FREE ACCESSWe have developed a fully automated spinner magnetometer which can operate, under the control of a personal computer, the measurement of remanence as well as thermal demagnetization of paleomagnetic samples. A fluxgate sensor and an electric furnace is housed within a three-layer magnetic shield equipped with a solenoid coil. The measurement of the three components of magnetization is made possible by sampling the magnetic field generated by the sample which is both rotating around and moving along an axis parallel to the axis of the magnetic shield. Thermal demagnetization is carried out in an electric furnace placed further away along the axis of translation but still inside the magnetic shield. The rotation and translation of the sample, the measurement of the magnetic field by a ring-core fluxgate sensor, heating and cooling of the electric furnace, and switching on and off of the solenoid magnetic field are all controlled by a personal computer. This instrument is ideally suited for performing paleointensity experiments by the Thellier method because a series of measurements after heating cycles with or without an applied field can be programmed and carried out automatically by computer control.View full abstractDownload PDF (1585K)
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