Reliability of palaeointensity methods using alternating field demagnetization and anhysteretic remanence

Abstract

Using natural volcanic rocks which acquired thermoremanence (TRM) in known fields, reliability of various palaeointensity methods using alternating field (AF) demagnetization were evaluated. Natural remanence (NRM), TRM and anhysteretic remanences (ARM'S) before and after heating were stepwisely AF demagnetized following Shaw's method.It was found that the coercivity spectra of TRM and ARM in these samples are very similar, and that even when changes occurred during heating, the changes for two remanences are quite similar in many samples. Therefore, Shaw's method of palaeointensity determination, which incorporates ARM checks to the conventional comparison of NRM and TRM coercivity spectra, gives results as reliable as those obtained by the Thellier method. Many examples were demonstrated in which TRM and ARM intensities changed substantially by heating, but without changes in the shape of their coercivity spectra. Such changes cannot normally be detected and erroneous palaeointensities with apparent internal consistency would be obtained by usual AF demagnetization methods.Although ARM is quite similar to TRM, the rate of acquisition of ARM and TRM in weak fields varies by a factor of five among the present samples. To determine palaeointensities from the linear relation between ARM and TRM, it is necessary to determine experimentally the relative acquisition rate of these remanences. Therefore, methods based only on NRM-ARM relations would not give palaeointensities with acceptable errors.

Comparison between the Thelliers' and Shaw's palaeointensity methods using basalts less than 5 million years old.

“…It is already well known that ARM has characteristics similar to TRM, including the observation that ARM is proportional to the magnitude of the steady field applied and that a law of additivity of partial ARM applies (PATTON and FITCH, 1962). KoNo (1978) has made an investigation of the reliability of palaeointensity methods using alternating field demagnetization and ARM techniques. Using volcanic rocks whose TRM was acquired in known fields, he compared the results obtained by the Thelliers' and by Shaw's method.…”

“…Suppose that the slope of the NRM-TRM plot in Shaw's method is m1 and the slope of the corresponding ARM 1-ARM2 plot is m2, then following KoNo (1978), the palaeointensity Fa is given by Fa=lFI. (2) KoNo (1978) subdivided the response of samples to demagnetization of ARM as follows. Class 1 samples are those for thich the ARM 1-ARM2 relation is linear with gradient unity, Class 2 are those for which the ARM 1-ARM2 relation is linear but the gradient is not unity, and Class 3 are those for thich the ARM 1-ARM2 relation is non-linear.…”

K-Ar Ages and Paleosecular Variation of Direction and Intensity from Quaternary Lava Sequences in the Ruapehu Volcano, New Zealand.

“…Rolph and Shaw (1985) further developed this method to make the correction to ONRM/ATRM at each demagnetization step by AARM1/AARM2. All Ruapehu lava samples showed some change in ARM capacity as anticipated from the non-repeatable X-T curves, and the correction of Kono (1978) was applied throughout the experiment.…”

“…As some degree of rock magnetic alteration almost always occurs during heating, this criterion would give an extremely low success rate. To overcome this problem, Kono (1978) introduced a correction method in which the ratio of NRM/TRM was corrected by the ratio of ARM1/ARM2, supposing that the TRM capacity changes linearly with ARM capacity. Rolph and Shaw (1985) further developed this method to make the correction to ONRM/ATRM at each demagnetization step by AARM1/AARM2.…”

“…Paleointensity experiments, using mainly the Shaw method, were made on 15 lavas which showed good stability to thermal demagnetization. In the analysis of the Shaw results, the correction method of Kono (1978) was applied, since most samples showed changes in ARM capacity after heating. The double heating technique of Tsunakawa and Shaw…”