Reliability of paleointensity method using ARM from volcanic rocks

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Reliability of the Shaw method of paleointensity determination from volcanic rocks was examined. An AF demagnetizing system with high performance of giving ARM was newly introduced.Samples from present-day lava were used to test correction methods involved in the Shaw method. Almost samples showed change in ARM capacity up to 20% after heating. Such samples were rejected in the orginal Shaw method in which only samples giving no change in ARM capacity were treated as successful. However, all the samples in this study gave true "paleointensities" if the apparent paleointensity was corrected according to the ratio of ARMs before and after heating. These results suggest that the correction methods using ARM ratio work as far as the lava samples used in this study are concerned.Shaw method was applied to 39 samples collected from andesite lava sequences at Ruapehu Volcano, New Zealand. Some samples were also examined by the Thellier method in which only thermal demagnetization was involved. Most samples showed some amount of change of ARM,and apparently reliable results were obtained after introducing the correction by ARM change. Although there is no absolute way to validate the obtained results, they showed very good within lava consistencies. Good agreement with the results from the Thelliers' method were also observed.Rock magnetic background was examined to understand this apparent success of ARM correction in the Shaw method. Some rock magnetic parameters such as initial magnetic susceptibility were compared with ARM intensities. More change in magnetic susceptibility than ARM intensity was usually observed after heating. This was accompanied by strong negative correlation of ratios of ARM and magnetic susceptibility. Simple interpretation is that the laboratory heating affects only larger magnetic grains which do not contribute to remanences.