磁性からみた蛇紋岩化度：北海道岩内岳超苦鉄質岩体を例として

摘要

This paper is intended to show the relationship between susceptibilities and degrees of serpentinizationof serpentinized peridotite. The susceptibility, magnetization and bulk density of 79 serpentinized peridotites weremeasured. Moreover, rock magnetic analyses, i.e., acquisition of IRM, thermal demagnetization of composite IRM,thermomagnetic analysis, and low-temperature magnetometry, were applied to selected samples obtained fromthe Iwanai-dake ultramafic rock body in Hokkaido, Japan. Samples with similar peridotite contents were chosento detect the serpentinization effects clearly. Results show that the magnetic carrier is mainly magnetite. Lineartrends fell between 0.1 % and 0.4 % of the predicted volume of magnetite when observed susceptibilities wereplotted against densities. The study results show that, if the magnetic carrier is magnetite, the relationship betweensusceptibility and density reveals the variation of serpentinization reactions. A significant spread of the data isapparent, but it remaines along each linear line of reactions. The volume of magnetite produced by serpentinizationof other ultramafic rock bodies is presumed to be similar in samples for which the magnetic carrier is magnetite.The different susceptibility is inferred to result from the volume of water reaction when these rock bodies comefrom the same peridotite series. The results suggest that comparable amounts of reacting water affect the ultramaficbodies. The water reaction was found to be an important approach to solving many tectonic problems. Therefore,we recommend that serpentinite, which has the same basic reaction should be used to elucidate tectonic problems.