Magnetic properties of hemoilmenite single crystals in Haruna dacite pumice revealed by the Bitter technique, with special reference to self-reversal of thermoremanent magnetization

In order to reconsider the mechanism of the self-reversal of TRM (SRTRM) of hemoilmenite in dacite pumice, magnetic properties of newly collected Haruna dacite pumice near the historical sampling site (more than 50 years ago) were investigated again. All Haruna dacite pumice samples collected showed SRTRM or at least partial SRTRM. This time, the methods of microprobe analysis and the Bitter technique which were not available 50 years ago, were added to examine hemoilmenite crystals. Contrary to the result which was first reported by Hoffmann and Fehr on the basis of the micromagnetic observation of hemoilmenite crystals in Pinatubo dacite pumice, our hemoilmenite crystals are considerably more homogeneous in TiO₂-content in a single crystal than those of Hoffmann and Fehr. Also, we could not find any “shell structure” with two phases having different magnetism and compositions in a single crystal of hemoilmenite in Haruna dacite pumice. We came to the conclusion that the intergrown shell-structure itself in Hoffmann and Fehr’s model may not be essential but only the coexistence of the intergrown two phases in a single crystal of hemoilmenite must be essential for the acquisition of SRTRM of hemoilmenite. Both models, one by Ishikawa and Syono and the other by Hoffmann and Fehr are basically similar to each other, regardless of the structure involved.