Hemoilmenite as a carrier of SRTRM in dacitic pumice from Akagi, Ontake and Sambe Volcanoes, Japan

Hemoilmenite crystals (0.7>x>0.4, x: ilmenite molecular fraction in ilmenite-hematite solid solution series) in dacite pumice in Japanese arc volcanoes are common. However, only a small percentage of those pumices show classic self-reversed thermo-remanent magnetization (SRTRM), such as that first found in a Haruna dacite pumice. The dacite pumice samples from Akagi Volcano in central Japan, about 110 km NW of Tokyo (N36°33′, E139°12′), however, acquired an intense SRTRM like that of the Haruna Volcano samples. Bitter patterns of the oriented hemoilmenite crystals from both the Haruna and Akagi volcanoes indicate that ordered ferrimagnetic and disordered weak-ferromagnetic phases are inter-grown in relatively comparable amounts, with an irregular texture. This pattern is consistent with rapid quenching of these pumices. We conclude that not only the chemical composition (TiO2-content or x) of the hemoilmenite, but also the cooling speed or thermal condition on cooling control SRTRM-acquisition in hemoilmenite of appropriate composition.