Magnetic properties of pyroclastic rocks from the later stage of the eruptive activity of Haruna Volcano in relation to the self-reversal of thermo-remanent magnetization

Focusing to the self-reversal of Thermo-Remanent Magnetization (TRM), we examined magnetic properties of 32 pieces of unoriented pumice samples, which were systematically collected from eight layers of pumice-fall and pumice-flow deposits at Haruna Volcano, Japan in the two stages of the eruption, i.e., Futatsu-dake stage (the 5th stage) and caldera-forming stage (the 4th stage). The magnetic behaviors of TRM well correlated with the primary chemical composition (TiO2-content or x (molecular fraction value of ilmenite in xFeTiO3-(1−x)Fe2O3)) of the hemoilmenite phenocrysts in the samples. That is, the samples of which chemical composition of hemoilmenite is TiO2 ∼ 30 wt% (x ∼ 0.582) showed various types of TRM including typical intense Self-Reversed Thermo-Remanent Magnetization (SRTRM), whereas the samples of which chemical composition of hemoilmenite is TiO2 ∼ 32 wt% (x ∼ 0.620) showed one type of TRM; weak intensity of self-reversed component of TRM. This result harmonizes with the well-known diagram by Uyeda, where the intensity of SRTRM of hemoilmenite (quenched) is an explicit function of the chemical composition, and only in the restricted narrow range of the chemical composition, intense SRTRM is acquired. Moreover, the hemoilmenite samples of which chemical composition is less than TiO2 ∼ 31 wt% (x ∼ 0.60) are capable to acquire various intensities of SRTRM on annealing according to its condition. In our TiO2-poorer (or smaller-x) samples, the variety in the behavior of TRM is well explained with this annealing effect in this compositional range.