Identification of magnetic minerals carrying NRM in pyroclastic-flow deposits

The Ikeshiro pyroclastic-flow deposit at Yufu volcano, Kyushu in Japan, is a typical block-and-ash flow generated by collapse of the Ikeshiro lava dome erupted ca. 2000 years ago. Clasts in the Ikeshiro pyroclastic-flow deposit were found to have a low-temperature magnetic component that is parallel to the present Earth’s magnetic field. We carried out paleomagnetic and rock magnetic experiments in order to examine the origin of the component by identifying magnetic minerals. As a result, the clasts were classified into two types based upon magnetic mineralogy. Type A is carried by titanomagnetite (Tc=480–485°C), whose grain size is mainly MD. Type B is carried by Ti-poor titanomagnetite (Tc=500–580°C), hematite and titanohematite (Fe2−yTiyO3) with y≈0.5 (Tc=215–220°C). The low-temperature magnetic component of Type A was carried by titanomagnetite with very low coercivity, whereas that of Type B was principally carried by titanohematite. The viscous remanent magnetization (VRM) acquisition and demagnetization results indicated that titanomagnetite with low coercivity in Type A acquires VRM easily, whereas titanohematite in Type B hardly acquires VRM. Therefore, the low-temperature magnetic component of Type B originates not from VRM but from thermoremanent magnetization, whereas a part of the low-temperature component of Type A originates from VRM. Emplacement temperature of the Ikeshiro pyroclastic-flow deposit, which is estimated from the unblocking temperature of Type B samples, is between 220 and 400°C. The different magnetic mineral assemblage between the two types reflects the difference in oxidation state: samples of Type B are strongly oxidized, whereas those of Type A are not. Oxidation had probably occurred at the Ikeshiro lava dome before generation of the Ikeshiro pyroclastic flow.