[S-VC31] 火山噴火のメカニズム

[SVC31-06] Rock-magnetic characterization of fine-grained Fe-Ti oxides in volcanic rocks using the coercivity-unblocking temperature diagram below 300 K

*佐藤 雅彦^1,2、無盡 真弓³、中村 美千彦³ (1.東京大学、2.宇宙航空研究開発機構、3.東北大学)

キーワード：Rock-magnetism、Fe-Ti oxides、Microlite, nanolite, and ultrananolite、Shinmoedake 2011 eruption

Fine-grained crystals in groundmass of volcanic rock such as microlite, nanolite, and ultrananolite record the late-stage groundmass differentiation, and then, knowledge of these crystals give a critical constraint on the shallow magmatic processes (e.g., Mujin and Nakamura 2014, 2020; Mujin et al. 2017). Here we report newly developed technique to evaluate the fine-grained Fe-Ti oxides, which is one of the major constituents of the microlite, nanolite, and ultrananolite, using isothermal remanent magnetization (IRM) spectra in coercivity and unblocking temperature plane (hereafter referred to as coercivity-unblocking temperature diagram). Experimental procedures are as follows: (1) cooling in zero-field from 300 to 10 K, (2) imparting IRM at 10 K and a certain DC field, (3) demagnetizing the IRM during zero-field warming from 10 to 300 K, (4) repeating steps 1–3 with IRM at higher DC field, and (5) calculating second derivative of IRM on the coercivity and unblocking temperature plane. Three volcanic rock samples from a series of eruption events in the Shinmoedake 2011 eruption (a pumice of the sub-Plinian eruption, a pumice of the Vulcanian explosions, and a dense juvenile fragment of the Vulcanian explosions) were used for the coercivity-unblocking temperature diagram experiments. In the cases of the pumices of the sub-Plinian eruption and the Vulcanian explosions, the coercivity-unblocking temperature diagrams show the similar pattern, and two components are clearly recognized in these diagrams. The coercivity and unblocking temperature of these components are below 100 mT and at approximately 50 K, respectively. The rock-magnetic characteristics is consistent with the grain sizes and ulvöspinel contents of microlite and nanolite in the pumices (Mujin et al. 2017). By contrast, the dense juvenile fragment of the Vulcanian explosions shows a distinct high-coercivity component in addition to the components of microlite and nanolite. The unblocking temperature of the distinct component continuously distributes up to 100 K, while it lacks the low-temperature demagnetization due to the Verwey transition. Then, the high-coercivity component can be interpreted as the nearly stoichiometric magnetite with the grain size significantly lower than 100 nm. The interpretation is consistent with the mineralogical characteristics of ultrananolite revealed in the microscopic observations for the dense juvenile fragment (Mujin and Nakamura 2014, 2020; Mujin et al. 2017). Thus, the microlite, nanolite, and ultrananolite are successfully recognized on the coercivity-unblocking temperature diagram. On the basis of coercivity-unblocking temperature diagrams and additional magnetic measurements, we will discuss the detailed rock-magnetic characteristics of microlite, nanolite, and ultrananolite and its relation to the late-stage groundmass differentiation.