10:45 AM - 12:15 PM
[SEM15-P03] Preparation and magnetic mapping of thin sections for micro-magnetic analysis from Nishinoshima 2014-2015 lava
Volcanic rocks (historical lava) formed after 1900, when the International Geomagnetic Reference Field (IGRF) has been determined, are useful as "natural reference samples" to assess the reliability of absolute paleointensity (API) estimation methods. Nishinoshima Island, one of the Izu-Ogasawara Islands, has been particularly active in recent years, repeatedly forming new lava flows. Yamamoto et al. (2021 SGEPSS) are conducting API measurements using the "Tsunagawa-Shaw method" on whole rock samples cut from rock block samples collected from eight sites on the western shore of Nishinoshima Island for lava formed during 2014-2015. The average API estimated from all block samples was 41.3 μT with a standard deviation of 4.4 μT, concluding that the model field value of 41.7 μT at the Nishinoshima Island during 2014-2015 deduced from the IGRF-13 model (Alken et al., 2021) was estimated within about 10 percent accuracy. However, to further improve the accuracy of estimation, it is necessary to work on the development of API estimation methods that also focus on micro scales. Therefore, thin sections for micro-magnetic analysis (micromag thin section) were prepared from these block samples, and magnetic mapping was performed utilizing the SQUID microscope developed by Oda et al. (2016), which can measure the magnetic field on the sample surface with high sensitivity and spatial resolution of ~100 μm.
Magnetic mapping is done by scanning a magnetic field generated vertically from surface of a micromag thin section. Thus, to facilitate later analysis, it is necessary to prepare a micromag thin section by cutting in a direction perpendicular to the direction of natural remanent magnetization (NRM) of the sample to be measured. For this purpose, small cubic-shaped blocks of about 10 mm per side were first cut from adjacent samples of the same rock block after the NRM orientation was ascertained. Next, the small blocks were attached to a 20 mm square synthetic quartz glass plate using a cementitious adhesive with a heat resistance of 600 degC or higher so that the NRM orientation of the small block faces upward to the glass surface. Final polishing was performed to obtain a thickness of about 30 μm, and micromag thin sections were prepared. In these processes, we received assistance from the Thin Slice Preparation Room of the Geological Museum, Geological Survey of Japan, AIST. We are in the process of completing magnetic mapping of one NRM for each of the micromag thin section prepared from the block samples at the two sites. In this presentation, we will report on these results.
Magnetic mapping is done by scanning a magnetic field generated vertically from surface of a micromag thin section. Thus, to facilitate later analysis, it is necessary to prepare a micromag thin section by cutting in a direction perpendicular to the direction of natural remanent magnetization (NRM) of the sample to be measured. For this purpose, small cubic-shaped blocks of about 10 mm per side were first cut from adjacent samples of the same rock block after the NRM orientation was ascertained. Next, the small blocks were attached to a 20 mm square synthetic quartz glass plate using a cementitious adhesive with a heat resistance of 600 degC or higher so that the NRM orientation of the small block faces upward to the glass surface. Final polishing was performed to obtain a thickness of about 30 μm, and micromag thin sections were prepared. In these processes, we received assistance from the Thin Slice Preparation Room of the Geological Museum, Geological Survey of Japan, AIST. We are in the process of completing magnetic mapping of one NRM for each of the micromag thin section prepared from the block samples at the two sites. In this presentation, we will report on these results.