5:15 PM - 6:45 PM
[SEM13-P01] Magnetic Imaging of the Nishinoshima 2014 Lava using Scanning SQUID Microscopy and anisotropy of remanent magnetization in thin section
Keywords:paleomagnetism, SQUID microscopy, geomagnetism
The volcanic rocks are believed to acquire thermal remanent magnetization proportional to the Earth's magnetic field strength at the time of formation (cooling). Therefore, they serve as valuable samples for estimating the Absolute Paleo Intensity (API) of ancient geomagnetic strength. However, volcanic rocks with older formation ages often undergo alteration, imposing limitations on achieving reliable API estimates based on bulk measurements. To address this issue, a method is proposed in this study to select unaffected micro-regions and conduct API estimation specifically for these regions. To achieve this, a scanning SQUID microscope (Oda et al., 2016) from the National Institute of Advanced Industrial Science and Technology is utilized to perform magnetic mapping on thin section samples, aiming to develop a technique for API estimation from micro-regions.
The target sample is the 2014 lava from Nishinoshima, with a known magnetic field strength at the time of formation (41.7 μT). The API estimate from bulk analysis is reported as 41.3±4.4 μT (Yamamoto et al., 2021SGEPSS). Thin-section samples (30 μm thickness) for magnetic mapping have been prepared and the preliminary results of magnetic mapping are reported by Yamamoto et al. (2023JpGU). In this study, a stepwise Alternating Field Demagnetization (AFD) is applied to the Natural Remanent Magnetization (NRM) of the samples at 0, 10, 20, 40, 70, and 180 mT, followed by magnetic mapping using the scanning SQUID microscope at each step. Subsequently, following the latest API estimation protocol, the Tsunakawa method (Yamamoto et al., 2003), Anhysteretic Remanent Magnetization (ARM), and Thermoremanent Magnetization (TRM) are induced in the thin-section samples successively. Similarly, stepwise AFD was performed, and magnetic mapping is carried out at each step.
The obtained magnetic images assume a consistent uniform direction of magnetic moments for the entire dipole and distributions within the thin section, employing the method of Weiss et al. (2007) for inversion calculations through residual sum of squares minimization. The magnetization intensity distribution is then determined.
For each AFD step of NRM, ARM, and TRM, the calculated magnetization intensity distribution, particularly for NRM intensity, is confirmed to be consistent with the magnetization of bulk sample during stepwise AFD. As the magnetization of ARM and TRM is induced by a vertically applied magnetic field to the thin-section samples, an investigation of remanent magnetization anisotropy reveals that the influence of anisotropy cannot be ignored for achieving reliable API estimates. Further analysis and discussion of these results are carried out and reported.
The target sample is the 2014 lava from Nishinoshima, with a known magnetic field strength at the time of formation (41.7 μT). The API estimate from bulk analysis is reported as 41.3±4.4 μT (Yamamoto et al., 2021SGEPSS). Thin-section samples (30 μm thickness) for magnetic mapping have been prepared and the preliminary results of magnetic mapping are reported by Yamamoto et al. (2023JpGU). In this study, a stepwise Alternating Field Demagnetization (AFD) is applied to the Natural Remanent Magnetization (NRM) of the samples at 0, 10, 20, 40, 70, and 180 mT, followed by magnetic mapping using the scanning SQUID microscope at each step. Subsequently, following the latest API estimation protocol, the Tsunakawa method (Yamamoto et al., 2003), Anhysteretic Remanent Magnetization (ARM), and Thermoremanent Magnetization (TRM) are induced in the thin-section samples successively. Similarly, stepwise AFD was performed, and magnetic mapping is carried out at each step.
The obtained magnetic images assume a consistent uniform direction of magnetic moments for the entire dipole and distributions within the thin section, employing the method of Weiss et al. (2007) for inversion calculations through residual sum of squares minimization. The magnetization intensity distribution is then determined.
For each AFD step of NRM, ARM, and TRM, the calculated magnetization intensity distribution, particularly for NRM intensity, is confirmed to be consistent with the magnetization of bulk sample during stepwise AFD. As the magnetization of ARM and TRM is induced by a vertically applied magnetic field to the thin-section samples, an investigation of remanent magnetization anisotropy reveals that the influence of anisotropy cannot be ignored for achieving reliable API estimates. Further analysis and discussion of these results are carried out and reported.