10:45 AM - 12:15 PM
[SEM15-P06] Preliminary SQUID microscopic analysis of remanent magnetization components recorded in Plio-Pleistocene sedimentary rocks distributed in the Boso Peninsula
Keywords:Paleomagnetism, Rock-magnetism, SQUID Microscope, Secondary magnetization
The Plio-Pleistocene strata are distributed on the Boso Peninsula, Chiba Prefecture, Japan. Because these formations retain stable paleomagnetic signals, paleomagnetic studies have been conducted extensively (e.g., Kotake et al., 1995). However, it has been pointed out that progressive thermal demagnetization (pThD) and progressive alternating field demagnetization (pAFD) alone do not sufficiently remove secondary magnetization and do not pass the reversal test (e.g., Okada et al., 2012).
Secondary magnetization is an obstacle to reconstructing detailed paleomagnetic variations and discussing deformation due to structural motion, therefore clarifying and isolating its details is a very important task in paleomagnetic studies. Recently, attempts have been made to remove secondary magnetization of sedimentary rocks by demagnetization methods combining ThD and pAFD (e.g., Okada el al., 2017; Konishi and Okada, 2020; Tanimoto et al. 2022). The secondary magnetization demagnetized in these studies is presumed to be chemical or (thermal) viscous remanent magnetization (CRM or VRM). Although the details are not known, the presence of greigite formed in the early sedimentary stage and Feroxyhyte [δ-Fe3+O(OH)] formed after terrestrialization of sedimentary layers has been indicated at the Terasaki outcrop on the Boso Peninsula as an example of magnetic minerals CRM acquired (Oda et al., 2022). Although there have been studies focusing on the demagnetization of secondary magnetization, there have been few cases in which the contribution of secondary magnetization to remanent magnetization has been evaluated by comprehensively examining the magnetic minerals responsible for the demagnetization, their form, and the origin of the secondary magnetization. Therefore, we investigated the possibility of quantitative evaluation by imaging the magnetization distribution of a sample surface using a SQUID microscope.
Up to the present, we have prepared thin sections of siltstone samples from the Kokumoto Formation of the Kazusa Group, the Hata Formation, and the Mera Formation of the Chikura Group for SQUID microscopic analysis using a heat-resistant resin (RO-8659; SANYU REC) with a glass transition temperature of 285°C for comparison of magnetization distributions before and after ThD at 250 to 300°C. In this presentation, the magnetization distribution on the sample surface at the time of thin section preparation (heat cure at 150°C) was mapped and imaged by SQUID microscopy, and the preliminary results are reported. In the future, we will try to isolate the components of the remanent magnetization and evaluate their contribution to the magnetization by performing ThD and remeasuring the sample.
References
Kotake, N., Koyama, M., Kameo, K. (1995) Magnetostratigraphy and biostratigraphy of the Plio-Pleistocene Chikura and Toyofusa Groups, southernmost part of the Boso Peninsula, central Japan. Journal of Geological Society of Japan 101:7 (in Japanese with English abstract). Konishi, T., Okada, M. (2020) A paleomagnetic record of the early Matuyama chron including the Réunion subchron and the onset Olduvai boundary: High-resolution magnetostratigraphy and insights from transitional geomagnetic fields. Progress in Earth and Planetary Science, 7:35. Oda, H., Nakazato, H., Nanayama, F., Harigane, Y. (2022) Matuyama–Brunhes geomagnetic reversal record and associated key tephra layers in Boso Peninsula: extraction of primary magnetization of geomagnetic fields from mixed magnetic minerals of depositional, diagenesis, and weathering processes. Earth Planets Space 74:80. Okada, M., Suganuma, Y., Haneda, Y., Kazaoka, O. (2017) Paleomagnetic direction and paleointensity variations during the Matuyama–Brunhes polarity transition from a marine succession in the Chiba composite section of the Boso Peninsula, central Japan. Earth, Planets and Space, 69:45. Okada, M., Yoshimi Tokoro, Y., Uchida, Y., Arai, Y., Saito, K. (2012) An integrated stratigraphy around the Plio-Pleistocene boundary interval in the Chikura Group, southernmost part of the Boso Peninsula, central Japan, based on data from paleomagnetic and oxygen isotopic analyses. Journal of Geological Society of Japan 118:2 (in Japanese with English abstract). Tanimoto, A., Okada, M., Hayashi, R. (2022) Magnetostratigraphy of the Mera Formation, Chikura Group, southernmost part of the Boso Peninsula. Abstract of Japan Geoscience Union Meeting 2022.
Secondary magnetization is an obstacle to reconstructing detailed paleomagnetic variations and discussing deformation due to structural motion, therefore clarifying and isolating its details is a very important task in paleomagnetic studies. Recently, attempts have been made to remove secondary magnetization of sedimentary rocks by demagnetization methods combining ThD and pAFD (e.g., Okada el al., 2017; Konishi and Okada, 2020; Tanimoto et al. 2022). The secondary magnetization demagnetized in these studies is presumed to be chemical or (thermal) viscous remanent magnetization (CRM or VRM). Although the details are not known, the presence of greigite formed in the early sedimentary stage and Feroxyhyte [δ-Fe3+O(OH)] formed after terrestrialization of sedimentary layers has been indicated at the Terasaki outcrop on the Boso Peninsula as an example of magnetic minerals CRM acquired (Oda et al., 2022). Although there have been studies focusing on the demagnetization of secondary magnetization, there have been few cases in which the contribution of secondary magnetization to remanent magnetization has been evaluated by comprehensively examining the magnetic minerals responsible for the demagnetization, their form, and the origin of the secondary magnetization. Therefore, we investigated the possibility of quantitative evaluation by imaging the magnetization distribution of a sample surface using a SQUID microscope.
Up to the present, we have prepared thin sections of siltstone samples from the Kokumoto Formation of the Kazusa Group, the Hata Formation, and the Mera Formation of the Chikura Group for SQUID microscopic analysis using a heat-resistant resin (RO-8659; SANYU REC) with a glass transition temperature of 285°C for comparison of magnetization distributions before and after ThD at 250 to 300°C. In this presentation, the magnetization distribution on the sample surface at the time of thin section preparation (heat cure at 150°C) was mapped and imaged by SQUID microscopy, and the preliminary results are reported. In the future, we will try to isolate the components of the remanent magnetization and evaluate their contribution to the magnetization by performing ThD and remeasuring the sample.
References
Kotake, N., Koyama, M., Kameo, K. (1995) Magnetostratigraphy and biostratigraphy of the Plio-Pleistocene Chikura and Toyofusa Groups, southernmost part of the Boso Peninsula, central Japan. Journal of Geological Society of Japan 101:7 (in Japanese with English abstract). Konishi, T., Okada, M. (2020) A paleomagnetic record of the early Matuyama chron including the Réunion subchron and the onset Olduvai boundary: High-resolution magnetostratigraphy and insights from transitional geomagnetic fields. Progress in Earth and Planetary Science, 7:35. Oda, H., Nakazato, H., Nanayama, F., Harigane, Y. (2022) Matuyama–Brunhes geomagnetic reversal record and associated key tephra layers in Boso Peninsula: extraction of primary magnetization of geomagnetic fields from mixed magnetic minerals of depositional, diagenesis, and weathering processes. Earth Planets Space 74:80. Okada, M., Suganuma, Y., Haneda, Y., Kazaoka, O. (2017) Paleomagnetic direction and paleointensity variations during the Matuyama–Brunhes polarity transition from a marine succession in the Chiba composite section of the Boso Peninsula, central Japan. Earth, Planets and Space, 69:45. Okada, M., Yoshimi Tokoro, Y., Uchida, Y., Arai, Y., Saito, K. (2012) An integrated stratigraphy around the Plio-Pleistocene boundary interval in the Chikura Group, southernmost part of the Boso Peninsula, central Japan, based on data from paleomagnetic and oxygen isotopic analyses. Journal of Geological Society of Japan 118:2 (in Japanese with English abstract). Tanimoto, A., Okada, M., Hayashi, R. (2022) Magnetostratigraphy of the Mera Formation, Chikura Group, southernmost part of the Boso Peninsula. Abstract of Japan Geoscience Union Meeting 2022.