[S-EM16] Geomagnetism, paleomagnetism, and rock magnetism

[SEM16-P11] Evaluation of Remanent magnetization and Paleointensity of 300-800 Ma igneous and plutonic rocks in the Zavkhan Terrane, Mongolia

*Chisato Anai¹, Hirokuni Oda², Noriko Hasebe³, Hitoshi Hasegawa⁴ (1.Aso Volcanological Laboratory, Kyoto University, 2.Geological Survey of Japan, 3.Institute of Nature and Environmental Technology, Kanazawa University, 4.Faculty of Science and Technology, Kochi University)

Keywords:Paleointensity , Mongolia, Tsunakawa-Shaw method

The purpose of this study is to conduct fundamental research to evaluate the remanent magnetization of igneous and plutonic rock samples from 500-800 Ma in multiple data and to estimate the timing of the inner core growth of the Earth. According to the recent paleomagnetic studies (e.g. Zhou et al., 2024), the inner core began to form 400-570 Ma. The timing of the growth of the inner core estimated in the previous studies coincides with major events in Earth history, such as the third “Snowball Earth” (approximately 600 Ma) and the transition from the extinction of the Ediacara fauna (multicellular mollusks: 530-570 Ma) to the Cambrian explosion. Although it is still unclear whether these events are closely related to the inner core growth, an accurate estimation of the timing of the inner core growth is extremely important for the history of the Earth, especially for the global environment history. To estimate the timing of the growth of the inner core from paleomagnetic studies, it is important to evaluate the reliability of the record. In this study, we try to extract remanent magnetization from igneous and plutonic rocks in the Zavkhan Terrane, Mongolia with age of 300-800 Ma, and evaluate the acquisition mechanism and its timing. In addition, we will estimate the paleointensity and discuss the strength of the Earth's magnetic field.
The samples were approx. 770 Ma Granite (Bold et al., 2016) and approx. 446 Ma Rhyolite (Kilian et al.,2016). Rock magnetic and paleomagnetic orientation measurements and paleointensity analysis were performed on these pilot samples. Magnetic mapping using a SQUID microscope at the AIST was also conducted to confirm the magnetic properties for the extraction of paleointensity data.
The results of magnetic mapping by SQUID microscopy and hysteresis measurements indicate that the granite of 770 Ma contains many multi-domain (MD) particles, and the magnetic minerals are heterogeneous in size and magnetic domains. In contrast, the 446 Ma Rhyolite is relatively homogeneous and consists of single domain (SD) to pseudo single domain (PSD) grains. Progressive AF demagnetization and progressive thermal demagnetization results also show that granite has at least three components, whereas rhyolite records two components. The IZZI-Thellier method and Tsunakawa-Shaw method were applied to the pilot samples for paleointensity measurements. The results for granite (770 Ma) were approximately 3.3 μT for the IZZI-Thellier method and 3.0 μT for the Tsunakawa-Shaw method, which are harmonic values. For Rhyolite (446 Ma), the IZZI-Thellier method and Tsunakawa-Shaw method yielded relatively similar values of approximately 12.5 μT and 17.6 μT, respectively. However, these values do not satisfy some of the selection criteria employed by the respective methods. This appears to be due in part to the multiplicity and heterogeneity of the magnetic domains of the magnetic particles contained in the samples. In the future, it is necessary to investigate a method to make selective paleointensity measurements on stable SD-like magnetic particles from magnetic mapping with SQUID microscopy.
Acknowledgements
This study was supported by JSPS KAKENHI Grant No. 23K13188 and 21H04523. We also thank Prof. Davaadorj Davaasuren, Prof. Niiden Ichinnorov, Dr. Shuukhaaz Ganbat and local drivers for their assistance in collecting samples in Mongolia.