[S-EM16] 地磁気・古地磁気・岩石磁気・環境磁気

[SEM16-P09] Paleointensity study of diorite form the subordinate tonalitic unit in the Tanzawa plutonic complex

*今木 陵太¹、佐藤 雅彦¹ (1.東京理科大学)

キーワード：古地磁気強度、閃緑岩、低温消磁二回加熱ショー法

Knowledge of long-term variations of geomagnetic field provides key insights into the evolution of interior dynamics. Numerical dynamo model predicts that an increase in the averaged core-mantle boundary (CMB) heat flow reduces the geomagnetic field dipole moment and increases the reversal frequency(Courtillot and Olson [2007]). A systematic change in the reversal rate has been observed over a timescale of 10⁸ years, corresponding to the expected change in the CMB heat flow associated with mantle convection. By contrast, change in the dipole moment for such a time scale has not yet reached a clear conclusion due to the difficulties in paleointensity measurements. Plutonic rocks could provide excellent records of long-term variations of geomagnetic field because of their extended cooling history of about 10^5-6 years. However, paleointensity studies of plutonic rocks are generally challenging due to the presence of multi-domain (MD) magnetite. In this study, we conducted the paleointensity measurements using diorite samples collected from subordinate tonalitic unit at the center of Tanzawa plutonic complex. We conducted paleointensity measurements using the double heating technique of the Shaw method combined with low-temperature demagnetization (LTD-DHT Shaw method). As a result, two samples passed the necessary criteria broadly used in the LTD-DHT Shaw method (Yamamoto and Tsunakawa [2005]), providing a time-averaged paleointensity of (11.2 ± 3.0) µT. A time-averaged virtual dipole moment (VDM) and virtual axial dipole moment (VADM) are calculated as (2.36 ± 0.63) × 10²² Am² and (3.56 ± 0.95) × 10²² Am², respectively, for a period about 4~5 Ma. This VDM value is smaller than those of the mid-Cretaceous normal superchron in previous studies(Kato et al. [2018]), and is consistent with the averaged VADM value over the past 5 million years(Yamamoto and Tsunakawa [2005]). Additionally, we plan to conduct the single-crystal paleointensity mesurement for the diorite sample. In this paper, we will discuss the paleointensity values at around 5 Ma on the basis of whole rock and single-crystal paleointensity measurements.