5:15 PM - 6:45 PM
[SEM13-P11] Paleomagnetic study of the Cretaceous Yezo Group, Kotanbetsu area, Hokkaido, Japan -Estimation of magnetic properties and polarity of calcareous nodule samples-
Keywords:Yezo group, Santonian-Campanian boundary
When conducting paleomagnetic studies, it is necessary to identify primary magnetization components and secondary magnetization components resulting from thermal alteration or chemical reactions, and to selectively demagnetize them according to the purpose. Especially when older samples are the subject, they are expected to be composed of more complicated magnetization components, and it is difficult to separate characteristic residual magnetization components using general demagnetization methods.
This study focuses on calcareous nodule samples collected from the Upper Cretaceous Haborogawa Formation of the Yezo Group in the Kotanbetsu area. The Kotanbetsu section is expected to determine an important reversal boundary (Santonian-Campanian boundary) in the Upper Cretaceous deposits. However, standard progressive Alternating field demagnetization (AFD) does not work due to the presence of magnetic minerals with high coercivity components. Thermal demagnetization (TD) has the property that a small amount of primary magnetization is overwritten by the crystallization of new magnetic minerals and the acquisition of magnetization due to thermal alteration in a certain temperature range (around 250-350°C). Therefore, in this study, we attempted to extract polarity using hybrid demagnetization combining TD+AFD proposed by Okada et al. (2017, EPS).
First, TD was performed in 18 steps for one sample at each site to confirm the thermal alteration temperature. The thermal alteration temperatures varied from sample to sample. Among the contained magnetic minerals estimated from the results of the 3axial IRM-TD experiments, highly coercive components such as goethite and iron sulfide were considered to the cause of the alteration. Due to these high coercivity components, AFD and TD were predicted to be ineffective. As next step, we heated the sister specimens from several sites to the demagnetization step temperature immediately before the thermal alteration temperature (e.g., 325°C for sites where increased magnetization was observed at 350°C), selectively demagnetized the highly coercive magnetic particles, and then performed to measure using the 2G-SRM with stationary triaxial step AFD (12 steps). As a result, it was observed that new magnetization was acquired as the alternating magnetic field increased, and the primary magnetization was overlapped and became unextractable.
This unstable behavior due to AFD may involve multidomain (MD) grains. Primary magnetization is recorded by magnetic minerals that are similar to stable single-domain (SD) grains. In a sample dominated by SD-like grains, unstable magnetization components such as MD grains are demagnetized in the initial stage of AFD, and their subsequent behavior does not affect the extraction of primary magnetization components. However, for samples with only a few SD grains, the behavior of MD grains with respect to AFD may be responsible for the unstable demagnetization results (Anai and Oda, 2022, JpGU ). These results clearly indicate the need to develop a demagnetization method that takes into account the behavior of MD grains.
This study focuses on calcareous nodule samples collected from the Upper Cretaceous Haborogawa Formation of the Yezo Group in the Kotanbetsu area. The Kotanbetsu section is expected to determine an important reversal boundary (Santonian-Campanian boundary) in the Upper Cretaceous deposits. However, standard progressive Alternating field demagnetization (AFD) does not work due to the presence of magnetic minerals with high coercivity components. Thermal demagnetization (TD) has the property that a small amount of primary magnetization is overwritten by the crystallization of new magnetic minerals and the acquisition of magnetization due to thermal alteration in a certain temperature range (around 250-350°C). Therefore, in this study, we attempted to extract polarity using hybrid demagnetization combining TD+AFD proposed by Okada et al. (2017, EPS).
First, TD was performed in 18 steps for one sample at each site to confirm the thermal alteration temperature. The thermal alteration temperatures varied from sample to sample. Among the contained magnetic minerals estimated from the results of the 3axial IRM-TD experiments, highly coercive components such as goethite and iron sulfide were considered to the cause of the alteration. Due to these high coercivity components, AFD and TD were predicted to be ineffective. As next step, we heated the sister specimens from several sites to the demagnetization step temperature immediately before the thermal alteration temperature (e.g., 325°C for sites where increased magnetization was observed at 350°C), selectively demagnetized the highly coercive magnetic particles, and then performed to measure using the 2G-SRM with stationary triaxial step AFD (12 steps). As a result, it was observed that new magnetization was acquired as the alternating magnetic field increased, and the primary magnetization was overlapped and became unextractable.
This unstable behavior due to AFD may involve multidomain (MD) grains. Primary magnetization is recorded by magnetic minerals that are similar to stable single-domain (SD) grains. In a sample dominated by SD-like grains, unstable magnetization components such as MD grains are demagnetized in the initial stage of AFD, and their subsequent behavior does not affect the extraction of primary magnetization components. However, for samples with only a few SD grains, the behavior of MD grains with respect to AFD may be responsible for the unstable demagnetization results (Anai and Oda, 2022, JpGU ). These results clearly indicate the need to develop a demagnetization method that takes into account the behavior of MD grains.