11:00 〜 13:00
[SEM15-P10] High resolution monitoring of paleomagnetic experiments with reef limestones - Clarification of magnetic behavior of MD magnetic grains -
キーワード：多磁区粒子、SQUID顕微鏡、礁性石灰岩
Reef limestone has very week and unstable remanent magnetization. Based on our previous research, magnetite seems to be the main carrier of the weak magnetism for reef limestones, which is superposed by other magnetic minerals crystallized secondarily (e.g. hematite and/or goethite). Reef limestones are composed of coral and other calcareous fossils with weak magnetizations, and the samples are significantly heterogeneous. For this reason, standard magnetization measurement using superconducting rock magnetometer is quite difficult for some reef limestones. Against this background, reef limestone has been considered to be unsuitable for general paleomagnetic studies. On the other hand, reef limestone is known to be an excellent recorder of paleoenvironment, such as sea-level change and/or climate change. Thus, successful measurements on weakly magnetized reef limestones should provide rare opportunities of combined studies on paleomagnetism and paleoenvironments. In order to retrieve high sensitivity and reliable paleomagnetic and rock magnetic information from reef limestones at sub-millimeter scales, we conducted analyses using a scanning SQUID microscope (SSM) on a thin section with AF and reductive chemical demagnetizations (RCD).
Typical characteristic behavior of reef limestones during AF demagnetization (AFD) is the instability showing the inconsistencies among multiple trials at each demagnetization step. We considered the possibility that this could be caused by unstable domain wall movements of multi-domain (MD) particles with non-reproducible magnetizations against AFD. From the results of SSM, the particles showing non-reproducible magnetizations (possible MD) seem to be located in the cavities of the samples. To confirm the behavior of MD particles against AFD, AFD in X, Y, and Z axes, and that by tumbling (rotating in 3D along two axes) were applied at 10 mT and 20 mT, respectively. The results show that the presumed MD grains are more unstable against AFD by tumbling than static AFD. RCD was conducted between AFD at 10 mT and 20 mT trying to remove secondary magnetic minerals, while monitoring magnetic particles including MD grains by XRF analysis as well as SSM. The XRF analysis before RCD show that minerals rich in iron are concentrated on the fossil surface and on the walls of the cavities, indicating the presence of secondary magnetic minerals attached to the water channels in the reef limestones. Since the locations of MD grains identified are those where effective RCD is expected, it is assumed that they are secondary origin and could be selectively removed. XRF and SSM results after RCD suggest that some MD particles attached to the voids were removed by RCD. However, MD particles with larger sizes remained unremoved. The behavior of MD particles depending on their size needs to be studied in the future.
Typical characteristic behavior of reef limestones during AF demagnetization (AFD) is the instability showing the inconsistencies among multiple trials at each demagnetization step. We considered the possibility that this could be caused by unstable domain wall movements of multi-domain (MD) particles with non-reproducible magnetizations against AFD. From the results of SSM, the particles showing non-reproducible magnetizations (possible MD) seem to be located in the cavities of the samples. To confirm the behavior of MD particles against AFD, AFD in X, Y, and Z axes, and that by tumbling (rotating in 3D along two axes) were applied at 10 mT and 20 mT, respectively. The results show that the presumed MD grains are more unstable against AFD by tumbling than static AFD. RCD was conducted between AFD at 10 mT and 20 mT trying to remove secondary magnetic minerals, while monitoring magnetic particles including MD grains by XRF analysis as well as SSM. The XRF analysis before RCD show that minerals rich in iron are concentrated on the fossil surface and on the walls of the cavities, indicating the presence of secondary magnetic minerals attached to the water channels in the reef limestones. Since the locations of MD grains identified are those where effective RCD is expected, it is assumed that they are secondary origin and could be selectively removed. XRF and SSM results after RCD suggest that some MD particles attached to the voids were removed by RCD. However, MD particles with larger sizes remained unremoved. The behavior of MD particles depending on their size needs to be studied in the future.