10:45 〜 11:00
[SEM16-07] クロスラインレーザーによる普遍的に活用可能な新たな試料定方位化の方法
★招待講演
キーワード：クロスラインレーザー、しんかい6500、定方位試料、古地磁気
Paleomagnetism has played a crucial role in unraveling processes such as plate motion and the emplacement of volcanic products. Paleomagnetic experiments rely on oriented samples, which are typically collected using methods such as clinocompasses with specialized tripod attachments or sun compasses. However, these methods have notable limitations. For instance, the size of the compass prevents the collection of small samples, and fragile samples may crumble when pressed with the compass. Additionally, magnetic compasses cannot be used near strongly magnetized rock samples. The sun compass method, on the other hand, requires direct sunlight near the outcrop to determine orientation, making it unsuitable in shaded or overcast conditions. Collecting oriented samples from the seafloor poses additional challenges. Various researchers have attempted to develop orientation methods for seafloor rocks using human-occupied vehicles (HOVs) and remotely operated vehicles (ROVs).
To address these challenges, we developed a rapid and straightforward technique for determining orientation using a cross-line laser system, which can be applied to both terrestrial and seafloor samples. This method is particularly effective for collecting small or fragile samples in terrestrial environments.
The cross-line laser system consists of two orthogonal line lasers. During sample collection, the center of the cross-line laser is aligned and focused on the target sample. The laser orientation, including the azimuth of a vertical line's trajectory and the tilt angle of the trajectory perpendicular to the vertical line, must be recorded for reconstruction of the sample orientation. The laser trajectory is then marked on the sample using a paint marker. For seafloor samples, trajectory is marked after retrieving the samples based on the photographs and movies taken during the sampling process. By combining the marked laser path with the recorded laser orientation data, researchers can accurately reconstruct the original orientation of the sample.
Using this method, we successfully collected oriented samples of pillow lavas and sediment cores from the seafloor, as well as lapilli from pyroclastic flow deposits and fragile small pumice from pyroclastic fall deposits in terrestrial environments. Paleomagnetic measurements conducted on these samples revealed paleomagnetic directions that are broadly consistent with the Earth's present-day magnetic field direction, aligning with the expected orientations of the samples. These findings demonstrate the effectiveness of the cross-line laser method in preserving sample orientation (Furukawa et al., 2024, Tectonophysics; Kanamaru and Furukawa, 2024, VSJ Fall Meeting; Kawamura et al., 2024, JpGU).
To address these challenges, we developed a rapid and straightforward technique for determining orientation using a cross-line laser system, which can be applied to both terrestrial and seafloor samples. This method is particularly effective for collecting small or fragile samples in terrestrial environments.
The cross-line laser system consists of two orthogonal line lasers. During sample collection, the center of the cross-line laser is aligned and focused on the target sample. The laser orientation, including the azimuth of a vertical line's trajectory and the tilt angle of the trajectory perpendicular to the vertical line, must be recorded for reconstruction of the sample orientation. The laser trajectory is then marked on the sample using a paint marker. For seafloor samples, trajectory is marked after retrieving the samples based on the photographs and movies taken during the sampling process. By combining the marked laser path with the recorded laser orientation data, researchers can accurately reconstruct the original orientation of the sample.
Using this method, we successfully collected oriented samples of pillow lavas and sediment cores from the seafloor, as well as lapilli from pyroclastic flow deposits and fragile small pumice from pyroclastic fall deposits in terrestrial environments. Paleomagnetic measurements conducted on these samples revealed paleomagnetic directions that are broadly consistent with the Earth's present-day magnetic field direction, aligning with the expected orientations of the samples. These findings demonstrate the effectiveness of the cross-line laser method in preserving sample orientation (Furukawa et al., 2024, Tectonophysics; Kanamaru and Furukawa, 2024, VSJ Fall Meeting; Kawamura et al., 2024, JpGU).