17:15 〜 19:15
[SEM16-P15] クロスラインレーザー法により沖縄トラフの深海底から採取された珪長質岩の古地磁気分析
キーワード：クロスラインレーザー法、古地磁気学、沖縄トラフ
Between September 15 and 29, 2023, a research cruise was conducted in the Okinawa Trough using the R/V Yokosuka and HOV SHINKAI6500 (YK23-16S cruise). We conducted oriented volcanic rock sampling from the seafloor during three dives (#1725, #1726, and #1731). A novel cross-line laser system was employed to ensure precise orientation of the samples (Furukawa et al., 2024, Tectonophysics), allowing for detailed paleomagnetic and rock magnetic analyses. This study aims to examine the relationship between vesicularity, cooling processes, and depositional mechanisms of submarine volcanic deposits, focusing on vesiculated rhyolite and well-vesiculated pumice.
Stepwise thermal demagnetization experiments were conducted on two types of volcanic rocks, vesiculated rhyolite (6K#1725-R02, -R04, -R06, -R11) and well-vesiculated rhyolitic pumice (6K#1726-R02, 6K#1731-R01), to determine a paleomagnetic directions and discuss the emplacement processes based on it
The results revealed that vesiculated rhyolites exhibited linear high temperature components, with their low temperature components considered to be a secondary magnetization. The distribution of high temperature components of all samples indicates no evidence of significant rotation or displacement during and after cooling. This suggests that the rocks solidified in situ, likely as part of lava flows or lava domes, maintaining their original orientation throughout the cooling process.
On the other hand, the well-vesiculated rhyolitic pumice of the 6K#1731-R01 displays linear low temperature components and high temperature components with random magnetic directions. In contrast, the pumice of 6K#1726-R02 has random magnetic directions at low temperatures although it shows a similar magnetic directional character to that observed in the pumice of 6K#1731-R01 at high and middle temperature ranges. Here, the pumice of 6K#1726-R02 was collected in a hydrothermal vent area. This suggests that the random magnetic directions of the low-temperature components of #1726-R02 were acquired by secondary hydrothermal alteration. Furthermore, if a pumice sustaining high temperature was ejected into water and underwent turbulent movement, its high temperature component would exhibit random magnetic directions. This means that the linear components observed in the well-vesiculated rhyolitic pumices are stable components acquired after settling on the seafloor, and the random high temperature component most likely reflects the turbulent movement during transport.
These findings indicate that vesicularity strongly influences the cooling and depositional processes of deep-sea volcanic deposits, and that paleomagnetic analysis results are consistent with rock textures. Furthermore, this study highlights the effectiveness of the cross-line laser system for collecting oriented volcanic rock samples from the deep-sea floor. The application of this method provides valuable insights into the eruption and emplacement history of submarine volcanic deposits and contributes to a deeper understanding of submarine volcanic activity and eruption mechanisms in the Okinawa Trough.
Stepwise thermal demagnetization experiments were conducted on two types of volcanic rocks, vesiculated rhyolite (6K#1725-R02, -R04, -R06, -R11) and well-vesiculated rhyolitic pumice (6K#1726-R02, 6K#1731-R01), to determine a paleomagnetic directions and discuss the emplacement processes based on it
The results revealed that vesiculated rhyolites exhibited linear high temperature components, with their low temperature components considered to be a secondary magnetization. The distribution of high temperature components of all samples indicates no evidence of significant rotation or displacement during and after cooling. This suggests that the rocks solidified in situ, likely as part of lava flows or lava domes, maintaining their original orientation throughout the cooling process.
On the other hand, the well-vesiculated rhyolitic pumice of the 6K#1731-R01 displays linear low temperature components and high temperature components with random magnetic directions. In contrast, the pumice of 6K#1726-R02 has random magnetic directions at low temperatures although it shows a similar magnetic directional character to that observed in the pumice of 6K#1731-R01 at high and middle temperature ranges. Here, the pumice of 6K#1726-R02 was collected in a hydrothermal vent area. This suggests that the random magnetic directions of the low-temperature components of #1726-R02 were acquired by secondary hydrothermal alteration. Furthermore, if a pumice sustaining high temperature was ejected into water and underwent turbulent movement, its high temperature component would exhibit random magnetic directions. This means that the linear components observed in the well-vesiculated rhyolitic pumices are stable components acquired after settling on the seafloor, and the random high temperature component most likely reflects the turbulent movement during transport.
These findings indicate that vesicularity strongly influences the cooling and depositional processes of deep-sea volcanic deposits, and that paleomagnetic analysis results are consistent with rock textures. Furthermore, this study highlights the effectiveness of the cross-line laser system for collecting oriented volcanic rock samples from the deep-sea floor. The application of this method provides valuable insights into the eruption and emplacement history of submarine volcanic deposits and contributes to a deeper understanding of submarine volcanic activity and eruption mechanisms in the Okinawa Trough.