5:15 PM - 6:30 PM
[SVC30-P08] An attempt to detect volcanic materials of Ogasawara Ioto (Iwojima) Volcano from the core sample of the turbidite layers deposited in the Palace Bella Basin
Ogasawara Ioto (Iwojima) Volcano is a volcanic Island with a 10 km-width caldera, and intense post-caldera volcanic activity ongoing. There are many unclear points about the products deposited on the surrounding seafloor, and little is known about the entire history of volcanic development. Therefore, not so much information is available on the timing, scale, eruption style, and impact on the surrounding environment, which is necessary for evaluating the disaster potential of Ioto volcano.
We focused on existing seafloor core samples and examined whether products from Ioto could be confirmed. The subject is the piston core KR98-01 P1 collected in the Palace Bella Basin. This core sample was analyzed in detail by Ikehara et al.(1998) and Kawamura et al.(1999 ; 2002), and considered to alternating layers of the turbidite layer and the pelagic clay layer. The depositional age was estimated to be younger than 460,000 years ago by microfossil analysis. Since the turbidite layer contains large amounts of volcanic glass particles and calcareous microfossils even though it was a core sample located deeper than CCD, it was probable that pyroclastic materials and microfossils once deposited shallower than the CCD were reworked by turbidity current. From the examination of paleocurrent direction and the seafloor topography, it was considered that the source was the relatively shallower seafloor around Ioto volcano at the southern end of the Izu-Ogasawara arc, which is more than 300 km away from the core site. However, the origin of pyroclastic material particles has not been investigated in detail. On the other hand, volcanoes in Ioto and surrounding submarine volcanoes are known to produce more alkaline rocks than adjacent areas, and have properties that are advantageous for identifying source volcanoes based on petrological characteristics.
This time, we mainly report the results of the major element composition analysis of volcanic glass particles by SEM-EDS using the core samples stored in the Kochi Core Center, Kochi University / JAMSTEC. According to the analysis results, most of the particles are rich in alkali elements and range from trachyandesite to trachydacite. In all the SiO2 variation diagram of the measured elements, there is a part that overlaps with the composition range of the known Ioto volcanic products, and it is considered that particles originating from Ioto volcano are included in core samples. On the SiO2-Na2O diagram, there are two major composition trends. On the SiO2-Na2O diagram, there are two major composition trends. The Na2O-rich composition trend is more consistent with Ioto volcano, and the Na2O-poor trend is in the position slightly poorer in Na2O than the whole rock composition of Fukutoku-Okanoba volcano (Kato, 1988 ; Ossaka et al., 1990 ; Nakano and Kawanabe, 1992 etc.), and the corresponding volcano is unknown at present. Because each turbidite layers always contain particles that match both trends, it is thought that the direct origin is mixture of the products from Ioto and unknown volcanoes. In addition to the above, trace amounts of high-silica rhyolite glass particles similar to AT tephra have been confirmed from the pelagic clay layer, and there is a possibility that the depositional age can be further narrowed down.
The results of this study support the estimation of the source area by Ikehara et al.(1998) and suggest that large-scale mass transport were repeated in seafloor around Ioto area. It is considered that the components of each turbidite layer reflected the surface deposits in the area around the volcanoes at the time of the turbidity current had occurred, so by analyzing each layer in detail, temporal changes such as eruption magma properties might be obtained.
Acknowledgment : We thank JAMSTEC for permitting the use of the piston core samples. We also thank Dr. Toshio Hisamitsu and other staff members of the Kochi Core Center for support of collection of analytical samples.
We focused on existing seafloor core samples and examined whether products from Ioto could be confirmed. The subject is the piston core KR98-01 P1 collected in the Palace Bella Basin. This core sample was analyzed in detail by Ikehara et al.(1998) and Kawamura et al.(1999 ; 2002), and considered to alternating layers of the turbidite layer and the pelagic clay layer. The depositional age was estimated to be younger than 460,000 years ago by microfossil analysis. Since the turbidite layer contains large amounts of volcanic glass particles and calcareous microfossils even though it was a core sample located deeper than CCD, it was probable that pyroclastic materials and microfossils once deposited shallower than the CCD were reworked by turbidity current. From the examination of paleocurrent direction and the seafloor topography, it was considered that the source was the relatively shallower seafloor around Ioto volcano at the southern end of the Izu-Ogasawara arc, which is more than 300 km away from the core site. However, the origin of pyroclastic material particles has not been investigated in detail. On the other hand, volcanoes in Ioto and surrounding submarine volcanoes are known to produce more alkaline rocks than adjacent areas, and have properties that are advantageous for identifying source volcanoes based on petrological characteristics.
This time, we mainly report the results of the major element composition analysis of volcanic glass particles by SEM-EDS using the core samples stored in the Kochi Core Center, Kochi University / JAMSTEC. According to the analysis results, most of the particles are rich in alkali elements and range from trachyandesite to trachydacite. In all the SiO2 variation diagram of the measured elements, there is a part that overlaps with the composition range of the known Ioto volcanic products, and it is considered that particles originating from Ioto volcano are included in core samples. On the SiO2-Na2O diagram, there are two major composition trends. On the SiO2-Na2O diagram, there are two major composition trends. The Na2O-rich composition trend is more consistent with Ioto volcano, and the Na2O-poor trend is in the position slightly poorer in Na2O than the whole rock composition of Fukutoku-Okanoba volcano (Kato, 1988 ; Ossaka et al., 1990 ; Nakano and Kawanabe, 1992 etc.), and the corresponding volcano is unknown at present. Because each turbidite layers always contain particles that match both trends, it is thought that the direct origin is mixture of the products from Ioto and unknown volcanoes. In addition to the above, trace amounts of high-silica rhyolite glass particles similar to AT tephra have been confirmed from the pelagic clay layer, and there is a possibility that the depositional age can be further narrowed down.
The results of this study support the estimation of the source area by Ikehara et al.(1998) and suggest that large-scale mass transport were repeated in seafloor around Ioto area. It is considered that the components of each turbidite layer reflected the surface deposits in the area around the volcanoes at the time of the turbidity current had occurred, so by analyzing each layer in detail, temporal changes such as eruption magma properties might be obtained.
Acknowledgment : We thank JAMSTEC for permitting the use of the piston core samples. We also thank Dr. Toshio Hisamitsu and other staff members of the Kochi Core Center for support of collection of analytical samples.