17:15 〜 18:30
[SVC31-P11] Petrological characteristics of the Kawago-daira rhyolite lava, Izu Peninsula, Japan
キーワード：流紋岩、黒曜石、軽石、赤外顕微分光法、伊豆半島
The explosive and effusive rhyolitic eruptions were occurred in Kawago-daira in 3000 years ago1), which relates to the Amagi volcanic group in the Izu peninsula, Japan2). The eruptions began with pyroclastic density currents, which were followed by the lava effusion. The Kawago-daira lava is composed of obsidian and pumiceous lithofacies.
In this study, the whole-rock chemical compositions and water contents of the Kawago-daira lava were provided to reveal the development process of the lithofacies. All samples were collected from the distal part of the lava. Major and trace elements were determined by wavelength dispersive X-ray fluorescence spectrometry (WDXRF), and rare earth elements (REE) were determined by inductively coupled plasma mass spectrometry (ICP-MS). Water contents were estimated using micro-Fourier-transformed infrared spectroscopy (m-FT-IR). Thickness of double polished thin sections for m-FT-IR analysis were 108-208 µm. The absorbance at 1630 cm-1 (bending vibration of H2O molecule) and 3580 cm-1 (stretching vibration of OH group) was measured 25 times for groundmass of each sample. The water content was calculated from the absorbance, using the Lambert-Beer’s law. Molar absorption coefficients used are 55 L/mol・cm and 67 L/mol・cm for 1630 cm-1 and 3580 cm-1, respectively3).
The whole-rock SiO2contents of both lithofacies show 70.18-71.41 wt.%. In Harker diagrams and chondrite-normalized REE patterns, the results of pumiceous and obsidian lithofacies were indistinguishable from each other. A total amount of H2O molecules and OH groups ranges from 0.23 to 0.25 wt.% for pumiceous lithofacies and from 0.18 to 0.31 wt.% for obsidian lithofacies. While the water contents of both lithofacies appear to be consistent, it is needed to assess the amount of secondary water to evaluate the accurate water contents. We will discuss the development process of the lithofacies from the chemical characteristics and the textures in thin sections.
1) Shigeru Shimada, The Quaternary Research, 39, (2000) 151-164.
2) Konosuke Sawamura, EXPLANATORY TEXT OF THE GEOLOGICAL MAP OF JAPAN Scale1:50,000 SHUZENJI, Tokyo, 100, (1954) 34-37.
3) Yohei Yoshimura, Kazuhiko Kano, Daizo Ishiyama, Bulletin of the Volcanological Society of Japan, 57, (2012) 159-176.
In this study, the whole-rock chemical compositions and water contents of the Kawago-daira lava were provided to reveal the development process of the lithofacies. All samples were collected from the distal part of the lava. Major and trace elements were determined by wavelength dispersive X-ray fluorescence spectrometry (WDXRF), and rare earth elements (REE) were determined by inductively coupled plasma mass spectrometry (ICP-MS). Water contents were estimated using micro-Fourier-transformed infrared spectroscopy (m-FT-IR). Thickness of double polished thin sections for m-FT-IR analysis were 108-208 µm. The absorbance at 1630 cm-1 (bending vibration of H2O molecule) and 3580 cm-1 (stretching vibration of OH group) was measured 25 times for groundmass of each sample. The water content was calculated from the absorbance, using the Lambert-Beer’s law. Molar absorption coefficients used are 55 L/mol・cm and 67 L/mol・cm for 1630 cm-1 and 3580 cm-1, respectively3).
The whole-rock SiO2contents of both lithofacies show 70.18-71.41 wt.%. In Harker diagrams and chondrite-normalized REE patterns, the results of pumiceous and obsidian lithofacies were indistinguishable from each other. A total amount of H2O molecules and OH groups ranges from 0.23 to 0.25 wt.% for pumiceous lithofacies and from 0.18 to 0.31 wt.% for obsidian lithofacies. While the water contents of both lithofacies appear to be consistent, it is needed to assess the amount of secondary water to evaluate the accurate water contents. We will discuss the development process of the lithofacies from the chemical characteristics and the textures in thin sections.
1) Shigeru Shimada, The Quaternary Research, 39, (2000) 151-164.
2) Konosuke Sawamura, EXPLANATORY TEXT OF THE GEOLOGICAL MAP OF JAPAN Scale1:50,000 SHUZENJI, Tokyo, 100, (1954) 34-37.
3) Yohei Yoshimura, Kazuhiko Kano, Daizo Ishiyama, Bulletin of the Volcanological Society of Japan, 57, (2012) 159-176.