[S-VC31] Mechanism of volcanic eruptions

[SVC31-P01] Magma ascent and pyroclastic flow formation in 7.6 ka Mashu eruption :Insights from textural and synchrotron analyses of pumice deposit

*Kyohei Sano¹, Tetsuo Harada², Keiji Wada³, Eiichi Sato³ (1.Graduate School of Regional Resource Management, University of Hyogo, 2.Laboratory of Advanced Science and Technology for Industry, University of Hyogo, 3.Earth Science Laboratory, Course of Subjects Education, Graduate School of Education, Hokkaido University of Education at Asahikawa)

Keywords:pyroclastic flow, oxidation, pumice, texture

Pyroclastic flow has a huge influence on human society, and it is important to reveal the formation process. For the formation of pyroclastic flow, the stability of the eruption cloud is an essential factor¹. The entrainment of air into the eruption cloud originates the buoyancy because of the heating and thermal expansion of air. Such entrainment also causes the oxidation of pyroclasts. Thus, by analyzing the degrees of oxidation based on the Fe³⁺/Fe²⁺ ratio in volcanic products, we can reveal the temperature condition in volcanic clouds, and discuss the formation process of pyroclastic flow based on the temperature of volcanic clouds.
Approximately 7.6 ka Mashu Volcano eruption started as phreatomagmatic eruption (Ma-j) and followed Plinian falls (Ma-i, -h, -g) and pyroclastic flow (Ma-f)^{2, 3}, thus it is suitable to reveal the formation process of pyroclastic flow during eruption. In this study, we performed the textural and synchrotron analyses for the pumice from Mashu 7.6 ka activity and discuss the magma ascent and pyroclastic flow formation processes.
In this study, we conducted the field survey and collected samples. We sieved samples and picked up the pumices larger than 4 mm. Samples were washed by a sonic wave washer and dried at 110°C. For the synchrotron analysis, we used the powdered pumices. To determine the Fe³⁺/Fe²⁺ ratio of pumice samples, we collected the absorption spectra between 650 eV and 750 eV by using the standard materials at room temperature using the BL-10 beamline at the NewSUBARU, Laboratory of Advanced Science and Technology for Industry, University of Hyogo, Japan. The soft X-rays were focused on a 2.5 mm ×1.2 mm area. For the standard material, pumice from the 7.6 ka Mashu eruption and obsidian from Akaishiyama lava, Shirataki, Hokkaido, Japan were analyzed by wet chemical analysis at JFE Tecno-Research Corporation. Samples were mounted on the analytical stage by carbon tape.
In the spectrum, peaks around 709 eV and 723 eV reflect the L₃ and L₂ of Fe, respectively⁴. Furthermore, we identified the small peak around 713 eV, and we considered that this peak reflects the Fe³⁺ of L₃⁴. For the construction of the calibration curve to determine the Fe³⁺/Fe²⁺ ratio, we extracted the three Gaussian peaks on 709, 713, 723 eV, and collected the peak intensity and area of each Gaussian peak. Using the intensity and area values, we calculate the peak character as follows;

Peak character = (I₇₀₉ / I₇₁₃ ) × A₇₂₃ × A₇₁₃

where I is the intensity, and A is the area of each Gaussian peak. By compiling the peak character values and Fe³⁺/Fe²⁺ ratio determined by wet chemical analysis, we construct the calibration curve for Fe³⁺/Fe²⁺ ratio by using the absorption spectra.
In the Ma-i layer, Fe³⁺/Fe²⁺ ratio changes from 10.9 to 24.5 from lower to the upper Ma-i pumice. Fe³⁺/Fe²⁺ ratio in Ma-h layer ranges from 9.2 to 13.1, and 5.9 to 27.5 in Ma-g. Fe³⁺/Fe²⁺ ratio in pumice from pyroclastic flow (Ma-f) show 9.6. Furthermore, we analyzed the vesicle texture by EPMA and estimate the magma ascent rate. We discuss the magma ascent and formation process of pyroclastic flow based on the oxidation of pumice and the texture.

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