11:00 AM - 11:15 AM
[SVC28-08] Significance of the nanolite in the black pumice of 2021 Fukutoku-Oka-no-Ba eruption
Keywords:Fukutoku-Oka-no-Ba, drift pumice, nanolite, XANES
Fukutoku-Oka-no-Ba (FOB) is an undersea volcano which is located at 24°17.1’N/141°28.9’E, ~5 km north-east to Minami-Ioto Island, south of mainland Japan. In August 13, 2021 (JST), an explosive eruption occurred and a large amount of pumice clasts were erupted, drifting westward and arriving at Nansei Islands in mid-October and subsequently at Kanto area in mid-November. The drifted pumice exhibited various characteristics in color and texture. The majority of the pumice clasts are highly-vesiculated gray pumice containing black materials, whose texture resembles “chocolate-chip cookie” (gray type), whereas some exhibited “black pumice” with the similar whole-rock composition to the gray ones. Petrographic study of Yoshida et al. (2022 Isl.Arc) showed that the groundmasses of the gray and black pumice consisted of vesiculated glasses of colorless-transparent and brown-transparent, respectively. Raman microscopy of the brown-glass yielded a broad peak at around 670 cm-1 that is attributed to (titano)magnetite nanolite (e.g., Caceres et al., 2021). The black and gray pumice that coexisted in a single clast show a clear textural boundary, where the gray and black pumice exhibited strongly elongated and rather spherical vesicles, respectively.
The present study shows more detailed characteristics of the nanolite-bearing and -free glasses and discusses their roles in the explosive 2021 FOB eruption.
Fe3+/Fe-total ratios of the colorless and brown-glass were determined by micro-XANES at BL4A of the Photon Factory, resulting in a similar range of approximately 25%.
Transmitted electron microscope (TEM) observation was performed on an area containing a sharp boundary between the colorless and brown glasses. Although no crystals were observed under an optical microscope, TEM observation detected very fine particles (nanolite) only in the domain of the brown glass, showing ~10 nm magnetite, ~50 nm possible biotite, and ~200 nm clinopyroxene.
We also performed a rhyolite-MELTS calculation using the glass composition and P-T conditions determined by Yoshida et al. (2022), and the fO2 values estimated from the Fe3+ ratio determined by XANES. We obtained spinel (magnetite) as a liquidus phase at ~1000 oC and 250 MPa when we assume a relatively hydrous conditions (~3 mass% H2O), and subsequently spinel and biotite at 890 oC and 250 MPa.
Yoshida et al. (2022) found magnesian olivine (~Fo92) from the black pumice and some black enclaves in the gray pumice, proposing that hidden mafic magma triggered the explosive 2021 FOB eruption. The new results of TEM, XANES, and phase equilibrium calculation proposed that the observed nanolite minerals can precipitate under the conditions of the proposed FOB magma reservoir conditions with considerable amount of water. This suggests that the injected mafic magma under the FOB magma reservoir provided not only heat, but also water to the trachytic magma of the FOB. As a result, the nanolite precipitation in the trachyte magma occurred and corresponding increase in magma viscosity could have enhanced the explosive 2021 FOB eruption.
The present study shows more detailed characteristics of the nanolite-bearing and -free glasses and discusses their roles in the explosive 2021 FOB eruption.
Fe3+/Fe-total ratios of the colorless and brown-glass were determined by micro-XANES at BL4A of the Photon Factory, resulting in a similar range of approximately 25%.
Transmitted electron microscope (TEM) observation was performed on an area containing a sharp boundary between the colorless and brown glasses. Although no crystals were observed under an optical microscope, TEM observation detected very fine particles (nanolite) only in the domain of the brown glass, showing ~10 nm magnetite, ~50 nm possible biotite, and ~200 nm clinopyroxene.
We also performed a rhyolite-MELTS calculation using the glass composition and P-T conditions determined by Yoshida et al. (2022), and the fO2 values estimated from the Fe3+ ratio determined by XANES. We obtained spinel (magnetite) as a liquidus phase at ~1000 oC and 250 MPa when we assume a relatively hydrous conditions (~3 mass% H2O), and subsequently spinel and biotite at 890 oC and 250 MPa.
Yoshida et al. (2022) found magnesian olivine (~Fo92) from the black pumice and some black enclaves in the gray pumice, proposing that hidden mafic magma triggered the explosive 2021 FOB eruption. The new results of TEM, XANES, and phase equilibrium calculation proposed that the observed nanolite minerals can precipitate under the conditions of the proposed FOB magma reservoir conditions with considerable amount of water. This suggests that the injected mafic magma under the FOB magma reservoir provided not only heat, but also water to the trachytic magma of the FOB. As a result, the nanolite precipitation in the trachyte magma occurred and corresponding increase in magma viscosity could have enhanced the explosive 2021 FOB eruption.