Petrography
The pumice raft from the 2021 FOB eruption consists mostly of gray pumice with a small amount of black and different colored (including amber and brown) pumice. Yoshida et al. (2022a) demonstrated that the black and brown pumice consist of brown glass with a magnetite nanolite Raman signature, with a peak at ~ 670 cm− 1. In contrast, the gray and amber pumice are composed of colorless and nanolite-free glass. Visible microlites have not been identified in the gray pumice, whereas the black pumice contains rare clinopyroxene and olivine microlites (Yoshida et al., 2022a). The black pumice occurs either as individual clasts or mingled with the gray pumice (Yoshida et al., 2022a, b).
The gray and black pumice often exhibit different textures (Fig. 1a; Yoshida et al., 2022a). Smaller, more elongated vesicles were observed in the groundmass of the gray pumice, whereas those in the black pumice were larger and more spherical (Fig. 1d, e). The major axes of the bubbles were estimated using ellipsoid fitting. Although large bubbles (> 500 µm) were identified in both types of glass, most bubbles in the colorless glass were < 50 µm. The mean lengths of the bubbles in the colorless and brown glass were 73 and 128 µm, respectively. The boundaries between the gray and black pumice varied: some clasts contained sharp boundaries between the brown (black pumice) and colorless (gray pumice) glass under the optical microscope (Fig. 1b), whereas others exhibited a gradual change from the brown to colorless glass (Fig. 1c). Phenocryst mineral assemblages are similar in both pumice types (clinopyroxene, plagioclase, and minor magnetite and olivine), and most minerals have similar compositions, except for those that likely originated in a mafic magma (Yoshida et al., 2022a, b). For example, two types of olivine are observed in the FOB pumice: one relatively Fe-rich (Mg# = molar Mg/[Mg + Fe] ~ 65) without compositional zonation and one with a high-Mg (Mg# ~ 90) plateau and decreasing Mg contents toward the rims (Yoshida et al., 2022a). The latter type of olivine is observed in or closely associated with the black pumice (Yoshida et al., 2022a, b).
We performed TEM analyses on the sharp boundary between the two glasses to identify the differences between the brown nanolite-bearing glass and the colorless nanolite-free glass (Fig. 1b).
TEM analysis
TEM analysis revealed three types of nanolite in the brown glass. In contrast, the colorless glass was crystal-free even at the scale of TEM analysis (Fig. 1f). The largest grains were clinopyroxene, with long axes of < 300 nm. In contrast, the abundant < 20 nm blocky grains were magnetite (Fig. 1g). Occasional tabular grains < 100 nm in length were observed, which yielded K, Al, and Mg EDS signatures, suggesting that they were biotite.
The magnetite nanolites were randomly orientated; however, the elongated clinopyroxene and biotite grains were weakly aligned (sub)parallel to the boundary between the brown and colorless glass. The solid phase was ~ 12 vol% of the sample based on the TEM image (Fig. 1g).
XANES analyses
Representative XANES spectra obtained by spot analyses of the colorless and brown glasses are shown in Fig. 2a–d with the calculated Fe3+/ΣFe ratios. The Fe3+/ΣFe ratios of the colorless and brown glasses were 0.24–0.28 (n = 4) and 0.31–0.36 (n = 8), respectively. In addition, the XANES spectra of the brown glass had a relatively sharp peak at ~ 7129.5 eV that can be attributed to the magnetite (Lerner et al., 2021). The presence of magnetite nanolites may invalidate the Fe XANES centroid energy used in the calibration of the Fe3+/ΣFe ratio; therefore, these values should be interpreted with caution. Although the true Fe3+/ΣFe ratio of the brown glass is uncertain, it should be noted that the brown glass is rich in Fe3+ and is more oxidized than the colorless nanolite-free glass.
2D XANES analysis also showed that the brown glass in the black pumice had higher Fe3+/ΣFe ratios than the colorless glass in the gray pumice (Fig. 2e).
MELTS modeling
The stable mineral assemblage for the FOB pumice composition was calculated using rhyolite-MELTS v.1.2.x model (Gualda and Ghiorso, 2015). The FOB pumice has a narrow range of whole-rock compositions, despite its appearance (Yoshida et al., 2022a). The whole rock composition of FOB-JMA-18 reported by Yoshida et al. (2022a) was used in the modelling.
The oxygen fugacity (fO2) of the colorless and brown glasses were calculated using the formula of Kress and Carmichael (1991), the composition of FOB-JMA-18, and the reported pressure and temperature of the magma reservoir (930°C and 250 MPa; Yoshida et al., 2022a). Under these conditions and with the measured Fe3+/ΣFe ratio, the log(fO2) values of the colorless glass relative to the QFM (quartz–fayalite–magnetite) buffer is QFM + 0.98. Although the XANES spectra of the brown glass includes a signal from magnetite nanolites, we use the estimated Fe3+/ΣFe ratio to calculate an apparent fO2 of QFM + 2.04 for the brown glass.
To model the appearance of nanolites and phenocrysts in the magma reservoir, we used a fixed temperature of 930°C and a pressure of 250 MPa and changed the fO2 and water content, as summarized in Fig. 3a. Magnetite is stable under all modelled conditions. Olivine, with a Mg# of ~ 60, was found to be stable only under reduced (QFM − 0.5) and wet (H2O = 6 mass%) conditions, whereas the other phenocryst minerals (clinopyroxene, plagioclase, and magnetite) were stable under more oxidized (QFM + 1.5 and + 2) conditions with relatively high water contents (5 mass%).
We also modelled the phase relationships with changing temperature and fO2 at a constant pressure of 250 MPa and a fixed water content of 5 mass% (Fig. 3b). The liquidus temperature reaches > 1100°C at QFM + 2, whereas more reduced conditions yield lower temperatures (< 1000°C). Biotite crystallizes at relatively high fO2 (> QFM + 0) and low temperatures (< 925°C).
To further evaluate the stability of biotite, we modelled variable pressures and fO2 at a constant temperature of 900°C and a water content of 5 mass% (Fig. 3c). At low pressures (< 150 MPa), H2O becomes saturated. Biotite becomes stable at higher pressures (> 100 MPa) and fO2 (> QFM + 1). The required oxygen fugacity for the biotite stability becomes lower at higher pressures.