11:15 〜 11:30
[SVC30-09] Experimental evidence for the shallow origin of nanolite-bearing black pumice from the Fukutoku-Oka-no-Ba 2021 eruption
★Invited Papers
キーワード：ナノライト結晶化、福徳岡ノ場、浅部マグマプロセス、爆発的噴火、結晶化実験
Nanolite crystallization influences the physical properties of magma and, hence, controls eruption dynamics, drawing significant attention in recent years. Mujin and Nakamura (2014) demonstrated that the variation in nanolite mineral assemblage and crystal number density may record the differences in shallow magmatic processes. Di Genova et al. (2017) proposed that the crystallization of Fe-Ti oxide nanolites may increase melt viscosity and thus determine the compositional tipping point at which rhyolitic melts fluidize or stiffen and that separates effusive from explosive eruptions.
The pumices that erupted from the 2021 Fukutoku-Oka-no-Ba (FOB) eruption exhibit color variations without chemical composition differences, with Yoshida et al. (2022, 2023) attributing the black-colored pumice's appearance to the presence of nanolites. They proposed an eruption model in which nanolite crystallization triggered explosive activity at FOB after the intrusion of oxidized melt into the magma reservoir. However, the high nanolite number density observed in the black pumice requires a large undercooling, which is unlikely to occur in a magma reservoir. Furthermore, the basis of Yoshida's model relies on the thermodynamic calculation that assumes biotite nanolites crystallize under stable equilibrium conditions. This interpretation is inconsistent with a common petrological interpretation of groundmass biotite and amphibole to be crystallized during magma ascent as metastable phases. To solve these problems, we experimentally determined the nanolite crystallization conditions and revisited nanolite-bearing pumices' role in the FOB eruption.
We used the nanolite-free groundmass glass of white-colored pumice from the eruption as starting material. Firstly, a crystallization experiment was conducted with an externally heated pressure vessel at 200 MPa, corresponding to the assumed magma reservoir depth. We then conducted a series of experiments at lower pressures corresponding to the shallow conduit using the jacketed silica-glass tube methods at the conditions of 10 – 2 MPa, 900 – 600°C, and fo2 of NNO buffer. The experimental products were quenched and analyzed using FE-SEM and TEM-EDS.
In the magma reservoir conditions, microlite-sized clinopyroxene and plagioclase were crystallized with crystal number density (NA) of 4.2 – 9.2×103/mm2, which is by far lower than that of FOB nanolites (7.9×106 – 1.6×108/mm2), producing significantly different microstructures from the natural FOB pumices. Conversely, the low-pressure run products showed systematic variations of the nanolite crystallization structure with experimental parameters. The petrographic characteristics of the natural black pumice were best reproduced at 6 MPa and 800°C. Besides magnetite and pyroxene nanolites, we confirmed the presence of biotite nanolite with TEM-EDS. In addition, the NA of the experimental product, 5.7 (±0.68) ×107/mm2, matches the range of natural samples. These results show that the nanolite crystallization of the black pumices occurred in the shallow conduit, not in the magma reservoir, requiring total reconsideration of the model by Yoshida et al. (2022, 2023).
The pumices that erupted from the 2021 Fukutoku-Oka-no-Ba (FOB) eruption exhibit color variations without chemical composition differences, with Yoshida et al. (2022, 2023) attributing the black-colored pumice's appearance to the presence of nanolites. They proposed an eruption model in which nanolite crystallization triggered explosive activity at FOB after the intrusion of oxidized melt into the magma reservoir. However, the high nanolite number density observed in the black pumice requires a large undercooling, which is unlikely to occur in a magma reservoir. Furthermore, the basis of Yoshida's model relies on the thermodynamic calculation that assumes biotite nanolites crystallize under stable equilibrium conditions. This interpretation is inconsistent with a common petrological interpretation of groundmass biotite and amphibole to be crystallized during magma ascent as metastable phases. To solve these problems, we experimentally determined the nanolite crystallization conditions and revisited nanolite-bearing pumices' role in the FOB eruption.
We used the nanolite-free groundmass glass of white-colored pumice from the eruption as starting material. Firstly, a crystallization experiment was conducted with an externally heated pressure vessel at 200 MPa, corresponding to the assumed magma reservoir depth. We then conducted a series of experiments at lower pressures corresponding to the shallow conduit using the jacketed silica-glass tube methods at the conditions of 10 – 2 MPa, 900 – 600°C, and fo2 of NNO buffer. The experimental products were quenched and analyzed using FE-SEM and TEM-EDS.
In the magma reservoir conditions, microlite-sized clinopyroxene and plagioclase were crystallized with crystal number density (NA) of 4.2 – 9.2×103/mm2, which is by far lower than that of FOB nanolites (7.9×106 – 1.6×108/mm2), producing significantly different microstructures from the natural FOB pumices. Conversely, the low-pressure run products showed systematic variations of the nanolite crystallization structure with experimental parameters. The petrographic characteristics of the natural black pumice were best reproduced at 6 MPa and 800°C. Besides magnetite and pyroxene nanolites, we confirmed the presence of biotite nanolite with TEM-EDS. In addition, the NA of the experimental product, 5.7 (±0.68) ×107/mm2, matches the range of natural samples. These results show that the nanolite crystallization of the black pumices occurred in the shallow conduit, not in the magma reservoir, requiring total reconsideration of the model by Yoshida et al. (2022, 2023).