RESUMO
Nanometer-sized crystals (nanolites) play an important role in controlling eruptions by affecting the viscosity of magmas and inducing bubble nucleation. We present detailed microscopic and nanoscopic petrographic analyses of nanolite-bearing and nanolite-free pumice from the 2021 eruption of Fukutoku-Oka-no-Ba, Japan. The nanolite mineral assemblage includes biotite, which is absent from the phenocryst mineral assemblage, and magnetite and clinopyroxene, which are observed as phenocrysts. The boundary between the nanolite-bearing brown glass and nanolite-free colorless glass is either sharp or gradational, and the sharp boundaries also appear sharp under the transmitted electron microscope. X-ray absorption fine structure (XAFS) analysis of the volcanic glass revealed that the nanolite-free colorless glass records an oxygen fugacity of QFM + 0.98 (log units), whereas the nanolite-bearing brown glass records a higher apparent oxygen fugacity (~ QFM + 2). Thermodynamic modelling using MELTS indicates that higher oxygen fugacities increase the liquidus temperature and thus induced the crystallization of magnetite nanolites. The hydrous nanolite mineral assemblage and glass oxygen fugacity estimates suggest that an oxidizing fluid supplied by a hot mafic magma induced nanolite crystallization in the magma reservoir, before the magma fragmentation. The oxidation-induced nanolite crystallization then enhanced heterogeneous bubble nucleation, resulting in convection in the magma reservoir and triggering the eruption.
RESUMO
Aging, change in property depending on the elapsed time from preparation, is known to affect the rheological behavior of various materials. Therefore, whether magma ages must be examined to characterize potentially widespread volcanic phenomena related to the transition from rest to flow. To achieve this, we performed rheological measurements and microstructural analyses on basaltic andesite lava from the 1986 Izu-Oshima eruption. The rheology shows an initial overshoot of shear stress during start-up flow that correlates with the duration and the shear rate of a pre-rest time. This indicates that the yield stress of magma and lava increases with aging. The microstructure shows that original aggregates of crystals, which may grow during crystallization, coalesce during the pre-rest period to form clusters without changing the crystal volume fraction. We conclude that the clusters are broken by shear in the start-up flow, which induces the stress overshoot. Thus, aging in magma rheology will impact the understanding of dynamic flow.