[SEM22-01] Nanolitic magnetic minerals in volcanic pyroclasts
Keywords:Nanolite, remnant magnetization, magnetic susceptibility, single domain, nanocrystals, volcanic rock
Nanoscale crystals are often included in the rhyodacitic groundmass matrix of the andesitic pyroclasts from Shinmoedake, which is commonly identified as typical volcanic “glasses” using an optical microscope and a scanning electron microscope with a tungsten filament. In almost all the pumice clasts, the minimum crystal sizes are typically 100 nm. In a dense juvenile fragment, the size of titanomagnetite crystals in the interstices of microlites ranges down to 10–20 nm. Fe-rich oxide spots with a diameter of 1–2 nm, which are most likely amorphous, are also observed. Fe-Ti oxide shows size gaps; that is, crystals with diameters of 2–10 nm and widths of 20–100 nm are absent, or their number densities are too low to be measured. We defined groundmass crystals with 30 nm–1 µm in width as “nanolites” and those smaller than 30 nm in diameter as “ultrananolites” (Mujin et al., 2017). In addition to magnetite, we found pyroxene with nanolite and ultrananolite sizes. The number densities of nanolites and ultrananolites of the Fe-Ti oxides were eight orders of magnitude greater than those of microlite. These observations show that nucleation of nanocrystals almost paused in the late stage of crystallization, whereas crystal growth was mostly continuous. The occurrence of nanoscale crystals is conspicuous in the basaltic pyroclasts. Basaltic ashes with real glass on an atomic scale comprise only 10% of the juvenile ashes corrected from the March 2015 ash plumes of the Aso volcano. The rest of the ashes are composed mostly of nanoscale crystals. Nanolites and ultrananolites are assumed to crystallize under high undercooling conditions in the shallow conduit after the crystallization of microlites.