Eruption style and crystal size distributions: Crystallization of groundmass nanolites in the 2011 Shinmoedake eruption

Abstract

Crystallization of groundmass minerals may record the physicochemical conditions of magmatic processes upon eruption and is thus a topic of interdisciplinary research in the disciplines of mineralogy, petrology, and volcanology. Recent studies have reported that the groundmass crystals of some volcanic rocks exhibit a break in their crystal size distribution (CSD) slopes that range from a few micrometers to hundreds of nanometers. The crystals consisting of the finer parts of the break were defined as nanolites. In this study, we report the presence of nanometer-scale crystals down to 1 nm in the pyroclasts of the 2011 eruption of Shinmoedake, the Kirishima volcano group, based on field emission-scanning electron microscopy (FE-SEM) and transmission electron microscopy (TEM).We discovered a gap (hiatus) from ~100 to ~30 nm in the size distribution of pyroxene in a dense juvenile fragment of a vulcanian explosion. The pyroxene crystals ~20-30 nm on a diameter were ferroaugite (C2/c), while those a few hundred nanometers in width had a composite structure consisting of the domains of orthopyroxene (Pbca), augite (C2/c), and sub-calcic augite (C2/c). In high-angle annular dark-field scanning TEM images of the same sample, bright spots ~1-2 nm in diameter were recognized with a gap in size from ~10-20 nm titanomagnetite (Fd3m). They are presumed to have Fe-rich compositions, although their phases were too small to be determined. In addition, we found that crystals smaller than a few tens of nanometers for pyroxene and 100 nm for plagioclase did not exist or their number densities were too low for accurate determination. This indicates that there are practical minimum sizes of the crystals. These observations show that nucleation of the nanoscale crystals almost paused (froze) in the late stage of groundmass crystallization, possibly due to a decrease in undercooling, increase in interfacial free energy, and decrease in diffusivity in a dehydrated melt, whereas crystal growth was mostly continuous. In this paper, we introduce the novel term "ultrananolite," to refer to crystals smaller than 30 nm in diameter, and redefine "nanolite" simply as those 30 nm to 1 μm in width, complementing the size interval of crystals in volcanic groundmass smaller than microlites (1-30 μm). In the transient nucleation process, the presence of subcritical size clusters is required. The observed ultrananolite-sized particles might partly include subcritical clusters. The difference in the slope of CSDs, presence of gaps in size distribution, and minimum crystal size among the eruption styles of the 2011 Shinmoedake eruption may be interpreted by considering the difference in magma residence time and fragmentation pressure in the shallow conduit, and possibly the rewelding process in the crater.

The effect of initial H2O concentration on decompression-induced phase separation and degassing of hydrous phonolitic melt

“…Another driving mechanism for decompression-induced vesicle formation in hydrous melt might be heterogeneous nucleation at nanometer-sized Fe-oxide crystals (ultrananolites smaller than 30 nm and nanolites 30 nm to 1 µm as classified by Mujin et al 2017), as suggested by, e.g., Di Genova et al (2017a,b; and Shea (2017). Such nanolites are observed in natural volcanic products.…”

“…Post-eruptive Fe-oxide nanolites are observed in glass particles of welded tuff (Schlinger and Smith 1986;Schlinger et al 1988) and in natural obsidian glasses (Sharp et al 1996). Fe-oxide nanolites related to syneruptive processes are observed in dense juvenile fragments of lava derived from a 2011 sub-plinian andesitic-dacitic eruption of Shinmoedake Nakamura 2014 andMujin et al 2017). Fe-oxide nanolites at and near vesicle surfaces of Mt Etna ashes might be related to syn-and posteruptive processes (Barone et al 2016).…”

In situ observation of nanolite growth in volcanic melt: A driving force for explosive eruptions

“…At a different size scale, crystalline nanoparticles (usually referred to as nanolites) are increasingly being discovered in geological samples over a wide range of environments of Earth’s surface ( 28 ) and in quenched glasses from erupted volcanic rocks ( 29 – 31 ) as well as experimental run products ( 32 , 33 ) using transmission electron microscopy (TEM) and Raman spectroscopy. Mujin et al .…”

Degassing-induced chemical heterogeneity at the 2011-2012 Cordón Caulle eruption

“…Enclaves can be fractured and all have been observed to have uneven boundaries. Phenocrysts are rare, and light-coloured glassy bands are largely devoid of crystals whilst darker bands have higher proportions of oxides and pyroxenes <1 µm in Degassing-induced chemical heterogeneity at Cordón Caulle Paisley et al, 2019 size, or 'nanolites' as coined by Mujin et al [2017]. Needle-shaped microlites of plagioclase and pyroxene are <50 µm in length, if present.…”