11:45 〜 12:00
[SVC31-05] 苦鉄質火砕物中にみられるスピノーダル分解とナノ結晶
キーワード：スピノーダル分解、バイノーダル分解、ナノ結晶、ウルトラナノライト、苦鉄質、爆発的噴火
Ultrananolite, a <30 nm crystal with high number density, was first defined in the andesitic dense juvenile fragment [1]. Although some studies have reported that ultrananolites affect the eruption style during volcanic activity [2], the crystallization conditions and mechanism of ultrananolite are unclear. In the field of soft matter research, experimental research using triphenyl phosphite, which has a liquid-liquid transition, revealed that the crystal nucleation rate increased drastically once conditioned to near but above the spinodal temperature and then maintained at the crystallization temperature [3]. This is assumed to be due to the formation of so-called crystal precursors (the presence of critical-like order parameter fluctuations) in the liquid upon holding near the spinodal temperature [3]. In igneous rock petrology, liquid immiscibility has long been observed in basalt lava [4]. A recent study has reported that immiscible liquids occur in the compositional boundary layers of plagioclase in tholeiitic basalt lava [5]. In addition, nanoscale liquid immiscibility and nanolites have been observed in submarine basanitic lava [6]. In this study, to investigate ultrananolite crystallization, liquid immiscibility, and their relationship, we observed mafic pyroclasts whose cooling rate was faster than that of lava.
The samples in this study were ejected by various eruption styles in Japan: 32 scoriae from the Plinian Hoei eruption of Mt. Fuji (hereinafter “Hoei”); 6 scoriae from Nippana Shinzan which was produced by phreatomagmatic eruption in Miyake-Jima (hereinafter “Nippana”); 3 scoriae from Aso 2014 Strombolian eruptions (hereinafter “Aso”). The polished samples were observed using a field-emission scanning electron microscope. Based on the crystallinity, scoriae were divided into two types: high crystallinity and low to moderate crystallinity. In the high-crystallinity scoria type, some scoriae included immiscible liquid droplets, which may have been caused by binodal decomposition in the groundmass whose glass area was low due to the crystallization of plagioclase and pyroxene microlites. Because high-crystallinity scoriae are considered lava-like, we analyzed low-to-moderate crystallinity scoriae using Raman spectroscopy and transmission electron microscopy (TEM). Ultrathin sections were prepared for TEM using a focused ion-beam system. A high-crystallinity scoria-type was observed only in Hoei.
A bright film and <50 nm spots were observed at the interface of plagioclase in the backscattered electron (BSE) images. The film was on the plagioclase, and the spots were on the outside of the film. Scanning TEM-energy-dispersive X-ray spectroscopy mapping indicated that the blight film and spots were relatively Fe-rich. The size of the spots decreased with increasing distance from the plagioclase. Raman spectroscopy and TEM revealed the nanocrystals crystallized in some spots in Nippana and Hoei, whereas no crystals were observed in Aso, although the spot size in Aso was large.
Bright films and spots in the BSE images may be generated by spinodal decomposition in the compositional boundary layers of plagioclase, as mentioned in [5]. The Fe-rich areas may be structurally and compositionally easy to form crystals. In contrast, no nanocrystals were observed in the spots of Aso, even though the spot size was large. Spinodal decomposition is believed to enhance crystallization; however, an event that triggers crystallization may be necessary for the crystallization of nanocrystals. The conditions of spinodal decomposition and physical properties of the decomposed magma during magma ascent should be investigated in the future.
[1] Mujin et al., (2017) Am. Mineral., [2] Hajimirza et al., (2021) Nat. Commun., [3] Kurita and Tanaka (2019) PNAS, [4]Fujii et al., (1980) Jour. Geol. Soc. Japan, [5]Honour, V. et al., (2019) Nat. Commun., [6]Thivet et al (2023) Commun. Earth & Environ.
The samples in this study were ejected by various eruption styles in Japan: 32 scoriae from the Plinian Hoei eruption of Mt. Fuji (hereinafter “Hoei”); 6 scoriae from Nippana Shinzan which was produced by phreatomagmatic eruption in Miyake-Jima (hereinafter “Nippana”); 3 scoriae from Aso 2014 Strombolian eruptions (hereinafter “Aso”). The polished samples were observed using a field-emission scanning electron microscope. Based on the crystallinity, scoriae were divided into two types: high crystallinity and low to moderate crystallinity. In the high-crystallinity scoria type, some scoriae included immiscible liquid droplets, which may have been caused by binodal decomposition in the groundmass whose glass area was low due to the crystallization of plagioclase and pyroxene microlites. Because high-crystallinity scoriae are considered lava-like, we analyzed low-to-moderate crystallinity scoriae using Raman spectroscopy and transmission electron microscopy (TEM). Ultrathin sections were prepared for TEM using a focused ion-beam system. A high-crystallinity scoria-type was observed only in Hoei.
A bright film and <50 nm spots were observed at the interface of plagioclase in the backscattered electron (BSE) images. The film was on the plagioclase, and the spots were on the outside of the film. Scanning TEM-energy-dispersive X-ray spectroscopy mapping indicated that the blight film and spots were relatively Fe-rich. The size of the spots decreased with increasing distance from the plagioclase. Raman spectroscopy and TEM revealed the nanocrystals crystallized in some spots in Nippana and Hoei, whereas no crystals were observed in Aso, although the spot size in Aso was large.
Bright films and spots in the BSE images may be generated by spinodal decomposition in the compositional boundary layers of plagioclase, as mentioned in [5]. The Fe-rich areas may be structurally and compositionally easy to form crystals. In contrast, no nanocrystals were observed in the spots of Aso, even though the spot size was large. Spinodal decomposition is believed to enhance crystallization; however, an event that triggers crystallization may be necessary for the crystallization of nanocrystals. The conditions of spinodal decomposition and physical properties of the decomposed magma during magma ascent should be investigated in the future.
[1] Mujin et al., (2017) Am. Mineral., [2] Hajimirza et al., (2021) Nat. Commun., [3] Kurita and Tanaka (2019) PNAS, [4]Fujii et al., (1980) Jour. Geol. Soc. Japan, [5]Honour, V. et al., (2019) Nat. Commun., [6]Thivet et al (2023) Commun. Earth & Environ.