Salt shell fallout during the ash eruption at the Nakadake crater, Aso volcano, Japan: evidence of an underground hydrothermal system surrounding the erupting vent

The Aso volcano is one of the largest caldera volcanoes in the world, with the size of 25 km north–south and 18 km east–west diameter, and repeated gigantic pyroclastic flow eruptions from 270 to 90 ka. Several post-caldera cones, with compositions ranging from basalt to rhyolites, formed near the center of caldera after the last caldera-forming eruption (Ono and Watanabe 1985). The Nakadake cone is a stratocone with compositions ranging from basalt to basaltic-andesite and is the only active cone of the Aso volcano. During the past 6000 years, the dominant eruptive products at the Nakadake cone are black ash with some scoria, indicating that the foremost eruption style was that of continuous ash emission, that is called “ash eruption” (Ono et al. 1995). The Nakadake cone is currently quite active with repeated ash eruptions in 1974, 1979, 1984–1985, 1985–1995 and 2014–2015 (Ikebe et al. 2008; Ono et al. 1995; Yokoo and Miyabuchi 2015).

Another feature of the volcano is the hot and acid crater lake, located within the summit crater of the Nakadake cone. The crater lake is one of the hottest and most acidic crater lakes in the world, with water temperatures ranging from 40 to 80 °C and a pH level between − 1 and + 1 (Ohsawa et al. 2003, 2010; Miyabuchi and Terada 2009). The crater lake water submerges the crater bottom during a quiet period, and the water level decreases with an increase in the volcanic activity (Kawakatsu et al. 2000; Sudo et al. 2006). The bottom of Nakadake crater was filled with the crater lake water until 2009 (Shinohara et al. 2015), and the water volume decreased to almost zero in late 2011, when a few small phreatic eruptions occurred. The water volume recovered in 2012 and then decreased again to almost zero in late 2013 following several small phreatic eruptions. The crater bottom became hotter, and the incandescence was observed even in the daylight on November 18, 2014. The eruption started from the high-temperature vent on November 25, becoming more intensive with the enlargement of the vent (Yokoo and Miyabuchi. 2015). The eruption style was the continuous ash eruption with sporadic Strombolian activity, similar to past eruptions. The ash eruption culminated in late January, discharging ~ 2 × 10⁶ tons of ash until February 2, 2015 (Yokoo and Miyabuchi 2015; Marumoto et al. 2017). The eruption continued with some temporal pauses until May 21, 2015.

Various non-silicate (salt) fallouts were observed at the Aso volcano during periods when the crater lake had almost or completely disappeared. The salt fallouts were first identified in May 2011, when the crater lake had almost dried out and a brownish plume emitted from a high-temperature vent of 585 °C at the center of the crater bottom. A small amount of water remained, surrounding the high-temperature vent (Fig. 1a). The salt-rich materials of a flaky shape fell from the plume and were deposited on the flank. The fallouts are yellowish-gray flakes, several mm in width, hereafter referred to as “salt flakes” (Fig. 1b). Salt flakes are fragile and are for the most part water-soluble. The brownish color of the plume is likely induced by the salt flakes. The salt flakes were observed on May 18, 19, and 31, 2011, and June 6, of the same year by people visiting the crater rim on the leeward side of the plume. Although the fallouts were observed only on limited occasions, the salt flakes are likely released for 2 weeks since the emission of the brownish plume was continuously observed from mid-May to early June. Eruptions and significant ash falls were not observed during this period.

The salt fallouts of different shapes were observed several times during the eruptive period of 2014–2015. The salts were discharged with the ashes during an ash eruption on January 13, 2015 (Fig. 1c). The salt fallouts had a hollow yellowish-white-colored shell structure, with a maximum size of ~ 5 cm, hereafter referred to as “salt shell” (Fig. 1d). The shells were thin and fragile, commonly occurring round, elongated, or coalesced round shapes. On March 17 and 20, 2015, the salts were released from the gas plume approximately 1 h after the temporal cease of the weak ash eruption (Fig. 1e). These fallouts were hollow, fragile, spherical shells of a yellowish-white color, with a maximum size of ~ 1 cm (Fig. 1f). The shape and color of these salt shells were quite similar to the hollow sulfur spherules observed at several acid crater lakes such as Kusatsu-Shirane and Poás volcanoes (Ohashi 1919; Oppenheimer and Stevenson 1989; Takano et al. 1994). These salt shells, however, were for most part water-soluble and not sulfur spherules. They absorb moisture and were recrystallized into a denser structure after being stored in a box that was not sealed against the moisture (Fig. 1g). On March 26, 2015, similar dense salt materials (hereafter referred to as “salt lump”), a few cm in size, were found on the ground (Fig. 1h). These salt lumps look similar to the recrystallized salt shells. A volcanic bomb covered with salts of a white color (hereafter referred to as “white salt”) was found on the eastern flank, ~ 300 m from the vent, on December 27, 2014, after the occurrence of Strombolian activities (Fig. 1i). The salts covering the bomb were a layer of fine fibers, of a few mm in length (Fig. 1j). The bomb appeared porous and consisted of consolidated tuff without signs of a significant hydrothermal alteration. The bomb is less likely the product of the Strombolian activity, as it is likely to be a block of the non-juvenile lithics beneath the crater floor, which was ejected by the associated explosions. A bomb with the white salts was only found on this occasion.

All the salt materials, including the salt flakes, the salt shells, the salt lumps, and the white salts, were analyzed. In order to evaluate the origin of the salt materials, the composition of the crater lake waters and leachate of the ashes emitted by the ash eruptions was also analyzed. The water-soluble components were analyzed using ion chromatography, and the bulk composition was measured through energy-dispersive X-ray spectrometry (EDS). For the analyses of the water-soluble components, the salt materials were dissolved in pure water and filtered through a 0.45-µm membrane filter, diluted, and analyzed without any further pre-treatment. Some solid materials, such as mineral and rock fragments, and elemental sulfur, remained settled after dissolution in the pure water. The concentrations of F, Cl, SO₄, Na, K, Mg, and Ca were determined. Fluoride forms complex chemical compounds under conditions of high Fe and Al content; therefore, the results of the F analyses of the crater lake waters by the ion chromatography are not accurate (Takano et al. 2000). An EDS (Oxford X-Max 20) on a scanning electron microscope (JEOL JSM6610LV) was used for the chemical composition analysis on the surface of the salt grains. Fragments of the salt samples were attached to an aluminum plate with double-side carbon tape and were then coated by graphite. Because the salt sample cannot be polished, only flat and smooth surfaces that could be oriented perpendicular to the microprobe were selected for analysis. The acceleration voltage of the electron microprobe was 15 kV, and the probe current was ~ 1 nA. The X-ray signal lower than 10 keV was analyzed.

Crater lake water samples were collected using the same method employed by Miyabuchi and Terada (2009). Chemical analyses of the lake water samples were performed by the Chikyu Kagaku Kenkyusho Co., Ltd (Nagoya, Japan). Concentrations of F, Cl, and SO₄ were determined using ion chromatography, SiO₂ was analyzed through a molecular absorption spectrometry method, and the cation concentrations were measured by an atomic absorption spectroscopy technique. The crater lake waters are very acidic, and some components, such as HCl, are likely lost through evaporation (Shinohara et al. 2015; Capaccioni et al. 2016). In order to examine the effect of the evaporation, the crater lake water was dried overnight at 90 °C, and the dried material was then dissolved in pure water and analyzed using the same ion chromatography method as was used for the salt materials.

Water-soluble components of the volcanic ashes emitted in 2014–2015 were analyzed. The ashes, collected shortly after the deposition on a clean surface, were used for the analyses. These ashes were mixed with pure water by 1:50 weight ratio and agitated for about 60 min at a temperature of ~ 40 °C in an ultrasonic bath, shaken intermittently. The solution was filtered using a 0.45-µm membrane filter after decantation and analyzed for the anions and cations using the ion chromatography method (Witham et al. 2005).

Various salt fallouts were observed during the intensive gas emission and ash eruption stages of the Nakadake crater of the Aso volcano. Chemical compositions of the salt fallouts were analyzed and compared to those of the ash leachates, the crater lake waters, and the dried samples of the crater lake water. Most of the salt fallouts have composition similar to those of the dried crater lake water samples and are quite different from those of the ash leachates. Flakes of salts were observed in May 2011 when only a small volume of the acid crater lake water remained, and a brownish plume intensively emitted from a high-temperature vent. The chemical composition indicates that the salt flakes are likely formed by the evaporation of the crater lake water sprayed by the fumarolic jet. Hollow spherical salt shells were observed at several times during the eruptive stage of 2014–2015, when the ash eruption with intermittent Strombolian explosions continued with temporal pauses. The salts are released from the plume during weak ash eruptions or shortly after the temporal cessation of the ash eruptions. Although the crater lake completely dried up prior to the eruptive stage, the similarity in chemical composition suggests that the shell salts are also formed by the evaporation of an underground hydrothermal solution pouring into the conduit of a volcanic plume. The hollow structure suggests that the salts were formed by the heating of hydrothermal solution droplets suspended in the plume stream. The preservation of the fragile hollow shell structure, even after transport by the plume, suggests that the hydrothermal solution pours into the conduit at a shallow depth well above the level of magma fragmentation. The salt shells indicate a close distribution of the hydrothermal system surrounding the erupting vent, even during the continuous magmatic eruption stage. The perturbation of the magmatic–hydrothermal interface can cause phreatomagmatic explosions, which, at this volcano, often occur at the end of an eruptive period.