Emission of water-soluble salts from volcanoes is not rare. Volcanic ashes commonly contain water-soluble salts (ash leachates) with variable contents (Witham et al. 2005). Water-soluble salts, such as NaCl and KCl, are often found in the high-temperature fumarolic sublimates (Symonds 1993; Africano et al. 2002). However, the observation of the salt fallouts from a plume is an uncommon phenomenon, particularly from an ash plume. This study is the first report of such a case, as far as the authors know. Compositions of the salt fallouts, with the exception of white salt, are similar to the dried lake water samples, indicating the salt fallouts formation caused by the evaporation of the lake water. The salt fallouts have large Mg/Na ratios (close to one), similar to the ratios observed from the dried lake water samples, but different from the ash leachates. Although there are no samples of the fumarolic sublimates at Aso volcano, such sublimates rarely contain Mg-containing salts (Symonds 1993; Africano et al. 2002). The Mg-rich composition of the salt fallouts is different from the Mg-poor fumarolic sublimates.
The salt flakes were observed when a brown-colored plume emitted from a high-temperature vent, surrounded by the remnant of the crater lake (Fig. 1a). The salt flakes are likely formed by evaporation of the crater lake water sprayed from the vent, which caused the brown color observed in the plume. The compositions of the salt shells and lumps are also similar to the crater lake water, indicating that these salts were formed by evaporation of hydrothermal fluids, similar to the crater lake water. The salt shells and lumps were observed during a period when the crater lake did not exist on the surface (Fig. 1c, e). The hydrothermal fluids forming the salt shells and lumps need to be derived from an underground hydrothermal system surrounding the erupting vent (Fig. 3a).
The salt shells, particularly those from of March 2015, have the fairly specific structure of a hollow spherule with a thin shell (Fig. 1f). This structure resembles the hollow sulfur spherules floating on other crater lakes (Ohashi 1919; Takano et al. 1994). Ohashi (1919) proposed that sulfur spherules are formed by the upward passage of gas bubbles through liquid sulfur pools at the bottom of the crater lakes. By analogy, gas bubbles passing through a liquid salt pool can create the salt spherules. In contrast to the common occurrence of elemental sulfur in the fumarolic fields, massive deposits of salts are rarely observed in active volcanoes, and the salt spherule formation at a liquid salt pool is rendered less likely.
The hollow structure of the salt shells can be formed by evaporation of droplets of hydrothermal solution during their transportation within the plume. The hydrothermal solutions pour into the plume forming droplets, which then are transported in the plume by a flow stream of gas–ash mixture. The droplets are heated in the plume, because plume temperature is likely much higher than that of the hydrothermal system. Heating of the droplets causes the water to evaporate and the salt to precipitate at the droplet surface, forming the salt shells, and the complete drying of the droplet makes these shells hollow (Fig. 3b). A thin shell implies that the salt volume was much smaller than the droplet volume. The crater lake water contains 8–25 wt.% of dissolved solids (Table 1). If we assume that the densities of the crater lake water and the precipitated solids are 1.0 and 2.5 (g/cm3), respectively, the precipitated salt volume is estimated to be 3.2–10 vol.% of the original droplet, requiring the shell thickness to be 0.01–0.03 times the droplet radius. The estimated thickness of a shell with a 5 mm radius is 0.05–0.15 mm. Although no quantitative measurements for the volume and weight ratios of the salt shells were conducted, these estimates look reasonable from the appearance (Fig. 1f). Preservation of the fragile structure of the hollow shells during the transport indicates that the salt shells were not damaged by any strong turbulent flows with ashes, and that the salt formation occurred after the magma fragmentation. A hydrothermal solution layer is likely located near the surface, at a depth shallower than the magma fragmentation level.
Formation of salt shells requires subtle conditions of hydrothermal solution flowing into a hot gas or eruption plume, but can occur at other crater lake bearing volcanoes, such as the Poás and the Copahue volcanoes. The salt fallouts, however, will not be preserved for a long time, as these will easily be crushed to a powder and get blown away or get dissolved by rain water. The salt fallouts can be observed only by frequent examinations at the crater rim on the leeward of the plume during eruptions or active degassing. Such examinations, however, can be risky. The rim of the Nakadake crater is easily accessible by road, enabling frequent exploration of the crater rim with a low risk, even during the active stages. This frequent monitoring has made the observation of rare salt fallouts possible at the Nakadake crater.
Compositions of the salt flakes collected in 2011 are almost identical to those of the dried crater lake water samples (Fig. 2). Compositions of the salt shells and lumps collected in March 2015 are similar to, but have larger Cl/SO4 ratios than, the salt flakes. The salt shells and lumps have an identical composition which indicates that the salt lumps are recrystallized materials from the salt shells. The salt shells collected in January 2015 have an intermediate Cl/SO4 ratio of the salts than those collected in 2011 and March 2015, suggesting a gradual increase in the Cl/SO4 ratios with time. The salt shells are likely derived from the subsurface hydrothermal solutions. The temporal increase in Cl/SO4 ratios can be caused by gradual changes in the hydrothermal solution composition under the subsurface conditions. However, no evidence of the composition of subsurface hydrothermal solutions currently exists, and the cause for compositional variation of the salt fallouts remains unclear.
The origin of the white salts, with an almost purely NaCl composition, is also unclear. The salts have a fibrous texture covering the bomb surface. Such a fibrous texture resembles that of a fumarolic sublimate precipitated from a vapor, such as fibrous sulfur. High-temperature fumarolic gases contain a relatively high concentration of Na, approximately 10–100 ppm at 800–900 °C (Hedenquist et al. 1994; Taran et al. 1995), which sublimates as NaCl by cooling. The fumarolic sublimates, however, are not pure NaCl but a mixture of various species depending on the sublimation temperature (Symonds 1993; Africano et al. 2002). The lack of the alteration on the bombs does not suggest a high-temperature origin for the white salts. Similar fibrous salt textures can be found also on rocks through which the seawater slowly seeps out at warm conditions (Murakami 1999). If a porous bomb contained a salt solution, a fibrous salt can form from the salt solution seeping out from the interior through warming by the sunshine. The NaCl-rich composition is a feature of neutral chloride waters commonly found in mature geothermal systems (Giggenbach 1996). Large Na/K ratios can be obtained under low-temperature equilibrium conditions (Giggenbach 1988). The NaCl-rich salt can form from a mature low-temperature equilibrated hydrothermal solution. If the white salt is derived from such a neutral pH solution, this neutral pH solution would be distributed by the acidic hydrothermal system and the erupting vent, implying a heterogeneous and complex structure of the magmatic–hydrothermal interface.
The difference in the ash leachate compositions compared to those of the salt fallouts and the crater lake water indicate that the salt components of the ashes are not derived from the hydrothermal solution, but rather from magma degassing. Several ash samples were collected on the same day of the salt shell collection, such as January 13, March 17, and March 20, 2015 (Table 1). Even these ash samples have larger Ca/Na and smaller Mg/Na ratios than the salt shells, suggesting that the contribution of the salt shell components to the ash leachate is not significant. The absence of any hydrothermal components in the ash leachate indicates that the hydrothermal solution fed into the erupting or degassing conduit was not continuous and likely infrequent.
Instability of the magmatic–hydrothermal interface may cause phreatomagmatic eruptions (Morrissey et al. 2000). Phreatic or phreatomagmatic explosions are a common feature at the end of the eruption cycles of the Aso volcano (Sudo et al. 2006). During the continuous ash eruptions, from November 2014 to May 2015, phreatic to phreatomagmatic eruptions did not occur and the interface seemed stable. The occasional hydrothermal solution input to the degassing conduit, forming the salt fallouts, did not cause further perturbation of the interface. The southern part of the crater floor, 100 m wide, suddenly subsided by 50 m on May 3, 2015, without any significant eruptive activity. The frequency and intensity of the ash eruptions decreased thereafter, and the eruption ceased on May 21, 2015. The crater lake water subsequently recovered by June 6, 2015. The immediate recovery of the crater lake is consistent with the continuous existence of a hydrothermal system.
The lack of phreatomagmatic explosion during the recent eruption cycle (2014–2015) suggests that the magmatic–hydrothermal interface has remained stable in this case. During the eruption periods of 1984–1985 and 1989–1991, frequent phreatomagmatic explosions occurred during the late stages, followed shortly by a crater lake recovery. Ono et al. (1995) suggested that these processes can be caused by water from a heavy precipitation. However, the continuing existence of a hydrothermal system surrounding the eruptive vent provides a condition for spontaneous phreatomagmatic explosions at the Aso volcano, with possible instability at the magmatic–hydrothermal interface occurring even during a continuous magmatic eruption, without any heavy precipitation.