Origins and significance of alteration textures in hydrovolcanic products from active volcanoes

doi:10.21203/rs.3.rs-3802511/v1

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Origins and significance of alteration textures in hydrovolcanic products from active volcanoes

https://doi.org/10.21203/rs.3.rs-3802511/v1

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While mineralization processes in hydrothermal systems have been studied intensively from the viewpoint of economic geology, the composition, and textures of hydrothermally altered materials in hydrovolcanic ash have not been fully analyzedin active volcanic systems. In particular, criteria for describing and interpreting textural characteristics based on observed ash components have not been established. We used X-ray diffraction and scanning electron microscopy to examine ash grains from hydrovolcanic activities of the Meakandake, Ontaka, Aso, and Kuchinoerabujima volcanoes. The grains were then classified into 15 types based on their mineral assemblages and textural characteristics. The textural variations are indicative of the temperature and acidity of the hydrothermal fluid, and the reaction time between the hydrothermal fluid and host rocks. Grains featuring “acid” type alteration exhibit co-existing silica and sulfate minerals, suggesting precipitation from acidic hydrothermal fluid at a shallow depth beneath active volcanoes, with inflow of volcanic gas directly from the magma. Grains featuring “neutral” type alteration consist of muscovite, adularia, K-feldspar (orthoclase), and miner biotite, and were formed within a higher-pH hydrothermal fluid, which underwent a neutralization process from an acidic fluid through rock-water interaction. Acid-type grains were typically found in ash from the Ontake, Aso, Kuchinoerabujima, and Meakandake Volcanoes. Neutral-type alteration was recognized in products from the 2014 Ontake eruption and some tephra units of Meakandake. The temporal change in the proportion of alteration types, along the Meakandake tephra sequence, suggests the neutralization of a hydrothermal fluid from the timing of magmatic–phreatomagmatic eruptions to the following period of phreatic eruptions, indicating maturation of the hydrothermal fluid. This study demonstrates how specific alteration textures can be used as indicators of boiling, replacement, hydrothermal fluid conditions, and reaction times between volcanic rocks and hydrothermal fluids.

Phreatomagmatic eruption

Phreatic eruption

hydrothermal eruption

hydrothermal alteration

epithermal alteration

volcanic hazards

porphyry copper system

hydrovolcanic ash

Hydrovolcanism, including phreatic and phreatomagmatic eruptions (Németh and Kósik 2020), is an eruption style driven directly or indirectly by the interaction between magmatic heat and liquid-phase water beneath a volcano. Hydrovolcanism involves a wide spectrum of interactions between magma and water within a volcanic system. Based on varying interaction between the water and intruding magma, hydrovolcanism can be divided into hydrothermal, phreatic, and phreatomagmatic eruptions (Figure 1). Furthermore, the water reacting with magma can be non-volcanic cold water, such as seawater or groundwater, or thermal water sealed inside the volcano. In this study, we focused on hydrovolcanism involving hydrothermal systems within volcanoes.

A phreatomagmatic eruption occurs when magma directly intrudes into seawater, groundwater, or a hydrothermal system (Figure 1, right). The amount of juvenile material in phreatomagmatic products varies greatly, from less than 1% to almost 100% (e.g., Tarff and Day 2013; Alvarado et al. 2016), indicating that the contribution of magma to a phreatomagmatic eruption depends on the varying volumetric ratio of the ascending magma to the hydrothermal fluid.

Phreatic and hydrothermal eruptions (Browne and Lawless 2001) represent hydrovolcanism triggered by indirect or partial interaction between a hydrothermal system and magma, without ejection of juvenile magmatic material (Figure 1, left and middle). The total energy of an eruption is supplied only by the hydrothermal system (Hedenquist and Hemley 1985; Browne and Lawless 2001). A key difference between phreatic and hydrothermal eruptions is the temperature of the hydrothermal fluid or groundwater (Browne and Lawless 2001). Phreatic eruptions are due to the heating and flashing of cold water by magma, resulting in the ejection of only country rock or overburden. In a phreatic eruption, the hydrothermal or circulation system of groundwater only provides a pathway for heat energy originating from the ascending magma (Figure 1). In contrast, hydrothermal eruptions that eject at least some solid material involve energy derived solely from heat loss and phase changes in a convection- or steam-dominated hydrothermal system (Figure 1: Browne and Lawless 2001). Hydrothermal eruption processes are governed by the temperature of hydrothermal fluids and by lithostatic pressure (Hedenquist and Hemley, 1985).

Evaluating the fluid temperature and composition a hydrothermal system beneath a volcano is key to understanding the mechanisms driving hydrovolcanism, comprising phreatomagmatic, phreatic, and hydrothermal eruptions. Hydrothermally altered materials generally found in the ash produced by hydrovolcanism are direct evidence of a hydrothermal system beneath the volcano (Ohba and Kitade 2005; White and Ross 2011; Minami et al. 2016; White and Valentine 2016; Geshi and Itoh 2018). The products of phreatomagmatic, phreatic, and hydrothermal eruptions are crucial for reconstructing the conditions of a subvolcanic hydrothermal system, such as acidity, temperature, source depth, and alteration sequence (e.g., Christenson et al. 2010; Piochi et al. 2015; Minami et al. 2016, 2022; Takahashi and Yahata 2018; Imura et al. 2019). Mineral assemblages can be used to evaluate the chemical properties of hydrothermal fluids because they reflect the chemical composition and temperature of the hydrothermal fluid, the rock/water ratio, and the development of alteration sequences. The mineralization process in hydrothermal systems has been studied intensively from the viewpoint of ore mineralogy (e.g. Hemley and Ellis 1983; Kimbara 1983; Stoffregen 1987; Heald et al. 1987; Hedenquist et al. 2000; Sillitoe 2010).

However, the composition and textures of hydrothermally altered materials in hydrovolcanic ash have not been fully analyzed in active volcanic systems. In particular, criteria for describing and interpreting textural characteristics based on observed ash components have not been established. This study focuses on the alteration textures and mineral assemblages in phreatomagmatic, phreatic, and hydrothermal eruption products from several volcanoes in Japan and outlines a framework for interpreting their origins. Therefore, the focus of this study was to investigate both alteration textures and mineral assemblages, and to provide interpretations of alteration materials from recent eruption products based on their chemical and physical conditions. Additionally, the geological record of the Meakandake Volcano was assessed to investigate the temporal variation of a sub-volcanic hydrothermal system at an active volcano. Based on these approaches, we propose that a more comprehensive understanding of the temporal variation in subvolcanic hydrothermal systems can be established by considering the chemical and physical properties of the altered materials in eruption products. The interpretations proposed in this study are important for advancing our understanding of the mechanisms involved in phreatomagmatic, phreatic, and hydrothermal eruptions, and our ability to forecast future eruption hazards.

Chemical and physical variation of hydrothermal system and relation with mineral assemblages and alteration textures

Differences in the water/rock ratio cause varying chemical reactions within a hydrothermal system (Stoffregen 1987; Heald et al. 1987; Giggenbach 1997). If a hydrothermal fluid contains abundant volcanic gas components, compared with the amount of fresh volcanic rock (high water/rock ratio condition), cations in the volcanic rock will be extracted by the highly acidic hydrothermal fluids (Figure 1), and only silica minerals and some Al-Si clay minerals can survive there (Giggenbach 1997; Sillitoe 2010). Such conditions (advanced argillic alteration zones) tend to form in the shallow subsurface because the accumulation and condensation of volcanic gas components advances upward. In contrast, if volcanic gas components in a hydrothermal fluid are low compared to the amounts of fresh volcanic rock (low water/rock ratio condition), the fluids tend to be neutralized (Figure 1) because excess anions in the volcanic gas escape upward or cannot react with the country rock, due to thermal equivalence (Giggenbach 1997; Sillitoe 2010). Such conditions tend to occur in the deep or far lateral regions of hydrothermal systems (Giggenbach 1997; Sillitoe 2010). Under neutral conditions, alteration materials mostly comprise K-mica and K-feldspar (Giggenbach 1997; Sillitoe 2010). Therefore, dominant mineral assemblages provide valuable information for estimating the chemical and thermal properties of hydrothermal fluids and can be correlated with their general formation depth, thermal gradation, and water/rock ratio (Sillitoe 2010).

Alteration texture characterization is another key tool used to understand the properties of subvolcanic hydrothermal systems. Eruption products with advanced argillic alteration (silica minerals + kaolin group minerals + pyrophyllite) occur with different textures such as partly altered glass, completely altered dense fabric, or completely altered porous fabric (Minami et al., 2016; Minami et al., 2022; Imura et al., 2019). Additionally, in some cases where the same mineral assemblage was present throughout the eruption sequence, its mineral assemblage and degree of alteration varied with eruption sequence. During the early stage of the 2018–2019 eruption of the Kuchinoerabujima Volcano, sulfate minerals in the ash were observed as phenocryst-sized crystals, which then became fine free crystals during the later stage (Minami et al., 2022). Minami et al. (2022) attributed this grain-size variation in sulfate minerals to a change in the degree of supersaturation of the hydrothermal fluid. In ore-forming hydrothermal systems, textural changes in silica minerals and K-feldspar (adularia) are used to infer changes in physical properties, such as pressure, temperature, and crystal growth rate (Dong et al. 1995; Dong and Morrison 1995). Therefore, alteration texture characterization is an important tool for understanding the physical conditions of hydrothermal systems.

Samples and their source eruptions

In recent years, phreatomagmatic, phreatic, and hydrothermal eruptions have occurred frequently in Japan. The authors collected products from the to 2018–2019 eruption of Kuchinoerabujima Volcano, the 2014 eruption of Ontake Volcano, and the 2019–2020 eruptions of Aso Volcano. Additionally, eruption products were also collected from the geological record (20 eruption deposits) of the Meakandake Volcano wand compared with ejecta from the modern volcanic activities listed above (Figure 2 and Table 1).

The 2018–2019 eruption of the Kuchinoerabujima Volcano changed from the continuous ejection of ash-laden plumes between October 21 and the middle of December, to intermittent explosive activity accompanied by several pyroclastic density currents which continued until January 2019 (Sugai et al. 2019; Minami et al. 2022). This eruption was precisely observed from July to October 2018 and was characterized by inflation and increased degassing activity. Inflation was detected by a global navigation satellite system (GNSS) during July–August following gradual deflation after the 2015 eruption (Japan Meteorological Agency, JMA, 2018). The flux of SO₂ increased from <500 tons/day in July to ~1500 tons/day in mid-August and then reached ~1700 tons/day in September. Seismic activity beneath the summit crater increased from the second week of October. Subsequent to these precursor events, the 2018–2019 eruption of the Kuchinoerabujima Volcano began on October 21, with magmatic activity dominating due to onset of juvenile magma ascent. Activities from mid-November to the end of the sequence were driven by the interaction between magma and hydrothermal fluids (Minami et al. 2022). From the northern flank of the edifice, we collected the sample of the ejecta produced by explosions which occurred from December 28, 2018 and was classified as late phreatomagmatic activities, accompanied by a pyroclastic density current (sample ID 18K01 in Table 1).

During the last 50 years, Ontake Volcano erupted in 1979, 1991, late March 2007, and late September 2014 (Oikawa et al. 2014); of these, we reviewed the eruption sequences from the latest two eruptions. Intense seismic activity occurred beneath the summit area from November 2006 to January 2007 (Nakamachi et al. 2009) during which time ground deformation was also detected (Takagi and Onizawa 2016) and a drop in groundwater pressure was observed ~10 km from the summit of the volcano (Koizumi et al. 2016). These phenomena clearly indicated the onset of unrest prior to the eruption in late March 2007. In contrast, precursory unrest signals for the 2014 eruption were unclear. Seismicity began with 12 events from September 6 to 15, 2014. Subsequently, the seismicity fluctuated with an average of 30 events/day, but there was a clear decreasing trend from September 16 to 26, 2014 (Zhang and Wen 2015). Stacking of the GNSS data revealed that a small volume change occurred immediately beneath the summit one month prior to the eruption, as well as a small volume increase at depth, 1.5 months before the eruption (Miyaoka and Takagi 2016). Other phenomena such as volcanic tremors or crustal deformation were not detected (Yamaoka et al., 2016). Intense seismicity, comprising 29 events, occurred 7 min before the eruption, which occurred at 11:52 on September 27 (Zhang and Wen 2015; Takagi and Onizawa 2016; Yamaoka et al. 2016). The authors collected volcanic ash immediately after the eruption on the eastern flank of the edifice (on September 28); accordingly, the sample was not affected by rainfall (sample ID:14_On_1 in Table 1).

The Aso Volcano erupted during 2019–2020 and in 2021. The former event began with small eruptions between April 16 and early May 2019 at the Nakadake First Crater (Miyabuchi et al. 2021). Major precursory phenomena of the 2019 eruption comprised increases in volcanic gas flux and volcanic tremor amplitudes from February 2019. The summit crater lake showed a drop in water level from mid-February 2019 and had almost dried up by April 2019. The most explosive period of the 2019 eruption occurred between May 3 and 9. The eruption column reached 1400 m above the crater rim and a volcanic glow was observed. Miyabuchi et al. (2021) reported a change in eruptive products, from dominantly altered material in the early stage (April 16 and early May) to dominantly juvenile fragments in the late stage characterized by continuous magmatic eruptive activity (May 2019 to mid-June 2020). The authors collected volcanic ash on April 16 from the southern flank of the edifice, which was classified as a product of the early small-scale eruptive stage (sample ID:19_A02 in Table 1).

Volcanic ejecta from activities at Ponmachineshiri over the last 1000 years were collected as examples of pre-historical eruption products. Ponmachineshiri is one of several volcanic bodies that form the Meakandake stratovolcano in eastern Hokkaido (Figure 2). At least four remarkable layers of pyroclastic fall deposits derived from Ponmachineshiri during the last 1,000 years have been recognized, with Volcanic Explosivity Index (VEI) levels ranging from 1 to 2 (Minami et al. 2021). The major pyroclastic fall deposits consist of Pon-1 (10th to 12th century; VEI 2), Pon-2 (13th to 14th century; VEI 2), Pon-3 (15th to 17th century; VEI 1), and Pon-4 (post-1739; VEI 1), with small-scale (VEI <1) phreatic, hydrothermal, and phreatomagmatic eruption deposits intercalated within the Pon-1, Pon-2, and Pon-3 pyroclastic fall deposits (Minami et al., 2021). The presence of scoria and minor pumice in the Pon-1, Pon-2, and Pon-3 pyroclastic fall deposits suggests that these resulted from phreatomagmatic events. In particular, Pon-1 and Pon-2 are relatively large-scale phreatomagmatic eruption deposits containing more than 25% of juvenile material. However, the absence of juvenile materials in the Pon-4 pyroclastic fall deposits suggests a phreatic/hydrothermal eruption. Small-scale (VEI <1) phreatic, hydrothermal, and phreatomagmatic eruption deposits are intercalated within the Pon-1, Pon-2, and Pon-3 pyroclastic fall deposits. As described above, the recent activities of Meakandake Volcano particularly suited to textural and mineral assemblage classification because the juvenile contents have already been reported, details of the eruption chronology have been established, and magmatic and non-magmatic eruptions occurred alternately (Minami et al., 2021).

Details of the samples the authors used in this study were listed in Table 1 and references therein. Samples from recent eruptions (2018–2019 eruption of Kuchinoerabujima Volcano, the 2014 eruption of Ontake Volcano, and the 2019–2020 eruptions of Aso Volcano) were collected just a few days after the eruption. Samples from Meakandake Volcano were collected geological record. Totally 15 samples were collected. The author collected more than 50 g and dried up 12 hours with 40 C°. Dried samples used for an X-ray diffractometer (XRD) and scanning electron microscope with energy dispersive spectrometers (SEM-EDS) to examine the mineral assemblages and alteration textures of each eruption product. For SEM-EDS observation the author useed samples sieved into 250 – 500 μm .

Mineral assemblages were identified using the XRD (MiniFlex; Rigaku Corporation) installed at Hokkaido University. A randomly oriented sample and an oriented sample were prepared from each volcanic product. Ethylene glycol and HCl treatments were applied to some samples to determine the clay mineral species. Measurements were carried out at a rate of 1° per minute from 2° to 60° using a CuKα target X-ray tube with an acceleration voltage of 30 kV and a filament current of 16 mA. The mineralogy of individual ash particles was investigated by stereoscopic microscope observation (on particles >125 μm) and SEM-EDS semi-quantitative analysis focusing on textures, shapes, colors, and chemical compositions. The ash morphology was observed using a stereoscopic microscope and SEM. Quantitative microprobe analysis was performed at Hokkaido University using an Oxford Energy200 EDS system attached to a JEOL JSM-5310 SEM. Polished sections were prepared for the SEM-EDS measurements. Atomic proportions measured by SEM-EDS semi-quantitative analysis were used to identify the mineral species, in combination with XRD measurements of the bulk samples. Mineral particles were identified using a combination of results from XRD measurements and SEM-EDS semi-quantitative measurements of atomic proportions, following the method of Minami et al. (2016).

Resulting classification of alteration textures

We recognized fifteen alteration texture types in volcanic ashes produced by the historic phreatomagmatic, phreatic, and hydrothermal eruptions of these volcanoes.

Fluidal native sulfur

Description: This texture is characterized by several millimeter- to centimeter-scale native sulfur crystals with a spherical to droplet-shape (Figure 3A). They occasionally form tube-like to hair-like shapes. This texture was observed in products of the 2019 eruption of Aso Volcano (sample ID:19_A02 in Table 1).

Interpretation: The fluidal native sulfur texture was newly named in this study, although similar products were reported from the 1995–1996 and 2007 eruptions at Ruapehu Volcano (Christenson 2000; Christenson et al. 2010). Products of both the Aso and Ruapehu Volcanos show droplet, tube-like, and hair-like shapes, indicating that they were formed by quenching of fluid-phase native sulfur or evaporation of sulfur-oversaturated fluid. The latter processes are more common because they are typically observed in volcanoes with slurry-filled crater lakes (Christenson 2000; Christenson et al. 2010).

Massive

Description: The texture “massive” is recognized in volcanic ash particles of the Meakandake, Kuchinoerabu, and Ontake Volcanoes (Tables 1 and 3). These particles consist of aggregates of very fine crystals (<1 µm) with unclear crystal boudaries (Figure 4A, B). Particles exhibiting this texture usually contain silica polymorphs (amorphous silica, trydimite, crystoballite, and quartz) and/or simple sulfates (gypsum and anhydrite) with clay minerals (kaolin group minerals, pyrophillite, and muscovite), pyrite, adularia, and rutile (Table 2). These particles exhibit a completely altered, dense fabric.

Interpretation: Massive texture was first described by Smirnov (1962) and defined by Dong et al. (1995) and is formed via two mechanisms: (1) crystallization directly from hydrothermal fluids (P-type massive texture) as reported for the 2018–2019 eruption at Kuchinoerabujima Volcano (Minami et al. 2022) and epithermal ore deposits (Fournier 1985); and (2) recrystallization of amorphous silica gel (R-type massive texture) as reported in the 2014 eruption at Ontake Vocano (Minami et al. 2016) and epithermal ore deposits (Fournier 1985).

Particles with massive textures in epithermal ore deposits usually consist of single crystals or fine submicron aggregates of silica minerals (Fournier 1985). In contrast, volcanic ash particles with P-type textures can contain simple sulfates (gypsum and anhydrite) with clay minerals (kaolin group minerals, pyrophillite, and muscovite) and pyrite with minor silica minerals. The co-occurrence of other minerals with the silica polymorphs may indicate that a hydrothermal fluid the was oversaturated not only in the silica mineral, but also in some other minerals. Such complex mineral assemblages indicate hydrothermal fluids that were not chemically matured with acid and are thus uncommon in the stable hydrothermal systems of epithermal ore deposits (Fournier 1985). When observed in volcanic ash from active volcanoes, this feature may indicate shorter reaction times for crystal precipitation and growth in subvolcanic hydrothermal systems than those in ore-forming hydrothermal systems.

Particles showing R-type massive textures usually contain silica polymorphs (amorphous silica, trydimite, crystoballite, and quartz), with clay minerals (kaolin group minerals, pyrophillite, and muscovite), pyrite, adularia, and rutile. This type of particle is formed by the recrystallization of silica gel, indicating a relatively long reaction time under a stable hydrothermal system, such as an epithermal ore deposit (Fournier 1985).

Vein

Description: Continuous veins of alteration minerals cutting through the host rock (Figure 3C, 4C, and 4D) were observed in samples from the Aso, Ontake, Kuchinoerabu, and Meakandake volcanoes (Tables 1 and 3). These veins are several micrometers to one centimeter wide and sometimes fill crystal boundaries or the cleavage of phenocrysts. The vein consists of silica polymorphs (amorphous silica, trydimite, crystoballite, and quartz), simple sulfates (gypsum and anhydrite), pyrite, and adularia with clay minerals (kaolin group minerals, pyrophillite, and muscovite) and rutile (Ontake Volcano: Table 2).

Interpretation: Although not strictly a texture, veins represent a key feature that can be used to determine cross-cutting relationships in an altered particle. As such, we classified it as a distinct feature, although previous researchers such as Smirnov (1962) and Dong et al. (1995) classified the vein texture along with massive texture. Veins often crosscut other alteration textures and can be interpreted as late-stage alterations, as reported for late-stage porphyry copper systems (Sillitoe 2010). The vein-forming minerals described above show greater variability than those of ore-forming hydrothermal systems (e.g. Gifkins et al., 2005). This implies that hydrothermal fluids in active volcanoes are more immature than those in stable ore-forming hydrothermal systems, in terms of chemical differentiation. Veins must be formed by quenching of hydrothermal fluids, indicating a relatively short reaction time with the host rocks.

Amygdaloidal

Description: Amygdaloidal texture is characterized by alteration minerals that fill vesiculation voids and/or vuggy voids in host rocks (Figure 4E and 4F), as observed in samples from the Meakandake Volcano (Tables 1 and 3). The inner parts of the host rock are sometimes unaltered. The alteration minerals associated with this texture include smectite, chlorite, kaolin-group minerals, silica polymorphs, pyrite, and simple sulfate minerals (Table 2).

Interpretation: Amygdaloidal is a commonly used term (e.g. McPhie et al. 1993; Le Maitre et al. 2002). This texture, in which crystallization occurs in pores of particles, indicates that there has been no chemical extraction from the host rock, with direct crystallization from hydrothermal fluids occurring instead. Accordingly, this texture has been described as “partially altered particles” in some studies (Tokachidake Volcano: Imura et al. 2019; Kuchinoerabujima Volcano: Minami et al. 2022). The texture was limited to reaction surfaces, indicating mineral precipitation on the reaction surface between the host rock and the hydrothermal fluid, under fluid-filling (hydrostatic) conditions. This incomplete equilibrium between the hydrothermal fluid and host hock indicates an immature hydrothermal system, possibly caused by the short reaction times between the host rocks and hydrothermal fluids when compared with stable ore-forming hydrothermal systems (e.g. Gifkins et al., 2005; Ridley 2013).

Druse

Description: Druse texture is characterized by alteration minerals that cover the surface of a host rock (Figure 4G and 4H), as observed in samples from the Kuchinoerabu, Ontake, and Meakandake Volcanoes (Tables 1 and 3). Observed alteration minerals are predominantly smectite, chlorite, kaolin-group minerals, silica polymorphs, pyrite, and simple sulfate minerals (Table 2). The inner parts of host rock particles are sometimes unaltered.

Interpretation: This is the first description of a druse texture in volcanic ash products from the Kuchinoerabujima, Ontake, and Meakandake Volcanoes or similar. The unaltered inner parts of the host rock particles indicate that crystallization occurred on the surface of the particles, with no chemical extraction from the host rock, via direct crystallization from hydrothermal fluids. Accordingly, this texture is described as “partially altered particles” in some studies (Tokachidake Volcano: Imura et al. 2019; Kuchinoerabujima Volcano: Minami et al. 2022). Similar to the amygdaloidal texture, this also indicates that mineral precipitation occurred on the reaction surface between the host rock and the hydrothermal fluid under hydrostatic conditions. This could be attributed to a short reaction time between the host rock and hydrothermal fluid, compared with stable ore-forming hydrothermal systems (e.g. Gifkins et al., 2005; Ridley 2013).

Bladed

Description: The bladed texture is characterized by alteration minerals that show euhedral and blade-to-sheet-like shapes within an altered particle (Figure 5A). This texture was observed in samples from the Meakandake Volcano (Tables 1 and 3). Usually, bladed textures are observed in altered minerals in the absence of other minerals. This texture typically consists of alunite and simple sulfate minerals (Table 2).

Interpretation: Bladed texture was first described by Adams (1920) and defined by Dong et al. (1995). The euhedral shape of the crystals indicates that they precipitated in free space, and the blade-like shapes imply rapid crystal growth under a high degree of supersaturation (Otálora and García-Ruiz 2014). The typical mineral assemblage of alunite and simple sulfate minerals and such a high degree of supersaturation are similar to those formed by the rapid evaporation of sulfate-acid hydrothermal fluids, as reported for the 2018–2019 eruption at Kuchinoerabujima (Minami et al. 2022). If the hydrothermal fluid evaporates rapidly enough, sulfate minerals can precipitate as fine-grained crystals (Minami et al. 2022). However, in this case, the blade texture indicates that they grew from the hydrothermal fluid.

Colloform

Description: Particles with colloform texture consist of micrometer-width wavy K-feldspar rich bands (bright bands) and a fine-grained mixture of silica mineral with K-feldspar (darker matrix in Figure 5B). A colloform texture was observed in ash samples from the Kuchinoerabujima, Ontake, and Meakandake volcanoes (Tables 1 and 3). Particles with this texture typically contained void veins and irregularly shaped pores (Figure 5B; Table 2).

Interpretation: Colloform texture was first described by Rogers (1917) and defined by Dong et al. (1995). Minami et al. (2016) reported some grains of Ontake volcanic ash exhibiting K-feldspar-bearing alteration (colloform texture), which could represent low-temperature conditions typical of epithermal ore deposits (Fournier 1985; Saunders 1990). K-feldspar in the colloform-textured grains features silica polymorphs and Or-endmember compositions that imply precipitation from a low-temperature neutral fluid under epithermal conditions (Fournier 1985; Saunders 1990; Giggenbach 1997). The colloform texture was interpreted to have formed through the recrystallization of silica gel in a free space filled with fluids, under hydrostatic conditions (Fournier 1985; Saunders 1990; Dong et al. 1995). This recrystallization texture requires a relatively long mineralization time in the hydrothermal fluid.

Pit

Description: The pit texture is characterized by the formation of small pits on the surface of glassy host rock particles (Figure 5C). Pits are usually several micrometers to several tens of micrometers in diameter. This texture can be recognized by surface observation with an SEM, and the authors carried out surface observations of the products of the 2019 eruption at the Aso Volcano (Table 1).

Interpretation: This is the first description of a pit texture in volcanic ash products from the Aso Volcano and can be interpreted as a leaching texture formed by chemical reaction on the particle surface, between volcanic glass and a highly acidic fluid or fumaroles over a short reaction time. Pit textures are commonly observed in recycled particles from the products of previous eruptions (Houghton and Smith 1993; Graettinger et al. 2016).

Vuggy

Description: The vuggy texture is characterized by pore (vug) formation in a host rock particle (Figure 3B and 5D). This texture was observed in all samples in this study (Tables 1 and 3). The vugs were observed in both groundmass and phenocrysts minerals, such as plagioclase and pyroxene. Immature alteration results in a vuggy texture with discontinuous pores that are a few micrometers in diameter. With increasing degree of alteration, the pores become larger (tens of centimeters) and the altered part predominantly contains a silica component. Particles with a vuggy texture usually contain clay minerals such as kaolin-group minerals and pyrophyllite (Table 2). In such particles, the shapes of the vugs can vary from platy, reflecting plagioclase leaching, and vein-like to irregular shapes within the volcanic glass. These particles exhibit completely altered porous fabrics.

Interpretation: The term “vuggy” textures have traditionally been used for describing porous white altered rocks in the field of economic geology (e.g. Stoffregen 1987; Hedenquist et al. 2000). Abundant pores in volcanic ash grains are presumably formed by the destruction of original crystals, such as plagioclase and pyroxene, during chemical leaching by highly acidic fluids. Previous studies have attributed similar textures to steam-heated volcanic environments and high-sulfidation ore systems(e.g. Giggenbach 1975; Stoffregen 1987; Rye et al 1992), wherein cation leaching is accompanied by sulfate and sulfide precipitation. The textures and dominance of the silica component imply that major cations, other than Al and Si, were leached from the original rock, a process that has been termed vuggy silica alteration (Stoffregen 1987; Hedenquist et al. 2000). The variability from the micrometer to centimeter scale can be attributed to varying reaction times between hydrothermal fluids and host rock.

Mosaic

Description: A mosaic texture was predominantly observed in the groundmass and crystal aggregates of the ash samples (Figure 5E). This texture was observed in samples from the Ontake and Meakandake volcanoes (Tables 1 and 3) and typically consists of irregularly shaped fine- to medium-sized (several tens of micrometers) silica minerals with minor clay minerals, simple sulfate minerals, and/or adularia (Table 2).

Interpretation: Mosaic texture was first described by Lovering (1972) and defined by Dong et al. (1995). This texture has been previously described in ash from other volcanoes (Akita-Yakeyama: Ohba et al., 2007) and is equivalent to a jigsaw texture, which is one of the most common microtextures in jasperoids (Lovering, 1972). The texture is formed in a similar way to the colloform texture (Dong et al., 1995), and can be interpreted as the recrystallization of silica gel (Fournier 1985; Dong et al., 1995). Although colloform-textured particles consist of fine aggregates of silica minerals, the mosaic texture consists of coarser crystals of silica minerals (Dong et al., 1995), indicating a longer reaction time with hydrothermal fluids than that required to form the colloform texture.

Saccharoidal

Description: This texture usually appears in the groundmass of ash particles or as aggregates of crystals (Figure 6A) and was observed in samples from the Ontake and Meakandake volcanoes (Tables 1 and 3). It typically consists of abundant elongated subhedral crystals of alteration minerals, such as silica minerals, simple sulfate minerals, alunite, and/or adularia (Table 2). The crystals are commonly randomly distributed in a matrix of smaller anhedral grains (Figure 6A). Some particles exhibited remaining phenocrysts of rock-forming minerals such as pyroxene.

Interpretation: The saccharoidal texture was first described in an ore-forming hydrothermal system by Lindgren (1901) and defined by Dong et al. (1995). This texture can be interpreted as the replacement of volcanic glass (Ontake Volcano: Minami et al., 2016; Akita-Yakeyama Volcano: Ohba et al., 2007); accordingly, a long reaction time with hydrothermal fluids is required. This texture is equivalent to “retiform structure” (Lindgren 1901; Adams 1920) and “reticulated texture” (Lovering 1972).

Equigranular

Description: The texture is recognized in volcanic ash particles of Ontake and Meakandake Volcanoes (Table 1 and 3) and consists of aggregates of anhedral fine crystals (micrometer scale) with clear crystal boudaries (Figure 6B). Particles with this texture usually contain silica polymorphs (amorphous silica, trydimite, crystoballite, and quartz) and/or simple sulfates (gypsum and anhydrite) with clay minerals (kaolin group minerals, pyrophillite, and muscovite), pyrite, adularia, and rutile (Table 2).

Interpretation: Equigranular texture is newly named in this study. This texture is attributed to the recrystallization of amorphous silica gel, as reported for the 2014 eruption at Ontake Volcano (Minami et al. 2016). Particles exhibiting this texture form over long reaction times under stable hydrothermal system conditions.

Pseudomorph

Description: This texture is recognized in phenocrysts and microlites in ash particles from the Ontake and Meakandake volcanoes (Tables 1 and 3). Within these particles, the major crystal phases of plagioclase, pyroxene, and olivine have been replaced by adularia, silica polymorphs, clay minerals, and a mixture of these minerals within their original shapes (Figure 6C, Table 2). Occasionally, volcanic glass remains in the ground mass.

Interpretation: The term pseudomorph is well-established (e.g. McPhie et al. 1993; Le Maitre et al. 2002) and this texture has been described in ash from Ontake Volcano (Minami et al., 2016), Akita-Yakeyama Volcano (Ohba et al., 2007), Tokachidake Volcano (Imura et al. 2019), and Kuchinoerabujima Volcano (Minami et al. 2022). In cases where volcanic glass has remained in the groundmass, this texture is regarded as “partial alteration” (Ontake Volcano: Minami et al., 2016; Tokachidake Volcano: Imura et al. 2019). This texture can be formed through chemical leaching by hydrothermal fluids (e.g. Giggenbach 1975; Stoffregen 1987; Rye et al 1992). The textures and dominance of the silica component imply the leaching of major cations other than Al and Si from the original rock (Stoffregen 1987; Hedenquist et al. 2000). Some particles contained adularia, indicating that Na and Ca had been chemically extracted from plagioclase. This indicates that chemical extraction can occur not only in acidic hydrothermal systems but also in neutral hydrothermal fluids.

Cryptic

Description: This texture is similar to the pseudomorph texture type but includes alteration in the groundmass of volcanic glass and microlites within the particles (Figure 6D). This texture was observed in samples from the Ontake and Meakandake volcanoes (Tables 1 and 3). The major original crystals of P plagioclase, pyroxene, and olivine have been chemically replaced by adularia, silica polymorphs, clay minerals, and mixtures of these minerals within their original shapes (Table 2).

Interpretation: Cryptic texture is newly named in this study. This texture has also been described in products from other volcanoes (Ontake Volcano: Minami et al., 2016; Akita-Yakeyama: Ohba et al., 2007; Tokachidake Volcano: Imura et al. 2019; Kuchinoerabujima Volcano: Minami et al. 2022). Particles exhibiting this texture were formed by chemical exchange between the host rock and the hydrothermal fluid, indicating a relatively long reaction time under a stable hydrothermal system.

Dissemination

Description: This texture was observed in all the ash particles (Figure 6E and 6F). This texture was also observed in samples from the Meakandake Volcano (Tables 1 and 3). Dot-like or irregularly shaped alteration mineral patches (several hundreds to tens of micrometers) occur throughout the ash particles. Chlorite and muscovite, with minor adularia, were typically observed as alteration minerals (Table 2).

Interpretation: Dissemination texture is newly named in this study. Patches of altered minerals are formed by chemical diffusion via chemical exchange between the host rock and hydrothermal fluid, indicating a relatively short reaction time under a stable hydrothermal system. Dot-like or irregularly shaped alteration mineral patches indicate that a chemical reaction between the hydrothermal fluid and host rock occurred within the inner particles. This evidence implies that this texture formed via hydrothermal fluids circulating as a percolative flow, probably under lithostatic pressure.

Temporal variation of sub-volcanic hydrothermal system imferred by geological records of Meakandake

Intensely altered fragments identified in eruption products from the Meakandake Volcano (Figure 7 and Table 5) were classified into the texture types described above (Figure 8 and Table 2). In addition, altered grains were classified as either “acid” or “neutral”, based on their mineral assemblages. For example, grains with alteration textures comprising silica polymorphs, sulfate minerals, and some clay minerals (kaolin and pyrophyllite), which are stable in acidic fluids, were classified as “acid”. In contrast, grain textures comprising minerals such as adularia, muscovite, and biotite, which are stable in neutral fluids, were classified as “neutral” (Figs. 8, 9, and Tables 2, 3).

The acid-type vuggy texture was the most common texture in all units obtained from Ponmachineshiri. In brief, alteration textures are highly variable in units containing lower amounts of visible juvenile material, such as scoria. Units Pa, Pe, Pf, Ph, Pi, Pj, and Pon-4 did not contain obvious juvenile grains (Figure 7, Table 4) and can be interpreted as products of phreatic or hydrothermal eruptions (Minami et al. 2021). Of these units, Pon-4 showed the highest variation in alteration textures, including cryptic, vuggy, druse, massive, colloform, and vein textures with acid-type mineral assemblages, and Pseudomorph, mosaic, saccharoidal, and dissemination textures with neutral-type mineral assemblages (Table 3). Similarly, a large variety of alteration textures was recognized in Pa (nine texture types), Pf (eight texture types), Pj (seven texture types), and Pl (seven texture types ). In contrast, units Pi (two texture types) and Pe (four texture types) exhibited lower variability in alteration texture.

The units Pb, Pon-1, Pd, Pon-2, Pon-3, and Pl contain significant amounts of juvenile material (>5%; Figure 7, Table 5) and can be interpreted as products of magmatic or phreatomagmatic eruptions (Minami et al. 2021). These units clearly show a trend of increasing juvenile content with decreasing variation in alteration texture (Figure 7 and Table 3). Units Pb, Pon-1, Pd, Pon-2, and Pon-3 displayed only vuggy-textured grains. Unit Pl contained 13% scoria and altered grains, featuring seven texture types. Grains with druse, bladed, massive, colloform, and cryptic textures occurred in units containing lower amounts of juvenile materials. Druse, bladed, massive, colloform, and cryptic textures were not observed in grains from phreatomagmatic eruptions or eruptions that occurred immediately after a phreatomagmatic eruption. Eruptions that produced more than 20% juvenile material are interpreted to have destroyed mature hydrothermal system and thus the altered texture association of volcanic ashes were decreased.

These variations exhibit a progression within the geological sequence. Units resulting from phreatomagmatic eruptions (Pb, Pon-1, Pd, Pon-2, and Pon-3) exhibited less variation in alteration textures; this variation increased within subsequent units, from Pon-1 (one type) to Pd (one type), Pe (four types), and Pf (nine types). The same trend was observed after the Pon-2 eruption: Pon-2 (one-type), Ph (two-types), Pi (seven-types). A limited number of alteration textures were recognized in the products of phreatomagmatic eruptions accompanied by large-scale magma intrusions (Pb, Pon-1, Pd, Pon-2, and Pon-3). In contrast, the number of texture types increased with each phreatic/hydrothermal eruption following a magmatic eruption (Pe, Pf, Pj, Pl, and Pon-4). The most recent phreatic/hydrothermal eruption product, Pon-4, exhibited the highest variation in alteration textures. In terms of mineral assemblage, any units that contain more than 20% of juvenile materials, and subsequent units immediately following a juvenile-rich eruption, contain no or just a few types of “neutral” alteration textures. The “neutral” alteration textures became more common higher in the geological record (Table 3).

For altered fragments in units containing more and less than 20% juvenile material, we used SEM-EDS to measure the chemical compositions of aggregates or amorphous parts composed of submicron-scale alteration minerals replacing volcanic glass (Tables 4, 5). The values of the Ishikawa Alteration Index (AI; Ishikawa et al., 1976) are plotted against the values of the Chlorite-Carbonate-Pyrite Index (CCPI; Large et al., 2001) in Figure 10.

In this plot, altered fragments of products containing less than 20% juvenile material tend to plot near the sericite and muscovite end-member fields (lower right in Figure 9), which corresponds with the “sericitic trend” identified by Large et al. (2001). Sericite and muscovite are common minerals formed under high temperature (>230 °C: Giggenbach 1997) and neutral alteration conditions (Sillitoe, 2010). In contrast, no altered fragments plotted near the chlorite end-member, which represents common minerals formed under lower temperatures and neutral alteration conditions (Giggenbach 1997; Sillitoe, 2010). Some fragments plotting in the top left of Figure 10 indicate acidic alteration, comprising abundant Si and Al with minor amounts of other cations.

Overlapping alteration textures

Some hydrothermally altered materials exhibited multiple overlapping textures, which are best represented by the fragments shown in Figure 11. At least eight types of alteration texture were recognized within a single particle, including both “acid” and “neutral” types. The particle provides evidence that the hydrothermal fluids circulating in subvolcanic hydrothermal systems have quite unstable chemical compositions. Such intense variation in the hydrothermal fluid is possibly a unique aspect of subvolcanic hydrothermal systems, where frequent magma intrusions disturb the formation of a stable hydrothermal system. Such overlapping alteration textures are uncommon in ore and geothermal field hydrothermal systems (Gifkins et al., 2005).

Interpretations of micro-textures

Fifteen types of alteration texture were observed in this study (Fig. 8). Additionally, the microscale alteration can be classified into 15 types based on their texture, as indicated by their mineral assemblages (Fig. 8). These alteration types can be grouped according to four forming mechanisms: (1) primary growth, (2) leaching, (3) recrystallization, and (4) replacement (Fig. 7: Dong et al. 1995; Dong and Morrison 1995). The microscale textures described above reflect two different formation environments (Table 4). The first is reflected in the primary growth textures (massive, vein, amygdaloidal, druse, and bladed) and leaching textures (pit and vuggy). These textures are characterized by interactions between the host rock and hydrothermal fluid in an open space because they are observed only on the surface of the ash particles (Fig. 8) and represent critical evidence that the hydrothermal fluid circulated through the fractures and was completely separated from the solid rock. Thus, such textures can be interpreted as having formed under hydrostatic conditions within shallow fractures in a volcanic edifice.

Recrystallization textures (mosaic and colloform textures) and replacement textures (saccharoidal, equigranular, pseudomorph, cryptic, and dissemination textures) were characterized by patched or whole-particle alteration, indicating that they formation conditions in which hydrothermal fluid circulated via a percolative flow under lithostatic pressure (Fig. 8). The crystal shapes observed in these textures were always anhedral, indicating growth under pressure. Therefore, microscale textures can be indicative of the relative depth of an alteration environment: primary growth and leaching textures reflect alteration at shallow levels of a volcanic edifice, whereas recrystallization and replacement textures reflect alteration occurring at a deeper level.

Alteration textures and mineral assemblages

“Acid” alteration types were most frequently recognized in both historical eruption products and geological units (Table 3), with primary growth and leaching textures indicating that a shallow environment was the most common. This is probably because most of the rock fragments released by phreatic and phreatomagmatic eruptions originate from the acidic alteration zone that develops at shallow depths in active volcanoes (Sillitoe, 2010). Usually, acidic sulfate fluid is the dominant hydrothermal fluid at shallow depths in active volcanoes hosting hydrothermal systems due to the inflow of volcanic gas from a magma reservoir into the shallow hydrothermal system (Rye et al., 1992). Therefore, eruption products derived from shallow hydrothermal systems mostly consist of silica polymorphs, sulfate minerals, sulfide minerals, and clay minerals that are stable under acidic conditions. Thus, magmatic eruptions and juvenile-rich phreatomagmatic eruptions lead to increasing volcanic gas inflow and the formation of acid hydrothermal systems. Altered materials in the Pon-1 and Pon-2 units of the Meakandake Volcano and the 2018–2019 eruption at Kuchinoerabujima Volcano mainly consist of silica polymorphs and sulfate minerals (Table 3). Pon-1 and Pon-2 also contain high proportions of juvenile material. This, together with clear records of precursory phenomena of magmatic intrusion during the 2018–2019 eruption at the Kuchinoerabujima Volcano (inflation and increased degassing activity; Sugai et al. 2019; Minami et al. 2022), indicate that magma intrusion accompanied these eruptions. This evidence implies that magma intrusion into the shallow levels of a hydrothermal system may cause minor contact metamorphism, capable of destroying the previously stable hydrothermal system and forming silica polymorphs from clay minerals (massive texture: Fig. 12A).

If the volume of ascending magma is not sufficiently large to displace the hydrothermal system, a phreatic or hydrothermal eruption will occur (Fig. 12B). Such phreatic eruptions may eject hydrothermally altered materials that formed in shallow, low-temperature, and highly acidic conditions, such as pyrophyllite, Kaolin-group minerals, and silica minerals. The 1926 eruption of the Tokachidake Volcano is a good example of this type of eruption: juvenile material was ejected, coated with pyrophyllite, kaolin-group minerals, and silica minerals (druse texture: Imura et al. 2019). Additionally, the 1997 eruption at the Akita-Yakeyama Volcano ejected only hydrothermally altered materials consisting of pyrophyllite, kaolin-group minerals, and silica minerals (massive and mosaic texture: Ohba et al. 2007). The latter can be interpreted as a phreatic eruption preceded by a short reaction time between the ascending magma and hydrothermally altered zones.

“Neutral” type alteration is rarely recognized in products from the 2014 Ontake eruption and units Pf, Pi, Pj, Pl, and Pon-4 of Meakandake Volcano. (Table. 3). This alteration type mostly appears as a recrystallization texture (mosaic texture) and replacement textures (saccharoidal, equigranular, pseudomorph, cryptic, and dissemination textures), indicative of formation conditions wherein hydrothermal fluid circulated as a perculative flow under lithostatic pressure. These eruption products contain muscovite, adularia, K-feldspar (orthoclase), and minor biotite (Table 2; Minami et al. 2016), and this assemblage can form in an environment with temperatures exceeding 230°C and neutral conditions (Giggenbach, 1997). This high-temperature and high-pressure condition indicated by products from the 2014 eruption of Ontake and units Pf, Pi, Pj, Pl, and Pon-4 of Meakandake Volcano correspond to an environment which may cause a hydrothermal eruption (> 230°C and under lithostatic pressure: Hedenquist and Hemley 1985). Products exhibiting “neutral” type mineral assemblages and recrystallization and replacement alteration textures, therefore, imply a hydrothermal eruptive origin.

A comparison of the precursory phenomena of the 2014 eruption at Ontake Volcano and other recent eruptions described in this study (the 2018–2019 eruption at Kuchinoerabujima Volcano, and the 2019 eruptions at Aso Volcano) reveals differences in timing and type. Precursory phenomena were recognized seven minutes prior to the 2014 eruption at Ontake Volcano and several months prior to eruptions at the Kuchinoerabujima and Aso volcanoes (Zhang and Wen 2015; Takagi and Onizawa 2016; Yamaoka et al. 2016; Miyabuchi et al. 2021; Sugai et al. 2019; Minami et al. 2022). No edifice inflation was observed prior to the Ontake eruption but was clearly recognized prior to the Kuchinoerabujima and Aso eruptions (Miyabuchi et al. 2021; Sugai et al. 2019; Minami et al. 2022). New magma intrusion is not necessary for hydrothermal eruptions in which energy is derived solely from heat loss and phase changes in convection or steam-dominated hydrothermal systems (Browne and Lawless 2001). The unclear precursory phenomena of the 2014 Ontake Volcano eruption are consistent with the characteristics of a hydrothermal eruption. As such, the products show a high degree of variation in terms of mineral assemblages and alteration textures (Fig. 12D). Because similar “neutral” type alteration is recognized in the units of Pf, Pi, Pj, Pl, and Pon-4 from Meakandake Volcano, it is possible that these products also originated from hydrothermal eruptions, for which precursory ground deformation and volcanic tremors are usually unclear (Dempsey et al. 2020; Caudron et al. 2021).

Reaction time estimated from alteration textures

Neutral hydrothermal fluids that form adularia and muscovite are relatively rare in active volcanoes. Because the production of neutral hydrothermal fluids requires a high rock/water ratio, they can form in the far lateral or deep regions of hydrothermal systems in ore fields (Sillitoe, 2010; Giggenbach, 1997; Hedenquist et al., 2000). Hydrothermal eruption products at Ontake in 2014 contain “neutral” type altered materials derived from a region with a high rock/water ratio (Minami et al., 2016). Miyagi et al. (2020) reported the energy source of the eruption as a shallow magma batch that intruded seven years prior to the eruption. This may indicate that a neutral hydrothermal fluid can form within several years after a magmatic intrusion. The textural and mineralogical changes observed in the eruption sequences of Pon-1 to Pf and Pon-2 to Pj at Meakandake also support the concept that hydrothermal fluids can change from acidic to neutral in less than a hundred years (Tables 1, 2, and 3). Pon-1 and Pon-2 were erupted less than 300 years apart (Pon-1 = ca. 1000 AD and Pon-2 = ca. 1300 AD: Minami et al. 2021), indicating that the eruption sequences of Pon-1 to Pf and Pon-2 to Pj varied over an even shorter time. A neutral fluid could form from deep ascending magma batches in fewer than several decades (Fig. 12C).

Beneath active volcanoes, the volume and depth of intrusive magma batches can vary widely. It is positive effect, therefore, to form inhomogeneous hydrothermally alteration zones showing gradational halo of deep “neutral” hydrothermal zone to shallow “acid” alteration zone. Although deep hydrothermal zonation is difficult to observe directly, it has been defined for some volcanoes, such as Mount Rainier (John et al. 2008) and Mount Adams (Finn et al. 2007). These structures occur as dendritic systems centered around the main conduit, which is consistent with the hydrothermal system model proposed in this study. Such mature hydrothermal systems contain variably altered materials, from low-temperature acidic to high-temperature neutral types (Fig. 12D). The 2014 eruption at Ontake Volcano ejected variably altered material (Tables 1, 2, 3; Minami et al. 2016), indicating that a highly developed hydrothermal system beneath the volcanic edifice. High-temperature hydrothermal fluids circulate in a mature hydrothermal system, providing significant potential for hydrothermal and phreatic eruptions.

Some alteration textures observed only on the rims of the ash particles, such as amygdaloidal and druse textures, are commonly interpreted as having formed over short reaction times (Imura et al. 2019). In contrast, alteration textures that cover entire ash particles, such as mosaic and colloform textures, are commonly interpreted as having formed over long reaction times (Minami et al. 2022). This study has summarized the estimated reaction times based on the cutting relationships and development sequences of alteration textures, using ore-forming hydrothermal system studies as a framework (Dong et al. 1995; Dong and Morrison 1995). Alteration textures formed within a short reaction time can also be formed within an eruption sequence, as shown in the 2018–2019 eruption at Kuchinoerabujima (Minami et al. 2022). Such textures are formed by the evaporation of hydrothermal fluid and the coating of altered minerals in a shallow hydrothermal system (Minami et al. 2022; Imura et al. 2019). Therefore, analyzing these textures is helpful for understanding the hydrothermal alteration processes during an ongoing eruption sequence.

In contrast, alteration textures that form over long reaction times cannot be interpreted as having formed within an eruption sequence. These altered materials exhibit the chemical and thermal characteristics of previous hydrothermal systems. Although this information is usually overlooked in studies of short-term eruption sequences, when placed within a geological record, they aid in understanding long-term variations in subvolcanic hydrothermal systems and the future potential of eruptions driven by hydrothermal systems. For example, a volcano characterized by ash fall deposits consisting of “neutral” mineral assemblages and long-term hydrothermal textures, such as recrystallization and replacement textures, has a high potential of hosting a mature hydrothermal system. Such volcanoes have a high risk of producing sudden hydrothermal eruptions without new magmatic intrusions and a low potential for producing recognizable precursory activity (e.g. Ontake Volcano; Minami et al. 2016; Dempsey et al. 2020; Caudron et al. 2021).

A descriptive classification of alteration textures was developed based on a review of literature and examination of sampled hydrovolcanic products in Japan. Fifteen textural types were defined, interpreted genetically, and grouped into four major classes: (1) primary growth textures, (2) leaching textures, (3) recrystallization textures, (4) replacement textures. Specific alteration textures can be used as indicators of new magma intrusion, replacement, hydrothermal fluid conditions, and reaction times between volcanic rocks and hydrothermal fluids. This study provides the groundwork for further systematic evaluation of the reaction kinetics between volcanic rocks and hydrothermal fluids during eruption sequences or prior hydrothermal alterations.

Availability of data and materials

All datasets used in this study are available from the corresponding author upon request.

Competing interests

The authors declare that they have no competing interests.

Funding

This study is supported by the project “Geological map of Meakandake Volcano” and the project "Integrated Program for Next Generation Volcano Research and Human Resource Development.”

Authors' contributions

YM conceived the study, carried out the XRD and SEM–EDS analyses, and drafted the manuscript. NG conceived the study, participated in its design and coordination, and drafted the manuscript.

Acknowledgements

We thank Dr. Chris Conway of the Geological Survey of Japan and Editage (www.editage.jp) for English language editing. We thank Dr. Akiko Tanaka of the Geological Survey of Japan for arranging the figures. Discussions with Dr. Junichi Itoh, Dr. Hiroshi Shinohara, Dr. Keiko Matsumoto, and Dr. Teruki Oikawa of the Geological Survey of Japan; Dr. Jenielyn T Padrones of the University of the Philippines; and members of the “Petrology and Volcanology Research Group (Hokudai Black)” of Hokkaido University were helpful for drafting the manuscript. This study is supported by the project “Geological map of Meakandake Volcano” and the project "Integrated Program for Next Generation Volcano Research and Human Resource Development.”

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Table 1 List of samples used in this study.

Volcano	Sample ID	Sample type	Eruption age/year	Reference
Aso	19_A02	volcanic ash	2019	This study
Kuchinoerabujima	18_K01	volcanic ash	2018-2019	Minami et al., 2022
Ontake	14_On_1	volcanic ash	2014	Minami et al., 2016
Meakandake	Pa	volcanic ash	>1000 AD	Minami et al., 2021
Meakandake	Pb	volcanic ash	>1000 AD	Minami et al., 2021
Meakandake	Pon-1(Pc)	volcanic ash	ca. 1000 AD	Minami et al., 2021
Meakandake	Pd	volcanic ash	1000-700 yr BP	Minami et al., 2021
Meakandake	Pe	volcanic ash	1000-700 yr BP	Minami et al., 2021
Meakandake	Pf	volcanic ash	1000-700 yr BP	Minami et al., 2021
Meakandake	Pon-2(Pg)	volcanic ash	ca. 700 yr BP	Minami et al., 2021
Meakandake	Pi	volcanic ash	700-380 yr BP	Minami et al., 2021
Meakandake	Pj	volcanic ash	700-380 yr BP	Minami et al., 2021
Meakandake	Pon-3(Pk)	volcanic ash	380 yr BP	Minami et al., 2021
Meakandake	Pl	volcanic ash	<380 yr BP	Minami et al., 2021
Meakandake	Pon-4(Pm)	volcanic ash	< 1739 AD	Minami et al., 2021

Table 2 Mineral compositions of volcanic ash from the Meakandake, Aso, and Ontake Volcanos.

Layer

Qtz

Crt

Trd

Fsp

Smct

Opx

Cpx

mica

Pyl

Kao

Chl

Alu

Jrs

Hem

Pyr

Anh

Gyp

***

Pon-1

***

Pom-2

***

Pon-3

***

Pon-4

***

18_K01

***

14_On_1

***

The intensities of the X-ray peaks in the XRD analysis are shown as: ***, intense; **, weak; *, minor; and n.p., no peaks observed.

Mineral names are abbreviated as Qtz: Quartz, Crt: Cristobalite, Trd: Tridymite, Fsp: Feldspars, Smct: Smectite, Opx: Orthopyroxene, Cpx: Clinopyroxene mica: mica minerals, Pyl: Pyrophyllite, Kao: 7 Å-kaolin group minerals, Chl: Chlorite, Alu: Alunite, Natroalunite and Minamiite, Jrs: Jarosite, Hem Hematite, Pyr: Pyrite, Ahd: Anhydrite, and Gyp: Gypsum.

Table 3 Alteration textures in volcanic ash from the Meakandake, Aso, and Ontake Volcanos.

Layer

Acid

Replacement or Leaching

Accretion and Recrystallization

Cryptic

Pseudomorph

Saccharoidal

Equigranular

Vuggy

Amygdaloidal

Druse

Bladed

Massive

Colloform

Mosaic

Vein

***

Pon-1

***

Pom-2

***

Pon-3

***

Pon-4

***

18_K01

***

14_On_1

***

Table 3 continue

Layer	Neutral
	Replace or Leaching						Accretion and Recrystallization
	Cryptic	Pseudomorph	Vuggy	Mosaic	Saccharoidal	Dissemination	Vein	Colloform
Pa		*		*
Pb
Pon-1
Pd
Pe
Pf						***	***
Pom-2
Pi		***
Pj	*	**						*
Pon-3
Pl		***	***
Pon-4		**		**	***	***
18_K01
14_On_1	***	**	*	**			*	***

Table 4 Proportion of matrix components in representative samples of pyroclastic fall deposits at Ponmachineshiri, part of Meakandake Volcano (Minami et al. 2021).

Phase	Scoria (%)	Pumice (%)	Lithic (%)	Intensely altered	Weakly altered	Free crystal (%)	n=
Phase	Scoria (%)	Pumice (%)	Lithic (%)	fragment (%)	fragment (%)	Free crystal (%)	n=
Pa	0	0	15.3	17.4	63.9	3.4	327
Pb	4.7	54	17.7	14.3	4.7	4.7	300
Pon-1	78.4	2	4	10.1	0	5.5	398
Pd	0	1	3.3	67.4	28.3	0	389
Pe	0	0	6.9	41.6	51.4	0	449
Pf	0	0	23.3	35.3	41.4	0	343
Pon-2	20.5	1.5	31.8	32.5	13.7	0	459
Ph	0	0.2	15.2	39.9	43.4	1.3	454
Pi	0	0	3.1	38.2	58.7	0	511
Pj	14.4	1.7	32	43.1	7.1	1.7	353
Pon-3	1.9	0	26.8	65.9	3.2	2.2	370
Pl	13	0	20.1	55.7	0	11.2	393
Pon-4	0	0	17.3	64.4	15.8	2.4	329

Table 5 Chemical composition of the amorphous portions of ash grains (including glass) from Meakandake Volcano. Values have been normalized to totals of 100 wt. %.

Phase	sample name	SiO₂	TiO₂	Al₂O₃	FeO*	MnO	MgO	CaO	Na₂O	K₂O	P₂O₅	total	FeO*/MgO	AI	CCPI
Pa	P2-5F-1	78.65	0.77	11.70	2.34	0.00	0.21	0.76	0.40	5.18	0	100	10.94	82.38	31.38
Pa	P2-7F-1	66.52	1.40	13.33	8.64	0.12	1.13	4.35	1.92	2.58	0	100	7.62	37.22	68.46
Pa	P2-8-1	76.02	0.61	12.04	2.14	0.00	0.25	1.09	2.54	5.31	0	100	8.50	60.50	23.31
Pb	P3-5F-1	57.75	1.63	12.99	12.75	0.10	3.35	8.05	1.91	1.47	0	100	3.81	32.61	82.67
Pb	P3-5F-2	57.52	1.58	12.85	12.88	0.25	3.25	8.31	1.88	1.48	0	100	3.97	31.71	82.77
Pb	P3-7F-1	81.44	0.55	11.72	2.05	0.09	0.10	0.29	0.49	3.28	0	100	21.36	81.21	36.26
Pb	P3-9F-1	81.17	0.81	11.53	2.17	0.00	0.00	0.40	0.42	3.49	0	100	n.d.	80.81	35.70
Pb	P3-11-1	76.84	0.53	13.22	2.14	0.15	0.36	2.08	1.19	3.49	0	100	6.00	54.01	34.82
Pb	P3-13-1	81.95	0.49	11.53	1.63	0.07	0.04	0.36	0.44	3.48	0	100	42.19	81.37	29.91
Pb	P13-14F-1	57.21	1.48	13.60	11.95	0.14	3.65	8.66	2.05	1.26	0	100	3.27	31.47	82.49
Pb	P13-14F-2	57.66	1.51	13.72	12.00	0.18	3.42	8.26	1.99	1.27	0	100	3.51	31.38	82.54
Pc	Ps2-1F-1	64.22	1.13	14.59	8.33	0.27	1.69	4.95	3.56	1.27	0	100	4.93	25.83	67.46
Pc	Ps2-1F-2	63.97	1.19	14.55	8.85	0.18	1.73	4.76	3.58	1.19	0	100	5.11	25.92	68.93
Pc	Ps2-3F-1	65.09	1.10	14.53	7.81	0.02	1.85	4.95	3.08	1.56	0	100	4.22	29.80	67.54
Pc	Ps2-7F-1	63.55	1.20	15.29	7.91	0.03	1.48	5.64	3.31	1.58	0	100	5.35	25.49	65.74
Pc	Ps2-7F-2	63.19	1.31	14.78	8.96	0.03	1.52	5.28	3.47	1.47	0	100	5.90	25.43	67.96
Pc	Ps2-11F-2	63.70	1.26	14.72	8.16	0.00	1.75	5.83	3.30	1.28	0	100	4.67	24.92	68.39
Pc	Ps2-12	64.78	1.05	15.03	7.36	0.08	1.65	4.98	3.32	1.74	0	100	4.47	29.00	64.05
Pc	Ps2-16	63.45	1.12	15.30	8.27	0.08	1.43	5.68	3.35	1.33	0	100	5.77	23.42	67.47
Pd	Po4g-2F-1	65.95	1.25	15.66	7.09	0.00	0.94	4.93	1.81	2.38	0	100	7.54	33.04	65.71
Pd	Po4-B1F-1	59.07	1.59	13.13	12.14	0.07	3.81	8.43	0.71	1.06	0	100	3.19	34.76	90.03
Pd	Po4-B1F-2	57.79	1.54	13.77	12.51	0.15	3.93	8.52	0.79	1.00	0	100	3.19	34.60	90.19
Pd	Po4-B2F-1	66.18	1.25	15.98	6.48	0.15	0.70	5.33	2.01	1.92	0	100	9.24	26.32	64.65
Pd	Po4-B4F-1	65.83	1.27	15.05	7.81	0.19	1.16	4.69	1.53	2.47	0	100	6.74	36.86	69.19
Pd	Po4-B5F-1	67.18	1.44	13.48	8.56	0.12	1.50	4.53	1.14	2.04	0	100	5.72	38.45	75.98
Pd	Po4-B8-1	64.46	1.00	16.09	7.66	0.13	1.19	5.52	1.80	2.15	0	100	6.43	31.37	69.15
Pd	Po4-B9	67.95	1.20	14.44	6.74	0.10	1.36	4.42	1.36	2.44	0	100	4.94	39.75	68.08
Pd	Po4-B10-1	58.63	1.65	13.66	13.00	0.28	2.41	7.69	1.34	1.34	0	100	5.40	29.31	85.19
Pd	Po4-B10-2	58.72	1.49	13.91	12.55	0.23	2.55	7.79	1.40	1.36	0	100	4.93	29.81	84.52
Pd	Po4-B11	72.94	0.81	12.99	4.85	0.09	0.73	2.92	1.61	3.06	0	100	6.61	45.57	54.41
Pf	P6-2F-1	78.65	0.50	11.79	0.89	0.20	0.06	0.28	1.55	6.08	0	100	14.64	77.02	11.12
Pg	Po-5 G-3	61.54	0.93	18.55	5.86	0.11	1.41	7.38	3.16	1.05	0	100	4.14	18.97	63.36
Pg	Po-5 G-6	77.66	0.31	13.74	0.59	0.00	0.22	3.44	3.33	0.71	0	100	2.66	12.08	16.67
Pg	Po-5 G-8	64.51	0.87	17.26	6.27	0.07	0.78	5.84	2.30	2.09	0	100	8.06	26.02	61.62
Pg	Po-5 G-9	64.15	1.26	15.87	7.40	0.00	1.18	5.69	2.39	2.07	0	100	6.27	28.66	65.78
Pg	Po-5 G-11	58.39	1.61	13.29	12.92	0.37	2.47	7.80	1.83	1.31	0	100	5.22	28.22	83.07
Pg	Po-5 G-12-1	64.31	0.68	18.96	4.30	0.08	0.62	7.05	2.34	1.67	0	100	6.99	19.57	55.08
Pg	Po-5 G-12-2	69.20	1.03	13.58	6.49	0.12	0.97	4.07	1.60	2.92	0	100	6.72	40.70	62.27
Pg	Po-5 G-14F-1	64.00	1.20	12.93	9.71	0.21	2.70	4.92	2.61	1.71	0	100	3.60	36.92	74.16
Pg	Po-5 G-14F-2	64.25	1.02	17.74	5.74	0.03	0.75	5.96	2.64	1.87	0	100	7.62	23.35	59.02
Pg	Po6-16F-1	66.88	1.35	14.38	8.36	0.00	1.27	4.62	1.42	1.73	0	100	6.58	33.15	75.36
Pg	Po6-g-17-1	56.94	1.72	16.46	8.75	0.16	2.96	8.89	2.84	1.28	0	100	2.96	26.54	73.98
Pg	Po6-g-17-2	57.33	1.52	16.49	9.89	0.15	2.72	8.35	2.34	1.21	0	100	3.64	26.89	78.01
Pg	Po6-g-18F-1	67.52	1.11	14.98	6.30	0.14	1.25	4.87	1.32	2.50	0	100	5.04	37.73	66.38
Pg	Po6-g-19	65.35	1.42	14.41	8.23	0.19	1.40	4.58	2.06	2.34	0	100	5.86	36.07	68.62
Pg	Po6-g20F	62.59	1.10	17.86	5.93	0.06	0.92	6.60	3.35	1.58	0	100	6.45	20.07	58.13
Pg	Po6-g21F	62.85	1.04	17.41	6.91	0.12	1.73	6.47	1.87	1.60	0	100	3.99	28.59	71.34
Pg	Ps6-1F-1	65.89	1.24	13.68	8.28	0.17	1.23	4.73	2.22	2.56	0	100	6.75	35.22	66.55
Pg	Ps6-2F-1	57.80	1.66	13.39	12.37	0.20	3.36	7.87	1.89	1.46	0	100	3.68	33.05	82.45
Pg	Ps6-2F-1	57.25	1.48	13.63	12.48	0.37	3.54	8.36	1.71	1.19	0	100	3.52	31.97	84.69
Pg	Ps6-3F-1	64.75	1.07	14.81	7.94	0.16	1.58	5.24	2.23	2.23	0	100	5.04	33.77	68.07
Pg	Ps6-3F-2	64.05	1.31	14.86	7.93	0.21	1.77	5.29	2.28	2.30	0	100	4.49	34.92	67.93
Pg	Ps6-4F-1	64.51	1.27	14.23	8.37	0.18	1.61	5.14	2.35	2.32	0	100	5.19	34.46	68.11
Pg	Ps6-5F-2	65.76	1.18	14.97	7.29	0.14	0.80	4.79	2.63	2.43	0	100	9.09	30.32	61.49
Pg	Ps6-6F-2	63.63	1.14	15.43	8.08	0.04	1.66	5.25	2.41	2.37	0	100	4.86	34.47	67.11
Pg	Ps6-7F-1	65.23	1.24	14.43	8.08	0.04	1.03	4.90	2.51	2.54	0	100	7.86	32.49	64.35
Pi	P7-9F-1	78.74	0.73	11.06	1.65	0.00	0.03	0.32	1.92	5.56	0	100	57.20	71.39	18.33
Pi	P7-8F-1	76.61	0.61	12.77	1.63	0.18	0.23	1.22	1.89	4.87	0	100	7.10	62.14	21.54
Pi	P7-5F-1	78.54	0.58	11.05	1.53	0.14	0.03	0.51	2.23	5.40	0	100	55.72	66.49	16.95
Pi	P7-4F-1	77.94	0.61	11.73	1.93	0.11	0.41	0.42	0.98	5.88	0	100	4.67	81.79	25.42
Pl	P8-12F-1	58.56	2.14	13.36	13.60	0.12	1.54	6.63	2.24	1.82	0	100	8.84	27.47	78.85
Pl	P8-7F-1	80.09	0.13	12.16	0.42	0.00	0.00	2.80	2.61	1.79	0	100	85.42	24.89	8.80
Pl	P8-3P-1	57.04	1.98	11.97	15.41	0.23	2.55	7.71	1.36	1.76	0	100	6.05	32.18	85.22
Pm	P9-1F-1	85.16	0.55	10.58	0.80	0.03	0.13	0.18	0.64	1.94	0	100	5.94	71.73	26.58
Pm	P9-5-1	69.59	1.16	13.67	7.69	0.18	0.94	4.04	0.48	2.24	0	100	8.17	41.29	76.03
Pm	P9-5F-1	68.09	1.49	13.96	8.16	0.05	1.09	4.67	0.56	1.93	0	100	7.51	36.56	78.76
Pm	P9-7F-1	60.34	1.60	13.92	11.59	0.41	2.68	6.85	0.72	1.88	0	100	4.32	37.64	84.60
Pm	P9-9F-1	78.54	0.72	12.62	1.35	0.00	0.00	1.20	1.25	4.32	0	100	n.d.	63.83	19.52

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Origins and significance of alteration textures in hydrovolcanic products from active volcanoes

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Abstract

Figures

Introduction

Methods

Discussion

Interpretations of micro-textures

Alteration textures and mineral assemblages

Reaction time estimated from alteration textures

Conclusion

Declarations

References

Tables

Supplementary Files

Status:

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