Parental magmas for the Nishiyama volcanic products
The Sr, Nd, and Pb isotopic compositions of the Nishiyama volcanic products, particularly those of the Fujitozando stage, are mostly homogeneous within analytical uncertainty (Fig. 5), but some basaltic samples from the Senjojiki stage have slightly less radiogenic Pb isotopic compositions (Figs. 5c and 6d). These samples are also characterized by lower La/Sm (Fig. 5d) and Zr/Y ratios and lower K2O contents than those of the other main samples. These observations suggest that the magmas of the Fujitozando stage were derived from a single parental magma, but another parental magma with distinct compositions (i.e., lower La/Sm, Zr/Y, and Pb isotopic ratios) might have been involved in the Nishiyama magma system before the Fujitozando stage. The presence of multiple primary magmas in a single volcano has also been recognized in other arc volcanoes, such as that of Pegan volcano in the Mariana arc (Tamura et al. 2014) and Me-akan volcano in the Kurile arc (Kuritani et al. 2021b).
Ishizuka et al. (2008) suggested that the subaerial Nishiyama volcanic products, as well as submarine products from the Hachijo NW chain and NE edifices (Fig. 1b), were derived from a common primary magma, while some magmas from subaerial satellite cones and those from submarine satellite cones from the NE edifices might have experienced slight crustal assimilation. However, the Hachijo NW chain samples, that are among the most primitive volcanic products in Hachijojima, are characterized by low K2O contents (< 0.15 wt.%; Ishizuka et al. 2008). For example, the K2O/TiO2 ratios of the most primitive Hachijo NW chain samples are about 0.1 (Ishizuka et al. 2008), which is much lower than those of the samples from the Fujitozando stage (> 0.24; Additional file 1: Table S1). Considering that titanomagnetite phenocryst is absent in the mafic Nishiyama samples, it is unlikely that the Fujitozando-stage magmas with the K2O/TiO2 ratios of > 0.24 were derived from the primitive Hachijo NW chain magma with the K2O/TiO2 ratios of 0.1. Therefore, we concluded that the parental magma for the Fujitozando-stage samples was different from that for the Hachijo NW chain samples.
As discussed above, the lower La/Sm, Zr/Y, and Pb isotopic ratios of some Senjojiki-stage samples (Fig. 5d) might have been attributed to the involvement of another parental magma in the Nishiyama magma system. Considering that these samples have the lowest K2O/TiO2 ratios of ~ 0.16 among the subaerial Nishiyama samples, the parental magma for these samples, which is expected to have low-K2O content and low Zr/Y and La/Sm ratios, might have been similar to the primitive Hachijo NW chain magma with the K2O/TiO2 ratios of 0.1. This scenario is consistent with the observations that the primitive Hachijo NW chain samples have lower Zr/Y ratios of about 1.3 and lower La/Sm ratios of about 0.6 (Ishizuka et al. 2008) than those of the subaerial Nishiyama samples.
Origin of whole-rock compositional variation
The whole-rock compositions of the samples collected from the Nishiyama volcano are variable (49.4–54.9 wt.% SiO2) and are scattered on Harker variation diagrams (Fig. 3). However, except for some low-Zr/Y samples, the Nishiyama magmas, particularly those of the Fujitozando stage, were derived from a single parental magma and that compositional variations were established essentially by crystal–melt separation, without a significant contribution from crustal assimilation. If the Nishiyama magmas were differentiated by a series of fractional crystallization in a single magma chamber, the compositional evolution would have followed a single liquid line of descent. Therefore, the scattered compositional data on Harker variation diagrams suggest that the formation of compositional variations involved multiple processes occurring at different depths.
Figure 10 shows the whole-rock compositions of the Nishiyama samples in the Al2O3–MgO diagram. In the figure, the compositions of some volcanic units, such as FjS–FjL, IuL, FjD, AzL, and a part of OkS–OkL, exhibit tight linear trends that correspond to plagioclase-controlled lines (broken lines in Fig. 10). This observation suggests that the trends were primarily established by the accumulation or fractionation of plagioclase phenocrysts (Nakano et al. 1991; Tsukui and Hoshino 2002; Aizawa et al. 2020). This is supported by the observation that the whole-rock FeO*/MgO ratios, which do not change with fractionation or accumulation of plagioclase, are mostly constant for some units, irrespective of the SiO2 content (Fig. 3d). The subaerial Nishiyama magmas are significantly differentiated from mantle-derived primary magma because of the low MgO contents (< 5 wt.%). This observation, along with the tight plagioclase-controlled trends, suggests that magmatic differentiation from the parental magma was followed by the accumulation or fractionation of plagioclase phenocrysts.
Depth of magma chamber
As discussed above, the compositional variations of the subaerial Nishiyama products were essentially established by crystal–melt separation. Therefore, there must have been a magma chamber in which differentiation from the parental primitive magma to the magmas of the subaerial volcanic products occurred. To estimate the depth of the magma chamber, the crystallization pressures of the pyroxene phenocrysts were estimated by applying the two-pyroxene geothermobarometer of Putirka (2008) to the clinopyroxene–orthopyroxene pairs found in three samples (#121-2, SL; #7-4, DbL; and #101-1, FjS; Additional file 1: Table S5). The clinopyroxene–orthopyroxene pairs in #121-2 and #7-4 show evidence of contemporaneous growth, but crystal aggregates consisting of orthopyroxene and clinopyroxene were not found in #101-1. Therefore, for this sample, the pressure conditions were estimated using the core compositions of the euhedral clinopyroxene and orthopyroxene phenocrysts in the same thin section. The Fe–Mg exchange coefficients KD(Fe–Mg)cpx−opx are all within the range of 1.09 ± 0.14 (Additional file 1: Table S5), which ensures that the pyroxene pairs were in equilibrium (Putirka 2008).
The result shows that the pressure conditions for the pyroxene crystallization in the samples range 3–4 kbar (Fig. 11). Considering the typical uncertainty of this method of 2.8 kbar (Putirka 2008), the difference in the estimated pressure conditions of the three volcanic stages is not significant. Therefore, we suggest that the magma chamber in which pyroxene crystallization occurred was located at 3–4 kbar, equivalent to a depth of 9–12 km. Ishizuka et al. (2008) considered that the magma plumbing system at the Nishiyama volcano consists of a deep magma chamber (> 20 km depth) from which the main parental magmas were supplied, a middle magma chamber (10–20 km depth) from which the Hachijo NW chain magmas branched off, and a shallow magma chamber (< 5 km depth) in which crystal fractionation and plagioclase accumulation occurred. In this case, the magma chamber located at a depth of 9–12 km, the presence of which is suggested by the pyroxenes in this study, may correspond to the middle magma chamber of Ishizuka et al. (2008). Ishizuka et al. (2008) estimated the depth of the middle magma chamber based on the depth range of the earthquake swarms that occurred in 2002 (Kimata et al. 2004). The estimated magma chamber depth of 9–12 km is also consistent with the dike injection depth of about 12 km estimated from the geodesic ground deformation (Kimata et al. 2004).
The water content of melt in the magma chamber at a depth of 9–12 km was estimated for sample #7-4 (DbL) which was used above. The melt composition was calculated using the whole-rock composition (Additional file 1: Table S1) and phenocryst modal abundances (Additional file 1: Table S4), assuming the average An content of plagioclase phenocrysts of 83 and the Mg# of orthopyroxene phenocrysts of 70. The water content was estimated using the constraint that the melt was saturated with plagioclase at 1030 °C and 3 kbar (obtained above from the two-pyroxene geothermobarometry), and the water content of 4 wt.% was obtained using Eq. 26 of Putirka (2008).
Crystallization of plagioclase phenocrysts
Previous petrological studies on Nishiyama volcanic products have suggested that crystal–melt separation involving high-An plagioclase phenocrysts (> An80) played a primary role in producing the whole-rock compositional variations of the Nishiyama products (e.g., Nakano et al. 1991; Tsukui and Hoshino 2002; Aizawa et al. 2020). In this section, the origin and crystallization processes of plagioclase phenocrysts are discussed.
Aizawa et al. (2020) recently suggested that the high-An plagioclase phenocrysts formed primarily in a shallow-level magma chamber at < 5 km depth under supersaturated conditions by increasing the liquidus temperatures due to decompression-induced vapor exsolution from the melt. However, we found that high-An plagioclase phenocrysts in some subaerial Nishiyama samples show evidence of simultaneous growth with pyroxene phenocrysts (Fig. 7a and d). Because the pyroxene phenocrysts crystallized at 9–12 km depth (Fig. 11), high-An plagioclase crystals are suggested to have been present in the middle-crustal magma chamber.
In some plagioclase phenocrysts, a glass inclusion-poor inner-core region is mantled by a glass inclusion-rich region (Fig. 8c and d). Such glass inclusion-rich plagioclase crystals can be formed by either rapid growth (honeycomb texture) or partial dissolution (dusty texture) of the crystals (e.g., Kawamoto 1992). The size of the glass inclusions in the plagioclase phenocrysts from the Nishiyama volcano is larger than those of plagioclase formed by partial dissolution experiments, which are characterized by fine glass inclusions (e.g., Tsuchiyama 1985; Nakamura and Shimakita 1998). In addition, if the glass inclusion-rich mantle was formed by partial dissolution, the dissolution should invade the inner glass inclusion-poor core region. However, the inner cores always show euhedral outlines (Fig. 8c and d), and no glass inclusions that cut the outlines of the inner cores are found. Therefore, it is suggested that the glass inclusion-rich mantle was formed by rapid overgrowth of the inner-core plagioclase. In this case, the change in the texture within the individual plagioclase phenocrysts (Fig. 8c and d) suggests that there were at least two growth stages; plagioclase formed under a relatively static condition (i.e., glass inclusion-poor inner core) followed by rapid overgrowth under a supersaturated conditions to form the glass inclusion-rich mantle. Considering that glass inclusions are rare in plagioclase phenocrysts that coexist with pyroxene phenocrysts (Fig. 7c and d), it is likely that the inclusion-poor plagioclase crystals formed in the magma chamber at a depth of 9–12 km. On the other hand, the rapid growth of plagioclase is likely to have occurred during magma ascent by increasing the liquidus temperatures due to water exsolution from the melt (e.g., Kuritani 1999; Taniuchi et al. 2021), as discussed by Aizawa et al. (2020).
As estimated above, the H2O content of the melt in some Nishiyama magmas was about 4 wt.% in the magma chamber at a depth of 9–12 km. Considering that the water solubility in the basaltic melt is approximately 4 wt.% at 1.6 kbar (Newman and Lowenstern 2002), the degassing-induced rapid overgrowth of plagioclase phenocrysts is considered to have occurred in the magmas at shallower depths than 5 km.
Accumulation of plagioclase phenocrysts
Ishizuka et al. (2008) and Aizawa et al. (2020) considered that the accumulation of plagioclase phenocrysts occurred in a shallow magma chamber located at < 5 km depth; however, neither explicitly discussed how the depth of < 5 km was constrained. If the magmas resided in a static magma chamber, it is expected that overgrowth of the phenocrysts would have occurred there. In some samples, plagioclase phenocrysts have thick rims with low An content surrounding the high-An core regions (Fig. 8c). It is unlikely that thick rims developed during cooling after the eruption, because rims are also found in quenched scoria samples. Therefore, the thick low-An rims would have grown during storage in a shallow magma chamber. However, in many other samples considered to have experienced plagioclase accumulation, thick rims are not present in the plagioclase phenocrysts (Fig. 8b). This contradicts the inference that accumulation of plagioclase occurred in a static magma chamber.
The presence of a shallow-level static magma chamber is also questioned by the variations in whole-rock compositions among the subaerial Nishiyama volcanic products, which consist of several discrete groups exhibiting tight plagioclase-controlled trends in the Al2O3–MgO diagram (Fig. 10). If plagioclase accumulation occurred in a static magma chamber at shallow levels, convection must not have been effective, because the homogenization of the magmas resulting from the convective current would have inhibited the accumulation or fractionation of plagioclase phenocrysts. Without vigorous convection, when a new magma is injected into the magma chamber in which the magmas exhibiting plagioclase-controlled compositional variation reside (Fig. 12a), the magmas would not mix effectively. The heterogeneous mixing between the two magmas and the subsequent plagioclase accumulation or fractionation would result in the formation of a compositional area in the Al2O3–MgO diagram (Fig. 12b and c), instead of the compositional groups with distinct plagioclase-controlled trends (Figs. 10 and 12d).
Based on these considerations, we conclude that a static magma chamber was not present at shallow levels (< 5 km) beneath the Nishiyama volcano. Alternatively, we suggest that plagioclase accumulation and fractionation occurred in the magmas during ascent (Fig. 13). This scenario can explain the observed whole-rock compositional variations of the subaerial Nishiyama samples (i.e., consisting of discrete compositional groups with tight plagioclase-controlled trends; Fig. 12d), because each magma batch with a distinct composition (i.e., Al2O3/MgO ratio), ascending from the 9–12 km magma chamber to the surface, did not necessarily interact with each other before plagioclase fractionation and accumulation. The absence or presence of the thick rims of the plagioclase phenocrysts can also be explained by different ascent paths: magma batches that temporarily stalled during the ascent may have resulted in the formation of plagioclase rims, whereas those that ascended without stalling for a significant time did not develop thick rims (Fig. 13).
Figure 14 compares the density of the melt with that of plagioclase with An80–92. The melt was represented by the interstitial melt of sample #7-4 (Additional file 1: Table S1), as used above. The variation in the melt density with pressure was calculated using the model of Iacovino and Till (2019) assuming that the temperature was constant at 1030 °C (obtained above from the two-pyroxene geothermometry). As discussed above, the water content of the melt was about 4 wt.%, and the melt was saturated with water at about 1.6 kbar. The H2O content of the water-saturated melt as a function of pressure was obtained using the water solubility model of Newman and Lowenstern (2002). The density of plagioclase with An80–92 at 1030 °C as a function of pressure was calculated using the model and parameters of Berman (1988). The comparison shows that the density of plagioclase was higher than that of the melt at < 4 kbar (Fig. 14). This suggests that, during the ascent of magma from the 9–12 km magma chamber, plagioclase phenocrysts would not have floated in the melt. Thus, accumulating and fractionating plagioclase settled relative to the surrounding melt as the melt ascended (Fig. 13). Effective plagioclase–melt separation might have occurred at deeper levels, where the density difference between the melt and plagioclase was larger. For example, the simple Stokes’s law suggests that a hypothetical spherical plagioclase crystal with a radius of 2 mm would settle at a velocity of 1.3 m/d at 1030 °C and 1.5 kbar, where the melt water content is estimated to be 3.8 wt.% and the melt viscosity is calculated to be 102.0 Pa·s (using the model of Giordano et al. 2008). As plagioclase phenocrysts settled in an ascending magma batch, it is expected that the abundance of plagioclase phenocrysts in the magmas would increase as the eruption progresses. Unfortunately, however, we could not successfully test this hypothesis because each volcanic unit (Fig. 2) commonly consists of many lava flow units and it was difficult to determine the eruption sequence of individual samples collected from the each volcanic unit.
Magma plumbing system
The magma plumbing system and pre-eruption magmatic processes inferred from the results of this study are summarized in Fig. 13. Parental magmas for the Nishiyama volcano were homogeneous during the Fujitozando stage (< 0.7 ka), while another parental magma with distinct geochemical features, which may have erupted from the Hachijo NW chain, was also present in the magmatic system before 1 ka. The parental primitive magmas were supplied from deep levels to the magma chamber at a depth of 9–12 km. Magmatic differentiation resulting from the fractional crystallization of olivine, pyroxenes, and plagioclase occurred in the magma chamber. The temporal variation in the whole-rock FeO*/MgO ratios, that are not affected by plagioclase accumulation and fractionation, is rather complex, and the ratios do not increase systematically with time (Fig. 6a), suggesting that primitive magmas were intermittently supplied to the magma chamber in which fractional crystallization occurred. During the ascent of magmas from the magma chamber, significant overgrowth of plagioclase phenocrysts is suggested to have occurred at levels shallower than about 5 km. And then, gravitational plagioclase–melt separation occurred in the ascending magmas, resulting in the accumulation and fractionation of plagioclase phenocrysts in the magmas.
Some magmas are considered to have stalled temporarily during their ascent to the surface, resulting in the crystallization of the microphenocrysts and thick rims of the plagioclase phenocryst (Fig. 8c). For sample #7–4, the pressure conditions for the crystallization were estimated assuming that the interstitial melt (Additional file 1: Table S1) was in equilibrium with An69 plagioclase (the mode of the An content of the rims; Fig. 9) at 1030 °C. The plagioclase–melt hygrometer (Eq. 25b of Putirka 2008) yields a melt H2O content as 1.8 wt.%. Because the melt was saturated with H2O at shallow levels, as discussed above, the pressure was estimated to be about 0.3 kbar using the water solubility model of Newman and Lowenstern (2002). Therefore, it is suggested that the magmas stalled temporarily at a depth of approximately 1 km (Fig. 13).