Bull. Volcanol. Soc. Japan
Vol. 58 (2013) No. 1, pp. 43-58

ArticleGeochemical and Sr-Nd-Pb Isotopic Constraints on the Origin andMagmatic Evolution of Quaternary Lavas of
Sakurajima Volcano, Southern Kyushu Island, Japan
Tomoyuki SHIBATA＊, Jun SUZUKI＊, Masako YOSHIKAWA＊, Tetsuo KOBAYASHI＊＊,
Daisuke MIKI＊＊＊ and Keiji TAKEMURA＊(Received October 5, 2011 ; Accepted January 7, 2013)
We present the results of a detailed petrogenetic study employing newly determined whole-rock major and traceelement geochemical analyses and Sr-Nd-Pb isotopic compositions of andesitic and dacitic Quaternary lavas of
Sakurajima volcano, a post-caldera volcano situated within the Aira caldera of Japan. Similar geochemical and isotopic
investigations are also carried out on basaltic rocks from pre-caldera stage and monogenetic volcanoes from near
Sakurajima volcano. Quaternary lavas of Sakurajima volcano analyzed in this study are classified as porphyritic
andesites or dacites that contain a mineral assemblage of orthopyroxene, clinopyroxene, and plagioclase, with orwithout olivine, in a groundmass exhibiting either hyalo-ophitic or hyalopilitic textures. The trace element
characteristics of these samples are similar to those of typical island arc magmas, showing clear evidence of Nb
depletion along with enrichments in Rb, K, and Pb, which suggests the addition of aqueous fluids to the mantle wedge
during melt generation. The Sr, Nd, and Pb isotopic compositions plot close to a mixing curve between MORB-type
mantle and sediments of the Philippine Sea Plate, but displaced a bit towards more radiogenic compositions. Plots of Zr
versus Nb concentration in these lavas yield a linear trend that falls on a compositional mixing line between the values
for mid-ocean ridge basalt (MORB) and average continental crust. Collectively, these observations indicate that the
primary source magmas for the Quaternary lavas of Sakurajima volcano were initially generated by partial melting of
MORB-type mantle wedge that had already been hydrated by fluids derived from the subducting Philippine Sea Plate.
The additional contribution of significant amounts of crustal material during magma evolution is also evident from theZr/Nb ratios and Sr-Nd-Pb isotopic compositions of the analyzed andesites and dacites of Sakurajima lava samples.
From the mixing relation of Sr-Nd-Pb isotopic compositions, it is suggested that the sedimentary rocks of Shimanto
Group can be a source of the crustal materials. Although most of the major element oxide compositions of these lavas
show a single linear trend on each of the Harker diagrams, two different trends are clearly discernible on each of theP2O5, and TiO2 versus silica variation diagrams, and are subdivided into low-P and high-P geochemical groups. These87
86
two groups can also be distinguished when comparing their P2O5 and TiO2 contents and Sr/ Sr ratios, relative to theirphenocrystic plagioclase modal abundances. The magma mixing trends of Sakurajima lavas, which seem to beextended from mono andesitic end-member to two different deictic end-members, are observed from the relationships
87
86
of major element contents and Sr/ Sr ratios. In addition, the low-P versus high-P groups of lavas show distinctive
distribution patterns, whereby the high-P lavas are surrounded by low-P lavas in the central to southern parts of the
Sakurajima volcano study area. These observations indicate that mixing of andesitic and dacitic magmas played an
important role in the genesis of Quaternary lavas of Sakurajima volcano, and that multiple dacitic magma chamberswith different geochemical characteristics once existed beneath the Sakurajima area at relatively shallow levels in thecrust. From the relations between SiO2 and Sr isotope ratios, an assimilation and fractional crystallization process isrequired to originate the andesite and dacite end-members.
Key words : Sakurajima volcano; Aira; Kyushu; caldera; arc magmas; trace elements; Sr-Nd-Pb isotopes＊

Beppu Geothermal Research Laboratory, Institute for
Geothermal Sciences, Kyoto University, Beppu, Oita
874-0903, Japan.
＊＊
Graduate School of Science and Engineering, Kagoshima
University, Kagoshima, Kagoshima 890-0065, Japan.

＊＊＊

Sakurajima Volcano Research Center, Disaster Prevention Research Institute, Kyoto University, Sakurajima,
Kagoshima 891-1419, Japan.
Corresponding author: Tomoyuki Shibata
e-mail: tomo@bep.vgs.kyoto-u.ac.jp

44

Tomoyuki SHIBATA, Jun SUZUKI, Masako YOSHIKAWA,
Tetsuo KOBAYASHI, Daisuke MIKI and Keiji TAKEMURA

1．Introduction
Volcanic eruptions and associated caldera formation
represent some of the most catastrophic geological phenomena on Earth, and hence many geological, petrological, geochemical, and geophysical studies have sought
to understand these phenomena (e.g., Smith and Bailey,
1968; Lipman, 1984; Furukawa et al., 2009; Gregg et al.,2012). From these studies, it is now well established thathuge volumes of silicic magmas accumulate beneath thevolcano prior to caldera formation, and that eruptions ofpyroclastic material are typically caused by some sort ofobvious trigger (e.g., Hildreth and Wilson, 2007). However, the origin of the silicic magmas themselves is more
controversial, and several petrogenetic models have been
proposed, as follows; 1) fractional crystallization of mafic
magma with crustal assimilation (AFC model; e.g., DePaolo,
1981; Caffe et al., 2002; Reagan et al., 2003), 2) partial
melting of basaltic lower crust (e. g., Takahashi, 1986;
Bead and Lofgren, 1991) and 3) partial melting of basaltic
rocks that were emplaced into the lower crust a long timeprior to the actual silicic magma activity itself (e.g., Sisson
et al., 2005; Ban et al., 2007). Investigations into thenature and origin of different magmas that may have been
generated pre-, syn-, or post-caldera formation, areparticularly useful for understanding the genesis of thesetypes of silicic magmas, especially because there is thedistinct possibility that such silicic magmatic and volcanic
activity could be the harbinger of the next caldera-forming
event (e.g., Furukawa et al., 2009; Miyoshi et al., 2011).
Sakurajima volcano represents a post-caldera volcano
linked with the Aira caldera, which itself is situated along
the volcanic front of the Ryukyu arc, at the southern end of
Kyushu Island, Japan (Fukuyama, 1978), where thePhilippine Sea Plate (PSP) is subducting beneath theEurasian Plate (Fig. 1). After the formation of the Airacaldera at ca. 29 cal ka BP, volcanism associated with
Sakurajima volcano at about 26 cal ka BP, which to thisday remains an active volcano (Okuno, 2002). For thisreason, Sakurajima volcano is considered to be one of themost suitable volcanoes for studying the links between
caldera formation and catastrophic silicic magmatic
activity. It has been proposed that the relative contributions of basaltic, andesitic, and dacitic magmas arereflections of the assimilation of variable amounts ofcrustal materials, and this relationship is thought to explain
the entire process of magmatic evolution of the Sakurajima
lavas subsequent to their initial formation by slab-fluidrelated partial melting in the mantle wedge (Yanagi et al.,
1991; Arakawa et al., 1998; Uto et al., 2005; Nakagawa etal., 2011). Yanagi et al. (1991) emphasized that a binary
magma-mixing process must have played an importantrole in producing the observed linear trends of the
Sakurajima lavas on plots of major element oxides versus
SiO2 content (Fig. 2), as well as the commonly observed
bimodal anorthite contents within the phenocrystic

plagioclase cores of these lavas. Arakawa et al. (1998)
reported an observed increase in 87 Sr/86 Sr ratio withincreasing SiO2 content for the volcanics of the Airacaldera (including Sakurajima lavas), and suggested thatthe contribution of crustal materials to the magma at somestage of its evolution can explain these geochemical/
isotopic relationships. They also emphasized that magma
mixing of mantle-derived basaltic magma with dacitic
magma that had interacted with lower crustal materials,
may have taken place during the magmatic evolution ofthese magmas. Recently, Uto et al. (2005) noted that two
distinct geochemical trends are discernible on several plotsof major oxide compositions versus SiO2 content, especially with regards to TiO2 and P2O5 concentrations. They
also identified two distinct trends on diagrams displaying
SiO2 content versus 87 Sr/86 Sr ratio; i. e., both low andhigh 87Sr/86Sr ratios for different rocks with the same SiO2
concentrations, and therefore claimed that more complicated magmatic evolutionary processes are required to
explain the data. Therefore, geochemical evolution of
Sakurajima lavas is still controversial. However, theavailable geochemical data on lavas erupted from Sakurajima volcano are currently somewhat limited in terms of
elucidating robust petrogenetic models of silicic magma
generation linked with this volcano-caldera system.
Although considerable amounts of major and trace element
geochemical data have been reported for this volcano, andseveral Sr-Nd isotopic datasets are also available (e. g.,
Arakawa, 1998; Uto et al., 2005), data on Pb isotopic
ratios of these lavas, which would be useful in deciphering
the magma source materials (e.g., Shibata and Nakamura,1997) are currently rather scarce.
In the present study, we collected several volcanic rocksamples (mainly lavas), from almost all of the individual
volcanostratigraphic units (Table 1; Fig. 1) previously
defined by Fukuyama and Ono (1981), for a detailed
whole-rock major and trace element geochemical and SrNd-Pb isotopic petrogenetic investigation of Quaternarylavas of Sakurajima volcano. In addition to these Sakurajima samples, we studied basaltic volcanic rocks collectedfrom a region adjacent to the Aira caldera, in order toobtain information of the primary magmas linked with theformation of Sakurajima volcano.
2．Sample descriptions
Sakurajima volcano consists of two adjoined stratovolcanoes; Kitadake and Minamidake. Kitadke volcano is
further subdivided into two separate volcanic edifices
known as “Old” Kitadake volcano and “Young” Kitadake
volcano, which were active from 〜26 to 24 cal ka BP andfrom 12. 8 to 5 cal ka BP, respectively (Okuno, 2002).
Minamidake volcano is an active stratocone that has been
growing on the southern slope of Kitadake since about 4.5
cal ka BP (Okuno, 2002). In addition, a number of smaller
parasitic volcanoes are observed that have been describedGeochemical and Sr-Nd-Pb Isotopic Constraints on the Origin and Magmatic Evolutionof Quaternary Lavas of Sakurajima Volcano, Southern Kyushu Island, Japan

45

Fig. 1. Geological context and sampling localities. The volcanostratigraphic succession of Sakurajima volcano is simplified
from Fukuyama and Ono (1981). Solid and open circles represent low-P and high-P groups, respectively (see main text
for details).

as the lateral cones and lava flows from lateral vents,
respectively, by (Fukuyama, 1978), with the latter having
erupted in historical times (except for the Atagoyama
lavas). From all of these various volcanic structures andflows, the volcanics of Sakurajima volcano are classified
into four distinct groups; Kitadake, Minamidake, lateralcones, and lateral vents. Fukuyama and Ono (1981)
subdivided these four groups into 27 units on the basis of

46

Tomoyuki SHIBATA, Jun SUZUKI, Masako YOSHIKAWA,
Tetsuo KOBAYASHI, Daisuke MIKI and Keiji TAKEMURA

Fig. 2. Plots of SiO2 versus major element oxides (and FeO*/MgO ratio). All oxides are presented in wt.%. TH and CA
denote tholeiite and calc-alkaline rock series, respectively. See the main text for additional information on TH, CA, lowK, medium-K, and high-K series.

lithologic, geographic and stratigraphic relationships.
Fukuyama and Ono (1981) also described that 22 of the 27
volcanostratigraphic units of Sakurajima volcano arecomposed of lava flows and/or lava domes, whereas theother 5 units (including the Harutayama unit) are com-posed of pyroclastics. Subsequent to that study, Kobayashi(1982) revealed that the Harutayama unit is actually madeup of a lava dome rather than pyroclastics. Thus, there are
now 23 units that are considered to be composed of lava
flows and/or lava domes; all of these were sampled in theGeochemical and Sr-Nd-Pb Isotopic Constraints on the Origin and Magmatic Evolutionof Quaternary Lavas of Sakurajima Volcano, Southern Kyushu Island, Japan
Table 1.

Stratigraphic succession and sample localities.

47

48

Tomoyuki SHIBATA, Jun SUZUKI, Masako YOSHIKAWA,
Tetsuo KOBAYASHI, Daisuke MIKI and Keiji TAKEMURA

present geochemical and isotopic study of Sakurajima
volcano. Because the rock types represented by all ofthese lava samples fall in a restricted range of compositions from andesitic to dacitic, we also sampledbasaltic rocks from pre-caldera stage lavas of the Airacaldera and from associated monogenetic volcanoes
adjacent to Sakurajima volcano, in order to investigate thechemical characteristics of the mantle wedge beneath thestudy area. All of these sample localities are shown in Fig. 1.
Petrographic observations of these historical lava samples,
as made in the present study, compare favorably with thosepresented by Kubotera (1981). Because no remarkablepetrographic differences are observed amongst the various
volcanic stages/units of Sakurajima volcano, the descriptions of the lava samples analyzed in this study are simply
made as a broad description for each rock type, although
the modal abundances (of both the mineralogical constituents and groundmass) for each individual sample are listed
in Table 2.
2-1 Dacites
The analyzed dacite samples comprise gray- to blackcolored porphyritic volcanic rocks that exhibit hyaloophitic or hyalopilitic groundmasses with exceptions of the
Matsura and K8 lavas, which show hyalopilitic groundmasses that often contain vesicles. Several dacite samplesof the Matsura and Atagoyama lavas show a distinct
fluidal structure, while a number of others exhibit only aweak fluidal structure. The phenocrysts within thesedacites exhibit a wide range of grain sizes from <0.1 to 2.0mm across, which to some degree correlates with crystal
habit, whereby the relatively large phenocrysts tend to beeuhedral and the smaller ones subhedral to anhedral. The
phenocrysts consist of orthopyroxene (Opx), clinopyroxene(Cpx), plagioclase (Pl), and opaque minerals (Opq).
According to Fukuyama (1981), the Opq that occur within
the historic lavas commonly contain titanomagnetite
crystals. The groundmass consists primarily of brown todark brown volcanic glass, with lesser amounts of microcrystalline Pl, Opx, Cpx, and Opq. Brown to dark brownglass inclusions often occur within the Pl phenocrysts.
2-2 Andesites
The andesite samples analyzed in this study are mainly
gray to black in color, exhibiting porphyritic textures withhyalo-ophitic groundmasses, and are often vesiculated.
Similarly to those in the dacite samples, the phenocrysts inthese rocks show a wide range of grain sizes from <0.1 to
2. 0 mm across, with relatively large phenocrysts being
euhedral and smaller phenocrysts being subhedral toanhedral. The andesites contain discrete phenocrystic
grains of Opx, Cpx, Pl, and Opq, although the rocksamples collected from the Taisho I and Atogoyama lava
flows additionally contain small amounts of olivine (Ol)phenocrysts (Table 2). Kubotera (1981) reported Ol
microphenocrysts in the Bunmei andesite lavas, although
in the present study we observed no such Ol micro-

phenocrysts in our Bunmei II thin section. The groundmass in these Sakurajima andesites consists predominantlyof dark brown glass, along with lesser amounts of Pl, Opx,
Cpx, and Opq. Brown to dark brown inclusions of glass
are common within Pl phenocrysts.
2-3 Basalts
The Shirahama basalt unit was one of the most dominant
types among the lavas which built up the pre-Aira-caldera
volcano. The Aojiki and Sumiyoshiike basalts form a partof a suite of monogenetic volcanoes located 〜20 km
north-northwest of Sakurajima volcano. Phenocrysts in
the Shirahama and Sumiyoshiike basalts are relatively
coarse-grained at around 2 mm across, whereas those in the
Aojiki basalt are comparatively fine-grained at < 1 mm
across. The basalts sampled in this study commonly show
a porphyritic texture with phenocrysts of Ol, Opx, Cpx, Pl,
and Opq. The groundmass within the Aojiki basalt showsa discernible intergranular texture consisting of Pl, Cpx,
and Opq crystals, whereas the other basalt samples exhibits
an intersertal texture made up of Pl, Cpx, Opq, and glass.
The plagioclase phenocrysts in the Aojiki basalt exhibit a
distinct poikilitic texture defined by numerous small
inclusions of glass and anhedral Cpx grains.
3．Whole-rock analytical methods
Prior to whole-rock geochemical and Sr-Nd-Pb isotopic
analyses, all rock samples were crushed to coarse chips(≈0.5 mm3), and then fresh pieces were handpicked foreach whole-rock aliquot. To avoid surface contamination,
the rock chips selected for analysis were then leached withethanol, 0.5 M HCl, and submerged and rinsed with MilliQ® water in an ultrasonic bath. The clean rock chips werethen comminuted to a grain-size of less than 200 meshusing a vibrating tungsten carbide puck mill for the major
element chemical analysis, whereas a puck mill made ofalumina ceramics was employed to the samples for the
trace element and Sr-Nd-Pb isotopic analysis.
Major and trace element whole-rock compositions were
determined using a RIGAKU 3070® X-Ray Fluorescence(XRF) system by employing the glass beads and pressedpellets methods, respectively, following closely the procedures and instrumental set-up outlined in Sugimoto et al.
(2007). Whole-rock Pb, Nd, and Sr isotopic compositions
were measured using a Finnigan MAT 262® Thermal
Ionization Mass Spectrometer (TIMS) after the methods
outlined in Shibata and Yoshikawa (2004), Shibata andYoshikawa (2002), and Shibata et al. (2007). All of thewhole-rock geochemical and isotopic analyses in thisstudy were carried out at the Institute for Geothermal
Sciences (IGS), Kyoto University, Japan. The normalizing
factors used to correct isotope fractionation during mass
spectrometry, in the course of Sr, Nd, and Pb isotopic
measurements, were: 86 Sr/88 Sr=0.1194, 146 Nd/144 Nd=
0.7219, and 0.001 % per atomic mass unit, respectively.
The measured isotopic ratios of the employed standard

Geochemical and Sr-Nd-Pb Isotopic Constraints on the Origin and Magmatic Evolutionof Quaternary Lavas of Sakurajima Volcano, Southern Kyushu Island, Japan

49

Table 2. Whole-rock major oxide and trace element concentrations, and modal mineralogy of Sakurajima volcano
lavas and nearby basalts.

Total amount of Fe oxide expressed as Fe2 O3 *. The abbreviations Ol, Opx, Cpx, Pl, Opq, and Gm denote olivine,orthopyroxene, clinopyroxene, plagioclase, opaque minerals, and groundmass, respectively. “tr.” and dash “-” denote‘trace’ and ‘not observed’, respectively.

materials were: 87 Sr/86 Sr=0.710279±0.000028 (2σ) for
NIST SRM987; 143Nd/144Nd=0.511851±0.000013 (2σ)
for La Jolla; and 206 Pb/204 Pb = 16. 944 ± 0. 004 (2 σ),207
Pb/204Pb = 15. 499 ± 0. 004 (2 σ), and 208 Pb/204 Pb =36.721±0.010 (2σ) for NIST SRM981.

4．Results
The whole-rock major element compositions of thestudied andesitic to dacitic rock samples from Sakurajima
volcano (and nearby basalts) are listed in Table 2 and alsoplotted on silica variation diagrams in Fig. 2. The obtained
range of the SiO2 and MgO contents, and FeO*/MgO

Tomoyuki SHIBATA, Jun SUZUKI, Masako YOSHIKAWA,
Tetsuo KOBAYASHI, Daisuke MIKI and Keiji TAKEMURA

50
Table 3.

Sr, Nd, and Pb isotopic compositions of Sakurajima volcano lavas and nearby basalts.

Analytical errors are quoted at 2σ.

ratios are 61.33-68.56 wt. %, 1.15-2.72 wt. %, and 2.40-4.
12, respectively. Based on the relationships between SiO2
contents and FeO*/MgO ratios, these lavas are classified
into the calc-alkaline series defined by Miyashiro (1974).
The K2O contents in these lavas range from 1.65 to 2.55
wt. %, and fall within the medium-K classification field of
Le Maitre et al. (1989). The basalt samples are alsoclassified as medium-K volcanic rocks. Overall, the obtained whole-rock major element compositions are in good
agreement with the relevant appeared in the previous geochemical studies on these rock suites (e.g., Kubotera, 1981;
Yanagi et al., 1991; Arakawa et al., 1998; Uto et al., 2005).
The whole-rock trace element compositions determinedin this study are listed in Table 2 and also plotted together
with the potassium (K) and phosphorus (P) contents as
MORB-normalized spider diagrams in Fig. 3. Pronounced
negative Nb spikes are clearly recognized for all thesamples, and Rb and K tend to be the most enrichedelements relative to MORB. In addition, the Rb/Nb andK/Nb ratios are relatively high, being consistent with thevalues observed in island arc volcanic rocks (e.g., Wood etal., 1979; Perfit et al., 1980).
The whole-rock Sr-Nd-Pb isotopic compositions for theabove mentioned samples are listed in Table 3 and presented in Figs 5 and 6. The Sr-Nd-Pb isotopic compositions of the Sakurajima post-caldera lavas are 18.355-18.
402 for 206 Pb/204 Pb; 15.534-15.591 for 207 Pb/204 Pb; 38.
485-38. 667 for 208 Pb/204 Pb; 0. 705136-0. 705556 for87
Sr/86Sr; and 0.512489-0.512561 for 143Nd/144Nd (Table3). In contrast, the isotopic compositions determined forthe pre-caldera basalt samples in this study are lessradiogenic than those determined for the andesitic todacitic post-caldera lavas (Table 3).

5．Discussion
5-1 Constraints on the origin of primary magmas
of mantle origin
One of the main reasons for investigating the traceelement compositions of the lavas is to elucidate themechanisms behind the generation of the primary magmas
for the Sakurajima volcano. The MORB-normalized traceelement patterns for these lavas display substantial enrichments of fluid mobile elements such as Rb, K, and Pb,
while showing also pronounced depletions in Nb (Fig. 3).
These characteristics are generally recognized within volcanic rocks originating from arc magmas, occurring above
subduction zones, and is often explained by the addition of
aqueous fluid from the subducting oceanic plate to theoverlying mantle wedge; the source region of arc magmas(e.g., Miller et al., 1994; Shibata and Nakamura, 1997). In
the case of Sakurajima volcano, it seems likely that
aqueous fluids released from the subducting PSP slab
would have lowered the solidus of the overlying mantle
wedge, causing partial melting to take place at depth there,
and ultimately leading to the surface/near-surface magmatic
activity to develop the Sakurajima volcano.
Similarly to the case with the NE Japan (e.g. Shibata
and Nakamura, 1997; Sakuyama and Nesbitt, 1986), thegeochemical characteristics for the mantle wedge beneaththe Sakurajima also resemble to those for the MORB type
mantle. As shown in Fig. 4, the Zr/Nb ratios for the mafic
magma of the Sakurajima volcano and its adjacent area
mostly come within the values for the lavas of the NE
Japan. Indeed, the ratios are somewhat lower than thevalues for the MORB, but the obtained values are muchcloser to the MORBs (ave; 32) than the relevant for the
OIBs (5.8; Fig. 4), although the ratio of basalt from Aojikishow the similarity to that of the OIBʼs.
The tendency that Zr/Nb ratio of the andesites anddacites from Sakurajima volcano decrease slightly with

Geochemical and Sr-Nd-Pb Isotopic Constraints on the Origin and Magmatic Evolutionof Quaternary Lavas of Sakurajima Volcano, Southern Kyushu Island, Japan

Fig. 3. MORB-normalized trace element patterns. The
normalizing values are from Sun and McDonough(1989).

increasing Nb content might be due to incorporation ofcontinental crustal materials into the magma source region,or assimilation of such crustal materials into the basaltic
parental magma. As shown in Fig. 4, all the Sakurajima
andesitic to dacitic volcanic rocks plot on or close to themixing line (thick black line) between average continental
crust and pre-caldera basalts in this region
The Sr-Nd-Pb isotopic compositions of all of Sakurajima lavas plot close to the two-component mixing curves
between MORB-type mantle and sediments of the PSP(Fig. 5). This is well reconciled with the view based on
trace element compositions that PSP slab-derived fluids
were added to the mantle wedge prior to magma
generation. However, from the view more in detail, the51isotopic compositions for the basalt samples fall nearly on
the above mentioned mixing curves, with distinctively
high 143Nd/144Nd and low 208Pb/204Pb ratios relative to thepost-caldera lavas (Figs 5a and 5b). This observationsuggests that considerable amounts of continental crustmaterials mixed with basaltic parental magma. This viewis compatible with our interpretation for the whole rock Zr
and Nb contents and also for Sr-Nd-Pb isotopic
compositions.
5-2 Sr-Nd-Pb isotope constraint for the crustal
materials
The Sr-Nd-Pb isotope mixing relationships of thebasement sedimentary rocks (Cretaceous Shimanto Group)with basaltic and andesitic magmas of Sakurajima volcano
are examined to better understanding for the crustal
materials contributed to the genesis of Sakurajima magma(Fig. 5). The sedimentary rocks of Shimanto Group are
considered as one of the possible crustal source materials
in the addition to the lower and upper crust (Yanagi et al.,
1991; Arakawa et al., 1989). Hosono et al. (2003) reported major, trace and Sr-Nd-Pb isotopic compositions of
Shimanto sedimentary rocks. The isotope ratios of the
Shimanto sedimentary rocks shows wide ranges ofvariations, so we used the data of most depleted (lowest Sr
and Pb and highest Nd isotope ratios) sample (4 W4,Hosono et al., 2003) and enriched (highest Sr and Pb andlowest Nd isotope ratios) sample (1 WL4, Hosono et al.,2003) for the mixing calculations (Fig. 5).
The different two isotopic components are required as
the crustal source of the Sakurajima magma from the SrNd-Pb isotope mixing relationships. It is clear that
andesite to dacite lavas of Sakurajima volcano are affectedwith crustal materials, because the Sr and Nd isotope ratios
are differed from the mixing line of the subducted slab andthe mantle wedge and are plotted near on the mixing lineof 4 W4 and Sumiyoshiike basalt, which can represent the
primary magma of Sakurajima magmas (line S in Fig. 5a).
However, the trend of Sr-Nd isotope ratios of Sakurajima
lava, especially two dacite lavas (10102109 and
10102112), seems to differ from the line S, and are closed
to the mixing curve of Showa andesite and 1 WL4 (line Sʼ
in Fig 5a). Although most of the Pb isotope ratios of
Sakurajima lavas lie on closed to the mixing line S, like as
Sr-Nd isotope system, the ratios are also plotted on theslab-mantle wedge mixing line (Fig. 5b). Furthermore, thePb isotope rations of two dacite lavas (10102109 and
10102112) show similar trend to the mixing line Sʼ (Fig.
5b). In the Sr-Nd isotope system, it can be possible todraw mixing lines of S and Sʼ by single isotopic componentof crustal material with changing Sr/Nd abundance ratio,
but it is impossible in the Pb isotope system (e.g., Shibata
and Nakamura, 1997). Therefore, at least two different
enriched isotopic components are required to explain thevariations of Sr-Nd-Pb isotopic compositions. Although
lower and upper crustal materials have to be examined as aTomoyuki SHIBATA, Jun SUZUKI, Masako YOSHIKAWA,
Tetsuo KOBAYASHI, Daisuke MIKI and Keiji TAKEMURA

52

Fig. 4. Nb versus Zr concentration showing data for Sakurajima volcano (and nearby basalts). Symbols are the same as inFig. 2. The compositional ranges of Zr and Nb from arc lavas from the Mariana (MRI), Kurile (KRL), South Sandwich(SSW), and Northeastern Japan (NEJ) arcs are shown as geochemical domains, and reference lines depicting MORB andOIB compositional trends are also shown (data are from Shibata and Nakamura (1997) and references therein). The solid
black line represents a mixing line between the pre-caldera basalt composition and average continental crust (Weaver
and Tamey, 1984).

crustal source, no available comprehensive data of theisotopic composition from the area closed to the
Sakurajima volcano can be found. Therefore, we
concluded that the Shimanto sedimentary rocks, of which
Sr-Nd-Pb isotopic composition heterogeneous, can be acrustal source for Sakurajima lavas.
5-3 Genetic relationship between high- and low-PgroupsThe dacites of 10102109 and 10102112 show different
trends of major element compositions from those of theother lavas from Sakurajima volcano, especially P2O5 andTiO2. To help elucidate detailed investigation of geochemical evolution of Sakurajima magma, here after, wehave subdivided our samples into high-P (P2O5>0.15 wt.
%) and low-P (P2O5 <0.15 wt.%) geochemical groups.
The variations of major and modal compositions can be
explained by mixing between andesite magma and dacite
magma. This is consistent with the previous studies (e.g.,
Yanagi et al., 1991; Arakawa et al., 1998). It can besuggested that the lavas of high- and low-P groups are
generated by different magmatic processes judged fromthe variations of SiO2, TiO2 and P2 O5 concentrations

and 87Sr/86Sr ratios against to the modal abundance of Pl(Fig. 6). The modal abundance of Pl in lavas of Sakurajima volcano strongly correlates with major oxide contents
and isotopic ratios, and the low-P and high-P groups also
appear to show distinctly different trends (commonly
sloping in opposite directions; Fig. 6). As shown in Fig. 6,
the concentrations of TiO2 and P2 O5 decrease systematically with decreasing modal Pl in the low-P group,
whereas the corresponding concentrations for the high-Pgroup either increase or remain at constant values withdecreasing modal Pl. Both the low-P and high-P lavas
show less distinctive and comparatively broader negative
correlations of SiO2 versus modal abundance of Pl, and the
SiO2 contents of the low-P rocks are slightly higher thanthose of high-P rocks at >20 % modal abundance of Pl.
Furthermore, the Sr isotopic ratios of the low-P and high-Plavas show similar trends with respect to SiO2 content,
whereby low-P samples that have <20 modal% of Pl havedistinctly high ratios (87Sr/86Sr>0.7055), while the highP rocks have lower ratios (87 Sr/86 Sr<0.7053) (Fig. 7).
These observations indicate that evolution both of highand low-P groups had started from similar andesite magma,

Geochemical and Sr-Nd-Pb Isotopic Constraints on the Origin and Magmatic Evolutionof Quaternary Lavas of Sakurajima Volcano, Southern Kyushu Island, Japan

Fig. 5. Variation diagrams displaying 87 Sr/86 Sr versus 143 Nd/144 Nd (a) and 208 Pb/204 Pb versus 206 Pb/204 Pb (b). The
compositional ranges of MORB and PSP sediments are also shown for reference. Symbols are the same as in Fig. 2. The
dashed lines represent mixing curves between MORB and PSP sediment compositions. Tick marks along the mixing
curves represent proportions of PSP sediment. Data for MORB and PSP sediments are from Shibata and Nakamura(1997) and Shimoda et al. (1998).

53

54

Tomoyuki SHIBATA, Jun SUZUKI, Masako YOSHIKAWA,
Tetsuo KOBAYASHI, Daisuke MIKI and Keiji TAKEMURA

Fig. 7. SiO2 (wt. %) versus 87 Sr/86 Sr isotopic ratio.
Symbols are the same as in Fig. 2.

Fig. 6. Modal abundance of Pl versus SiO2, TiO2, andP2 O5 concentrations, as well as 87 Sr/86 Sr isotopic
ratios.

and the andesite magma mixed with the different acidic
magma resulting in two different paths of magmatic
processes.
The 87Sr/86Sr values of the volcanic rocks increase withincreasing SiO2 contents. However, the data are not plotted on simple mixing curve (line S in Fig. 7) between the
Shimanto sedimentary rock (1 WL4, Hosono et al., 2003)
and basalt (Fig. 7). Therefore, it is unlikely that theandesite end-member magma of Sakurajima volcano were
simply produced by mixing between the basaltic magma
and the crustal materials. Alternatively, the process, suchas assimilation and fractional crystallization (AFC,DePaolo, 1981) from the basaltic magma is required to
produce the andesite magma.
The relation of SiO2 and 87 Sr/86 Sr ratio (Fig. 7) alsosuggests that the low-P group can be generated by themixing between high-P andesite magma and crust derived
material. The low-P group lies on the mixing line between
andesite (high-P) and 4 W4 of Shimanto sedimentary rock(line Sʼ in Fig. 7). Therefore, sedimentary rocks in theupper crust can be an end-member of the mixing. However, high-P group trend is defer from the mixing line Sʼ,but high-P group cannot be explained by this binary
mixing. Arakawa et al. (1998) reported that the mixing
line between basalt and granitic rock beneath Sakurajima
volcano shows similar trend of line S. This indicates thatthe granitic rocks or melt generated from the granitic rockalso cannot be an end-member of high-P group. Therefore,
alternative process, such as assimilation and fractional
crystallization (AFC, DePaolo, 1981) is again required to
produce dacite end-member to produce the trend of high-Pgroup.
5-4 Diversity and genesis of low- and high-P groups
Both the low-P and high-P lavas are observed within allof the volcanostratigraphic successions, with the exceptionof Kitadake, and the abundance of low-P lavas seems todecrease systematically with time (Table 1). Lavas of thehigh-P group are mostly distributed in the central andsouthern part of Sakurajima volcano, and to some extent
are surrounded by low-P lavas (Fig. 1). Arakawa et al.
(1998) proposed that dacitic magmas intruded an andesitic
magma chamber, which was situated at shallower depths
than the dacitic magmas, in order to enable the mixing of
andesite and dacite magmas. However, the present results
indicate that the dacitic magmas that occur beneath
Sakurajima volcano are heterogeneous (low- and high-P),

Geochemical and Sr-Nd-Pb Isotopic Constraints on the Origin and Magmatic Evolutionof Quaternary Lavas of Sakurajima Volcano, Southern Kyushu Island, Japanand that the heterogeneity shows clear systematic spatiogeochemical variations (Fig. 1). If these dacitic magmas
first originated from deeper levels and then interacted witha single andesitic magma, then it seems unlikely that thedistinctive spatial distributions of low-P and high-P rocks(Fig. 1) would actually arise. Furthermore, dacitic magmas
are more buoyant than andesitic magmas.
On the other hand, the observed heterogeneity (low- andhigh-P) of the dacitic magmas might have arisen due toheterogeneity within a mono-dacite magma chamber, or by
the occurrence of multiple dacite magmas. The distinctive
spatial distributions of low-P and high-P lavas (Fig. 1)
support the existence of multiple dacitic magma chambers
beneath Sakurajima volcano. Therefore, we conclude thatat least two different dacitic magma chambers exist
beneath Sakurajima volcano at relatively shallow levels in
the crust and that andesitic magmas arose from deeper
levels and then intruded into the dacitic magma chambers.
A similar petrogenetic mechanism was recently proposed
by Nakagawa et al. (2011) for the origin of the Sakurajima lavas. Ultimately, the inferred different geochemical
characteristics of the low- and high-P dacitic magma
chambers might have arisen by AFC processes taking
place within heterogeneous crust (DePaolo, 1981).
To explain the genesis of such silicic magmas in thistectonic setting, several petrogenetic models have been
proposed; 1) fractional crystallization of mafic magmawith crustal assimilation (AFC model) (e. g., DePaolo,
1981; Caffe et al., 2002; Reagan et al., 2003), 2) partial
melting of basaltic lower crust (e. g., Takahashi, 1986;
Bead and Lofgren, 1991), and 3) partial melting of basaltic
rocks that were emplaced in the lower crust a long timeprior to the actual silicic magma activity (e.g., Sisson et al.,2005; Ban et al., 2007). Ban et al. (2007) invoked model
3 in order to explain arc magmatism in Northeastern Japanon the basis of relatively homogeneous Sr isotopic
compositions recorded throughout the entire sequence of
volcanic activity. In contrast, relatively large variations of
Sr-Nd-Pb isotopic compositions are observed throughout
the Sakurajima volcanostratigraphic succession, suggestingthat the lower crust beneath the studied area is similar to
relatively old crust having high Sr and Pb, and low Nd
isotopic ratios. It has also been suggested that lower
crustal materials are eventually consumed, if continual
long-lived magma production occurs nearby (e.g., Hildreth
and Wilson, 2007). If this view is correct, then old crust
beneath Sakurajima volcano will become increasingly
consumed, and eventually replaced by solidified basaltic
magmas. This would also seem to suggest that the stage ofvolcanic activity taking place at Sakurajima volcano issomewhat younger than that of Northeastern Japan.
Furthermore, AFC processes, in addition to crustal
assimilation processes, appear to be the dominant factors
controlling silicic magma production, based on the
documented petrological, geochemical, and isotopic data

55

recorded in the post-caldera lavas of Sakurajima volcano.
6．Conclusions
The depletion of Nb and enrichment of Rb, K, and Pb,
which are typical geochemical features of arc magmas
formed in subduction zone settings, are documented here
in lavas from Sakurajima volcano. On the basis of theseobserved similar trace element compositions, we havededuced that the magmatism associated with this Sakurajima volcanic activity was triggered by the addition of
fluids to the mantle wedge by the underlying subducting
oceanic plate. We also conclude that the mantle wedgethat occurs beneath the studied area is composed of
MORB-type mantle, based on the similar Zr/Nb ratios inthe mafic magma to that of MORB. The concentrations ofZr and Nb of andesite and dacite of the Sakurajima lavas
are plotted along a mixing line of the basaltic magma andaverage continental crust. In addition, the Sr, Nd, and Pb
isotopic compositions of the mafic magmas plot close to amixing curve connecting MORB-type mantle and sediments of the PSP, and those of andesite to dacite lavas
depart from the mixing curve along a direction of
becoming more radiogenic. Collectively, these observations
have led us to the following petrogenetic interpretations; 1)
subducted sediments contributed to the chemical composition of the aqueous fluids derived from the PSP that
infiltrated the overlying mantle wedge, 2) along with theirinherited trace element compositions, the primary magmas
of Sakurajima volcano were generated by the partial
melting of MORB-type mantle wedge, which was metasomatized by aqueous fluids originating from the dehydrating subducting PSP slab and 3) the assimilation ofcrustal materials also played a role in contributing to thechemical and magmatic evolution of the studied lavas.
From the Sr-Nd-Pb isotopic compositions, at least two
isotopic components are required as crustal materials to
explain the isotope variations of Sakurajima andesitic todacitic magmas. Although one of the crustal component is
not well constraint, but the sedimentary rocks of Shimanto
Group can be the other crustal component.
The linear relationships observed on plots of SiO2
versus most of other major element oxides suggest thatmagma mixing was involved in the production of the postcaldera lavas of Sakurajima volcano. Furthermore, thegeochemical relationships observed on plots of SiO2
versus TiO2, P2O5, and 87Sr/86Sr suggest that the observed
variations of major elements cannot be explained using
only a simple binary mixing model, because two distinct
trends can be observed in these diagrams. Similar
conclusions were presented in previous studies (e. g.,
Yanagi, 1991; Arakawa et al., 1998; Uto et al., 2005). To
explain the compositional variations, three end-members
are necessary, high-P andesite and dacite, and low-P
dacite, and the AFC process is required for generating
andesite and dacite end-members. Another important

56

Tomoyuki SHIBATA, Jun SUZUKI, Masako YOSHIKAWA,
Tetsuo KOBAYASHI, Daisuke MIKI and Keiji TAKEMURA

observation made in this study is that rock samples
grouped as low-P (low P2O5 content) versus high-P (highP2O5 content) exhibit remarkably different trends on plotsof plagioclase modal abundance versus SiO2, P2 O5, andTiO2 concentrations and 87Sr/86Sr ratios. The Sr isotopic
ratios of the low-P rocks are also high compared with thoseof the high-P suite of lavas. These contrasting low-P andhigh-P lavas also show distinctive spatial (map pattern)
distributions across the Sakurajima study area. From thesecollective observations, we suggest that multiple dacitic
magma chambers are required in the petrogenesis of thesegeochemically and isotopically diverse lava successions.
Acknowledgments
We thank J. Yamamoto, Y. Shitaoka, Y. Kameishi and
M. Miyoshi for their valuable help and advice with regards
to the experiments. We also appreciate the suggestions
provided by N. Tsuchiya pertaining to petrographic
studies. In addition, we are grateful to H. M. Helmy forhelping to improve the manuscript. We also greatly appreciate the critical and constructive comments provided
by A. Fujinawa and an anonymous reviewer. M. Ban isdeeply acknowledged for his helpful and efficient handlingof our manuscript. Part of this work was facilitated by aGrant-in-Aid for Scientific Research (Ministry of Education, Culture, Sports, Science and Technology, Japan)
awarded to T. Shibata, as well as a Grant for Institution
Collaboration (IGS, Kyoto University).
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(Editorial handling Masao Ban)

58

Tomoyuki SHIBATA, Jun SUZUKI, Masako YOSHIKAWA,
Tetsuo KOBAYASHI, Daisuke MIKI and Keiji TAKEMURA

桜島火山第四紀溶岩の地球化学及び Sr-Nd-Pb 同位体組成を用いたマグマ起源の解明
柴田知之・鈴木淳・芳川雅子・小林哲夫・味喜大介・竹村恵二

姶良カルデラの後カルデラ期の桜島火山に産する溶岩，及び，桜島周辺に産する前カルデラ期と単成火山の玄武岩について，主要元素・微量元素，ストロンチウム (Sr)・ネオジム (Nd)・鉛 (Pb) 同位体組成を測定した．桜島火山の溶岩は斜方輝石，単斜輝石，斜長石（稀にかんらん石）の斑晶を持つ，安山岩及びデイサイ
トである．微量元素組成から，島弧マグマの典型である Nb の枯渇，Rb・K・Pb の富化が観察され，マント
ルウェッジに水に富む流体が付加されたことを示す．Zr/Nb 比は MORB の比に類似し，また，Sr・Nd・Pb 同位体組成は MORB 型マントルとフィリピン海プレート上の堆積物の混合曲線付近にプロットされる．これ
らは，桜島及びその周辺のマグマは，MORB 型のマントルが沈み込んだフィリピン海プレート起源の流体の付加を受け部分溶融して生成したことを示す．さらに，Zr/Nb 比や同位体組成が，前者は MORB の値から，後者は MORB とフィリピン海プレート上の堆積物との混合曲線から，それぞれ地殻物質の値へ向かう傾向を示すことから，地殻物質がマグマの化学組成に影響を与えた可能性が指摘される．Sr-Nd-Pb 同位体組成から，少なくとも二つの地殻物質が桜島のマグマの起源に関与しており，そのうち一つは四万十層群の堆積岩を起源とすると考えられる．主要元素組成はハーカー図において，P2O5と TiO2を除き，直線的な傾向を示す．この事から，P2O5濃度をもとに，試料を low-P と high-P に区分した．Low-P と high-P は斜長石の斑晶8786
モード組成に対する SiO2，P2O5，TiO2濃度，及び， Sr/ Sr 比との間で，それぞれ異なるマグマ混合の関係を示す．また．主要元素と同位体組成の関係から，マグマ混合の両端成分を生成するには，地殻同化と結晶分化作用が同時に進行する過程(AFC)などの過程が必要であることが示された．