Bull. Volcanol. Soc. Japan
Vol. 58 (2013) No. 1, pp. 19-42

ArticleTemporal Variation for Magmatic Chemistry of the Sakurajima Volcano
and Aira Caldera Region, Southern Kyushu, Southwest Japan since 61 ka
and Its Implications for the Evolution of Magma Chamber System
Masaki TAKAHASHI＊, Tadashi OTSUKA＊＊, Hisashi SAKO＊＊, Hiroshi KAWAMATA＊＊＊,
Maya YASUI＊, Tatsuo KANAMARU＊, Mei OTSUKI＊＊＊＊, Tetsuo KOBAYASHI＊＊＊＊＊,
Kazuhiro ISHIHARA＊＊＊＊＊＊ and Daisuke MIKI＊＊＊＊＊＊(Received October 5, 2011 ; Accepted December 19, 2012)
The temporal variation of magmatic chemistry and the evolution of magma chamber system of the Sakurajima
volcano and Aira caldera region since 61 ka are studied based on the whole-rock major element, incompatible traceelement and rare earth element chemistry of the eruptive products. The magmas of the Sakurajima volcano and Airacaldera region since 61 ka consist of four groups: (1) basaltic to basaltic andesitic magma of the mantle origin, (2)rhyolitic to high silica rhyolitic magma of the crustal origin, (3) dacitic magma and (4) andesitic magma produced bymagma mixing of the mafic magma of mantle origin and the crustal felsic magma. Around 61 to 60 ka, basaltic tobasaltic andesitic, andesitic and rhyolitic magmas were active in the Aira caldera region, and the Shikine andesite andthe Iwato pyroclastic flow deposit were erupted. After a dormant period of about twenty-four thousands of years, therhyolitic magmatism resumed and the voluminous high silica rhyolitic magma erupted at 29 ka to form the large-scale
Osumi pumice fall and Ito pyroclastic flow deposits. The felsic magma produced the Iwato pyroclastic flow deposit
and the Osumi pumice fall and Ito pyroclastic flow deposits were similar in composition; the latter high silica rhyolite
can be derived from the former rhyolite by crystallization differentiation. The rhyolitic to high silica rhyolitic magma
chamber system was stable and long-lived with duration of about thirty thousands of years. The magmatic activity of
the Sakurajima volcano began at 26 ka after a quiescent period of about three thousands of years. The Moeshimarhyolitic magma discharged at 13.8 ka in the Aira caldera constitute another magma chamber system different from thatof the Sakurajima volcano. The magma chamber system of the Sakurajima volcano was composed of the low Ti-P typeand high Ti-P type dacitic and andesitic magmas. The magma chamber system of the low Ti-P type, which was activefrom about 14 to 4 ka, comprises at least the three sub-systems based on the whole-rock chemistry, while that of thethhistorical eruption since 8 C is restricted to the high Ti-P type and consists of the three sub-systems, the youngest ofwhich has been active since the subaqueous An-ei eruption at 1779AD. The duration of the activity of each magma
chamber sub-system of the Sakurajima volcano is rather short, the time span of which is thousands to several hundredsof years.
Key words : Sakurajima volcano, Aira caldera, felsic magma, magmatic chemistry, magma chamber

1．Introduction
The Sakurajima volcano in southern Kyushu is one of
the representative active dacitic to andesitic volcanoes in
the late Quaternary Japanese islands, which is constructedon the southern margin of the large-scale Aira caldera with＊

Department of Geosystem Sciences, College of Humanities
and Sciences, Nihon University, 3-25-40 Sakurajyosui
Setagayaku, Tokyo 156-8550, Japan.
＊＊
Graduate School of Science and Technology, Ibaraki
University, 2-1-1 Bunkyo, Mito 310-8512, Japan.
＊＊＊
Department of Earth Sciences, Faculty of Science, Ibaraki
University, 2-1-1 Bunkyo, Mito 310-8512, Japan.
＊＊＊＊
Graduate School of Integrated Basic Sciences, Nihon University, 3-25-40 Sakurajyosui Setagayaku, Tokyo 1568550, Japan.

a dimension of 24×20 km. The eruption of the Sakurajima volcano is now continuing from the Showa crater atthe upper slope of the eastern flank of the volcano. The
magma of the Sakurajima volcano is estimated to be being
fed from the magma chamber beneath the Aira caldera, to＊＊＊＊＊Graduate School of Science and Technology, Kagoshima
University, 1-21-35 Korimoto, Kagoshima 890-0055,
Japan.
＊＊＊＊＊＊
Sakurajima Volcano Research Center, Disaster Prevention Research Institute, Kyoto University, SakurajimaYokoyamacho, Kagoshima 891-1419, Japan.
Corresponding author: Masaki Takahashi
e-mail: takahashi.masaki@nihon-u.ac.jp

20

Masaki TAKAHASHI, Tadashi OTSUKA, Hisashi SAKO, Hiroshi KAWAMATA, Maya YASUI,
Tatsuo KANAMARU, Mei OTSUKI, Tetsuo KOBAYASHI, Kazuhiro ISHIHARA and Daisuke MIKI

Fig. 1. Map showing the distribution of the late Quaternary calderas in southern Kyushu and the location of
the Aira caldera and Sakurajima volcano. Arrows
indicate that the N-S trending Kagoshima graben is
extending in E-W direction at rates of less than 7 to 8
mm/year.

which the magma is supplied at a rate of about 0.01 km3/10years (Iguchi, 2007; Hyadati et al., 2008), showing thatthe magma chamber system of the Sakurajima volcano is
closely related to that of the Aira caldera. Although theformation of the Aira caldera was completed by the largescale eruption of the Osumi pumice fall, Ito pyroclastic
flow and AT ash fall deposits at 29 ka, the onset of theformation of the Aira caldera goes back to at least 100 ka.
The explosive eruptions of the felsic magma have been
repeated since 100 ka in the Aira caldera. In this paper, thewhole-rock major element, incompatible trace element andrare earth element chemistry of the eruptive products of the
Sakurajima volcano and Aira caldera region since 61 ka
are summarized and the evolution of magma chambersystems beneath the Aira caldera including the Sakurajima
volcano since 61 ka will be discussed based on the wholerock chemistry.
The ages mentioned in this paper include both thereported calendar/calibrated 14C ages and those by the KAr radiometric datings.
2．Eruptive history of the Sakurajima Volcano andAira Caldera
The Aira caldera is situated at the northern end of the
Kagoshima (Kinkowan) bay, which comprises the southernhalf of the Kagoshima graben. The north-south trending
Kagoshima graben extends about 100 km in length with awidth of about 20 km, having been formed under the east-

Fig. 2. The topographic relief map of the Aira caldera and
Sakurajima volcano (courtesy of Chiba, T.). The
Sakurajima volcano is located on the southern rim of
the Aira caldera. The depression in the northeast
portion of the caldera floor is the Wakamiko caldera. A
rise in the western rim of the Aira caldera is the
Yoshinodai plateau which consists of Quaternary
volcanic rocks. A large hill near the north of the
Wakamiko caldera comprises thick lavas of the
Shikine andesite.

west extensional tectonics (Nagaoka, 1988), and its active
extension is ongoing at rates of less than 7 to 8 mm/year(Wallace et al., 2009) (Fig. 1).
The volcanic activity of the Aira caldera region since 1
Ma began with the eruption of andesitic to rhyolitic lavas
at the Yoshinodai plateau in the western margin of the Airacaldera (Fig. 2), which was active during the time spanfrom 1. 0 to 0. 8 Ma; they are the Kennooka pyroxene
andesite, the Muregaoka pyroxene andesite, the Ryugamizu
pyroxene andesite, the Amagahana hornblende dacite andthe Mifune pyroxene rhyolite (Sudo et al., 2000a; 2000b;2001). The Yuwandake pyroxene andesite erupted from 0.
9 to 0.5 Ma at the Kajiki area in the northern margin of the
Aira caldera (Sudo et al., 2000) (Fig. 2).
Around 0. 5 Ma, the Osakibana pyroxene dacite, theHiramatsu pyroxene andesite and the Shirahama basalt
effused in the Yoshinodai plateau. The Yoshino hornblende pyroxene dacitic pyroclastic flow deposit erupted at
0. 40 to 0. 45 Ma near the Yoshinodai plateau, which isoverlain by the Nanayashiro basalt effused at 0. 35 Ma(Sudo et al., 2000a; 2000b; 2001). The Ushine basalt
erupted at 0. 37 Ma at the Ushine area in the southern
margin of the Aira caldera, and the Okogashima pyroxenerhyolite, now constructing a small island in the Kagoshimabay, effused at 0.38-0.25 Ma in the south of the present
Sakurajima volcano (Tatsumi and Inoue, 1993) (Fig. 2).

Temporal Variation for Magmatic Chemistry of the Sakurajima Volcano and Aira Caldera Region, Southern
Kyushu, Southwest Japan since 61 ka and Its Implications for the Evolution of Magma Chamber System

Fig. 3. History of the volcanic activity of the Sakurajima
volcano and Aira caldera region since 61 ka.

The volcanic activity was nearly quiescent from 0.30 to
0.10 Ma and resumed at 0.10 Ma in the Aira caldera area;
the eruptive style dramatically changed from effusive to
explosive since 0. 10 Ma (Nagaoka et al., 2001). The
eruption of the Hikiyama andesitic scoria fall deposit(103-95 ka) and the Kongoji pyroclastic surge deposit(95-86 ka) were followed by the large-scale explosive
eruption of the Fukuyama hornblende pyroxene dacitic
pumice fall deposit (40 km3) at about 90 ka (Nagaoka etal.,2001).
The history of volcanic activity in the Sakurajima
volcano-Aira caldera region since 61 ka is briefly summarized in Fig. 3. At the Kokubu area in the northeastern
margin of the Aira caldera, the Shikine andesite (olivine
bearing orthopyroxene clinopyroxene andesite to basaltic
andesite) effused to produce thick lava flows with a total
volume of 1.3 km3 at 61 ka (Nagaoka et al., 2001; Sudo etal., 2000ab). The Shikine andesite includes basaltic
magmatic inclusions. The Iwato pyroclastic flow deposit(18-23 km3) with pyroxene rhyolitic pumice and andesitic
scoria, consisting of the three pumice fall and five
pyroclastic flow deposits, erupted at 60 ka probably fromthe northeastern portion in the Aira caldera (Nagaoka etal., 2001). The scoria of the Iwato pyroclastic flow
deposit is olivine bearing pyroxene andesite (Takahashi etal., 2011).
The Shimizu hornblende pyroxene rhyolitic lava effused
at 36 ka in the northern margin of the Aira caldera near the
Kajiki city (Sudo et al., 2000a; 2000b). The Ushine
pyroxene rhyolitic lava erupted at 33 ka at the Ushine areain the southern margin of the Aira caldera (Sudo et al.,2000a; 2000b). In the boring core samples drilled at thesouthern and southeastern portion of the Sakurajima
volcano, the pyroxene andesite of 38 ka was found in the21

depth of − 98 m at the Furusato drill hole site and thepyroxene andesite of 30 ka was collected in the depth of −105 m at the Kurokami drill hole site (Miki et al., 1999;
2000; 2003) (Fig. 4).
The intermediate scale eruption of the Otsuka pyroxenerhyolitic pumice fall deposit (32.5 ka; 0.9 km3), theFukaminato-Arasaki pyroxene rhyolitic pumice fall andpyroclastic flow deposits (31 ka; 7.5 km3), and the Kenashino
pyroxene rhyolitic pumice fall deposit (30 ka; 0. 4 km3)
were the precursors of the forthcoming climactic largescale eruption (Nagaoka et al., 2001).
At 29 ka, the climactic large-scale eruption of thepyroxene rhyolitic magma produced the Osumi pumice
fall, Tarumizu pyroclastic flow, Tsumaya pyroclastic flow,
Ito pyroclastic flow and AT ash fall deposits with a total
volume of 450 km3 (Machida and Arai, 2003; Fukushima
and Kobayashi, 2000) which completed the formation of
the present Aira caldera (Aramaki, 1984). Although theprecise location of the center of the climactic eruption has
not yet been revealed, the Osumi pumice fall and Tarumizu
pyroclastic flow deposits are estimated to have erupted
from the location of the present Sakurajima volcano(Aramaki, 1984; Fukushima and Kobayashi, 2000), and
the Wakamiko caldera, occupied the northeastern portionof the Aira caldera, is one of the probable candidates of thevent area of the Tsumaya and Ito pyroclastic flow deposits.
The construction of the Sakurajima volcano began at 26
ka in the southern margin of the Aira caldera after adormant period of about three thousands of years(Kobayashi and Ezaki, 1997; Okuno, 1997 and 2002) (Fig.
4). The Sakurajima volcano is composed of the two main
composite volcanic cones, the Kitadake situated in thenorth and the Minamidake in the south (Fig. 4). The
volcanic history of the Sakurajima volcano comprises thethree main stages, the older Kitadake (26 to 24 ka),younger Kitadake (13 to 4 ka) and Minamidake stages (4ka to present) (Kobayashi and Ezaki, 1997; Okuno, 1997;
2002).
The three Plinian pumice fall deposits, P17 (26 ka), P16(25 ka) and P15 (24 ka), erupted during the older Kitadakestage (Okuno, 2002). At 19 ka, the Takano base surge
deposit was discharged in the Aira caldera (Okuno, 2002).
After a dormant period of about eleven thousands of years,
the younger Kitadake stage began with the eruption of theSatsuma pumice fall deposit (P14) at 12.8 ka, the eruptive
volume of which is 2. 5 km3 and it is the largest single
eruptive event of the Sakurajima volcano (Moriwaki,1992). Concurrently, the subaqueous eruption of theShinshima (Moejima) pyroxene rhyolitic pumice occurredin the Aira caldera at 12.8 ka (Kano et al., 1996; Okuno,2002).
In the younger Kitadake stage, the nine Plinian pumice
fall deposits, P13, P12, P11 (8 ka), P10 (7.7 ka), P9 (7.5ka), P8 (6.5 ka), P7 (5 ka), P6 (3.8 ka), P5 (5.6 ka) erupted
during the time span from 12.8 to 3.8 ka (Kobayashi and22

Masaki TAKAHASHI, Tadashi OTSUKA, Hisashi SAKO, Hiroshi KAWAMATA, Maya YASUI,
Tatsuo KANAMARU, Mei OTSUKI, Tetsuo KOBAYASHI, Kazuhiro ISHIHARA and Daisuke MIKI

Fig. 4. Geologic sketch map showing the distribution of lavas and volcanic edifices of the Sakurajima volcano. 1: volcanic
rocks of younger Kitadake; 2: lateral volcanoes of younger Kitadake; 3: Minamidake older lavas; 4: Minamidake
younger, Nakadake and Tenpyohoji volcanic rocks; 5: Bunmei, An-ei terrestrial, Taisho and Showa volcanic rocks; 6:An-ei subaqueous volcanic rocks.

Ezaki, 1997; Okuno, 2002). The essential part of thepresent Kitadake volcanic cone was built during theyounger Kitadake stage. Some of the younger Kitadake
pyroxene dacitic lavas flowed down at about 10 ka andspread over the northern foot of the Kitadake volcanic
cone (Miki et al., 2003). The Gongenyama pyroxene
dacitic lava dome in the eastern flank of the Kitadake
volcanic cone was formed at 8 ka (Kobayashi and Ezaki,1997), and the Harutayama pyroxene dacitic lava dome
(including the Yunohira and Furihatayama lava domes) in
the western flank of the Kitadake volcanic cone emerged at
7 ka (Uto et al., 1999) (Fig. 4). One of the youngest
pyroclastic flow deposit of the Kitadake volcanic cone
erupted at 5 ka (Kobayashi and Ezaki, 1997), indicatingthat the construction of the main portion of the Kitadake
volcanic cone was almost completed until 5 ka.
The Sakurajima volcanic ash deposit shows that theconstruction of the Minamidake volcanic cone by thefrequent Vulcanian eruptions commenced at 4. 5 ka andcontinued to 1. 6 ka (Kobayashi and Ezaki, 1997). The
older Minamidake andesitic lavas in the southern foot of
the Minamidake pyroclastic cone effused during the period

from 4.0 to 3.7 ka (Miki, 1999) (Fig. 4).
The historical volcanic activity of the Minamidake
volcano started from the Tenpyohoji eruption at 764 to
766AD, which occurred in the eastern flank of the
Minamidake volcanic cone and produced the Tenpyohoji
pumice fall deposoit (P4), the Nabeyama pumice cone andthe Nagasakibana pyroxene andesitic lava flow (Fig. 4).
The Minamidake younger pyroxene dacitic lavas and the
Nakadake volcanic edifice located between the Kitadakeand Minamidake craters erupted between 8th and 12th
centuries (Kobayashi and Ezaki, 1997; Kobayashi, 2010).
The Bunmei eruption at 1471 to 1476AD gave rise to
the Bunmei pumice fall deposit (P3) and Bunmei pyroxene
dacitic lava flows, the vent of which located in the northernand southern flanks of the Sakurajima volcano (Fig. 4).
The An-ei eruption at 1779 to 1782AD comprises both theterrestrial and subaqueous eruptions; the An-ei terrestrial
eruption produced the An-ei pumice fall deposit (P2) andAn-ei pyroxene dacitic lava flows, while the An-ei
subaqueous eruption formed the subaqueous pyroxene
dacitic lava domes and cryptodomes, some of whichappeared above the sea level as the An-ei islets

Temporal Variation for Magmatic Chemistry of the Sakurajima Volcano and Aira Caldera Region, Southern
Kyushu, Southwest Japan since 61 ka and Its Implications for the Evolution of Magma Chamber System(Kobayashi, 2009) (Fig. 4). The An-ei terrestrial eruption
occurred in the northern and southern flanks of the
Sakurajima volcano.
The Taisho eruption at 1914 to 1915AD began with thePlinian eruption producing the Taisho pumice fall deposit(P1) and followed by the outpouring of the Taisho olivine
bearing pyroxene andesitic lava flows, some of which isclastogenic in origin (Yasui et al., 2007; 2013) (Fig. 4).
The Taisho eruption occurred in the eastern and western
flanks of the Sakurajima volcano. At 1946AD, the Showa
olivine bearing pyroxene andesitic lava flow effused fromthe newly formed Showa crater located at the eastern upper
slope of the Minamidake volcanic cone (Fig. 4). In thehistory of the Sakurajima volcano, the effusion of the
Showa olivine bearing pyroxene andesitic lava is the latest
event erupting lava flow.
3．Previous works
The whole-rock chemistry of the eruptive products of
the Sakurajima volcano was firstly summarized by Yamaguchi(1974). Yamaguchi (1974) gave the 75 analytical data ofmajor element chemistry of the volcanic rocks of the
Sakurajima volcano, but did not discuss their petrology.
Kurasawa et al. (1984) reported for the first time the Sr
isotopic ratios of the Ito pyroclastic flow deposit, the AT
ash fall deposit and some of the eruptive products of the
Sakurajima volcano. According to Kurasawa et al.
(1984), the 87Sr/86Sr ratios of the Ito pyroclastic flow and
AT ash fall deposits are 0.7060 to 0.7062, while those of
the younger Kitadake lavas, the Nabeyama pyroclastic
cone of the Tenpyohoji eruption, the Bunmei and An-ei
lavas, and the Taisho and Showa lavas of the Sakurajima
volcano are 0.7054 to 0.7057, 0.7052, 0.7054 and 0.7053,
respectively. Tsukui and Aramaki (1990) studied thewhole-rock major and trace element chemistry of theeruptive products of the Osumi pumice fall and Itopyroclastic flow deposits. They concluded that the erupted
voluminous magma is high silica rhyolite and monotonousin composition.
Yanagi et al. (1991) examined in detail about thechemical composition of phenocrystic minerals of thelavas of the historic large-scale eruption of the Sakurajima
volcano. The core anorthite compositions (An) of phenocrystic plagioclase of the Taisho and Showa lavas show
bimodal distribution, while those of the Bunmei and An-ei
lavas are almost unimodal. The orthopyroxene and clinopyroxene phenocryst of the Taisho and Showa lavas show
reverse zoning, but those of the Bunmei and An-ei lavas
mostly exhibit normal zoning. On the basis of above data,
they advocated that the magmas of the Taisho and Showa
eruptions suffered magma mixing; the dacitic magma with
low An phenocrystic plagioclase and low Mg-number
phenocrystic pyroxenes are mixed with the basaltic magmawith high An phenocrystic plagioclase, high Mg-number
phenocrystic pyroxenes and olivine. Contrarily, there are23

no positive evidences of magma mixing in the magmas of
the Bunmei and An-ei eruptions.
Arakawa et al. (1998) investigated the Sr and Nd
isotopic ratios of the Osumi pumice fall and Ito pyroclastic
flow deposits, the historical eruptive products of the
Sakurajima volcano, and some of the volcanic rocks of the
pre-Aira caldera, and discussed their origin. The 87Sr/86Sr
isotopic ratios of the high-silica rhyolite producing theOsumi pumice fall and Ito pyroclastic flow deposits are
0.7052 to 0.7076, and those of the pre-caldera basalt andandesite are 0.7047 to 0.7054. They also threw light on
the 87Sr/86Sr ratios of the historical lavas of the Sakurajima
volcano; those of the Bunmei, An-ei, Taisho and Showalavas are 0.7052 to 0.7053, 0.7052 to 0.7053, 0.7050 to
0.7053 and 0.7051 to 0.7052, respectively. Arakawa et al.
(1998) concluded that the high-silica rhyolitic magma of
the Osumi pumice fall and Ito pyroclastic flow deposits
with relatively high 87Sr/86Sr ratios were produced by thepartial melting of the lower crust, while the historicalandesitic lavas of the Sakurajima volcano are the mixing
products of the dacitic magma of lower crustal origin andthe basaltic magma of mantle origin.
Arakawa et al. (1998) also presented the rare earth
element (REE) contents of the Osumi pumice fall and Itopyroclastic flow deposits, the historical eruptive productsof the Sakurajima volcano, and some of volcanic rocks ofthe pre-Aira caldera stage. The pumice of the Osumi
pumice fall and Ito pyroclastic flow deposits are enrichedin light REE with high LREE/HREE ratio and shows
negative Eu anomaly in the chondrite normalized diagram.
The LREE/HREE ratio is modest in the andesite of the
Sakurajima volcano, which shows slightly negative Eu
anomaly, and the LREE/HREE ratio is the least in thebasalt of the pre-Aira caldera stage, which has no Eu
anomaly. The results of the REE analysis are consistent
with their conclusion based on the Sr and Nd isotopic
compositions that the andesite was formed by magma
mixing between the crustal felsic magma and the basaltic
magma of mantle origin.
Uto et al. (2005) studied the generation process ofmagmas of the Sakurajima volcano based on the Sr
isotope, major and trace element compositions. Uto et al.
(2005) showed that the eruptive products of the
Sakurajima volcano older than 2 ka are poorer in TiO2, P2
O5, FeO*, Na2O, Zr and Y, and lower in Nb/Th and Zr/Th
ratios than those younger than 2 ka. The 87Sr/86Sr ratios ofthe volcanic rocks older than 2 ka are 0.7055 to 0.7057 andthose younger than 2 ka are 0.7051 to 0.7054. The rhyoliteof the Ito pyroclastic flow deposit is higher in 87 Sr/86 Srratio (0.7060) and lower in Nb/Th and Zr/Th ratios, while
the basaltic magmatic inclusion in the lavas of the
Sakurajima volcano and the basalt to andesite of the precaldera stage are lower in the 87 Sr/86 Sr ratio (0.7049 to
0.7051) and higher in Nb/Th and Zr/Th ratios. Theirconclusions are as follows; (1) magma mixing between theMasaki TAKAHASHI, Tadashi OTSUKA, Hisashi SAKO, Hiroshi KAWAMATA, Maya YASUI,
Tatsuo KANAMARU, Mei OTSUKI, Tetsuo KOBAYASHI, Kazuhiro ISHIHARA and Daisuke MIKI

24

Fig. 5. Silica variation diagrams for TiO2 ,P2 O5, Na2 O and K2 O contents of the eruptive products of the Sakurajima
volcano. 1: Showa lava; 2: Taisho lava; 3: An-ei subaqueous lava; 4: An-ei terrestrial lava; 5: Bunmei lava; 6: younger
Minamidake lava; 7: Tenpyohoji lava; 8: older Minamidake lava; 9: younger Kitadake pyroclastic flow deposit; 10:
younger Kitadake lava; 11: Furihatayama-Harutayama lava; 12: Hikinohira lava; 13: Gongenyama lava; 14: andesitic
magmatic inclusion of the An-ei terrestrial lava (Yanagi et al., 1991); 15: basaltic andesitic magmatic inclusion of theyounger Kitadake lava.

felsic magma, being erupted as the Ito pyroclastic flow
deposit, and the mafic magma, producing the basaltic
magmatic inclusions in the lavas of the Sakurajima
volcano and the basalt of the pre-caldera stage, gave rise to
the andesite and dacite of the Sakurajima volcano, and (2)the mixing ratio of the mafic magma to felsic magma
increased with time from the eruptions older than 2 ka to
those younger than 2 ka.
4．Major element
The whole-rock major element chemistry of the eruptive
products was obtained using the X-ray fluorescence
spectrometry (XRF). All data were recalculated to 100 %water-free. The table of analytical data and maps ofsampling localities, in addition to the details of accuracy
and method of chemical analysis, are shown in Takahashi
et al. (2011). The data of the chemical analysis include
those of the boring core samples of the Kurokami (theTenpyohiji Nagasakibana lava), Shirahama (the An-ei
terrestrial and younger Kitadake lavas), Shinjima (the Anei subaqueous lava) and Harutayama (the Harutayamalava) drill-hole sites.

4-1 Sakurajima volcano
The eruptive products of the Sakurajima volcano areclassified into the two types based on the TiO2 and P2O5
contents; they are the high Ti-P and low Ti-P types (Fig.
5). The historic eruptive products are the high Ti-P types,
while the pre-historic ones belong to the low Ti-P types.
The historic eruptive products include the Tenpyohoji(Nagasakibana) (58 to 64 wt. %SiO2), younger Minamidake (64 to 65 wt.%SiO2), Bunmei (66 to 68 wt.%SiO2),
An-ei terrestrial (63 to 66 wt.%SiO2), An-ei subaqueous(63 to 65 wt. %SiO2), Taisho (59 to 62 wt. %SiO2) andShowa (61 to 62 wt.%SiO2) lavas. On the other hand, thepre-historic eruptive products include the older Minamidake (61 to 63 wt.%SiO2), younger Kitadake (63 to 65 wt.
%SiO2), Hikinohira (66 wt.%SiO2), Furihatayama-Harutayama (67 to 68 wt. %SiO2) and Gongenyama (68 wt.
%SiO2) lavas.
The high Ti-P types are enriched in TiO2, P2O5, Na2O
and FeO* and depleted in MgO, Al2O3 and CaO, while thelow Ti-P types are lower in TiO2, P2O5, Na2O and FeO*,
and higher in MgO, Al2O3 and CaO (Fig. 5 and 6). The
K2O contents of both types are similar and belong to theTemporal Variation for Magmatic Chemistry of the Sakurajima Volcano and Aira Caldera Region, Southern
Kyushu, Southwest Japan since 61 ka and Its Implications for the Evolution of Magma Chamber System

25

Fig. 6. Silica variation diagrams for MgO, FeO*, Al2O3 and CaO of the eruptive products of the Sakurajima volcano. 1:
Showa lava; 2: Taisho lava; 3: An-ei subaqueous lava; 4: An-ei terrestrial lava; 5: Bunmei lava; 6: younger Minamidake
lava; 7: Tenpyohoji lava; 8: older Minamidake lava; 9: younger Kitadake pyroclastic flow deposit; 10: younger Kitadake
lava; 11: Furihatayama-Harutayama lava; 12: Hikinohira lava; 13: Gongenyama lava; 14: andesitic magmatic inclusionof the An-ei terrestrial lava (Yanagi et al., 1991); 15: basaltic andesitic magmatic inclusion of the younger Kitadake
lava.

Gillʼs medium-K series (Fig. 5). The TiO2 contents of thehigh Ti-P types are nearly constant with increase of SiO2,
while those of the low Ti-P types decrease. The P2 O5
contents of the high Ti-P types increase with increase of
SiO2, but those of the low Ti-P types are nearly constant.
In both types, MgO, FeO*, Al2O3, CaO, and MnO contents
decrease but K2 O and Na2 O contents increase with increasing SiO2 in the silica variation diagrams (Fig. 5 and 6).
Mafic magmatic inclusions (MMI) are found in somelavas, such as the An-ei and younger Kitadake lavas; thosein the high Ti-P type An-ei lava are mafic andesitic (ca.56
wt.%SiO2) (Yanagi et al.,1991) and those in the low Ti-P
type younger Kitadake lava are basaltic andesitic (ca. 54
wt.%SiO2) in composition.
The high Ti-P types are further classified into the three
sub-groups. The Tenpyohoji lava is the first sub-groupand the second comprises the younger Minamidake,
Bunmei and terrestrial An-ei lavas, and the third consistsof the subaqueous An-ei, Taisho and Showa lavas. The
third sub-group is the highest in TiO2, P2 O5 and FeO*,
while the first sub-group is the lowest in P2O5 and Na2O

(Figs. 5 and 6). The younger Minamidake lava of thesecond sub-group is slightly lower in Na2 O than othermembers of the same sub-group. Among the low Ti-P
types, the Gongenyama lava is lower in TiO2 and P2 O5than others (Fig. 5).
In the FeO*/MgO vs. SiO2 diagram, most lavas belong
to the calc-alkaline rock-series except for the younger
Minamidake, Bunmei, An-ei terrestrial, and An-ei subaqueous lavas, which are the tholeiitic rock-series (Fig. 7)
4-2 Iwato pyroclastic flow, Osumi pumice fall andIto pyroclastic flow deposits and related pyroclastic rocks.
The pumice of the Iwato pyroclastic flow deposit (72to74 wt.%SiO2) and the Moeshima pumice (71 to 73 wt.
%SiO2) are rhyolite in composition, and that of the Osumi
pumice fall and Ito pyroclastic flow deposits is high-silica
rhyolite (74 to 76 wt.%SiO2) (Figs.8 and 9). The scoria of
the Iwato pyroclastic flow deposit is andesite (57 to 59 wt.
%SiO2) (Figs. 8 and 9). The felsic essential clasts of the
Iwato pyroclastic flow, Osumi pumice fall and Ito pyroclastic flow deposits show the same trend of compositional

26

Masaki TAKAHASHI, Tadashi OTSUKA, Hisashi SAKO, Hiroshi KAWAMATA, Maya YASUI,
Tatsuo KANAMARU, Mei OTSUKI, Tetsuo KOBAYASHI, Kazuhiro ISHIHARA and Daisuke MIKI

of the Shikine andesite is nearly the same as that of thescoria of the Iwato pyroclastic flow deposit except for P2
O5 and K2 O contents (Figs.8 and 9). The basaltic MMI(about 52 wt.%SiO2) is included in the Shikine andesite.

Fig. 7. SiO2 vs. FeO*/MgO diagram for the eruptive
products of the Sakurajima volcano.
1: Showa lava; 2: Taisho lava; 3: An-ei subaqueous lava;
4: An-ei terrestrial lava; 5: Bunmei lava; 6: younger
Minamidake lava; 7: Tenpyohoji lava; 8: older Minamidake lava; 9: younger Kitadake pyroclastic flow deposit;
10: younger Kitadake lava; 11: Furihatayama-Harutayama lava; 12: Hikinohira lava; 13: Gongenyama lava;
14: andesitic magmatic inclusion of the An-ei terrestrial lava (Yanagi et al., 1991); 15: basaltic andesitic
magmatic inclusion of the younger Kitadake lava.

variation in the silica variation diagrams. On the contrary,
the Moeshima pumice is lower in TiO2, P2O5, MgO, Al2O3
and CaO, and higher in Na2O, K2O, FeO* and MnO thanthe Iwato pyroclastic flow, Osumi pumice fall and Itopyroclastic flow deposits (Figs. 8 and 9). The andesitic
scoria of the Iwato pyroclastic flow deposit is higher in K2
O and lower in Na2O, CaO and MnO than the andesite of
the Sakurajima volcano (Figs.8 and 9).
In the FeO*/MgO vs. SiO2 diagram, the pumice andscoria of the Iwato pyroclastic flow deposit are the calcalkaline rock-series and the pumice of the Osumi pumice
fall and Ito pyroclastic flow deposits and the Moeshima
pumice belong to the tholeiitic rock- series (Fig. 10). The
pumice and scoria of the Iwato pyroclastic flow deposit,
and the pumice of the Osumi pumice fall and Itopyroclastic flow deposits are the Gillʼs medium-K series(Fig. 8).
4-3 Shikine andesite
The SiO2 content of the Shikine andesite is 53 to 60 wt.
%, which includes the basaltic MMI (51 wt.%SiO2). The
Shikine andesite is lower in P2 O5, Na2 O and MnO,
compared with the andesite of the Sakurajima volcano(Fig. 8 and 9). The Shikine andesite belongs to the Gillʼsmedium-K series on the K2 O vs. SiO2 diagram and thecalc-alkaline rock-series in the FeO*/MgO vs. SiO2 diagram (Fig. 10). The whole-rock major element chemistry5．Incompatible trace element
The whole-rock incompatible trace element chemistryof the eruptive products is obtained by using the X-ray
fluorescence spectrometry (XRF). The details of theanalytical method and accuracy of the chemical analysis
are presented in Takahashi et al. (2011).
The content of incompatible trace elements excluding Sr
increases with increase of SiO2 in the silica variation
diagram.
The HFSE such as Zr and Nb, and Y contents of the highTi-P types of the Sakurajima volcano are higher than thoseof the low Ti-P types in the silica variation diagram (Fig.
11). The rhyolitic pumice of the Iwato pyroclastic flow,
Osumi pumice fall and Ito pyroclastic flow deposits are
depleted in Zr, Nb and Y, compared with those of theMoeshima rhyolitic pumice (Fig. 11). The Shikine
andesite and the andesitic scoria of the Iwato pyroclastic
flow deposit are enriched in Rb, Ba and Nb, and depleted
in Sr in the silica variation diagram, in comparison withother volcanic rocks of the Sakurajima volcano and Airacaldera region (Fig. 11).
The boring core samples consist of the Tenpyohoji (theKurokami drill-hole site), An-ei terrestrial (the Shirahama
drill-hole site), An-ei subaqueous (the Shinjima drill-holesite), younger Kitadake (the Shirahama drill-hole site) andHarutayama (the Harutayama drill-hole site) lavas. The Srcontent of the Harutayama lava is the highest and that of
the Tenpyohoji lava is the lowest, but the inclination ofcompositional trend of the Tenpyohoji lava is rather gentle
in the silica variation diagram (Fig. 12). The lavas of thehigh Ti-P types, such as the Tenpyohoji, An-ei terrestrial,and An-ei subaqueous lavas, are rather enriched in Zr, Nb
and Y in the silica variation diagrams, but some of theTenpyohoji lava are lower in Nb (Fig. 12). The Zr content
of the Harutayama lava, a member of the low Ti-P types, ishigher than that of the younger Kitadake lava which is alsoa member of the low Ti-P types (Fig. 12). On the other
hand, the Zr and Y contents of the An-ei subaqueous lava
are higher than those of the An-ei terrestrial lava, althoughboth are members of the high Ti-P types (Fig. 12).
The Rb/Zr, Rb/Y, Ba/Y and Zr/Y ratios of the pumice of
the Iwato pyroclastic flow, Osumi pumice fall and Itopyroclastic flow deposits are similar and higher than thoseof other volcanic rocks of the Sakurajima volcano and Airacaldera region since 61 ka (Fig. 13).
The Rb/Zr, Rb/Y, Ba/Y and Zr/Y ratios of the high Ti-P
types of the Sakurajima volcano are lower than those of the
low Ti-P types and the Moeshima rhyolitic pumice (Fig.
13). The Rb/Zr, Rb/Y, Ba/Y, and Zr/Y ratios of thebasaltic MMI of the Shikine andesite and basaltic andesitic

Temporal Variation for Magmatic Chemistry of the Sakurajima Volcano and Aira Caldera Region, Southern
Kyushu, Southwest Japan since 61 ka and Its Implications for the Evolution of Magma Chamber System

27

Fig. 8. Silica variation diagrams for TiO2 and P2O5 Na2O and K2O contents of the eruptive products of the Sakurajima
volcano and Aira caldera region since 61 ka. 1: Minamidake historical lavas; 2: older Minamidake lava, lavas andpyroclastic flow deposit of Kitadake; 3: Moeshima pumice; 4: Osumi pumice fall deposit; 5: Ito pyroclastic flow deposit;
6: Iwato pyroclastic flow deposit (pumice); 7: Iwato pyroclastic flow deposit (scoria); 8: Shikine andesite; 9: basaltic
magmatic inclusion of the Shikine andesite; 10: basaltic andesitic magmatic inclusion of the younger Kitadake lava.

MMI of the younger Kitadake lava, and those of the mafic
andesite of the Shikine andesite are nearly constant and thelowest among the eruptive products of the Sakurajima
volcano and Aira caldera region since 61 ka (Fig. 13). On
the contrary, the Rb/Zr, Rb/Y, Ba/Y and Zr/Y ratios of theandesite of the Shikine andesite and the andesitic scoria of
the Iwato pyroclastic flow deposit show wide variations(Fig. 13).
The boring core samples of the low Ti-P types, theyounger Kitadake and Harutayama lavas, are higher in
Rb/Y, Rb/Zr, Ba/Y and Zr/Y ratios than those of thehistorical Tenpyohoji, An-ei terrestrial and An-ei subaqueous lavas which belong to the high Ti-P types (Fig.
14). The boring core samples of the An-ei subaqueous
lava are lower in Rb/Zr, Rb/Y, Ba/Y and Zr/Y ratios thanthose of the An-ei terrestrial lava.
6．Rare earth element
The whole-rock chemistry of rare earth elements is
obtained by the inductively coupled plasma (ICP) analytical apparatus. The details of the analytical method andthe accuracy of chemical analysis are presented in Tagiri
and Fujinawa (1988).
The chondrite-normalized abundance of rare earth

elements (REE) of the high Ti-P and low Ti-P types of the
Sakurajima volcano are intermediate between those of thepumice of the Moeshima pumice deposit and the basaltic
MMI in the younger Kitadake lava of the Sakurajima
volcano (Fig. 15). The LREE (Light REE)/HREE (Heavy
REE) ratios are the highest in the Moeshima pumice
deposit and the lowest in the basaltic andesitic MMI of theyounger Kitadake lava (Fig. 15). The LREE/HREE ratiosof the low Ti-P types are higher than those of the high TiP types of the Sakurajima volcano (Fig. 15).
The chondrite-normalized abundance of REE is lower inthe pumice of the Iwato pyroclastic flow, Osumi pumice
fall and Ito pyroclastic flow deposits and higher in thescoria of the Iwato pyroclastic flow deposit, the high Ti-P
types and low Ti-P types of the Sakurajima volcano, andthe pumice of the Moeshima pumice deposit (Fig. 16).
The LREE/HREE ratios of the pumice of the Iwato
pyroclastic flow, Osumi pumice fall and Ito pyroclastic
flow deposits are higher than those of the scoria of the
Iwato pyroclastic flow deposit, the high Ti-P types and low
Ti-P types of the Sakurajima volcano, and the pumice ofthe Moeshima pumice deposit, although all of them show
negative Eu anomaly (Fig. 16).

28

Masaki TAKAHASHI, Tadashi OTSUKA, Hisashi SAKO, Hiroshi KAWAMATA, Maya YASUI,
Tatsuo KANAMARU, Mei OTSUKI, Tetsuo KOBAYASHI, Kazuhiro ISHIHARA and Daisuke MIKI

Fig. 9. Silica variation diagrams for MgO, FeO*, Al2 O3 and CaO contents of the eruptive products of the Sakurajima
volcano and Aira caldera region since 61 ka. 1: Minamidake historical lavas; 2: older Minamidake lava, lavas andpyroclastic flow deposit of Kitadake; 3: Moeshima pumice; 4: Osumi pumice fall deposit; 5: Ito pyroclastic flow deposit;
6: Iwato pyroclastic flow deposit (pumice); 7: Iwato pyroclastic flow deposit (scoria); 8: Shikine andesite; 9: basaltic
magmatic inclusion of the Shikine andesite; 10: basaltic andesitic magmatic inclusion of the younger Kitadake lava.

7．Discussion
7-1 Origin of basaltic to basaltic andesitic magmasof the Sakurajima volcano and Aira caldera
region since 61 ka
Basaltic to basaltic andesitic rocks are rare in the
Sakurajima volcano and Aira caldera region since 61 ka;
they occur only as the basaltic MMI and basaltic andesitic
lava in the Shikine andesite and the basaltic andesitic MMIof the younger Kitadake lava of the Sakurajima volcano.
They have nearly the same incompatible trace element
ratios, and the basaltic andesite can be derived from thebasaltic magma produced the basaltic MMI by the simple
crystallization differentiation of olivine, pyroxenes andplagioclase, because MgO and CaO decrease and TiO2,
P2O5, K2O, Rb, Ba, Nb and Y increase, while Al2O3, Na2O
and Sr are nearly constant as SiO2 increases in the silica
variation diagrams (Fig. 8, 9 and 11). The basaltic andesitic
MMI in the younger Kitadake lava is, however, slightly
higher in Na2O, FeO*and MnO, and lower in MgO and Nb
than the basaltic andesite of the Shikine andesite. Since
the differences are minimal, it may be plausible that they
were derived from the similar more primitive parental
basaltic magma. The 87 Sr/86 Sr isotopic ratio of thebasaltic andesitic MMI of the younger Kitadake lava is not

so high (0.70505), hence they were considered to be themantle origin in the previous study (Uto et al., 2005).
Although the occurrence of the basaltic to basaltic
andesitic rocks are rare in the Sakurajima volcano and Airacaldera region since 61 ka, they must have underplated
voluminously to the lower crust, supplying enough heat tomelt the lower crust and to produce felsic magmas in the
Aira caldera region since 61 ka. The presence of abundantfelsic magma in the shallow crust may block the ascent of
the basaltic to basaltic andesitic magma from the deeper
level in the crust to the surface, which may be the main
reason why basaltic to basaltic andesitic rocks are rare in
the Sakurajima volcano and Aira caldera region.
7-2 Origin of andesitic magma of the Sakurajima
volcano and Aira caldera region since 61 ka
The andesites with 56 to 63 wt% SiO2 are not abundantin the Sakurajima volcano and Aira caldera region since 61
ka. Significant amount of andesite occurred in the Shikine
andesite as lavas and in the Iwato pyroclastic flow deposit
as scoria. In the Sakurajima volcano, most volcanic rocks
are dacite (SiO2 > 63 wt. %) excluding the P5 air-fallpumice deposit and some of the older Minamidake,
Tenpyohoji, Taisho and Showa lavas, which are andesitic
in composition (Takahashi et al., 2011). The andesites ofTemporal Variation for Magmatic Chemistry of the Sakurajima Volcano and Aira Caldera Region, Southern
Kyushu, Southwest Japan since 61 ka and Its Implications for the Evolution of Magma Chamber System

Fig. 10. SiO2 vs. FeO*/MgO diagram for the eruptive
products of the Sakurajima volcano and Aira caldera
region since 61 ka. 1: Minamidake historical lavas; 2:older Minamidake lava, lavas and pyroclastic flow
deposit of Kitadake; 3: Moeshima pumice; 4: Osumi
pumice fall deposit; 5: Ito pyroclastic flow deposit; 6:
Iwato pyroclastic flow deposit (pumice); 7: Iwato
pyroclastic flow deposit (scoria); 8: Shikine andesite;
9: basaltic magmatic inclusion of the Shikine andesite;
10: basaltic andesitic magmatic inclusion of theyounger Kitadake lava.

the Sakurajima volcano show the mineralogical evidencesof magma mixing, such as reverse compositional zoning ofplagioclase and pyroxenes (Yanagi et al., 1991). The
incompatible trace element ratios of andesites, such as
Rb/Zr, Rb/Y, Ba/Y, and Zr/Y, show a wide range and theirtrends of variation are almost linear, suggesting that theirtrend of variation caused not by crystallization differentiation but by magma mixing of the end-member magmas
with different incompatible trace element ratios.
For the Taisho and Showa lavas of the Sakurajima
volcano, Yanagi et al.(1991) estimated that the mafic endmember magma is basaltic with 52 wt. %SiO2 and thefelsic end-member magma is dacitic with 69 wt. %SiO2.
The inclination of negative linear compositional trends of
MgO content of the andesite of the high and low Ti-P types
in the MgO vs. SiO2 diagram are larger than those of thedacite of the high and low Ti-P types. Both compositional
trends cross at around 65 wt%SiO2 (Fig. 6), implying thatthe felsic end-member magmas of the andesite of the
Sakurajima volcano are the dacite with around 65 wt.
%SiO2. Namely, the felsic end-member magma of theolder Minamidake lavas, the Tenpyohoji lavas and the
Taisho and Showa lavas are the dacite of the younger

29

Kitadake lava, the younger Minamidake lava, and the Anei subaqueous lava, respectively, the SiO2 contents ofwhich are around 65 wt.%.
The compositions of mafic end-member magmas ofandesitic magmas are inferred from the compositional
trend of andesites in the silica variation diagrams. The
extension of the linear compositional trends to the SiO2
content corresponding to basalt to basaltic andesite suggests the composition of the mafic end-member magma.
The inferred compositions of mafic end-member magma inthe silica variation diagrams for Na2 O and Al2 O3 arevariable (Fig. 17). The estimated mafic end-member magmawith high Na2O is also high in Al2O3, suggesting that theircompositional variations are mainly controlled by a seriesof crystallization differentiation of mafic minerals from asimilar parental basaltic magma (Fig. 17).
The rhyolitic magma yielding the pumice of the Iwato
pyroclastic flow deposit is a plausible candidate for thefelsic end-member magma of the Shikine andesite (Fig.
18), but the felsic end-member magma of andesitic scoriaof the Iwato pyroclastic flow deposit is not the rhyolitic
magma of the Iwato pumice. The felsic end-member
magma of the Iwato scoria must be more enriched in K2O
and Rb than the Iwato pumice. On the other hand, themafic end-member magma of the andesitic scoria of the
Iwato pyroclastic flow deposit was probably the mafic
andesite or basaltic andesite of the Shikine andesite.
In the La/Yb vs. La diagram, it is also difficult tointerpret the rhyolitic magma of the Iwato pyroclastic flow
deposit as a candidate for felsic end-member magma of theandesitic scoria of the Iwato pyroclastic flow deposit (Fig.
19).
It may be concluded that the andesites with 56 to 63
wt%SiO2 of the Sakurajima volcano and Aira caldera
region since 61 ka are the products of mgama mixing. The
felsic end-member magma of the andesite of the Sakurajima volcano is dacitic, and that of the Shikine andesite areinferred to be the rhyolitic magma which produced the
pumice of the Iwato pyroclastic flow deposit. The felsic
end-member magma of the Iwato scoria is probably moreenriched in K2 O and Rb than the Iwato pumice. The
compositions of felsic end-member magma of andesite in
the Sakurajima volcano and Aira caldera region since 61
ka are variable in each stage of volcanic activity.
7-3 Origin of felsic magmas of the Sakurajima
volcano and Aira caldera region since 61 ka
The incompatible trace element ratios of the dacite andrhyolite of the Sakurajima volcano and Aira caldera region
since 61 ka exhibit a wide range. The Rb/Y ratio, forexample, is the highest in the pumice of the Iwato
pyroclastic flow, Osumi pumice fall and Ito pyroclastic
flow deposits, and the lowest in the dacitic members of thehigh Ti-P types of the Sakurajima volcano (Fig. 13). It isdifficult to derive this wide range of Rb/Y ratios and otherincompatible trace element ratios by simple crystallization

30

Masaki TAKAHASHI, Tadashi OTSUKA, Hisashi SAKO, Hiroshi KAWAMATA, Maya YASUI,
Tatsuo KANAMARU, Mei OTSUKI, Tetsuo KOBAYASHI, Kazuhiro ISHIHARA and Daisuke MIKI

Fig. 11. Silica variation diagrams for Rb, Ba, Sr, Zr, Nb and Y contents of the eruptive products of the Sakurajima volcano
and Aira caldera region since 61 ka. 1: Showa lava; 2: Taisho lava; 3: Anei terrestrial lava; 4: Bunmei lava; 5: younger
Minamidake lava; 6: Tenpyohoji lava; 7: older Minamidake lava; 8: younger Kitadake lava; 9: Hikinohira lava; 10:
Furihatayama lava; 11: Gongenyama lava; 12: mafic magmatic inclusion of the younger Kitadake lava; 13: Moeshima
pumice; 14: Osumi pumice fall deposit; 15: Ito pyroclastic flow deposit; 16: Iwato pyroclastic flow deposit (pumice); 17:
Iwato pyroclastic flow deposit (scoria): 18: Shikine andesite lava; 19: basaltic magmatic inclusion of the Shikine
andesite.

differentiation of basaltic to basaltic andesitic magma; inthe case of the Sakurajima volcano, the dacitic magmas
cannot be produced by simple crystallization differentiation of basaltic andesitic magma, the composition ofwhich is the same as the basaltic andesitic MMI of theyounger Kitadake lava (Fig. 13). The wide range of the
Rb/Y ratios also cannot be explained by magma mixing
between the two end-members, for example the rhyolitic tohigh-silica rhyolitic magma producing the pumice of the
Iwato pyroclastic flow, Osumi pumice fall and Itopyroclastic flow deposits, and the basaltic to basaltic
andesitic magma yielding the Shikine andesite or thebasaltic andesitic MMI of the younger Kitadake lava of the

Sakurajima volcano (Fig. 13). The wide range of Rb/Y
ratios of the felsic magmas suggests that their sources aredifferent.
The Rb/Y ratios of felsic magmas decrease in thefollowing order, (1) the rhyolitic to high-silica rhyolitic
pumice of the Iwato pyroclastic flow, Osumi pumice falland Ito pyroclastic flow deposits, (2) the Gongenyama
dacitic lava of the younger Kitadake stage, (3) the
Furihatayama-Harutayama dacitic lavas of the younger
Kitadake stage, (4) the Moeshima rhyolitic pumice
deposit, (5) the Hikinohira-Kitadake dacitic lavas of theyounger Kitadake stage, (6) the dacitic lavas of the
Minamidake stage (Fig. 13). The dacitic lavas of theTemporal Variation for Magmatic Chemistry of the Sakurajima Volcano and Aira Caldera Region, Southern
Kyushu, Southwest Japan since 61 ka and Its Implications for the Evolution of Magma Chamber System

31

Fig. 12. Silica variation diagrams for Rb, Ba, Sr, Zr, Nb and Y contents of the lavas of boring core samples of drill-holesites. 1: An-ei subaqueous lava; 2: An-ei terrestrial lava; 3: Tenpyohoji lava; 4: younger Kitadake lava; 5: Harutayama
lava.

Minamidake stage are further classified into three subgroups with different compositional trends based on Rb/Zr,
Rb/Y, Ba/Y and Zr/Y ratios; they are (6-1) the An-ei
terrestrial lavas, (6-2) the Tenpyohoji lava, and (6-3) theAn-ei subaqueous lava (Fig. 14).
The high silica rhyolitic magma of the Osumi pumice
fall and Ito pyroclastic flow deposits can be produced fromthe rhyolitic magma of the Iwato pyroclastic flow depositby crystallization differentiation, because the incompatible
trace element ratios of both magmas are similar and thecontent of incompatible trace element is lower in therhyolitic magma of the Iwato pyroclastic flow deposit.
The chondrite-normalized patterns of REE of thepumice of the Iwato pyroclastic flow deposit and those of
the Osumi pumice fall and Ito pyroclastic flow deposit areparallel and the REE is enriched in the Osumi pumice falland Ito pyroclastic deposit, also indicating that the highsilica rhyolitic magma of the Osumi pumice fall and Itopyroclastic flow deposit can be derived from the rhyolitic
magma producing pumice of the Iwato pyroclastic flow
deposit by crystallization differentiation (Fig. 16).
The felsic magmas in the Sakurajima volcano and Airacaldera region since 61 ka probably comprise at least
above eight types with different Rb/Y ratios, which could
not be derived from the same source by the same process.
As regard to the Rb/Zr, Rb/Y, Ba/Y and Zr/Y ratios, it isalso difficult to produce the felsic and andesitic magmas
by crystallization differentiation of basaltic to basaltic
andesitic magma, the composition of which is the same as
the basaltic MMI of the Shikine andesite or the basaltic
andesitic MMI of the younger Kitadake lava (Fig. 13).
The La/Yb ratios also range widely in the La/Yb vs. La
diagram (Fig. 19). The La/Yb ratio is the highest in the
pumice of the Iwato pyroclastic flow, Osumi pumice falland Ito pyroclastic flow deposits, next in the low Ti-P
types, and the lowest in the high Ti-P types of the32

Masaki TAKAHASHI, Tadashi OTSUKA, Hisashi SAKO, Hiroshi KAWAMATA, Maya YASUI,
Tatsuo KANAMARU, Mei OTSUKI, Tetsuo KOBAYASHI, Kazuhiro ISHIHARA and Daisuke MIKI

Fig. 13. Diagrams showing the incompatible trace element ratios of the eruptive products of the Sakurajima volcano andAira caldera region since 61 ka with calculated crystallization differentiation trend started from the composition of thebasaltic andesitic magmatic inclusion of the younger Kitadake lava. The partition coefficients of Rb, Zr and Y between
minerals and basaltic magma are cited from Rollinson (1993). The crystallization differentiation trends are calculated
based on the Rayleigh fractionation model using above partition coefficients and the modal composition of phenocrystsof the basaltic andesitic magmatic inclusion ( plagioclase: 8.1vol.%; clinopyroxene: 7.3vol.%; orthopyroxene: 2.7vol.%;magnetite: 4.7vol.%; groundmass: 77.3vol.% ). The simple crystallization differentiation of basaltic andesitic magma,
represented by the composition of the magmatic inclusion of the younger Kitadake lava, cannot explain the wide rangeof ratios of the andesitic to felsic eruptive products of the Sakurajima volcano and Aira caldera region since 61 ka. A:
Rb/Zr; B: Rb/Y; C: Ba/Y; D: Zr/Y. 1: Showa lava; 2: Taisho lava; 3: Anei terrestrial lava; 4: Bunmei lava; 5: younger
Minamidake lava; 6: Tenpyohoji lava; 7: older Minamidake lava; 8: younger Kitadake lava; 9: Hikinohira lava; 10:
Furihatayama lava; 11: Gongenyama lava; 12: mafic magmatic inclusion of the younger Kitadake lava; 13: Moeshima
pumice; 14: Osumi pumice fall deposit; 15: Ito pyroclastic flow deposit; 16: Iwato pyroclastic flow deposit (pumice); 17:
Iwato pyroclastic flow deposit (scoria); 18: Shikine andesite lava; 19: basaltic magmatic inclusion of the Shikine
andesite.

Temporal Variation for Magmatic Chemistry of the Sakurajima Volcano and Aira Caldera Region, Southern
Kyushu, Southwest Japan since 61 ka and Its Implications for the Evolution of Magma Chamber System

33

Fig. 14. Diagrams showing the incompatible trace element ratios of the lavas of boring core samples of drill-hole sites. A:
Rb/Zr; B: Rb/Y; C: Ba/Y; D: Zr/Y. 1: An-ei subaqueous lava; 2: An-ei terrestrial lava; 3: Tenpyohoji lava; 4: younger
Kitadake lava; 5: Harutayama lava.

Sakurajima volcano. The compositional trends of the low
and high Ti-P types cannot be explained by magma mixing
between the rhyolitic magma producing the pumice of the
Iwato pyroclastic flow, Osumi pumice fall and Itopyroclastic flow deposits and the basaltic andesitic magma
yielded the MMI of the younger Kitadake lava, or by
crystallization differentiation of the basaltic to basaltic
andesitic magma produced the MMI of the younger
Kitadake lava. This is consistent with the inference fromthe incompatible trace element ratios such as Rb/Zr, Rb/Y ,Ba/Y and Zr/Y.
The 87Sr/86Sr isotopic ratios of the felsic magma of the
Sakurajima volcano and Aira caldera region since 61 ka
are variable. The 87Sr/86Sr isotopic ratio of the high-silica
rhyolitic magmas of the Osumi pumice fall and Itopyroclastic flow deposit are 0.7058 to 0.7076, while thoseof the low Ti-P type dacitic magma (0.7054 to 0.7057) arehigher than those of the high Ti-P type dacitic magma(0.7052 to 0.7054) (Kurasawa et al., 1984; Arakawa et al.,
1998; Uto et al., 2005). The 87Sr/86Sr isotopic ratios also

34

Masaki TAKAHASHI, Tadashi OTSUKA, Hisashi SAKO, Hiroshi KAWAMATA, Maya YASUI,
Tatsuo KANAMARU, Mei OTSUKI, Tetsuo KOBAYASHI, Kazuhiro ISHIHARA and Daisuke MIKI

Fig. 16. Chondrite-normalized compositional pattern ofrare earth elements for the eruptive products of the
Aira caldera region. 1: average of high Ti-P types of
the Sakurajima volcano; 2: average of low Ti-P typesof the Sakurajima volcano; 3: average of the Itopyroclastic flow deposit; 4: average of pumice of the
Iwato pyroclastic flow deposit; 5: average of scoria of
the Iwato pyroclastic flow deposit; 6: Moeshima
pumice.
Fig. 15. Chondrite-normalized compositional pattern ofrare earth elements for the eruptive products of the
Sakurajima volcano. A: high Ti-P types; 1: Showa
lava; 2: Taisho lava; 3: An-ei terrestrial lava; ; 4:
Bunmei lava; 5: younger Minamidake lava; 6: mafic
magmatic inclusion of the younger Kitadake lava; 7:
Moeshima pumice; B: low Ti-P types; 1: older Minamidake lava; 2: Furihatayama-Harutayama lava; 3: Gongenyama lava; 4: Hikinohira lava; 5: younger Kitadake
lava; 6: mafic magmatic inclusion of younger Kitadake
lava; 7: Moeshima pumice.

support the conclusion that the felsic magmas consist ofvarious types with different origin. On the view point of
the 87 Sr/86 Sr isotopic ratios, the rhyolitic to high-silica
rhyolitic magmas with relatively high 87 Sr/86 Sr isotopic
ratios are considered to be the crustal origin (Arakawa etal., 1998; Uto et al., 2005).
The combination of magma mixing and crystallizationdifferentiation is another alternative explanation. The
crystallization differentiation of the mixed andesitic
magma with various mixing ratios will give rise to theevolved felsic magmas with variable incompatible traceelement ratios such as Rb/Y (Fig. 20); the andesitic magma
was produced by magma mixing between the rhyolitic
magma of the Iwato pyroclastic flow deposit or the highsilica rhyolitic magma of the Osumi pumice fall and Itopyroclastic flow deposits, and the basaltic to basaltic
andesitic magma yielding the magmatic inclusions of the
Shikine andesite and the younger Kitadake lava of the
Sakurajima volcano (Fig. 20). The felsic magma withhigher Rb/Y ratio (the low Ti-P types) may have been

Table 1. The contents of rare earth elements of eruptive
products of the Sakurajima volcano and Aira caldera
region (in ppm). 1: Showa lava; 2: Taisho lava; 3: Anei terrestrial lava; 4: Bunmei lava; 5: younger Minamidake lava; 6: older Minamidake lava; 7: younger
Kitadake lava; 8: basaltic andesitic magmatic inclusionof the younger Kitadake lava; 9: Hikinohira lava; 10:
Furihatayama lava; 11: Gongenyama lava; 12: Moeshima
pumice; 13*: average of the Osumi air-fall pumice;
14*: average of pumice of the Ito pyroclastic flow
deposit; 15*: average of pumice of the Iwato pyroclastic flow deposit; 16*: average of scoria of the Iwato
pyroclastic flow deposit.

derived from the mixed andesitic magma enriched in thefelsic end-member component, while that with lower Rb/Y
ratio (the high Ti-P types) is from the mixed andesitic
magma depleted in the felsic end-member component (Fig.

Temporal Variation for Magmatic Chemistry of the Sakurajima Volcano and Aira Caldera Region, Southern
Kyushu, Southwest Japan since 61 ka and Its Implications for the Evolution of Magma Chamber System

35

Fig. 17. Silica variation diagrams for Na2O and Al2O3 contents of andesites of the Sakurajima volcano and Aira caldera
region produced by the magma mixing. 1: Taisho lavas of the Sakurajima volcano; 2: Tenpyo-hoji (Nagasakibana) lavasof the Sakurajima volcano; 3: older Minamidake lavas of the Sakurajima volcano; 4: MMI in the younger Kitadake lavaof Sakurajima volcano; 5: MMI in the Shikine andesite.

Fig. 18. Silica variation diagrams for K2O and Rb contents of the Shikine andesite, andesitic scoria and rhyolitic pumice of
the Iwato pyroclastic flow deposit. 1: pumice of the Iwato pyroclastic flow deposit; 2: scoria of the Iwato pyroclastic
flow deposit; 3: Shikine andesite; 4: MMI in the Shikine andesite.

20).
The high Ti-P type felsic magma is higher in TiO2, P2
O5, Na2 O and FeO*, and lower in MgO and CaO,
compared with the low Ti-P type felsic magma. It may be
explained by assuming that the mafic end-member magmaof the original mixed andesitic magma of the high Ti-P
type is more differentiated and enriched in TiO2, P2 O5,
Na2O and FeO*, and depleted in MgO and CaO, while thatof the low Ti-P types is less differentiated and depleted in
TiO2, P2 O5, Na2 O and FeO*, and enriched in MgO andCaO (Fig. 21). On the other hand, the original mixed
andesitic magma of the low Ti-P types is more enriched in
the felsic end-member component than that of the high TiP types, because the Rb/Y ratio of the low Ti-P types ishigher than that of the high Ti-P types (Fig. 20).
The 87 Sr/86 Sr isotopic ratios of the felsic end-member
magma is high (0. 7058 to 0. 7076), while that of thebasaltic andesitic MMI of the younger Kitadake lava,
which is one of the plausible candidates for the mafic end-

member magma, is low (0.7050) (Kurasawa et al., 1984;
Arakawa et al., 1998; Uto et al., 2005). The 87Sr/86Sr isotopic ratios of the high Ti-P type felsic magma (0.7052 to
0.7054) are lower than those of the low Ti-P type felsicmagma (0.7054 to 0.7057) (Kurasawa et al., 1984; Arakawa
et al., 1998; Uto et al., 2005). The lower 87Sr/86Sr isotopic
ratios of the high Ti-P type felsic magmas imply that their
original magmas are enriched in the mafic end-member
component with low 87Sr/86Sr isotopic ratios.
Kay et al. (2010) proposed the La/Sm vs. Sm/Yb discrimination diagram to distinguish the constituent mineralsof the source rocks of the felsic magmas. The La/Sm ratiomeans the degree of enrichment of incompatible elementin the source rocks, while the Sm/Yb ratio indicates thesource rock mineralogy reflecting the pressure. As the
Sm/Yb ratio increases, the constituent mafic minerals of
the source rocks change from pyroxene, hornblende togarnet, which indicate the increase of pressure. The
source rock mineralogy of the rhyolitic to high-silica

36

Masaki TAKAHASHI, Tadashi OTSUKA, Hisashi SAKO, Hiroshi KAWAMATA, Maya YASUI,
Tatsuo KANAMARU, Mei OTSUKI, Tetsuo KOBAYASHI, Kazuhiro ISHIHARA and Daisuke MIKI

Fig. 19. La/Yb vs. La diagram for the eruptive products of
the Sakurajima volcano and Aira caldera region. The
partition coefficients of La and Yb between minerals
and basaltic melt are cited from Rollinson, (1993). The
crystallization differentiation trends are calculated basedon the Rayleigh fractionation model using above partition coefficients and the modal composition of phenocrysts of the basaltic andesitic magmatic inclusion(plagioclase: 8.1 vol.%; clinopyroxene: 7.3 vol.%; orthopyroxene: 2. 7 vol. %; magnetite: 4. 7 vol. %; groundmass: 77.3 vol.% ). The trends of crystallization differentiation are variable mainly due to the wide range ofpartition coefficient of magnetite. The four cases ofbulk partition coefficient of La and Yb are presented.
The crystallization differentiation of basaltic andesitic
magma, represented by the composition of the magmatic inclusion of the Kitadake lava, cannot explain
the ratios of the andesitic to felsic eruptive products of
the Sakurajima volcano and Aira caldera region since
61 ka. Numerals in the line showing the trend of
crystallization differentiation denote the percent crystallization. 1: Showa lava; 2: Taisho lava; 3: An-ei
terrestrial lava; 4: Bunmei lava; 5: younger Minamidake lava; 6: older Minamidake lava; 7: younger
Kitadake lava; 8: Hikinohira lava; 9: Furihatayama
lava; 10: Gongenyama lava; 11: mafic magmatic
inclusion of younger Kitadake lava; 12: Moeshima
lava, 13: Osumi pumice fall deposit; 14; Ito pyroclast
flow deposit; 15: pumice of the Iwato pyroclastic flow
deposit; 16: scoria of the Iwato pyroclastic flow
deposit.

rhyolitic magma of the Iwato pyroclastic flow, Osumi
pumice fall and Ito pyroclastic flow deposits and dacitic
magma of the Sakurajima volcano is pyroxene bearing,
judging from the discrimination diagram (Fig. 22).
The excess alumina ratios (Al2O3/(CaO+Na2O+K2O)
molar ratio) of the rhyolitic to high-silica rhyolitic magma

Fig. 20. Schematic diagram of the Rb/Y ratio showing thegenetic process of dacitic magmas produced by crystallization differentiation of andesitic magma formedby magma mixing between the basaltic to basaltic
andesitic magma and the rhyolitic to high-silica
rhyolitic magma (dashed lines). The parental mixed
andesitic magma of high Ti-P type dacite is moreenriched in the mafic end-member component than thatof the low Ti-P type dacite. The erupted andesitic
magma of high Ti-P types, for example, are theproducts of magma mixing between the basaltic tobasaltic andesitic magma and the dacitic magma
produced by the crystallization differentiation ofparental mixed andesitic magma (dashed line).

of the Iwato pyroclastic flow, Osumi air fall and Itopyroclastic flow deposits and the dacitic magmas of the
Sakurajima volcano (>63 wt%SiO2) are mostly less than
1.1 and metaluminous (Fig. 23). The source rocks of therhyolitic to high-silica rhyolitic magma of the Iwato
pyroclastic flow, Osumi air fall and Ito pyroclastic flow
deposits and the dacitic magmas of the Sakurajima volcano
are not pelitic but igneous in composition.
The temperature of the high-silica rhyolitic magma
yielding the Ito pyroclastic flow deposit, which is estimated by the hydrothermal experiment, is 770℃ (Aramaki,1971).The temperatures of the high Ti-P type and low TiP type dacitic magmas based on the two pyroxenes
geothermometer by Lindsley (1983) at 5 kb are about
1000℃ to 1100℃ and 900℃ to 1000℃, respectively (Fig.
24). The temperature of the high Ti-P type dacitic magma
is slightly higher than that of the low Ti-P type dacitic
magma. If the source rocks were different, the high Ti-P
dacitic magma may have been formed under the higher
temperature condition than the low Ti-P dacitic magma.
On the other hand, if both types of dacitic magma wereTemporal Variation for Magmatic Chemistry of the Sakurajima Volcano and Aira Caldera Region, Southern
Kyushu, Southwest Japan since 61 ka and Its Implications for the Evolution of Magma Chamber System

Fig. 21. Schematic silica variation diagram showing thegenetic process of dacitic magmas produced by the
crystallization differentiation of andesitic magma formedby the magma mixing between the basaltic to basaltic
andesitic magma and the rhyolitic to high-silica rhyolitic magma. Arrows indicate the trend of crystallization differentiation and solid lines show the magma
mixing line. A: TiO2, P2O5, Na2O and FeO* contents;
B: MgO and CaO contents; star: felsic end-member
magma (rhyolite); solid square: high Ti-P type andesitic magma; open square: low Ti-P type andesitic
magma.

produced by the crystallization differentiation of mixed
andesitic magmas, the high Ti-P type mixed andesitic
magma is enriched in the component of high temperature
mafic end-member magma. The core compositions oforthopyroxene phenocryst of the low Ti-P types are moremagnesian than those of the high Ti-P types (Fig. 24),suggesting that the mafic end-member magma of the low
Ti-P types was less evolved and the mixed magma wasmore magnesian, which probably caused the relatively
magnesian nature of the dacitic magma of the low Ti-P
types (Fig. 24).
The two different explanations for the chemicalvariations of the eruptive products of the Sakurajima
volcano and Aira caldera region since 61 ka are briefly
summarized and schematically shown in Fig. 25.
7-4 Evolution of the magma chamber system of the
Sakurajima volcano and Aira caldera region
since 61 ka
The evolution of the magma chamber system of the
Sakurajima volcano and Aira caldera region since 61 ka issummarized in Fig. 4.

37

Fig. 22. La/Sm vs.Sm/Yb diagram by Kay et al. (2010).
The La/Sm ratio shows the degree of source enrichment and the Sm/Yb ratio indicates the source rockmineralogy. The Sm/Yb ratios of the felsic eruptive
products of the Sakurajima volcano and Aira caldera
region suggest that their source rocks are pyroxene
bearing. 1: low Ti-P types; 2: high Ti-P types; 3:
Osumi pumice fall and Ito pyroclastic flow deposits; 4:
Iwato pyroclastic flow deposit; PXY: pyroxene bearing; HBL: hornblende bearing; GAR: garnet bearing.

Fig. 23. Histogram showing the Al2O3/(CaO+Na2O+K2O) ratios. The Al2 O3 / (CaO + Na2 O + K2 O) molar
ratios of felsic eruptive products (> 63 %SiO2) aremostly less than 1.1 and metaluminous, showing that
their source rocks are not pelitic but igneous incomposition. 1: high Ti-P types; 2: low Ti-P types; 3:
Moeshima pumice; 4: Osumi pumice fall and Itopyroclastic flow deposits; 5: Iwato pyroclastic flow
deposit (pumice).

Masaki TAKAHASHI, Tadashi OTSUKA, Hisashi SAKO, Hiroshi KAWAMATA, Maya YASUI,
Tatsuo KANAMARU, Mei OTSUKI, Tetsuo KOBAYASHI, Kazuhiro ISHIHARA and Daisuke MIKI

38

Fig. 24. The estimation of temperature of the high Ti-P and low Ti-P type magmas by the pyroxene geothermometer ofLindsley (1981) at 5 kb. The core compositions of phenocrystic orthopyroxenes coexisting with phenocrystic clinopyroxenes are presented. The numerals indicate the temperature in ℃. The temperature of the high Ti-P types estimatedfrom the core composition of phenocrystic orthopyroxenes is higher than that of the low Ti-P types. The estimatedtemperatures of the high Ti-P types are 1000℃ to 1100℃ and those of the low Ti-P types are 900℃ to 1000℃,
respectively. Di: diopside; En: enstatite; Fs: ferrosilite.

Around 61 to 60 ka, the andesitic magma chambersystem was active beneath the Aira caldera region, and the
Shikine andesite lavas (53 to 60 wt%SiO2) and the scoriaof the Iwato pyroclastic flow deposit (57 to 59 wt%SiO2)
were erupted. Concurrently, the large-scale felsic magma
chamber was formed, which produced the pumice of the
Iwato pyroclastic flow deposit (72 to 74 wt%SiO2) with avolume of exceeding 20 km3.
Most andesitic magma in this stage was thought to beformed by magma mixing between the basaltic to basaltic
andesitic magmas of mantle origin and the felsic magmas
of crustal origin, some of which yielding the pumice of the
Iwato pyroclastic flow deposit.
Bachman and Bergantz (2008) classified the style ofmagma chambers, which gives rise to the large-scale
eruption of voluminous felsic magma, into the threegroups. The Group 1 comprises the upper crystal-poor
rhyolitic magma grading to the lower crystal-rich lessdifferentiated felsic magma. The Group 2 consists of thefelsic magma having almost no compositional gradient.
The Group 2 is further subdivided into (a) the crystal-poor
rhyolitic magma and (b) the crystal-rich dacitic magma.
The Group 3 is composed of the upper crystal-poor
rhyolitic magma shift abruptly to the lower crystal-richless differentiated magma.
The magma chamber of the Iwato pyroclastic flow
deposit probably belongs to the Group 3; the rhyolitic
pumice with phenocryst content of about 10vol. % wasderived from the upper crystal-poor rhyolitic portion andthe andesitic scoria with phenocryst content of about
30vol.% originated from the lower crystal-rich andesitic
part of the magma chamber, though both magmas had no
genetic relations in chemistry. Otherwise, the magma
chamber of the Iwato rhyolitic pumice was independentFig. 25. Schematic diagram showing the mode of magma
generation in the Sakurajima volcano and Aira caldera
region. A: explanation in which felsic magmas (including dacitic magma) were formed by melting of thedifferent source rocks (the case for both the Sakurajima
volcano and Aira caldera region); B: explanation inwhich dacites were produced by the crystallizationdifferentiation of mixed andesitic magma (the case for
the Sakurajima volcano).

Temporal Variation for Magmatic Chemistry of the Sakurajima Volcano and Aira Caldera Region, Southern
Kyushu, Southwest Japan since 61 ka and Its Implications for the Evolution of Magma Chamber Systemfrom that of the Iwato andesitic scoria; the eruption of the
Iwato rhyolitic pumice triggered the eruption of othermagma chamber filled with the andesitic magma of the
Iwato scoria.
After a long dormant period of about twenty-four
thousands of years, the felsic volcanic activity resumed at
36 ka in the Aira caldera region. The climactic gigantic
eruption at 29 ka with a volume of 450 km3 produced the
pumice and ash of high-silica rhyolite (74 to 76 wt.
%SiO2), the magma chamber of which is nearly uniform
and monotonous in composition and classified as theGroup 2(a) of Bachman and Bergantz (2008). The lowtemperature high-silica rhyolitic magma of the Osumi
pumice fall and Ito pyroclastic flow deposits can bederived from the rhyolitic magma erupted as the Iwato
pyroclastic flow deposit by crystallization differentiation.
Both felsic magmas are similar in composition andprobably comprise the same magma chamber system. The
rhyolitic magma chamber system which produced the
Iwato pyroclastic flow, Osumi pumice fall and Itopyroclastic flow deposits was long-lived, the duration ofwhich was as long as thirty thousands of years. The long
duration probably resulted in the storing of voluminous
felsic magma and the formation of gigantic-scale felsicmagma chamber beneath the Aira caldera region at thetime of the climactic eruption. It was difficult for basaltic
to basaltic andesitic magmas to penetrate the gigantic
felsic magma chamber, causing the lack of basaltic toandesitic volcanic activity in the Aira caldera region at thisperiod, except for the southern margin of the Aira caldera
where small amount of andesitic lavas erupted. The
magma chamber system of the voluminous high-silica
rhyolite was destroyed completely by the large catastrophic eruption at 29 ka.
After a quiescent period of about three thousands ofyears, the volcanic activity resumed and the magma
chamber system of the Sakurajima volcano began to feed
the daicitic to andesitic magmas to the surface.
The magma chamber system of the Sakurajima volcano
is not simple on the view point of the whole-rockchemistry; it is composed of the low and high Ti-P type
dacitic magmas. During the period from 26 to 13 ka, themagma chamber systems of the rhyolite (the Moeshimarhyolite) and dacite coexisted beneath the Sakurajima
volcano and Aira caldera region. The magma chambersystem of the Moeshima rhyolite was probably independent from that of the Sakurajima volcano. The magma
chamber systems with the relatively low temperature low
Ti-P type daicitic magma were active during the period
from 13 to 4 ka beneath the Sakurajima volcano and Airacaldera region, which further comprised at least more than
three different sub-systems based on the whole-rockchemistry of felsic volcanic rocks; they are represented by
the Gongenyama, Furihatayama-Harutayama, and younger
Kitadake-Hikinohira dacitic lava groups. The andesite

39

formed by the magma mixing was not dominant during
this period. The low Ti-P type older Minamidake andesite
lavas are akin to the lavas of the younger Kitadake stage inchemical composition; they belong to either the last stageof the Kitadake volcano or the earliest member of the
Minamidake volcano.
The magma chamber system of the historical eruptions
with the relatively high temperature high Ti-P type dacitic
magma consists of the three sub-systems; they are represented by the Tenpyohoji, younger Minamidake-BunmeiAn-ei terrestrial, and An-ei subaqueous lava groups mainlyon the basis of the whole-rock chemistry. Among them,both the Tenpyohoji and An-ei subaqueous magma
chamber sub-systems erupted andesitic magma producedby magma mixing; they are the Nagasakibana, Taisho andShowa lavas.
The each magma chamber sub-system of the Sakurajima
volcano is short-lived with duration of thousands to several
hundreds of years. The onset of the magma chamber subsystem of the present volcanic activity of the Sakurajima
volcano was probably the An-ei subaqueous eruption at
1779AD.
8．Summary and conclusion(1) The eruptive products of the Sakurajima volcano
consist of the high Ti-P and low Ti-P type dacite toandesite. The high Ti-P types are enriched in Na2O, FeO*,
Zr, Nb and Y, and depleted in MgO, Al2 O3 and CaO,
compared with the low Ti-P types. The incompatible traceelement ratios, such as Rb/Y, Rb/Zr, Ba/Y and Zr/Y, of thehigh Ti-P types are lower than those of the low Ti-P types.
The LREE/HREE ratios such as La/Yb are lower in thehigh Ti-P types. The 87Sr/86Sr isotopic ratios of the highTi-P types (0.7051 to 0.7054) are lower than those of the
low Ti-P types (0.7055 to 0.7057). It is difficult to deriveboth types from the same parental mafic magma by simple
crystallization differentiation.
(2) The high Ti-P and low Ti-P types of the Sakurajima
volcano are classified into at least the six sub-groups basedon the whole-rock chemical compositions, especiallymajor element chemistry. The high Ti-P types are composed of the three sub-groups: the Tenpyohoji, the yonger
Minamidake-Bunmei-An-ei terrestrial, and the An-ei
subaqueous-Taisho-Showa lava groups, while the low TiP types consist of the three sub-groups: the Gongenyama,
the Furihatayama-Harutayama, and the younger Kitadake Hikinohira lava groups.
(3) The Moeshima rhyolitic pumice erupted in the Airacaldera is lower in TiO2, P2O5, MgO, Al2O3 and CaO, andhigher in Na2 O, K2 O, FeO* and MnO than those of the
Iwato pyroclastic flow, Osumi pumice fall and Itopyroclastic flow deposits. The incompatible trace element
ratios, such as Rb/Y, Rb/Zr, Ba/Y and Zr/Y, of theMoeshima rhyolite are lower than those of the Iwato
pyroclastic flow, Osumi pumice fall and Ito pyroclastic

40

Masaki TAKAHASHI, Tadashi OTSUKA, Hisashi SAKO, Hiroshi KAWAMATA, Maya YASUI,
Tatsuo KANAMARU, Mei OTSUKI, Tetsuo KOBAYASHI, Kazuhiro ISHIHARA and Daisuke MIKI

flow deposits. The Moeshima pumice is more enriched in
REE content and lower in LREE/HREE ratios, such as
La/Yb, than the Iwato pyroclastic flow, Osumi pumice falland Ito pyroclastic flow deposits.
(4) The high-silica rhyolitic magma producing theOsumi pumice fall and Ito pyroclastic flow deposits can bederived from the rhyolitic magma of the Iwato pyroclastic
flow deposit by crystallization differentiation.
(5) The mafic volcanic rocks are present as lavas in the
Shikine andesite and as MMIs of the Shikine andesite andthe Kitadake lava of the Sakurajima volcano. They are
medium-K basalt to basaltic andesite. The basaltic andesite of the Shikine andesite can be produced by the crystallization differentiation of basalt of the Shikine andesite.
The basaltic magma continued to ascend constantly fromthe mantle to the crust and probably to supply enough heatto the lower crust to produce felsic magmas since 61 ka.
(6) The andesite of the Sakurajima volcano and Airacaldera region are the products of magma mixing. The
magma mixing between the magma of the Shikine andesite
and felsic end-member magma more enriched in K2O andRb than the rhyolitic magma of the Iwato pyroclastic flow
deposit gave rise to the andesitic scoria of the Iwato
pyroclastic flow deposit. The low Ti-P type andesites of
the Sakurajima volcano were produced by magma mixing
between the basaltic to basaltic andesitic magma giving
rise to the MMI of the younger Kitadake lava and the low
Ti-P type dacitic magma, while the high Ti-P type
andesites can be produced by magma mixing between thebasaltic to basaltic andesitic magma being akin to the MMIof the younger Kitadake lava and the high Ti-P type dacitic
magma.
(7) The felsic magmas of the Sakurajima volcano andAira caldera region consist of at least eight different types.
They may be generated independently by the melting ofpyroxene-bearing crustal source rocks with different
compositions. The alternative explanation is that they
were derived from the andesitic magma with various
mixing ratios by crystallization differentiation, the endmember magmas of which were the basaltic to basaltic
andesitic magma and the rhyolitic to high-silica rhyolitic
magma of the Iwato pyroclastic flow, Osumi pumice falland Ito pyroclastic flow deposits. The low Ti-P type
dacitic magma of the Sakurajima volcano is derived fromthe andesitic magma by crystallization differentiation, which
is produced by magma mixing between the less differentiated basaltic magma and the rhyolitic to high-silica
rhyolitic magma; the low Ti-P type mixed andesitic
magma is more enriched in the felsic end-member component. Contrarily, the high Ti-P type dacite is derived
from the andesitic magma by crystallization differentiation, which is formed by magma mixing between the
evolved basaltic to basaltic andesitic magma and therhyolitic to high-silica rhyolitic magma; the high Ti-P typemixed andesitic magma is more depleted in the felsic end-

member component.
(8) In the Aira caldera region, the andesite magma
chamber system was active around 61 ka, in which themagma mixing between basaltic magma and rhyolitic
magma was taken place, feeding the Shikine andesite lava
to the surface. In 60 ka, the relatively large-scale magma
chamber system of the rhyolite was developed, which was
accompanied by the andesitic magma produced by magma
mixing, erupting the pumice and scoria of the Iwato
pyroclastic flow deposit. The size of the rhyolitic magma
chamber increased during a quiescent period of twentyfour thousands of years, constructing a gigantic magma
chamber filled with voluminous high-silica rhyolitic
magmas, resulted in the catastrophic eruption which
formed the Osumi pumice fall, Ito pyroclastic flow, andAT ash fall deposits. During the period from 35 ka to 29
ka, no andesite erupted in the Aira caldera region; it isbecause the basaltic to basaltic andesitic magma could not
penetrate the gigantic felsic magma chamber beneath theAira caldera and could not reach the surface. The magma
chamber system of the rhyolitic to high-silica rhyolitic
magma was long-lived and maintained for as long as thirty
thousands of years.
After a dormant period of three thousands of years, thesmall to intermediate-scale magma chamber systemcomprising dacitic magma started to feed magmas to thesurface. The supply of basaltic to basaltic andesitic
magma to the dacitic magma chamber produced the mixed
andesitic magma. At 13.8 ka, the subaqueous eruption of
the Moeshima rhyolitic magma occurred in the Airacaldera, which was not directly related to the volcanic
activity of the Sakurajima volcano and probably comprised
the independent magma chamber system. The low Ti-P
type magma chamber system was active during 13.8 ka to
4 ka in the Sakurajima volcano, which consists of at least
three sub-systems. The high Ti-P type magma chambersystem was dominant since the historical Tenpyohoji
eruption at 8th C and continued until now, which
comprised three sub-systems. The onset of the youngest
sub-system is the An-ei subaqueous eruption at 1779AD.
The duration of the activity of each magma chamber subsystems of the Sakurajima volcano is rather short, which isthousands to hundreds of years.
Acknowledgement
We are grateful for Profs. Tagiri, M. and Fujinawa, A.
of the Ibaraki University for their discussions and help ofchemical analysis by XRF and ICP apparatus. We also
thank Shimada, J., Atsuchi, T., Umezawa, T., Shiroishi, T.,
Ichiki, Y., and Satake, S., the undergraduate students of the
Department of Geosystem Sciences of the Nihon
University, for their assistance of sampling and chemical
analysis. We appreciate Prof. Nakada, S. of the Earthquake
Research Institute of University of Tokyo for permitting
the use of XRF apparatus. We thank the Sakurajima Vol-

Temporal Variation for Magmatic Chemistry of the Sakurajima Volcano and Aira Caldera Region, Southern
Kyushu, Southwest Japan since 61 ka and Its Implications for the Evolution of Magma Chamber Systemcano Research Center of Disaster Prevention ResearchInstitute of Kyoto University for providing the boring coresamples of drill- hole sites and permitting us to use themfor the chemical analysis. We also appreciate the review
and helpful advices of Prof. Nakamura, M. of the Tohoku
University and another anonymous reviewer, which was
useful for improving the manuscript.
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(Editorial handling Masao Ban)

桜島火山および姶良カルデラ地域における 61 ka 以降のマグマ化学組成の時間変化とマグマ溜りシステムの進化
高橋正樹・大塚 匡・迫寿・川俣博史・安井真也・金丸龍夫・
大槻 明・小林哲夫・石原和弘・味喜大介

桜島火山および姶良カルデラ地域における 61 ka 以降のマグマ化学組成の時間変化とマグマ溜りシステムの進化について噴出物の全岩化学組成に基づいて検討した．61 ka 以降の桜島火山および姶良カルデラ地域のマグマは，(1) マントル起源の玄武岩質および玄武岩質安山岩質マグマ，(2) 地殻起源の流紋岩質および高シリカ流紋岩質マグマ，(3) デイサイト質マグマ，(4)マグマ混合によって形成された安山岩質マグマ，の 4グループに分けられる．姶良カルデラ地域では，61 ka 頃に玄武岩質，玄武岩質安山岩質，安山岩質そして流紋岩質マグマの活動があり，敷根安山岩や安山岩質スコリアおよび流紋岩質軽石からなる岩戸火砕流堆積物が噴出した．24,000 年ほどの静穏期の後に流紋岩質マグマの活動が再開し，29 ka には大量の高シリカ流紋岩質マグマが噴出して，大隅降下軽石堆積物および入戸火砕流堆積物が形成された．60 ka の岩戸火砕流堆積物の流紋岩質マグマと，29 ka の大隅降下軽石堆積物および入戸火砕流堆積物の高シリカ流紋岩質マグマは同源と考えられ，前者の結晶分化作用によって後者が形成されたと考えられる．姶良カルデラ地域では30,000 年余りの期間にわたって，流紋岩質〜高シリカ流紋岩質からなるマグマ溜りが長期に安定であったものと推定される．3,000 年余りの静穏期の後，桜島火山の活動が 26 ka に開始された．13.8 ka には姶良カル
デラ内で燃島軽石の海底噴火が生じたが，これは桜島火山とは別のマグマ溜りシステムからもたらされたものと考えられる．桜島火山のマグマ溜りシステムは，高 Ti-P タイプおよび低 Ti-P タイプのデイサイト質および安山岩質マグマからなる．このうちの安山岩質マグマは，玄武岩質〜玄武岩質安山岩質マグマと高 TiP タイプおよび低 Ti-P タイプのデイサイト質マグマのマグマ混合によって形成された．低 Ti-P タイプのマ
グマ溜りシステムは 14 ka から 4 ka に活動的であり，全岩化学組成からは少なくとも 3 つのサブシステムからなる．8 世紀以降の歴史時代噴出物は高 Ti-P タイプであり，全岩化学組成からは 3 つのサブシステムからなるが，最新のものは 1779 年 AD の安永海底噴火以降に噴出したものである．桜島火山のそれぞれのマ
グマ溜りシステムの持続時間は数 100 年から数 1,000 年程度と短い．