JOURNAL OF PETROLOGY
VOLUME 49
NUMBER 11
PAGES 1971^2008
2008
doi:10.1093/petrology/egn055
New Insights into Andesite Genesis: the Role of
Mantle-derived Calc-alkalic and Crust-derived
Tholeiitic Melts in Magma Differentiation
beneath Zao Volcano, NE Japan
Y. TATSUMI1*, T. TAKAHASHI1, Y. HIRAHARA1, Q. CHANG1,
T. MIYAZAKI1, J.-I. KIMURA1, M. BAN2 AND A. SAKAYORI3
1
INSTITUTE FOR RESEARCH ON EARTH EVOLUTION (IFREE), JAPAN AGENCY FOR MARINE^EARTH SCIENCE AND
TECHNOLOGY (JAMSTEC), YOKOSUKA 237-0061, JAPAN
2
DEPARTMENT OF EARTH AND ENVIRONMENTAL SCIENCES, YAMAGATA UNIVERSITY, YAMAGATA 990-8560, JAPAN
3
DEPARTMENT OF EARTH SCIENCES, KANAZAWA UNIVERSITY, KANAZAWA 920-1192, JAPAN
RECEIVED APRIL 5, 2008; ACCEPTED OCTOBER 10, 2008
ADVANCE ACCESS PUBLICATION DECEMBER 4, 2008
Two distinctive differentiation trends, tholeiitic and calc-alkalic,
are recognized in Zao volcano, which is located immediately behind
the volcanic front of the NE Japan arc. The genetic relationship
between these two magma series is critical for a better understanding
of andesite genesis, because they often coexist in close spatial and
temporal proximity in arc volcanoes. Petrographic features indicative
of ‘disequilibrium’, such as reversely zoned pyroxene phenocrysts,
the wide and bimodal compositional distribution in Ca/(Ca þ Na)
of plagioclase phenocrysts, honeycomb textures and dusty zones that
these plagioclase phenocrysts often exhibit, and the presence of olivine^pyroxene pairs with different Mg/Fe, are observed exclusively
in calc-alkalic rocks. In tholeiitic rocks the Sr isotopic ratios of
plagioclase phenocrysts, determined by both micromilling combined
with thermal ionization mass spectrometry, and laser-ablation
inductively coupled plasma mass spectrometry techniques, are constant at 07042^07044. On the other hand, those in calc-alkalic
rocks (07033^07042) show more complex characteristics, which
can be best understood if at least three end-member components,
a calc-alkalic basaltic melt, a tholeiitic basaltic melt and a tholeiitic
felsic melt, contribute to the production of mixed calc-alkalic
magmas. The 87Sr/86Sr and trace element compositions of the leastdifferentiated basalt magmas, which are inferred from the composition of the calcic plagioclase [Ca/(Ca þ Na) 409], suggest that
two types of basaltic magma, calc-alkalic and tholeiitic, exist
beneath the volcano. The tholeiitic basalt magma has a higher
*Corresponding author. E-mail: [email protected]
87
Sr/86Sr than the calc-alkalic magma (07042 vs 07038) and a
characteristic trace element signature consistent with the presence
of plagioclase and amphibole as melting residues. This suggests that
the tholeiitic magmas are produced via anatexis of amphibolitic crust
caused by underplating and/or intrusion of mantle-derived calcalkalic basalt magmas into the sub-Zao crust. The mantle-derived
calc-alkalic basalt magma mixes with crust-derived tholeiitic melts
to form calc-alkalic andesite magmas. The hypothesis proposed here
requires revision (or even abandonment) of the general consensus
that calc-alkalic magmas have greater contributions of a crustal
component than tholeiitic magmas.
KEY WORDS:
andesite; calc-alkalic: crust; mantle; tholeiitic
I N T RO D U C T I O N
How andesite is generated has long been a central question
of igneous petrology. The reason for this is two-fold. First,
andesite erupts in more than 80% of arc volcanoes, typifies
subduction zone magmatism that creates over 20% of current terrestrial magmatic products, and is the dominant
volcanic rock in mature continental arcs. Second, the continental crust, the most differentiated end-member among
components within the solid Earth, is overall andesitic or
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JOURNAL OF PETROLOGY
VOLUME 49
intermediate in composition (e.g. Rudnick, 1995; Taylor,
1995; Rudnick & Gao, 2003). Knowledge of andesite genesis should therefore provide key constraints on the origin
of continental crust and differentiation processes during
the evolution of the solid Earth. The following five models
are currently favored for the production of andesitic
magmas (sensu lato).
(1) Crystallization differentiation of mantle-derived
basaltic magmas either in shallow crustal reservoirs or in
the deep crust close to the Moho (Sisson & Grove, 1993;
Mu«ntener et al., 2001; Annen & Sparks, 2002; Pichavant
et al., 2002; Prouteau & Scaillet, 2003).
(2) Anatexis of mafic lower crust by intrusion or underplating of mantle-derived basaltic magma (Takahashi,
1986; Smith & Leeman, 1987; Petford & Atherton, 1996;
Kimura et al., 2002; Annen et al., 2006; Tatsumi et al., 2008).
(3) Open-system differentiation, such as mixing
between felsic and mafic magmas, and crustal assimilation
(Eichelberger, 1975; DePaolo, 1981; Sakuyama, 1981;
Hildreth & Moorbath, 1988; Clynne, 1999; Tatsumi &
Kogiso, 2003; Dungan & Davidson, 2004).
(4) Generation of melts in equilibrium with mantle
peridotite under hydrous conditions as a result of either
direct fluxing of slab-derived fluids or slab-melting and
subsequent melt^mantle interaction (Kay, 1978; Tatsumi,
1981; Crawford et al., 1989; Pearce et al., 1992; Yogodzinski
et al., 1994; Kelemen, 1995; Blatter & Carmichael, 2001;
Tatsumi & Hanyu, 2003; Parman & Grove, 2004).
(5) Production of andesitic to more felsic melts by
dehydration melting of the subducted oceanic crust
(Kay, 1978; Martin, 1986; Stern & Kilian, 1986; Defant &
Drummond, 1990).
Two distinctive differentiation trends, tholeiitic and
calc-alkalic, are recognized in the sub-alkalic volcanic
rocks, denoting the presence or absence of relative iron
enrichment during magmatic differentiation (Wager &
Deer, 1939; Nockolds & Allen, 1953; Kuno, 1959; Irvine &
Baragar, 1971; Miyashiro, 1974). It has been well documented that tholeiitic rocks are dominant in juvenile oceanic
arcs, whereas calc-alkalic rocks are the major magmatic
products in mature continental arcs with thicker crust
(e.g. Miyashiro, 1974; Ewart, 1982). In several arc^trench
systems, however, tholeiitic and calc-alkalic andesites to
dacites have been observed to coexist in close temporal
and spatial proximity (Kuno, 1959; Sakuyama, 1981; Grove
& Baker, 1984; Fujinawa, 1988; Brophy, 1990; Hunter &
Blake, 1995; Hunter, 1998). Resolution of the genetic relationship between these two types of andesitic magmas
should, therefore, provide a better understanding of andesite genesis and arc crust evolution.
The primary aim of this paper is to investigate the
mechanism that produces tholeiitic and calc-alkalic
magmas at a single volcano, by combining petrographical
and geochemical data, including micro-analyses of the
NUMBER 11
NOVEMBER 2008
isotopic and trace element compositions of plagioclase
phenocrysts.
O V E RV I E W O F T H O L E I I T I C A N D
CA LC -A L K A L IC A N DESI T E
Definition and chemical characteristics
Bowen (1928) proposed that silica content increases and
iron content decreases with differentiation of sub-alkalic
magmas. On the other hand, Fenner (1929) emphasized
that some suites show iron enrichment during magmatic
differentiation. These two differentiation trends have been
referred to as calc-alkalic and tholeiitic, respectively
(Wager & Deer, 1939). The identification of these two
trends is commonly based on a ternary plot of Na2O þ
K2O, FeO (total iron as FeO), and MgO (Irvine &
Baragar, 1971). For a more quantitative distinction of the
two magma series, FeO/MgO vs SiO2 variation plots
(Miyashiro, 1974) are often used; calc-alkalic and tholeiitic
rock series show steeper and gentler slopes, respectively,
than the straight line: SiO2 (wt %) ¼ 64 FeO/
MgO þ 428. Unfortunately, however, Miyashiro’s discriminant line is frequently misused and applied as a simple
compositional discriminant outside this range rather than
its intended use as a ‘trend slope’ comparison.
The terms medium-K and calc-alkalic series are often
used interchangeably (Fig. 1). This misusage may be due to
poor understanding of the following two observations.
First, concentrations of incompatible elements such as K,
Rb, and Nb in lavas at a constant SiO2 content increase
with distance from the volcanic front or the height of a
volcano above the slab surface (Fig. 1), which is generally
known as the K^h relationship (Dickinson, 1975). Second,
calc-alkalic series rocks are generally more enriched
in incompatible elements than tholeiitic series rocks
(e.g. Masuda & Aoki, 1979; Kimura & Yoshida, 2006).
A schematic illustration showing the across-arc variation
in magma series from tholeiitic, via calc-alkalic, to highK or alkalic with distance from the volcanic front (Fig. 1a)
is then often cited (e.g. Hess, 1989; Wilson, 1989), although
calc-alkalic rocks do exist along the volcanic front and tholeiitic rocks belonging to the medium-K series often erupt
at volcanoes behind the volcanic front as shown in Fig. 1b
(Kuno, 1960; Kawano et al., 1961; Gill, 1981; Yoshida & Aoki,
1984; Tatsumi & Eggins, 1995; Tatsumi & Kogiso, 2003).
The tholeiitic and calc-alkalic series should be defined by
the presence and absence of iron enrichment, respectively,
whereas low-, medium-, and high-K series should be
defined on the basis of K2O concentrations.
Yoder & Tilley (1962) emphasized the presence of at
least two types of basalt magmas, one with normative
hypersthene differentiating to silica-saturated and the
other with normative nepheline differentiating to silicaundersaturated liquids, and coined the terms olivine
1972
TATSUMI et al.
(a)
CALC-ALKALIC VS THOLEIITIC SERIES
An awkward scheme
(b)
4
An unfailing scheme
4
Shoshonitic
(Alkalic)
Shoshonitic
(Alkalic)
Hig
h-K
3
K2O (wt.%)
2
lic
-alka
Calc
2
Med
Low-K
Low
60
65
0
50
70
55
SiO2 (wt.%)
Volcanic
Front
Forearc
Volcanic Arc
Medium-K
(CA/TH)
High-K(CA/TH)
Increasing K, H2O
Trench
Calc-alkalic
High-K
Backarc
Backarc
Forearc
Volcanic Arc
Mantle Wedge
s
tho
ere
ere
ph
i
s
tho
i
dL
te
uc
bd
Su
70
Mantle Wedge
ph
dL
65
SiO2 (wt.%)
Low-K tholeiitic
Increasing K, H2O
60
Trench
55
K
ium-
1
iitic)
-K (thole
0
50
h-K
Volcanic
Front
1
Hig
Low-K (CA/TH)
K2O (wt.%)
3
te
uc
bd
Su
Fig. 1. Two schemes for the classification and spatial distribution of arc magmas. (a) The interchangeable usage of the terms medium-K and
calc-alkalic series leads to an awkward understanding of across-arc variation; that is, magma types change from low-K tholeiitic via calc-alkalic
to high-K series. (b) Low-, medium-, and high-K series should be defined by K2O contents, whereas calc-alkalic and tholeiitic series should be
identified based on the absence and presence of iron enrichment, respectively. Concentrations of incompatible elements such as K increase
towards the back-arc side of a volcanic arc. Calc-alkalic rocks (CA) do occur in the low-K zone along the volcanic arc and tholeiitic rocks
(TH) often coexist with calc-alkalic rocks in volcanoes behind the volcanic front.
tholeiite and alkali basalt for these magmas, respectively.
Tholeiites can be clearly defined based on normative compositions; however, it should be noted that Yoder & Tilley’s
tholeiites are identical to sub-alkalic magmas and thus
include both tholeiitic and calc-alkalic rock series defined
based on the presence or absence of iron enrichment
during differentiation of sub-alkalic magmas.
Calc-alkalic and tholeiitic series can be well identified
for rocks with intermediate compositions; in other words,
it may be difficult to classify mafic (basaltic) or felsic
(rhyolitic) rocks into these two rock series. We tentatively
use the term calc-alkalic or tholeiitic for basalts and rhyolites, if they, together with andesites, form calc-alkalic or
tholeiitic trends, respectively.
Petrographical characteristics
The calc-alkalic vs tholeiitic series should be defined exclusively on the basis of differentiation trends. However, the
identification of such chemical trends is, in some cases,
difficult because of a lack of sufficient data for defining the
1973
JOURNAL OF PETROLOGY
VOLUME 49
chemical trend. As a result, this can be supplemented with
petrographical characteristics to identify these two magma
series. On the basis of groundmass mineralogy Kuno
(1950, 1959, 1968) divided sub-alkalic volcanic rocks into
two series: hypersthenic and pigeonitic. These are distinguished by the presence or absence of orthopyroxene in
the groundmass, and are synonymous with the calc-alkalic
and tholeiitic series, respectively. Although it may not be
easy to identify fine-grained groundmass pyroxenes under
the microscope, orthopyroxene phenocrysts with a reaction rim of clinopyroxene, which occur solely in the pigeonitic rock series, can be recognized (e.g. Kawano et al.,
1961). Furthermore, phenocrysts of hornblende and biotite
are limited to the hypersthenic rock series (Kuno, 1950).
It has been well established that Kuno’s scheme is valid for
Quaternary arc volcanoes along the trench-side volcanic
chain in the NE Japan and Izu^Bonin arcs (Kuno, 1950;
Kawano et al., 1961; Wada, 1981, 1985; Fujinawa, 1988, 1990),
where tholeiitic rocks are broadly equivalent to the low-K
series of Gill (1981).
Sakuyama (1981) examined the petrographical characteristics of volcanic rocks from Myoko^Kurohime volcanoes, Central Japan, where both calc-alkalic and tholeiitic
magmas have erupted from single vents, and divided these
rocks into two types, N-type and R-type, on the basis of the
absence and presence of reversely zoned mafic phenocrysts,
respectively. The following ‘disequilibrium’ petrographical
features characterize the R-type volcanic rocks:
(1) the presence of reversely zoned pyroxene phenocrysts
with a lower Mg-number [¼100 Mg/(Mg þ Fe)] core
surrounded by a higher Mg-number rim;
(2) the presence of groundmass pyroxenes with higher
Mg-number than the phenocryst cores;
(3) bimodal distribution in the core compositions of
plagioclase phenocrysts;
(4) disequilibrium phenocryst assemblages such as
Mg-rich olivine and quartz;
(5) patchy groundmass with different colors and/or
amount of mafic minerals.
These disequilibrium features are not observed in
N-type rocks. It was emphasized by Sakutyama (1981)
that the N- and R-type rocks are broadly equivalent to
Kuno’s pigeonitic and hypersthenic rock series, and hence
more generally to tholeiitic and calc-alkalic series,
respectively.
The consistency between Sakuyama’s petrographical
classification and bulk-rock chemical characteristics (i.e.
N- vs R-type and tholeiitic vs calc-alkalic, respectively)
has been well established at least for Quaternary NE
Japan arc volcanoes (Wada, 1981; Sakuyama, 1983;
Fujinawa, 1988, 1990). However, applied elsewhere there
are exceptions. Particular calc-alkalic andesites (high-Mg
andesites) from the Setouchi volcanic belt, SW Japan,
characterized by unusually high MgO contents or high
NUMBER 11
NOVEMBER 2008
Mg-number and hence believed to represent leastdifferentiated mantle-derived magmas, are petrographically classified as N-type rocks (Tatsumi, 2006). On the
other hand, high-Mg andesite from Mt. Shasta, USA,
which is considered as representative of primitive andesite
(Baker et al., 1994; Grove et al., 2002), has now been identified as R-type andesite, and hence is likely to be the product of magma mixing (Streck et al., 2007).
Occurrence
It has long been known that some volcanic arcs are characterized by either tholeiitic or calc-alkalic magmatism
(Jakes & Gill, 1970; Plank & Langmuir, 1988). Calc-alkalic
rocks are clearly dominant in continental arcs rather than
oceanic arcs (Miyashiro, 1974; Ewart, 1982), and calcalkalic/tholeiitic volume ratios tend to increase with
increasing age or arc maturity (Baker, 1973) and crustal
thickness (Gill, 1981). It should be stressed, however, that
tholeiitic and calc-alkalic magmas do coexist in some
single volcanic systems; e.g. Mt. Shasta, USA (Baker et al.,
1994), Chichontepec, El Salvador (Bau & Knittel, 1993),
Aso in SW Japan (Hunter, 1998), and Myoko^Kurohime in
Central Japan (Sakuyama, 1981). Furthermore, along the
volcanic front of the NE Japan arc about one-third of
Quaternary volcanoes erupt both tholeiitic and calcalkalic rocks (Kawano et al.,1961). Examining the geochemistry of the two coexisting magma series in the above
volcanoes reveals that the calc-alkalic rocks are generally
more enriched in both compatible elements such as Mg, Ni
and Cr and incompatible elements such as Rb, K, Th and
U (e.g. Masuda & Aoki,1979; Kimura & Yoshida, 2006).
The misunderstanding caused by the interchangeable
usage of the terms medium-K and calc-alkalic series
(Fig. 1a) may further mislead us to a ‘likely’ conclusion
that calc-alkalic magmas are more hydrous than tholeiitic
magmas, because the H2O content in mantle-derived primary magmas may increase together with incompatible
elements towards the back-arc side within a volcanic arc
(Sakuyama, 1979; Tatsumi et al., 1983). The experimental
constraints that calc-alkalic trends can be reproduced
under hydrous conditions but tholeiitic trends under
H2O-poor conditions (e.g. Grove & Baker, 1984; Sisson &
Grove, 1993; Hamada & Fujii, 2008) may further reinforce
this idea. However, it should be stressed here that there are
no data available that demonstrate the difference in H2O
contents in these magma series occurring in a single
volcano.
G E O L O G I C A L B AC KG RO U N D
O F Z AO VO L C A N O
NE Japan arc
The NE Japan arc is formed by the subduction of the
Pacific plate beneath the North American and Eurasian
1974
TATSUMI et al.
44°N
Eurasia
Plate
CALC-ALKALIC VS THOLEIITIC SERIES
(3) the number of volcanoes and the eruptive volume are
greater in the trench-side volcanic chain;
(4) volcanic rocks in the back-arc side chain tend to be
more enriched in incompatible elements than those in the
trench-side chain.
These characteristics are well documented in the central
to northern NE Japan arc, but not in the southern arc
where the Philippine Sea plate is being subducted into
the mantle wedge above the descending Pacific plate and
the Izu^Bonin^Mariana arc is colliding with the Japanese
islands (Fig. 2). This complicated tectonic setting may cause
the unusual characteristics of the magmatism at the
southern end of the NE Japan arc.
From north to south the pre-Tertiary basement rocks of
the NE Japan arc consist of Cretaceous to Jurassic metamorphic rocks, Cretaceous sedimentary rocks and granitoids, and subduction zone and metamorphic complexes
of Ordovician to Cretaceous age. Granitoids in these basements show spatial variations in isotopic composition, with
more enriched isotopic signatures southwards (Kagami,
2005). Kersting et al. (1996) and Kimura & Yoshida (2006)
further demonstrated the correlation between the Sr^Nd
isotopic compositions of volcanic front lavas and their
underlying basement rocks.
km
200
m
0k
15
N American
Plate
J a p
a n
A r c
m
rc
0k eA
10 Kuril
60 km
40 km
20
km
ana
in-Mari
Izu-Bon
n
Pacific
Plate
Zao volcano
Arc
Philippine Sea
Plate
32°N
136°E
o
Ja
e-
pa
I
II
Zon
36°N
N
Zo
ne
-II
Zao Volcano
Tr
enc
h
rt
h e
a s
t
Z o n
e - I
40°N
140°E
144°E
Fig. 2. Distribution of Quaternary volcanoes in the NE Japan arc
and the adjacent Kurile and Izu^Bonin^Mariana arcs. The NE
Japan volcanic arc can be subdivided into three zones (filled circles,
Zone I; half-filled circles, Zone 2; open circles, Zone 3) based on
Sr^Nd isotopic characteristics and basement geology (Kimura &
Yoshida, 2006). Zao volcano straddles the volcanic front, which lies
110 km above the top of the subducting Pacific Plate (dashed lines,
depth contours to the slab surface; numbers, depths). Small filled
circles indicate volcanoes in the Kurile and Izu^Bonin^Mariana arcs.
plates at the Japan Trench (Fig. 2) at a rate of 10 cm/year.
The volcanic front of this arc, which is the trenchward
boundary of the volcanic arc, runs parallel to the trench
axis. The NE Japan arc exhibits the following tectonic and
magmatic characteristics, which are common to most arc^
trench system (Tatsumi & Eggins, 1995):
(1) dual volcanic chains are present, one trench-side and
one on the back-arc side, known as the Nasu and Chokai
chains, respectively;
(2) these chains are 110 km and 150^170 km above the
top of the subducted oceanic lithosphere, respectively, at
least in the central portion of the arc (Fig. 2);
A total of 55 volcanoes are distributed in the NE Japan
arc. Zao volcano is one such Quaternary volcano, and is
situated immediately behind the volcanic front in the
Nasu trench-side chain (Fig. 2). The following summary
of the geological history of Zao volcano is based on a
synthesis of previously published studies (Oba & Konda,
1989; Takaoka et al., 1989; Sakayori, 1992).
The basement around Zao consists of Cretaceous
granitoids of the Abukuma Terrane and Tertiary volcanic
rocks. Magmatic activity, which commenced at 10 Ma
and has formed a volcanic edifice with a current volume
of 25 km3, can be separated into four major stages
(Fig. 3): Stage 1 (10^06 Ma): formation of a small volcano
consisting of basaltic to basaltic andesite pyroclastic
rocks and dykes; Stage 2 (03 Ma): formation of a stratovolcano composed of andesitic to dacitic lavas and pyroclastic deposits; Stage 3 (03^01Ma): eruption of basaltic
andesite to basalt lava flows and pyroclastic deposits
from two vents near the summit; Stage 4 (501Ma):
caldera-forming eruption (522 km in diameter), after
which a pyroclastic cone composed of basaltic andesites
built up within the caldera. The rocks of Stage 1 are
classified as low-K tholeiitic series, whereas those of
Stages 2^4 belong to the medium-K, calc-alkalic series.
This study is based on 39 volcanic samples collected at
14 sites from Stages 1 and 3 of the evolution of Zao
volcano (Fig. 3).
1975
JOURNAL OF PETROLOGY
ZA3011, ZA3012,
ZA3013, ZA3014
ZA3041, ZA3042
VOLUME 49
NUMBER 11
NOVEMBER 2008
ZA3021, ZA3022,
ZA3023,ZA3024, ZA3031, ZA3032
ZA3025
ZA1041,
ZA1021 ZA1042,
ZA1043,
ZA1044 ZA3081,
ZA3082
2 km
Stage 1 (<0.1Ma)
Stage 2 (0.3-0.1Ma)
Stage 3 (~0.3Ma)
ZA3051
Stage 4 (1.0-0.6Ma)
ZA3071,
ZA3061, ZA3062
ZA3072
ZA3063, ZA3064
ZA1031, ZA1032, ZA1033
ZA3091, ZA3101,ZA3102-1 ZA1011,
ZA3093 ZA3102-2, ZA3103 ZA1012, ZA1013
Basement &
other volcanics
Crater &
caldera
Fig. 3. Simplified geological map of Zao volcano after Sakayori (1992), showing sampling localities.
A N A LY T I C A L M E T H O D S
Bulk-rock analyses
Rock samples for trace element and Sr^Nd^Pb isotope
analyses were crushed to coarse chips (505 mm3) and
fresh pieces were hand picked. To avoid surface contamination, the rock chips were washed with ethanol and then
leached with 10M HCl at room temperature for 1h.
These chips were rinsed three times with Milli-Q water
an then dried in an oven. The chips were pre-pulverized
using a tungsten carbide mortar. At this step, any remaining altered portions were removed. Finally, the coarse
grain samples were pulverized in a tungsten carbide vibration mill.
Major and trace element (Ni, Cu, Zn, Rb, Sr, Y, Zr, Nb,
Ba, Pb, and Th) compositions were measured on fused
glass beads and pressed powder pellets, respectively, using
RIGAKUÕ Simultix 12 and RIX3000 X-ray fluorescence
(XRF) spectrometers. Analytical procedures, precision
(mostly 51%), and accuracy (mostly 51%) have been
fully described by Tani et al. (2006).
Concentrations of rare earth and 17 other trace elements
(Sc, Co, Ni, Cu, Rb, Sr, Y, Zr, Nb, Cs, Ba, Hf, Ta, Tl, Pb,
Th, and U) were analyzed using by inductively coupled
plasma mass spectrometry (ICP-MS) using an Agilent
7500ce system fitted with PFA sample introducing and a
Pt-inject torch system. The ICP-MS system was operated
in no collision gas and multi-tune acquisition mode.
This combination allowed a wide range of elements to be
precisely determined using pulse counting detection with
the hydrofluoric acid containing sample solution being
delivered directly into the plasma. Sample dissolution,
preparation and measurement were described by Chang
et al. (2003) and Nakamura & Chang (2007). Analytical
accuracy and precision for ICP-MS analyses, estimated
from repeated measurements of international reference
rocks (JB-1a of GSJ, BCR-2 and BIR-1 of USGS) were
mostly better than 5% and 3%, respectively.
The analytical procedure used for chemical separation
and mass spectrometry for Sr, Nd and Pb isotope determinations was outlined by Miyazaki et al. (2007). Total procedural blanks for Sr, Nd and Pb were less than 15 pg, 3 pg
and 6 pg, respectively. Mass spectrometry was performed
on a Thermo-FinniganÕ Triton TI equipped with nine
Faraday cups, using a static multi-collection mode. Normalizing factors used to correct for isotopic fractionation
in Sr, Nd and Pb isotope analyses were 86Sr/88Sr ¼ 01194,
146
Nd/144Nd ¼ 07219, and 0148% per atomic mass unit,
respectively. Measured isotopic ratios for standard materials were 87Sr/86Sr ¼ 0710262 14 (2s) for NIST 987
(n ¼ 40), 143Nd/144Nd ¼ 0512098 13 (2s), for JNdi-1
(n ¼ 31), and 208Pb/204Pb ¼ 36717 7 (2s), 207Pb/204Pb ¼
15497 2 (2s) and 206Pb/204Pb ¼16940 2 (2s) for
NIST 981 (n ¼ 23).
Micro-analyses
Mineral compositions of major and minor elements were
analyzed by electron-probe micro-analysis (EPMA) using
a JEOL JXA-8800 instrument, following the method
described by Shukuno (2003). For all elements the excitation potential, specimen current, and analytical time of
peak and background were: on olivine 20 kV, 25 nA, 20 s,
and 10 s (except for Mn, Ca, and Ni, for which 25 kV,
20 nA, 100 s and 50 s were used); on spinel 15 kV, 12 nA,
1976
TATSUMI et al.
CALC-ALKALIC VS THOLEIITIC SERIES
and 20 s; on pyroxene and plagioclase 15 kV, 15 nA,
and 20 s. ZAF correction procedures were employed.
Trace element [Rb, Sr, Y, Zr, Nb, Ba, rare earth elements
(REE), Hf, Ta, Pb, Th, and U] micro-analyses of plagioclase were performed by laser-ablation (LA)-ICP-MS on
the same thick sections that were used for the Sr isotope
analyses (see below). The 193 nm excimer laser system
aerosol line was connected to a VG ElementalÕ PQ3
quadrupole-type ICP-MS system (Kimura et al., 2000).
We used the 43Ca peak for internal standardization,
which corrected any variation in the ablated sample
volume. CaO contents were determined by EPMA prior
to the LA-ICP-MS analysis. NIST 612 synthetic glass was
used as a standard, adopting the reference values of
Pearce et al. (1996). The element concentrations measured
in NIST 612 were within 85% of the reference values.
The overall homogeneity of the glass for most elements
was better than 10% (2 SD), when 20 mm laser spots were
used (Kimura et al., 2000). The LA conditions were set at
200 mJ laser source energy, 50 mm crater size, and repetition rate at 5 Hz, yielding signals of 2000 c.p.s./ppm at
115
In. Ar gas blank was measured for 60 s for blank subtraction prior to sample analysis. Three spots on the NIST
612 standard were analyzed and averaged to minimize
errors caused by heterogeneity. Typical analytical time for
a single crater was 60 s, generating craters about 30 mm
deep. Accuracy of the results can be affected by the
matrix effect between the synthetic glass standard and the
silicate minerals. However, accuracy is generally better
than 15% when the aerosol loading to the plasma is controlled by 50% (Kimura et al., 2000). Such accuracy is
sufficient for most geochemical and petrological purposes.
Sr isotopes were analyzed in the plagioclases using two
techniques; a combined micromilling followed by chemical
separation and thermal ionization mass spectrometry
(MM-TIMS), and in situ laser ablation multi-collector
inductively coupled plasma source mass spectrometry
(LA-MC-ICP-MS).
The MM-TIMS techniques we used were similar to
those of Davidson & Tepley (1997), Tepley et al. (1999) and
Charlier et al. (2006), and were fully described byTakahashi
et al. (2005). The rock sample was cut into a wafer with
a thickness of 10 mm, which was then bonded onto a
glass slide and polished. Micromilling of the sample was
carried out using a New WaveTM MicroMillTM. The diameter at the tip of the drill used for sampling is 027 mm.
After micromilling, the collected sample powder was
decomposed with HF, HCl and HNO3. Sr selective extraction resin (Sr Resin from EICHROM Technologies Inc.)
was used for chemical separation of Sr (Horwitz et al.,
1992). Resin was charged into a modified pipette tip
column with quartz wool filter. Bedded resin volume was
005 ml. Total procedural blanks for Sr in the MM-TIMS
procedure were less than 10 pg. Methods used for Sr
isotope MM-TIMS analyses are identical to those used
for the bulk-rock analyses. Repeated analyses of NIST
987 (10 ng Sr) gave 87Sr/86Sr ¼ 0710260 22 (2s, n ¼ 8).
Additionally, NIST 610 glass was used for estimating the
analytical precision. Analysis of NIST 610 by the bulkrock analysis procedure and the MM-TIMS techniques
gave 87Sr/86Sr ¼ 0709681 07 (2s, n ¼ 4) and 87Sr/86Sr ¼
0709696 29 (2s, n ¼ 5), respectively. These values are
almost identical to the value (87Sr/86Sr ¼ 0709699 18)
reported for NIST 610 (Woodhead & Hergt, 2001).
In situ Sr isotope micro-analyses were also performed
by LA-MC-ICP-MS using a VG ElementalÕ Plasma 54
MC-ICP-MS system equipped with a dry and solution
aerosol dual sample intake system. The dual intake system
consists of an in-house 193 nm ArF excimer laser ablation
(Kimura et al., 2000) with He carrier gas and an AridusÕ
desolvating nebulizer using Ar (þ trace N2) carrier gas.
The two carrier gas lines were mixed in a 50 cm3
volume TeflonÕ mixing chamber prior to the ICP torch.
While the laser aerosol alone was analyzed, the Milli-QÕ
deionized water was taken up by the AridusÕ solution line,
for which blanks were negligible. Standard solutions were
introduced into the ICP torch while LA was unfired with
the LA carrier gas on, for determination of instrumental
mass bias and isobaric overlap correction factors (see
below). We hereafter call this the ‘dual intake system’.
An on-peak background method was applied to subtract
the blank from the Kr impurity (82Kr, 83Kr, 84Kr, and
86
Kr) in the Ar plasma gas (Woodhead et al., 2005). This
is necessary rather than peak stripping using mass bias
factor by monitoring one of the Kr peaks, because mass
bias for Kr cannot be determined because of complex
interferences during the sample analyses (Vroon et al.
2008). Background signals were acquired for the first 15 s
on the gas blank and the signal from the sample or standard was then collected, typically for 5 min, depending on
the sample.
The 87Rb interference on 87Sr was corrected for by
monitoring 85Rb using an empirical overlap correction
factor, determined from a Rb-doped NIST 987 Sr isotope
standard solution (20 ppb Rb in 100 ppb Sr, Rb/Sr ¼ 02)
from the AridusÕ aerosol line. The mass bias factor of Rb
is not identical to that of Sr (Hirata, 1996) and Rb has only
two stable isotopes, which prevents internal mass bias
correction. The mass bias factor for Rb may be different
between solution and LA modes (Vroon et al., 2008).
However, our simultaneous dual intake system cancels
out the mass bias difference caused by the different ICP
operating parameters in discrete LA aerosol or solution
aerosol introduction. Therefore, our empirical Rb overlap
correction method is an alternative to correction factor
determination using a synthetic glass standard (Davidson
et al., 2001). The difference between LA and AridusÕ solution analyses with our dual intake system is in the presence
1977
JOURNAL OF PETROLOGY
VOLUME 49
or absence of matrix elements for the LA and the AridusÕ
aerosols, respectively. The non-spectral matrix effect
(Barling & Weis, 2008) should be present and is noted
below. The rate of 87Rb overlap on 87M during plagioclase
analyses was typically less than 1% (Rb/Sr 5 000025)
and occasionally exceeded 10% (Rb/Sr 4 0025) with
plagioclase containing glass inclusions. However, we did
not see any problems caused by the change in the Rb overlap correction factor on the 87Sr/86Sr ratios of plagioclase
within analytical precision (87Sr/86Sr ¼ 000005).
The CaAr and Ca dimmer molecular ion interferences
on masses 82M, 84M, 86M, and 88M were subtracted by
monitoring 82M using the correction method proposed
by Woodhead et al. (2005). The effect of the correction
was monitored by the most interference-sensitive isotope
ratio, 84Sr/86Sr, the value of which was typically 00565
(Woodhead et al., 2005). We have confirmed the validity of
the correction method by analyzing a Ca-doped NIST 987
standard solution. The reference isotope ratio of NIST 987
was reproducible within the analytical precision with
a standard solution containing 50 ppm Ca and 100 ppb Sr
(Ca/Sr ¼ 500), in which Ca/Sr was more than that in the
natural plagioclase (50^200) and comparable with
natural carbonates (500).
Other interferences from doubly charged ions such
as Yb2þ are not considered. Plagioclase crystals have low
element abundances of REE (see trace element compositions of the plagioclase crystals below). The effect of the
doubly charged heavy ions is negligible (Vroon et al., 2008).
Mass bias, which was not considered by internal correction using 86Sr/88Sr ¼ 01194, caused by day-to-day basis
changes in interface cone or operating parameters (gas
flows, sampling depths, etc., Woodhead et al., 2001), was
further corrected for using the beta factor (Pachett et al.,
1981; Iizuka & Hirata, 2005) determined by analyses of a
solution of NIST 987 with 87Sr/86Sr ¼ 070125 from the
AridusÕ line. The dual sample intake system was again
advantageous for performing this complex correction
procedure, as it allows immediate switching between the
two introduction lines without changing the condition of
the plasma. Absence and presence of matrix elements may
cause change in Sr mass bias between the LA and the standard solution aerosols. However, increase of the matrix element by the addition of 50 ppm Ca to the NIST 987
solution did not make any detectable change in 87Sr/86Sr,
confirming previous reports (Woodhead et al., 2005;
Vroon et al., 2008). The NIST 987 standard bracketing
method was used to perform the beta correction after a
series of 10 analyses on the plagioclase.
A source energy of 200 mJ was used for the laser, with
a repetition rate of 20 Hz and a diameter of 200 mm.
This yielded a stable signal of 2^3 V on 88Sr for plagioclase with 300 ppm Sr over 5 min by our LA system,
with crater penetration to about 400^500 mm in the
NUMBER 11
NOVEMBER 2008
thick section. All the plagioclase analyses were performed
in single spot mode rather than raster mode. The 87Sr/86Sr
ratios of individual plagioclase spots were measured for
5 s each. The downhole Sr isotopic zoning was carefully
rejected by observing the time resolving profiles. Integrated data from the homogeneous section of the profile
were used for further statistical treatment, including average and two standard error calculations. Downhole
element fractionation has been reported between U and
Pb in a zircon crystal (Horn et al., 2000). On the other
hand, such fractionation was not detectable during Hf
isotope analysis of zircon crystal including the overlap
correction factors of Lu and Yb on 176Hf (Woodhead
et al., 2004). The downhole elemental fractionation may
occur between Rb and Sr, likewise between Pb and U.
However, we did not see any detectable change in the Sr
isotopic ratio in homogeneous plagioclase crystals with
sporadic melt inclusions at different depth levels. In fact,
overall precision in the single spot analyses of the homogeneous crystals was better than 000005 (2 SE), irrespective of the presence or absence of melt inclusions that
contain Rb. This indicates that isotopic fractionation
during downhole laser ablation is unlikely or at least not
detectable with our analytical precision, for both Sr and
Rb, which is similar to the case of Pb, Hf, Yb, and Lu isotopes in zircon (Horn et al., 2000; Woodhead et al., 2004).
The accuracy of the LA-ICP-MS plagioclase method
was tested by comparing MM-TIMS data from the
same homogeneous plagioclase (J.-I. Kimura, unpublished
data, and the present study); the results agreed within
00001, which is almost the same level as those reported
elsewhere for LA-MC-ICP-MS Sr isotope analyses (Vroon
et al., 2008). This accuracy is acceptable for the purpose
of this study.
R E S U LT S
Bulk-rock chemistry
Major and trace element concentrations and Sr^Nd^Pb
isotopic compositions of Zao volcanic rocks are reported
in Table 1.
The volcanic rocks define two distinct chemical trends;
tholeiitic and calc-alkalic (Fig. 4). These are generally
regarded as equivalent to Kuno’s pigeonitic and hypersthenic rock series, and Sakuyama’s N-type and R-type rocks,
respectively. Tholeiitic magmatism occurred solely during
Stage 1, whereas the rocks of Stages 2^4 are classified as
calc-alkalic. It should be stressed that the Zao volcanic
rocks form chemical trends similar to those that typify the
tholeiitic and calc-alkalic rocks of the trench-side Nasu volcanic chain (Fig. 4), suggesting that the genetic relation
between the two magma series at Zao may be applied to
the NE Japan arc in general. Among the major elements,
the calc-alkalic rocks tend to be more depleted in Fe and
more enriched in Mg and K than the tholeiitic rocks.
1978
TATSUMI et al.
CALC-ALKALIC VS THOLEIITIC SERIES
Table 1: Major and trace element, isotopic and modal compositions of Zao volcanic rocks
Rock type:
Tholeiitic
Sample:
ZA1013
ZA1011
ZA1033
ZA1032
ZA1021
ZA1042
ZA1012
ZA1043
ZA1044
ZA1031
ZA1041
5531
SiO2 (wt %)
5198
5202
5215
5241
5261
5274
5275
5416
5472
5512
TiO2
080
078
095
093
081
080
078
083
100
094
104
Al2O3
1798
1774
1814
1792
1786
1781
1790
1802
1718
1705
1719
Fe2O3
1095
1109
1066
1111
1111
1026
1034
1087
962
1066
1010
MnO
019
018
019
019
020
018
018
017
019
018
019
MgO
549
597
475
494
563
582
547
465
408
411
394
779
CaO
973
1028
946
946
985
961
1024
924
862
891
Na2O
197
204
239
239
217
228
212
241
261
267
256
K2O
028
026
026
032
037
028
025
044
041
048
043
P2O5
009
011
016
016
016
014
009
016
020
018
011
Total
9960
10004
9956
9983
9992
10000
10065
9970
9967
9974
9951
Ni (ppm)
20
22
15
15
25
31
17
19
8
9
8
Cu
45
38
36
39
41
41
44
39
37
35
31
Zn
87
89
99
96
89
87
79
89
103
97
102
Rb
64
57
50
62
70
41
50
100
90
11
97
Sr
248
249
274
274
263
258
255
269
263
271
257
Y
26
18
18
20
21
20
16
19
20
20
28
Zr
44
46
54
56
55
55
43
65
69
72
73
Nb
Ba
Pb
11
105
40
Th
17
104
21
97
20
120
34
19
49
15
07
06
24
135
23
134
32
32
16
129
40
24
156
29
161
32
167
43
46
58
20
13
21
30
163
42
08
Sc (ppm)
362
375
335
329
319
345
342
Co
396
461
377
388
407
335
343
Ni
208
250
177
184
288
Cu
405
364
337
355
380
Rb
Sr
610
247
550
248
460
272
578
274
857
344
672
859
263
258
853
289
929
253
Y
264
187
186
207
221
199
278
Zr
459
482
565
578
563
714
745
Nb
147
166
225
229
225
309
Cs
0429
0361
0255
0354
0344
0348
Ba
La
Ce
Pr
Nd
102
663
141
229
112
987
447
111
106
482
120
117
563
136
127
160
649
638
135
158
156
173
196
205
214
760
85
942
972
993
319
0284
167
104
193
286
132
Sm
331
226
251
269
268
279
345
Eu
111
0825
0968
102
0982
104
126
Gd
433
295
313
341
346
335
439
Tb
074
051
0536
0581
0576
0573
0719
Dy
472
335
345
375
369
367
456
Ho
100
0721
0737
081
0794
0779
0988
Er
295
217
220
240
236
234
293
Tm
0416
031
0317
0342
0335
0336
041
Yb
270
206
212
229
220
229
272
(continued)
1979
JOURNAL OF PETROLOGY
VOLUME 49
NUMBER 11
NOVEMBER 2008
Table 1: Continued
Rock type:
Tholeiitic
Sample:
ZA1013
ZA1011
ZA1033
ZA1032
ZA1021
ZA1042
ZA1012
ZA1043
ZA1044
ZA1031
ZA1041
Lu
0414
0321
0329
0352
0342
0348
0425
Hf
144
146
167
173
166
207
215
Ta
0081
0086
0106
0109
011
0154
0155
Tl
0063
0048
0031
0082
0049
0046
0124
Pb
353
320
301
326
329
425
385
U
0649
0555
0537
0598
0664
0943
0973
Th
0171
015
0139
0158
0171
0245
87
0704189
0704227
0704361
0704351
0704332
Sr/86Sr
2s
143
Nd/144Nd
2s
9
8
0512797
9
10
0512793
10
7
0512760
9
9
0512770
10
0704327
9
0512777
10
0704176
8
0512779
9
0704236
10
0512818
10
0219
0704465
9
0512784
0704439
8
0512724
10
10
0704465
9
0512758
10
0512768
10
206
Pb/204Pb
18472
18472
18454
18452
18453
18462
18473
18451
18459
18465
18465
207
Pb/204Pb
15582
15582
15582
15575
15577
15588
15583
15576
15580
15586
15582
208
Pb/204Pb
38511
38506
38503
38482
38490
38526
38513
38486
38509
38528
10
17
10
09
07
08
01
olivine
02
—
—
38515
—
clinopyroxene
15
39
22
21
43
11
32
15
14
24
orthopyroxene
59
59
09
15
30
39
24
13
62
27
05
plagioclase
281
313
264
237
265
223
279
191
313
195
125
quartz
—
—
—
—
—
—
—
—
opaque
—
—
groundmass
643
589
Rock type:
Calc-alkalic
Sample:
ZA3093
ZA3091
SiO2 (wt %)
01
694
ZA3032
—
—
—
—
—
—
710
662
717
ZA3031
ZA3064
ZA3102-1
01
05
655
769
ZA3103
ZA3101
ZA3011
—
603
ZA3102-2
ZA3012
33
02
02
751
835
ZA3013
ZA3014
5245
5258
5562
5590
5643
5677
5679
5688
5689
5699
5709
5718
TiO2
085
086
082
082
085
084
079
083
081
084
081
082
5739
082
Al2O3
1739
1738
1705
1706
1698
1726
1740
1728
1663
1729
1702
1670
1670
Fe2O3
982
980
872
865
879
862
830
865
815
861
810
846
844
MnO
016
016
015
015
016
015
014
014
014
015
013
015
015
MgO
698
673
500
486
505
439
425
435
414
443
400
413
411
CaO
888
874
833
832
770
706
732
687
736
705
769
695
700
Na2O
247
251
253
260
271
258
262
259
259
258
270
253
253
K 2O
088
091
112
115
114
124
122
126
124
124
117
121
121
P 2O5
018
018
013
014
016
015
015
016
013
015
012
013
013
Total
10008
9985
9946
9965
9996
9905
9897
9901
9807
9932
9883
9824
9849
Ni (ppm)
58
27
31
21
21
21
Cu
36
42
25
38
38
36
Zn
70
68
66
77
77
72
Rb
27
36
32
37
37
36
Sr
391
317
340
306
306
277
Y
20
23
23
26
26
23
Zr
81
101
100
103
103
108
Nb
Ba
22
323
26
325
30
389
30
363
30
363
24
350
(continued)
1980
TATSUMI et al.
CALC-ALKALIC VS THOLEIITIC SERIES
Table 1: Continued
Rock type:
Calc-alkalic
Sample:
ZA3093
ZA3091
ZA3032
ZA3031
ZA3064
ZA3102-1
ZA3103
ZA3101
ZA3011
ZA3102-2
ZA3012
ZA3013
Pb
49
59
60
74
74
Th
26
44
32
40
40
Sc (ppm)
271
273
274
282
257
Co
422
454
343
302
410
Ni
682
322
368
224
240
Cu
352
390
241
344
322
Rb
266
349
309
362
Sr
396
Y
214
Zr
851
312
229
106
333
239
104
258
263
320
321
186
113
105
La
329
957
Ce
950
224
Pr
225
296
Nd
295
133
132
374
120
251
261
328
232
110
276
0884
378
109
145
338
268
273
0519
315
34
105
Nb
Ba
175
299
Cs
339
960
228
370
299
164
132
Sm
342
339
367
41
344
Eu
107
0951
108
115
0974
Gd
386
389
412
466
387
Tb
0631
0659
0687
0763
0658
Dy
391
414
429
472
417
Ho
0817
0875
0901
0995
0884
Er
242
261
270
296
267
Tm
0345
038
039
0424
039
Yb
230
254
261
283
262
Lu
0354
0386
0405
0435
0402
Hf
249
320
310
313
339
Ta
0161
0176
0211
0206
0193
Tl
0081
0113
0168
0243
0205
Pb
385
595
540
766
U
258
348
340
338
Th
0641
0863
0834
0831
091
87
0703596
0703786
0703733
0703815
0703850
Sr/86Sr
2s
8
143
Nd/144Nd
9
0512882
2s
0512864
10
Pb/204Pb
10
8
0512853
10
164
370
9
10
0512840
0512851
10
15
206
18389
18415
18410
18416
18420
207
15551
15558
15555
15561
15565
208
38357
38398
38394
38414
Pb/204Pb
Pb/204Pb
olivine
47
23
03
01
03
clinopyroxene
45
69
85
69
76
orthopyroxene
31
40
23
16
plagioclase
196
290
232
quartz
—
—
—
opaque
groundmass
—
ZA3014
38424
03
03
—
63
90
81
63
63
70
71
20
24
35
21
21
24
23
24
22
297
227
221
230
209
303
221
261
270
234
—
—
—
—
—
—
—
06
03
—
—
58
06
04
01
06
02
11
03
07
08
07
09
16
08
19
13
07
675
576
646
614
667
678
631
676
597
678
624
622
679
(continued)
1981
JOURNAL OF PETROLOGY
VOLUME 49
NUMBER 11
NOVEMBER 2008
Table 1: Continued
Rock type:
Calc-alkalic
Sample:
ZA3072
ZA3041
SiO2 (wt %)
ZA3071 ZA3042 ZA3051
ZA3061 ZA3062 ZA3025 ZA3063
ZA3021 ZA3081
ZA3024 ZA3023 ZA3022
ZA3082
6273
5741
5750
5752
5755
5861
5922
5966
5998
6060
6133
6161
6174
6187
6196
TiO2
081
082
082
083
081
077
076
079
076
073
071
073
072
074
070
Al2O3
1698
1656
1692
1668
1633
1680
1708
1605
1626
1602
1609
1560
1559
1546
1563
Fe2O3
839
879
849
879
868
794
765
849
757
748
720
748
753
747
698
MnO
015
015
015
015
014
014
014
014
013
013
013
013
013
013
013
MgO
469
437
475
434
413
393
365
364
359
326
334
321
309
316
319
CaO
707
753
702
751
713
634
609
665
610
617
609
606
570
575
571
Na2O
276
267
274
267
268
277
287
271
289
282
300
285
284
285
305
K 2O
123
124
124
124
129
120
113
149
145
156
151
173
175
179
163
P 2O5
015
013
016
013
012
013
013
013
013
011
012
011
012
012
012
Total
9963
9974
9981
9987
9991
9925
9915
10006
9946
9962
9980
9964
9932
9942
9987
Ni (ppm)
30
22
19
21
19
14
Cu
30
21
21
15
36
27
Zn
66
65
61
60
58
57
Rb
38
37
38
44
46
58
Sr
330
278
252
273
271
236
Y
23
24
24
29
26
29
Zr
102
110
116
122
124
146
Nb
Ba
28
372
24
338
25
31
350
418
32
415
30
485
Pb
63
84
97
75
67
Th
33
46
48
48
47
66
Sc (ppm)
269
259
260
223
217
243
Co
308
391
336
282
282
374
Ni
349
255
234
235
209
173
Cu
284
213
204
144
329
275
Rb
361
359
371
423
442
Sr
Y
Zr
318
243
104
273
240
114
246
262
247
293
120
120
262
265
117
68
558
231
288
122
Nb
316
276
290
358
365
375
Cs
106
0846
0916
125
121
155
Ba
382
La
109
Ce
244
Pr
Nd
321
141
341
960
224
293
130
346
424
970
225
294
447
125
125
279
270
283
396
130
427
134
172
359
154
368
161
Sm
360
343
344
422
382
406
Eu
105
0942
0914
111
101
098
Gd
407
396
402
484
433
463
Tb
0679
067
0681
0801
0729
0784
Dy
426
426
434
499
454
492
Ho
090
0899
0925
106
0968
105
Er
270
270
277
313
290
314
(continued)
1982
TATSUMI et al.
CALC-ALKALIC VS THOLEIITIC SERIES
Table 1: Continued
Rock type:
Calc-alkalic
Sample:
ZA3072
ZA3041
ZA3071 ZA3042 ZA3051
ZA3061 ZA3062 ZA3025 ZA3063
ZA3021 ZA3081
ZA3024 ZA3023 ZA3022
Tm
0393
0393
0405
0457
0421
0457
Yb
262
264
271
301
283
303
Lu
0405
0407
0419
0466
0441
0465
Hf
311
348
362
364
359
377
Ta
0211
0184
0196
0242
0244
0259
Tl
0149
0155
0155
0218
0177
0125
Pb
597
691
860
631
674
645
U
350
375
391
434
442
536
Th
0847
0926
0958
105
103
128
87
0703792
0703938
0704101
0703935
0703733
0703982
Sr/86Sr
2s
143
Nd/144Nd
2s
Pb/204Pb
9
0512838
12
10
9
0512847
7
0512837
11
8
0512820
10
8
0512798
9
0512834
10
11
206
18436
18409
18424
18437
18423
18422
207
15585
15555
15564
15580
15561
15563
208
38498
38388
38424
38485
38422
Pb/204Pb
Pb/204Pb
olivine
09
—
02
clinopyroxene
55
101
67
orthopyroxene
—
76
00
71
—
73
01
—
59
64
00
65
—
56
03
38
ZA3082
38422
—
53
—
83
—
71
02
61
25
10
34
25
21
17
15
29
11
27
29
24
08
16
12
plagioclase
209
253
215
237
263
245
226
239
236
307
250
303
297
290
288
quartz
—
—
—
—
21
opaque
groundmass
03
05
02
00
06
04
17
16
11
07
09
09
01
11
10
16
04
14
17
19
10
09
24
13
14
695
627
672
651
635
646
690
652
671
585
666
594
572
599
602
Total iron as Fe2O3.
Ni–Th by XRF; Sc–Th by ICP-MS.
As a result they belong to the medium-K series, whereas
the tholeiitic rocks belong to the low-K series (Fig. 4).
The normal mid-ocean ridge basalt (N-MORB) normalized incompatible trace element patterns of the Zao
volcanic rocks are shown in Fig. 5. Although elements
with higher incompatibility during mantle melting tend to
be more enriched, the high field strength elements such as
Nb, Ta and Zr do not show such enrichment. This selective
enrichment of particular elements results in strongly spiked
patterns (Fig. 5), which have also been observed in other
arc lavas (e.g. Tatsumi & Eggins, 1995). Calc-alkalic rocks
are characterized by higher concentrations of incompatible
trace elements than tholeiitic rocks.
The Sr^Nd^Pb isotope compositions of the Zao volcanic
rocks are also well within the range of the trench-side Nasu
lavas (Fig. 6). It has been well established that volcanic rocks
along the NE Japan volcanic front exhibit more enriched
Sr^Nd isotopic signatures towards the south (Notsu, 1983;
Kimura & Yoshida, 2006). Zao volcano is situated in the
southernmost part of Zone I of Kimura & Yoshida (Fig. 2)
and has Sr^Nd isotopic characteristics typical of this zone
(Fig. 6). The Pb isotopic ratios of Zao, and more generally
of Quaternary volcanic rocks from the NE Japan arc, form
a broad trend towards the compositions of Pacific sediments
from the MORB field (Fig. 6).
Calc-alkalic and tholeiitic rocks from Zao volcano exhibit systematic differences in both major/trace element and
isotopic characteristics. To emphasize the difference in
element concentrations, element abundances are compared
in Fig. 7 by normalizing the calc-alkalic compositions to
the tholeiitic composition at 55 wt % SiO2. It should be
stressed that the calc-alkalic rocks are more enriched in
both highly incompatible elements (U, Rb, Th, and K)
and compatible elements (Ni and Mg). Isotopically, the
calc-alkalic rocks exhibit more depleted characteristics in
terms of Sr, Nd and Pb isotopes (Fig. 6).
Petrography
Modal proportions of phenocrysts and representative mineral compositions are given in Tables 1^7.
1983
JOURNAL OF PETROLOGY
VOLUME 49
NUMBER 11
NOVEMBER 2008
SiO2 (wt.%)
5
45
50
55
60
SiO2 (wt.%)
65
70
45
12
Zao Tholeiitic
55
60
65
70
11
Zao Calc-alkalic
iitic lic
a
ole
Th c-alk
l
a
C
3
10
FeO* (wt.%)
FeO*/MgO
4
50
2
9
8
7
Nasu Chain TH
Nasu Chain CA
6
0
5
1.4
10
1.2
8
MgO (wt.%)
TiO2 (wt.%)
1
1.0
0.8
6
4
0.6
2
0.4
0
22
2.5
2.0
K2O (wt.%)
Al2O3 (wt.%)
20
18
16
14
45
1.5
-K
dium
1.0
Me
K
Low-
0.5
0
50
55
60
65
70
45
SiO2 (wt.%)
50
55
60
65
70
SiO2 (wt.%)
Fig. 4. Major element vs SiO2 variation diagrams for volcanic rocks of Zao and the Nasu trench-side volcanic chain. Zao volcano is composed
of two magma series, tholeiitic (TH) and calc-alkalic (CA), which form chemical trends that are typical of those of the magma series of the
trench-side Nasu volcanic chain. Calc-alkalic and tholeiitic rocks in Zao volcano belong to the medium- and low-K series, respectively.
Subsequent petrographic description of the Zao volcanic
rocks (below) highlights the differences between the calcalkalic and tholeiitic rocks.
Olivine phenocrysts (usually51vol. %) occur as euhedral
to subhedral crystals unrimmed or rarely mantled by orthopyroxene in basaltic andesites to andesites from both the
calc-alkalic and tholeiitic series. Although there is little difference in the Mg-number of the core compositions between
the two magma series (65^85), NiO contents are higher in
the calc-alkalic rocks at a constant Mg-number (Fig. 8).
Orthopyroxene phenocrysts are ubiquitous in the Zao
volcanic rocks. Those in rocks from the tholeiitic series
1984
TATSUMI et al.
CALC-ALKALIC VS THOLEIITIC SERIES
Plagioclase is the most abundant phenocryst phase and
generally makes up 10^30 vol. % of the analyzed samples.
The plagioclase phenocrysts in the tholeiitic rocks have a
limited compositional range, whereas in the calc-alkalic
rocks they exhibit a broader range of compositions with
a bimodal distribution (Fig. 10). Notably, the plagioclase
phenocrysts in the calc-alkalic rocks commonly have
honeycomb textures and dusty zones. Representative textures of plagioclase phenocrysts are shown in Fig. 11,
together with compositional profiles across the crystal for
Ca/(Ca þ Na) ratio and Sr concentration.
N-MORB Normalized
100
10
Calc-alkalic
Plagioclase microanalyses
1 Tholeiitic
Cs Rb Th Nb K Ce Pr Nd Hf Eu Tb Y Er Yb
Tl Ba U Ta La Pb Sr Zr Sm Gd Dy Ho Tm Lu
Incompatibility during mantle melting
Chondrite Normalized
100
50
Calc-alkalic
Tholeiitic
10
La Ce Pr Nd
Sm Eu Gd Tb Dy Ho Er Tm Yb Lu
Fig. 5. Incompatible trace element and REE characteristics of
the Zao volcanic rocks. (a) MORB-normalized; (b) chondritenormalized. Normalizing values are from Sun & McDonough (1989).
exhibit little or normal zoning with a higher Mg-number
core mantled by a lower Mg-number rim (Fig. 9). In
contrast, calc-alkalic rocks contain both normally and
reversely zoned orthopyroxene phenocrysts (Fig. 9), with
the cores of normally zoned orthopyroxene phenocrysts
having higher Mg-number (70^80) than those of the reversely zoned phenocrysts (60). Orthopyroxene phenocrysts
with reactions rims of clinopyroxene (pigeonite) occur
only in the tholeiitic rocks. Only in the calc-alkalic series
is orthopyroxene present as a groundmass phase.
Clinopyroxene (augite) phenocrysts occur in all Zao
volcanic rocks, although they are less abundant than
orthopyroxene. Augite phenocrysts have a rather limited
compositional range, both in tholeiitic and calc-alkalic
rocks, (Mg-number 70; Fig. 9). Reversely zoned clinopyroxene phenocrysts are found only in calc-alkalic rocks
(Fig. 9), whereas pigeonite exists only in the groundmass
of tholeiitic rocks.
Representative trace element concentrations and Sr isotopic compositions of plagioclase phenocrysts are listed in
Table 5. The correlations of selected trace element abundances and 87Sr/86Sr with Ca/(Ca þ Na) are shown in
Figs 12 and 13.
Plagioclase phenocrysts in calc-alkalic rocks tend to be
more enriched in Ba and Sr than those in tholeiitic rocks
(Fig. 12), consistent with the chemical characteristics of
the bulk-rocks (Fig. 7). Plagioclases from the two magma
series show little difference in REE concentrations,
whereas Yconcentrations in Ca-rich plagioclase from calcalkalic rocks are lower than those from tholeiitic rocks
(Fig. 12).
Sr isotopic ratios in plagioclases from tholeiitic rocks are
constant at 07042^07044 and show little correlation with
the anorthite content (Fig. 13), whereas those in the calcalkalic rocks show more complex characteristics. The core
compositions of plagioclase phenocrysts in calc-alkalic
rocks show a broad bimodal distribution in terms of
anorthite content (Fig. 10). Importantly, the high-An plagioclase from the most mafic calc-alkalic basaltic andesites
exhibit the lowest 87Sr/86Sr of 07034 (Fig. 13).
DISCUSSION
Closed vs open processes in the tholeiitic
series
Tholeiitic rocks from Zao volcano belong to the N-type
rocks of Sakuyama (1981) and Kuno’s pigeonitic rock
series, and exhibit little evidence for disequilibrium:
(1) phenocryst phases such as pyroxenes and plagioclase
are normally zoned (Figs 9 and 10); (2) olivine is in equilibrium with pyroxenes in terms of Fe^Mg partitioning
(Fig. 14) at temperatures of 10508C (Fig. 15). These observations, together with the relatively constant 87Sr/86Sr for
plagioclase phenocryst cores (Fig. 13), suggest that the Zao
tholeiitic magmas differentiate mainly in a closed system,
via fractional crystallization; this is a mechanism that has
been accepted as a general process of differentiation in arc
tholeiites (e.g. Sakuyama, 1981; Fujinawa, 1988, 1990;
Tatsumi & Kogiso, 2003; George et al., 2004; Villiger et al.,
2007). It should be stressed here that differentiation by
1985
JOURNAL OF PETROLOGY
VOLUME 49
NUMBER 11
NOVEMBER 2008
0.5129
15.60
38.5
0.5128
0.5134
15.55
39.5
38.3
18.4
18.5
18.4
15.7
18.5
39.0
15.6
Zao tholeiitic
0.5126
Zao calc-alkalic
15.5
NE Japan
38.5
Pb/204Pb
208
0.5130
Pb/204Pb
133
15.8
207
Nd/144Nd
0.5127
0.7035 0.7040 0.7045
38.0
Zone-I
0.5122
0.702
0.704
0.706
87
Sr/86Sr
0.708
Zone-II
17.8
18.2
Zone-III
206
18.6
204
Pb/
Pb
15.4
19.0
17.8
18.2
206
18.6
37.5
19.0
204
Pb/
Pb
Pacific MORB
Pacific sediments
Fig. 6. Sr^Nd^Pb isotope compositions of Zao volcanic rocks and Quaternary volcanoes in the trench-side Nasu chain. Data for NE Japan,
MORB and sediments are from Kimura & Yoshida (2006).
6
3
More enriched
in calc-alkalic
2
Comparable
More depleted in calc-alkalic
Calc-alkalic/Tholeiitic
4
1
the tholeiitic magmas. Assuming Abukuma granitic rocks
(Gr in Table 8, data from Kamei et al., 2003), which form
the basement of the Zao volcano, to be a possible upper
crustal contaminant, the observed variation in 87Sr/86Sr
for the tholeiitic rocks can be explained by simple bulk
contamination by upper crust (Fig. 16b).
Element Concentration
at 55% SiO2
5
Magma mixing in the calc-alkalic series
0.8
0.6
U Th K Pb Zr Pr Mg Sm Tb P Tm Na Al Eu Cu Ti Fe
Rb Ni Ba Ce Sr La Nd Yb Gd Lu Dy Ho Er Y Ca Nb Zn
Fig. 7. Geochemical differences between calc-alkalic and tholeiitic
rocks in Zao volcano. Element concentrations are normalized to the
tholeiitic composition at 55 wt % SiO2. Although for some of these
elements there is little difference between calc-alkalic and tholeiitic
rocks, calc-alkalic rocks are more enriched in both compatible
elements such as Ni and Mg and highly incompatible elements.
crystallization is not the only process that can occur
during closed-system differentiation. Partial melting, or
anatexis, for example, of basaltic lower crust could cause
differentiation from mafic to felsic compositions, which
will be discussed further below.
However, closed-system differentiation cannot solely
explain the geochemical characteristics of the Zao tholeiitic rocks. For example, plagioclase rims tend to show
higher 87Sr/86Sr (up to 07046) than plagioclase cores
(Fig. 13). Furthermore, a positive correlation between
SiO2 content and 87Sr/86Sr is observed for bulk-rock compositions (Fig. 16b). These observations suggest that an
open-system process, such as shallow-level crustal contamination, plays a role in controlling the final compositions of
‘Disequilibrium’ petrographic features in the calc-alkalic
rocks of Zao volcano indicate that they belong to the
R-type rocks of Sakuyama (1981). Such features include:
the occurrence of reversely zoned pyroxene phenocrysts
(Fig. 9); a wide compositional range in Ca/(Ca þ Na) of
plagioclase phenocrysts, which also show a bimodal compositional distribution (Fig. 10); the occurrence of plagioclase phenocrysts with honeycomb textures and dusty
zones; and the presence of disequilibrium olivine^pyroxene
pairs within a single specimen (Fig. 14).
Disequilibrium in the calc-alkalic rocks is further indicated by temperature estimates for pyroxene crystallization. Figure 15 summarizes the temperature estimates
from contiguous crystals of clinopyroxene and orthopyroxene using a two-pyroxene geothermometer (Wells,
1977). This thermometry cannot be applied to the pyroxenes in the groundmass of the tholeiitic rocks in Zao volcano, because they do not contain orthopyroxene; they
belong to Kuno’s pigeonitic rock series. Figure 15 indicates
that in calc-alkalic rocks the rim^rim pair and the groundmass pair of pyroxenes exhibit crystallization temperatures
higher than those obtained for the core^core pair.
One possible process that can provide a comprehensive
explanation of these disequilibrium textures would be
mixing of magmas having different compositions and temperatures (e.g. Eichelberger, 1975; Sakuyama, 1981, 1983;
1986
TATSUMI et al.
CALC-ALKALIC VS THOLEIITIC SERIES
Table 2: Representative compositions of olivine phenocrysts
Rock type:
Tholeiitic
Calc-alkalic
Sample:
ZA1011
Grain:
ol1-2
ol2-3
ol2-9
ol3-12
ol-12
ol-6
ol007
ol018
ol041
ol054
5ol-8
10ol-3
Position:
Core
Core
Core
Core
Core
Core
Core
Core
Core
Core
Core
Core
ZA1013
ZA1044
ZA3064
ZA3081
ZA3093
SiO2
4032
3888
3892
3882
3975
3786
4038
4034
3911
3786
3972
3864
FeO
1633
2115
2053
2645
1951
2915
1563
1589
197
3037
1803
2414
MnO
025
033
033
04
03
MgO
4504
4041
4085
3686
4241
CaO
012
013
013
017
013
017
012
012
013
01
015
011
NiO
009
003
004
004
004
004
017
014
011
003
010
005
10215
10093
10080
10274
10214
10206
10189
10179
10138
10234
10139
10190
Total
044
344
026
027
031
054
031
054
4533
4503
4202
3344
4308
3842
Si
0997
0996
0996
1000
0997
0997
0998
0999
0991
0999
0998
Fe
0338
0453
0439
0570
0409
0642
0323
0329
0417
0670
0379
0995
0520
Mn
0005
0007
0007
0009
0006
0010
0005
0006
0007
0012
0007
0012
Mg
1659
1542
1557
1415
1585
1349
1669
1661
1587
1315
1612
1473
Ca
0003
0003
0004
0005
0003
0005
0003
0003
0003
0003
0004
0003
Ni
0002
0001
0001
0001
0001
0001
0003
0003
0002
0001
0002
0001
Total
3004
3002
3004
3000
3001
3004
3001
3001
3007
3000
3002
3004
Mg/(Mg þ Fe)
0831
0773
0780
0713
0795
0678
0838
0835
0792
0662
0810
0739
Numbers of ions are calculated on the basis of four oxygens.
Wada, 1985; Fujinawa, 1988, 1990; Hunter & Blake, 1995;
Clynne, 1999; Streck et al., 2007). To better constrain the
magma mixing process, especially to decode the chemical
characteristics of the end-member components for the
mixed calc-alkalic magmas, the Sr isotopic compositions
recorded in plagioclase phenocrysts (Fig. 13) are considered in combination with the chemical composition of the
magma. Element concentrations for magmas that crystallize plagioclase in the Zao volcanic rocks are obtained
using element partitioning data (Bindeman et al., 1998;
Bindeman & Davis, 2000). The panels down the righthand side of Fig. 13 show the relationship between
87
Sr/86Sr and Sr concentration in the melt in equilibrium
with the plagioclase and allow the identification of
four melt components that mix to form the calc-alkalic
rocks. These are: a low 87Sr/86Sr (07034), Sr-rich
(650^700 ppm) melt (I in Fig. 13), probably having a
mafic/basaltic composition; and three components
(II, III, and IV in Fig. 13) with similar rather high
87
Sr/86Sr (07040), but with different Sr concentrations
(500^400, 300, 200^150 ppm, respectively), that are probably intermediate in composition. The above four melt
components may crystallize plagioclase with An490,
An80^90, An70, and An50^60, respectively (Figs 13 and 16a).
The next problem to address is how these four components are generated. Figure 16a summarizes the
geochemical characteristics of the four calc-alkalic melt
components (I^IV in Fig. 13) and the tholeiitic melts in
Zao volcano inferred from the core compositions of plagioclase phenocrysts. To understand the characteristics of
the Zao calc-alkalic melts, at least three end-member components may be required, L1, L2, and L3 in Fig. 16a and
Table 8. L1 and L2 are a calc-alkalic and a tholeiitic
basaltic melt, believed to be the host melts for Ca-rich
(An490) plagioclase found in the calc-alkalic and tholeiitic
basaltic andesites at Zao volcano, respectively. The SiO2
contents of these basaltic melts are assumed to be
50 wt %; Sr concentrations and 87Sr/86Sr are deduced
from plagioclase core compositions (Table 8). L3 is a tholeiitic felsic melt that is a liquid differentiated from L2 and
is assumed to contain 70 wt % SiO2. The 87Sr/86Sr of L3
is simply assumed to be identical to that of L2; the Sr concentration in L2 is 5200 ppm, and has been assumed to
be 100 ppm. The reason for this is that the identifiable
and the most differentiated tholeiitic melt in Zao volcano
contains 200 ppm Sr and crystallizes An60 plagioclase
(Fig. 13), which is more calcic than the plagioclase present
in the calc-alkalic intermediate component (An50^60;
Fig. 13). This suggests that a more differentiated tholeiitic
felsic melt with a Sr concentration lower than 200 ppm
is required as the felsic end-member for the calcalkalic rocks.
1987
JOURNAL OF PETROLOGY
VOLUME 49
NUMBER 11
NOVEMBER 2008
Table 3: Representative compositions of clinopyroxene
Rock type:
Tholeiitic
Calc-alkalic
Sample:
ZA1011
Grain:
px021 px021 pxg06 px042 px042 pxg58 px038 px038 pxg09 px014 px014 px024 px024 pxg030 px101 px101 px002 px002 pxg02
Position:
Core
ZA1013
Rim
GM
Core
ZA1044
Rim
GM
Core
ZA3064
Rim
GM
Core
ZA3093
Rim
Core
Rim
GM
Core
Rim
Core
Rim
GM
SiO2
5270
5294
5328
5285
5290
5268
5229
5261
5191
5313
5102
5287
5322
5403
5263
5277
5308
TiO2
042
034
036
033
040
031
042
035
044
038
063
035
068
044
027
031
067
057
070
Al2O3
275
207
157
174
186
140
195
212
168
251
282
097
349
102
088
090
182
211
166
Cr2O3
025
008
001
007
004
000
008
005
004
006
004
000
007
000
000
000
000
004
000
FeO
821
1038
1964
1176
1150 1929
992
1148
2111
777
881
1170
1028
2170
1067
2020
1252
1010
1056
5276 5240
MnO
023
026
045
033
047
015
021
032
020
025
044
030
066
037
057
035
031
039
MgO
1615
1649
1895
1548
1515 2111
1574
1570
1668
1618
1501
1380
1568
1830
1444
2311
1448
1473
1594
CaO
2050
1811
657
1834
1865
463
1943
1792
827
2091
2050
2080
1914
554
2052
193
1841
2055
1853
Na2O
021
023
007
023
028
008
022
021
017
024
026
025
024
014
020
007
023
024
032
Total
030
10142 10090 10090 10113 10094 9969 10081 10072 10100 10086 10023 10144 10090 10067 10057 10112 10111 10142 10118
Si
1922
1946
1973
1953
1952 1956
1949
1948
1960
1928
1923
1971
1887
1979
1979
1976
1952
1941
1951
Ti
0118
0090
0068
0076
0081 0062
0085
0092
0074
0108
0123
0043
0152
0045
0039
0039
0080
0092
0072
Al
0011
0009
0010
0009
0011 0009
0012
0010
0012
0010
0017
0010
0019
0012
0008
0009
0019
0016
0019
Cr
0007
0002
0000
0002
0001 0000
0002
0001
0001
0002
0001
0000
0002
0000
0000
0000
0000
0001
0000
Fe
0250
0319
0608
0363
0356 0602
0306
0355
0662
0238
0273
0363
0318
0679
0332
0618
0388
0311
0325
Mn
0007
0008
0014
0010
0009 0015
0005
0007
0010
0006
0008
0014
0009
0021
0012
0018
0011
0010
0012
Mg
0878
0903
1046
0852
0835 1174
0864
0865
0931
0884
0829
0763
0864
1021
0800
1259
0800
0807
0873
Ca
0801
0713
0261
0726
0739 0185
0767
0710
0332
0821
0814
0826
0758
0222
0818
0076
0731
0810
0730
Na
0015
0016
0005
0016
0020 0005
0015
0015
0012
0017
0019
0018
0017
0010
0014
0005
0017
0017
0023
4005
Total
4009
4006
3985
4007
4004 4008
4005
4003
3994
4014
4007
4008
4026
3989
4002
4000
3998
4005
Mg/(Mg þ Fe)
0778
0739
0632
0701
0701 0661
0739
0709
0585
0788
0752
0678
0731
0600
0707
0671
0673
0722
0729
Ca/(Ca þ Mg þ Fe)
0415
0368
0136
0374
0383 0094
0396
0368
0172
0423
0425
0423
0391
0116
0419
0039
0381
0420
0379
Mg/(Ca þ Mg þ Fe)
0455
0467
0546
0439
0433 0599
0446
0448
0484
0455
0433
0391
0445
0531
0410
0645
0417
0419
0453
Fe/(Ca þ Mg þ Fe)
0130
0165
0317
0187
0184 0307
0158
0184
0344
0122
0142
0186
0164
0353
0170
0316
0202
0161
0169
Total iron as FeO.
GM, groundmass. Numbers of ions are calculated on the basis of six oxygens.
Mixing between L1 and L2 (15% contribution of L1) produces a basaltic melt, L4, that has chemical characteristics
consistent with a melt component inferred from An80^90
plagioclase in the calc-alkalic rocks (Fig. 16a and Table 8).
A felsic melt component, L5, can be produced by 1:9
mixing of L1 and L3 (Fig. 16a and Table 8). Inferred intermediate calc-alkalic melt components can then be interpreted as mixing products between a felsic component
(L5), and a mafic component (L4) as shown in Fig. 16a
and Table 8.
Liquid mixing and cryptic mixing in
calc-alkalic magmas
Isotopic and elemental compositions can be used to
successfully identify the melt components that mixed to
produce the variety of calc-alkalic rocks in Zao volcano.
It should be stressed here that calc-alkalic andesites from
Zao volcano, whose end-member components are produced
by mixing between a calc-alkalic basaltic melt (L1 in
Fig. 16a), and tholeiitic basaltic and felsic melts (L2 and L3,
respectively), contain plagioclase crystallizing from mixed
end-member components (L4 and L5), but not from L1, L2,
nor L3 (see calc-alkalic andesite in Fig. 13). This observation
implies that mixing of melts or liquids, not magmas containing plagioclase crystals, and subsequent crystallization
of the mixed melts is the likely process that formed the
calc-alkalic end-member components. On the other hand,
the existing calc-alkalic andesites are produced by mixing
of these end-member ‘magmas’, which contain plagioclase
and other phenocryst phases.
The SiO2 contents and 87Sr/86Sr of the inferred mixed
magmas formed by the above processes can be calculated
1988
TATSUMI et al.
CALC-ALKALIC VS THOLEIITIC SERIES
Table 4: Representative compositions of orthopyroxene
Rock type:
Tholeiitic
Calc-alkalic
Sample:
ZA1011
Grain:
px032 px032 px002 px002 px009 px009 px004 px004 px004 px013 px013 px013 pxg017 px003 px003 px012 px012 pxg08 pxg12
Position:
Core
ZA1013
Rim
Core
ZA1044
Rim
Core
ZA3064
Rim
ZA3081
Core Mantle Rim
Core
Mantle Rim
GM
Core
Rim
Core
Rim
GM
GM
SiO2
5524
5511
5424
5466
5445
5512
5275
5456
5475
5470
5346
5333
5332
5416
5293
TiO2
022
025
024
018
019
036
021
012
033
020
024
022
025
020
020
015
016
037
038
Al2O3
136
133
143
121
133
086
062
082
073
165
092
076
131
062
066
130
090
254
271
Cr2O3
002
003
001
005
003
005
000
007
001
014
001
000
000
001
001
005
007
008
002
FeO
1600
1760
1842
1865
1779
2195 2370
1457
2193
1455
1646
1917
1863
2407
2440
1472 1608
1566
1940
MnO
043
043
042
045
024
MgO
2618
2520
2483
2437
2481
5325 5266
079
039
066
037
043
043
048
078
080
1955 2011
034
2749
1924
2779
2643
2453
2459
2092
2033
5568 5411
037
005
006
2751 2653
033
2579
2298
CaO
188
193
188
200
180
453
134
171
438
157
173
166
171
142
135
194
170
216
177
Na2O
002
004
001
002
004
004
006
004
007
003
004
005
004
001
004
003
002
004
004
Total
10135 10192 10148 10159 10068 10093 9949 10033 10010 10086 10101 10152 10047 10136 10111 10171 9994 10085 10029
Si
1973
1972
1959
1973
1973
1981 1991
1977
1983
1947
1968
1979
1955
1981
1987
1969 1964
1942
1940
Ti
0057
0056
0061
0051
0057
0038 0028
0035
0032
0069
0039
0032
0056
0027
0029
0054 0038
0107
0117
Al
0006
0007
0007
0005
0005
0010 0006
0003
0009
0005
0007
0006
0007
0006
0006
0004 0004
0010
0010
Cr
0001
0001
0000
0001
0001
0001 0000
0002
0000
0004
0000
0000
0000
0000
0000
0001 0002
0002
0001
Fe
0478
0526
0556
0563
0539
0683 0749
0437
0689
0434
0495
0580
0569
0747
0761
0435 0488
0469
0595
Mn
0013
0013
0013
0014
0007
0011 0025
0012
0021
0011
0013
0013
0015
0025
0025
0010 0011
0001
0002
Mg
1393
1344
1336
1311
1340
1084 1133
1469
1077
1478
1416
1323
1340
1158
1129
1450 1435
1378
1255
Ca
0072
0074
0073
0077
0070
0180 0054
0066
0176
0060
0067
0064
0067
0056
0054
0073 0066
0083
0070
Na
0002
0003
0001
0001
0003
0003 0005
0003
0005
0002
0003
0004
0003
0001
0003
0002 0001
0003
0003
Total
3995
3996
4006
3996
3995
3991 3991
4004
3992
4010
4008
4001
4012
4001
3994
3998 4009
3995
3993
Mg/(Mg þ Fe)
0745
0718
0706
0700
0713
0614 0602
0771
0610
0773
0741
0695
0702
0608
0598
0769 0746
0746
0679
Ca/(Ca þ Mg þ Fe)
0037
0038
0037
0039
0036
0092 0028
0033
0091
0030
0034
0033
0034
0029
0028
0037 0033
0043
0036
Mg/(Ca þ Mg þ Fe)
0717
0691
0680
0672
0688
0557 0585
0745
0555
0749
0716
0673
0678
0591
0581
0741 0721
0714
0654
Fe/(Ca þ Mg þ Fe)
0246
0271
0283
0289
0277
0351 0387
0222
0355
0220
0250
0295
0288
0381
0391
0222 0245
0243
0310
Total iron as FeO.
GM, groundmass. Numbers of ions are calculated on the basis of six oxygens.
using the compositions assumed for L1, L2, and L3 (Table 8
and Fig. 16b), and are plotted along the mixing curve
between L4 and L5 in Fig. 16b. The existing calc-alkalic
andesites have 87Sr/86Sr lower than the inferred mixed
magma (Fig. 16b). One possible mechanism to explain
this apparent dilemma would be that an above-liquidus
liquid, not a sub-liquidus magma, with low 87Sr/86Sr, such
as L1 (Fig. 16b), contributes to the existing calc-alkalic
andesite formation. Zao calc-alkalic rocks, however,
show no petrographic signs of such mixing. We thus propose to call this process ‘cryptic’ mixing of basaltic liquid
or melt. A calc-alkalic magma that experiences cryptic
mixing should not crystallize plagioclase phenocrysts.
If this is the case, then such cryptic mixing must have
taken place immediately before and could have caused
the eruption.
One petrographical observation that characterizes calcalkalic or Sakuyama’s R-type andesites is the presence of
pyroxenes in the groundmass that record higher temperatures than the phenocryst pyroxenes (e.g. Sakuyama, 1981,
1983; Wada, 1985; Fujinawa, 1988, 1990); this is the case for
Zao calc-alkalic rocks (Fig. 15). This observation can be
also explained by the cryptic mixing of a higher-T basaltic
melt with a lower-T magma.
Genesis of tholeiitic vs calc-alkalic basaltic
magmas: different fluid contributions
Petrological and geochemical observations suggest that at
least two basaltic magmas are simultaneously present in
the magma plumbing system of Zao volcano. One is a
magma that differentiates to form the tholeiitic series, and
the other a mafic end-member magma that mixes to form
1989
Rock type:
Tholeiitic
Sample:
ZA1011
Calc-alkalic
ZA1032
ZA3093
ZA3062: Andesite I
Grain:
bp002
bp002
ap001
ap001
cp001
cp001
cp003
cp003
bp001
bp001
ap002
ap002
cp001
cp001
cp001
cp001
Position:
Core
Rim
Core
Rim
Core
Rim
Core
Rim
Core
Rim
Core
Rim
Core
Rim
Core
Rim
4726
4505
4477
4438
4949
5355
5544
4626
4643
5040
5272
4652
4795
5467
000
001
000
000
002
005
004
005
000
001
001
007
000
004
001
5702
000
Al2O3
3469
3320
3418
3411
3423
3168
2920
2674
3389
3307
3040
2884
3281
3251
2876
2678
FeO
054
065
044
048
001
000
002
003
055
053
082
111
093
070
047
068
MnO
015
051
087
012
001
000
002
000
000
051
000
006
000
020
000
044
MgO
007
008
007
006
005
011
005
014
009
008
008
007
002
003
005
006
CaO
1861
1759
1862
1898
1903
1591
1267
1100
1794
1791
1481
1264
1807
1664
1238
1053
Na2O
060
144
067
064
061
258
426
437
080
108
307
405
105
177
449
513
K2O
002
001
000
002
001
003
009
023
004
005
011
025
002
007
017
028
Total
9949
10075
9990
9918
9835
9985
9990
9800
9957
9967
9970
9981
9942
9991
10100
10092
2082
2166
2091
2090
2084
2265
2425
2540
2139
2152
2314
2407
2161
2207
2452
2551
Ti
0000
0000
0000
0000
0001
0002
0001
0002
0000
0000
0000
0002
0000
0001
0000
0000
Al
1900
1793
1869
1876
1894
1709
1559
1443
1846
1806
1644
1551
1796
1763
1520
1411
Fe
0021
0025
0017
0019
0000
0000
0001
0001
0021
0020
0032
0042
0036
0027
0017
0026
Mn
0006
0020
0034
0005
0000
0000
0001
0000
0000
0020
0000
0002
0000
0008
0000
0017
Mg
0005
0006
0005
0004
0003
0008
0003
0010
0006
0005
0005
0005
0002
0002
0003
0004
Ca
0926
0863
0926
0949
0957
0780
0614
0540
0888
0889
0728
0618
0899
0820
0594
0505
Na
0054
0128
0061
0058
0056
0229
0374
0388
0071
0097
0273
0358
0095
0158
0390
0445
K
0001
0001
0000
0001
0000
0002
0005
0013
0003
0003
0006
0014
0001
0004
0010
0016
Total
4995
5002
5003
5002
4995
4995
4983
4937
4974
4992
5002
4999
4990
4990
4986
4975
O
8000
8000
8000
8000
8000
8000
8000
8000
8000
8000
8000
8000
8000
8000
8000
8000
Ca/(Ca þ Na)
0945
0871
0939
0942
0945
0773
0622
0582
0926
0902
0727
0633
0905
0839
0604
0532
Rb (ppm)
n.d.
0156
n.d.
0011
n.d.
0803
1420
2002
0031
0026
0127
4656
0015
3263
9061
1939
NOVEMBER 2008
Si
NUMBER 11
4481
TiO2
VOLUME 49
1990
SiO2
JOURNAL OF PETROLOGY
Table 5: Representative compositions of plagioclase phenocrysts
Sr
306
390
337
350
364
422
482
475
682
700
463
422
487
501
599
601
Y
0051
0145
0108
0161
0037
4903
0966
0598
0105
0105
0381
3575
0159
11593
3274
1402
Zr
0005
0259
0178
0262
0003
7914
2464
2025
0001
0006
0037
16299
0026
27001
21002
3996
Nb
n.d.
0009
0007
0004
n.d.
0559
0020
0180
n.d.
n.d.
0007
0730
0701
0782
Ba
504
487
585
436
3259
3548
La
0063
0308
0089
0129
0064
4262
1596
1457
0463
0552
1961
2688
0180
7970
6233
6598
Ce
0167
0534
0192
0267
0159
5788
3734
3405
0965
0993
3657
6018
0426
18418
12603
12409
1565
3610
10018
9081
7755
11493
n.d.
1223
17666
0236
25525
33913
0062
0020
0036
0018
1075
0380
0330
0092
0115
0378
0768
0054
1960
1301
1173
0247
n.d.
0162
0126
4548
1523
1229
0428
0447
1609
3274
0179
8068
4851
4223
Sm
0006
0056
0034
0052
0026
1009
0251
0309
0086
0102
0207
0792
0038
1994
0860
0580
Eu
0097
0191
0106
0108
0098
0584
1004
0853
0227
0204
0516
0802
0132
0710
1688
1978
Gd
0025
0047
0020
0025
0010
0883
0289
0217
0091
0110
0269
0732
0065
2131
1148
0914
Tb
0001
0008
0006
0006
0000
0132
0034
0022
0006
0010
0025
0114
0007
0367
0121
0071
Dy
0017
0038
0011
0036
0001
0823
0208
0127
0031
0039
0104
0733
0033
2353
0668
0405
Ho
n.d.
0003
0004
0003
0003
0161
0038
0024
0008
0009
0014
0124
0010
0436
0117
0057
Er
0006
0022
0006
0015
0008
0435
0120
0057
0003
0010
0029
0362
0023
1354
0352
0179
Tm
0002
0003
0001
0000
n.d.
0662
0009
0009
n.d.
0001
0003
0046
0000
0177
0054
0015
Yb
n.d.
0016
0004
0017
0008
0412
0114
0052
n.d.
0006
0016
0334
0017
1360
0358
0107
Lu
0001
0002
n.d.
n.d.
n.d.
0058
0014
0005
0003
n.d.
0001
0045
0002
0195
0059
0018
Hf
0002
0012
0011
n.d.
n.d.
0252
0060
0052
0002
n.d.
n.d.
0447
n.d.
0724
0582
0130
Ta
0001
0007
n.d.
n.d.
n.d.
0016
0007
0006
0001
n.d.
0001
0032
0003
0055
0057
0010
Pb
0113
0359
0161
0138
0112
0972
7406
6329
0291
0189
1533
1887
0407
4285
12205
6797
Th
n.d.
n.d.
0023
0004
n.d.
0119
0045
0039
n.d.
n.d.
0003
0584
n.d.
1044
0962
0169
U
0002
0003
0018
0001
0002
0053
0026
0033
0001
0001
0003
0150
n.d.
0658
0263
0073
0704204
0704486
0704374
0704469
0704549
070456
0703556
0703463
0704125
070398
0704165
0704193
0704094
87
Sr/86Sr
67
87
Sr/86Sr
0704119
15
85
89
0704202
40
0704328
25
0704260
15
71
0704396
42
61
82
70
0703418
33
72
80
0704033
26
75
59
0704205
20
54
57
0703971
17
(continued)
CALC-ALKALIC VS THOLEIITIC SERIES
0022
0102
TATSUMI et al.
1991
Pr
Nd
Rock type:
Calc-alkalic
Sample:
ZA3023: Andesite II
ZA3024: Andesite II
Grain:
ap003
ap003
ap004
ap004
ap004
ap004
ap005
ap005
ap007a
ap007a
Position:
Core
Rim
Core
Rim
Core
Rim
Core
Rim
Core
Rim
5418
5469
5093
4535
5576
5540
5577
5424
003
004
001
002
002
000
002
000
003
5657
001
Al2O3
3233
2797
2766
2996
3416
2738
2762
2735
2821
2684
FeO
048
042
041
044
000
003
001
001
000
001
MnO
040
000
000
033
000
002
000
001
000
000
MgO
003
003
004
003
002
002
000
000
004
003
CaO
1648
1172
1099
1417
1817
1021
1051
1024
1157
963
Na2O
162
492
491
348
096
533
514
539
477
560
K2O
004
026
026
015
002
030
027
029
023
035
Total
9879
9954
9897
9951
9870
9905
9897
9906
9909
9904
2203
2466
2495
2339
2115
2530
2517
2531
2471
Ti
0001
0001
0000
0001
0001
0000
0001
0000
0001
2562
0000
Al
1772
1500
1487
1621
1877
1464
1479
1463
1515
1432
Fe
0019
0016
0016
0017
0000
0001
0000
0000
0000
0000
Mn
0016
0000
0000
0013
0000
0001
0000
0000
0000
0000
Mg
0002
0002
0003
0002
0001
0001
0000
0000
0003
0002
Ca
0821
0571
0537
0697
0908
0496
0511
0498
0564
0467
Na
0146
0434
0434
0310
0087
0469
0452
0474
0421
0491
K
0002
0015
0015
0008
0001
0018
0016
0016
0013
0020
Total
4982
5005
4987
5008
4990
4980
4976
4982
4988
4974
O
8000
8000
8000
8000
8000
8000
8000
8000
8000
8000
Ca/(Ca þ Na)
0849
0568
0553
0692
0913
0514
0531
0512
0572
0487
Rb (ppm)
0405
6069
0326
0798
n.d.
0402
0402
0620
0344
0337
NOVEMBER 2008
Si
NUMBER 11
4738
TiO2
VOLUME 49
1992
SiO2
JOURNAL OF PETROLOGY
Table 5: Continued
Sr
584
507
480
642
389
490
477
466
545
490
Y
1198
0982
0875
1158
0060
0901
0785
1003
0908
0972
Zr
0499
2576
0150
1539
0017
0030
0119
0421
0536
0035
Nb
0031
0112
0004
0016
0001
n.d.
0013
0008
0020
Ba
14344
16266
18977
26710
763
29171
23557
29742
25335
0008
34454
3637
4008
6771
0127
5274
4887
5173
5065
6204
7117
7184
11140
0220
8611
9540
9595
7909
9609
Pr
0838
0699
0729
1105
0025
0883
0885
0873
0896
0961
Nd
2929
2411
2602
3951
0154
3177
3044
3537
3542
3824
Sm
0429
0360
0423
0588
0031
0521
0432
0465
0559
0553
Eu
1196
0980
1296
1675
0113
1701
1309
1600
1777
1919
Gd
0622
0524
0575
0911
0026
0751
0700
0815
0829
1044
Tb
0044
0035
0037
0051
0000
0043
0037
0063
0054
0050
Dy
0205
0171
0187
0275
0017
0239
0262
0243
0322
0248
Ho
0032
0031
0024
0032
0001
0026
0024
0044
0041
0033
Er
0096
0078
0055
0080
0006
0043
0080
0088
0125
0087
Tm
0010
0010
0006
0008
n.d.
0005
0008
0007
0011
0009
Yb
0081
0066
0037
0038
0012
0043
0060
0061
0046
0045
Lu
0013
0008
0004
0007
n.d.
0003
0005
0006
0005
0003
Hf
0027
0086
0019
0068
n.d.
0046
0042
0021
0062
0048
Ta
0001
0007
0002
0007
0000
0003
0000
0002
n.d.
n.d.
Pb
3894
3673
3794
5263
0399
5116
6187
7741
6013
6863
Th
0008
0101
0003
0038
0008
0002
0017
0085
0032
0013
U
0003
0027
0002
0010
0001
0006
0005
0015
0004
0001
0704198 62
0704144 63
0704056 69
0704058 65
0704042 38
0704052 68
0704007 67
0704164 62
0704079 44
0704168 57
87
Sr/86Sr
87
Sr/86Sr
LA-MC-ICP-MS,
0704018 19
MM-TIMS
0704035 29
0703945 17
0703942 33
CALC-ALKALIC VS THOLEIITIC SERIES
4339
8671
TATSUMI et al.
1993
La
Ce
JOURNAL OF PETROLOGY
VOLUME 49
NUMBER 11
NOVEMBER 2008
Table 6: Representative compositions of Fe^Ti oxides
Rock type:
Tholeiitic
Calc-alkalic
Sample:
ZA1011
Grain:
OQ-007
OQ-007c2
OQ-7
OQ-445
OQ-4
OQ-10
OQG-25
OQ-21
OQG-10
OQ-32
OQ-55
OQG-95
OQ-61
OQG-79
Mineral:
MT
MT
MT
MT
MT
MT
MT
IL
IL
MT
MT
MT
IL
IL
Position:
GM
GM
GM
GM
Core
Core
GM
Core
GM
Core
Core
GM
Core
GM
ZA1044
ZA3081
ZA3093
SiO2
012
020
016
012
005
006
010
001
007
005
005
007
007
011
TiO2
1181
1185
1564
1341
1099
1496
1564
4777
4813
1237
1219
981
4760
4261
Al2O3
381
302
214
281
149
176
118
007
010
222
224
161
009
011
FeO
7670
7733
7559
7391
8137
7755
7757
4934
4882
7621
7737
7993
4622
5302
MnO
025
032
034
042
035
046
040
066
053
033
037
045
126
039
MgO
267
180
089
148
092
101
105
148
147
221
238
090
302
086
Cr2O3
005
009
033
374
008
004
008
003
000
111
096
068
029
005
Total
9540
9463
9510
9589
9525
9584
9601
9936
9911
9449
9556
9345
9855
9715
Si
0004
0007
0006
0004
0002
0002
0004
0000
0002
0002
0002
0003
0002
0003
Ti
0325
0333
0444
0374
0311
0421
0441
0899
0906
0348
0339
0282
0896
0802
Al
0165
0133
0095
0123
0066
0077
0052
0002
0003
0098
0098
0073
0003
0003
Fe3þ
1174
1184
0995
1010
1307
1075
1057
0202
0180
1169
1193
1337
0209
0345
Fe2þ
1176
1229
1389
1284
1250
1352
1373
0830
0842
1216
1198
1219
0758
0764
Mn
0008
0010
0011
0013
0011
0015
0013
0014
0011
0010
0012
0015
0027
0008
Mg
0146
0100
0050
0082
0051
0056
0059
0055
0055
0123
0131
0051
0113
0032
Cr
0002
0003
0010
0110
0002
0001
0002
0001
0000
0033
0028
0020
0006
0001
Total
3000
3000
3000
3000
3000
3000
3000
2004
1998
3000
3000
3000
2013
1959
Total iron as FeO.
GM, groundmass; MT, titanomagnetite; IL, ilmenite. Fe3þ and Fe2þ are calculated assuming Fe–Ti oxides stoichiometry.
Numbers of ions are calculated on the basis of four oxygens.
the calc-alkalic series rocks. Although basaltic andesites in
Zao volcano are differentiated and do not represent primitive magma compositions, it is reasonable to assume that
calcic plagioclase (An490) in both tholeiitic and calcalkalic mafic andesites crystallizes from different primitive
magmas, as such plagioclase phenocrysts in the two rock
series exhibit different concentrations of some trace elements, especially of Sr (Fig. 12), If so, then the geochemical
characteristics of such primitive magmas, one tholeiitic
and other calc-alkalic, can be inferred from the plagioclase
compositions and element partitioning between plagioclase and silicate melt (Table 9 and Fig. 17).
Calc-alkalic rocks tend to be more enriched in incompatible trace elements than tholeiitic rocks, which is also
the case for the inferred primitive magma compositions
(Figs 7 and 17). This observation led Masuda & Aoki
(1979) to the conclusion that both calc-alkalic and tholeiitic
primary magmas tap a common mantle source but the
former is produced by lower degrees of melting. However,
this study on volcanic rocks from Zao volcano indicates
that the tholeiitic primitive magma has a higher 87Sr/86Sr
than the calc-alkalic magma, suggesting a more radiogenic
mantle source for the tholeiitic magmas. Such isotopic
heterogeneity within the mantle wedge could be caused
by larger contributions from isotopically enriched, slabderived fluids to the tholeiitic magma source than to the
calc-alkalic source.
To test this hypothesis quantitatively, geochemical modeling of element transport by slab-derived fluid addition to
the mantle wedge and subsequent incremental fractional
melting was conducted (Table 10 and Fig. 18). The model
compositions, including H2O contents of subducted altered
oceanic crust and terrigenous sediments, are from Tatsumi
& Hanyu (2003). Experimental data for the mobility of
Sr during dehydration of amphibolite (Kogiso et al., 1997)
and sediment (Aizawa et al., 1999) were used to estimate
the fluid compositions. Sr concentrations (357 and
664 ppm) and 87Sr/86Sr (070420 and 070343) in tholeiitic
and calc-alkalic primary magmas, respectively, are based
on the average compositions of melts in equilibrium
with An490 plagioclase phenocrysts (Fig. 13). The results
(Table 10 and Fig. 18) indicate that the geochemical
1994
TATSUMI et al.
CALC-ALKALIC VS THOLEIITIC SERIES
0.20
Table 7: Representative compositions of spinel inclusions
Rock type:
Tholeiitic
Calc-alkalic
Sample:
ZA1011
ZA3093
Grain:
OQ-10c1
OQ-10c2
OQ-7
OQ-445
Position:
Core
Core
Core
Core
Host mineral:
Cpx
Cpx
Ol
Ol
NiO (wt.%)
0.15
TH 1011
TH 1013
TH 1044
CA 3064
CA 3081
CA 3093
0.10
0.05
SiO2
006
008
010
010
TiO2
147
154
076
048
Al2O3
1961
1941
2821
3611
FeO
4334
4238
4383
3257
MnO
028
028
039
028
MgO
956
969
754
1236
Cr2O3
2447
2480
1785
1864
Total
9879
9816
9868
10053
0
60
65
70
75
80
85
100×Mg/(Mg+Fe)
Fig. 8. Compositions of olivine phenocrysts in tholeiitic (TH) and
calc-alkalic (CA) rocks of Zao volcano. Olivines in calc-alkalic rocks
tend to be more enriched in NiO than those in tholeiitic rocks.
Si
0002
0002
0003
0003
Ti
0035
0037
0018
0010
Al
0735
0732
1036
1232
Fe3þ
0576
0563
0482
0315
Fe2þ
0576
0570
0660
0473
Mn
0008
0008
0010
0007
Mg
0453
0462
0350
0533
Cr
0615
0627
0440
0427
Total
3000
3000
3000
3000
Cr/(Cr þ Al)
0456
0461
0298
0257
Mg/(Mg þ Fe2þ)
0440
0447
0347
0530
Cr/(Al þ Cr þ Fe3þ)
0319
0326
0225
0216
Al/(Al þ Cr þ Fe3þ)
0382
0381
0529
0624
Fe3þ/(Al þ Cr þ Fe3þ)
0299
0293
0246
0160
Total iron as FeO.
Cpx, clinopyroxene; Ol, olivine. Fe3þ and Fe2þ are calculated assuming spinel stoichiometry. Numbers of ions are
calculated on the basis of four oxygens.
characteristics of the tholeiitic and calc-alkalic primary
magmas can be explained by different contributions from
slab-derived fluids (1% vs 02%) and different degrees of
melting (40% vs 7%). It is suggested that these processes
produce primary magmas containing 263^265 wt %
H2O, which are acceptable values for arc magmas
(e.g. Hauri et al., 2006).
Although the different contributions from enriched slabderived fluids, together with different degrees of melting,
could generate two types of primary magma in the
mantle wedge, this may not be a plausible process to produce the tholeiitic and calc-alkalic magmas of Zao volcano.
This process does not explain why magma mixing plays
a major role in the differentiation of calc-alkalic magmas
but not in the tholeiitic magmas; the two types of
mantle-derived basalt magmas should have an equal
chance of magma mixing.
Genesis of tholeiitic vs calc-alkalic basaltic
magmas: crust vs mantle melts
When mantle-derived basaltic magmas are underplated
and/or intruded into the arc crust they transfer heat into
the overlying and surrounding crust, which can lead to
partial melting of the wall-rocks (e.g. Hildreth, 1981; Raia
& Spera, 1997; Annen & Sparks, 2002). However, herein
lies a problem: whether or not basaltic magmas emplaced
at the base of the lower crust could transfer enough heat to
continue to cause crustal anatexis; heat transfer from the
basaltic magma to the crust would cause a rapid temperature drop in the magma, which would lead to the magma
being unable to further melt the crust (e.g. Marsh, 1989;
Petford & Gallagher, 2001). This problem may be overcome
if the temperature of the pre-existing crust is high enough;
that is, the basaltic magma intrudes where crustal temperatures are near the basalt solidus or the crust is in a
partially molten state (Couch et al., 2001; Tatsumi et al.,
2006). Numerical simulations of heat transfer (Annen &
Sparks, 2002; Annen et al., 2006) further suggest a model
in which mantle-derived basalts emplaced as a succession
of sills into the lower crust generate a deep crustal ‘hot
zone’ where differentiated melts are produced from two
distinct sources: crystallization of mantle-derived magma,
and melting of crustal rocks.
A ‘hot zone’ is likely to occur beneath the NE Japan
arc. This is believed to be the case for the following
two reasons. First, the temperature of the NE Japanese
mantle-derived magma is much higher than the solidus
temperature of the lower crust. High-pressure melting
experiments (Tatsumi et al., 1983) suggest that primary
1995
JOURNAL OF PETROLOGY
VOLUME 49
NUMBER 11
Tholeiitic
70
60
60
50
90
50
90
70
70
60
60
50
50
90
60
70
80
100×Mg/ (Mg+Fe)
core
100×Mg/ (Mg+Fe)
rim
50
90
80
90
3093
clinopyroxene
60
70
3093
orthopyroxene
80
90
90
80
70
70
60
60
50
90
50
90
80
3064
clinopyroxene
3064
orthopyroxene
80
70
70
60
60
50
90
50
90
80
3062
clinopyroxene
3062
orthopyroxene
80
70
70
60
60
50
90
50
90
3081
clinopyroxene
80
70
reversely
zoned
60
normally
zoned
50
50
90
60
70
80
100×Mg/ (Mg+Fe)
core
3081
orthopyroxene
80
70
60
100×Mg/ (Mg+Fe)
rim
60
70
80
100×Mg/ (Mg+Fe)
core
70
100×Mg/ (Mg+Fe)
rim
80
100×Mg/ (Mg+Fe)
rim
1044
orthopyroxene
1044
clinopyroxene
100×Mg/ (Mg+Fe)
rim
80
100×Mg/ (Mg+Fe)
rim
1013
orthopyroxene
80
60
100×Mg/ (Mg+Fe)
rim
1013
clinopyroxene
100×Mg/ (Mg+Fe)
rim
50
90
70
80
50
90
100×Mg/ (Mg+Fe)
rim
100×Mg/ (Mg+Fe)
rim
80
60
50
90
100×Mg/ (Mg+Fe)
rim
90
90
70
reversely
zoned
normally
zoned
80
Calc-alkalic
100×Mg/ (Mg+Fe)
100×Mg/ (Mg+Fe)
core
core
100×Mg/ (Mg+Fe)
rim
60
1011
orthopyroxene
1011
clinopyroxene
80
70
100×Mg/ (Mg+Fe)
core
90
60
70
80
100×Mg/ (Mg+Fe)
rim
100×Mg/ (Mg+Fe)
rim
50
90
100×Mg/ (Mg+Fe)
core
60
70
80
NOVEMBER 2008
50
60
70
80
90
100×Mg/ (Mg+Fe)
core
Fig. 9. Compositions of pyroxene phenocrysts in tholeiitic and calc-alkalic rocks from Zao volcano. Tholeiitic rocks contains normally zoned
pyroxenes, whereas calc-alkalic rocks are characterized by the occurrence of both normally and reversely zoned orthopyroxene.
magmas beneath the volcanic front of this arc equilibrate
with the mantle at 14008C and 10 GPa, a pressure equivalent to the depth immediately beneath the Moho underlying the NE Japan arc. Second, low-frequency tremors
and micro-earthquakes, which may be caused by deformation associated with magma intrusion, are observed
at depths of 30^50 km only beneath the Quaternary volcanoes of the volcanic front of the NE Japan arc (Obara,
2002; Katsumata & Kamaya, 2003).
The petrographic and geochemical data for the Zao volcanic rocks presented here suggest the presence of two distinct basaltic magmas beneath a single volcano; one a high
87
Sr/86Sr tholeiitic magma and the other a low 87Sr/86Sr
calc-alkalic magma. If the formation of a ‘hot zone’,
resulting from basaltic underplating and subsequent
generation of both crust-derived and mantle-derived
magmas, is accepted, then it is reasonable to suggest that
the isotopically enriched tholeiitic and depleted calcalkalic magmas may be crust- and mantle-derived,
respectively.
Contributions from melting of crustal and mantle materials to generate tholeiitic and calc-alkalic magmas, respectively, are now examined on the basis of their geochemical
characteristics. Relative abundances of trace elements
between tholeiitic and calc-alkalic primitive magmas,
which are based on solid^melt partitioning between
Ca-rich plagioclase and silicate melts (Table 9), are plotted
as a function of the ionic radii of elements in Fig. 19.
Systematic patterns of enrichment and depletion of certain
elements can be observed for the two magma series.
1996
TATSUMI et al.
CALC-ALKALIC VS THOLEIITIC SERIES
Tholeiitic
Calc-allkalic
100xCa/(Ca+Na)
Freq.
15
50
60
70
80
100xCa/(Ca+Na)
90 100
40
20
1011
15
Freq.
40
20
10
5
20
20
15
15
10
10
5
5
0
20
0
20
1013
15
10
10
5
5
20
20
15
15
10
10
5
5
0
0
20
20
15
1044
15
10
10
5
5
20
20
15
15
10
10
5
5
0
40
50
60
70
80
60
70
80
90 100
3093
10
5
15
50
3064
3062C
0
20
90 100
100xCa/(Ca+Na)
3081
15
10
5
core
20
rim &
groundmass
15
10
5
0
40
50
60
70
80
90 100
100xCa/(Ca+Na)
Fig. 10. Compositions of plagioclase in Zao volcanic rocks. The plagioclase phenocrysts in the tholeiitic rocks have a much narrower range of
core compositions than those in the calc-alkalic rocks. Furthermore, the cores of plagioclase phenocrysts in calc-alkalic rocks exhibit a bimodal
compositional distribution.
1997
JOURNAL OF PETROLOGY
VOLUME 49
NUMBER 11
NOVEMBER 2008
A
100
0.704272±68
100×Ca/ (Ca+Na)
100×Ca/ (Ca+Na)
0.704296±66
0.704325±81
0.704262±16
B
800
80
600
TH-ZA1032bp005
60
Sr (ppm)
400
40
1000µm
0
1000
3000 µm
2000
0.703418±33
0.703690±96
0.703556±70
0.703463±72
A
B
80
600
CA basaltic andesite I
-ZA3093bp001
60
40
1000µm
800
0
Sr (ppm)
100×Ca/ (Ca+Na)
A
100
B
Sr (ppm)
A
B
400
4000 µm
2000
A
B
100
800
0.704035±29
0.704042±38
80
600
CA andesite II
-ZA3024ap004
Sr (ppm)
100×Ca/ (Ca+Na)
B
B
60
400
0.704052±68
40
1000µm
A
A
100
0
400
800
1600 µm
1200
A
B
800
CA basaltic andesite III
0.703914±28
A
80
60
400
1000µm
40
100
0.704105±75
0.704058±65
0.704056±69
B
0
500
1000
1500
A
2000 µm
B
800
CA andesite IV
-ZA3023ap004
80
600
Sr (ppm)
0.7041
19±15
100×Ca/ (Ca+Na)
A
600
Sr (ppm)
0.704113±116
-ZA3093bp002
100×Ca/ (Ca+Na)
B
0.704047±76
60
400
40
1000µm
0
500
1000
1500
2000 µm
Fig. 11. Scanning electron micrographs for representative plagioclase phenocrysts and compositional profiles of 100 Ca/Ca þ Na) (continuous line) and Sr concentration (dashed line) across the plagioclase crystals. The numbers (I^IV) labeled for plagioclase in calc-alkalic rocks
correspond to those given based on 87Sr/86Sr and Sr concentration (see Fig. 13). Large open circles and italic values on the images represent
sampling spots and 87Sr/86Sr by MM-TIMS, and large closed circles and regular-font values those by LA-MC-ICP-MS. Small grey circles are
sampling spots for LA-ICP-MS trace element analyses.
1998
TATSUMI et al.
CALC-ALKALIC VS THOLEIITIC SERIES
100×Ca/ (Ca+Na)
40
400
50
60
70
80
100×Ca/ (Ca+Na)
90
100
40
TH
60
70
80
90
100
50
60
70
80
90
100
700
CA
300
600
Sr
Ba
50
200
500
400
100
300
0
2.5
20
2.0
15
Eu
Ce
1.5
10
1.0
0.5
0
0
16
4
12
3
8
2
Y
Pb
5
1
4
0
40
50
60
70
80
90
0
40
100
100×Ca/ (Ca+Na)
100×Ca/ (Ca+Na)
Fig. 12. Trace element concentrations in plagioclase phenocrysts in Zao volcanic rocks as a function of Ca/(Ca þ Na). Plagioclase phenocrysts
in calc-alkalic rocks tend to be more enriched in Ba and Sr and more depleted in Y than those in tholeiitic rocks.
This pattern and a consideration of the crystal structure
control on trace element partitioning between melts and
solid phases (Matsui et al.,1977) may suggest that the tholeiitic basalt magma is more depleted in elements that are likely
to be partitioned into plagioclase and possibly amphibole
than the calc-alkalic basalt magma (Fig. 19). This can be
understood as the result of buffering of particular elements
by residual phases during partial melting; for example, Sr
and Eu by plagioclase, and K and Ba by amphibole. If so,
then the melting residue of the tholeiitic basalt magma is
distinct from that of the calc-alkalic basalt magmas in the
presence of plagioclase (and amphibole). These phases are
commonly observed in the melting residues of hydrous
mafic compositions such as amphibolite (e.g. Beard &
1999
JOURNAL OF PETROLOGY
VOLUME 49
NUMBER 11
NOVEMBER 2008
Sr (ppm) in Melt
100×Ca/(Ca+Na)
40
50
60
70
0.7048
90
0
100
0.7048
CA bulk TH bulk
Tholeiitic
0.7040
100 200 300 400 500 600 700 800
Tholeiitic
0.7044
87Sr/86Sr
0.7044
87Sr/86Sr
80
in equilibrium with plagioclase
MM-TIMS
0.7036
0.7040
Core TIMS
Rim TIMS
Core ICPMS
Rim ICPMS
0.7036
LA-ICP-MS
0.7032
0.7032
0.7048
Calc-alkalic
0.7048
Calc-alkalic
basaltic andesite
basaltic andesite
0.7044
87Sr/86Sr
87Sr/86Sr
0.7044
II
0.7040
0.7040
II
III
IV
IV
0.7036
0.7036
I
III
I
0.7032
0.7032
0.7048
0.7048
Calc-alkalic
Calc-alkalic
andesite
andesite
87Sr/86Sr
87Sr/86Sr
0.7044
II
0.7040
0.7044
0.7040
II
III
0.7032
IV
IV
0.7036
0.7036
I
40
50
60
70
80
90
III
I
0.7032
100
100×Ca/(Ca+Na)
0
100 200 300 400 500 600 700 800
Sr (ppm) in Melt
in equilibrium with plagioclase
Fig. 13. 87Sr/86Sr of plagioclase phenocrysts in tholeiitic and calc-alkalic rocks from Zao volcano as functions of the anorthite content and Sr
concentration in melts inferred from Sr partitioning between plagioclase and silicate melts (Bindeman et al., 1998; Bindeman & Davis, 2000).
Plagioclase in calc-alkalic rocks tends to have lower 87Sr/86Sr than in tholeiitic rocks; more importantly, calcic plagioclase in the most mafic calcalkalic andesites crystallizes from magmas with characteristically low 87Sr/86Sr (07034) and high Sr up to 700 ppm (I). At least four components can be identified (I^IV), based on plagioclase core compositions, for production of mixed calc-alkalic rocks. Accuracy of MM-TIMS
analyses (2s) and precision of LA-ICP-MS analyses are shown by bars.
Lofgren, 1991; Beard et al., 1993; Patin‹o Douce & Beard,
1995), but are unlikely to coexist with primary arc basalt
magmas in a peridotite system (e.g. Tatsumi et al., 1983).
Therefore, it is possible that the geochemical characteristics
of the tholeiitic and calc-alkalic basaltic magma series in
Zao volcano can be understood if they are produced by partial melting of amphibolitic lower crust and peridotitic
upper mantle, respectively.
2000
TATSUMI et al.
CALC-ALKALIC VS THOLEIITIC SERIES
100×Mg/(Mg+Fe) in olivine
90
50
60
70
80
100×Mg/(Mg+Fe) in olivine
90
50
90
Tholeiitic
70
80
90
Calc-alkalic
101 1
1044
70
1013
60
3064
80
100×Mg/(Mg+Fe)
in clinopyroxene
100×Mg/(Mg+Fe)
in clinopyroxene
80
60
3081
70
60
3093
50
90
50
90
Tholeiitic
Calc-alkalic
100×Mg/(Mg+Fe)
in orthopyroxene
100×Mg/(Mg+Fe)
in orthopyroxene
80
70
80
70
60
60
50 50
60
70
80
50
50
90
100×Mg/(Mg+Fe) in olivine
60
70
80
90
100×Mg/(Mg+Fe) in olivine
Fig. 14. Relationship between the core composition of olivine and pyroxene phenocrysts in Zao volcanic rocks. The compositional range for
each sample (numbered) is shown by a box. Broadly identical Mg/(Mg þ Fe) values in olivine and pyroxenes for tholeiitic rocks suggest that
these phases are in equilibrium. The pyroxenes with low Mg/(Mg þ Fe) that tend to be reversely zoned (Fig. 9) are not in equilibrium with
olivine with higher Mg/(Mg þ Fe).
1200
Temperature (°C)
1150
Higher-T Rim
in CA
Higher-T Rim &
Groundmass
in CA
1100
1050
1000
950
900
850
50
TH core
CA core
CA rim
CA gm
52
54
56
58
60
62
64
SiO2 (wt.%) in bulk rock
Fig. 15. Temperature estimates based on two-pyroxene geothermometry (Wells, 1977). Calc-alkalic rocks are characterized by the
occurrence of pyroxene phenocrysts with cores that show lower
temperatures than their rims and pyroxenes in the groundmass. CA,
calc-alkalic; TH, tholeiitic.
Subduction zone tholeiitic magmas have been considered to form from mantle-derived basaltic magmas via differential crystallization (e.g. Wada, 1981; Sakuyama, 1983;
Fujinawa, 1988, 1990; Tatsumi & Kogiso, 2003) for the
following reasons: (1) tholeiitic rocks show little evidence
of disequilibrium textures; (2) they exhibit systematic
changes in both phenocryst compositions and assemblages; (3) the tholeiitic trend can be explained by the fractionation of phenocryst phases. On the other hand, these
petrographic and compositional characteristics are also
consistent with inverse differential crystallization of a
parental basaltic magma (i.e. partial melting of a basaltic
source) having affected the tholeiitic rocks. As a basaltic
parental magma for the Zao tholeiitic rocks is generated
by melting of mafic lower crust caused by heat transfer
from an underplating mantle-derived, calc-alkalic basaltic
magma, it is reasonable to suggest that differentiated
tholeiitic melts are also created via crustal anatexis
rather than crystallization of a mantle-derived basaltic
magma.
2001
JOURNAL OF PETROLOGY
VOLUME 49
Sr (ppm) in melt
in equilibrium with plagioclase
(a)
0.7046
0
100
200
300
400
500
87Sr/ 86Sr
0.7042
An50-60
in CA-A
L2
0.7040
0.7036
Magma type Plagioclase SiO2
0.7034
L1
0.7032
Tholeiitic basalt
Tholeiitic rhyolite
Calc-alkalic basalt
Calc-alkalic mafic endmember
(b)
87
Sr/86Sr
L1
calc-alkalic
An490
50
664
070342
L2
tholeiitic
An490
50
357
070425
L3
tholeiitic
An550
70
100
070425
Gr
upper crust
70
300
070512
L4 (¼ 015L1 þ 085L3) calc-alkalic
An90–80
50
403
070401
L5 (¼ 01L1 þ 09L2)
An50
68
156
070386
calc-alkalic
Calc-alkalic felsic endmember
Upper crust
Tholeiitic
Calc-alkalic
0.7055
n
atio
in
tam
on
0.7050
Cru
0.7045
L3
L2
0.7040
0.7035
0.7030
45
Gr
lC
sta
87Sr/ 86Sr
Sr
(wt %) (ppm)
An>90
in CA-BA
An~70
in CAA & BA
L5
800
An80-90
in CA-A
L4
0.7038
700
NOVEMBER 2008
Table 8: Characteristics of end-member and mixed
components for Zao magmas
An>90 in TH
0.7044
L3
600
NUMBER 11
L4
L5
L1
Cryptic Mixing
50
55
60
65
70
75
SiO2 (wt.%)
Fig. 16. Compositions of end-member components that contribute
to the production of Zao magmas. (a) Variations in 87Sr/86Sr and Sr
concentrations for the melt components inferred from the plagioclase
compositions (see Fig. 13). Three principal end-member components
(L1, L2, and L3) are required to explain the calc-alkalic melt components, L4 and L5 , which further mix to produce calc-alkalic melts that
are able to crystallize An-poor plagioclase. TH, tholeiitic; CA, calcalkalic; BA, basaltic andesite; A, andesite. (b) Relationship between
87
Sr/86Sr and SiO2 contents inferred for the end-member components
of Zao magmas. The compositional characteristics in tholeiitic rocks
can be explained by contamination from basement granitic rock
(Gr), whereas those in calc-alkalic rocks suggest a contribution from
basaltic melt, L1, which forms an end-member component for cryptic
mixing (see text).
Magma plumbing system beneath
Zao volcano
The model for the generation of the two types of magmas,
tholeiitic and calc-alkalic, beneath Zao volcano described
below is shown schematically in Fig. 20.
A mantle-derived basaltic magma (L1), which finally
equilibrates with the upper mantle immediately below the
Moho (10 GPa; Tatsumi et al., 1983), underplates and
transfers heat to the base of the crust, causing both its
own crystallization and partial melting of the lower crust.
Although the temperature and melt fraction gradually
decrease upwards within the partially molten ‘hot zone’
formed at the base of the lower crust, the hot zone is simplified to consist of two sub-zones with higher and lower
melt fractions that generate basaltic and felsic melts.
These are melts L2 and L3, respectively, inferred from the
isotopic compositions of plagioclase phenocrysts in the tholeiitic rocks (Figs 16 and 20). The boundary between the
two sub-zones may be defined by the breakdown and melting of amphibole, which causes an abrupt increase in melt
fraction (e.g. Foden & Green, 1992; Annen et al., 2006).
If so, then the temperature at this boundary would be
10758C (Mu«ntener et al., 2001). This temperature estimate
is consistent with the following petrographic and experimental constraints; first, the highest temperature estimate
for tholeiitic magmas based on a two-pyroxene geothermometer is 10758C (Fig. 15); second, experiments at
03 GPa on a basalt (SiO2 49 wt %; Al2O3 18 wt %; FeO
11wt %; MgO 7 wt %) in the presence of 05 wt %
H2O yield a partial melt with a composition similar to
that of the tholeiitic andesitic basalts (SiO2 52 wt %) at
11008C by 40^50% of partial melting (Tatsumi &
Suzuki, in preparation).
A variable contribution of L2 vs L3 through liquid^
liquid mixing yields melts with mafic to felsic compositions, which crystallize in shallow-level magma reservoirs
to form the tholeiitic rocks (Fig. 20). Alternatively, differential crystallization of L2 may also contribute to the production of tholeiitic magmas with intermediate compositions.
In addition to these processes, contamination of the
magmas by upper crustal granitic rocks plays a role in
controlling the Sr isotopic compositions of the tholeiitic
magmas (Fig. 20), which is suggested by (1) the observation
that plagioclase rims tend to have higher 87Sr/86Sr
than cores in tholeiitic rocks (Fig. 13) and (2) bulk compositions exhibiting higher 87Sr/86Sr with increasing SiO2
(Fig. 16).
2002
TATSUMI et al.
CALC-ALKALIC VS THOLEIITIC SERIES
Table 9: Trace element compositioins of calcic plagicolase and inferred primitive melt
Tholeiitic
Plagioclase
Rb
00779
K
76
Ba
578
Sr
357
Calc-alkalic
1s
0123
50
140
19
Melt
D
001
1s
902
Plagioclase
145
004–005
1649
1065
007–005
109
23
118–094
358
25
1s
00872
0106
250
001
86
376
827
005
77
691
Melt
D
24
1s
101
4780
1711
007–006
601
138
109–100
665
38
La
0102
0040
014–013
0779
0298
0552
0110
014
Ce
0215
0080
008–007
291
105
107
033
008
Pr
00274
00118
007–006
0428
0181
0118
0049
007
163
068
Nd
0124
0051
009–008
157
063
0461
0271
009
523
307
Sm
00301
00162
006–005
0534
0283
00971
00939
007–006
152
147
Eu
0118
0110
008–007
167
154
0234
0059
008
297
077
Pb
0206
0369
050–045
0783
0326
0141
049–048
Y
0152
0149
002–001
0302
0714
002–001
0442
122
123
3899
130
0674
198
0781
40
0297
467
Distribution coefficients (D) are after Bindeman et al. (1998) and Bindeman & Davis (2000).
N-MORB Normalized
100
Primitive CA
CA
Primitive TH
TH
Table 10: Modeling for primary magma generation via
different fluid contributions
87
Sr/86Sr
Sr
10
Fluid
Melt
H2O
fraction (f)
fraction (F)
(wt %)
Altered oceanic crust
amphibolite
1
mobilityy
fluidz
500
070450
041
13667
Sediment
terriginous
0.1
Rb Ba K
mobilityy
La Ce Pb Pr Sr Nd Sm Eu Y
Fig. 17. N-MORB-normalized trace element patterns for average
calc-alkalic (CA) and tholeiitic (TH) rocks from Zao volcano and
those inferred for primitive magma compositions. Calc-alkalic rocks
tend to be more enriched in incompatible trace elements than
tholeiitic rocks, which is also the case for inferred primitive magma
compositions.
250
fluidz
2000
Slab fluidx
11333
Original mantle
070600
012
070455
25
070250
Tholeiitic
357
070420
00105
04
263
Calc-alkalic
664
070343
000182
0068
265
The generation of calc-alkalic magmas is distinct from
tholeiitic magmas in that it involves a mantle-derived
basaltic component, L1, either as a liquid or magma
(Fig. 20). This contributes to the generation of mafic (L4)
and felsic (L5) end-member magmas for andesites via
mixing with crust-derived, tholeiitic basalt (L2) and felsic
(L3) melts. Furthermore, the calc-alkalic primitive liquid
L1 plays a role in the production of all calc-alkalic
Tatsumi & Hanyu (2003).
yKogiso et al. (1997) and Aizawa et al. (1999).
zH2O in amphibolite and sediment is assumed to be 15 wt %.
xTwenty per cent sediment fluid contribution.
magmas via cryptic mixing (Fig. 16). On the other hand,
a magma with the composition of L1, containing Ca-rich
and low 87Sr/86Sr (07034) plagioclase phenocrysts, can
be identified in the most mafic basaltic andesite in Zao
volcano (Figs 16 and 20).
2003
JOURNAL OF PETROLOGY
VOLUME 49
0.7050
0.7025
0.7020
1
Primitive Melt
Bulk Rock
amph
Slab-derived
fluid
5.0
0.005
0.003
0.002
Y
amph
Rb
plag
f =0.001
Original
mantle
wedge
10
102
103
Tholeiitic/Calc-alkalic
0.7030
0.02
0.01
F=0.4
0.2
0.1
0.05
87Sr/86Sr
0.7035
NOVEMBER 2008
10.0
0.7045
0.7040
NUMBER 11
Tholeiitic
Calc-alkalic
104
105
106
Sr (ppm)
Fig. 18. Compositions of magmas produced by different contributions
from slab-derived fluid (f) and different degrees of partial melting (F).
The geochemical characteristics of tholeiitic and calc-alkalic primary
magmas in Zao volcano can be explained by different contributions
from slab-derived fluids (1% vs 02%) and different degrees of melting (40% vs 7%).
Pb
3+
1+
2+
Sm
1.0
Eu
Nd
Pr
Ce
K
0.5
La Sr
Ba
4+
CONC LUSIONS
It is generally accepted that the differentiation of calcalkalic magmas involves variable contributions from crustal components; for example, via wall-rock assimilation, or
mixing with crust-derived felsic magma (Eichelberger,
1975; DePaolo, 1981; Sakuyama, 1981; Hildreth &
Moorbath, 1988; Clynne, 1999; Dungan & Davidson, 2004;
Tatsumi & Kogiso, 2003), whereas tholeiitic magmas show
more pristine mantle signatures. The hypothesis presented
here, which proposes a crustal origin for the tholeiitic
magmas and a mantle origin for the calc-alkalic basaltic
magmas, requires that these models be revised and even
in some cases discarded.
One aspect that we need to re-examine concerns the
geochemical characteristics of the ‘mantle-derived’ basalt
magmas that are used to understand the contribution
of slab-derived components to arc magma generation. To
minimize the effect of shallow-level crustal contamination
and to assess the magma source characteristics, basaltic
rocks and/or tholeiitic rocks tend to be examined (e.g.
Notsu, 1983; Sakuyama & Nesbitt, 1986; Shibata &
Nakamura, 1997; Kimura & Yoshida, 2006). We suggest
that the tholeiitic basalt magmas in Zao volcano are
derived from melting of mafic lower crust via underplating
of calc-alkalic, mantle-derived basalt magmas and subsequent crustal anatexis. If so, then the tholeiitic basalt,
although it is relatively primitive, does influence the geochemical signatures of Quaternary mantle-derived arc
magmas, contributing to the production of calc-alkalic
magmas. However, only by examining the phenocryst
phases that crystallized from the least differentiated
0.1
60
80
100
120
140
160
Ionic Radius (pm)
Fig. 19. Relative abundances of elements between tholeiitic and
calc-alkalic primitive magmas in Zao volcano as a function of
ionic radius. The tholeiitic magma is more depleted in elements that
are likely to be partitioned into plagioclase (plag) and possibly
amphibole (amph) than the calc-alkalic basalt magma, suggesting
the presence of these phases in the melting residue of the tholeiitic
magma.
mantle-derived magmas can the effect of shallow-level
magma mixing processes on the mantle signatures be
hinted at. Therefore, analysis and examination of the compositions of minerals that crystallize from the primitive
calc-alkalic basalt magma could provide the only chance
to fully understand the geochemical characteristics of a
mantle-derived magma, and hence the source mantle and
slab-derived components.
AC K N O W L E D G E M E N T S
We thank Bogdan Vaglarov for analytical assistance, Miki
Fukuda for preparing the manuscript and figures, and
Richard Price, Bruce Charlier, Gene Yogodzinski, Alex
Nichols, and the editor John Gamble for their critical and
constructive comments on the manuscript. This work is
partially supported by Grant-in-Aid for Creative
Scientific Research (19GS0211).
2004
TATSUMI et al.
CALC-ALKALIC VS THOLEIITIC SERIES
Tholeiitic
Calc-alkalic
Basaltic Andesitic
Andesitic Basaltic
Mixing
Crystallization
Contamination
L5
Partially Molten
Hot Zone
L4
Crystallizing
Magma Reservoirs
Lower-T Partial Melt L3
+ Restite
Higher-T Partial Melt L2
+ Restite
Underplating & Crystallizing
Basaltic Magma (L1)
that causes crustal anatexis
Upper Mantle
Contribution of melt
Contribution of melt+crystal (magma)
Fig. 20. A schematic model for the magma plumbing system and the magma differentiation process in Zao volcano. The compositional
characteristics of melts (L1 to L5) contributing to tholeiitic and calc-alkalic magmas have been given in Figs 13 and 16a and Table 10.
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