JOURNAL OF PETROLOGY
VOLUME 46
NUMBER 8
PAGES 1543–1563
2005
doi:10.1093/petrology/egi025
Petrogenesis of Mafic Inclusions in Rhyolitic
Lavas from Narugo Volcano, Northeastern
Japan
MASAO BAN*, KOJI TAKAHASHI, TAKEHIRO HORIE AND
NARUHISA TOYA
DEPARTMENT OF EARTH AND ENVIRONMENTAL SCIENCES, YAMAGATA UNIVERSITY,
4-12 KOJIRAKAWA-MACHI 1-CHOME, YAMAGATA 990-8560, JAPAN
RECEIVED MARCH 1, 2003; ACCEPTED FEBRUARY 11, 2005
ADVANCE ACCESS PUBLICATION MARCH 18, 2005
Mafic inclusions present in the rhyolitic lavas of Narugo volcano,
Japan, are vesiculated andesites with diktytaxitic textures mainly
composed of quenched acicular plagioclase, pyroxenes, and interstitial glass. When the mafic magma was incorporated into the silicarich host magma, the cores of pyroxenes and plagioclase began to
crystallize (>1000 C) in a boundary layer between the mafic and
felsic magmas. Phenocryst rim compositions and interstitial glass
compositions (average 78 wt % SiO2) in the mafic inclusions are the
same as those of the phenocrysts and groundmass glass in the host
rhyolite. This suggests that the host felsic melt infiltrated into the
incompletely solidified mafic inclusion, and that the interstitial melt
composition in the inclusions became close to that of the host melt
(c. 850 C). Infiltration was enhanced by the vesiculation of the
mafic magma. Finally, hybridized and density-reduced portions of
the mafic magma floated up from the boundary layer into the host
rhyolite. We conclude that the ascent of mafic magma triggered the
eruption of the host rhyolitic magma.
Fine-grained mafic inclusions are common in andesitic to
rhyolitic rocks from subduction-related tectonic settings
(e.g. Eichelberger, 1980; Bacon, 1986; Koyaguchi,
1986a). These inclusions appear as clots or blobs ranging
from a few millimeters to over 1 m in diameter. Many
researchers have concluded, on the basis of features such
as ellipsoidal shapes, chilled textures preserved in groundmass minerals and high vesicularity, that these inclusions
are the quenched products of mafic magma in contact
with a cooler and more silicic host magma (e.g.
Eichelberger, 1980; Bacon, 1986). The process that produces such incompletely mixed magmas is commonly
called ‘magma mingling’ and is distinguished from processes that produce well-mixed, blended magma, called
‘hybridization’ (e.g. Murphy et al., 2000). Practically,
when magma mingling is the dominant process, smallscale mixing (hybridization) can also take place, especially
around the boundary area between the end-member
magmas (Bacon, 1986).
Among the many mechanisms proposed to create
mingled or hybrid magmas, the following three mechanisms are the most likely to result in the formation of mafic
inclusions: (1) forcible injection of mafic magma into a
cooler, felsic magma chamber (Campbell & Turner,
1989; Pallister et al., 1992; Nakamura, 1995a); (2) turbulent mixing of contrasting magmas during eruption of
zoned magma chambers (Koyaguchi, 1985; Blake &
Ivey, 1986; Cioni et al., 1995); (3) flotation of vesiculated
mafic magma up to the top of a more silica-rich magma
body (Eichelberger, 1980). Recently, some studies have
suggested that the mechanism might change temporally
with evolution of the magma chamber system (Feeley
et al., 1998; Murphy et al., 2000). Murphy et al. (2000)
proposed a scenario in which mafic inclusions are originally emplaced as fragments of disrupted dykes that
intrude into a highly crystalline, lower-temperature,
magma body; subsequently, as the temperature and
*Corresponding author. Telephone: þ81-23-628-4642. Fax: þ81-23628-4661. E-mail: [email protected]
# The Author 2005. Published by Oxford University Press. All
rights reserved. For Permissions, please e-mail: journals.permissions@
oupjournals.org
mafic inclusion; stratified magma chamber; magma
mixing; mingling; Narugo volcano; Japan
KEY WORDS:
INTRODUCTION
JOURNAL OF PETROLOGY
an
200km
130°E
NUMBER 8
AUGUST 2005
40°N
Osore V.R.
an
a
ap
Pacific Oce
Se
J
of
VOLUME 46
35°N
Hakkoda
-Towada V.R.
40°N
140°E
Sea of
Japan
Sengan V.R.
Kurikoma
-Onikobe V.R.
Narugo
volcano
Zao-Funagata
V.R.
Pacific
Ocean
Bandai-Adatara
V.R.
Aizu V.R.
Quaternary volcano
Quaternary ignimbrite
0
38°N
50km
140°E
Fig. 1. Locality map of Narugo volcano. Distribution of volcanic regions after Umeda et al. (1999). V.R., volcanic region.
mobility of the host magma increases, the mafic magma
floats as ‘blobs’. From these studies, it appears that the
mechanisms that form mafic inclusions can be varied,
depending on specific circumstances.
In Narugo volcano, 1–2% mafic inclusions are present
in most of the rhyolitic lavas; these are usually round in
form, up to 70 cm in diameter, and vesicular. The inclusions are andesitic in composition (55–60 wt % SiO2),
with acicular plagioclase and pyroxene microlites, and
interstitial glass in the groundmass; some inclusions have
a minor amount of phenocryst quartz. Groundmass
glasses in both the inclusions and the host lavas are well
preserved. We have carefully examined these glasses in
both the mafic inclusions and the hosts, as well as the textures, mineral compositions and whole-rock compositions.
We describe the solidification sequences accompanying
the formation of the mafic inclusions, and provide constraints for the timing of solidification of the inclusions
based on glass compositions as well as other petrological data.
ERUPTIVE SEQUENCE OF NARUGO
VOLCANO
Narugo volcano, situated in the central part of the Nasu
volcanic zone (Figs 1 and 2) is one of 17 active volcanoes
in the northeastern Japan arc. According to Sakaguchi &
Yamada (1988) and Soda & Yagi (1991), the activity of
Narugo began with eruptions of pyroclastic flows: the
Nizaka pyroclastic flow (c. 73 ka) and the Yanagizawa
1544
BAN et al.
SOLIDIFICATION PROCESS OF MAFIC INCLUSIONS
Fig. 2. Geological sketch map and simplified stratigraphy of Narugo volcano. , Sample localities.
pyroclastic flow (c. 45 ka). A large caldera, c. 5 km in
diameter, was formed by these paroxysmal eruptions.
Lake deposits found in the caldera indicate that water
ponded in the caldera after c. 45 ka. Afterwards, lava
flows and lava domes were formed in the inner part of
the caldera. These lavas can be divided into three units:
Ogatake–Kurumigatake lava dome, Matsugamine lava,
and Toyagamori lava. The Toyagamori lava is younger
than 11 830 555 years BP based on a 14C date of a
buried tree (Omoto, 1993). Craters observed on the lava
surface were formed by phreatic eruptions. One of these is
regarded as a historical event (AD 837) (Murayama, 1978).
SAMPLES AND ANALYTICAL
PROCEDURES
Samples were collected from the lava flows and lava
domes of the three units of Narugo volcano.
Whole-rock major element and selected trace element
analyses were made on fused disks by X-ray fluorescence
spectrometry (XRF) (Rigaku Rix2000) at Yamagata
University. The glass disks were prepared using the
method of Yamada et al. (1995). The calibration method
followed that of Yamada et al. (1995) for major and trace
elements. The standards used in the analyses are the
Geological Survey of Japan (GSJ) igneous rock series.
Analytical uncertainties for XRF trace elements are
<5% for Nb, Zr, Y, Sr, Rb, and Ni; <10% for V and
Cr; 5–15% for Ba. The range of uncertainties for a single
element is based on the concentration range observed in
the standards.
Mineral analyses were carried out with a JEOL 8600SS
electron microprobe at Yamagata University using a
wavelength-dispersive technique. Operating conditions
were 15 kV accelerating voltage, 10 nA (plagioclase and
glasses) to 20 nA (pyroxenes) beam current, and 8–20 s
counting time for each element. Glasses were analyzed
using a 10 mm defocused beam. All analyses were corrected using the oxide ZAF method.
SAMPLE DESCRIPTIONS AND
PETROGRAPHY
The petrographic characteristics of rocks from Narugo
volcano are summarized in Table 1. Sixty-two samples
were studied using a photomicroscope, and point counting to determine mineral modes was performed for
40 samples. Most samples are porphyritic rhyolites with
1545
JOURNAL OF PETROLOGY
VOLUME 46
Table 1: Modal proportions (vol. %) of phenocrysts in lavas
and their mafic inclusions from Narugo volcano
Average (number
Phenocryst (vol. %)
of samples) and
representative
samples
qtz
plg
opx
cpx
ox
0.3
0.3
14.5
11.8
1.3
1.6
0.8
0.7
0 .8
0 .9
T-3
0.1
0.3
13.2
14.6
1.6
1.5
0.3
0.3
0 .9
0 .8
T-4
0.1
15.9
1.4
1.0
0 .6
2.5
3.0
14.2
14.4
1.4
1.3
0.5
0.6
0 .5
0 .4
M-3
2.1
2.5
12.0
14.9
1.4
1.4
0.5
0.9
0 .6
0 .5
M-4
2.6
14.8
1.5
0.5
0 .4
OgatakeKurumigatake lava dome
Average (n ¼ 24)
1.5
21.5
.
OK-1
17
18.7
1.6
2.1
1.1
1.0
1 .0
1 .5
Toyagamori lava
Average (n ¼ 12)
T-1
T-2
Matsugamine lava
Average (n ¼ 14)
M-1
M-2
OK-3
1.6
1.2
17.4
23.5
1.6
2.0
0.6
1.1
1 .0
1 .6
OK-4
1.4
20.2
1.4
0.8
0 .8
trace
n.d.
n.d.
n.d.
n.d.
OK-2
NUMBER 8
AUGUST 2005
plagioclase, quartz, and Fe–Ti oxides. Clinopyroxene
and orthopyroxene phenocrysts (1 mm), and plagioclase phenocrysts (3 mm) are subhedral to euhedral
in shape. Most of the plagioclase phenocrysts are clear,
but some plagioclase phenocrysts have a dusty zone
(e.g. Tsuchiyama, 1985), or patchy zoning (Vance,
1965). Both types show oscillatory zoning, but the range
of compositions between zones is usually within 10 mol %
anorthite component. Quartz (2 mm) phenocrysts
always have a resorbed margin. The modal amounts of
phenocrystic clinopyroxene, orthopyroxene, and Fe–Ti
oxides are similar in each of the three geological
units. However, the modes of phenocrystic plagioclase
and quartz are variable between units. Phenocrystic
plagioclase ranges from 17 to 25% in the Ogatake–
Kurumigatake lava dome, and from 11 to 16% in the
Matsugamine and Toyagamori lavas. Quartz ranges
from 2 to 3% in the Matsugamine lava, from 1 to 2%
in the Ogatake–Kurumigatake lava dome, and is <05%
in the Toyagamori lava. The groundmass is mainly composed of glass with a hyalopilitic texture. Tiny Fe–Ti
oxide grains are scattered throughout glass. The amount
of Fe–Ti oxides is greater in the reddened type than in
the gray type. Minor amounts of acicular to prismatic
clinopyroxene, orthopyroxene and plagioclase can be
seen. In the groundmass of some samples, perlitic textures
or flow structure are observed. The glasses are clear in the
gray type and are usually reddish to brownish in the
reddened type.
Mafic inclusions
Mafic inclusions
qtz, quartz; plg, plagioclase; opx, orthopyroxene; cpx,
clinopyroxene; ox, FeTi oxides; n.d., not detected.
15–30 vol. % phenocrysts; some are weakly vesiculated. Two lithological types of mafic inclusion can be recognized within the rhyolites. One is gray to dark gray and
the other is reddened. These mafic inclusions are usually
in rounded form and up to c. 70 cm in diameter; they are
moderately vesiculated, and occur in most of the lavas.
Mafic inclusions that are included in gray to dark gray
hosts are gray to whitish gray in color, whereas those that
are reddened are included in reddened hosts. Some of the
dark gray rhyolites from the Toyagamori lava are highly
glassy in nature and weakly vesiculated.
Silicic hosts
The hosts to the inclusions are rhyolitic in composition
(705–75 wt % SiO2). These samples have porphyritic
textures with 15 to 30 vol. % phenocrysts. Photomicroscope images of the representative gray and reddened
hosts are shown in Fig. 3a and b. The phenocryst
assemblage consists of clinopyroxene, orthopyroxene,
The mafic inclusions have a diktytaxitic texture and are
andesitic in composition (55–60 wt % SiO2). Photomicrographs of representative gray and reddened inclusions are
shown in Fig. 3c and d. Subspherical vesicles are abundant. The mafic inclusions only rarely have phenocrysts
and these are solely of quartz. Quartz phenocrysts
(15 mm) occur only in the mafic inclusions within the
Ogatake–Kurumigatake lava dome, and are subhedral
in shape. These crystals are larger than the groundmass
minerals but sometimes include some of the groundmass
phases, indicating that some quartz crystallized contemporaneously with the groundmass minerals. The groundmass is composed of acicular minerals (clinopyroxene,
orthopyroxene, olivine, and plagioclase), granular
Fe–Ti oxides, and interstitial silica minerals and glass.
Groundmass olivine is rare and found only in the
Toyagamori lava. Some of the groundmass crystals
have dendritic textures. Tiny acicular plagioclase and
pyroxene grains and tiny granular Fe–Ti oxide grains
are more frequently found in the reddened type than in
the gray type. Relatively larger granular Fe–Ti oxides are
found in the reddened type. The volume of interstitial
glasses varies from sample to sample and is c. 10–15%.
1546
BAN et al.
SOLIDIFICATION PROCESS OF MAFIC INCLUSIONS
Fig. 3. Photomicrographs (in plane-polarized light) of representative host rocks and their mafic inclusions from Narugo volcano. (a) Gray host;
(b) reddened host; (c) gray inclusion; (d) reddened inclusion. gl, glass; pl, plagioclase; qtz, quartz; opx, orthopyroxene.
Usually vesicles can be found in glass of the inclusion.
The volume percent of vesicles varies between samples
from c. 10 to 25 vol. %. Glasses in the gray type of
inclusion are usually clear and sometimes brownish.
On the other hand, glasses in the reddened inclusions
look dirty because most of the glass is speckled with tiny
indistinguishable grains, most of which are probably
magnetite.
WHOLE-ROCK COMPOSITIONS
Representative whole-rock analyses of host lavas and
mafic inclusions are given in Table 2. The host lavas
and mafic inclusions belong to the low-K calc-alkaline
series according to the classification scheme of Gill
(1981) and to the medium-Fe suite of Arculus (2003)
(Fig. 4).
Major oxide and trace element variation diagrams are
shown in Figs 5 and 6. The silica contents of the host
rocks are in the range 705–75 wt %. In most diagrams,
the compositions of the host rocks define linear trends.
MgO, FeO*, CaO, Al2O3, TiO2, MnO, P2O5, Sr and V
decrease with increasing SiO2 content, whereas K2O,
Na2O, Rb, Ba, Nb, Zr and Y increase. Data are scattered
in the Cr diagram; Cr contents tend to decrease with
increasing SiO2 content. Ni contents are low and show no
systematic variation with respect to SiO2 content. The
bulk-rock silica content of each geological unit is distinct:
705–715 wt % for the Ogatake–Kurumigatake lava
dome; 73–75 wt % for the Toyagamori lava; 715–
725 wt % for the Matsugamine lava.
The bulk-rock silica contents of the mafic inclusions are
in the range 55–60 wt %. In most diagrams, the data for
the mafic inclusions define linear trends. MgO, FeO*,
CaO, Al2O3, TiO2, MnO, P2O5 and V decrease with
increasing SiO2 content, whereas K2O, Na2O, Rb, Ba,
Nb, Zr and Y increase. Sr contents slightly decrease with
increasing silica content. Data are slightly scattered in the
Cr and Ni diagrams. However, the Cr and Ni contents
tend to decrease with increasing silica content. Extrapolations of the mafic inclusion trends to high-SiO2 contents
do not coincide with the host rock trends in some
diagrams (e.g. MgO, Na2O vs SiO2 diagrams in Fig. 5,
and Cr, Ni vs SiO2 diagrams in Fig. 6).
1547
JOURNAL OF PETROLOGY
VOLUME 46
NUMBER 8
AUGUST 2005
Table 2: Selected chemical analyses of bulk-rocks from Narugo volcano
Unit:
Toyagamori lava host
Matsugamine lava host
OgatakeKurumigatake l.d. host
Sample:
T-1
T-2
T-3
T-4
M-1
M-2
M-3
M-4
OK-1
OK-2
OK-3
OK-4
G or R:
G
R
G
G
G
R
G
R
G
G
R
R
TiO2
71.47
0.41
71.51
0.42
71.42
0.41
71.10
0.41
72.09
0.37
72.03
0.38
72.95
0.35
73.30
0.39
70.17
0.48
70.15
0.47
70.20
0.45
70.20
0.45
Al2O3
FeO*
14.28
3.11
14.55
3.05
14.35
3.11
14.43
3.07
14.24
2.45
14.04
2.73
14.08
2.57
14.15
2.48
14.95
3.22
14.58
3.56
15.24
3.42
15.06
3.29
MnO
0.09
0.87
0.09
0.83
0.09
0.87
0.09
0.85
0.08
0.68
0.08
0.70
0.08
0.68
0.08
0.72
0.09
1.07
0.10
1.07
0.11
1.04
0.09
0.90
3.59
4.36
1.16
3.57
4.30
1.07
3.60
3.46
4.38
1.17
3.09
4.40
1.25
3.08
4.35
1.25
3.68
4.04
1.06
4.03
4.23
1.08
3.86
4.34
1.23
3.27
4.32
1.22
3.24
4.36
1.17
4.14
1.00
3.75
4.18
1.05
0.09
99.43
3.57
0.08
0.09
0.06
0.08
99.01
3.61
98.65
3.60
99.67
3.78
98.80
3.00
0.10
99.37
3.33
0.10
99.47
3.57
0.06
99.86
3.44
0.06
99.47
3.67
0.07
98.84
3.90
0.04
Total
FeO*/MgO
99.52
3.29
99.07
3.66
V
39
44
43
n.d.
30
33
34
32
49
54
53
49
Cr
3
5
4
n.d.
6
4
1
5
3
6
5
3
Ni
0
0
0
n.d.
0
0
0
0
0
0
0
0
SiO2
MgO
CaO
Na2O
K2O
P2O5
0.07
Rb
27
21
27
n.d.
27
28
30
28
23
25
22
23
Sr
200
204
201
n.d.
198
186
187
182
198
209
209
215
Y
31
30
30
n.d.
31
31
32
31
33
30
28
27
Zr
126
128
125
n.d.
133
134
134
134
121
119
124
119
Nb
5
4
5
n.d.
4
5
4
5
4
4
4
4
Ba
328
327
340
n.d.
355
354
353
387
300
315
317
320
Toyagamori lava inclusion
M.l. inclusion
O-K. l.d. inclusion
Unit:
Sample:
T-R1
T-R2
T-R3
T-R4
T-G1
T-G2
T-G3
T-G4
M-G1
M-G2
OK-R1
OK-R2
G or R:
R
R
R
R
G
G
G
G
G
G
R
R
TiO2
57.37
1.01
54.77
1.08
55.00
1.02
54.78
1.06
59.09
0.87
59.29
0.81
56.74
0.91
58.36
0.83
55.41
0.94
55.38
0.93
56.42
0.99
56.06
0.98
Al2O3
FeO*
16.49
9.17
16.72
10.28
17.15
10.07
16.88
10.26
16.59
8.10
16.48
7.88
16.98
8.96
16.90
8.05
17.09
9.43
17.07
9.40
16.56
9.84
16.53
9.87
MnO
0.19
3.42
0.20
4.07
0.20
4.16
0.20
4.20
0.17
2.87
0.16
3.24
0.18
3.82
0.17
3.34
0.19
4.28
0.19
4.27
0.19
3.93
0.18
3.88
7.57
3.29
8.24
2.81
8.17
2.98
8.41
2.90
7.12
3.38
6.90
3.47
7.51
3.15
7.52
3.28
8.24
2.93
8.25
2.94
7.88
2.82
8.22
2.92
0.37
0.15
0.39
0.13
0.32
0.12
0.32
0.13
0.58
0.13
0.69
0.11
0.50
0.12
0.52
0.13
0.36
0.16
0.36
0.16
0.42
0.09
0.40
0.11
99.03
2.68
98.69
2.52
99.19
2.42
99.14
2.44
98.90
2.82
99.03
2.43
98.87
2.35
99.10
2.41
99.03
2.20
98.95
2.20
99.14
2.50
99.15
2.54
SiO2
MgO
CaO
Na2O
K2O
P2O5
Total
FeO*/MgO
V
209
259
248
238
174
183
216
166
230
234
254
262
Cr
5
9
9
9
3
12
15
11
14
18
10
12
Ni
0
2
3
3
0
3
4
5
10
16
2
2
Rb
7
8
5
5
14
15
11
9
8
7
10
9
1548
BAN et al.
Unit:
SOLIDIFICATION PROCESS OF MAFIC INCLUSIONS
Toyagamori lava inclusion
M.l. inclusion
O-K. l.d. inclusion
Sample:
T-R1
T-R2
T-R3
T-R4
T-G1
T-G2
T-G3
T-G4
M-G1
M-G2
OK-R1
OK-R2
G or R:
R
R
R
R
G
G
G
G
G
G
R
R
Sr
269
265
277
274
268
254
265
266
274
275
247
246
Y
26
23
26
24
27
25
23
24
23
24
28
26
Zr
64
55
58
53
71
75
64
69
62
61
66
72
Nb
3
2
2
2
3
2
2
3
2
2
2
2
Ba
154
124
140
115
188
209
158
185
142
134
144
153
*Total
Fe reported as FeO.
l.d., lava dome; M.l., Matsugamine lava; O-K. l.d., Ogatake-Kurumigatake lava dome; G, gray; R, reddened; n.d., not
determined.
6.0
-K
ium w-K
d
e
m
lo
1.5
FeO*/MgO
K2O,wt%
2.0
1.0
0.5
0.0
50
55
60
65
70
75
medium-Fe
2.0
low-Fe
55
60
65
70
75
80
SiO2,wt%
Matsugamine L
SiO2,wt%
Toyagamori L
host
mafic inclusion(gray)
mafic inclusion(reddened)
CA
4.0
0.0
50
80
TH
high-Fe
host
mafic inclusion(gray)
Ogatake-Kurumigatake LD
host
mafic inclusion(reddened)
Fig. 4. K2O vs SiO2 and FeO*/MgO vs SiO2 diagrams for rocks from Narugo volcano. The boundaries defining the low-K and medium-K fields
are from Gill (1981), and that between TH (tholeiitic) and CA (calc-alkalic) fields is from Miyashiro (1974). Boundaries between low-, mediumand high-Fe suites are quoted from Arculus (2003). FeO* is total iron calculated as FeO.
MINERAL COMPOSITIONS
Representative chemical compositions of clinopyroxene,
orthopyroxene, plagioclase, olivine and Fe–Ti oxides in
both hosts and mafic inclusions are given in Tables 3
and 4. The compositions of pyroxenes in both hosts and
inclusions are illustrated in terms of their position in the
pyroxene quadrilateral for each unit in Fig. 7.
The 100 Mg/(Mg þ Fe) (Mg-value) of most of
the phenocryst orthopyroxene cores, as well as rim
compositions, in the host rocks is between 58 and 64
in each sample. Orthopyroxene phenocrysts with higher
Mg-value cores are rarely found, and those that show
apparent reverse zoning in Mg-value are uncommon.
Mg-values of most of the groundmass orthopyroxene in
the host rocks are 58–64. However, minor proportions of
phenocrysts with lower Mg-values ( 56) are also present.
The Wo contents of orthopyroxene phenocryst core, rim
and groundmass are <3 mol %. Although small differences can be seen, the core, rim and groundmass orthopyroxene compositions of the host rocks are similar
between units.
In the mafic inclusions, the Mg-values of the cores
compositions of the groundmass orthopyroxene are
higher (65–78). The Mg-values of orthopyroxene rims
are lower, ranging from 58 to 64; this is similar to the
range for the phenocryst and groundmass orthopyroxene
in the host rocks. The Wo content of groundmass cores
is c. 3–4 mol %, and is higher than that of the rims
(c. 2–3 mol %). The core and rim of groundmass
orthopyroxene compositions in the mafic inclusions are
similar between units as well as between gray and
reddened types.
1549
JOURNAL OF PETROLOGY
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wt.%
NUMBER 8
AUGUST 2005
wt.%
6
8
MgO
4
6
Na2O
4
2
2
0
50
55
60
65
70
10
75
80
85
FeO*
50
4
2
host rocks
50
55
60
65
70
22
75
80
85
Al2O3
18
50
60
65
70
80
85
80
85
glass in grey type
low-K glass
in
K2O reddened type
55
60
65
70
75
TiO2
1.0
14
75
high-K glass
in reddened type
6
6
2
55
0.5
10
50
55
60
65
70
8
75
80
85
CaO
6
50
55
60
65
70
80
85
MnO
0.2
4
75
0.1
2
50
55
60
65
70
75
80
85 50
SiO2,wt%
Toyagamori L
55
60
65
70
75
80
85
SiO2,wt%
host
mafic inclusion(gray)
mafic inclusion(reddened)
groundmass glass in host
interstitial glass in inclusion
host
mafic inclusion(gray)
groundmass glass in host
interstitial glass in inclusion
host
mafic inclusion(gray)
groundmass glass in host
interstitial glass in inclusion
Matsugamine L
Ogatake-Kurumigatake LD
Fig. 5. SiO2 variation diagrams showing abundances of major oxides for rocks and glasses from Narugo volcano.
Clinopyroxene in the host lavas is augite; compositions are similar among phenocryst cores, rims and
groundmass (Wo43En41). The Mg-values of the core
compositions are around 70 (68–74). Although small
differences are present, the core, rim and groundmass
clinopyroxene compositions of the host rocks are similar
between units.
Mg-values of clinopyroxene core and rim compositions
in mafic inclusions are around 70 (65–75), which is
similar to the phenocryst clinopyroxene composition of
1550
BAN et al.
SOLIDIFICATION PROCESS OF MAFIC INCLUSIONS
ppm
450
400
Ba
300
300
150
200
0
50
30
55
60
65
70
75
ppm
100
80 50
Rb
Sr
55
60
65
70
20
75
80
Cr
20
10
10
0
50
150
55
60
65
70
75
80
Zr
0
50
20
60
65
70
75
80
Ni
15
100
10
50
0
50
40
55
5
55
60
65
70
75
0
80 50
300
30
55
60
65
70
75
80
V
200
20
Y
10
0
50
55
60
65
70
75
100
0
50
80
55
60
65
70
75
80
SiO2,wt%
6
4
Nb
2
0
50
55
60
65
70
75
80
SiO2,wt%
Fig. 6. SiO2 variation diagrams showing abundance of trace elements for the rocks from Narugo volcano. Symbols are as in Fig. 4.
the host rocks. The Wo contents of rims tend to be higher
(40–44 mol %) than those of cores (35–43 mol %). The
rim compositions are similar to those of the clinopyroxene in the host rocks.
Using the two-pyroxene thermometer of Lindsley
(1983), the magmatic temperature deduced from
core compositions of clinopyroxene and orthopyroxene phenocrysts in the host rhyolites is about
1551
JOURNAL OF PETROLOGY
VOLUME 46
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AUGUST 2005
Table 3: Representative chemical compositions of orthopyroxene and clinopyroxene in rocks from Narugo volcano
Orthopyroxene
Unit:
Toyagamori lava
Matsugamine lava
OgatakeKurumigamine l.d.
Sample:
host
host
inc.
inc.
host
host
inc.
inc.
host
host
inc.
inc.
Position:
ph-c
ph-r
gm-c
gm-r
ph-c
ph-r
gm-c
gm-r
ph-c
ph-r
gm-c
gm-r
TiO2
52.70
0.55
52.88
0.45
53.35
0.25
52.59
0.34
52.70
0.57
52.47
0.54
54.37
0.20
52.85
0.14
53.18
0.45
53.54
0.53
52.51
0.30
53.66
0.10
Al2O3
FeO*
0.13
23.69
0.15
23.60
1.91
17.15
2.56
22.58
0.11
22.65
0.13
23.75
1.68
16.30
1.11
22.42
0.14
23.20
0.10
23.02
2.80
17.20
0.50
22.78
MnO
1.26
21.09
1.27
21.09
0.63
24.29
0.87
20.74
1.35
20.95
1.37
20.14
0.52
25.41
1.18
20.60
1.25
21.25
1.31
21.41
0.46
24.43
1.07
21.50
1.07
0.09
0.97
0.00
2.00
0.00
1.38
0.04
1.20
0.00
0.94
0.02
1.97
0.05
1.27
0.02
1.04
0.04
1.04
0.00
2.07
0.05
0.99
0.03
0.03
100.61
0.03
100.44
0.05
99.62
0.00
101.09
0.00
99.53
0.00
99.36
0.00
100.51
0.00
99.60
0.01
100.55
0.00
100.95
0.06
99.87
0.00
100.61
SiO2
MgO
CaO
Na2O
Cr2O3
Total
(O ¼ 6)
Si
Ti
Al
Fe
Mn
Mg
Ca
Na
Cr
Total
Mg-value
1.976
0.016
1.983
0.013
1.955
0.007
1.945
0.009
1.988
0.016
1.992
0.015
1.964
0.005
1.985
0.004
1.987
0.013
1.990
0.015
1.922
0.008
1.994
0.003
0.006
0.743
0.007
0.740
0.083
0.526
0.112
0.698
0.005
0.715
0.006
0.754
0.071
0.492
0.049
0.704
0.006
0.725
0.004
0.716
0.121
0.526
0.022
0.708
0.040
1.179
0.040
1.179
0.019
1.327
0.027
1.143
0.043
1.178
0.044
1.140
0.016
1.368
0.038
1.153
0.039
1.183
0.041
1.186
0.014
1.333
0.034
1.191
0.043
0.006
0.039
0.000
0.079
0.000
0.055
0.003
0.049
0.000
0.038
0.001
0.076
0.003
0.051
0.002
0.042
0.003
0.041
0.000
0.081
0.003
0.039
0.002
0.001
4.009
0.001
4.001
0.001
3.996
0.000
3.992
0.000
3.993
0.000
3.991
0.000
3.997
0.000
3.987
0.000
3.999
0.000
3.993
0.002
4.011
0.000
3.993
61
61
72
62
62
60
74
62
62
62
72
63
Clinopyroxene
Unit:
Toyagamori lava
Matsugamine lava
OgatakeKurumigamine l.d.
Sample:
host
host
inc.
inc.
host
host
inc.
inc.
host
host
inc.
inc.
Position:
ph-c
ph-r
gm-c
gm-r
ph-c
ph-r
gm-c
gm-r
ph-c
ph-r
gm-c
gm-c
51.94
0.90
49.44
0.88
51.22
0.64
51.85
1.62
52.36
0.92
49.64
1.02
52.18
0.42
0.21
4.52
10.27
0.34
3.12
9.56
0.33
0.15
9.74
5.11
1.51
10.19
0.57
52.50
0.74
52.17
1.15
50.22
0.79
52.85
0.30
52.18
1.33
0.15
10.07
0.66
0.18
10.16
0.63
4.47
9.91
0.29
9.28
0.34
1.19
10.14
0.40
0.57
10.45
0.55
14.29
21.39
14.09
21.47
13.80
19.33
14.10
20.87
14.24
21.41
13.70
21.32
14.01
19.94
0.42
13.99
20.26
0.21
0.01
Total
100.00
0.31
0.01
100.16
0.25
0.00
99.11
0.26
0.02
100.13
0.28
0.02
99.58
0.30
Cr2O
0.00
99.36
0.25
0.02
99.66
0.22
0.00
99.43
SiO2
TiO2
Al2O3
FeO*
MnO
MgO
CaO
Na2O
(O ¼ 6)
1552
10.06
0.52
0.66
10.23
0.47
13.70
21.20
13.95
21.63
14.25
18.88
13.98
20.68
0.30
0.07
99.65
0.30
0.00
99.70
0.37
0.00
99.96
0.31
0.00
99.85
BAN et al.
SOLIDIFICATION PROCESS OF MAFIC INCLUSIONS
Clinopyroxene
Unit:
Toyagamori lava
Sample:
host
host
inc.
inc.
host
host
inc.
inc.
host
host
inc.
inc.
Position:
ph-c
ph-r
gm-c
gm-r
ph-c
ph-r
gm-c
gm-r
ph-c
ph-r
gm-c
gm-c
Si
Ti
Al
Fe
Mn
Mg
Ca
Na
Cr
Total
Mg-value
Matsugamine lava
OgatakeKurumigamine l.d.
1.971
0.021
1.960
0.032
1.888
0.022
1.973
0.008
1.962
0.038
1.968
0.026
1.859
0.025
1.920
0.018
1.956
0.046
1.972
0.026
1.854
0.029
1.957
0.012
0.006
0.316
0.008
0.319
0.198
0.311
0.052
0.317
0.013
0.292
0.009
0.331
0.200
0.323
0.138
0.300
0.015
0.317
0.007
0.307
0.225
0.320
0.067
0.320
0.021
0.800
0.020
0.789
0.011
0.773
0.013
0.785
0.018
0.798
0.018
0.774
0.011
0.785
0.013
0.782
0.017
0.770
0.021
0.783
0.015
0.793
0.018
0.782
0.861
0.015
0.864
0.023
0.778
0.018
0.835
0.019
0.863
0.020
0.866
0.022
0.803
0.018
0.814
0.016
0.857
0.022
0.873
0.022
0.756
0.027
0.831
0.023
0.000
4.012
0.000
4.015
0.000
4.000
0.001
4.002
0.001
4.004
0.000
4.013
0.000
4.025
0.000
4.001
0.002
4.001
0.000
4.010
0.000
4.018
0.000
4.009
72
71
71
71
73
70
71
72
71
72
71
71
*Total
Fe reported as FeO.
ph, phenocryst; gm, groundmass; c, core; r, rim; inc., mafic inclusion; l.d., lava dome.
Fig. 7. Pyroxene compositions of representative rocks and mafic inclusions from Narugo volcano. Data are recalculated and plotted relative to
isotherms using the approach of Lindsley (1983).
800–850 C (Fig. 7). Magmatic temperatures deduced
from core compositions of clinopyroxene and orthopyroxene in mafic inclusions are higher than 1000 C,
whereas those based on rim compositions are about
850 C, which is similar to the temperature of the host
rhyolites (Fig. 7).
The Mg-values of groundmass olivine cores in the
mafic inclusions of the Toyagamori lava are 69–73, and
1553
JOURNAL OF PETROLOGY
VOLUME 46
NUMBER 8
AUGUST 2005
Table 4: Representative chemical compositions of plagioclase, olivine, and Fe–Ti oxides in rocks from Narugo volcano
Plagioclase
Unit:
Toyagamori lava
Sample:
host
host
inc.
inc.
host
host
inc.
inc.
host
host
inc.
inc.
Position:
ph-c
ph-r
gm-c
gm-r
ph-c
ph-r
gm-c
gm-r
ph-c
ph-r
gm-c
gm-r
SiO2
56.91
26.92
48.04
32.79
Al2O3
Fe2O3*
MgO
CaO
Na2O
K2O
Total
0.41
0.02
9.60
6.18
0.10
100.13
Matsugamine lava
56.30
46.64
55.76
55.61
56.82
27.26
0.45
33.21
0.73
27.28
0.44
27.49
0.41
26.34
0.37
0.03
9.66
0.05
17.30
0.03
10.14
0.03
9.86
0.02
9.17
5.95
0.12
99.76
1.35
0.00
99.29
5.40
5.75
6.28
0.23
99.29
0.10
99.24
0.13
99.12
OgatakeKurumigamine l.d.
54.98
56.22
56.10
0.74
28.16
0.40
27.44
0.33
27.26
0.39
49.43
31.60
0.71
56.30
26.59
0.44
0.08
16.49
0.06
10.74
0.02
10.36
0.02
9.77
0.08
15.37
0.04
9.39
1.98
5.32
0.01
99.67
5.68
0.10
100.16
5.94
0.02
100.15
0.14
99.64
2.59
0.06
99.83
5.93
0.28
98.97
(O ¼ 8)
Si
Al
Fe
Mg
Ca
Na
K
Total
An
2.554
1.424
0.014
2.537
1.448
0.015
2.160
1.813
0.025
2.527
2.520
2.573
2.201
2.486
1.457
0.015
1.469
0.014
1.406
0.012
1.771
0.026
1.500
0.014
0.001
0.462
0.537
0.002
0.466
0.520
0.004
0.858
0.122
0.002
0.002
0.001
0.006
0.004
0.493
0.475
0.479
0.506
0.445
0.552
0.810
0.176
0.520
0.466
0.005
4.998
0.007
4.995
0.000
4.982
0.013
0.006
0.008
0.001
0.001
4.981
4.994
4.997
4.990
4.991
46
47
88
50
48
44
Olivine
FeTi oxides
Unit:
Toyagamori lava
Toyagamori lava
Sample:
host
host
inc.
inc.
Position:
gm-c
gm-r
gm
gm
SiO2
FeO*
38.83
23.67
38.22
26.32
TiO2
MnO
0.38
37.73
0.14
0.56
Fe2O3
35.46
0.13
Cr2O3
0.06
100.82
0.03
100.72
MgO
CaO
NiO
Total
(O ¼ 4)
Si
Fe
Mn
Mg
Ca
Ni
Total
Mg-value
Al2O3
1.006
0.579
0.008
1.459
0.004
0.013
1.392
0.004
0.001
2.993
0.001
2.994
74
53
1.451
0.013
2.533
0.002
0.499
0.495
0.473
0.520
0.006
4.992
4.999
0.002
0.008
50
47
2.264
1.706
0.024
2.557
1.423
0.015
0.005
0.754
0.230
0.003
0.457
0.522
0.003
4.987
0.016
4.993
76
46
Matsugamine l.
O-K. l.d.
host
host
host
host
host
host
ph-c
ph-c
ph-c
ph-c
ph-c
ph-c
8.81
1.57
44.28
0.14
10.34
1.71
44.44
0.12
9.35
1.00
46.98
0.04
8.03
1.83
43.57
0.15
50.44
0.00
17.75
0.00
46.32
0.00
16.85
0.02
49.35
0.02
13.13
0.01
51.18
0.03
17.56
0.00
V2O3
0.61
0.34
0.42
0.24
0.45
0.33
37.81
0.31
36.02
0.76
37.28
0.65
0.31
35.18
0.89
0.38
FeO
37.02
1.04
36.96
1.59
35.89
0.67
34.57
0.88
0.96
100.51
1.70
100.97
1.62
98.33
2.16
99.97
1.03
99.18
1.97
100.92
1.36
99.42
2.05
99.10
MnO
MgO
1.007
0.513
82
2.526
1.453
0.011
Total
Xusp
Xilm
0.26
0.30
0.83
0.27
0.83
0.24
0.87
0.82
71
*Total Fe reported as Fe O for plagioclase and FeO for olivine.
2 3
Ferric iron, Xusp and Xilm calculated after Stormer (1983). ph, phenocryst; gm, groundmass; c, core; r, rim; inc., mafic
inclusion; l, lava; O-K. l.d., OgatakeKurumigamine lava dome.
1554
BAN et al.
SOLIDIFICATION PROCESS OF MAFIC INCLUSIONS
Fig. 8. Chemical compositions of plagioclase of representative host rocks and mafic inclusions from Narugo volcano.
the rim compositions are 67–73. These values are similar
to the Mg-values of the groundmass clinopyroxene cores.
Thus groundmass olivine appears to be in equilibrium
with the groundmass clinopyroxene core, as Obata et al.
(1974) showed that the Mg-value of olivine is nearly equal
to or slightly lower than that of clinopyroxene when these
minerals coexist in equilibrium.
The plagioclase compositions of both host lavas and
inclusions are shown in Fig. 8 as histograms of anorthite
content. The compositional range of the phenocryst cores
from the host rocks is An40–65 and the peak position is
always around An45–50. Plagioclase phenocrysts with
anorthite contents >An55 are rare, and are always of
the clear type. The subordinate amounts of dusty-type
plagioclases always have low An contents. The rim compositions are similar to those of the core composition, but
all compositions are <An55. The compositional ranges
and the peak positions are similar between units.
The groundmass plagioclases in the mafic inclusions
exhibit strong normal compositional zoning, with
core compositions of An65–90, and rim compositions of
An40–60. The larger crystals tend to have higher anorthite
contents.
Using the iron–titanium oxide thermobarometer
(Stormer, 1983), the magmatic temperature and oxygen
fugacity ( fO2) indicated by coexisting magnetite–
ulvöspinel and ilmenite–hematite solid solutions in the
Fig. 9. Fe–Ti oxide estimates of temperature and oxygen fugacity in
host rocks and mafic inclusion from Narugo volcano. HM, hematite–
magnetite; NNO, nickel–nickel oxide; FMQ, fayalite–magnetite–
quartz; WM, w€
ustite–magnetite.
host rhyolites are calculated to be c. 850 C and fO2
slightly lower than the nickel–nickel oxide (NNO) buffer
(Fig. 9). The data plot in the lower temperature area
of the field of island arc calc-alkaline rocks of Takahashi
et al. (1995). This temperature is consistent with that
obtained by pyroxene thermometry.
1555
JOURNAL OF PETROLOGY
VOLUME 46
NUMBER 8
AUGUST 2005
Table 5: Chemical compositions of groundmass glasses in host rocks and interstitial glasses in mafic inclusions from
Narugo volcano
Type
Note
SiO2
TiO2
Al2O3
FeO*
MnO
MgO
CaO
Na2O
K2O
P2O5
Total
Sample name
(bulk wt % SiO2)
Toyagamori lava, gl in host
gray
78.41
0.26
0.23
12.02
12.09
1.54
0.42
0.05
0.03
0.25
0.03
1.58
2.06
4.52
4.97
1.50
0.78
0.03
0.04
100.17
100.18
T-1 (71.47)
T-2 (71.51)
0.27
11.32
0.78
0.11
0.00
0.26
2.25
6.88
0.03
99.61
T-2 (71.51)
gray
78.36
0.28
0.28
11.60
11.61
1.49
1.48
0.07
0.07
0.25
0.22
1.49
1.53
4.19
4.17
1.34
1.57
0.04
0.06
99.40
99.36
T-G1 (59.0)
T-R1 (59.29)
reddened LK
reddened HK
79.81
76.76
0.21
0.20
11.50
11.64
0.56
1.17
0.03
0.06
0.04
0.06
2.16
0.58
4.70
2.96
0.50
5.59
0.03
0.00
99.54
99.02
T-R3 (55.00)
T-R3 (55.00)
reddened LK
79.96
0.17
11.42
0.50
0.04
0.03
1.90
4.90
0.76
0.02
99.71
T-R4 (54.78)
0.23
0.23
11.75
11.75
1.48
0.49
0.06
0.03
0.21
0.05
1.36
1.65
4.42
4.90
1.85
0.90
0.03
0.03
100.66
99.22
M-1 (73.11)
M-2 (73.33)
0.23
11.40
1.12
0.07
0.04
0.27
2.61
6.52
0.03
99.70
M-2 (73.33)
0.24
0.29
11.09
11.11
1.49
1.56
0.04
0.09
0.16
0.15
1.13
1.21
4.44
4.67
1.28
1.37
0.02
0.06
99.09
99.92
M-G1 (55.41)
M-G2 (55.38)
0.18
0.21
12.16
11.62
0.41
1.17
0.02
0.04
0.03
0.05
1.72
0.21
4.82
2.70
0.86
6.37
0.04
0.02
99.90
99.91
M-R6 (n.d.)
0.18
11.52
0.49
0.03
0.02
1.79
4.88
0.69
0.05
99.39
M-R6 (n.d.)
OgatakeKurumigatake lava dome, gl in host
reddened LK
79.87
0.22
11.93
.
.
reddened LK
79 87
0 22
11.93
0.34
0.34
0.01
0.01
0.02
0.02
1.87
1.87
4.40
4.40
0.73
0.73
0.01
0.01
99.42
99.42
OK-3 (70.20)
OK-3 (70.20)
0.35
0.55
0.02
0.02
0.02
0.02
1.53
0.08
4.76
1.87
0.59
7.67
0.01
0.01
99.09
99.23
OK-R1 (56.42)
OK-R1 (56.42)
0.37
0.53
0.02
0.04
0.04
0.12
1.65
0.49
4.67
3.05
0.71
4.96
0.02
0.02
99.30
100.26
OK-R2 (56.06)
OK-R2 (56.06)
reddened LK
reddened HK
79.54
77.72
Toyagamori lava, gl in inc.
gray
78.64
Matsugamine lava, gl in host
gray
79.28
reddened LK
reddened HK
79.20
77.41
Matsugamine lava, gl in inc.
gray
79.20
gray
reddened LK
reddened HK
reddened LK
79.39
79.66
77.50
79.73
OgatakeKurumigatake lava dome, gl in inc.
reddened LK
80.17
0.18
11.47
reddened HK
77.27
0.21
11.53
reddened LK
79.94
0.19
11.68
reddened HK
78.84
0.23
11.98
M-R6 (n.d.)
*Total
Fe reported as FeO.
gl, glass; inc., mafic inclusion; LK, low-K; HK, high-K; n.d., not determined. These analyses are averages of 10 points.
COMPOSITION OF GROUNDMASS
GLASSES
Representative compositions of groundmass glasses are
shown in Table 5. The SiO2 content of the groundmass
glass ranges from 76 to 80 wt %. In the gray inclusions,
the glass composition within the inclusions is very similar
to that of their host. On the other hand, the groundmass
glass composition of both reddened hosts and their inclusions is split into two groups. One group has c. 79–80 wt %
SiO2, but lower K2O (c. 07–09 wt %), higher Na2O
(c. 45–49 wt %), and higher CaO (c. 12–21 wt %)
contents. The other type has c. 76 wt % SiO2, higher
K2O (c. 50–78 wt %), lower Na2O (c. 20–30 wt %), and
lower CaO (c. 01–05 wt %). Back-scattered electron
images of representative occurrences of these two groups
are shown in Fig. 10. The ratio of the high-SiO2 to the
low-SiO2 type is approximately 8:2. The low-SiO2
glasses occur as isolated blebs in the high-SiO2 glasses
or as colloidal skins of perlitic glass particles (Fig. 10). The
average glass composition in the reddened type host and
its inclusions is similar to the glass composition of the gray
type host and inclusions. In most of the variation diagrams (e.g. Fig. 5), the compositions of the glass in both
the gray type host and its inclusions plot towards the
SiO2-rich extrapolation of the host-rock trends.
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BAN et al.
SOLIDIFICATION PROCESS OF MAFIC INCLUSIONS
100 m
100 m
100 m
100 m
10 m
100 m
Fig. 10. Back-scattered electron images showing occurrences of glasses in rocks from Narugo volcano. Glasses in gray type host and inclusion (a),
gray type inclusion (b), reddened type hosts (c and e), and reddened type inclusions (d and f ). gl, glass; gl(HK), high-K glass; gl(LK), low-K glass;
pl, plagioclase; qtz, quartz; px, pyroxene.
DISCUSSION
Explanation of host rock trends
As described above, the chemical compositions of both
phenocrysts and groundmass glass in the host rocks are
similar between units, whereas the bulk chemical compositions and modal characteristics of each unit are distinct. These data suggest that differences in the bulk-rock
compositions between units are controlled by the modal
proportions of phenocrysts in each unit.
Many workers have suggested that magma mixing
plays an important role in producing the compositional
variations of volcanic rocks (e.g. Sakuyama, 1981; Feeley
& Davidson, 1994; Ban & Yamamoto, 2002). Mafic
inclusions, which provide evidence for magma mingling,
are commonly observed in the Narugo lavas. When such
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VOLUME 46
mafic inclusions are incorporated, the composition of the
host magma must be modified to some degree by mixing
with the mafic magma. In the case of Narugo, however,
the lavas lack petrographic evidence for magma
mixing (hybridization) such as a disequilibrium phenocryst assemblages. Moreover, the features of the
whole-rock chemical trends of the Narugo lavas are not
consistent with a magma mixing process. If the compositional trends of the host rocks were created by differences
in the degree of mixing between mafic magma and the
host rhyolitic magma, the mafic inclusions should plot on
the extension of the host rock trends to lower SiO2 contents. However, on some variation diagrams (e.g. Figs 5
and 6), the mafic inclusions do not plot on the extensions
of the host lava trends. Thus, in the case of Narugo, we
consider that magma mingling took place but that
magma mixing was not important in producing variations in the bulk-rock chemical composition of the
host rocks.
In Fig. 12, estimated bulk phenocryst compositions,
based on the average phenocryst modal abundances
(Table 1) for each unit and the mineral composition
data reported in Tables 3 and 4, are shown. The
estimated bulk phenocryst compositions lie on the lowSiO2 extrapolation of the host rock trends. Thus it is
possible that the variations in whole-rock composition
between units were caused by differences in the
amount of the different phenocryst phases between the
units.
Possible processes to produce the chemical
trends among the mafic inclusions and
the compositional characteristics of the
glasses in both the mafic inclusions
and their hosts
It seems clear that the particular type of mafic inclusion
found in the Narugo lavas is the quenched product of
mafic magma that intruded into cooler felsic magma (e.g.
Eichelberger, 1980; Bacon, 1986; Koyaguchi, 1986a).
Features such as the ellipsoidal shapes of the inclusions
and the textural evidence for rapid crystallization of the
groundmass minerals are further evidence of quenching
(Eichelberger, 1980; Bacon, 1986).
Several processes can be considered that might have
influenced the bulk-rock chemical characteristics of the
mafic inclusions. One possibility is that their characteristics reflect mixing between the host magma and the
intruding mafic magma. When considering two endmember mixing between mafic and felsic magmas, the
mafic end-member can easily change its composition as a
result of mixing with the felsic end-member (e.g. Kouchi
& Sunagawa, 1985). In this case, the host lava composition should plot on the high-SiO2 extrapolation of
the mafic inclusion trends in major and trace element
NUMBER 8
AUGUST 2005
variation diagrams (e.g. Figs 11 and 12). Although this is
broadly correct for some major and trace elements, it is
not the case in the MgO, Na2O, Cr, Ni vs SiO2 diagrams
in Figs 6, 11 and 12.
The second possibility is that the compositional trends
defined by the mafic inclusions result from in situ fractionation of the mafic magma, as suggested by Bacon
(1986) to explain the compositional variations of mafic
inclusions in the silicic andesites of Mount Mazama. In
this case, when the bulk composition of the mafic inclusions becomes more felsic, the residual glass compositions
should also be more felsic. However, in the case of
Narugo, the SiO2 contents of the residual glasses are
similar in all the mafic inclusions, regardless of the bulk
inclusion SiO2 content (Table 2).
The third possibility is that the inclusion trends are the
result of interdiffusion of elements between two compositionally distinct magmas, as experimentally proposed by
Baker (1991). In this case, alkalis diffuse much
more rapidly during interdiffusion of silicate melts and
can be decoupled from SiO2. However, in the case of
Narugo volcano, such decoupling cannot be detected
(Fig. 5).
An important feature of the mafic inclusions in the
Narugo lavas is that the chemical compositions of the
interstitial glasses are similar to those of the groundmass
glasses of the host rocks. This is the key to explaining
the chemical trends of the mafic inclusions, and strongly
indicates that prior to the final stage of solidification of
the mafic and felsic magmas, the residual melts of both
the mafic inclusions and their hosts became chemically
homogeneous. It is likely that these two melts infiltrated
each other and consequently their compositions became
similar. The amount of felsic melt that was involved
in mixing and magma mingling may have been larger
than that of the mafic melt, assumed because of the
low proportion (1–2%) of the inclusions based on field
observations. The bulk composition of the host lavas
was not changed significantly during this process. This
mechanism can explain the similarity in the chemical
compositions of glasses within the mafic inclusions and
their hosts. However, this process alone cannot explain
the variation in compositions of the mafic inclusions,
because the glass compositions do not plot on the highSiO2 extrapolations of the mafic inclusion trends (e.g.
Fig. 11).
Accordingly, a composite process must be considered
to explain the chemical trends of the mafic inclusions. We
conclude that before the final stage of solidification, the
felsic melt infiltrated the incompletely solidified mafic
inclusions, changing the bulk chemical composition of
the mafic inclusion towards the groundmass glass composition in the host lava. The estimated chemical trends
of this infiltration process and the inferred original
variation trends of the mafic inclusions are illustrated
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BAN et al.
SOLIDIFICATION PROCESS OF MAFIC INCLUSIONS
Fig. 11. Estimated bulk phenocryst compositions plotted on SiO2 variation diagrams for selected major elements. Symbols are as in Fig. 5. The
bulk phenocryst compositions are calculated using the average modal abundance (Table 1) and the mineral composition data (Tables 3
and 4). Volume ratio of magnetite vs ilmenite is assumed to be c. 5:1. The calculated bulk phenocryst compositions are as follows: SiO2 5208,
TiO2 115, Al2O3 2168, FeO* 875, MgO 265, CaO 929, Na2O 439, K2O 001 for the Toyagamori lava; SiO2 5815, TiO2 082, Al2O3 1971,
FeO* 685, MgO 238, CaO 809, Na2O 399, K2O 002 for the Matsugamine lava; SiO2 5433, TiO2 101, Al2O3 2121, FeO* 774, MgO 236,
CaO 906, Na2O 429, K2O 001 for the Ogatake–Kurumigatake lava dome. FeO* is total iron calculated as FeO.
Fig. 12. MgO (wt %) and Cr (ppm) vs SiO2 (wt %) diagram showing a possible mechanism that explains the chemical trends of the mafic
inclusions. Symbols are as in Fig. 5.
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JOURNAL OF PETROLOGY
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in Fig. 12. The original variation may have been caused
by variable degrees of differentiation, which took place
prior to the magma mingling process.
Solidification sequence of minerals in the
mafic inclusions and the environment in
which the residual melts in the inclusion
and the host lavas became homogeneous
Based on the above discussion, it is inferred that the mafic
inclusions experienced chilling by a cooler host magma
(Stage 1), and the composition of their residual interstitial
melt became more felsic by the crystallization of cores of
the acicular minerals (Stage 2). Prior to the final stage of
solidification (Stage 3), the interstitial melt composition
approached that of the silicic host magmas. A schematic
representation of the creation of the mafic inclusions is
shown in Fig. 13.
The possible crystallization sequences and temperatures at each stage are shown as follows. In Stage 1, the
cores of acicular groundmass plagioclase (An70–90), clinopyroxene (Wo35–43), and orthopyroxene with Mg-value
65–78 crystallized. The elongated crystals possibly
formed a network structure, which later became the
framework of a diktytaxitic texture (Fig. 10). The
magmatic temperature when this framework was established is estimated to be higher than 1000 C based on
pyroxene thermometry. Subsequently, the rims of the
acicular groundmass plagioclase (<An60), clinopyroxene
(Wo40–44), and orthopyroxene with Mg-value 58–64 crystallized from a more differentiated melt (Stage 2). The
magmatic temperature is estimated to have decreased
to c. 850 C on the basis of pyroxene thermometry. This
temperature is similar to the crystallization temperature of
pyroxene phenocrysts in the host rhyolites. The rim compositions of the groundmass plagioclase and pyroxene are
similar to the phenocryst plagioclase and pyroxenes of the
host rhyolites. Moreover, the groundmass glasses of both
the mafic inclusions and the host rhyolites have similar
chemical compositions. These observations suggest that
from the initial chilling stage (Stage 1) to crystallization of
groundmass minerals (Stage 2), the host rhyolitic magma
infiltrated into the framework of the mafic inclusions, and
became well mixed with the existing interstitial melt
(Stage 3). The average SiO2 content of all groundmass
glasses is high (78 2 wt %). This means that the amount
of host rhyolitic melt involved in mixing (on the interstitial melt scale) and magma mingling was much larger
than the volume of residual melt in the mafic inclusions.
Thus the mixed melts have a high SiO2 content. This
estimation is supported by the field observation that the
approximate proportion of the inclusions is 1–2%. A
similarity between the chemical composition of groundmass glasses in both mafic inclusions and their host lavas
was shown for silicic rocks in Dikii Greben’ volcano
NUMBER 8
AUGUST 2005
(Kamchatka) by Bindeman & Bailey (1994). They used
this observation to deduce the similarity in density of both
magmas. However, they did not propose a mechanism
for the similarity. On the other hand, Bacon (1986)
reported chemical compositions of residual glasses in
mafic inclusions that are different from those of the
glass in the host Mt. Mazama andesite. The difference
between these two examples may be in the relative
amounts of felsic and mafic magmas involved in mixing
and magma mingling.
Several ideas have been advanced to define where
mafic and felsic magmas might contact and mingle in a
magma feeding system. Eichelberger (1980) showed that
exsolution of volatiles during crystallization of basaltic
magma can cause a density inversion in a stratified
basalt–rhyolite magma chamber, and that the vesiculated
mafic magma can float up into the silicic magma to form
mafic inclusions. Koyaguchi (1985) proposed that mixing
and mingling can take place during magma ascent
through a conduit. He showed that this largely depends
on the difference in the mixing ratio of the end-member
magmas, and whether the mafic magma becomes a mafic
inclusion in a felsic host or the two magmas mix to
become a compositionally homogeneous mixed magma
(Koyaguchi, 1986a). Bacon (1986) suggested that the
mechanism largely depends on the thermal and compositional contrasts between the two magmas, and that, in
most cases, the mafic inclusions originate in a hybrid
layer in a stratified basalt–rhyolite magma chamber.
Such hybridization can take place in the boundary layer
between the mafic and felsic magmas. This might be
established during the injection and ponding of hot
mafic magma at the base of a silicic magma chamber
(e.g. Eichelberger, 1975; Koyaguchi, 1986b; Bacon &
Druitt, 1988; Feeley & Davidson, 1994; Tomiya &
Takahashi, 1995; Feeley & Dungan, 1996).
In the case of Narugo volcano, the following scenario is
likely. Most inclusions lack olivine or Mg-rich pyroxene,
which shows that the mafic magma was itself already
differentiated, perhaps in a boundary layer within the
stratified magma chamber. When the temperature
dropped below the liquidus (Stage 1), acicular minerals
began to crystallize and the composition of the residual
melt evolved towards that of the felsic magma and mixed
with the chamber magma, especially near the boundary
(Stage 2). Volatile components may be continually supplied to the crystallizing boundary layer from deeper
parts of the mafic magma layer. Although some of the
volatiles might be released to the felsic layer along with
the melts in the mafic layer, the volatile phases would be
diluted in the larger felsic layer. Finally, the density of
some parts of the boundary layer became lower than the
overlying felsic magma and detached blobs (Eichelberger,
1980) or boudin-shaped waves (Koyaguchi, 1986a) of the
mafic magma began to float up into the felsic magma.
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BAN et al.
SOLIDIFICATION PROCESS OF MAFIC INCLUSIONS
Fig. 13. Schematic representation of the magma feeding system under Narugo volcano. (a) Stage 1: crystallization of cores of acicular minerals in
the boundary layer between mafic and felsic magmas, when the mafic magma was cooled. (b) Stage 2: crystallization of rims of acicular minerals
in the boundary layer, when the residual melts changed their composition to felsic through the crystallization and infiltration of felsic melts from
the overlying felsic magma. (c) Stage 3: solidification of the interstitial melts, after the mafic blob erupted to the surface with host felsic magma.
Mg-v, Mg-value.
In the magma conduit during ascent to the surface, these
mafic magma blobs were not completely solidified and
thus may have become disrupted into smaller pieces. In
the case of Narugo, it is deduced that the mafic inclusions
did not solidify completely until the host magma solidified, because the glass compositions can be split into two
types, high-K and low-K in both reddened type host lava
and inclusions. One of the possibilities to produce the two
kinds of glasses is a local difference of mineral assemblage
and proportion of crystallized groundmass phases; however, such a difference cannot be observed. Another possibility is that some kind of liquid immiscibility may be
induced at high oxidation states; however, we do not have
sufficient data to prove this possibility. Regardless, the
existence of high-K and low-K glasses in both reddened
type host lava and inclusions suggests that the residual
melts of the mafic inclusions solidified after the host
magma was extruded at the Earth’s surface.
Timing of solidification of the
mafic inclusions
Pallister et al. (1992) estimated that in the case of the
Pinatubo 1991 eruption, injection of basaltic magma
from deeper levels into a shallower felsic magma chamber
caused magma mixing and triggered the eruption. They
considered that the preservation of a disequilibrium mineral assemblage suggested that magma mixing took place
shortly before eruption. Murphy et al. (2000) suggested
that the 1995–1999 eruption of the Soufriere Hills volcano, Montserrat, was triggered by a recent influx of hot
mafic magma, based on petrological observations as well
as data on seismicity, extrusion rate, and SO2 fluxes.
However, these petrological observations do not provide
direct constraints on the timing of mafic magma intrusion
events beneath Pinatubo and the Soufriere Hills volcano.
If the magmas mixed well and some phenocrysts were out
of equilibrium with the surrounding melt, the residence
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JOURNAL OF PETROLOGY
VOLUME 46
time of these phenocrysts in the mixed (hybrid) magma
could be estimated using geo-speedometers, such as the
Ni content in olivine (Nakamura, 1995b), or the dissolved
width of plagioclase (Nakamura & Shimakita, 1998).
However, in the case of (complete) magma mingling,
such disequilibrium phenocrysts cannot be found. Practically, when magma mingling is a dominant process,
small-scale mixing can also take place especially along
the boundary between the end-member magmas (Bacon,
1986). However, even in such cases, disequilibrium
phenocrysts are rare. In these cases, it is difficult to use geospeedometers to estimate the time lapse between initiation of the mingling event and a subsequent eruption.
In the case of Narugo, disequilibrium phenocrysts are
rare, and the major evidence for magma mingling is
solely the existence of the mafic inclusions. As already
discussed, it is widely accepted that mafic inclusions are
the products of quenched mafic magma. Thus it must
take only a short time from the initial chilling to the
complete solidification of the mafic magma to form the
mafic inclusions. We conclude that the mafic inclusions
and their host rhyolites completely solidified almost simultaneously after the eruption, based on the chemical
compositions of the residual glass in both the reddened
type of mafic inclusion and the host rhyolite. In other
words, shortly after the mafic magma invaded the host
rhyolitic magma chamber, these magmas mingled and
erupted to the surface.
ACKNOWLEDGEMENTS
We are grateful to R. J. Arculus and Marjorie Wilson for
many constructive comments and suggestions on the
manuscript. We express our thanks to Professors
T. Yoshida at Tohoku University and Y. Yamaguchi
at Shinshu University for helpful suggestions,
Dr. R. W. Jordan at Yamagata University for correcting
the English in this paper, and Professor H. Tanaka
at Yamagata University for his continual support of
this research. Technical advice on XRF analyses
from Y. Yamada at Rigaku Co., Ltd. was very helpful.
We also appreciate the financial support from the
Japanese Ministry of Education.
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