Download Melting of the Uppermost Metasomatized Asthenosphere Triggered

JOURNAL OF PETROLOGY
VOLUME 55
NUMBER 3
PAGES 499^528
2014
doi:10.1093/petrology/egt074
Melting of the Uppermost Metasomatized
Asthenosphere Triggered by Fluid Fluxing from
Ancient Subducted Sediment: Constraints from
the Quaternary Basalt Lavas at Chugaryeong
Volcano, Korea
TETSUYA SAKUYAMA1*, SHINJI NAGAOKA2, TAKASHI MIYAZAKI1,
QING CHANG1, TOSHIRO TAKAHASHI1, YUKA HIRAHARA1,
RYOKO SENDA1, TETSUMARU ITAYA3, JUN’ICHI KIMURA1 AND
KAZUHITO OZAWA4
1
INSTITUTE FOR RESEARCH ON EARTH EVOLUTION, JAPAN AGENCY FOR MARINE^EARTH SCIENCE AND
TECHNOLOGY, YOKOSUKA, 237-0061, JAPAN
2
DEPARTMENT OF EDUCATION, NAGASAKI UNIVERSITY, NAGASAKI 852-8521, JAPAN
3
RESEARCH INSTITUTE OF NATURAL SCIENCES, OKAYAMA UNIVERSITY OF SCIENCE, OKAYAMA 700-0005, JAPAN
4
DEPARTMENT OF EARTH AND PLANETARY SCIENCE, GRADUATE SCHOOL OF SCIENCE, UNIVERSITY OF TOKYO,
TOKYO, 113-0033, JAPAN
RECEIVED AUGUST 2, 2012; ACCEPTED NOVEMBER 13, 2013
Major and trace element and Sr^Nd^Pb isotope data for wholerocks and major element data for minerals within basalt samples
from the Chugaryeong volcano, an intra-plate back-arc volcanic
centre in the central part of the Korean Peninsula, are used to
address the process of magma genesis in the deep back-arc region
of eastern Asia. There are two lava flow units at Chugaryeong volcano: the Chongok (0·50 Ma) and the Chatan (0·15 Ma) basalts.
These basalts have similar MgO (9·1^10·4 wt %) but exhibit
differences in their major and trace element and isotope compositions. The Chongok basalt has higher TiO2, Al2O3, Na2O, K2O,
P2O5, Cr2O3, large ion lithophile elements (LILE), high field
strength elements (HFSE), and rare earth elements (REE), and
lower FeO*, SiO2, and CaO than the Chatan basalt. In addition,
the Chongok basalt has more radiogenic 143Nd/144Nd and
206
Pb/204Pb, and less radiogenic 87Sr/86Sr and 208Pb/204Pb than
the Chatan basalt. Chi-square tests for the major elements indicate
that crystal fractionation can explain the chemical variations
within each basalt suite; intra-crustal processes, including crystal fractionation and assimilation of continental crust, cannot
result in the formation of one basalt suite from the other. The Sr^
Nd^Pb isotopic compositions of the Chongok and Chatan basalts
plot on mixing hyperbolae between peridotite mantle xenoliths from
the area and a fluid flux derived from a mixture of ancient and
recent sediments. The trace element compositions of the estimated
primary melts for the two basalt suites suggest different degrees of
partial melting of a common enriched mantle source that was metasomatized by a Ba-, K-, Pb-, and Sr-rich fluid. The estimated
degree of melting increased with time from 7·5% for the
Chongok basalt to 10% for the Chatan basalt. The source
mantle for the Chatan basalt is more enriched in Ba and Pb, indicating a greater fluid flux than for the Chongok basalt. This suggests that melting of the source mantle increased with time,
sustained by an increased sediment-derived fluid flux from the
deeper upper mantle.
*Corresponding author. Telephone: þ81-46-867-9785.
þ81-46-867-9625. E-mail: [email protected]
ß The Author 2014. Published by Oxford University Press. All
rights reserved. For Permissions, please e-mail: journals.permissions@
oup.com
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JOURNAL OF PETROLOGY
VOLUME 55
WORDS: back-arc intraplate volcanism; alkaline basalt;
Chugaryeong volcano; Korean Peninsula; Quaternary; fluid flux; subducted sediment; mantle melting
KEY
I N T RO D U C T I O N
Transportation of aqueous fluids from subducting slabs is
known to play an important role in the generation of arc
volcanism (Gill, 1981; Tatsumi et al., 1983; Stolper &
Newman, 1994). Relative enrichment of fluid-mobile elements, such as large ion lithophile elements (LILE) and
light rare earth elements (LREE), and depletion in high
field strength elements (HFSE) in volcanic rocks from subduction zones has been thought to be the result of a supply
of water-rich fluid from the subducting slab. The effects of
fluids from subducting oceanic plates have also been identified in many back-arc regions (e.g. Nakamura et al., 1985;
McCulloch & Gamble, 1991; Woodhead et al., 1993; Taylor
& Martinez, 2003; Tian et al., 2008; Bertotto et al., 2009;
Kuritani et al., 2011), even though the relative enrichment
of LILE and LREE in comparison with HFSE is minor in
the volcanic rocks of these back-arc regions.
Back-arc volcanism in the Japan Sea area has been
described as the ‘CircumJapan Sea Alkaline Rock Province’
(Tomita, 1935). Because of this volcanism, the eastern
margin of the Eurasian Plate was named the ‘hot region’ by
Miyashiro (1986). Several mechanisms have been proposed
for the back-arc volcanism in this region: hot asthenospheric
injection related to large-scale mantle upwelling beneath
NE China and Far Eastern Russia (Tatsumi & Eggins,1995);
a mantle plume from the upper mantle^lower mantle or
core^mantle boundary (Nakamura et al.,1985); mantle convection induced by collision of the Indian Plate (Liu et al.,
2004); eastward asthenospheric flow (Flower et al., 2001; Niu,
2005); or convection of a‘Big MantleWedge’ structure above
a stagnant slab (Zhao et al., 2007).
Although large-scale asthenospheric injection is still an
attractive model, recent studies have focused more on the
roles of fluids released from a subducted slab of Pacific
lithosphere. These have been inspired by the progress of
seismic tomography, which has revealed the existence of a
stagnant slab extending continuously from the subducting
Pacific plate to beneath the deep back-arc volcanoes in
eastern Asia (Fukao et al., 1992; Huang & Zhao, 2006).
Recently, Kuritani et al. (2011) proposed a model for
magma generation induced by hydrous mantle upwelling,
which explains the observed variations in the isotopic and
trace element compositions of young volcanic rocks in
northeastern China. Although they suggested the possibility of decompression melting of the hydrated mantle, detailed models of the melting process in relation to the
generation of the volcanism were not presented. Kuritani
et al. (2009) investigated the trace element and isotopic
composition of the Changbaishan volcano (Fig. 1), which
is regarded as a major centre of activity for hydrous
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mantle upwelling, to constrain the melting processes in
the mantle. However, the relatively differentiated compositions of the Changbaishan volcanic rocks prevented
Kuritani et al. from discussing the melting process in detail.
The Quaternary Chugaryeong volcano, located in the
centre of the Korean Peninsula, is one of a number of alkaline volcanoes at the eastern margin of the Eurasian
Plate. Because some basaltic lavas from the Chugaryeong
volcano have less differentiated chemical compositions, we
can use them to investigate melting processes in the
mantle. Here, we present the results of an integrated
geochemical and petrological study of two Chugaryeong
basalt lava flows and the minerals therein, to provide constraints on the melting process in the back-arc mantle. In
the following discussion, we will consider the effects of processes that modify the primary magma compositions at
shallow crustal depths, such as crustal contamination and
crystal fractionation, followed by an estimation of the primary magma compositions for the Chongok and Chatan
basalts. The melting conditions are then estimated following the approach of Sakuyama et al. (2009). Finally, the
source mantle characteristics and melting processes
involved in the formation of the Chugaryeong basalts are
discussed, based on their trace element and Sr^Nd^Pb isotope compositions. Systematic differences between the two
basalts allow us to discuss fluid-fluxed melting processes
in the mantle induced by the addition of fluid from deeper
levels. Contributions from both recent and ancient fluid
sources are considered.
G E O L O G Y O F C H U G A RY E O N G
VO L C A N O
The Korean Peninsula is located in the eastern part of the
Eurasian Plate. Here, Cenozoic volcanism is sparse, concentrated in four discrete Quaternary volcanic provinces; from
north to south these are Changbaishan, Chugaryeong,
Ulleung-do, and Jeju (Cheju) (Fig. 1). Strongly alkaline basalts that differ from typical subduction-related magma types
occur in these provinces. The Chugaryeong graben on the
Korean Peninsula trends NNE^SSW, and is bordered by a
set of normal faults (Lee et al., 1983). The Chugaryeong volcano erupted within this fault system, and lava flows filled
the graben over a distance of 100 km along the Hantan
River (Fig.1b).The main eruptioncentre of the Chugaryeong
volcanic province is Mt. Ori, located inthe North Koreanterritory (Fig.1b; Park & Park,1996).
Basalt lavas from the Chugaryeong volcano are exposed in
a narrow, long plateau and range from10 to 50 m inthickness.
The lavas rest on basement rocks composed of Precambrian
gneiss andJurassic granite. In contrast to the NNE^SSWdistribution of the lavas confined by the Chugaryeong graben,
the alignment of volcanic vents is nearly east^west.
Geological information is currently limited to South Korean
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SAKUYAMA et al.
METASOMATIZED ASTHENOSPHERE MELTING
Fig. 1. (a) Distribution of Quaternary volcanism on the Korean Peninsula, indicated by black filled areas: Changbaishan, Chugaryeong,
Ulleung-do island, and Jeju Island. The Chugaryeong Graben Zone is indicated by parallel dashed lines. (b) Detailed map of the study area
of the Chugaryeong basalts. (c) Geological cross-sections. Lines of section are indicated in (b).
territory and therefore the complete eruption sequence of the
Chugaryeong volcano remains unknown. From the stratigraphic sequence observed inthe available sections, the eruption products can be divided into at least two sub-units,
based on their geological, geochronological, and geochemical characteristics. In stratigraphically ascending order,
these are the Chongok and Chatan basalts. Both basalt units
vary from a few to around 30 m in thickness. Lavas with columnar cooling joints are ubiquitous and clinkers develop
above and below the lava flows. Paleosol layers are occasionally developed beneath both the Chongok and the Chatan
basalt lava flows. Ryu et al. (2011) reported K^Ar ages for the
two basalt lavas and zircon fission-track (reset) ages from a
baked paleosol immediately underneath the lavas. Their results showed that the Chongok and the Chatan basalts
erupted at 0·50 and 0·15 Ma, respectively (Ryu etal.,2011).
A N A LY T I C A L M E T H O D S
Major elements
Rock samples were sliced into chips using a diamond saw,
polished with No. 180 abrasive powder to remove the
tracks of the cutter and washed in an ultrasonic bath. The
chips were crushed to fragments a few millimetres in diameter using a tungsten carbide mortar. The fragments were
then ground in an automatic agate mortar for around 2 h.
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JOURNAL OF PETROLOGY
VOLUME 55
Whole-rock major and trace element contents were determined by X-ray fluorescence (XRF) spectrometry using a
Phillips PW-1480 system at the Department of Earth and
Planetary Science, University of Tokyo. Fused glass beads
for major element analysis were prepared from an aliquot
of 0·4 g rock powder mixed with 4·0 g of lithium tetraborate flux. Details of the analytical method have been previously reported by Kushiro (1994). Reproducibility of the
XRF data is better than 1%, except for MnO (1·5%),
P2O5 (2%), and Na2O (4%). Several samples were discarded from the dataset: those that gave a total of less
than 97 wt %, and strongly altered samples, in which olivine was totally replaced by a secondary mineral such as
iddingsite.
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Pb isotope ratios of leached powder samples were determined by multicollector-inductively coupled plasma-mass
spectrometry (MC-ICP-MS; Neptune Thermo Fisher,
Bremen). Pb was isolated using AG1-X8 200^400 mesh
anion exchange resin. Procedural Pb blanks were 530 pg
(Kimura & Nakano, 2004; Miyazaki et al., 2009). The
mean 206Pb/204Pb, 207Pb/204Pb, and 208Pb/204Pb ratios of
NIST 981 were 16·9309 (0·0013; 2s, n ¼11), 15·4840
(0·0013; 2s, n ¼11), and 36·6749 (0·0038; 2s, n ¼11), respectively. The Pb isotope data presented in this study are
re-calibrated using NIST 981 values of 206Pb/204Pb ¼
16·9416, 207Pb/204Pb ¼15·5000, and 208Pb/204Pb ¼ 36·7262,
as reported by Baker et al. (2004).
Mineral compositions
Trace elements
Trace elements (Sc, Co, Ni, Cu, Cs, Tl, Cr, Ga, Zn, and V)
were determined by XRF on pressed powder pellets.
Concentrations of other trace elements (Rb, Sr, Y, Zr, Nb,
Ba, Lu, Hf, Ta, Pb, Th, and U) and rare earth elements
(REE) (La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm,
and Yb) in selected samples (2403, 2404, 2603, 2607, 2608,
and 2704) were analysed by inductively coupled plasmamass spectrometry (ICP-MS: Agilent 7500ce). Pulverized
whole-rock samples were digested in an HClO4^HF mixture. After drying, the samples were dissolved in HNO3,
with a small amount of HF. Indium and bismuth were
added to the solution as internal standards (Chang et al.,
2003). The precision and reproducibility were generally
better than 1% and 5%, respectively. The analysis reference standard (JB-1a) was in good agreement with the reference value.
Sr^Nd^Pb isotopes
Prior to sample digestion, powder sample splits were leached with 6N HCl at room temperature for 1h, rinsed
with Milli-Q water, and then dried prior to Sr^Nd^Pb isotope analysis. The analytical procedure used in the chemical separation has been previously outlined by Kimura &
Nakano (2004), Hirahara et al. (2009), Miyazaki et al.
(2009), and Takahashi et al. (2009). Sr and Nd isotope
ratios were measured by thermal ionization mass spectrometry (TIMS: Triton; Thermo-Finnigan, Bremen)
using an instrument equipped with nine Faraday cups in
static multi-collection mode. Sr was isolated using Sr resin
(Eichrom Industries, IL, USA) (Takahashi et al., 2009).
For Nd isotopic analysis, the REE were initially separated
by cation exchange before isolating Nd on Ln resin
(Eichrom Industries) columns (Hirahara et al., 2009). The
mean 87Sr/86Sr and 143Nd/144Nd values in NIST987 and
JNdi-1 are 0·710239 (0·000004; 2s, n ¼ 4) and 0·512096
(0·000008; 2s, n ¼ 6), respectively, which are in good
agreement with the published values (NIST 987 87Sr/86Sr
¼ 0·710252^0·710256; JNdi-1 143Nd/144Nd ¼ 0·512101) (Weis
et al., 2006, 2007).
The compositions of olivine phenocrysts and spinel inclusions in olivine phenocrysts were analysed by electron
microprobe (JEOL JCMA-733MKII) at the University of
Tokyo. The analytical procedures have been given by
Nakamura & Kushiro (1970), with a correction procedure
by Bence & Albee (1968). Accelerating voltage, beam current, and counting time were 15 kV, 12 nA, and 30 s, respectively. Olivine compositions were determined at 25 kV
accelerating voltage with a 20 nA beam current and 30 s
counting time with ZAF correction.
P E T RO G R A P H Y A N D M I N E R A L
COMPOSITIONS
The modal proportions of phenocrysts (4150 mm) were
determined by point counting (45000 points in each thin
section) and are shown in Fig. 2 and Table 1 and Supplementary Data Table A1 (supplementary data are available
for downloading at http://www.petrology.oxfordjournals.
org). The chemical compositions of olivine phenocrysts
and spinel inclusions are listed in Tables 2 and 3, and
Fig. 2. Phenocryst modal proportions in representative samples from
the Chongok and Chatan basalts. Values in parentheses next to the
sample number represent whole-rock MgO concentrations (in wt %).
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METASOMATIZED ASTHENOSPHERE MELTING
Table 1: Representative whole-rock major element, trace element, isotope, and modal compositions of the Chugaryeong basalt
and analysis by ICP-MS at Japan Agency for Marine^Earth Science and Technology, and recommended value of GSJ
basalt (JB-1) standard
Unit:
Chongok
Chongok
Chongok
Chatan
Chatan
Chatan
Standard
Name:
2403
2704
2607
2404
2603
2608
JB-1
1s
Reference
Major elements (wt %)
SiO2
47·79
TiO2
2·07
Al2O3
15·75
Fe2O3
47·62
48·03
48·42
48·21
2·03
1·69
1·91
1·92
15·39
15·74
15·41
15·58
15·58
11·14
11·15
11·1
11·33
11·51
11·21
MnO
0·16
0·16
0·17
0·17
0·17
0·16
MgO
9·05
9·94
9·4
10·32
9·28
9·15
CaO
8·25
8·24
8·15
8·37
8·44
8·57
Na2O
3·12
3·6
3·75
3·19
3·09
2·99
1·86
2
48·23
FeO
K2O
2·06
1·96
2·04
1·54
1·68
P2O5
0·45
0·43
0·45
0·34
0·38
0·39
Total
99·85
100·48
100·85
100·77
100·25
100·07
Trace elements (ppm)
Rb
Ba
23·6
312
24·3
287
24·7
292
17·7
257
18·5
254
21·8
269
36·8
496
0·3
39·2
3
504
Th
3·39
3·2
3·3
2·61
2·72
2·94
9·16
0·02
9·03
U
0·925
0·9
0·877
0·542
0·7
0·811
1·65
0·01
1·57
Ta
1·9
1·81
1·88
1·39
1·62
1·69
1·64
0·004
Nb
30·4
28·9
30
22·7
25·9
26·9
15·9
0·2
26·9
La
24·4
23·1
23·9
19·5
21·3
22·6
37·5
0·3
37·6
Ce
49·9
47·7
49·2
40·1
43·8
46·4
66·7
0·5
65·9
Pb
3·18
2·94
3·13
2·51
2·69
2·95
6·15
0·02
Pr
5·97
5·7
5·86
4·81
5·2
5·51
7·04
0·04
Sr
Nd
Zr
622
24·6
194
576
23·7
186
627
24·2
190
492
530
20
21·4
157
172
547
22·7
182
435
26·3
136
1·93
6·76
7·3
3
442
0·1
26
0·2
144
Hf
4·4
4·22
4·34
3·61
3·99
4·18
3·62
0·01
3·41
Sm
5·48
5·26
5·38
4·56
4·89
5·12
5·06
0·01
5·07
Eu
1·83
1·77
1·8
1·54
1·65
1·69
1·48
0·01
1·46
Gd
5·57
5·41
5·45
4·69
5·1
5·29
4·73
0·04
4·67
Tb
0·86
0·836
0·847
0·739
0·801
0·824
0·734
0·004
0·69
4·15
0·01
Dy
Y
4·88
23·3
4·75
22·7
4·77
22·9
4·27
20·7
4·6
22·2
4·76
22·8
20·8
3·99
0·1
24
Ho
0·939
0·915
0·924
0·832
0·9
0·927
0·810
0·003
0·71
Er
2·62
2·56
2·56
2·32
2·49
2·57
2·31
0·004
2·18
Tm
0·346
0·34
0·338
0·313
0·339
0·346
0·319
0·000
0·33
Yb
2·21
2·17
2·17
2
2·17
2·2
2·08
0·02
2·1
0·32
0·30
0·002
0·33
Lu
Sc*
Co*
0·322
22
46·1
Ni*
184
Cu*
55
0·312
22·3
48·4
213
52·9
0·315
21·7
47·1
194
53·5
0·29
21·7
52
0·311
22·2
47·9
225
181
51·5
52·8
22·2
46·7
187
53·7
(continued)
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Table 1: Continued
Unit:
Chongok
Chongok
Chongok
Chatan
Chatan
Chatan
Standard
Name:
2403
2704
2607
2404
2603
2608
JB-1
Cs*
0·16
0·296
0·237
0·135
0·314
Tl*
0·061
0·076
0·074
0·039
0·088
Cr*
240
282
262
307
212
Ga*
19·2
19·6
18·4
18·7
19
Zn*
80·6
81·5
82·1
86·4
86·9
V*
192
192
189
176
1s
Reference
0·295
0·076
231
18·4
88·2
160
178
Isotope compositions
87
Sr/86Sr
143
Nd/144Nd
Pb/204Pb
0·704451(7)
0·704430(7)
0·704430(7)
0·705076(7)
0·704672(8)
0·70468(7)
0·512663(10)
0·512684(8)
0·512677(8)
0·512570(7)
0·512606(7)
0·512579(7)
206
17·8507(5)
17·8650(6)
17·8462(6)
17·7210(5)
17·7895(5)
207
15·5491(5)
15·5490(5)
15·5475(6)
15·5567(5)
15·5486(5)
15·5457(5)
208
38·5616(16)
38·5568(15)
38·5524(17)
38·5214(14)
38·5100(16)
38·4845(14)
Pb/204Pb
Pb/204Pb
17·7646(6)
Phenocryst mode (vol. %)
Ol
2
7·8
6·4
6·2
4·3
Pl
0
0
0
0
0
0
98
92·2
93·6
93·8
95·7
94·8
Gm
5·2
The numbers in parentheses are one standard deviation in terms of the last digit(s). Recommended values are from Imai
et al. (1995).
*Concentration of those elements were determined by XRF.
Table 2: Representative compositions of olivine phenocrysts
Unit:
Chatan
Chatan
Chatan
Chatan
Chatan
Chatan
Chongok
Chongok
Chongok
Chongok
Chongok
Chongok
Host rock:
2404
2404
2606
2404
2703
2606
2704
2704
2704
2704
2704
2605
Name:
OL15
OL17
OL19
OL6
OL19
OL6
OL1C
OL24
OL18
OL12
OL3
OL25
Position:
core
core
core
core
core
rim
core
core
core
core
core
rim
39·39
Major elements (wt %)
SiO2
39·115
39·195
38·537
38·759
37·546
38·167
38·916
38·759
38·15
37·482
37·167
MnO
0·208
0·19
0·237
0·219
0·285
0·307
0·151
0·206
0·236
0·25
0·315
0·351
NiO
0·318
0·195
0·202
0·21
0·181
0·138
0·338
0·216
0·188
0·184
0·147
0·13
FeO*
13·914
15·047
16·083
17·362
18·648
20·32
10·376
13·847
15·775
17·172
20·306
20·206
39·66
39·983
MgO
46·253
45·467
43·306
43·761
41·145
48·585
45·822
44·078
43·078
40·573
CaO
0·268
0·26
0·253
0·243
0·274
0·382
0·077
0·222
0·243
0·243
0·339
0·427
Cr2O3
0·035
0·076
0·039
0·061
0·025
0·03
0·01
0·042
0·032
0·031
0·038
0·036
Total
100·111
100·43
98·658
100·615
98·105
99·005
98·926
99·272
99·312
99·108
99·2
98·3
Fo#
85·6
84·3
79·7
77·7
89·3
85·5
83·3
81·7
78·1
77·9
82·8
81·8
*Total iron as FeO.
Supplementary Data Tables A2 and A3 respectively. It is
difficult to distinguish between the Chongok and the
Chatan basalts based on petrographic observations. Both
basalts are almost aphyric (5 vol. % phenocrysts) and
have normally zoned olivine phenocrysts containing
spinel (10 mm) and melt inclusions (5 mm); several samples (2407, 2406, 2504, 2601, and 2701) have a trace
amount (1vol. %) of plagioclase with strongly corroded
cores (Fig. 3) and clinopyroxene microphenocrysts. Clear
melt inclusions were not found in either the rims or the
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Table 3: Representative compositions of spinel inclusions in
olivine phenocryst
Unit:
Chongok Chongok Chongok Chatan Chatan Chatan
Host rock:
2609
2704
2704
2404
2407
2404
Name:
SP1
SP4
SP2
SP2
SP1
SP2
Host olivine:
OL16
OL17
OL44
OL24
OL2
OL3
Fo#:
87·1
84·2
82·5
84·5
82·1
83·7
Major elements (wt %)
SiO2
Al2O3
TiO2
FeO*
MnO
MgO
0·091
0·053
27·9
40·46
3·218
0·136
36·83
1·221
32·28
22·29
0·293
1·427
23·37
0·171
10·53
14·63
0·191
13·74
0·102
32·92
1·236
22·3
0·229
14·21
0·039
26·84
2·701
24·94
0·231
10·45
0·105
34·3
1·369
21·63
0·177
14·3
CaO
0·025
0·022
0·023
0
0·01
0·01
Na2O
0·003
0·02
0
0·072
0·083
0·073
K2O
Cr2O3
0
0
21·17
0·019
21·25
23·37
0
28·06
0·019
24·04
V2O3
0·328
0·126
0·206
0·166
0·417
NiO
0·177
0·256
0·211
0·086
0·105
Total
96·01
100·5
99·53
99·38
89·88
Fe2þ
1·779
1·793
1·847
1·669
1·991
Fe3þ
2·545
1·095
1·225
1·341
1·442
100Cr3þ/(Cr3þ þ 26·7
23·7
26·8
32·4
32·9
0·013
27·33
0·203
0·184
99·71
1·721
1·121
31·6
dusty cores of the plagioclase. There is no systematic correlation between phenocryst mode and bulk chemical composition (Fig. 2). Many samples contain olivine xenocrysts
with distinctive kink bands (Fig. 3), although these are
rare, amounting at most to a few grains in a single thin section. No non-equilibrium minerals such as quartz or lowmagnesium orthopyroxene were observed.
The groundmass of both the Chongok and the Chatan
basalts is composed of plagioclase, Ti-augite, olivine,
alkali-feldspar, magnetite, and ilmenite. A small amount
of calcite is present as an intergranular mineral in sample
2702. Samples from the central part of the lava flows show
high crystallinity and an ophitic texture, whereas those
from the margin have a porphyritic texture.
The chemical compositions of olivine phenocryst cores
from the two basalt units are almost equivalent to one another in terms of their Fo# (Fig. 4a), NiO (Fig. 4b),
MnO, and CaO contents (Table 2). However, spinel inclusions in the olivine phenocrysts show obvious differences
in Cr# between the two basalt groups: spinel in the
Chatan basalt has higher Cr# [¼ 100 Cr3þ/
(Cr3þ þ Al3þ)] than that of the Chongok basalt (Fig. 4c).
The Fo#^Cr# relationships of the two basalts define subparallel trends away from the olivine^spinel mantle array
(OSMA) proposed by Arai (1987), showing slight increase
in Cr# with a decrease of host olivine Fo#.
Fe3þ þ Al3þ)
Unit:
Chatan Chatan Chatan Chatan Chatan Chatan Chatan Chatan
Host rock:
2404
2407
2404
2404
2606
2404
2407
2606
Name:
SP2
SP1
SP2
SP1
SP1
SP1
SP4
SP2
Host olivine:
OL24
OL2
OL3
OL6
OL39
OL14
OL10
OL1
Major elements (wt %)
SiO2
Al2O3
TiO2
FeO*
MnO
MgO
0·102
32·92
1·236
22·30
0·229
14·21
0·039
26·84
2·701
24·94
0·231
10·45
0·105
34·30
1·369
21·63
0·177
14·30
0·087
30·05
1·965
26·64
0·242
11·69
0·100
28·59
2·213
27·29
0·287
11·30
0·099
30·87
1·827
27·69
0·285
11·47
2·778
21·66
4·529
32·63
0·296
7·31
0·069
26·58
2·683
28·02
0·272
10·47
CaO
0·000
0·010
0·010
0·035
0·018
0·000
0·068
0·025
Na2O
0·072
0·083
0·073
0·065
0·003
0·080
0·838
0·002
K2O
Cr2O3
0·000
28·06
0·019
24·04
0·013
27·33
0·000
28·10
0·017
28·13
0·008
26·96
0·061
26·20
V2O3
0·166
0·417
0·203
0·309
0·311
0·297
0·588
NiO
0·086
0·105
0·184
0·165
0·122
0·115
0·072
Total
99·38
89·88
99·71
99·35
98·38
99·70
97·04
Fe2þ
1·669
1·991
1·721
2·101
2·089
2·163
2·407
Fe3þ
1·341
1·442
1·121
1·464
1·571
1·590
1·625
100Cr3þ/(Cr3þ þ 32·4
32·9
31·6
33·9
34·6
32·1
38·0
0·015
27·32
0·403
0·146
96·00
2·111
1·660
35·1
Fe3þ þ Al3þ)
*Total iron as FeO.
Fe3þ and Fe2þ are calculated assuming stoichiometry for
spinel.
W H O L E - RO C K C H E M I C A L
COM POSITIONS
Representative whole-rock major element compositions of
the Chugaryeong basalts are listed in Table 1 and shown
in Fig. 5. Both the Chongok and the Chatan basalts are
alkali basalts (Miyashiro, 1978) and trachybasalts (Le Bas
et al., 1986) (Fig. 5a); however, there are systematic differences between the two basalts. The Chongok basalts (9·1^
10·0 wt % MgO) are higher in TiO2 (Fig. 5b), Na2O,
K2O P2O5, Cr2O3, Rb, Ba, Nb, Sr, and Zr, and lower in
FeO*, SiO2 (Fig. 5c), and CaO (Fig. 5d), for a given
MgO content, compared with the Chatan basalts
(9·2^10·4 wt % MgO). The Chongok basalts are also
higher in K2O/TiO2 (Fig. 5e) and Zr/Y, and lower in
CaO/Al2O3 (Fig. 5f), Zr/Rb and Zr/Nb at the same MgO
contents, compared with the Chatan basalts. The increase
in Al2O3 content and the decrease in the CaO/Al2O3
ratio with decreasing MgO of the Chongok basalt is
greater than that of the Chatan basalt.
Figure 6 shows multi-element incompatible trace element patterns for the Chugaryeong basalt samples normalized to the primitive mantle composition (Fig. 6a) and
REE normalized to the CI chondrite composition
(Fig. 6b; Sun & McDonough, 1989). The patterns of both
the Chongok and the Chatan samples are similar to one
another and show characteristics of enrichment typical of
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Fig. 3. Photomicrograph of olivine phenocrysts (a, b) and xenocryst (c, d) in the Chongok basalt under crossed polars (a, c) and planepolarized light (b, d). Dashed white lines in (c) represent kink bands in an olivine xenocryst. (e) Back-scattered electron image of olivine phenocryst. (f) Photomicrograph of plagioclase phenocryst in a Chongok basalt under crossed polars. ol, olivine; sp, spinel; aug, augite; pl, plagioclase.
oceanic island alkali basalts (OIB; e.g. Hofmann, 1997),
with positive spikes at Ba, K, Pb, and Sr (Fig. 6a). The
Chatan basalts have slightly higher Ba/Th and Pb/U
(Fig. 6c) and lower Sm/Yb and Ce/Yb (Fig. 6d) than the
Chongok basalts. There are no Eu anomalies, suggesting
that fractionation or accumulation of plagioclase does not
play an important role in the petrogenesis of these basalts.
The Sr^Nd^Pb isotope compositions of the Chugaryeong basalts are shown in Fig. 7 and reported in Table 1.
In the 87Sr/86Sr vs 143Nd/144Nd diagram they plot in a
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relatively enriched compositional field, near Bulk Earth,
(Fig. 7a), and in terms of 7/4 { ¼ 100[(207Pb/204Pb)sample
^ 0·1084(206Pb/204Pb)sample ^ 13·491]; Hart, 1984} and
8/4 {¼ 100[(208Pb/204Pb)sample ^ 1·209(206Pb/204Pb)sample
^ 15·627]; Hart, 1984} they are the highest among Cenozoic
basalts in eastern Asia (Fig. 7b and c). There is, however,
a systematic difference between the Chongok and the
Chatan basalts. The Chatan basalt has slightly more radiogenic 87Sr/86Sr and less radiogenic 143Nd/144Nd,
206
Pb/204Pb, and 208Pb/204Pb (Fig. 7).
DISCUSSION
Shallow magmatic process in the crust
Fig. 4. (a) Forsterite (Fo) content of the cores of olivine phenocrysts
plotted against whole-rock Mg# [¼ 100 Mg/(MgO þ FeO*)], (b)
NiO content of cores of olivine phenocrysts plotted against Fo content
(Takahashi 1986), and (c) relationships between Cr/(Cr þAl þ Fe3þ)
atomic ratio of spinel inclusions and the Fo content of the host olivine
phenocrysts in the Chongok and Chatan basalts. Lherzolite and harzburgite xenolith data for the Korean Peninsula and Hannuoba are
from Choi et al. (2005, 2008).
If the geochemical differences between the two Chugaryeong basalts were solely a result of intra-crustal processes,
such as magma mixing, crustal assimilation, or crystal
fractionation, it would be difficult to isolate the mantle
processes that formed the primary magmas. Mixing of different magmas often results in disequilibrium petrographic
and compositional characteristics (e.g. Eichelberger, 1975;
Sakuyama, 1979). These include (1) the presence of reversely
zoned mafic phenocrysts such as olivine and pyroxene and
(2) the coexistence of disequilibrium phenocryst assemblages such as olivine and quartz. Phenocrysts in the Chugaryeong basalts always show normal zoning, and there is
no disequilibrium phenocryst assemblage, except for the
dissolved cores in some plagioclase phenocrysts (Fig. 3f).
However, these plagioclases are small (51mm in diameter)
and occur very occasionally. The dusty cores are surrounded by overgrowths of clear plagioclase and the internal dusty structure appears to originate from an albite
twin, which could have been derived from felsic crustal
rocks. Thus we can infer that some of these rare plagioclase
crystals could be derived from fragments of the local crustal wall-rock. We therefore conclude that magma mixing
is not a significant process contributing to the compositional variability in the Chugaryeong basalts.
The compositional relationship between the olivine
phenocrysts and their host magma is illustrated in Fig. 4a,
to examine the possible effects of crystal accumulation
(Iwamori, 1989). The range of forsterite contents that are
in equilibrium with the bulk-rock compositions are indicated by the continuous and dashed curves, assuming a
certain range for the Fe^Mg partition coefficient
(KD ¼ 0·3 0·03) between olivine and melt (Roeder &
Emslie, 1970). Whole-rock Mg# [¼ 100 Mg/
(Mg þ Fe2þ) in mole per cent] is calculated by assuming
Fe3þ/(Fe3þ þ Fe2þ) ¼ 0·15 in the magma (Kelley &
Cottrell, 2009). The most magnesian cores of the olivine
phenocrysts in the samples are in equilibrium with the
host magma, with the whole-rock chemical composition
consistent with Fe^Mg partitioning, suggesting that the
effect of olivine accumulation is negligible.
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Fig. 5. Whole-rock MgO variation diagrams for representative major elements in the Chugaryeong basalts. Major element analyses are recalculated to 100%. Crosses represent data for the Chugaryeong basalts reported by other studies (Won et al., 1990; Park & Park, 1996; Wee, 1996).
The dashed line in (a) represents the boundary between alkaline and sub-alkaline basalts after Miyashiro (1978). Rock classification and nomenclature shown by continuous lines in (a) are from Le Bas et al. (1986).
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Fig. 6. (a) Primitive mantle normalized patterns of trace elements; (b) chondrite-normalized (Sun & McDonough, 1989) patterns of rare earth
elements. Relationships between (c) Ba/Th and Pb/U and (d) Ce/Yb and Sm/Yb are shown for the Chugaryeong basalts. Analytical errors
(2s) in all the diagrams are smaller than the symbols.
Possibility of crustal assimilation
Heat transfer from a hot magma to the surrounding cold
crust results in cooling of the magma, which enhances
crystal fractionation. At the same time, heat from the
magma, including latent heat of crystallization, causes
melting of crustal materials (e.g. DePaolo, 1981; Bohrson
& Spera, 2003). As noted above, because tiny plagioclase
crystals with strongly corroded cores may have originated
from crustal materials, the effect of crustal assimilation
should be examined.
Crustal materials and their partial melts usually have distinct major and trace element and isotopic compositions
compared with primary basaltic magmas; thus, assimilation
of wall-rock melts or bulk crustal materials can be identified.
The Sr and Nd isotope compositions of plausible mafic to silicic crustal materials below the Chugaryeong volcano are
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Fig. 7. Sr^Nd^Pb isotopic compositions of the Chongok and Chatan basalts. (a) 143Nd/144Nd, vs 87Sr/86Sr; (b) 207Pb/204Pb vs 206Pb/204Pb; (c)
208
Pb/204Pb vs 206Pb/204Pb; (d) 143Nd/144Nd vs 206Pb/204Pb. Peridotite xenoliths exhumed in Korean basalts (Choi et al., 2005, 2008),
Changbaishan and Kuandian basalts (Kuritani et al., 2011), and depleted mantle (DM), recent sediment, and inferred ancient sediment compositions (Kuritani et al., 2011) are also plotted. Analytical errors (2s) in all diagrams are smaller than the symbols.
shown in Fig. 7a. Because these all have Sr and Nd isotope
compositions that are more enriched than the Chugaryeong
basalts, the Chatan basalts, which have more radiogenic Sr
isotope compositions than the Chongok basalts, should be
more affected by crustal contamination if the difference between the two basalts resulted from wall-rock assimilation.
Here we test the possibility of an assimilation and fractional
crystallization (AFC) model, as described by DePaolo
(1981). When the mass ratios of crystallization (Mc) and assimilation (Ma) are not equivalent, but constant (r Ma/
Mc 6¼ 1), the mass-balance equation for an element in the
AFC process is described by equation (6a) of DePaolo (1981):
r C
Cm
a
z
¼
F
þ
ð1 F z Þ
ð1Þ
0
0
r 1 zCm
Cm
where
rþD1
r1
ð2Þ
Mm
Mc
¼ 1 þ ðr 1Þ 0 ,
0
Mm
Mm
ð3Þ
z¼
F¼
0
Cm
is the original concentration of an element in the
magma, Cm is the current concentration of the element in
the magma, Ca is the concentration of the element in the
assimilant, D is the bulk solid/liquid partition coefficient
0
for the element, Mm
is the initial mass of the magma
body, Mm is the current mass of the magma body, and
Mc is the total mass of material fractionated. The
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Fig. 8. Trajectories of AFC model calculation for the Chugaryeong basalts. (a) 143Nd/144Nd vs 87Sr/86Sr; (b) 87Sr/86Sr vs Sr; (c) 87Sr/86Sr vs Nb;
(d) 87Sr/86Sr vs Zr. Bold and fine dashed curves are trajectories of the AFC model calculation when 30% and 10% partial melts of mafic crust
are used respectively, and bold and fine continuous curves are trajectories of the AFC model calculation when 30% and 10% partial melts of
a silicic crust are used respectively. The dotted curve is an AFC trajectory when r ¼ 0·8 and 30% partial melt of a silicic crust are used. The
0
) is not marked on the fine dashed and continuous curves or the dotted curve.
total mass of assimilant (Ma) divided by the initial mass (Mm
Partial melts generated by 10% melting of mafic and silicic crust are not plotted in (c) and (d) because they are beyond the range of the figures.
parameter Mc/Mm0 represents the degree of crystallization
(i.e. the fraction of initial liquid mass converted into solid).
We selected mafic and silicic crustal materials reported
from Gyeonggi massif in the centre of the Korean
Peninsula (Williams et al., 2009) as a proxy for the crustal
assimilant. The melting stoichiometries for mafic and silicic
crust were taken from Beard & Lofgren (1991) and Beard
et al. (1993), respectively. The reaction stoichiometries and
partition coefficients between minerals and melt are
assumed to be constant during the AFC process.The crystallization mode for basalt is assumed to be olivine, clinopyroxene, and plagioclase in the weight ratio 0·33:0·33:0·34. As
will be discussed in the following section, this mode of
crystallization may not be appropriate for the
Chugaryeong basalts. However, we used the crystallization
mode above to evaluate as best as possible the effect of
plagioclase crystallization on Sr concentration. Kobayashi
& Nakamura (2001) suggested a higher assimilation rate for
lower crust than for upper crust because the higher temperature conditions of the lower crust make it possible to reach
the solidus temperature more easily than in the upper crust.
Assimilation rates for mafic and silicic crust are assumed to
be 0·2 and 0·8, respectively.
An AFC process can reproduce the differences in Sr^Nd
isotope composition and Sr concentration between the
Chongok and Chatan basalts (Fig. 8a and b) whereas the
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abundances of other incompatible trace elements cannot be
consistently reproduced by an AFC model (Fig. 8c and d).
In terms of the Sr^Nd isotope system, the Chatan basalts
can be modeled by 2 and 12 wt % assimilation of partial melts of mafic and silicic crustal materials, respectively
(7217-5A and 7628-2B; Williams et al., 2009) together with
2·5 wt % and 60 wt % of crystallization, when the assimilation rate r is 0·2 and 0·8 for mafic and silicic crust,
respectively, and a degree of partial melting of the crust of
30% (Fig. 8a and b). Because partial melts of crustal materials are poor in Sr and rich in other incompatible trace
elements, the melt composition will be more affected by
an assimilant when the assimilation rate r is high. As a
result, the trajectory of AFC modal curves is steeper in
the case of r ¼ 0·8 (dotted curve), compared with r ¼ 0·2
(bold continuous curve) and deviates from the trend of
the Chugaryeong basalts (Fig. 8a). When the degree of
melting of the crust is small (10%), the AFC trajectories
for either mafic or silicic crust do not fit the trend of the
Chugaryeong basalts (Fig. 8a). This is because the concentration of Sr in the partial melt is lower at lower extents of
melting than at higher extents of melting. As AFC progresses, the Sr concentration decreases in both mafic and
silicic crust because Sr is compatible in plagioclase and
alkali-feldspar. Although the estimated mass of the assimilant is inconsistent with that estimated for the Sr^Nd isotope system, the trajectory of AFC for silicic crust is able
to better reproduce the Sr concentrations than that for
mafic crust (Fig. 8b). This is because the abundance of Sr
in a partial melt of mafic crust is lower than that in silicic
crust. However, assimilation of such isotopically enriched
crustal materials cannot consistently reproduce the difference in other trace element contents between the
Chongok and Chatan basalts, using the isotope compositions of either mafic or silicic crust. Because a partial
melt of crustal material is richer in incompatible trace
elements, which are incompatible in plagioclase and
alkali-feldspar, than the Chugaryeong basalts, the abundance of these incompatible trace elements in the melt
should increase with progress of the AFC process (Fig. 8c
and d). The abundance of Sr in the melt would increase
with continuing AFC if plagioclase was not considered as
a fractional crystallization phase. In this case, all the incompatible trace elements in the melt increase with AFC
while the melt develops a more enriched Sr and Nd isotope
composition. This is inconsistent with the difference between the Chongok and Chatan basalts, as the isotopically
more enriched Chatan basalts are poorer in incompatible
trace elements than the Chongok basalts.
Bohrson & Spera (2001) pointed out that incompatible
trace elements do not necessarily increase monotonically
with the progress of AFC. Because fractional melting
depletes incompatible trace elements in the residual
country rock with an increase in the degree of melting,
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the anatectic melt becomes depleted in such incompatible
trace elements. Therefore, the concentration of such elements in the contaminated magma can decrease with progressive AFC, despite their incompatibility. However, in
this case, a decrease in Sr concentration without a large increase of 87Sr/86Sr becomes difficult to achieve because
the concentration of Sr in a partial melt of the crust increases with increasing degree of melting. Assimilation of
such a melt will increase 87Sr/86Sr with a decrease or even
an increase in Sr concentration. This trend does not fit
the trend seen at Chugaryeong; that is, that of a decrease
in Sr concentration without a large increase in 87Sr/86Sr
(Fig. 8b).
All of these lines of evidence indicate that crustal assimilation is not the process that results in the geochemical
variation between the Chongok and Chatan basalts. We
cannot dismiss the possibility that the Chongok and
Chatan basalts originated from two distinct primary melts
that underwent AFC independently. However, it is too ad
hoc an explanation to invoke AFC of two independent primary melts to explain the systematic difference between
the Chongok and Chatan basalts. We therefore conclude
that the geochemical variations in the Chugaryeong basalts reflect variation in the primary melts produced in the
upper mantle and not assimilation of crustal material.
Possibility of assimilation of lithospheric mantle
Assimilation of wall-rock may occur not only in the continental crust but also in the mantle during migration of the
melt from its source region. Peridotite^melt interaction
has been reported from mantle samples in ophiolites
(Kelemen, 1990; Zhou et al., 1996), ultramafic intrusive
complexes (Quick, 1981), and peridotite xenoliths (Arai &
Abe, 1995). Because reactions between primary melts and
mantle peridotite at pressures less than the pressure of
melting involve reaction between a melt undersaturated
in low-Ca pyroxene and peridotite containing low-Ca
pyroxene, the interaction of a primary basaltic melt with
lherzolite or harzburgite generally increases the SiO2
content in the derivative liquids as interaction progresses
(e.g. Kelemen et al., 1990). Because mantle xenoliths from
northeastern Asia have depleted Sr^Nd isotopic compositions (Fig. 7a), melts that undergo greater degrees of interaction with the lithospheric mantle are expected to be
richer in SiO2 and isotopically more depleted than those
with less contribution from the lithospheric mantle.
However, the Chongok basalts, which are more depleted
in their isotopic composition, are lower in SiO2 content
than the Chatan basalts. This geochemical difference between the Chongok and Chatan basalts suggests that assimilation of lithospheric mantle during melt migration
does not play an important role in producing the geochemical variations between the two basalts.
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Crystal fractionation
Crystal fractionation could be a dominant process in
the generation of the major element compositions and
the variation in the abundances of trace elements in the
Chugaryeong basalts, as magma mixing, crystal accumulation, and crustal assimilation have been shown not to play
an important role. We examine whether or not the major
element compositions in the basalts can be explained by
crystal fractionation from a common undifferentiated
parent basalt melt on the basis of probability Q, which is the
probability in a w2 test and is a quantitative measure for the
goodness of fit calculated from the w2 value at its minimum
and the number of degrees of freedom in a least-squares optimization (Press et al.,1992).
Olivine and clinopyroxene are used as potential fractionated crystal phases, because they are likely to be the
major phenocryst minerals in the Chugaryeong basalts.
The major elements Si, Al, Fe, Mg, and Ca, which are
compatible in these minerals, are investigated.
To constrain the liquid line of descent for the Chugaryeong basalts, we calculated groundmass compositions by
subtracting the observed phenocryst compositions from
the whole-rock compositions (Fig. 5). Parent and daughter
magmas are represented by the average compositions of
the least fractionated and most fractionated groundmass
compositions respectively, as shown in Fig. 5. Starting
from the two parental magmas (Cho-1GM and Cha-1GM;
Fig. 9), differentiated magmas (Cho-2GM and Cha-2GM)
can be derived from the parental magma in the same
group with the criterion Q40·05 (Fig. 9 and Table 4).
The compositional variations in the Chongok basalts
can be explained by 3·2 wt % of olivine and 1·5 wt %
of clinopyroxene fractionation, whereas that of the
Chatan basalt is explained by 1·8 wt % of olivine fractionation (Table 4). This difference in fractionated phase
proportions is consistent with the lower CaO contents in
the Chongok basalts than the Chatan basalts, for a given
relatively differentiated MgO content of 8 wt % (Fig. 9b
Fig. 9. Results of mass-balance calculations for the Chongok and Chatan basalts. Major element variations for these basalts can be reasonably
reproduced by a fractional crystallization process. Arrows pointing towards the reduced MgO direction indicate the removal of minerals.
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Table 4: Results of mass-balance calculation and its w2 test
for Chongok and Chatan basalt
clinopyroxene
olivine
melt
w2
Chatan
0·0
1·9
98·1
2·32
0·80
Chongok
1·4
3·2
95·4
1·12
0·95
Q
Q is probability in w2 test and a quantitative measure for the
goodness of fit calculated from the w2 value at its minimum
and the number of degrees of freedom in least-squares
optimization (Press et al., 1992). w2 is a normalized sum
of squared deviations between observed and calculated
element concentrations.
and d), because the fractionation of clinopyroxene effectively removes CaO from the melt. The difference in major
elements between the least fractionated Chongok and
Chatan basalts (Cho-1GM and Cha-1GM) cannot be
derived from the least differentiated samples of the other
basalt by any crystal fractionation process. We therefore
conclude that the primary magmas of the Chongok and
Chatan basalts are different and their differences cannot
be generated solely by intra-crustal processes.
To check the probability of the mass-balance model
above, we also performed MELTS calculations (Ghiorso
& Sack, 1995; Asimow & Ghiorso, 1998; Smith & Asimow,
2005) to investigate whether the major element trends for
the Chongok and the Chatan basalts can be reproduced
by isobaric fractional crystallization starting from the
least differentiated compositions of the two basalts
(Fig. 10). Water contents used in the initial melts were 0·5,
1, and 2 wt %, and crystallization pressures were set to
0·05 and 0·5 GPa. Isobaric crystallization paths were calculated with incremental decreases in temperature of 2 K
under a fayalite^magnetite^quartz (FMQ) oxygen fugacity buffer.
Liquid lines of descent for both the Chongok and the
Chatan basalts can be reasonably reproduced by fractionation of olivine followed by clinopyroxene (Fig. 10) although there is very little sign of clinopyroxene as a
phenocryst phase. As pressure increases, clinopyroxene
begins to crystallize and the MgO content of the melt becomes higher; however, the effect of water on the liquid
line of descent is larger than that of pressure. As the water
content in the melt increases, the liquidus temperature of
clinopyroxene decreases, and accordingly the MgO content at which the CaO content and CaO/Al2O3 ratio start
decreasing with a decrease in MgO content decreases. For
the CaO/Al2O3 of the Chongok basalts to decrease, the
water content in the starting melt must be less than 2 wt
%. In contrast, 2 wt % of water is required for the starting melt of the Chatan basalt to reproduce its trend,
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MARCH 2014
which maintains a constant CaO/Al2O3 for all MgO
contents.
Kuritani et al. (2013) estimated the water content of the
basalts from the Wudalianchi volcanic field in northeastern
China to be more than 1·1wt %, based on thermodynamic calculations, although the upper limit of water
contents in the Wudalianchi magmas has not yet been constrained. Our estimation of the water content in the
Chugaryeong magma is slightly higher than the lower
limit of the water content in the Wudalianchi basalts.
Estimation of primary melt compositions
Because the stability field of olivine increases as pressure
decreases, primitive basaltic magmas normally start to
crystallize olivine followed by clinopyroxene or plagioclase
with increasing cooling (e.g. Kushiro, 1969). Even the two
least differentiated samples (2704 and 2404) from the
Chugaryeong basalts have fractionated to a certain extent
as shown by their elevated FeO*/MgO 1·0, which is not
in equilibrium with the olivine in the potential source
mantle peridotite (FeO*/MgO40·7). Therefore, the primary basalt compositions need to be estimated by the addition of olivine. Here, we calculate the primary magma
compositions for the Chongok and the Chatan basalts
using an olivine maximum fractionation model (e.g.
Tatsumi et al., 1983). This involves the stepwise addition of
olivine in equilibrium with the melt until the calculated
melts reach equilibrium with mantle olivine. The ferric/ferrous iron ratio of the host basalt affects the calculation of
olivine composition, so we assumed Fe3þ/(Fe3þ þ Fe2þ) in
the melt to be constant at 0·1 (Tatsumi et al., 2005). The
Fo# of olivine in the residual peridotite is assumed to be
Fo# ¼ 90, the reasons for which are described below.
As peridotite becomes depleted owing to melt extraction, the Cr# of spinel becomes elevated (Arai, 1987). In
addition, the Cr# in spinel at the transition between lherzolite and harzburgite with increased melting is known to
depend on the fertility of the peridotite: the Cr# of the
lithological change becomes higher as the peridotite becomes more fertile (Pickering-Witter & Johnston, 2000;
Schwab & Johnston, 2001). Mantle xenoliths from eastern
Asia show a narrow range of Mg# from 89 to 91·5 (Arai
et al., 2007), and the Cr# of spinel in peridotite xenoliths
at the transition from lherzolite to harzburgite is 20
(Choi et al., 2005, 2008). This suggests that the peridotite
mantle beneath the Korea Peninsula and Hannuoba is
moderately depleted in clinopyroxene (Fig. 4c). Provided
that the peridotite source mantle of the Chugaryeong basalts follows the local OSMA as indicated by the mantle
xenoliths, the Fo# at the intercept of the back-extrapolation of the fractionation trends of spinel inclusions should
indicate the Fo# of the residual source peridotite in equilibrium with the primary melt (Fig. 4c). The trends of the
Chongok and the Chatan basalts intercept the OSMA at
Fo# 90. We therefore use Fo# ¼ 90 for the mantle
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SAKUYAMA et al.
METASOMATIZED ASTHENOSPHERE MELTING
Fig. 10. Representative major element compositions plotted versus MgO for calculated groundmass compositions for the Chongok and Chatan
basalts and fractional crystallization paths modelled by the MELTS program for the conditions indicated in (c). (See text for details.)
olivine composition in equilibrium with the primary melts
for both the Chongok and Chatan basalts. The validity of
this assumption will be discussed below. The estimated
major and trace element compositions of the primary
melts, and the amounts of added olivine are given in Table
5. The estimated primary basalt compositions differ from
one another, suggesting that two distinct Chongok and
Chatan primary magmas existed at 0·5 and 0·15 Ma,
respectively.
Possibility of mafic source material
Mafic materials, such as pyroxenite or eclogite, have been
considered as plausible components in the sources of some
continental flood basalts and ocean island basalts (e.g.
Yasuda et al., 1994; Takahashi et al., 1998; Sobolev et al.,
2007). Herzberg & Asimow (2008) proposed a procedure
to distinguish between mafic source and ultramafic source
basalts based on the CaO and MgO contents of the basalts.
Their discrimination diagram is built upon a melting experiment conducted in the high-pressure garnet stability
field on fertile peridotite KR4003 (Walter, 1998). When
the Chugaryeong basalts are plotted on this discrimination
diagram, they fall in the mafic source region, suggesting a
contribution of mafic material to the generation of the
Chugaryeong basalts. However, partial melts of peridotite
KLB1 (Hirose & Kushiro, 1993), PHN1611 (Kushiro, 1996),
MPY (Robinson & Wood, 1998; Robinson et al., 1998),
FER (Pickering-Witter & Johnston, 2000), and INT
(Schwab & Johnston, 2001) at pressures less than 3 GPa
can be lower in CaO at a given MgO content than at
high pressure and plot within the mafic-source pyroxenite
region defined by Herzberg & Asimow (2008). As discussed below, the Chugaryeong primary basalts probably
originate in the spinel stability field, suggesting that the
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Table 5: Estimated primary melt compositions for the
Chongok and Chatan basalts
Chatan (0·15 Ma)
Chongok (0·50 Ma)
Amount of olivine added:
18 wt %
18 wt %
NiO in Fo90 olivine:
0·31 wt %
0·31 wt %
Major elements (wt %)
SiO2
47·24
TiO2
1·42
46·81
1·65
Al2O3
12·90
13·01
FeO*
10·42
10·17
MnO
0·16
0·16
MgO
16·55
16·21
CaO
7·01
6·95
Na2O
2·67
2·99
K2O
1·29
1·64
P2O5
0·28
0·35
NiO
0·06
Total
100
Fig. 11. Normative compositions of the estimated primary melts of
the Chongok and Chatan basalts projected on the Ne’^Ol’^Qtz’
plane. Arrows on each isobaric curve represent the direction of increase of the degree of melting. The projection scheme is after Irvine
& Baragar (1971): Ne’ ¼Ne þ 0·6Ab, Qtz’ ¼Qtz þ 0·4Ab þ 0·25Opx,
Ol’ ¼Ol þ 0·75Opx. (See text for details.)
0·05
100
application of the discrimination diagram of Herzberg &
Asimow (2008) is not appropriate for constraining their
petrogenesis. In addition, the FeO contents of the
Chugaryeong basalts are not as high as those of basalts
that cannot be derived from partial melting of peridotite
(Dasgupta et al., 2007). We therefore consider that peridotite is a reasonable source lithology for the Chugaryeong
basalts.
Trace elements (ppm)
Rb
Ba
14·7
208
20·1
255
Th
2·13
2·77
U
0·52
0·75
Ta
1·19
Nb
1·55
19·1
24·8
La
16·1
19·9
Ce
33·2
40·9
Pb
Pr
Sr
Nd
Zr
2·11
3·96
483
16·4
130
4·89
574
20·2
159
Hf
3·01
3·62
Sm
3·73
4·51
Eu
1·34
1·58
Gd
3·86
4·60
Tb
0·61
0·71
Dy
3·50
Y
16·9
Melting pressure and temperature of the
source mantle
2·60
4·03
19·3
Ho
0·68
0·78
Er
1·89
2·16
Tm
0·26
0·29
Yb
1·63
1·83
Lu
0·24
0·27
Value of Fe3þ/(Fe3þ þ Fe2þ) ratio was assumed as 0·1.
*Total iron as FeO calculated after estimation of primary
melt.
High pressure^temperature melting experiments on peridotites with various degrees of fertility have revealed that
the SiO2 contents of the partial melts monotonously increase with decreasing pressure (e.g. Kushiro & Kuno,
1963; Walter, 1998). First, we estimate melting pressure
under anhydrous conditions by using the inferred primary
basalt compositions and data from partial melting experiments at high pressure conducted by Sakuyama et al.
(2009). Melting pressures for the Chongok and Chatan basalts under anhydrous conditions are estimated to be 2·7
and 2·6 GPa (80 km depth), respectively (Fig. 11).
In contrast, the addition of water to the melting system
is also known to increase SiO2 in partial melts at a given
pressure (e.g. Kushiro, 1972; Hirose & Kawamoto, 1995).
As shown in Fig. 5a and Table 5, the systematic difference
in SiO2 between the Chongok and the Chatan basalts
could reflect a difference in melting pressure or in the
amount of water in the mantle source. As the effects of
water and pressure on the partial melt composition of peridotite are opposed to one another, to examine melting conditions it is necessary to estimate the water contents in the
primary basalts. Tatsumi et al. (1983) conducted high-
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SAKUYAMA et al.
METASOMATIZED ASTHENOSPHERE MELTING
Fig. 12. Estimated melting pressure and temperature for the
Chongok and Chatan basalts. Anhydrous peridotite solidi are shown
by continuous and dashed curves (Hirschmann, 2000; Katz et al.,
2003). Adiabatic P^T path for solid peridotite (178C GPa^1) for a
mantle potential temperature of 14008C is shown by the wide gray
line. The lithological boundary between garnet and spinel stability
(Robinson & Wood, 1998) and the xenolith-derived geotherm (Xu,
2007) are shown by the fine dot^dash line and bold continuous
curve, respectively. The thickness of the lithosphere beneath the
Korean Peninsula (An & Shi, 2006; Zhu, 2007) is shown by the
double-headed arrow.
pressure experiments to find the conditions at which a primary melt is multiply saturated with lherzolite or harzburgite assemblages. They showed that an addition of 1·5
and 3 wt % of water into the melt increases the saturation
pressure by 0·2 GPa and 0·5 GPa, respectively. This relationship can be formulated as
P wet ¼ P dry þ H2 Oprimary 05
30
where Pwet and Pdry are the melting pressure under hydrous and anhydrous conditions, respectively, and
H2Oprimary is the water content in primary melt. We utilize
this relationship to estimate melting pressure under hydrous conditions. If we assume 2 wt % of water in the primary melts of both the basalts, the melting pressure is
estimated to be 3 GPa. The equivalent depths are slightly
deeper than or almost equal to the depth of the base of
the lithosphere beneath the Korean Peninsula (An & Shi,
2006; Zhu, 2007).
Next, we estimated the melting temperature using the
relationship between temperature, pressure, and MgO
content in melts saturated with olivine and/or clinopyroxene (Maaloe, 2004). An equation for the depression of the
liquidus temperature of olivine-saturated primitive basaltic
melt by the effect of water (Medard & Grove, 2008) was
used for estimation of the hydrous melting temperature.
Using these equations, the melting temperature for the
Chugaryeong basalts was estimated to be 14908C under
anhydrous conditions and 14208C under hydrous conditions (with 2 wt % of water in the primary melt) (Fig. 12).
For comparison, we estimated melting pressures and temperatures using the primary melts with 2 wt % of water
estimated in this study and the thermobarometer of Lee
et al. (2009). Pressures and temperatures of melting were
estimated to be 2·67 GPa and 14168C for the Chongok
basalt and 2·52 GPa and 14198C for the Chatan basalt.
Pressures estimated by the Lee et al. (2009) model are also
within the spinel peridotite field but 0·4 GPa lower than
our estimation, whereas temperatures show good agreement with our calculations.
Our estimates of the pressure^temperature conditions of
magma generation for the Chugaryeong basalts are restricted (2·5^3·0 GPa and 1420^14908C), even when we
consider a variation of water content (2 wt %) in the primary magmas. The estimated pressures correspond to the
depth of the uppermost asthenospheric mantle within the
spinel stability field. The absence of garnet peridotite xenoliths from Cenozoic basalts in the Korean Peninsula (Arai
et al., 2007) is also consistent with melting of asthenospheric spinel peridotite. The elevated concentrations of
fluid-mobile elements in the Chatan basalts may reflect an
increased fluid flux in these basalts (Fig. 6c). If these elements originated in a fluid flux, then the degree of melting
of the source mantle might have increased progressively
as a result of the additional fluid input in the mantle
source at constant pressure. This progressive melting
model is consistent with the estimated spinel Cr# in the
source mantle. The residue of the older Chongok source
would have had a lower Cr# than that of the younger
Chatan source, suggesting a more depleted residual source
mantle for the Chatan basalt (Fig. 4c).
Source mantle compositions of the
Chongok and the Chatan basalts
We have attributed the systematic differences in whole-rock
and mineral major element compositions between the
Chongok and the Chatan basalts to differences in melting
conditions in the mantle. Here we test the origin of these
systematic differences in whole-rock trace element compositions between the two basalts.
The trace element compositions of partial melts of peridotite depend mainly on the residual mineral assemblage,
the melting mode, and the partition coefficient between
minerals and melts. The pressure-dependent garnet^spinel
transition is one of the most important phase changes in
the upper mantle (Robinson & Wood, 1998), and can drastically change the chemical composition of the partial
melts. Mineral^melt partition coefficients for the heavy
rare earth elements (HREE) are greatly controlled by the
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JOURNAL OF PETROLOGY
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presence of residual garnet, because these partition coefficients are greater than unity in contrast to those for spinel
for which the HREE partition coefficients are extremely
low (e.g. Green, 1994). Hence, the abundance of HREE in
the partial melts increases with increasing melt fraction in
the garnet field, whereas it decreases in the spinel field.
If both the Chongok and Chatan basalts were generated
by differing degrees of partial melting of a common
source mantle in the garnet stability field, the Chongok
basalts should have been generated by a lower degree of
melting than the Chatan basalts, because the LREE/
HREE and middle REE (MREE)/HREE ratios of the
Chongok basalts are higher than those of the Chatan basalts (Fig. 6d). Concurrently, in the case of garnet peridotite
melting, the abundance of HREE in the Chongok primary
basalt should be lower than that of the Chatan primary
basalt, whereas the abundances of LREE and MREE of
the Chongok basalt should be higher than those of the
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Chatan basalt (Fig. 13a). In other words, the REE patterns
of the primary melt of Chongok basalt cross over those of
the Chatan basalt at an element with Dsolid/melt 41 (Fig.
13a).
The abundance of trace elements in the estimated primary melts depends on amount of olivine added to the
least differentiated samples; the amount of olivine addition
is constrained by the assumed Fo# of residual olivine in
equilibrium with the primary melt. If the amount of olivine added to the least differentiated basalts was almost
identical in the Chongok and Chatan basalts, the abundance of all trace elements in the Chongok basalt would
be higher than that of the Chatan basalt (Fig. 13a). For
the trace element patterns of the Chongok and Chatan primary basalts to cross over one another at an element with
Dsolid/melt 41, a further 8 wt % addition of olivine to the
Chongok basalt is required compared with the Chatan
basalt. Thus, the Fo# of olivine in equilibrium with the
Fig. 13. (a) Schematic chondrite-normalized REE patterns of the least differentiated Chongok and Chatan basalts and estimated primary
melts of the two basalts. (b) Relative abundance of La, Dy, and Yb divided by the estimated primary melt composition of the Chatan basalt
in equilibrium with olivine with Fo# ¼ 90 plotted against the assumed Fo# of olivine in equilibrium with the Chongok primary melt. (c)
Averaged plausible primary melt compositions for the Chongok and Chatan basalts, estimated source mantle compositions for the Chongok
and Chatan basalts, respectively, and range of mantle xenolith compositions (gray fields) reported from the Korean Peninsula (Choi et al.,
2005) and eastern China (Rudnick et al., 2004) normalized to primitive mantle (Sun & McDonough, 1989). (d) Difference of estimated source
mantle compositions between the Chatan and Chongok basalts normalized to the averaged value of the estimated source compositions. Open
diamonds (Ta, Nb, Nd, Zr, Hf, Sm, Eu, Gd, Tb, Dy, Y, Ho, Er, Yb, and Lu) indicate those elements that are used for inversion calculation.
Error bars are calculated on the basis of 1s of the estimated primary melts for the Chongok and Chatan basalts shown in (a).
(continued)
518
SAKUYAMA et al.
METASOMATIZED ASTHENOSPHERE MELTING
Fig. 13. Continued.
primary melt of Chongok is required to be greater than
that of the Chatan basalt (Fig. 13a and b).
According to high-pressure melting experiments on peridotite, an increase by 10% in the degree of melting will
increase olivine Fo# by approximately one unit (e.g.
Kushiro, 1996). This positive relationship between the Fo#
of olivine and the degree of melting suggests that a residual
mantle source with a higher Fo# of olivine should have
experienced a higher degree of melting than one with
lower Fo#. Thus, a basalt in equilibrium with Fo# ¼ 91
olivine would be generated by a higher degree of melting
of peridotite than a basalt with Fo# ¼ 90. This contradicts
the inference from the trace element ratios of the two basalts as mentioned above: the Chongok basalt should have
been generated by a lower degree of partial melting than
the Chatan basalt. We therefore argue that partial melting
of peridotite in the spinel stability field is more appropriate
to reproduce the geochemical variation in the Chongok
and Chatan basalts than melting within the garnet stability
field, and that the assumption of Fo# ¼ 90 is reasonable
for both the major and trace element compositions of the
two basalts to be consistent with one another, provided
that the difference in trace element compositions between
the two basalts originated from different degrees of melting
of common source mantle. Because the estimated pressures
of the Chongok and Chatan primary magmas (2·7 GPa;
see above discussion) are lower than the pressures of the
garnet stability field (43 GPa; Robinson & Wood, 1998),
we argue for partial melting of spinel lherzolite in the following discussion.
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JOURNAL OF PETROLOGY
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Figure 13c shows estimated trace element compositions
of the primary basalts for the Chongok and Chatan units
(see also Table 5 and discussions above). The LREE/
MREE and LREE/HREE ratios of the primary Chongok
basalt are slightly higher than those for Chatan, and the
trace element abundances of the Chongok basalt are also
higher than those of Chatan. The difference in the trace
element compositions between the Chongok and the
Chatan primary basalts could, therefore, reasonably be explained by minor differences in the degree of melting of a
common mantle source in the spinel stability field. The
Chongok basalt must have been generated by a lower
degree of partial melting. This is also supported by the
fractionation trends in the Cr#^Fo# plots as noted
above (Fig. 4c).
Once we assume a common original source mantle for
the two basalts, both the extent of melting and the source
mantle composition can be estimated by a series of inversion calculations using the two primary basalt compositions. Fluid-mobile elements, such as Ba, Pb, and Sr,
show clear positive spikes in comparison with neighbouring elements in Primitive Mantle normalized multi-element diagrams (Fig. 13c), suggesting the involvement of
fluid components probably derived from dehydration of
the stagnant slab sediments lying beneath northeastern
Asia (Kuritani et al., 2011). If this is the case, it is not appropriate to discuss these trace elements in relation to the
extent of melting. We therefore first investigated 15 fluidimmobile elements (Ta, Nb, Nd, Zr, Hf, Sm, Eu, Gd, Tb,
Dy, Y, Ho, Er, Yb, and Lu) representing the HFSE,
MREE and HREE to constrain the degree of melting. We
estimate the extent of melting for the Chongok and
Chatan basalts and the common source mantle composition by adopting a non-modal, batch-melting model
(Shaw, 2000). Then, we estimate the concentration of
fluid-mobile elements, such as Rb, Ba, K, Pb, and Sr, in
the source mantle using the extent of melting estimated
based on fluid-immobile elements. We then evaluate the
contribution of the fluid component to the source mantle
by comparing concentrations of fluid-mobile elements
with those of the neighbouring fluid-immobile elements in
a multi-element plot. The initial bulk and melting mode
for spinel lherzolite are assumed to be clinopyroxene, olivine, orthopyroxene, and spinel in the weight ratios
0·15:0·58:0·25:0·02 (Hirose & Kushiro, 1993) and 0·82:^
0·30:0·40:0·08 (Kinzler & Grove, 1992), respectively.
Variable parameters are the extent of partial melting and
the source mantle composition. The inversion calculations
involved (1) varying both the extent of melting and the
composition of the source mantle for the Chongok and
the Chatan primary basalts separately, while (2) simultaneously minimizing the differences between the estimated
source mantle compositions from the two estimates, and
(3) eventually deriving the minimum conversions in the
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errors of the common mantle composition. We used averaged values of the estimated primary melt compositions
for the Chongok and the Chatan basalts, taking the standard deviations of samples from each group as the errors
(Table 5 and Fig. 13c).
Estimated values for the degrees of melting for the
Chongok and the Chatan sources are 7·5% and 10·0%, respectively. The best-fitting source mantle composition is
enriched in LILE and LREE, depleted in HREE, and
characterized by positive spikes in Ba, K, Pb, and Sr
(Fig. 13c and Table 6). This LILE^LREE-enriched trace
element pattern of the source mantle for the Chugaryeong
Table 6: Partition coefficients and chemical compositions
(ppm) of estimated source mantle compositions for the
Chugaryeong basalt
Ol
Opx
Cpx
Sp
Estimated source
composition
Chongok
1·519
Chatan
Rb
0·000179
0·0006
0·00175
0
Ba
0·000032
0·0035
0·0006
0
Th
0·000052
0·013
0·00531
0
0·219
0·221
U
0·000018
0·0017
0·00361
0
0·057
0·052
Ta
0·00007
0·003
0·0102
0·02
0·123
0·124
Nb
0·0001
0·003
0·0077
0·02
1·958
K
0·000177
0·0003
0·0072
0
La
0·000028
0·0025
0·0536
0·01
1·627
1·703
Ce
0·000038
0·005
0·0858
0·01
3·514
3·621
Pb
0·000479
0·0013
0·072
0
0·200
0·214
Pr
0·0008
0·0048
0·15
0·01
0·456
Sr
0·0015
0·007
0·1283
0
Nd
0·00042
0·0068
0·1873
0·01
Zr
0·007
0·021
0·123
0·02
Hf
0·0038
0·01
0·256
0·02
0·391
Sm
0·0013
0·01
0·291
0·008
0·497
0·482
Eu
0·0016
0·013
0·31
0·007
0·178
0·176
Gd
0·0055
0·016
0·3
0·006
0·528
0·516
Tb
0·0041
0·019
0·31
0·009
0·083
0·082
Dy
0·01
0·022
0·33
0·01
0·493
0·491
Y
0·007
0·06
0·421
0·0023
2·719
2·643
Ho
0·007
0·026
0·31
0·009
0·093
0·094
Er
0·0087
0·03
0·29
0·01
0·258
0·261
Tm
0·009
0·12
0·255
0·01
0·040
0·041
Yb
0·017
0·049
0·28
0·008
0·234
0·239
Lu
0·02
0·06
0·28
0·02
0·035
0·036
19·38
1066
52·58
1·976
15·57
1·473
21·03
1·978
1084
0·456
54·98
1·944
15·65
0·385
Ol, olivine; Opx, orthopyroxene; Cpx, clinopyroxene; Sp,
spinel.
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SAKUYAMA et al.
METASOMATIZED ASTHENOSPHERE MELTING
basalts is more similar to that of peridotite mantle xenoliths from Hannuoba, in eastern China (Rudnick et al.,
2004), than to that of xenoliths from South Korea (Choi
et al., 2005).
The prominent positive spikes in fluid-mobile elements,
such as Rb, Ba, K, Pb, and Sr, cannot, however, be reproduced even by melting of Hannuoba-type mantle. This
strongly suggests the influence of a fluid flux, in addition
to an incompatible trace element-enriched source mantle,
in the generation of the Chugaryeong basalts.
There are systematic differences between the Chongok
and the Chatan source mantle. Figure 13d shows the differences in the estimated source mantle compositions for
Chongok and Chatan, including fluid-mobile elements. It
should be noted that we did not apply any constraints to
Rb, Ba, Th, U, K, La, Ce, Pb, and Sr for the inversion calculations. The Ba, La, Pb, and Sr contents in the estimated
source mantle of the Chatan basalt are slightly higher
than for the Chongok basalt. In particular, Ba and Pb concentrations in the source mantle of the Chatan basalt are
clearly higher than for the Chongok basalt (Fig. 13d).
It is difficult to invoke the lithospheric mantle as a direct
source for the Chugaryeong basalts because of the discrepancy between the estimated melting temperature and the
lithospheric geotherm. From petrological studies of
mantle xenoliths, the estimated geotherm beneath eastern
Asia (Xu, 2007) is much lower than the necessary melting
conditions of the Chugaryeong basalts (Fig. 12), suggesting
that the Chugaryeong basalt originated in the asthenospheric mantle. Rudnick et al. (2004) proposed that an injection of incompatible trace element-rich silicate melts
into the lithosphere beneath eastern China during the
Tertiary produced the enriched mantle xenolith suite in
that region. Although the origin of this Tertiary metasomatic event is beyond the scope of this study, it could have
been responsible for the production of an uppermost part
of the asthenospheric mantle with enriched trace element
characteristics, similar to the Hannuoba-type lithospheric
mantle beneath the Korean Peninsula. Here, we assume
the source mantle of the Chugaryeong basalts to be asthenospheric mantle with a LILE^LREE-enriched trace
element chemical composition.
Origin of the fluid flux
Fluids in subducted oceanic plates are released and
recycled to the surface via subduction-related volcanism
(Hacker, 2008; Kimura et al., 2010; van Keken et al., 2011),
although a small amount of water is thought to be transported to the deep mantle, captured in nominally anhydrous minerals (e.g. Iwamori, 2007) and hydrous minerals
(e.g. Ono, 1998). Kuritani et al. (2011) suggested that the
fluids from subducted sediments on the oceanic plate play
an important role in influencing the geochemical characteristics of the basalts in northeastern China. They attributed the selective enrichment in Ba, Pb, and Sr observed in
northeastern Chinese basalts to the breakdown of K-hollandite held in the hydrous sediment layer of the subducted
slab, because those elements are known to be highly partitioned into K-hollandite, which can be broken down at
the depth of the mantle Transition Zone under hydrous
conditions (Rapp et al., 2008).
The general geochemical features of the source mantle of
the Chugaryeong basalts, such as positive spikes in Rb, Ba,
K, Pb, and Sr, are similar to those proposed for the
Changbaishan volcano and can be attributed to the breakdown of a phase such as K-hollandite, as suggested by
Kuritani et al. (2011). Rapp et al. (2008) reportedthat partition
coefficients of Sr, Pb, and Ba into K-hollandite are higher
under hydrous conditions than under dry conditions. In particular, Ba and Pb are the most strongly partitioned into Khollandite under hydrous conditions (Rapp et al., 2008). In
turn, the compatibility of U, Th, and Rb is lower under hydrous conditions than under dry conditions. U in particular
becomes incompatible in K-hollandite under water-saturated conditions, probably because the higher water content
causes a higher oxidation state, hence making U occur as
U6þ, which is incompatible in K-hollandite and compatible
in an aqueous fluid (Rapp et al., 2008). These experimental
results suggest that a fluid released by the breakdown of Khollandite under hydrous conditions would be enriched in
Ba and Pb, moderately enriched in Sr and K, but not enriched in Th or U. The estimated source mantle for the
Chugaryeong basalts shows positive anomalies in Ba, K, Pb,
and Sr, but no significant anomalies in Rb and U, and Th
(Fig.13c).The trace element characteristics of this estimated
source mantle suggest that K-hollandite, which can supply
fluids to the upper mantle by its breakdown, was formed
under hydrous conditions rather than under dry conditions.
In addition, the inference of higher Ba and Pb contents in
the source mantle for the Chatan basalt than for the
Chongok basalt is consistent with the idea that a larger
amount of fluid, released by the breakdown of K-hollandite
under hydrous conditions, was supplied to the source
mantle of the Chatan basalt.
We therefore suggest that the positive spikes in fluidmobile elements and the systematic differences observed
between the Chatan and the Chongok mantle sources are
readily attributable to fluids derived from deep breakdown
of K-hollandite under hydrous conditions. If this is the
case, the melting sequence of the source mantle is as follows: (1) the asthenospheric source mantle beneath the
Chugaryeong area was already rich in incompatible trace
elements; (2) the first partial melting occurred by the addition of a deep fluid flux from the stagnant Pacific Plate,
owing to hydrous breakdown of K-hollandite in the sediment, generating the Chongok basalts at 0·50 Ma; (3) further upwelling of mantle that was more hydrated by fluid
from the sediment yielded partial melt at a higher degree
of melting to form the Chatan basalts at 0·15 Ma (Fig. 14a).
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JOURNAL OF PETROLOGY
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Fig. 14. (a) Schematic illustrations showing the melting process involved in the production of the Chongok and Chatan basalts. At 0·50 Ma,
decompression melting of the upwelling mantle hydrated by fluid released from subducted sediment occurred to generate the Chongok basalt
(1), then further upwelling of the mantle yielded a partial melt to produce the Chatan basalt at 0·15 Ma (2). (b) Schematic diagram illustrating
the inferred upper mantle conditions beneath the Korean Peninsula and northeastern China.
Source mantle characteristics and origin
The upper mantle beneath northeastern China and the
Korean Peninsula has been recognized to have a strong affinity to Enriched Mantle 1 (EM1) (Zou et al., 2000;
Flower et al., 2001; Choi et al., 2005), although the origin of
the enrichment has not yet been identified. The classical
view was that of isotopic ingrowth at the base of the continental mantle lithosphere over 43 Gyr (Tatsumoto &
522
SAKUYAMA et al.
METASOMATIZED ASTHENOSPHERE MELTING
Nakamura, 1991). However, this model is negated by the
evidence of erosion of the continental lithosphere beneath
this area in the late Cenozoic (Gao et al., 2004).
Recently, Kuritani et al. (2011) proposed a model that
attributed the EM1 component to an ancient fluid derived
from subducted sediment 1·5 Gyr ago, which was subsequently trapped in the upper mantle Transition Zone, on
the basis of a correlation between the spatial distribution
of basalt geochemistry and a plume-like low-velocity zone
rising from the Transition Zone across the upper mantle
identified based on seismic tomography data. They
showed a concentric spatial variation in basalt geochemistry, centred on the Changbaishan volcano. With increasing
distance from Changbaishan, the basalts show a decrease
in Ba/Th, Pb/U, and EM1 affinity in Pb isotopes.
The Chugaryeong volcano is located in the middle of the
Korean Peninsula, which is 450 km away from Changbaishan (Fig. 1a), so the contribution of the EM1 component
or ancient fluid here is expected to be less than in the
Changbaishan basalts if the model of Kuritani et al. (2011)
can be extended to the middle of the Korean Peninsula.
The Chugaryeong basalts, however, have more radiogenic
Pb isotope compositions than the Changbaishan basalts
(Fig. 7), whereas the Ba/Th of the Chugaryeong basalts is
much lower than that of Changbaishan. Kuritani et al.
(2011) assumed that the slab sediment flux has two components, one an ancient sediment flux [EM1: modelled by isotopic ingrowth in ancient sediment based on Stacey &
Kramers (1975)], residing for41·5 Gyr in the mantle Transition Zone, and the other an immediate supply from the present-day Pacific Plate subducted sediment (recent sediment
flux; see Fig. 7). The composition of the fluid flux released
from the sediment on the Pacific Plate stagnant slab can be
approximated by the current sediment composition on the
ocean floor, based on studies by Ben Othman et al. (1989),
Plank & Langmuir (1998), Shimoda et al. (1998), Hauff
et al. (2003), and Plank et al. (2007). In general, such sediments have enriched Sr^Nd isotope compositions and have
radiogenic Pb isotope compositions. We can therefore attribute the isotopic composition of the Chugaryeong basalts
to a flux less influenced by the ancient fluid component
and perhaps more influenced by the sediment flux from
the Pacific Plate stagnant slab.
The mantle into which the mixture of ancient and recent
fluids infiltrated is asthenospheric mantle with an LILE^
LREE-enriched trace element compositions, as inferred
from the discussions above. The isotopic compositions of
the mantle xenoliths reported from eastern Asia (Choi
et al., 2005, 2008) are plotted in Fig. 7. They have more
depleted Sr^Nd isotope compositions (Fig. 7a) and have
more radiogenic Pb (Fig. 7b and c) than the Chugaryeong
basalts. The Chatan basalts, which are generated by a
higher degree of melting and are more affected by the
fluid flux, plot further away from the compositional range
of the xenoliths than the Chongok basalts. Kuritani et al.
(2011) assumed the presence of depleted mantle (DM)
(e.g. Zindler & Hart, 1986), represented by the asthenospheric mantle composition beneath the Sea of Japan
(Cousens & Allan, 1992). Mantle xenoliths from the eastern margins of the Asian continent show a very wide
range in isotopic composition, in which the DM component is also included; as such this material is also a plausible source mantle for the Chugaryeong basalts. Based
upon our study of trace elements using an inversion
method, we suggest that the source mantle for the
Chugaryeong basalts is enriched in trace elements similar
to the sampled mantle xenoliths in China. It is reasonable
to assume a source mantle that is enriched in incompatible
trace elements but depleted in terms of its Sr^Nd^Pb
isotope composition. By implication, this suggests that the
enrichment event was relatively recent.
We test the validity of our three end-member model by
using the isotopic compositions of (1) a fluid flux from ancient subducted sediment (Kuritani et al., 2011), (2) a fluid
flux from the stagnant Pacific Plate sediment (see above
for details), and (3) the lithospheric mantle as an analogue
of enriched asthenospheric mantle that may be present beneath the Chugaryeong volcano (Fig. 15). The isotopic
compositions of the Chugaryeong basalts can be reasonably reproduced by mixing between the three components
with 50^60% of the ancient sediment component (Fig. 15).
There are two other Quaternary basalt provinces close to
Chugaryeong: Changbaishan and Kuandian (Fig. 1). The
Changbaishan basalts, which have less radiogenic isotopic
compositions than the Chugaryeong basalts, may be more
influenced by the ancient sediment component. However,
the Kuandian basalts cannot be reproduced by mixing between the three components assumed here. This discrepancy might be attributable to heterogeneity in the mantle,
as suggested above.
There is a clear plume-like low-velocity zone in the
upper mantle and a high-velocity zone in the mantle
Transition Zone beneath Changbaishan volcano (Zhao
et al., 2009). Kuritani et al. (2011) attributed the low-velocity
zone to a hydrous mantle upwelling, which is modified by
mixed fluxes of ancient and recent sediment-derived
fluids. No reliable mantle seismic tomography model is
available for the upper mantle beneath Chugaryeong volcano. However, the geochemical evidence discussed above
suggests contributions of sediment-derived fluid at various
times in the petrogenesis of the Chugaryeong basalts.
Although the source mantle composition would probably
vary between the different basalt centres, the strong EM1
geochemical affinities and enrichment in fluid-mobile
elements common to the two basalts studied could be attributable to subducted sediment fluid fluxes at various
times (Fig. 14b). Detailed examination of each basalt
centre would provide information both on local
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MARCH 2014
Fig. 15. Sr^Nd^Pb isotopic compositions of the Chongok and Chatan basalts and results of mixing calculations among plausible end-member
components. (a) 87Sr/86Sr vs 143Nd/144Nd; (b) 207Pb/204Pb vs 206Pb/204Pb; (c) 208Pb/204Pb vs 206Pb/204Pb; (d) 143Nd/144Nd vs 206Pb/204Pb. The
dashed line indicates mixing (with percentages indicated) between fluids from ancient sediment and recent sediment as proposed by Kuritani
et al. (2011). The fine lines indicate mixing between the mixed fluid and plausible lithosphere (star). Percentages along the dashed line are the
percentage of the ancient sediment component. The composition of the Chugaryeong basalt can be reasonably reproduced by 0·2^1·0% fluid
component that consists of 50^60% of the ancient sediment component. (See text for details.) Cross-hatched field shows the range of mantle
xenolith compositions from the region (Fig. 7) Also plotted are fields for Kuandian and Changbaishan basalts.
heterogeneities and the bigger picture for the origin of basaltic volcanism in the deep back-arc.
(2)
CONC LUSIONS
Comprehensive whole-rock major element, trace element,
and Sr^Nd^Pb isotope data and mineral compositions for
the Chongok (0·5 Ma) and Chatan (0·15 Ma) basalts of
the Chugaryeong volcano are presented to provide insights
into magmatic differentiation processes within the crust,
melting processes in the mantle, and source mantle characteristics underlying the Korean Peninsula. The main conclusions are as follows.
(1) Major element variations within each basalt can be
reproduced by crystal fractionation, whereas the differences between the two basalts cannot be reproduced by any crystal fractionation or crustal
524
(3)
(4)
(5)
assimilation processes, suggesting that the two lavas
originated from different primary melts.
Estimated melt segregation pressure for the Chatan
basalt is lower than that for the Chongok basalt.
Estimated melt segregation temperatures for the
Chugaryeong basalts are higher than the temperature
of the lithospheric mantle beneath the Korean
Peninsula based on studies of mantle xenoliths.
The source mantle for both basalts exhibits an incompatible element-enriched multi-element pattern with
positive spikes at Ba, K, Pb, and Sr, when normalized
to Primitive Mantle.
The Chongok basalt has higher 143Nd/144Nd and
206
Pb/204Pb, and lower 87Sr/86Sr and 208Pb/204Pb
than the Chatan basalt, the trend of which is from
the field of local mantle xenoliths towards a mixture
of ancient and recent sediment-derived components.
SAKUYAMA et al.
METASOMATIZED ASTHENOSPHERE MELTING
These trace element and isotopic characteristics suggest an origin of the Chugaryeong basalts by flux
melting of metasomatized asthenospheric mantle
induced by fluid released from subducted sediments.
The younger Chatan basalt was generated by a
higher degree of melting of the source mantle peridotite that had been more affected by sediment-derived
fluid than the source of the Chongok basalt.
(6) Isotopic differences between the Chugaryeong and
Changbaishan volcanoes can be attributed to a lesser
contribution of the ancient sediment component at
the Chugaryeong volcano than the Changbaishan volcano, whereas isotopic heterogeneity of the metasomatized mantle is required to explain the isotopic
composition of Kuandian volcano.
AC K N O W L E D G E M E N T S
This study was carried out in collaboration with two
archaeologists: Dr Kazuto Mastufuji of Doshisya
University and Dr Kidong Bae of Hanyang University
(South Korea). We would like to express our thanks to
Hikaru Iwamori and Hiroko Nagahara for constructive
discussions. We are grateful to Hideto Yoshida for EPMA
and XRF analyses at the Department of Earth and
Planetary Science, University of Tokyo. We thank John
Gamble, Takashi Sano and an anonymous reviewer for
constructive and thoughtful comments on the paper.
FUNDING
This work was supported by funds from Japan Society for
the Promotion of Science (21840067 and 23740398) to T. S.
and funds from the Ministry of Education, Culture,
Sports, Science and Technology (16251005) to Kazuto
Matsufuji.
S U P P L E M E N TA RY DATA
Supplementary data for this paper are available at Journal
of Petrology online.
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