Tracing magma sources in an arc-arc collision zone:Helium and carbon isotope and relative abundancesystematics of the Sangihe Arc, Indonesia
Lillie A. Jaffe and David R. HiltonFluids and Volatiles Laboratory, Scripps Institution of Oceanography, University of California San Diego, La Jolla,California 92093-0244, USA ([email protected]; [email protected])
Tobias P. FischerDepartment of Earth and Planetary Sciences, University of New Mexico, Albuquerque, New Mexico 87131, USA([email protected])
Udi HartonoGeological Research and Development Centre, Jl. Diponegoro 57, Bandung 40122, Indonesia
[1] The Sangihe Arc is presently colliding with the Halmahera Arc in northeastern Indonesia, forming the
world’s only extant example of an arc-arc collision zone. We report the first helium and carbon isotopic and
relative abundance data from the Sangihe Arc volcanoes as a means to trace magma origins in this
complicated tectonic region. Results of this study define a north-south trend in 3He/4He, CO2/3He, and
d13C, suggesting that there are variations in primary magma source characteristics along the strike of the
arc. The northernmost volcanoes (Awu and Karangetang) have higher CO2/3He and d13C (up to 179 � 109
and �0.4%, respectively) and lower 3He/4He (�5.4 RA) than the southernmost volcanoes (Ruang,
Lokon, and Mahawu). Resolving the arc CO2 into component structures (mantle-derived, plus slab-derived
organic and carbonate CO2), the northern volcanoes contain an unusually high (>90%) contribution of CO2
derived from isotopically heavy carbonate associated with the subducting slab (sediment and altered
oceanic basement). Furthermore, the overall slab contribution (CO2 of carbonate and organic origin)
relative to carbon of mantle wedge origin is significantly enhanced in the northern segment of the arc.
These observations may be caused by greater volumes of sediment subduction in the northern arc, along-
strike variability in subducted sediment composition, or enhanced slab-derived fluid/melt production
resulting from the superheating of the slab as collision progresses southward.
Components: 9627 words, 3 figures, 2 tables.
Keywords: arc geochemistry; carbon isotopes; helium isotopes; mantle cycling; Sangihe Arc; volatiles.
Index Terms: 1030 Geochemistry: Geochemical cycles (0330); 1040 Geochemistry: Isotopic composition/chemistry; 1025
Geochemistry: Composition of the mantle; 8499 Volcanology: General or miscellaneous.
Received 6 November 2003; Revised 25 February 2004; Accepted 3 March 2004; Published 7 April 2004.
Jaffe, L. A., D. R. Hilton, T. P. Fischer, and U. Hartono (2004), Tracing magma sources in an arc-arc collision zone: Helium
and carbon isotope and relative abundance systematics of the Sangihe Arc, Indonesia, Geochem. Geophys. Geosyst., 5,
Q04J10, doi:10.1029/2003GC000660.
————————————
Theme: Trench to Subarc: Diagenetic and Metamorphic MassGuest Editors: Gray Bebout, Jonathan Martin, and Tim Elliott
G3G3GeochemistryGeophysics
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Published by AGU and the Geochemical Society
AN ELECTRONIC JOURNAL OF THE EARTH SCIENCES
GeochemistryGeophysics
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Article
Volume 5, Number 4
7 April 2004
Q04J10, doi:10.1029/2003GC000660
ISSN: 1525-2027
Copyright 2004 by the American Geophysical Union 1 of 17
1. Introduction
[2] The study of helium and carbon in arc-related
volcanic emissions has provided a wealth of infor-
mation on the subduction process and its involve-
ment in geochemical cycling between the terrestrial
mantle and the crust, hydrosphere and atmosphere.
For example, the flux of various volatile species
(including CO2) from subduction zones can be
estimated through knowledge of the arc-related
primordial 3He flux [Torgersen, 1989; Allard,
1992] and measurement of the relevant elemental
ratio (xi/3He) where xi = element of interest (see
review by Hilton et al. [2002]). In the case of CO2,
it is possible to resolve the output flux into con-
stituent components (slab-related versus mantle
wedge contributions) through the use of modeled
end-member compositions with specific He and
CO2 isotopic and relative abundance characteristics
[Marty et al., 1989; Varekamp et al., 1992; Sano
and Marty, 1995]. Helium and carbon (isotopes
and/or relative abundances) can also be used to
identify regional tectonic controls on magma gen-
esis, as in the case of the Sunda-Banda arcs of
Indonesia [Hilton and Craig, 1989; Hilton et al.,
1992; Varekamp et al., 1992], including recogniz-
ing crustal influences and their contribution to arc-
related magmatism [Gasparon et al., 1994].
[3] In this contribution, we apply the He-C ap-
proach to geothermal fluids collected at active
volcanic centers along the Sangihe Arc of north-
eastern Indonesia. The Sangihe Arc is unusual
because it is part of a conjugate pair of arcs, located
on either side of the Molucca Sea Plate, which are
in the process of colliding (Figure 1). The Sangihe
and Halmahera arcs are the only extant example of
an arc-arc collision zone. Although data are avail-
able on major element chemistry of arc rocks in
this region [e.g., Tatsumi et al., 1991; Elburg and
Foden, 1998; Macpherson et al., 2003], there is
little information on the composition of subducted
components. For example, there are no major or
trace element studies on Molucca Sea sediment,
nor is there any record of the composition of the
Molucca Sea basement (as it has been subducted).
Therefore, to identify various contributions to
magmagenesis in the region, prior studies have
had to approximate the composition of (a) Molucca
Sea sediments (major and trace elements and iso-
topes), using sedimentary analogs from the Philip-
pines, SW Pacific and Banda Arc, and (b) crustal
basement, using material from the Celebes Sea [see
Elburg and Foden, 1998]. In this work, we target
He and CO2 characteristics in the volcanic arc
output of this remote arc setting, with the aim of
characterizing slab-related sources by considering
the gross (volatile) systematics of the subducted
components (both the oceanic crustal basement and
its sedimentary veneer). In this respect, the present
study represents an attempt to utilize geothermal
fluids of the Sangihe Arc to reveal the character-
istics of volatiles from the underlying source re-
gion. A related aim is to assess if along-strike
variations exist in the He-C characteristics. Such
variations may reflect heterogeneity in the compo-
sition of slab-related inputs, and thus provide
information related to the tectonic development
of the region.
2. Geological and Tectonic Background
[4] There are four major lithospheric plates which
interact in the Molucca Sea region of northern
Indonesia: the Eurasian Plate to the west, the
Philippine Plate to the east, the Australian Plate
to the south, and the Molucca Sea Plate trapped in
the center (Figure 1a). The Molucca Sea Plate is
highly unusual in that it is being subducted beneath
both the Eurasian and Philippine Sea plates giving
Figure 1. Map of the Molucca Sea region, southeast Asia (modified from Macpherson et al. [2003]). Small, solidtriangles are active volcanoes from the Smithsonian Institution’s database. Bathymetric contours are at 200, 2000,4000, and 5000 m. (b) Detailed map of the Sangihe Arc. Open triangles representing volcanoes sampled duringthe course of this work. (c) Cross section of the collision zone [from Macpherson et al., 2003]. Note that in subfigures(a) and (b), barbs on each side of the Molucca Sea represent (1) direction of thrust in the Sangihe fore-arc region and(2) direction of subduction of the Halmahera Arc beneath the Sangihe Arc (see section 2 and Macpherson et al.[2003] for further explanation).
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Figure 1.
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rise to two converging, subparallel volcanic arcs: the
Sangihe Arc in the west and the Halmahera Arc in
the east (Figures 1b and 1c). Seismic data indicate
that the Benioff-zone extends as much as 600 km
below the Sangihe Arc, at an approximate dip angle
of 45�, and up to 250 km below the younger
Halmahera Arc subducting in the opposite direction
at approximately 40� [Hatherton and Dickinson,
1969; Silver and Moore, 1978; Lallemand et al.,
1998]. The mean convergence rates of the Sangihe
and Halmahera arcs are �4 cm/yr and �3 cm/yr,
respectively, on the basis of seismic and chronolog-
ical constraints [Lallemand et al., 1998].
[5] The origin of the Molucca Sea Plate can be
traced to the collision of the Philippine Sea Plate
with the Australian Plate approximately 25 Ma
ago, trapping a piece of Indian Ocean lithosphere
that attached to, and moved with, the Philippine
Sea Plate: this fragment became the Molucca Sea
Plate [Hall, 1996]. Subduction on the western
side, forming the Sangihe Arc, probably began
soon after collision. Eastward subduction of the
Molucca Sea Plate began about 15 Ma ago when
collision of the Snellius Plateau with the Sangihe-
Philippine Arc system forced a subduction rever-
sal, breaking the Molucca Sea Plate away from the
Philippine Sea Plate and forming the Halmahera
Arc [Hall and Nichols, 1990; Hall, 1996].
[6] There is presently no surface remnant of the
Molucca Sea Plate basement other than the thick
collision complex consisting of the melange wedges
of the two arcs [Silver and Moore, 1978]. This
wedge of unconsolidated and deformed Tertiary
sediments, up to 15 km in thickness, is composed
of volcaniclastic and continental debris, including
peridotite, serpentinite, gabbro, basalt, chert, lime-
stone, and greywacke [Silver and Moore, 1978;
Hamilton, 1979; Sukamto, 1979], generated primar-
ily by the continuous and ongoing collision of the
Halmahera and Sangihe arcs [Silver and Moore,
1978; McCaffrey et al., 1980].
[7] The Sangihe Arc is the oldest extant subduction
zone in the Philippine-Indonesia region: subduc-
tion-related rocks about 25 Ma old [Dow, 1976;
Effendi, 1976; Apandi, 1977; Priadi, 1993] have
been reported in the northeastern arm of Sulawesi.
Whereas the southern portion of the arc ends at the
left-lateral Sorong Fault (Figure 1a), the northern
termination is marked by the collision of eastern
and western Mindanao about 4–5 Ma ago [Pubel-
lier et al., 1991]. The present-day active arc ceases
at �4�N; however, the extinct margin extends to
5.5�N [Lallemand et al., 1998;Widiwijayanti et al.,
2003], with lavas in central Mindanao thought to be
of Sangihe origin [Pubellier et al., 1991]. This
northern collision region is further complicated by
back-arc thrusting along the Cotobato Trench in the
west and the Philippine Trench in the east, leaving
dissected remnants of the Sangihe (and Halmahera)
arcs on Mindanao Island and the Talaud Ridge
[Morrice et al., 1983]. Post-collisional volcanism
remains active in central Mindanao (i.e., the relic
Sangihe Arc) as a result of the continuing subduc-
tion of portions of the Halmahera Arc [Pubellier et
al., 1991, 1999]. While the active portions of the
Sangihe and Halmahera arcs are currently several
hundred kilometers apart, the northern collision is
propagating southward and will continue until the
Halmahera Arc accretes onto the Eurasian margin
[Hall and Nichols, 1990].
[8] The active Sangihe Arc can be divided into two
segments: four volcanic islands north of the large
island province, Sulawesi, and eight volcanoes in
northeastern Sulawesi (see Figure 1b). The active
volcanoes in the northern segment of the arc (Awu,
BanuaWahu, Karangetang, and Ruang) occur about
50 km apart along a north-south transect. The
onshore active Sangihe Arc volcanoes (Tongkoko,
Mahawu, Lokon-Empung, and Soputan) are spaced
less regularly. Four other volcanoes, morphologi-
cally young and hydrothermally active (Duasudara,
Klabat, Manado Tua, and Ambang), also lie along
the southern portion of the arc. Lavas erupted along
the chain are mostly typical island arc volcanics
(basaltic andesite to andesite) with the exception of
Soputan that produces olivine-rich basalt [Morrice
et al., 1983]. The active Sangihe Arc is bounded by
Awu volcano in the north and Ambang in the south.
3. Sampling and Analytical Protocols
[9] In order to characterize the helium and carbon
systematics of the Sangihe Arc, we collected geo-
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thermal fluids (fumaroles, gas emanations from
bubbling springs, and thermal spring waters), and
olivine- and/or pyroxene-rich lavas from seventeen
different localities covering seven active volcanic
centers along the north-south strike of the arc.
Collection of geothermal samples was facilitated
by use of a Ti-tube or inverted plastic funnel [after
Giggenbach and Goguel, 1989], and samples were
stored in either AR-glass bottles or cold-welded
annealed copper tubes for transfer back to the
laboratory. Normal sampling precautions were
taken to minimize the possibility of air contamina-
tion [see Hilton et al., 2002].
[10] In the laboratory, samples were extracted us-
ing instrumentation and procedures described pre-
viously [Kulongoski and Hilton, 2002; Shaw et al.,
2003]. Briefly, all fluid samples were released into
an ultra-high vacuum system consisting of a series
of traps held at different temperatures, thus sepa-
rating water vapor from the condensable (mainly
CO2) and noncondensable gases. The noncondens-
able fraction was aliquoted into an AR-glass break-
seal for transfer to a noble gas mass spectrometer
(MAP215) for He isotopic and He and Ne relative
abundance analyses. The condensable fraction was
transferred to a separate vacuum line for further
purification, and the total amount of CO2 was
measured manometrically. A fraction of the puri-
fied CO2 was then collected in another breakseal
for isotopic analysis (using a VG Prism mass
spectrometer).
[11] Rock samples were prepared by crushing and
sieving into various grain sizes. Olivine and py-
roxene mineral grains, ranging in diameter from
850–1700 mm, were hand-picked from the appro-
priate size fraction, and ultrasonically cleaned
using a 50/50 methanol/acetone solution. Olivine
and pyroxene grains were loaded separately into
on-line, electro-magnetic crushers (see description
by Scarsi [2000]) attached to the MAP215 to
release volatiles for measurement.
[12] Prior to inlet into the noble gas mass spec-
trometer, all samples underwent a purification pro-
cedure designed to isolate the He and Ne fractions
by a combination of active gas gettering, sorption
onto charcoal at liquid nitrogen temperatures and
cryogenic separation using a liquid He cooled trap.
Isotopic measurements were made in static mode,
and either an air or a 3He-rich standard (Murdering
Mudpots (Yellowstone) = 16.45 RAwhere RA = air3He/4He) was used for normalization.
4. Results
[13] Helium and carbon isotopic and relative abun-
dance characteristics of 28 samples (26 geothermal
fluids and 2 phenocrysts) from the Sangihe volca-
nic arc are given in Table 1. Samples cover
7 distinct volcanic centers (3 offshore and 4 on-
shore). Results from each volcanic center, listed
from north to south, are discussed in turn.
4.1. Awu Volcano (Sangihe Island)
[14] Two different crater localities were sampled.
The fumarole samples (from location 1) have air-
like 3He/4He (�1 RA) and lowHe/Ne values (X� 2,
where X is the air-normalized He/Ne value multi-
plied by the ratio of the Bunsen coefficients; see
Table 1 footnote). The second locality (location 2)
was a bubbling spring located in the crater lake
close to the shore: this gas sample has a He isotope
value of 6.2 RAwith an X value of 55. The CO2/3He
value of the spring is 117 � 109. Its d13C value is
�0.4%.
4.2. Karangetang Volcano (Siau Island)
[15] Two coastal volcano flank thermal spring
localities were sampled, with both Temboko
(6.4 RA) and Lehi (5.4 RA) showing a dominantly
magmatic He input, albeit with a significant air
correction (X � 12). One sample (IND-17) has an
anomalously high d13C value of +1.3%; however,
the other two samples haveCO2/3He values between
6.4 and 17.9 (� 109) and have similar d13C values
of ��2%. The pyroxene mineral separate from
Sang’01-40 has a 3He/4He ratio �4.9 RA; slightly
lower than the thermal spring samples.
4.3. Ruang Volcano (Tagulandang IslandGroup)
[16] Two summit fumarole localities were sampled,
with one characterized by a 3He/4He ratio of 7.0 RA
(X = 470). The other locality has a greater degree
GeochemistryGeophysicsGeosystems G3G3
jaffe et al.: tracing magma sources 10.1029/2003GC000660
5 of 17
Table
1.
Helium
andCarbonResultsAlongtheN-S
StrikeoftheSangiheArc
(Sam
pledin
2001)
Volcano
Sam
plea
Typeb
Lat,�N
Long,�E
Elev,
mT,�C
RM/R
Ac
Xd
RC/R
Ae,f
CO2/3He(�
109)f
d13C,%
f,g
[4He]/g
h
(�10�9cm
3STP)
Awu
CraterLoc.
1IN
D-15
fm3�40.4890
125�27.2320
1278
96.6
1.00±0.01
2.42
1.00±0.01
31.3
±0.4
�2.0
–IN
D-16
fm"
""
"1.22±0.01
2.27
1.39±0.02
108±1
�1.5
–I-048
fm"
""
"1.08±0.02
2.18
1.14±0.02
96±2
�1.3
–CraterLoc.
2I-043
sg"
""
–6.12±0.07
55.0
6.22±0.08
117±2
�0.4
–Karangetang
Tem
boko
IND-18
sf2�46.3370
125�22.1110
068.7
5.49±0.06
5.77
6.43±0.08
63.8
±0.9
�2.0
309.1
Lehi
IND-17
sf2�45.4870
125�22.6030
048.8
4.73±0.06
11.8
5.08±0.07
27.5
±0.4
1.3
312.2
I-021
sf"
""
"4.99±0.07
11.3
5.37±0.08
179±3
�2.0
206.2
Sang’01-40
px
""
"–
4.42±0.39
8.72
4.86±0.50
––
0.28
Ruang
CraterLoc.
1IN
D-19
sl2�18.1870
125�22.1150
691
97.9
3.55±0.03
6.41
4.02±0.04
1.49±0.08
�3.3
–CraterLoc.
2IN
D-20
sl2�18.2200
125�22.1000
688
140
6.97±0.07
470
6.99±0.07
2.83±0.04
�3.1
–Lokon
Crater
IND-1
fm1�21.8270
124�47.9820
1109
96.1
7.11±0.06
833
7.12±0.07
4.41±0.05
––
I-159
fm"
""
"7.27±0.06
885
7.27±0.06
8.4
±0.1
�3.6
–Mahaw
uKakaskasen
I-150
sf1�20.8710
124�50.8750
897
35.9
7.33±0.11
37.7
7.50±0.12
53.8
±1
�6.8
256.0
IND-3
sf"
""
"7.11±0.08
68.9
7.20±0.08
6.44±0.08
�5.7
2190
Lahendong1
IND-4
sg1�16.1490
124�49.3230
832
101.1
7.32±0.07
2741
7.33±0.10
5.07±0.09
�3.5
–IN
D-5
sg"
""
"7.04±0.08
2159
7.04±0.10
5.37±0.09
�3.4
–Lahendong2
IND-6
sg1�15.9710
124�49.0580
777
61.9
7.23±0.09
2557
7.23±0.14
5.8
±0.1
�3.0
–IN
D-7
sg"
""
"6.90±0.09
1062
6.91±0.10
5.8
±0.1
�3.3
–Soputan
Crater
IND-12
fm1�7.1220
124�44.2880
1708
72.9
1.07±0.02
2.66
1.11±0.02
0.69±0.01
�3.5
–I-016
fm"
""
"1.04±0.01
2.38
1.07±0.01
––
–Aeseput
IND-13
fm1�7.5290
124�44.6460
1426
86.9
0.95±0.02
2.59
0.93±0.02
0.113±0.002
�19.9
–(flankcone)
IND-14
fm"
""
"1.02±0.01
2.50
1.03±0.01
0.143±0.002
�20.1
–SOP-1
ol
""
""
5.33±0.15
78.7
5.39±0.34
––
3.05
Ambang
CraterLoc.
1IN
D-9
fm0�44.9060
124�25.2800
1317
97.6
4.63±0.07
924
4.64±0.07
5.7
±0.1
�5.2
–I-198
fm"
""
"4.59±0.04
713
4.60±0.04
6.45±0.08
�4.5
–CraterLoc.
2IN
D-10
sf"
""
89.7
3.68±0.09
30.0
3.77±0.10
590±20
�2.3
88.5
I-193
sf"
""
"3.27±0.10
349
3.27±0.22
140±10
�4.3
97.3
Bonkurai
IND-11
sf0�42.9400
124�22.1820
494
46.9
3.97±0.06
27.8
4.08±0.06
292±5
�4.7
252.7
aIN
D-##=AR-glass
bottle,I-###=Copper
tube,
othersarerock
samples.
bAbbreviations:sf,thermal
springfluid
phase;
sg,thermal
springgas
phase;
fm,fumarole;sl,solfatarafumarole;ol,olivine;
px,pyroxene.
cRM/R
A=measured
3He/4He(R
M)in
sample
relativeto
air3He/4He(R
A).Erroris1s.
dX
=[(He/Ne)
sample/(He/Ne)
air]�
1.209(i.e.,theair-norm
alized
He/Neratiomultiplied
byb N
e/b
He=1.209-theratiooftheBunsencoefficientsat
17�C
).eRC/R
A=air-corrected
3He/4He(R
C)in
sample
relativeto
air(R
A).RC/R
A=[(RM/R
A)X
�1]/(X
�1).
fValues
initalicshavebeenremoved
from
thediscussion;they
areconsidered
unrepresentativeofprimarymagmatic
source(see
section5.1).
gErrors
ond1
3C
areless
than
±0.5%,based
onreplicate
analyses.
h[4He]
per
gram
ofwater
influid
samples,andper
gram
ofmineral
inrock
samples.
GeochemistryGeophysicsGeosystems G3G3
jaffe et al.: tracing magma sources 10.1029/2003GC000660
6 of 17
of air contamination (X = 6.41) and its 3He/4He
ratio is significantly lower (3.6 RA). Although the
air-corrected 3He/4He ratios are dissimilar (4.0 and
7.0 RA), there is good agreement between d13Cvalues (��3%) for the two localities. In addition,
the two CO2/3He ratios 1.5–2.8 (�109) fall within
a narrow range.
4.4. Lokon Volcano (North Sulawesi)
[17] 3He/4He values of duplicate samples from a
single fumarole locality at Lokon volcano showed
excellent agreement (�7.2 ± 0.1 RA). Air contam-
ination in each case was negligible (X > 800).
CO2/3He ratios are 4–8 (�109) and the single d13C
value is �3.6%.
4.5. Mahawu Volcano (North Sulawesi)
[18] Two different localities in the vicinity of
Mahawu volcano were sampled: the Lahendong
geothermal complex (including thermal resort) and
a remote thermal spring locality at Kakaskasen.
Consistent results were obtained for all samples
with the exception of one duplicate from Kakaska-
sen (see below). 3He/4He ratios are 6.9–7.5 RA and
CO2/3He values lie between 5 and 6 (�109). One
sample from Kakaskasen has an anomalously high
CO2/3He ratio of 54 � 109. d13C values for
Lahendong are tightly constrained at ��3.3%,
whereas Kakaskasen values are significantly lower
(�5.8 to �6.8%).
4.6. Soputan Volcano (North Sulawesi)
[19] All fumarole samples from the summit of
Soputan have air-like values (3He/4He � 1 RA;
X � 2.5). The flank cinder cone Aeseput samples
also had air-like He isotopes (3He/4He � 1 RA;
X � 2.5). The olivine crystals, separated from the
ash sample, gave a 3He/4He ratio of �5.4 RA.
4.7. Ambang Volcano (North Sulawesi)
[20] Two localities were sampled on the summit of
Ambang Volcano, and one on the volcano flank.
The summit fumarole locality gave consistent3He/4He ratios of 4.6 RA for 2 samples, with good
agreement in both CO2/3He (5.7 � 109 and 6.4 �
109) and d13C values (�4.5% and �5.2%). The
summit thermal fluid locality gave lower 3He/4He
ratios (3.3 RA and 3.8 RA), and higher d13C(�4.3% and �2.3%) and CO2/
3He (140 � 109
and 590 � 109) values. The flank thermal spring
locality at Bonkurai has intermediate 3He/4He (4.1
RA), CO2/3He (292 � 109) and d13C (�4.7%)
values.
5. Discussion
[21] There are three principal contributors to the
inventory of magmatic volatiles at arc-related set-
tings: the subducted crustal basement, its overlying
sedimentary veneer and the mantle wedge above
the slab. The atmosphere as well as the arc litho-
sphere through which magma erupts are also po-
tential sources of volatiles; although these sources
are generally regarded as contamination and unre-
lated to the magma source of volatiles. Therefore,
in order to quantify the various contributors of
volatiles to these magmas, and to recognize any
extraneous additions to, or modifications from,
source characteristics, it is essential to adopt vari-
ous criteria that differentiate samples that possess
intrinsic magma characteristics from those that
have been modified by air contamination, crustal
assimilation, degassing, and/or sampling error. To
this end, we adopt a number of criteria in the
following section aimed at assessing the integrity
of each individual sample to preserve its primary
source signature. Only after we have applied this
filter can we relate the He-C systematics of the
Sangihe Arc to the tectonic framework of the
region.
5.1. Identifying Samples With ModifiedHe-C Characteristics
5.1.1. Air-Like 3He//4He and He//Ne Values
[22] There are a total of 7 geothermal samples with
air-like 3He/4He and He/Ne values (Table 1). These
are: all three samples from Awu crater fumaroles
(location 1; IND-15, IND-16, I-048) and all four
samples from Soputan fumaroles (from the main
crater and the Aeseput flank cone; IND-12, I-016,
IND-13, IND-14). In both cases, the diffusely flow-
ing fumaroles could have entrained air through the
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highly altered (Awu) or porous and blocky (Sopu-
tan) material covering the fumarole discharge sites.
5.1.2. Low 3He//4He: Indications ofRadiogenic Crustal Influence
[23] Helium isotopic ratios in arc-related settings
around the circum-Pacific region generally fall
between 6–8 RA [Poreda and Craig, 1989]. A
more recent compilation of arc 3He/4He data
[Hilton et al., 2002] has produced an average value
of �5.4 RA for all arcs worldwide. As He isotopes
are a sensitive tracer of volatiles of crustal prove-
nance [Hilton et al., 1993; Gasparon et al., 1994],
we suspect that 3He/4He less than the average value
cited above may have incorporated a significant
radiogenic He contribution, thus masking the mag-
matic source ratio. For this reason, we reject all five
Ambang samples (IND-9, IND-10, IND-11, I-198,
and I-193), as their 3He/4He are less than 4.6 RA.
We reject the pyroxene separate from Karangetang
(Sang’01-40), and the olivine sample from Soputan
(SOP-1) as they also have low 3He/4He values.
5.1.3. High CO2//3He Values: Indications ofFractionation in the Hydrothermal System
[24] The average CO2/3He ratio of arc-related vol-
canism is 15 ± 11 (�109) [Sano and Marty, 1995;
Sano and Williams, 1996]. Significant deviations
from this value are usually indicative of fraction-
ation of He from C [van Soest et al., 1998; Hilton
et al., 2002]. In the case of geothermal (aqueous)
fluids, the greater solubility of C relative to He [see
Stephen and Stephen, 1963] can lead to higher
CO2/3He values in the residual phase following
vapor formation or other gas loss event. Both van
Soest et al. [1998] and Shaw et al. [2003] report
significantly higher CO2/3He ratios in gases dis-
solved in thermal waters compared to gas phase
samples collected at the same locality. This obser-
vation is consistent with the notion that free gas
samples provide a more robust means of sampling
the intrinsic CO2/3He value of a degassing magma
body whereas gases dissolved in waters may rep-
resent (fractionated) volatiles residual from a
degassing event. The current data set allows us to
make the same comparison between liquid and gas
phase CO2/3He values.
[25] Unusually high CO2/3He ratios are measured
in Ambang and Karangetang geothermal fluid
samples. Three Ambang geothermal fluid phase
samples (IND-10, I-193, and IND-11; average
CO2/3He � 340 � 109) had significantly higher
CO2/3He values than their gas-phase counterparts
from the same locality (IND-8, IND-9, and I-198;
average CO2/3He � 4.5 � 109). Likewise, one
Karangetang thermal spring fluid phase sample (I-
021) has a high CO2/3He value (179 � 109) when
compared with the other fluid phase sample from
the same locality (IND-17 CO2/3He = 27.5 � 109).
These differences between gas and fluid phase
CO2/3He values suggest that the fluid phase
CO2/3He represents volatiles that have been mod-
ified due to fractionation most likely by vapor
formation in the geothermal fluid system. There-
fore we omit four samples with anomalously high
CO2/3He values from further consideration.
5.1.4. Atypical D13C Values: AdditionalIndications of Alteration in theHydrothermal System
[26] Although agreement in d13C between dupli-
cate samples is generally good (see Table 1), there
is one locality (Karangetang volcano) with samples
showing markedly different d13C values. One sam-
ple (IND-17) has an unusual d13C value of +1.5%that is very different from its duplicate (I-021)
(d13C = �2.0%). We can find no analytical reason
for the discrepancy. However, we note that given
the observation [e.g., Sano and Williams, 1996]
that almost all arc-related d13C values are negative
(reflecting end-member mixtures of CO2 which are
all �0%) then this positive value is clearly anom-
alous. Consequently, we reject it from further
consideration.
[27] Likewise, we measure exceptionally low d13Cvalues in two samples from Soputan volcano: the
two Aeseput samples (IND-13 and IND-14) have
exceptionally low d13C values of ��20%. This
observation, coupled with air-like He-isotope
ratios, leads us to believe that these samples have
been compromised. Interestingly, the carbon is not
air-like (air CO2 has a d13C ��8% [Keeling,
1984]). Prolonged outgassing may be responsible
for these extraordinarily light C isotope ratios
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[Gerlach and Taylor, 1990]. Again, we remove
these values from further consideration.
5.1.5. Poor Agreement Between DuplicateSamples
[28] There are a number of samples that show poor
agreement between duplicate samples. For exam-
ple, the 3He/4He ratio of IND-19 from location 1 in
Ruang crater is rejected as a result of its low He/Ne
and 3He/4He ratios compared to a duplicate sample
(IND-20) collected at the same locality. Similarly,
sample I-150 from Kakaskasen (Mahawu volcano)
has an anomalously high CO2/3He value (53.8 �
109), approximately ten times that of a duplicate
from the same locality (6.4 � 109), and other
samples from the same volcano (�5.5 � 109).
Additionally, its d13C value, and the d13C value
of its duplicate (IND-3), is much lower than other
samples from Mahawu (average d13C = �3.3%). It
seems reasonable therefore to assume that both
samples from Kakaskasen do not reflect the pri-
mary magma sources in the region.
5.1.6. Summary
[29] Out of a total of 26 geothermal fluid samples
that were collected along the strike of the Sangihe
Arc, 15 samples have experienced sufficient mod-
ification that both their He and C systematics no
longer reflect primary magma characteristics.
These samples are: IND-15, IND-16, I-048
(Awu); IND-17 (Karangetang); I-150, IND-3
(Mahawu); IND-12, I-016, IND-13, IND-14
(Soputan); and IND-9, I-198, IND-10, I-193,
IND-11 (Ambang). Of the remaining 11 samples,
there is no evidence of modification for either He
or C for the following nine: I-043 (Awu); IND-18
(Karangetang); IND-20 (Ruang); IND-1, I-159
(Lokon); IND-4, IND-5, IND-6, IND-7 (Mahawu).
The remaining 2 samples have unmodified data for
either He or C (CO2/3He and d13C) but not both.
5.2. Along-Strike Variations in He-CCharacteristics
[30] In this section, all interpretations are based
on the filtered data from section 5.1. Figure 2
shows 3He/4He, CO2/3He and d13C as a function
of latitude along the Sangihe Arc. Here, we
consider both variations along-strike as well as
the magnitude of the absolute values in relation to
averages of arc-related volcanism and MORB
worldwide.
5.2.1. The 3He//4He Variations
[31] The majority of arc-related volcanism is char-
acterized by 3He/4He values coincident with that
found in MORB mantle [Poreda and Craig, 1989;
Hilton et al., 2002]. Some localities, however,
record the addition of radiogenic He, either through
slab-derived contributions [Hilton and Craig,
1989; Hilton et al., 1992; Marty et al., 1994]
and/or contamination by arc crust [Hilton et al.,
1993; Gasparon et al., 1994]. If the filtered data-
base (section 5.1) allows consideration of magmat-
ic 3He/4He characteristics only, then the Sangihe
Arc comprises relatively low values in the northern
section of the arc (Awu and Karangetang; 5.4–
6.4 RA) versus more typical arc-like values in the
southern segment (Ruang, Lokon and Mahawu;
6.9–7.5 RA). Addition of a small radiogenic He
component to magmas of the northern Sangihe Arc
may reflect contributions from either the subducted
sedimentary veneer, its underlying oceanic base-
ment or the overlying arc crust. It seems unlikely
that sediments have the transport capacity (due to
diffusional losses) to subduct He [Hilton et al.,
1992; Hiyagon, 1994] so we can discount this
possibility with a fair degree of confidence. Pre-
liminary trace element and Sr-Nd-Pb data do not
show any evidence for upper-crustal contamination
along the entire strike of the Sangihe Arc [van der
Meer et al., 2002]. This observation is in contrast
to the southern Lesser Antilles Arc where low3He/4He ratios are accompanied by radiogenic Sr
and Pb isotope ratios [van Soest et al., 1998, 2002].
In the absence of any other constraints, therefore,
we conclude that the source of radiogenic He in
northern Sangihe Arc magmas may be the sub-
ducted crustal basement which modifies 3He/4He
values to below typical arc values (see further
discussion in section 5.4).
5.2.2. The CO2//3He Variations
[32] In a manner similar to the He isotope distri-
bution, CO2/3He values of geothermal samples
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from Mahawu and Lokon 4–8 (�109) are typical
of arc-related volcanism: all samples fall close to
the worldwide arc average of 15 ± 11 � 109 [Sano
and Williams, 1996]. Ruang has a slightly lower
than average value (CO2/3He � 2 ± 1 � 109);
nevertheless, its value is close to the lower range of
arc lavas. Therefore the three southern volcanoes
show similar, arc-like CO2/3He characteristics.
[33] In contrast, the northern arc samples (Awu and
Karangetang) have CO2/3He values significantly
higher than the southern arc (Figure 2). The
Figure 2. Latitudinal variations (from south to north) in (a) air-corrected He-isotope ratios (RC/RA notation),(b) CO2/
3He ratios, and (c) C-isotope ratios (% relative to PDB) for geothermal fluids and phenocrysts from theSangihe Arc, Indonesia. All errors fall within symbols except where indicated by error bars. Worldwide arc 3He/4Heaverage of 5.4 RA (dotted line) and MORB range (8 ± 1 RA) from Hilton et al. [2002]. Worldwide arc averageCO2/
3He value (15.7 ± 11.0 � 109) from Sano and Williams [1996] and average MORB d13C of �6.5 ± 2.3% fromSano and Marty [1995]. Volcano location listed by letter: Am, Ambang; S, Soputan; M, Mahawu; L, Lokon; R,Ruang; K, Karangetang; Aw, Awu. Large (colored) symbols represent data points considered representative ofprimary magmatic values, whereas small (gray) symbols are deemed modified by crustal, geothermal, and/or otherprocesses (see discussion in section 5.1).
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CO2/3He values are 64–180 (� 109), or between 1
and 2 orders of magnitude greater than the southern
segment of the arc. Higher CO2/3He values in the
northern arc would suggest a diminished mantle3He input, a greater subducted slab influence, or
both. A diminished mantle 3He input and/or a
greater slab flux might also be expected to result
in a lower 3He/4He ratio, as observed for this
section of the arc.
5.2.3. The D13C Variations
[34] Although d13C values of all samples are higher
than typical mantle values (��6.5% [Sano and
Marty, 1995]), there is a marked distinction in d13Cvalues between the northern and southern segments
of the arc. The two volcanoes in the northern arc
(Awu and Karangetang) have d13C values ��2%,
whereas all three southern segment volcanoes
(Ruang, Lokon and Mahawu) are characterized by
d13C values ��3%. It is interesting to note that if
some of the rejected data from Soputan and Ambang
are included, then the pattern between the northern
and southern segments remains unchanged as these
two volcanoes also have low values of d13C.
5.2.4. Along-Strike Variations in He-C:Summary
[35] Along-strike profiles in 3He/4He, CO2/3He and
d13C of Sangihe Arc volatiles show a consistent
pattern: the southern Sangihe Arc volcanoes
(Ruang, Lokon, and Mahawu) show typical arc
values in all three He-C parameters whereas the
northern volcanoes (Awu and Karangetang) have
higher CO2/3He and d13C, and lower 3He/4He
ratios relative to the southern segment. The results
are consistent with either addition of a strong
crustal input (presumably slab-derived) in the
northern segment of the arc or a reduction in
the mantle contribution. The boundary between
the two segments lies between the islands of Ruang
and Karangetang.
5.3. Differentiating CO2 Sources inSangihe Arc Magmas
[36] Following the methodology of Marty et al.
[1989] and Sano and Marty [1995], we can
resolve the total CO2 observed in any particular
Sangihe Arc sample into its component structures.
We adopt the same end-member compositions of
previous workers: namely, MORB mantle
(CO2/3He = 2 � 109; d13C = �6.5%); limestone
(sedimentary carbonate and altered oceanic base-
ment) (CO2/3He = 1013; d13C = 0%); and organic
sediment (CO2/3He = 1013; d13C = �30%). In
Figure 3 we plot He-C results from the present
study along with the end-member compositions
joined by two binary mixing trajectories with
mantle-derived carbon common to both mixtures.
It is clear that Sangihe Arc samples do not fall on
either of the binary mixing trajectories. This would
imply that all three end-members must contribute
to the total CO2. As the majority of samples lie
close to the M-L binary mixing line, an alternative
explanation is that the Sangihe samples are indeed
binary mixtures of mantle- and limestone-derived
carbon with some samples experiencing isotopic
fractionation to lower d13C [Snyder et al., 2001].
Dewatering of metabasalts at the slab interface
and/or precipitation of calcite within the hydro-
thermal systems are two possibilities that may lead
to lower d13C values [Snyder et al., 2001]. Given
the observation that some samples (e.g., Lehi and
Temboko) appear to plot subparallel to the M-L
mixing line (Figure 3), then the extent of isotopic
fractionation must be approximately equal in all
cases irrespective of possible along-strike varia-
tions in thermal regime at the slab interface (see
next section) or differences in fluid chemistry at
the various volcanic geothermal systems. Both of
these possibilities seem unlikely. A more compel-
ling argument comes from the N-isotope system-
atics which show some volatile contributions from
organic sedimentary material along the entire
strike of the Sangihe Arc (L. E. Clor et al., Volatile
and N-isotopic chemistry of colliding island arcs:
Tracing source components along the Sangihe
Arc, Indonesia, submitted to Geochemistry Geo-
physics Geosystems, 2004) (hereinafter referred to
as Clor et al., submitted manuscript, 2004). We
conclude therefore that addition of sedimentary-
derived C is responsible for d13C values lower
than predicted by simple binary M-L mixing.
[37] In this case, the contribution of each end-
member to the total CO2 can be quantified by use
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of the following equations [Sano and Marty,
1995]:
13C=12C� �
o¼ fM
13C=12C� �
Mþ fL
13C=12C� �
Lþ fS
13C=12C� �
S
ð1Þ
1= 12C=3He� �
o¼ fM=
12C=3He� �
Mþ fL=
12C=3He� �
L
þ fS=12C=3He
� �S
ð2Þ
fM þ fS þ fL ¼ 1 ð3Þ
where o = observed and f is the fraction contributed
by L, S and M to the total carbon output.
[38] In Table 2, we detail the fractional contribu-
tions of the L-, M- and S-components to the total
CO2 inventory as well as the ratios of (a) limestone
(carbonate) to organic sediment carbon input (L/S),
and (b) slab-derived carbon (L + S) to carbon of
mantle derivation (M). The first and most obvious
point to note is that carbon is predominantly of
carbonate derivation throughout the Sangihe Arc.
This could reflect contributions from both sedi-
mentary carbonate in subducted sedimentary
sequences as well as carbonate contained within
the oceanic basement (e.g., as calcite veins). It is
not unusual for arc-type lavas to be dominated by a
carbonate component [Sano and Marty, 1995;
Hilton et al., 2002]. However, it is significant that
the proportion of carbonate-derived carbon is con-
sistently >90% in the northern section of the arc,
and considerably lower in the southern segment.
Indeed, the southern Sangihe arc has a carbonate-
derived carbon contribution similar to the average
value seen at arcs worldwide (�75%).
[39] Although there is an enhanced contribution of
carbonate-derived carbon to the total carbon in-
ventory in the northern Sangihe Arc, there is (with
the exception of Awu volcano) little difference in
the ratio of L/S in the volcanic output, i.e., the
fraction of carbon of carbonate derivation versus
organic (sedimentary) carbon. We note that the
same observation characterizes the Nicaragua seg-
ment of the Central America Arc compared to the
adjacent Costa Rica sector [Shaw et al., 2003].
This may simply reflect uniformity in this param-
Figure 3. Plot of CO2/3He versus d13C showing (a) data points considered unmodified from primary magmatic
values (see section 5.1 and caption to Figure 2) and (b) binary mixing trajectories involving representative end-member compositions of mantle wedge (M), and slab-derived organic sediment (S) and limestone (L). End-membercompositions are from Sano and Marty [1995]: namely, MORB mantle (CO2/
3He = 1.5 � 109; d13C = �6.5 ± 2.5%),limestone (CO2/
3He = 1013; d13C = 0 ± 2%), and organic sediment (CO2/3He = 1013; d13C = �30 ± 10%).
Uncertainties in CO2/3He values are as shown. The box denotes the worldwide average for arc-related geothermal
fluids [Sano and Marty, 1995].
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eter in the sedimentary input. In the case of Central
America, organic carbon is dispersed throughout
the subducted sedimentary pile so that it is insu-
lated against thermal loss during early stages of
subduction [Shaw et al., 2003, and references
therein]. The same may hold true in the Sangihe
Arc input.
[40] Finally, the Sangihe Arc presents a consistent
picture of an enhanced slab-derived input in the
north relative to carbon from the mantle wedge.
The ratio (L + S)/M is considerably higher in the
northern volcanoes (>40) compared the southern
section of the arc (<4.3). Again, the same obser-
vation was made for the Nicaragua segment of the
Central America Arc compared to the adjacent
Costa Rica sector [Shaw et al., 2003]. In Central
America, the difference in (L + S)/M was ascribed
to differences in thermal regime along the strike of
the arc with steeper subduction in Nicaragua lead-
ing to a colder slab retaining its carbon inventory
until subarc depths. In contrast, the warmer thermal
regime in Costa Rica leads to loss of carbon from
the slab at shallower (fore-arc) depths thus lower-
ing the relative amount of available slab-derived
carbon. This possibility, i.e., a thermal control on
the relative contribution of slab-derived carbon,
plus other alternative explanations for along-strike
differences in relative carbon contributions, is dis-
cussed in more detail in the following section.
5.4. Implications for TectonicDevelopment of the Sangihe Arc
[41] In this section, we explore the possibility that
differences in the volatile systematics observed
between the northern and southern sections of
Sangihe Arc ultimately have a tectonic origin and
are related to the collision and development of the
arc. In this context, we note that the Sangihe and
Halmahera arcs have already collided in the north
causing volcanism to cease north of 4�N [e.g.,
Pubellier et al., 1991], and that the collision is
propagating southward. This has caused the sub-
duction rate to decrease along the active arc
[Elburg and Foden, 1998]. Therefore we might
anticipate that any variability in volatile chemistry
related to the collision may be more noticeable in
the northernmost section of the arc, closer to the
locus of collision. We speculate that the observed
along-strike variations in He-C characteristics
could therefore reflect (1) an increase in the
volume of subducted sediment in the northern
Table 2. Limestone-Mantle-Sediment Contributions to CO2 Inventory of Sangihe Arc Geothermal Fluids
Volcano Samplea Typea Lb Sb Mb L/S (L + S)/M
AwuCrater Loc. 2 I-043 sg 97.6 1.1 1.3 87.4 74.4
KarangetangTemboko IND-18 sf 91.4 6.2 2.4 14.8 39.9Lehi I-021 sf 92.7 6.4 0.9 14.4 114.9
RuangCrater Loc. 2 IND-20 sl 46.4 0 55.4 – 0.81
LokonCrater I-159 fm 73.3 7.9 18.8 9.2 4.3
MahawuLahendong Loc.1 IND-4 sg 64.2 4.8 30.9 13.3 2.2
IND-5 sg 65.7 5.1 29.2 13.0 2.4Lahendong Loc.2 IND-6 sg 68.7 4.0 27.3 17.1 2.7
IND-7 sg 67.6 5.1 27.3 13.3 2.7Worldwide Averagec 75 ± 10 13 ± 8 13 ± 6 5.8 6.8
aSee Table 1 footnote for explanation.
bL, S, and M (in %) are calculated using the same end-members compositions as Sano and Marty [1995] (see text for details).
cAverage values for arcs worldwide [from Sano and Marty, 1995].
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arc; (2) variability in the composition of subducted
sediment chemistry; or (3) a change in thermal
regime experienced by the subducting slab related
to the onset of collision. These possibilities are
discussed in turn.
[42] 1. Volume of subducted sediment: The mas-
sive accretionary wedge (�15 km thick) formed by
the collision of the Sangihe and Halmahera arcs is
thickest in the northern Molucca Sea where colli-
sion is oldest [e.g., Hall, 1996]. These sedimentary
sequences represent the fraction of the total sedi-
ment load involved in the collision that have
obducted onto the fore-arc region. Studies at other
accretionary arcs worldwide [von Huene and
Scholl, 1991] estimate that only �20% of the
sediment on the incoming plate forms the prism,
whereas 80% is subducted into the mantle. Al-
though the He-C systematics of this work are
consistent with recycling of sediment-derived car-
bon through the entire arc system, we speculate
that the thicker incoming sequences in the northern
arc translates into a greater absolute volume of
sediments that is subducted. Under these circum-
stances, then it is conceivable that the contribution
of mantle wedge derived carbon could be over-
whelmed by the large sediment-derived carbon
contribution in the north, and this would lead to
the enhanced CO2/3He and (L + S)/M ratios
observed at Awu and Karangetang.
[43] 2. Variability in sediment composition: Given
the relatively high C isotopic ratios measured in the
northern arc, we suggest that the subducted sedi-
ment in this region contains little hemi-pelagic,
organic-rich sediment. This conclusion is rein-
forced by nitrogen isotope data from the same
Awu locality that was sampled for the present
sample suite. The d15N value of ��3.3% (Clor
et al., submitted manuscript, 2004) indicates that
there is some but not much organic-derived sedi-
mentary N contributing to the volatile flux in this
region (organic N is characterized by d15N values
�+6 to +7% [Peters et al., 1978; Kienast, 2000]).
Thus N in the northern arc has a predominantly
upper mantle origin. Carbonate is essential devoid
of N [see also Fischer et al., 2002], and therefore
slab carbonate addition would not mask a mantle-
like N isotope signature. In combination with the
high CO2/3He and (L + S)/M, it is reasonable to
conclude that there is a strong slab contribution in
the north, dominated by pelagic or crustal carbon-
ate. In contrast, the southern arc samples have
heavier N isotope signature (average d15N��2.5%) and higher N2/He, consistent with a
more pronounced contribution of hemipelagic
sediment derived N2 (Clor et al., submitted man-
uscript, 2004). Thus there is geochemical evi-
dence for some degree of heterogeneity in the
slab composition between the two segments of
the arc.
[44] One possibility to account for the heterogene-
ity in sediment composition along the strike of the
Sangihe Arc is the process of off-scraping; i.e., loss
of the uppermost sedimentary veneer during sub-
duction. Fischer et al. [2002] interpreted variations
in d15N between Costa Rica and other segments of
the Central America Arc by loss of hemipelagic
(N-bearing) and shallow subducted sediments by
accretion to the over-riding plate. If this scenario is
applicable to the Sangihe Arc, then loss of the
hemipelagic sediment would have to be greater in
the northern segment of the arc, consistent with the
increased thickness of the accretionary wedge
toward the collision. In support of this possibility
is the observation that the L/S ratio is extremely
high at Awu volcano (L/S = 87.4) and then
becomes more constant to the south (L/S � 13).
The relatively large distance between Awu and
Karangetang volcanoes may account for the fact
that the effect of off-scraping is not yet observed at
volcanoes other than Awu.
[45] 3. Enhanced fluid/melt generation: An en-
hanced slab signature (i.e., high (L + S)/M) may
result from more efficient heating of the slab rather
than the addition of larger volumes or different
compositions of sediment. In this scenario, the
slow-down or cessation of collision in the north-
ernmost arc (as seen from seismic evidence [e.g.,
McCaffrey, 1983; Pubellier et al., 1991]) would
lead to enhanced heating of the slab [Peacock et
al., 1994], thereby promoting greater production
of melt and/or fluid. Additionally, on the basis
of mineral chemistry, Morrice and Gill [1986]
propose a northward increase in the depth of
melting.
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[46] Along the northern extension of the Sangihe
Arc (central Mindanao, The Philippines), there is
extensive volcanic activity formed by post-colli-
sional magmatism. Sajona et al. [2000] describe
these magmas as subducted basaltic crust-derived
melts, or adakites. Their study attributes the slab
melting to thermal rebound of previously depressed
geotherms upon cessation of subduction. In the
northernmost Sangihe Arc (south of Mindanao),
where subduction-related volcanism is still active
but where the subduction rate is slowing [e.g.,
Pubellier et al., 1991], an analogous situation of
more efficient heating of the subducting slab would
be expected to enhance the slab contribution rela-
tive to the (mantle) wedge input. Elburg and Foden
[1998] have suggested that the Sangihe Arc has
evolved from a system of fluid-derived contribu-
tions to magmagenesis to a system dominated by
sediment-derived melts. They indicate that this
temporal change in apparent source composition
is due to the reduced rate of subduction and
therefore superheating of sediments on the slab.
The CO2/3He and (L + S)/M values in both Awu
and Karangetang geothermal fluids, 1–2 orders of
magnitude higher than average volcanic arcs, are
consistent with the notion of a much more efficient
transfer of a slab component to the northern San-
gihe volcanics. Furthermore, the anomalously high
L/S value of Awu could indicate that the thermal
regime is sufficiently hot so that a significant
fraction of CO2 is derived from a marine carbonate
component within the oceanic basement (i.e., the
source of the L-component). Kerrick and Connolly
[2001] show that significant slab decarbonation is
limited to localities with high-T geotherms; there-
fore an enhanced CO2 flux in the northern segment
of the arc may indicate initiation of slab melting, as
proposed by Sajona et al. [2000]. In this scenario,
the helium-carbon results are tracing changes in the
thermal regime of the subducted slab as the colli-
sion complex propagates south along the Molucca
Sea Plate margin.
6. Conclusions
[47] This work presents the first He and C results
of geothermal fluids from the Sangihe Arc, Indo-
nesia, giving insight into how volatiles are mobi-
lized in an arc-arc collision zone. The following
conclusions are emphasized:
[48] 1. The 26 geothermal and 2 mineral samples
collected along the north-south strike of the
Sangihe Arc contain air-, crustal-, subducted-slab
and/or mantle-derived volatiles. Samples showing
either air contamination (throughout the arc) or
a significant crustal signature (e.g., Ambang
volcano) are identified using a number of criteria
(low 3He/4He, lowHe/Ne values, highCO2/3He, and
extreme d13C), and are distinguished from samples
that characterize the parental source magmas.
[49] 2. He-C isotope and relative abundance results
of this investigation suggest that the oblique colli-
sion of the Halmahera and Sangihe arcs has de-
fined a distinctive pattern of variability in these
parameters along the north-south strike of the
Sangihe Arc. The northern volcanoes (Awu and
Karangetang) show higher CO2/3He and d13C, and
lower 3He/4He than the southern volcanoes
(Ruang, Lokon and Mahawu): the transition in
He-C systematics occurs between Karangetang
and Ruang volcanoes.
[50] 3. Resolving the CO2 output into primary
source component structures (mantle- carbonate-
and organic-derived carbon) indicates that there is
a dominant carbonate influence (>90%) in magmas
supplying the northernmost volcanoes (Awu and
Karangetang). In contrast, the southern arc volca-
noes (Ruang, Lokon, and Mahawu) contain car-
bonate contributions (�63%) more typical of
average arc magmas (�75%).
[51] 4. The northern arc volcanoes record an en-
hanced slab contribution to the carbon inventory
relative to that derived from the mantle wedge. We
discuss three possible reasons for this observation:
(1) increase in sediment volume contributing to arc
volcanism; (2) variations in sediment composition;
and (3) enhanced heating of the slab resulting from
cessation of collision in the northernmost portion
of the arc.
Acknowledgments
[52] This work was supported by the National Science Foun-
dation (grant EAR-0100881 to DRH). Additional funds for field
expenses came from UCSD (Earth Sciences Program to LJ)
GeochemistryGeophysicsGeosystems G3G3
jaffe et al.: tracing magma sources 10.1029/2003GC000660
15 of 17
and UNM (Research Allocations Committee to TF). We thank
Justin Kulongoski and Pat Castillo for valuable support in the
field, Mark Erdmann for arranging accommodations in
Manado, and Purnama Hilton for help with LIPI in Jakarta.
Alison Shaw, Martin Walhen and Bruce Deck all helped
with laboratory analyses. Colin Macpherson, Robert Hall
and the SE Asia Research Group, London, kindly supplied
Figure 1. We thank J. Varekamp, J. Foden, G. Bebout
(Guest Editor) and one anonymous referee for useful com-
ments. We also thank GRDC director Bambang Dwiyanto
(Bandung) for his support of the project.
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