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 (ljaffe@ucsd.edu; drhilton@ucsd.edu)

Tobias P. FischerDepartment of Earth and Planetary Sciences, University of New Mexico, Albuquerque, New Mexico 87131, USA(fischer@unm.edu)

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

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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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