He-Ar and Nd-Sr isotopic compositions of ultramafic xenoliths and

341

Geochemical Journal, Vol. 39, pp. 341 to 356, 2005

*Corresponding author (e-mail: [email protected])

Copyright © 2005 by The Geochemical Society of Japan.

He-Ar and Nd-Sr isotopic compositions of ultramafic xenoliths andhost alkali basalts from the Korean peninsula

KYU HAN KIM,1* KEISUKE NAGAO,2 TSUYOSHI TANAKA,3 HIROCHIKA SUMINO,2 TOSHIO NAKAMURA,4

MITSURU OKUNO,5 JIN BAEG LOCK,6 JEUNG SU YOUN7 and JEEHYE SONG1

1Department of Science Education, Ewha Womans University, Seoul 120-750, South Korea2Laboratory for Earthquake Chemistry, Graduate School of Science, University of Tokyo, Tokyo 113-0033, Japan

3Division of Earth and Environmental Sciences, Nagoya University, Nagoya 464-8602, Japan4Center for Chronological Research, Nagoya University, Nagoya 464-8602, Japan

5Department of Earth System Science, Faculty of Science, Fukuoka University, Fukuoka 814-0180, Japan6Qiaonguang Science and Technology Research Institute, Sichuan Province, China

7Department of Oceanography, Cheju National University, Cheju 690-756, South Korea

(Received July 24, 2004; Accepted February 7, 2005)

Noble gas, Nd and Sr isotopic ratios and major and trace element compositions were determined for ultramafic xenolithsand their host Cenozoic alkali basalts from Baegdusan, Baegryongdo, Jogokri, Jejudo in the Korean peninsula, and LongQuan, close to the Baegdusan in northeastern China, to characterize the lithospheric mantle and the source of alkalibasaltic magmatism beneath the active continental margin of the southeastern part of the Eurasian plate. The xenolithsamples yield significantly variable 3He/4He ratios ranging from <0.2 to 16.8 RA, with most samples (3.5–7.9 RA) lowerthan the MORB value (~8 RA). Among them, high 3He/4He ratios obtained by melting the samples are considered to reflectcosmogenic contribution. The 40Ar/36Ar ratios are much lower than the MORB values. Enriched Nd-Sr isotopic composi-tions in xenoliths and host basalts from the Baegdusan and Baegryongdo areas suggest assimilation of EMII lithosphereand/or continental crust. Widely ranging trace element concentrations in the xenoliths and highly saturated incompatibleelements in the host alkali basalts are observed. K-Ar age data show that Cenozoic alkali volcanism in the Korean penin-sula occurred intermittently, ranging in age from 21 Ma through 11.5–5.0 Ma to 0.1 Ma, and becoming gradually youngertoward the south of the peninsula. Our geochemical and isotopic data suggest the presence of heterogeneously metasomatizedenriched lithospheric mantle generated at an ancient subduction zone within the continental margin of the southeasternend of the Eurasian plate. Degree of enrichments by the metasomatism is discussed based on the observed 3He/4He ratiosin the xenolithic olivines.

Keywords: noble gas isotopes, ultramafic xenoliths, Nd-Sr isotopes, alkali basalts, metasomatized mantle wedge

volcanism (Park and Park, 1996); differing degrees ofpartial melting of an EMI type lithospheric mantle source(Hsu et al., 2000); and melting of a depleted subconti-nental lithospheric mantle source (Kim et al., 2002). Inaddition to these models, evidence of continental rift zonemagmatism has been found in the Boun area, where aperidotite characterized by high Na-clinopyroxene hasbeen reported by Arai et al. (2001). Recently, Sumino etal. (2000) found evidence of mantle plume type He inmantle xenoliths from Takashima, Japan, which is onlyca. 200 km from the Jejudo area of the Korean peninsula(Fig. 1).

Most lherzolites found in the peninsula arecompositionally close to the subcontinental mantleperidotite with equilibrium temperatures and pressuresestimated to be 820–1210°C and 11–28 kb, respectively(Lee, 1995; Choi, 1998; Yun et al., 1998; Choi et al., 2000;Shim, 2003).

INTRODUCTION

Cenozoic alkali basalts with abundant ultramaficxenoliths are distributed in the Long Quan, Baegdusan,Baegryongdo, Gansung, Jogokri (Boun) and Jejudo areasof the southeast margin of the Eurasian continental plate(Fig. 1). Most alkali basalts in the Korean peninsula showOIB geochemical signatures, particularly in REE (Kimet al., 1999). Eruptive ages for the alkali basalts contain-ing the ultramafic xenoliths range from 21 Ma to 0.1 Maas shown in Table 1.

Several genetic models for the origin of the ultramaficxenolith-bearing alkali basaltic magma of the Koreanpeninsula have been proposed. These include: hot spot

342 K.-H. Kim et al.

Until recently, very little was known about the originof Cenozoic alkali basaltic magmatism of the Koreanpeninsula and about the composition of upper mantle be-neath the Korean peninsula. In general, isotopic compo-sitions of noble gases in mantle xenoliths and basalticglasses from MORB and OIB have been used asgeochemical constraints on models for the formation andevolution of the mantle-crust-atmosphere system (e.g.,Allègre et al., 1987; Hart et al., 1985; Hanyu andKaneoka, 1997). The He-Ar-Nd-Sr isotopic compositionsof the ultramafic xenoliths and host alkali basalts of theKorean peninsula will provide valuable information thatcan be used to decipher the origin and evolution of thealkali basaltic magma. This information can also be usedto detect a lower mantle component and to decipher theevolution of the upper mantle as well as the possible in-teraction between crustal and upper mantle materials.

We measured noble gas and Nd-Sr isotopic composi-tions as well as the chemical compositions of mantlexenoliths and host basaltic rocks from the Korean penin-sula to elucidate the origin of alkali basaltic volcanic rocksand to gain insight into the composition of the uppermantle beneath the active continental margin of the Eura-sian plate. Whole rock K-Ar ages were also determinedfor eighteen host basaltic rock samples.

GEOLOGICAL SETTING

Continental ultramafic xenoliths have been found atonly a few localities in the Cenozoic alkali basalts in-truded into Precambrian basement rock of the Koreanpeninsula. Samples collected in this study come from theBaegdusan, Baegryongdo, Gansung, Boun, and Jejudoareas of Korea and from Long Quan in northeastern China.The Korean peninsula is located on the southeast conti-nental margin of the Eurasian plate, which comes in con-tact with the island arcs of Japan. The peninsula repre-sents an important tectonic link between China and theJapanese islands. Furthermore, the Korean peninsula isnoted for Tertiary and Quaternary K-series alkalivolcanism generated when it was an active continentalmargin in the past. The peninsula consists of several tec-tonic provinces from north to south; the Nangrim massif,Pyeongnam basin, Imjingang belt, Kyonggi massif,Ogcheon belt and Yeongnam massif (Ree et al., 1996;Chough et al., 2000). Three Precambrian massifs thatconstitute the basement of the peninsula are composedmainly of high grade metamorphic rocks (Na and Lee,1973). The mobile Ogcheon belt is an intra-cratonic riftzone between the cratonic Kyonggi and Yongnam mas-sifs (Cluzel et al . , 1990). The belt consists of

Fig. 1. Distribution of ultramafic xenolith bearing Cenozoic alkali volcanic rocks in the Korean peninsula of the southeastmargin of the Eurasian continental plate. Sampling sites are included.

He-Ar and Nd-Sr isotopic compositions of ultramafic xenoliths 343

metasedimentary rocks of unknown age.The ultramafic xenoliths appear in alkali basaltic dikes

extruded into the Precambrian Nangnim (Baegdusan andBaegryoungdo), Kyonggi (Gansung) massifs and theOgcheon belt (Jogokri and Jejudo). In the case of theBaegdusan and Jejudo areas, which are noted for largescale successive volcanic activity from Miocene to his-toric time, alkali basalt volcanism is characterized byvoluminous intrusions of ultramafic xenoliths. However,the Baegryongdo and Jogokri (Boun) areas contain onlysmall scale intrusions of xenolith bearing basaltic dikesintruded into the so-called Imjingang collision zone andOgcheon geosynclinal belt. For comparison, peridotitexenoliths were also collected from the vicinity of a craterlake in the Long Quan volcanic area, east China, ca. 100km north of the Baegdusan.

The Baegdusan ultramafic xenoliths range from 2 to30 cm in diameter and are comprised of spinel lherzoliteconsisting mainly of olivine, orthopyroxene,clinopyroxene and chromium spinel. The spinel lherzoliteis characterized by a high Mg value (100∗Mg/(Mg + ∑Fe))of 90% (Shim, 2003). Whole rock K-Ar dating of the hostbasalt from Baegdusan yields ages of 18 to 21 Ma(Table 1). The Baegryongdo xenoliths are coarsely granu-lar (0.5–5 mm in grain diameter) spinel peridotites, rep-resented by Mg-olivine- and clinopyroxene-dominantspinel lherzolite. Olivine is kinked. The host basaltic rocks(4.7–5.0 Ma, Park and Park, 1996) consist mainly of ba-salt-mugearite and basaltic andesite (Lee, 1995; Kim etal., 2002). The basaltic rocks are composed of olivine,pyroxene, plagioclase and minor magnetite and apatite.

Alkali basalt in the Jogokri, Boun area of the Ogcheonbelt contains predominantly peridotites and gabbroicxenoliths ranging in size from 0.5 to 30 cm, and a fewgrass-green olivine and clinopyroxene megacrysts. Notethat olivine is slightly kinked. Most xenoliths show signsof post-eruptive weathering. Hydrous minerals, however,are absent. The peridotite xenoliths are medium grainedlherzolite with a clinopyroxene/pyroxene ratio of 0.2 to0.3 (Arai et al., 2001). The 11 Ma host alkali basalt isfree from feldspathoids and consists mainly of fine-grained olivine and plagioclase. The Jejudo ultramaficxenoliths are 5–40 cm in diameter and are represented byspinel lherzolite, websterites and clinopyroxenites (Choiet al., 2001). The host alkali basalt has been dated by theK-Ar method yielding ages of 0.1–0.2 Ma (Table 1).Ultramafic xenoliths from the Long Quan, east China,consist mainly of Mg-olivine, clinopyroxene andorthopyroxene minerals. Olivines are thick grass-greenmegacrysts up to 10 mm in grain diameter and have Mgvalue range from 90 to 93 %. Notably, phlogopite andamphibole were found in spinel lherzolite xenoliths inthe Wangching area in the vicinity of the Long Quan area(Ou and Chao, 1987), which provides support for the sug-gestion of mantle metasomatism in this area. The K-Arage of the host basaltic rock was determined to be 0.6 Ma(Table 1).

ANALYTICAL PROCEDURES

For noble gas analyses, lherzolite samples werecrushed using a stainless-steel pestle and olivine miner-

Sample No. Province K(Wt.%)

36Ar(10–10 cc/g)

40Ar-rad(10–8 cc/g)

K-Ar age(Ma)

Air-frac(%)

PD1 Baegdusan 1.40 2.61 ± 0.13 108.7 ± 5.4 19.9 ± 1.4 6.6BPD1 Baegdusan 1.22 4.12 ± 0.21 98.3 ± 4.9 20.6 ± 1.5 11.1BPD2 Baegdusan 1.68 4.46 ± 0.22 117.0 ± 5.9 17.8 ± 1.3 10.2BPD3 Baegdusan 2.04 5.44 ± 0.27 167.2 ± 8.4 21.0 ± 1.5 8.8BPD4 Baegdusan 1.65 4.89 ± 0.24 133.4 ± 6.7 20.7 ± 1.5 9.8BR3 Baegryongdo 1.82 12.63 ± 0.38 50.0 ± 1.3 7.1 ± 0.3 42.7BR5U Baegryongdo 2.57 8.00 ± 0.19 64.4 ± 0.6 6.4 ± 0.2 26.8GS1 Songji, Gansung 0.74 9.01 ± 0.45 20.1 ± 1.0 7.0 ± 0.5 57.0GS2 Songji, Gansung 0.94 4.57 ± 0.23 16.9 ± 0.9 4.6 ± 0.3 44.5GS3 Songji, Gansung 0.64 3.29 ± 0.17 17.8 ± 0.9 7.2 ± 0.5 35.3GW1 Unbong, Gansung 1.41 7.09 ± 0.35 39.4 ± 2.0 7.2 ± 0.5 34.8GW2 Unbong, Gansung 0.67 2.19 ± 0.11 19.5 ± 1.0 7.5 ± 0.5 24.9GW3 Unbong, Gansung 1.34 8.86 ± 0.44 37.6 ± 1.9 7.2 ± 0.5 41.1OC1 Jogokri, Boun 0.82 24.5 ± 1.2 35.5 ± 1.8 11.1 ± 0.8 67.1BCJ1 Jejudo 1.45 3.27 ± 0.16 0.99 ± 0.05 0.175 ± 0.013 90.7BCJ2 Jejudo 1.32 3.01 ± 0.15 0.76 ± 0.04 0.149 ± 0.011 92.1BCJ3 Jejudo 1.59 3.06 ± 0.15 0.71 ± 0.04 0.115 ± 0.008 92.7CY1 Long Quan, China 1.43 4.16 ± 0.21 3.67 ± 0.19 0.66 ± 0.05 77.0

Table 1. The K-Ar ages of ultramafic xenolith bearing basalts from the Korean peninsula

Two samples (sample no. BR3 and BR5U) were analyzed at Korea Basic Science Institute.


als were selected by hand picking. Noble gases were ex-tracted from the olivine mineral grains (grain size 0.5–2mm) by in vacuo crushing and heating methods appliedby Nagao et al. (1996) and Sumino et al. (2001). In thecrushing experiments, about 1 g of sample was crushedin a stainless steel tube using a nickel rod driven fromoutside the vacuum by using a solenoid magnet. About0.5 g of the crushed samples was completely melted at1800°C to extract all noble gases. The isotopic composi-tions of the released noble gases were measured using asector-type mass spectrometer, a modified-VG5400 (MS-III) in the Laboratory for Earthquake Chemistry (LEC) atthe University of Tokyo. Sensitivities and mass discrimi-nation correction factors of the mass spectrometer sys-tem were determined by measuring known amounts ofatmosphere with the same procedure applied for samples.The discrimination factor for 3He/4He was determinedwith the HESJ, (He Standard of Japan) with 3He/4He =(28.88 ± 0.14) × 10–6 (Matsuda et al., 2002). Experimen-tal uncertainties in the noble gas concentrations were es-timated to be about 10% based on the reproducibility ofmeasurements of the standard gas. Errors on isotopic ra-tios are one standard deviation, including errors in blankcorrection and mass discrimination.

For whole rock K-Ar dating of the host basalts, frag-ments of 60–80 mesh were prepared by excluding anyvisible phenocrysts under a binocular microscope. Ar iso-tope analysis was carried out by the sensitivity methodwithout a 38Ar-spike, using the modified-VG5400 (MS-III) mass spectrometer in the LEC at the University ofTokyo. K-concentrations and trace elements wereanalyzed by a V.G. Plasmaquad inductively coupledplasma mass spectrometer (ICP-MS) with 5–10% analyti-cal precision at Kings College, University of London,England.

The Nd-Sr isotopic ratios were measured on a VG sec-tor thermal ionization mass spectrometer at Nagoya Uni-versity. The 143Nd/144Nd and 87Sr/86Sr were normalizedto 143Nd/144Nd =0.7219 and 87Sr/86Sr =0.1194, respec-tively. NBS987 yielded a 87Sr/86Sr value of 0.7102417 ±(11). 143Nd/144Nd measurements of La Jolla and JNdi-1yielded 0.511857 ± (18) and 0.512115 ± (17), respectively.Reported uncertainties are 1σ of the mean of 200 ratios.

RESULTS

K-Ar age of host alkali basaltWhole rock K-Ar ages of several ultramafic xenolith

bearing alkali basalts from the Korean peninsula showeda wide spectrum ranging from 0.1 to 21 Ma: Baegdusan(18–21 Ma), Baegryongdo (6.4–7.1 Ma), Boun (11.1 Ma),Gansung (5.0–7.5 Ma) and Jejudo (0.1–0.2 Ma)(Table 1). Our age data for the Boun basalt (11.1 ± 0.8Ma) is in good agreement with the results of Arai et al.

(2001) (11.0 ± 0.6 Ma). However, our ages for basaltsfrom the Baegryongdo area are slightly older than theresults of Park and Park (1996) who have reported the K-Ar ages of host basalts in the Baegryongdo area as 4.7 to5.0 Ma. In addition, a basalt sample from Long Quan,China, shows a relatively young eruptive age of 0.7 Ma(Table 1). Cenozoic alkali volcanism characterized byabundant ultramafic xenoliths in the Korean peninsulaerupted intermittently over a scattered area, ranging inage from 21 Ma through 11–5 Ma to 0.1 Ma (Table 1).The radiometric age data indicate that the alkali volcanismbecomes gradually younger toward the south of the pe-ninsula.

Geochemical compositions of host basalt and xenolithThe SiO2 contents in the host alkali basalts range from

40.7 to 51.6 wt%. Alkali contents (Na2O + K2O) fall inthe relatively high value range of 4.2 to 5.0 wt%, com-pared with average basalts (4.01 wt%, Le Maitre, 1976).High concentrations of incompatible elements with Cr(244–388 ppm) and Ni (172–263 ppm) suggest that themantle in this area is slightly enriched in these elements(Table 2). Furthermore, incompatible elements (Rb, Ba,Sr) are also highly saturated in the host basalts with 250–558 ppm (av. 451 ppm) for Ba, 525–1274 ppm (av. 921ppm) for Sr and 13–54 ppm for Rb (Rb/Sr ratio = 0.01–0.08). The saturation of incompatible elements in basal-tic magma in this area is presumably due to the preferen-tial incorporation of these elements in the subcontinentallithosphere.

The bulk rock analysis of ultramafic xenoliths indi-cates that Mg-rich olivine (36–47 wt%). Ni (1713–2726ppm, av. 2029 ppm) contents in the xenoliths are a littlehigher than undepleted primitive upper mantle (Ni: 1890ppm), whereas the contents of Cr (255–1807 ppm, av.1198 ppm), Sc (2–15 ppm, av. 9.2 ppm) and V (7–69 ppm,av. 39.3 ppm) are lower than those of primitive uppermantle (Cr: 2625, Sc: 15.5 ppm, V; 82 ppm, McDonoughand Sun, 1995). Trace elements in the ultramafic xenolithsfrom several localities in the Korean peninsula show awide range in concentrations. The wide range of traceelement concentrations in the ultramafic xenoliths indi-cates a significant heterogeneity in trace element compo-sition in the source, i.e., the subcontinental lithospherebeneath the Korean peninsula, possibly due to mantlemetasomatism as suggested by Sumino et al. (2004) formantle xenoliths from Jejudo (Cheju Island) area.

Noble gas isotopic compositions of ultramafic xenolithsNoble gas (He, Ne, Ar, Kr and Xe) concentrations and

isotopic ratios obtained by crushing and heating experi-ments for the olivine separates from ultramafic xenolithsare presented in Table 3. They are expected to providesome constraints on the source of the Cenozoic alkali


Sam

ple

No.

Loc

atio

nX

PD1

(Xen

olit

h)B

aegd

usan

XPD

3(X

enol

ith)

Bae

gdus

an

XPD

5(X

enol

ith)

Bae

gdus

an

XB

R1

(Xen

olit

h)B

aegr

yong

do

XB

R2

(Xen

olit

h)B

aegr

yong

do

XC

J1(X

enol

ith)

Jeju

do

XC

J3(X

enol

ith)

Jeju

do

XC

Y2

(Xen

olit

h)L

ong

Qua

n

XC

Y4

(Xen

olit

h)L

ong

Qua

n

PD1

Bas

alt

Bae

gdus

an

BR

2B

asal

tB

aegr

yong

do

CJ1

Bas

alt

Jeju

do

OC

1B

asal

tJo

gokr

i

CY

1B

asal

tL

ong

Qua

n

Maj

or e

lem

ents

(w

t.%)

SiO

243

.19

42.6

942

.38

43.0

643

.38

41.2

842

.32

41.7

43.2

45.8

151

.63

48.6

040

.68

46.7

2A

l 2O

30.

931.

032.

200.

170.

291.

632.

562.

152.

1014

.04

12.8

014

.34

12.1

514

.48

Fe 2

O3

6.93

7.59

7.15

9.01

9.97

9.33

8.71

7.94

8.23

11.9

113

.10

11.5

112

.21

10.9

9M

gO41

.05

43.3

436

.447

.29

45.8

39.7

138

.51

36.1

337

.69

9.61

7.90

8.14

7.76

8.17

CaO

0.42

0.64

2.60

0.25

0.31

1.85

3.33

2.63

3.46

7.48

6.56

7.85

8.56

7.76

Na 2

O0.

030.

050.

190.

050.

050.

160.

310.

200.

243.

203.

833.

213.

273.

30K

2O0.

050.

030.

110.

040.

040.

020.

030.

010.

011.

691.

081.

750.

991.

72T

iO2

0.02

0.02

0.05

0.01

0.02

0.07

0.10

0.08

0.10

2.38

2.43

2.18

1.74

2.05

P2O

50.

010.

020.

010.

010.

010

0.01

0.01

0.01

0.41

0.52

0.33

0.77

0.32

MnO

0.11

0.11

0.11

0.11

0.13

0.13

0.13

0.12

0.12

0.15

0.15

0.15

0.17

0.15

Sum

92.7

495

.52

91.2

010

0.00

100.

0094

.18

96.0

190

.97

95.1

896

.68

100.

0098

.06

88.3

095

.66

Tra

ce e

lem

ents

(pp

m)

Ba

911

97

76

76

848

425

045

151

355

8C

o87

9081

9191

9585

8987

5148

4648

46C

r12

0014

8912

4927

025

510

4616

6617

9618

0738

824

430

035

226

3C

u6

616

32

911

1413

3837

3844

39L

i5

44

32

44

36

1212

1213

10N

i19

3920

9517

2327

2625

7919

5117

1317

2718

0526

319

417

523

517

2Sc

77

122

210

1513

1516

1219

1319

Sr21

1337

22

610

1317

1274

1149

582

1076

525

V26

2151

78

4869

6163

179

161

171

226

162

Zn

4145

4151

4755

5153

5410

512

411

012

882

Zr

2017

202

321

2125

4128

929

726

326

219

0U

0.1

0.08

0.08

ND

ND

0.08

0.13

0.08

0.14

0.85

ND

1.25

0.42

0.94

Th

0.37

0.34

0.29

ND

ND

0.44

0.55

0.35

0.64

4.85

ND

6.05

6.39

5.40

Rb

01

25

42

12

454

3437

1342

Nb

1.3

0.4

0.8

ND

ND

0.7

1.3

1.1

1.2

40.1

ND

46.2

63.1

44.3

Cs

0.02

0.01

0.00

ND

ND

0.00

0.00

0.01

0.00

1.51

ND

0.22

0.28

0.42

Hf

0.17

0.13

0.11

ND

ND

0.33

0.19

0.24

0.44

6.46

ND

56.5

6.01

4.50

Ta

0.12

0.13

0.13

ND

ND

0.12

0.10

0.12

0.22

2.56

ND

2.77

4.08

2.71

Y0.

30.

42.

20

02

3.3

2.4

3.2

23.1

1821

.724

.227

.2

Tabl

e 2.

M

ajor

and

tra

ce e

lem

ent

com

posi

tion

s of

man

tle

xeno

lith

s an

d th

eir

host

bas

alts

fr o

m t

he K

orea

n pe

nins

ula


Sam

ple

No.

(Pro

vinc

e)M

etho

dW

eigh

tg

4 He

10−9

cc/

g

3 He/

4 He

(R/R

A)

2 0N

e10

−12 c

c/g

36A

r10

−9 c

c/g

36A

r/36

Ar

4 0A

r/3 6

Ar

84K

r10

−12 c

c/g

13

2X

e10

−12 c

c/g

XC

Y2

Lon

g Q

uan,

Chi

naC

rush

1800

°C0.

8617

0.70

9910

113

75.

90 ±

0.2

0

6.18

± 0

.15

16.4

126

0.05

350.

559

0.19

08 ±

0.0

007

0.18

88 ±

0.0

005

2953

± 2

4

399.

0 ±

0.7

1.6

44.3

0.01

18.6

XC

Y3

Lon

g Q

uan,

Chi

naC

rush

1800

°C0.

3471

0.30

4823

.922

.66.

00 ±

0.4

3

6.88

± 0

.54

14.9

212

0.06

111.

340.

1893

± 0

.001

9

0.18

91 ±

0.0

005

1038

.1 ±

5.1

319.

3 ±

0.3

1.8

114

0.16

42.8

XC

Y5

Lon

g Q

uan,

Chi

naC

rush

1800

°C1.

0408

0.73

882.

24.

95.

87 ±

0.8

3

6.21

± 0

.59

5.3

65.1

0.02

710.

438

0.18

71 ±

0.0

008

0.18

88 ±

0.0

006

583.

7 ±

1.2

321.

9 ±

0.7

0.4

39.7

0.04

14.1

XPD

1B

a egd

usa n

Cru

sh18

00°C

0.99

470.

8987

0.3

29.2

ND

2.76

± 0

.17

6.3

64.4

0.06

621.

010.

1885

± 0

.001

4

0.18

87 ±

0.0

004

312.

7 ±

1.0

303.

6 ±

0.3

2.6

144

0.18

146

XPD

3B

a egd

usa n

Cru

sh18

00°C

1.19

130.

6782

5.6

22.3

7.71

± 0

.49

7.96

± 0

.30

7.9

52.8

0.06

060.

816

0.18

84 ±

0.0

008

0.18

87 ±

0.0

004

321.

8 ±

0.4

305.

1 ±

0.3

1.6

94.3

0.16

83.8

XPD

5B

a egd

usa n

Cru

sh18

00°C

0.97

240.

6521

2.8

10.0

7.14

± 1

.00

11.5

0 ±

0.86

30.1

112

0.16

41.

080.

1893

± 0

.000

7

0.18

85 ±

0.0

004

317.

6 ±

0.8

300.

1 ±

0.3

4.1

95.8

0.31

78.2

XB

R(O

L-7

)B

aegr

yong

doC

rush

1800

°C0.

7764

0.47

531.

42.

64<

0.6

16.8

± 3

.19.

411

.00.

0968

0.32

50.

1908

± 0

.001

4

0.18

88 ±

0.0

007

500.

6 ±

1.5

320.

4 ±

0.6

0.3

11.7

0.08

3.69

XB

R(O

L8-

1)B

aegr

yong

doC

rush

1800

°C0.

9541

0.61

121.

59.

67<

0.6

5.0

± 1.

130

.359

.90.

215

0.23

40.

1887

± 0

.002

4

0.18

89 ±

0.0

010

312.

8 ±

0.8

301.

2 ±

0.5

0.5

9.4

0.06

3.31

Kim

et a

l. (2

002)

XB

R(O

L8-

2)B

aegr

yong

doC

rush

1800

°C0.

9938

0.44

490.

58.

26<

0.2

6.7

± 1.

325

.681

.60.

184

0.28

50.

1883

± 0

.001

6

0.18

86 ±

0.0

008

299.

9 ±

1.2

299.

4 ±

0.5

0.36

10.9

0.04

3.56

Kim

et a

l. (2

002)

XO

C2

Jogo

kri,

Bou

nC

rush

1800

°C0.

4297

0.30

670.

613

.4N

D5.

91 ±

0.5

66.

738

50.

325

7.65

0.18

91 ±

0.0

009

0.18

87 ±

0.0

005

307.

7 ±

0.6

296.

9 ±

0.4

16.5

612

1.19

176

Kim

et a

l. (2

002)

XO

C3

Jogo

kri,

Bou

nC

rush

1800

°C0.

5034

0.48

342.

127

.04.

64 ±

1.4

3

3.47

± 0

.22

14.6

114

0.34

71.

030.

1891

± 0

.001

1

0.18

88 ±

0.0

005

320.

3 ±

0.4

300.

9 ±

0.3

15.8

86.3

0.80

36.6

Tabl

e 3.

N

oble

gas

con

cent

rati

ons

and

isot

opic

rat

ios

of u

ltra

maf

ic x

enol

iths

fr o

m t

he K

orea

n pe

nins

ula

(R/ R

A):

Nor

mal

i zed

to

t he

atm

osph

eri c

rat

i o =

1.4

× 1

0–6 (

Ozi

ma

and

Pod

osek

, 20

02).

ND

: no

t de

t erm

i ned

.


basaltic magma and hence the composition of the uppermantle beneath the Korean peninsula.

Helium isotopes: Helium abundances were obtainedby crushing olivines at room temperature and then fusingthem at 1800°C, with abundances ranging from 0.3(XPD1) to 101 (XCY2) × 10–9 cm3STP/g and from 5(XCY5) to 137 (XCY2) × 10–9 cm3STP/g for crushingand fusion, respectively (Fig. 2). Fusion of samples yieldsover 50% more helium than the amount obtained by crush-ing, suggesting the majority of the helium in these sam-ples resides in very small inclusions and/or mineral lat-tice that remained closed systems upon crushing. On theother hand, samples XCY2 and XCY3 from Long Quan,China, yield similar helium abundances by both fusionand crushing methods. 3He/4He values for the samplesfrom China (XCY-samples) indicate a relatively homo-geneous helium isotopic ratio distribution in the sample.Even though different He contents were obtained fromthe olivine by different extraction methods (crushing andheating), the helium isotope measurement (3He/4He)yields similar results. However, olivines from the LongQuan, China have variable, and high content of 4He up to137 × 10–9 cm3STP/g for the sample XCY2.

Although MORBs have relatively uniform 3He/4Heratio of 8 RA (RA = 1.4 × 10–6 for atmospheric 3He/4Heratio; Porcelli and Wasserburg, 1995; Sarda and Graham,1990), OIB (hot spot volcanism) show wide variation in3He/4He, ranging from 8 to 37 RA (Kurz et al., 1982;Honda et al., 1993; Hilton et al., 1999). On the other hand,arc related volcanic rocks in subduction zones are char-acterized by heterogeneous helium isotopic compositionsranging from 0.01 RA to a highest reported value of 8.90

RA (av. 5.4 RA) (Hilton et al., 1992, 1993, 2002). As shownin Fig. 2, most ultramafic xenoliths from the Korean pe-ninsula have 3He/4He ratios between 3.5 and 7.9 RA, whichare lower than MORB and similar to those found in sub-duction zone volcanism (<8 RA). The low and wide vari-ation of 3He/4He ratios compared with the MORB valueimplies that the lithospheric mantle source beneath theKorean peninsula has a higher (U + Th)/3He ratio thanthe MORB mantle source as pointed out for the xenolithdata from Jejudo by Sumino et al. (2004). In the case ofthe Jejudo area, lower 3He/4He ratio (6.51 ± 0.05 RA) and40Ar/36Ar ratios (40Ar/36Ar < 5500) than MORB valueshave been obtained. Sumino et al. (2004) explained the3He/4He and 40Ar/36Ar values as due to a trace ofmetasomatism from a slab-derived component.

However, unusually high He isotopic values in olivineshave been obtained from the Baegdusan (11.5 RA) andBaegryongdo (16.8 RA) areas. The high values were ob-tained by the heating method. This indicates that the he-lium source is possibly from post-eruptive cosmogenic3He as reported by Hilton et al. (1993) and Dodson andBrandon (1999). This is well expressed in Fig. 3, where3He/4He ratios obtained by crushing and heating werecompared. Because cosmogenic 3He accumulates in thelattice and is released only by heating, 3He/4He ratios byheating higher than that by crushing can be attributed tothe post eruptive product by cosmic-ray irradiation.Hence, 3He/4He ratio by crushing is regarded as more rep-

Fig. 2. 3He/4He ratios plotted against 4He concentrations forolivine separates from mantle xenoliths. He by heating for someolivines from Baegdusan and Baegryongdo exhibit cosmogeniccontribution of 3He (see Fig. 3). XCY (Long Quan, China), XPD(Baegdusan), XBR (Baegryongdo) and XOC (Jogokri, Boun).

Fig. 3. Comparison of 3He/4He ratios obtained by crushingand heating methods for the same sample of the lherzolitexenolith from the Korean peninsula. The dashed line gives the1:1 correlation line. Four samples are plotted on the 1:1 cor-relation line, indicating neither post eruptive addition of ra-diogenic 4He nor cosmogenic 3He. However, some olivines fromthe Baegdusan (XPD) and Baegryongdo (XBR) contain thecosmogenic 3He.


resentative for the value of the source region. From thepoint of view, 3He of the samples (XBR) fromBaegryongdo is mostly of cosmogenic origin, and 3He ofXPD5 from Baegdusan is partly cosmogenic. In the fol-lowing discussion, 3He/4He ratios obtained by crushingis used as the source signature if the ratio by heating isclearly higher than that by crushing.

Argon isotopes: Most olivine samples from the Ko-rean peninsula show relatively homogeneous 40Ar/36Arratios by crushing, ranging from 300 to 500. In contrast,the ratios in olivines from Long Quan, China, show muchhigher values of 2953 and 1038 (XCY2 and XCY3) bycrushing. However, the same samples extracted by heat-ing yield lower 40Ar/36Ar ratio (Table 3). The argon ra-tios by heating for most olivine samples are close to theatmospheric value of 296, which may be atmospheric Arcontamination adsorbed on the crushed powder samples.

As shown in Table 3, the 38Ar/36Ar ratios in the olivinesamples varies from 0.1871 to 0.1908, which are close tothe atmospheric composition (0.1880, Ozima andPodosek, 2002).

In the case of Jejudo area, the 40Ar/36Ar ratios ofultramafic xenoliths show high values of 650 to 4400 bycrushing method, and the highest value up to 5500 bystepwise heating method (Sumino et al., 2004). However,the values are significantly lower than those of the MORBmantle source (>30000, Graham, 2002).

The olivine with 40Ar/36Ar ratios much lower than theupper mantle value suggest that the lithospheric mantlebeneath the Korean peninsula situated at the eastern mar-gin of the Eurasian continental plate could have experi-enced mantle metasomatism by a slab-derived componentinvolved in the Cretaceous subduction zone located be-neath the Korean peninsula (Maruyama et al., 1989).

Sample No. Province Rb(ppm)

Sr (ppm) Rb/Sr ratio 87Sr/86Sr εSr Nd(ppm)

Sm(ppm)

Sm/Nd ratio 143Nd/144Nd εNd

Alkali basaltBPD1 Baegdusan 61.03 1305 0.05 0.70577 18.0 16.18 1.79 0.11 0.512690 1.0BPD2 Baegdusan 36.96 1775 0.02 0.70568 16.7 32.08 3.56 0.11 0.512793 3.0BPD3 Baegdusan 32.41 1069 0.03 0.70576 17.9 33.75 3.74 0.11 0.512687 1.0BPD4 Baegdusan 39.95 827 0.05 0.70571 17.2 18.57 2.06 0.11 0.512690 1.0BR1 Baegryongdo 20.00 592 0.03 0.70432 −2.6 45.05 6.34 0.14 0.512729 1.8

BR6u Baegryongdo 23.00 1160 0.02 0.70336 −16.2 22.24 3.38 0.15 0.512939 5.5

BR10 Baegryongdo 13.00 1050 0.01 0.70370 −11.4 35.00 5.20 0.15 0.512897 5.1

GS1 Songji, Gansung 26.39 513 0.05 0.70378 −10.2 17.96 1.99 0.11 0.512868 4.5



GW1 Unbong, Gansung 31.44 543 0.06 0.70537 12.3 18.36 2.03 0.11 0.512636 0.0GW2 Unbong, Gansung 11.56 435 0.03 0.70410 −5.6 18.73 2.07 0.11 0.512797 3.1

GW3 Unbong, Gansung 27.97 536 0.05 0.70508 8.2 18.21 2.02 0.11 0.512671 0.6BCJ1 Jejudo 51.51 629 0.08 0.70434 −2.2 28.29 3.20 0.11 0.512785 2.9

BCJ2 Jejudo 64.59 541 0.12 0.70428 −3.1 17.43 1.93 0.11 0.512809 3.3

BCJ3 Jejudo 58.95 629 0.09 0.70421 −4.2 17.61 1.95 0.11 0.512969 6.5

CY1 Long Quan, China 56.05 559 0.10 0.70409 −5.8 17.83 1.98 0.11 0.512650 0.2

Ultramafic xenolithXPD1 Baegdusan 2.56 21.02 0.12 0.70564 16.1 0.16 0.03 0.19 0.512694 1.1XPD2 Baegdusan 1.98 12.42 0.16 0.70558 15.3 0.10 0.04 0.40 0.513231 11.6XPD3 Baegdusan 1.45 9.82 0.15 0.70599 21.1 0.08 0.02 0.25 0.512526 −2.2

XPD4 Baegdusan 2.36 27.09 0.09 0.70589 19.8 0.15 0.04 0.27 0.512675 0.7XPD5 Baegdusan 3.90 33.91 0.12 0.70577 18.0 0.36 0.11 0.31 0.512818 3.5XBR3 Baegryongdo 9.00 1.34 6.72 0.70750 42.6 0.32 0.08 0.25 0.512173 −9.1

XBR8-1 Baegryongdo 0.23 9.34 0.02 0.70452 0.3 0.37 0.08 0.22 0.512846 4.0XCJ1 Jejudo 0.17 1.95 0.09 0.70443 −1.0 0.11 0.04 0.36 0.512867 4.5

XCJ2 Jejudo 0.48 2.22 0.22 0.70431 −2.8 0.07 0.01 0.14 0.512787 2.9

XCJ3 Jejudo 0.52 7.46 0.07 0.70417 −4.7 0.48 0.14 0.29 0.512819 3.5

XCY2 Long Quan, China 1.62 9.36 0.17 0.70378 −10.2 0.47 0.15 0.32 0.512938 5.8

XCY4 Long Quan, China 1.14 7.15 0.16 0.70409 −5.8 0.36 0.10 0.28 0.512832 3.8

Table 4. Sr-Nd isotopic compositions of alkali basalts and mantle xenoliths from the Korean peninsula

Rb, Sr, Sm and Nd concentrations were measured by isotopic dilution method.


Nd-Sr isotopesCompared to their host basalts, the xenoliths show

considerable scatter in Nd and Sr isotopic compositions(Table 4 and Fig. 4). The host alkali basalt ranges from143Nd/144Nd = 0.51264 to 0.51297 (εNd = 0.0 to 6.5) and87Sr/86Sr = 0.7034 to 0.7058 (εSr = –16.2 to 18.0)(Table 4). On the other hand, the Nd and Sr isotopic com-positions of xenoliths vary widely with 143Nd/144Nd from0.51217 to 0.51323 (εNd = –9.1 to 11.6) and 87Sr/86Srfrom 0.7038 to 0.7075 (εSr = –10.2 to 21.1) (Table 4).The general isotopic characteristics of host basalts areconsistent with xenoliths in the same localities. This sug-gests that the alkali basaltic magma has been generatedfrom melting of in situ lithospheric mantle. The Nd-Srisotopic signature of the basalts shows slightly enrichedmantle characteristics compared to HIMU and PREMA(Fig. 4).

However, xenoliths and their host basalts from theBaegdusan area show enriched isotopic compositions withmore extreme 87Sr/86Sr isotopic ratios. One xenolith sam-ple (sample no. XBR 3) from the Baegryongdo area has

unusually enriched isotopic compositions (Fig. 4). Thisisotopic enrichment trend could potentially be generatedby contamination of magma by EMII materials and/or U-and Rb-enriched continental crust. The extremely highRb/Sr (0.09 to 6.72) and Sm/Nd ratios (0.11 to 0.40) inthe xenoliths are also likely to be related to contamina-tion from an ancient subducted slab, whereby oceaniccrustal material is injected into the lithospheric mantlebeneath the Korean peninsula.

These chemical and Nd-Sr isotopic signatures can begenerated by an ancient metasomatic event, probably re-lated to old subduction zone processes, causing LIL ele-ment enrichment of the lithospheric mantle.

DISCUSSION

In the Korean peninsula, some alkali basaltic magmashost lherzolite xenoliths. These alkali basalts and theirxenoliths are emplaced through thick, old continentallithosphere and may tell us about magmatic processes inthe active continental margin of the Eurasian plate. He-

Fig. 4. Variations of 87Sr/86Sr versus 143Nd/144Nd for host alkali basalts and ultramafic xenoliths from the Korean peninsula.Shaded area shows the main oceanic mantle reservoirs of Zindler and Hart (1986). EMI and EMII, enriched mantle; DM, de-pleted mantle; BSE, bulk silicate Earth; HIMU, mantle with high U/Pb ratio; PREMA, frequently observed PRE valent mantlecomposition. Xenoliths and host basalts from the Baegdusan indicate enriched isotopic signatures.


Ar and Nd-Sr isotopic data represent a powerful tool forsuch genetic studies. Based on the noble gas isotopic com-positions of xenolith minerals and their host basalts, thexenoliths were found to have equilibrated with their hosts(Bernatowicz, 1981; Dunai and Baur, 1995; Dodson andBrandon, 1999). Therefore, noble gas data from ultramaficxenoliths can be used to discuss characteristics of the hostalkali basaltic magmatism as well as providing informa-tion about the character of the upper mantle.

Noble gases in the mantle source region of basalts andxenoliths at the continental margin arise from threesources: (1) an elementally unfractionated component,introduced by bulk transfer from the deeper mantle viaplumes (Harrison et al . , 1999), (2) a radiogenic/nucleogenic component arising from in situ radioactivedecay of U, Th and K, and accumulated within the uppermantle over a residence time (Reid and Graham, 1996),(3) a subducted atmospheric component, possibly re-stricted to the heavy noble gases (Kurz et al., 1982).

He isotopic data for xenoliths from the Baegdusan area(XPD) are almost the same as the MORB values. On theother hand, unusually low 3He/4He ratios (<0.2 RA) weredetected in the Baegryongdo xenoliths, which may be dueto the low concentrations of He (Table 3) and the produc-tion of radiogenic 4He from the enriched U and Th due toa metasomatic event which occurred in the source mate-

rial as will be discussed later. The very low 3He/4He ra-tios for Baegryongdo (XBR) may not be caused by a con-tamination from crustal materials such as old graniticrocks on the way to the surface. If the crustal contamina-tion is responsible for the very low 3He/4He ratios, therelatively low 40Ar/36Ar ratios (300–500) by both crush-ing and heating methods are difficult to explain. Nd andSr isotopes ratios partly indicate an EMII signature(Fig. 4), which may be related with subducted slab mate-rials. Hence the He and Ar isotopic compositions mayrepresent noble gas signature of the source region. Simi-larly low 3He/4He and 40Ar/36Ar ratios have been reportedfor mantle xenoliths from Sikhote-Alin, Far Eastern Rus-sia, by Yamamoto et al. (2004), in which the noble gasdata are attributed to a signature of old subduction-re-lated metasomatism. Xenoliths from other localities haveintermediate 3He/4He ratios, i.e., 6 RA (XCY3) and 4.6RA (XOC3) for Long Quan and Jogokri, respectively.

Crush released 3He/4He ratios for the xenoliths fromLong Quan (XCY), Baegdusan (XPD), Baegryongdo(XBR) and Jejudo area (Sumino et al., 2004) are presentedagainst the 87Sr/86Sr ratios in Fig. 5, along with someendmembers for comparison. Though the high 87Sr/86Srisotopic ratios for Baegdusan (XPD) indicate a highlyenriched character compared with other two localities,the 3He/4He ratios of the olivines from the same locality

Fig. 5. Diagram of 3He/4He versus 87Sr/86Sr ratios for ultramafic xenoliths from the Korean peninsula. Data sources: Iceland(Condomines et al., 1983), Loihi (Kurz et al., 1983; Staudigel et al., 1984), Jejudo (Sumino et al., 2004) and remainder summa-rized in Lupton (1983) and Porcelli et al. (1986).


xenoliths studied in this work would reflect the involve-ment of a pre-existing metasomatized mantle wedge inthe genesis of the ultramafic xenoliths.

Figure 6 shows the tectonic setting at the easternboundary of the Eurasian plate at around 100 Ma andpresent. In Cretaceous the Izanagi oceanic plate subductedbeneath the Eurasian continental plate which induced arcmagmatism at the Korean peninsula leaving ametasomatized subcontinental lithospheric mantle(Maruyama et al., 1989). The 4He has likely been pro-duced by the decay of U and Th in the metasomatizedmantle just above the subducting slab as suggested byReid and Graham (1996) and Dodson and Brandon (1999).

Based on the geochemical characteristics of incom-patible elements, Arai et al. (2001) considered that theorigin of alkali basalt volcanism in the Korean peninsulais intraplate as well as mantle plume volcanism. How-ever, noble gas isotope and geochemical data in this studyprovide no evidence for the plume volcanism in the stud-ied area, but instead indicate the metasomatized charac-

show the least contribution of radiogenic 4He to the origi-nally trapped He in olivines. The decoupling observationcommonly asserted as justification for the magma over-printing phenomenon (Porcelli et al., 1986; Dunai andBaur, 1995; Barfod et al., 1999; Hilton et al., 1993;Dodson and Brandon, 1999). In our samples, thedecoupling between helium and strontium for theBaegdusan (XPD) olivines reflect the different processescontrolling the volatile versus the lithophile element char-acteristics of the ultramafic xenoliths. In the plot for Ndand Sr isotope ratios (Fig. 4), the Baegdusan data plottedabove the trend of the mantle array, suggesting an addi-tional EMII-like component to the xenoliths and hostbasalts. This may be a result of crustal assimilation to thesamples, whereas He was not affected by the process.

Because the xenoliths should have been brought tosurface within a short time with the alkali basalt magma,crustal contamination seems to be unlikely to explain thelower and variable 3He/4He ratios trapped in the olivinecrystals. Accordingly, the wide variations in 3He/4He in

Fig 6. A mantle model depicting the noble gas isotopic compositions of ultramafic xenoliths and host alkali volcanic rocks fromthe Korean peninsula of the southeast continental margin of the Eurasian plate. The area with V marks indicates the active zoneof arc magmatism (Maruyama et al., 1989). Thick solid lines and black areas indicate plate boundaries and distribution of alkalibasalt, respectively. SK: Sino-Korean craton, Y: Yangtze craton. 40Ar/36Ar = 5500 for Jejudo (Sumino et al., 2004).


teristics of mantle xenoliths. These data can also lead tothe interpretation that the origin of alkali basalt volcanismin this region is closely related to subduction zonemagmatism, rejuvenated by old (Mesozoic) subductedslab of the Izanagi plate (Maruyama et al., 1989) beneaththe Korean peninsula. Min et al. (1982, 1988) reportedevidence from the peninsula for the operation of platetectonics during the Cretaceous. They pointed out an in-crease in the depth of the subducted plate towards theinland side of the peninsula based on the K2O variationand the southward decrease of K-Ar ages of andesiticrocks. Geobarometry (820–1210°C, 11–28 kb) suggeststhe existence of thick continental crust above the sourceregion for the basaltic magmatism (Lee, 1995; Choi, 1998;Yun et al., 1998; Choi et al., 2001; Shim, 2003). A sig-nificant lithospheric contribution to the alkali basalticmagma is further indicated by the enriched signature ofNd-Sr isotopic compositions for some samples as indi-cated in Fig. 4.

Noble gas isotopic and geochemical examination ofthe ultramafic xenoliths and host alkali basalts indicatesthat the subcontinental lithosphere is a metasomatizedmantle wedge possibly generated by the subduction ofthe Mesozoic Izanagi plate (Maruyama et al., 1989) asillustrated in Fig. 6. Based on the K-Ar age data, whichis young (11 to 0.1 Ma) relative to the age of the openingof the East Sea and Sea of Japan (ca. 15 Ma), and thetectonic framework around the Korean peninsula, the al-

kali volcanism is possibly related to the opening of theEast Sea (Sea of Japan). This interpretation is consistentwith the results of Iwamori (1991) and Uto (1990) whoreported that the asthenospheric mantle diapirism thatresulted in the opening of the Japan Sea was responsiblefor the Cenozoic volcanism in the area including theSouthwest Japan arc where xenolith-bearing alkali basaltserupted.

Figure 7 is a plot of 4He/40Ar-rad versus 4He concen-tration for the xenoliths. The sample XBR (OL-7) withthe large contribution of cosmogenic 3He was omitted inthe plot. Radiogenic 40Ar concentration was calculatedby subtracting atmospheric Ar (see caption for Fig. 7).Data points for XCY, XPD and XOC form their owntrends, where the points for heating are plotted close tothe area for 4He/40Ar-rad values and 4He concentrationsobserved in MORBs (e.g., Graham, 2002), while thepoints by crushing are plotted in the area of low 4He/40Ar-rad ratio and low 4He concentration. The He isotopic ra-tios by crushing and heating methods are roughly identi-cal for each locality. If the observed trend represents dif-ferent noble gas compositions trapped in inclusions andlattice, the variable concentration of He among the dif-ferent sites would have resulted in difference in 3He/4Heratios among the trapping sites. This is not observed inthe olivine samples. The trend can be explained as a pref-erential loss of He from the crush-release trapping sitesduring the transfer from the lithospheric mantle source to

Fig. 8. Growth curve of 3He/4He ratio in the lithosphere afterthe metasomatic event at 100 and 200 Ma. The 3He/4He ratio(R/RA) observed at present is shown for each locality. Enrich-ment factors defined as η = ([U]/[3He]0)sample/([U]/[3He]0)upper

mantle are; 29 (XCR), 4 (XPD), 64 (XOC) and 3400 (1600 for200 Ma) (XBR). The [U]/[3He] ratio for present-day uppermantle adopted here was 5 × 107 ppm/(cm3STP/g).

Fig. 7. Plot of 4He/40Ar-rad versus 4He concentration for thexenoliths. Concentration of radiogenic 40Ar was calculated bythe formula,

[40Ar]rad = {(40Ar/36Ar)meas – 296.0} × [36Ar]meas

where subscripts “rad” and “meas” mean radiogenic and mea-sured, respectively. Atmospheric 40Ar/36Ar of 296.0 was adopted(caption of table 1.3 in Ozima and Podosek, 2002).


the surface. The He loss must have been preferentiallylost from inclusions, resulting in lower 4He/40Ar-rad ra-tios and 4He concentrations in crush-release sites com-pared with the gases in melt-release trapping sites. Apartfrom the noble gas signatures for XCY, XPD and XOC,the data points for XBR (Baegryongdo) have 4He/40Ar-rad ratios which are in the range for the upper mantle.Although the 4He concentrations vary by more than anorder of magnitude, this may not mean that the He losshas occurred on the passage to the surface, rather, it rep-resents heterogeneous concentrations of noble gases inthe samples.

The noble gas data by heating experiment have closeaffinities with MORB values, and this may indicate thatthe noble gas compositions in the metasomatizedlithosphere were originally similar to those of depletedupper mantle. Though the plot suggests that thelithospheric mantle beneath the Korean peninsula is acommon source of noble gases, the observed 3He/4He ra-tios in the xenoliths are largely variable from 7.7 RA to<0.2 RA as already noted above.

If we adopt the metasomatized lithospheric mantle asa source of the alkali basalts and the xenoliths, the differ-ent 3He/4He ratios should be due to the accumulation of4He from the radioactive decay of U and Th in thelithosphere. The difference in the 3He/4He ratios amongthe regionally separated sampling sites would representdifferent contributions of the in situ produced radiogenic4He to the originally trapped He prior to the occurrenceof mantle metasomatic event. Figure 8 represents thegrowth curve of 3He/4He ratio in the lithosphere with timeafter the metasomatization, which imported U (and Th)with fluids from a subducting slab. The amount of radio-genic 4He derived from U and Th was calculated by theformulae;

[4He]rad (cm3STP/g) = [238U] (ppm) g(t, β)

and

g(t, β) = 10–4 {7.53(exp(λ238t) – 1)+ 0.0483(exp(λ235t) – 1) + 5.79β(exp(λ232t) – 1)}

where [4He]rad and [238U] are the present-day concentra-tions of radiogenic 4He and 238U in units of cm3STP/gand ppm, respectively, λ238 (1.55 × 10–10 yr–1), λ235 (9.85× 10–10 yr–1) and λ232 (4.95 × 10–11 yr–1) are the decayconstants of 238U, 235U and 232Th, respectively (Steigerand Jäger, 1977), β is 232Th/238U ratio, and t (yr) is timeinterval for accumulation of the radiogenic 4He. Usingthe formulae, 4He/3He ratio is expressed as a function oft, β and [U]/[3He]0 ratio ([U] ≈ [238U]);

(4He/3He)t = (4He/3He)0 + ([U]/[3He]0) g(t, β).

The ratio (4He/3He)0 is an initial value at the beginningof radiogenic 4He accumulation, [3He]0 an initial concen-tration of 3He, which is approximately constant in a closedsystem, and the ratio [U]/[3He]0 is expressed in the unitof ppm/(cm3STP/g).

Evolution of 3He/4He ratio with time in lithosphericmantle has been discussed by several authors (e.g., Reidand Graham, 1996; Gautheron and Moreira, 2002). Theytreated relatively old subcontinental lithospheric mantleas a He reservoir, and calculated a contribution of radio-genic 4He from U and Th to the 3He/4He ratio assumingclosed or open systems. In the models, metasomatismcauses U and Th enrichments in the subcontinentallithospheric mantle.

We have estimated the enrichments of U relative to3He for each sampling localities, following principallythe same method presented by the authors. At first, wecalculated 3He/4He for depleted upper mantle, which isassumed to be an initial value in the lithospheric mantlejust before the occurrence of metasomatization. U con-centration and [U]/[3He] ratio for present-day upper man-tle adopted here were 0.005 ppm and 5 × 107 ppm/(cm3STP/g) respectively (e.g., Allègre et al., 1986;O’Nions and MacKenzie, 1993; O’Nions and Tolstikhin,1994; Reid and Graham, 1996; Gautheron and Moreira,2002). The values corresponding to the He concentrationsin the upper mantle are [3He] = 1.0 × 10–10 and [4He] =8.9 × 10–6 cm3STP/g. Based on the [U]/[3He] value, 3He/4He ratios at 100 m.y. and 200 m.y. ago are calculated as8.09 RA and 8.19 RA, respectively. In the calculation, asthe β value does not affect the resulting 4He/3He ratiossignificantly, a constant value of 3 was adopted (Allègreet al., 1986; O’Nions and McKenzie, 1993). If we as-sume 100 Ma as the onset of radiogenic 4He accumula-tion (Fig. 6), the required [U]/[3He]0 ratios to producethe observed 3He/4He ratios are 1.47 × 109 (XCY, 6.0 RA),2.10 × 108 (XPD, 7.7 RA), 3.21 × 109 (XOC, 4.6 RA), and>1.70 × 1011 (XBR, <0.2 RA). For XBR, the calculationwas also carried out assuming an onset time of 200 Ma,and this resulted in 8.1 × 1010 as the [U]/[3He]0 ratio. Thetime dependent variation of 3He/4He ratios starting fromthe upper mantle values are presented in Fig. 8 for thedifferent localities studied in this work. The calculationindicates that the U enrichment factors defined as η =([U]/[3He]0)sample/([U]/[3He]0)MORB are 29 (XCR), 4(XPD), 64 (XOC) and 3400 (1600 for 200 Ma) (XBR).The enrichment factor can be enhanced by a He loss fromthe lithospheric reservoir as well as the low concentra-tion of initially trapped He. Gautheron and Moreira (2002)pointed out that a closed system evolution of He cannotmaintain the relatively high 3He/4He (av. 6.1 RA) observedin subcontinental lithospheric mantle for more than 1 Ga,and that the He residence time in the subcontinental man-tle is about 100 Ma due to a supply of He from the under-


lying asthenosphere and a loss to the surface. We madeour calculations assuming a closed system model for theeastern margin of the Eurasian plate. The time span treatedhere was much shorter (≈0.1 Ga) than that for the oldsubcontinental lithospheric mantle beneath the central partof continents. Hence the obtained results would quantita-tively represent circumstances in this area. The large vari-ation of enrichment factors obtained for the mantle-de-rived xenoliths in the Korean peninsula probably repre-sent heterogeneously metasomatized lithospheric mantlebeneath the Eurasian continental margin.

CONCLUSIONS

We have measured noble gas, Nd and Sr isotopic ra-tios and major and trace element compositions forultramafic xenoliths and their host Cenozoic alkali basaltsfrom Baegdusan, Baegryongdo, Jogokri and Jejudo in theKorean peninsula, and Long Quan, China. From the re-sults and the discussion described above, we can drawthe following conclusions:

1) Alkali basalts containing abundant ultramaficxenoliths are characterized by highly saturated incompat-ible elements and were erupted periodically duringCenozoic time (0.1 to 21 Ma) through the thick continen-tal crust of the Korean peninsula.

2) Characteristic features revealed by the noble gas,Nd and Sr isotopic ratios and major and trace elementcompositions determined for ultramafic xenoliths andtheir host Cenozoic alkali basalts from the Korean penin-sula can be best understood by the presence ofmetasomatized subcontinental lithosphere beneath thearea.

3) A wide variation in trace element concentration forthe ultramafic xenoliths suggests heterogeneity of theirlithospheric mantle source due to the metasomatic proc-esses.

4) Significantly variable 3He/4He ratios ranging from<0.2 RA to 7.7 RA and the 40Ar/36Ar ratios are distinctlyand/or slightly lower than the MORB values (~8 RA). Thewide and low ratios probably represent the heterogene-ously metasomatized source materials beneath the Ko-rean peninsula.

5) Nd and Sr isotopic compositions for the ultramaficxenoliths and their host basalts lie on the mantle arraytoward enriched mantle rather than HIMU or PREMA.However, extreme Nd and Sr isotopic compositions forthe Baegdusan and Baegryongdo samples reflect the as-similation of EMII lithosphere materials and/or U- andRb-enriched continental crust by the basaltic magma.

6) Based on the isotopic and geochemical data, wecan conclude that the ultramafic xenoliths and their hostalkali basalts originated from the metasomatizedlithospheric mantle wedge formed by an ancient subduc-

tion zone (≥100 Ma) located at the southeastern corner ofthe Eurasian plate.

Acknowledgments—This work was supported by a KoreanResearch Foundation Grant (KRF-2003-015-C00659) to KimKyu Han. Our thanks to Dr. Y. Asahara (Nagoya University)for his help in isotopic and chemical analyses by isotopic dilu-tion method and to Mi Young Kim, Hun Kong Choi and HyunKyung Jang (Ewha Womans University) for drawing the fig-ures. Reviews by Drs. David R. Hilton, Tobias P. Fischer andTakeshi Hanyu are very much appreciated.

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He-Ar and Nd-Sr isotopic compositions of ultramafic xenoliths and