The Petrology and Geochemistry of Volcanic Rocks on Jeju Island

The Petrology and Geochemistry of VolcanicRocks on Jeju Island: Plume Magmatismalong the Asian Continental Margin

YOSHIYUKI TATSUMI1*, HIROSHI SHUKUNO1,MASAKO YOSHIKAWA1,2, QING CHANG1, KEIKO SATO1

AND MOON WON LEE3

1INSTITUTE FOR RESEARCH ON EARTH EVOLUTION (IFREE), JAPAN AGENCY FOR MARINE–EARTH SCIENCE

AND TECHNOLOGY ( JAMSTEC), YOKOSUKA 237-0061, JAPAN

2INSTITUTE FOR GEOTHERMAL SCIENCES, KYOTO UNIVERSITY, BEPPU 974-0907, JAPAN

3DEPARTMENT OF SCIENCE EDUCATION, COLLEGE OF EDUCATION, KANGWON NATIONAL UNIVERSITY,

CHUNCHEON 200-701, SOUTH KOREA

RECEIVED OCTOBER 31, 2003; ACCEPTED OCTOBER 1, 2004ADVANCE ACCESS PUBLICATION NOVEMBER 24, 2004

The incompatible element signatures of volcanic rocks forming Jeju

Island, located at the eastern margin of the Asian continent, are

identical to those of typical intraplate magmas. The source of these

volcanic rocks may be a mantle plume, located immediately behind

the SW Japan arc. Jeju plume magmas can be divided into three

series, based on major and trace element abundances: high-alumina

alkalic, low-alumina alkalic, and sub-alkalic. Mass-balance

calculations indicate that the compositional variations within each

magma series are largely governed by fractional crystallization of

three chemically distinct parental magmas. The compositions of

primary magmas for these series, using inferred residual mantle

olivine compositions, suggest that the low-alumina alkalic and sub-

alkalic magmas are generated at the deepest and shallowest depths by

lowest and highest degrees of melting, respectively. These estimates,

together with systematic differences in trace element and isotopic

compositions, indicate that the upper mantle beneath Jeju Island is

characterized by an increased degree of metasomatism and a change

in major metasomatic hydrous minerals from amphibole to phlogopite

with decreasing depth. The original plume material, having rather

depleted geochemical characteristics, entrained shallower meta-

somatized uppermost mantle material, and segregated least-enriched

low-alumina alkalic, moderately enriched high-alumina alkalic, and

highly enriched sub-alkalic magmas, with decreasing depth.

KEY WORDS: Jeju Island; magma genesis; mantle plume; subcontinental

mantle

INTRODUCTION

The eastern margin of the Asian continent is a site ofintensive Cenozoic volcanism characterized bysubduction-related arc–back-arc basin and mantleplume-related intraplate magmatism. The simultaneousoccurrence of magmatism in association with both man-tle downwelling and upwelling in the region provides arare opportunity for investigating material circulationwithin the Earth’s mantle. Issues that may be addressedby analysing such magmatism include: (1) the contribu-tion of subduction components extracted from thefoundering oceanic lithosphere to the back-arc basinand further intraplate magmatism; (2) the contributionto continental margin magmatism of continental compo-nents, such as continental crust and subcontinental litho-spheric mantle, with geochemical characteristics that aredifferent from asthenospheric mantle; (3) the role ofupwelling, asthenospheric mantle materials in causingsuch magmatism and controlling magma composition.Jeju Island is located between the Korean Peninsula of

the Asian continent and Kyushu of the SW Japan arc, atthe western margin of the Sea of Japan (or East Sea); theSea of Japan is a back-arc basin built behind the SW andNE Japan arc–trench systems (Fig. 1). Jeju magmatism isthus distinct in that it takes place near the boundarybetween arc–back-arc basin and continental tectonic set-tings. Analytical study of Jeju magmatism may provide

*Corresponding author. E-mail: [email protected]

# The Author 2004. Published by Oxford University Press. All

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(a) Tectonics

(b) Geology

(c) Stratigraphy and samples

CJ1

CJ2

CJ3

CJ4

CJ5CJ6

CJ7

CJ8CJ9

CJ10

C11CJ12

CJ13CJ30

CJ31 CJ42

CJ14

CJ38

CJ29CJ15 CJ32

CJ33

CJ37

CJ40

CJ36

CJ35

CJ34

CJ26

CJ25CJ27

CJ28

CJ16

CJ17

CJ18CJ20

CJ24CJ23

CJ19CJ21

CJ22

N

10 kmStage 2

Stage 3-1

Stage 3-2

Stage 3-3

Stage 4

Shell-sand Formation

Scoria volcanic cones

Backlockdam basalt

Hallasan trachyte

Hallasan basalt

Seongpanak basalt

Shiungri basalt

Beobjeongri trachyandesite

Hahyori basalt

Sumangri basalt

Jeju basalt

Sinyangri Formation

Sanbangsan trachyte

Jungmun trachyandesite

Hwasun-Seongsan hyaloclastite

Seoguipo trachyandesite

Pyosunri basalt

Seoguipo Formation

Basal basalt

GraniteBasement

Stage 1

Stage 2

Stage 3

3-1

3-2

3-3

3-4Stage 4 5, 33, 38, 42

30.130.2, 3115, 16, 17, 26.2, 289, 10, 18, 19, 36, 40

6, 202711, 12, 13, 14, 23, 24, 29

22

7, 8, 211, 2, 3, 25, 26.1, 32, 34, 35, 37

439, 41

115 120 125 130 135 140 145 150°E25

30

35

40

45°N

Kurile T

rench

Philippine Sea Plate

Eurasian Plate

Pacific Plate

KuriieArc

S

WJap

an Arc

NE

Jap

an

Arc

JejuIsland

Sea of Japan

Izu

-Bo

nin

-Maria

na

Arc

N

ankai Trough

Izu-B

onin

-Maria

na

Tre

nch

Japan

Tre

nch

Ryu

kyu

Tre

nch

Fig. 1. Tectonic and geological framework for Jeju Island. (a) Quaternary volcanism along the eastern margin of the Asian continent. Volcanicfronts, the trenchward limit of a volcanic arc, are shown by bold continuous lines. Along the convergent plate margins, extensive subduction-related arc magmatism is taking place, whereas intraplate, possibly mantle-plume-related, volcanoes (stars) are built within the Eurasian plate. (b)Generalized geological map of Jeju Island after Lee (1982), showing sample localities for this study. (c) Stratigraphic relationships of Jeju volcanicrocks and samples for this study.

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constraints for assessment of the above-mentioned topicsof continental margin magmatism.Voluminous volcanic piles accumulated on Jeju Island

during the Late Cenozoic and include at least two chem-ically distinct magma series: alkalic and sub-alkalicseries (Lee, 1982; Park, 1994). This paper presents petro-graphy, major and trace elements, and Sr–Nd–Pb iso-topic compositions for Jeju volcanic rocks. On the basisof this comprehensive dataset, the occurrence of threemagma series, processes of magmatic differentiation, con-ditions of mantle melting, and the geochemical andmineralogical characteristics of the upper-mantle sourcesof the magmas will be discussed.

GEOLOGY

The eastern margin of the Asian continent is character-ized by the occurrence of extensive magmatism. MostQuaternary volcanoes in this region are built along theconvergent plate margins where Pacific and PhilippineSea Plates are being subducted beneath the EurasianPlate (Fig. 1a). These volcanic arcs form 100–200 kmabove the dipping seismic zone located near the surfaceof the foundering oceanic lithosphere (e.g. Tatsumi &Eggins, 1995). Far behind the volcanic arcs, on theother hand, are sporadically distributed ‘intraplate’ vol-canoes such as Jeju volcano (Fig. 1a). Although no seis-mically active slab is observed, recent tomography resultshave revealed the presence of horizontally lying, sub-ducted lithospheric material near the upper–lower-mantle boundary beneath the intraplate volcanoes(Fukao et al., 2001).East of Jeju Island is the Sea of Japan, a back-arc basin

formed behind the Japanese Islands at 30–15Ma(Tamaki et al., 1992) with clockwise and anti-clockwiserotations of SW and NE Japan arc slivers respectively at�15Ma (Otofuji et al., 1985). Although the principalcause of the back-arc rifting is controversial, back-arcbasin formation results in, or is caused by, upwelling ofasthenospheric material that ultimately creates newoceanic crust. It is therefore interesting to compare thechemical characteristics of upwelling asthenosphericmaterial beneath the back-arc basin and the intraplateregions.Jeju Island is roughly elliptical in shape (80 km �

40 km) and mainly comprises Holocene volcanic rocks.It is composed of thick piles of lava flows, minor pyro-clastic rocks, hyaloclastites, and numerous parasitic scoriacones (Fig. 1b). These volcanic rocks are believed to haveerupted onto a granitic basement, although granitic rocksare found only as xenoliths in both lavas and pyroclastics.The volcanic activity on this island can be divided intofour stages (Fig. 1b and c), based on stratigraphic relation-ships (Lee, 1982). Radiometric age determinations forJeju lavas indicate that the volcanic activity commenced

at �800 ka and continued to historical times (Lee, 1982).Stage 1 began with the eruption of basaltic lava flows thatformed a shield volcano growing from the sea floor. Oncethe shield volcanic activity ended, the Stage 1 volcanicrocks were unconformably overlain by volcaniclastic sedi-ments (Seoguipo Formation; Fig. 1c). The Stage 2 basal-tic lavas (Pyosunri basalts; Fig. 1c) form the bulk of theexposed volcanic rocks as a lava plateau. Minor lava flowscomposed of trachyandesites and trachytes were alsoerupted during this stage. The Stage 3 volcanic rocksform the Halla shield volcano, with a peak height of1950m, and can be subdivided into four substagesbased on stratigraphic relationships and petrographicalcharacteristics. The final volcanic activity on this island,Stage 4, yielded more than 360 parasitic scoria cones thatare distributed along the axis of the island (Fig. 1b).Forty volcanic samples collected from the island cover

almost all volcanic stages (Fig. 1b and c). To evaluate therole of the granitic basement in the formation of the Jejumagmas, two granitic xenoliths included in pyroclasticrocks were also analysed.

ANALYTICAL METHODS

Major and trace element (Ni to Th in Table 1) composi-tions were measured using RIGAKU1 Simaltics 3550and Rix 3000 X-ray fluorescence (XRF) spectrometerson fused glass beads and pressed powder pellets, respect-ively. Detailed analytical procedures have beendescribed by Goto & Tatsumi (1994, 1996). Concentra-tions of rare earth elements (REE) and 11 other traceelements (Rb to U in Table 2) were determined by induc-tively coupled plasma mass spectrometry (ICP-MS) usinga VG Elemental1 PQ3 system enhanced with a chicanelens system, following the procedures described by Changet al. (2003). Trace element data, except for the highfield strength elements (HFSE: Zr, Nb, Hf and Ta),were obtained from HF–HClO4–HNO3 digestion. ForHFSE, alkali fusion (LiBO2–Li2B4O7, SpectrofluxR100B of Johnson Matthey) was applied to ensure a com-plete decomposition of refractory minor phases. Analyti-cal accuracy and precision estimated from repeatedmeasurements of international reference rocks werebetter than �10% and 2–5%, respectively.Rock samples for Sr–Nd–Pb isotope analysis were

crushed to coarse chips (<0�5mm3) and fresh pieceswere hand picked. To avoid surface contamination, therock chips were washed with ethanol and then leachedwith 0�5M HCl at room temperature for 1 h. Finally thechips were rinsed with Milli-Q water. The chips wereground to less than 200 mesh size using a vibration millmade of alumina ceramic. The analytical procedure forchemical separation and mass spectrometry for Sr, Ndand Pb isotope determinations has been outlined byYoshikawa et al. (2001), Miyazaki et al. (2003) and Shibata

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Table 1: Major and trace element and modal compositions of Cheju volcanic rocks

Rock type: High-Al ALK series Low-Al ALK series

Stage: 4 2 2 2 2 1 3-2 3-1 4 2 2

Sample: CJ42 CJ08 CJ22 CJ21 CJ07 CJ04 CJ10 CJ12 CJ33 CJ01 CJ34

wt %

SiO2 48.62 49.09 53.71 56.13 56.41 60.31 47.14 47.87 48.24 48.25 48.40

TiO2 2.46 2.27 1.71 1.26 1.18 0.77 2.39 3.14 2.74 2.21 2.13

Al2O3 17.77 18.62 18.95 19.20 19.53 19.22 13.84 16.25 15.60 14.39 14.55

Fe2O3* 11.64 10.46 8.30 7.14 6.47 4.61 12.42 13.48 12.86 12.36 11.99

MnO 0.17 0.22 0.20 0.19 0.25 0.28 0.16 0.15 0.15 0.16 0.16

MgO 3.95 2.82 2.00 1.37 1.17 0.61 9.85 5.42 6.61 9.36 9.14

CaO 7.34 8.30 5.67 4.46 5.49 3.54 9.55 8.30 7.83 9.72 9.35

Na2O 4.08 4.11 5.23 5.66 5.86 6.51 2.81 3.65 2.82 2.82 3.08

K2O 2.33 2.37 3.40 3.81 3.21 3.98 1.36 1.33 1.43 1.03 1.16

P2O5 1.01 1.05 0.65 0.45 0.38 0.17 0.43 0.56 0.51 0.34 0.40

Total 99.36 99.32 99.82 99.66 99.94 100.00 99.96 100.13 98.80 100.65 100.35

ppm (XRF)

Ni 19 1 1 —— —— 1 178 47 94 156 166

Cu 18 7 3 2 —— —— 48 30 44 28 45

Zn 113 112 110 111 111 79 97 113 129 102 97

Rb 54 58 96 111 104 112 32 26 36 18 27

Sr 1150 1179 1219 1154 1040 909 488 544 541 386 451

Y 23 28 26 24 32 31 18 20 23 18 19

Zr 381 403 545 595 566 590 202 219 263 170 185

Nb 65 68 92 97 97 100 33 34 42 24 27

Ba 756 637 995 1242 941 1069 363 339 366 326 335

Th 8.4 8.3 15 16 14.9 14.0 4.3 4.7 5.3 4.6 3.2

vol. %

olivine 1 —— —— —— —— —— 12 2 —— 5 5

clinopyroxene 2 —— —— —— —— —— 3 1 —— —— ——

amphibole 5 —— —— —— —— —— —— —— —— —— ——

plagioclase 9 2 —— —— 3 2 —— 3 —— —— ——

K-feldspar 1 —— —— —— —— 3 —— —— —— —— ——

groundmass 82 98 100 100 97 95 85 94 100 95 95

Rock type: Low-Al ALK series

Stage: 2 3-1 3-3 3-1 3-1 3-1 3-2 4 2 3-1 3-1 3-1

Sample: CJ25 CJ11 CJ26.2 CJ29 CJ20 CJ18.2 CJ16 CJ38 CJ26.1 CJ06 CJ24 CJ18.1

wt %

SiO2 48.47 48.55 48.63 48.67 48.68 48.85 48.86 48.86 48.94 48.95 49.06 49.11

TiO2 2.68 2.92 2.64 2.71 2.74 2.99 2.58 2.65 2.61 3.18 2.64 2.96

Al2O3 15.34 16.51 15.63 16.61 15.84 16.06 16.04 16.30 13.73 16.63 15.47 16.16

Fe2O3* 12.95 12.71 12.22 12.38 12.56 12.88 12.19 12.35 12.36 13.41 12.80 12.84

MnO 0.15 0.15 0.15 0.14 0.14 0.16 0.15 0.15 0.15 0.15 0.15 0.16

MgO 6.88 5.35 6.79 5.85 6.64 4.67 6.04 5.21 9.00 3.95 6.28 4.80

CaO 9.82 8.41 7.87 9.25 8.56 7.75 7.96 7.63 8.82 7.75 8.48 7.71

Na2O 3.01 3.69 3.65 3.35 3.34 3.94 3.62 3.92 3.04 3.89 3.54 3.85

K2O 0.62 1.22 1.88 0.91 1.00 1.74 1.91 1.85 1.14 1.43 1.21 1.75

P2O5 0.33 0.57 0.60 0.46 0.43 0.89 0.68 0.64 0.41 0.63 0.51 0.89

Total 100.26 100.08 100.05 100.34 99.93 99.93 100.02 99.56 100.21 99.98 100.14 100.23

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Stage: 2 3-1 3-3 3-1 3-1 3-1 3-2 4 2 3-1 3-1 3-1

Sample: CJ25 CJ11 CJ26.2 CJ29 CJ20 CJ18.2 CJ16 CJ38 CJ26.1 CJ06 CJ24 CJ18.1

ppm (XRF)

Ni 84 51 117 61 88 40 74 68 178 12 87 38

Cu 54 32 63 37 32 31 29 29 54 25 41 31

Zn 104 116 122 99 107 124 114 121 104 113 119 120

Rb 10 20 47 19 17 35 54 37 29 27 25 38

Sr 435 556 625 558 498 598 674 574 459 538 486 596

Y 19 21 19 18 19 23 20 22 19 22 20 23

Zr 164 236 278 175 200 265 298 284 206 246 219 268

Nb 22 35 45 25 28 45 50 45 30 37 33 45

Ba 231 336 445 255 270 446 495 479 297 384 328 487

Th 3.4 4.8 7.1 3.7 4.1 6.1 8.5 6.4 4.7 5.9 5.4 6.6

vol. %

olivine 8 5 2 5 5 8 5 5 2 1 8 8

clinopyroxene —— —— —— —— 3 1 5 —— —— —— 5 ——

amphibole —— —— —— —— —— —— —— —— —— —— —— ——

plagioclase —— 8 —— 20 20 20 3 —— —— 15 20 20

K-feldspar —— —— —— —— —— —— —— —— —— —— —— ——

groundmass 92 87 98 75 72 71 87 95 98 84 67 72


Stage: 3-2 3-2 3-2 3-2 3-2 3-2 3-2 3-2 2 3-1 4 3-2

Sample: CJ28 CJ09 CJ17 CJ40 CJ14 CJ36 CJ31 CJ30.2 CJ32 CJ19 CJ05 CJ15

wt %

SiO2 49.30 49.46 49.53 49.58 49.62 49.63 49.86 49.93 49.95 50.12 50.98 51.81

TiO2 2.44 3.03 2.54 2.42 2.67 2.42 2.58 2.56 2.45 2.54 2.22 2.19

Al2O3 15.25 16.56 16.47 15.58 16.54 15.61 16.55 16.45 15.93 16.47 16.40 16.89

Fe2O3* 12.08 12.76 11.90 11.61 12.78 11.61 11.71 12.08 11.85 11.51 11.89 12.03

MnO 0.15 0.16 0.15 0.14 0.17 0.14 0.15 0.15 0.15 0.15 0.16 0.17

MgO 7.63 3.89 5.27 6.70 3.61 6.69 4.56 5.42 6.01 4.83 4.71 3.07

CaO 8.50 7.27 7.56 8.16 7.21 8.15 8.41 8.00 7.55 8.63 6.94 6.13

Na2O 3.37 4.11 4.12 3.62 4.28 3.72 3.88 3.85 3.88 3.83 4.28 4.52

K2O 1.44 1.93 2.00 1.57 1.83 1.60 1.77 1.50 1.97 1.71 2.13 2.14

P2O5 0.49 0.74 0.69 0.54 1.17 0.52 0.58 0.56 0.61 0.56 0.76 0.98

Total 100.65 99.90 100.23 99.93 99.87 100.09 100.04 100.50 100.35 100.35 100.46 99.93

ppm (XRF)

Ni 123 2 54 100 —— 100 31 59 77 40 46 ——

Cu 43 20 28 41 14 40 32 22 33 37 31 14

Zn 103 113 110 115 138 111 118 102 113 102 129 138

Rb 35 49 57 31 40 36 43 37 46 35 56 39

Sr 532 599 683 589 594 577 521 560 595 566 612 539

Y 19 23 20 19 26 19 22 20 20 21 24 27

Zr 228 283 313 266 309 267 275 236 315 269 341 352

Nb 35 48 51 40 51 40 40 36 46 39 54 53

Ba 363 493 527 402 494 385 431 403 478 489 506 565

Th 5.3 6.1 10 6.3 7.6 7.2 5.9 5.3 7.0 5.6 7.3 8.4

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Table 1: continued


Stage: 3-2 3-2 3-2 3-2 3-2 3-2 3-2 3-2 2 3-1 4 3-2

Sample: CJ28 CJ09 CJ17 CJ40 CJ14 CJ36 CJ31 CJ30.2 CJ32 CJ19 CJ05 CJ15

vol. %

olivine 5 1 5 8 —— 5 10 8 2 5 3 ——

clinopyroxene —— —— 5 2 —— 3 2 1 —— 5 1 ——

amphibole —— —— —— —— —— —— —— —— —— —— —— ——

plagioclase —— —— 8 5 5 8 20 20 —— 30 1 1

K-feldspar —— —— —— —— —— —— —— —— —— —— —— ——

groundmass 95 99 82 85 95 84 68 71 98 60 95 99

Rock type: Low-Al ALK series Sub-ALK series Granite

Stage: 3-1 3-1 3-2 2 2 2 2

Sample: CJ23 CJ13 CJ30.1 CJ35 CJ37 CJ02 CJ03 CJ41.2 CJ41.1

wt %

SiO2 53.76 55.55 59.02 51.14 51.44 51.49 51.95 67.09 71.26

TiO2 1.77 1.53 1.00 2.16 2.14 1.86 2.00 0.53 0.30

Al2O3 16.90 16.88 17.41 14.84 14.30 14.56 14.51 17.34 15.80

Fe2O3* 11.31 10.39 7.97 11.87 11.88 12.35 12.53 2.87 1.54

MnO 0.17 0.16 0.13 0.14 0.15 0.15 0.15 0.05 0.03

MgO 2.60 2.09 1.02 7.10 7.62 8.21 7.18 0.81 0.58

CaO 5.58 4.90 3.15 8.85 8.47 8.64 8.64 3.58 2.07

Na2O 4.85 5.10 5.65 3.21 3.16 2.81 2.99 4.27 3.80

K2O 2.46 2.84 3.70 0.93 0.96 0.44 0.38 2.37 3.40

P2O5 0.84 0.81 0.33 0.33 0.34 0.22 0.22 0.18 0.20

Total 100.25 100.25 99.39 100.58 100.45 100.72 100.54 99.08 98.99

ppm (XRF)

Ni 7 1 5 112 149 164 147 —— ——

Cu 11 9 10 50 45 56 51 1 2

Zn 137 139 129 103 110 108 118 67 36

Rb 55 71 100 22 26 11 8 80 92

Sr 518 467 397 388 362 304 271 729 593

Y 29 30 36 18 19 17 18 10 9

Zr 391 439 582 179 184 123 129 236 164

Nb 58 61 76 20 20 11 10 8 4

Ba 632 733 954 242 257 133 115 443 1067

Th 10 12 15 3.4 3.3 1.7 2.0 8.7 8.4

vol. %

olivine 3 5 —— 3 8 3 5

clinopyroxene —— —— —— —— —— —— 2

amphibole —— —— —— —— —— —— ——

plagioclase 5 8 —— —— —— —— ——

K-feldspar —— —— —— —— —— —— ——

groundmass 92 87 100 97 92 97 93

*Total iron as Fe2O3.

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et al. (2003). Total procedural blanks for Sr, Nd and Pbwere about 10 pg, 10 pg and 5 pg, respectively. Massspectrometry was performed on a Thermo-Finnigan1

Triton TI equipped with nine Faraday cups, using a staticmulti-collection mode. Normalizing factors used to cor-rect for isotopic fractionation in the Sr, Nd and Pbisotope analyses were 86Sr/88Sr ¼ 0�1194, 146Nd/144Nd ¼ 0�7219, and 0�147% per atomic mass unit,

respectively. Measured isotopic ratios for standard mater-ials were 87Sr/86Sr ¼ 0�710268 � 19 (2s) for NIST 987(n¼ 10), 143Nd/144Nd¼ 0�511844� 11 (2s), for La Jolla(n ¼ 11), and 208Pb/204Pb ¼ 36�721 � 13 (2s),207Pb/204Pb ¼ 15�498 � 4 (2s) and 206Pb/204Pb ¼16�001 � 3 (2s) for NIST 981 (n ¼ 28).Mineral compositions were analysed using JEOL JXA-

8800 and -8900 electron-probe micro-analysers following

Table 2: Trace element and isotopic compositions of Cheju volcanic rocks

Rock type: High-Al ALK series Low-Al ALK series

Stage: 4 2 2 1 3-2 3-1 2 3-1 3-2 3-1 3-1

Sample: CJ42 CJ08 CJ21 CJ04 CJ10 CJ14 CJ32 CJ19 CJ15 CJ23 CJ13

ppm (ICP-MS)

Rb 57.8 61.7 113 101 31.9 43.2 45.7 34.6 41.1 —— 66.2

Sr 1160 1096 1072 817 503 628 597 568 575 —— 443

Y 17.3 30.8 20.9 30.4 17.1 24.5 16.6 17.7 24.5 —— 26.7

Zr —— —— 601 646 195 303 307 267 353 384 451

Nb —— —— 109 111 37.5 56.0 49.8 44.1 59.7 64.4 66.8

Ba 827 699 1237 1126 394 556 518 509 616 —— 731

La 31.8 47.4 66.2 63.9 20.6 33.1 29.1 24.4 35.1 40.6 42.2

Ce 64.4 99.5 145 128 41.6 65.5 55.8 47.8 67.9 78.5 77.6

Pr 7.88 11.6 13.2 14.5 5.14 8.07 6.78 5.93 8.24 9.58 9.52

Nd 32.2 46.1 45.8 54.1 21.7 33.9 26.7 24.3 33.6 38.8 37.9

Sm 6.41 8.9 7.74 9.80 5.05 7.72 5.78 5.60 7.58 8.71 8.44

Eu 2.90 3.05 3.10 3.44 1.83 2.97 2.24 2.37 3.09 3.23 3.23

Gd 5.53 7.83 5.82 7.71 4.89 7.39 5.23 5.29 7.13 8.13 7.82

Tb 0.768 1.12 0.873 1.15 0.705 1.05 0.763 0.794 1.05 1.20 1.19

Dy 4.03 6.17 4.61 6.46 3.91 5.58 4.04 4.26 5.58 6.45 6.41

Ho 0.7 1.14 0.872 1.22 0.695 1.002 0.733 0.781 1.02 1.19 1.20

Er 1.78 3.10 2.37 3.47 1.80 2.53 1.82 1.97 2.58 3.01 3.11

Tm 0.225 0.412 0.336 0.485 0.218 0.316 0.238 0.257 0.343 0.394 0.428

Yb 1.40 2.62 2.15 3.20 1.40 1.90 1.43 1.58 2.06 2.38 2.59

Lu 0.201 0.378 0.323 0.479 0.188 0.271 0.204 0.227 0.299 0.344 0.380

Hf —— —— 12.6 14.12 4.85 7.21 7.07 6.40 8.36 9.14 10.7

Ta —— —— 6.74 7.55 2.34 3.64 3.18 2.71 3.64 3.93 4.11

Pb 3.75 4.52 8.07 5.28 2.30 3.48 3.06 2.14 4.01 4.45 4.81

Th 6.65 7.34 14.8 12.3 4.28 7.62 6.98 4.98 8.48 9.42 11.1

U 0.677 1.50 3.41 3.20 0.882 1.61 1.58 0.590 1.87 2.00 2.63

87Sr/86Sr 0.704494 0.704928 0.704629 0.705019 0.704493 0.704128 0.704223 0.704202 0.704142 0.704198 0.704164

2s 0.000008 0.000009 0.000006 0.000010 0.00001 0.000006 0.000012 0.000006 0.000009 0.000010 0.000007

143Nd/144Nd 0.512741 0.512756 0.512741 0.512752 0.512795 0.512789 0.512759 0.512810 0.512795 0.512794 0.512764

2s 0.000008 0.00001 0.000011 0.000013 0.000006 0.000005 0.000007 0.000009 0.000014 0.000007 0.000011

208Pb/204Pb 39.812 39.451 39.996 39.428 39.394 39.687 39.697 39.520 39.639 39.584 39.415

2s 0.012 0.010 0.014 0.004 0.010 0.008 0.004 0.012 0.014 0.008 0.026

207Pb/204Pb 15.670 15.670 15.680 15.673 15.643 15.651 15.654 15.641 15.651 15.650 15.655

2s 0.004 0.018 0.006 0.002 0.004 0.002 0.002 0.006 0.008 0.004 0.010

206Pb/204Pb 19.070 19.004 19.179 18.904 18.868 19.053 19.061 18.963 19.032 18.998 18.836

2s 0.006 0.004 0.006 0.002 0.004 0.004 0.002 0.006 0.006 0.002 0.012

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the method described by Shukuno (2003). The excitationpotential, specimen current, and analytical time were:15 kV, 15 nA and 20 s (25 kV, 20 nA and 100 s for Mn,Ca, and Ni analyses) for olivine; 15 kV, 12 nA and 20 s forspinel; 15 kV, 15 nA and 20 s for pyroxene and plagio-clase. ZAF correction procedures were employed.

RESULTS

Major and trace element compositions

Major and trace element abundances, and isotopic com-positions of the Jeju lavas are listed in Tables 1 and2 together with the modal compositions of phenocrysts.

Table 2: continued

Rock type: Sub-ALK series Granite

Stage: 2 2 2 2

Sample: CJ35 CJ37 CJ02 CJ03 CJ41.1

ppm (ICP-MS)

Rb 22.4 27.0 11.7 9.02 77.0

Sr 375 373 312 269 530

Y 14.3 15.0 11.8 10.8 3.05

Zr 176 176 126 130 154

Nb 23.8 22.4 12.5 11.6 5.38

Ba 252 263 142 115 1118

La 13.1 11.1 5.14 3.46 24.6

Ce 26.0 22.8 11.3 7.65 45.7

Pr 3.32 2.89 1.50 1.10 5.25

Nd 14.2 12.9 7.21 5.72 18.4

Sm 3.76 3.51 2.27 1.97 2.99

Eu 1.59 1.51 1.19 1.24 0.929

Gd 3.95 3.75 2.69 2.47 1.82

Tb 0.624 0.575 0.420 0.396 0.209

Dy 3.51 3.33 2.55 2.42 0.853

Ho 0.669 0.609 0.479 0.456 0.130

Er 1.74 1.64 1.29 1.23 0.308

Tm 0.232 0.206 0.162 0.162 0.038

Yb 1.43 1.34 1.11 1.06 0.213

Lu 0.207 0.185 0.147 0.150 0.031

Hf 4.51 4.50 3.47 3.72 4.28

Ta 1.49 1.45 0.809 0.756 0.404

Pb 2.16 2.19 1.52 1.14 20.2

Th 3.17 2.93 1.22 1.05 5.56

U 0.686 0.691 0.266 0.235 1.35

87Sr/86Sr 0.704886 0.704757 0.705350 0.705305 0.717073

2s 0.000008 0.000008 0.000008 0.000007 0.000008

143Nd/144Nd 0.512720 0.512729 0.512697 0.512679 0.511666

2s 0.000007 0.000009 0.000018 0.000008 0.000010

208Pb/204Pb 39.417 39.530 39.397 39.736 38.593

2s 0.018 0.010 0.014 0.014 0.004

207Pb/204Pb 15.653 15.642 15.698 15.716 15.745

2s 0.006 0.004 0.004 0.004 0.002

206Pb/204Pb 18.736 18.973 18.850 19.094 18.371

2s 0.008 0.004 0.004 0.006 0.002

530


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Major and selected trace elements are shown in SiO2-variation diagrams in Figs 2 and 3. Jeju volcanic rockspossess a wide range of compositions, with SiO2 contentsfrom 48 to 60 wt %, i.e. from basalt to felsic andesite.Two magma types can be recognized on the basis ofrelative abundance of total alkali elements (Fig. 2): oneclearly belongs to the sub-alkalic and the other mostly tothe alkalic series of Le Bas & Streckeisen (1991). Althoughsome basalts with SiO2 contents <50 wt % are classifiedas sub-alkalic based on the classification of Le Bas &Streckeisen (1991), these rocks are grouped, hereafter,into the alkalic series for the following two reasons(Fig. 2). First, they form a continuous chemical trendwith typical alkalic rocks having higher SiO2 contents.Second, they can be classified as alkalic series based onthe classification scheme of Miyashiro (1978). Althoughthe sub-alkalic series rocks are hypersthene normative,some ‘alkalic’ series rocks are also hypersthene normative(Fig. 2). To avoid confusion in terminology, therefore, theidentification of distinctive magma series on Jeju Island,hereafter referred to as alkalic (ALK) and sub-alkalic(Sub-ALK) series, are based on total alkali contents.The alkalic series can be further subdivided into twogroups based on Al2O3 content (Fig. 2): High-Al ALKand Low-Al ALK. High-Al ALK rocks tend to havehigher concentrations of K, Rb, Sr, Zr, Nb, Ba, andTh, and lower abundances of Fe, Mg, and Cu (Figs 2and 3). It should be stressed that all Sub-ALKrocks belong to the Pyosunri basalt of Stage 2, andmost High-Al ALK rocks are also found in Stage 2(Sanbangsan trachyte and Seoguipo trachyandesite).Normal mid-ocean ridge basalt (N-MORB)-normal-

ized (Sun & McDonough, 1989) multi-element diagramsfor the Jeju volcanic rocks are shown in Fig. 4. Althoughrelative enrichment in highly incompatible elements,which typifies ocean-island basalts, can be seen, the Jejusamples are further characterized by a relative depletionof Nb and overabundances of Pb and Sr. It has been wellestablished that the production of magmas with thesedistinctive chemical characteristics typifies magmatismat convergent plate boundaries (e.g. Pearce, 1983;Hawkesworth et al., 1993). To identify this ‘subductionflavour’ in magmas more quantitatively, Tatsumi et al.(2000) demonstrated that K/Y and K/Nb can be used todistinguish the overabundance of highly incompatibleelements and depletion of HFSE relative to large ionlithophile elements (LILE) for arc magmas. In a K/Y vsK/Nb diagram, subduction-zone magmas are distinctfrom those from other tectonic settings, with high ratiosof both K/Nb and K/Y, whereas intraplate rocks havehigh K/Y but low K/Nb (Fig. 5). If we accept thisdistinction, it implies that Jeju magmas have chemicalcharacteristics identical to those of intraplate (hotspot)rather than subduction-zone magmas. The geochemicalcharacteristics of the Jeju volcanic rocks, therefore,

strongly suggest the location of a mantle plume beneaththe island, although neither a hotspot track on the surfacenor a low-velocity anomaly in the mantle have beendocumented in the eastern margin of the Asian continentincluding the NE China and Jeju regions (Fukao et al.,2001).Although concentrations of particular incompatible

elements differ between High- and Low-Al ALKs,and Sub-ALK series, the element patterns in MORB-normalized trace element variation diagrams are broadlyidentical for all three series (Fig. 4). However, Sub-ALKlavas are distinct from the other two series in that theyhave rather flat REE patterns (Fig. 4).Jeju lavas have positive Eu anomalies in chondrite-

normalized (Sun & McDonough, 1989) REE diagrams(Fig. 4). A possible cause for this anomaly would be thecontribution of plagioclase, into which Eu is preferen-tially partitioned compared with other REE, to theformation and/or differentiation of the magmas. Thepositive Sr spikes observed in the MORB-normalizedtrace element variation diagrams also support this. Theorigin of these plagioclase-related characteristics will bediscussed below.Granitic basement rocks on Jeju Island are also highly

depleted in Nb relative to Th and K, and are furtherdistinct in having a steep REE pattern and strong deple-tion in the heavy REE (HREE; Fig. 4).

Isotopic compositions

Jeju volcanic rocks show a wide range of 87Sr/86Sr valuesfrom 0�704128 to 0�705350, whereas 143Nd/144Nd valuesare rather limited, from 0�512810 to 0�512679 (Fig. 6aand b). The Sr–Nd isotopic compositions of the Jeju lavasoverlap with those of Cenozoic intraplate basalts, espe-cially those of sub-alkalic basalts from NE China (e.g.Hannuoba basalts; Song & Frey, 1989; Song et al.,1990), and are identical to those of relatively enrichedback-arc basin basalts from the Sea of Japan floor(Cousens et al., 1994) (Fig. 6a). It has been well establishedfor the NE Chinese intraplate magmatism that sub-alkalic basalts typically possess more enriched Sr–Ndisotopic signatures than coexisting alkalic basalts (Zhou& Armstrong, 1982; Zhou et al., 1988; Song et al., 1990),as is documented for Hannuoba magmatism (Fig. 6a).Such compositional differences are also broadly observedfor the Jeju samples: the Low-Al ALK series exhibit thelowest Sr and highest Nd isotopic ratios, and theSub-ALK series are characterized by more enrichedsignatures (Fig. 6a and b). However, lavas showingvery depleted Sr–Nd isotopic signatures close to those ofMORB, such as the Hannuoba alkalic basalts, are notdocumented on Jeju Island.Jeju volcanic rocks are, however, distinct from basalts

from both the Sea of Japan back-arc basin and NE China

531


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0.8

4

7

0

2

4

6

8

10

12

10

12

alkalic

sub-alkalic

High-Al ALKLow-Al ALKSub-ALK

0

10

20

30

10

hyp

ers

the

ne

ne

ph

elin

eN

a2O

+K

2O

(w

t.%

)A

l 2O

3 (

wt.

%)

TiO

2 (

wt.

%)

Fe

O*

(wt.

%)

Mg

O (

wt.

%)

Ca

O (

wt.

%)

Na

2O

(w

t.%

)K

2O

(w

t.%

)P

2O

5 (

wt.

%)

SiO2 (wt.%) SiO2 (wt.%)

46 48 50 52 54 56 58 60 62 46 48 50 52 54 56 58 60 62

46 48 50 52 54 56 58 60 62 46 48 50 52 54 56 58 60 62


4

6

8

10

12

2

12

14

16

18

20

22

1

2

3

4

0

4

6

8

10

14

2

(wt.

%)

(wt.

%) 4

6

8

12

2

3

4

5

6

8

2

1

2

3

5

0

0.2

0.4

0.6

1.0

1.2

1.4

0

Fig. 2. Variations in major element and normative compositions of the Jeju volcanic rocks. Continuous and dot–dashed lines in the SiO2 vsNa2O þ K2O diagram indicate the boundaries between alkalic and sub-alkalic series after Le Bas & Streckeisen (1991) and Miyashiro(1978), respectively. CIPW normative compositions are calculated by assuming Fe2þ/(Fe2þ þ Fe3þ) ¼ 0�9 in the magma. Jeju volcanic rockscan be broadly divided into alkalic and subalkalic series on a SiO2 vs total alkalis diagram; the alkalic series are further subdivided into two series,high-Al and low-Al series (abbreviated as High-Al ALK and Low-Al ALK, respectively), based on Al2O3 contents. Compositional trends definedby High- and Low-Al ALKs can be reasonably explained by fractionation of existing minerals, including olivine, plagioclase, clinopyroxene,magnetite, and apatite, from the magma.

532


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in that they possess much more radiogenic Pb isotopiccompositions (Fig. 6c). Further, the Pb isotopic character-istics differ from the Sr–Nd isotopic systematics in that theJeju volcanic rocks do not show systematic compositional

differences between the alkalic and sub-alkalic series(Fig. 6f ).The mantle geochemical reservoirs required for

explaining the isotopic signatures of oceanic hotspot


Ni (p

pm

)C

u (

pp

m)

Zn

(p

pm

)

Zr

(ppm

)

Rb

(p

pm

)S

r (p

pm

)

Nb

(p

pm

)B

a (

pp

m)

Th

(p

pm

)

50

100

150

200

0

46 48 50 52 54 56 58 60 62

10

20

30

40

50

60

70

0

80

100

120

140

20

40

60

80

100

120

0

400

600

800

1000

1200

1400

20046 48 50 52 54 56 58 60 62

0

100

200

300

400

500

600

700

20

40

60

80

100

120

0

200

400

600

800

1000

1200

1400

0

5

10

15

20

046 48 50 52 54 56 58 60 62

SiO2 (wt.%)SiO2 (wt.%)

SiO2 (wt.%)46 48 50 52 54 56 58 60 62

SiO2 (wt.%)

Y (

ppm

)

15

20

25

30

35

40

10

Fig. 3. Trace element abundances vs SiO2 in Jeju volcanic rocks.

533


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basalts or ocean island basalts (OIB) can be identified by aSr–Pb isotopic diagram (Fig. 6g). The Sr–Pb isotopiccompositions of Jeju volcanic rocks are within the rangeof OIBs and suggest the contribution of HIMU and EMIIreservoirs to their source, whereas the NE Chinese hot-spot basalts show isotopic characteristics close to EMI(Fig. 6g).

Petrography

Representative compositions of phenocrysts are given inTables 3–7. Olivine phenocrysts are ubiquitous in the

mafic lavas of Jeju Island, although the amount of olivineis <10 vol. % (Table 1). The olivine phenocrysts inrelatively undifferentiated rocks (MgO > �5 wt %)tend to have a narrow compositional range with a peakcomposition (Fig. 7) that is in equilibrium with the bulkrock in terms of Fe–Mg exchange partitioning assumingFe2þ/(Fe2þ þ Fe3þ) ¼ 0�9 in the magma. By contrast,Mg-poor or differentiated samples contain ‘disequili-brium’ olivine phenocrysts (Fig. 7).Minor amounts of clinopyroxene phenocrysts

(<5 vol. %) occur in some Jeju samples. Clinopyroxenesin the Sub-ALK series lavas are more depleted in the

1

10

100

1000Rb Ba Th Nb K Sr Zr Y

High-Al ALK

0.1

N-M

OR

B N

orm

alis

ed

Low-Al ALK

1

10

100

1000

0.1

N-M

OR

B N

orm

alis

ed

Sub-ALK

1

10

100

1000

0.1

N-M

OR

B N

orm

alis

ed

Granite

1

10

100

1000

0.1Rb Ba Th Nb K Sr Zr Y

N-M

OR

B N

orm

alis

ed

1

10

100

1000

0.1

N-M

OR

B N

orm

alis

ed

RbBa

ThNb

KLa

CePb

PrSr

NdZr

SmEu

GdTb

DyY

HoEr

TmYb

Lu

1

10

100

1000

0.1

N-M

OR

B N

orm

alis

ed

1

10

100

1000

0.1

N-M

OR

B N

orm

alis

ed

1

10

100

1000

0.1N-M

OR

B N

orm

alis

ed

0.01Rb

BaTh

NbK

LaCe

PbPr

SrNd

ZrSm

EuGd

TbDy

YHo

ErTm

YbLu

10

100

1000La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu

1

Ch

on

drite

No

rma

lise

d

10

100

1000

1

Ch

on

drite

No

rma

lise

d

10

100

1000

1

Ch

on

drite

No

rma

lise

d

10

100

1000

La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu1

Ch

on

drite

No

rma

lise

d

High-Al ALK

Low-Al ALK

Sub-ALK

Granite

High-Al ALK

Low-Al ALK

Sub-ALK

Granite

Fig. 4. N-MORB-normalized multi-element and chondrite-normalized REE diagrams for Jeju volcanic rocks and basement granites. AlthoughJeju lavas show incompatible element patterns broadly similar to those for hotspot magmas, they exhibit a relative depletion of Nb that is generallyconsidered more typical of subduction-zone magmas. Normalization values from Sun & McDonough (1989).

534


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diopside component and show stronger normal zoningthan those in the ALK series lavas (Fig. 8). Orthopyr-oxene is rarely found in the Sub-ALK series lavas asmicrophenocryst and groundmass phases (Fig. 8).Abundant plagioclase phenocrysts (>20 vol. %) are

observed in some Low-Al ALK series lavas (Table 1).They are characterized by rather limited compositionalranges and do not show disequilibrium textures such asreverse zoning (Fig. 9), or dusty and/or honeycombstructures.Spinel crystals are often included in olivine phenocrysts

and are characterized by rather high Cr/Al values andlow Fe3þ contents compared with Cenozoic stronglyalkalic basalts from SW Japan that are not related tosubduction (Shukuno & Arai, 1999) (Fig. 10a). Thisfeature of the spinel compositions may be confirmed byan olivine–spinel compositional relationship diagram(Arai, 1994), which indicates that Jeju basalt magmaswould be in equilibrium with the residual spinel in themantle with higher Cr/Al values than those from SWJapan (Fig. 10b). Mantle xenoliths in the Jeju basaltscontain olivine and spinel crystals (Choi et al., 2002) thatplot well within the olivine–spinel mantle array of Arai(1994): spinel in lherzolite xenoliths has much lowerCr/Al values than those in harzburgite xenoliths, andthe latter possess Cr/Al values identical to spinelinclusions in the Jeju olivine phenocrysts (Fig. 10b).Ballhaus et al. (1990, 1991) and Ballhaus (1993) exam-

ined Cr–Al-rich spinel compositions and demonstratedthat Fe3þ in spinel may provide a reasonable estimate of

the fO2relative to the FMQ (fayalite–magnetite–quartz)

buffer for a magma that crystallizes spinel. Figure 10cindicates fO2

estimates based on spinel crystals in Jejubasalts. It should be stressed that the fO2

of the Jejumagmas may be higher than that of ocean island basaltsand close to that of island-arc basalt magmas.

DISCUSSION

Magmatic differentiation

Processes of differentiation of the three magma series ofJeju Island are examined. One commonly occurring pro-cess for terrestrial magmas is mixing of compositionallydifferent magmas. Evidence indicative of magma mixingincludes the following disequilibrium petrographiccharacteristics (Eichelberger, 1975; Sakuyama, 1979;Bloomfield & Arculus, 1989; Kawamoto, 1992; Yanget al., 1999): (1) the presence of plagioclase phenocrystswith a dusty zone containing fine melt inclusions and witha wide range of compositions; (2) the presence of reverselyzoned pyroxene phenocrysts with rounded cores mantledby rims of higher Mg-number; (3) the presence of dis-equilibrium phenocryst assemblages such as olivine andquartz. Jeju volcanic rocks, however, do not exhibit suchpetrographic features (Figs 8 and 9), suggesting thatmagma mixing may not have been a significant processaffecting these magmas.Mantle-derived, high-temperature, basaltic magmas

are likely to cause partial melting of crustal rocks withlower solidus temperatures, and hence could becontaminated by crust-derived felsic melts. The basementof Jeju volcano is likely to be granite, because such rocksare found as xenoliths in the volcanic rocks. To evaluatethe geochemical contribution of crustal contamination toJeju magma differentiation, the results of simple mixingcalculations using compositions of a basaltic magma(Low-Al ALK CJ-10) and a total granitic melt, areshown in Fig. 11. It is clearly demonstrated that a con-taminated magma possesses increasingly enriched iso-topic signatures with increasing contribution of graniticmelt. However, Jeju volcanic rocks, especially those of theHigh-Al and Low-Al ALK series, possess rather constantisotopic compositions with increasing SiO2 content(Fig. 11), indicating at most only minor involvement ofgranite-derived melts during magmatic differentiationprocesses. On the other hand, two Sub-ALK seriessamples that are characterized by unusually enrichedisotopic signatures appear to have been contaminatedsignificantly by basement granitic rocks (Fig. 11).The above considerations suggest that the observed

wide range of magma compositions of Jeju volcanicrocks may be largely governed by fractional crystall-ization. Phases involved in such differentiation processes,as inferred from major element variations (Fig. 2) and

MORB

NE China

Japan Sea

NE Japan

Sikhote Alin

10 100 1000

10000

100

1000

K/Y

K/N

b


PolynesiaHawaii

Fig. 5. K/Y–K/Nb relationships for Jeju volcanic rocks, Quaternaryarc lavas from the NE Japan arc, back-arc basin basalts from the Sea ofJapan, intraplate basalts from NE China and Polynesia, and MORB.Data from this study, Kogiso et al. (1997b) and Tatsumi et al. (2000).Jeju volcanic rocks share characteristics common to intraplate basaltsrather than subduction-related magmas, although they show ‘Nbdepletion’ in multi-element MORB-normalized REE diagrams inFig. 4.

535


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petrographic observations, include olivine, clinopyrox-ene, plagioclase, magnetite, and apatite. To examinethis quantitatively, least-squares mixing calculationsusing the compositions of the coexisting phases were

conducted (Table 8 and Fig. 12). In these calculations,three Low-Al ALK series samples (CJ-10, 29, and 14)and two High-Al ALK series samples (CJ-42 and 21)were selected as representative magmas (Fig. 12).

0.5126

0.5127

0.5128

0.5129

0.5130

0.5131

0.5132

0.7035 0.7040 0.7045 0.7050 0.7055

14

3N

d/1

44N

d

87Sr/86Sr

15.35

15.45

15.55

15.65

15.75

17.0 17.5 18.0 18.5 19.0 19.5

20

7P

b/2

04P

b

206Pb/204Pb

37

38

39

40

17.0 17.5 18.0 18.5 19.0 19.5

0.701

0.703

0.705

0.707

0.709

16 17 18 19 20 21 22

87S

r/8

6S

r2

08P

b/2

04P

b

0.51265

0.51270

0.51275

0.51280

0.51285

0.7040 0.7044 0.7048 0.7052

15.64

15.66

15.68

15.70

15.72

15.74

18.2 18.4 18.6 18.8 19.0 19.2 19.4

39.4

39.6

39.8

40.0

18.7 18.8 18.9 19.0 19.1 19.2

HIMU

EMII

EMI

DMM

Hannuobaalkalic

Hannuobasub-alkalic

Ocean IslandBasalts

NHRL

NHRL

BE

206Pb/204Pb

206Pb/204Pb

20

8P

b/2

04P

b

206Pb/204Pb

14

3N

d/1

44N

d

87Sr/86Sr

20

7P

b/2

04P

b206Pb/204Pb

High-Al ALKLow-Al ALKSub-ALKgranite

(a) (b)

(c) (d)

(e) (f)

(g)

crustal contamination

Sea of Japan

Fig. 6. Sr–Nd–Pb isotopic compositions of Jeju volcanic rocks and related lavas. A mixing line between the least differentiated Low-Al ALK seriesmagma and granite (dashed line) is shown in (d).

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The calculations confirm that the magmatic differentia-tion for these series can be reproduced by separation ofthe above minerals (Table 8 and Fig. 12). The effect offractional crystallization on the compositional variationwas further examined by using Rayleigh fractionationmodels for REE elements. Crystal–liquid partition coeffi-cients used in this modelling (Table 9) are after Tatsumi(2001), and are based on the compilation of experimentaldata by Green (1994) and a consideration of the crystalstructure control in trace element partitioning betweenmelts and solid phases (Matsui et al., 1977). The fractionof phases separated from the magma is based on themass-balance calculations (Table 8). The modellingresults are illustrated in Fig. 12, and indicate that REEconcentrations in the Jeju magmas can also be largelyreproduced by fractional crystallization processes.

Melting regime

Genetic relations between the three chemically distinctmagma series are now examined. A possible mechanismfor producing sub-alkalic magmas from alkalic parentalmagmas is contamination by silica-oversaturated, incom-patible-element-enriched crustal rocks. As mentionedabove, isotopic compositions, especially 207Pb/206Pb, oftwo of the Sub-ALK series rocks could be explained by

this mechanism (Fig. 11). However, this process cannotreasonably reproduce the isotopic characteristics of theother Sub-ALK series lavas. On Pb–Pb isotopic dia-grams, for example, Sub-ALK rocks do not lie on themixing line defined by ALK and granitic rocks (Fig. 6cand d). Thus, it may be concluded that crustal contam-ination is not a likely process for producing the sub-alkalicmagmas from alkalic magmas on Jeju Island, and thatboth alkalic and sub-alkalic magmas were derived fromthe mantle beneath the island.The High-Al ALK magmas are distinct from the

Low-Al ALK series in that the former are more enrichedin Al and Sr, consistent with involvement of a plagioclasecomponent. It could be possible, therefore, that High-AlALK series magmas were derived from Sub-ALK mag-mas by selective accumulation of plagioclase phenocrysts.Positive Eu anomalies observed in some High-Al ALKlavas also support this (Fig. 4). However, plagioclaseaccumulation is unlikely to be responsible for producingthe High-Al ALK magmas. The reasons for believingso are twofold. First, the modal amount of plagioclasephenocrysts in High- and Low-Al ALKs does not corre-late with the plagioclase component in the magmas, suchas Al and Sr concentrations, Sr/Ba values and degreesof positive Eu anomalies (Fig. 13). Second, the positiveEu anomaly is most clearly observed for the most

Table 3: Representative compositions of olivine phenocrysts

Rock type: High-Al ALK Low-Al ALK Sub-ALK

Sample: CJ-42 CJ-1 CJ-10 CJ-34 CJ-37 CJ-2

Position: core rim core rim core rim core rim core rim core rim

SiO2 38.39 38.10 40.40 39.34 40.59 39.87 40.10 38.67 39.45 37.84 39.36 38.31

FeO* 24.38 27.09 14.10 18.50 13.61 18.47 13.99 21.31 16.93 24.46 17.22 24.42

MnO 0.44 0.52 0.18 0.26 0.17 0.25 0.18 0.31 0.22 0.26 0.21 0.29

MgO 36.99 34.51 45.51 41.90 45.51 41.62 45.65 39.25 42.97 36.72 43.03 37.13

CaO 0.18 0.19 0.19 0.20 0.18 0.27 0.18 0.26 0.18 0.27 0.15 0.22

NiO 0.03 0.02 0.24 0.19 0.26 0.18 0.30 0.17 0.26 0.18 0.34 0.23

Total 100.40 100.42 100.61 100.39 100.32 100.66 100.41 99.96 100.12 99.79 100.31 100.59

Si 1.004 1.010 1.004 1.001 1.009 1.010 0.999 1.002 1.000 0.999 0.997 1.001

Fe 0.533 0.600 0.293 0.394 0.283 0.391 0.292 0.462 0.359 0.540 0.365 0.534

Mn 0.010 0.012 0.004 0.006 0.004 0.005 0.004 0.007 0.005 0.006 0.005 0.006

Mg 1.443 1.363 1.686 1.589 1.686 1.572 1.696 1.516 1.623 1.444 1.625 1.447

Ca 0.005 0.005 0.005 0.006 0.005 0.007 0.005 0.007 0.005 0.008 0.004 0.006

Ni 0.001 0.000 0.005 0.004 0.005 0.004 0.006 0.003 0.005 0.004 0.007 0.005

Total 2.996 2.990 2.996 2.999 2.991 2.990 3.001 2.998 3.000 3.001 3.003 2.999

Mg/(Mg þ Fe) 0.730 0.694 0.852 0.801 0.856 0.801 0.853 0.766 0.819 0.728 0.817 0.730

*Total iron as FeO.Numbers of ions are calculated on the basis of four oxygens.

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REE-depleted, least differentiated rocks and tends tobecome weak with increasing degrees of magmatic differ-entiation (Fig. 4). It is therefore more likely that thecharacteristic enrichment of the plagioclase componentin the High-Al ALK magmas is a primary signaturegained during mantle melting, not derived from intra-crustal processes.The above considerations suggest the production of the

three types of magmas in the upper mantle beneath JejuIsland. To examine the P–T conditions of magmageneration for the three magma series, compositionsof primary magmas for each series were estimated andcompared with those of partial melts produced in

peridotite melting experiments. The first step was toestimate the composition of residual mantle olivine inequilibrium with such primary magmas. Some Low-AlALK and Sub-ALK lavas contain Mg-rich (Mg-number >80) olivine phenocrysts. The NiO–Mg-number relationships for those olivine phenocrysts inMg-rich samples are demonstrated in Fig. 14, togetherwith an inferred olivine composition that would be inequilibrium with the bulk rock, estimated on the basis ofFe–Mg–Ni exchange partitioning between olivine andsilicate melts (Roeder & Emslie, 1970; Kinzler et al.,1990) and assumption of Fe2þ/(Fe2þ þ Fe3þ) ¼ 0�9 inthe magma. It is shown in Fig. 14 that the inferred olivine

Table 4: Representative compositions of pyroxene phenocrysts

Rock type: High-Al ALK Low-Al ALK Sub-ALK

Sample: CJ-42 CJ-10 CJ-16 CJ-2 CJ-3 CJ-3


SiO2 48.63 50.08 49.34 49.24 49.13 50.82 52.92 51.22 52.38 49.90 54.69 53.33

TiO2 1.90 1.66 1.82 2.09 1.74 1.68 0.73 1.37 0.85 0.88 0.29 0.40

Al2O3 7.12 4.85 6.11 5.58 6.47 2.82 1.88 1.60 1.20 1.69 1.97 0.97

Cr2O3 0.21 0.00 0.89 0.26 0.74 0.03 0.84 0.05 0.08 0.00 0.39 0.30

FeO* 7.49 8.25 6.33 7.44 6.58 8.59 11.19 13.16 16.46 20.43 11.61 17.93

MnO 0.14 0.22 0.11 0.13 0.14 0.18 0.27 0.25 0.31 0.35 0.21 0.31

MgO 13.63 13.24 14.37 13.60 14.50 14.47 20.14 15.19 20.00 11.58 29.05 23.75

CaO 20.61 21.96 20.85 21.36 20.17 20.77 12.04 16.63 7.73 13.21 1.83 2.17

Na2O 0.62 0.49 0.45 0.44 0.60 0.42 0.18 0.26 0.16 0.24 0.04 0.07

K2O 0.00 0.02 0.00 0.03 0.03 0.03 0.01 0.00 0.00 0.03 0.01 0.02

NiO 0.02 0.01 0.02 0.01 0.02 0.00 0.05 0.05 0.04 0.01 0.11 0.04

Total 100.36 100.77 100.29 100.17 100.12 99.80 100.26 99.77 99.21 98.31 100.20 99.28

Si 1.797 1.854 1.818 1.827 1.812 1.897 1.936 1.929 1.957 1.955 1.941 1.970

Ti 0.053 0.046 0.050 0.058 0.048 0.047 0.020 0.039 0.024 0.026 0.008 0.011

Al 0.310 0.212 0.265 0.244 0.281 0.124 0.081 0.071 0.053 0.078 0.082 0.042

Cr 0.006 0.000 0.026 0.008 0.021 0.001 0.024 0.001 0.002 0.000 0.011 0.009

Fe 0.231 0.255 0.195 0.231 0.203 0.268 0.342 0.414 0.514 0.669 0.345 0.554

Mn 0.004 0.007 0.003 0.004 0.004 0.006 0.008 0.008 0.010 0.012 0.006 0.010

Mg 0.751 0.731 0.789 0.752 0.798 0.805 1.099 0.853 1.114 0.676 1.537 1.308

Ca 0.816 0.871 0.823 0.849 0.797 0.830 0.472 0.671 0.309 0.554 0.069 0.086

Na 0.044 0.035 0.032 0.031 0.043 0.031 0.013 0.019 0.011 0.018 0.003 0.005

K 0.000 0.001 0.000 0.001 0.001 0.001 0.001 0.000 0.000 0.001 0.000 0.001

Ni 0.001 0.000 0.001 0.000 0.001 0.000 0.002 0.001 0.001 0.000 0.003 0.001

Total 4.014 4.012 4.002 4.005 4.010 4.010 3.998 4.006 3.997 3.990 4.006 3.996

Mg/(Mg þ Fe) 0.764 0.741 0.802 0.765 0.797 0.750 0.762 0.673 0.684 0.503 0.817 0.702

Ca/(Ca þ Mg þ Fe) 0.454 0.469 0.455 0.463 0.443 0.436 0.247 0.346 0.160 0.292 0.036 0.044

Mg/(Ca þ Mg þ Fe) 0.418 0.394 0.437 0.411 0.444 0.423 0.574 0.440 0.575 0.356 0.788 0.671

Fe/(Ca þ Mg þ Fe) 0.129 0.138 0.108 0.126 0.113 0.141 0.179 0.214 0.265 0.352 0.177 0.284

*Total iron as FeO.Numbers of ions are calculated on the basis of six oxygens.

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in equilibrium with the bulk rock mostly has NiO–Mg-number compositions within the range of the olivinephenocrysts. On the other hand, the Sub-ALK sampleCJ-2 contains olivine that is more nickeliferous thanexpected. Such unusually nickeliferous olivine pheno-crysts are not uncommon in basalts and andesites(e.g. Sato & Banno, 1983; Nabelek & Langmuir, 1986;Nakamura, 1995; Tatsumi et al., 2002, 2003). Nakamura(1995) examined the compositional zoning of olivinephenocrysts in andesites from the Yatsugatake volcano,Central Japan, by using a growth and diffusion model inthe Mg–Fe–Ni system. He indicated that the character-istic compositions of unusual olivine phenocrysts can beexplained by diffusion processes within normally zonedolivines causing Fe enrichment without a marked depres-sion in the Ni content. This is due to a greater Fe–Mg

interdiffusion coefficient than the Ni tracer diffusion coef-ficient in olivine. It is thus concluded, at least for the CJ-2magma, that a long residence time of olivine phenocrystsmay cause such unusual compositions.It has been well established that olivine in the upper

mantle possesses a rather constant NiO content whereasits Mg-number is variable (Sato, 1977; Takahashi, 1990)(Fig. 14). If we accept 0�4 wt % NiO for mantle olivine,then it is possible to estimate the Mg-number of olivinethat was once in equilibrium with the primary magma, byback-calculating the equilibrium olivine composition.The results of such calculations, assuming Fe2þ/(Fe2þ þFe3þ) ¼ 0�9 in the magma and using Fe–Mg–Niexchange partition coefficients of Roeder & Emslie(1970) and Kinzler et al. (1990), are shown in Fig. 14and suggest that the observed compositional variation of

Table 5: Representative compositions of plagioclase phenocrysts

Rock type: High-Al ALK Low-Al ALK

Sample: CJ-42 CJ-8 CJ-7 CJ-36 CJ-30 . 2 CJ-23


SiO2 53.18 54.34 50.85 53.05 53.65 54.34 52.61 52.81 54.16 56.49 54.38 57.60

TiO2 0.07 0.04 0.06 0.10 0.04 0.08 0.05 0.08 0.10 0.18 0.08 0.14

Al2O3 29.71 28.93 30.76 28.90 28.59 27.85 29.43 28.58 28.79 26.19 27.72 25.52

Cr2O3 0.00 0.02 0.00 0.00 0.00 0.01 0.00 0.00 0.00 0.00 0.04 0.03

FeO* 0.42 0.41 0.49 0.69 0.53 0.73 0.48 0.70 0.44 0.66 0.43 0.63

MnO 0.00 0.01 0.00 0.04 0.02 0.02 0.02 0.01 0.00 0.00 0.00 0.00

MgO 0.05 0.06 0.08 0.08 0.03 0.05 0.11 0.15 0.09 0.07 0.05 0.08

CaO 12.77 11.63 13.95 12.16 11.51 10.75 13.36 12.59 11.88 9.30 10.94 8.67

Na2O 4.20 4.55 3.38 4.26 4.88 5.15 3.95 4.20 4.70 5.94 5.57 6.32

K2O 0.33 0.41 0.23 0.47 0.41 0.34 0.25 0.37 0.33 0.59 0.40 0.73

NiO 0.00 0.00 0.00 0.00 0.03 0.01 0.00 0.00 0.01 0.01 0.01 0.00

Total 100.73 100.39 99.80 99.73 99.67 99.33 100.25 99.50 100.50 99.42 99.61 99.71

Si 2.399 2.449 2.323 2.418 2.442 2.477 2.389 2.416 2.443 2.563 2.475 2.602

Ti 0.002 0.001 0.002 0.003 0.001 0.003 0.002 0.003 0.004 0.006 0.003 0.005

Al 1.579 1.537 1.657 1.553 1.534 1.496 1.575 1.541 1.530 1.401 1.487 1.359

Cr 0.000 0.001 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.001 0.001

Fe 0.016 0.015 0.019 0.026 0.020 0.028 0.018 0.027 0.017 0.025 0.017 0.024

Mn 0.000 0.000 0.000 0.001 0.001 0.001 0.001 0.001 0.000 0.000 0.000 0.000

Mg 0.003 0.004 0.005 0.005 0.002 0.004 0.007 0.010 0.006 0.004 0.003 0.005

Ca 0.617 0.562 0.683 0.594 0.561 0.525 0.650 0.617 0.574 0.452 0.534 0.420

Na 0.367 0.398 0.299 0.376 0.431 0.455 0.348 0.372 0.411 0.523 0.492 0.554

K 0.019 0.023 0.013 0.027 0.024 0.020 0.014 0.022 0.019 0.034 0.023 0.042

Ni 0.000 0.000 0.000 0.000 0.001 0.000 0.000 0.000 0.000 0.000 0.000 0.000

Total 5.003 4.991 5.002 5.004 5.017 5.009 5.003 5.008 5.004 5.009 5.035 5.011

Ca/(Ca þ Na) 0.627 0.586 0.695 0.612 0.566 0.536 0.651 0.624 0.583 0.464 0.520 0.431

*Total iron as FeO.Numbers of ions are calculated on the basis of eight oxygens.

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olivine phenocrysts may be reasonably explained by oli-vine fractionation from the magma. The back-calculationmay suggest that the Mg-numbers of the residual mantleolivine are �88 and �85 for Low-Al ALK and Sub-ALKseries, respectively (Fig. 14). Fairly low Mg-numbers ofthe residual olivine for Sub-ALK series magmas may becaused by extensive metasomatism by silicate–carbonatemelts, as discussed below. On the basis of the composi-tions of these residual mantle olivines and the bulk rocks,the major element compositions of the primary magma

for each magma series can be back-calculated by assum-ing maximum olivine fractionation from the magma. Thesamples used for these calculations contain phenocrystssolely of olivine (excepting CJ-1, which contains a minoramount of clinopyroxene), providing the basis for assum-ing that the only major phase fractionated from theprimary magma during the differentiation processeswas olivine. The inferred primary magma compositionsfor the Low-Al ALK and Sub-ALK series are listedin Table 10.On the other hand, the High-Al ALK samples do not

contain Mg-rich olivine and these methods cannotbe used to estimate the primary magma composition. Toovercome this, the following two simplified methods wereapplied. The first is based on the systematic compositional

Table 6: Representative compositions of spinel

inclusions

Rock type: Low-AL ALK Sub-Alk

Sample: CJ-10 CJ-1 CJ-34 CJ-35 CJ-37

SiO2 0.08 0.06 0.06 0.06 0.02

TiO2 2.36 2.43 2.46 3.19 4.29

Al2O3 19.17 19.30 19.88 18.85 15.66

Cr2O3 33.37 33.50 32.64 35.27 35.54

FeO* 31.87 30.77 32.31 32.39 33.59

MnO 0.21 0.23 0.22 0.24 0.26

MgO 10.66 11.24 10.68 9.57 8.79

CaO 0.01 0.02 0.00 0.01 0.04

NiO 0.00 0.00 0.00 0.00 0.00

Total 97.72 97.55 98.24 99.58 98.18

Si 0.003 0.002 0.002 0.002 0.001

Ti 0.057 0.058 0.059 0.076 0.106

Al 0.724 0.727 0.745 0.708 0.607

Cr 0.846 0.847 0.821 0.889 0.924

Fe3þ a 0.311 0.305 0.312 0.247 0.256

Fe2þ b 0.544 0.518 0.548 0.617 0.668

Mn 0.006 0.006 0.006 0.007 0.007

Mg 0.510 0.536 0.507 0.455 0.431

Ca 0.000 0.001 0.000 0.000 0.001

Ni 0.000 0.000 0.000 0.000 0.000

Total 3.000 3.000 3.000 3.000 3.000

Cr/(Cr þ Al) 0.539 0.538 0.524 0.557 0.604

Mg/(Mg þ Fe2þ) 0.469 0.518 0.462 0.369 0.323

Cr/(Cr þ Al þ Fe3þ) 0.450 0.451 0.437 0.482 0.517

Al/(Cr þ Al þ Fe3þ) 0.385 0.387 0.397 0.384 0.340

Fe3þ/(Cr þ Al þ Fe3þ) 0.165 0.162 0.166 0.134 0.143

olivineb 0.834 0.839 0.834 0.804 0.804

Dlog(fO2)FMQ

c 1.73 1.71 1.77 1.16 1.25

*Total iron as FeO.aFe3þ and Fe2þ are calculated assuming stoichiometry forspinel.bMg/(Mg þ Fe) of coexisting olivine.cfO2

(relative to FMQ, DFMQ) is calculated assuming P ¼0�1GPa after Ballhaus et al. (1991).Numbers of ions are calculated on the basis of four oxygens.

Table 7: Representative compositions of amphibole

phenocrysts

High-Al ALK

Sample: CJ-42

Position: core core core

SiO2 40.54 40.66 40.79

TiO2 5.27 5.44 4.94

Al2O3 13.79 13.83 13.35

Cr2O3 0.03 0.01 0.01

FeO* 12.56 12.08 13.45

MnO 0.21 0.15 0.18

MgO 11.80 12.20 11.61

CaO 11.00 10.97 10.62

Na2O 2.76 2.60 2.59

K2O 1.01 1.02 1.11

NiO 0.01 0.03 0.00

Total 98.98 98.98 98.64

Si 5.946 5.944 6.017

Ti 0.582 0.598 0.549

Al 2.383 2.383 2.321

Cr 0.004 0.001 0.001

Fe 1.541 1.477 1.659

Mn 0.026 0.018 0.023

Mg 2.581 2.658 2.553

Ca 1.728 1.719 1.678

Na 0.784 0.736 0.739

K 0.190 0.190 0.208

Ni 0.001 0.004 0.000

Total 15.765 15.729 15.747

Mg/(Mg þ Fe) 0.626 0.643 0.606

K/(Na þ K) 0.121 0.129 0.141

*Total iron as FeO.Numbers of ions are calculated on the basis of 23 oxygens.

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Fig. 7. Frequency distribution diagrams for olivine core compositions in Jeju volcanic rocks. The number in parentheses shows MgO content ofthe bulk-rock sample. Dashed lines indicate Mg-number of olivine in equilibrium with the bulk composition, which is estimated based on Fe–Mgexchange partitioning. Arrows indicate the compositional range of olivine phenocryst rims.

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difference between Low- and High-Al ALKs. As demon-strated in Figs 2 and 12, High-Al ALK rocks tend topossess �2 and �0�5 wt % higher Al2O3 and Na2O,and �2 and �0�5 wt % lower FeO* and MgO than theLow-Al ALK series lavas, respectively. By assuming thesedifferences for the primary magmas, the inferred primarymagma compositions (A-1, B and C in Table 10) can bededuced from the Low-Al primary magma compositions.The second method is based on (1) the composition of themost primitive High-Al ALK sample (CJ-42) and (2) theamounts of minerals fractionated from the primarymagma to produce the CJ-42 magma, assumed to beidentical to those fractionated during magmatic differen-tiation from the Low-Al ALK primary magma, via CJ-10,to CJ-14 magma (Fig. 12). The composition of thisinferred magma is listed as A-2 in Table 10.Developments in experimental methods, particularly

the peridotite–basalt ‘sandwich’ technique (Takahashi &Kushiro, 1983; Fujii & Scarfe, 1985; Falloon & Green,1987, 1988; Falloon et al., 1988), have facilitated thesuccessful quenching and determination of peridotitepartial melt compositions at high pressures. Peridotitemelting studies using the sandwich technique have pro-duced a consistent set of data despite the use of differentperidotite compositions. The consistent and systematicbehaviour of melt compositions when projected fromdiopside onto the plagioclase–olivine–quartz plane ofthe basalt tetrahedron allowed Falloon & Green (1988)to construct a pressure-sensitive melting grid, which iscapable of establishing the depth of magma separation forprimary magma compositions within at least the silica-saturated portion of the basalt tetrahedron. However, thisgrid has not been calibrated using data for degrees ofmelting smaller than �10%, where elevated and variableincompatible element concentrations, arising from varia-tions in degree of melting and starting compositions(basalt layer and peridotite sandwich) may cause signific-ant changes to phase relationships. A new technique,which employs aggregates of diamond embeddedbetween peridotite layers, has improved the small-degree

melt problem significantly (Baker et al., 1992; Johnson &Kushiro, 1992; Kushiro & Hirose, 1992). In these experi-ments, small-degree partial melts can be extracted fromthe peridotite layers into the pore space between dia-mond aggregates, and therein analysed by electronmicroprobe. The compositions of these partial meltsprobably provide the best available estimates of primarymagma compositions formed as a function of varyingupper-mantle P–T conditions. The results of Hirose &Kushiro (1993) confirm previous assertions based on boththe basalt–peridotite sandwich technique and simple sys-tem experiments, that more alkaline magmas will form athigher pressure given an identical degree of partial melt-ing (Fig. 15). It should be thus possible to use these gridsand the primary magma compositions of the Jeju mag-mas (Table 10) to estimate the P–T conditions for the lastequilibration between the upper-mantle residue and themagmas. Figure 15 clearly shows that the Sub-ALK seriesmagmas were produced at lower pressures and by greaterdegrees of partial melting than ALK series magmas. Itmay be further suggested from Fig. 15 that the Low-AlALK magmas formed at greater depths than High-AlALK magmas.Differences in the degree of mantle partial melting

between the three types of Jeju magmas are also sug-gested by the abundances of incompatible trace elements(Figs 3 and 4): the High-Al ALK series are the mostenriched and the Sub-ALK magmas are the leastenriched in incompatible elements. If we assume anidentical peridotite source for the three magma types,then higher degrees of partial melting for the Sub-ALKseries are likely to formmoreMg-rich olivine as a residualphase. However, this is not the case for the Jeju magmas,as mentioned above. Greater degrees of partial melting ofrelatively fertile or enriched mantle peridotite wereresponsible for production of the Sub-ALK magmas.It has been well established that the presence of vola-

tiles such as H2O and CO2 in magmas causes significantchanges in P–T conditions of magma generation.At present, however, no estimates for the abundances

Di

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Fs

Fig. 8. Composition of core (filled symbols) and rim (open symbols) of pyroxenes in Jeju volcanic rocks. No reversely zoned pyroxene wasobserved.

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Fre

q.

Fre

q.

Fre

q.

Fre

q.F

req.

Fre

q.

Fre

q.

Fre

q.

304050607080

304050607080304050607080

100×Ca/(Ca+Na)

100×Ca/(Ca+Na) 100×Ca/(Ca+Na)

100×Ca/(Ca+Na) 100×Ca/(Ca+Na)100×Ca/(Ca+Na)

Fig. 9. Frequency distribution diagrams for core compositions of plagioclase phenocrysts in Jeju volcanic rocks. Arrows indicate the compositionalrange of plagioclase phenocryst rims.

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of volatiles in the Jeju magmas are available, so thatthe above-mentioned estimates for the conditions ofmagma production assuming a ‘dry’ peridotite sourceare tentatively accepted here.

Source characteristics

Petrographic and geochemical studies of mantle-derivedxenoliths and magmas from the continental regions haveshown the presence of variably metasomatized peridotite

1.0-1

0

1

2

3

4

95 90 85 80 75 700

0.5

1.0

Cr/(C

r + A

l)spin

el

Inferred mantle for SW Japaneseand ocean island alkaline basalts

olivin

e-s

pin

el

mantle

arra

y

ALKSub-ALKharzburgitelherzolite

100×Mg/(Mg+Fe)olivine

(b) olivine-spinel

SW Japanesealkaline basalts

Cr

Al Fe3+

(a) spinel

ALK

Sub-ALK

0 0.2 0.4 0.6 0.8

∆lo

g(f

O2)

Cr/(Cr+Al)spinel

ALKSub-ALK

(c) oxygen fugacities

Island arcbasalts

Ocean islandbasalts

FMQ

Fig. 10. (a) Compositions of spinel inclusions in olivine phenocrysts ofJeju volcanic rocks. (b) Relationship between spinel inclusions and theirhost olivine phenocrysts. (c) fO2

relative to the FMQ buffer inferredfrom spinel compositions of Jeju and other volcanic rocks (afterBallhaus et al., 1990, 1991; Ballhaus, 1993).

SiO2 (wt.%)

87S

r/86S

r207P

b/2

04P

b143N

d/1

44N

d

SiO2 (wt.%)

High-Al ALKLow-Al ALKSub-ALKgranite

0.706

0.710

0.714

0.71845 50 55 60 65 70 75

0.704

0.2

0.4

0.6

0.8

0.5116

0.5118

0.5120

0.5122

0.5124

0.5126

0.5128

0.5130

15.62

15.64

15.66

15.68

15.70

15.72

15.74

15.76

45 50 55 60 65 70 75

Fig. 11. SiO2 vs isotopic ratios for Jeju volcanic rocks and basementgranite. The composition of an undifferentiated basalt that is contamin-ated by a granitic melt is shown by the continuous lines, with thefraction of granitic melt indicated. Most Jeju volcanic rocks exhibitrather constant isotopic ratios with increasing SiO2 content, suggestingat most only a minor role for upper-crustal contamination in thedifferentiation of Jeju magmas.

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in the subcontinental upper mantle (e.g. Menzies et al.,1987). Because the Jeju volcano is built upon the Asiancontinental lithosphere, the source regions for theJeju magmas are likely to include metasomatizedsubcontinental upper mantle. The geochemical andpetrographical characteristics of the Jeju subcontinentalmantle are examined here on the basis of the Jeju magmacompositions.The NiO–Mg-number relationships for olivine pheno-

crysts suggest that the source (residual) upper-mantlematerial was more fertile for Sub-ALK magmas thanfor ALK magmas, in terms of olivine composition(Fig. 14). This is consistent with the observation thatthe Sub-ALK series lavas tend to show more enrichedSr–Nd isotopic signatures than the ALK series lavas(Fig. 6a and b), although a systematic difference betweenthe magma series cannot be demonstrated for Pb isotopes(Fig. 6c–f ). Furthermore, among the ALK series, Low-AlALK rocks are distinct in their rather depleted Sr–Nd

isotopic characteristics (Fig. 6a and b). Such systematicdifferences in isotopic signatures for alkalic and sub-alkalic rocks have been documented for Cenozoic intra-plate magmas in NE China and may represent differentdegrees of metasomatism within the subcontinentalupper mantle (e.g. Zhou et al., 1988; Song & Frey,1989; Song et al., 1990; Nohda et al., 1991). Acceptingthis and the above-mentioned differences in depth ofmagma separation from the upper mantle, we proposethat the upper mantle beneath Jeju Island is variablymetasomatized, with increasing degrees of metasomaticenrichment with decreasing depth. An upwelling mantleplume, probably with depleted isotopic and majorelement signatures, could have produced Low-Al ALKmagmas at deeper levels, caused partial melting of moremetasomatized upper mantle to form the High-Al ALKmagmas, and finally resulted in higher degrees of partialmelting of shallow-level, highly metasomatized uppermantle to produce the Sub-ALK magmas.

Table 8: Results of mixing calculations

Sample SiO2 TiO2 Al2O3 FeO* MgO CaO Na2O K2O P2O5 Contrib.

(wt %)

parent CJ-10 47.83 2.43 14.04 11.34 10.00 9.69 2.86 1.38 0.44 100.0

olivine 40.38 0.03 0.00 13.37 45.96 0.25 0.00 0.00 0.00 11.2

clinopyroxene 50.75 1.35 4.91 5.70 15.17 21.71 0.42 0.00 0.00 18.7

plagioclase 52.96 0.00 29.72 0.70 0.00 12.28 4.21 0.13 0.00 14.6

magnetite 0.00 20.49 1.48 75.60 2.43 0.00 0.00 0.00 0.00 3.6

calculated 50.78 3.03 16.97 11.90 3.79 7.35 4.04 1.41 0.73 51.9

daughter CJ-14 50.42 2.71 16.81 11.68 3.67 7.32 4.35 1.85 1.19

residual �0.37 �0.32 �0.16 �0.21 �0.12 �0.03 0.30 0.45 0.46 R2 ¼ 0.83

parent CJ-14 50.42 2.71 16.81 11.68 3.67 7.32 4.35 1.85 1.19 100.0

olivine 40.38 0.03 0.00 13.37 45.96 0.25 0.00 0.00 0.00 3.0

clinopyroxene 50.75 1.35 4.91 5.70 15.17 21.71 0.42 0.00 0.00 4.8

plagioclase 52.96 0.00 29.72 0.70 0.00 12.28 4.21 0.13 0.00 16.8

apatite 0.00 0.00 0.00 0.00 0.00 57.04 0.00 0.00 42.96 1.6

magnetite 0.00 20.49 1.48 75.60 2.43 0.00 0.00 0.00 0.00 6.2

calculated 55.99 2.04 16.97 9.21 2.10 4.88 5.35 2.71 0.75 67.6

daughter CJ-13 56.08 1.54 17.04 9.44 2.11 4.95 5.15 2.87 0.82

residual 0.09 �0.50 0.07 0.23 0.01 0.07 �0.20 0.16 0.07 R2 ¼ 0.39

parent CJ-42 49.60 2.51 18.13 10.69 4.03 7.49 4.16 2.37 1.03 100.0

olivine 40.38 0.03 0.00 13.37 45.96 0.25 0.00 0.00 0.00 4.6

clinopyroxene 50.75 1.35 4.91 5.70 15.17 21.71 0.42 0.00 0.00 6.0

plagioclase 52.96 0.00 29.72 0.70 0.00 12.28 4.21 0.13 0.00 20.8

apatite 0.00 0.00 0.00 0.00 0.00 57.04 0.00 0.00 42.96 1.7

magnetite 0.00 20.49 1.48 75.60 2.43 0.00 0.00 0.00 0.00 7.6

calculated 56.76 1.47 19.45 6.47 1.41 4.49 5.49 3.95 0.52 59.3

daughter CJ-21 56.73 1.27 19.40 6.49 1.38 4.50 5.72 3.85 0.45

residual �0.03 �0.20 �0.05 0.03 �0.02 0.01 0.23 �0.10 �0.06 R2 ¼ 0.11

*Total iron as FeO.

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14

16

18

20

Al 2

O3 (

wt.

%)

46 48 50 52 54 56 58 60 62

0

5

10

Fe

O*

(wt.

%)

0

4

8

12

High-Al ALKLow-Al ALKInferred

2

4

6

8

10

12

0

0.3

0.6

0.9

1.2

1.5

46 48 50 52 54 56 58 60 62

0

20

40

60

80

100

La (

ppm

)

46 48 50 52 54 56 58 60 62

0

20

40

60

80

Nd (

ppm

)E

u (

ppm

)

0

1

2

3

4

5

0

2

4

6

8

10

Dy (

ppm

)

0

1

2

3

4

5

Yb (

ppm

)

46 48 50 52 54 56 58 60 62

12

15

CJ-10

CJ-14

CJ-13

CJ-21CJ-42

MgO

(w

t.%

)C

aO

(w

t.%

)P

2O

5 (

wt.

%)



High-Al ALK

Low-Al ALK

Fig. 12. Results of mass-balance calculations for High-Al and Low-Al ALK series magmas. The compositional variation for these magma seriescan be reasonably explained by fractional crystallization processes. Arrows towards the more SiO2-rich direction indicate the fractional crystal-lization trend and those towards the SiO2-poor direction show the back-calculation of an inferred parental magma for the High-Al ALK series(see text).

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To further reveal the characteristics of the meta-somatized upper mantle beneath Jeju Island, especiallyto identify metasomatic minerals, the trace elementabundances in the Jeju magmas were examined. First,relatively undifferentiated samples were selected: CJ-8and 42 for the High-Al ALK series; CJ-10, 14, 15, 19,and 32 for the Low-AL ALK series; CJ-2, 3, 35, and37 for the Sub-ALK series. Second, the trace elementconcentrations were normalized to Nb to minimizethe effect of crystallization and partial melting onelement abundances. Finally, Nb-normalized element

concentrations were further normalized to those of theLow-Al ALK series magmas, because these magmas maybe derived from a less metasomatized source than theother magmas, as discussed above. The averaged valuesfor each magma series are plotted in Fig. 16 as a functionof ionic radius. Different but systematic patterns ofenrichment and depletion of certain elements can beobserved for the two magma types. It may be suggestedfrom both these patterns and inferred sizes of cation sitesfor minerals that the Sub-ALK magma source, which isthe most metasomatized source in terms of Mg/Fe ratios

200

400

600

800

1000

1200

1400

0

0.5

1.0

1.5

2.0

2.5

13

14

15

16

17

18

19

20

Plagioclase phenocryst (vol.%)

Al 2

03 (

wt.

%)

Sr

(pp

m)


Sr/

Ba

2.0

1.8

1.6

1.4

1.2

1.0

Eu

*



0 5 10 15 20 25 30 350 5 10 15 20 25 30 35

0 5 10 15 20 25 30 350 5 10 15 20 25 30 35


Fig. 13. Relationship between the percentage of plagioclase phenocrysts and the ‘plagioclase components’ in the Jeju magmas. Eu* ¼ 2 � (Eu)N/[(Sm)N þ (Gd)N], where (i )N denotes the chondrite-normalized concentration of an element i. No obvious correlation can be seen for theseparameters, suggesting that accumulation of plagioclase phenocrysts played only a minor role in the differentiation of the Jeju magmas.

Table 9: Partition coefficients used for REE modelling

La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu

olivine 0.00006 0.00009 0.00014 0.00022 0.00066 0.00085 0.0012 0.002 0.0031 0.005 0.008 0.012 0.02 0.03

clinopyroxene 0.02 0.04 0.07 0.10 0.15 0.16 0.18 0.18 0.19 0.19 0.19 0.20 0.20 0.20

plagioclase 0.040 0.035 0.030 0.023 0.017 1.000 0.011 0.009 0.008 0.007 0.006 0.005 0.004 0.003

apatite 3.0 4.0 5.5 7.0 8.5 9.0 8.5 8.0 7.0 6.0 5.0 4.2 3.5 3.0

magnetite 0.0002 0.0002 0.0002 0.0002 0.0002 0.0002 0.0002 0.0002 0.0002 0.0002 0.0002 0.0002 0.0002 0.0002

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and Sr–Nd isotopic signatures, is enriched in elementsthat are likely to be partitioned into plagioclase andamphibole, whereas the High-Al ALK source is enrichedin both plagioclase and phlogopite components (Fig. 16).One possible explanation for these observations is thecontribution of these phases to producing the magmasas melted-out phases, not melting residues. However, thisexplanation is unlikely, as plagioclase is likely to reactwith orthopyroxene at the pressures at which the JejuALK magmas were produced (>2�0GPa; Fig. 15).

Alternatively, the characteristic incompatible elementpatterns in Fig. 16 can be understood as the result ofbuffering by residual phases. The High-Al ALK and Sub-ALK magmas are depleted in amphibole and phlogopitecomponents, respectively, suggesting the presence ofthose minerals as melting residues.The geochemical and petrographical characteristics of

the Jeju magma source regions in the upper mantle, andtheir contributions to the generation of three differentmagma series, are schematically illustrated in Fig. 17.

90 85 80 75 70

100×Mg/(Mg+Fe)

0.1

0.2

0.3

0.4

0.5

NiO

(w

t.%)

0

mantlelherzolite

Low-Al ALKCJ-1(88.5, 0.4)

0.1

0.2

0.3

0.4

0.5

NiO

(w

t.%)

0

Low-Al ALKCJ-10(88.0, 0.4)

0.1

0.2

0.3

0.4

0.5

NiO

(w

t.%)

0

Low-Al ALKCJ-34(87.5, 0.4)

0.1

0.2

0.3

0.4

0.5

NiO

(w

t.%)

0

Sub-ALKCJ-37(85.0, 0.4)

0.1

0.2

0.3

0.4

0.5

NiO

(w

t.%)

0

(85.5, 0.4)Sub-ALKCJ-2

90 85 80 75 70

100×Mg/(Mg+Fe)

100×Mg/(Mg+Fe)

100×Mg/(Mg+Fe)

90 85 80 75 70

90 85 80 75 70

Fig. 14. Ni–Mg-number relationships for olivine phenocrysts in the Jeju magmas. Mantle lherzolite values are from Takahashi (1990). Opendiamonds and stars indicate olivine compositions in equilibrium with the bulk rocks and mantle lherzolite, respectively. The number inparentheses indicates the Mg-number and NiO contents in the residual mantle olivine during partial melting of the Jeju primary magmas.

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The uppermost mantle beneath the region has beenvariably metasomatized and exhibits isotopically moreenriched signatures with decreasing depths. Hydrousphases that probably crystallized as a result of the meta-somatism include phlogopite, at rather shallow levelswhere Sub-ALK magmas segregated from the upwellingmantle material, and amphibole, at deeper levels fromwhich the High-AL ALK magmas were derived. Themajor component of the Jeju mantle plume is likely to be

isotopically depleted, unmetasomatized asthenosphericmaterial, which contributed significantly to producingLow-Al ALK magmas.What is the nature of the metasomatic agents that

enrich the subcontinental upper mantle? This is animportant but unsolved problem concerning the evolu-tion of the continents. Hydrous phases such as amphiboleand phlogopite that are inferred as metasomatic mineralsin the magma source region in the upper mantle beneathJeju Island may result from infiltration of either H2O-richfluids or silicate–carbonate melts, because both agentscan readily transport LILE. However, Fe/Mg valuescannot be changed by fluid-dominant metasomatism,because of the fairly low solubility of these elements inan aqueous fluid. The vertical variation in the Mg-num-ber of olivine in the upper mantle, which is inferred fromthe Jeju magma compositions, may, therefore, suggest themelt-dominant metasomatism for the Jeju upper mantle.

CONCLUSION

Although Jeju Island is located along the eastern marginof the Asian continent in the vicinity of the arc–trenchsystem, the Jeju volcanic rocks were produced in associa-tion with intraplate, mantle plume-related magmatism.The major process responsible for differentiation of theJeju magmas was fractional crystallization of mineralphases such as olivine, clinopyroxene, plagioclase, apatiteand magnetite. The systematic difference in both incom-patible element abundances and isotopic compositions

Table 10: Inferred primary magma compositions

Low-Al ALK High-AL ALK Sub-ALK

CJ-10 CJ-01 CJ-34 A-1 A-2 B C CJ-37 CJ-2

SiO2 47.18 47.75 48.09 47.18 47.07 47.75 48.09 50.91 50.97

TiO2 2.24 2.01 1.96 2.24 2.16 2.01 1.96 1.99 1.74

Al2O3 12.95 13.11 13.42 14.95 13.89 15.11 15.42 13.31 13.65

FeO* 11.44 11.36 11.06 9.44 10.99 9.36 9.06 11.09 11.42

MnO 0.15 0.15 0.15 0.15 0.08 0.15 0.15 0.14 0.14

MgO 12.80 12.95 12.42 12.30 12.20 12.45 11.92 10.54 10.74

CaO 8.93 8.85 8.62 8.93 9.23 8.85 8.62 7.88 8.10

Na2O 2.63 2.57 2.84 2.63 2.69 2.57 2.84 2.94 2.64

K2O 1.27 0.93 1.07 1.77 1.18 1.43 1.57 0.89 0.41

P2O5 0.40 0.31 0.37 0.40 0.50 0.31 0.37 0.31 0.21

Total 100.00 100.00 100.00 100.00 99.99 100.00 100.00 100.00 100.00

FeO*/MgO 0.89 0.88 0.89 0.77 0.90 0.75 0.76 1.05 1.06

olivine1 0.88 0.88 0.88 0.90 0.88 0.90 0.90 0.86 0.86

*Total iron as FeO.1Mg-number of olivine in equilibrium with the primary magma.

3.0 GPa

2.5 GPa

2.0 GPa

1.5 GPa1.0 GPa

plagioclase

olivine quartz

High-Al ALKHigh-Al ALK (A-2)Low-Al ALKSub-ALK

Fig. 15. Normative [using the method of Walker et al. (1979)] compo-sitions of the Jeju primary magmas projected onto the plagioclase–olivine–quartz plane from the diopside apex. Compositions of A-2 andother primary magmas are given in Table 10. Partial melt composi-tions obtained in peridotite melting experiments at various pressures(Hirose & Kushiro, 1993) are also shown.

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observed for the Jeju magmas appears to reflect verticalcompositional and mineralogical heterogeneity in themagma source region of the upper mantle: the uppermantle beneath Jeju Island tends to possess moreenriched and metasomatized signatures with decreasingdepth. Processes including upwelling of a rather depletedmantle plume into such a metasomatized and enrichedupper mantle, subsequent interaction and mixingbetween these mantle components, and separation ofmagmas at different depths may reasonably explain thegeochemical characteristics of the Jeju magmas.The presence of mantle plumes with depleted isotopic,

especially Sr–Nd, signatures has been proposed as thecause of intraplate magmatism in NE China (Zhou et al.,1988; Song et al., 1990), as well as Jeju Island. However,the origin and location of such an isotopically depletedmantle component are unknown. Tatsumi & Eggins(1995) speculated that the harzburgitic portion of the

subducting lithosphere, which is the residue after extrac-tion of oceanic crust MORB magmas and hence is likelyto possess very depleted isotopic characteristics, could risefrom the upper–lower-mantle boundary region owing tothe density contrast between harzburgitic slab and fertilelherzolitic mantle material at those depths.A significant difference in the isotopic compositions of

the Jeju and NE Chinese intraplate magmas is the ratherhigh Pb isotopic ratios for the Jeju magmas, suggestingthe contribution of a HIMU-like geochemical reservoirto the Jeju mantle plume. One plausible mechanism forcreating a HIMU reservoir in the deep mantle is theaccumulation of both fresh and dehydrated oceaniccrust (Chauvel et al., 1992; Hauri & Hart, 1993; Brenanet al. 1995; Kogiso et al. 1997a, 1997b; Tatsumi & Kogiso,2003). The location and the origin of such a HIMUreservoir in the Jeju mantle plume system, however, is afuture problem to be addressed.

60 80 100 120 140 160

60 80 100 120 140 160

0.6

0.8

1.0

1.2

1.4

1.6

0.5

1.0

1.5

2.0

2.5

3.0

Rb

K

Na

Sr

Ba

Lu

La

Ti

Zr

Th

High-Al ALK

Sub-ALK

RbK

Na

Ba

Sr

La

LuTi

Zr

Th

1+

1+

2+

2+

3+

3+

4+

4+

Ionic Radius (pm)

Ionic Radius (pm)

Nb-n

orm

alis

ed

Low

-Al A

LK

-norm

alis

ed

Nb-n

orm

alis

ed

Low

-Al A

LK

-norm

alis

ed

PlPhAm

Fig. 16. Trace element characteristics of the Jeju volcanic rocks suggesting the residual mineral phases in the magma source region (see text fordiscussion). Sizes of cation sites for minerals (Matsui et al., 1977) are indicated by arrows. Pl, plagioclase; Ph, phlogopite; Am, amphibole.

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ACKNOWLEDGEMENTS

We thank Takashi Sano, Ken Itoh and In-Seok Son fortheir help in sampling on Jeju Island, Yuka Yonezawaand Bogdan Vaglarov for analytical assistance, MikiFukuda for preparing the manuscript and figures, andRichard Arculus and Monica Handler for constructivecomments on the manuscript.

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The Petrology and Geochemistry of Volcanic Rocks on Jeju Island

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