Geochemistry of Cenozoic basalts in the Fukuoka district (northern Kyushu, Japan): implications for asthenosphere and lithospheric mantle interaction

Chemical Geology 198 (2003) 249 – 268 www.elsevier.com/locate/chemgeo

Geochemistry of Cenozoic basalts in the Fukuoka district (northern Kyushu, Japan): implications for asthenosphere and lithospheric mantle interaction Nguyen Hoang *, Kozo Uto Geological Survey of Japan, National Institute of Advanced Industrial Science and Technology, 1-1-1-Higashi, Tsukuba Central 7th, Tsukuba 305-8567, Japan Received 2 August 2002; accepted 24 December 2002

Abstract Fukuoka volcanic field in northern Kyushu (Japan) is comprised of scattered small, monogenetic volcanoes with ages ranging from 1.1 to 4.4 Ma. A set of samples from the area, together with some from nearby localities, was collected and analyzed for major and trace element abundances and Sr, Nd and Pb isotope compositions. The basalts, unlike lavas from other nearby centers in northern Kyushu, show the highest FeO*, TiO2 and lowest SiO2 characteristics, which are interpreted to reflect high melting temperature and pressure; whereas high Sr, Sm and high-field-strength elements (HFSE) such as Zr and Nb, high light rare earth element (LREE), relatively low Ba, Rb, and broadly oceanic island basalt (OIB)-like primitive mantle normalized incompatible trace element patterns are interpreted to reflect source characteristics. In addition to lead isotopic compositions that are the most radiogenic yet analyzed from northern Kyushu and the Sea of Japan, strontium and neodymium isotopic compositions of Fukuoka lavas free from crustal contamination are among the highest and lowest, respectively (average, 0.7052 and 0.5126), in the region. The samples show the signature of enriched mantle type 2 (EM2), differing from most of the other Dupal anomaly bearing lavas reported from the Sea of Japan and elsewhere in northern Kyushu. The EM2-like characteristics and relatively low concentrations of large ionic lithophile elements (LILE), low LILE/LREE and LREE/highfield-strength elements (HFSE), and mid-ocean ridge basalt (MORB)-like Rb/Sr and Nb/Zr ratios in the Fukuoka lavas are explained by melts from an asthenospheric source that experienced previous melt extraction. Because the chemical characteristics of Fukuoka basalts are strictly, geographically localized, we suggest that, while the mantle beneath most of northern Kyushu is very much similar to that of the Sea of Japan, represented by a spectrum of depleted MORB-EM1 (Dupallike) hybrids, the Fukuoka EM2-rich component may have been added from shallower levels. D 2003 Elsevier Science B.V. All rights reserved. Keywords: Kyushu; Basalt; Isotope; Lithospheric mantle; Dupal anomaly

1. Introduction * Corresponding author. Tel.: +81-298-61-3558; fax: +81-29856-8725. E-mail address: [email protected] (N. Hoang).

Two oceanic plates are subducting beneath the Japanese islands, the Pacific in the northeast and the

0009-2541/03/$ - see front matter D 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0009-2541(03)00031-7

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Philippine in the southwest. Subducting slabs, depending on the direction, dip angles and depths control the spatial distribution of Cenozoic volcanism on the islands (Uyeda and Kanamori, 1979; Uto, 1989; Uto and Tatsumi, 1996). Cenozoic volcanic activity in the southwest of Japan, including northern Kyushu, however, is believed to not directly relate to any of the above subducting slabs but rather associated more with post-opening of the Japan Sea (Uto, 1989; Uto and Tatsumi, 1996). Intraplate basalts in southwestern Japan show many chemically similar characteristics, including relatively low ratios between large ionic lithophile elements and high-field-strength elements (LILE/HFSE) (Uto, 1989; Nakamura et al., 1990; Uto and Tatsumi, 1996), elevated 87Sr/86Sr, low-206Pb/204Pb and high-208Pb/204Pb (Tatsumoto and Nakamura, 1991). The similarity has not changed

significantly over about 12 my eruption period (Uto and Tatsumi, 1996). The lavas, while showing little effect of subduction-related contamination and displaying many chemical features similar to Cenozoic lavas from China, Korea and especially the Japan Sea (Uto, 1989; Nakamura et al., 1990; Uto and Tatsumi, 1996), differ fundamentally from the contemporary subduction-related lavas in the northeast Japan volcanic front (e.g. Uto and Tatsumi, 1996; Ikeda et al., 2001). Despite a relatively large chemical database for the Sea of Japan and southwest Japan lavas, thanks to intensive studies that have been carried out in the recent years, there are still many volcanic centers left unknown. In this study, a set of samples was collected in and around Fukuoka district and analyzed for age, major and trace elements and Sr, Nd and Pb isotope

Fig. 1. Scheme of Cenozoic basalt distribution in and around Fukuoka; sampling sites, sample labels are indicated. Italicized numbers below sample names are radiometric ages from Uto et al. (1993). Samples FUK9315 and FUK9316 are located southwest of Fukuoka and north of large Higashi Matsuura volcanic field (not shown).

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compositions. The data are interpreted in terms of temporal and spatial evolution, melting conditions, possible crustal contamination and mantle isotopic signature in the context of mantle dynamics.

2. Basalts in the Fukuoka district Radiometric ages of Cenozoic intraplate basalts in northern Kyushu are about 9 – 1 Ma (Uto, 1989; Matsumoto et al., 1992; Uto et al., 1993). For the Fukuoka district, age data reveal at least three eruptive episodes as follows: 4.35– 3.4, 2.6 – 2.4 and 1.6 – 1.1 million years ago (Matsumoto et al., 1992; Uto et al., 1993) (Fig. 1). These three episodes of volcanic activity may reflect renewed extension triggered by plate kinematic adjustments following the opening of the Japan Sea (e.g. Seno, 1999). In general, volcanic activity in Fukuoka shows the following features: (1) small, monogenetic volcanoes most likely formed by a single eruption, (2) single volcanoes being within 10 –25 km apart from each other, (3) short-lived (ca. 1 –2 my) active periods with about 1 my of quiescence and (4) chemical homogeneity over a relatively long period (Uto et al., 1993) (Fig. 1). In addition to the samples collected in the Fukuoka district, two samples from Kurose Island, northwest of Fukuoka district in the Japan Sea (KRH-1 and KRH2: 1.13 Ma), and two samples belonging to large Higashi Matsuura volcanic field, west of Fukuoka (FUK9315 and FUK9316: 3.19 Ma), were added with the aim to assess temporal and spatial evolution of the samples. Massive olivine alkali basalts are the dominant rock types in Fukuoka. They are aphyric to moderately phyric, with olivine being the only phenocryst (3– 7%). The phenocrysts represent several generations judging from sizes that range from 3  3 to less than 0.5  0.5 mm and compositions. Olivine is fresh, euhedral to sub-euhedral and sometimes aggregated. The groundmass consists of plagioclase, olivine, clinopyroxene, magnetite and interstitial glass. Magnetite occurs as euhedral micro-phenocrysts up to 0.5  0.5 mm (samples FUK9303, FUK9304, FUK9308 and FUK9313). Two alkali basalts from Kurose Island (KRH-1 and KRH-2) are the only samples bearing mantle xenoliths in the region (Arai et al., 2000; Ikeda et al., 2001).

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3. Analytical procedures All chemical analyses were conducted at the Institute of Geoscience, Geological Survey of Japan (GSJ). Powdered samples used for acquiring major and trace element data were made from fresh parts of whole rock samples that were crushed to < 1cm size and pulverized in agate mills. Major elements were obtained on fused lithium tetra-borate glass disks, and trace elements such as Rb, Sr, Ba, Nb, Zr, Y, Zn, Cu, Ni, Cr and V were obtained on pressed pellets using a Philip PW1404 X-ray Fluorescence spectrometer. GSJ standards (JB-1a and JB-1) were routinely measured as unknown during the measurements. Rare earth elements (REE), Sc, Hf, Ta and Th, were obtained using instrumental neutron activation analysis (INAA) equipped with an automatic sampler and measured using a germanium detector (ORTEC GEM20180) and a multichannel analyzer (SEIKO EG&G 7800-8A2). For the analysis, about 50 mg of powdered sample were sealed in a quartz tube and irradiated at Japan Atomic Energy Research Institute’s reactor with a thermal neutron flux of 8  1013 cm2 s 1 for 40 min. Samples were measured twice in 7 and 30 days after irradiation with integration time of, respectively, 7500 and 15 000 s. During the analysis, JB-1, a GSJ standard, was used as a standard and JB-1a, another standard, was measured as an unknown to verify the accuracy of the analysis. In general, data obtained by INAA are reported in Table 1. The analytical precision and accuracy, together with the accuracy of the XRF method relative to JB-1a standard, are reported in Table 2. Sr, Nd and Pb isotope compositions were also acquired at the Geological Survey of Japan. All the analyzed samples were fresh. Rock chips were crushed to pieces of 1 –2 mm in size and washed ultrasonically in ultrapure water for about 30 min, followed by multiple rinses with the water before being ground in an agate mill. All the acids and water used during chromatographic work were certified TAMA-Pure AA-10 grade (e.g. concentrations of Sr, Pb and Nd are less than 5 pg/ml) and kept under 20 jC condition. About 50 mg of the powder (estimate about 300 ng of lead to be analyzed) was dissolved in concentrated HNO3 and HF (ratio, 1:3), repeated with HNO3 and followed with HCl. Pb and

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Table 1 Chemical compositions of Fukuoka basalts Sample

FUK9303

Age SiO2 TiO2 Al2O3 FeO* MnO MgO CaO Na2O K2O P2O5 Sum Mg-number Ba Rb Sr Zr Y Nb Ni Cr Sc La Ce Nd Sm Eu Tb Yb Lu Hf Ta Th 87 Sr/86Sr 143 Nd/144Nd eNd 206 Pb/204Pb 207 Pb/204Pb 208 Pb/204Pb D8/4Pb D7/4Pb

3.74 46.38 3.07 14.18 13.46 0.20 8.58 9.08 2.84 1.49 0.73 100 53.2 331 27 711 189 27 30 94 213.6 25.5 30.5 78.6 37.1 8.9 2.5 1.4 2.1 0.3 4.8 1.9 2.7 0.705173

Sample

FUK9312

Age SiO2 TiO2 Al2O3 FeO* MnO MgO CaO Na2O

FUK9304 FUK9305 3.89 45.56 3.19 14.09 13.85 0.19 7.88 10.02 3.05 1.39 0.79 100 50.4 314 23 777 196 27 33 72 169.1 26.9 33.5 85.2 61.1 10.0 2.9 2.2 0.3 4.8 1.9 2.5

18.390 15.578 38.528 66.7 9.4

3.39 43.70 3.60 14.69 15.91 0.23 6.79 9.72 3.23

3.98 49.93 2.38 16.02 10.99 0.16 5.79 8.82 3.89 1.23 0.78 100 48.4 307 20 899 199 25 24 25 104.5 30.4 50.3 111.8 60.7 10.0 3.0 1.1 2.5 0.4 6.0 1.6 5.8 0.705290

18.401 15.592 38.569 69.6 10.7 FUK9313 FUK9314 3.52 44.49 3.73 13.57 15.18 0.21 7.20 9.82 3.13

3.51 44.88 3.56 14.18 14.56 0.22 7.28 9.57 3.21

FUK9306

FUK9307

2.49 2.49 48.66 45.23 2.55 3.34 15.67 14.86 11.66 15.04 0.17 0.18 6.07 7.12 8.94 9.18 4.02 3.19 1.32 1.26 0.94 0.58 100 100 48.1 45.8 309 205 24 17 974 1063 198 91 22 18 28 14 38 37 130.2 74.9 28.6 29.9 56.0 24.6 124.9 63.4 77.4 45.2 11.5 8.6 3.2 2.7 1.2 0.9 2.6 1.7 0.3 0.2 6.2 3.5 2.1 1.0 6.4 1.6 0.705213 0.705245 0.512662 0.512624 0.47 0.27 18.426 18.410 15.601 15.611 38.619 38.614 71.4 73.0 11.3 12.4 FUK8602 1.62 51.39 2.43 16.52 9.99 0.12 4.90 8.15 4.07

R64030 1.62 51.15 2.15 17.57 10.48 0.2 4.06 8.07 4.11

FUK9308

FUK9309

FUK9310

FUK9311

2.49 48.00 3.07 14.65 12.47 0.17 6.68 8.74 3.70 1.57 0.93 100 48.9 363 26 943 192 27 28 41 156.6 29.4 56.7 117.2 70.7 11.7 3.5 1.1 2.8 0.4 6.1 2.5 4.2 0.705322 0.512628 0.20 18.333 15.518 38.416 62.5 4.0

2.49 47.72 3.11 14.97 12.23 0.18 6.74 8.91 3.37 1.85 0.92 100 49.6 367 31 979 189 25 27 38 160.5 24.6 40.8 101.6 42.2 10.5 2.7 1.1 2.0 0.2 5.2 1.9 3.7

2.63 47.14 3.05 14.81 13.29 0.18 6.81 9.12 3.31 1.39 0.90 100 47.7 346 21 967 179 22 28 40 139.2 30.4 49.7 115.4 67.5 11.4 3.4 1.2 2.4 0.3 6.0 2.4 4.5 0.705265

2.49 46.56 2.99 14.81 13.26 0.18 6.91 9.28 3.72 1.33 0.95 100 48.2 307 20 942 175 23 28 43 149.6 24.4 38.4 92.4 57.1 9.2 2.8

NOK-1

KRH-1

KRH-2

1.13 47.95 2.74 15.51 10.86 0.17 7.39 8.90 4.66

1.13 48.82 2.59 15.70 10.07 0.16 7.91 8.76 4.48

1.8 0.2 4.5 1.9 3.4

18.413 15.605 38.601 71.3 11.8 FUK9316 3.19 49.55 1.74 14.98 10.19 0.16 8.90 9.47 2.99

FUK9315 3.19 48.41 1.77 15.01 11.45 0.16 8.96 9.36 3.04

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Table 1 (continued ) Sample

FUK9312

FUK9313 FUK9314

FUK8602

K2O P2O5 Sum Mg-number Ba Rb Sr Zr Y Nb Ni Cr Sc La Ce Nd Sm Eu Tb Yb Lu Hf Ta Th 87 Sr/86Sr 143 Nd/144Nd eNd 206 Pb/204Pb 207 Pb/204Pb 208 Pb/204Pb D8/4Pb D7/4Pb

1.07 1.06 100 43.2 205 10 701 199 35 31 22 51.5 27.5 27.2 76.4 40.3 10.9 2.8 1.4 2.4 0.3 5.1 1.7 0.9 0.705118 0.512713 1.46 18.375 15.589 38.514 67.1 10.6

1.43 1.24 100 45.8 341 18 1033 241 35 40 28 136.6 28.7 51.7 124.3 85.5 14.1 3.6

1.60 1.49 0.77 0.75 100 100 46.7 40.9 368 31 1037 206 20 24 21 106.5 20.3 20.7 43.7 47.6 96.8 95.0 43.5 42.7 7.5 7.8 2.2 2.2 1.1 1.3 1.7 0.2 0.3 4.8 3.9 1.3 1.3 5.2 5.2 0.705399 0.705496

2.6 0.4 5.1 2.7 2.9

1.40 1.13 100 47.1 293 18 884 220 30 35 29 143.0 28.5 41.1 108.3 52.7 12.0 3.2 1.4 2.4 0.3 5.6 2.2 2.3 0.705120

18.390 15.567 38.499 63.9 8.3

18.375 15.579 38.525 68.3 9.7

R64030

18.366 15.562 38.491 65.9 8.0

Sr extractions used Sr-spec resin from Eichrom, following the procedure described by Deniel and Pin (2001). For the procedure, samples were loaded in 1.5 ml of 2 M HNO3 into 1-ml pipette tip columns with a resin bed of about 0.05 – 0.07 ml. The samples were rinsed with 1.5 ml of 2 M HNO3 then with 1 ml of cold 7.5 M HNO3. To reduce possible Rb interference and Sr impurity, about 0.2 ml of ultrapure water was added before Sr being collected in 1 ml of 0.05 M HNO3. After Sr, the columns were washed with 0.5 ml of 0.05 M HNO3 followed by 2 ml of 2 M HCl, and Pb was collected in 1.5 ml of 6 M HCl. The Pb samples were dried on a hot plate under a lamp in nitrogen gas flow tank for about 2 h. Solutions after Sr and Pb elution were used for rough extraction of rare earth ele-

NOK-1

KRH-1 1.05 0.77 100 54.8 730

KRH-2

0.85 0.65 100 58.3 706 41 672 216 28 61 143 181 208.1 23.2 21.4 21.4 36.8 38.9 40.6 77.0 75.3 78.9 35.8 40.0 34.3 7.8 7.9 7.7 2.2 2.7 2.4 1.4 1.8 2.4 2.2 0.3 0.3 0.3 3.7 2.9 4.9 1.4 4.2 3.9 4.7 6.0 6.2 0.705142 0.704141 0.704416 0.512843 4.00 18.384 18.356 18.387 15.585 15.600 15.609 38.547 38.667 38.697 69.3 84.7 84.0 10.1 11.9 12.5

FUK9316

FUK9315

1.58 0.43 100 60.9 593 38 477 133 22 33 166 473.5 28.0 29.9 56.0 25.8 5.8 1.7 0.8 2.1 0.3 3.5 1.9 4.7 0.704195 0.512708 1.37 17.915 15.515 38.266 97.9 8.2

1.47 0.36 100.00 58.2 527 36 513 125 24 30 162 489 27.4 29.3 52.3 19.6 5.3 1.8 1.1 2.2 0.3 3.3 1.8 4.4 0.704366

17.776 15.434 38.033 91.5 1.6

ments using conventional AG50W-X8 resin in small quartz columns (resin bed is about 4 mm (i.d.) by 50 mm height), followed by Ln-resin (Eichrom) using 0.2 N HCl as eluant to extract Nd, following the procedure described by Pin and Santos Zalduegui (1997). Nd, Sr and Pb isotope ratios were measured on a multi-collector VG Sector 54 thermal ionization mass spectrometer at GSJ. Sr and Pb isotopes were obtained on single Ta and Re filaments, respectively, while Nd was measured as metal on triple Re filaments. The 87 Sr/86Sr was normalized to 86Sr/88Sr = 0.1194 and the 143 Nd/144Nd was normalized to 146Nd/144Nd = 0.7219. The within-run precision (2r) for 87Sr/86Sr was F 0.000006 to F 0.000009 and F 0.000007 to F 0.000012 for 143Nd/144Nd. During the period of

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Table 2 Analytical results of JB-1a standard

4. Analytical results

XRF

JB-1ameasured F r, n = 6

Ref.a

SiO2 TiO2 Al2O3 FeO* MnO MgO CaO Na2O K2O P2O5 Sum Ba Rb Sr Zr Y Nb Ni Cr

53.31 F 0.65 1.32 F 0.02 14.71 F 0.09 8.68 F 0.43 0.14 F 0.00 7.84 F 0.08 9.42 F 0.09 2.81 F 0.05 1.43 F 0.02 0.27 F 0.01 100 496.1 F 1.89 39.0 F 0.46 439.3 F 2.51 145.3 F 0.50 26.1 F 1.86 26.3 F 1.04 141.0 F 1.80 406.9 F 1.92

53.40 1.30 14.72 8.49 0.15 7.98 9.49 2.78 1.43 0.26 100 504 39.2 442 144 24 26.9 139 392

INAA

F r, n = 3

Ref.a

Sc La Ce Nd Sm Eu Tb Yb Lu Hf Ta Th

28.2 F 0.3 39.5 F 0.4 66.8 F 1.5 23.4 F 3.0 5.04 F 0.1 1.47 F 0.02 0.69 F 0.17 2.27 F 0.05 0.34 F 0.01 3.32 F 0.10 1.92 F 0.11 9.04 F 0.11

27.9 37.6 65.9 26 5.07 1.46 0.69 2.1 0.33 3.41 1.93 9.03

a Values and analytical sources of the standard may be found at http://www.aist.go.jp/RIODB/geostand/igneous.html. Major elements are normalized to 100% volatile-free.

measurement, 87Sr/86Sr of the NBS 987 Sr standard was 0.71025 F 0.00001 (1r, n = 18) and 143Nd/144Nd for the JNdi-1 (GSJ) Nd standard (Tanaka et al., 2000) was 0.512105 F 0.000005 (1r, n = 10). Lead isotopic compositions were corrected for mass fractionation and are reported relative to the NBS 981 Pb standard values of (mean, 1r, n = 16) 36.564 F 0.025, 15.453 F 0.010 and 16.908 F 0.009 for 208Pb/204Pb, 207 Pb/204Pb and 206Pb/204Pb, respectively. Internal precision of the Pb ratios (2r) is less than 0.01%, and total blank is smaller than 50 pg. The data are shown in Table 1.

4.1. Major and trace elements Data for the Fukuoka lavas are reported and compared with data from other northern Kyushu centers (Hoang and Uto, 2003). Major elements from Fukuoka basalts show many features that are not observed in other northern Kyushu basalt centers. For example, they have the highest FeO*, TiO2, lowest SiO2 and relatively low K2O accompanied by moderate to low MgO contents (Table 1, Fig. 2). However, other major elements are within the range of the latter. Except for a basalt with SiO2 of 51.39 (wt.%) and FeO* of 10 (wt.%), olivine and alkali basalts range in SiO2 from ca. 43 to 50 (most less than 47%), FeO* from about 12 to 16 (wt.%) and TiO2 from 2.35 to 3.7 (wt.%) with MgO at 6– 8 (wt.%). Plots of MgO against major element oxides for Fukuoka basalts (Fig. 2) reveal a broadly positive correlation with FeO* and CaO, and a clearly negative correlation with SiO2, Al2O3 and Na2O and little correlation with TiO2 and K2O (not shown), indicating possible olivine fractionation. Two samples from Kurose Island, except those showing higher TiO2 and Na2O and slightly lower FeO*, plot within the fields of other northern Kyushu lavas, including the two samples from Higashi Matsuura, differing from other Fukuoka samples (Fig. 2). Compared with other northern Kyushu lavas (Hoang and Uto, 2003), Fukuoka basalts have the highest Sr (from 700 to 1100 ppm) and Sm (8– 12 ppm) and relatively low Rb and Ba with average values of, respectively, 20 and 280 ppm compared with Rb and Ba in olivine alkali basalts from other nearby centers of 40 –600 ppm, respectively, which are more comparable with reported worldwide alkali basalts (Weaver, 1991; Chauvel et al., 1995; Hofmann, 1997). The basalts show the lowest Ni (most below 50 ppm) and low Cr (mean value of 135 compared with >300 ppm in other northern Kyushu centers) (Fig. 3, Table 1). However, the Fukuoka La (and other light rare earth element [LREE]), Nb (25 ppm average) and Zr (mean value, 190 ppm) are within the range of other northern Kyushu lavas. Relative depletion of Rb and Ba and the elevation of Sr abundances result in low N-MORB-like Ba/Zr and Rb/Sr (mean value of 1.7 and 0.025), and low Ba/ La and Nb/Zr (Fig. 3). High HREE and HFSE

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255

Fig. 2. Plots of wt.% major elements vs. wt.% MgO for samples shown in Fig. 1 (Table 1); data for other northern Kyushu basalt centers are from Hoang and Uto, 2003.

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Fig. 3. Plots of wt.% MgO against trace element abundances and ratios, data from Table 1.

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257

Fig. 3 (continued).

accompanied by relatively high LREE concentration lead to generally higher LREE/HREE, but LREE/ HFSE is slightly lower relative to other northern Kyushu basalts (Fig. 3). MgO contents are correlated negatively with La (and other LREE) and Sr, and positively with Ni, but are scattered with other trace elements such as Ba, Rb, Zr and Sm (Fig. 3). Incompatible element distributions normalized to average primitive mantle (Hofmann, 1988) are broadly oceanic island basalt (OIB)like (Fig. 4). Except the slight depletion of Rb, Th and

Ba, however, the LREE to HREE slope is much gentler than normally observed for typical OIB under garnet control. For instance, average La/Sm in the Fukuoka basalts is 4.1 compared to 5.4 for OIB (Hofmann, 1988). In general, while the behavior of some incompatible trace elements in Fukuoka basalts is different from those of other northern Kyushu basalts and may be used to discriminate from the latter, the concentration of many of the trace elements in basalts from northern Kyushu centers, including those in the Fukuoka district, is in the range that is

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Fig. 4. Incompatible element distribution normalized to primitive mantle for representative basalts from Fukuoka, Higashi Matsuura (filled diamond), Kurose Island (open diamond). Note slight depletion of LILE relative to LREE of the Fukuoka basalts. Also shown is an OIB sample from Hawaii (dashed line, data from Frey et al., 2000) for comparison. Normalizing data from Hofmann (1988).

normally observed for intraplate basalts (Nakamura et al., 1990; Tatsumoto and Nakamura, 1991; Hoang et al., 1996). Note that samples from Higashi Matsuura and Kurose Island show significantly higher Ba, slightly higher Rb, but lower Sr and Sm than the other. In addition, samples from Kurose Island have the highest Nb and Ta (Figs. 3 and 4). High FeO* and TiO2 contents accompanying relatively low MgO may reflect olivine fractionation as illustrated in Fig. 2. For example, the contents of FeO* and MgO in sample FUK9312 are 15.9 and 6.67, respectively, which theoretically is in equilibrium with an olivine of Fo72, assuming K ol/liq d (Fe/Mg) is 0.30 (Fe2O3/FeO = 0.10; Roeder and Emslie, 1970). Thus, Mg-number is too low for a primitive basalt (see Hirose and Kushiro, 1993; Kushiro, 1996). Moreover, the Fukuoka samples have low Cr (average, 135 ppm) and especially Ni abundances (21 –96, but mostly less than 40 ppm) (Fig. 3) too low to be considered as primitive. In spite of broadly positive correlation between MgO and FeO*, the latter does not form a clear trend with FeO*/MgO (Fig. 2), suggesting that olivine may not be the only crystallizing phase. Evidence of clinopyroxene and possibly plagioclase crystallization may be illustrated by positive correlation between MgO and CaO (Fig. 2). In general, the major element variations indicate that olivine (and possibly pyroxene and magnetite) was the primary crystallizing phase in these basalts.

4.2. Strontium, neodymium and lead isotopes Mantle components have been identified from isotopic studies of mid-ocean ridge basalt (MORB) and OIB magmas (Zindler and Hart, 1986), including (1) depleted MORB (DM) mantle, interpreted to represent depleted asthenosphere feeding mid-ocean ridge magmas, (2) enriched mantle (EM1) with relatively low 206 Pb/ 204 Pb and 87 Sr/ 86 Sr, but high 208 Pb/204Pb and 207Pb/204Pb, which may represent ancient Pb-enriched continental crust and (3) a second enriched mantle (EM2) component with high 206 Pb/204Pb, 207Pb/204Pb, 208Pb/204Pb and 87Sr/86Sr, interpreted as Phanerozoic continental crust and/or crust-derived sediments. Northern Kyushu samples lie well above and generally sub-parallel to the Northern Hemisphere Reference Line (NHRL) (Hart, 1984), showing collinear trends between high- (EM2-like) and low-206Pb/204Pb (EM1-like) extremes and being sandwiched between high- and low-208Pb/204Pb Ulreung-Dog and Sea of Japan back-arc basalts, respectively, at the same 206 Pb/204Pb ratios (Fig. 5a –c). Fukuoka lavas show the highest 206Pb/204Pb (18.4 – 18.5) yet analyzed for basalts in the back-arc side of the Japanese islands, and among the highest in 208Pb/204Pb (ca. 38.6) and 207 Pb/204Pb (ca. 15.6) observed from northern Kyushu lavas (Nakamura et al., 1990; Tatsumoto and Nakamura, 1991; Cousens and Allan, 1992) (Fig. 5b– c). The combination of moderately high 208Pb/204Pb but

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Fig. 5. Plots of (a) 207Pb/204Pb and (b) 208Pb/204Pb vs. 206Pb/204Pb and (c) D7/4 Pb vs. D8/4 Pb for the studied samples compared with data fields for Ulreung-Dog islands and the Japan Sea Basin (data from Tatsumoto and Nakamura, 1991; Cousens and Allan, 1992). Northern Hemisphere Reference Line (NHRL) and calculation for D7/4 Pb and D8/4 Pb from Hart (1984), mantle components EM1, EM2 and N-MORB from Zindler and Hart (1986). Data for other northern Kyushu lavas are from Hoang and Uto, 2003.

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Fig. 6. Plots of 87Sr/86Sr vs. 143Nd/144Nd (a), 206Pb/204Pb (b) and (c) D8/4 Pb for Fukuoka and offshore basalts compared with data fields of Ulreung-Dog islands, the Japan Sea Basin (data from Tatsumoto and Nakamura, 1991; Cousens and Allan, 1992) and mantle xenoliths from southwest Japan (data from Ikeda et al., 2001). Mantle isotopic components are from Zindler and Hart (1986). Data for other northern Kyushu lavas are from Hoang and Uto, 2003. Note that in all figures, Fukuoka basalts tend toward EM2 relative to the other. See text for details.

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very high 206Pb/204Pb results in the Fukuoka basalts having the lowest D8/4Pb (a deviation from NHRL and the indicator of Dupal anomaly (Hart, 1984)). Thus, the Fukuoka samples show the most EM2-like characteristics of northern Kyushu lavas (Table 1, Fig. 5c). In addition to high lead isotopic ratios, 87Sr/86Sr isotopes in the Fukuoka basalts are high, ranging from 0.70519 to 0.70532, and are among the most radiogenic in the region. High 87Sr/86Sr accompanied by low 143Nd/144Nd (0.5126 –0.5127) trends toward the EM2 extreme (Fig. 6a). Plots of isotope data from Fukuoka, together with those from the Sea of Japan and northern Kyushu, show a complicated, threecomponent correlation (Fig. 6b – c). Low 87Sr/86Sr, low 208Pb/204Pb and D8/4Pb basalts from the Japan Sea Basin are viewed as the most depleted end member, forming one of the apexes, high 87Sr/86Sr, 208 Pb/ 204 Pb and D8/4Pb EM1-like Ulreung-Dog island lavas define the second, and Fukuoka basalts having the highest 87Sr/86Sr and 208Pb/204Pb and lowest D8/4Pb occupy the third EM2-like extreme. Meanwhile, other northern Kyushu basalts are embedded within the triangle (Figs. 5 and 6). Therefore, any explanation for the isotopic characteristics of the basalts should involve at least three above representative end members. Note that the Kurose (KRH-1,KRH-2: 1.1 Ma) and especially Higashi Matsuura samples (FUK9315– 16: 3.2 Ma) tend more toward EM1-like, differing from the rest of the Fukuoka samples. However, Fukuoka samples FUK8602 and R64030 (1.6 Ma) and FUK9312 – 14 (3.4 Ma) have similar isotopic compositions and plot within the field of other Fukuoka samples. Therefore, there does not appear to be a temporally related change in source.

5. Discussion 5.1. Crustal contamination The strontium isotopic compositions of the Fukuoka samples are high (most >0.705), although many are in the range of those reported for the Sea of Japan and elsewhere in southwest Japan (Kurasawa, 1968; Nohda et al., 1988; Morris and Kagami, 1989; Nakamura et al., 1990; Tatsumoto and Nakamura, 1991),

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but the test for any crustal contamination is essential. The extent of wall rock contamination in continental basalts is controversial and difficult to identify unless chemical compositions of both contaminant and magmatic source are independently known (Carlson and Hart, 1988). In general, the addition of crustal material to basaltic magmas or their source region is expected to result in a positive covariance of 87Sr/86Sr with parameters such as SiO2, Rb/Sr and K2O/P2O5 (Carlson and Hart, 1988), although this relationship may be complicated by assimilation-fractional crystallization and partial melting effects (DePaolo, 1981). Fig. 7a,b shows that 87Sr/86Sr and 206Pb/204Pb change only slightly over the range of MgO, suggesting that assimilation-fractional crystallization (AFC, DePaolo, 1981) process is unlikely to have been responsible for the enrichment of the Fukuoka lavas. Plots of 87Sr/86Sr against Rb/Sr for the Fukuoka samples shown in relation to N-MORB and continental crust compositions (Fig. 8a) show that the samples cluster around a narrow range of Rb/Sr (0.01 –0.04) apart from other northern Kyushu centers. In addition, crustal involvement results in increasing Ba/Zr, Rb/Zr and Sr/Zr relative to Ti/Zr (Hoang and Flower, 1998). Fig. 8b shows Ba/Zr ratios of the Fukuoka samples, which are within the N-MORB range, plot within the mantle array providing further indication that crustal contamination is minimal. 5.2. Mantle signature inferred from major and trace element compositions Cenozoic OIB-like basalts along the Sea of Japan margin in southwest Japan are characterized by low LILE/HFSE and high LREE/HFSE, and are different from those lavas influenced by the subduction-related dehydrated fluids (Uto, 1989; Morris and Kagami, 1989; Uto and Tatsumi, 1996). Experimentally determined compositions of basaltic melts (Hirose and Kushiro, 1993; Baker and Stolper, 1994; Kushiro, 1996) have shown that their SiO2 contents are primarily pressure-dependent and decrease with increasing pressure. Concentrations of FeO also strongly depend on pressure (see Hirose and Kushiro, 1993 and references therein), decreasing with increasing melting pressure and, unlike MgO, decreasing with increasing melt fraction. In addition, FeO contents increase with increasing melt-

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Fig. 7. Plots of wt.% MgO vs. (a) 87Sr/86Sr and (b) 206Pb/204Pb for the studied samples. Note no significant variation of the isotopic compositions observed for Fukuoka samples over a range of MgO. Samples from Kurose Island and Higashi Matsuura plot outside the range of the Fukuoka.

ing temperature (Hirose and Kushiro, 1993). Kogiso et al.’s (1998) experiments showed that melting of peridotite – basalt mixtures tends to produce silicaundersaturated magmas enriched in Fe and Ti (Kogiso et al., 1998). Generally, FeO (TiO2)-rich (OIB-like) basalts in continental settings are commonly interpreted to reflect melting from a fertile asthenospheric source (e.g. Hawkesworth et al., 1988; Gallagher and Hawkesworth, 1992; Turner and Hawkesworth, 1995).

In general, other than high FeO abundances, major element characteristics of Fukuoka basalts, while differing from other northern Kyushu samples, are closely similar to many intraplate alkali basalts and basanites both from oceanic (Hawaiian) and continental (northeast China and elsewhere) settings (e.g. Uto, unpublished data; Tu et al., 1991; Turner and Hawkesworth, 1995), and moreover, regardless of the anomaly in abundance of some trace elements that may not be readily explained by fractionation from each other

Fig. 8. (a) Plots of 87Sr/86Sr vs. Rb/Sr and (b) Ti/Zr vs. Ba/Zr for the studied samples (Fig. 1, Table 1) in relation to N-MORB (Regelous et al., 1999), continental crust (CC) (Taylor and McLennan, 1981), OIB hypothetical distribution line extrapolated based on data from Kogiso et al. (1997) and Frey et al. (2000) and primitive mantle (PM) (Hofmann, 1988). See text for detailed discussion.

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by melting, the incompatible trace element distribution patterns are generally OIB-like (Fig. 4); we propose that major and trace element compositions of Fukuoka alkali basalts are consistent with basaltic melts derived from an asthenospheric source that is fertile to slightly depleted. 5.3. Asthenosphere dynamics Asthenosphere dynamic effects responsible for partial melting may include secondarily induced convection at passive margins (Mutter et al., 1988) or a plume ‘head’ impacting at the base of the lithosphere (McKenzie and Bickle, 1988). When high temperature asthenosphere rises from lower levels by convection, following lithosphere stretching, it will melt partially, and the melting degree will increase because of the pressure decrease. Some molten parts of the asthenospheric mass flow may obtain buoyancy and Rayleigh – Taylor instability that happens because the density of the upwelling mass becomes lower than the overlying mantle (Kerr and Lister, 1988 cf. Mutter et al., 1988). Thus, the small dimension mass flow may start to penetrate the overlying mantle as diapirs. Mantle diapirs should stop when their density is about the same as of the surrounding material or when they are forced to stop at the base of the mantle lithosphere acting as rigid wall. Partial melt will then segregate from the diapirs and pool before erupting to the surface, with or without interaction with the mantle lithosphere and/or crust. The concept of the formation and rise of mantle diapirs appears to explain the volcanic activity in Fukuoka in terms of the small volume and areal distribution of the volcanoes, and possibly the periodicity of eruption, on the one hand, and source homogeneity and the similarity of melting conditions, on the other. However, there are several aspects that need to be accounted for. Is the chemical heterogeneity among Cenozoic basalts in northern Kyushu due to differences in the depth of origin of the mantle diapirs (asthenosphere) or to interaction with the overlying mantle (continental lithospheric mantle) where they stop rising, which may be further complicated by crustal contamination? Asthenosphere is believed to be fertile but depleted in the most incompatible trace elements. It is hot and chemically well mixed due to vigorous convection. In

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contrast, the continental lithospheric mantle (thermal boundary layer) is not involved in convection and is believed to be variably refractory but enriched in incompatible elements (Anderson, 1995). As reported elsewhere, heterogeneity is commonly observed for the continental lithospheric mantle, which records histories of melt addition and removal (e.g. Carlson and Irving, 1994), and ancient and recent metasomatism (e.g. Menzies et al., 1987), and leaves heterogeneous and complexly enriched, diversified EM1-, EM2-rich reservoirs. The Fukuoka lavas, which appear to be free from crustal contamination, have features that are consistent with their derivation from a fertile and variably depleted asthenospheric source. We therefore assume that the EM2-like isotopic enrichment of the Fukuoka basalts is an asthenospheric characteristic of these magmas, assuming that the EM2 component (recycled sediment?) introduced into the asthenosphere lowers the solidus, following the breakdown of hydrous phases that allows decompression melting to commence deeper, and this component happens to be beneath the region for the last several million years. If this is true, and because the enrichment is geographically localized, we need to explain how the supposedly small-sized, deep and enriched source still remains beneath Fukuoka and survives despite the asthenospheric convection over several millions of years. Tatsumoto and Nakamura (1991) observed that 206 Pb/204Pb from southwest Japan alkaline rocks shows a smooth decrease from northeast to southwest Japan and then to inland China. Most northeast China, southwest Japan and Ulreung-Dog island lavas have D8/4Pb values higher than 60, indicating that rocks from the Eurasian margin were derived from a source having high Th/U for a long time. Because within the Japan Sea and southwest Japan there are a number of continental remnants believed to be eastward extensions of the Korean Peninsula, including Yamato Bank, Ulreung and Oki flanks (Ludwig et al., 1975), the regional variation of 206Pb/204Pb may be interpreted to reflect either contamination effects of the EM1-rich (Sino-Korean) cratonic lithospheric remnants (e.g. Ulreung-Dog islands) or heterogeneity of the underlying asthenosphere (e.g. Tatsumoto and Nakamura, 1991; Cousens and Allan, 1992). Nohda et al. (1988) observed a temporal shift in basalts from

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enriched Nd and Sr characters during the preopening stage of back-arc spreading of the Sea of Japan to a depleted signature during the post-opening stage, and interpreted the temporal shifts as a result of decreasing involvement of subcontinental lithosphere as it becomes thinner during back-arc spreading. This model implies that melts of homogeneous MORB-like asthenosphere could be variably enriched depending on the age and type of the penetrated lithosphere. However, the EM1 component affects all Japan-area basalts including those from the Japan Sea Basin (Tatsumoto and Nakamura, 1991; Cousens and Allan, 1992), and thus appears to be an intrinsic component of the asthenospheric source in this area. In contrast, we observe that the isotopic compositions of basalts from different centers (localities) in northern Kyushu plot within distinct fields and show a general correlation between low-206Pb/204Pb, low-87Sr/86Sr (EM1-like) and high- 206Pb/ 204 Pb, high- 87 Sr/ 86 Sr (EM2-like) among the others, regardless of eruption age (Figs. 5– 7; Hoang and Uto, 2003), suggesting that, beside possibly a common (EM1-like) source shared by the centers, there may be a spatial factor that controls the EM2-like chemical diversity. Various peridotites recovered in southwest Japan, including northern Kyushu, reveal a heterogeneous upper mantle in terms of degree of depletion and enrichment (Arai et al., 2000; Ikeda et al., 2001). Isotopic data reported for recovered xenoliths are limited, but the data from Ikeda et al. (2001) show a large range of Sr and Nd isotopic compositions (Fig. 6a), suggesting EM2-like presents in the lithosphere mantle (Ikeda et al., 2001) that controls the local-scale chemical diversity. Therefore, we suggest that the mantle heterogeneity inferred beneath southwest Japan might reflect both the depth of origin of asthenospheric diapirs and/or the effects of their interaction with overlying lithospheric mantle. 5.4. Isotopic mixing model Finally, reports of the existence of the Dupal-like anomaly in East and Southeast Asia are not new (Mukasa et al., 1987; Tatsumoto and Nakamura, 1991; Tu et al., 1991; Hoang et al., 1996). The anomaly (pervasive EM1) defined by Hart (1984) as an enriched component with low 206Pb/204Pb, high 208 Pb/204Pb, 87Sr/86Sr>0.705 and D8/4Pb> + 60 orig-

inally was applied to basalts from the Indian Ocean and was believed to belong to mantle domains in the Southern Hemisphere (Hart, 1984). Several workers have observed the similarity of Dupal-like East Asian and western Pacific (WPAC) asthenosphere to Indian Ocean (I)-MORB (Mahoney et al., 1992) and suggested that it reflects a common mantle reservoir formed by northward flow of the Indian Ocean mantle (Mukasa et al., 1987; Hickey-Vargas et al., 1995; Castillo, 1996). Of endogenous enrichment, Tatsumoto and Nakamura (1991) appealed that it is a distinct reservoir generated by (deep) mantle plumes. In contrast, Tu et al. (1991) and Hoang et al. (1996) followed by Flower et al. (1998) proposed a delaminated Sino-Korean cratonic mantle for the following reasons. Firstly, WPAC thermal (low velocity) anomalies are shallow and not indicative of deep mantle plume (Zhang and Tanimoto, 1993). Secondly, mantle contamination is strongest beneath the Sea of Japan, proximal to the Sino-Korean craton, where volcanic rocks and mantle-derived xenoliths show extreme enrichment in an EM1-like contaminant (Basu et al., 1991; Tatsumoto and Nakamura, 1991), and to lesser extent, Taiwan and Indochina, and concentration gradients inconsistent with either north –south flow or provenance beneath WPAC basins. Thirdly, there are strong indications that Archean lithospheric mantle has been removed from the Sino-Korean craton since the Mesozoic (Griffin et al., 1992; Tatsumoto et al., 1992). Thus, asthenospheric EM1 may have been incorporated by east-flowing asthenosphere associated with Tethyan closure (Hoang et al., 1996; after McKenzie and O’Nions, 1983). In explaining the triangular relationship of isotopic compositions of the Fukuoka basalts with respect to other southwest Japan intraplate lavas, including the Sea of Japan, we adopt the isotopic mixing model reported by Hoang et al. (1996) and Flower et al. (1998). Based on existing geophysical evidence and EM1 concentration gradients relative to the SinoKorean craton, they proposed that the East Asian ‘low velocity component’ (LVC) might be identified assuming that the mantle isotopic variation may be by variable EM1-like enrichment of DM and HIMU (high 238U/204Pb mantle component, Zindler and Hart, 1986) hybrids, and followed by contamination of the asthenosphere (or partial melts) by crust-derived EM2. Using end members defined in terms of Sr

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and Pb elemental contents and isotopic ratios of 87 Sr/86Sr and 206Pb/204Pb from the literature, these were calculated as a basis for understanding LVC mass balances and illustrated by plots of 87Sr/86Sr vs. 206 Pb/204Pb (Fig. 9). Mixing of HIMU and DM (curve A) appears to be a fundamental constraint on global suboceanic mantle (Hart et al., 1992). East Pacific Rise (13 – 23j) N-MORB compositions lie on the HIMU/DM mixing line, and an average of these is taken to be the N-MORB end member for the East Asian WPAC asthenosphere. Curve B illustrates development of the East Asian –WPAC domain by addition of EM1 to the N-MORB (Mukasa et al., 1987; Tu et al., 1991) prior to its contamination by, or mixing with, lithospheric EM2, as presented by curves C1 – C6 (Fig. 9). East Asian– WPAC asthenosphere may thus reflect EM1-enriched N-MORB mantle with, for example, small, subducting slab-derived additions of fluid and sediment melt.

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According to the model, the most depleted samples from the Sea of Japan spreading center that form one of the apexes of the triangle (Figs. 5 and 6) are consistent with addition from 1% to 2% of the defined EM1 component to the N-MORB before EM2 was added. The configuration of other northern Kyushu centers is almost similar to the Japan Sea Basin basalts, which reflects EM2 addition to EM1-rich compositions (e.g. Tatsumoto and Nakamura, 1991), with maximum additions of ca. 2% EM1 and 5% of EM2. Note that, while northern Kyushu and the Japan Sea Basin variation may be consistent with the addition of EM2 to EM1-rich melts, Ulreung-Dog compositions that make the second apex, showing the highest EM1-rich addition, may be explained by reaction with EM1-rich wall rocks as suggested by Cousens and Allan (1992). Meanwhile, Fukuoka compositions, which form the third apex, showing the highest EM2 addition, about 7%, and being differ-

Fig. 9. Plots of 87Sr/86Sr against 206Pb/204Pb for the studied samples compared with data from other northern Kyushu (Hoang and Uto, 2003), data fields of Ulreung-Dog islands, the Sea of Japan spreading center (data source is in Figs. 5 and 6). Hypothetical mixing lines are constructed. Line A: N-MORB (DM/HIMU hybrid) with 87Sr/86Sr = 0.70265 and 206Pb/204Pb = 18.45 (average values of East Pacific Rise MORB, data from Mahoney et al., 1994), Sr = 20 (ppm), Pb = 0.05 (ppm); mixing line B: the N-MORB + EM1 (87Sr/86Sr = 0.707, 206Pb/204Pb = 16.84, Sr = 180 (ppm) and Pb = 17 (ppm)) giving rise to heterogeneous, EM1-rich sub-Asian WPAC asthenosphere; mixing line C1 – C6: N-MORB/EM1 hybrids (increments of 0.1%, 0.5%, 1%, 2%, 5% and 10% EM1) + EM2 (87Sr/86Sr = 0.710 and 206Pb/204Pb = 18.82, Sr = 180 (ppm) and Pb = 17 (ppm)) reflecting variable addition of EM2 to the Asian WPAC asthenospheric melts. EM1 and EM2 elemental and isotopic compositions are modified from Taylor and McLennan (1981) and Zindler and Hart (1986).

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ent from the others, may be explained by reaction with EM2-rich material in the lithospheric mantle.

6. Conclusions (1) Olivine and alkali basalts occurred periodically in three major episodes, at 4.35 – 3.4, 2.6 – 2.4 and 1.6 – 1.1 Ma, forming scattered small, monogenetic volcanoes in the Fukuoka district, one of the centers of northern Kyushu intraplate basalts. The composition of the lavas appears to be relatively homogenous and temporally insensitive and different significantly from center to center, suggesting a spatial factor may be more important. (2) High FeO* and TiO2 and low SiO2 concentrations possibly indicate melting at high temperature and pressure from a fertile asthenospheric source. Primitive mantle normalized trace element patterns are OIB-like; however, relatively low LILE contents, low LILE/HFSE and LREE/HFSE and N-MORB Rb/Sr and Ba/Zr ratios suggest that the source may have experienced previous melt extraction. (3) The high 87Sr/86Sr, low 143Nd/144Nd of the crustal contamination-free Fukuoka basalts, and the facts that they have the highest lead isotope ratios among the northern Kyushu lavas may reflect the addition of EM2-rich material from wall rocks probably in the lithosphere mantle to EM1-rich contaminated asthenospheric melts. The latter is a Dupal-like component believed to be present throughout the East Asian asthenosphere.

Acknowledgements We thank Japan International Science and Technology Exchange Center (JISTEC) for their financial support. T. Kamioka, A. Matsumoto and T. Kani are thanked for assisting in clean laboratory and mass spectrometer work. We are grateful to Richard Carlson (DTM) and Martin Flower (UIC) for patiently reading and commenting on an earlier version that helped improve the manuscript significantly. We thank Yasuo Ikeda and Ryuichi Shinjo for their constructive criticism. Editorial comments by R. Rudnick are acknowledged. [RR]

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