* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
Download Melting of the Uppermost Metasomatized Asthenosphere Triggered
Survey
Document related concepts
History of Earth wikipedia , lookup
Geomorphology wikipedia , lookup
Future of Earth wikipedia , lookup
Supercontinent wikipedia , lookup
Post-glacial rebound wikipedia , lookup
Age of the Earth wikipedia , lookup
Plate tectonics wikipedia , lookup
Mackenzie Large Igneous Province wikipedia , lookup
Baltic Shield wikipedia , lookup
Igneous rock wikipedia , lookup
Tectonic–climatic interaction wikipedia , lookup
Mantle plume wikipedia , lookup
Transcript
JOURNAL OF PETROLOGY VOLUME 55 NUMBER 3 PAGES 499^528 2014 doi:10.1093/petrology/egt074 Melting of the Uppermost Metasomatized Asthenosphere Triggered by Fluid Fluxing from Ancient Subducted Sediment: Constraints from the Quaternary Basalt Lavas at Chugaryeong Volcano, Korea TETSUYA SAKUYAMA1*, SHINJI NAGAOKA2, TAKASHI MIYAZAKI1, QING CHANG1, TOSHIRO TAKAHASHI1, YUKA HIRAHARA1, RYOKO SENDA1, TETSUMARU ITAYA3, JUN’ICHI KIMURA1 AND KAZUHITO OZAWA4 1 INSTITUTE FOR RESEARCH ON EARTH EVOLUTION, JAPAN AGENCY FOR MARINE^EARTH SCIENCE AND TECHNOLOGY, YOKOSUKA, 237-0061, JAPAN 2 DEPARTMENT OF EDUCATION, NAGASAKI UNIVERSITY, NAGASAKI 852-8521, JAPAN 3 RESEARCH INSTITUTE OF NATURAL SCIENCES, OKAYAMA UNIVERSITY OF SCIENCE, OKAYAMA 700-0005, JAPAN 4 DEPARTMENT OF EARTH AND PLANETARY SCIENCE, GRADUATE SCHOOL OF SCIENCE, UNIVERSITY OF TOKYO, TOKYO, 113-0033, JAPAN RECEIVED AUGUST 2, 2012; ACCEPTED NOVEMBER 13, 2013 Major and trace element and Sr^Nd^Pb isotope data for wholerocks and major element data for minerals within basalt samples from the Chugaryeong volcano, an intra-plate back-arc volcanic centre in the central part of the Korean Peninsula, are used to address the process of magma genesis in the deep back-arc region of eastern Asia. There are two lava flow units at Chugaryeong volcano: the Chongok (0·50 Ma) and the Chatan (0·15 Ma) basalts. These basalts have similar MgO (9·1^10·4 wt %) but exhibit differences in their major and trace element and isotope compositions. The Chongok basalt has higher TiO2, Al2O3, Na2O, K2O, P2O5, Cr2O3, large ion lithophile elements (LILE), high field strength elements (HFSE), and rare earth elements (REE), and lower FeO*, SiO2, and CaO than the Chatan basalt. In addition, the Chongok basalt has more radiogenic 143Nd/144Nd and 206 Pb/204Pb, and less radiogenic 87Sr/86Sr and 208Pb/204Pb than the Chatan basalt. Chi-square tests for the major elements indicate that crystal fractionation can explain the chemical variations within each basalt suite; intra-crustal processes, including crystal fractionation and assimilation of continental crust, cannot result in the formation of one basalt suite from the other. The Sr^ Nd^Pb isotopic compositions of the Chongok and Chatan basalts plot on mixing hyperbolae between peridotite mantle xenoliths from the area and a fluid flux derived from a mixture of ancient and recent sediments. The trace element compositions of the estimated primary melts for the two basalt suites suggest different degrees of partial melting of a common enriched mantle source that was metasomatized by a Ba-, K-, Pb-, and Sr-rich fluid. The estimated degree of melting increased with time from 7·5% for the Chongok basalt to 10% for the Chatan basalt. The source mantle for the Chatan basalt is more enriched in Ba and Pb, indicating a greater fluid flux than for the Chongok basalt. This suggests that melting of the source mantle increased with time, sustained by an increased sediment-derived fluid flux from the deeper upper mantle. *Corresponding author. Telephone: þ81-46-867-9785. þ81-46-867-9625. E-mail: [email protected] ß The Author 2014. Published by Oxford University Press. All rights reserved. For Permissions, please e-mail: journals.permissions@ oup.com Fax: JOURNAL OF PETROLOGY VOLUME 55 WORDS: back-arc intraplate volcanism; alkaline basalt; Chugaryeong volcano; Korean Peninsula; Quaternary; fluid flux; subducted sediment; mantle melting KEY I N T RO D U C T I O N Transportation of aqueous fluids from subducting slabs is known to play an important role in the generation of arc volcanism (Gill, 1981; Tatsumi et al., 1983; Stolper & Newman, 1994). Relative enrichment of fluid-mobile elements, such as large ion lithophile elements (LILE) and light rare earth elements (LREE), and depletion in high field strength elements (HFSE) in volcanic rocks from subduction zones has been thought to be the result of a supply of water-rich fluid from the subducting slab. The effects of fluids from subducting oceanic plates have also been identified in many back-arc regions (e.g. Nakamura et al., 1985; McCulloch & Gamble, 1991; Woodhead et al., 1993; Taylor & Martinez, 2003; Tian et al., 2008; Bertotto et al., 2009; Kuritani et al., 2011), even though the relative enrichment of LILE and LREE in comparison with HFSE is minor in the volcanic rocks of these back-arc regions. Back-arc volcanism in the Japan Sea area has been described as the ‘CircumJapan Sea Alkaline Rock Province’ (Tomita, 1935). Because of this volcanism, the eastern margin of the Eurasian Plate was named the ‘hot region’ by Miyashiro (1986). Several mechanisms have been proposed for the back-arc volcanism in this region: hot asthenospheric injection related to large-scale mantle upwelling beneath NE China and Far Eastern Russia (Tatsumi & Eggins,1995); a mantle plume from the upper mantle^lower mantle or core^mantle boundary (Nakamura et al.,1985); mantle convection induced by collision of the Indian Plate (Liu et al., 2004); eastward asthenospheric flow (Flower et al., 2001; Niu, 2005); or convection of a‘Big MantleWedge’ structure above a stagnant slab (Zhao et al., 2007). Although large-scale asthenospheric injection is still an attractive model, recent studies have focused more on the roles of fluids released from a subducted slab of Pacific lithosphere. These have been inspired by the progress of seismic tomography, which has revealed the existence of a stagnant slab extending continuously from the subducting Pacific plate to beneath the deep back-arc volcanoes in eastern Asia (Fukao et al., 1992; Huang & Zhao, 2006). Recently, Kuritani et al. (2011) proposed a model for magma generation induced by hydrous mantle upwelling, which explains the observed variations in the isotopic and trace element compositions of young volcanic rocks in northeastern China. Although they suggested the possibility of decompression melting of the hydrated mantle, detailed models of the melting process in relation to the generation of the volcanism were not presented. Kuritani et al. (2009) investigated the trace element and isotopic composition of the Changbaishan volcano (Fig. 1), which is regarded as a major centre of activity for hydrous NUMBER 3 MARCH 2014 mantle upwelling, to constrain the melting processes in the mantle. However, the relatively differentiated compositions of the Changbaishan volcanic rocks prevented Kuritani et al. from discussing the melting process in detail. The Quaternary Chugaryeong volcano, located in the centre of the Korean Peninsula, is one of a number of alkaline volcanoes at the eastern margin of the Eurasian Plate. Because some basaltic lavas from the Chugaryeong volcano have less differentiated chemical compositions, we can use them to investigate melting processes in the mantle. Here, we present the results of an integrated geochemical and petrological study of two Chugaryeong basalt lava flows and the minerals therein, to provide constraints on the melting process in the back-arc mantle. In the following discussion, we will consider the effects of processes that modify the primary magma compositions at shallow crustal depths, such as crustal contamination and crystal fractionation, followed by an estimation of the primary magma compositions for the Chongok and Chatan basalts. The melting conditions are then estimated following the approach of Sakuyama et al. (2009). Finally, the source mantle characteristics and melting processes involved in the formation of the Chugaryeong basalts are discussed, based on their trace element and Sr^Nd^Pb isotope compositions. Systematic differences between the two basalts allow us to discuss fluid-fluxed melting processes in the mantle induced by the addition of fluid from deeper levels. Contributions from both recent and ancient fluid sources are considered. G E O L O G Y O F C H U G A RY E O N G VO L C A N O The Korean Peninsula is located in the eastern part of the Eurasian Plate. Here, Cenozoic volcanism is sparse, concentrated in four discrete Quaternary volcanic provinces; from north to south these are Changbaishan, Chugaryeong, Ulleung-do, and Jeju (Cheju) (Fig. 1). Strongly alkaline basalts that differ from typical subduction-related magma types occur in these provinces. The Chugaryeong graben on the Korean Peninsula trends NNE^SSW, and is bordered by a set of normal faults (Lee et al., 1983). The Chugaryeong volcano erupted within this fault system, and lava flows filled the graben over a distance of 100 km along the Hantan River (Fig.1b).The main eruptioncentre of the Chugaryeong volcanic province is Mt. Ori, located inthe North Koreanterritory (Fig.1b; Park & Park,1996). Basalt lavas from the Chugaryeong volcano are exposed in a narrow, long plateau and range from10 to 50 m inthickness. The lavas rest on basement rocks composed of Precambrian gneiss andJurassic granite. In contrast to the NNE^SSWdistribution of the lavas confined by the Chugaryeong graben, the alignment of volcanic vents is nearly east^west. Geological information is currently limited to South Korean 500 SAKUYAMA et al. METASOMATIZED ASTHENOSPHERE MELTING Fig. 1. (a) Distribution of Quaternary volcanism on the Korean Peninsula, indicated by black filled areas: Changbaishan, Chugaryeong, Ulleung-do island, and Jeju Island. The Chugaryeong Graben Zone is indicated by parallel dashed lines. (b) Detailed map of the study area of the Chugaryeong basalts. (c) Geological cross-sections. Lines of section are indicated in (b). territory and therefore the complete eruption sequence of the Chugaryeong volcano remains unknown. From the stratigraphic sequence observed inthe available sections, the eruption products can be divided into at least two sub-units, based on their geological, geochronological, and geochemical characteristics. In stratigraphically ascending order, these are the Chongok and Chatan basalts. Both basalt units vary from a few to around 30 m in thickness. Lavas with columnar cooling joints are ubiquitous and clinkers develop above and below the lava flows. Paleosol layers are occasionally developed beneath both the Chongok and the Chatan basalt lava flows. Ryu et al. (2011) reported K^Ar ages for the two basalt lavas and zircon fission-track (reset) ages from a baked paleosol immediately underneath the lavas. Their results showed that the Chongok and the Chatan basalts erupted at 0·50 and 0·15 Ma, respectively (Ryu etal.,2011). A N A LY T I C A L M E T H O D S Major elements Rock samples were sliced into chips using a diamond saw, polished with No. 180 abrasive powder to remove the tracks of the cutter and washed in an ultrasonic bath. The chips were crushed to fragments a few millimetres in diameter using a tungsten carbide mortar. The fragments were then ground in an automatic agate mortar for around 2 h. 501 JOURNAL OF PETROLOGY VOLUME 55 Whole-rock major and trace element contents were determined by X-ray fluorescence (XRF) spectrometry using a Phillips PW-1480 system at the Department of Earth and Planetary Science, University of Tokyo. Fused glass beads for major element analysis were prepared from an aliquot of 0·4 g rock powder mixed with 4·0 g of lithium tetraborate flux. Details of the analytical method have been previously reported by Kushiro (1994). Reproducibility of the XRF data is better than 1%, except for MnO (1·5%), P2O5 (2%), and Na2O (4%). Several samples were discarded from the dataset: those that gave a total of less than 97 wt %, and strongly altered samples, in which olivine was totally replaced by a secondary mineral such as iddingsite. NUMBER 3 MARCH 2014 Pb isotope ratios of leached powder samples were determined by multicollector-inductively coupled plasma-mass spectrometry (MC-ICP-MS; Neptune Thermo Fisher, Bremen). Pb was isolated using AG1-X8 200^400 mesh anion exchange resin. Procedural Pb blanks were 530 pg (Kimura & Nakano, 2004; Miyazaki et al., 2009). The mean 206Pb/204Pb, 207Pb/204Pb, and 208Pb/204Pb ratios of NIST 981 were 16·9309 (0·0013; 2s, n ¼11), 15·4840 (0·0013; 2s, n ¼11), and 36·6749 (0·0038; 2s, n ¼11), respectively. The Pb isotope data presented in this study are re-calibrated using NIST 981 values of 206Pb/204Pb ¼ 16·9416, 207Pb/204Pb ¼15·5000, and 208Pb/204Pb ¼ 36·7262, as reported by Baker et al. (2004). Mineral compositions Trace elements Trace elements (Sc, Co, Ni, Cu, Cs, Tl, Cr, Ga, Zn, and V) were determined by XRF on pressed powder pellets. Concentrations of other trace elements (Rb, Sr, Y, Zr, Nb, Ba, Lu, Hf, Ta, Pb, Th, and U) and rare earth elements (REE) (La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, and Yb) in selected samples (2403, 2404, 2603, 2607, 2608, and 2704) were analysed by inductively coupled plasmamass spectrometry (ICP-MS: Agilent 7500ce). Pulverized whole-rock samples were digested in an HClO4^HF mixture. After drying, the samples were dissolved in HNO3, with a small amount of HF. Indium and bismuth were added to the solution as internal standards (Chang et al., 2003). The precision and reproducibility were generally better than 1% and 5%, respectively. The analysis reference standard (JB-1a) was in good agreement with the reference value. Sr^Nd^Pb isotopes Prior to sample digestion, powder sample splits were leached with 6N HCl at room temperature for 1h, rinsed with Milli-Q water, and then dried prior to Sr^Nd^Pb isotope analysis. The analytical procedure used in the chemical separation has been previously outlined by Kimura & Nakano (2004), Hirahara et al. (2009), Miyazaki et al. (2009), and Takahashi et al. (2009). Sr and Nd isotope ratios were measured by thermal ionization mass spectrometry (TIMS: Triton; Thermo-Finnigan, Bremen) using an instrument equipped with nine Faraday cups in static multi-collection mode. Sr was isolated using Sr resin (Eichrom Industries, IL, USA) (Takahashi et al., 2009). For Nd isotopic analysis, the REE were initially separated by cation exchange before isolating Nd on Ln resin (Eichrom Industries) columns (Hirahara et al., 2009). The mean 87Sr/86Sr and 143Nd/144Nd values in NIST987 and JNdi-1 are 0·710239 (0·000004; 2s, n ¼ 4) and 0·512096 (0·000008; 2s, n ¼ 6), respectively, which are in good agreement with the published values (NIST 987 87Sr/86Sr ¼ 0·710252^0·710256; JNdi-1 143Nd/144Nd ¼ 0·512101) (Weis et al., 2006, 2007). The compositions of olivine phenocrysts and spinel inclusions in olivine phenocrysts were analysed by electron microprobe (JEOL JCMA-733MKII) at the University of Tokyo. The analytical procedures have been given by Nakamura & Kushiro (1970), with a correction procedure by Bence & Albee (1968). Accelerating voltage, beam current, and counting time were 15 kV, 12 nA, and 30 s, respectively. Olivine compositions were determined at 25 kV accelerating voltage with a 20 nA beam current and 30 s counting time with ZAF correction. P E T RO G R A P H Y A N D M I N E R A L COMPOSITIONS The modal proportions of phenocrysts (4150 mm) were determined by point counting (45000 points in each thin section) and are shown in Fig. 2 and Table 1 and Supplementary Data Table A1 (supplementary data are available for downloading at http://www.petrology.oxfordjournals. org). The chemical compositions of olivine phenocrysts and spinel inclusions are listed in Tables 2 and 3, and Fig. 2. Phenocryst modal proportions in representative samples from the Chongok and Chatan basalts. Values in parentheses next to the sample number represent whole-rock MgO concentrations (in wt %). 502 SAKUYAMA et al. METASOMATIZED ASTHENOSPHERE MELTING Table 1: Representative whole-rock major element, trace element, isotope, and modal compositions of the Chugaryeong basalt and analysis by ICP-MS at Japan Agency for Marine^Earth Science and Technology, and recommended value of GSJ basalt (JB-1) standard Unit: Chongok Chongok Chongok Chatan Chatan Chatan Standard Name: 2403 2704 2607 2404 2603 2608 JB-1 1s Reference Major elements (wt %) SiO2 47·79 TiO2 2·07 Al2O3 15·75 Fe2O3 47·62 48·03 48·42 48·21 2·03 1·69 1·91 1·92 15·39 15·74 15·41 15·58 15·58 11·14 11·15 11·1 11·33 11·51 11·21 MnO 0·16 0·16 0·17 0·17 0·17 0·16 MgO 9·05 9·94 9·4 10·32 9·28 9·15 CaO 8·25 8·24 8·15 8·37 8·44 8·57 Na2O 3·12 3·6 3·75 3·19 3·09 2·99 1·86 2 48·23 FeO K2O 2·06 1·96 2·04 1·54 1·68 P2O5 0·45 0·43 0·45 0·34 0·38 0·39 Total 99·85 100·48 100·85 100·77 100·25 100·07 Trace elements (ppm) Rb Ba 23·6 312 24·3 287 24·7 292 17·7 257 18·5 254 21·8 269 36·8 496 0·3 39·2 3 504 Th 3·39 3·2 3·3 2·61 2·72 2·94 9·16 0·02 9·03 U 0·925 0·9 0·877 0·542 0·7 0·811 1·65 0·01 1·57 Ta 1·9 1·81 1·88 1·39 1·62 1·69 1·64 0·004 Nb 30·4 28·9 30 22·7 25·9 26·9 15·9 0·2 26·9 La 24·4 23·1 23·9 19·5 21·3 22·6 37·5 0·3 37·6 Ce 49·9 47·7 49·2 40·1 43·8 46·4 66·7 0·5 65·9 Pb 3·18 2·94 3·13 2·51 2·69 2·95 6·15 0·02 Pr 5·97 5·7 5·86 4·81 5·2 5·51 7·04 0·04 Sr Nd Zr 622 24·6 194 576 23·7 186 627 24·2 190 492 530 20 21·4 157 172 547 22·7 182 435 26·3 136 1·93 6·76 7·3 3 442 0·1 26 0·2 144 Hf 4·4 4·22 4·34 3·61 3·99 4·18 3·62 0·01 3·41 Sm 5·48 5·26 5·38 4·56 4·89 5·12 5·06 0·01 5·07 Eu 1·83 1·77 1·8 1·54 1·65 1·69 1·48 0·01 1·46 Gd 5·57 5·41 5·45 4·69 5·1 5·29 4·73 0·04 4·67 Tb 0·86 0·836 0·847 0·739 0·801 0·824 0·734 0·004 0·69 4·15 0·01 Dy Y 4·88 23·3 4·75 22·7 4·77 22·9 4·27 20·7 4·6 22·2 4·76 22·8 20·8 3·99 0·1 24 Ho 0·939 0·915 0·924 0·832 0·9 0·927 0·810 0·003 0·71 Er 2·62 2·56 2·56 2·32 2·49 2·57 2·31 0·004 2·18 Tm 0·346 0·34 0·338 0·313 0·339 0·346 0·319 0·000 0·33 Yb 2·21 2·17 2·17 2 2·17 2·2 2·08 0·02 2·1 0·32 0·30 0·002 0·33 Lu Sc* Co* 0·322 22 46·1 Ni* 184 Cu* 55 0·312 22·3 48·4 213 52·9 0·315 21·7 47·1 194 53·5 0·29 21·7 52 0·311 22·2 47·9 225 181 51·5 52·8 22·2 46·7 187 53·7 (continued) 503 JOURNAL OF PETROLOGY VOLUME 55 NUMBER 3 MARCH 2014 Table 1: Continued Unit: Chongok Chongok Chongok Chatan Chatan Chatan Standard Name: 2403 2704 2607 2404 2603 2608 JB-1 Cs* 0·16 0·296 0·237 0·135 0·314 Tl* 0·061 0·076 0·074 0·039 0·088 Cr* 240 282 262 307 212 Ga* 19·2 19·6 18·4 18·7 19 Zn* 80·6 81·5 82·1 86·4 86·9 V* 192 192 189 176 1s Reference 0·295 0·076 231 18·4 88·2 160 178 Isotope compositions 87 Sr/86Sr 143 Nd/144Nd Pb/204Pb 0·704451(7) 0·704430(7) 0·704430(7) 0·705076(7) 0·704672(8) 0·70468(7) 0·512663(10) 0·512684(8) 0·512677(8) 0·512570(7) 0·512606(7) 0·512579(7) 206 17·8507(5) 17·8650(6) 17·8462(6) 17·7210(5) 17·7895(5) 207 15·5491(5) 15·5490(5) 15·5475(6) 15·5567(5) 15·5486(5) 15·5457(5) 208 38·5616(16) 38·5568(15) 38·5524(17) 38·5214(14) 38·5100(16) 38·4845(14) Pb/204Pb Pb/204Pb 17·7646(6) Phenocryst mode (vol. %) Ol 2 7·8 6·4 6·2 4·3 Pl 0 0 0 0 0 0 98 92·2 93·6 93·8 95·7 94·8 Gm 5·2 The numbers in parentheses are one standard deviation in terms of the last digit(s). Recommended values are from Imai et al. (1995). *Concentration of those elements were determined by XRF. Table 2: Representative compositions of olivine phenocrysts Unit: Chatan Chatan Chatan Chatan Chatan Chatan Chongok Chongok Chongok Chongok Chongok Chongok Host rock: 2404 2404 2606 2404 2703 2606 2704 2704 2704 2704 2704 2605 Name: OL15 OL17 OL19 OL6 OL19 OL6 OL1C OL24 OL18 OL12 OL3 OL25 Position: core core core core core rim core core core core core rim 39·39 Major elements (wt %) SiO2 39·115 39·195 38·537 38·759 37·546 38·167 38·916 38·759 38·15 37·482 37·167 MnO 0·208 0·19 0·237 0·219 0·285 0·307 0·151 0·206 0·236 0·25 0·315 0·351 NiO 0·318 0·195 0·202 0·21 0·181 0·138 0·338 0·216 0·188 0·184 0·147 0·13 FeO* 13·914 15·047 16·083 17·362 18·648 20·32 10·376 13·847 15·775 17·172 20·306 20·206 39·66 39·983 MgO 46·253 45·467 43·306 43·761 41·145 48·585 45·822 44·078 43·078 40·573 CaO 0·268 0·26 0·253 0·243 0·274 0·382 0·077 0·222 0·243 0·243 0·339 0·427 Cr2O3 0·035 0·076 0·039 0·061 0·025 0·03 0·01 0·042 0·032 0·031 0·038 0·036 Total 100·111 100·43 98·658 100·615 98·105 99·005 98·926 99·272 99·312 99·108 99·2 98·3 Fo# 85·6 84·3 79·7 77·7 89·3 85·5 83·3 81·7 78·1 77·9 82·8 81·8 *Total iron as FeO. Supplementary Data Tables A2 and A3 respectively. It is difficult to distinguish between the Chongok and the Chatan basalts based on petrographic observations. Both basalts are almost aphyric (5 vol. % phenocrysts) and have normally zoned olivine phenocrysts containing spinel (10 mm) and melt inclusions (5 mm); several samples (2407, 2406, 2504, 2601, and 2701) have a trace amount (1vol. %) of plagioclase with strongly corroded cores (Fig. 3) and clinopyroxene microphenocrysts. Clear melt inclusions were not found in either the rims or the 504 SAKUYAMA et al. METASOMATIZED ASTHENOSPHERE MELTING Table 3: Representative compositions of spinel inclusions in olivine phenocryst Unit: Chongok Chongok Chongok Chatan Chatan Chatan Host rock: 2609 2704 2704 2404 2407 2404 Name: SP1 SP4 SP2 SP2 SP1 SP2 Host olivine: OL16 OL17 OL44 OL24 OL2 OL3 Fo#: 87·1 84·2 82·5 84·5 82·1 83·7 Major elements (wt %) SiO2 Al2O3 TiO2 FeO* MnO MgO 0·091 0·053 27·9 40·46 3·218 0·136 36·83 1·221 32·28 22·29 0·293 1·427 23·37 0·171 10·53 14·63 0·191 13·74 0·102 32·92 1·236 22·3 0·229 14·21 0·039 26·84 2·701 24·94 0·231 10·45 0·105 34·3 1·369 21·63 0·177 14·3 CaO 0·025 0·022 0·023 0 0·01 0·01 Na2O 0·003 0·02 0 0·072 0·083 0·073 K2O Cr2O3 0 0 21·17 0·019 21·25 23·37 0 28·06 0·019 24·04 V2O3 0·328 0·126 0·206 0·166 0·417 NiO 0·177 0·256 0·211 0·086 0·105 Total 96·01 100·5 99·53 99·38 89·88 Fe2þ 1·779 1·793 1·847 1·669 1·991 Fe3þ 2·545 1·095 1·225 1·341 1·442 100Cr3þ/(Cr3þ þ 26·7 23·7 26·8 32·4 32·9 0·013 27·33 0·203 0·184 99·71 1·721 1·121 31·6 dusty cores of the plagioclase. There is no systematic correlation between phenocryst mode and bulk chemical composition (Fig. 2). Many samples contain olivine xenocrysts with distinctive kink bands (Fig. 3), although these are rare, amounting at most to a few grains in a single thin section. No non-equilibrium minerals such as quartz or lowmagnesium orthopyroxene were observed. The groundmass of both the Chongok and the Chatan basalts is composed of plagioclase, Ti-augite, olivine, alkali-feldspar, magnetite, and ilmenite. A small amount of calcite is present as an intergranular mineral in sample 2702. Samples from the central part of the lava flows show high crystallinity and an ophitic texture, whereas those from the margin have a porphyritic texture. The chemical compositions of olivine phenocryst cores from the two basalt units are almost equivalent to one another in terms of their Fo# (Fig. 4a), NiO (Fig. 4b), MnO, and CaO contents (Table 2). However, spinel inclusions in the olivine phenocrysts show obvious differences in Cr# between the two basalt groups: spinel in the Chatan basalt has higher Cr# [¼ 100 Cr3þ/ (Cr3þ þ Al3þ)] than that of the Chongok basalt (Fig. 4c). The Fo#^Cr# relationships of the two basalts define subparallel trends away from the olivine^spinel mantle array (OSMA) proposed by Arai (1987), showing slight increase in Cr# with a decrease of host olivine Fo#. Fe3þ þ Al3þ) Unit: Chatan Chatan Chatan Chatan Chatan Chatan Chatan Chatan Host rock: 2404 2407 2404 2404 2606 2404 2407 2606 Name: SP2 SP1 SP2 SP1 SP1 SP1 SP4 SP2 Host olivine: OL24 OL2 OL3 OL6 OL39 OL14 OL10 OL1 Major elements (wt %) SiO2 Al2O3 TiO2 FeO* MnO MgO 0·102 32·92 1·236 22·30 0·229 14·21 0·039 26·84 2·701 24·94 0·231 10·45 0·105 34·30 1·369 21·63 0·177 14·30 0·087 30·05 1·965 26·64 0·242 11·69 0·100 28·59 2·213 27·29 0·287 11·30 0·099 30·87 1·827 27·69 0·285 11·47 2·778 21·66 4·529 32·63 0·296 7·31 0·069 26·58 2·683 28·02 0·272 10·47 CaO 0·000 0·010 0·010 0·035 0·018 0·000 0·068 0·025 Na2O 0·072 0·083 0·073 0·065 0·003 0·080 0·838 0·002 K2O Cr2O3 0·000 28·06 0·019 24·04 0·013 27·33 0·000 28·10 0·017 28·13 0·008 26·96 0·061 26·20 V2O3 0·166 0·417 0·203 0·309 0·311 0·297 0·588 NiO 0·086 0·105 0·184 0·165 0·122 0·115 0·072 Total 99·38 89·88 99·71 99·35 98·38 99·70 97·04 Fe2þ 1·669 1·991 1·721 2·101 2·089 2·163 2·407 Fe3þ 1·341 1·442 1·121 1·464 1·571 1·590 1·625 100Cr3þ/(Cr3þ þ 32·4 32·9 31·6 33·9 34·6 32·1 38·0 0·015 27·32 0·403 0·146 96·00 2·111 1·660 35·1 Fe3þ þ Al3þ) *Total iron as FeO. Fe3þ and Fe2þ are calculated assuming stoichiometry for spinel. W H O L E - RO C K C H E M I C A L COM POSITIONS Representative whole-rock major element compositions of the Chugaryeong basalts are listed in Table 1 and shown in Fig. 5. Both the Chongok and the Chatan basalts are alkali basalts (Miyashiro, 1978) and trachybasalts (Le Bas et al., 1986) (Fig. 5a); however, there are systematic differences between the two basalts. The Chongok basalts (9·1^ 10·0 wt % MgO) are higher in TiO2 (Fig. 5b), Na2O, K2O P2O5, Cr2O3, Rb, Ba, Nb, Sr, and Zr, and lower in FeO*, SiO2 (Fig. 5c), and CaO (Fig. 5d), for a given MgO content, compared with the Chatan basalts (9·2^10·4 wt % MgO). The Chongok basalts are also higher in K2O/TiO2 (Fig. 5e) and Zr/Y, and lower in CaO/Al2O3 (Fig. 5f), Zr/Rb and Zr/Nb at the same MgO contents, compared with the Chatan basalts. The increase in Al2O3 content and the decrease in the CaO/Al2O3 ratio with decreasing MgO of the Chongok basalt is greater than that of the Chatan basalt. Figure 6 shows multi-element incompatible trace element patterns for the Chugaryeong basalt samples normalized to the primitive mantle composition (Fig. 6a) and REE normalized to the CI chondrite composition (Fig. 6b; Sun & McDonough, 1989). The patterns of both the Chongok and the Chatan samples are similar to one another and show characteristics of enrichment typical of 505 JOURNAL OF PETROLOGY VOLUME 55 NUMBER 3 MARCH 2014 Fig. 3. Photomicrograph of olivine phenocrysts (a, b) and xenocryst (c, d) in the Chongok basalt under crossed polars (a, c) and planepolarized light (b, d). Dashed white lines in (c) represent kink bands in an olivine xenocryst. (e) Back-scattered electron image of olivine phenocryst. (f) Photomicrograph of plagioclase phenocryst in a Chongok basalt under crossed polars. ol, olivine; sp, spinel; aug, augite; pl, plagioclase. oceanic island alkali basalts (OIB; e.g. Hofmann, 1997), with positive spikes at Ba, K, Pb, and Sr (Fig. 6a). The Chatan basalts have slightly higher Ba/Th and Pb/U (Fig. 6c) and lower Sm/Yb and Ce/Yb (Fig. 6d) than the Chongok basalts. There are no Eu anomalies, suggesting that fractionation or accumulation of plagioclase does not play an important role in the petrogenesis of these basalts. The Sr^Nd^Pb isotope compositions of the Chugaryeong basalts are shown in Fig. 7 and reported in Table 1. In the 87Sr/86Sr vs 143Nd/144Nd diagram they plot in a 506 SAKUYAMA et al. METASOMATIZED ASTHENOSPHERE MELTING relatively enriched compositional field, near Bulk Earth, (Fig. 7a), and in terms of 7/4 { ¼ 100[(207Pb/204Pb)sample ^ 0·1084(206Pb/204Pb)sample ^ 13·491]; Hart, 1984} and 8/4 {¼ 100[(208Pb/204Pb)sample ^ 1·209(206Pb/204Pb)sample ^ 15·627]; Hart, 1984} they are the highest among Cenozoic basalts in eastern Asia (Fig. 7b and c). There is, however, a systematic difference between the Chongok and the Chatan basalts. The Chatan basalt has slightly more radiogenic 87Sr/86Sr and less radiogenic 143Nd/144Nd, 206 Pb/204Pb, and 208Pb/204Pb (Fig. 7). DISCUSSION Shallow magmatic process in the crust Fig. 4. (a) Forsterite (Fo) content of the cores of olivine phenocrysts plotted against whole-rock Mg# [¼ 100 Mg/(MgO þ FeO*)], (b) NiO content of cores of olivine phenocrysts plotted against Fo content (Takahashi 1986), and (c) relationships between Cr/(Cr þAl þ Fe3þ) atomic ratio of spinel inclusions and the Fo content of the host olivine phenocrysts in the Chongok and Chatan basalts. Lherzolite and harzburgite xenolith data for the Korean Peninsula and Hannuoba are from Choi et al. (2005, 2008). If the geochemical differences between the two Chugaryeong basalts were solely a result of intra-crustal processes, such as magma mixing, crustal assimilation, or crystal fractionation, it would be difficult to isolate the mantle processes that formed the primary magmas. Mixing of different magmas often results in disequilibrium petrographic and compositional characteristics (e.g. Eichelberger, 1975; Sakuyama, 1979). These include (1) the presence of reversely zoned mafic phenocrysts such as olivine and pyroxene and (2) the coexistence of disequilibrium phenocryst assemblages such as olivine and quartz. Phenocrysts in the Chugaryeong basalts always show normal zoning, and there is no disequilibrium phenocryst assemblage, except for the dissolved cores in some plagioclase phenocrysts (Fig. 3f). However, these plagioclases are small (51mm in diameter) and occur very occasionally. The dusty cores are surrounded by overgrowths of clear plagioclase and the internal dusty structure appears to originate from an albite twin, which could have been derived from felsic crustal rocks. Thus we can infer that some of these rare plagioclase crystals could be derived from fragments of the local crustal wall-rock. We therefore conclude that magma mixing is not a significant process contributing to the compositional variability in the Chugaryeong basalts. The compositional relationship between the olivine phenocrysts and their host magma is illustrated in Fig. 4a, to examine the possible effects of crystal accumulation (Iwamori, 1989). The range of forsterite contents that are in equilibrium with the bulk-rock compositions are indicated by the continuous and dashed curves, assuming a certain range for the Fe^Mg partition coefficient (KD ¼ 0·3 0·03) between olivine and melt (Roeder & Emslie, 1970). Whole-rock Mg# [¼ 100 Mg/ (Mg þ Fe2þ) in mole per cent] is calculated by assuming Fe3þ/(Fe3þ þ Fe2þ) ¼ 0·15 in the magma (Kelley & Cottrell, 2009). The most magnesian cores of the olivine phenocrysts in the samples are in equilibrium with the host magma, with the whole-rock chemical composition consistent with Fe^Mg partitioning, suggesting that the effect of olivine accumulation is negligible. 507 JOURNAL OF PETROLOGY VOLUME 55 NUMBER 3 MARCH 2014 Fig. 5. Whole-rock MgO variation diagrams for representative major elements in the Chugaryeong basalts. Major element analyses are recalculated to 100%. Crosses represent data for the Chugaryeong basalts reported by other studies (Won et al., 1990; Park & Park, 1996; Wee, 1996). The dashed line in (a) represents the boundary between alkaline and sub-alkaline basalts after Miyashiro (1978). Rock classification and nomenclature shown by continuous lines in (a) are from Le Bas et al. (1986). 508 SAKUYAMA et al. METASOMATIZED ASTHENOSPHERE MELTING Fig. 6. (a) Primitive mantle normalized patterns of trace elements; (b) chondrite-normalized (Sun & McDonough, 1989) patterns of rare earth elements. Relationships between (c) Ba/Th and Pb/U and (d) Ce/Yb and Sm/Yb are shown for the Chugaryeong basalts. Analytical errors (2s) in all the diagrams are smaller than the symbols. Possibility of crustal assimilation Heat transfer from a hot magma to the surrounding cold crust results in cooling of the magma, which enhances crystal fractionation. At the same time, heat from the magma, including latent heat of crystallization, causes melting of crustal materials (e.g. DePaolo, 1981; Bohrson & Spera, 2003). As noted above, because tiny plagioclase crystals with strongly corroded cores may have originated from crustal materials, the effect of crustal assimilation should be examined. Crustal materials and their partial melts usually have distinct major and trace element and isotopic compositions compared with primary basaltic magmas; thus, assimilation of wall-rock melts or bulk crustal materials can be identified. The Sr and Nd isotope compositions of plausible mafic to silicic crustal materials below the Chugaryeong volcano are 509 JOURNAL OF PETROLOGY VOLUME 55 NUMBER 3 MARCH 2014 Fig. 7. Sr^Nd^Pb isotopic compositions of the Chongok and Chatan basalts. (a) 143Nd/144Nd, vs 87Sr/86Sr; (b) 207Pb/204Pb vs 206Pb/204Pb; (c) 208 Pb/204Pb vs 206Pb/204Pb; (d) 143Nd/144Nd vs 206Pb/204Pb. Peridotite xenoliths exhumed in Korean basalts (Choi et al., 2005, 2008), Changbaishan and Kuandian basalts (Kuritani et al., 2011), and depleted mantle (DM), recent sediment, and inferred ancient sediment compositions (Kuritani et al., 2011) are also plotted. Analytical errors (2s) in all diagrams are smaller than the symbols. shown in Fig. 7a. Because these all have Sr and Nd isotope compositions that are more enriched than the Chugaryeong basalts, the Chatan basalts, which have more radiogenic Sr isotope compositions than the Chongok basalts, should be more affected by crustal contamination if the difference between the two basalts resulted from wall-rock assimilation. Here we test the possibility of an assimilation and fractional crystallization (AFC) model, as described by DePaolo (1981). When the mass ratios of crystallization (Mc) and assimilation (Ma) are not equivalent, but constant (r Ma/ Mc 6¼ 1), the mass-balance equation for an element in the AFC process is described by equation (6a) of DePaolo (1981): r C Cm a z ¼ F þ ð1 F z Þ ð1Þ 0 0 r 1 zCm Cm where rþD1 r1 ð2Þ Mm Mc ¼ 1 þ ðr 1Þ 0 , 0 Mm Mm ð3Þ z¼ F¼ 0 Cm is the original concentration of an element in the magma, Cm is the current concentration of the element in the magma, Ca is the concentration of the element in the assimilant, D is the bulk solid/liquid partition coefficient 0 for the element, Mm is the initial mass of the magma body, Mm is the current mass of the magma body, and Mc is the total mass of material fractionated. The 510 SAKUYAMA et al. METASOMATIZED ASTHENOSPHERE MELTING Fig. 8. Trajectories of AFC model calculation for the Chugaryeong basalts. (a) 143Nd/144Nd vs 87Sr/86Sr; (b) 87Sr/86Sr vs Sr; (c) 87Sr/86Sr vs Nb; (d) 87Sr/86Sr vs Zr. Bold and fine dashed curves are trajectories of the AFC model calculation when 30% and 10% partial melts of mafic crust are used respectively, and bold and fine continuous curves are trajectories of the AFC model calculation when 30% and 10% partial melts of a silicic crust are used respectively. The dotted curve is an AFC trajectory when r ¼ 0·8 and 30% partial melt of a silicic crust are used. The 0 ) is not marked on the fine dashed and continuous curves or the dotted curve. total mass of assimilant (Ma) divided by the initial mass (Mm Partial melts generated by 10% melting of mafic and silicic crust are not plotted in (c) and (d) because they are beyond the range of the figures. parameter Mc/Mm0 represents the degree of crystallization (i.e. the fraction of initial liquid mass converted into solid). We selected mafic and silicic crustal materials reported from Gyeonggi massif in the centre of the Korean Peninsula (Williams et al., 2009) as a proxy for the crustal assimilant. The melting stoichiometries for mafic and silicic crust were taken from Beard & Lofgren (1991) and Beard et al. (1993), respectively. The reaction stoichiometries and partition coefficients between minerals and melt are assumed to be constant during the AFC process.The crystallization mode for basalt is assumed to be olivine, clinopyroxene, and plagioclase in the weight ratio 0·33:0·33:0·34. As will be discussed in the following section, this mode of crystallization may not be appropriate for the Chugaryeong basalts. However, we used the crystallization mode above to evaluate as best as possible the effect of plagioclase crystallization on Sr concentration. Kobayashi & Nakamura (2001) suggested a higher assimilation rate for lower crust than for upper crust because the higher temperature conditions of the lower crust make it possible to reach the solidus temperature more easily than in the upper crust. Assimilation rates for mafic and silicic crust are assumed to be 0·2 and 0·8, respectively. An AFC process can reproduce the differences in Sr^Nd isotope composition and Sr concentration between the Chongok and Chatan basalts (Fig. 8a and b) whereas the 511 JOURNAL OF PETROLOGY VOLUME 55 abundances of other incompatible trace elements cannot be consistently reproduced by an AFC model (Fig. 8c and d). In terms of the Sr^Nd isotope system, the Chatan basalts can be modeled by 2 and 12 wt % assimilation of partial melts of mafic and silicic crustal materials, respectively (7217-5A and 7628-2B; Williams et al., 2009) together with 2·5 wt % and 60 wt % of crystallization, when the assimilation rate r is 0·2 and 0·8 for mafic and silicic crust, respectively, and a degree of partial melting of the crust of 30% (Fig. 8a and b). Because partial melts of crustal materials are poor in Sr and rich in other incompatible trace elements, the melt composition will be more affected by an assimilant when the assimilation rate r is high. As a result, the trajectory of AFC modal curves is steeper in the case of r ¼ 0·8 (dotted curve), compared with r ¼ 0·2 (bold continuous curve) and deviates from the trend of the Chugaryeong basalts (Fig. 8a). When the degree of melting of the crust is small (10%), the AFC trajectories for either mafic or silicic crust do not fit the trend of the Chugaryeong basalts (Fig. 8a). This is because the concentration of Sr in the partial melt is lower at lower extents of melting than at higher extents of melting. As AFC progresses, the Sr concentration decreases in both mafic and silicic crust because Sr is compatible in plagioclase and alkali-feldspar. Although the estimated mass of the assimilant is inconsistent with that estimated for the Sr^Nd isotope system, the trajectory of AFC for silicic crust is able to better reproduce the Sr concentrations than that for mafic crust (Fig. 8b). This is because the abundance of Sr in a partial melt of mafic crust is lower than that in silicic crust. However, assimilation of such isotopically enriched crustal materials cannot consistently reproduce the difference in other trace element contents between the Chongok and Chatan basalts, using the isotope compositions of either mafic or silicic crust. Because a partial melt of crustal material is richer in incompatible trace elements, which are incompatible in plagioclase and alkali-feldspar, than the Chugaryeong basalts, the abundance of these incompatible trace elements in the melt should increase with progress of the AFC process (Fig. 8c and d). The abundance of Sr in the melt would increase with continuing AFC if plagioclase was not considered as a fractional crystallization phase. In this case, all the incompatible trace elements in the melt increase with AFC while the melt develops a more enriched Sr and Nd isotope composition. This is inconsistent with the difference between the Chongok and Chatan basalts, as the isotopically more enriched Chatan basalts are poorer in incompatible trace elements than the Chongok basalts. Bohrson & Spera (2001) pointed out that incompatible trace elements do not necessarily increase monotonically with the progress of AFC. Because fractional melting depletes incompatible trace elements in the residual country rock with an increase in the degree of melting, NUMBER 3 MARCH 2014 the anatectic melt becomes depleted in such incompatible trace elements. Therefore, the concentration of such elements in the contaminated magma can decrease with progressive AFC, despite their incompatibility. However, in this case, a decrease in Sr concentration without a large increase of 87Sr/86Sr becomes difficult to achieve because the concentration of Sr in a partial melt of the crust increases with increasing degree of melting. Assimilation of such a melt will increase 87Sr/86Sr with a decrease or even an increase in Sr concentration. This trend does not fit the trend seen at Chugaryeong; that is, that of a decrease in Sr concentration without a large increase in 87Sr/86Sr (Fig. 8b). All of these lines of evidence indicate that crustal assimilation is not the process that results in the geochemical variation between the Chongok and Chatan basalts. We cannot dismiss the possibility that the Chongok and Chatan basalts originated from two distinct primary melts that underwent AFC independently. However, it is too ad hoc an explanation to invoke AFC of two independent primary melts to explain the systematic difference between the Chongok and Chatan basalts. We therefore conclude that the geochemical variations in the Chugaryeong basalts reflect variation in the primary melts produced in the upper mantle and not assimilation of crustal material. Possibility of assimilation of lithospheric mantle Assimilation of wall-rock may occur not only in the continental crust but also in the mantle during migration of the melt from its source region. Peridotite^melt interaction has been reported from mantle samples in ophiolites (Kelemen, 1990; Zhou et al., 1996), ultramafic intrusive complexes (Quick, 1981), and peridotite xenoliths (Arai & Abe, 1995). Because reactions between primary melts and mantle peridotite at pressures less than the pressure of melting involve reaction between a melt undersaturated in low-Ca pyroxene and peridotite containing low-Ca pyroxene, the interaction of a primary basaltic melt with lherzolite or harzburgite generally increases the SiO2 content in the derivative liquids as interaction progresses (e.g. Kelemen et al., 1990). Because mantle xenoliths from northeastern Asia have depleted Sr^Nd isotopic compositions (Fig. 7a), melts that undergo greater degrees of interaction with the lithospheric mantle are expected to be richer in SiO2 and isotopically more depleted than those with less contribution from the lithospheric mantle. However, the Chongok basalts, which are more depleted in their isotopic composition, are lower in SiO2 content than the Chatan basalts. This geochemical difference between the Chongok and Chatan basalts suggests that assimilation of lithospheric mantle during melt migration does not play an important role in producing the geochemical variations between the two basalts. 512 SAKUYAMA et al. METASOMATIZED ASTHENOSPHERE MELTING Crystal fractionation Crystal fractionation could be a dominant process in the generation of the major element compositions and the variation in the abundances of trace elements in the Chugaryeong basalts, as magma mixing, crystal accumulation, and crustal assimilation have been shown not to play an important role. We examine whether or not the major element compositions in the basalts can be explained by crystal fractionation from a common undifferentiated parent basalt melt on the basis of probability Q, which is the probability in a w2 test and is a quantitative measure for the goodness of fit calculated from the w2 value at its minimum and the number of degrees of freedom in a least-squares optimization (Press et al.,1992). Olivine and clinopyroxene are used as potential fractionated crystal phases, because they are likely to be the major phenocryst minerals in the Chugaryeong basalts. The major elements Si, Al, Fe, Mg, and Ca, which are compatible in these minerals, are investigated. To constrain the liquid line of descent for the Chugaryeong basalts, we calculated groundmass compositions by subtracting the observed phenocryst compositions from the whole-rock compositions (Fig. 5). Parent and daughter magmas are represented by the average compositions of the least fractionated and most fractionated groundmass compositions respectively, as shown in Fig. 5. Starting from the two parental magmas (Cho-1GM and Cha-1GM; Fig. 9), differentiated magmas (Cho-2GM and Cha-2GM) can be derived from the parental magma in the same group with the criterion Q40·05 (Fig. 9 and Table 4). The compositional variations in the Chongok basalts can be explained by 3·2 wt % of olivine and 1·5 wt % of clinopyroxene fractionation, whereas that of the Chatan basalt is explained by 1·8 wt % of olivine fractionation (Table 4). This difference in fractionated phase proportions is consistent with the lower CaO contents in the Chongok basalts than the Chatan basalts, for a given relatively differentiated MgO content of 8 wt % (Fig. 9b Fig. 9. Results of mass-balance calculations for the Chongok and Chatan basalts. Major element variations for these basalts can be reasonably reproduced by a fractional crystallization process. Arrows pointing towards the reduced MgO direction indicate the removal of minerals. 513 JOURNAL OF PETROLOGY VOLUME 55 Table 4: Results of mass-balance calculation and its w2 test for Chongok and Chatan basalt clinopyroxene olivine melt w2 Chatan 0·0 1·9 98·1 2·32 0·80 Chongok 1·4 3·2 95·4 1·12 0·95 Q Q is probability in w2 test and a quantitative measure for the goodness of fit calculated from the w2 value at its minimum and the number of degrees of freedom in least-squares optimization (Press et al., 1992). w2 is a normalized sum of squared deviations between observed and calculated element concentrations. and d), because the fractionation of clinopyroxene effectively removes CaO from the melt. The difference in major elements between the least fractionated Chongok and Chatan basalts (Cho-1GM and Cha-1GM) cannot be derived from the least differentiated samples of the other basalt by any crystal fractionation process. We therefore conclude that the primary magmas of the Chongok and Chatan basalts are different and their differences cannot be generated solely by intra-crustal processes. To check the probability of the mass-balance model above, we also performed MELTS calculations (Ghiorso & Sack, 1995; Asimow & Ghiorso, 1998; Smith & Asimow, 2005) to investigate whether the major element trends for the Chongok and the Chatan basalts can be reproduced by isobaric fractional crystallization starting from the least differentiated compositions of the two basalts (Fig. 10). Water contents used in the initial melts were 0·5, 1, and 2 wt %, and crystallization pressures were set to 0·05 and 0·5 GPa. Isobaric crystallization paths were calculated with incremental decreases in temperature of 2 K under a fayalite^magnetite^quartz (FMQ) oxygen fugacity buffer. Liquid lines of descent for both the Chongok and the Chatan basalts can be reasonably reproduced by fractionation of olivine followed by clinopyroxene (Fig. 10) although there is very little sign of clinopyroxene as a phenocryst phase. As pressure increases, clinopyroxene begins to crystallize and the MgO content of the melt becomes higher; however, the effect of water on the liquid line of descent is larger than that of pressure. As the water content in the melt increases, the liquidus temperature of clinopyroxene decreases, and accordingly the MgO content at which the CaO content and CaO/Al2O3 ratio start decreasing with a decrease in MgO content decreases. For the CaO/Al2O3 of the Chongok basalts to decrease, the water content in the starting melt must be less than 2 wt %. In contrast, 2 wt % of water is required for the starting melt of the Chatan basalt to reproduce its trend, NUMBER 3 MARCH 2014 which maintains a constant CaO/Al2O3 for all MgO contents. Kuritani et al. (2013) estimated the water content of the basalts from the Wudalianchi volcanic field in northeastern China to be more than 1·1wt %, based on thermodynamic calculations, although the upper limit of water contents in the Wudalianchi magmas has not yet been constrained. Our estimation of the water content in the Chugaryeong magma is slightly higher than the lower limit of the water content in the Wudalianchi basalts. Estimation of primary melt compositions Because the stability field of olivine increases as pressure decreases, primitive basaltic magmas normally start to crystallize olivine followed by clinopyroxene or plagioclase with increasing cooling (e.g. Kushiro, 1969). Even the two least differentiated samples (2704 and 2404) from the Chugaryeong basalts have fractionated to a certain extent as shown by their elevated FeO*/MgO 1·0, which is not in equilibrium with the olivine in the potential source mantle peridotite (FeO*/MgO40·7). Therefore, the primary basalt compositions need to be estimated by the addition of olivine. Here, we calculate the primary magma compositions for the Chongok and the Chatan basalts using an olivine maximum fractionation model (e.g. Tatsumi et al., 1983). This involves the stepwise addition of olivine in equilibrium with the melt until the calculated melts reach equilibrium with mantle olivine. The ferric/ferrous iron ratio of the host basalt affects the calculation of olivine composition, so we assumed Fe3þ/(Fe3þ þ Fe2þ) in the melt to be constant at 0·1 (Tatsumi et al., 2005). The Fo# of olivine in the residual peridotite is assumed to be Fo# ¼ 90, the reasons for which are described below. As peridotite becomes depleted owing to melt extraction, the Cr# of spinel becomes elevated (Arai, 1987). In addition, the Cr# in spinel at the transition between lherzolite and harzburgite with increased melting is known to depend on the fertility of the peridotite: the Cr# of the lithological change becomes higher as the peridotite becomes more fertile (Pickering-Witter & Johnston, 2000; Schwab & Johnston, 2001). Mantle xenoliths from eastern Asia show a narrow range of Mg# from 89 to 91·5 (Arai et al., 2007), and the Cr# of spinel in peridotite xenoliths at the transition from lherzolite to harzburgite is 20 (Choi et al., 2005, 2008). This suggests that the peridotite mantle beneath the Korea Peninsula and Hannuoba is moderately depleted in clinopyroxene (Fig. 4c). Provided that the peridotite source mantle of the Chugaryeong basalts follows the local OSMA as indicated by the mantle xenoliths, the Fo# at the intercept of the back-extrapolation of the fractionation trends of spinel inclusions should indicate the Fo# of the residual source peridotite in equilibrium with the primary melt (Fig. 4c). The trends of the Chongok and the Chatan basalts intercept the OSMA at Fo# 90. We therefore use Fo# ¼ 90 for the mantle 514 SAKUYAMA et al. METASOMATIZED ASTHENOSPHERE MELTING Fig. 10. Representative major element compositions plotted versus MgO for calculated groundmass compositions for the Chongok and Chatan basalts and fractional crystallization paths modelled by the MELTS program for the conditions indicated in (c). (See text for details.) olivine composition in equilibrium with the primary melts for both the Chongok and Chatan basalts. The validity of this assumption will be discussed below. The estimated major and trace element compositions of the primary melts, and the amounts of added olivine are given in Table 5. The estimated primary basalt compositions differ from one another, suggesting that two distinct Chongok and Chatan primary magmas existed at 0·5 and 0·15 Ma, respectively. Possibility of mafic source material Mafic materials, such as pyroxenite or eclogite, have been considered as plausible components in the sources of some continental flood basalts and ocean island basalts (e.g. Yasuda et al., 1994; Takahashi et al., 1998; Sobolev et al., 2007). Herzberg & Asimow (2008) proposed a procedure to distinguish between mafic source and ultramafic source basalts based on the CaO and MgO contents of the basalts. Their discrimination diagram is built upon a melting experiment conducted in the high-pressure garnet stability field on fertile peridotite KR4003 (Walter, 1998). When the Chugaryeong basalts are plotted on this discrimination diagram, they fall in the mafic source region, suggesting a contribution of mafic material to the generation of the Chugaryeong basalts. However, partial melts of peridotite KLB1 (Hirose & Kushiro, 1993), PHN1611 (Kushiro, 1996), MPY (Robinson & Wood, 1998; Robinson et al., 1998), FER (Pickering-Witter & Johnston, 2000), and INT (Schwab & Johnston, 2001) at pressures less than 3 GPa can be lower in CaO at a given MgO content than at high pressure and plot within the mafic-source pyroxenite region defined by Herzberg & Asimow (2008). As discussed below, the Chugaryeong primary basalts probably originate in the spinel stability field, suggesting that the 515 JOURNAL OF PETROLOGY VOLUME 55 NUMBER 3 MARCH 2014 Table 5: Estimated primary melt compositions for the Chongok and Chatan basalts Chatan (0·15 Ma) Chongok (0·50 Ma) Amount of olivine added: 18 wt % 18 wt % NiO in Fo90 olivine: 0·31 wt % 0·31 wt % Major elements (wt %) SiO2 47·24 TiO2 1·42 46·81 1·65 Al2O3 12·90 13·01 FeO* 10·42 10·17 MnO 0·16 0·16 MgO 16·55 16·21 CaO 7·01 6·95 Na2O 2·67 2·99 K2O 1·29 1·64 P2O5 0·28 0·35 NiO 0·06 Total 100 Fig. 11. Normative compositions of the estimated primary melts of the Chongok and Chatan basalts projected on the Ne’^Ol’^Qtz’ plane. Arrows on each isobaric curve represent the direction of increase of the degree of melting. The projection scheme is after Irvine & Baragar (1971): Ne’ ¼Ne þ 0·6Ab, Qtz’ ¼Qtz þ 0·4Ab þ 0·25Opx, Ol’ ¼Ol þ 0·75Opx. (See text for details.) 0·05 100 application of the discrimination diagram of Herzberg & Asimow (2008) is not appropriate for constraining their petrogenesis. In addition, the FeO contents of the Chugaryeong basalts are not as high as those of basalts that cannot be derived from partial melting of peridotite (Dasgupta et al., 2007). We therefore consider that peridotite is a reasonable source lithology for the Chugaryeong basalts. Trace elements (ppm) Rb Ba 14·7 208 20·1 255 Th 2·13 2·77 U 0·52 0·75 Ta 1·19 Nb 1·55 19·1 24·8 La 16·1 19·9 Ce 33·2 40·9 Pb Pr Sr Nd Zr 2·11 3·96 483 16·4 130 4·89 574 20·2 159 Hf 3·01 3·62 Sm 3·73 4·51 Eu 1·34 1·58 Gd 3·86 4·60 Tb 0·61 0·71 Dy 3·50 Y 16·9 Melting pressure and temperature of the source mantle 2·60 4·03 19·3 Ho 0·68 0·78 Er 1·89 2·16 Tm 0·26 0·29 Yb 1·63 1·83 Lu 0·24 0·27 Value of Fe3þ/(Fe3þ þ Fe2þ) ratio was assumed as 0·1. *Total iron as FeO calculated after estimation of primary melt. High pressure^temperature melting experiments on peridotites with various degrees of fertility have revealed that the SiO2 contents of the partial melts monotonously increase with decreasing pressure (e.g. Kushiro & Kuno, 1963; Walter, 1998). First, we estimate melting pressure under anhydrous conditions by using the inferred primary basalt compositions and data from partial melting experiments at high pressure conducted by Sakuyama et al. (2009). Melting pressures for the Chongok and Chatan basalts under anhydrous conditions are estimated to be 2·7 and 2·6 GPa (80 km depth), respectively (Fig. 11). In contrast, the addition of water to the melting system is also known to increase SiO2 in partial melts at a given pressure (e.g. Kushiro, 1972; Hirose & Kawamoto, 1995). As shown in Fig. 5a and Table 5, the systematic difference in SiO2 between the Chongok and the Chatan basalts could reflect a difference in melting pressure or in the amount of water in the mantle source. As the effects of water and pressure on the partial melt composition of peridotite are opposed to one another, to examine melting conditions it is necessary to estimate the water contents in the primary basalts. Tatsumi et al. (1983) conducted high- 516 SAKUYAMA et al. METASOMATIZED ASTHENOSPHERE MELTING Fig. 12. Estimated melting pressure and temperature for the Chongok and Chatan basalts. Anhydrous peridotite solidi are shown by continuous and dashed curves (Hirschmann, 2000; Katz et al., 2003). Adiabatic P^T path for solid peridotite (178C GPa^1) for a mantle potential temperature of 14008C is shown by the wide gray line. The lithological boundary between garnet and spinel stability (Robinson & Wood, 1998) and the xenolith-derived geotherm (Xu, 2007) are shown by the fine dot^dash line and bold continuous curve, respectively. The thickness of the lithosphere beneath the Korean Peninsula (An & Shi, 2006; Zhu, 2007) is shown by the double-headed arrow. pressure experiments to find the conditions at which a primary melt is multiply saturated with lherzolite or harzburgite assemblages. They showed that an addition of 1·5 and 3 wt % of water into the melt increases the saturation pressure by 0·2 GPa and 0·5 GPa, respectively. This relationship can be formulated as P wet ¼ P dry þ H2 Oprimary 05 30 where Pwet and Pdry are the melting pressure under hydrous and anhydrous conditions, respectively, and H2Oprimary is the water content in primary melt. We utilize this relationship to estimate melting pressure under hydrous conditions. If we assume 2 wt % of water in the primary melts of both the basalts, the melting pressure is estimated to be 3 GPa. The equivalent depths are slightly deeper than or almost equal to the depth of the base of the lithosphere beneath the Korean Peninsula (An & Shi, 2006; Zhu, 2007). Next, we estimated the melting temperature using the relationship between temperature, pressure, and MgO content in melts saturated with olivine and/or clinopyroxene (Maaloe, 2004). An equation for the depression of the liquidus temperature of olivine-saturated primitive basaltic melt by the effect of water (Medard & Grove, 2008) was used for estimation of the hydrous melting temperature. Using these equations, the melting temperature for the Chugaryeong basalts was estimated to be 14908C under anhydrous conditions and 14208C under hydrous conditions (with 2 wt % of water in the primary melt) (Fig. 12). For comparison, we estimated melting pressures and temperatures using the primary melts with 2 wt % of water estimated in this study and the thermobarometer of Lee et al. (2009). Pressures and temperatures of melting were estimated to be 2·67 GPa and 14168C for the Chongok basalt and 2·52 GPa and 14198C for the Chatan basalt. Pressures estimated by the Lee et al. (2009) model are also within the spinel peridotite field but 0·4 GPa lower than our estimation, whereas temperatures show good agreement with our calculations. Our estimates of the pressure^temperature conditions of magma generation for the Chugaryeong basalts are restricted (2·5^3·0 GPa and 1420^14908C), even when we consider a variation of water content (2 wt %) in the primary magmas. The estimated pressures correspond to the depth of the uppermost asthenospheric mantle within the spinel stability field. The absence of garnet peridotite xenoliths from Cenozoic basalts in the Korean Peninsula (Arai et al., 2007) is also consistent with melting of asthenospheric spinel peridotite. The elevated concentrations of fluid-mobile elements in the Chatan basalts may reflect an increased fluid flux in these basalts (Fig. 6c). If these elements originated in a fluid flux, then the degree of melting of the source mantle might have increased progressively as a result of the additional fluid input in the mantle source at constant pressure. This progressive melting model is consistent with the estimated spinel Cr# in the source mantle. The residue of the older Chongok source would have had a lower Cr# than that of the younger Chatan source, suggesting a more depleted residual source mantle for the Chatan basalt (Fig. 4c). Source mantle compositions of the Chongok and the Chatan basalts We have attributed the systematic differences in whole-rock and mineral major element compositions between the Chongok and the Chatan basalts to differences in melting conditions in the mantle. Here we test the origin of these systematic differences in whole-rock trace element compositions between the two basalts. The trace element compositions of partial melts of peridotite depend mainly on the residual mineral assemblage, the melting mode, and the partition coefficient between minerals and melts. The pressure-dependent garnet^spinel transition is one of the most important phase changes in the upper mantle (Robinson & Wood, 1998), and can drastically change the chemical composition of the partial melts. Mineral^melt partition coefficients for the heavy rare earth elements (HREE) are greatly controlled by the 517 JOURNAL OF PETROLOGY VOLUME 55 presence of residual garnet, because these partition coefficients are greater than unity in contrast to those for spinel for which the HREE partition coefficients are extremely low (e.g. Green, 1994). Hence, the abundance of HREE in the partial melts increases with increasing melt fraction in the garnet field, whereas it decreases in the spinel field. If both the Chongok and Chatan basalts were generated by differing degrees of partial melting of a common source mantle in the garnet stability field, the Chongok basalts should have been generated by a lower degree of melting than the Chatan basalts, because the LREE/ HREE and middle REE (MREE)/HREE ratios of the Chongok basalts are higher than those of the Chatan basalts (Fig. 6d). Concurrently, in the case of garnet peridotite melting, the abundance of HREE in the Chongok primary basalt should be lower than that of the Chatan primary basalt, whereas the abundances of LREE and MREE of the Chongok basalt should be higher than those of the NUMBER 3 MARCH 2014 Chatan basalt (Fig. 13a). In other words, the REE patterns of the primary melt of Chongok basalt cross over those of the Chatan basalt at an element with Dsolid/melt 41 (Fig. 13a). The abundance of trace elements in the estimated primary melts depends on amount of olivine added to the least differentiated samples; the amount of olivine addition is constrained by the assumed Fo# of residual olivine in equilibrium with the primary melt. If the amount of olivine added to the least differentiated basalts was almost identical in the Chongok and Chatan basalts, the abundance of all trace elements in the Chongok basalt would be higher than that of the Chatan basalt (Fig. 13a). For the trace element patterns of the Chongok and Chatan primary basalts to cross over one another at an element with Dsolid/melt 41, a further 8 wt % addition of olivine to the Chongok basalt is required compared with the Chatan basalt. Thus, the Fo# of olivine in equilibrium with the Fig. 13. (a) Schematic chondrite-normalized REE patterns of the least differentiated Chongok and Chatan basalts and estimated primary melts of the two basalts. (b) Relative abundance of La, Dy, and Yb divided by the estimated primary melt composition of the Chatan basalt in equilibrium with olivine with Fo# ¼ 90 plotted against the assumed Fo# of olivine in equilibrium with the Chongok primary melt. (c) Averaged plausible primary melt compositions for the Chongok and Chatan basalts, estimated source mantle compositions for the Chongok and Chatan basalts, respectively, and range of mantle xenolith compositions (gray fields) reported from the Korean Peninsula (Choi et al., 2005) and eastern China (Rudnick et al., 2004) normalized to primitive mantle (Sun & McDonough, 1989). (d) Difference of estimated source mantle compositions between the Chatan and Chongok basalts normalized to the averaged value of the estimated source compositions. Open diamonds (Ta, Nb, Nd, Zr, Hf, Sm, Eu, Gd, Tb, Dy, Y, Ho, Er, Yb, and Lu) indicate those elements that are used for inversion calculation. Error bars are calculated on the basis of 1s of the estimated primary melts for the Chongok and Chatan basalts shown in (a). (continued) 518 SAKUYAMA et al. METASOMATIZED ASTHENOSPHERE MELTING Fig. 13. Continued. primary melt of Chongok is required to be greater than that of the Chatan basalt (Fig. 13a and b). According to high-pressure melting experiments on peridotite, an increase by 10% in the degree of melting will increase olivine Fo# by approximately one unit (e.g. Kushiro, 1996). This positive relationship between the Fo# of olivine and the degree of melting suggests that a residual mantle source with a higher Fo# of olivine should have experienced a higher degree of melting than one with lower Fo#. Thus, a basalt in equilibrium with Fo# ¼ 91 olivine would be generated by a higher degree of melting of peridotite than a basalt with Fo# ¼ 90. This contradicts the inference from the trace element ratios of the two basalts as mentioned above: the Chongok basalt should have been generated by a lower degree of partial melting than the Chatan basalt. We therefore argue that partial melting of peridotite in the spinel stability field is more appropriate to reproduce the geochemical variation in the Chongok and Chatan basalts than melting within the garnet stability field, and that the assumption of Fo# ¼ 90 is reasonable for both the major and trace element compositions of the two basalts to be consistent with one another, provided that the difference in trace element compositions between the two basalts originated from different degrees of melting of common source mantle. Because the estimated pressures of the Chongok and Chatan primary magmas (2·7 GPa; see above discussion) are lower than the pressures of the garnet stability field (43 GPa; Robinson & Wood, 1998), we argue for partial melting of spinel lherzolite in the following discussion. 519 JOURNAL OF PETROLOGY VOLUME 55 Figure 13c shows estimated trace element compositions of the primary basalts for the Chongok and Chatan units (see also Table 5 and discussions above). The LREE/ MREE and LREE/HREE ratios of the primary Chongok basalt are slightly higher than those for Chatan, and the trace element abundances of the Chongok basalt are also higher than those of Chatan. The difference in the trace element compositions between the Chongok and the Chatan primary basalts could, therefore, reasonably be explained by minor differences in the degree of melting of a common mantle source in the spinel stability field. The Chongok basalt must have been generated by a lower degree of partial melting. This is also supported by the fractionation trends in the Cr#^Fo# plots as noted above (Fig. 4c). Once we assume a common original source mantle for the two basalts, both the extent of melting and the source mantle composition can be estimated by a series of inversion calculations using the two primary basalt compositions. Fluid-mobile elements, such as Ba, Pb, and Sr, show clear positive spikes in comparison with neighbouring elements in Primitive Mantle normalized multi-element diagrams (Fig. 13c), suggesting the involvement of fluid components probably derived from dehydration of the stagnant slab sediments lying beneath northeastern Asia (Kuritani et al., 2011). If this is the case, it is not appropriate to discuss these trace elements in relation to the extent of melting. We therefore first investigated 15 fluidimmobile elements (Ta, Nb, Nd, Zr, Hf, Sm, Eu, Gd, Tb, Dy, Y, Ho, Er, Yb, and Lu) representing the HFSE, MREE and HREE to constrain the degree of melting. We estimate the extent of melting for the Chongok and Chatan basalts and the common source mantle composition by adopting a non-modal, batch-melting model (Shaw, 2000). Then, we estimate the concentration of fluid-mobile elements, such as Rb, Ba, K, Pb, and Sr, in the source mantle using the extent of melting estimated based on fluid-immobile elements. We then evaluate the contribution of the fluid component to the source mantle by comparing concentrations of fluid-mobile elements with those of the neighbouring fluid-immobile elements in a multi-element plot. The initial bulk and melting mode for spinel lherzolite are assumed to be clinopyroxene, olivine, orthopyroxene, and spinel in the weight ratios 0·15:0·58:0·25:0·02 (Hirose & Kushiro, 1993) and 0·82:^ 0·30:0·40:0·08 (Kinzler & Grove, 1992), respectively. Variable parameters are the extent of partial melting and the source mantle composition. The inversion calculations involved (1) varying both the extent of melting and the composition of the source mantle for the Chongok and the Chatan primary basalts separately, while (2) simultaneously minimizing the differences between the estimated source mantle compositions from the two estimates, and (3) eventually deriving the minimum conversions in the NUMBER 3 MARCH 2014 errors of the common mantle composition. We used averaged values of the estimated primary melt compositions for the Chongok and the Chatan basalts, taking the standard deviations of samples from each group as the errors (Table 5 and Fig. 13c). Estimated values for the degrees of melting for the Chongok and the Chatan sources are 7·5% and 10·0%, respectively. The best-fitting source mantle composition is enriched in LILE and LREE, depleted in HREE, and characterized by positive spikes in Ba, K, Pb, and Sr (Fig. 13c and Table 6). This LILE^LREE-enriched trace element pattern of the source mantle for the Chugaryeong Table 6: Partition coefficients and chemical compositions (ppm) of estimated source mantle compositions for the Chugaryeong basalt Ol Opx Cpx Sp Estimated source composition Chongok 1·519 Chatan Rb 0·000179 0·0006 0·00175 0 Ba 0·000032 0·0035 0·0006 0 Th 0·000052 0·013 0·00531 0 0·219 0·221 U 0·000018 0·0017 0·00361 0 0·057 0·052 Ta 0·00007 0·003 0·0102 0·02 0·123 0·124 Nb 0·0001 0·003 0·0077 0·02 1·958 K 0·000177 0·0003 0·0072 0 La 0·000028 0·0025 0·0536 0·01 1·627 1·703 Ce 0·000038 0·005 0·0858 0·01 3·514 3·621 Pb 0·000479 0·0013 0·072 0 0·200 0·214 Pr 0·0008 0·0048 0·15 0·01 0·456 Sr 0·0015 0·007 0·1283 0 Nd 0·00042 0·0068 0·1873 0·01 Zr 0·007 0·021 0·123 0·02 Hf 0·0038 0·01 0·256 0·02 0·391 Sm 0·0013 0·01 0·291 0·008 0·497 0·482 Eu 0·0016 0·013 0·31 0·007 0·178 0·176 Gd 0·0055 0·016 0·3 0·006 0·528 0·516 Tb 0·0041 0·019 0·31 0·009 0·083 0·082 Dy 0·01 0·022 0·33 0·01 0·493 0·491 Y 0·007 0·06 0·421 0·0023 2·719 2·643 Ho 0·007 0·026 0·31 0·009 0·093 0·094 Er 0·0087 0·03 0·29 0·01 0·258 0·261 Tm 0·009 0·12 0·255 0·01 0·040 0·041 Yb 0·017 0·049 0·28 0·008 0·234 0·239 Lu 0·02 0·06 0·28 0·02 0·035 0·036 19·38 1066 52·58 1·976 15·57 1·473 21·03 1·978 1084 0·456 54·98 1·944 15·65 0·385 Ol, olivine; Opx, orthopyroxene; Cpx, clinopyroxene; Sp, spinel. 520 SAKUYAMA et al. METASOMATIZED ASTHENOSPHERE MELTING basalts is more similar to that of peridotite mantle xenoliths from Hannuoba, in eastern China (Rudnick et al., 2004), than to that of xenoliths from South Korea (Choi et al., 2005). The prominent positive spikes in fluid-mobile elements, such as Rb, Ba, K, Pb, and Sr, cannot, however, be reproduced even by melting of Hannuoba-type mantle. This strongly suggests the influence of a fluid flux, in addition to an incompatible trace element-enriched source mantle, in the generation of the Chugaryeong basalts. There are systematic differences between the Chongok and the Chatan source mantle. Figure 13d shows the differences in the estimated source mantle compositions for Chongok and Chatan, including fluid-mobile elements. It should be noted that we did not apply any constraints to Rb, Ba, Th, U, K, La, Ce, Pb, and Sr for the inversion calculations. The Ba, La, Pb, and Sr contents in the estimated source mantle of the Chatan basalt are slightly higher than for the Chongok basalt. In particular, Ba and Pb concentrations in the source mantle of the Chatan basalt are clearly higher than for the Chongok basalt (Fig. 13d). It is difficult to invoke the lithospheric mantle as a direct source for the Chugaryeong basalts because of the discrepancy between the estimated melting temperature and the lithospheric geotherm. From petrological studies of mantle xenoliths, the estimated geotherm beneath eastern Asia (Xu, 2007) is much lower than the necessary melting conditions of the Chugaryeong basalts (Fig. 12), suggesting that the Chugaryeong basalt originated in the asthenospheric mantle. Rudnick et al. (2004) proposed that an injection of incompatible trace element-rich silicate melts into the lithosphere beneath eastern China during the Tertiary produced the enriched mantle xenolith suite in that region. Although the origin of this Tertiary metasomatic event is beyond the scope of this study, it could have been responsible for the production of an uppermost part of the asthenospheric mantle with enriched trace element characteristics, similar to the Hannuoba-type lithospheric mantle beneath the Korean Peninsula. Here, we assume the source mantle of the Chugaryeong basalts to be asthenospheric mantle with a LILE^LREE-enriched trace element chemical composition. Origin of the fluid flux Fluids in subducted oceanic plates are released and recycled to the surface via subduction-related volcanism (Hacker, 2008; Kimura et al., 2010; van Keken et al., 2011), although a small amount of water is thought to be transported to the deep mantle, captured in nominally anhydrous minerals (e.g. Iwamori, 2007) and hydrous minerals (e.g. Ono, 1998). Kuritani et al. (2011) suggested that the fluids from subducted sediments on the oceanic plate play an important role in influencing the geochemical characteristics of the basalts in northeastern China. They attributed the selective enrichment in Ba, Pb, and Sr observed in northeastern Chinese basalts to the breakdown of K-hollandite held in the hydrous sediment layer of the subducted slab, because those elements are known to be highly partitioned into K-hollandite, which can be broken down at the depth of the mantle Transition Zone under hydrous conditions (Rapp et al., 2008). The general geochemical features of the source mantle of the Chugaryeong basalts, such as positive spikes in Rb, Ba, K, Pb, and Sr, are similar to those proposed for the Changbaishan volcano and can be attributed to the breakdown of a phase such as K-hollandite, as suggested by Kuritani et al. (2011). Rapp et al. (2008) reportedthat partition coefficients of Sr, Pb, and Ba into K-hollandite are higher under hydrous conditions than under dry conditions. In particular, Ba and Pb are the most strongly partitioned into Khollandite under hydrous conditions (Rapp et al., 2008). In turn, the compatibility of U, Th, and Rb is lower under hydrous conditions than under dry conditions. U in particular becomes incompatible in K-hollandite under water-saturated conditions, probably because the higher water content causes a higher oxidation state, hence making U occur as U6þ, which is incompatible in K-hollandite and compatible in an aqueous fluid (Rapp et al., 2008). These experimental results suggest that a fluid released by the breakdown of Khollandite under hydrous conditions would be enriched in Ba and Pb, moderately enriched in Sr and K, but not enriched in Th or U. The estimated source mantle for the Chugaryeong basalts shows positive anomalies in Ba, K, Pb, and Sr, but no significant anomalies in Rb and U, and Th (Fig.13c).The trace element characteristics of this estimated source mantle suggest that K-hollandite, which can supply fluids to the upper mantle by its breakdown, was formed under hydrous conditions rather than under dry conditions. In addition, the inference of higher Ba and Pb contents in the source mantle for the Chatan basalt than for the Chongok basalt is consistent with the idea that a larger amount of fluid, released by the breakdown of K-hollandite under hydrous conditions, was supplied to the source mantle of the Chatan basalt. We therefore suggest that the positive spikes in fluidmobile elements and the systematic differences observed between the Chatan and the Chongok mantle sources are readily attributable to fluids derived from deep breakdown of K-hollandite under hydrous conditions. If this is the case, the melting sequence of the source mantle is as follows: (1) the asthenospheric source mantle beneath the Chugaryeong area was already rich in incompatible trace elements; (2) the first partial melting occurred by the addition of a deep fluid flux from the stagnant Pacific Plate, owing to hydrous breakdown of K-hollandite in the sediment, generating the Chongok basalts at 0·50 Ma; (3) further upwelling of mantle that was more hydrated by fluid from the sediment yielded partial melt at a higher degree of melting to form the Chatan basalts at 0·15 Ma (Fig. 14a). 521 JOURNAL OF PETROLOGY VOLUME 55 NUMBER 3 MARCH 2014 Fig. 14. (a) Schematic illustrations showing the melting process involved in the production of the Chongok and Chatan basalts. At 0·50 Ma, decompression melting of the upwelling mantle hydrated by fluid released from subducted sediment occurred to generate the Chongok basalt (1), then further upwelling of the mantle yielded a partial melt to produce the Chatan basalt at 0·15 Ma (2). (b) Schematic diagram illustrating the inferred upper mantle conditions beneath the Korean Peninsula and northeastern China. Source mantle characteristics and origin The upper mantle beneath northeastern China and the Korean Peninsula has been recognized to have a strong affinity to Enriched Mantle 1 (EM1) (Zou et al., 2000; Flower et al., 2001; Choi et al., 2005), although the origin of the enrichment has not yet been identified. The classical view was that of isotopic ingrowth at the base of the continental mantle lithosphere over 43 Gyr (Tatsumoto & 522 SAKUYAMA et al. METASOMATIZED ASTHENOSPHERE MELTING Nakamura, 1991). However, this model is negated by the evidence of erosion of the continental lithosphere beneath this area in the late Cenozoic (Gao et al., 2004). Recently, Kuritani et al. (2011) proposed a model that attributed the EM1 component to an ancient fluid derived from subducted sediment 1·5 Gyr ago, which was subsequently trapped in the upper mantle Transition Zone, on the basis of a correlation between the spatial distribution of basalt geochemistry and a plume-like low-velocity zone rising from the Transition Zone across the upper mantle identified based on seismic tomography data. They showed a concentric spatial variation in basalt geochemistry, centred on the Changbaishan volcano. With increasing distance from Changbaishan, the basalts show a decrease in Ba/Th, Pb/U, and EM1 affinity in Pb isotopes. The Chugaryeong volcano is located in the middle of the Korean Peninsula, which is 450 km away from Changbaishan (Fig. 1a), so the contribution of the EM1 component or ancient fluid here is expected to be less than in the Changbaishan basalts if the model of Kuritani et al. (2011) can be extended to the middle of the Korean Peninsula. The Chugaryeong basalts, however, have more radiogenic Pb isotope compositions than the Changbaishan basalts (Fig. 7), whereas the Ba/Th of the Chugaryeong basalts is much lower than that of Changbaishan. Kuritani et al. (2011) assumed that the slab sediment flux has two components, one an ancient sediment flux [EM1: modelled by isotopic ingrowth in ancient sediment based on Stacey & Kramers (1975)], residing for41·5 Gyr in the mantle Transition Zone, and the other an immediate supply from the present-day Pacific Plate subducted sediment (recent sediment flux; see Fig. 7). The composition of the fluid flux released from the sediment on the Pacific Plate stagnant slab can be approximated by the current sediment composition on the ocean floor, based on studies by Ben Othman et al. (1989), Plank & Langmuir (1998), Shimoda et al. (1998), Hauff et al. (2003), and Plank et al. (2007). In general, such sediments have enriched Sr^Nd isotope compositions and have radiogenic Pb isotope compositions. We can therefore attribute the isotopic composition of the Chugaryeong basalts to a flux less influenced by the ancient fluid component and perhaps more influenced by the sediment flux from the Pacific Plate stagnant slab. The mantle into which the mixture of ancient and recent fluids infiltrated is asthenospheric mantle with an LILE^ LREE-enriched trace element compositions, as inferred from the discussions above. The isotopic compositions of the mantle xenoliths reported from eastern Asia (Choi et al., 2005, 2008) are plotted in Fig. 7. They have more depleted Sr^Nd isotope compositions (Fig. 7a) and have more radiogenic Pb (Fig. 7b and c) than the Chugaryeong basalts. The Chatan basalts, which are generated by a higher degree of melting and are more affected by the fluid flux, plot further away from the compositional range of the xenoliths than the Chongok basalts. Kuritani et al. (2011) assumed the presence of depleted mantle (DM) (e.g. Zindler & Hart, 1986), represented by the asthenospheric mantle composition beneath the Sea of Japan (Cousens & Allan, 1992). Mantle xenoliths from the eastern margins of the Asian continent show a very wide range in isotopic composition, in which the DM component is also included; as such this material is also a plausible source mantle for the Chugaryeong basalts. Based upon our study of trace elements using an inversion method, we suggest that the source mantle for the Chugaryeong basalts is enriched in trace elements similar to the sampled mantle xenoliths in China. It is reasonable to assume a source mantle that is enriched in incompatible trace elements but depleted in terms of its Sr^Nd^Pb isotope composition. By implication, this suggests that the enrichment event was relatively recent. We test the validity of our three end-member model by using the isotopic compositions of (1) a fluid flux from ancient subducted sediment (Kuritani et al., 2011), (2) a fluid flux from the stagnant Pacific Plate sediment (see above for details), and (3) the lithospheric mantle as an analogue of enriched asthenospheric mantle that may be present beneath the Chugaryeong volcano (Fig. 15). The isotopic compositions of the Chugaryeong basalts can be reasonably reproduced by mixing between the three components with 50^60% of the ancient sediment component (Fig. 15). There are two other Quaternary basalt provinces close to Chugaryeong: Changbaishan and Kuandian (Fig. 1). The Changbaishan basalts, which have less radiogenic isotopic compositions than the Chugaryeong basalts, may be more influenced by the ancient sediment component. However, the Kuandian basalts cannot be reproduced by mixing between the three components assumed here. This discrepancy might be attributable to heterogeneity in the mantle, as suggested above. There is a clear plume-like low-velocity zone in the upper mantle and a high-velocity zone in the mantle Transition Zone beneath Changbaishan volcano (Zhao et al., 2009). Kuritani et al. (2011) attributed the low-velocity zone to a hydrous mantle upwelling, which is modified by mixed fluxes of ancient and recent sediment-derived fluids. No reliable mantle seismic tomography model is available for the upper mantle beneath Chugaryeong volcano. However, the geochemical evidence discussed above suggests contributions of sediment-derived fluid at various times in the petrogenesis of the Chugaryeong basalts. Although the source mantle composition would probably vary between the different basalt centres, the strong EM1 geochemical affinities and enrichment in fluid-mobile elements common to the two basalts studied could be attributable to subducted sediment fluid fluxes at various times (Fig. 14b). Detailed examination of each basalt centre would provide information both on local 523 JOURNAL OF PETROLOGY VOLUME 55 NUMBER 3 MARCH 2014 Fig. 15. Sr^Nd^Pb isotopic compositions of the Chongok and Chatan basalts and results of mixing calculations among plausible end-member components. (a) 87Sr/86Sr vs 143Nd/144Nd; (b) 207Pb/204Pb vs 206Pb/204Pb; (c) 208Pb/204Pb vs 206Pb/204Pb; (d) 143Nd/144Nd vs 206Pb/204Pb. The dashed line indicates mixing (with percentages indicated) between fluids from ancient sediment and recent sediment as proposed by Kuritani et al. (2011). The fine lines indicate mixing between the mixed fluid and plausible lithosphere (star). Percentages along the dashed line are the percentage of the ancient sediment component. The composition of the Chugaryeong basalt can be reasonably reproduced by 0·2^1·0% fluid component that consists of 50^60% of the ancient sediment component. (See text for details.) Cross-hatched field shows the range of mantle xenolith compositions from the region (Fig. 7) Also plotted are fields for Kuandian and Changbaishan basalts. heterogeneities and the bigger picture for the origin of basaltic volcanism in the deep back-arc. (2) CONC LUSIONS Comprehensive whole-rock major element, trace element, and Sr^Nd^Pb isotope data and mineral compositions for the Chongok (0·5 Ma) and Chatan (0·15 Ma) basalts of the Chugaryeong volcano are presented to provide insights into magmatic differentiation processes within the crust, melting processes in the mantle, and source mantle characteristics underlying the Korean Peninsula. The main conclusions are as follows. (1) Major element variations within each basalt can be reproduced by crystal fractionation, whereas the differences between the two basalts cannot be reproduced by any crystal fractionation or crustal 524 (3) (4) (5) assimilation processes, suggesting that the two lavas originated from different primary melts. Estimated melt segregation pressure for the Chatan basalt is lower than that for the Chongok basalt. Estimated melt segregation temperatures for the Chugaryeong basalts are higher than the temperature of the lithospheric mantle beneath the Korean Peninsula based on studies of mantle xenoliths. The source mantle for both basalts exhibits an incompatible element-enriched multi-element pattern with positive spikes at Ba, K, Pb, and Sr, when normalized to Primitive Mantle. The Chongok basalt has higher 143Nd/144Nd and 206 Pb/204Pb, and lower 87Sr/86Sr and 208Pb/204Pb than the Chatan basalt, the trend of which is from the field of local mantle xenoliths towards a mixture of ancient and recent sediment-derived components. SAKUYAMA et al. METASOMATIZED ASTHENOSPHERE MELTING These trace element and isotopic characteristics suggest an origin of the Chugaryeong basalts by flux melting of metasomatized asthenospheric mantle induced by fluid released from subducted sediments. The younger Chatan basalt was generated by a higher degree of melting of the source mantle peridotite that had been more affected by sediment-derived fluid than the source of the Chongok basalt. (6) Isotopic differences between the Chugaryeong and Changbaishan volcanoes can be attributed to a lesser contribution of the ancient sediment component at the Chugaryeong volcano than the Changbaishan volcano, whereas isotopic heterogeneity of the metasomatized mantle is required to explain the isotopic composition of Kuandian volcano. AC K N O W L E D G E M E N T S This study was carried out in collaboration with two archaeologists: Dr Kazuto Mastufuji of Doshisya University and Dr Kidong Bae of Hanyang University (South Korea). We would like to express our thanks to Hikaru Iwamori and Hiroko Nagahara for constructive discussions. We are grateful to Hideto Yoshida for EPMA and XRF analyses at the Department of Earth and Planetary Science, University of Tokyo. We thank John Gamble, Takashi Sano and an anonymous reviewer for constructive and thoughtful comments on the paper. FUNDING This work was supported by funds from Japan Society for the Promotion of Science (21840067 and 23740398) to T. S. and funds from the Ministry of Education, Culture, Sports, Science and Technology (16251005) to Kazuto Matsufuji. S U P P L E M E N TA RY DATA Supplementary data for this paper are available at Journal of Petrology online. R EF ER ENC ES An, M. J. & Shi, Y. L. (2006). Lithospheric thickness of the Chinese continent. Physics of the Earth and Planetary Interiors 159, 257^266. Arai, S. (1987). An estimation of the least depleted spinel peridotite on the basis of olivine^spinel mantle array. Neues Jahrbuch fu«r Mineralogie Monatshefte 8, 347^354. Arai, S. & Abe, N. (1995). Reaction of orthopyroxene in peridotite xenoliths with alkali-basalt melt and its implication for genesis of alpine-type chromitite. American Mineralogist 80, 1041^1047. Arai, S., Abe, N. & Ishimaru, S. (2007). Mantle peridotites from the Western Pacific. Gondwana Research 11, 180^199. Asimow, P. D. & Ghiorso, M. S. (1998). Algorithmic modifications extending MELTS to calculate subsolidus phase relations. American Mineralogist 83, 1127^1132. Baker, J., Peate, D., Waight, T. & Meyzen, C. (2004). Pb isotopic analysis of standards and samples using a 207Pb^204Pb double spike and thallium to correct for mass bias with a double-focusing MCICP-MS. Chemical Geology 211, 275^303. Beard, J. S. & Lofgren, G. E. (1991). Dehydration melting and watersaturated melting of basaltic and andesitic greenstones and amphibolites. Journal of Petrology 32, 365^401. Beard, J. S., Abitz, R. R. & Lofgren, G. E. (1993). Experimental melting of crustal xenolith from Kilbourne Hole, New Mexico and implications for the contamination and genesis of magmas. Contributions to Mineralogy and Petrology 115, 88^102. Bence, A. E. & Albee, A. L. (1968). Empirical correction factors for the electron microanalysis of silicates and oxides. Journal of Geology 76, 382^403. Ben Othman, D., White, W. M. & Patchett, J. (1989). The geochemistry of marine sediments, island arc magma genesis, and crust^ mantle recycling. Earth and Planetary Science Letters 94, 1^21. Bertotto, G. W., Cingolani, C. A. & Bjerg, E. A. (2009). Geochemical variations in Cenozoic back-arc basalts at the border of La Pampa and Mendoza provinces, Argentina. Journal of South American Earth Sciences 28, 360^373. Bohrson, W. & Spera, F. (2001). Energy-constrained open-system magmatic processes II: Application of energy-constrained assimilation^fractional crystallization (EC-AFC) model to magmatic systems. Journal of Petrology 42, 1019^1041. Bohrson, W. & Spera, F. (2003). Energy-constrained open-system magmatic processes IV: Geochemical, thermal and mass consequences of energy-constrained recharge, assimilation and fractional crystallization (EC-RAFC). Geochemistry, Geophysics, Geosystems 4, doi:10.1029/2002GC000316. Chang, Q., Shibata, T., Sinotsuka, K., Yoshikawa, M. & Tatsumi, Y. (2003). Precise determination of trace elements in geological standard rocks using inductively coupled plasma mass spectrometry (ICP-MS). Frontier Research on Earth Evolution 1, IFREE Report for 2001^2002 357^362. Choi, S. H., Kwon, S. T., Mukasa, S. B. & Sagong, H. (2005). Sr^Nd^ Pb isotope and trace element systematics of mantle xenoliths from Late Cenozoic alkaline lavas, South Korea. Chemical Geology 221, 40^64. Choi, S. H., Mukasa, S. B., Zhou, X. H., Xian, X. H. & Androniko, A. V. (2008). Mantle dynamics beneath East Asia constrained by Sr, Nd, Pb and Hf isotopic systematics of ultramafic xenoliths and their host basalts from Hannuoba, North China. Chemical Geology 248, 40^61. Cousens, B. L. & Allan, J. F. A. (1992). Pb, Sr, and Nd isotopic study of basaltic rocks from the Sea of Japan, Legs 127/128. In: Tamaki, K., Suyehiro, K., Allan, J. & McWilliams, M. (eds) Proceedings of the Ocean Drilling Program, Scientific Results 127/128. College Station, TX: Ocean Drilling Program, pp. 805^817. Dasgupta, R., Hirschmann, M. M. & Smith, N. D. (2007). Partial melting experiments of peridotite CO2 at 3 GPa and genesis of alkalic ocean island basalts. Journal of Petrology 48, 2093^2124. DePaolo, D. J. (1981). Trace element and isotopic effects of combined wallrock assimilation and fractional crystallization. Earth and Planetary Science Letters 53, 189^202. Eichelberger, J. C. (1975). Origin of andesite and dacite: Evidence of mixing at Glass Mountain in California and at other circum-Pacific volcanoes. Geological Society of America Bulletin 86, 1381^1391. Flower, M. F. J., Russo, R. M., Tamaki, K. & Hoang, N. (2001). Mantle contamination and the Izu^Bonin^Mariana (IBM) ‘hightide mark’: evidence for mantle extrusion caused byTethyan closure. Tectonophysics 333, 9^34. 525 JOURNAL OF PETROLOGY VOLUME 55 Fukao, Y., Obayashi, M., Inoue, H. & Nenbai, M. (1992). Subducting slabs stagnant in the mantle transition zone. Journal of Geophysical Research 97, 4809^4822. Gao, S., Rudnick, R. L., Yuan, H. L., Liu, X. M., Liu,Y. S., Xu, W. L., Ling, W. L., Ayers, J., Wang, X. C. & Wang, Q. H. (2004). Recycling lower continental crust in the North China craton. Nature 432, 892^897. Ghiorso, M. S. & Sack, R. O. (1995). Chemical mass transfer in magmatic processes IV. A revised and internally consistent thermodynamic model for the interpolation and extrapolation of liquid solid equilibria in magmatic systems at elevated temperatures and pressures. Contributions to Mineralogy and Petrology 119, 197^212. Gill, J. B. (1981). Origin of Igneous Rocks:The Isotopic Evidence. New York: Springer, 390 p. Green, T. H. (1994). Experimental studies of trace-element partitioning applicable to igneous petrogenesisçSedona 16 years later. Chemical Geology 117, 1^36. Hacker, B. R. (2008). H(2)O subduction beyond arcs. Geochemistry, Geophysics, Geosystems 9, doi:10.1029/2007gc001707. Hart, S. R. (1984). A large-scale isotope anomaly in the Southern Hemisphere mantle. Nature 309, 753^757. Hauff, F., Hoernle, K. & Schmidt, A. (2003). Sr^Nd^Pb composition of Mesozoic Pacific oceanic crust (Site 1149 and 801, ODP Leg 185): Implications for alteration of ocean crust and the input into the Izu^Bonin^Mariana subduction system. Geochemistry, Geophysics, Geosystems 4, doi:10.1029/2002GC000421. Herzberg, C. & Asimow, P. D. (2008). Petrology of some oceanic island basalts: PRIMELT2.XLS software for primary magma calculation. Geochemistry, Geophysics, Geosystems 9, doi:10.1029/ 2008GC002057. Hirahara, Y., Takahashi, T., Miyazaki, T., Vaglarov, B. S., Chang, Q., Kimura, J.-I. & Tatsumi, Y. (2009). Precise Nd isotope analysis of igneous rocks using cation exchange chromatography and thermal ionization mass spectrometry (TIMS). JAMSTEC Report of Research and Development Special Issue 65^72. Hirose, K. & Kawamoto, T. (1995). Hydrous partial melting of lherzolite at 1 GPaçThe effect of H2O on the genesis of basaltic magmas. Earth and Planetary Science Letters 133, 463^473. Hirose, K. & Kushiro, I. (1993). Partial melting of dry peridotites at high pressures: Determination of compositions of melts segregated from peridotite using aggregates of diamond. Earth and Planetary Science Letters 114, 477^489. Hirschmann, M. M. (2000). Mantle solidus: Experimental constraints and the effects of peridotite composition. Geochemistry, Geophysics, Geosystems 1, doi:10.1029/2000GC000070. Hofmann, A. W. (1997). Mantle geochemistry: The message from oceanic volcanism. Nature 385, 219^229. Huang, J. & Zhao, D. (2006). High-resolution mantle tomography of China and surrounding regions. Journal of Geophysical Research 111, doi:10.1029/2005JB004066. Imai, N., Terashima, S., Itoh, S. & Ando, A. (1995). 1994 compilation of analytical data for minor and trace-elements in 17 GSJ geochemical reference samples, ‘Igneous rock series’. Geostandards Newsletter 19, 135^213. Irvine, T. N. & Baragar, W. R. (1971). A guide to the chemical classification of the common volcanic rocks. Canadian Journal of Earth Sciences 8, 523^548. Iwamori, H. (1989). Compositional zonation of Cenozoic basalts in the central Chugoku district, southwestern Japan: Evidence for mantle upwelling. Bulletin of the Volcanological Society of Japan, Second Series 34(2), 105^123. NUMBER 3 MARCH 2014 Iwamori, H. (2007). Transportation of H2O beneath the Japan arcs and its implications for global water circulation. Chemical Geology 239, 182^198. Katz, R. F., Spiegelman, M. & Langmuir, C. H. (2003). A new parameterization of hydrous mantle melting. Geochemistry, Geophysics, Geosystems 4(9), doi:10.1029/2002GC000433. Kelemen, P. B. (1990). Reaction between ultramafic rock and fractionating basaltic magma. 1. Phase relations, the origin of calc-alkaline magma series, and the formation of discordant dunite. Journal of Petrology 31, 51^98. Kelemen, P., Johnson, K. T. M., Kinzler, R. J. & Irving, A. J. (1990). High-field-strength element depletions in arc basalts due to mantle^magma interaction. Nature 345, 521^524. Kelley, K. A. & Cottrell, E. (2009). Water and the oxidation state of subduction zone magmas. Science 325, 605^607. Kimura, J.-I. & Nakano, N. (2004). Precise lead isotope analysis using multiple collector-inductively coupled plasma-mass spectrometry (MC-ICP-MS): Analytical technique and evaluation of mass fractionation during Pb separation. Geoscience Report of Shimane University 23, 9^15. Kimura, J. I., Kent, A. J. R., Rowe, M. C., Katakuse, M., Nakano, F., Hacker, B. R., van Keken, P. E., Kawabata, H. & Stern, R. J. (2010). Origin of cross-chain geochemical variation in Quaternary lavas from the northern Izu arc: Using a quantitative mass balance approach to identify mantle sources and mantle wedge processes. Geochemistry, Geophysics, Geosystems 11, doi:10.1029/2010GC003050. Kinzler, R. J. & Grove, T. L. (1992). Primary magmas of mid-ocean ridge basalts. 1. Experiments and methods. Journal of Geophysical Research 97, 6885^6906. Kobayashi, K. & Nakamura, E. (2001). Geochemical evolution of Akagi volcano, NE Japan: Implications for interaction between island-arc magma and lower crust, and generation of isotopically various magmas. Journal of Petrology 42, 2303^2331. Kuritani, T., Kimura, J. I., Miyamoto, T., Wei, H. Q., Shimano, T., Maeno, F., Jin, X. & Taniguchi, H. (2009). Intraplate magmatism related to deceleration of upwelling asthenospheric mantle: Implications from the Changbaishan shield basalts, northeast China. Lithos 112, 247^258. Kuritani, T., Ohtani, E. & Kimura, J.-I. (2011). Intensive hydration of the mantle transition zone beneath China caused by slab stagnation. Nature Geoscience 4, 713^716. Kuritani, T., Kimura, J. I., Ohtani, E., Miyamoto, H. & Furuyama, K. (2013). Transition zone origin of potassic basalts from Wudalianchi volcano, northeast China. Lithos 156, 1^12. Kushiro, I. (1969). The system forsterite^diopside^silica with and without water at high pressures. American Journal of Science 267-A, 269^294. Kushiro, I. (1972). Effect of water on the composition of magmas formed at high pressures. Journal of Petrology 13, 311^334. Kushiro, I. (1994). Analysis of major and minor elements in silicate rocks with X-ray fluorescence. Evolution of the Crust in Island Arcs. Grand Report for the Ministry of Education of Japan 1^22. Kushiro, I. (1996). Partial melting of a fertile mantle peridotite at high pressures: an experimental study using aggregates of diamond. In: Basu, A. & Hart, S. (eds) Earth Processes: Reading the Isotopic Code. Washington, DC: American Geophysical Union, pp. 109^122. Kushiro, I. & Kuno, H. (1963). Origin of primary basalt magmas and classification of basaltic rocks. Journal of Petrology 4, 75^89. Le Bas, M. J., Le Maitre, R. W., Streckeisen, A. & Zanettin, B. (1986). A chemical classification of volcanic rocks based on the total alkali^silica diagram. Journal of Petrology 27, 745^750. Lee, C. T. A., Luffi, P., Plank, T., Dalton, H. & Leeman, W. P. (2009). Constraints on the depths and temperatures of basaltic magma 526 SAKUYAMA et al. METASOMATIZED ASTHENOSPHERE MELTING generation on Earth and other terrestrial planets using new thermobarometers for mafic magmas. Earth and Planetary Science Letters 279, 20^33. Lee, D. S., Ryu, K. J. & Kim, G. H. (1983). Geotectonic interpretation of Choogaryong rift valley, Korea. Journal of the Geological Society of Korea 19, 19^38. Liu, M., Cui, X. & Liu, F. (2004). Cenozoic rifting and volcanism in eastern China: a mantle dynamic link to the Indo-Asian collision? Tectonophysics 393, 29^42. Maaloe, S. (2004). The PT-phase relations of an MgO-rich Hawaiian tholeiite: the compositions of primary Hawaiian tholeiites. Contributions to Mineralogy and Petrology 148, 236^246. McCulloch, M. T. & Gamble, J. A. (1991). Geochemical and geodynamical constraints on subduction zone magmatism. Earth and Planetary Science Letters 102, 358^374. Medard, E. & Grove, T. (2008). The effect of H2O on the olivine liquidus of basaltic melts: experiments and thermodynamic models. Contributions to Mineralogy and Petrology 155, 417^432. Miyashiro, A. (1978). Nature of alkalic volcanic rock series. Contributions to Mineralogy and Petrology 66, 91^104. Miyashiro, A. (1986). Hot regions and the origin of marginal basins in the western Pacific. Tectonophysics 122, 195^216. Miyazaki, T., Kanazawa, N., Takahashi, T., Hirahara, Y., Vaglarov, B. S., Chang, Q., Kimura, J.-I. & Tatsumi, Y. (2009). Precise Pb isotope analysis of igneous rocks using fully-automated double spike thermal ionization mass spectrometry (FA-DS-TIMS). JAMSTEC Report of Research and Development, Special Issue 73^80. Nakamura, E., Campbell, I. H. & Sun, S.-S. (1985). The influence of subduction processes on the geochemistry of Japanese alkaline basalts. Nature 316, 55^58. Nakamura, Y. & Kushiro, I. (1970). Compositional relations of coexisting orthopyroxene, pigeonite and augite in a tholeiitic andesite from Hakone volcano. Contributions to Mineralogy and Petrology 26, 265^275. Niu, Y. (2005). Generation and evolution of basaltic magmas: Some basic concepts and a new view on the origin of Mesozoic^ Cenozoic basaltic volcanism in eastern China. Geological Journal of China Universities 11, 9^46. Ono, S. (1998). Stability limits of hydrous minerals in sediment and mid-ocean ridge basalt compositions: Implications for water transport in subduction zones. Journal of Geophysical ResearchçSolid Earth 103, 18253^18267. Park, J. B. & Park, K. H. (1996). Petrology and petrogenesis of the Cenozoic alkali volcanic rocks in the middle part of Korean Peninsula (I): petrography, mineral chemistry and whole rock major element chemistry. Journal of Geological Society of Korea 32, 223^249. Pickering-Witter, J. & Johnston, A. D. (2000). The effects of variable bulk composition on the melting systematics of fertile peridotitic assemblages. Contributions to Mineralogy and Petrology 140, 190^211. Plank, T. & Langmuir, C. (1998). The chemical composition of subducting sediment and its consequences for the crust and mantle. Chemical Geology 145, 325^394. Plank, T., Kelley, K., Murray, R. & Stern, L. (2007). Chemical composition of sediments subducting at the Izu^Bonin trench. Geochemistry, Geophysics, Geosystems 8(4), 2006GC001444. Press, W. H., Teukolsky, S. A., Vetterling, W. T. & Flannery, B. P. (1992). Numerical Recipes in C, The Art of Scientific Computing. Cambridge: Cambridge University Press. Quick, J. E. (1981). The origin and significance of large, tabular dunite bodies in the Trinity Peridotite, Northern California. Contributions to Mineralogy and Petrology 78, 413^422. Rapp, R. P., Irifune, T., Shimizu, N., Nishiyama, N., Norman, M. D. & Inoue, J. (2008). Subduction recycling of continental sediments and the origin of geochemically enriched reservoirs in the deep mantle. Earth and Planetary Science Letters 271, 14^23. Robinson, J., Wood, B. & Blundy, J. (1998). The beginning of melting of fertile and depleted peridotite at 1·5 GPa. Earth and Planetary Science Letters 155, 97^111. Robinson, J. A. C. & Wood, B. J. (1998). The depth of the spinel to garnet transition at the peridotite solidus. Earth and Planetary Science Letters 164, 277^284. Roeder, P. & Emslie, R. (1970). Olivine^liquid equilibrium. Contributions to Mineralogy and Petrology 29, 275^289. Rudnick, R. L., Shan, G., Ling, W. L., Liu, Y. S. & McDonough, W. F. (2004). Petrology and geochemistry of spinel peridotite xenoliths from Hannuoba and Qixia, North China craton. Lithos 77, 609^637. Ryu, S., Oka, M., Yagi, K., Sakuyama, T. & Itaya, T. (2011). K^Ar ages of the Quaternary basalts in the Jeongok area, the central part of Korean Peninsula. GeosciencesJournal 15, 1^8. Sakuyama, M. (1979). Evidence of magma mixing: Petrological study of Shirouma-Oike calc-alkaline andesite volcano, Japan. Journal of Volcanology and Geothermal Research 5, 179^208. Sakuyama, T., Ozawa, K., Sumino, H. & Nagao, K. (2009). Progressive melt extraction from upwelling mantle constrained by the Kita-Matsuura basalts in NW Kyushu, SW Japan. Journal of Petrology 50, 725^779. Schwab, B. E. & Johnston, A. D. (2001). Melting systematics of modally variable, compositionally intermediate peridotites and the effects of mineral fertility. Journal of Petrology 42, 1789^1811. Shaw, D. M. (2000). Continuous (dynamic) melting theory revisited. Canadian Mineralogist 38, 1041^1063. Shimoda, G., Tatsumi, Y., Nohda, S., Ishizaka, K. & Jahn, B. (1998). Setouchi high-Mg andesites revisited: geochemical evidence for melting of subducting sediments. Earth and Planetary Science Letters 160, 479^492. Smith, P. M. & Asimow, P. D. (2005). Adiabat_1ph: A new public frontend to the MELTS, pMELTS, and pHMELTS models. Geochemistry, Geophysics, Geosystems 6, doi:10.1029/2004gc000816. Sobolev, A., Hofmann, A., Kuzmin, D., Yaxley, G., Arndt, N., Chung, S., Danyushevsky, L., Elliott, T., Frey, F., Garcia, M., Gurenko, A., Kamenetsky, V., Kerr, A., Krivolutskaya, N., Matvienkov, V., Nikogosian, I., Rocholl, A., Sigurdsson, I., Sushchevskaya, N. & Teklay, M. (2007). The amount of recycled crust in sources of mantle-derived melts. Science 316, 412^417. Stacey, J. S. & Kramers, J. D. (1975). Approximation of terrestrial lead isotope evolution by a 2-stage model. Earth and Planetary Science Letters 26, 207^221. Stolper, E. & Newman, S. (1994). The role of water in the petrogenesis of Mariana trough magmas. Earth and Planetary Science Letters 121, 293^325. Sun, S.-S. & McDonough, W. F. (1989). Chemical and isotopic systematics of oceanic basalts: implications for mantle composition and processes. In: Saunders, A. D. & Norry, M. J. (eds) Magmatism in the Ocean Basins, Geological Society, London, Special Publications 42, 313^345. Takahashi, E. (1986). Origin of basaltic magmas -Implications from peridotite melting experiments and an olivine fractionation model (in Japanese). Bulletin of the Volcanological Society of Japan 30, s17^s40. Takahashi, E., Nakajima, K. & Wright, T. L. (1998). Origin of the Columbia River basalts: melting model of a heterogeneous plume head. Earth and Planetary Science Letters 162, 63^80. Takahashi, T., Hirahara, Y., Miyazaki, T., Vaglarov, B. S., Chang, Q., Kimura, J.-I. & Tatsumi, Y. (2009). Precise determination of Sr isotope ratios in igneous rock samples and application to 527 JOURNAL OF PETROLOGY VOLUME 55 micro-analysis of plagioclase phenocrysts. JAMSTEC Report of Research and Development, Special Issue 59^64. Tatsumi, Y. & Eggins, S. (1995). Subduction Zone Magmatism. Tokyo: Blackwell, 211 p. Tatsumi, Y., Sakuyama, M., Fukuyama, H. & Kushiro, I. (1983). Generation of arc magmas and thermal structure of the mantle wedge in subduction zones. Journal of Geophysical Research 88, 5815^5825. Tatsumi, Y., Shukuno, H., Yoshikawa, M., Chang, Q., Sato, K. & Lee, M. (2005). The petrology and geochemistry of volcanic rocks on Jeju Island: Plume magmatism along the Asian continental margin. Journal of Petrology 46, 523^553. Tatsumoto, M. & Nakamura, Y. (1991). Dupal anomaly in the Sea of JapançPb, Nd, and Sr isotopic variations at the eastern Eurasian continental margin. Geochimica et Cosmochimica Acta 55, 3697^3708. Taylor, B. & Martinez, F. (2003). Back-arc basin basalt systematics. Earth and Planetary Science Letters 210, 481^497. Tian, L., Castillo, P. R., Hawkins, J. W., Hilton, D. R., Hannan, B. B. & Pietruszka, A. J. (2008). Major and trace element and Sr^Nd isotope signatures of lavas from the Central Lau Basin: Implications for the nature and influence of subduction components in the back-arc mantle. Journal of Volcanology and Geothermal Research 178, 657^670. Tomita, T. (1935). On the chemical composition of the Cenozoic alkaline suite of the circum-Japan Sea region. Journal of the Shanghai Science Institute 1, 227^306. van Keken, P. E., Hacker, B. R., Syracuse, E. M. & Abers, G. A. (2011). Subduction factory: 4. Depth-dependent flux of H2O from subducting slabs worldwide. Journal of Geophysical ResearchçSolid Earth 116, doi:10.1029/2010jb007922. Walter, M. J. (1998). Melting of garnet peridotite and the origin of komatiite and depleted lithosphere. Journal of Petrology 39, 29^60. Wee, S. M. (1996). Geochemical characteristics of the Quaternary Jungok baalt in Choogaryong rift valley, Mid-Korean Peninsula. Journal of Korean Economic and Environmental Geology 29, 171^182. Weis, D., Kieffer, B., Maerschalk, C., Barling, J., de Jong, J., Williams, G. A., Hanano, D., Pretorius, W., Mattielli, N., Scoates, J. S., Goolaerts, A., Friedman, R. M. & Mahoney, J. B. (2006). High-precision isotopic characterization of USGS reference materials by TIMS and MC-ICP-MS. Geochemistry, Geophysics, Geosystems 7, doi:10.1029/2006GC001283. NUMBER 3 MARCH 2014 Weis, D., Rhodes, J. M. & Garcia, M. O. (2007). The structure of the Hawaiian plume conduit from high-precision isotopic studies of Mauna Loa lavas. Geochimica et Cosmochimica Acta 71, A1099^A1099. Williams, I. S., Cho, D. L. & Kim, S. W. (2009). Geochronology, and geochemical and Nd^Sr isotopic characteristics, of Triassic plutonic rocks in the Gyeonggi Massif, South Korea: Constraints on Triassic post-collisional magmatism. Lithos 107, 239^256. Won, C. K., Kim, Y. K. & Lee, M. W. (1990). The study on the geochemistry of Choogaryong alkali basalt. Journal of Geological Society of Korea 26, 70^81. Woodhead, J., Eggins, S. & Gamble, J. (1993). High-field strength and transition element systematics in island-arc and back-arc basin basaltsçevidence for multiphase melt extraction and a depleted mantle wedge. Earth and Planetary Science Letters 114, 491^504. Xu, Y. G. (2007). Diachronous lithospheric thinning of the North China Craton and formation of the Daxin’anling^Taihangshan gravity lineament. Lithos 96, 281^298. Yasuda, A., Fujii, T. & Kurita, K. (1994). Melting phase-relations of an anhydrous mid-ocean ridge basalt from 3 to 20 GPa: implications for the behavior of subducted oceanic crust in the mantle. Journal of Geophysical Research 99, 9401^9414. Zhao, D., Maruyama, S. & Omori, S. (2007). Mantle dynamics of Western Pacific and East Asia: Insight from seismic tomography and mineral physics. Gondwana Research 11, 120^131. Zhao, D. P., Tian, Y., Lei, J. S., Liu, L. C. & Zheng, S. H. (2009). Seismic image and origin of the Changbai intraplate volcano in East Asia: Role of big mantle wedge above the stagnant Pacific slab. Physics of the Earth and Planetary Interiors 173, 197^206. Zhou, M. F., Robinson, P. T., Malpas, J. & Li, Z. J. (1996). Podiform chromitites in the Luobusa ophiolite (southern Tibet): Implications for melt^rock interaction and chromite segregation in the upper mantle. Journal of Petrology 37, 3^21. Zhu, J. (2007). The structural characteristics of lithosphere in the continent of Eurasia and its marginal seas. Earth Science Frontiers 14, 1^20. Zindler, A. & Hart, S. (1986). Chemical geodynamics. Annual Review of Earth and Planetary Sciences 14, 493^571. Zou, H. B., Zindler, A., Xu, X. S. & Qi, Q. (2000). Major, trace element, and Nd, Sr and Pb isotope studies of Cenozoic basalts in SE China: mantle sources, regional variations, and tectonic significance. Chemical Geology 171, 33^47. 528