Journal of Petrology Advance Access originally published online on November 24, 2004
Journal of Petrology 2005 46(3):523-553; doi:10.1093/petrology/egh087
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The Petrology and Geochemistry of Volcanic Rocks on Jeju Island: Plume Magmatism along the Asian Continental Margin
1 INSTITUTE FOR RESEARCH ON EARTH EVOLUTION (IFREE), JAPAN AGENCY FOR MARINE–EARTH SCIENCE AND TECHNOLOGY (JAMSTEC), YOKOSUKA 237-0061, JAPAN
2 INSTITUTE FOR GEOTHERMAL SCIENCES, KYOTO UNIVERSITY, BEPPU 974-0907, JAPAN
3 DEPARTMENT OF SCIENCE EDUCATION, COLLEGE OF EDUCATION, KANGWON NATIONAL UNIVERSITY, CHUNCHEON 200-701, SOUTH KOREA
RECEIVED OCTOBER 31, 2003; ACCEPTED OCTOBER 1, 2004
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONGEOLOGYANALYTICAL METHODSRESULTSDISCUSSIONCONCLUSIONREFERENCES |
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The incompatible element signatures of volcanic rocks forming Jeju Island, located at the eastern margin of the Asian continent, are identical to those of typical intraplate magmas. The source of these volcanic rocks may be a mantle plume, located immediately behind the SW Japan arc. Jeju plume magmas can be divided into three series, based on major and trace element abundances: high-alumina alkalic, low-alumina alkalic, and sub-alkalic. Mass-balance calculations indicate that the compositional variations within each magma series are largely governed by fractional crystallization of three chemically distinct parental magmas. The compositions of primary magmas for these series, using inferred residual mantle olivine compositions, suggest that the low-alumina alkalic and sub-alkalic magmas are generated at the deepest and shallowest depths by lowest and highest degrees of melting, respectively. These estimates, together with systematic differences in trace element and isotopic compositions, indicate that the upper mantle beneath Jeju Island is characterized by an increased degree of metasomatism and a change in major metasomatic hydrous minerals from amphibole to phlogopite with decreasing depth. The original plume material, having rather depleted geochemical characteristics, entrained shallower metasomatized uppermost mantle material, and segregated least-enriched low-alumina alkalic, moderately enriched high-alumina alkalic, and highly enriched sub-alkalic magmas, with decreasing depth.
KEY WORDS: Jeju Island; magma genesis; mantle plume; subcontinental mantle
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONGEOLOGYANALYTICAL METHODSRESULTSDISCUSSIONCONCLUSIONREFERENCES |
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The eastern margin of the Asian continent is a site of intensive Cenozoic volcanism characterized by subduction-related arc–back-arc basin and mantle plume-related intraplate magmatism. The simultaneous occurrence of magmatism in association with both mantle downwelling and upwelling in the region provides a rare opportunity for investigating material circulation within the Earth's mantle. Issues that may be addressed by analysing such magmatism include: (1) the contribution of subduction components extracted from the foundering oceanic lithosphere to the back-arc basin and further intraplate magmatism; (2) the contribution to continental margin magmatism of continental components, such as continental crust and subcontinental lithospheric mantle, with geochemical characteristics that are different from asthenospheric mantle; (3) the role of upwelling, asthenospheric mantle materials in causing such magmatism and controlling magma composition.
Jeju Island is located between the Korean Peninsula of the Asian continent and Kyushu of the SW Japan arc, at the western margin of the Sea of Japan (or East Sea); the Sea of Japan is a back-arc basin built behind the SW and NE Japan arc–trench systems (Fig. 1). Jeju magmatism is thus distinct in that it takes place near the boundary between arc–back-arc basin and continental tectonic settings. Analytical study of Jeju magmatism may provide constraints for assessment of the above-mentioned topics of continental margin magmatism.
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Voluminous volcanic piles accumulated on Jeju Island during the Late Cenozoic and include at least two chemically distinct magma series: alkalic and sub-alkalic series (Lee, 1982
; Park, 1994). This paper presents petrography, major and trace elements, and Sr–Nd–Pb isotopic compositions for Jeju volcanic rocks. On the basis of this comprehensive dataset, the occurrence of three magma series, processes of magmatic differentiation, conditions of mantle melting, and the geochemical and mineralogical characteristics of the upper-mantle sources of the magmas will be discussed. |
GEOLOGY |
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TOPABSTRACTINTRODUCTIONGEOLOGYANALYTICAL METHODSRESULTSDISCUSSIONCONCLUSIONREFERENCES |
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The eastern margin of the Asian continent is characterized by the occurrence of extensive magmatism. Most Quaternary volcanoes in this region are built along the convergent plate margins where Pacific and Philippine Sea Plates are being subducted beneath the Eurasian Plate (Fig. 1a). These volcanic arcs form 100–200 km above the dipping seismic zone located near the surface of the foundering oceanic lithosphere (e.g. Tatsumi & Eggins, 1995
). Far behind the volcanic arcs, on the other hand, are sporadically distributed ‘intraplate’ volcanoes such as Jeju volcano (Fig. 1a). Although no seismically active slab is observed, recent tomography results have revealed the presence of horizontally lying, subducted lithospheric material near the upper–lower-mantle boundary beneath the intraplate volcanoes (Fukao et al., 2001).
East of Jeju Island is the Sea of Japan, a back-arc basin formed behind the Japanese Islands at 30–15 Ma (Tamaki et al., 1992) with clockwise and anti-clockwise rotations of SW and NE Japan arc slivers respectively at 
15 Ma (Otofuji et al., 1985). Although the principal cause of the back-arc rifting is controversial, back-arc basin formation results in, or is caused by, upwelling of asthenospheric material that ultimately creates new oceanic crust. It is therefore interesting to compare the chemical characteristics of upwelling asthenospheric material beneath the back-arc basin and the intraplate regions.
Jeju Island is roughly elliptical in shape (80 km x 40 km) and mainly comprises Holocene volcanic rocks. It is composed of thick piles of lava flows, minor pyroclastic rocks, hyaloclastites, and numerous parasitic scoria cones (Fig. 1b). These volcanic rocks are believed to have erupted onto a granitic basement, although granitic rocks are found only as xenoliths in both lavas and pyroclastics. The volcanic activity on this island can be divided into four stages (Fig. 1b and c), based on stratigraphic relationships (Lee, 1982). Radiometric age determinations for Jeju lavas indicate that the volcanic activity commenced at 800 ka and continued to historical times (Lee, 1982). Stage 1 began with the eruption of basaltic lava flows that formed a shield volcano growing from the sea floor. Once the shield volcanic activity ended, the Stage 1 volcanic rocks were unconformably overlain by volcaniclastic sediments (Seoguipo Formation; Fig. 1c). The Stage 2 basaltic lavas (Pyosunri basalts; Fig. 1c) form the bulk of the exposed volcanic rocks as a lava plateau. Minor lava flows composed of trachyandesites and trachytes were also erupted during this stage. The Stage 3 volcanic rocks form the Halla shield volcano, with a peak height of 1950 m, and can be subdivided into four substages based on stratigraphic relationships and petrographical characteristics. The final volcanic activity on this island, Stage 4, yielded more than 360 parasitic scoria cones that are distributed along the axis of the island (Fig. 1b).
Forty volcanic samples collected from the island cover almost all volcanic stages (Fig. 1b and c). To evaluate the role of the granitic basement in the formation of the Jeju magmas, two granitic xenoliths included in pyroclastic rocks were also analysed.
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ANALYTICAL METHODS |
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TOPABSTRACTINTRODUCTIONGEOLOGYANALYTICAL METHODSRESULTSDISCUSSIONCONCLUSIONREFERENCES |
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Major and trace element (Ni to Th in Table 1) compositions were measured using RIGAKU® Simaltics 3550 and Rix 3000 X-ray fluorescence (XRF) spectrometers on fused glass beads and pressed powder pellets, respectively. Detailed analytical procedures have been described by Goto & Tatsumi (1994
, 1996). Concentrations of rare earth elements (REE) and 11 other trace elements (Rb to U in Table 2) were determined by inductively coupled plasma mass spectrometry (ICP-MS) using a VG Elemental® PQ3 system enhanced with a chicane lens system, following the procedures described by Chang et al. (2003). Trace element data, except for the high field strength elements (HFSE: Zr, Nb, Hf and Ta), were obtained from HF–HClO4–HNO3 digestion. For HFSE, alkali fusion (LiBO2–Li2B4O7, SpectrofluxR 100B of Johnson Matthey) was applied to ensure a complete decomposition of refractory minor phases. Analytical accuracy and precision estimated from repeated measurements of international reference rocks were better than ±10% and 2–5%, respectively.
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Rock samples for Sr–Nd–Pb isotope analysis were crushed to coarse chips (<0·5 mm3) and fresh pieces were hand picked. To avoid surface contamination, the rock chips were washed with ethanol and then leached with 0·5 M HCl at room temperature for 1 h. Finally the chips were rinsed with Milli-Q water. The chips were ground to less than 200 mesh size using a vibration mill made of alumina ceramic. The analytical procedure for chemical separation and mass spectrometry for Sr, Nd and Pb isotope determinations has been outlined by Yoshikawa et al. (2001)
, Miyazaki et al. (2003) and Shibata et al. (2003). Total procedural blanks for Sr, Nd and Pb were about 10 pg, 10 pg and 5 pg, respectively. Mass spectrometry was performed on a Thermo-Finnigan® Triton TI equipped with nine Faraday cups, using a static multi-collection mode. Normalizing factors used to correct for isotopic fractionation in the Sr, Nd and Pb isotope analyses were 86Sr/88Sr = 0·1194, 146Nd/144Nd = 0·7219, and 0·147% per atomic mass unit, respectively. Measured isotopic ratios for standard materials were 87Sr/86Sr = 0·710268 ± 19 (2
) for NIST 987 (n = 10), 143Nd/144Nd = 0·511844 ± 11 (2), for La Jolla (n = 11), and 208Pb/204Pb = 36·721 ± 13 (2), 207Pb/204Pb = 15·498 ± 4 (2) and 206Pb/204Pb = 16·001 ± 3 (2) for NIST 981 (n = 28).
Mineral compositions were analysed using JEOL JXA-8800 and -8900 electron-probe micro-analysers following the method described by Shukuno (2003). The excitation potential, specimen current, and analytical time were: 15 kV, 15 nA and 20 s (25 kV, 20 nA and 100 s for Mn, Ca, and Ni analyses) for olivine; 15 kV, 12 nA and 20 s for spinel; 15 kV, 15 nA and 20 s for pyroxene and plagioclase. ZAF correction procedures were employed.
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RESULTS |
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TOPABSTRACTINTRODUCTIONGEOLOGYANALYTICAL METHODSRESULTSDISCUSSIONCONCLUSIONREFERENCES |
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Major and trace element compositions
Major and trace element abundances, and isotopic compositions of the Jeju lavas are listed in Tables 1 and 2 together with the modal compositions of phenocrysts. Major and selected trace elements are shown in SiO2-variation diagrams in Figs 2 and 3. Jeju volcanic rocks possess a wide range of compositions, with SiO2 contents from 48 to 60 wt %, i.e. from basalt to felsic andesite. Two magma types can be recognized on the basis of relative abundance of total alkali elements (Fig. 2): one clearly belongs to the sub-alkalic and the other mostly to the alkalic series of Le Bas & Streckeisen (1991)
. Although some basalts with SiO2 contents <50 wt % are classified as sub-alkalic based on the classification of Le Bas & Streckeisen (1991), these rocks are grouped, hereafter, into the alkalic series for the following two reasons (Fig. 2). First, they form a continuous chemical trend with typical alkalic rocks having higher SiO2 contents. Second, they can be classified as alkalic series based on the classification scheme of Miyashiro (1978). Although the sub-alkalic series rocks are hypersthene normative, some ‘alkalic’ series rocks are also hypersthene normative (Fig. 2). To avoid confusion in terminology, therefore, the identification of distinctive magma series on Jeju Island, hereafter referred to as alkalic (ALK) and sub-alkalic (Sub-ALK) series, are based on total alkali contents. The alkalic series can be further subdivided into two groups based on Al2O3 content (Fig. 2): High-Al ALK and Low-Al ALK. High-Al ALK rocks tend to have higher concentrations of K, Rb, Sr, Zr, Nb, Ba, and Th, and lower abundances of Fe, Mg, and Cu (Figs 2 and 3). It should be stressed that all Sub-ALK rocks belong to the Pyosunri basalt of Stage 2, and most High-Al ALK rocks are also found in Stage 2 (Sanbangsan trachyte and Seoguipo trachyandesite).
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Normal mid-ocean ridge basalt (N-MORB)-normalized (Sun & McDonough, 1989
) multi-element diagrams for the Jeju volcanic rocks are shown in Fig. 4. Although relative enrichment in highly incompatible elements, which typifies ocean-island basalts, can be seen, the Jeju samples are further characterized by a relative depletion of Nb and overabundances of Pb and Sr. It has been well established that the production of magmas with these distinctive chemical characteristics typifies magmatism at convergent plate boundaries (e.g. Pearce, 1983; Hawkesworth et al., 1993). To identify this ‘subduction flavour’ in magmas more quantitatively, Tatsumi et al. (2000) demonstrated that K/Y and K/Nb can be used to distinguish the overabundance of highly incompatible elements and depletion of HFSE relative to large ion lithophile elements (LILE) for arc magmas. In a K/Y vs K/Nb diagram, subduction-zone magmas are distinct from those from other tectonic settings, with high ratios of both K/Nb and K/Y, whereas intraplate rocks have high K/Y but low K/Nb (Fig. 5). If we accept this distinction, it implies that Jeju magmas have chemical characteristics identical to those of intraplate (hotspot) rather than subduction-zone magmas. The geochemical characteristics of the Jeju volcanic rocks, therefore, strongly suggest the location of a mantle plume beneath the island, although neither a hotspot track on the surface nor a low-velocity anomaly in the mantle have been documented in the eastern margin of the Asian continent including the NE China and Jeju regions (Fukao et al., 2001).
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Although concentrations of particular incompatible elements differ between High- and Low-Al ALKs, and Sub-ALK series, the element patterns in MORB-normalized trace element variation diagrams are broadly identical for all three series (Fig. 4). However, Sub-ALK lavas are distinct from the other two series in that they have rather flat REE patterns (Fig. 4).
Jeju lavas have positive Eu anomalies in chondrite-normalized (Sun & McDonough, 1989) REE diagrams (Fig. 4). A possible cause for this anomaly would be the contribution of plagioclase, into which Eu is preferentially partitioned compared with other REE, to the formation and/or differentiation of the magmas. The positive Sr spikes observed in the MORB-normalized trace element variation diagrams also support this. The origin of these plagioclase-related characteristics will be discussed below.
Granitic basement rocks on Jeju Island are also highly depleted in Nb relative to Th and K, and are further distinct in having a steep REE pattern and strong depletion in the heavy REE (HREE; Fig. 4).
Isotopic compositions
Jeju volcanic rocks show a wide range of 87Sr/86Sr values from 0·704128 to 0·705350, whereas 143Nd/144Nd values are rather limited, from 0·512810 to 0·512679 (Fig. 6a and b). The Sr–Nd isotopic compositions of the Jeju lavas overlap with those of Cenozoic intraplate basalts, especially those of sub-alkalic basalts from NE China (e.g. Hannuoba basalts; Song & Frey, 1989; Song et al., 1990), and are identical to those of relatively enriched back-arc basin basalts from the Sea of Japan floor (Cousens et al., 1994) (Fig. 6a). It has been well established for the NE Chinese intraplate magmatism that sub-alkalic basalts typically possess more enriched Sr–Nd isotopic signatures than coexisting alkalic basalts (Zhou & Armstrong, 1982; Zhou et al., 1988; Song et al., 1990), as is documented for Hannuoba magmatism (Fig. 6a). Such compositional differences are also broadly observed for the Jeju samples: the Low-Al ALK series exhibit the lowest Sr and highest Nd isotopic ratios, and the Sub-ALK series are characterized by more enriched signatures (Fig. 6a and b). However, lavas showing very depleted Sr–Nd isotopic signatures close to those of MORB, such as the Hannuoba alkalic basalts, are not documented on Jeju Island.
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Jeju volcanic rocks are, however, distinct from basalts from both the Sea of Japan back-arc basin and NE China in that they possess much more radiogenic Pb isotopic compositions (Fig. 6c). Further, the Pb isotopic characteristics differ from the Sr–Nd isotopic systematics in that the Jeju volcanic rocks do not show systematic compositional differences between the alkalic and sub-alkalic series (Fig. 6f).
The mantle geochemical reservoirs required for explaining the isotopic signatures of oceanic hotspot basalts or ocean island basalts (OIB) can be identified by a Sr–Pb isotopic diagram (Fig. 6g). The Sr–Pb isotopic compositions of Jeju volcanic rocks are within the range of OIBs and suggest the contribution of HIMU and EMII reservoirs to their source, whereas the NE Chinese hotspot basalts show isotopic characteristics close to EMI (Fig. 6g).
Petrography
Representative compositions of phenocrysts are given in Tables 3 –7. Olivine phenocrysts are ubiquitous in the mafic lavas of Jeju Island, although the amount of olivine is <10 vol. % (Table 1). The olivine phenocrysts in relatively undifferentiated rocks (MgO>5 wt %) tend to have a narrow compositional range with a peak composition (Fig. 7) that is in equilibrium with the bulk rock in terms of Fe–Mg exchange partitioning assuming Fe2+/(Fe2+ + Fe3+) = 0·9 in the magma. By contrast, Mg-poor or differentiated samples contain ‘disequilibrium’ olivine phenocrysts (Fig. 7).
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Minor amounts of clinopyroxene phenocrysts (<5 vol. %) occur in some Jeju samples. Clinopyroxenes in the Sub-ALK series lavas are more depleted in the diopside component and show stronger normal zoning than those in the ALK series lavas (Fig. 8). Orthopyroxene is rarely found in the Sub-ALK series lavas as microphenocryst and groundmass phases (Fig. 8).
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Abundant plagioclase phenocrysts (>20 vol. %) are observed in some Low-Al ALK series lavas (Table 1). They are characterized by rather limited compositional ranges and do not show disequilibrium textures such as reverse zoning (Fig. 9), or dusty and/or honeycomb structures.
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Spinel crystals are often included in olivine phenocrysts and are characterized by rather high Cr/Al values and low Fe3+ contents compared with Cenozoic strongly alkalic basalts from SW Japan that are not related to subduction (Shukuno & Arai, 1999
) (Fig. 10a). This feature of the spinel compositions may be confirmed by an olivine–spinel compositional relationship diagram (Arai, 1994), which indicates that Jeju basalt magmas would be in equilibrium with the residual spinel in the mantle with higher Cr/Al values than those from SW Japan (Fig. 10b). Mantle xenoliths in the Jeju basalts contain olivine and spinel crystals (Choi et al., 2002) that plot well within the olivine–spinel mantle array of Arai (1994): spinel in lherzolite xenoliths has much lower Cr/Al values than those in harzburgite xenoliths, and the latter possess Cr/Al values identical to spinel inclusions in the Jeju olivine phenocrysts (Fig. 10b).
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Ballhaus et al. (1990
, 1991) and Ballhaus (1993) examined Cr–Al-rich spinel compositions and demonstrated that Fe3+ in spinel may provide a reasonable estimate of the fO2 relative to the FMQ (fayalite–magnetite–quartz) buffer for a magma that crystallizes spinel. Figure 10c indicates fO2 estimates based on spinel crystals in Jeju basalts. It should be stressed that the fO2 of the Jeju magmas may be higher than that of ocean island basalts and close to that of island-arc basalt magmas. |
DISCUSSION |
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TOPABSTRACTINTRODUCTIONGEOLOGYANALYTICAL METHODSRESULTSDISCUSSIONCONCLUSIONREFERENCES |
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Magmatic differentiation
Processes of differentiation of the three magma series of Jeju Island are examined. One commonly occurring process for terrestrial magmas is mixing of compositionally different magmas. Evidence indicative of magma mixing includes the following disequilibrium petrographic characteristics (Eichelberger, 1975
; Sakuyama, 1979; Bloomfield & Arculus, 1989; Kawamoto, 1992; Yang et al., 1999): (1) the presence of plagioclase phenocrysts with a dusty zone containing fine melt inclusions and with a wide range of compositions; (2) the presence of reversely zoned pyroxene phenocrysts with rounded cores mantled by rims of higher Mg-number; (3) the presence of disequilibrium phenocryst assemblages such as olivine and quartz. Jeju volcanic rocks, however, do not exhibit such petrographic features (Figs 8 and 9), suggesting that magma mixing may not have been a significant process affecting these magmas. Mantle-derived, high-temperature, basaltic magmas are likely to cause partial melting of crustal rocks with lower solidus temperatures, and hence could be contaminated by crust-derived felsic melts. The basement of Jeju volcano is likely to be granite, because such rocks are found as xenoliths in the volcanic rocks. To evaluate the geochemical contribution of crustal contamination to Jeju magma differentiation, the results of simple mixing calculations using compositions of a basaltic magma (Low-Al ALK CJ-10) and a total granitic melt, are shown in Fig. 11. It is clearly demonstrated that a contaminated magma possesses increasingly enriched isotopic signatures with increasing contribution of granitic melt. However, Jeju volcanic rocks, especially those of the High-Al and Low-Al ALK series, possess rather constant isotopic compositions with increasing SiO2 content (Fig. 11), indicating at most only minor involvement of granite-derived melts during magmatic differentiation processes. On the other hand, two Sub-ALK series samples that are characterized by unusually enriched isotopic signatures appear to have been contaminated significantly by basement granitic rocks (Fig. 11).
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The above considerations suggest that the observed wide range of magma compositions of Jeju volcanic rocks may be largely governed by fractional crystallization. Phases involved in such differentiation processes, as inferred from major element variations (Fig. 2) and petrographic observations, include olivine, clinopyroxene, plagioclase, magnetite, and apatite. To examine this quantitatively, least-squares mixing calculations using the compositions of the coexisting phases were conducted (Table 8 and Fig. 12). In these calculations, three Low-Al ALK series samples (CJ-10, 29, and 14) and two High-Al ALK series samples (CJ-42 and 21) were selected as representative magmas (Fig. 12). The calculations confirm that the magmatic differentiation for these series can be reproduced by separation of the above minerals (Table 8 and Fig. 12). The effect of fractional crystallization on the compositional variation was further examined by using Rayleigh fractionation models for REE elements. Crystal–liquid partition coefficients used in this modelling (Table 9) are after Tatsumi (2001)
, and are based on the compilation of experimental data by Green (1994) and a consideration of the crystal structure control in trace element partitioning between melts and solid phases (Matsui et al., 1977). The fraction of phases separated from the magma is based on the mass-balance calculations (Table 8). The modelling results are illustrated in Fig. 12, and indicate that REE concentrations in the Jeju magmas can also be largely reproduced by fractional crystallization processes.
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Melting regime
Genetic relations between the three chemically distinct magma series are now examined. A possible mechanism for producing sub-alkalic magmas from alkalic parental magmas is contamination by silica-oversaturated, incompatible-element-enriched crustal rocks. As mentioned above, isotopic compositions, especially 207Pb/206Pb, of two of the Sub-ALK series rocks could be explained by this mechanism (Fig. 11). However, this process cannot reasonably reproduce the isotopic characteristics of the other Sub-ALK series lavas. On Pb–Pb isotopic diagrams, for example, Sub-ALK rocks do not lie on the mixing line defined by ALK and granitic rocks (Fig. 6c and d). Thus, it may be concluded that crustal contamination is not a likely process for producing the sub-alkalic magmas from alkalic magmas on Jeju Island, and that both alkalic and sub-alkalic magmas were derived from the mantle beneath the island.
The High-Al ALK magmas are distinct from the Low-Al ALK series in that the former are more enriched in Al and Sr, consistent with involvement of a plagioclase component. It could be possible, therefore, that High-Al ALK series magmas were derived from Sub-ALK magmas by selective accumulation of plagioclase phenocrysts. Positive Eu anomalies observed in some High-Al ALK lavas also support this (Fig. 4). However, plagioclase accumulation is unlikely to be responsible for producing the High-Al ALK magmas. The reasons for believing so are twofold. First, the modal amount of plagioclase phenocrysts in High- and Low-Al ALKs does not correlate with the plagioclase component in the magmas, such as Al and Sr concentrations, Sr/Ba values and degrees of positive Eu anomalies (Fig. 13). Second, the positive Eu anomaly is most clearly observed for the most REE-depleted, least differentiated rocks and tends to become weak with increasing degrees of magmatic differentiation (Fig. 4). It is therefore more likely that the characteristic enrichment of the plagioclase component in the High-Al ALK magmas is a primary signature gained during mantle melting, not derived from intra-crustal processes.
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The above considerations suggest the production of the three types of magmas in the upper mantle beneath Jeju Island. To examine the P–T conditions of magma generation for the three magma series, compositions of primary magmas for each series were estimated and compared with those of partial melts produced in peridotite melting experiments. The first step was to estimate the composition of residual mantle olivine in equilibrium with such primary magmas. Some Low-Al ALK and Sub-ALK lavas contain Mg-rich (Mg-number >80) olivine phenocrysts. The NiO–Mg-number relationships for those olivine phenocrysts in Mg-rich samples are demonstrated in Fig. 14, together with an inferred olivine composition that would be in equilibrium with the bulk rock, estimated on the basis of Fe–Mg–Ni exchange partitioning between olivine and silicate melts (Roeder & Emslie, 1970
; Kinzler et al., 1990) and assumption of Fe2+/(Fe2+ + Fe3+) = 0·9 in the magma. It is shown in Fig. 14 that the inferred olivine in equilibrium with the bulk rock mostly has NiO–Mg-number compositions within the range of the olivine phenocrysts. On the other hand, the Sub-ALK sample CJ-2 contains olivine that is more nickeliferous than expected. Such unusually nickeliferous olivine phenocrysts are not uncommon in basalts and andesites (e.g. Sato & Banno, 1983; Nabelek & Langmuir, 1986; Nakamura, 1995; Tatsumi et al., 2002, 2003). Nakamura (1995) examined the compositional zoning of olivine phenocrysts in andesites from the Yatsugatake volcano, Central Japan, by using a growth and diffusion model in the Mg–Fe–Ni system. He indicated that the characteristic compositions of unusual olivine phenocrysts can be explained by diffusion processes within normally zoned olivines causing Fe enrichment without a marked depression in the Ni content. This is due to a greater Fe–Mg interdiffusion coefficient than the Ni tracer diffusion coefficient in olivine. It is thus concluded, at least for the CJ-2 magma, that a long residence time of olivine phenocrysts may cause such unusual compositions.
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It has been well established that olivine in the upper mantle possesses a rather constant NiO content whereas its Mg-number is variable (Sato, 1977
; Takahashi, 1990) (Fig. 14). If we accept 0·4 wt % NiO for mantle olivine, then it is possible to estimate the Mg-number of olivine that was once in equilibrium with the primary magma, by back-calculating the equilibrium olivine composition. The results of such calculations, assuming Fe2+/(Fe2+ + Fe3+) = 0·9 in the magma and using Fe–Mg–Ni exchange partition coefficients of Roeder & Emslie (1970) and Kinzler et al. (1990), are shown in Fig. 14 and suggest that the observed compositional variation of olivine phenocrysts may be reasonably explained by olivine fractionation from the magma. The back-calculation may suggest that the Mg-numbers of the residual mantle olivine are 88 and 85 for Low-Al ALK and Sub-ALK series, respectively (Fig. 14). Fairly low Mg-numbers of the residual olivine for Sub-ALK series magmas may be caused by extensive metasomatism by silicate–carbonate melts, as discussed below. On the basis of the compositions of these residual mantle olivines and the bulk rocks, the major element compositions of the primary magma for each magma series can be back-calculated by assuming maximum olivine fractionation from the magma. The samples used for these calculations contain phenocrysts solely of olivine (excepting CJ-1, which contains a minor amount of clinopyroxene), providing the basis for assuming that the only major phase fractionated from the primary magma during the differentiation processes was olivine. The inferred primary magma compositions for the Low-Al ALK and Sub-ALK series are listed in Table 10.
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On the other hand, the High-Al ALK samples do not contain Mg-rich olivine and these methods cannot be used to estimate the primary magma composition. To overcome this, the following two simplified methods were applied. The first is based on the systematic compositional difference between Low- and High-Al ALKs. As demonstrated in Figs 2 and 12, High-Al ALK rocks tend to possess
2 and 0·5 wt % higher Al2O3 and Na2O, and 2 and 0·5 wt % lower FeO* and MgO than the Low-Al ALK series lavas, respectively. By assuming these differences for the primary magmas, the inferred primary magma compositions (A-1, B and C in Table 10) can be deduced from the Low-Al primary magma compositions. The second method is based on (1) the composition of the most primitive High-Al ALK sample (CJ-42) and (2) the amounts of minerals fractionated from the primary magma to produce the CJ-42 magma, assumed to be identical to those fractionated during magmatic differentiation from the Low-Al ALK primary magma, via CJ-10, to CJ-14 magma (Fig. 12). The composition of this inferred magma is listed as A-2 in Table 10.
Developments in experimental methods, particularly the peridotite–basalt ‘sandwich’ technique (Takahashi & Kushiro, 1983; Fujii & Scarfe, 1985; Falloon & Green, 1987, 1988; Falloon et al., 1988), have facilitated the successful quenching and determination of peridotite partial melt compositions at high pressures. Peridotite melting studies using the sandwich technique have produced a consistent set of data despite the use of different peridotite compositions. The consistent and systematic behaviour of melt compositions when projected from diopside onto the plagioclase–olivine–quartz plane of the basalt tetrahedron allowed Falloon & Green (1988) to construct a pressure-sensitive melting grid, which is capable of establishing the depth of magma separation for primary magma compositions within at least the silica-saturated portion of the basalt tetrahedron. However, this grid has not been calibrated using data for degrees of melting smaller than 10%, where elevated and variable incompatible element concentrations, arising from variations in degree of melting and starting compositions (basalt layer and peridotite sandwich) may cause significant changes to phase relationships. A new technique, which employs aggregates of diamond embedded between peridotite layers, has improved the small-degree melt problem significantly (Baker et al., 1992; Johnson & Kushiro, 1992; Kushiro & Hirose, 1992). In these experiments, small-degree partial melts can be extracted from the peridotite layers into the pore space between diamond aggregates, and therein analysed by electron microprobe. The compositions of these partial melts probably provide the best available estimates of primary magma compositions formed as a function of varying upper-mantle P–T conditions. The results of Hirose & Kushiro (1993) confirm previous assertions based on both the basalt–peridotite sandwich technique and simple system experiments, that more alkaline magmas will form at higher pressure given an identical degree of partial melting (Fig. 15). It should be thus possible to use these grids and the primary magma compositions of the Jeju magmas (Table 10) to estimate the P–T conditions for the last equilibration between the upper-mantle residue and the magmas. Figure 15 clearly shows that the Sub-ALK series magmas were produced at lower pressures and by greater degrees of partial melting than ALK series magmas. It may be further suggested from Fig. 15 that the Low-Al ALK magmas formed at greater depths than High-Al ALK magmas.
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Differences in the degree of mantle partial melting between the three types of Jeju magmas are also suggested by the abundances of incompatible trace elements (Figs 3 and 4): the High-Al ALK series are the most enriched and the Sub-ALK magmas are the least enriched in incompatible elements. If we assume an identical peridotite source for the three magma types, then higher degrees of partial melting for the Sub-ALK series are likely to form more Mg-rich olivine as a residual phase. However, this is not the case for the Jeju magmas, as mentioned above. Greater degrees of partial melting of relatively fertile or enriched mantle peridotite were responsible for production of the Sub-ALK magmas.
It has been well established that the presence of volatiles such as H2O and CO2 in magmas causes significant changes in P–T conditions of magma generation. At present, however, no estimates for the abundances of volatiles in the Jeju magmas are available, so that the above-mentioned estimates for the conditions of magma production assuming a ‘dry’ peridotite source are tentatively accepted here.
Source characteristics
Petrographic and geochemical studies of mantle-derived xenoliths and magmas from the continental regions have shown the presence of variably metasomatized peridotite in the subcontinental upper mantle (e.g. Menzies et al., 1987). Because the Jeju volcano is built upon the Asian continental lithosphere, the source regions for the Jeju magmas are likely to include metasomatized subcontinental upper mantle. The geochemical and petrographical characteristics of the Jeju subcontinental mantle are examined here on the basis of the Jeju magma compositions.
The NiO–Mg-number relationships for olivine phenocrysts suggest that the source (residual) upper-mantle material was more fertile for Sub-ALK magmas than for ALK magmas, in terms of olivine composition (Fig. 14). This is consistent with the observation that the Sub-ALK series lavas tend to show more enriched Sr–Nd isotopic signatures than the ALK series lavas (Fig. 6a and b), although a systematic difference between the magma series cannot be demonstrated for Pb isotopes (Fig. 6c–f). Furthermore, among the ALK series, Low-Al ALK rocks are distinct in their rather depleted Sr–Nd isotopic characteristics (Fig. 6a and b). Such systematic differences in isotopic signatures for alkalic and sub-alkalic rocks have been documented for Cenozoic intraplate magmas in NE China and may represent different degrees of metasomatism within the subcontinental upper mantle (e.g. Zhou et al., 1988; Song & Frey, 1989; Song et al., 1990; Nohda et al., 1991). Accepting this and the above-mentioned differences in depth of magma separation from the upper mantle, we propose that the upper mantle beneath Jeju Island is variably metasomatized, with increasing degrees of metasomatic enrichment with decreasing depth. An upwelling mantle plume, probably with depleted isotopic and major element signatures, could have produced Low-Al ALK magmas at deeper levels, caused partial melting of more metasomatized upper mantle to form the High-Al ALK magmas, and finally resulted in higher degrees of partial melting of shallow-level, highly metasomatized upper mantle to produce the Sub-ALK magmas.
To further reveal the characteristics of the metasomatized upper mantle beneath Jeju Island, especially to identify metasomatic minerals, the trace element abundances in the Jeju magmas were examined. First, relatively undifferentiated samples were selected: CJ-8 and 42 for the High-Al ALK series; CJ-10, 14, 15, 19, and 32 for the Low-AL ALK series; CJ-2, 3, 35, and 37 for the Sub-ALK series. Second, the trace element concentrations were normalized to Nb to minimize the effect of crystallization and partial melting on element abundances. Finally, Nb-normalized element concentrations were further normalized to those of the Low-Al ALK series magmas, because these magmas may be derived from a less metasomatized source than the other magmas, as discussed above. The averaged values for each magma series are plotted in Fig. 16 as a function of ionic radius. Different but systematic patterns of enrichment and depletion of certain elements can be observed for the two magma types. It may be suggested from both these patterns and inferred sizes of cation sites for minerals that the Sub-ALK magma source, which is the most metasomatized source in terms of Mg/Fe ratios and Sr–Nd isotopic signatures, is enriched in elements that are likely to be partitioned into plagioclase and amphibole, whereas the High-Al ALK source is enriched in both plagioclase and phlogopite components (Fig. 16). One possible explanation for these observations is the contribution of these phases to producing the magmas as melted-out phases, not melting residues. However, this explanation is unlikely, as plagioclase is likely to react with orthopyroxene at the pressures at which the Jeju ALK magmas were produced (>2·0 GPa; Fig. 15).
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Alternatively, the characteristic incompatible element patterns in Fig. 16 can be understood as the result of buffering by residual phases. The High-Al ALK and Sub-ALK magmas are depleted in amphibole and phlogopite components, respectively, suggesting the presence of those minerals as melting residues.
The geochemical and petrographical characteristics of the Jeju magma source regions in the upper mantle, and their contributions to the generation of three different magma series, are schematically illustrated in Fig. 17. The uppermost mantle beneath the region has been variably metasomatized and exhibits isotopically more enriched signatures with decreasing depths. Hydrous phases that probably crystallized as a result of the metasomatism include phlogopite, at rather shallow levels where Sub-ALK magmas segregated from the upwelling mantle material, and amphibole, at deeper levels from which the High-AL ALK magmas were derived. The major component of the Jeju mantle plume is likely to be isotopically depleted, unmetasomatized asthenospheric material, which contributed significantly to producing Low-Al ALK magmas.
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What is the nature of the metasomatic agents that enrich the subcontinental upper mantle? This is an important but unsolved problem concerning the evolution of the continents. Hydrous phases such as amphibole and phlogopite that are inferred as metasomatic minerals in the magma source region in the upper mantle beneath Jeju Island may result from infiltration of either H2O-rich fluids or silicate–carbonate melts, because both agents can readily transport LILE. However, Fe/Mg values cannot be changed by fluid-dominant metasomatism, because of the fairly low solubility of these elements in an aqueous fluid. The vertical variation in the Mg-number of olivine in the upper mantle, which is inferred from the Jeju magma compositions, may, therefore, suggest the melt-dominant metasomatism for the Jeju upper mantle.
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CONCLUSION |
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TOPABSTRACTINTRODUCTIONGEOLOGYANALYTICAL METHODSRESULTSDISCUSSIONCONCLUSIONREFERENCES |
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Although Jeju Island is located along the eastern margin of the Asian continent in the vicinity of the arc–trench system, the Jeju volcanic rocks were produced in association with intraplate, mantle plume-related magmatism. The major process responsible for differentiation of the Jeju magmas was fractional crystallization of mineral phases such as olivine, clinopyroxene, plagioclase, apatite and magnetite. The systematic difference in both incompatible element abundances and isotopic compositions observed for the Jeju magmas appears to reflect vertical compositional and mineralogical heterogeneity in the magma source region of the upper mantle: the upper mantle beneath Jeju Island tends to possess more enriched and metasomatized signatures with decreasing depth. Processes including upwelling of a rather depleted mantle plume into such a metasomatized and enriched upper mantle, subsequent interaction and mixing between these mantle components, and separation of magmas at different depths may reasonably explain the geochemical characteristics of the Jeju magmas.
The presence of mantle plumes with depleted isotopic, especially Sr–Nd, signatures has been proposed as the cause of intraplate magmatism in NE China (Zhou et al., 1988; Song et al., 1990), as well as Jeju Island. However, the origin and location of such an isotopically depleted mantle component are unknown. Tatsumi & Eggins (1995) speculated that the harzburgitic portion of the subducting lithosphere, which is the residue after extraction of oceanic crust MORB magmas and hence is likely to possess very depleted isotopic characteristics, could rise from the upper–lower-mantle boundary region owing to the density contrast between harzburgitic slab and fertile lherzolitic mantle material at those depths.
A significant difference in the isotopic compositions of the Jeju and NE Chinese intraplate magmas is the rather high Pb isotopic ratios for the Jeju magmas, suggesting the contribution of a HIMU-like geochemical reservoir to the Jeju mantle plume. One plausible mechanism for creating a HIMU reservoir in the deep mantle is the accumulation of both fresh and dehydrated oceanic crust (Chauvel et al., 1992; Hauri & Hart, 1993; Brenan et al. 1995; Kogiso et al. 1997a, 1997b; Tatsumi & Kogiso, 2003). The location and the origin of such a HIMU reservoir in the Jeju mantle plume system, however, is a future problem to be addressed.
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ACKNOWLEDGEMENTS |
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We thank Takashi Sano, Ken Itoh and In-Seok Son for their help in sampling on Jeju Island, Yuka Yonezawa and Bogdan Vaglarov for analytical assistance, Miki Fukuda for preparing the manuscript and figures, and Richard Arculus and Monica Handler for constructive comments on the manuscript.
* Corresponding author. E-mail: tatsumi{at}jamstec.go.jp
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