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Journal of Petrology | Volume 43 | Number 4 | Pages 631-661 | 2002
© Oxford University Press 2002
Origin of Low-K Intermediate Lavas at Nekoma Volcano, NE Honshu Arc, Japan: Geochemical Constraints for Lower-Crustal Melts
1DEPARTMENT OF GEOSCIENCE, SHIMANE UNIVERSITY, MATSUE CITY, 690-8504 JAPAN
2INSTITUTE OF MINERALOGY, PETROLOGY, AND ECONOMIC GEOLOGY, SCHOOL OF SCIENCE, TOHOKU UNIVERSITY, AOBA-KU, SENDAI CITY, 980-8578 JAPAN
Received April 30, 2000; Revised typescript accepted October 18, 2001
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONGEOLOGICAL BACKGROUNDSAMPLES AND ANALYTICAL...GEOCHEMISTRY OF LAVASELEMENT DISTRIBUTION BETWEEN...DISCUSSIONCONCLUSIONREFERENCES |
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Extremely low-K basaltic andesite to andesite lavas at Nekoma volcano, situated in the frontal volcanic zone of the NE Honshu arc, were produced from melts that originated in the lower crust. Multiple incompatible trace element model calculations suggest that extremely low-K basalt found in the same arc is a natural analog for the source composition. However, fractional crystallization, magma mixing, and crustal contamination models of primary low-K basalt cannot reproduce the Nekoma chemical composition. Derivation of melts from an extremely low-K amphibolitic lower-crustal rock with the residual mineral phases hornblende, olivine, pyroxenes, plagioclase, and magnetite is plausible. Major element compositions of Nekoma lavas are very similar to those of experimental melts of amphibolite dehydration melting, which further support the proposal. Light rare earth elements are slightly enriched, but total rare earth element abundances are relatively low, suggesting a high degree of partial melting of the source. Ba/Th ratios are low for frontal arc lavas, reflecting modification of the ratio during partial melting. Zr/Hf and Nb/Ta ratios are significantly greater than is usual for arc lavas, suggesting an anomalous source composition. Markedly low K, Rb, Cs contents in the extremely low-K lavas are attributed to an extremely low-K source. Underplating of an extremely low-K basalt originating from a hydrous depleted mantle wedge could form such an amphibolite. In contrast, Nd and Sr isotope ratios fall close to Bulk Earth values, indicating an isotopically enriched source. Hornblende-bearing rocks may predominate in the lower crust of the NE Honshu arc, based on the observation of crustal xenoliths. The presence of large low-Vp regions at lower-crustal depths beneath the frontal arc is suggested by geophysical observations. These observations further support lower-crustal melting beneath Nekoma as the origin of the intermediate low-K lavas.
KEY WORDS: amphibolite source; crustal melting; low-K andesite; Sr–Nd isotopes; trace element
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONGEOLOGICAL BACKGROUNDSAMPLES AND ANALYTICAL...GEOCHEMISTRY OF LAVASELEMENT DISTRIBUTION BETWEEN...DISCUSSIONCONCLUSIONREFERENCES |
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Interest in andesite lavas has been sustained because: (1) they make up the majority of lavas erupted at convergent plate boundaries; (2) most recent volcanic eruptions are andesitic and explosive, and thus constitute geologic hazards; (3) andesite bulk compositions are similar to those estimated for the average Earth’s crust (e.g. Gill, 1981
). Recent work on oceanic arcs has questioned the existence of an andesitic crust in island arcs in general, because basaltic to basaltic andesite lavas predominate in these settings (e.g. Foden, 1983; Woodhead et al., 1993; Peate et al., 1997). In terms of this, andesite does not necessarily represent the average product of global volcanism or the common rock type within arc crusts. However, andesite is still a major component of arcs with thicker crust.
Despite this level of interest, however, the origins of andesite lavas are not well constrained, partly because of variable formation processes including: (1) partial melting of hydrous upper mantle (e.g. Green, 1973); (2) partial melting of crustal rocks (e.g. Beard & Lofgren, 1991); (3) differentiation from parental basalt magma; (4) magma mixing between basalts and dacites (e.g. Anderson, 1976; Sakuyama, 1981); (5) mixing between basalts and crustal melts (e.g. Eichelberger, 1978). Additionally, some andesites may be formed by (6) melting of the subducted slab (e.g. Defant & Drummond, 1990).
Lateral chemical variation of lavas exists across subduction zones. In many early studies, this was thought to be simply related to slab depth (e.g. Kuno, 1966; Hutchinson, 1982). Recent models for arc magma genesis invoke material transfer in subduction zones based on trace element and isotopic evidence (e.g. Sakuyama & Nesbitt, 1986; Hawkesworth et al., 1993; Pearce & Peate, 1995; Tatsumi & Eggins, 1995; Davidson, 1996; Shibata & Nakamura, 1997). Currently, much interest is being shown in material exchange between the subducting slab, the mantle wedge, and the arc crust by fluid or silicate melt fluxes; nevertheless, many uncertainties remain (e.g. Turner et al., 1998). To elucidate arc magma generation processes, many studies have focused on the petrogenesis of basalts, to examine the chemical processes in the mantle wedge. All arc basalts, however, must ascend from the mantle wedge through the overlying arc crust and mantle lithosphere. This process clearly modifies the chemistry of primary basalts in continental margin arcs where thicker lithosphere exists (e.g. Hawkesworth et al., 1993; Davidson, 1996). Young oceanic arcs are not exceptions as these arcs can also have relatively thick crust (e.g. Izu–Mariana–Bonin arc; Suyehiro et al., 1996). Isotopic evidence suggests that crustal assimilation occurs in some oceanic island arc lavas, such as those in the Lesser Antilles (Davidson, 1996).
Lateral chemical variation in andesites is observed in the NE Honshu arc lavas (e.g. Sakuyama & Nesbitt, 1986), suggesting that most andesite lavas there were derived by differentiation of mantle-derived basalts. In contrast, isotopic studies of the NE Honshu arc lavas have confirmed interaction between upper mantle derived basalts and various crustal rocks (e.g. Kersting et al., 1996; Gust et al., 1997). In view of this crustal contribution, the NE Honshu arc is well suited for evaluation of the role of the crust in magma genesis. Kersting et al. (1996) and Gust et al. (1997) discussed along-arc variations of Pb, Nd, and Sr isotopes in basaltic to andesitic lavas in the area. The isotopic compositions differ between different basement terranes, and isotopically differing segments are bounded by major faults (Shibata & Ishizaka, 1979; Kimura et al., 2001). Shibata & Nakamura (1997) documented across-arc variation of trace elements and Pb, Sr, Nd isotopes in basalts from the northern part of the NE Honshu arc, and discussed the contribution of subducted slab components. Kimura et al. (2001) also described arc-wide and within-center variations in K contents and Sr isotope ratios in andesites, and showed that large isotopic variations exist between frontal arc and back-arc volcanic centers, although K contents gradually increase from the trench side to the back arc. The relationship between K and Sr isotope composition in single volcanoes is also complex. This complexity is introduced by the varied processes producing the andesites.
Nekoma volcano is unusual amongst the Quaternary volcanoes of the NE Honshu arc, because it has produced extremely low-K basaltic andesite to dacite lavas with isotopically enriched signatures. The major element compositions of the lavas are similar to published experimental melt compositions produced from hornblende-bearing crustal rocks (e.g. Beard & Lofgren, 1991; Rapp & Watson, 1995; Johannes & Holz, 1996), and are therefore well suited for examination of the potential role of crustal agents (Kimura et al., 2001). Sr and Nd isotopic compositions and incompatible element ratios of the lavas are very homogeneous, whereas highly incompatible elements, such as Cs, Rb and K, vary considerably. Here we describe the geochemical and isotopic characteristics of Nekoma lavas, and model trace element behavior that may have occurred during source melting and magma differentiation. Published mantle–crust seismic structures beneath the volcano are also examined. The role of contamination by the lower crust in producing the chemical diversity of arc lavas has been emphasized in the literature (e.g. Arculus & Johnson, 1981). In this paper, we propose that melting of an extremely low-K, lower-crustal amphibolite, which may have formed by crustal underplating of an extremely low-K basalt, is a significant process for the generation of low-K intermediate lavas in the island arc.
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GEOLOGICAL BACKGROUND |
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TOPABSTRACTINTRODUCTIONGEOLOGICAL BACKGROUNDSAMPLES AND ANALYTICAL...GEOCHEMISTRY OF LAVASELEMENT DISTRIBUTION BETWEEN...DISCUSSIONCONCLUSIONREFERENCES |
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The NE Honshu volcanic arc is formed on the North America Plate. The Pacific Plate is being subducted beneath both the North American and Eurasian plates (Fig. 1a and b). Nekoma volcano is located in the southern NE Honshu about 35 km from the volcanic front, forming part of a volcanic cluster near Fukushima (Kimura, 1996
; Fig. 1c). Pre-Tertiary basement rocks beneath this section of the NE Honshu Quaternary arc consist of Carboniferous to Jurassic metamorphic rocks of the Tamba–Ashio–Mino Terrane, Cretaceous sedimentary rocks and granitoids of the Abukuma Terrane, and an Ordovician to Cretaceous complex (Kitakami Terrane). The Tamba–Ashio–Mino and Abukuma Terranes are in contact along the Tanakura Tectonic Line, which is the main boundary fault between NE and SW Japan. The Abukuma and Kitakami Terranes are separated by the Hatagawa Tectonic Line (Ichikawa, 1990; Fig. 1b). Nekoma, Bandai, and West Azuma, Nasu, and Takahara volcanoes are situated on Tamba–Ashio–Mino Terrane basement, whereas Middle Azuma, East Azuma and Adatara volcanoes sit on Abukuma basement.
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, Quaternary volcanoes. Bold dashed lines are volcanic fronts. Figure adapted from Kimura & Yoshida (1999b)
Nekoma volcano first erupted at 
1·1 Ma, and activity ceased at 0·35 Ma. Activity is divisible into Stage 1 (1·1 Ma), Stage 2a (0·9–0·7 Ma), Stage 2b (0·7–0·6 Ma), and Stage 3 (0·45–0·35 Ma) (Kimura et al., 2001; Fig. 2). Lavas in each stage range from basaltic andesite to dacite, but most are andesitic (54–67 SiO2 wt %) occurring as thick lava flows or lava domes with block and ash flow deposits. A collapse of caldera occurred at the beginning of Stage 3. This destroyed the summit of the former volcanic cone, and Stage 3 domes grew within the caldera. Magma volumes erupted were >5 km3 in Stage 1, 30 km3 in Stage 2, and 1·6 km3 in Stage 3, with averaged eruption rates of 0·013–0·017 km3/ka. Low-K lavas were erupted throughout all stages, with contemporaneous eruption of extremely low-K lavas in Stage 2. Medium-K lavas are also associated with low-K lavas in Stage 3 (Kimura et al., 2001). After cessation of Nekoma activity, the eruption center migrated to the east, and the Bandai center began to erupt. Medium-K was produced there between 0·35 and 0·02 Ma (Nakamura, 1978; Kimura et al., 2001; Fig. 2).
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SAMPLES AND ANALYTICAL TECHNIQUES |
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TOPABSTRACTINTRODUCTIONGEOLOGICAL BACKGROUNDSAMPLES AND ANALYTICAL...GEOCHEMISTRY OF LAVASELEMENT DISTRIBUTION BETWEEN...DISCUSSIONCONCLUSIONREFERENCES |
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Samples
The samples analyzed are those described by Kimura et al. (2001)
. Samples were collected from lava flows, lava domes, and lava blocks in block and ash flow deposits. All samples are pyroxene basaltic andesite, andesite, and dacite, varying from aphyric to porphyritic. Plagioclase is ubiquitous, and orthopyroxene (opx) is usually also present. Clinopyroxene (cpx) does occur in basaltic andesite, but is usually rare. Hornblende (hb) occurs as a constituent of plagioclase–opx–magnetite crystal clots in a lava flow, but was not found in the other lavas.
Analytical techniques
Major elements and 14 trace elements were analyzed by X-ray fluorescence (XRF) spectrometry in 134 samples (Kimura et al., 2001). Samples from each stage and K suite were selected for bulk-rock inductively coupled plasma-mass spectrometry (ICP-MS) analysis and Sr and Nd isotope measurements. Samples were crushed manually in an iron pestle, rinsed with distilled water in an ultrasonic bath, and dried for 2 h at 110°C. Dried chips were then ground in an agate mortar for 30 min. The powder produced was ignited at 1100°C for 3 h. Glass disks with a 2:1 flux:sample ratio were prepared following the method of Kimura & Yamada (1996). The resulting glass disks were analyzed for major and 14 trace elements at Fukushima University using a Rigaku RIX 2000 wavelength-dispersive XRF spectrometer.
Additional trace and ultra-trace element analyses were carried out on selected samples using solution ICP-MS, following the methods described by Kimura et al. (1995). Acid reagents used were EL-grade nitric (Kanto Chemicals) and hydrofluoric acid (Tama Chemicals), and precise analysis grade perchloric acid (Wako Chemicals). Experimental water was distilled and subsequently ion exchanged with a Milli Q filter (Millipore). Procedural blanks for elements determined were all <1 ppt. The ICP-MS used was a Thermo ELEMENTAL VG PQ3 at Shimane University, equipped with a normal concentric nebulizer and a water-chilled impact bead-type nebulizer. Instrument settings were fundamentally those reported by Kimura et al. (1995) (Table 1).
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Isotope analysis
The analytical procedure for Sr and Nd isotope analyses follows Iizumi et al. (1994, 1995). Acid reagents were Suprapra grade hydrofluoric and nitric acids (Merck), and precise measurement grade hydrochloric acid (Wako Chemicals). Distilled-ion exchanged water and hydrochloric acid were simmered before use, and procedural blanks were <1 ppt for both Nd and Sr. Samples were analyzed by the Ta–Re double filament method for both Nd and Sr, using a Finnigan MAT 262 thermal ionization mass spectrometer equipped with five collectors. NIST NBS987 Sr standard and La Jolla Nd standard were analyzed before and after each of 12 unknowns (Table 1). Standard values during the analyses were 87Sr/86Sr = 0·710245 ± 0·000010 (n = 3) and 143Nd/144Nd = 0·511847 ± 0·000010 (n = 3) for Nd (errors in 2
), respectively. Typical 2 internal errors for sample analyses are ±0·000010 for Sr and ±0·000015 for Nd.
Mineral analyses
Four representative andesites from the low-K suite were selected for plagioclase and cpx analysis. Samples were cut into 1 mm thick, 25 mm square slabs, glued to slides, polished, and carbon coated. Major element compositions were determined by electron microprobe analysis (EPMA) using a JEOL JXA-8800M system at Shimane University, with an accelerating voltage of 15 kV and a beam current of 2·00 x 10-8 A. Correction followed the ZAF method (e.g. Reed, 1975).
The same sample slabs were utilized for laser ablation ICP-MS (LA-ICP-MS) analysis at Fukushima University, using an in-house 1064 nm wavelength Nd–YAG laser probe equipped with a VG PQ2+ plus high-efficiency rotary pump (Kimura & Chuman, 1996). The laser source was a DCR 11 (Spectra Physics), and the laser beam was attenuated, shaped, and focused through a modified infra-red microscope. Instrumentation and performance have been described in detail by Kimura et al. (1997). A pulsed Q-switch laser was used at a 10 Hz repetition rate, and laser power after the attenuator was set at 4 mJ. Laser craters produced by these settings were 40–50 µm in diameter, with the same depth after 60 s ablation. The standard material used was NIST SRM612 synthetic glass, and the working values of Perkins et al. (1992). The 29Si peak was monitored for all standard and unknown measurements to correct instrumental drift and laser sampling efficiency. Correction factors for Si were issued by NIST for the SRM612 glass, and Si contents measured by EPMA were used for unknown minerals. Representative analyses are given in Table 2.
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GEOCHEMISTRY OF LAVAS |
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TOPABSTRACTINTRODUCTIONGEOLOGICAL BACKGROUNDSAMPLES AND ANALYTICAL...GEOCHEMISTRY OF LAVASELEMENT DISTRIBUTION BETWEEN...DISCUSSIONCONCLUSIONREFERENCES |
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Major elements
Nekoma lavas range between 54 and 67 wt % SiO2, and almost all are andesites with a lesser amount of basaltic andesite and dacite. SiO2 ranges are similar over the three stages: Stage 1, 57–63 wt %; Stage 2, 54–67 wt %; Stage 3, 57–60 wt % (Fig. 3). K2O contents range from 0·1 to 1·3 wt %, and Stage 1 and 2 lavas are classified as low-K (LK) (Fig. 3; Peccerillo & Taylor, 1976
). Some Stage 3 lavas are classed as medium-K (MK) and the remainder plot close to the boundary between low- and medium-K lavas. Extremely low-K (ELK) lavas (K2O 0·1–0·5 wt %) with 54–60 SiO2 wt % occur in Stage 2. K contents gradually increase with SiO2 in all three K suites. These trends are sub-parallel to the boundary between low- and medium-K suites, and have very similar slopes (Fig. 3).
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Total alkali contents all fall within the low-alkali field of Kuno (1966) (Nakamura, 1978; Kimura et al., 2001), as is typical of volcanic front lavas in the NE Honshu arc (e.g. Nakagawa et al., 1988). ELK lavas have the lowest total alkalis, but little contrast is observed between the LK and MK suites (Fig. 3). ELK and LK lavas all fall in the TH (tholeiitic) field, and MK lavas alone plot within the CA (calc-alkaline) field on a FeO*/MgO vs SiO2 plot (Fig. 3). The slight iron enrichment in the tholeiites is typical of almost all such lavas in the NE Honshu arc (e.g. Miyashiro, 1974; Nakagawa et al., 1988; Gust et al., 1997). The extent of iron enrichment does not compare with the marked iron enrichment typical of tholeiites, however (e.g. Grove & Baker, 1984). MgO contents in the LK and ELK suites range from 1 to 5 wt %, and decrease with increase in SiO2. The MK suite is slightly enriched in MgO at a given SiO2 content. TiO2 and P2O5 contents are commonly greater in the ELK and LK suites than in the MK suite. Little contrast in CaO and Al2O3 is seen between the suites (Fig. 3).
Trace elements
Rb contents of Nekoma lavas are correlated positively with K. Rb contents of the ELK lavas are very low, MK lavas are the most enriched, and LK lavas fall between the two (Fig. 4). Other incompatible elements, such as Ba, Ce, Y, Zr and Nb, all gradually increase with increasing SiO2, and differences between the suites are indistinguishable, notwithstanding the marked differences in Rb and K (Fig. 4). Ga and Sr contents, however, are almost unchanged throughout the compositional range, although ELK lavas contain a little more Ga than the other suites. Cr and Ni contents are low in all samples, but decrease markedly in the range 54–60 wt % SiO2, and are near zero at 60–67 wt % SiO2 in ELK and LK lavas. MK lavas have relatively high Cr and Ni, and behave similarly to MgO. Co and V decrease linearly over the entire compositional range. Co and V contents of the MK suite lavas are greater than LK equivalents, and ELK lavas are poorer in Co and V than both at low silica contents.
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N-MORB normalized trace element variation diagrams (Pearce & Parkinson, 1993) highlight several features in the Nekoma lavas (Fig. 5a). Normalized plots of arc rocks typically are enriched in the segment Cs to Zr with negative Nb and Ta anomalies (e.g. Pearce & Parkinson, 1993). In this respect, all Nekoma lavas have island-arc signatures. Positive Li anomalies may also be exhibited by arc lavas (Pearce & Parkinson, 1993). This is true for MK lavas but is not apparent for ELK lavas. A negative Li anomaly is even seen in one of the ELK lavas. Two out of three low-K lavas show positive Li anomalies (Fig. 5a).
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Two of the mafic (SiO2 55·8 wt %) and intermediate (59·3 wt %) ELK lavas analyzed are markedly depleted in Cs, Tl, Rb, and K compared with the LK and MK suites (Fig. 5a and b). One ELK dacite sample has slightly lower concentrations of these particular elements but otherwise differs little from LK lavas. These elements increase in abundance as SiO2 increases (Figs 3 and 4). LK lavas are similar to ELK equivalents in Ba, Th, U, Pb, high field strength elements (HFSE: Ta, Nb, Zr, Hf, Y), Li, and rare earth element (REE) contents. Small negative Rb anomalies are found in the LK suite. Basalts and basaltic andesites from the northern segment of the NE Honshu arc (Shibata & Nakamura, 1997) show across-arc chemical variation (Fig. 5d). Direct comparison of their trace element abundances with the Nekoma lavas is not possible, because of differing differentiation states. However, it is notable that negative Rb anomalies are present in the frontal volcanic arc basalts (Gust et al., 1997). This is also seen in JA-1 (Mt. Hakone: Imai et al., 1995) andesite (Fig. 5c). Anomalous large ion lithophile elements seem to be a characteristic of frontal arc lavas (Gust et al., 1997), but the Nekoma case is extreme.
For the ELK and LK suites, N-MORB normalized Ta concentrations are lower than Nb concentrations, in contrast to usual LK andesites in the Japan arc (Fig. 5a and c). MK lavas at neighboring Bandai volcano are typical of frontal arc lavas in central Japan and NE Honshu. At Nekoma, Nb/Ta ratios are lower in ELK-LK than in MK suite lavas. However, the highest ratio is even lower than that seen in Bandai. The above features show that the ELK lavas are extremely anomalous, and LK and MK lavas at Nekoma are transitional between Bandai MK lavas and ELK Nekoma lavas.
REE patterns of Nekoma lavas also have interesting features. ELK and LK lavas have relatively linear C1 chondrite-normalized REE patterns (Fig. 6a). Negative Ce anomalies are present in some lavas of both suites. In contrast, MK lavas at Nekoma have more concave-upwards curved patterns. Neither Ce nor negative Eu anomalies are observed in Nekoma MK lavas (Fig. 6b). Bandai MK lavas are richer in REE, except for Eu, which is comparable with Nekoma equivalents. This results in negative Eu anomalies in the Bandai lavas. A very small anomaly is seen in Mt. Hakone LK andesite JA-3 (Imai et al., 1995; Fig. 6c), and similar values have also been reported from the Izu–Mariana arc (White & Patchett, 1984). Such anomalies are not observed in the northern NE Honshu arc basalts (Fig. 6d), which also show curved patterns with flatter heavy REE (HREE) and steeply inclined light REE (LREE) segments, particularly in back-arc basalts and some frontal arc basalts. A basalt lava from Iwate volcano has a positive Eu anomaly and an LREE-depleted pattern and one from Akita-Koma volcano has a flatter LREE region, comparable with those observed in the Izu–Mariana arc. Overall, negative Ce anomalies and more linear REE patterns distinguish Nekoma LK and ELK lavas from equivalents in the rest of the NE Honshu arc.
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All NE Honshu lavas plot on a linear array on a La/Yb vs La plot (Fig. 7a). This could reflect the partial melting trend of a common source rock (Minster & Allègre, 1978; Sakuyama & Nesbitt, 1983; Edy & Kimura, 1997). However, this linear relationship is not shown by other incompatible element ratio plots. Differing behavior between large ion lithophile elements (LILE) is shown by Ba/Th vs La/Yb plots. Variation of Ba/Th is large in frontal arc lavas, but is reduced and values are low in back-arc lavas. Ba/Th of mafic ELK and LK lavas are as low as back-arc lavas, and those of mafic Nekoma and Bandai MK lavas are actually even lower. As La/Yb and Th are linearly correlated (figure not shown), the large variation is due to Ba variation. Pb/U–Th plots also exhibit similar features. Pb exhibits positive anomalies in the N-MORB normalized plot (Fig. 5a). Anomalous Pb behavior is also apparent on Pb/U plots. Nekoma and Bandai Pb/U are intermediate between frontal and back-arc lavas (Fig. 7d). U and Th are positively correlated (not shown), and so the distinct Pb/U characteristics between frontal and back-arc lavas are due to the behavior of Pb.
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NE Honshu frontal arc basalts have low Zr/Hf at the same Zr, whereas Nekoma lavas have high ratios, similar to those of back-arc basalts. Zr/Hf ratios in Izu–Mariana frontal arc lavas are similar to those in the NE Honshu frontal arc (Fig. 7c). Nekoma lavas also have extremely high Nb/Ta ratios compared with other Japan arc lavas. Izu–Mariana lavas (JA-1 and JA-3) have lower Nb/Ta ratios (11–13), as do low-K NE Honshu frontal arc lavas (13–17, J.-I. Kimura & T. Yoshida, unpublished data, 1999) along with low Nb (0·3–4·2 ppm) (Fig. 8). The Nb/Ta ratio of a basalt from Aoso volcano in NE Honshu (Fig. 1) compares with Nekoma ELK. Bandai MK lavas have high Nb (5·5–7 ppm) and low Nb/Ta (9–14). Bandai MK lavas thus have Nb/Ta ratios comparable with those of Izu–Mariana andesites but greater Nb contents. In contrast, Nekoma MK lavas have higher Nb/Ta comparable with NE Honshu frontal arc LK, but greater Nb contents. ELK and LK lavas have extremely high Nb/Ta ratios (19–21), although Nb contents are similar to Bandai MK or Nekoma LK. Three ELK lavas have the highest Nb/Ta ratios (20–20·5). Overall, Nekoma LK, ELK lavas, and Aoso basalt are extreme in terms of Nb/Ta, not only in NE Honshu, but also over central Japan. High Nb/Ta and Zr/Hf ratios are thus peculiar to the Nekoma lavas.
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Isotopes
Sr and Nd isotopic compositions of Nekoma lavas fall within very narrow ranges (Fig. 9). 87Sr/86Sr ratios range from 0·70492 to 0·70524, and 143Nd/144Nd from 0·51261 to 0·51273. In Nd–Sr systematics, Nekoma lavas plot close to Bulk Earth composition (Zindler & Hart, 1986
), and are also close to the Adatara and Nasu centers in the south NE Honshu arc (Fujinawa, 1988, 1991, 1992; Notsu, 1993; Kersting et al., 1996). A number of researchers have suggested that the Tanakura Tectonic Line separates low Sr isotope ratios in the northern segment from higher ratios to the south (Kersting et al., 1996; Gust et al., 1997; Kimura et al., 2001), although Iwate volcano spans the entire range. A compositional gap between frontal and back-arc lavas is also obvious. The NE Honshu arc can therefore be divided into three segments based on Sr isotope data (Kimura et al., 2001; Fig. 1b). Nd–Sr isotope systematics also shows these three distinct clusters: back arc, north frontal arc and south frontal arc (Fig. 9). Isotopic variations within centers are usually small, but Iwate lavas in the north frontal arc possess both northern and southern frontal arc characteristics. Large isotopic differences are also shown by LK and MK andesites at Adatara volcano (Fujinawa, 1991); in this case LK andesites have lower Sr isotope ratios (Fig. 9).
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Nekoma MK lava is isotopically enriched relative to the LK suites. The difference between the ELK and LK suites is small, but the ELK lavas have the lowest 87Sr/86Sr, and Sr isotope variation is relatively large between the suites (Fig. 9, inset). However, isotopic variation in the ELK and LK suites is very small compared with the large variation in some LILE contents.
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ELEMENT DISTRIBUTION BETWEEN MINERALS AND MELT |
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TOPABSTRACTINTRODUCTIONGEOLOGICAL BACKGROUNDSAMPLES AND ANALYTICAL...GEOCHEMISTRY OF LAVASELEMENT DISTRIBUTION BETWEEN...DISCUSSIONCONCLUSIONREFERENCES |
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Trace element distribution between minerals and host silicate melts is important in modeling igneous processes (e.g. Green, 1994
). Green & Pearson (1985) examined experimentally determined REE partition coefficients (DREE) between melt and cpx, and concluded that compositional dependence is the major cause of varying DREE. Pressure and temperature also affect the DREE and have been modeled by Blundy & Wood (1994) and Wood & Blundy (1997). Natural lavas have varying chemical composition with phenocrysts forming over a wide range of pressures and temperatures. It is thus necessary to determine mineral and groundmass compositions in situ. Phenocryst minerals contain very low abundances of incompatible trace elements, and high instrumental sensitivity is required for successful trace element analysis. Distribution coefficients between minerals can now be determined accurately using LA-ICP-MS and EPMA. We have determined phenocryst plagioclase compositions in LK suite lavas using these techniques. These minerals are major hosts of trace elements, and are abundant in the Nekoma lavas. Samples were selected at representative SiO2 contents (57, 60, 63, 64 SiO2 wt %) in LK suite lavas. Unfortunately, plagioclase phenocrysts in the ELK lavas are very small, and were not suitable for trace element analysis with the existing equipment. Major element compositions in the LK and ELK suites are similar, and thus the plagioclase–melt distribution coefficients determined for LK are considered to represent both suites. Some cpx phenocrysts and cpx in crystal clots in mafic LK lavas were also analyzed.
Plagioclase
Plagioclase in LK suite lavas varies widely in composition, according to the SiO2 content of the host lava. Phenocryst plagioclase is typically 1–2 mm in diameter, and homogeneous under the microscope. Compositions vary from An93 (in 57 SiO2 wt % lava) to An49 (64 SiO2 wt %). Zoning is weak (usually <10% An), and thus the phenocrysts were in equilibrium with their host magmas. The groundmass is usually fine but is heterogeneous. Consequently, >50 groundmass spots were analyzed for each sample and the results averaged. For most trace elements, the derived distribution coefficients (Dplag/melt) have D < 1 except for Li, Be, and B (D > 1–10), Sr (D = 2–6), and Ga (D = 1–3) (Fig. 10). REE and HFSE (Zr, Nb, Hf, Ta) and hygromagmatophile elements (HME; Th and U) are consistent with the compositional differences, showing decreases in middle REE (MREE)–HREE region with increasing SiO2 in the host lavas, which might relate to decreasing magma temperatures (Blundy & Wood, 1994). HFSE and HME almost follow the HREE (Fig. 10). Eu shows anomalous behavior in that D increases with increasing SiO2. Increases in D values are also observed in moderately compatible or incompatible elements for plagioclase, such as Sr, Ga, Ba, and K. LILE, such as Rb, in plagioclase does not differ greatly. In fact DRb in two of the three plagioclases does not differ and D values are between 0·02 and 0·03. One calcic plagioclase has significantly high DRb (0·3), possibly as a result of analysis of a melt inclusion during ablation.
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Clinopyroxenes
Cpx is the dominant mafic phenocryst mineral in less evolved LK andesites. LILE, HFSE, and HME could not be analyzed because of small phenocryst sizes (<0·5 mm in diameter) and very low D between cpx and melt. DREE in cpx is 0·1 for La, and gradually increases to 1 between Dy and Lu (Fig. 11). This DREE pattern is consistent with that reported elsewhere. For HREE, D values approach unity, which is also reported for cpx in andesite to dacite (Green & Pearson, 1985). A cpx hosted in a crystal clot exhibited values of 0·5 for all REE, suggesting crystallization from more mafic melt in the magma chamber (Fig. 11).
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) or cpx in cognate inclusions ((). Errors bars are 1 |
DISCUSSION |
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TOPABSTRACTINTRODUCTIONGEOLOGICAL BACKGROUNDSAMPLES AND ANALYTICAL...GEOCHEMISTRY OF LAVASELEMENT DISTRIBUTION BETWEEN...DISCUSSIONCONCLUSIONREFERENCES |
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Major, trace and isotopic compositions of Nekoma lavas have characteristics differing from other frontal arc andesite lavas in the NE Honshu arc. These are: (1) occurrence of extremely low-K lavas coeval with low-K lavas; (2) anomalous LILE (Cs, Tl, Rb, K) behavior in ELK lavas; (3) high Nb/Ta and Zr/Hf; (4) anomalous Pb and Ba behavior on spidergrams; (5) near-linear C1 chondrite normalized REE patterns; (6) restricted ELK and LK Sr and Nd isotopic compositions. Kimura et al. (2001)
suggested that ELK lavas were possibly produced by lower-crustal melting. We further examine these distinctive Nekoma lavas below.
Effect of fractional crystallization
The ELK and LK lavas have distinct LILE characteristics. On the Rb/K–Rb process identification diagram of Minster & Allègre (1978), the ELK suite fall on a steeply inclined trend, whereas the medium-K trend is horizontal, and LK lavas fall between the two (Fig. 12). It is also notable that two dacite lavas and a felsic ELK andesite plot in the LK region of this diagram. Basaltic andesite to mafic andesite ELK, which are significantly depleted in LILE, plot separately, close to the origin.
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, LK; Plagioclase is the only mineral that can affect Rb/K ratios during fractional crystallization. As D(K)plag/melt changes with the composition of the host magma (Fig. 10), this must be taken into account in modeling fractional crystallization. D(K)plag/melt can be expressed as a function of Rb. The relation equation is deduced from the regression curve shown in Fig. 10. Calculated model melt compositions are shown in Fig. 12. Although large uncertainties exist in D(K), model melt compositions do not change much because of low D(K). Starting melt compositions were selected to represent various Rb/K2O values, and mafic ELK, mafic LK, and intermediate LK compositions were chosen. The intermediate LK at 62 wt % SiO2 was selected because a small kink is seen in the SiO2–Rb trend (Fig. 4). Parental magma compositions for each suite are difficult to assume. Instead, these three starting melt compositions yield the general trend of fractional crystallization. The initially horizontal and progressively curved models exhibit the fractionation effect of plagioclase. Olivine, cpx, opx, and magnetite fractionation have also probably taken place in Nekoma lavas. Removal of these phases drives melt compositions to the right (arrowed). Actual fractionation trends, however, are combined vectors between plagioclase and olivine, cpx, opx, magnetite vectors. The modeled trend almost fits for the LK suite, but does not for the ELK suite. The MK suite trend is almost horizontal, suggesting extensive fractionation of mafic minerals. D(Eu) for plagioclase increases gradually as the melt composition becomes more Si rich (Fig. 10). Nekoma MK lavas do not exhibit negative Eu anomalies and show parallel REE patterns. These features are supporting evidence for extensive mafic mineral and limited plagioclase fractionation at intermediate compositions. In contrast, some evolved LK and ELK suite lavas have negative Eu anomalies, suggestive of plag fractionation. ELK lavas have a very steep linear trend (Fig. 12), which cannot be produced by fractional crystallization of the observed minerals. Equally large Rb/K variation is also seen in the basic LK lavas, which must also have been subjected to a process other than fractional crystallization.
Roles of magma mixing
Magma mixing is another common cause of chemical variation in andesitic magmas (e.g. Eichelberger, 1978; Sakuyama, 1981). Mixing between ELK and MK magmas could produce LK andesites, because LK lava compositions are intermediate between the two, as shown in Fig. 12. MK and LK lavas erupted contemporaneously in Stage 3, and the possibility of magma mixing thus needed to be examined. If magma mixing occurred, mixing trends should be observed between the MK and ELK suites. No mixing trends are apparent on K–SiO2 or Rb–SiO2 plots (Figs 3 and 4). Furthermore, no petrographic evidence of magma mixing, such as disequilibrium mineral assemblages (Sakuyama, 1981), is seen in the ELK or LK suites. MK lavas do contain petrographic evidence of magma mixing, such as dusty plagioclases or reverse-zoned pyroxenes (Eichelberger, 1978). However, incompatible element contents show parallel trends with increasing differentiation (Figs 3 and 4), suggesting any mixing is internal, between differentiated and less differentiated magmas of the same K suite.
One possibility for generation of the LK suite magma by mixing between MK and ELK magmas is that the process took place at more mafic compositions, and subsequent fractional crystallization produced the chemical variations we now see in LK lavas. The intermediate LK suite trace elements contents may be explained by this scheme. However, the above suggestion cannot solve the origin of ELK lavas, because the suite is an end-member. The steeply inclined trend shown on the Rb/K–Rb plot (Fig. 12) could be produced by mixing, but the basalt end-member magma required would need to have extremely low K (<0·1 K2O wt %) and low Rb (<0·5 Rb ppm). Such LILE-depleted basalts have only been reported from the NE Honshu arc at Aoso volcano (Rb 1·81 ppm, K2O 1·04 wt %: Togashi et al., 1992; this paper), but their LILE compositions are still slightly greater than the expected ELK source. MORB compositions (e.g. Pearce & Parkinson, 1993) or very low-K tholeiitic South Sandwich Island arc basalts (e.g. Pearce et al., 1995) could be alternative candidates for the mafic mixing end-member. However, this mixing model still requires a felsic ELK end-member, because silica vs LILE plots show a distinctive very low-K trend (Fig. 3).
Crustal assimilation
Crustal assimilation can account for much of the chemical diversity in andesite magmas (Gill, 1981; Kurasawa et al., 1986; Kersting et al., 1996; Kimura & Yoshida, 1999a, 1999b). Isotopes are useful tools to detect assimilation (e.g. DePaolo, 1981). Notwithstanding their differing K contents, Nekoma lavas have very narrow Sr and Nd isotope compositional ranges, suggesting that they were derived from isotopically similar sources. Very small but clear isotopic contrasts are seen between the K suites. MK lavas have the most enriched Sr isotopic compositions, and ELK the lowest; LK lavas lie between (Fig. 13). Sr and Nd isotope diagrams show that Sr isotope ratios increase with increasing SiO2 whereas there are no apparent changes in Nd isotope ratios in ELK, but an increase is seen in MK. This suggests that the magmas assimilated enriched crustal rocks during ascent. Isotopic data for basement granitoids in this area are few, but available ratios are all enriched (87Sr/86Sr = 0·705–0·706; Shibata & Ishizaka, 1979). The isotopic compositions of the granitoids are similar to those of the Nekoma lavas, and evaluation of crustal assimilation is therefore difficult. Silicic crustal rocks usually have high K contents and can produce high-K silicic melts by anatexis (e.g. Johannes & Holz, 1996). Progressive mixing of such melts with differentiating magmas causes a curvilinear increase in K (Kimura & Yoshida, 1999a). This may not be the case for the Nekoma LK suites, because the increase in K is linear (Fig. 3). ELK lavas have a curvilinear trend (Fig. 3), and this might be caused by progressive crustal assimilation. However, the almost flat trend in SiO2 vs Sr isotopes (Fig. 13) does not support extensive assimilation. On SiO2 vs K and Rb and Rb/K–Rb plots (Figs 3, 4 and 12), felsic ELK have characteristics similar to LK lavas, suggesting that they could in fact be classified as LK.
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Overall, we conclude that crustal assimilation may have occurred, but it was not the major process accounting for the origin of the differing K suites at Nekoma. Additionally, magma mixing between the K suites is also precluded by their contrasting isotopic compositions. Considering that anomalous behavior of LILE is limited to the mafic ELK (Fig. 5a), hereafter our discussion will concentrate on the origin of these anomalous mafic ELK.
Roles of slab-derived fluid and oceanic sediments
Tatsumi & Eggins (1995) proposed fluid-fluxed melting of the upper mantle beneath volcanic fronts as a means of producing LK basalts. Because Nekoma volcano is in a near frontal arc setting, the role of slab-derived fluid needs to be evaluated. Turner et al. (1997) evaluated the role of slab-derived fluid using Ba/Th–Sr isotope space (Fig. 14). Ba/Th ratios in basalts and andesites from the NE Honshu arc are generally lower than those in the Tonga–Kermadec (Turner et al., 1997), Kamchatka, and Aleutians arcs (Turner et al., 1998). However, some NE Honshu volcanic front basalts have Ba/Th ratios that do compare with those from Tonga–Kermadec or the Aleutians. Ba/Th ratios in high-K back-arc and MK Moriyoshi basalts are as low as Pacific MORB, and fall within the range of Lau back-arc basin basalts (Turner et al., 1997; Fig. 14). Pb is also highly mobile in fluids (Brenan et al., 1995; Keppler, 1996; Kogiso et al., 1997), and Pb/U plotted against less mobile Th (Fig. 7) also suggests fluid addition in frontal arc lavas. Slab fluid addition is plausible for volcanic front lavas in NE Honshu, as Shibata & Nakamura (1997) argued using Pb–Sr isotope data. The fluid addition is, however, smaller than that in the Tonga–Kermadec and Kamchatka–Aleutians arcs, based on the Ba/Th systematics.
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Nekoma mafic lavas have low Ba/Th ratios compared with the rest of the NE Honshu arc, or even compared with other arcs (Fig. 14). Notably, the mafic ELK lava has lowest Ba/Th, even when compared with mafic LK lavas. Sr isotopic compositions are more radiogenic than other lavas. Nekoma Nd–Sr isotope systematics are also similar to Samoan lavas. From their Ba/Th–Sr isotope systematics, the mafic ELK Nekoma lavas cannot be derived from basalts affected by fluid-mobile elements. This character also negates contamination by slab sediment flux, because oceanic sediments are rich in both K and Rb (e.g. Taylor & McLennan, 1985) and these elements are mobile in both fluids and silicate melts. Consequently, Nekoma ELK lavas should be little affected by slab components, if chemical compositions are not strongly modified by other processes during ascent.
Partial melting: consideration of source rock
Nekoma ELK and LK lavas may not be linked to each other by fractional crystallization of anhydrous silicate minerals or by magma mixing, and neither slab-derived fluid addition nor sediment contamination can explain their origin. Intensive crustal contamination of basalt magma is also unlikely. An origin as direct mantle-derived andesite (e.g. Mysen & Boettcher, 1974a,1974b; Tatsumi, 1982; Shimoda et al., 1998) is also unlikely because of the low MgO contents. Melting of the subducted basaltic slab can also produce intermediate magmas (e.g. Defant & Drummond, 1990). This can be excluded due to the low 143Nd/144Nd, and the absence of residual garnet signatures in the Nekoma lavas. Assimilation–fractional crystallization cannot explain the homogeneous isotopic compositions over the basaltic andesite to dacite compositional range. Rather, isotopic similarity with the crustal rocks suggests that the Nekoma lower-K lavas were derived from lower-crustal melts. The isotopic compositions of Nekoma lavas are similar to underlying upper-crustal granitoids, which potentially originated from melting of the middle or lower crust. The granitoid isotopic compositions probably reflect those of the lower to middle crust. Hornblende-dominated fractional crystallization of extremely low-K hydrous basalt magma underplating the crust at Moho depths (e.g. Foden & Green, 1992) may be an alternative way of deriving the ELK lavas, because hornblende fractionation would not change K contents greatly. This possibility will be examined in a later section.
Crustal melting has been shown to be important for the generation of intermediate magmas (e.g. Gill, 1981), but is most often proposed for low-K plutonic rocks such as the tonalite–trondhjemite–granodiorite suite (TTG) (e.g. Barker & Arth, 1976; Beard & Lofgren, 1991; Johannes & Holtz, 1996; Springer & Seck, 1997; Kawate & Arima, 1998; Nakajima & Arima, 1998). Partial melts of TTG composition can be produced in basaltic systems at a wide variety of pressures (up to 3 GPa), under either water-saturated or undersaturated conditions (Martin, 1987; Wedepohl et al., 1991; van der Laan & Wyllie, 1992; Springer & Seck, 1997). Dehydration melting of amphibolitic source rocks can also yield such melts, with or without residual hornblende (Beard & Lofgren, 1991; Rapp et al., 1991; Rushmer, 1991; Wolf & Wyllie, 1994; Rapp & Watson, 1995).
Springer & Seck (1997) examined the phase petrology of basaltic source rock melting experiments and calculated REE patterns based on mineral compositions and partition coefficients. The major mineral phases examined were hornblende, opx, cpx, and plagioclase, with or without garnet. Garnet occurs in high-pressure (>1 GPa) experiments, replacing hornblende by volume (Beard & Lofgren, 1991; Rapp et al., 1991; Rushmer, 1991; Wolf & Wyllie, 1994; Rapp & Watson, 1995). Estimated melt REE patterns are dependent largely on the residual phases with which they are in equilibrium, suggesting flatter patterns for hornblende-rich lower-pressure (<1 GPa) partial melts, whereas HREE-depleted patterns occur for garnet-rich higher-pressure melts. Positive, negative, or nil Eu anomalies can occur in various pressure ranges, dependent on both plagioclase abundance and melt composition. Nekoma LK and ELK lavas show flat (La/Lu < 2) patterns with very small Eu anomalies (Fig. 6). This may compare with the low-pressure melting of metagabbro [see S37 of Springer & Seck (1997)], but the source composition must be more LREE depleted, because the La/Lu ratio of the S37 word melt at 0·5 GPa is 5 (La/Yb = 3). REE patterns of experimental melts thus suggest that lower-K suite Nekoma lavas can be derived from melting of hornblende-bearing rocks without garnet in the residue.
Johannes & Holtz (1996) compiled the major element compositions of products of amphibolite dehydration melting. K contents of the experimental melts range from ELK to MK. LK melts persist in the pressure range >1 GPa, with garnet in the residue (Fig. 15). K contents of Nekoma lavas compare with melt compositions produced between 0·8 and 1 GPa. Other major element contents including total alkalis, Ti, Ca, Al, Mg and Fe also fall between the 0·8 and 1 GPa experimental melt compositions. High FeO*/MgO tholeiitic characteristics are attained when melts are produced at lower pressures. The experimental results suggest that the pressure for the Nekoma lavas was as high as 1 GPa. Lower K contents found in the Nekoma lavas are closer to the garnet residue experimental melts, but such an origin is negated by flatter REE patterns. This discrepancy could be due to a lower-K source amphibolite composition for the Nekoma lavas. This will be discussed in a later section. The preferred pressure range equates to lower-crustal depths in the area, i.e. it is unlikely to be granitoids (Hasegawa et al., 1991). Hornblende gabbros or hornblende granulite xenoliths frequently occur at Ichinomegata in the NE Honshu back arc, and are considered to be lower crustal in origin (Aoki, 1971,1987; Arai & Saeki, 1980; Zashu et al., 1980). Most of them have major and trace element compositions consistent with an origin as cumulates from underplated basalt magmas (e.g. Arai & Saeki, 1980), and therefore may be suitable as sources for the Nekoma lavas. Although the Ichinomegata volcanic center is located on the Abukuma terrane, differing from Nekoma, hornblende-bearing lower-crustal rocks exist beneath NE Honshu. Hornblende-bearing lower-crustal rocks are thus the most plausible Nekoma lava sources. Intensive fractionation of hornblende from basalt magma also causes the same effect. Because hornblende has a higher K content than anhydrous silicate minerals, intensive hornblende fractionation still maintains the low-K characteristics of the parental basalt. However, the fact that hornblende phenocrysts or their breakdown products are absent in the Nekoma lavas may support crustal melting, rather than hornblende fractionation at lower-crustal depths. This petrographic evidence is the reverse case of that at Sangeang Api volcano, Indonesia, which is regarded as an example of hornblende fractionation (Foden & Green, 1992).
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, bulk-rock compositions of Nekoma ELK and LK andesites. All Nekoma lavas fall between the high-pressure (>1 GPa) and lower-pressure (<0·8 GPa) experimental melt compositions except for FeO. TH (tholeiitic) low to extremely low-K suite low-alkali andesites can be produced directly from hornblende-bearing crustal rocks by dehydration melting.
Modeling of partial melting of hornblende-bearing lower crust
Consideration of partition coefficients between minerals and melts and phase stabilities of minerals is necessary before partial melting modeling. Because the composition of hornblende can vary, trace element partitioning between hb and melts is complex, as a result of the factors of composition, pressure and temperature. Amphibole–melt element partitioning has recently been determined in basaltic melts over a wide range of pressure and SiO2 content (e.g. Adam & Green, 1994; Tiepolo et al., 2000). Cpx–melt partitioning also largely controls the REE composition of melts produced in basaltic systems, because of relatively higher D values in the HREE region. Cpx–basaltic melt trace element partitioning has been determined for experimental run products using highly sensitive analytical techniques such as secondary ion source mass spectrometry (SIMS) (e.g. Blundy & Wood, 1994; Green et al., 2000).
Both hb and cpx D values for a range of compositions need to be considered to model the generation of Nekoma LK and ELK lavas. The experimental results of Adams & Green (1994) and Tiepolo et al. (2000) show that inter-mineral partition coefficients for amphibole–cpx for REE, Ti, Zr, Hf are similar (Damp/cpx = 1–5), but are high for Ta, Nb, K, Ba, and Sr (Damp/cpx = 20–400). As noted above, REE, U and Th concentrations in Nekoma ELK lavas are similar to other NE Honshu arc lavas, whereas Nb, Ta, K, Rb and Cs are anomalous. K is more compatible in hb than in cpx. D(Rb) and D(Cs) are inferred to be two or three orders of magnitude higher in hb (e.g. Green, 1994). Although D values for LILE between melts and hb are greater than one, these differences largely affect the chemical characteristics of the melt produced by melting of hb-bearing lower-crustal rocks. Biotite is also a potential host of K, Rb and Ba, but does not occur in melting experiments of various source rocks at andesite magma temperatures (>800°C; e.g. Takahashi, 1986; Johannes & Holtz, 1996; Takahashi et al., 1997). Biotite also does not affect the REE abundances and is thus neglected here because of its low thermal stability. Plagioclase is also the residual phase in melting experiments of hb-bearing crustal rocks (e.g. Johannes & Holtz, 1996), and has high D values for Sr, Ba, and Eu as shown in Fig. 10. This is also taken into account in modeling of melting or fractional crystallization.
We modeled partial melting of hb-bearing crustal rocks using the following steps:
- changes in mineral assemblages and melt chemical compositions over various degrees of partial melting (F = 0·19–0·53) were assumed following the 0·8 GPa lower K metabasalt dehydration melting experiment of Rapp & Watson (1995).
- Compositional differences in hb D values were estimated based on the observed relationships between melt SiO2 and D values reported by Tiepolo et al. (2000). Molar proportions of Ti in hb are necessary to determine D values, and in this case were estimated by using the determined D(Ti) = 1·5 at 0·8 GPa after Adam & Green (1994). D(Nb/Ta) was determined using equation (2) of Tiepolo et al. (2000), and D(Nb/La) was estimated by applying the relationship between mg-number and D(Nb/La) shown in Fig. 10 of the same paper. D(Nb) was calculated based on the melt SiO2–D(Nb) relationship shown in fig. 5 of Tiepolo et al. (2000), and D(Ta) and D(La) were then calculated based on the D(Nb/Ta) and D(Nb/La) relationships above. D values for Zr, Rb, Sr, Ba, and K were adopted from the compilation of Rollinson (1993). D values for cpx Y and Ho were calculated based on the SiO2–Ho,Y relationship reported by Green & Pearson (1985). Other D values for cpx and those for olivine, opx, plagioclase were adopted from Rollinson (1993) (Table 3).
- At a degree of partial melting F = 0·39, the experimental melt composition is close to the most silica-poor ELK lava. Source rock incompatible element composition was estimated using the inverse equation of batch melting (Shaw, 1970) and bulk D values. At this step, we applied the K2O content from the Nekoma ELK lavas.
- With the estimated source composition, forward melting calculations were conducted stepwise, using different mineral assemblages and composition dependent D(bulk) (Table 3) at F = 0·53, 0·39 (inverse of step 3), 0·25, 0·22, 0·21 or 0·19. This iterative method allows us to simulate the entire compositional range of the Nekoma lavas. Estimated compositions are displayed in Table 3.
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Calculated amphibolite source composition and partial melt compositions at various degrees of partial melting are indicated in Fig. 16, along with the compositions of Nekoma ELK and LK lavas and Aoso basalt. D(Ce) is assumed to be equal to D(La). It is notable that estimated trace element source compositions are close to those of Aoso basalt. This is consistent with the extremely low LILE contents and very high Nb/Ta ratios apparent in both ELK and Aoso lavas. These extreme features suggest a close genetic link between the two suites.
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At lower degrees of partial melting (F = 0·19–0·24), LILE such as Ba, Rb, K, and Sr in Nekoma ELK felsic andesites to dacites plot well above the trend of the amphibolite melting model. This is also true for the HFSE and Ce. The melting trend path is close to the mafic ELK chemical groups at high degrees of melting (F = 0·25–0·45), but is not for the felsic group. As noted above, silicic ELK lavas have major and trace element compositions closer to the LK suite, and may belong to that group.
The above results suggest that amphibolite melting at lower-crustal depth (0·8 GPa) is a plausible mechanism for the origin of the mafic Nekoma ELK lavas. Also, Aoso basalt may be a natural analog of the source amphibolite composition. However, the estimated degrees of melting for the Nekoma lavas range from 25% to 45%, and are very high for a natural system. Fractional crystallization of the extremely low-K Aoso basalt could also account for the origin of the Nekoma mafic ELK lavas as discussed below.
Partial melting or fractional crystallization?
The REE composition of the source was estimated based on D(La)bulk and D(Y,Ho)bulk by using the same D(Y, Ho)bulk for the HREE, and by interpolating a smooth curve between D(Gd)bulk and D(La)bulk for the LREE. The primitive mantle normalized pattern of the estimated composition of the source amphibolite (Table 4) is almost flat, but is slightly LREE enriched (Fig. 17). The amphibolite is also very depleted in Nb, Ta, Rb and K, comparable with N-MORB. The Aoso basalt has a very similar trace element pattern to the amphibolite source, except for negative Ce and Zr anomalies (Fig. 17).
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Very large positive spikes for Ba and Sr occur in the estimated source composition, but these are almost eliminated at a high degree of partial melting (F = 0·39). This finding contradicts the apparently small slab fluid flux component in ELK lavas, as identified from Ba/Th systematics. If the source amphibolite was formed by underplating of Aoso-like basalt, the basalt itself had a significant fingerprint of slab-derived fluid. Sediment flux addition is also possible, because of the intense positive Sr anomaly in the source material and enriched Sr isotopic signature. Nb and Ta are depleted relative to LILE in both the source and in the ELK lavas, suggesting that the amphibolite already possessed an arc signature. However, very high Nb/Ta ratios are also inherited from the source material (see modeled results in Fig. 8). There is no change in Nb/Ta between source amphibolite and mafic ELK lavas at higher degrees of partial melting (F = 0·25–0·40), and the source ratio is very similar to that of Aoso basalt. This further suggests that extremely high Nb/Ta basalt and amphibolite exist beneath the NE Honshu arc.
Extremely low-K Aoso basalt can also be the source magma for Nekoma ELK lavas through intensive fractional crystallization. To test this possibility, mass balance calculations were applied using the major element composition of the Aoso basalt as the parent and that of mafic ELK as the daughter. Amphibole was not included in this calculation because it is not an actual phenocryst phase in the Aoso and Nekoma lavas. Estimated fractionation phases are olivine:cpx:plagioclase:magnetite = 14:18:58:9 at 30% solidification. The calculated results fit very well, with residual sum of squares of 0·068 (Table 4).
Using the mineral assemblage with estimated Dminerals for basaltic compositions (D values at F 0·53 in Table 3), we modeled trace element behavior during fractional crystallization from Aoso basalt at F = 0·9, 0·8, 0·7 up to 30% solidification. Estimation of D values for the REE region follows the same method as in the ELK source calculation. The results obtained are shown in a mantle-normalized variation diagram in Fig. 17. The result cannot reproduce the composition of the ELK basaltic andesite, particularly for the LREE and HFSE. As a result of intensive fractionation of plagioclase and cpx, Sr and HREE are almost the same as those in mafic ELK. If we neglect the major element mass balance, 60% solidification with certain mineral phases and D values as used in the melting calculation at F 0·39 (Table 3) can reproduce the ELK trace element pattern from the Aoso basalt (see Fig. 16, F 0·4). This is the reverse course of the amphibolite melting calculation. However, this possibility is negated by the major element mass balance if we assume Aoso basalt as a source. Concequently, fractional crystallization of the Aoso basalt cannot produce ELK-like compositions.
As we cannot be certain of the major and trace element source compositions of melted amphibolite for the Nekoma ELK because of uncertainties in D values, we equally cannot be unequivocal in concluding that low-K amphibolite melting was the major process responsible for generation of the ELK mafic andesite at Nekoma. However, the coincidence of major and trace element compositions between the Aoso basalt and the calculated amphibolite gives a realistic basis for this proposal. We conclude, at this stage, that partial melting of lower-crustal low-K amphibolite was a potential source of the ELK mafic andesites. The source of the amphibolite itself could be an Aoso-like basalt. In this case, underplating and remelting of an extremely low-K basalt is the most plausible origin of the Nekoma ELK lavas.
Geophysical constraints and genetic relationship between different K suites
The seismic Moho depth beneath Nekoma volcano is 35 km (Hasegawa et al., 1991). Our geobarometric estimation is thus well within crustal depths. Muro et al. (1997) estimated from seismic data that a 2–3% partial melt zone exists beneath Nekoma volcano just below the Moho. This geophysical evidence suggests that a heat source exists close to the Moho, and intrusion of mantle-derived basalt causes crustal melting. The heat energy necessary for crustal melting in the NE Honshu mantle–crust section is small because of high ambient temperatures near the Moho beneath the volcanoes (Takahashi, 1986). The mantle-derived heat source and high thermal gradient in the area allow a high degree of crustal partial melting to occur.
The ELK, LK, and MK suites at Nekoma are thought to originate from discrete sources, because their Sr isotopic compositions differ (Fig. 13). Differing degrees of partial melting of a common source cannot produce the range of K suites found in Nekoma, although trace element patterns closely resemble each other except for certain key elements. These three K suites coexist in stages, such as ELK and LK in Stage 2 and LK and MK in Stage 3 (see Fig. 2), suggesting variable sources existed beneath the volcano. However, the overall trend of K-level increase with stage may reflect changes in the source over a period of 0·7 my. ELK lavas occur only in the climactic Stage 2 activity, and the erupted volume was small. In this context, ELK is a minor end-member, and probably originated by remelting of lower crust by heating as a result of intensive input of LK source magma. MK suite lavas differ from those of the LK suite in terms of both trace elements and isotopes, and are more similar to lavas from Izu–Mariana. MK lavas may be derived from a medium-K basalt parent by intensive fractional crystallization with lesser crustal assimilation.
Significance of crustal melting for intermediate to silicic arc magmatism
If our underplating and remelting model is correct, some significant points are raised that bear on our understanding of intermediate to silicic arc magmatism. These are: (1) intensive modification of certain incompatible trace element ratios during melting; (2) limitation of compositional changes between underplated basalt and resultant crustal melt; (3) existence of extremely low-K basalt beneath the frontal arcs and its role in the origin of low-K TTG arc crust.
It is notable that the prominent Sr spikes in the source hb-bearing lower crust are almost completely eliminated by the partial melting process (see Fig. 17). As we discussed above, lower Ba/Th ratios and small positive Sr spikes found in the Nekoma ELK lavas are inherited from the mantle-derived basalt, but are the result of modification in the underplating–remelting process. The Aoso basalt is compositionally a natural analog of the source. In this context, either Aoso-type magma generated beneath the NE Honshu arc or amphibolite formed at lower-crustal depths by underplating of mantle-derived basalt must have already possessed a strong arc signature, possibly derived from fluid-flux addition (e.g. Pearce & Parkinson, 1993). This element ratio modification might also be one cause of the decoupling in behavior between Sr isotopic compositions and Sr concentrations in arc lavas. This can be caused by Sr fractionation during mantle melting (e.g. Arculus & Powell, 1986; Green et al., 2000), but the intracrustal process discussed here is another option. Although lateral variation in incompatible trace element ratios of andesite lavas occurs across the arc (e.g. Gill, 1981; Sakuyama & Nesbitt, 1986, for NE Honshu arc), modification during crustal processes should be reconciled, as we see large differences between the source basalt and the remelted magmas, not only in Ba/Th but also in REE (see Fig. 17).
Foley & Wheller (1990), Green (1995), Stolz et al. (1996), Münker (1998) and others examined Nb/Ta fractionation in mantle and igneous rocks. Green (1995) and Münker (1998) compiled D(Nb) and D(Nb)/D(Ta) for various mineral–melt and mineral–fluid pairs. Münker (1998) examined various mantle process models and emphasized the importance of high D(Nb)/D(Ta) residual minerals, such as hb or rutile, for generation of the low Nb–Ta abundances commonly found in arc lavas. He emphasized the role of fluid, coupled with residual rutile. Rutile is not stable in supersolidus conditions in other magmatic processes (e.g. Ryerson & Watson, 1987; Ayers & Watson, 1993). Major residual phases in partial melting experiments of amphibolite are olivine, pyroxene, magnetite, and plagioclase, because of consumption of hb by incongruent melting. The ELK and LK Nekoma lavas and the Aoso basalt have higher Nb/Ta ratios than any other Quaternary arc lavas in Japan (Fig. 8). The low-K amphibolite source partial melting model does not change Nb/Ta greatly at high degrees of melting (>25% melting), suggesting an extremely high Nb/Ta ratio basalt source. The modeled result also shows that once hb occurs in the residue, significant lowering of Nb/Ta ratios and HFSE concentrations takes place in the lower melting range between 25 and 19% (Figs 8 and 16). This suggests that melting of lower-crustal amphibolite depletes HFSE and modifies Nb/Ta in felsic magmas. This leads to the important observation that arc characteristics in intermediate and evolved lavas can become more pronounced as a consequence of intra-crustal processes, such as underplating–remelting and assimilation of lower-crustal melts into mantle-derived basalts.
Another significant result from our work is that extremely low-K intermediate lava could be derived either by underplating–remelting (this paper) or by fractionation of extremely low-K basalt (e.g. Pearce et al., 1995). In both cases extremely low-K basalt with a K content comparable with MORB is required. Such basalt has been reported very occasionally. However, it may be common not only in oceanic arcs (e.g. South Sandwich Island arc) but also in old arcs such as NE Honshu (Aoso basalt and Nekoma ELK source). The highly LILE-depleted nature may be due to melting of a depleted MORB source mantle (Pearce et al., 1995), but more enriched REE abundances and isotopic signatures close to Bulk Earth at Nekoma suggest an isotopically enriched mantle source. Isotopic evolution of the source could be achieved by a longer residence time as amphibolite in the lower crust, but the time necessary for such evolution is unrealistic (more than several billion years). The NE Honshu arc is more complex than previously expected (e.g. Tatsumi & Eggins, 1995), possibly reflecting multiple enrichment–depletion processes in the mantle wedge (or mantle lithosphere) through subduction for a lengthy period, and including crustal formation processes in the arc.
The partial melting model examined in this paper (Figs 15 and 16) suggests that silicic melts with very low incompatible element concentrations can be produced by partial melting of lower-crustal amphibolite at lower degrees of melting (F = 0·20–0·22 for SiO2 at 72–64 wt %). Even in this case, the depleted incompatible element characteristics would be inherited from the source amphibolite itself (see Fig. 16). This suggests that TTGs could be produced by remelting of amphibolites with a low-K basalt composition. Such amphibolites occur in young oceanic arcs associated with low-K basalts [e.g. the Izu–Mariana–Bonin arc: Kawate & Arima, 1998; Nakajima & Arima, 1998]. This geological coincidence of TTG and low-K basalt further suggests that formation of low-K amphibolitic lower crust can be achieved by underplating of low-K basalts. Formation of TTGs may be closely related to this process, which could operate intensively in young oceanic arcs (e.g. Izu–Mariana–Bonin) and in old arcs with thicker crust such as NE Honshu.
Finally, we propose giving more attention to the modification of mantle-derived basalts at crustal depths when discussing models for andesite genesis and arc crust generation. The above results show that the identification of the fingerprints of crustal and mantle melts demonstrates the importance of crustal melting in the generation of arc magmas (e.g. Arculus & Powell, 1986; Kersting et al., 1996; Gust et al., 1997; Kimura et al., 2001). The question of the genetic relationships between ELK, LK, and MK remains open, and needs to be examined further. Studies of the MK suite from Bandai or other MK and HK suite lavas in the NE Honshu arc (Kimura et al., 2001) will be relevant, and will be presented in a subsequent paper.
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CONCLUSION |
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TOPABSTRACTINTRODUCTIONGEOLOGICAL BACKGROUNDSAMPLES AND ANALYTICAL...GEOCHEMISTRY OF LAVASELEMENT DISTRIBUTION BETWEEN...DISCUSSIONCONCLUSIONREFERENCES |
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Major, trace element and isotopic compositions of ELK Nekoma lavas suggest that they were derived from lower-crustal melts produced initially by dehydration melting of hb-bearing rocks. ELK lavas have anomalously low Cs, Rb, and K contents, and high Nb/Ta and Zr/Hf ratios without positive Sr and Ba spikes. These features are considered to be controlled by the composition of the hb-bearing source rock, which was formed by crustal underplating of low-K basalt, and remelted at lower-crustal pressures. Flat REE patterns negate the possibility that garnet was present in the residual phase assemblage, and constrain melting to pressures <1 GPa. Major element compositions of the ELK lavas are similar to experimental melts produced by amphibolite dehydration melting. This view is further supported by modeling based on trace elements. A basalt from Aoso volcano in the same arc is a natural analog of the source amphibolite composition. The existence of this analog lends support to the underplating and two-stage remelting model. Intermediate Low-K lavas predominate along frontal volcanic arrays in island arcs, and some may have been derived by such a process.
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ACKNOWLEDGEMENTS |
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Our thanks go to Professor J. P. Davidson at UCLA for helpful comments on the manuscript. Critical and constructive reviews by Professors R. J. Arculus and T. H. Green improved this manuscript considerably. Professor K. Manabe at Fukushima University provided free access to XRF facilities, and Mr S. Chiba of Fukushima–Higashi High School provided Bandai lava samples. Dr Y. Takaku and Mr S. Scott of Thermo VG Elemental Co., and Mr K. Ohki of OK Laboratory Ltd. provided technical support for laser ablation ICP-MS analysis. Dr B. Roser of Shimane University helped us with the English and thoughtful comments. This study was supported by grants-in-aid from the Ministry of Education, Science, Culture, and Sports 10304038 and 10304038 (Rep.: J.-I. Kimura), 07304041 (Rep.: H. Kagami), 11440150 (Rep.: A. Takasu), 11640454 (Rep.: S. Sano), 11640451 (Rep. S. Iizumi), 12304031 (Rep. T. Yoshida).
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FOOTNOTES |
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*Corresponding author. Telephone: 81-852 32 6462. Fax: 81-852 32 6469. E-mail: jkimura{at}riko.shimane-u.ac.jp
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REFERENCES |
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