Journal of Petrology Advance Access originally published online on April 15, 2005
Journal of Petrology 2005 46(9):1769-1803; doi:10.1093/petrology/egi033
| ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
Are Arc Basalts Dry, Wet, or Both? Evidence from the Sumisu Caldera Volcano, Izu–Bonin Arc, Japan
1 INSTITUTE FOR RESEARCH ON EARTH EVOLUTION (IFREE), JAPAN AGENCY FOR MARINE–EARTH SCIENCE AND TECHNOLOGY (JAMSTEC), YOKOSUKA 237-0061, JAPAN
2 INSTITUTE OF GEOSCIENCE, GEOLOGICAL SURVEY OF JAPAN/AIST, TSUKUBA 305-8567, JAPAN
3 SMITHSONIAN INSTITUTION, WASHINGTON, DC 20560, USA
RECEIVED APRIL 20, 2004; ACCEPTED MARCH 8, 2005
 |
ABSTRACT |
|---|
Basalt–basaltic andesite (<55 wt % SiO2) and dacite–rhyolite (66–74 wt % SiO2) are the predominant eruptive products in the Sumisu caldera volcano, Izu–Bonin arc, Japan. The most magnesian basalt (8·5% MgO), as well as some of the other basalts, has a low Zr content (20–25 ppm), and cannot yield basalts with higher Zr contents (29–40 ppm) through fractionation and/or assimilation. The high- and low-Zr basalts have different phenocryst assemblages, olivine, plagioclase and pyroxene phenocryst chemistries, REE (rare earth element) patterns, and fluid-mobile element/immobile element ratios. Estimated primary olivine compositions are more magnesian (>Fo91) in the low-Zr basalts compared with those in high-Zr basalts (<Fo89). The low-Zr basalts contain up to 11 vol. % augite, but many high-Zr basalts are free of augite, which appears only in their more differentiated products. The low-Zr basalts are considered to be hydrous magmas in which olivine crystallizes first followed by augite and plagioclase, whereas the high-Zr basalts are dry. The low-Zr basalts have higher U/Th ratios than the high-Zr basalts. We suggest that both dry and wet primary basalts existed in the Sumisu magmatic system, each having different trace element concentrations, mineral assemblages and mineral chemistry. The lower contents of Zr and light REE and magnesian primary olivines in the wet basalts could have resulted from a higher degree of partial melting (
20%) of a hydrous source mantle compared with 10% melting of a dry source mantle. The Sr, Nd and Pb isotope compositions of the wet and dry basalts are similar and are limited in range. These lines of evidence indicate that a mantle diapir model might be applicable to satisfy the configuration of such a mantle source region beneath a single volcanic system such as Sumisu. KEY WORDS: degree of melting; hot fingers; isotopes; mantle diapir; mantle wedge
|
INTRODUCTION |
|---|
Fluxing of water released from the subducted slab into the overlying mantle wedge lowers the mantle solidus, triggering magma generation that results in the formation of arc-front volcanoes. Thus we expect arc basalts to be wet, and there are many examples containing a few percent of water to support this (e.g. Sisson & Layne, 1993
; Roggensack et al., 1996, 1997; Sobolev & Chaussidon, 1996; Newman & Stolper, 2000). For example, Sisson & Layne (1993) have undertaken an ion and electron microprobe study of basalt and basaltic andesite glass inclusions in olivine phenocrysts from Quaternary eruptions of arc volcanoes and directly demonstrated that typical arc basalts and basaltic andesites can have high H2O contents, sometimes in excess of 6 wt % H2O. Roggensack et al. (1997) showed that Cerro Negro's 1992 magma evidently retained high concentrations of volatiles (H2O and CO2), up to 6·1 wt % H2O and 1039 ppm CO2, and erupted explosively. On the other hand, substantial mantle upwelling and pressure-release melting may also take place beneath arc-front volcanoes. Sisson & Bronto (1998) presented measurements of the volatile content of primitive basalts from Galunggung volcano in the Indonesian arc. Inclusions of mafic glass in olivine phenocrysts in these basalts are characterized by uniformly low H2O concentrations (0·21–0·38 wt %) but relatively high levels of CO2 (up to 750 ppm), indicating that the low H2O concentrations are primary and not due to magma degassing. Such nearly H2O-free basaltic magmas, which are also found along the Cascade arc, were considered to be derived by pressure-release melting caused by upwelling in the sub-arc mantle (Sisson & Bronto, 1998). Similarly, some basalt magmas at the front of the Izu–Bonin arc are either anhydrous or nearly so (Aramaki & Fujii, 1988; Fujii et al., 1988; Nakano et al., 1988; Nakano & Yamamoto, 1991). Significantly, there is petrological evidence that plagioclase phenocrysts accumulated in the upper parts of magma chambers beneath the Izu-Oshima volcano in this arc. The plagioclase phenocryst content in the Izu-Oshima lavas varies between 0 and 20% by volume and mafic phenocrysts are rare, yet all of the lavas have similar groundmass (melt) compositions. For plagioclase to float in a basaltic liquid, the H2O content must be less than 0·7% (Aramaki & Fujii, 1988; Fujii et al., 1988; Nakano et al., 1988; Nakano & Yamamoto, 1991), and so the abundance of phenocryst plagioclase and the rarity of mafic phenocrysts suggest dry conditions at depth.
A fundamental question remains, therefore, as to whether arc basalts are dry, wet or both. What are the relationships between fluid-flux melting and pressure-release melting beneath arc volcanoes? The example we discuss here is the Sumisu caldera volcano in the Izu–Bonin arc. We present evidence that there are two kinds of basalt erupted from the volcano, which have petrographic, mineralogical and geochemical differences. These systematic and interrelated differences could have resulted from different water contents in the parental magmas and differing degrees of partial melting in the source mantle. We propose that the wet basalts were derived from a more depleted mantle source than the dry basalts, and resulted from different degrees of melting, 20% and 10%, respectively. We develop a model for a mantle diapir beneath the volcano, which could produce both dry and wet basalts simultaneously in the same magmatic system. Our findings may have relevance to magma genesis models in other subduction zones.
|
GEOLOGICAL SETTING AND SAMPLE COLLECTION |
|---|
 TOPABSTRACTINTRODUCTION GEOLOGICAL SETTING AND SAMPLE... ANALYTICAL METHODSROCK TYPESPETROGRAPHY AND MINERAL...FRACTIONATION MODELSRARE EARTH ELEMENT (REE)...DEGREE OF MELTINGISOTOPESDISCUSSIONCONCLUSIONSREFERENCES |
|---|
The northern Izu–Bonin arc consists of 11 Quaternary volcanoes and eight Quaternary submarine caldera volcanoes (Fig. 1a and b). Sumisu (31·5°N, 140°E) (Fig. 2a), about 500 km south of Tokyo, is one of these arc-front volcanoes characterized by a well-formed caldera, 8 km x 9 km in diameter with 600–700 m high inner walls (Maritime Safety Agency, 1997). The 136 m high spire of Sumisu Island stands on the southern caldera rim (Fig. 2b). Some parts of the caldera rim and Sumisu Knoll No. 2, just west of the caldera, have flat tops about 100–200 m below sea level, possibly reflecting wave planation during periods of Pleistocene sea-level lowering. Thus, these flat-topped bathymetric surfaces may be older than 20 ka BP (Maritime Safety Agency, 1997; Iwabuchi, 1999
). On the other hand, the summit of the conical Shirane volcano is only about 7 m below sea level, and there is no evidence of planation. Shirane is, therefore, the newest volcanic feature in the Sumisu area. After Hakone volcano, Sumisu is the largest volcanic complex in the Izu–Bonin arc. We sampled the Sumisu area in September–October and December 2002 using JAMSTEC's manned submersible Shinkai 2000 and the ROV Dolphin 3K. Additional surveys included SeaBeam mapping and single-channel seismic traverses.
|
|
Between 27° and 34°N a series of narrow grabens, a few tens of kilometres wide, rift the Izu–Bonin arc adjacent to the active volcanic front edifices. Most of the grabens lie west of the frontal arc volcanoes. The Sumisu Rift (Fig. 1b) is one of these grabens and provides a present-day example of the initial stages of back-arc basin evolution (Ikeda & Yuasa, 1989
; Fryer et al., 1990; Hochstaedter et al., 1990). Ocean Drilling Program (ODP) Leg 126 drilled two sites 2·4 km apart in the Sumisu Rift (Sites 790 and 791) (Taylor, 1992). The basement basalts are similar to the younger Sumisu Rift basalts, which are exposed on the Sumisu Rift sea floor (Gill et al., 1992).
Sumisu Island
The sharp spire of Sumisu Island is 270 m long, 100 m wide and rises 136 m above sea level. The island has no flat ground and is surrounded by a narrow boulder beach, where the surf pounds incessantly. Sumisu Island stands on a flat submarine pedestal, whose 2 km x 5 km area lies about 100 m below sea level. Only one chemical analysis (major element) of dolerite has previously been reported from this island (Ossaka et al., 1985). We collected 12 new samples along the steep eastern side of the island (Fig. 2b); these include augite–plagioclase dolerite, olivine–plagioclase basalt, and plagioclase basalt.
Submersible and dredge hauls
The ROV Dolphin 3K and the manned submersible Shinkai 2000 were used to obtain samples from the post-caldera central cones, caldera walls, and caldera floor. Dive tracks and locations of samples are shown in Fig. 3. Most samples were collected from lava breccias, jointed dykes and/or sills, and nearly all contained fresh glass.
|
Eight dredge hauls (D1–D8) were collected in the Sumisu area in December 2002 (Fig. 2a). Varying amounts of pumice were collected from the flanks of the Sumisu caldera volcano, but none was recovered from the caldera floor or walls. Thus, these pumices pre-date the caldera or were erupted at the time caldera formed. On the basis of the chemical composition and petrography of the lava blocks and pumice, the samples fall into two groups. Those produced during the Sumisu caldera-forming eruption (D1, D2, D3, D5, D8) range from basalt, through andesite, to dacite lavas and rhyolite pumice, whereas those from the flanks of Sumisu Knoll No. 1 (D6) and Sumisu Knoll No. 2 (D4) and D7 are mostly high-SiO2 pumice. The Sumisu Knolls (No. 1, No. 2) and other bathymetric highs west of Sumisu caldera probably postdate the Sumisu caldera. Pumices from D4, D6, and D7 are white high-silica rhyolite (
76% SiO2) with phenocrysts in decreasing order of abundance of plagioclase, quartz, orthopyroxene and opaque minerals. In contrast, the pumices associated with the Sumisu caldera are aphyric yellowish rhyolite (70–73 wt % SiO2). |
ANALYTICAL METHODS |
|---|
After initial splitting and jaw crushing, all samples were pulverized in an agate ball mill. Major and trace elements were determined by X-ray fluorescence (XRF) at IFREE, JAMSTEC. Trace elements were analysed on pressed powder discs, and major elements were determined on fused glass discs. A mixture of
0·4 g of each powdered sample and 4 g of anhydrous lithium tetraborate (Li2B4O7) was used; no matrix correction was applied because of the high dilution involved. Rare earth elements (REE) were determined by inductively coupled plasma mass spectrometry (ICP-MS) using a VG Elemental® PQ3 instrument enhanced with a chicane lens system, following the procedures described by Chang et al. (2003). Whole-rock powder was digested with an acid mixture of HF–HClO4–HNO3, finally dissolved in 2% HNO3 and spiked with 115In and 209Bi for internalization of signal in ICP-MS measurements. The analytical precision of REE determinations, as estimated from repeated analysis of well-established reference standards (JB-2 and BHVO-1), is better than 2% (1
). Major and trace element data and modal mineralogy for basaltic rocks from the Sumisu caldera volcano are reported in Table 1.
|
Sr, Nd and Pb isotope ratios were measured at the Geological Survey of Japan/AIST on a seven-collector VG Sector 54 mass spectrometer. About 250–300 mg of hand-picked rock chips with a grain size of 0·5–1 mm were leached with 6N HCl for 30–45 min at 100°C prior to dissolution; Sr and Pb were isolated using Sr resin (Eichrom Industries, Darien, IL, USA) following the procedure described by Hoang & Uto (2003)
. Procedural Pb blanks were <50 pg, and considered negligible relative to the amount of sample analysed. For Nd isotopic analysis, the REE were initially separated from major elements and Ba by cation exchange, before isolation of Nd using Ln resin (Eichrom Industries). Sr and Nd isotope ratios were determined as the average of 200 ratios by measuring ion intensities in multi-dynamic collection mode. Isotope ratios were normalized to 86Sr/88Sr = 0·1194 and 146Nd/144Nd = 0·7219. Measured values for NBS SRM-987 and JNdi-1 were 87Sr/86Sr = 0·710276 ± 6 (2 SD, n = 4) and 143Nd/144Nd = 0·512104 ± 12 (2 SD, n = 4), respectively, during the measurement period. Pb isotopic measurements were made in static mode using four Faraday detectors. Natural (unspiked) run measurements were made on 65–75% of the collected Pb, giving beam intensities of 0·5–1 x 10–11 A of 208Pb. The rest of the collected Pb was spiked on a filament with the Southampton–Brest–Lead 207–204 spike (SBL74), such that the 204natural/204spike is 0·1 (Ishizuka et al., 2003). The true Pb isotopic compositions were obtained from the natural and mixture runs by iterative calculation adopting a modified linear mass bias correction (Johnson & Beard, 1999). The reproducibility of this Pb isotopic measurement (external error: 2 SD) by double spike is <200 ppm for all 20xPb/204Pb ratios. Internal errors were estimated by propagating within-run 2 SE through a closed-form linear double-spike deconvolution (Johnson & Beard, 1999). Measured values for NBS SRM-981 during the measurement period were 206Pb/204Pb = 16·9390 ± 19, 207Pb/204Pb = 15·5013 ± 18 and 208Pb/204Pb = 36·719 ± 4 (2 SD, n = 8), respectively. Radiogenic isotope ratios of representative basalts from the Sumisu caldera volcano and the internal errors are reported in Table 2.
|
Microprobe analyses were carried out on the JAMSTEC JEOL JXA-8900 Superprobe equipped with five wavelength-dispersive spectrometers (WDS). Olivine analyses were made with a counting time of 100 s, using an accelerating voltage of 20 kV, a beam current of 25 nA and a probe diameter of 5 µm to ensure reliable Ni values. Pyroxene and plagioclase analyses were made with a counting time of 20 s, using an accelerating voltage of 15 kV and a beam current of 15 nA. Representative mineral compositions in basalts and basaltic andesites from Sumisu caldera volcano are given in Table 3.
|
|
ROCK TYPES |
|---|
The northern part of the Izu–Bonin arc is characterized by bimodal magmatism of basalt and dacite–rhyolite (Tamura & Tatsumi, 2002
); the Sumisu caldera volcano is a good example of this bimodal volcanism. All discussions in this paper refer to analyses that have been normalized to 100% on a volatile-free basis with total iron calculated as FeO. Figure 4a shows the frequency distribution of the SiO2 content of the samples collected by submersible and by dredging (D1, D2, D3, D5, and D8) (Figs 2 and 3). Most samples collected west of Sumisu caldera (D4, D6, and D7) are high-silica rhyolite (>75% SiO2) and are not included in this histogram.
|
Three groups of rocks, basalts (<55 wt % SiO2), andesites (56–61 wt % SiO2) and dacites–rhyolites (>66 wt % SiO2) occur in the Sumisu area, with a distinct SiO2 gap of
6 wt % between the andesites and the dacites (Fig. 4b). The relatively high MgO content of the andesites (2–8 wt %) is not consistent with crystal fractionation from parental basalts, and the SiO2 gap between the andesites and the dacites–rhyolites also discourages the use of crystal fractionation models to explain these relationships. Plots of selected major and trace elements and Y/Zr vs SiO2 for the Sumisu caldera volcano are shown in Fig. 5. Data from the entire northern Izu–Bonin arc (Tamura & Tatsumi 2002) are also plotted in Fig. 5a for comparison. The compositional trends in the Sumisu caldera volcano are much the same as those along the entire Izu–Bonin arc and emphasize the regional nature of this bimodal magmatism (Tamura & Tatsumi, 2002).
|
) (Tamura & Tatsumi, 2002
). (b) Rb, Ba, Sr, Y, Zr and Y/Zr vs wt % SiO2 in rocks from Sumisu caldera volcano.Here, we discuss the basalts of the Sumisu caldera volcano (data presented in Tables 1 and 2); the petrogenesis of the associated andesites, dacites, and rhyolites will be discussed elsewhere.
Two types of basalts
Two types of basalt exist at the Sumisu caldera volcano, which have different Zr contents; these are defined here as low-Zr basalt and high-Zr basalt, respectively. These exhibit characteristic differences in phenocryst assemblage and phenocryst content, mineral chemistry, bulk-rock trace element contents and REE patterns.
Low-Zr and high-Zr basalts
Figure 6 shows the interpreted boundary between low-Zr basalts and high Zr basalts. The low-Zr basalts are represented by only four samples; these include samples with the lowest SiO2 (D3-R16) and the highest MgO contents (575R4). D3-R16 was dredged from the sea floor just west of Sumisu caldera (Fig. 2). Samples 575R4, 575R5 and 1391R4 are from the SE caldera wall, collected by ROV Dolphin 3K and Shinkai 2000, respectively (Fig. 3). These rocks probably came from the jointed dykes and sills that intrude (and thus post-date) the pre-caldera edifice. In contrast, all basalts from Sumisu Island are high-Zr basalts, together with rocks from the SE slope of the Sumisu caldera volcano (D1 and D2) (Fig. 2). ROV Dolphin 3K also collected high-Zr basalts from the same dive traversing the southeastern caldera wall (3K575); these samples are thought to be from brecciated lava flows cropping out in the caldera wall and thus represent samples of the pre-caldera edifice (Fig. 3). The available evidence therefore suggests the high-Zr basalts pre-date the low-Zr basalts.
|
PETROGRAPHY AND MINERAL CHEMISTRY |
|---|
Mineral assemblages
Low-Zr and less-evolved high-Zr basalts have different phenocryst assemblages and phenocryst contents. Figure 6 shows the mode of phenocrysts, Y/Zr and U/Th ratios of low-Zr basalts and representative high-Zr basalts. All the low-Zr basalts contain olivine + clinopyroxene + plagioclase, and one of these (575R4) contains more than 10 vol. % of clinopyroxene. Many high-Zr basalts, on the other hand, contain only olivine + plagioclase. Augite appears only in the more evolved lavas of the high-Zr basalt type. Evolved high-Zr basalts contain clinopyroxene ± orthopyroxene up to 1 vol. % (Table 1), but these phenocryst contents are dramatically less in volume compared with the low-Zr basalts. The total volume of phenocrysts is also greater in the low-Zr basalts (23–38 vol. %) than most of the high-Zr basalts (2–31 vol. %) (Table 1). Because the low-Zr basalts are more magnesian than the high-Zr basalts, the large percentages of clinopyroxene in the more mafic rocks (low-Zr basalts) and the virtual absence of clinopyroxene in the less mafic rocks (high-Zr basalts) are not consistent with fractionation models.
|
Representative mineral compositions in low-Zr and high-Zr basalts and basaltic andesites are presented in Table 3 and shown in Figs 7–10.
|
|
|
|
Olivine
Olivine phenocrysts occur in both the low-Zr basalts and high-Zr basalts. Plots of NiO vs Fo [100 Mg/(Mg + Fe)] for olivines from eight representative samples of Sumisu basalts are shown in Fig. 7a and b. There are interesting differences between the olivines in the low-Zr, clinopyroxene-bearing basalts and those in the high-Zr, clinopyroxene-free basalts. Fo contents in olivines in the high-Zr basalts are commonly around Fo75–79 (complete range Fo70–79; Fig. 7a), whereas those in the low Zr basalts vary widely from Fo71 to Fo91 (Fig. 7b). Some olivines from the low-Zr basalts overlap the distribution of olivines from the high-Zr basalts. Olivines in the low-Zr basalts have lower wt % NiO and/or higher Fo contents than those in the high-Zr basalts at given Fo and at given NiO contents (Fig. 7a and b). We can consider the relationships between the host basalts and phenocryst olivine following the approach of Tamura et al. (2000)
.
Measured vs calculated olivine compositions
A series of equilibrium olivine and basalt compositions were calculated from an original basalt composition using the olivine fractionation model of Tamura et al. (2000), using estimated KD(Fe/Mg)ol/liq values in the range 0·30–0·31, and DNiol/liq from Kinzler et al. (1990). Equilibrium olivines in the low-Zr basalts have higher Fo contents than those in the high-Zr basalts at a given NiO content. These calculated olivines are similar to the actual olivine phenocrysts and, generally, the calculated olivines in equilibrium with both low-Zr basalts and high-Zr basalts also show similar compositional contrasts to the actual olivine phenocrysts; the former are more magnesian at a given NiO content than the latter. The relationships between actual olivine phenocrysts and calculated olivines are, however, different between the low-Zr and high-Zr basalts.
Figure 7 shows plots of NiO vs Fo for olivine phenocrysts, calculated equilibrium olivines and olivine fractionation trends for four high-Zr basalts (SI10, SI14, 575R7, and D2-R4; Fig. 7c) and four low-Zr basalts (1394R4, 575R4, 575R5 and D3-R16; Fig. 7d). High-Zr basalts contain olivine phenocrysts whose compositions overlap those of the calculated equilibrium olivines, and continuously extend towards more iron-rich compositions (Fig. 7c). On the other hand, the low-Zr basalts contain many olivine phenocrysts that are more iron-rich and/or more Ni-rich than the calculated equilibrium olivines, and also a few olivines that are more magnesian (Fo90) and lower in NiO (0·15 wt %) than the calculated olivines. In other words, the distribution of observed olivine compositions in the low-Zr basalts cuts across the calculated olivine fractionation trends (Fig. 7d).
Calculated primary olivines
Sato (1977) pointed out that the first olivines crystallizing from primary magmas should have NiO contents of 0·4 wt %, because the olivines in upper-mantle lherzolites have uniform NiO contents of 0·4 wt %. Current understanding, however, suggests that peridotite olivines do not always have a regionally constant NiO content of 0·4 wt %. Abyssal peridotite olivines, for example, fall in the range of 0·22–0·4 wt % NiO (Dick, 1989). It is, however, reasonable to assume that basalts from a single volcano, such as Sumisu, were derived from local mantle sources, which have similar and constant NiO contents in their constituent olivine. Thus we assume that olivine in the source mantle of the Sumisu basalts has a uniform NiO content of 0·4 wt %. On this basis a series of equilibrium olivine compositions were calculated from selected low-Zr and high-Zr basalts following the method of Tamura et al. (2000) until the calculated equilibrium olivines had NiO contents of 0·4 wt %. We assume that the forsterite contents of these olivines are equal to that of olivines in the primary magmas and their source-mantle peridotites. Although these calculations ignore the effect of clinopyroxene fractionation, the effect of clinopyroxene fractionation in the low-Zr ol–cpx-bearing basalts is to increase the difference between the primary low-Zr and high-Zr basalts. Thus the differences between the primary low-Zr and high-Zr magmas based on olivine fractionation models represent minimum values of the actual differences between the respective mantle source peridotites.
The primary olivine compositions calculated for the low-Zr ol–cpx basalts (575R4, 575R5, 1391R4 and D3-R16) range from Fo88·7 to Fo91·9. In contrast, those calculated for the high-Zr ol basalts (SI10, SI14, 575R7 and D2-R4) range from Fo86·0 to Fo89·0. This suggests that the source mantle of the low-Zr basalts is more magnesian, and thus more depleted, than that of the high-Zr basalts.
Plagioclase
Plagioclase is the dominant phenocryst phase in both high-Zr and low-Zr basalts and basaltic andesites, ranging in volume from 14 to 29%, except for one aphyric andesite (D2-R2). Figure 8 shows frequency histograms of An content [100 x Ca/(Ca + Na)] in the low-Zr basalts and high-Zr basalts and basaltic andesites. Less evolved high-Zr basalts and low-Zr basalts have similar distribution patterns, which peak between An85 and An90. Most plagioclase rim compositions in these rocks extend to lower An contents than the core compositions (Fig. 8), indicating the weak zoning patterns in both basalt types. Plagioclases with An>95, however, appear only in the low Zr basalts (especially in D3-R16 and 575R4); those with An>90 are rare in the high-Zr basalts compared with those in low Zr basalts.
Evolved high-Zr rocks (D1-R9, 1018R2, 575R1 and 575R2) exhibit distinct normal zoning of their plagioclase phenocrysts (Fig. 8); cores have An contents ranging from An80 to An90, whereas the rims have lower An contents ranging from An70 to An80.
Augite
As shown in Fig. 6 and Table 1, augite phenocrysts appear at an earlier, more primitive stage in the low-Zr basalts and at a more evolved stage in the high-Zr basalts. Figure 9a shows the XMg [100 Mg/(Mg + Fe)] values of phenocryst cores of coexisting augite and olivine in both low-Zr and high-Zr basalts. Generally, augites have higher maximum XMg values than coexisting olivines, but the range of augite XMg values is greater than that of the olivines. Olivines and augites in the low-Zr basalts have much wider ranges in composition than those in the more evolved high-Zr basalts.
Olivine–augite geothermometry
Figure 9b shows magmatic temperatures estimated using the olivine–augite Mg–Fe-exchange geothermometer of Loucks (1996). To use this geothermometer, we need an equilibrium pair of compositions of olivine and augite. Temperatures for each sample were estimated using pairs of average, maximum, and minimum Mg-values of olivine and augite cores. The high-Zr basalt samples yielded consistent temperatures of 1150–1200°C. In contrast, the three pairs of olivine–augite compositions from the low-Zr basalts show varying, inconsistent, temperatures in each sample. For example, the temperature estimated using the maximum Mg-values of olivine and augite cores in 575R4 is 1300°C, but the temperature estimated by using the average Mg-values is below 1100°C. Moreover, temperatures estimated by using minimum Mg-values (1200°C) fall near the midpoint between the other two estimates. We therefore conclude that this geothermometer does not perform satisfactorily in the low-Zr basalts.
Two-pyroxene thermometry
Coexisting augite–hypersthene phenocryst pairs are rare in the evolved high-Zr basalts, limiting the use of the two-pyroxene thermometer of Lindsley & Andersen (1983) (Fig. 10). Pyroxenes in the evolved basalt 1018R2 indicate magmatic temperatures of 1100–1200°C, whereas those in andesite 575R1 clearly indicate a lower temperature of 1100°C. As olivines and augites in the same sample of 1018R2 indicate a temperature of 1200°C, the two-pyroxene thermometry is consistent with that estimated by the olivine–augite thermometer (Fig. 9).
Based on the temperatures indicated by the olivine–augite and two-pyroxene thermometers, we conclude that the high-Zr basalts and andesites had temperatures ranging from 1200°C to 1100°C, and that the andesites had slightly lower temperatures.
|
FRACTIONATION MODELS |
|---|
To understand the role of fractional crystallization in producing the range of magma compositions observed at Sumisu, many parent–daughter pairs were modelled using least-squares mass-balance calculations and the observed phenocryst phases. Two low-Zr basalts, 575R4 (8·5 wt % MgO) and 575R5 (6·2 wt % MgO), were chosen as parental magmas and the feasibility of deriving high-Zr basalts from low-Zr parents was evaluated. In some cases, the sum of the squares of residuals (
r2) becomes much lower if the parent basalts were to assimilate small amounts of rhyolite, together with fractional crystallization (AFC). For example, removal of phenocryst olivine, augite and plagioclase, and the addition of rhyolite can account for the major-element differences between the low-Zr basalts (575R4 and 575R5) and basaltic andesite (575R1). Table 4 shows representative results of these calculations.
|
The low-Zr basalt 575R5 (6·2 wt % MgO) can be derived from the low-Zr basalt 575R4 (8·5 wt % MgO) through crystal fractionation. Generally, fractionation of phenocryst phases from these low-Zr basalts can explain the major element differences between the low-Zr basalts and many high-Zr basalts (Table 4, Fig. 11a). Zr and other trace element enrichments are, however, not compatible with closed-system fractionation from the low-Zr basalts (Table 4 and Fig. 11b). Although the fractionation models need only 10–30% removal of phenocrysts from the magnesian low-Zr basalts to produce high-Zr basalts with lower MgO (3–6 wt % MgO), many high-Zr basalts have 1·5–2 times more Zr than the low-Zr basalts over a small range of SiO2 contents (51–53 wt %). The rhyolites in the Sumisu caldera contain up to 127 ppm Zr (Fig. 5b), which, however, could not produce the steep trend in the Zr–SiO2 diagram if they had been assimilated by mixing with the low-Zr basalts (Table 4). In conclusion, the high-Zr basalts in the Sumisu caldera volcano cannot be produced from the low-Zr basalts by fractional crystallization and/or assimilation (Table 4). This is also consistent with petrographic observations that the low-Zr basalts contain up to 11 vol. % augite, but many of the high-Zr basalts are free of augite (Fig. 6).
|
|
RARE EARTH ELEMENT (REE) PATTERNS |
|---|
Figure 12 shows a C1 chondrite-normalized REE plot for the Sumisu basalts. All the basalts are strongly depleted in the more incompatible light rare earth elements (LREE) compared with middle and heavy REE (MREE and HREE). Such a pattern of upward-convex curvature is characteristic of mid-ocean ridge basalt (MORB) worldwide and reflects the broad pattern of trace element concentrations in the mantle source region. For example, Eocene MORB from the West Philippine Basin (Pearce et al., 1999
), also on the Philippine Sea Plate, has a similar REE pattern to the Sumisu basalts. Importantly, the low-Zr basalts are more LREE depleted than the high-Zr basalts. Fractionation of augite from low-Zr basalts would increase the abundances of LREE compared with MREE and HREE. Thus it might be considered that the differing REE patterns between the low- and high-Zr basalts could have resulted from crystal fractionation of augite from the low-Zr basalts. This scenario, however, is not consistent with petrographic observations that these high-Zr basalts are free of augite phenocrysts; additionally, mass-balance calculations are not consistent with crystal fractionation from a low-Zr basalt parent to a high-Zr basalt. Thus, the more LREE-depleted patterns in the Sumisu basalts may suggest higher degrees of partial melting of the source mantle. Thus basalts produced by different degrees of partial melting of the source mantle may coexist in the same magmatic system. It is therefore pertinent to determine whether similar REE patterns exist in basalts from other Bonin arc volcanoes.
|
REE patterns of basalts from the Izu–Bonin arc
Taylor & Nesbitt (1988) presented extensive data for other Izu–Bonin arc volcanoes, which include the five Quaternary arc-front volcanoes of Oshima, Miyakejima, Hachijojima, Aogashima and Torishima. Figure 13 shows La/Sm–Sm/Yb and La/Sm–Y/Zr plots for basalts from Sumisu caldera volcano (this study) and the five other Quaternary volcanoes from the dataset of Taylor & Nesbitt (1988). La/Sm is positively correlated with Sm/Yb and is negatively correlated with Y/Zr, especially in the Sumisu, Torishima and Hachijojima areas. La/Sm values at Sumisu range from 0·7 to 1·05; the low-Zr basalts have lower ratios ranging from 0·7 to 0·9. La/Sm values at Hachijojima and Torishima range from 0·59 to 0·79 and from 0·57 to 0·88, respectively. As shown by Taylor & Nesbitt (1988), variation in Nd isotopes along the length of the frontal Izu–Bonin arc indicates that heterogeneity existed within the mantle wedge prior to enrichment by subduction fluids. Thus, comparison of La/Sm ratios among the different frontal volcanoes may reflect the heterogeneity in the mantle wedge and not different degrees of partial melting of the source mantle. It is, however, important to note that a range of La/Sm values exists within individual volcanoes. Basalts from Hachijojima and Torishima have ranges of 0·2 and 0·31, respectively, similar to that of Sumisu (0·35). This may suggest that the basalts of these individual volcanoes might have resulted from differing degrees of partial melting of the mantle source along the Izu–Bonin arc.
|
Figure 14 shows a La/Sm–Sm/Yb plot for the Izu–Bonin arc basalt field based on the data from this study and Taylor & Nesbitt (1998)
, the NE Japan arc basalt field after Shibata & Nakamura (1997), SW Japan Daisen basalts from Tamura et al. (2000), and the average enriched and normal MORB (E-MORB and N-MORB) and ocean island basalt (OIB) compositions of Sun & McDonough (1989). Izu–Bonin arc basalts have similar (or more depleted) REE patterns than average N-MORB.
|
|
DEGREE OF MELTING |
|---|
Differences in olivine chemistry, incompatible trace element concentrations, and REE patterns cannot be explained by crystal fractionation models from the same primary magma. On the other hand, the combination of depleted olivine chemistry, low content of incompatible elements, and LREE-depleted pattern of the low-Zr basalts from the Sumisu caldera volcano suggest that higher degrees of melting of the source mantle could have produced these magmas. However, we need a way to quantify the melt fraction.
Stolper & Newman (1994) presented a widely cited fluid-fluxed melting model and concluded that for a given set of solid/melt partition coefficients (D values), the best-fit degree of melting by which Mariana trough basalts are generated is positively and approximately linearly correlated with the amount of H2O in the mantle source. One possible approach would be to apply the Stolper & Newman fluid-fluxed melting model to see if it reproduces the trace element patterns observed in the Sumisu high- and low-Zr basalts. There are, however, three assumptions in their model that we find questionable and must be commented upon. First, they used a given set of solid/melt D values for batch melting of a source comprising 60% olivine, 30% orthopyroxene and 10% clinopyroxene. Generally, this batch modal melting cannot adapt to the typically higher degrees of melting required to produce arc lavas, in which the residual modal mineralogy of the source is expected to change drastically. The degrees of melting required to produce the Mariana arc magmas, for example, are much higher than those required to generate the Mariana trough basalts (Stolper & Newman, 1994). Second, all of their compositions were reconstructed from actual glass or lava compositions by addition of olivine until the liquidus olivine is Fo90; the effect of fractionation of high-calcium clinopyroxene was neglected (Stolper & Newman, 1994). For the Sumisu caldera volcano, as shown in Fig. 7 and discussed below, we concluded that the low-Zr basalts were derived from a more magnesian, and thus more depleted mantle source than the high-Zr basalts. Thus the assumption that the sources of the high- and low-Zr basalts have the same fertility is not valid. Third, the smallest degree of partial melting of the source of the Mariana trough magmas was assumed to be 5% (partial melting of N-MORB source). This is acceptable for the MORB-like Mariana trough basalts, but to assume such a small degree of melting for arc magmas, such as at Sumisu, seems artificial. Thus we do not apply the Stolper & Newman (1994) fluid-fluxed melting model.
The degree of partial melting of the source of the primary magmas can be estimated using the ratios of trace element concentrations in lavas with different compositions (Maaløe, 1994; Maaløe & Pedersen, 2003). To accomplish this these workers assumed (1) the likely modal mineralogy of the source mantle, and (2) the relative proportions of the mineral constituents that enter the melt. Based on these assumptions, the estimated degree of melting depends on the trace element ratios in two selected lava compositions and not on their absolute values.
As shown in Fig. 14, the Izu–Bonin arc basalts are among the most depleted on Earth, and the two assumptions of Maaløe (1994) and Maaløe & Pedersen (2003) would be difficult to justify. In particular, the clinopyroxene content in the source mantle is unknown. Instead, we use the bulk distribution coefficient between the primary magma and the residual solid as a variable. Thus, the mode of the residual mantle, consisting of olivine, orthopyroxene and clinopyroxene, will change according to the degree of partial melting. As we do not make the two assumptions listed above, degrees of melting cannot be obtained uniquely. To constrain the degree of melting, however, we use the three ratios La/Sm, Sm/Yb, and Zr/Y.
We develop this line of reasoning as follows. In the context of batch partial melting, the variation in the concentration of a trace element, CL, in the melt is given by
 |
). We consider a partially molten multi-mineral assemblage, consisting of n – 1 residual mineral phases and a liquid phase. Let Cj be the concentration of an element in a residual phase and Xj the mass-fraction of each phase j relative to the partially molten source (and not to the residue). Mass balance requires that
 |
 |
 |
 |
 |
):
 |
, 
, 
, 
, 
) and (
, 
, 
, 
, 
), and F1 and F2, respectively;
 |
 |
 |
 |
 |
 |
 |
 |
 |
 |
If the likely modes of the residual solids of the two primary magmas are initially assumed, then the degree of melting of one primary magma, F2, will be a function of that of another magma, F1. Following the development of equations for La–Sm (as above), additional relationships can be considered for Sm–Yb and Y–Zr. The modes of the residual solids can then be varied until the three sets of equations intersect. The point of intersection indicates the degree of partial melting.
Although, generally, we cannot obtain unique solutions, some consistent values are obtained, as shown in Table 5. The degrees of partial melting required to produce the primary magmas of the low-Zr basalts are 20–22%, whereas those of the high-Zr basalts are 10–12%. Residual clinopyroxene plays a major role in fractionating La from Sm and Zr from Y. Thus, to produce lower La/Sm and higher Y/Zr in the primary low-Zr basalts than in the high-Zr basalts, the content of residual clinopyroxene in the source of the low-Zr primary magmas should be much smaller than that in the source of the primary high-Zr magmas. Our results suggest that the residual mantle should contain only 4–6 wt % of clinopyroxene after production of the low-Zr primary magmas but should contain 10% clinopyroxene after production of the high-Zr magmas.
xsx
|
|
ISOTOPES |
|---|
Sr, Nd and Pb isotope data for the Sumisu basalts are listed in Table 2. Samples 575R4 and 575R5 and 575R7 and SI14 are representative low-Zr basalts and high-Zr basalts, respectively. Importantly, the low-Zr and high-Zr basalts overlap in isotopic composition. These new Sr, Nd and Pb isotopic data for basalts from Sumisu caldera volcano are integrated with previously published data for Quaternary frontal volcanoes in the Izu–Bonin arc (Taylor & Nesbitt, 1998
; Ishizuka et al., 2003) and the Sumisu Rift (Hochstaedter et al., 1990). Figure 15 shows along-arc Sr–Nd–Pb isotopic and Ba/Zr variations of lavas from frontal volcanoes and the Sumisu Rift. Each frontal volcano has a limited range of isotope compositions compared with those from the Sumisu Rift. Similar systematic along-arc variations for the lavas of the Izu–Bonin arc have already been reported by Taylor & Nesbitt (1998) and Ishizuka et al. (2003); these are coherent between the volcanic front and the back-arc (Ishizuka et al., 2003).
|
|
DISCUSSION |
|---|
Were the high-Zr Sumisu basalts generated by fractionation of the low-Zr basalts?
Olivine phenocrysts occur in both the low-Zr and high-Zr Sumisu basalts. The low-Zr basalts, however, differ in a striking way from the high-Zr basalts in containing 2–11 vol. % of clinopyroxene phenocrysts (Fig. 6, Table 1). Thus, simply from a petrographic point of view, it is not reasonable to produce cpx-free high-Zr basalts from the cpx-rich low-Zr basalts. Moreover, although fractionation of phenocryst phases from the low-Zr basalts can explain the major element variations of the high-Zr basalts, Zr and other incompatible element concentrations are not consistent with fractionation from the low-Zr basalts (Table 4). It is interesting to note that the Sr–Nd–Pb isotope data overlap between the low-Zr and high-Zr basalts and are inconclusive.
Were the low-Zr basalts derived from a more depleted mantle source than the high-Zr basalts?
The crystallization and fractionation of olivine, or olivine plus clinopyroxene, will produce less magnesian magmas. Ni, however, is concentrated more in olivine than in clinopyroxene: most cpx phenocrysts in the low-Zr basalts contain <0·01 wt % NiO when they are analysed using an accelerating voltage of 20 kV and a beam current of 25 nA. Thus the fractionation of olivine alone would result in a drastic decrease of NiO compared with the fractionation of an equal volume of olivine plus clinopyroxene. For example, low-Zr basalt 575R5 can be produced by crystal fractionation of 4% olivine, 7% augite and 1% plagioclase from low-Zr basalt 575R4 (Table 4). The calculated olivine in equilibrium with basalt 575R5, however, has a much higher NiO content than the olivine fractionation trend expected from 575R4 (Fig. 7e). Thus, generally, the discrepancy between actual olivines and calculated olivines for low-Zr basalts (Fig. 7d) could have resulted from the crystallization and fractionation of augite phenocryst together with olivine. Clinopyroxene fractionation cannot, however, be responsible for the differences between the two types of basalt magma in the Sumisu area; indeed, it tends to blur the differences between them. It should be emphasized that the original differences in olivine chemistry between the two primitive magmas may have been greater than that now observed in their more differentiated products (Fig. 7a and b). We conclude that the sources of high- and low-Zr basalts differed in fertility, and that the low-Zr basalts were derived from a more magnesian, and thus more depleted mantle source than the high-Zr basalts.
Did the low-Zr and high-Zr Sumisu basalts have different mantle sources, or do they reflect different degrees of partial melting of the same source?
Olivine chemistry and bulk-rock REE patterns both suggest that the low-Zr basalts were derived from a more depleted mantle source than the high-Zr basalts. It is possible that the source mantle beneath the Sumisu caldera volcano consists of a mixture of depleted and less-depleted peridotite (note that in this context we are using the word depleted to indicate a previous partial melting event or events). Melting of a depleted source at high temperatures and melting of a less-depleted source at lower temperatures produces depleted and less-depleted basalts, respectively. Another possibility, however, is that the distribution of water in a homogeneous source mantle was variable, resulting in different degrees of partial melting at similar temperatures. The Sr–Nd–Pb isotope data suggest some heterogeneity, which could have been introduced by slab fluid. Although these two models are compatible, we develop a line of evidence showing that the two kinds of basalt, and their mantle sources, had different water contents.
Differences in water content in the source mantle beneath Sumisu caldera volcano
Green (1982) showed that, under dry conditions in the 0–5 kbar pressure range, plagioclase is the major liquidus or near-liquidus phase in a tholeiitic basalt; this is subsequently joined by olivine and later by pyroxene. When 5% H2O is added to a tholeiitic basalt, however, the appearance of plagioclase is markedly depressed, and pyroxene together with olivine are important near-liquidus phases from 2 to c. 14 kbar (Green, 1982). Sisson & Grove (1993) demonstrated that at 2 kbar, with addition of H2O, the assemblage plagioclase + olivine + high-Ca pyroxene appears at high melt fractions, apparently close to the liquidi of high-alumina basalts. On the other hand, at low to moderate pressures, anhydrous high-alumina basalts crystallize plagioclase or olivine and plagioclase over extended temperature intervals before being joined by high-Ca pyroxene (Grove et al., 1982; Baker & Eggler, 1983; Bartels et al., 1991). Cotectic formation of augite is probably a consequence of elevated magmatic water content that promotes expansion of the clinopyroxene phase volume. These experimental results suggest that the phenocryst assemblage olivine + plagioclase crystallizes under dry conditions and the appearance of high-Ca pyroxene together with olivine and plagioclase requires a few percent of water. This suggests that the low-Zr basalts could have had higher water contents than the high-Zr basalts.
KD Fe/Mgol/aug provides another indirect indicator of water content in magmas. The successful application of the olivine–augite Mg–Fe-exchange geothermometer of Loucks (1996) to the evolved high-Zr basalts contrasts markedly with its poor performance for the low-Zr basalts (Fig. 9). What causes the erratic performance of this thermometer? Loucks (1996) stated that KD Fe/Mgol/aug shows no systematic correlation with temperature in systems having more than a few weight percent H2O in the melt because, in these cases, the phase assemblage olivine + augite + plagioclase + melt no longer behaves as an isobarically univariant assemblage. Thus the contrasting poor performance of the olivine–augite thermometer in the low-Zr basalts may suggest that the low-Zr basalt magmas contained more than a few weight percent of water.
Figure 16 shows the variation of U/Th and Y/Zr vs SiO2 for the Sumisu basalts. The low-Zr basalts are enriched in the fluid mobile element U compared with the less mobile element Th. Because Y/Zr has a negative correlation with La/Sm (Fig. 13b), Y/Zr could be a measure of the degree of partial melting. Higher fluid contents seem necessary to produce greater degrees of partial melting. Furthermore, the basalts inferred to be the products of higher degrees of melting (low-Zr basalts) contain olivine + augite + plagioclase phenocrysts, which suggests higher water content in the basalts themselves.
|
Figure 17a shows the variation of Ba/Zr vs Y/Zr and the dominant phenocryst assemblages in the Sumisu basalts. Some high-Zr basalts have Ba/Zr values similar to, or even higher than, those of low-Zr basalts. There are two possible explanations for this overlap. First, it may suggest that higher water contents in the source, which result in the enrichment of the fluid mobile element Ba, may not always result in higher degrees of partial melting. Second, Ba is more incompatible than Zr during normal mantle melting, and thus, lower degrees of melting of the source of the high-Zr basalts could increase Ba/Zr ratios and blur the differences between the high-Zr and low-Zr basalts. Thus Ba/Zr may not be a reliable measure of the water content of the source when degrees of melting are not the same. Nevertheless, the broad positive correlation between Ba/Zr and Y/Zr suggests that higher water contents generally result in higher degrees of melting.
|
Figure 17b plots (La/Sm)N [primitive mantle normalized after Sun & McDonough (1989)
] versus the weight ratio Ba/Th for the Sumisu low-Zr and high-Zr basalts. The concept for this diagram and the fields for island arc basalts and MORB fields are from Elliott (2003). Basalts from the Sumisu caldera volcano, and, more generally, in the Izu arc (Taylor & Nesbitt, 1998), occupy the LREE-depleted end of the island arc array. Elliott (2003) noted that ‘highly depleted arcs (where minor sediment addition does not mask the effects of prior melt depletion) such as Izu, New Britain and Tonga show variable Ba/Th at similar degrees of LREE enrichment which clearly cannot be accounted for by variable sediment (involvement) and near constant addition of a second component’. We concur with this comment and note that the Sumisu basalts have highly variable Ba/Th values (Fig. 17b). The low-Zr basalts exhibit a wide range of Ba/Th ratios whereas the high-Zr basalts are characterized by a much more restricted range.
An aqueous fluid derived from subducted oceanic crust will significantly affect the budgets of Ba, Sr and Pb, less markedly influence those of K, Rb and U, and does not perceptibly contribute LREE, Th, Nb, Zr (Elliott, 2003). Thus, it is not at present clear why U/Th and Ba/Th ratios do not show the same differences between the low-Zr and high-Zr basalts of the Sumisu caldera volcano (Figs 16a and 17b). Nevertheless, the low-Zr basalts are clearly enriched in the fluid mobile elements U and Ba compared with the less mobile elements Th and Zr.
Water and degrees of melting
Stolper & Newman (1994) studied Mariana Trough glasses and concluded that the degree of melting of the mantle source is positively correlated with the amount of H2O in the source. Based mainly on results from the Vanuatu arc, Eiler et al. (2000) suggested that the extent of melting in the mantle source is related to the supply of 18O-enriched, slab-derived oxygen. They proposed that mantle sources containing relatively large amounts of slab-derived fluid (or melt) are both relatively 18O enriched and also experience relatively large degrees of partial melting. On the basis of these data, it can be concluded that, from a regional point of view, a positive correlation exists between the proportion of water in the source mantle and its degree of melting.
Our results suggest that within a single volcanic complex, the water content in the mantle source region can be heterogeneous, which results in different degrees of partial melting of this source. An alternative possibility is that the mantle source was variably melt depleted before the addition of the water and that the extent to which the fluid component was added is a function of the degree of prior melt depletion. The next question then is how the upper mantle can be made locally heterogeneous beneath a single volcanic system such as Sumisu.
Mantle diapirs
Hochstaedter et al. (1990) suggested that the Sumisu Rift basalts are like E-MORB with a moderate slab-derived overprint, whereas arc lavas are like N-MORB with a greater slab fluid component. However, each frontal arc volcano exhibits a limited range of Sr–Nd–Pb isotope compositions compared with those from the back-arc basin basalts of the Sumisu Rift (Fig. 15). The mantle source for Sumisu Rift basalts is clearly not uniform.
We favour a model in which arc magmas are derived from a relatively homogeneous mantle source that is processed to produce isotopically uniform magmas, and thus prefer the mantle diapir model for arc magma genesis (Tamura, 1994; Tamura & Nakamura, 1996; Hall & Kincaid, 2001; Gerya & Yuen, 2003). Tamura (1994) proposed a zoned diapir model that involved the following two-step process: (1) a hydrated peridotite diapir, generated above the cool subducting slab, ascends and is heated as it penetrates the overlying hot mantle wedge; (2) temperatures and H2O gradients established in the diapir result in simultaneous generation of basalt in hotter and drier regions and magnesian andesite (boninite) in cooler and wetter regions (Tamura, 1994). Hall & Kincaid (2001) presented the results of laboratory experiments that indicated that the interaction between buoyantly upwelling mantle diapirs and subduction-induced mantle flow creates a network of low-density, low-viscosity conduits, which facilitate the rapid melt transport required by recent U–Th disequilibrium studies (e.g. Hawkesworth et al., 1997). Gerya & Yuen (2003) used a high-resolution, two-dimensional numerical model to simulate such diapiric upwelling, in which rising hydrated peridotite diapirs cooler than the asthenosphere by 300–400°C were driven upward by compositional buoyancy. They assumed a slow melt percolation rate and a large volume fraction of dense supercritical water–silicate fluid (melt) in the hydrated peridotite, implying a high (1–3 wt %) bulk-rock water content and a large (100–200 kg/m3) density contrast.
Mantle wedge convection
Subduction of a rigid oceanic slab into the viscous mantle induces convection in the mantle wedge. Asthenospheric flow and upwelling within the wedge may take the form of hot, finger-like regions in the mantle wedge that move toward the volcanic front (Tamura et al., 2002). A low-viscosity mantle wedge coupled with a large-scale flow induced by the subducting slab may allow the formation of roll-like small-scale convection cells whose axis is normal to the strike of the plate boundary (Honda & Saito, 2003). Such a roll structure may explain the origin of along-arc variations in mantle temperature or ‘hot fingers’ (Tamura et al., 2002). Figure 18a shows a proposed model for dynamic convection along the axis of a ‘hot finger’ within the mantle wedge and the genesis of arc magmas after Tamura (2003). The upper upwelling part of the system is absent in the section lying between two hot fingers, but in both sections, conveyor-like return flow is interpreted to carry the remnants of these fingers to depth along the top of the subducting slab (Tamura, 2003). Figure 18b shows a schematic cross-section along the axis of a hot finger. Significant quantities of aqueous fluid are expelled from the downgoing crust and subducted sediments as they encounter increasing pressure and temperature. Thus a highly hydrated and serpentinized forearc region should be ubiquitous in subduction zones (Kamiya & Kobayashi, 2000; Bostock et al., 2002). The positive buoyancy and weak rheology of serpentine should serve to isolate most of the hydrated forearc wedge from the mantle-wedge corner flow system, whereas basal portions could be dragged down by the subducting plate (Fig. 18b and c, Bostock et al., 2002). At depths >60 km and temperatures >850°C a dense supercritical water–silicate fluid (melt) is the only hydrous phase stable in peridotite (Schmidt & Poli, 1998), and the dragged hydrated serpentinized mantle will change into hydrated serpentine-free mantle containing a large volumetric fraction of this supercritical fluid (Gerya & Yuen, 2003), implying a large density contrast with surrounding dry mantle.
|
Mantle diapirs are considered to be formed in the deeper part of the mantle wedge above the top surface of the subducted slab (Tamura, 1994
; Gerya & Yuen, 2003; Fig. 18c), and must therefore rise through the ‘hot fingers’ in the mantle wedge (Fig. 18a).
Mantle diapirs and genesis of arc magmas
As indicated in Fig. 18a, a hydrated peridotite diapir generated above the cool subducting slab ascends and is heated as it penetrates the overlying hot mantle wedge (Tamura, 1994). Such a mantle diapir model might be applicable to the Sumisu area if we make minor changes in the location of the basalt production region. The series of diagrams in Fig. 18c show a mantle diapir, as originally envisaged, producing dry basalt and hydrous magnesian andesite magma below an arc volcano. [In the paper by Tamura (1994), magnesian andesite was equated with boninite, but this term is not used here.] The result of our study of Sumisu basalts shows that basalts are not always dry, in contrast to the conclusion of Tamura (1994); wet, strongly depleted basalts can coexist with dry, depleted basalts. Thus, we infer that the mantle diapir must have a hot rind, whose water content is not uniform (as shown schematically in Fig. 18c). The variation of water content in this rind results in the production of (1) dry, depleted basalt, yielding moderately depleted residues (9–13% cpx), and (2) wet, strongly depleted basalts yielding strongly depleted residues (4–6% cpx). We adhere to the interpretation of Tamura (1994) that a diapir's wet and cool interior produces magnesian andesites.
|
CONCLUSIONS |
|---|
- Two types of basalt magma exist at Sumisu caldera volcano. The first contains less Zr (20–25 ppm) than the second (29–40 ppm).
- The high-Zr basalts cannot be derived from the low-Zr basalts through crystal fractionation or AFC processes.
- All low-Zr basalts contain olivine + clinopyroxene + plagioclase phenocrysts, whereas many high-Zr basalts contain only olivine + plagioclase. Low-Zr basalts have higher U/Th and Ba/Th than high-Zr basalts. These lines of evidence suggest that the low-Zr basalts have higher water contents than the high-Zr basalts.
- Low-Zr basalts are depleted in LREE and contain olivine phenocrysts with higher Mg-values and lower Ni content, suggesting that they are the products of higher degrees of partial melting of a previously melt depleted source.
- Concentration ratios of La/Sm, Sm/Yb and Y/Zr differ between high-Zr and low-Zr basalts, suggesting that degrees of melting for the primary magmas of the high-Zr basalts and low-Zr basalts are 10% and 20%, respectively.
- The low-Zr and high-Zr basalts are not distinct in terms of their Sr, Nd, and Pb isotopic composition. Each frontal volcano in the Izu–Bonin arc has a limited range of isotope compositions compared with those from the Sumisu Rift, suggesting that the mantle source of the arc frontal volcano is processed to produce isotopically uniform magmas.
- Basalts erupted from the Sumisu caldera volcano range from highly depleted, wet basalts to moderately depleted dry basalts. Higher degrees of partial melting in the source mantle are closely related to higher water contents. A mantle diapir model can explain the petrogenesis of both high-Zr and low-Zr basalts at Sumisu.
|
ACKNOWLEDGEMENTS |
|---|
Samples used in this study were collected during JAMSTEC cruises in September–October 2002 (NT02-10) and December 2002 (KR02-16). We are indebted to Captains F. Saito of R.V. Natsushima and O. Yukawa of R.V. Kairei, their crews, Commander H. Shibata, and the operation team of the research submersible Shinkai 2000. We also thank Tom Sisson and Dima Kamenetsky for careful and insightful reviews, and Richard Arculus for editorial help. Marjorie Wilson has read carefully through our manuscript and greatly assisted in improving the style and content of the final product.
* Corresponding author. Telephone: +81-46-867-9761. Fax: +81-46-867-9625. E-mail: tamuray{at}jamstec.go.jp
|
REFERENCES |
|---|
Albarède, F. (1995). Introduction to Geochemical Modeling. Cambridge: Cambridge University Press, 543 pp.
Aramaki, S. & Fujii, T. (1988). Petrological and geological model of the 1986–1987 eruption of Izu-Oshima volcano. Bulletin of the Volcanological Society of Japan 33, S297–S306 (in Japanese with English abstract and figure captions).
Baker, D. R. & Eggler, D. H. (1983). Fractionation paths of Atka (Aleutians) high-alumina basalts: constraints from phase relations. Journal of Volcanology and Geothermal Research 18, 387–404.[CrossRef][Web of Science]
Bartels, K. S., Kinzler, R. J. & Grove, T. L. (1991). High-pressure phase relations of primitive high-alumina basalt from Medicine Lake volcano, northern California. Contributions to Mineralogy and Petrology 108, 253–270.[CrossRef][Web of Science]
Bostock, M. G., Hyndman, R. D., Rondenay, S. & Peacock, S. M. (2002). An inverted continental Moho and serpentinization of the forearc mantle. Science 417, 536–538.
Chang, Q., Shibata, T., Shinotsuka, K., Yoshikawa, M. & Tatsumi, Y. (2003). Precise determination of trace elements in geological standard rocks using inductively coupled plasma mass spectrometry. Frontier Research on Earth Evolution (IFREE Report for 2001–2002) 1, 357–362.
Dick, H. J. B. (1989). Abyssal peridotites, very slow spreading ridges and ocean ridge magmatism. In: Saunders, A. D. & Norry, M. J. (eds) Magmatism in the Ocean Basins. Geological Society, London, Special Publications 42, 71–105.
Eiler, J. M., Crawford, A., Elliott, T., Farley, K. A., Valley, J. W. & Stolper, E. M. (2000). Oxygen isotope geochemistry of oceanic-arc lavas. Journal of Petrology 41, 229–256.
Elliott, T. (2003). Tracers of the slab. In: Eiler, J. (ed.) Inside the Subduction Factory. Geophysical Monograph, American Geophysical Union 138, 23–45.
Fryer, P., Taylor, B., Langmuir, C. H. & Hochstaedter, A. G. (1990). Petrology and geochemistry of lavas from the Sumisu and Torishima backarc rifts. Earth and Planetary Science Letters 100, 161–178.[CrossRef][Web of Science]
Fujii, T., Aramaki, S., Kaneko, T., Ozawa, K., Kawanabe, Y. & Fukuoka, T. (1988). Petrology of the lavas and ejecta of the November, 1986 eruption of Izu-Oshima volcano. Bulletin of the Volcanological Society of Japan 33, S224–S254 (in Japanese with English abstract and figure captions).
Gerya, T. V. & Yuen, D. A. (2003). Rayleigh–Taylor instabilities from hydration and melting propel ‘cold plumes’ at subduction zones. Earth and Planetary Science Letters 212, 47–62.[CrossRef][Web of Science]
Gill, J. B., Seales, C., Thompson, P., Hochstaedter, A. G. & Dunlap, C. (1992). Petrology and geochemistry of Pliocene–Pleistocene volcanic rocks from the Izu arc, Leg 126. In: Taylor, B., Fujioka, K., et al. (eds) Proceeding of the Ocean Drilling Program, Scientific Results, 126. College Station, TX: Ocean Drilling Program, pp. 383–404.
Green, T. H. (1982). Anatexis of mafic crust and high pressure crystallization of andesite. In: Thorpe, R. S. (ed.) Andesites. New York: John Wiley, pp. 465–487.
Grove, T. L., Gerlach, D. C. & Sando, T. W. (1982). Origin of calc-alkaline series lavas at Medicine Lake volcano by fractionation, assimilation and mixing. Contributions to Mineralogy and Petrology 80, 160–182.[Web of Science]
Hall, P. S. & Kincaid, C. (2001). Diapiric flow at subduction zones: a recipe for rapid transport. Science 292, 2472–2475.
Hawkesworth, C. J., Turner, S. P., McDermott, F., Peate, D. W. & van Calsteren, P. (1997). U–Th isotopes in arc magmas: implications for element transfer from the subducted crust. Science 276, 551–555.
Hoang, N. & Uto, K. (2003). Geochemistry of Cenozoic basalts in the Fukuoka district (northern Kyushu, Japan): implications for asthenosphere and lithospheric mantle interaction. Chemical Geology 198, 249–268.[CrossRef][Web of Science]
Hochstaedter, A. G., Gill, J. B. & Morris, J. D. (1990). Volcanism in the Sumisu Rift, II. Subduction and non-subduction related components. Earth and Planetary Science Letters 100, 195–209.[CrossRef][Web of Science]
Honda, S. & Saito, M. (2003). Small-scale convection under the back-arc occurring in the low viscosity wedge. Earth and Planetary Science Letters 216, 703–715.[CrossRef][Web of Science]
Ikeda, Y. & Yuasa, M. (1989). Volcanism in nascent back-arc basins behind the Shichito ridge and adjacent areas in the Izu–Ogasawara arc, northwest Pacific: evidence for mixing between E-type MORB and island arc magmas at the initiation of back-arc rifting. Contributions to Mineralogy and Petrology 101, 377–393.[CrossRef][Web of Science]
Ishizuka, O., Taylor, R. N., Milton, J. A. & Nesbitt, R. W. (2003). Fluid–mantle interaction in an intra-oceanic arc: constraints from high-precision Pb isotopes. Earth and Planetary Science Letters 211, 221–236.[CrossRef][Web of Science]
Iwabuchi, Y. (1999). Sumisu caldera. JAMSTEC Journal of Deep Sea Research 15, 83–94.
Johnson, C. M. & Beard, B. L. (1999). Correction of instrumentally produced mass fractionation during isotopic analysis of Fe by thermal ionization mass spectrometry. International Journal of Mass Spectrometry 193, 87–99.[CrossRef]
Kamiya, S. & Kobayashi, Y. (2000). Seismological evidence for the existence of serpentinized wedge mantle. Geophysical Research Letters 27, 819–822.[CrossRef][Web of Science]
Kinzler, R. J., Grove, T. L. & Recca, S. I. (1990). An experimental study on the effect of temperature and melt composition on the partitioning of nickel between olivine and silicate melt. Geochimica et Cosmochimica Acta 54, 1255–1265.[CrossRef][Web of Science]
Lindsley, D. H. & Andersen, D. J. (1983). A two-pyroxene thermometer. Proceedings of 13th Lunar and Planetary Science Conference. Journal of Geophysical Research 88, Supplement, 887–906.
Loucks, R. R. (1996). A precise olivine–augite Mg–Fe-exchange geothermometer. Contributions to Mineralogy and Petrology 125, 140–150.[CrossRef][Web of Science]
Maaløe, S. (1994). Estimation of the degree of partial melting using concentration ratios. Geochimica et Cosmochimica Acta 58, 2519–2525.[CrossRef][Web of Science]
Maaløe, S. & Pedersen, R. B. (2003). Two methods for estimating the degree of melting and trace element concentrations in the sources of primary magmas. Chemical Geology 193, 155–166.[CrossRef][Web of Science]
Marine Safety Agency (1997). Sumisu Sima. Basic Map of the Sea in Coastal Waters (1:50,000). Tokyo: Marine Safety Agency.
Nakano, S. & Yamamoto, T. (1991). Chemical variations of magmas at Izu-Oshima volcano, Japan: plagioclase-controlled and differentiated magmas. Bulletin of Volcanology 53, 112–120.[Web of Science]
Nakano, S., Togashi, S. & Yamamoto, T. (1988). Bulk and mineral chemistry of products of the 1986 eruption of Izu-Oshima volcano. Bulletin of the Volcanological Society of Japan 33, S255–S264 (in Japanese with English abstract and figure captions).
Newman, S. & Stolper, E. M. (2000). H2O and CO2 in magmas from the Mariana arc and back arc systems. Geochemistry, Geophysics, Geosystems 1, 1999GC000027.
Ossaka, J., Hirabayashi, J., Konto, S. & Tsuchide, M. (1985). Chemical compositions of rocks erupted from submarine volcanoes and adjacent areas in the Izu–Ogasawara arc. Bulletin of the Volcanological Society of Japan 30, 98–99 (in Japanese).
Pearce, J. A., Kempton, P. D., Nowell, G. M. & Noble, S. R. (1999). Hf–Nd element and isotope perspective on the nature and provenance of mantle and subduction components in Western Pacific arc–basin systems. Journal of Petrology 40, 1579–1611.[CrossRef][Web of Science]
Roggensack, K., Williams, S. N., Schaefer, S. J. & Parnell, R. A. J. (1996). Volatiles from the 1994 eruptions of Rabaul: understanding large caldera systems. Science 273, 490–493.[Abstract]
Roggensack, K., Hervig, R. L., McKnight, S. B. & Williams, S. N. (1997). Explosive basaltic volcanism from Cerro Negro volcano: influence of volatiles on eruptive style. Science 277, 1639–1642.
Sato, H. (1977). Nickel content of basaltic magmas: identification of primary magmas and a measure of the degree of olivine fractionation. Lithos 10, 112–120.
Schmidt, M. W. & Poli, S. (1998). Experimentally based water budgets for dehydrating slabs and consequences for arc magma generation. Earth and Planetary Science Letters 163, 361–379.[CrossRef][Web of Science]
Shibata, T. & Nakamura, E. (1997). Across-arc variations of isotope and trace element compositions from Quaternary basaltic volcanic rocks in northeastern Japan: implications for interaction between subducted oceanic slab and mantle wedge. Journal of Geophysical Research 102, 8051–8064.[CrossRef]
Sisson, T. W. & Bronto, S. (1998). Evidence for pressure-release melting beneath magmatic arcs from basalt at Galunggung, Indonesia. Nature 391, 883–886.[CrossRef]
Sisson, T. W. & Grove, T. L. (1993). Experimental investigations of the role of H2O in calc-alkaline differentiation and subduction zone magmatism. Contributions to Mineralogy and Petrology 113, 143–166.[CrossRef][Web of Science]
Sisson, T. W. & Layne, G. D. (1993). H2O in basaltic and basaltic andesite glass inclusions from four subduction-related volcanoes. Earth and Planetary Science Letters 117, 619–637.[CrossRef][Web of Science]
Sobolev, A. V. & Chaussidon, M. (1996). H2O concentrations in primary melts from suprasubduction zones and mid-oceanic ridges: implications for H2O storage and recycling in the mantle. Earth and Planetary Science Letters 137, 45–55.[CrossRef][Web of Science]
Stolper, E. & Newman, S. (1994). The role of water in the petrogenesis of Mariana trough magmas. Earth and Planetary Science Letters 121, 293–325.[CrossRef][Web of Science]
Sun, S.-s. & McDonough, W. F. (1989). Chemical and isotopic systematics of oceanic basalts: implications for mantle composition and processes. In: Saunders, A. D. & Norry, M. J. (eds) Magmatism in the Ocean Basins. Geological Society, London, Special Publications 42, 313–345.
Tamura, Y. (1994). Genesis of island arc magmas by mantle-derived bimodal magmatism: evidence from the Shirahama Group, Japan. Journal of Petrology 35, 619–645.
Tamura, Y. (2003). Some geochemical constraints on hot fingers in the mantle wedge: evidence from NE Japan. In: Larter, R. D. & Leat, P. T. (eds) Intra-Oceanic Subduction Systems: Tectonic and Magmatic Processes. Geological Society, London, Special Publications 219, 221–237.
Tamura, Y. & Nakamura, E. (1996). The arc lavas of the Shirahama Group, Japan: Sr and Nd isotopic data indicate mantle-derived bimodal magmatism. Journal of Petrology 37, 1307–1319.
Tamura, Y. & Tatsumi, Y. (2002). Remelting of an andesitic crust as a possible origin for rhyolitic magma in oceanic arcs: an example from the Izu–Bonin arc. Journal of Petrology 43, 1029–1047.
Tamura, Y., Yuhara, M. & Ishii, T. (2000). Primary arc basalts from Daisen volcano, Japan: equilibrium crystal fractionation versus disequilibrium fractionation during supercooling. Journal of Petrology 41, 431–448.
Tamura, Y., Tatsumi, Y., Zhao, D., Kido, Y. & Shukuno, H. (2002). Hot fingers in the mantle wedge: new insights into magma genesis in subduction zones. Earth and Planetary Science Letters 197, 105–116.[CrossRef][Web of Science]
Taylor, B. (1992). Rifting and the volcanic–tectonic evolution of the Izu–Bonin–Mariana arc. In: Taylor, B., Fujioka, K., et al. (eds) Proceeding of the Ocean Drilling Program, Scientific Results, 126. College Station, TX: Ocean Drilling Program, pp. 627–651.
Taylor, R. N. & Nesbitt, R. W. (1998). Isotopic characteristics of subduction fluids in an intra-oceanic setting, Izu–Bonin arc, Japan. Earth and Planetary Science Letters 164, 79–98.[CrossRef][Web of Science]
CiteULike 
Connotea 
Del.icio.us What's this?
This article has been cited by other articles:
|
|
 |
|
  Y. Tamura, J. B. Gill, D. Tollstrup, H. Kawabata, H. Shukuno, Q. Chang, T. Miyazaki, T. Takahashi, Y. Hirahara, S. Kodaira, et al. Silicic Magmas in the Izu-Bonin Oceanic Arc and Implications for Crustal Evolution J. Petrology, April 15, 2009; (2009) egp017v1. [Abstract] [Full Text] [PDF]
|
|
|
|
 |
|
 S. M. Straub Uniform processes of melt differentiation in the central Izu Bonin volcanic arc (NW Pacific) Geological Society, London, Special Publications, January 1, 2008; 304(1): 261 - 283. [Abstract] [Full Text] [PDF]
|
|
|
|
|
|
Y. Tamura, K. Tani, Q. Chang, H. Shukuno, H. Kawabata, O. Ishizuka, and R. S. Fiske Wet and Dry Basalt Magma Evolution at Torishima Volcano, Izu Bonin Arc, Japan: the Possible Role of Phengite in the Downgoing Slab J. Petrology, October 1, 2007; 48(10): 1999 - 2031. [Abstract] [Full Text] [PDF]
|
|
| ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||