G 3. AN ELECTRONIC JOURNAL OF THE EARTH SCIENCES Published by AGU and the Geochemical Society

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1 Geosystems G 3 AN ELECTRONIC JOURNAL OF THE EARTH SCIENCES Published by AGU and the Geochemical Society Article Volume 4, Number 9 30 September , doi: /2003gc ISSN: Geochemical modeling of dehydration and partial melting of subducting lithosphere: Toward a comprehensive understanding of high-mg andesite formation in the Setouchi volcanic belt, SW Japan Yoshiyuki Tatsumi and Takeshi Hanyu Institute for Frontier Research on Earth Evolution (IFREE), Japan Marine Science and Technology Center (JAMSTEC), Yokosuka , Japan (tatsumi@jamstec.go.jp; hanyut@jamstec.go.jp) [1] Possible mechanisms for the production of mantle-derived, high-mg andesite magmas, including (1) partial melting of mantle wedge peridotite by addition of aqueous fluids from the subducting lithosphere and (2) partial melting of the subducting sediments and altered oceanic crust, and subsequent melt-mantle interaction, were examined by geochemical formulation of dehydration, partial melting and melt-solid reactions. The modeling results demonstrate that both mechanisms can reasonably explain the incompatible trace element characteristics of high-mg andesites in the Setouchi volcanic belt, SW Japan. However, simple hydrous melting of mantle wedge peridotite cannot account for the Sr-Nd-Pb-Hf isotopic compositions of such andesites. By contrast, the latter mechanism, which is consistent with thermal structures beneath the Setouchi volcanic belt, can well reproduce the isotopic signature of those high-mg andesites. Components: 10,146 words, 4 figures, 6 tables. Keywords: High-Mg andesite; SW Japan; dehydration; slab melting. Index Terms: 3640 Mineralogy and Petrology: Igneous petrology; 1065 Geochemistry: Trace elements (3670); 1040 Geochemistry: Isotopic composition/chemistry. Received 2 March 2003; Revised 12 August 2003; Accepted 13 August 2003; Published 30 September Tatsumi, Y., and T. Hanyu, Geochemical modeling of dehydration and partial melting of subducting lithosphere: Toward a comprehensive understanding of high-mg andesite formation in the Setouchi volcanic belt, SW Japan, Geochem. Geophys. Geosyst., 4(9), 1081, doi: /2003gc000530, Introduction [2] Magmas formed in subduction zones are noted for their distinct chemistry compared with those in other tectonic setting such as mid-oceanic ridge basalts (MORBs) and intraplate magmas. In particular, they are characterized by an over-abundance of large-ion-lithophile elements and a depletion in high-field-strength elements. It is generally accepted that metasomatic overprinting of particular elements from the subducting lithosphere is responsible for this distinctive geochemical signature [e.g., Gill, 1981; Pearce, 1983; Hawkesworth, 1982]. Aqueous fluids and silicate melts, which are produced by dehydration and partial melting of the sinking oceanic lithosphere, respectively, are potential metasomatic agents for such overprinting. The identification of which metasomatic agents are responsible for enriching the source region of subduction zone magmas, i.e., aqueous fluids or Copyright 2003 by the American Geophysical Union 1 of 19

2 silicate melts, is a topic of great importance not only for subduction zone processes, but also for material circulation within the Earth s mantle, as the composition of the residual slab will differ depending on whether melts or fluids were extracted. A significant majority of researchers currently favor the hypothesis that the metasomatic agent is not a melt but an aqueous fluid [e.g., Tatsumi and Eggins, 1995]. However, in subduction zones where rather young, and hence hot, oceanic crust is being subducted into the mantle, as is the case for the Northern Cascade, Patagonia, Panama-Costa Rica, and possibly Archean arcs, partial melting of the foundering oceanic lithosphere could play a significant role in magma production in the subarc mantle. Furthermore, the contribution of slab melts to producing some geochemical signatures, such as Th/Nb ratios, has been emphasized even in cool subduction zones (e.g., the Mariana arc [Elliott et al., 1997]). [3] One of the major products that typify magmatism in subduction zones, especially those along the active continental margins, is andesite. Furthermore, the continental crust, one of the major geochemical reservoirs within the solid Earth, possesses an andesitic composition and is believed to have been created in ancient subduction zones [e.g., Taylor and McLennan, 1995; Rudnick and Fountain, 1995]. The majority of andesites are derived essentially from parental basaltic magmas through variable differentiation processes such as fractional crystallization, mixing of basaltic and felsic magmas, crustal contamination, and anatexis of pre-existing crustal materials [Gill, 1981; Sakuyama, 1981; Hildreth, 1981; Couch et al., 2001]. However, andesitic magmas produced by partial melting of upper mantle peridotite do exist in arcs. Such mantle-derived andesites are referred to as high-mg andesites (HMAs), as they are characterized by high MgO contents and/or Mg# (=100 Mg/(Mg + Fe)), which suggest that they equilibrated with Mg-rich mantle olivine. Examples of HMAs include boninites on the Bonin Islands [Kuroda et al., 1978], bajaites in Baja California [Saunders et al., 1987], and sanukitoids in SW Japan [Tatsumi and Ishizaka, 1981]. [4] Partial melting experiments on both simple and natural peridotite systems [e.g., Kushiro, 1969; Hirose, 1997] have established that hydrous melting of upper mantle peridotite is a possible mechanism for HMA magma production. It has been further demonstrated that some HMAs are multiply saturated with peridotitic phases under hydrous conditions at mantle pressures [e.g., Kushiro and Sato, 1978; Tatsumi, 1981, 1982; Umino and Kushiro, 1989; van der Laan et al., 1989]. These experimental results led Crawford et al. [1989] and Tatsumi and Maruyama [1989] to the conclusion that the direct addition of an aqueous fluid produced by slab dehydration into the mantle wedge is a likely mechanism for HMA generation. However, this rather simple process may not be the only way to attain equilibration between hydrous HMA magmas and mantle peridotite. A process including partial melting of subducting lithosphere and the subsequent reaction of hydrous felsic slab melts with mantle wedge peridotite can also achieve such conditions [Kay, 1978; Pearce et al., 1992; Yogodzinski et al., 1994; Kelemen, 1995]. However, a comprehensive quantitative examination of fluid addition vs. meltmantle interaction that includes the major element, trace element, and isotopic characteristics of HMAs, has not yet been completed. Such an examination should provide key insights into the genesis of HMA magmas, and more generally, of mantlederived magmas in both modern and Archean subduction zones. [5] This paper assesses the two above mentioned, potential mechanisms for HMA magma formation in the Setouchi volcanic belt, SW Japan, through geochemical modeling of dehydration and partial melting processes in both the subducting lithosphere and the overlying mantle wedge. The reasons for selecting these HMAs are threefold. First, the Setouchi volcanic belt is a typical warm subduction zone where relatively hot lithosphere is being subducted. In this context, it may be analogous to Archean subduction zones, which may have been the major site of andesitic continental crust formation by processes including either slab-melting and subsequent melt-mantle interaction [e.g., Martin, 1986; Kelemen, 1995], or slab-dehydration-induced basalt magma forma- 2of19

3 Geosystems G 3 tatsumi and hanyu: high-mg andesite formation /2003GC tion and subsequent delamination of mafic restites produced during the anatexis of the initial basaltic crust [Kay and Kay, 1993; Tatsumi, 2000a]. Furukawa and Tatsumi [1999] demonstrated on the basis of numerical simulation that the temperature of the surface of the subducting lithosphere beneath the Setouchi volcanic belt was high enough for partial melting of both the subducting sediments and the oceanic crust. Second, a comprehensive data set, including geology, chronology, petrography, experimental petrology and geochemistry, has been accumulated for these HMAs. Third, based on Sr-Nd-Pb isotopic and trace element characteristics, Shimoda et al. [1998] and Tatsumi [2001] suggested the involvement of subducting sediment-derived melts in the HMA magma formation, but did not examine the role of subducting oceanic crust. 2. Setouchi Volcanic Belt [6] The Setouchi volcanic belt (SVB) is located in the present forearc region, 80 km trenchward of the Quaternary volcanic front of the SW Japan arc, where the Philippine Sea plate is being subducted beneath the Eurasian plate. The SVB extends for 600 km with five major volcanic regions: Shitara, Osaka, NE and NW Shikoku and NE Kyushu (Figure 1). New K-Ar age data for Setouchi volcanics [Tatsumi et al., 2001, 2003] confirm an earlier suggestion [Tatsumi, 1983; Tatsumi et al., 1983a] that Setouchi magmatism took place within a short period, from 11 to 15 Ma. Felsic volcanoplutonic complexes are distributed in the neartrench region, to the south of Median Tectonic Line of the SW Japan arc (Figure 1). K-Ar ages of these felsic rocks concentrate at Ma [Shibata, 1978; Sumii, 2000], synchronous with the Setouchi magmatism. [7] Schematic diagrams indicating the tectonic evolution of SW Japan during the Miocene period are shown in Figure 2, after Hibbard and Karig [1990] and Yamaji and Yoshida [1998]. The Shikoku Basin, situated to the south of the SW Japan arc, is a backarc basin that was created behind the Izu-Bonin-Mariana arc by rifting at Ma [Okino et al., 1994, 1998, 1999]. Through this backarc rifting, the Izu-Bonin-Mariana arc sliver was separated from the paleo-kyushu-palau arc and migrated toward the east (Figure 2). [8] Miocene magmatism, both in the Setouchi and near-trench regions of the SW Japan arc, is largely synchronous with the timing of a clockwise rotation of the arc sliver at Ma that was caused by the rifting of the Japan Sea backarc basin [Otofuji et al., 1991]. It is thus inferred that the southward drift of the SW Japan arc caused by the clockwise rotation of the arc sliver, resulted in subduction of the very young (<15 m.y.) oceanic lithosphere of the Shikoku Basin (Figure 2). The youngest part (15 Ma) of the Shikoku Basin lithosphere is situated along the paleo-ridge system, which is currently located to the southeast of Shikoku Island (Figure 1). It may be thus suggested that, among the regions of Setouchi magmatism, the youngest (1 m.y.) and hence the hottest, lithosphere was being subducted in the Shikoku region. [9] HMAs are distributed widely in the SVB, excepting the easternmost Shitara region of the volcanic belt. Although these HMAs possess a limited range of major element compositions, the incompatible element and isotopic compositions of the Setouchi HMAs are highly variable [Shimoda et al., 1998; Tatsumi et al., 2003]. In order to overcome the effect of such regional heterogeneity in the HMAs and probably in the source mantle composition, HMAs and basalts from a small island in the NE Shikoku region, Shodo-Shima Island, were used for the following geochemical modeling and discussions. 3. Geochemical Modeling [10] Shimoda et al. [1998] first emphasized the role of melts from subducting sediments interacting with overlying mantle peridotite in Setouchi HMA magma formation, based on the Sr-Nd-Pb isotopic signatures of those rocks. However, they assumed simple mixing rather than reactions between sediment-melts and mantle wedge peridotite. A more realistic melt-mantle interaction modeling was conducted by Tatsumi [2001] and successfully reproduced the incompatible trace 3of19

4 Figure 1. Distribution of the Setouchi volcanic belt (red stars), which formed in the present forearc region of the SW Japan arc, i.e., at the trench side of the Quaternary volcanic front (Q.V.F.). Formation of the Setouchi volcanic belt parallel to the arc-trench system may indicate the involvement of the subducting lithosphere of the Shikoku Basin, which is bordered by the Kyushu-Palau ridge in the west and the Izu-Bonin-Mariana arc in the east, in producing Setouchi magmas. The Shikoku Basin was created by backarc opening along a spreading center currently located as a paleo-ridge in central part of the Shikoku Basin. Felsic volcano-plutonic complexes, which formed synchronously with the Setouchi volcanic belt, are distributed along the near-trench region of the SW Japan arc (yellow circles). EUR, Eurasian plate; NA, North American plate; PHS, Philippine Sea plate; PAC, Pacific plate; MTL, Median Tectonic Line. element characteristics of Setouchi HMAs. However, that modeling is incomplete, as it did not examine the isotopic signatures of HMAs. In addition to these flaws, the previous modeling did not take into account the role of melts from the subducting oceanic crust, however, partial melting of the oceanic crust is likely to have occurred [Furukawa and Tatsumi, 1999]. The present geochemical modeling therefore aims for a comprehensive understanding of HMA magma production in terms of both trace elements and isotopic compositions Mantle Wedge and Subduction Components [11] The oceanic crust that was subducted beneath the SVB was that of the Shikoku Basin. The chemical composition of Shikoku Basin basalts has been reported by Wood et al. [1979] and 4of19

5 Figure 2. A possible tectonic evolution model for the formation of the Setouchi volcanic belt. Opening of the Japan Sea backarc basin and associated southward migration of the SW Japan arc sliver caused the obduction of that arc onto the Shikoku Basin lithosphere, which was also formed by backarc spreading processes immediately before the obduction. Young lithosphere created by backarc rifting is shown in pink. KPR, Kyushu-Palau ridge; IBM, Izu- Bonin-Mariana arc; PAC, Pacific plate; PHS, Philippine Sea plate. Hickey-Vargas [1998]. Hydrothermal alteration or amphibolization of oceanic crust at oceanic ridge systems redistributes elements through water-rock interaction and has a profound effect on its chemical composition. Although such processes are complex, Tatsumi [2000a] provided a simple estimate of the amount of trace element overprinting during oceanic ridge amphibolization by comparing the compositions of a fresh NMORB and the inferred amphibolite that is the source of both aqueous fluids and specific trace elements for typical, modern island arc basalt magmas. Accepting this estimate allows the trace element composition of the altered oceanic crust or amphibolite that was subducted beneath the SVB to be obtained (Table 1). [12] Shimoda et al. [1998] reported the composition of terrigenous, trench-fill sediments that were recovered from the floor of the Nankai Tough at Site 582 of DSDP (Deep Sea Drilling Project) Leg 87, and suggested that such sediments were likely to be subducted and involved in the production of Setouchi HMA magmas, at least for Sr-Nd-Pb isotopic compositions. Furthermore, Tatsumi [2001], based on geochemical modeling of meltmantle interactions, demonstrated that such terrigenous sediments may play the major role in forming the incompatible trace element characteristics of Setouchi HMAs. Nankai Trough terrigenous sediment (Table 1) was therefore taken as representative of the subducting sediments contributing to the Setouchi magmatism. [13] Original, pre-subduction mantle has been generally accepted as being identical to the NMORBsource mantle, because of the rather uniform chemistry of MORBs and the similarity in highfield-strength element ratios between MORBs and arc lavas [e.g., Sakuyama and Nesbitt, 1986; McCulloch and Gamble, 1991; Pearce, 1993; Tatsumi and Eggins, 1995]. Although this assumption may be valid for oceanic arc settings, variably metasomatized subcontinental mantle material may be an important component in the original mantle wedge beneath continental arcs such as the SW Japan arc [Pearce, 1993]. However, the composition of the subcontinental mantle and the degree of its contribution to the pre-subduction, subarc mantle are unknown. In order to overcome this, we assumed that the pre-subduction mantle composition beneath the SVB was close to the mantle composition from which Setouchi basalt magmas were derived. This assumption is supported by the results of melting experiments for HMAs and basalts [Tatsumi, 1982], 5of19

6 Table 1. Compositions of Slab and Mantle Components a K Rb Sr Pb Ba Y Nd Zr Th Nb Hf Altered Oceanic Crust Shikoku Basin b basalt a EMORB c Amphibolization d Amphibolite Sediment e Terrigenous max Terrigenous min Distribution Coefficient Olivine Orthopyroxene Clinopyroxene Garnet Quartz Sillimanite Rutile Original Mantle Wedge Basalt SD438 e Bulk distribution coefficient Degree of melting Source mantle a Concentrations are in ppm. b Hickey-Vargas [1998], Wood et al. [1979]. c Sun and McDonough [1989]. d Tatsumi [2000a]. 6of19

7 Table 2. Results of Trace Element Modeling for HMA Magma Generation by Addition of Slab Fluids a K Rb Sr Pb Ba Y Nd Zr Th Nb Hf Altered oceanic crust Amphibolite Mobility b H 2 O Fluid Sediment Terrigenous max Mobility c H 2 O (wt.%) Fluid Slab fluid Sediment contribution Fluid Original mantle wedge Source mantle HMA generation H 2 O in the source HMA source Degree of melting Setouchi HMA max. d Setouchi HMA min. d Terrigenous max. (0.2 d ) (1.0 d ) Terrigenous min. (0.2 d ) (1.0 d ) a Element concentrations are in ppm. Sediment contribution is defined as the fraction of sediment that contributes to producing fluid from the slab composed of altered oceanic crust and sediment. b Kogiso et al. [1998]. c Aizawa et al. [1999]. d Shimoda et al. [1998]; Tatsumi, unpublished data, which suggest that the Setouchi basaltic magma source contained less H 2 O and, hence, was much less metasomatized by slab-derived components, than the HMA source. It should be stressed that the basalt source was metasomatized by slabderived components and, in a strict sense, should have a composition more enriched in such components than the pre-subduction mantle. Therefore by using the basalt source to represent pre-subduction mantle in the HMA magma modeling, the amount of slab-derived components overprinted onto the mantle wedge will be underestimated. By accepting that, at least initially, the magma source for the HMAs was affected by the same slab-derived components as those responsible for the basalts, the composition of the HMA magmas can be reasonably reproduced. [14] The trace element compositions of a Mg-rich basalt, SD-438, from the NE Shikoku region (Table 1), which represents a possible primary basaltic magma in the SVB [Tatsumi, 1982], and 15% incremental fractional melting of a peridotite composed of 55% olivine, 30% orthopyroxene, and 15% clinopyroxene, together with partition coefficients, were used to provide an estimate for the original mantle composition (Table 1). [15] The crystal-liquid partition coefficients used in the calculations (Table 1) are after Tatsumi [2000a], and are based on the compilation of experimental data by Green [1994] and consideration of the crystal structure control in trace element partitioning between melts and solid phases [Matsui et al., 1977] Trace Element Modeling of Slab Dehydration [16] A certain amount of trace elements should dissolve in the ambient aqueous fluid under upper 7of19

8 mantle conditions. Migration of fluids in the upper mantle will therefore result in the formation of chemically distinct mantle components. It is widely accepted that the element fluxes caused by migration of fluids derived from subducting oceanic lithosphere are largely responsible for the distinctive chemistry of modern arc magmas [Morris et al., 1990; Hawkesworth et al., 1993; Tatsumi and Eggins, 1995]. The experimental basis for invoking a major role for slab-derived aqueous fluids in generating arc magma chemistry was established by measurements of both the degree of element mobilization during dehydration processes [Tatsumi et al., 1986; Tatsumi and Isoyama, 1988; Kogiso et al., 1997; Aizawa et al., 1999], and mineral-fluid partitioning of trace elements [Brenan and Watson, 1991; Brenan et al., 1994, 1995a, 1995b; Johnson and Plank, 1999]. In the modeling presented here, the trace element composition of Setouchi magmas generated by slab-derived fluid fluxing was calculated by using the mobility data of Kogiso et al. [1997] for amphibolite (altered oceanic crust) and Aizawa et al. [1999] for sediments (Table 2). The reasons for this are threefold. First, the starting materials used in their experiments are natural amphibolite and sediment from SW Japan, with compositions similar to the oceanic crust and subducting sediments contributing to the Setouchi magmatism, respectively. Second, element transport during open-system dehydration operating in the sinking slab may be better reproduced in such dehydration experiments than in equilibrium partitioning experiments. Third, the incompatible trace element composition of oceanic arc basalts can be explained quantitatively by such mobility data together with a plausible multi-step process of arc magma production [Tatsumi and Kogiso, 1997, 2003]. The element mobilities listed in Table 2 are the fraction of an element released during dehydration against that originally present in the starting material. For the modeling, 1.5 wt.% H 2 O in both amphibolite and sediment was assumed. The degree of contribution of sediment relative to amphibolite to the production of slab-derived fluids was varied from 0.2 to 1.0. The trace element concentrations in the slabderived fluids were then obtained (Table 2). [17] The process of overprinting slab-derived fluids onto the mantle wedge is likely to be complex. The fluids may interact with peridotite at the base of the mantle wedge to form hydrous peridotite that is subsequently dragged downward on the slab, releasing secondary fluids beneath the volcanic arc [Tatsumi, 1989; Tatsumi and Eggins, 1995]. Such multi-step processes were examined geochemically by Tatsumi and Kogiso [1997]. However, this approach relies on unconstrained parameters such as the Na content in slab-derived fluids and the amount of phlogopite, chlorite, and amphibole crystallizing in the downdragged hydrous peridotite. For the present assessment, therefore, the net effect of fluid fluxing was deduced using a single-step metasomatic process in which slab-derived fluids directly overprint the original mantle wedge and cause partial melting. [18] The fraction of slab-derived H 2 O in the HMA magma source was set at 1 wt.% (Table 2). We used this value based on the following considerations. First, H 2 O content in a HMA magma is assumed to be 5 wt.%, which is largely equivalent to the minimum H 2 O content in HMA magmas produced in the uppermost mantle immediately below the crust/mantle boundary, 30 km depth beneath the SVB [Tatsumi, 1981, 1982; Umino and Kushiro, 1989]. Second, 20% incremental fractional melting is assumed for HMA magma production. The reason for applying such a mode of melting instead of simple batch melting is that hydrous HMA magmas are likely to have a low viscosity and may therefore be readily separated from the magma source region. Tatsumi [1981, 1982] demonstrated that the residual mantle mineral assemblages for Setouchi HMAs are olivine + orthopyroxene ± clinopyroxene. These experimental results, together with the change in the residual mineral assemblages with increasing degrees of partial melting [e.g., Hirose and Kushiro, 1993], provide the estimate of the degree of partial melting. The trace element compositions of the HMA magma source mantle and the inferred HMA magma could then be estimated (Table 2). [19] The trace element compositions of the calculated HMA magmas, with different contributions and compositions of subducting sediments, are 8of19

9 Figure 3. The results of geochemical modeling of addition of slab-derived aqueous fluids to the mantle wedge. The broken lines in the isotope diagram indicate mixing trajectory between terrigenous sediments (TS) and altered oceanic crusts (AOC). The solid lines are mixing lines between the mantle wedge (MW) and slab-derived fluids. The results obtained by changing the fraction of sediment (shown as TS = 1/5, 1/2, or 1) that contributes to producing the slabderived fluid composed of TS- and AOC-derived fluids. Although the trace element compositions of the Setouchi HMAs can be reproduced, the Sr-Nd-Pb-Hf isotopic characteristics of the Setouchi HMAs are not consistent with this process. Element abundances in N-MORB are after Sun and McDonough [1989]. plotted on a NMORB-normalized multielement diagram (Figure 3), together with those of the inferred original mantle, slab-fluids, and HMAs from Shodo-Shima island in the NE Shikoku region of the SVB [Shimoda et al., 1998]. Figure 3 demonstrates that the distinctive incompatible element characteristics of Setouchi HMAs (e.g., positive spikes of Rb, Ba, Th, K, and Pb) are well reproduced by the modeling, which includes element fluxing by fluid overprinting of the mantle 9of19

10 Table 3. Results of Isotope Modeling for HMA Magma Generation by Addition of Slab Fluids a Sr Nd Pb Hf 87 Sr/ 86 Sr 143 Nd/ 144 Nd 206 Pb/ 204 Pb 207 Pb/ 204 Pb 208 Pb/ 204 Pb 176 Hf/ 177 Hf AOC-fluid b Sediment-fluid c Sediment contribution Slab-fluid Original mantle Results Fluid fraction Sr Nd Pb Hf 87 Sr/ 86 Sr 143 Nd/ 144 Nd 206 Pb/ 204 Pb 207 Pb/ 204 Pb 208 Pb/ 204 Pb 176 Hf/ 177 Hf a Element concentrations are in ppm. b Isotope ratios of altered oceanic crust (AOC) are from Hickey-Vargas [1998], Wood et al. [1979], Tatsumi [2000]. c On the basis an average value of terrigenous sediments in Shimoda et al. [1998]. wedge and subsequent partial melting of the metasomatized subarc mantle. [20] As mentioned before, Setouchi HMAs are multiply saturated with mantle minerals under hydrous conditions, indicating that the major element compositions of these HMAs can be explained by aqueous fluid addition to the mantle. It is thus concluded that overprinting of slab-derived fluids onto the mantle wedge is a viable mechanism for producing the characteristic major and trace element compositions of the Setouchi HMAs Isotopic Modeling of Slab Dehydration [21] Sr, Nd, Pb, and Hf isotopic compositions of HMA, the original mantle wedge, and the subduction components used in the modeling are after Shimoda et al. [1998], Hanyu et al. [2002] and Hickey-Vargas [1998] and are listed in Table 3. No 176 Hf/ 177 Hf ratios have been reported for Shikoku basin basalts and terrigenous sediments, so values were instead taken from basalts from the Mariana Trough, located immediately east of the Shikoku Basin [Woodhead et al., 2001], and deduced from the Nd-Hf correlation trend of sediments [Vervoort et al., 1999], respectively. [22] The results of mixing calculations with 20%, 50% and 100% sediment contribution, relative to amphibolite, to slab-fluid formation (TS = 1/5, 1/2, and 1 in the figure, respectively) are shown in Figure 3. Sr and Nd isotopic ratios of Setouchi HMAs can be explained by 0.5 to 1% addition of sediment-derived aqueous fluids. This addition of H 2 O to the original mantle would produce HMA magmas with 2.5 wt.% H 2 O, a slightly smaller value than the inferred H 2 O contents of the HMAs (5 7 wt.%), but probably still an acceptable value. [23] However, a significantly small amount of the sediment-derived fluid (<0.1%) is enough to explain the rather enriched Pb isotopic characteristics of Setouchi HMAs (Figure 3). Such a small amount of H 2 O in the magma source mantle would produce a magma containing <0.5 wt.% H 2 O, which would be basaltic rather than andesitic in composition even if it forms in the uppermost mantle at around 1.0 GPa [e.g., Tatsumi et al., 1983b]. Furthermore, the degree of contribution of the slab-derived aqueous fluids to forming the 10 of 19

11 HMA magma source required for explaining the Pb isotopic characteristics (<0.1% assuming 100% sediment contribution) is more than five times smaller than that required for the Sr and Nd isotopes (0.5%; Figure 3). This confirms the previous suggestion by Shimoda et al. [1998], even though their isotopic modeling was based on unrealistically low concentrations of Sr, Nd, and Pb in the sediment-derived fluids. Lower amounts of fluid addition are required to explain the Pb isotopic compositions of the HMA than the Sr-Nd isotopes as dehydration processes will more readily transport Pb, resulting in higher concentrations of Pb in slab-derived fluids than Sr and Nd (Table 2). In order for the Setouchi HMA Pb isotopic signatures to be consistent with the 0.5% addition of slab-derived fluid inferred from the Sr-Nd isotopic compositions, the mobility of Pb during dehydration would have to be smaller than that of Sr and Nd. However, this is not consistent with existing experimental data [Brenan et al., 1995a; Kogiso et al., 1997; Aizawa et al., 1999]. [24] It is generally accepted that high-field-strength elements such as Nb, Ta, Zr, and Hf, remain largely immobile during the dehydration processes that occur in subduction zones [e.g., Pearce, 1983; Tatsumi et al., 1986]. Hf is distinct among this group in its potential as an isotopic tracer. Hanyu et al. [2002] determined 176 Hf/ 177 Hf isotopic ratios for HMAs and basalts in the SVB and demonstrated that Setouchi rocks have distinctly low 176 Hf/ 177 Hf compared with both other arc basalts and MORBs. Such Hf isotopic characteristics may be best explained by a significant contribution of sediment-derived melts, rather than aqueous fluids, to the Setouchi magma source [Hanyu et al., 2002]. The 176 Hf/ 177 Hf and 143 Nd/ 144 Nd ratios of HMA may be accounted for if the contribution from sediment fluid against AOC fluid is 4:1 (TS = 4/5 in Figure 3). The ratio of mixing between slab-fluids and the original mantle required for these isotopes is, however, around 5%, which is again inconsistent with the estimated mixing ratios based on Pb and Sr- Nd isotope data (<0.1% and 0.5%, respectively). [25] The modeling therefore indicates that although the major and trace element composition of the HMAs can be reproduced by slab-dehydration processes and associated element transport, the Sr, Nd, Pb and Hf isotopic compositions cannot be comprehensively understood solely by such a mechanism Trace Element Modeling of Slab Melting and Subsequent Melt-Mantle Interaction [26] An alternative potential mechanism responsible for HMA magma production, other than the direct addition of slab-derived fluid, is partial melting of the subducting lithosphere and subsequent interaction of slab melts with overlying mantle peridotite [Kay, 1978; Pearce et al., 1992; Yogodzinski et al., 1994; Shimoda et al., 1998; Hanyu et al., 2002]. Tatsumi [2001] examined this with geochemical modeling of partial melting of subducting sediments and melt-mantle reactions, and concluded that the trace element characteristics of the Setouchi HMAs can be successfully reproduced by such processes. As demonstrated by numerical simulations of temperature distribution along the downgoing lithosphere, however, partial melting of the subducting oceanic crust, in addition to subducting sediments, may have taken place during the Setouchi magmatism [Furukawa and Tatsumi, 1999]. The surface temperature of the subducting 5 m.y.-old Shikoku Basin lithosphere at 50 km depth is estimated to be 800 C, which is well above the solidus temperature of both terrigenous sediments and hydrous oceanic crust. The modeling presented here therefore takes into account melting of the subducting Shikoku Basin oceanic crust as well as subducting sediments. [27] Partial melting of subducting sediments was modeled based on the high-pressure experimental results of Vielzeuf and Holloway [1988], as the experimental starting material used, a Carino gneiss, possesses a major element composition almost identical to that of the averaged sample of the Nankai Trough terrigenous sediments. Although Nankai Trough terrigenous sediments possess variable incompatible trace element compositions (Table 1), they have rather limited major element compositions [Shimoda et al., 1998]. The modeling used melt and restite compositions at 1.0 GPa and 1050 C (Table 4), as these condi- 11 of 19

12 Table 4. Major Element Compositions of Slab-Derived Melts and (Reacted) Magmas a Relative Contribution, wt.% SiO 2 Al 2 O 3 FeO* MgO CaO Na 2 O Slab Melt Oliv. Opx. Cpx Basalt-melt Sediment-melt Olivine Orthopyroxene Clinopyroxene Setouchi HMA :1 mixture (R 2 = 1.70) Slab-melt r = F = :2 mixture (R 2 = 0.98) Slab-melt r = F = :4 mixture (R 2 = 0.43) Slab-melt r = F = a Element concentrations are in wt.%. 1:1, 1:2, and 1:3 indicate the weight ratios between amphibolite- and sediment-melts. tions are reasonable for the surface of the subducting lithosphere beneath the SVB [Furukawa and Tatsumi, 1999]. These conditions result in 75% batch partial melting of the sediment, leaving a restite composed of 40% garnet, 40% quartz, and 20% sillimanite. [28] Experimental results of dehydration melting of a low-k magmatic amphibolite, with a similar major element composition to the Shikoku Basin basalts, at 1.6 GPa and 1050 C by Rapp and Watson [1995] were used for estimating the composition and the fraction of partial melt from the subducting oceanic crust (Table 4). These conditions produce 25% batch melting, and leave a restite consisting of 33% garnet, 66% clinopyroxene, and 1% rutile. [29] The relative contribution of sediment- and amphibolite-derived melts to slab-melt formation is not well constrained. We thus used three different values for these contributions: 1:1 (amphibolite: sediment), 1:2, and 1:4 (Table 4). Slab-derived melts are saturated with the SiO 2 component and should react with mantle olivine and crystallize orthopyroxene during their ascent through the mantle wedge. Although such processes take place continuously, a simple least squares mixing calculation was used for obtaining the final melt composition from the simplified slab-derived melts, mantle and crystallizing phases, and an HMA from the NE Shikoku region (Table 4). Amphibole will not crystallize during the reaction in the mantle wedge as the temperature of final equilibration of the HMA magmas with mantle peridotite (1050 C [Tatsumi, 1982]) is higher than the amphibole-out temperature in HMA melts [Tatsumi, 1981, 1982]. The results of calculations (Table 4) indicate that slab-derived partial melts dissolve olivine and clinopyroxene and crystallize orthopyroxene, which is consistent with the reaction relation, to form melts with compositions identical to HMAs with the sum of the squares of the residuals (R 2 in Table 4) below the acceptable upper limit [Mann, 1983]. [30] The reaction model used in this study is the AFC (assimilation-fractional crystallization) formulation of DePaolo [1981] and Albarède [1995]: C i liq ¼ Ci 0 F D i 1 r 1 þ r r þ D i 1 Ca i C0 i 1 F D i 1 r 1 i where C liq, C i 0, and C i a are the concentration of an element in the reacted melt, initial melt, and the country rock (mantle wedge peridotite), respectively, and D i is the bulk distribution coefficient of the element. Phases dissolved into the melt 12 of 19

13 Figure 4. The results of geochemical modeling of melting of subducting sediments and altered oceanic crust, and subsequent melt-mantle interaction. The ranges of isotopic compositions of HMA calculated using different terrigenous sediment compositions (TS min. and max.) and different sediment contributions (shown as TS = 1/2, 2/3, or 4/5) are shown. Broken lines indicate the mixing lines of terrigenous sediment (TS)-derived and altered oceanic crust (AOC)-derived melts using the terrigenous max. composition. This mechanism can comprehensively explain both major/trace element and isotopic compositions of Setouchi HMAs. Symbols are as for Figure 3. through the reaction are olivine and clinopyroxene (Table 4). This indicates that in a strict sense, element concentrations in these minerals, rather than in peridotite, should be used for the modeling. However, the elements modeled here have much lower concentrations in olivine, clinopyroxene, and other peridotitic minerals than in the initial slab melt. It is thus suggested that the present modeling may reasonably reproduce the reaction processes. F is defined as the residual melt fraction relative to the initial amount of magma M 0 and is given by F = M liq /M o. The 13 of 19

14 Table 5. Results of Trace Element Modeling for HMA Magma Generation by Slab Melting and Melt-Mantle Interaction a K Rb Sr Pb Ba Y Nd Zr Th Nb Hf Altered oceanic crust Amphibolite Degree of melting Bulk distribution coefficient Melt Sediment Terrigenous max Degree of melting Bulk distribution coefficient Melt Slab-melt 1:1 mixture Original mantle wedge Source mantle Melt-mantle reaction Distribution coefficient (opx) F r Setouchi HMA max Setouchi HMA min Terrigenous max. (1:1 mixture) (1:2 mixture) (1:4 mixture) Terrigenous min. (1:1 mixture) (1:2 mixture) (1:4 mixture) a Element concentrations are in ppm ratio, r, of assimilation and crystallization increments is defined as r ¼ dm a dm sol where M a and M sol represent the amount of solid phases assimilated and crystallized during the reaction, respectively. F and r values in the formulation were calculated on the basis of the amounts of dissolving and crystallizing phases obtained from the mixing calculations that involve the compositions described (Table 4). [31] The trace element compositions of calculated HMA melts are listed in Table 5. The range of HMA compositions obtained from varying the composition of the subducting sediments while keeping a constant sediment contribution (sediment:amphibolite = 2:1), are shown in a N-MORB-normalized spidergram, together with the compositions of the Setouchi HMAs, the original mantle, and the slab-derived melt. Figure 4 clearly demonstrates that the composition of the slab-melt, rather than the original peridotite, governs the geochemical characteristics of the final 14 of 19

15 Table 6. Results of Isotope Modeling for HMA Magma Generation by Slab Melt-Mantle Reactions Sr Nd Pb Hf 87 Sr/ 86 Sr 143 Nd/ 144 Nd 206 Pb/ 204 Pb 207 Pb/ 204 Pb 208 Pb/ 204 Pb 176 Hf/ 177 Hf AOC-melt a Sediment-melt Terrigenous max Sediment contribution Slab-melt Original mantle wedge Melt-mantle reaction Distribution coefficient (opx) r Terrigenous max. (1:1 mixture) (1:2 mixture) (1:4 mixture) Terrigenous min. (1:1 mixture) (1:2 mixture) (1:4 mixture) a AOC, altered oceanic crust reaction product. Furthermore, even if we use the N-MORB source mantle as the original mantle, the calculated HMA compositions are close to those obtained by using the SD-438 basalt source compositions (calculated HMA-2 in Table 5). The main reason for this is that the elements considered here have much higher concentrations in slab-melts than in peridotites. The calculated HMA melts possess trace element characteristics similar to those of the Setouchi HMAs, although the concentrations of some elements, such as Ba, Th, and K, are higher in the calculated melts than in the Setouchi HMAs (Figure 4). A possible reason for this discrepancy in trace element abundances is the uncertainty in the composition of the subducting oceanic crust; i.e., the degree of amphibolization for the Shikoku basin oceanic crust. In order to examine this, HMA compositions were calculated assuming an unaltered oceanic crust as an extreme case (calculated HMA-3 in Table 5). The trace element concentrations obtained from such calculations are much closer to the natural HMA compositions. The geochemical modeling therefore reasonably reproduces the characteristic incompatible element patterns of the Setouchi HMAs. It should be further stressed that in the present modeling the calculated trace element patterns do not change significantly even if the amphibolite/sediment contribution ratios change from 1:4 to 1:1 (Table 5), and that they are largely governed by the chemical characteristics of the slab melts, not the mantle wedge, because of the low concentrations of incompatible elements in the mantle Isotopic Modeling of Slab Melting and Subsequent Melt-Mantle Interaction [32] The AFC formulation of DePaolo [1981] and Albarède [1995] was used for the isotopic modeling: C i2 C i1 ¼ liq Ci2 C i1 þ a Ci2 C i1 Ci2 0 C i1 a r 1 r þ D i 1 r 1 r þ D i 1 C i1 a C i1 liq where i1 and i2 represent two isotopes of the same element. C i1 a C i of 19

16 [33] The element concentrations used in the modeling were obtained from the aforementioned trace element modeling (Tables 5 and 6). These data, together with the isotopic compositions of the end-member components (Table 6), provide the isotopic ratios for the calculated HMA magmas produced by reactions between slab melts and the original mantle wedge (Table 6 and Figure 4). The range of isotopic compositions of the HMAs calculated using different terrigenous sediment compositions (Terrigenous max. and min. in Table 1) and different sediment contributions are shown in Figure 4. As indicated in the zoom-up Sr-Nd isotopic diagram in Figure 4, the calculated HMA magma composition is close to that of the slab melt. [34] The modeling results demonstrate that the subducting altered oceanic crust (amphibolite) component, in addition to partial melts from subducting sediments, is significant (a relative amphibolite-sediment contribution of 1:1 to 1:2) for reproducing the isotopic characteristics of the Setouchi HMAs. It should be further stressed that the isotopic compositions of both calculated and measured Setouchi HMAs plot close to a mixing line between altered oceanic crust- and sedimentderived melts (broken lines in Figure 4). This indicates that the isotopic characteristics, as well as the trace element characteristics, of the Setouchi HMAs are largely governed by slab melts rather than the mantle wedge. [35] Hanyu et al. [2002] demonstrated that the 176 Hf/ 177 Hf ratios of Setouchi HMAs are distinctly lower than those for MORBs and Izu-Mariana arc basalts, and attributed these characteristics to the contribution of slab-derived melts rather than aqueous fluids to HMA magma generation. The present results support this. In contrast to the addition of aqueous fluids, the calculated HMA magmas are distinct in their low 176 Hf/ 177 Hf ratios, essentially the result of much higher concentrations of Hf in slab-melts than in aqueous fluids. However, the modeling based on the estimated concentration of Hf and 176 Hf/ 177 Hf in the subducting components does not completely reproduce the Hf isotopic compositions of the Setouchi HMAs, which show slightly higher 176 Hf/ 177 Hf ratios than the calculated magmas. One possible reason for this discrepancy is if HFSE-bearing minerals like rutile play a role by partitioning Hf, but not Nd, during slab melting. Figure 4 illustrates the mixing lines between melts from terrigenous sediments and altered oceanic crust by changing the amount of rutile in the molten oceanic crust. As residual rutile reduces, Hf is distributed more into the melts of oceanic crust, thereby calculated 176 Hf/ 177 Hf ratios of the HMAs are elevated. Furthermore, the involvement of subducted pelagic sediments, which may possess higher 176 Hf/ 177 Hf than terrigenous trough sediments (Figure 4), may be responsible for determining the 176 Hf/ 177 Hf characteristics of Setouchi HMAs. 4. Conclusions [36] HMA is one of the characteristic rocks that typify subduction zone magmatism. However, the limited occurrence of HMAs in modern arc-trench systems suggests that rather unusual tectonic conditions may be needed for the production of these magmas. The geochemical modeling results presented here indicate that addition of slab-derived, aqueous fluids to the mantle wedge, which may be a likely mechanism for magma production in most modern subduction zones, is not viable for forming the Sr-Nd-Pb-Hf isotopic characteristics of the Setouchi HMAs. Instead, partial melting both of subducting sediments and altered oceanic crust, and subsequent melt-mantle interaction can reasonably and comprehensively explain major and trace element, and isotopic compositions of these HMAs. This unusual environment that resulted in slab melting would have been caused by the subduction of the young (<10 m.y.), and hence hot, lithosphere of the Shikoku Basin that was created by backarc spreading processes. [37] The tectonic conditions under which the Setouchi HMAs were produced may be analogous to those existing more widely during Earth s early history. If so, partial melting of the subducting lithosphere could have significantly contributed to magma production, and possibly continental crust formation, during the Archean [e.g., Martin, 1986; Kelemen, 1995]. Tatsumi [2000b] demonstrated that the Sr-Nd-Pb isotopic compositions of melting 16 of 19