Journal of Volcanology and Geothermal Research 200 (2011) 255–266

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Journal of Volcanology and Geothermal Research

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Subduction components in Pleistocene to recent Kurile arc magmas inNE Hokkaido, Japan

Nguyen Hoang ⁎, Jun'ichi Itoh, Isoji MiyagiGeological Survey of Japan, Higashi 1-1-1, Tsukuba Central 7th, Tsukuba, 305-8567, Japan

⁎ Corresponding author. Tel./fax: +81 29 861 3637.E-mail address: hoang-nguyen@aist.go.jp (N. Hoang

0377-0273/$ – see front matter © 2011 Elsevier B.V. Aldoi:10.1016/j.jvolgeores.2011.01.002

a b s t r a c t

a r t i c l e i n f o

Article history:Received 17 May 2010Accepted 5 January 2011Available online 19 January 2011

Keywords:Kurile arcHokkaidoarc magmaaltered oceanic crusthydrous fluid

Samples of Kurile arc lavas erupted between 1.6 Ma and ca. 30,000 years were collected from the Kutcharo,Mashu and Akan caldera area in NE Hokkaido, about 150 km west of the Kurile trench. The samples includerhyolitic pumice, rhyolite, dacite, andesite and, rare, tholeiitic basalt, and show ‘medium’ potassic calc-alkaline affinity. Except for relatively high concentrations of large ionic lithophile elements (LILE), Th andespecially Pb, other trace elements, including the rare earths (REE) and high field strength elements (HFSE),show relatively low abundances when compared with those of normal mid-ocean ridge basalts (N-MORB).Their Sr, Nd isotopic compositions are relatively depleted, with 87Sr/86Sr ranging from 0.7033 to 0.7034and 143Nd/144Nd from 0.51295 to 0.51230. Pb isotopic compositions are also relatively unradiogenic, with206Pb/204Pb at about 18.4 and 208Pb/204Pb ranging from 38.3 to 38.4, significantly more depleted than otherQuaternary lavas in NE Japan. The Kurile lavas show typical subduction-type element distributions, with highratios of fluid-mobile incompatible elements over fluid-immobile HFSE, Ba/Nb, for example, ranging betweenca. 200 and 450. The lack of covariance between (e.g.) Ba/Nb and Ba/Th with 87Sr/86Sr, and Nd/Pb with Pbisotopic ratios suggests minimal involvement of sediment-derived metasomatism of the magmatic source.Geochemical character of the latter probably reflects contamination by hydrous fluids derived from alteredoceanic crust (AOC). This is indicated by the coupling of relatively depleted, MORB-like Sr and Pb isotopiccompositions and high Sr and Pb contents. Thus, given their N-MORB-type isotopic compositions, the LILE,and HFSE-like character, evidenced by high ratios of Ba/Nb, and variable Nd/Pb and Th/Nd, suggests NEHokkaido arc magma genesis is best explained in terms of a binary mixing model involving: a dominantlyN-MORB-like (i.e. depleted) convecting mantle ‘wedge’, contaminated by hydrous AOC-derived fluid. Thusmagmatic sources for the NE Hokkaido sector appear to have been minimally affected by sedimentary melt-derived metasomatism, consistent with their association with the Kurile arc system.

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1. Introduction

Island arc magmas are most likely generated by partial melting in asubduction-related mantle wedge in response, directly or indirectly, tothe addition of metasomatic components released from subductingoceanic lithosphere (Gill, 1981; Arculus and Powell, 1986; Tatsumi et al.,1986; Crawford et al., 1987; McCulloch and Gamble, 1991). Metaso-matic fluxes may consist of hydrous (supercritical) fluid or incipientpartial melts of subducted sediment and/or basaltic crust (e.g. Shimodaet al., 1998, and references therein). Fluids may be released directly tothe convecting asthenosphere (e.g. Kay et al., 1978; Morris and Hart,1983; Ben Othman et al., 1989; McCulloch and Gamble, 1991), eitherfrom breakdown of hydrous minerals in subducting basaltic andsedimentary crust (Staudigel et al., 1981; Peacock, 1990; Cousenset al., 1994; Hawkesworth et al., 1994; Peacock et al., 1994; Brenan et al.,

1995; Staudigel et al., 1996; Kogiso et al., 1997; Woodhead et al., 1998;Hochstaedter et al., 2001), refractory mantle lithosphere in theoverridingplate, hydrated at relatively lowpressures then progressivelydehydrating at depth, following delamination and ‘down-dragging’ bythe subduction plate, (Tatsumi, 1989). Other possible mechanismsmayinvolve the incorporation of previously enriched mantle lithosphereand/or continental crust components (e.g. Miller et al., 1994; Kerstinget al., 1996). The distinctive enrichment of H2O-soluble incompatibleelements (e.g. large ionic lithophile (LIL) elements and Pb) and relativedepletions in fluid-immobile high-field strength (HFSE) and rare earthelements (REE) (Gill, 1981; Kay, 1984; Tatsumi, 1989; McCulloch andGamble, 1991; Brenan et al., 1994, 1995; Elliot et al., 1997; Woodheadet al., 1998) are key attributes of subduction-related magmatic sources.Themost likely source of hydrousfluid and silicatemetasomatismof themantle wedge is subducting altered oceanic crust and sediment. Whilearc basalts shows light differences in Sr, Pb and, especially, Nd isotopiccompositions compared to altered MORB, the major constituent ofaltered oceanic crust (AOC) (e.g. Staudigel et al., 1996), these contrastsignificantly with those of most subducted sedimentary components

256 N. Hoang et al. / Journal of Volcanology and Geothermal Research 200 (2011) 255–266

(e.g. Cousens et al., 1994; Plank and Langmuir, 1998). Given such dif-ferences it is possible to distinguish the respective contributions ofAOC, sediment-derived fluid and silicate melt metasomatism.

Kersting and Arculus (1995) reported the most highly depleted Pbisotopic compositions observed in arc magmas from the Klyuchevskoyvolcano in Kamchatka (a northern continuation of theKurile arc system)and suggested the Kamchatka magma may represent an ‘end member’involving little or no sediment. Data reported by Kepezhinskas et al.(1997) for Kamchatka volcanoes, including Klyuchevskoy, also indicatedsubduction-related affinity, which they interpreted as due to interactionof slab melts and/or fluids with depleted mantle wedge peridotite.

Arc volcanic samples, including rhyolitic pumices and lavas rangingfrom rhyolite to basalt, were collected from the Kutcharo caldera andsurrounding areas of NE Hokkaido for geochemical and isotopic studywith the object of distinguishing subduction-derived contributions toKurilemagmatic sources. The NEHokkaido data are comparedwith thoseof contemporaneous intraplate (i.e. ‘back-arc’) magmas in northeastern

Fig. 1. Sketch map showing the distribution of the calderas in the Kutcharo area. Kitami badacite — rhyolite, respectively) is simplified from Yamaguchi et al. (1983) and Takagi et al.

Hokkaido, arc magmas in southern Kamchatka and NE Japanese arcs, andare interpreted in the context of regional Kurile Arc source character.

2. Geology and petrography

In the late Oligocene (ca. 30 Ma) theOkhotsk Terrain began to rotateas the southern part of the (back-arc) Sea of Okhotsk began opening tothe west of the Kurile Arc (Kimura and Tamaki, 1986; Baranov et al.,2002). Between the late Oligocene and Middle Miocene, opening of theOkhotsk basin was accompanied by southwestward rollback of theKurile arc to its present location, at which point it collided with the NEJapan Arc which had hitherto been moving northwestward followingthe opening of the Japan Sea (Kimura, 1986; Kimura and Tamaki, 1986;Komatsu et al., 1989; Baranov et al., 2002) (Fig. 1). This allowed forcontinuing oblique subduction of the Pacific plate beneath the southernpart of NE Hokkaido (Kimura, 1986; Kimura and Tamaki, 1986),accommodated by left-lateral strike-slip faults allowing for widespread

ck-arc volcanic field (filled colors: black, gray, white representing basalt, andesite and(1999).

257N. Hoang et al. / Journal of Volcanology and Geothermal Research 200 (2011) 255–266

west-southwestward migration of the Kurile Arc. The Late Miocenecollision of theKurile arcwith the Eurasianplate (Kimura, 1986; Kimuraand Tamaki, 1986; Bazhenov and Burtman, 1994) formed the Hidakamountain belt in central Hokkaido, thereby consolidated the present-daylocation of the southern Kuriles.

The Hokkaido region is divided into three major geologic areas:southwestern, central, and eastern. The geologic units in southwestHokkaido are northward extensions of those found in northeastHonshu (Kimura and Tamaki, 1986). In Central Hokkaido, the HidakaMountains and their northward extension show geologic structurestrending parallel to the central Hidaka axis. Eastern Hokkaido ischaracterized by Jurassic volcanics, Cretaceous to Paleogene fore-arcsediments, and Neogene to Quaternary volcanics and sediments(Komatsu et al., 1989; Takashima et al., 2006 and references therein).These constitute the southern (Chishima) part of the Kurile Arc andare believed to have collided with geologic units of central Hokkaidoduring theMiddleMiocene (Kimura and Tamaki, 1986; Komatsu et al.,1989; e.g. Baranov et al., 2002; Ren et al., 2002).

Widespread intraplate Tertiary volcanism characterizes the ‘back-arc’region of the NE Hokkaido part of the Kurile system (Fig. 1) (Goto et al.,1995; Ikeda et al., 2000). The volcanic activity occurred between 14Maand the Quaternary, produced lavas ranging from olivine tholeiite,andesite to rhyolite, which are interpreted as typical back-arc magmaticproducts (Shibata et al., 1981; Ikeda, 1998, Aoki et al., 1999; Takagi et al.,1999; Yamashita et al., 1999; Ikeda et al., 2000; Shuto et al., 2004). Goto etal. (1995) and Ikeda et al. (2000) linked the magmatic activities either toasthenospheric upwelling, evidenced by the steep dip of the subductingPacific slab (Watanabe, 1995), or by collision of the Eurasian, Okhotsk andPacific plates (Okamura et al., 1998). In addition, based on the evidence ofSr and Nd isotopic depletion, several workers have argued that theTertiary back-arc magmas, including those with high SiO2, were mantle-derived melts (e.g. Takagi et al., 1999).

The NE Hokkaido Kurile volcanic front occupies an area, about150 kmwest of the Kurile trench (Fig. 1). Kutcharo, the largest group ofcalderas is 20 by 26 km in area and developed during a series of major

Table 1Reported reference and (GSJ) ICP-MS analyzed trace element data.

Element BIR-1 BIR-1* AGV1 AGV1*

Rb 0.09 69.76 67±1Sr 103.18 110±2 681.29 660±2Y 15.55 16±1 20.68 20±3Zr 15.77 18±1 238.70 227±1Nb 0.46 0.6±? 14.18 15±?Cs 0.00 n.a 1.34 n.aBa 5.54 7±? 1213.00 1230±La 0.55 n.a 38.04 38±2Ce 1.77 1.9±0.4 69.50 67±6Pr 0.33 n.a 8.08 7.6± ?Nd 2.07 2.5±0.7 31.65 33±3Sm 1.02 1.1±? 5.81 5.9±0.Eu 0.49 0.55±0.05 1.69 1.6±0.Gd 1.77 1.8±0.4 4.63 5.0±0.Tb 0.33 n.a 0.66 0.7±0.Dy 2.46 4±1 3.46 3.6±0.Ho 0.54 n.a 0.66 n.aEr 1.60 n.a 1.82 n.aTm 0.30 n.a 0.26 0.34±?Yb 1.58 1.7±0.1 1.62 1.72±0Lu 0.23 0.26±? 0.24 0.27±0Hf 0.54 0.6±0.08 5.16 5.1±0.Ta 0.04 n.a 0.94 0.9±0.Pb 2.62 3±? 36.53 36±5Th 0.04 n.a 6.43 6.5±0.U 0.00 n.a 1.96 1.92±0V 324.87 310±11 123.87 120±1Cr 379.51 370±8 10.72 10±3Ni 166.10 170±6 17.95 16±?

*: reported by USGS (http://minerals.cr.usgs.gov/geo_chem_stand/).†: reported by Geological Survey of Japan (GSJ) (http://riodb02.ibase.aist.go.jp/geostand/ign

eruptions between ca. 340,000 to 30,000 years agowhile the age of pre-caldera activity is estimated to be from ca. 1.6 to 1 Ma (Miyagi et al.,2008, submitted for publication; J. Itoh, unpublished data). Twoparticularly significant stages of caldera formation occurred at about120,000 and 35,000 Ka (KP4 and KP1, respectively) (Machida and Arai,2003) producing about 20 km3 and 48 km3 of felsic pyroclastic deposits,respectively. Lavas erupted in the Kutcharo lake area include rhyolite,andesite and a minor amount of tholeiitic basalt (Fig. 1). Many of thelavas aremassive, some tholeiites beingpartially vesicular. The rhyolitesare moderately phyric with phenocrysts of plagioclase, K-rich feldspar,and anhedral quartz with accessory mica. A microcrystalline ground-mass is set in volcanic glass. Some KP4 and KP1 rhyolitic pumicesinclude representative phenocrysts of plagioclase (An42.6Ab56.3Or1.1 —

An84.3Ab15.5Or0.1), euhedral orthopyroxene (Wo1.9En67.3Fs30.8,TiO2=0.11), clinopyroxene (Wo43.1En42.6Fs14.3, TiO2=0.45),magnetiteand trace amounts of ilmenite and euhedral olivine (Fo75–86). A largenumber of Ca-rich amphibole inclusions are found in clinopyroxenephenocrysts [SiO2: 49–50, TiO2: 1.5–1.8, Al2O3: 6.3–7.8, FeO: 11.5,MnO:0.2–0.45, MgO: 15.4–16.4, CaO: 10.9–11.3, Na2O: 1.4, K2O: 0.2] (Miyagiet al., submitted for publication). The andesites are porphyritic withphenocrysts (mostly plagioclase, in some cases orthopyroxene and asmall amount of Ca-rich amphibole) ranging from N10 to about 50 vol.%.Andesitic groundmass contains microcrystalline plagioclase and alkalifeldspar. The basalts are sparsely phyric tholeiite, with phenocrystsabout 3–5 vol.%, including small, idiomorphic olivine (about 0.5 by0.2 mm) and elongate plagioclase (about 1.5 by 0.5 mm). Thegroundmass comprises microlites of pyroxene and plagioclase (Miyagiet al., 2008).

3. Analytical procedures

The analytical work was conducted at the Geological Survey ofJapan (GSJ). Procedures for X-ray fluorescence (XRF) major elementsanalyses have been described elsewhere (Uto et al., 2004), reportedaccuracies being better than ±1.5% based on repeated measurements

JB-1a JB-1a† BHVO-2 BHVO-2*

37.40 39.2 8.93 9.8±19 443.74 442 385.57 389±2.3

23.97 24 26.67 26±28 136.79 144 166.40 172±11

27.44 26.9 17.43 18±21.24 1.31 0.10 n.a

16 492.39 504 125.64 130±1337.38 37.6 15.04 15±166.33 65.9 37.65 38±26.70 7.3 5.02 n.a

26.30 26 23.82 25±1.84 5.13 5.07 5.90 6.2±0.41 1.46 1.46 1.95 n.a6 4.53 4.67 6.08 6.3±0.21 0.72 0.69 0.92 0.9±?4 4.08 3.99 5.28 n.a

0.84 0.71 0.96 1.04±0.042.25 2.18 2.48 n.a0.33 0.33 0.33 n.a

.2 2.10 2.1 1.91 2±0.2

.03 0.30 0.33 0.25 0.28±0.014 3.49 3.41 4.29 4.1±0.309 1.96 1.93 1.29 1.4±?

5.92 6.76 1.55 n.a5 8.91 9.03 1.06 1.2±0.3.15 1.59 1.57 0.36 n.a1 205.96 205 334.33 317±11

423.47 392 269.81 280±19135.54 139 109.93 119±7

eous.html).

Table 2Geochemical and Sr, Nd, and Pb isotopic compositions of the Kutcharo volcanic rocks, NE Hokkaido.

Sample 060823-04-A 060823-06 060823-07 060820-01-E 060820-05-A 060820-03-D 060819-05-A 060820-02-A 060819-02-A 060819-01-C 060820-01-A 060819-03 070921-01E 060823-01-B 070923-02-A 070922-02B

Type Pumice Pumice Pumice KP-4 KP-7 KP-4 KP-6 KP-2/3 KP-4 KP-1 KP-5 KP-4 Dacite Welded tuff Aphyr. And. Andesite

Latitude 43°35′19.2′N 43°33′32.4″N 43°35′31.9″N 43°46′26.5″N 43°49′30.3″N 43°49′29.7″N 43°15′48.0″N 43°49′05.9″N 43°18′07.0″N 43°18′07.0″N 43°46′26.5″N 43°15′16.4″N 44°40′24.5″N 43°40′11.0″N 44°40′15.8″N 44°40′15.8″N

Longitude 144°23′54.3″E

144°22′10.1″E

144°26′03.4″E

144°26′15.6″E

144°18′29.4″E

144°28′39.9″E

144°15′40.0″E

144°27′20.9″E

144°32′57.0″E

144°32′57.0″E

144°26′15.6″E

144°20′27.8″E

144°22′42.2″E

144°10′29.8″E

144°23′48.0″E

144°23′48.0″E

SiO2 69.86 72.64 68.23 67.23 68.53 70.23 71.73 72.22 72.24 72.40 72.41 73.08 70.98 72.47 57.84 61.65TiO2 0.59 0.54 0.80 0.75 0.58 0.64 0.64 0.59 0.61 0.46 0.57 0.55 0.40 0.38 1.09 0.88Al2O3 15.45 14.09 14.70 14.68 16.48 14.63 14.20 14.25 14.73 14.39 14.24 13.99 15.43 14.45 15.49 16.47FeO* 3.49 2.62 4.89 5.11 3.09 3.55 3.13 2.93 2.71 2.86 2.87 2.57 3.06 2.63 9.60 7.02MnO 0.15 0.15 0.20 0.24 0.16 0.21 0.15 0.17 0.20 0.14 0.18 0.19 0.08 0.12 0.17 0.19MgO 1.09 0.71 1.45 1.81 0.90 1.12 0.83 0.75 0.73 0.79 0.76 0.72 0.21 0.72 3.91 2.23CaO 3.57 2.74 4.06 4.59 4.47 3.64 3.23 2.94 2.67 2.97 2.96 2.59 2.76 2.88 7.73 6.05Na2O 4.15 4.65 4.02 4.05 4.08 4.37 4.20 4.41 4.49 4.06 4.22 4.57 4.90 4.12 3.06 4.03K2O 1.59 1.80 1.53 1.36 1.65 1.48 1.82 1.65 1.56 1.89 1.72 1.68 2.09 2.20 0.94 1.26P2O5 0.10 0.11 0.18 0.23 0.12 0.19 0.12 0.12 0.11 0.09 0.12 0.11 0.10 0.08 0.16 0.21Sum 100 100 100 100 100 100 100 100 100 100 100 100 100 100 100 100Mg# 35.7 32.6 34.7 38.7 34.1 36.0 32.2 31.5 32.5 33.0 32.1 33.3 11.0 33.0 42.1 36.2Rb 35.16 43.82 32.63 28.44 34.44 32.86 41.08 38.55 36.29 44.57 37.70 29.18 49.27 60.32 25.14 27.25Sr 245.79 237.04 257.06 237.45 268.46 246.22 206.74 234.97 233.51 213.01 222.62 196.65 266.27 221.87 321.95 357.30Y 40.84 47.64 31.45 42.47 35.76 43.75 43.96 47.98 54.34 35.47 46.18 45.31 31.75 14.09 38.32 32.47Zr 165.85 176.33 137.55 124.01 135.61 138.80 157.57 155.64 168.95 165.32 150.67 153.86 146.23 40.27 104.63 90.20Nb 2.41 2.68 2.36 1.76 2.13 1.99 2.20 2.43 2.35 2.30 2.22 2.27 2.46 2.05 1.90 1.58Cs 1.96 2.37 0.68 1.84 2.07 1.94 2.38 2.12 2.11 2.43 2.25 2.12 1.27 2.31 1.57 0.56Ba 529.85 597.67 494.92 415.34 495.41 486.49 537.30 531.92 542.42 580.34 546.09 511.70 679.67 671.52 369.98 422.33La 12.20 13.98 7.29 10.92 11.57 12.19 13.12 12.27 13.68 12.21 12.95 11.59 17.10 13.78 10.82 11.96Ce 31.30 35.24 18.21 25.87 28.64 29.52 31.96 30.89 35.24 29.45 31.66 30.95 36.13 30.76 26.32 27.19Pr 3.86 4.36 2.27 3.40 3.27 3.74 3.84 3.97 4.25 3.43 3.85 3.66 4.96 2.90 3.59 3.40Nd 18.29 20.97 11.26 16.99 16.73 18.51 18.38 19.75 21.69 15.31 19.51 17.95 19.75 12.10 17.52 15.64Sm 4.79 5.27 3.12 4.55 4.45 4.99 4.80 5.23 6.04 3.78 5.03 4.82 4.60 2.39 5.02 4.15Eu 1.23 1.40 1.30 1.42 1.29 1.53 1.28 1.44 1.73 0.85 1.40 1.49 1.29 0.77 1.44 1.26Gd 5.18 5.90 3.58 5.10 4.79 5.44 5.46 6.14 6.89 4.12 5.65 5.41 4.40 2.18 5.50 4.66Tb 0.97 1.07 0.66 0.98 0.87 1.07 1.02 1.08 1.30 0.71 1.05 1.08 0.76 0.36 0.96 0.77Dy 5.94 6.79 4.35 6.15 5.49 6.43 6.19 6.65 7.83 4.51 6.83 6.78 4.94 1.88 6.25 5.23Ho 1.32 1.48 0.97 1.31 1.25 1.44 1.38 1.51 1.82 1.01 1.51 1.53 1.07 0.42 1.35 1.10Er 4.07 4.49 3.04 3.41 4.83 3.66 5.05 4.78 4.40 3.16 3.67 3.57 3.05 1.14 3.98 3.24Tm 0.64 0.71 0.57 0.55 0.98 0.57 0.80 0.78 0.65 0.51 0.49 0.53 0.49 0.16 0.60 0.51Yb 4.10 4.65 3.28 4.31 3.97 4.53 4.56 4.64 5.39 3.38 4.74 4.88 3.14 1.30 3.84 3.22Lu 0.67 0.75 0.53 0.67 0.60 0.75 0.69 0.73 0.84 0.54 0.73 0.80 0.46 0.21 0.59 0.47Hf 4.19 8.28 3.53 3.27 3.33 6.29 3.99 3.98 4.41 4.13 4.18 4.09 3.70 0.96 2.90 2.48Ta 0.21 0.23 0.19 0.18 0.20 0.19 0.19 0.21 0.21 0.19 0.18 0.18 0.16 0.23 0.12 0.10Pb 14.80 16.15 10.98 10.51 16.29 11.17 16.34 16.35 15.96 15.45 14.80 13.23 9.50 11.05 11.00 5.82Th 4.23 4.73 3.50 2.93 4.01 3.39 4.62 4.17 4.23 5.35 3.94 3.70 5.37 4.80 2.94 3.13U 1.26 1.40 1.04 0.94 1.31 1.06 1.39 1.30 1.24 1.62 1.27 1.10 1.48 1.09 0.86 0.82V 60.18 22.50 87.90 100.42 46.60 42.52 39.03 22.09 17.76 41.10 17.46 21.73 20.41 48.53 162.03 130.51Cr 1.22 0.40 6.59 1.06 5.12 1.64 4.38 4.21 0.19 1.04 0.15 0.10 0.15 1.39 0.76 1.64Ni 1.20 4.1287Sr/86Sr 0.703368 0.703341 0.703364 0.703454 0.703411 0.703381 0.703356 0.703364 0.703404 0.703374 0.703298 0.703419 0.703411 0.703398 0.703354 0.703415143Nd/

144Nd0.513035 0.513022 0.512979 0.513037 0.512987 0.512985 0.512958 0.512958 0.512997 0.512985 0.512990 0.512994 0.512901 0.512980 0.513007 0.513021

εNd 7.75 7.49 6.66 7.78 6.81 6.77 6.24 6.24 7.01 6.77 6.87 6.95 5.13 6.66 7.20 7.47206Pb/

204Pb18.405 18.422 18.417 18.403 18.389 18.398 18.401 18.398 18.418 18.418 18.406 18.410 18.367 18.399 18.397 18.426

207Pb/204Pb

15.515 15.534 15.530 15.520 15.506 15.510 15.510 15.511 15.531 15.534 15.517 15.519 15.511 15.519 15.514 15.543

208Pb/204Pb

38.269 38.322 38.321 38.295 38.279 38.257 38.278 38.284 38.317 38.335 38.301 38.281 38.293 38.305 38.277 38.388

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Sample 070922-03A 070922-02 C 070922-03B IM060915fb IM060915fc IM060915fa 060823-04 C 060820-03 C 060820-04D 060820-01 C 070921-02A 070921-02B 070922-03 C 070921-01D 070921-01A 070921-01B 070922-01

Type Andesite Andesite Andesite Andesite Andesite Andesite Andesite KP-4 KP-4 KP-4 Basalt Basalt Basalt Basalt Basalt Basalt Basalt

Latitude 44°40′13.5″N 44°40′15.8″N 44°40′13.5″N 43°35′00.2″N 43°34′48.4″N 43°34′55.6″N 43°35′19.2″N 43°49′29.7″N 43°50′05.5″N 43°46′26.5″N 44°40′24.5″N 44°40′24.5″N 44°40′13.5″N 44°40′24.5″N 44°40′24.5″N 44°40′24.5″N 43°40′13.4″N

Longitude 144°23′49.7″E

144°23′48.0″E

144°23′49.7″E

144°18′54.5″E

144°18′48.7″E

144°19′16.6″E

144°23′54.3″E

144°28′39.9″E

144°34′23.2″E

144°26′15.6″E

144°22′42.2″E

144°22′42.2″E

144°23′49.7″E

144°22′42.2″E

144°22′42.2″E

144°22′42.2″E

144°22′41.1″E

SiO2 61.65 61.71 62.29 56.44 59.24 59.44 58.44 57.53 61.84 62.63 52.62 52.63 52.84 53.10 53.24 53.39 53.47TiO2 0.88 0.88 0.85 0.83 0.79 0.85 1.16 1.11 1.04 0.91 0.99 0.99 0.82 0.99 0.98 1.01 0.99Al2O3 16.28 16.72 16.36 17.13 16.72 17.01 16.60 15.45 15.49 15.18 16.78 16.67 19.36 16.79 16.80 16.87 17.00FeO* 7.02 7.03 6.60 8.33 7.45 7.66 9.35 9.77 7.09 7.06 11.90 12.11 8.98 12.19 12.32 11.93 11.56MnO 0.19 0.17 0.19 0.28 0.25 0.25 0.32 0.35 0.32 0.29 0.30 0.19 0.16 0.24 0.35 0.25 0.22MgO 2.39 2.24 2.18 4.47 3.55 3.60 3.00 3.73 2.62 2.71 4.26 4.65 4.11 4.08 3.83 3.95 3.93CaO 6.14 5.89 5.81 8.89 7.74 7.20 6.84 8.05 6.52 6.27 9.84 9.38 10.49 9.22 9.11 9.20 9.39Na2O 3.97 3.90 4.18 2.80 3.13 3.10 3.39 2.90 3.57 3.65 2.57 2.54 2.43 2.56 2.52 2.56 2.56K2O 1.27 1.25 1.32 0.84 1.11 0.94 0.83 0.77 0.97 1.04 0.62 0.72 0.69 0.72 0.73 0.72 0.74P2O5 0.22 0.21 0.22 0.11 0.13 0.05 0.19 0.47 0.66 0.36 0.12 0.12 0.13 0.12 0.12 0.12 0.12Sum 100 100 100 100 100 100 100 100 100 100 100 100 100 100 100 100 100Mg# 37.8 36.2 37.0 48.9 45.9 45.6 36.4 40.5 39.8 40.7 39.0 40.6 45.0 37.3 35.7 37.1 37.8Rb 26.11 30.49 29.01 18.47 25.87 18.28 17.06 16.79 21.36 22.79 15.90 14.86 14.93 16.20 17.67 16.48 14.74Sr 375.77 359.79 387.99 314.01 317.11 335.49 327.58 280.66 307.76 276.69 324.26 311.53 384.01 292.05 302.46 302.76 326.88Y 32.84 33.10 35.25 25.04 28.44 30.04 27.90 29.41 38.28 34.47 23.94 23.29 19.73 27.89 28.80 27.67 24.11Zr 93.64 96.37 92.45 75.69 94.17 94.96 82.13 71.06 96.84 101.72 55.87 56.46 57.11 54.73 59.44 57.55 57.45Nb 1.56 1.57 1.69 1.12 1.38 1.43 1.43 1.10 1.51 1.49 0.84 0.78 0.95 0.72 0.74 0.75 0.81Cs 0.70 1.15 0.88 0.92 1.30 0.80 0.72 0.91 1.20 1.32 0.74 0.32 0.60 1.14 1.80 1.01 0.50Ba 414.14 420.57 428.85 263.58 344.41 399.14 299.63 241.61 317.07 338.83 219.54 221.95 226.00 213.04 305.39 251.94 224.66La 11.93 11.78 12.27 6.70 8.62 7.23 7.73 7.02 9.64 9.10 6.90 6.39 6.88 7.32 8.11 7.60 6.26Ce 27.64 27.26 28.05 17.56 21.55 18.38 19.34 17.64 22.17 22.78 16.14 15.29 15.86 15.33 16.60 16.09 15.68Pr 3.45 3.43 3.67 2.16 2.67 2.28 2.48 2.31 3.04 2.92 2.02 1.98 2.02 2.30 2.50 2.23 2.04Nd 15.48 15.53 16.55 10.38 13.03 10.61 12.19 11.97 15.55 14.52 9.37 8.81 9.54 10.69 11.55 10.76 9.42Sm 4.28 4.80 4.53 2.88 3.36 2.95 3.36 3.32 4.41 3.96 2.81 2.56 2.56 3.15 3.37 3.16 2.77Eu 1.34 1.24 1.32 0.87 1.02 1.02 1.17 1.14 1.42 1.29 0.84 0.81 0.77 0.94 0.88 0.86 0.84Gd 4.90 4.16 5.07 3.22 3.89 3.50 3.93 4.02 5.35 4.55 3.28 2.89 2.89 3.60 3.74 3.74 3.31Tb 0.74 0.74 0.85 0.58 0.68 0.68 0.70 0.72 0.93 0.86 0.53 0.51 0.50 0.64 0.66 0.62 0.57Dy 5.22 4.96 5.45 3.67 4.30 4.19 4.33 4.37 5.71 5.53 3.67 3.38 3.14 4.06 4.25 4.09 3.51Ho 1.13 1.10 1.25 0.77 0.92 0.93 0.95 1.03 1.24 1.16 0.80 0.73 0.68 0.94 0.95 0.89 0.79Er 3.38 3.10 3.58 2.35 2.83 2.86 2.81 2.55 3.30 6.02 2.46 2.21 2.03 2.88 2.78 2.70 2.33Tm 0.55 0.50 0.56 0.36 0.44 0.44 0.43 0.42 0.52 1.42 0.37 0.34 0.32 0.45 0.43 0.42 0.36Yb 3.20 3.04 3.45 2.34 2.71 2.97 2.69 2.87 3.78 3.71 2.23 2.03 1.94 2.77 2.51 2.62 2.27Lu 0.50 0.49 0.53 0.36 0.46 0.46 0.42 0.45 0.56 0.57 0.37 0.32 0.30 0.43 0.39 0.38 0.40Hf 2.44 2.50 2.66 3.49 2.48 2.49 2.19 5.57 2.66 2.61 1.74 1.45 1.44 1.50 1.52 1.52 1.59Ta 0.10 0.10 0.10 0.10 0.14 0.13 0.12 0.11 0.14 0.15 0.06 0.05 0.06 0.06 0.05 0.05 0.05Pb 5.30 5.20 5.01 6.76 8.36 8.48 8.04 8.62 10.73 8.89 4.92 4.63 3.71 3.74 4.65 4.63 4.02Th 3.17 3.38 3.59 2.03 2.75 2.86 1.73 1.59 2.24 2.38 1.69 1.62 1.74 1.63 1.70 1.69 1.76U 0.82 0.87 0.97 0.56 0.81 0.83 0.42 0.51 0.68 0.74 0.46 0.46 0.47 0.47 0.45 0.48 0.49V 125.91 128.37 95.61 237.44 215.58 223.02 243.68 264.24 134.98 159.60 447.61 453.01 311.78 435.45 482.91 449.89 464.75Cr 57.38 1.47 0.79 55.10 26.80 23.59 0.54 2.47 1.86 13.76 71.12 5.01 50.54 136.97 11.69 4.18 4.28Ni 41.76 32.82 7.31 32.54 68.77 8.62 6.86 7.2887Sr/86Sr 0.703415 0.703412 0.703409 0.703433 0.703393 0.703466 0.703332 0.703369 0.703369 0.703369 0.703420 0.703436 0.703314 0.703410 0.703429 0.703416 0.703430143Nd/

144Nd0.512967 0.512972 0.512975 0.512993 0.512958 0.512982 0.513035 0.512990 0.513025 0.512988 0.512952 0.512962 0.512957 0.512957 0.512976 0.512955 0.512976

εNd 6.42 6.52 6.57 6.92 6.25 6.70 7.75 6.87 7.55 6.82 6.13 6.33 6.23 6.22 6.58 6.18 6.60206Pb/

204Pb18.397 18.414 18.400 18.427 18.427 18.425 18.409 18.403 18.385 18.409 18.419 18.423 18.405 18.417 18.416 18.416 18.400

207Pb/204Pb

15.509 15.530 15.516 15.532 15.531 15.529 15.521 15.517 15.494 15.522 15.537 15.541 15.523 15.534 15.533 15.532 15.521

208Pb/204Pb

38.272 38.339 38.304 38.329 38.330 38.319 38.279 38.286 38.214 38.291 38.362 38.371 38.315 38.357 38.349 38.345 38.336

Table 2 (continued)

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Fig. 2. Plots of SiO2 vs. (a) K2O (wt.%) and (b) MgO (wt.%), for the Kutcharo volcanicsamples showing the distinction from low-K to high-K calc-alkaline field (after Gill,1981; Arculus, 2003). Compositional fields of north and NE Hokkaido (HKD) back-arcmagmas (Ikeda, 1998; Ikeda et al., 2000), the NE Japanese arc (Kimura and Yoshida,2006), and Kamchatka arc (Kepezhinskas et al., 1997) are shown for comparison.

260 N. Hoang et al. / Journal of Volcanology and Geothermal Research 200 (2011) 255–266

of GSJ standard JB-1a. Trace elements were analyzed by inductivelycoupled plasma mass spectrometry (ICP-MS) on a MicromassPlatform-ICP. Details of this procedure were reported in Ishizukaet al. (2003). ICP-MS reproducibility is better than ±4% (2σ) for rareearth elements, Rb, and Nb, and better than ±6% for other elements.Reported data for reference materials and our results are shown forcomparison in Table 1.

Analysis of Sr, Nd and Pb isotopic compositions was also conductedat the GSJ using techniques described in Hoang and Uto (2003). Nd, Srand Pb isotopic ratios were measured in a multi-collector VG Sector 54thermal ionization mass spectrometer (TIMS) at the GSJ with runningparameters as reported by Hoang and Uto (2003, 2006). Thewithin-runprecision (2σ) for 87Sr/86Sr was ±0.000007 and ±0.000006 for 143Nd/144Nd. The internal precision of Pb isotopic ratios (2σ) is less than 0.01%and for total blanks less than 70 pg. Measured Pb isotopic ratios werecorrected for instrumentalmass fractionation of 0.1amu−1 by referenceto repeated measurements of NBS981 lead isotope standard. The dataare shown in Table 2.

4. Analytical results

4.1. Major and trace element compositions

The relatively high K2O contents of basalt to rhyolite lavas, and alsopumice deposits indicate a medium-potassic calc-alkaline affinity(e.g. Gill, 1981; Arculus, 2003). Compared with NE Japanese arc lavas,most of those from Kutcharo show higher K2O (0.5 to 2 wt.%) and SiO2

(up to 75 wt.%) (Fig. 2a), low MgO (from 5 to 0.1 wt.%, for basalt andfelsic pumice, respectively) and resemble the Miocene back-arcvolcanics in terms of SiO2 for equivalent K2O and MgO (Fig. 2b) (e.g.Ikeda, 1998; Ikeda et al., 2000; Shuto et al., 2004). Chondrite-normalized rare earth element (REE) patterns (e.g. Anders andGrevesse, 1989) show a smooth transition from the basalts via andesiteto rhyolitic pumice (Fig. 3a). Compared with calc-alkaline andesitesfrom the NE Japan arc, the Kutcharo lava show significantly lowerlight (L)REE abundances and higher contents of heavy (H)REE. TheKutcharo HREE contents are significantly lower than those of ‘normal’mid-ocean ridge basalt (N-MORB) (Fig. 3a). N-MORB-normalizedincompatible element distributions (e.g. Sun and McDonough, 1989)for the Kutcharo samples show strong relative enrichments for (e.g.)highly incompatible LILEs such as Rb, Ba, Th, U and Pb, pronouncednegative Nb and Ta anomalies, andmoderate negative anomalies for Zr,Ti (in rhyolite) and HREE (Fig. 3b). This configuration is common inintra-oceanic arc magmas and generally considered to reflect hydrousslab-derived fluid contamination of mantle wedge magmatic sources(e.g. Gill, 1981; Ben Othman et al., 1989; Brenan et al., 1994). Theelement distribution patterns are progressively more abundant fromthe basalts to andesite and rhyolite. This progression probably indicatesfractional crystallization relationship; whereas strong enrichment of Srin andesites and depletion of Ti in rhyolites, respectively, may suggestplagioclase accumulation and the effects of fractionation of pyroxeneand hornblende (Fig. 3b) (see above; e.g. Ionov and Hofmann, 1995;Miyagi et al., submitted for publication).

4.2. Sr―Nd― and Pb isotopic compositions

Sr isotopic compositions are near-uniformly depleted, in allsamples, including pumice, rhyolite, and tholeiitic basalt, varyingbetween 0.7033 and 0.7034 and are matched by relatively depleted143Nd/144Nd ratios, which range between 0.51295 and 0.5130(Table 2). The values for both isotopes overlap closely with those forthe Kamchatka arc and the most depleted Neogene northern Hokkaidoback-arc basalt (e.g. Kersting and Arculus, 1995; Kepezhinskas et al.,1997; Takagi et al., 1999; Ikeda et al., 2000; Shuto et al., 2004),which aremostly distinct from more enriched NE Japanese arc magmas (e.g.Kimura and Yoshida, 2006) (Fig. 4). The Pb isotopic compositions,

however, are relatively unradiogenic, 206Pb/204Pb ranging from 18.4 to18.5 and 208Pb/204Pb from 38.3 to 38.5 (Table 2). Pb isotopecompositions partly overlap those for Kamchatka in terms of 206Pb/204Pb vs. 207Pb/204Pb (Fig. 5a), but are more radiogenic with respect to206Pb/204Pb, plotted against 208Pb/204Pb (Fig. 5b). In general, theKutcharo isotopic values are comparable with the most depleted lavasso far reported for NE Hokkaido and the most enriched Kurile arcincluding Southern Kamchatka, but significantly more depleted thanthose of NE Japanese Quaternary arc (Kersting et al., 1996).

5. Discussion

In general, volcanic arc isotopic compositions reflect MORB-typemantle element distributions, influenced by subducted slab compo-nents, including hydrous fluid released from altered oceanic basalt andsediment, and/or metasomatic sediment melts (e.g. Gill, 1981; Arculusand Powell, 1986; Crawford et al., 1987; Tatsumi, 1989; McCulloch andGamble, 1991; Woodhead and Johnson, 1993; Staudigel et al., 1996;Hawkesworth et al., 1997; Ayers, 1998; Plank and Langmuir, 1998;Shimoda et al., 1998; Hochstaedter et al., 2001; Ishizuka et al., 2003). Anadditional contributory factor thatmaypotentiallymodify arcmagmaticsource isotopic compositions is old, previously enriched mantle

Fig. 3. (a) Chondrite-normalized rare earth element (REE) distributions (e.g. Andersand Grevesse, 1989) for the Kutcharo lavas: rhyolite to dacite series are outlined bylight-gray, andesite by gray, and basalts by dark lines. (b) ‘Normal’ mid-ocean ridgebasalt- (N-MORB-) normalized element distributions (after Sun and McDonough,1989) show strong negative anomalies for high field strength elements such as Nb, Taand Ti, and strong positive anomaly for Pb. Overall REE abundances are low comparedwith N-MORB (Regelous et al., 1999).

Fig. 4. Variation of Sr and Nd isotopic ratios for lava samples from the Kutcharo volcaniccalderas. The data plot in the ‘depleted’ field partly overlapping that of the Kamchatkaarc magmas (data from Kersting and Arculus, 1995; Kepezhinskas et al., 1997) althoughcontrasting somewhat from the more ‘enriched’ NE Japanese arc magma (Kimura andYoshida, 2006). The field shown for Japan Sea basalts is from Tatsumoto and Nakamura(1991). The data for Pacific-MORB are from Regelous et al. (1999). The data fieldrepresenting altered oceanic crust (AOC) is from Staudigel et al. (1996).

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lithosphere (Staudigel et al., 1981; Hawkesworth et al., 1994) and/oroverlying crust (Miller et al., 1994; Kersting et al., 1996; Kepezhinskas etal., 1997). All such processes are contingent on differences in isotopiccompositions and incompatible trace element abundances in basaltsand sediment (e.g. Staudigel et al., 1996; Plank and Langmuir, 1998). Inparticular, elements that have similar partitioning coefficients andbehave similarly between peridotite and its partial melt products canhave very different solubility in hydrous fluid (for example, Th and Nb,e.g. Hofmann, 1988; Brenan et al., 1995, Class et al., 2000, and others).

5.1. Mantle wedge beneath NE Hokkaido (Kurile Arc)

Given the relative immobility of HFSE in hydrous fluids (McCullochand Gamble, 1991; Brenan et al., 1995; Kogiso et al., 1997) and theirlow concentrations in subduction-related melt products, relative toLILE in sediments (e.g. Cousens et al., 1994; Plank and Langmuir,1998), these element are not slab-derived contaminants of mantlewedge regions. Depleted HFSE character, as reflected by low Nb/Ta inarc magmas, may reflect previous melting of the mantle source,conceivably in contiguous back-arc basins (McCulloch and Gamble,1991; Woodhead et al., 1993; Hochstaedter et al., 2001). This concept

relies on implicit downward traction of convecting mantle bysubducting oceanic lithosphere, allowing for mantle residues fromback arc melting to become incorporated into the convection in thesub-arc mantle wedge region, where, following contamination byslab-derived components, it would be a potential source for primitivearc magmas (McCulloch and Gamble, 1991; Woodhead et al., 1993;Elliot et al., 1997).

Observing the similarity of Nb/Ta ratios in Umnak Island in theAleutian Arc to those of N-MORB (ca. 15.5) and the absence of back-arc spreading behind the Aleutians (in contrast, for example, to theMarianas), Class et al. (2000; after McCulloch and Gamble, 1991)concluded that the bulk of N-MORB-type mantle wedge sources hadnot been subject to significantly previous melt extraction (Class et al.,2000).

As already noted, Miocene to Quaternary intraplate basaltic torhyolitic magmatic products are widespread in the back-arc side of thepresent day NE Hokkaido sector of the Kurile Arc (Fig. 1) (Takagi et al.,1999; Ikeda et al., 2000; Shuto et al., 2004). These lavas, including theparent magma(s) of rhyolites, are mantle-derived and show slightlyenriched Sr and Nd isotopic compositions and incompatible elementabundances relative to N-MORB (e.g. White et al., 1987; Regelous et al.,1999) and the Kutcharo lavas (Figs. 3ab, 4). In commonwith Class et al.(2000), we observe that Nb/Ta ratios of most Kutcharo basalts overlapwith those of NE Hokkaido intraplate lavas, i.e. within the range of N-MORB, whereas most of the intermediate and silicic compositions aremore scattered, trending towards a depleted, significantly lower, Nb/Tafield (Fig. 6). Ratios of Nb/Ta are considered to be unaffected bysubduction components (e.g. Brenan et al., 1995) thereby representingthe magmatic source composition in the mantle wedge. Note that mostof the Kutcharo basalt compositions overlap those of NEHokkaido back-arc volcanics and have an average Nb/Ta ratio of 15.5, closely similar tothat of Pacific MORB. The lower Nb/Ta (more depleted) character ofsilicic and intermediate magmas compared to the inferred source maybe attributed to fractional crystallization of Ti-rich minerals with highNb/Ta ratios, such as amphibole and mica (e.g. Ionov and Hofmann,1995). Regardless of Miocene back-arc melting, the basalt data clearlysuggest that themantlewedge beneath the NEHokkaido arc is ofMORBaffinity, showing no significant depletion from prior melt extraction. Asarguedabove, the trace element and isotopic character of Kutcharo lavas(Figs. 3ab, 4, 5) suggests their parental melts evolved within a single

Fig. 5. Plots of (a) 206Pb/204Pb vs. 207Pb/204Pb and (b) 208Pb/204Pb for the Kutcharo arcvolcanics, data fields for several volcanic regions shown for comparison (data sources asin the Fig. 4), along with fields for Pacific MORB (White et al., 1987) and Indian MORB(Hamelin et al., 1985/1986). Binary mixing between a Pacific MORB-like source andnorthern Pacific oceanic sediments (Cousens et al., 1994) is consistent with thevariation of Kutcharo 207Pb/204Pb ratios but not for those of 208Pb/204Pb. Shown forcomparison is the data field for Kitami back-arc basalts from NE Hokkaido (Hoang et al.,in preparation; see Takagi et al. 1999 for geological details).

Fig. 6. Variation of 143Nd/144Nd versus Nb/Ta for Kutcharo volcanics. Pacific MORBrepresentatives from Regelous et al. (1999) are shown along with compositions ofnorthern Pacific Ocean sediments (Cousens et al., 1994). Shown for comparison are datafields for Kitami back-arc lavas (Takagi et al., 1999; Hoang et al., in preparation:continuous line) and NE Japan arc (Kimura and Yoshida, 2006). See text for explanation.Symbols are as in Fig. 5.

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magma supply system, without significant lithospheric input, asevidenced, for example, by Nb depletions relative to Ta (e.g. Ionov andHofmann, 1995; Ionov et al., 1995; cf. Ryerson and Watson, 1987).

5.2. Contribution from overlying lithosphere

Arc magmas are variously affected by interaction with theoverlying lithosphere, as evidenced by geochemical and isotopiccharacteristics of arc eruptions involving mantle wedge-equilibratedpartial melts (Gill, 1981; Sakuyama and Nesbitt, 1986). Lead isotopestudies of Quaternary volcanics from two distinct crustal terrainswithin the NE Japanese arc system led Kersting et al. (1996) toconclude that the isotopic heterogeneity observed in arc magmasmaybest be explained by mixing between a fluid-fluxed mantle wedge andcontinental crustal components characterizedbyhigher 87Sr/86Sr, 208Pb/204Pb, 207Pb/204Pb and lower 143Nd/144Nd. These authors concluded thatmixing of mantle-derived magmas with lower crustal components isprobably a critical process in arc volcanism. The variable extent offractionation observed in the Kutcharo lavas (see above; Figs. 2ab, 3b)

does not correlatewith their Sr andNd isotopic compositions (Fig. 7), anobservation that reinforces our conclusion that overlying crustalcontamination was minimal.

It is worth noting, however, that many Nd and Sr isotopic studiesof 15 Ma back-arc volcanics in northern and northeastern Hokkaidoshowed evidence of variation from ‘enriched’ to isotopically ‘depleted’MORB-like character. Such changes have been attributed to progres-sive changes in the magma source region beneath northeastern Japanwhere back-arc thinning of continental mantle lithosphere may havebeen replaced by upwelling, depleted asthenosphere following Kurileback-arc basin opening (Ikeda, 1998, Ikeda et al., 2000; Yamashita etal., 1999; Shuto et al., 2004; after Kimura and Tamaki, 1986; Komatsuet al., 1989; Maeda, 1990; Goto et al., 1995). Had the Kutcharo islandarc magmas passed through depleted lithospheric mantle and young,thin crust, as inferred for NE Hokkaido (Komatsu et al., 1989; Togashiet al., 2000) lithospheric imprints might be hard to identify.

5.3. Contributions to arc magma source from subducted sediment

Considering potential contaminants of crustal input to the mantlewedgemight include bulk sediment (White and Patchett, 1984;Miller etal., 1994), sediment melts (Kay, 1978; Elliot et al., 1997; Shimoda et al.,1998), and interstitial hydrousfluid expelled fromsediments (McCullochand Perfit, 1981; Miller, 1995). Typically, oceanic sediments have higher207Pb/204Pb and 208Pb/204Pb for a given 206Pb/204Pb and higher 87Sr/86Srwhen compared to MORB. They also have higher Ba, Pb, Th and Srcontents, such that these elements would be sensitive tracers ofsediment-related contamination (Ben Othman et al., 1989; Woodheadet al., 1998; see Plank and Langmuir, 1998, for references).

High Pb contents in the Kutcharo volcanics, match their Pb isotopicratios and are compositionally between Pacific MORB (White et al.,1987; Regelous et al., 1999) and northern Pacific sediments (Cousenset al., 1994) (Figs. 3b, 5a, b), suggesting possible binary mixing.Mixing of a MORB-like depleted source with partial melts of sedimentwould clearly form a positive correlation between (for example) 87Sr/86Sr and SiO2, and trace element ratios such as Ba/Th and Ba/Nb.However, the absence of any such correlations (Fig. 7) would indicatethat sediment melt contributions were negligible.

Even if sedimentary components are not directly involved in themagmatic source below NE Hokkaido, fluids released from thebreakdown of hydrous minerals in subducted sediments may

Fig. 8. Plots of Nb/Y versus Ba (ppm) showing a strong correlation between the two. Thetrends indicated for subducted slab-derived fluid and the effects of differential partialmelting are from (e.g.) Brenan et al. (1995), Kepezhinskas et al. (1997), Hochstaedter etal. (2001). Data fields shown are as in Figs. 4 and 5, and for Hawaiian ‘oceanic islandbasalts’ (OIB), from Frey et al. (2000), used to illustrate the effects of partial melting.Shown for comparison is the data field for Kitami back-arc basalts from NE Hokkaido(Hoang et al., in preparation).

Fig. 7. Variation of (a) 87Sr/86Sr versus SiO2 (wt.%) and (b) 143Nd/144Nd versus MgO (wt.%) shows little or no systematic correlation, consistent with the combined effectsexpected following fractional crystallization and possible magma mixing of magmastapping a common source.

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nonetheless play a role in the transfer of elements from a subductingslab to arc magmatic source regions. The trace element composition ofslab sediment-derived fluids depends on element partitioningbetween the fluid and residual solid, whether the latter were basalticor (at higher pressure) hydrated eclogite which may also liberatesubstantial fluid (Tatsumi, 1989; Saunders et al., 1991; Peacock et al.,1994; Kogiso et al., 1997). To investigate the chemical composition offluids liberated from the dehydrating subducted slab, Brenan et al.(1994, 1995) calculated bulk eclogite- and lherzolite-equilibratedaqueous fluid partitioning between experimentally produced mineral-aqueous fluid pairs at 2 GPa and 900 °C. Under these conditions Ba, Pband, to a lesser extent, Sr partition into the fluid phase relative to theirrespective solid residua. Thus, a fluid derived from the dehydration ofsubducted sediment would be expected to be enriched in fluid-mobileelements such as Pb, Ba, and Sr, relative tofluid-immobileHFSE (Brenanet al., 1994, 1995). Accordingly, a mantle source fluxed by such fluidwould give rise to melts with higher abundances of Pb, Ba, and Sr andsignificantly lower contents ofHFSE compared to those ofMORB (Fig. 3).The high Ba relative to the consistently low Nb/Y ratios in the Kutcharolavas (Fig. 8) therefore suggests hydrous fluid had a considerably

stronger influence over (metasomatic) melt in contaminating thesource of the Kutcharo arc magmas. Although incompatible elementcompositions in hydrous fluids derived from subducted sediment andAOC (altered oceanic crust) are difficult to distinguish, the distinction ofSr, Pb and Nd isotopic compositions of the two sources is unambiguous,isotopic tracers being the most reliable indicators of arc magma sourcecontamination (e.g. Class et al., 2000; Ishizuka et al., 2003).

5.4. Contributions from the subducted altered oceanic (basaltic) crust

Radiogenic Sr and Pb isotopes and those elements stronglypartitioned into hydrous fluid cf. their solid residua (Brenan et al.,1994, 1995; Class et al., 2000; see Hochstaedter et al., 2001; Ishizukaet al., 2003) would therefore be expected to correlate with each other,as the primary fluid-bearing contaminants of the convecting mantlewedge, in contrast to the fluid-immobile HFSE. To illustrate this, weconsider 87Sr/86Sr isotopic ratios for altered oceanic basalt (AOB)samples collected at DSDP Sites 417 and 418 (ranging from 0.703 to0.707, averaging about 0.70445; Staudigel et al., 1996), in contrast to87Sr/86Sr ratios in subducting slab hydrous mineral (for example,amphibole)-derived fluids estimated at about 0.70410 (Tatsumi andKogiso, 1997). Sr isotopic compositions of AOB and their derivativefluids are more radiogenic than unaltered MORB (e.g. White et al.,1987; Regelous et al., 1999). Whereas, both crust and fluids aresignificantly less radiogenic than subducting sediments, ‘GLOSS’, theglobal average, showing 87Sr/86Sr ratios of ca. 0.717 (Plank andLangmuir, 1998); they are also considerably less radiogenic thannorthern Pacific sediments (NPS), showing 87Sr/86Sr values of 0.7125(Cousens et al., 1994). Magma generated in a MORB-like mantlewedge contaminated by subducted sediment-derived fluid wouldshow a strong positive correlation between radiogenic Sr and traceelement ratios such as Ba/Th and Ba/Nb. It is also clear that such apositive correlation would be much weaker for the case of a fluidderived from altered basaltic (oceanic) crust. Hypothetical mixingcurves, calculated for 87Sr/86Sr and Ba/Th, between aMORB-type sourceand sediment-derived fluids from both the GLOSS average (87Sr/86Sr=0.715) and average NPS (87Sr/86Sr=0.713) are inconsistentwith the Kutcharo data, however (Fig. 9a). Amore likely explanation forthe Kutcharo variation is the interaction with a MORB-like mantlewedge of hydrous fluids released from a source with significantly lessradiogenic Sr, for example, AOC, characterized by 87Sr/86Srb0.707. This

Fig. 10. Plots of 206Pb/204Pb isotopic ratios and Nd/Pb for the Kutcharo volcanic. Arrowsindicate variation trend of Nd/Pb in hydrous fluid (e.g., Brenan et al., 1995).Hypothetical mixing lines illustrate two possible sources tapped by primitive Kutcharoarc magmas: (1) melting of a MORB-type mantle wedge (206Pb/204Pb=18.25 — 18.5,Nd/Pb=26, data from Regelous et al., 1999; White et al., 1987) mixed by North Pacificocean sediment-derived hydrous fluids (average 206Pb/204Pb=18.8, Nd/Pb=1.25,Cousens et al., 1994) (dashed lines), and (2) melting of a MORB-type mantle wedgesource infiltrated by hydrous fluids released from altered oceanic (basalt) crust (206Pb/204Pb=18.5, Nd/Pb=27.5, Staudigel et al., 1996; Ishizuka et al., 2003) (continuouslines). See text for details.

Fig. 9. Plots of (a) Ba/Nb and (b) Ba/Th versus 87Sr/86Sr isotopic ratios for the Kutcharovolcanics, showing the likely effect of fluid addition as inferred by Staudigel et al.(1996), Kepezhinskas et al. (1997), Hochstaedter et al. (2001) and Ishizuka et al.(2003). Data fields for the NE Japanese arc (Kimura and Yoshida, 2006), Kamchatka arc(Kepezhinskas et al., 1997) and Hokkaido back-arc magmas (Okamura et al., 1998,Ikeda et al., 2000) are shown for comparison. Hypothetical mixing curves, calculated for87Sr/86Sr and Ba/Th, between aMORB-type source and sediment-derived fluids (dashedlines) from the global average of subducting sediment (GLOSS, 87Sr/86Sr=0.715, Plankand Langmuir, 1998), average NPO sediment (87Sr/86Sr=0.713, Cousens et al., 1994),and altered oceanic crust (AOC), characterized by 87Sr/86Sr ratiosb0.707 (continuouslines). End-member compositions used in the mixing calculation are as follows: 87Sr/86Sr, Ba, Th, respectively, for MORB: (0.7028, 30, 0.65, and compositional effects ofadding potential subduction-related contaminants: (1) 0.707, 1500, 0.5, (2) 0.707,1000, 1, (3) 0.707, 500, 1, (4) 0.715, 1500, 0.5, (5) 0.715, 1000, 1, and (6) 0.715, 500, 0.5.

264 N. Hoang et al. / Journal of Volcanology and Geothermal Research 200 (2011) 255–266

process is the most consistent, with respect to the mixing curves(Fig. 9a). In general, the tight correlation between 87Sr/86Sr (whicheffectively remains constant) and Ba/Th and Ba/Nb ratios (Fig. 9ab),suggests that any contribution froma subducted sediment-derivedfluidwas minimal.

Additional evidence may include the Nd/Pb and corresponding Pbisotopic ratios (Fig. 10). The Nd/Pb ratio remains virtually constantduring fractional crystallization, given that neither element is signifi-cantly partitioned into the crystallizing silicate phases. However, due tovery slight differences in their (extremely low) partition coefficientsduringpartialmelting, a small degree ofNd/Pb ratio fractionationwouldbe expected at relatively low melt fractions (e.g. Hofmann, 1988), atwhich Nd/Pb ratios would be slightly lower than otherwise. Further-more, Brenan et al. (1995) showed that, in contrast to Nd, Pb is readilymobilized byhydrousfluid. Note the very small variation of Nd/Pb in theKutcharo volcanics (between 1 and 3) and their coherence with 206Pb/204Pb isotopic ratios, which vary within an extremely narrow range(averaging ca. 18.4; Fig. 10). Note also that many of the basalts havehigher Nd/Pb than the more silicic magmas (and presumably theirparent melts). Thus the near-constant lead isotopic ratios and narrowrange of Nd/Pb may be explained by (1) partial melting and/or(2) mixing between high- and low-Nd/Pb sources. If, as is most likely,the magmas share a common source, partial melting is the less likelyexplanation, especially if more SiO2-rich partial melts (parental to therhyolites) were to be interpreted as products of smaller degrees ofpartial melting. If we accept the likelihood of binary source mixing, thePb isotopic compositions do not vary significantly despite the small butsignificant Nd/Pb range; the two end-member sources must somehowhave closely similar Pb isotopic ratios. On this assumption, mantlecontamination by sediments of the type reported for the northernPacific (Cousens et al., 1994), whose 206Pb/204Pb ratios average 18.8,would be inconsistent with any model appealing to their subductionbeneathNEHokkaido (Figs. 9ab, 10). Alternatively,melting anN-MORB-type mantle wedge source infiltrated by a hydrous fluid phase releasedfrom altered oceanic (basalt) crust (AOC), which has near-MORB Pbisotopic compositions and a small range of Nd/Pb, could account for theKutcharo source character (Fig. 10).

6. Conclusions

To summarize, the evidence so far points to the absence of bothsubducted sediment-derived fluids as well as sediment melts

265N. Hoang et al. / Journal of Volcanology and Geothermal Research 200 (2011) 255–266

infiltrating the Kutcharo magmatic source region. We argue that theabsence of a positive correlation between Sr isotopic compositionswith hydrous fluid trace element ‘signatures’ such as the coupling ofBa/Nb with unradiogenic Sr and Pb appears to preclude theinvolvement of sediments in the genesis of the Kutcharo lavas.While sediments may or may not be subducted beneath NE Hokkaido(e.g., Matsuda and Isozaki, 1991), the fluid flux appears to bedominated by the dehydration of altered oceanic crust (after Classet al., 2000). In this respect, the NE Hokkaido Arc would appear closelysimilar to the Kamchatka Arc (Kersting and Arculus, 1995), represen-tative of an arc system where little or no sediment is involved incontaminating the mantle source of arc magmas.

Acknowledgements

We thank Martin Flower for polishing the English and helpfuldiscussions. Aki Matsumoto, Osamu Ishizuka and Kiyo Yamanobe arethanked for assisting in clean laboratory and ICP-MS analysis.Comments and helpful suggestions on an earlier version of thismanuscript by Nelson Eby and two anonymous reviewers helpedimprove themanuscript significantly. This paper has benefited greatlyfrom reviews by Pavel Kepezhinskas and an anonymous reviewer.Editorial corrections and helpful comments by M.J. Rutherford aregratefully acknowledged. This study is a regulatory support researchfunded by the Nuclear and Industrial Safety Agency, Ministry ofEconomy, Trade and Industry, Japan.

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