Across-arc variation of lava chemistry in the Izu-Bonin Arc: identification of subduction components

Journal of Volcanology and Geothermal Research, 49 (1992) 179-190

179

Elsevier Science Pubhshers B.V., Amsterdam

Across-arc variation of lava chemistry in the Izu-Bonin Arc: identification of subduction components Y. Tatsumi a, M. Murasaki a and S. Nohda b aDepartment of Geologyand Mmeralogy, Kyoto Untverstty, Kyoto 606, Japan bKyoto Sangyo Umverslty, Kamtgarno, Kyoto 603, Japan (Received March 4, 1991, revised and accepted August 12, 1991 )

ABSTRACT TatsumI, Y , Murasakl, M. and Nohda, S., 1992. Across-arc variation of lava chemistry in the lzu-Bonm Arc ~dentlfication ofsubductlon components J Volcanol Geotherm Res, 49: 179-190. In order to understand the role of the subducted hthosphere m producing the geochemical characteristics of arc magmas, major- and trace-element along with Sr- and Nd-lsotope compositions have been determined for Quaternary volcanic rocks from the Izu-Bonln lntra-oceanic arc. 87Sr/86Sr and ~43Nd/~44Nd ratxos decrease away from the volcanic front of this arc and lie on mixing hnes between the assumed isotopic compositions of fluid phases mainly derived from the basalt layer of the subducted hthosphere and upper-mantle materials m the sub-arc wedge. This across-arc variation can be explained through a simple sequence of processes involving initial release of fluid phases from the subducted oceanic crust to produce hydrous pendotlte at the base of the mantle wedge. Thxs hydrous perldotxte is dragged downward with the slab and releases a second-stage metasomatlzmg fluid beneath the volcamc arc. The higher concentrations of both Sr and Nd in the fluid beneath the volcamc front than those beneath the back-arc side may be a possible cause of the observed across-arc variation m Sr-Nd isotopic ratios. The difference in composxtlons of fluid phases is attributed to the different hydrous phases which decompose in the hydrous peridotlte layer; amphibole beneath the volcanic front and phlogopite beneath the back-arc sxde of the volcamc arc The mlneralog~cally controlled fluid addition may also be responsible for the across-arc variation in R b / K and R b / Z r ratios, increasing away from the volcanic front.

Introduction

The identification of subduction components and knowledge of the mechanism by which such components are introduced into the arc magma source region are essential in understanding the origin of geochemical characteristics of arc magmas. It has been demonstrated that subduction zone magmas are more enriched in large-ion lithophile elements than magmas in other tectonic settings (e.g., Perfit et al., 1980; Gill, 1981; Pearce, 1982). In addition, combined Sr- and Nd-isotopic studies indicate that many subduction zone magmas Correspondence to: Y. Tatsumx, Department of Geology and Mineralogy, Kyoto Umverslty, Kyoto 606, Japan.

0377-0273/92/$05.00

are displaced to higher 8 7 S r / 8 6 S r ratios relative to the main "mantle array" which encompasses MORB and intraplate basalts (e.g., DePaolo and Wasserburg, 1977; Hawkesworth, 1982). These geochemical characteristics have been attributed at least in part to contributions from the subducted oceanic lithosphere. However, the mechanism by which the subduction components are imprinted on the mantle wedge remains controversial (e.g., Pearce, 1982; Hawkesworth, 1982). This paper presents Sr- and Nd-isotopic compositions of Quaternary volcanic rocks from two across-arc sections of the Izu-Bonin intra-oceanic arc. We discuss the isotopic characteristics of subduction components, and propose a possible mechanism for transporta-

© 1992 Elsevier Scxence Pubhshers B.V. All rights reserved

180

Y T A P S t + M I E ] AL

tion of slab-derived components into the arc magma source regions.

angle of subduction (Katsumata and Sykes, 1969; Izutani et al., 1975); (2) the volume of volcanics and the number of volcanoes is greatest on the volcanic front and decreases towards the back-arc region (Suga and Fujioka+ 1990); (3) incompatible element contents in volcanic rocks increase (e.g., Onuma et al.. 1983 ), and 87Sr/86Sr ratios decrease ( Notsu et al., 1983) away from the volcanic front. As these are c o m m o n to many volcanic arcs on the Earth (e.g., Sugimura et al., 1963; Marsh, 1979: Gill, 1981; Tatsumi, 1989), the information obtained from this arc is likely to provide a general understanding of magma genesis at convergent margins. In order to better understand the geochemical characteristics of magma source regions beneath the arc, in particular to assess the acrossarc variation in chemistry of the mantle wedge, we have analyzed Quaternary rocks from both the volcanic front and the back-arc-side volcanic chains in two across-arc sections of the Izu-Bonin arc (Fig. 1 ).

Tectonic background and samples

The Izu-Bonin arc is built on the Philippine Sea plate and overlies the subducting Pacific plate (Fig. 1 ). The angle of subduction steepens towards the south (50 to 90°; e.g., Katsumata and Sykes, 1969 ), in conjunction with narrowing of the volcanic arc in that direction. The Shikoku Basin behind the volcanic arc was formed by back-arc spreading between 30 and 17 Ma (e.g., Kobayashi and Nakada, 1981 ). Furthermore, in the central part of this arc (Fig. 1 ), very young back-arc basins are now forming just behind the volcanic front (Karig, 1971; Tamaki, 1985). Volcanism in the Izu-Bonin arc includes the following characteristics: ( 1 ) the depth to the top of the dipping seismic zone is 100-120 km beneath the volcanic front and the width of the volcanic arc is inversely proportional to the

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Fig. 1. S a m p h n g localmes in the Izu-Bonm arc. Filled circles and triangles are trench-rode samples and open symbols are back-arc-side samples. Volcanoes on the volcamc front are also shown by d m m o n d s . Very young back-arc depressions are forming b e h i n d the volcanic front o f the southern part o f this arc. D e p t h to the top o f the dipping selsm]c zone ts shown m km for the Izu Peninsula region

ACROSS-ARC VARIATION OF LAVA CHEMISTRY IN THE IZU-BONIN ARC

Results Major- and trace-element concentrations were determined at Rigaku Corporation by XRF techniques on fused glass beads and pressed powder pellets, using Rigaku ® Symaltics 3530 and 3270 spectrometers, respectively• Details of counting times, operating conditions, detection limits, and calibration lines can be found in Goto and Tatsumi (1991). REE analyses were done following ion-exchange chromatography (Robinson et al., 1986). Sr- and Ndisotopic ratios were analyzed using a Finnigan ® 261E MAT mass spectrometer at Kyoto Sangyo University, following analytical procedures described in Nohda et al. (1988). Analytical results are listed in Tables 1 and 2. Concentrations of incompatible elements in the Izu-Bonin lavas show across-arc varia-

181

tions, with rocks from trench-side volcanoes being more depleted in these elements (Fig. 2 ). Also, there are marked across-arc var:~ations in some incompatible element ratios and REE patterns (Figs. 3 and 4). Sr- and Nd-isotopic compositions of these rocks also show clear across-arc variations (Fig. 5 ), in which a positive correlation between Sr- and Nd-isotopic 14

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Zr (ppm) Fig. 2. Variations m Rb and Zr concentrations in two across-arc sections on the Izu-Bonm arc Lavas in the trench-side volcanoes are more depleted in incompatible elements

Fig. 3. Correlations between Rb, K, Zr and Y. Symbols are same as Fig. 2. R b / K e O (a), R b / Z r (b) and Z r / Y (c) ratios are higher sn the back-arc-s~de lavas.

182

V TA[SUMI E'I AI.

TABLEI MaJor- and trace-element and isotopic compositions for lavas m the Izu-Bonln arc (to be continued) N trench side IZ-I 2 S~O2 T102 A1203 Fe203* MnO MgO CaO Na20 K20 P20~

48 13 l 02 18.72 11 63 016 5 18 11 89 1 97 0 105 0 09

Total

98 89

Nb Z1 Y Sr Rb Ba N1

30 41 17 217 14 121 19

87Sr/S6Sr 0.703416_+20 ~43Nd/J44Nd 0.513125_+23 ~s~ -15 4 ~No 9 50

N. back-arc-side OSO-2 51.72 0 73 16 16 11 88 021 612 11 39 1 56 0 177 0 07 100 02 27 13 160 26 111 15

OSF-I

IZ-10

IZ-19

SIO~ T102 AIzO3 Fe203* MnO MgO CaO Na20 K20 P205 Total Nb Zr Y Sr Rb Ba NI 87Sr/S6Sr ~43Nd/~44Nd ~sr ~Nd

47.04 0.78 18.23 12 56 0 21 6 88 12 54 1.52 0095 007 99.92 20 11 162 11

IZ-25 I

49 30 0 60 20.68 9 88 016 464 12.64 1 74 0.230 0 07

50 04 0 95 16 97 1004 0t7 7 11 11.03 2 77 0 404 0 18

48 57 0 99 17.91 10.26 017 7 78 11 00 2 73 0 108 017

43 67 1 42 14 89 II 14 0 20 10 57 12 90 2 76 0 766 0 50

52 53 0 90 t8 14 8 80 013 5 54 9 38 34 ~, 0 53;5 0lq

99 94

99.06

99 69

98 82

99 66

27 12 192 34 107 13

0.703623_+20 0 513089-+ 19 -12.4 8 80

38 69 18 344 133 64 53

34 70 18 35O 25 60 64

0.703001 _+21 0 513001 -+20 -21 3 7 08

0 703115_+25 0 703039_+ 26 0 513066+ 16 0 512994_+20 -20 7 -19,7 835 6 94

S trench side HCJ- 17

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31.l 112 22 613 27 5 398 132

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49.84 0 90 19 40 11 87 0 19 4 08 11 61 2.20 0 132 008 100.30

70 33 0 55 13 92 4 82 0 21 0 88 3 75 5 13 0 779 0 15 100 52

30 14 180 1.9

I2 100 40 175 84

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Major elements m wt.% and trace elements m ppm

142

D651-3 48 48 071 1921 1112 019 5 46 12 48 1 67 0 173 0 08 99 48 27 t3 184 28 _ L

D669-2 52 67 0 84 19 29 10.33 018 313 1022 3 48 0 395 012 100 66 53 23 198 53 72

18 0.703549_+ 22 0 513118_+21 -135 9 40

0 703438_+22 0513108_+16 -151 917

0 703442_+ 22 0513111_+21 -150 9 23

D630-1 52 97 081 18 25 825 018 497 8 94 312 1 290 0.31 99 10 31 111 24 491 328 95 14 0.702988 _+16 0 513028___ 18 -21 5 761

D036-3 50 54 0 66 1683 900 la 8 24 10o2 2 50 0,08'~ 021 98 03 60 22 235

109 0.703249 -+21 0 513060_+ 23 -176 823

ACROSS-ARCVARIATIONOF LAVACHEMISTRYIN THE IZU-BONINARC

183

TABLE 2

12

REE analyses

10

La Ce Pr Nd Sm Eu Gd Dy Er Yb

IZ-I.2

IZ-23

1 50 4.55 0.72 4.91 2.02 0 75 2.46 2.75 1.49 1 38

24 6 46.3 5.84 25.3 5 21 1.80 5 17 4.21 2.10 1.51

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Fig. 5. An ~Sr-~d diagram for rocks from the Izu-Bonm arc. Data from Nohda and Wasserburg ( 1981 ) and White and Patchett (1984) are also plotted. Symbols are same as Fig. 2. Positive correlation between Sr and Nd isotopic ratios are also documented in the Kurlle (Zhuravlev et al., 1987) and Sangihe (Tatsuml et al., 1991 ) arcs. Broken lines indicate the mantle array. VF=volcan]c front; BA = back-arc-s~de.

being essentially free from the complications derived from continental crustal components.

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ratios occurs with a trend toward decreasing both 87Sr/86Sr and 143Nd/144Nd away from the volcanic front. There are no systematic changes in these geochemical characteristics along the arc, although strong along-arc variations have been observed in the southern part of this arc (e.g., Lin et al., 1989). Discussion---origin of across-arc variations The Izu-Bonin arc is built on the oceanic lithosphere. Isotopic studies of many such tectonic settings have suggested that isotopic compositions of lavas from these regions are most likely to represent source compositions,

Difference in depth of magma segregation In order to explain the across-arc variation in concentrations of incompatible elements, several mechanisms have been advocated (e.g., Jakes and White, 1970; O'Hara, 1973; Miyashiro, 1974; Best, 1975 ). Among them, both experimental and geochemical data support the hypothesis that the difference in depth of magma segregation at a relatively constant temperature and resultant difference in degree of partial melting play a major role in causing the across-arc variation (e.g., Tatsumi et al., 1983; Morrice and Gill, 1986). Here this mechanism is examined for the variation observed in the Izu-Bonin arc. According to Kuno's definition (1959), basalt samples from volcanoes on the trench side are mainly classified into tholeiitic basalts and those from volcanoes on the back-arc-side, into high-alumina basalts (Fig. 6). Based on highpressure melting phase relations for primary basalt magma compositions, Tatsumi et al.

184

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is thus possible to consider the existence of residual garnet in the magma source region for back-arc-side magmas, an interpretation consistent with the idea that back-arc-side magmas are produced at higher pressures ( > 2.5 GPa; Takahashi and Kushiro, 1983) and at relatively small degrees of partial melting.

I 0 40

45

50

55

60

Difference in added subduction components

S i O 2 (wt.%)

Fig. 6 A SlO2-total alkah &agram for the Izu-Bonm basaltic rocks. Symbols are same as Fig. 2 A broken hne indicates the boundary between tholentes (THB) and high-alumina basalts ( H A B ) after Kuno (1959).

(1983) indicated that Kuno's three types of basalt magmas are produced in the mantle wedge by the difference in depth of magma segregation; tholeiites at 30 km, high-alumina basalt at 50 km, and alkali olivine basalts at 70 km. The present basalts along the volcanic front have similar major-element compositions to tholeiites which were used for estimating the primary magma compositions by Tatsumi et al. (1983). Also, the primary highalumina basalt compositions were deduced mainly from back-arc-side basalts on the Izu Peninsula. Thus, it may be suggested that experimental results by Tatsumi et al. ( 1983 ) are applicable to the origin of the present acrossarc variation in basalt magma types. A basalt lava occurring the furthest from the volcanic front in the northern section of this arc (IZ23) is quite undersaturated in SiO2; on the other hand, high-alumina basalts and tholeiites have normative hypersthene. This is consistent with the tendency that magmas are segregated at greater depths away from the volcanic front. Trace-element and REE compositions of lavas from the Izu-Bonin arc also support the above mechanism. Figure 3c indicates that back-arc-side rocks have systematically lower Y / Z r than those in trench-side volcanoes. The depletion of HREE relative to LREE is only observed in a back-arc-side basanite (Fig. 4). It

Differences in degrees of partial melting alone, however, cannot account for the acrossarc variation of isotope and incompatible element ratios. It follows that heterogeneity in the magma source region does exist beneath this arc. Assuming a homogeneous upper mantle before the initiation of subduction in this area, the source heterogeneity should be attributed to the difference in added subduction components. Here we focus our attention on a H20rich fluid phase, not a partial melt, as an agent for transporting the subduction component. This mechanism is strongly supported by both geochemical and petrological constraints ( e.g., Gill, 1981; Tatsumi et al., 1986; Woodhead, 1989), although additional quantitative examination must be required. We may here assume that both Sr and Nd can be transported with HzO fluids under upper-mantle P-T conditions. It has been suggested that REE including Nd may be immobile during the alteration process (e.g., Menzies et al., 1979; Michard et al., 1983 ) as well as highfield strength elements such as Zr and Nb. On the other hand, experimental data (Tatsumi et al., 1986) demonstrated that the degree of transportation of LREE including Nd with an aqueous fluid is same order of magnitude as that of Sr ( ~ 20% of Sr and Nd may be transported through dehydration processes of serpentine at 12 kbar and 850°C) and much higher than both HREE and Nb. The isotopic trends in Figure 5 are distinct from the Sr-Nd isotopic systematics of some arcs (e.g., north Chile, Hawkesworth et al., 1979a; Grenada, Hawkesworth et al., 1979b;

ACROSS-ARC VARIATION O F LAVA CHEMISTRY IN T H E IZU-BONIN ARC

northeast Japan, Nohda and Wasserburg, 1981 ). The present isotopic trends are perpendicular to, rather than subparallel to the "mantle array". This characteristic across-arc variation has been documented in at least two other intra-oceanic arcs; Kurile (Zhuravlev et al., 1987) and Sangihe (Tatsumi et al., 1991 ) arcs (Fig. 5 ). These variations in Sr- and Nd-isotopic ratios can be explained by two simple mixing models (Fig. 7). Possible geochemical reservoirs governing Sr- and Nd-isotopic compositions of intra-oceanic arc basalts are mantle wedge materials and subduction components; the latter include oceanic sediments (e.g., Eld-

T Model A

Z

T

% % %

-BE

%

Model B

" ESr Fig. 7. A schemahc ~sr-eJ~d diagram showing a possible mechamsm for producing across-arc variation m Sr-Nd ~sotopic ratios observed in the Izu-Bonin arc. M = N-type MORB source; A M = altered MORB; S = subducted se&ments, M W = a n enriched original mantle wedge; SDF= slab-derived fluid; HPDF= hydrous-perldotlte-derived fluid; B E = b u l k Earth. As it is not possible to characterlze Sr/Nd ratios in altered MORB- and sediment-derived rinds, mixing hnes between these are shown w~th broken hnes.

18 5

erfield et al., 1981) and altered MORB (e.g., Piepgras et al., 1979) composing the surface and the basaltic layers of the subducted slab, respectively. In Model A (Fig. 7), it is assumed that subducted sediments play a major role in determining isotopic characteristics of the slab-derived fluid. Isotopic compositions of slab-derived fluids can be shown only qualitatively, as its is difficult to estimate Sr/Nd ratios of fluids derived from altered MORa and sediments in the subducted slab. Tatsumi (1989) examined the stability of hydrous minerals in the oceanic lithosphere and emphasized that dehydration reactions in the subducted slab are limited beneath the forearc region to form hydrous peridotites at the base of the forearc mantle wedge. Hydrous peridotires are dragged downwards with the slab, that is a necessary consequence of the subduction of the oceanic lithosphere into the upper mantle. Therefore, then the isotopic composition of dragged hydrous peridotites and secondstage fluid phases released through breakdown of hydrous phases in the dragged layer lies on the mixing line between compositions of the mantle wedge and the slab-derived fluid. In this model, the mantle wedge with isotopic compositions identical to the N-type MORB source is assumed. This magma source has been suggested by several authors for the original mantle wedge composition (e.g., Perfit et al., 1980; Ellam and Hawkesworth, 1988). The acrossarc variation in isotopic compositions may be explained if higher Sr/Nd is attained for the hydrous-peridotite-derived fluid beneath the volcanic front (e.g., DePaolo and Wasserburg, 1979). Tatsumi ( 1989 ) suggested that amphibole and phlogopite are the main hydrous phases decomposing to release fluids beneath the trench-side and back-arc-side volcanic chains, respectively, through the following pressure-dependent reactions: K-bearing pargasitic amphibole + orthopyroxene-> phlogopite + garnet + clinopyroxene + olivine + fluid (H20) at 3.5 GPa (e.g., Millhollen et al., 1974) and phlogopite + clinopyroxene + or-

186

thopyroxene ~K-amphibole + garnet + olivine + fluid ( H 2 0 + K 2 0 ) at 6.0 GPa (Sudo and Tatsumi, 1990). If amphibole and also an amphibole-derived fluid has higher Sr/Nd than phlogopite and a phlogopite-derived fluid, then the mixing line shown in the Model A of Figure 7 could be reasonable. However, this may not be the case because of the following two reasons: ( 1 ) S r / N d ratios in amphiboles overlap or are rather smaller than those in phlogopite (10-100 vs. > 75, respectively; e.g., Basu and Tatsumoto, 1980; Menzies and Murthy, 1980); (2) amphibole contains both Sr and Nd in much greater than phlogopite (300-1000 vs. 150 ppm for Sr and 10-50 vs. < 2 ppm for Nd), suggesting that the isotopic compositions oflavas or their magma sources for trench-side volcanoes should plot much closer to compositions of hydrous-peridotite-derived fluid than in Figure 7. Here let us examine an alternative mechanism (Model B in Fig. 7) to elucidate the across-arc variation. In this model, the subducted altered MORB is, at least in terms of SrNd isotopes, assumed to the main contributor to the slab-derived fluids. Then, isotopic compositions of the mantle wedge must be different from those of the MORB source and are plotted between the MORB source and the Bulk Earth (Fig. 7). This isotopic composition is similar to an enriched ocean-island-basalt (ore) source, which has been proposed as a possible mantle wedge composition (e.g., Stern, 1981; Morris and Hart, 1983 ). Furthermore, the enriched magma source is expected for the Izu-Bonin magmatism from the following two geochemical constraints: (1) Behind this arc is situated the Shikoku Basin, which was formed by the process of backarc rifting. Therefore, it is possible to consider that the original mantle wedge beneath the arc has compositions identical to the upper mantle underlying the Shikoku Basin. Geochemistry of ocean floor basalts in this basin (Wood et al.,, 1980) suggests that the magma source for Shikoku Basin basalts is a slightly enriched source

~ TATSUMIET 4[

rather than a typical N-MORB source (Fig. 8 ). (2) Ikeda and Yuasa (1989) analyzed submarine rocks from southern part of the IzuBonin arc and proposed an enriched E-type MORB source as a possible geochemical reservoir for those magmas. If we adopt the enriched source, then the across-arc variation in Sr- and Nd°isotopic compositions may be reasonably understood through the process of twocomponent mixing between the slab-derived fluid and the mantle wedge because the amphibole-derived fluid beneath the volcanic front contains larger amounts of both Sr and Nd than the phlogopite-derived fluid beneath the backarc-side volcanic chain. It should again be stressed that the present model suggests the minor role of subducted sediments in forming the Sr- and Nd-isotopic characteristics at least for the Izu-Bonin lavas. One of the most compelling lines of evidence for subducted sediment incorporation in arc magma genesis is the occurrence of the cosmogenetic isotope 1°Be in a number of arc lavas (Tera et al., 1985; Morris and Tera, 1989: Morris et al., 1990). However, ~°Be concentrations in samples from the Mariana arc. the southward extension of the Izu-Bonin arc, are negligible (Woodhead, 1989 ), an observation consistent with the minimum involvement of sediments in forming the Sr- and Nd-isotopic characteristics of Izu-Bonin rocks. The present mechanism for producmg the

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ACROSS-ARC VARIATIONOF LAVACHEMISTRY IN THE IZU-BONINARC

isotopic heterogeneity in the magma source region may also be applicable to the trace-element characteristics observed in the Izu-Bonin rocks. Figure 3a indicates higher R b / K 2 0 in lavas from the back-arc-side volcanoes than those from the volcanic front, which is a characteristic common to many volcanic arcs (e.g., Gill, 1981; Morrice and Gill, 1986; Bailey et al., 1989). Following the logic of Matsui et al. ( 1977 ), it is possible to speculate that a similar pattern of relative element partitioning can be expected for hydrous fluid/mineral and melt/mineral. If so, then a H20-rich fluid phase beneath the volcanic front, released from pargasitic amphibole and more importantly coexisting with phlogopite, is likely to have lower R b / K than one beneath the backarc-side volcano, formed by decomposition of phlogopite and equilibrated with K-amphibole. Tatsumi et al. (1991) also attributed the across-arc variation in R b / K and Ba/Pb ratios observed in the Sangihe intra-oceanic arc to this mechanism. The across-arc variation in R b / Z r (Fig. 3b), more generally LIL/HFS variation, may also be caused by the change in hydrous phase assemblage in the down-dragged hydrous peridotite layer (Tatsumi et al., 1991 ). Potassium and Rb, and probably L I L elements in general, may be released together with H20 through the decomposition of phlogopite and the crystallization of K-amphibole (see the reaction mentioned before) and can be readily transported with fluid phases, although Zr, more generally HFS elements, are immobile during the dehydration process (Tatsumi et al., 1986; Tatsumi and Isoyama, 1988). Alternatively, the observed isotopic acrossarc variation could be caused by the difference in the amount of subduction components ovel-printed on the original mantle wedge. Notsu et al. (1983) observed decreasing 87Sr/86Sr of Izu-Bonin rocks away from the volcanic front and suggested that the contribution of subduction component becomes smaller towards the back-arc-side of the Izu-Bonin arc. Morris et

187

al. (1990) showed a high correlation coefficient between ~°Be/9Be and B/Be for lavas in a volcanic arc and attributed this to the greater contribution of a homogeneous subduction component beneath the volcanic front. Indeed, amphibole and chlorite decomposing beneath the volcanic front can supply fluid phases greater than phlogopite beneath the back-arcside volcanoes (Tatsumi, 1989). However, if the larger amount of fluid addition results in the formation of the greater number of mantle diapirs and volcanoes along the volcanic front (Tatsumi, 1989), then the amount of subduction component introduced into each mantle diapir may be rather constant between trenchand back-arc-side of a volcanic arc. Therefore, the mechanism including the difference in the amount of added subduction component alone cannot account for the observed isotopic across-arc variation. Zhuravlev et al. ( 1987 ) explained the acrossarc variation in the Kurile arc as the result of longer duration of magmatism, hence larger accumulation of subduction components beneath the volcanic front. However, this mechanism may not be the case, as the subduction component might not accumulate in the magma source region of the mantle wedge (Kay et al., 1986; Tatsumi et al., 1988). Conclusions

(1) Quaternary lavas from the Izu-Bonin intra-oceanic arc show across-arc variation in concentrations of incompatible elements, higher in back-arc-side rocks. This lateral variation is caused by decreasing degrees of partial melting resulting from increasing depth of magma segregation toward the back-arc region. (2) 8 7 5 r / 8 6 S r and 1 4 3 N d / 1 4 4 N d isotopic ratios and R b / K and R b / Z r ratios are lower in the magma source region beneath the back-arcside volcanoes. This across-arc variation may be explained if aqueous fluids added to the mantle wedge are produced by decomposition of amphibole and phlogopite beneath trench-

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and back-arc-side volcanic chains, respectively. This is consistent with the sequence of decomposition reactions in the down-dragged hydrous peridotite layer at the base of the mantle wedge. Acknowledgements We acknowledge Steve Eggins, Mike Perfit, Jon Woodhead and Kyoichi Ishizaka for their contributing discussions and reviewing the manuscript. Drs. M. Yuasa and N. Isshiki supplied some samples for this work. Critical reviews by Dr. I. Kaneoka and an anonymous referee are thanked. XRF analyses were done with the assistance of Dr. M. Murata of Rigaku Corporation. References Bailey, J.C, Frolova, T.I. and Burikova, I.A., 1989. Mineralogy, geochemistry and petrogenesis of Kunle ~sland-arc basalts. Contrib. Mineral. Petrol., 102: 265280. Basu, A.R. and Tatsumoto, M , 1980. Nd-lsotopes in selected mantle-derived rocks and minerals and their implication for mantle evolution. Contrlb. Mineral. Petrol., 75: 43-54. Best, M.G., 1975. Migration of hydrous fluids in the upper mantle and potassium variation m calc-alkahne rocks. Geology, 3: 429-432. BVTP, 1981. Basaltic Volcanism on the Terrestrial Planets. Pergamon Press, New York, NY, 1286 pp. DePaolo, D.J. and Wasserburg, G . J , 1977. The sources ff island arcs as indicated by Nd and Sr ISOtOpic studies, Geophys. Res. Lett., 4: 465-468. DePaolo, D.J. and Wasserburg, G.J., 1979. Petrogenetlc mixing models and Nd-Sr isotopic patterns. Geoch~m. Cosmochim. Acta, 43:615-627 Elderfield, H., Hawkesworth, C.J., Greaves, M. and Calvert, S.E., 1981. Rare earth element geochemistry of oceanic ferromanganese nodules and associated sediments. Geochlm. Cosmochlm. Acta, 45:513-528. Ellam, R.M. and Hawkesworth, C.J., 1988. Elemental and isotopic variations in subduction related basalts: evidence for a three component model, Contrib. Mineral. Petrol., 98: 72-80. Fujimaki, H., 1982. Basalt produced by mechanical mixing of andeslte magma and gabbroic fragments: Hokone volcano and adjacent area. Central Japan. J. Volcanol. Geotherm. Res., 12:111-132. Fujlmakl, H. and Kurasawa, H., 1980. Lateral variation

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