フィッション・トラック

ニュースレター第 21 号

49-52

2008 年

特別 講 演Arc Basalt Simulator, a simulation model for arc basalts: An attempt tounderstand mass balance beneath subduction zones
Jun-Ichi Kimura*1, Bradley Hacker *2, Peter van Keken *3,
Takeyoshi Yoshida*4 and Robert J. Stern *5

アーク・バサルト・シミュレーター，島弧玄武岩のシミュレーションモデル：
沈み込み帯マスバランスの理解を目指して
木村純一*1 ，ブラドリー・ハッカー*2 ，ピーター・ファン-ケケン*3 ，
吉田武義*4 ，ロバート・スターン*5*1 Institute for Research on Earth Evolution (IFREE), Japan Agency for Marine-Earth Science andTechnology (JAMSTEC), 2-15 Natsushima-cho, Yokosuka 237-0061, Japan*2 Department of Earth Science, University of California, Santa Barbara, California 93109-9630,
USA*3 Department of Geological Sciences, University of Michigan, 425 East University Avenue, AnnArbor, MI 48109-1063, USA*4 Institute of Mineralogy, Petrology and Economic Geology, Tohoku University, Aobaku, Sendai,980-8578, Japan*5 Geosciences Department, University of Texas at Dallas, Richardson, Texas 75083-0688, USA

和文要旨
島弧玄武岩の成因を(1)沈み込みスラブの累進変成脱水作用−(2)スラブフルイド・マントル反応−(3)フルイド付加・マントル融解反応の３段階のモジュールを用いたマスバランス計算によってシミュレーシ
ョンしたモデルを紹介する（ Kimura et al., under review ）．このモデルでは，島弧玄武岩の地球化学的組成多様性は，様々な温度で脱水したスラブフルイドの多様性と，スラブフルイドの付加率，さらにマントルの部分融解度で説明できる．

Overv iew: A BS h i stor y and c once pt
Anumericalmassofthe origin of the K-h relationship in the NE-

slab-dehydration, fluid-mantle reaction, andand SW-Japan arc basalts (Kimura and Stern,

fluid-fluxed

was generated

in press) and West Sunda arc basalt, Indonesia(Kimura et al., under review). The develop-(Sendjaja et al., in press). ABS ver.1 simply

ment of the model includes an earlier version

assumed mixing of sediment (SED) and alteredof the ABS (Arc Basalt Simulator) model, which

oceanic crust (AOC) for slab component andwas applied to the basalts from NE-Japan anddehydration of the slab with slab-fluid Ds de-result published in (Kimura and Yoshida, 2006).

termined for K-free AOC (Kessel et al., 2005).

An EXCEL® spreadsheet based calculator ABS

Here we outline a more comprehensive model

mantlebalancemeltingmodel

ver.1 was coded and applied to understanding- 49 -Fig. 1 Conceptual schematics of Arc Basalt Simulator (ABS ver.2) model equipped with Slab Dehydration Simulator (SDS).

including slab dehydration along the slab P-TboundH2Ocontentpath (Fig. 1). Prograde metamorphism of AOC

100-1400 ( C) and 0.5-6.0 GPa (Hacker, 2008).

and SED were simulated using Perple̲X ver.7

Slab P-T trajectories were calculated withthermodynamic model (Connolly and Kerrick,geodynamic

1987; Connolly and Petrini, 2002) for modal

pendent olivine rheology (Fig. 2; van-Keken,

composition of metamorphic minerals and2003). This information was combined withmodeloverusing

P-T

conditions

temperaturede-Fig. 2 Phase diagram and bound H2O isopreth of SED and MORB calculated by Perple̲X ver. 7. Perple̲X ver.7 calculations provide phase boundaries, modes, mineral compositions (not used for ABS),and bound H2O in the subducted slab. If P-T trajectory of subducted slab surface (SST) is given,
equilibrium changes in slab modes and fluid dehydration from the slab can be calculated. The SST
given on the panels (a)-(b) was calculated for Izu arc subduction zone. Dashed lines with +/- numbers
show P-T trajectories (in km) above and below the slab surface. Mineralogic modes and H2O content ata given P-T condition along SST are then taken into ABS ver.2 calculations.

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Fig. 3 Application of the ABS ver.2 to the N-Izu across arc variation. (a) trace element calculation results for volcanic front (VF) and rear arc (RA) lavas, (b) calculated results for Pb isotope variation between VF and RA.

partition coefficients between

metamorphic

sive parameters required are reasonable. The-minerals and aqueous fluid (Brenan et al.,se variables are: for VF basalt: slab dehydra-

1995; Feineman et al., 2007; Green and Adam,tion 2.9 GPa-685 ( C), 3x fluid mantle reaction,2003; Kessel et al., 2005; Stalder et al., 1998)

20% melting of mantle at 1 GPa with 4% fluid

to calculate the trace element compositions offlux; for RA basalt: slab dehydration at 6.0

slab-derived fluid.

GPa-813 ( C), no mantle reaction, 1% mantle
melting at 2.3 GPa with 1% fluid flux.

Mo del and ap pli ca ti on
Referen ces

Because ABS ver.2 is a forward model of theslab dehydration, fluid-mantle reaction, andBrenan, J.M., Shaw, H.F., Ryerson, F.J. and Phinney,

fluid-fluxed melting, adjusting each parameter

D.L., 1995, Mineral-aqueous fluid partitioning ofshould be done by comparing model results totrace elements at 900

the composition of natural basalts. Graphicalstraints on the trace element chemistry of man-fitting using multi-element plots and isotope

tle and deep crustal fluids. Geochimica et Cos-plots are used for the evaluations (Fig. 3). For

mochimica Acta 59, 3331-3350.

C and 2.0 GPa: con-application to the N-Izu basalts, we used sour-Connolly, J.A.D. and Kerrick, D.M., 1987, An algo-ce compositions DMM (a 3% MORB depleted

rithm and computer program for calculating

DMM (Workman and Hart, 2005), AOC ofcomposition phase diagrams. CALPHAD 11,

Pacific Plate altered oceanic crust (Hauff et al.,1-55.

2003; Kelley et al., 2003)), and SED of mixture

Connolly, J.A.D. and Petrini, K., 2002, An automat-

between pelagic clay and chert sediments ined strategy for calculation of phase diagram sec-

the Pacific Ocean (Plank and Langmuir, 1998).

tions and retrieval of rock properties as a func-

With the ABS model, fitting of trace elementtionabundance and isotope compositions wereMetamorphic Geology 20, 697-708.

achieved (see Fig. 3). All intensive and exten-- 51 -of

physicalconditions.

Journalof

Feineman, M.D., Ryerson, F.J., DePaolo, D.J. andPlank, T., 2007, Zoisite-aqueous fluid trace ele-application. Geochemistry Geophysics Geosystems (under review).

ment partitioning with implications for subduc-Kimura, J-I. and Stern, R.J., 2008, Neogene volcan-tion zone fluid composition. Chemical Geology

ism of the Japan island arc: The K-h relationship239, 250-265.

revisited

Green, T.H. and Adam, J., 2003, Experimentallydeterminedtraceelement

characteristics

Circum-PacificTectonics,

Geologic

Evolution, and Ore Deposits. Arizona GeologicalofSociety Digest (in press).

aqueous fluid from partially dehydrated mafic

Kimura, J-I. and Yoshida, T., 2006, Contributions ofoceanic crust at 3.0 GPa, 650-700 C. Europeanslab fluid, mantle wedge and crust to the origin

Journal of Mineralogy 15, 815-830.

of Quaternary lavas in the NE Japan arc. Journal

Hacker, B.R., 2008, H2O subduction beyond arcs.
Geochemistry

Geophysics

Geosystems9,

doi:10.1029/2007GC001707.

of Petrology 47, 2185-2232.
Plank, T. and Langmuir, C.H., 1998, The chemicalcomposition of subducting sediment and its

Hauff, F., Hoernle, K. and Schmidt, A., 2003,
Sr-Nd-Pb composition of Mesozoic Pacific oce-consequence for the crust and mantle. ChemicalGeology 145, 325-394.

anic crust (Site 1149 and 801, ODP Leg 185):

Sendjaja, Y.A., Kimura, J-I. and Sunardi, E., 2008,

Implications for alteration of ocean crust and theAcross-arc geochemical variation of Quaternary

input into the Izu-Bonin-Mariana subductionlavas in West Java, Indonesia: Mass-balancesystem. Geochemistry Geophysics Geosystems 4,

elucidation using Arc Basalt Simulator model.

doi:10.1029/2002GC000421.

Island Arc (in press).

Kelley, K.A., Plank, T., Ludden, J. and Staudigel, H.,

Stalder. R., Foley, S.F., Brey, G.P. and Horn, I., 1998,

2003, Composition of altered oceanic crust atMineral-aqueous fluid partitioning of trace ele-ODP

Sites

Geophysics

801andGeosystems

1149.

Geochemistryments at 900-1200 C and 3.0-5.7 GPa: New ex-4,

doi:10.1029/

perimental data for garnet, clinopyroxene, and2002GC000435.

rutile, and implications for mantle metasoma-Kessel, R., Schmidt, M.W., Ulmer, P. and Pettke, T.,2005, Trace element signature of subduction-tism. Geochimica et Cosmochimica Acta 62,
1781-1801.

zone fluids, melts and supercritical liquids at
120-180 km depth. Nature 439.

van-Keken, P., 2003, The structure and dynamics ofthe mantle wedge. Earth and Planetary Science

Kimura, J-I., Hacker, B.R., van-Keken, P., Yoshida, T.
and Stern, R.J., 2008, Arc Basalt Simulator

Letters 215, 323-338.
Workman, R.K. and Hart, S.R., 2005, Major and(ABS), a simulation model for slab dehydration,

trace

fluid-mantle reaction, and fluid fluxed mantle

MORB mantle (DMM). Earth and Planetary

melting for arc basalts: modeling scheme andScience Letters 231, 53-72.

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element compositionofthedepleted