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Assimilation of Ultramafic Rock in Subduction-Related Magmatic Arcs Author(S): Peter B

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Assimilation of Ultramafic Rock in Subduction-Related Magmatic Arcs Author(S): Peter B Assimilation of Ultramafic Rock in Subduction-Related Magmatic Arcs Author(s): Peter B. Kelemen Source: The Journal of Geology, Vol. 94, No. 6 (Nov., 1986), pp. 829-843 Published by: The University of Chicago Press Stable URL: http://www.jstor.org/stable/30071586 Accessed: 29/03/2010 09:42 Your use of the JSTOR archive indicates your acceptance of JSTOR's Terms and Conditions of Use, available at http://www.jstor.org/page/info/about/policies/terms.jsp. JSTOR's Terms and Conditions of Use provides, in part, that unless you have obtained prior permission, you may not download an entire issue of a journal or multiple copies of articles, and you may use content in the JSTOR archive only for your personal, non-commercial use. Please contact the publisher regarding any further use of this work. Publisher contact information may be obtained at http://www.jstor.org/action/showPublisher?publisherCode=ucpress. Each copy of any part of a JSTOR transmission must contain the same copyright notice that appears on the screen or printed page of such transmission. JSTOR is a not-for-profit service that helps scholars, researchers, and students discover, use, and build upon a wide range of content in a trusted digital archive. We use information technology and tools to increase productivity and facilitate new forms of scholarship. For more information about JSTOR, please contact [email protected]. The University of Chicago Press is collaborating with JSTOR to digitize, preserve and extend access to The Journal of Geology. http://www.jstor.org ASSIMILATION OF ULTRAMAFIC ROCK IN SUBDUCTION-RELATED MAGMATIC ARCS1 PETER B. KELEMEN Dept. of GeologicalSciences AJ-20, Universityof Washington,Seattle, WA 98195 ABSTRACT Assimilationof ultramaficrock by fractionatingmagma may be an importantprocess in the genesis of subduction-relatedmagmatic arcs. Physicalconditions, characterized by low viscosity, fractionatingmafic magma, and high wall rock temperatures,are favorablefor interactionbetween magmaand rock in the uppermantle. Kinetic and equilibriumconstraints determine the availabilityof wall rock for reactionand the ratioof mass assimilatedvs. mass crystallized(Ma/Mc). The exact value of Ma/Mc is difficultto predict but, in general, should vary from near 1.0 to about 0.4 for mafic magmareacting with peridotiteat high temperature.For any positive value of Ma/Mc, the effect of assimilationof magnesianrock in fractionating magmais to producea less iron-enriched,more alkaline derivative liquid than would be producedby crystal fractionationalone. A method is presentedfor evaluatingthe effect of combinedassimilation and crystal fractionation (AFC) which permits continuously changing values for the crystal/liquid distribution coefficient, Ma/Mc, and the composition of the assimilate, unlike modified versions of the integrated Rayleighequations in which these parametersmust be constant over the entire crystallizationinterval. A comparisonof the concentrationof compatiblevs. incompatibleelements shows that, in principle,even in the absence of isotopic or trace element discriminants,magma series producedby AFC may be distin- guished from those derived by crystal fractionationalone. In AFC, the concentrationof a compatible elementin many cases approachesa steady state value, while the concentrationof an incompatibleelement increases exponentially. INTRODUCTION uid will react with the liquid. Bowen ob- Although studies of assimilation of felsic served that the heats of mixing in silicate liq- material in mafic silicate liquids have become uid systems were negligible in comparison increasingly popular and fruitful in the past with the heats of fusion of most silicate min- few years, (e.g., Grove et al. 1982; DePaolo erals. The enthalpy change during assimila- 1981a; Watson 1982a; Watson and Jurewicz tion reactions at constant temperature can be 1984), little attention has been paid to the as- approximated by the difference between the similation of mafic and ultramafic rocks. heat of fusion of the assimilate and the heat However, such a process could be important of crystallization of phases with which the in the evolution of some rock series. Whereas magma is saturated. If the solid to be dis- assimilation of crustal material is limited by solved is below its melting temperature, and the thermal requirements of heating wall rock if the magma is saturated in less refractory to magmatic temperatures, assimilation of members of the same reaction series (Bowen ultramafic rock by fractionating magma in the 1922b), then dissolution results in crystalliza- upper mantle is subject to no such constraint. tion of an energetically equivalent mass of Nevertheless, there is an instinctive resis- saturated phases, the mass crystallized gen- tance to considering the effect of assimilation erally being larger than the mass dissolved. of refractory rock in more "felsic" liquids at These considerations are not path dependent. temperatures below the solidus for the rock. It is equally valid to view such reactions as As shown by Bowen (1922a), there is no addition of liquid components to a refractory theoretical basis for this reluctance. solid phase, decreasing the mass of liquid and Solid phases out of equilibrium with a liq- increasing that of the solid. As long as the mass assimilated is small relative to the mass crystallized, reactions between mantle pe- ' Manuscript received December 9, 1985; re- ridotite and basaltic magma involve a de- vised July 16, 1986. crease in the entropy and volume of the solid- liquid system; i.e., reactions will often be [JOURNALOF GEOLOGY, 1986, vol. 94, p. 829-843] © 1986 by The University of Chicago. All rights exothermic at constant temperature. (Note reserved. that this reasoning does not apply to assimila- 0022-1376/86/9406-001$1.00 tion of phases across a cotectic "valley" in 829 830 PETER B. KELEMEN the liquidus surface or to assimilationof less would affect the composition of the liquid. refractoryphases in a reactionseries. In gen- Variantsof this simple hypothesis have been eral, AH = Ma . AH - Mc . A1H, where AH proposed by Harris (1957), Green and Ring- is the reaction enthalpy, Ma is the mass as- wood (1967), Quick (1981), and Watson similated,Mc the mass crystallized,and AH (1982b)to account for the genesis of alkalic and A/H are the apparententhalpies of fusion, basalt. As these authors have pointed out, per gram, for the assimilate and precipitate such a process would lead to enrichmentin at the temperatureof reaction. Assimilation the melt of incompatibletrace elements, al- reactionsare exothermicwhen Ma/Mc AH / kalies, and volatiles, while preservingother- AHJ< 1, and endothermicwhen this term is wise very "primitive" major element chem- greaterthan one.) istry. In addition, the increasing volatile Formation of pyroxenite by reaction be- content of the liquid could cause significant tween mantle peridotite and more felsic changes in the minimummelting composition magmahas been proposed on a plate tectonic of the local mantle-magmasystem. Further- scale by Ringwood (1974), experimentally more, previous investigators have assumed demonstrated on a microscopic scale by that magmatictemperature remains constant Sekine and Wyllie (1982, 1983), and invoked from initial melting to eruption. Where to explain map and outcrop scale contact re- magmaand wall rock temperaturesare below lations by Ramp (1975), Kelemen and Son- the equilibriumtemperature of primarymelts nenfeld (1983), Evans (1985), and Kelemen in the mantle (perhaps 1200 or 1300°C),the and Ghiorso (1986). Irving (1980) and Men- result of solid-liquidreaction must be some- zies et al. (1985) presented evidence for what different.The effect of a combinationof cryptic and modal metasomatism of spinel assimilationand crystal fractionationon the lherzolite wall rock by magmain the mantle. composition of liquid ascending through Arculus et al. (1983) proposed that assimila- cooler upper mantlehas not been sufficiently tion of ultramaficmaterial in the crust played explored. an importantrole in the development of an Mantletectonite xenoliths are rarelyfound unusualmagma series in PapuaNew Guinea. in volcanic arcs, althoughstudy of the condi- Additional evidence for reaction between tions of formation of some "cumulate" magma and more mafic rocks beneath vol- xenoliths indicatethat they originateat upper canic arcs, preserved in composite or "hy- mantle temperaturesand pressures (Powell brid" xenoliths, has recently been presented 1978; Arculus and Wills 1980; Conrad and by Conradet al. (1983) and Conradand Kay Kay 1984). Unequivocal exceptions are (1984). They wrote (1984, p. 94) that "tex- Itinome-gata,Japan, Kanaga Island, Alaska, turalfeatures in composite xenolithsgive evi- and Grenada, Lesser Antilles, where tecto- dence for assimilationof wall rock or magma nite spinel lherzolite inclusions have been ('primary assemblage') by another magma found (Kuno 1967; DeLong et al. 1975; ('secondaryassemblage'), all of which occurs Takahashi 1980; Pope et al. 1981; Arculus prior to incorporationof the xenolith in the pers. comm. 1986).Thermal regimes inferred host lava . Assimilated phases in the pri- for subduction-relatedvolcanic arcs (Ander- mary assemblage are . Mg-, Cr-, and Ni- son et al. 1976, 1978, 1980; Blackwell et al. rich, and are presumablythe firstfractionates 1982;Tatsumi et al. 1983)have temperatures of mantle derived melts." near 1000°Cin the upper mantle. These con- ditions would lead to crystal fractionation HYBRIDIZATION OF THE MANTLE combinedwith infiltrationand
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