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Manufacturing Continental Crust in , Volume 4, a Quarterly Journal of Th 19, Number the Subduction Or Other Means Reposting, Is Only Photocopy Machine, Permitted Th or collective redistirbution of any portion article of any by of this or collective redistirbution SPECIAL ISSUE FEATURE articleis has been published in Oceanography Manufacturing Continental Crust in 19, Number journal of Th 4, a quarterly , Volume the Subduction permitted photocopy machine, only is reposting, means or other Factory e Oceanography Society. 2006 by Th Copyright BY YOSHIYUKI TATSUMI AND ROBERT J. STERN with the approval of Th WERE CONTINENTS BORN dehydration reactions and/or partial length of Earth’s 55,000 km of conver- IN THE OCEAN? melting during subduction dissolve par- gent margins (Stern, 2002) (Figure 2), Subduction zones have been consuming ticular elements and carry them into the provide vital information on how the gran is e Oceanography All rights Society. reserved. Permission oceanic lithosphere since the dawn of overlying mantle wedge, leading to gener- subduction factory works. They are or Th e Oceanography [email protected] Send Society. to: all correspondence plate tectonics on this planet (Figure 1). ation of chemically distinct arc magmas situated on thinner and dominantly Raw materials such as pelagic and ter- that differentiate, solidify, and produce more mafi c crust than systems at con- rigenous sediments, altered and fresh juvenile arc crust; the site of these trans- tinental margins and hence signifi cant basaltic oceanic crust, and lithosphere formations is known as the “subduction contamination of magmas by easily fus- are conveyed into this “factory” as the factory.” Collisional coalescence and ible felsic (light-colored rocks high in subducted plate sinks deeper into the magmatic thickening of juvenile arcs feldspar and silica, for example, gran- mantle. Assuming steady-state subduc- ultimately yield continental crust, an im- ite) crust may not occur. Along such tion of the entire 7-km-thick oceanic portant subduction-factory product. The margins, we can also better measure crust for 3 billion years, the accumulated factory inevitably emits waste materials, crustal growth rates, now estimated to ted to copy this article Repu for use copy this and research. to ted in teaching crustal materials occupy ~ 10 percent of such as chemically modifi ed, residual be 30–95 km3 km-1 Myr-1 (Dimalanta et the lower mantle. Aqueous fl uids and/or oceanic crust and sediments and delami- al., 2002). Intra-oceanic arcs, however, silicate melts that are progressively ex- nated arc mafi c (high in iron and mag- are signifi cantly less well studied than e Oceanography Society, PO Box 1931, Rockville, MD 20849-1931, USA. tracted from these raw materials through nesium) lower crust, which sink deeper continental arcs. The main reason for the into the mantle. These waste materials, paucity of research is that except for the Yoshiyuki Tatsumi (tatsumi@jamstec. because of their great mass and residual tops of the largest volcanoes, most of the go.jp) is Program Director, Geochemical crustal composition, contribute greatly arc is submerged below sea level. Ocean Evolution, Institute for Frontier Research on to the thermal and chemical evolution drilling at intra-oceanic arcs, therefore, Earth Evolution, Japan Marine Science and of the mantle and ultimately may return should provide key observations for Technology Center, Yokosuka, Japan. Robert to Earth’s surface as mantle plumes and comprehending the solid-Earth cycle and blication, systemmatic reproduction, reproduction, systemmatic blication, J. Stern ([email protected]) is Professor, related magmas (Figure 1). evolution. Because of the importance of Department of Geosciences, University of Intra-oceanic subduction systems, this problem, both the MARGINS Sci- Texas at Dallas, Dallas, TX, USA. which occupy ~ 40 percent of the global ence Plans (2003) and Integrated Ocean 104 Oceanography Vol. 19, No. 4, Dec. 2006 Figure 1. Role of the subduction factory in the evolution of the solid Earth. Raw materials, such as ocean- ic sediments, oceanic crust, mantle lithosphere, and wedge materials, are fed into the factory and are manufactured into arc magmas and continental crust. Th e waste ma- terials or residues processed in this factory, such as chemically modifi ed slab components (oceanic crust and sediments) and delaminated mafi c lower arc crust, are transported and stored in the deep mantle, and re- cycled as raw materials for mantle- plume-related hot-spot magmatism. Drilling Program (IODP) Initial Sci- component because it is the storehouse crust. The very distinctive composition ence Plan (ISP) (Coffi n, McKenzie et al., of most natural resources and serves as of the continental crust indicates that 2001) have identifi ed understanding the substrate for agriculture and civiliza- understanding how this crust forms is operation of Earth’s subduction factory tion. Chemically, the continental crust is essential for understanding solid Earth as a research priority. a reservoir for “incompatible” elements evolution. The IODP ISP includes conti- that do not fi t easily in mantle minerals, nental-crust formation and growth as a PERSPECTIVES ON such as K, Rb, U, Th, and the light Rare priority drilling target (Coffi n, McKenzie CONTINENTAL CRUST Earth Elements. Continental crust thus et al., 2001). FORMATION possesses “differentiated” compositions, It is generally accepted that conti- Continental crust occupies only 0.4 per- quite distinct from a chondritic (compo- nental crust has an andesitic bulk com- cent of the total mass of the solid Earth; sition similar to that of asteroids) bulk position (Rudnick and Fountain, 1995; however, it is by far the most important Earth, primitive mantle, or even oceanic Taylor and McLennan, 1995). Because Oceanography Vol. 19, No. 4, Dec. 2006 105 andesitic magmatism typifi es subduction Reconstructing crustal-growth processes to andesite and more felsic crustal com- zones, many researchers consider that for continental arcs is, however, challeng- positions typical of continental crust. continental crust has been created domi- ing. Low-density continental crust acts as Seismic profi ling of the Izu-Bonin-Mari- nantly in arc-trench settings. However, a density fi lter, trapping mantle-derived ana (IBM) (Figure 3) arc crust (Suyehiro modern-day, mantle-derived magmatism mafi c melts that then are likely to frac- et al., 1996; Takahashi et al., 2006) has in such settings is dominated by basalt. tionate, assimilate, and melt the crust, further demonstrated the presence of This dilemma faces anyone interested in strongly affecting the compositions of middle crust that has a P-wave velocity continental-crust formation. lavas and plutons. In contrast, intra-oce- of ~ 6 km s-1 (Figure 4a), interpreted to To explain the characteristic andesitic anic arcs are built on high-density, mafi c correspond broadly to intermediate-to- composition of the average continen- oceanic crust. Oceanic crust is much felsic “tonalitic” composition (Figure 4b). tal crust, several mechanisms have been less effective as a density fi lter and melts (A tonalite is an igneous intrusive rock proposed (e.g., Rudnick, 1995; Tatsumi, at a higher temperature, thus effects of of felsic composition.) Furthermore, 2005). These hypotheses have been as- pre-existing crust are minimized for tonalitic rocks are found as xenoliths in- sessed petrologically, geochemically, and intra-oceanic arcs. Nevertheless, intra- cluded in lavas and as exposures along geophysically at various continental arcs. oceanic arc crust appears to differentiate intra-oceanic arcs such as the IBM and 0˚ 60E˚ 120E˚ 180˚ 120W˚ 60W˚ 60˚ Eurasia W. Aleutians Kuriles N. America Lesser 30˚ Izu-Bonin Antilles Mariana Africa 0˚ New Britain Vanuatu S. America Tonga Australia New 30˚ Hebrides Kermadec Scotial/ S. Sandwich Arc 60˚ Antarctica Figure 2. Locations of modern intra-oceanic arc-trench systems. Trenches associated with these systems are shown by barbed lines. Intra-oceanic arcs are sites where the eff ects of pre-existing continental crust can be neglected and represent relatively simple natural laboratories for the study of modern crustal growth. 106 Oceanography Vol. 19, No. 4, Dec. 2006 Kyushu-Palau systems. This compelling now understand that the subduction fac- to the upper level of the subduction fac- evidence suggests that the ingredients tory has two “fl oors.” The lower level of tory—the arc crust—where it is further for continental crust are indeed created the factory, the subducted slab and man- processed by fractionation, partial melt- today in intra-oceanic arcs. IODP drill- tle wedge, extracts fl uids and/or melts ing, and delamination to generate tonal- ing in intra-oceanic arcs will “be a tre- from subducted materials. These fl uids itic/andesitic continental crust. mendous step towards understanding the and melts are enriched in the “continen- We cannot visit (by drilling) the lower origin of the andesitic continental crust” tal component.” This mixture is trans- fl oor of the subduction factory, so our (Coffi n, McKenzie et al., 2001). ferred upward, away from the slab, and information about its operations must mixes with normal asthenospheric man- be indirect. One method to evaluate the THE FLOOR PLAN OF THE tle, enriching it and lowering its melting processes in the lower fl oor of the facto- SUBDUCTION FACTORY temperature. This hybrid mantle melts ry is to analyze primitive, least-fraction- Like all factories, the subduction factory to form primitive (Mg-rich) arc basaltic ated arc lavas, which are relatively rare, follows specifi ed procedures. To under- magma that is enriched in continental- especially on large volcanic islands. It is stand them, one must know where the type incompatible elements. This primi- easier to fi nd primitive lavas by sampling various operations are carried out. We tive yet distinctive magma is then sent small submarine arc volcanoes, which 125˚E 130˚ 135˚ 140˚ 145˚ 150˚ Figure 3. Location map of the Philippine Sea Eurasia Plate region. Th e Izu-Bonin-Mariana (IBM) arc-trench system forms the convergent margin between 40˚N Pacifi c and Philippine Sea plates. Backarc ba- sins such as Shikoku Basin, Parece Vela Basin, and Mariana Trough were created by seafl oor spreading between the formerly contiguous remnant arc (Kyushu-Palau and West Mariana ridges) and the active IBM arc.
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