The subduction factory: How it operates in the evolving Earth Yoshiyuki Tatsumi, Institute for Research on Earth Evolution (IFREE), Japan Agency for Marine-Earth Science and Technology (JAMSTEC), Yokosuka 237-0061, Japan, [email protected] ABSTRACT The subduction factory processes raw materials such as oceanic sediments and oceanic crust and manufactures mag- mas and continental crust as products. Aqueous fluids, which are extracted from oceanic raw materials via dehydration reactions during subduction, dissolve particular elements and overprint such elements onto the mantle wedge to gener- ate chemically distinct arc basalt magmas. The production of calc-alkalic andesites typifies magmatism in subduction zones. One of the principal mechanisms of modern-day, calc-alkalic andesite production is thought to be mixing of two end- member magmas, a mantle-derived basaltic magma and an arc crust-derived felsic magma. This process may also have contributed greatly to continental crust formation, as the bulk continental crust possesses compositions similar to calc-alkalic andesites. If so, then the mafic melting residue after extrac- tion of felsic melts should be removed and delaminated from the initial basaltic arc crust in order to form “andesitic” crust compositions. The waste materials from the factory, such as chemically modified oceanic materials and delaminated mafic lower crust materials, are transported down to the deep man- tle and recycled as mantle plumes. The subduction factory has played a central role in the evolution of the solid Earth through creating continental crust and deep mantle geochemi- Figure 1. Role of the subduction factory in the evolution of the solid Earth. cal reservoirs. Raw materials, such as oceanic sediments, oceanic crust, mantle lithosphere, and wedge materials, are fed into the factory and are manufactured into arc INTRODUCTION magmas and continental crust. The waste materials or residues processed Subduction zones, where the oceanic plates sink into the in this factory, such as chemically modified slab components (oceanic crust mantle, have been “factories” since plate tectonics began and sediments) and delaminated mafic lower arc crust, are transported and stored in the deep mantle and recycled as raw materials for mantle plume– on Earth (Fig. 1). Oceanic materials such as pelagic and ter- related hotspot magmatism. EMI—enriched mantle type I; EMII—enriched rigenous sediments, altered and fresh basaltic oceanic crust, mantle type II; HIMU— high-μ mantle. and mantle lithosphere enter the factory as raw materials. These materials, together with mantle-wedge peridotites, during subduction, and possibly delaminated mafic lower- are processed into products, during which the entire factory arc crust. These waste materials founder into Earth’s deeper adjusts and deforms, causing magmatism and earthquakes. interior, reside somewhere in the deep mantle, and may The products of the factory are arc magmas, their solidified contribute greatly to the evolution of mantle because of their materials, and ultimately continental crust. Such products may significant mass and characteristic compositions; assuming be volumetrically small, as the continental crust occupies <1% steady-state subduction of the entire 7-km-thick oceanic of the total mass of solid Earth. However, they possess “differ- crust for 3 billion years, accumulated crust materials with entiated” compositions, quite distinct from a chondritic bulk basaltic compositions occupy ~10% of the lower mantle. Earth, which suggests that the origin of products from the This paper outlines how the subduction factory creates subduction factory should provide a clue to understanding products and discusses its role in the evolution of Earth’s the evolution of the solid Earth. mantle. The subduction factory, as do other factories, emits waste PRODUCTION PROCESSES materials, such as slab materials, which are chemically modi- Subduction zones are sites of intensive volcanism and are fied through complex dehydration and/or melting processes creating >20% of the current terrestrial magmatic products. GSA Today: v. 15, no. 7, doi: 10:1130/1052-5173(2005)015<4:TSFHIO>2.0.CO;2 4 JULY 2005, GSA TODAY Although the interaction of physical and Eggins (1995), the characteristic depths chemical processes occurring in subduc- correspond to pressures expected for tion zones is complex, arc magmatism the dehydration of amphibole and chlo- exhibits characteristics common to not rite (110 km) and phlogopite (170 km) in all but most arc-trench systems. One the hydrous peridotite layer at the base of these general characteristics is the of the mantle wedge, which is formed by presence of paired lines of volcanoes. addition of slab-derived aqueous fluids Another is the negative correlation beneath the forearc and dragged down- between the volcanic arc width and ward on the subducting plate. the subduction angle (Fig. 2A). In other Within any volcanic arc, lava chemis- words, the depth to the surface of the try tends to change systematically with subducting lithosphere is constant at distance from the volcanic front. The Figure 3. Normal mid-oceanic-ridge basalt (N- ~110 km and ~170 km, respectively, degree of silica saturation decreases MORB)-normalized (Sun and McDonough, beneath the trenchward limit of the toward the backarc (Kuno, 1966) with 1989) incompatible element characteristics volcanic arc (the volcanic front) and the increasing concentrations of incompat- of arc basalts (Tatsumi and Eggins, 1995), an backarc-side volcanoes (Fig. 2B). This ible elements such as K, Rb, and Zr, a arc basalt magma inferred from geochemical characteristic volcano distribution rela- relationship known as the K-h (K, potas- modeling on element transport by dehydration of subducting sediments and altered oceanic tive to subduction depth suggests an sium; h, height above the slab) rela- crust (Tatsumi and Kogiso, 2003), and a important role of pressure-dependent tionship (Dickinson, 1975). A possible hotspot-related oceanic-island basalt (Sun and processes such as dehydration reac- origin of this observation is separation McDonough, 1989). tions in magma generation in the subarc of a primary magma from an adiabati- mantle. As emphasized by Tatsumi and cally rising mantle diapir at a greater enriches the source region of subduc- depth by a lower degree of partial melt- tion zone magmas? Since the work of ing toward the backarc, which may be Nicholls and Ringwood (1973), many caused by the presence of a thicker petrologists have favored mechanisms lithosphere beneath the backarc side of including slab melting and subsequent a volcanic arc (e.g., Tatsumi et al., 1983; melt-mantle interaction. A majority of Zhao et al., 1992; Furukawa, 1993). researchers, however, currently believe Selective enrichment of particular that the subducting lithosphere dehy- incompatible elements in arc basalts has drates but does not melt except in arc- been well established since the pioneer trench systems where a young and hot work by Pearce and Cann (1973). This plate is being subducted (Peacock et al., can be intuitively attributed to addition of 1994). During the Archean, however, the slab-derived component to the origi- temperatures of both the upper mantle nal mantle wedge. Herein the geochemi- and the subducting crust might have cal characteristics of arc magmas and been higher, which could cause slab their possible causes including element melting, rather than slab dehydration transport from the downgoing oceanic (Martin, 1987). This possibility will be lithosphere will be further outlined. discussed later. Basaltic Magma Under upper mantle P-T conditions, Although basalts are not always the H2O exists as a supercritical fluid, which major surface products of the subduc- can dissolve additional components and tion factory, magmas generated in the elements to a certain extent. Therefore, mantle wedge are likely to be basal- aqueous fluid phases released by dehy- tic in composition. Basalts erupted in dration reactions within the subducting subduction zones are noted for their lithosphere may be a likely metasomatic distinct chemistry compared with those agent responsible for the characteristic in other tectonic settings. In particular, trace element signatures. To elucidate Figure 2. Tectonic characteristics of subduction they are elevated in large ion lithophile the geochemical characteristics of such zone magmatism (Tatsumi and Eggins, elements (e.g., Cs, Rb, K, Ba, Pb, Sr) slab-derived fluid phases, several exper- 1995). (A) Negative correlation between the and depleted in high field strength ele- imental studies have been conducted subduction angle and the volcanic arc width. on the distribution of elements between (B) Constant depths to the surface of the ments (e.g., Ta, Nb, Zr, Ti) (Fig. 3). Such subducting lithosphere beneath the trench- characteristic compositions likely arise aqueous fluids and solid minerals (e.g., side (volcanic front) and backarc-side volcanic from subducting oceanic lithosphere Tatsumi et al., 1986; Brenan et al., 1995; chains in volcanic arcs. Noted values are the through metasomatic reactions between Keppler, 1996). On the basis of these average depths and one standard deviation. the subducting lithosphere and the experimental data, together with rea- These observations suggest the primary role of sonable assumptions of H O contents pressure-dependent reactions in production of overlying mantle wedge. What is the 2 arc magmas. nature of the metasomatic process that (1.5