The Izu-Mariana arc system involves subduction of the ancient Pacific plate beneath the relatively young Philippine Sea plate with the resulting production of an island arc chain of volcanoes and marginal backarc basins (Fig. F3).

There are several reasons why the Izu-Mariana margin is favorable for studying material recycling in subduction zones. The first is that significant progress has already been made on many parts of the flux equation. Serpentine seamounts, which represent forearc sites of fluid outflow, have already been drilled (Leg 125; Fryer, 1992), as have most of the sedimentary components being subducted at the Mariana Trench (Leg 129; Lancelot, Larson, et al. 1990) (see "Western Pacific Stratigraphy"). The Izu and Mariana volcanic arcs and the Mariana Trough and Sumisu Rift backarcs are among the best characterized intraoceanic convergent margins, both in space and time (Legs 125 and 126; Gill et al., 1994; Arculus et al., 1995; Elliott, et al., 1997; Ikeda and Yuasa, 1989; Stern et al., 1990; Tatsumi et al., 1992; Woodhead and Fraser, 1985; Taylor and Nesbitt, 1998). Thus, major parts of the forearc, arc, and backarc output, as well as the sedimentary input, have already been characterized. The other advantage to the Izu-Mariana system is that the problem is simplified here because (1) the upper plate is oceanic and, therefore, upper crustal contamination is minimized; and (2) sediment accretion in the forearc is nonexistent (Taylor, 1992) and, therefore, sediment subduction is complete.

Despite the simple oceanic setting and the shared plate margin, there are clear geochemical differences between the Izu and Mariana arcs. The Mariana arc erupts basalts in which both subducted sedimentary and altered oceanic crustal components can be identified (e.g., Elliott et al., 1997), and the arc conforms well to the global trend in Ba sediment input vs. Ba arc output (Fig. F5A). On the other hand, the Izu arc erupts basalts that are among the most depleted of any arc on Earth in trace element concentrations (e.g., REEs, Ba, and Sr). In addition to the contrast in elemental concentrations, there are also clear differences in the isotopic composition of Mariana and Izu basalts, such as 207Pb/204Pb and 206Pb/204Pb, which may derive from isotopic differences in the input to the two trenches (Fig. F5B).

The divergence of compositions between the volcanics of these two oceanic arcs provides the simplest test for how the composition of the subducting crust affects them. The key missing information is the composition of the incoming crustal sections, specifically the basaltic basement subducting at the Mariana Trench and the sediment and basement sections subducting at the Izu Trench. The low trace element concentrations of Izu volcanics may derive from a lower flux of these elements at the trench, and their distinctive isotopic composition may be inherited from the composition of the sediments subducting there. These hypotheses can be tested by drilling the subducting sediment and basement sections feeding the two arc systems. Alternatively, differences in the fluxes cycled to the arcs may derive from different operations of the subduction factory in the two areas. For example, along-strike changes (e.g., dip, age, and depth) in the subducting slab could affect where material exits the slab and enters the arc melting regime. The changes in the geometry of the slab and its relationship to the volcanic arc may signal a change in where the volcanoes are sampling the slab material. Distinguishing between these two models—the input model vs. the slab model—requires good control on the subducted inputs, which was a primary objective of Leg 185.