IZU-MARIANA ARC
The Izu-Mariana arc system involves subduction of the ancient Pacific
plate beneath the relatively young Philippine Sea plate with the resulting
production of a classic island arc chain of volcanoes and marginal backarc
basins (Fig. 2).
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 below). 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). 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 the upper plate is oceanic‹therefore,
upper crustal contamination is minimized, and sediment accretion in the
forearc is nonexistent (Taylor, 1992)‹so 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. 4A). On the other hand, the Izu arc erupts basalts
that are among the most depleted of any global arc 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 207/204 and 206/204,
which may derive from isotopic differences in the input to the two
trenches (Fig. 4B).
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 fluids. Distinguishing between these two models‹the input model vs.
the slab model‹requires good control on the subducted inputs, which was
the primary objective of Leg 185.
To 185 Western Pacific Stratigraphy
To 185 Table of Contents