Figure 1. The crustal fluxes through a subduction zone (after von Huene and Scholl, 1993). Arrows indicate schematic flow, not detailed flow patterns. The return flux of crustal material to the mantle depends on many other fluxes: the partitioning of bulk sediment in the shallow part of the subduction zone (by accretion, underplating, and erosion), the chemical and fluid fluxes due to sediment dehydration and melting, and the chemical fluxes to the volcanic arc and backarc.
Figure 2. Map of the Western Pacific and Mariana, Izu-Bonin, and Northern Mariana Seamount Province volcanic arcs. Solid circles = existing drill holes; most coring that occurred prior to Sites 800-802 failed to penetrate the complete sedimentary section and/or had poor recovery. Shaded seafloor includes Pigafetta Basin (PB) and Magellan Seamount, where alkalic Cretaceous overprint predominates. Solid triangles = active volcanoes. Site BON-8A and Hole 801C are proposed drill sites. Ogasawara Fracture Zone-Magellan Seamount Flexural Moat (OFZ-MSM) after Abrams et al., 1992 (their fig. 2). Curved arrows = instantaneous trajectories for the Pacific Plate relative to the Philippine Plate, after Seno et al., (1993). Dashed curves = continuation of trajectories beneath the arc. EMB = East Mariana Basin. N. Smt. Prov. = Northern Seamount Province. Gray shaded boxes show Leg 125 and 126 drilling areas and the Izu Crosschains. The Izu Crosschains shaded box shows the location of a cruise in 1995 that conducted detailed sampling of the Izu Crosschain seamount volcanoes. These samples provide excellent controls on spatial and temporal variations in slab outputs across the Izu-Bonin arc.
Figure 3. A. Contrasting Pb isotopic composition of Marianas (open circle) and Izu-Bonin (solid circle) arc volcanics. Marianas volcanics form a mixing trend (arrow), almost perfectly coincident with mixtures of ODP Hole 801C sediment (open boxes) and basalt (solid boxes) averages. Drilling at Site BON-8A will test whether the Izu-Bonin Arc trend (arrow) is consistent with different subducted material than for the Marianas. Modern Indian MORB, Pacific MORB, and Honshu Arc data are shown for reference. Data sources: Elliott, et al., 1997; Gill, et al., 1994; Plank and Langmuir, 1998; Castillo et al., 1992a; and Gust et al., 1997. B. Correlation between Ba flux in subducted sediment and Ba enrichment of arc basalts for various arcs (Ant = Northern Antilles, Mar = Marianas, T= Tonga, Mex = Mexico, J = Java, Al = Aleutians, and G = Guatemala) around the world (after Plank and Langmuir, 1993). Open circles = three different sediment flux estimates for the Marianas, based on the three ODP Sites drilled during Leg 129 (800-802) (Plank and Langmuir, 1998). Although there are variations from site to site, the average sediment input to the Marianas is fairly well constrained (+/- 20%). Note Izu volcanics are lower in Ba/Na than Marianas volcanics by a factor of two. Drilling at Site BON-8A will help to test if the low Ba/Na of the Izu volcanics is related to a lower Ba sediment flux. Shown for reference are the average Mariana Ba sediment flux and the flux for a 600-m section of chert (with 125 ppm Ba, similar to the upper radiolarites in Hole 801C; Karl, et al., 1992).
Figure 4. Estimates of H2O input and output fluxes for the Marianas subduction zone. Height of bar gives the flux for each parameter (scale on the left); bars are placed side-by-side to show competing estimates (as for Site 801 vs. Site 800 sediment) and are stacked to show cumulative input (on the right) and output (on the left). Shaded bars represent "continental" fluxes; unshaded bars are pristine igneous fluxes. Note that continental inputs and outputs may be very closely balanced; however, the balance depends critically on the real alteration fluxes for Hole 801C, which can only be constrained by further drilling. Cretaceous overprint given for both the East Mariana Basin (EMB) and Pigafetta Basins (PB); lines show fluxes resulting from different layer thickness (100, 250, 400 m).
Figure 5. Theoretical
Mesozoic paleolatitude histories of ODP Sites 801 and BON-8A
based on the combined polar wander paths for the Pacific plate of
Sager and Pringle (1988) for 60-100 Ma and Larson and Sager
(1992) for 100-155 Ma. Great circle distances are measured at
5-Ma intervals on the combined polar wander path to the
present-day site locations and then converted to paleolatitudes
to construct these histories. The paleolatitudes of 155-165 Ma
for ODP Site 801 are based on measured remanent inclinations in
Jurassic core samples from that site.
Figure 6. Location of Leg 185 drill site BON-8A and alternate BON-9.
Figure 7. Portion of seismic Line 39 from Conrad 2005 crossing the Izu-Bonin Trench. These multichannel data have not been reprocessed. Note location of Leg 185 drill site BON-8A and alternate BON-9 on top of fault blocks.
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