The Izu-Bonin-Mariana (IBM) arc system extends for ~2800 km from near Tokyo, Japan, to south of Guam, Mariana Islands (Fig. F1), where the Mesozoic Pacific plate is being subducted west-northwestward beneath the West Philippine plate. The Mariana forearc region preserves a record of extensive vertical movements resulting from seamount collision and fracturing associated with the arc configuration and plate motions over time (Fryer, Pearce, Stokking, et al., 1992). The forearc basement of the IBM system (Fig. F1) was formed after the initiation of "infant" arc volcanism and is a consequence of either the trapping of old, most probably Philippine Sea oceanic crust or by intra-oceanic island arc rifting and volcanism (DeBari et al., 1999; Stern et al., 2004). Initial subduction began in the early Eocene almost simultaneously for the Mariana and the Izu-Bonin segments of the IBM. This is evidenced by the eruption of boninite and arc-tholeiite lavas on Chichi Jima and Bonin Islands (Bloomer, 1983; Stern et al., 2004).
Currently, the absolute velocities of subduction along the Mariana margin are ~40 mm/yr in a northwestward direction (Stern et al., 2004, and references therein). The Pacific plate descends beneath the West Philippine plate at an angle of ~20° to depths of 60 km, whereas, at depths >100 km it dramatically steepens (to near-vertical descent in the Marianas) (Fryer, Pearce, Stokking, et al., 1992). Based on evidence from multichannel seismic reflection data, Mrozowski et al. (1981) (in the Marianas) and Horine et al. (1990) (in the Izu-Bonin) proposed that only minor sediment accretion has occurred along the entire convergent margin. The lithology of the dredged samples from the slopes of the Mariana Trench (e.g., island arc tholeiites and boninites) suggests that almost no sediment or oceanic crustal accretion occurs along the Mariana margin as well (Bloomer, 1983; Fryer, Pearce, Stokking, et al., 1992).
Ocean Drilling Program (ODP) Leg 125 Sites 778 and 779 are located on the flanks of Conical Seamount (Fig. F1) (detailed descriptions can be found in Fryer, Pearce, Stokking, et al., 1992, and in Fryer et al., 1999). Site 1200 is situated on the summit knoll of South Chamorro Seamount (Fig. F1), known as a site combining active blueschist mud volcanism and associated megafaunal assemblages (Fryer and Mottl, 1997; Salisbury, Shinohara, Richter, et al., 2002). The site is ~90 km west of the Mariana Trench axis, and based on the recent subduction rate and dip of the Pacific plate underneath the Philippine plate, the part of the Pacific crust that lies directly underneath the seamount today was located at the trench ~2.5 m.y. ago (95 km of Pacific crust been subducted to a depth of 30 km at the current ~40 mm/yr subduction rate).
Several cores were drilled on the summit and flanks of South Chamorro Seamount. Hole 1200A was cored with the rotary core barrel (RCB) to a depth of 147.2 meters below seafloor (mbsf), and serpentinized ultramafic clasts with minor amounts of the host serpentinite matrix were recovered. Holes, 1200D, 1200E, and 1200F were cored using the advanced piston corer/extended core barrel (APC/XCB) to a maximum depth of 54.4 mbsf. The recovered soft serpentinite matrix contains metamorphic rock clasts, including blueschists (Fryer et al., 1999). Based on the presence of epidote and Na amphiboles (Gharib et al., 2002) and on similarities in the mineral paragenesis and mineral chemistry of the metabasalts studied by Maekawa et al. (1992, 1993), Gharib et al. (2002) characterized Leg 195 metabasic samples as recording low temperatures (<300°C) and high pressures.