The two sites drilled during this leg are in Mesozoic crust, in the oldest part of the Pacific Ocean Basin and in extreme water depths, which provided a challenge in terms of drilling technology. These sites are the most recent part of an unfolding drama of drilling by DSDP and ODP in the west Pacific abyssal plains over the past three decades.
It was apparent in 1968 after DSDP Leg 3 that the deep ocean basins were formed by seafloor spreading and, thus, were very young relative to the age of the Earth. The same evidence from rifted continental margins that led Wegner and DuToit to propose continental drift then could be used to infer that the Atlantic and Indian Ocean Basins were no older than ~200 m.y. and probably somewhat younger. However, no such clues applied to the Pacific basin, because it is geologically isolated from the surrounding continents by subduction zones. Thus, in the late 1960s it seemed possible that the world's oldest deep ocean rocks lay somewhere in the western Pacific more than 10,000 km away from the nearest spreading ridge.
DSDP Legs 6 and 7 in 1969 were the first to search the western Pacific for the Earth's oldest oceanic crust and sediments. The search ultimately took 20 yr and 10 legs of DSDP/ODP (Legs 6, 7, 17, 20, 32, 33, 60, 61, 89, and 129) to achieve the final goal. Many people were involved; the most persistent members of the "Old Pacific Club" include B.C. Heezen, E.L. Winterer, S.O. Schlanger, R. Moberly, I. Premoli Silva, W. Sliter, D. Bukry, R.G. Douglas, and H.P. Foreman. During the early legs, drilling sites were targeted with single-channel seismic records characterized by acoustically opaque chert layers that obscured the underlying volcanic basement. Often the coring was frustrated by these impenetrable cherts, as well as by volcaniclastic sediments and basalts of Cretaceous age. To those who went out repeatedly and came back with more questions than answers, what had started as an oceanographic exercise turned into an ongoing adventure.
Leg 129 brought the JOIDES Resolution, with improved station keeping and heave compensation that proved capable of penetrating the cherts, where the Glomar Challenger would have failed. Also, preparations for Leg 129 led by Yves Lancelot and Roger Larson included four multichannel seismic expeditions to the area searching for seismic "windows" through the Cretaceous volcaniclastic sediments and solid basalts. This combination of improved science and technology was finally successful 10 yr ago, in 1989, at Site 801 in the Pigafetta Basin where Jurassic sediments of Bajocian-Bathonian age were discovered overlying ~170-Ma oceanic crust (Lancelot, Larson, et al., 1990). If older material exists in the area, tectonic reconstructions suggest it would not exceed that at Site 801 by more than 10 m.y. By comparison, the next oldest deep-ocean sites are ~5-15 m.y. younger: Site 534 near the continental margin of North Carolina in the North Atlantic and Site 765 in the Argo Abyssal Plain, in the Indian Ocean (Table 1). Thus, the original suggestion that the Earth's oldest deep-ocean deposits lie in the western Pacific is correct but, coincidently, not by much.
Just before Leg 129 in the Pacific Ocean, Plank and Ludden (1992) completed the first attempt at quantifying the global geochemical budget in an ODP Leg 123 drill core. Drilling at Site 765 penetrated ~1000 m of sediment, derived from the northwestern Australian margin, and ~250 m into the basement. The main objective was to characterize the crust subducting into the Sunda Arc of Indonesia. The idea of characterizing the inputs to subduction zones arose from a proposition by J. Natland and C. Langmuir in 1987-1988. The idea was extensively debated in the ODP Planning Committee and the Indian Ocean and West Pacific Ocean Regional Panels. Despite the fact that Leg 123 had been successful, the idea took several years to root itself firmly within the panel structure as a viable scientific approach. One remark was particularly pointed: "How can you learn something about milk by studying grass when you don't know anything about cows?"
The "cow model" is in fact an excellent way to convey the competing models for subduction recycling studies. The grass control model is that the same breed of cow eating different flavors of grass will produce different flavors of milk (Fig. 3A). The cow control model emphasizes the cow over the grass‹different breeds will produce different flavors of milk, even if they eat the same kind of grass (Fig. 3B). In the context of subduction recycling studies, the cow is the subduction factory, the grass is the oceanic crust and sediment being subducted, and the milk is the volcanic output at the arc. The grass control model predicts that the "flavor" of the subducted input (carbonate-rich pelagic or volcaniclastic sediments) has a strong effect on the flavor of the volcanic output despite slight differences in subduction style from one margin to the next. The cow control model would predict that given similar subducted input, the different characteristics of the subduction zone (dip of the subducted plate, thermal structure, convergence rate) will lead to differences in the volcanic output. Thus, the central question is whether the grass or the cow is the dominant control on the flavor of the milk‹whether the subducted sediments or the physics of the subduction zone are the dominant control on the volume and composition of the volcanic output. Subduction factory studies need excellent control on the subducted input before we can answer this question.
Drilling into the serpentinite mounds and the ash record in the Izu Bonin forearc (Leg 126; Leg 125; Fryer, 1992) recorded the magmatic output of the arc and the early dewatering of the subducting plate in the Izu-Bonin arc. Leg 170 in the Costa Rica accretionary prism (Kimura, Silver, et al., 1997) focused on problems of sediment accretion and fluid evolution in an accretionary prism and was a continuation of initiatives over the past 10 yr by ODP in active margin accretionary assemblages (e.g., Barbados, Nankai, and Cascadia)
Thus, ODP has come to embrace the idea of using drilling to understand geochemical mass balance. Hopefully, Leg 185 will be the first of several legs dedicated to this problem in arc systems of different tectonic regime and sedimentary input. The U.S. MARGINS project and the international Geochemical Earth Reference Model (GERM) have adopted this approach as an essential part of their strategy.

To 185 Izu-Mariana Arc

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