OCEAN DRILLING PROGRAM LEG 185 SCIENTIFIC PROSPECTUS IZU-MARIANA MARGIN Dr. Terry Plank Co-Chief Scientist Department of Geology 120 Lindely Hall University of Kansas Lawrence, KS 66045 U.S.A. Dr. John Ludden Co-Chief Scientist Centre de Recherches Petrographiques et Geochimiques (CNRS) Nancy, 54501 France Dr. Carlota Escutia Staff Scientist Ocean Drilling Program Texas A&M University Research Park 1000 Discovery Drive College Station, TX 77845-9547 U.S.A. ___________________ __________________ Jack Baldauf Carlota Escutia Deputy Director Leg Project Manager of Science Operations Science Services ODP/TAMU ODP/TAMU November 1998 Material in this publication may be copied without restraint for library, abstract service, educational, or personal research purposes; however, republication of any portion requires the written consent of the Director, Ocean Drilling Program, Texas A&M University Research Park, 1000 Discovery Drive, College Station, TX 77845-9547, U.S.A., as well as appropriate acknowledgment of this source. Scientific Prospectus No. 85 First Printing 1998 Distribution D I S C L A I M E R This publication was prepared by the Ocean Drilling Program, Texas A&M University, as an account of work performed under the international Ocean Drilling Program, which is managed by Joint Oceanographic Institutions, Inc., under contract with the National Science Foundation. Funding for the program is provided by the following agencies: Australia/Canada/Chinese Taipei/Korea Consortium for Ocean Drilling Deutsche Forschungsgemeinschaft (Federal Republic of Germany) Institut Francais de Recherche pour l'Exploitation de la Mer (France) Ocean Research Institute of the University of Tokyo (Japan) National Science Foundation (United States) Natural Environment Research Council (United Kingdom) European Science Foundation Consortium for the Ocean Drilling Program (Belgium, Denmark, Finland, Iceland, Italy, The Netherlands, Norway, Portugal, Spain, Sweden, and Switzerland) People's Republic of China Any opinions, findings, and conclusions or recommendations expressed in this publication are those of the author(s) and do not necessarily reflect the views of the National Science Foundation, the participating agencies, Joint Oceanographic Institutions, Inc., Texas A&M University, or Texas A&M Research Foundation. This Scientific Prospectus is based on precruise JOIDES panel discussions and scientific input from the designated Co-chief Scientists on behalf of the drilling proponents. The operational plans within reflect JOIDES Planning Committee and thematic panel priorities. During the course of the cruise, actual site operations may indicate to the Co-chief Scientists and the Operations Manager that it would be scientifically or operationally advantageous to amend the plan detailed in this prospectus. It should be understood that any proposed changes to the plan presented here are contingent upon approval of the Director of the Ocean Drilling Program in consultation with the Science and Operations Committees (successors to the Planning Committee) and the Pollution Prevention and Safety Panel. Technical Editor: Karen K. Graber ABSTRACT Leg 185 will drill two deep-water (5674 and 6000 m) sites, one, an existing Ocean Drilling Program hole (Hole 801C) seaward of the Mariana Trench, and the other, a new site (Site BON- 8A), east of the Izu-Bonin Arc. The primary objectives are to investigate sediment subduction along this arc-trench system and to characterize the chemical fluxes during alteration of the oceanic crust. Despite the simple setting and shared subducting plate, there are still clear geochemical differences between the Marianas and Izu volcanic arc systems. Drilling the crustal inputs (sediments and basalts) will help test whether geochemical differences in the volcanics are derived from contrasts in the crustal inputs to the two trenches. Previous drilling has already provided sections through the sedimentary layer approaching the Mariana Trench. Coring during Leg 185 will provide samples of the remaining input fluxes to the subduction zones: the altered basaltic crust at Hole 801C and the sediments and altered basaltic crust at Site BON-8A. The specific drilling plan is to deepen Hole 801C an additional 250 m (to ~400 m total basement depth) to penetrate the upper oxidative alteration zone, and to core the entire 470 m sedimentary section at BON-8A, along with as much basement as possible (to a maximum of 430 m) in the time allowed. One outcome of Leg 185 will be the best existing mass balance of input and output fluxes for several key tracers (H2O, CO2, U, and Pb) cycled through the subduction factory. These data will permit a better understanding of the recycling of oceanic crust in the Earth's mantle, the formation of continental crust in arc systems and the role of recycled oceanic crust in the generation of mantle plume magmas. With Hole 801C, which penetrates the world's oldest in situ oceanic crust (~165 Ma), the science party will also provide the first reference site for the structure and composition of old, fast-spreading oceanic crust to compare with other crustal end-members. Based on the seismic structure of the East Pacific Rise crust formed at comparable spreading rates (160 mm/yr full rate), the upper 450 m at Hole 801C may include the entire extrusive section, or layer 2A, of the oceanic crust. Thus, it is possible that Leg 185 will provide the second example of in situ oceanic crust into the sheeted dike complex. Leg 185 has been targeted to conduct contamination testing and development of a standard sampling procedure for deep biosphere research. The objective of the contamination test is to determine the amount of mixing created by the coring process by introducing a tracer (latex beads) at the mouth of the core barrel as the core is cut. INTRODUCTION The State of Crust-Mantle Recycling Science Subduction zones are the modern sites of continental crust formation and destruction. Continental growth occurs today by accretion of island arcs and magmatic additions to the crust at arcs. Crustal destruction occurs by subduction of crustal material (seawater components, marine sediments, and basaltic crust) at oceanic trenches. Thus, the geochemical and physical evolution of the Earth's crust and mantle depends in large part on the fate of subducted material at convergent margins (Armstrong, 1968; Karig and Kay, 1981). The crustal material on the downgoing plate is recycled to various levels in the subduction zone. Some of it returns to the shallow crust during forearc accretion and dewatering, some returns to the arc crust via volcanism, some is mixed back into the deep mantle, and some may even re-emerge in mantle plumes. Despite strong evidence for these different types of crustal recycling (from seismic imaging, drilling, and isotopic tracers), and despite the important ramifications for mantle evolution, continent formation, and geochemical cycles, few studies have focused on quantifying crustal fluxes through subduction zones. Von Huene and Scholl (1991) calculated a large global flux of subducted sediment-as high as modern crustal growth rates. Their calculations, however, represent an upper limit on sediment fluxes into the mantle because some material is cycled back to the upper plate. It is a common misconception that the sediment contribution to the volcanic arc is trivial (~1%), based on isotopic mixing arguments, which constrain only the proportion of sediment to the mantle and not the proportion of the total subducting budget that contributes to the arc. To calculate the latter, estimates of input fluxes (sediment) and output fluxes (volcanic) are required. Earlier flux balances by Karig and Kay (1981) for the Marianas suggested that 10% of the sedimentary section contributes elements to the arc, whereas more recent calculations (Plank and Langmuir, 1993; Zheng et al., 1994) give values of 30%-50% globally. These estimates, however, have large uncertainties because none of them take into account all of the crustal outputs. Plank and Langmuir (1993) do not consider underplating or erosion at the forearc; von Huene and Scholl (1991) do not consider recycling to the arc; and neither study considers the mobile components dissolved in fluids that are lost to the forearc. It is entirely possible that the 50%-70% recycling efficiency to the deep mantle suggested by Plank and Langmuir (1993) could be reduced to 0% for many important element tracers, given the other shallower outputs that have yet to be taken into account. Clearly, the difference between 70% and 0% recycling would lead to vastly different outcomes for mantle evolution and structure. The Role of Drilling The recycling equation involves many variables-aging of the oceanic lithosphere, flow of material through accretionary prisms, and fluid circulation at active margins-that are linked across a convergent margin and can be explored in combination through a drilling transect across a margin (Fig. 1; Scholl et al., 1996). The incoming section of sediment and altered oceanic crust can be drilled near trenches. The extent of sediment accretion, underplating, erosion, and subduction can be determined by combining forearc drilling with seismic reflection images and material balance considerations. The fluids lost from the downgoing plate can be sampled by drilling into fault zones and serpentine seamounts. Output to the arc can be determined from the chemical composition of the volcanics and from arc growth rates. The flux of crustal material that is eventually recycled to the mantle is then the input minus the output. Because the bulk sediment is not conserved through the entire subduction process, chemical tracers must be used to track the sediment and deduce the recycling processes. Thus, the problem is impossible to solve by remote means and is completely dependent on drilling to recover material for chemical analysis. Determination of crustal fluxes into the mantle is not yet possible largely because the various parts of the problem are being investigated at different margins. Although this is a good way to understand individual processes, it is not a good way to determine the behavior of the entire system. The approach that we emphasize here is to try to solve the recycling equation at a few margins where significant progress can be made by a focused drilling effort. Recent studies have focused on tracers of the subduction process (fluids and chemical components from the subducted sediment and basalt). Subducted material has been successfully identified in forearc fluids and serpentinite mud volcanoes (Mottl, 1992; Fryer, 1992), in arc volcanoes (Tera et al., 1986; Plank and Langmuir, 1993), and in backarc volcanism (Stolper and Newman, 1994). Laboratory experiments have helped in understanding the dehydration and/or melting processes in the downgoing slab that send material back to the overlying crust. What is now needed is to move beyond the tracer approach to begin to try to mass balance the recycling process. How much subducted material returns to the arc lithosphere vs. how much is mixed back into the mantle? How is the subducted material chemically modified during this process? These are the questions behind Crustal Mass Balances, a theme that appears in the Ocean Drilling Program (ODP) Long Range Plan, the MARGINs Initiative Science Plan, and a JOI/USSAC Workshop on Crustal Recycling (Scholl et al., 1996). BACKGROUND Crustal Recycling at the Izu-Mariana Margin There are several reasons why the Izu-Mariana margin (Fig. 2) is ideal for studying subduction recycling. Significant progress has already been made on many parts of the flux equation. Forearc sites of fluid outflow (serpentine seamounts) 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). These volcanic arcs and backarcs are among the best characterized of 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), and the problem is simplified here because the upper crust is oceanic, so upper crustal contamination is minimized. 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 still clear geochemical differences between the Mariana and Izu Arcs (e.g., in Pb isotopes and elemental enrichments; Figs. 3A and 3B). 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, particularly the (altered) basement sections. Existing Crustal Inventory at the Izu-Mariana Margin Of the eight sites drilled in the seafloor immediately east of the Izu-Bonin Trench (not including the Shatsky Rise sites), only one penetrated basement, Site 197 (Leg 20; Fig. 2). No sediments, however, and only 1 m of basalt were recovered. Recovery was poor at most of the other sites: less than 25 m of sediment was recovered at all sites drilled during Leg 20 (Sites 194-198), and the recovered material was dominantly pelagic clays above resistant cherts. Although more was recovered at Sites 52 (45 m) and 578 (165 m), drilled during Deep Sea Drilling Project (DSDP) Legs 6 and 86, respectively, coring again was halted by chert layers, leaving hundreds of meters of unsampled sediment below. Prior drilling has provided us with many samples of the upper 50 to 150 m of the pelagic clay and ash unit, but almost nothing of the units below, including the oceanic crust. ¥ The main goal of Izu-Bonin coring is to sample all sedimentary units and the upper alteration zone (~300 m) of the oceanic crust below. The success rate in coring sediments and basement to the south, seaward of the Marianas, was just about as poor as that experienced to the north, until Leg 129, when three complete sections (ODP Sites 800-802) were sampled through the cherts to "basement." During Leg 129, sedimentary units were well sampled at Sites 800-802, but normal oceanic crust was sampled at only one site, Hole 801C. At the other two sites, off-axis Cretaceous sills and flows were encountered as "basement." The crustal inventory at the Mariana Trench includes (from top to bottom): pelagic clay, chert, and radiolarite (+ chalk), Cretaceous volcaniclastic turbidites, radiolarite, off-axis Cretaceous intrusives and extrusives, and Jurassic oceanic crust. Leg 129 coring and prior DSDP efforts provided adequate samples of the sedimentary units being subducted at the Mariana Trench (providing estimates of chemical fluxes with better than 30% precision for most elements [Plank and Langmuir, 1998]). However, the only sample of Jurassic oceanic crust, which must comprise the largest mass of crustal material being subducted at the Mariana Trench, comes from the lowest 63 m in Hole 801C (of the ~135 m total penetration into basement in Holes 801B and C, only the lower 63 m of drilling recovered 43 m of normal, Jurassic tholeiitic oceanic crust). ¥ Thus, the main goal of Mariana coring is to provide a more complete sampling of the upper alteration zones in the Jurassic seafloor, which constitutes a significant part of the budget for many geochemical tracers of the subduction process, including H2O, CO2, Rb, and U. Existing Crustal Mass Balance for the Marianas With information in hand, it is possible to calculate many of the input and output fluxes for a few chemical components through the Marianas subduction zone. We consider here a preliminary flux balance for H2O (Fig. 4). The sediment input is fairly well constrained by previous coring during Leg 129 (Sites 800-802), and by the extensive chemical analyses of the recovered material (Karl et al., 1992; Karpoff, 1992; Lees et al., 1992; France-Lanord et al., 1992) as well as the geochemical logs for the different holes (Pratson et al., 1992; Fisher et al., 1992). As a result, H2O flux estimates for sediments from Sites 800 and 801 are quite consistent with one another (within 15%). The other crustal input flux is the subducting oceanic crust, which is very poorly constrained because of a lack of significant penetration into the mid-ocean ridge (MOR) basement in this area (63 m in Hole 801C). The geochemical budget of elements in the oceanic crust has two sources: primary igneous and secondary alteration. The primary igneous composition is fairly well constrained, based on extensive sampling of modern mid-ocean ridge basalt (MORB) and on the relatively unaltered samples recovered from Hole 801C. The secondary alteration fluxes are virtually unknown, however, and can only be estimated from various other regions, compilations, and assumptions: the average global H2O flux in Peacock (1990), alteration studies of DSDP Hole 504B (Leg 69; Alt et al., 1986) and DSDP Sites 417/418 (Legs 51, 52, and 53; Staudigel et al., 1995), and assuming 10% interpillow material in Hole 801C (Castillo et al., 1992b). These estimates show that the alteration fluxes may be large, but are poorly known. The applicability of existing data (obtained for slow-spreading old crust at Sites 417 and 418 and medium-spreading young crust in Hole 504B) to the crust seaward of the Mariana Trench (old-fast spreading) remains to be seen and is, in fact, a major goal of Leg 185. Unique to the seafloor seaward of the Mariana Trench is an overprint of Cretaceous basement flows and sills. There are two sources of uncertainty in estimating this flux: the thickness of the Cretaceous "basement" layer and its chemical composition. Calculations based on sonobuoy velocities, reflection data, and drilling results from Leg 129 indicate a 100- to 400-m-thick layer of massive Cretaceous basalt, and possibly some interbedded sediments, overlying Jurassic oceanic crust (Abrams et al., 1993). Although this is not the case for water, the total flux of many elements depends critically on whether this Cretaceous basalt is alkalic (as for Site 800 basalts and various seamounts of the Pigafetta Basin [PB]) or tholeiitic (as for Site 802 basalts of the East Mariana Basin [EMB]). Although plate trajectories (Fig. 2) indicate that the seafloor subducting beneath the Marianas is largely the tholeiitic EMB, we consider both EMB (tholeiitic) and PB (alkalic) type basalts in estimating the water flux into the subduction zone (Fig. 4). Both estimates yield small water input fluxes relative to the sediment and altered Jurassic MORB. The first measurable outputs from the subduction zone are the forearc fluids, which have shown to be freshened and from a subducted source (Mottl, 1992). It is currently difficult to estimate rates based on the fluid flow itself. We, therefore, use a model based on the total (maximum) water generated during clay mineral breakdown in the subducted sediments (Plank et al., 1994). This calculation is model dependent, but further study of the nature of these fluids will help to identify the actual dehydration reactions that are occurring with depth during subduction. Figure 4 shows that the water outputs to the forearc may be a significant fraction of the sediment input. Magmatic outputs to the volcanic arc and backarc are determined from the chemical composition of arc and backarc basalts (assuming 5.7 and 1.25 wt% H2O above MORB background, respectively; Stolper and Newman, 1994) and from magmatic addition rates. The magmatic arc water flux is the largest of the crustal outputs from the subduction zone. These preliminary calculations provide some initial insights into the flux balance in subduction zones and reveal where the major uncertainties lie. If we ignore the igneous MORB and Cretaceous basalt contributions as no net gain from the mantle perspective, then the continental water inputs and outputs appear to be remarkably closely balanced across the subduction zone. The balance hinges critically, however, on the magnitude of the basement alteration fluxes. Present estimates are poor, and the actual flux balance could still go either way. ¥Drilling through the upper oceanic crust subducting beneath the Marianas can dramatically improve one key flux in the mass balance equation-the alteration flux. Existing Crustal Mass Balance for Izu-Bonin Because neither the sediments nor the altered oceanic crust seaward of the Izu Trench have been sampled to any significant extent, our ability to determine mass balance is much more limited here. However, we can make some predictions about the crustal inputs to the Izu Trench based on the Izu volcanic output. Izu basalts record almost half the K or Ba enrichment of Marianas basalts (Fig. 3B), whereas sediment mass fluxes into the two trenches are roughly comparable, or even greater, at Izu (600 m of sediment into Izu vs. 400 m into the Mariana Trench). Thus, Izu sediments should be much poorer in K and Ba than Marianas sediments. One way to explain this would be to replace the volcaniclastic sections in the Marianas sediment columns with cherts, which are barren of K and may be very poor in Ba (Karl et al., 1992). This makes some sense given what we know about the history of sedimentation in this part of the ocean-the Cretaceous overprint east of the Marianas may be absent to the north, east of Izu (Fig. 2), where the seafloor spent a longer time on average beneath equatorial zones of high biologic productivity (Fig. 5), possibly leading to greater sections of chert and/or carbonates. Drilling the seafloor east of Izu can directly test these predictions. Sediment layers are fairly uniform throughout the region, reflecting uniform pelagic sedimentation. Thus, a single hole should give us a fairly representative sampling of sediments being subducted at the Izu Trench. If the extra thickness of sediments off Izu is not dominantly barren cherts, this means that much of this sediment does not contribute to arc magmas, either because it is underplated (we can see that it is not accreted), or because it fails to dewater or melt beneath the arc. Thus, by drilling and sampling the crustal inputs, we can learn more about the process of sediment subduction and recycling. The geochemical differences between the Marianas and Izu arc volcanics could also be related to the chemical composition of the altered oceanic crust. The K or water contents in the altered basaltic sections may vary regionally, possibly explaining regional variations in K and the extent of melting reflected in Marianas and Izu lavas. This can be tested by drilling the upper oxidative alteration zone, which contains most of the alkali budget in the oceanic crust, in both regions. Finally, some of the differences between the Izu and Marianas lavas may have nothing to do with subducted input and may be explained by more enriched mantle beneath the Marianas. Evidence for enriched mantle in the region comes from enriched shoshonites at the adjacent volcanic arc (Bloomer et al., 1989; Lin et al., 1989). Although drilling cannot test whether enriched mantle exists beneath the Marianas, it can make invoking it unnecessary. SCIENTIFIC OBJECTIVES Previous drilling has already provided many parts of the crustal flux equation at the Izu and Mariana Margins and provides a strong rationale for continuing the effort to mass balance fluxes across the subduction zones. The missing part of the flux equation is largely the input: (1) both the incoming sediment and basaltic sections approaching the Izu-Bonin Trench, and (2) the altered oceanic crust seaward of the Mariana Trench. In order to provide this critical information on the crustal inputs to the subduction zone, drilling is planned at two sites: one seaward of the Mariana Trench (ODP Hole 801C), and one seaward of the Izu Trench (proposed Site BON-8A). Hole 801C The primary motivation for returning to ODP Hole 801C, seaward of the Mariana Trench (Fig. 2), is to sample the upper oxidative zone of alteration of this oldest in situ oceanic crust. Previous drilling during Leg 129 only penetrated 63 m into "normal" Jurassic basement. Based on Hole 504B and other basement sites with sufficient penetration, the upper oxidative zone of alteration, which contains the lion's share of some element budgets (e.g., K, B, etc.), lies in the upper 200- 300 m of the basaltic crust (Alt et al., 1986; Staudigel et al., 1995). The transition from volcanics to sheeted dikes may not lie much deeper: 500-600 m at Hole 504B (Detrick et al., 1994); 450 m to Layer 2b (Carbotte et al., 1997); and only a few 100 m at Hess Deep (Francheteau et al., 1992). We propose to deepen Hole 801C an additional 250 m (~400 m total basement penetration) into basement at Hole 801C to accomplish the following scientific objectives to 1.characterize the geochemical fluxes and geophysical aging attending the upper oxidative alteration of the oceanic crust in Hole 801C (as discussed above); 2.compare igneous compositions, structure, and alteration with other drilled sections of in situ oceanic crust, in particular 504B, contrasting a young site in Pacific crust with the oldest site in Pacific crust; 3.help constrain general models for seafloor alteration that depend on spreading rate and age. Hole 801C is in the world's oldest drilled oceanic crust, which is at 165 Ma and was formed at a fast- spreading ridge at ~160 mm/yr full-rate, therefore it embodies several end-member characteristics; and 4.test models for the Jurassic Magnetic "Quiet" Zone (JQZ). Hole 801C is located in an area of very low amplitude magnetic anomalies, usually called the Jurassic Magnetic "Quiet" Zone. The JQZ has been suggested to result from (1) oceanic crust of a single polarity with small anomalies due to intensity fluctuations, (2) oceanic crust with magnetic reversals so numerous as to "cancel each other out" when measured at the sea surface, or (3) oceanic crust with a more normal frequency of magnetic reversals acquired when the dipole field intensity was anomalously low. Deepening Hole 801C would allow testing of the above hypotheses, and in particular, of the third hypothesis of magnetic reversals during a period of anomalously low field intensity, if fresh unaltered volcanic glass could be obtained. Such material can yield reliable paleointensity information (Pick and Tauxe, 1993) on the very fine, single-domain grains of the titanium-free magnetite within the volcanic glass. Site BON-8A The primary motivation for proposed Site BON-8A, a site ~60 km seaward of the Izu Trench (Fig. 6), is to provide the first complete section of sediment and a significant section of altered oceanic crust that enters this subduction zone. Previous drilling failed to penetrate successfully through resistant cherts, so most of the sediment column is unsampled. Only 1 m of basalt has been recovered from basement in this vast area (at DSDP Site 197). We propose to drill and core the entire sedimentary sequence (470 m) at Site BON-8A, and as far into the upper oxidative alteration zone of the basaltic basement as possible to a maximum basement penetration of 430 m. The scientific objectives are to 1.provide estimates of the sediment inputs and altered basalt inputs (geochemical fluxes) into the Izu subduction zone (as discussed above); 2. contrast crustal budgets here with those for the Marianas, to test whether along-strike differences in the volcanics can be explained by along-strike variations in the crustal inputs (as discussed above); 3.compare basement alteration characteristics with those in Hole 801C (also in old Pacific crust); 4.provide constraints on the Early Cretaceous paleomagnetic time scale. Site BON-8A is approximately on magnetic anomaly M12 (Nakanishi et al., 1988). Its basement age should be about 135 Ma and should correspond to the Valanginian Stage of the Early Cretaceous according to recent time scale calibrations (Harland et al., 1990; Gradstein et al., 1994; Channell et al., 1995). However, those age estimates are poorly known and can be tested by drilling at Site BON-8A. Specifically, a reasonably precise date on M12 at Site BON-8A could test the proposed new time scale of Channell et al. (1995); and 5.provide constraints on mid-Cretaceous carbonate compensation depth (CCD) and equatorial circulation fluctuations. Based on its theoretical Cretaceous paleolatitude history, Site BON-8A may have formed at ~5¡S, drifted south to 10¡S in its early history and then gradually drifted north, crossing the paleoequator as the Pacific plate accelerated its northward motion about 85-90 Ma. A site such as BON-8A with an Early Cretaceous basement age (~135 Ma), an equatorial paleolatitude history during the mid-Cretaceous, and a predictable subsidence history for the Cretaceous is ideal for testing proposed CCD variations (Theirstein, 1979; Arthur et al., 1985). In addition, Erba (1992), following Roth (1981), has shown that certain species of nannoplankton can be characterized as "high fertility indices" and used as approximate indicators for the paleoequatorial upwelling zone. Using these nannoflora, potential fluctuations in the mid- Cretaceous equatorial circulation system could be studied at Site BON-8A when the site was nearly stationary near the paleoequator (especially from 115-95 Ma). Deep Biosphere During Leg 185, scientists will conduct contamination tests and develop a standard sampling procedure for deep biosphere research. The objective of the contamination test is to determine the amount of mixing created by the coring process by introducing a tracer (latex beads) at the mouth of the core barrel as the core is cut. Some limited shipboard microbiological analyses will be conducted during Leg 185 (i.e., direct bacterial counts using the existing epifluorescence microscope). Most of the samples, however, will be adequately stored for shore-based analysis. Diamond Core Barrel (DCB) Leg 185 has a total of two days allocated to conduct tests of the diamond core barrel (DCB) at both Sites 801C and Bon-8A. Comparison of DCB techniques, hardware, and results in varied lithologies has great value to longer-term ODP goals and the DCB development program. The greatest potential benefit of the DCB tests to Leg 185 is to optimize core recovery and quality. REGIONAL GEOLOGICAL SETTING OF PROPOSED SITES Hole 801C Although located almost 1000 km from the Mariana Trench, Hole 801C is the most promising site for penetrating Jurassic MORB in the region. Throughout much of the Pigafetta and East Mariana Basin (Fig. 2), "basement" consists of Cretaceous flows and sills that overlie the "normal" Jurassic crust. Because these Cretaceous units have already been sampled by Leg 129 drilling, the remaining goal is the MORB section. Hole 801C is the only location where Jurassic-aged material (165 Ma) was reached in a reasonable amount of drilling time, and that material should be essentially the same as what is now being subducted beneath the Marianas (also fast spreading and within 15 m.y. in age). It is necessary to penetrate several hundred meters into the upper oxidative layer of Jurassic basement to constrain that part of the crustal input equation, and that section is now available directly beneath the bottom of Hole 801C. Hole 801C was left clean with a reentry cone that is cased and cemented into basement, and is ready for more extensive basement drilling. Above basement, the 460 m sedimentary section is dominated by pelagic clays and cherts, with notable intervals of volcaniclastic material from the mid-Cretaceous, derived from the nearby Magellan Seamounts. Specifically, the sedimentary stratigraphy at Site 801 consists of a thin pelagic brown clay unit (64 m, Cenozoic-Campanian /Maestrichtian?) overlying 63 m of brown chert and porcellanite (Campanian-Cenomanian) overlying 192 m of volcaniclastic turbidites (Cenomanian-Albian) overlying 125 m of brown radiolarite (Valangian-Oxfordian), with a basal unit of alternating red radiolarite and claystone (19 m, Callovian). Further background on Site 801 can be found in Lancelot, Larson, et al. (1990). Site BON-8A Site BON-8A is ~60 km east of the Izu-Bonin Trench, where the plate surface is broken by normal faults as it bends into the subduction zone (Fig. 7). Avoiding some of this complexity, Site BON-8A is located on the top of a fault block in flat-lying sediments. Coring the complete sedimentary section as well as the altered oceanic crust at Site BON-8A will sample the main crustal components that enter the Bonin trench. Based on our assessment of regional variations in subducting sediments elsewhere (Plank and Langmuir, 1998), we feel confident that a single reference site will provide adequate constraints on the crustal inputs to the Izu-Bonin Trench. Even though the sedimentary stratigraphy will vary regionally, changes in unit thickness can be mapped more efficiently seismically than with multiple drill holes. Site BON-8A should provide us with samples of the largely pelagic sediments from the region. Of the ~470 m of sediment, we anticipate 170 m of pelagic clay and volcanic arc ash above 300 m of mid- to Early Cretaceous cherty porcellanites and chalks (Fig. 7). The basement should be Early Cretaceous MORB (135 Ma), with the upper 200-300 m of extrusives containing the oxidative alteration zone. DRILLING STRATEGY Hole 801C The objective at this site is to re-enter the cone at Hole 801C (Leg 129) and penetrate through the upper, oxidative alteration zone in basaltic basement, deepening Hole 801C at least another 250 m, to >850 mbsf or ~400 m sub-basement. A drill-string length of 6530 m will be deployed, which would be one of the longest drill strings that the JOIDES Resolution will have deployed, and drilling will be in basaltic rocks. Expected rates of penetration with the rotary core barrel (RCB) may be as low as 1.5 m/hr (as experienced during Leg 129) and trip times will be long. It should be possible with 20 days on site to achieve >250 m of penetration into basement, and conduct three logging runs. We will actively monitor the chemical composition of the cores on board the ship to determine the changes in the alteration zones. The most likely drilling problems to be encountered will be due to drilling materials (junk) left in the hole. In such an event some time will be spent fishing the junk, to continue the hole to >250 m depth into basement. A decision will be made on site as to the time to be dedicated to fishing and continued drilling, given the requirement for at least 15-20 days operations at Site BON-8A. If a new hole is needed for technological reasons, our first recourse will be to move a few 100 ft and wash to basement and start again. In the case where drilling problems are geological (an unfavorable formation) an alternate site has been designated (PIG-3B), ~75 km east of Site 801 (Fig. 2). Given the amount of time involved in washing down to the last depth of penetration at Hole 801C (~600 m) or starting a new hole at PIG-3B, it is likely that if such problems are encountered after the first week of operation at Hole 801C, further efforts will be shifted to Site BON-8A. Site BON-8A The objectives for Site BON-8A are to continuously core the sedimentary section (470 m) and the upper pillow alteration zone in the basement section (as deeply as possible). Drilling conditions and hole stability in the sedimentary section are predicted to be good at this site, but recovery will be moderate to poor in the anticipated chert-rich sediments. In order to optimize recovery, a combination of the advanced hydraulic piston/extended core barrel/motor-driven core barrel (APC/XCB/MDCB) corers will be used, ideally, to core the entire sedimentary section. Recovery within chert sequences, especially of soft, chalky sediments, was still a problem on Leg 129 because of the need to pump heavily during chert penetration to keep the hole clean. The dramatically increased recovery of hydrothermal sediments in the Trans-Atlantic Geotraverse (TAG) area on Leg 158 with the newly designed MDCB raises the encouraging possibility of similar enhanced recovery in chert/chalk sequences. Four logging runs will follow APC/XCB/MDCB drilling in the pilot hole. As stated above, if the drilling operation in Hole 801C fails, and there is insufficient time to achieve our stated objectives at this site, operations will be moved to Site BON-8A. Complete operations at this site involve coring of the sedimentary section, logging the sediments, installation of a reentry cone and casing and cementing into basement (470 m), and drilling with a combination of RCB, MDCB, and diamond coring system (DCS) to at least 200 m below the sediment/ basement interface. The basement will be logged following drilling operations. The entire operation involving the establishment, coring, and logging of a legacy hole will take 26-28 days. Depending on the time spent in Hole 801C, a series of scenarios are envisaged for coring the sediments and penetrating at least 50 m into basement, and logging the section at Site BON-8A. We stress the fact that it is essential that the sedimentary section at this site be logged to define the proportions of representative lithologies and compute the mass flux into the subduction zone. This operation could involve a minimum program, involving drilling a single hole using a single RCB bit, through the sediments and basement, dropping the bit, and logging through the hole through the drill-string (10-12) days. A more complete drilling operation (using APC/XCB/RCB/MDCB combinations) at this site would involve reentry of the hole by use of a Free-Fall Funnel (FFF). This would allow deeper coring into basement and complete logging of the section (15 to 22 days), depending on depth of penetration into basement and the number of logging operations. The logging operations will involve, either (1) a logging run after the sediment section is cored and a further run after basement is cored, thus creating maximum hole quality for logging the sediments, or (2) a single logging operation at the end of coring the sediments and basement. If hole quality is not good enough for logging, the possibility exists for drilling a separate hole using a tricone bit, followed by logging. In the case of loss of the hole at Site BON-8A (again this would most likely be because of drilling problems involving junk in the hole in the basement section), we propose to drill a hole adjacent to the lost hole. For this hole, we would wash down through the sedimentary section, install a second reentry cone, and drill as deep as possible within the time constraints; a minimum drilling operation as described above would be essential to the success of the project. In the case where the drilling problems are geological (an unfavorable formation) an alternate site has been designated (proposed Site BON-9), ~10 km east of BON-8A (Figs. 6, 7). LOGGING PLAN Recording downhole geochemical and physical properties data during Leg 185 is essential to filling recovery gaps in both sediment and basement sections, as well as enabling site-to-site comparisons of the geochemical signatures of the drilled sequences. Downhole Measurements in Hole 801C Downhole measurements were conducted in the upper 100 m of basement during Leg 144 to begin the characterization of typical old oceanic crust generated at a fast-spreading rate (Larson et al., 1993). The most surprising result from Leg 144 downhole measurements was the extremely high permeability measured below 501 mbsf in a hydrothermal alteration zone. This zone appears to act as an aquifer, an argument supported with the apparent bulk porosity profile. Below the hydrothermal zone and within the tholeiitic basalts, the logs begin to approximate more expected values for old oceanic crust. To further characterize the petrology, hydrogeology, structure, and physical properties of the old oceanic crust, the hole will be logged with the triple combo, Formation MicroScanner (FMS)/Sonic, and Geologic High-resolution Magnetometer Tool (GHMT) tool strings. The triple combo consists of the HNGS (Hostile Environment Natural Gamma Sonde), APS (Accelerator Porosity Sonde), HLDS (Hostile Environment Litho-Density Sonde) and DITE (Dual Induction Tool) with the Lamont Temperature Logging Tool (TLT) attached to the bottom of the string. The FMS/Sonic string will contain a standard four pad FMS tool combined with a DSI (Dipole Shear Sonic Imager) sonic tool. Lastly, to satisfy the time-scale objective (i.e., to determine the age of the basement), the magnetic susceptibility and total magnetic field measurements obtained by the GHMT will be deployed to provide a paleomagnetic reversal sequence of the overlying sediment.The anticipated logged interval will be from casing at 481 mbsf to the new total depth expected to be about 850 mbsf. In addition, if enough time is available, a packer experiment will be completed for the interval below the hydrothermal deposit drilled on Leg 129 at 501 mbsf and the bottom of the newly drilled basement section. This experiment will complement results on the permeability of the basement obtained on Leg 144 (Larson et al., 1993), and may provide important constraints on the alteration process and fluid flow in old oceanic crust. Downhole Measurements at Site BON-8A The oceanic crust subducted in the Izu-Bonin Trench has never been sampled or logged. Because determining the geochemical budgets in sediment and basement columns is central to the objectives of Leg 185, geochemical logging tool (GLT) will be extremely valuable. Geochemical logging on Leg 129 served as an excellent proxy for actual recovery of sediments similar to those expected at Site BON-8A (Fisher et al., 1992). To compare the sedimentary sequence and the upper oceanic crust at Site BON-8A with those in Hole 801C, the triple combo, FMS/Sonic, and GLT will be deployed. The GLT is typically combined with an AACT (Aluminum Activation Tool), which requires a Californium 252 source. Logging at Site BON-8A will occur in two stages to optimize data quality and logged depth. The first logging operation will be completed through the APC/XCB bottom-hole assembly (BHA) following the cessation of drilling near the sediment/basement contact. The second logging program will be completed through the RCB BHA after completion of the hole to basement. The standard tool-string deployment sequence is triple combo, FMS/Sonic, GLT, and GHMT. SAMPLING STRATEGY New sampling guidelines specify that a formal, leg-specific sampling strategy must be prepared by the Sample Allocation Committee (SAC: co-chiefs, staff scientist, and ODP curator onshore- curatorial representative on board ship) for each prospectus. Any modification of the sampling strategy during Leg 185 must be approved by the SAC following evaluation of sample requests by the shipboard party at each site. Nominal Sample Limit Leg 185 shipboard scientists may expect to obtain up to 100 samples of no more than 15 cm3 in size. Additional samples may be obtained upon written request onshore soon after the cores are sent to the ODP Repository. Depending on the penetration and recovery during Leg 185, the number of samples taken may be increased by the shipboard SAC. All sample requests must be made on the standard sample request form and approved by the SAC. Samples larger than 15 cm3 may be obtained with approval of the SAC, but will be considered as the equivalent of multiple samples in partial or complete increments of 15 cm3. Requests for large samples must be specified on the sample request form. Multiple Analysis of Samples Some redundancy of measurement is unavoidable, but minimizing redundancy of measurements among the shipboard party and identified shore-based collaborators will be a factor in evaluating sample requests. For Leg 185, we anticipate the preparation of samples for which splits will be analyzed in different laboratories. We stress the importance of having a coherent data set on the same powdered or solid samples. Shipboard Samples and Data Following core labeling, measuring nondestructive properties, and splitting, samples will be selected from core working halves by members of the Leg 185 shipboard party for routine measurement of physical and magnetic properties, bulk chemical analyses by X-ray fluorescence (XRF) and carbon-hydrogen-nitrogen-sulfur (CHNS) analyzer, and X-ray diffraction (XRD), as necessary. Polished thin sections will be prepared for identification of minerals, determination of mineral modes by point counting, and studies of texture and fabric. In most cases these samples will serve as the shore-based material for full analysis by multiple laboratories. Shipboard thin sections, selected from representative sections of the core and at some critical intervals, will remain the property of ODP. The thin-section chips from which the sections are made will be retained by ODP and should normally be thick enough to allow for the production of additional sections unless the sampling plan for a critical interval precludes this. Members of the Leg 185 shipboard party can request the production of a thin section from these thin-section chips for their personal use, at their own expense, and as part of their nominal 100-sample limit. Scientists must arrange for the prepaid manufacture of these thin sections with a third-party commercial service unless otherwise approved by the ODP core lab curator. The thin-section chip will then be sent directly to the commercial service and returned directly to ODP. Critical Intervals If critical intervals of unusual scientific interest are identified during the core description process, a sampling protocol will be established by the interested scientists and shipboard SAC. Small Samples Studies requiring only small-sample volumes (1 cm3 or less, e.g., for veins, fluid inclusions, etc.) may require more than 100 samples to characterize a long section of core. The SAC will review the appropriate sampling interval for such studies as the cores are recovered. Ideally, many of these studies will be coordinated with the shipboard and shore-based sampling protocols. REFERENCES Abrams, L.J., Larson, R.L., Shipley, T.H., and Lancelot, Y., 1993. Cretaceous volcanic sequences and Jurassic oceanic crust in the East Mariana and Pigafetta basins of the Western Pacific. In Pringle, M.S., Sager, W.W., Sliter, W.V., and Stein, S. (Eds.), The Mesozoic Pacific: Geology, Tectonics and Volcanism, Am. Geophys. Union, Geophysical Mon., 77:77-101. _____, 1992. The seismic stratigraphy and sedimentary history of the East Mariana and Pigafetta basins of the Western Pacific. In Larson, R., Lancelot, Y., et al., Proc. ODP, Sci. Results, 129: College Station, TX (Ocean Drilling Program), 551-570. Alt, J.C., Honnorez, J., Laverne, C., and Emmerman, R., 1986. Hydrothermal alteration of a 1 km section through the upper oceanic crust, DSDP 504B. J. Geophys. Res., 91:10309-10355. Arculus, R.J., Gill, J.B., Cambray, H., Chen, W., and Stern, R.J., 1995. Geochemical evolution of arc systems in the Western Pacific: the ash and turbidite record recovered by drilling. In Taylor, B., and Natland, J. (Eds.), Active Margins and Marginal Basins of the Western Pacific, Am. Geophys. Union, Geophysical Mon., 88:45-65. Armstrong, R.L., 1968. A model for Sr and Pb isotopic evolution in a dynamic earth. Rev. Geophys., 6:175-199. Arthur, M.A., Dean, W.E., and Schlanger, S.O., 1985. Variations in global carbon cycle during the Cretaceous related to climate volcanism and changes in atmospheric CO2. In Sundquist, E.T. and Broecker, W.S. (Eds.), The Carbon Cycle and Atmospheric CO2: Natural Variations Archean to Present, Am. Geophys. Union, Geophysical Mon., 32:504-529. Bloomer, S.H., Stern, R.J., Fisk, E., and Geschwind, C.H., 1989. Shoshonitic volcanism in the northern Mariana Arc 1. Mineralogic and major and trace element characteristics. J. Geophys. Res., 94:4469-4496. Carbotte, S.M., Mutter, J.C. and Xu, L., 1997. Contribution of volcanism and tectonism to axial and flank morphology of the southern East Pacific Rise, 17¡10'-17¡40'S, from a study of layer 2A geometry. J. Geophys. Res., 102:10165-10184. Castillo, P.R., Floyd, P.A., and France-Lanord, C., 1992a. Isotope geochemistry of Leg 129 basalt. In Larson, R.L., Lancelot, Y., et al., Proc. ODP., Sci. Results, 129: College Station, TX (Ocean Drilling Program), 405-414. _____, 1992b. Data Report: Summary of geochemical data for Leg 129 igneous rocks. In Larson, R.L., Lancelot, Y., et al., Proc. ODP, Sci. Results, 129: College Station, TX (Ocean Drilling Program), 653-670. Channell, J.E.T., Erba, E., Nakanishi, M., and Tamaki, K., 1995. Late Jurassic-Early Cretaceous time scales and oceanic magnetic anomaly block models. In Berggren, W.A., Kent, D.V., Aubry, M.P., Hardebol, J. (Eds.), Geochronology, Time Scales and Global Stratigraphic Correlations. SEPM Spec. Pub., 54:51-63. Detrick, R., Collins, J., Stephen, R. and Swift, S., 1994. In situ evidence for the nature of the seismic layer 2/3 boundary in oceanic crust. Nature, 370: 228-290. Elliott, T., Plank, T., Zindler, A., White, W., and Bourdon, B., 1997. Element transport from subducted slab to volcanic front at the Mariana Arc, J. Geophys. Res., 102:14991-15019. Erba, E., 1992. Middle Cretaceous calcareous nannofossils from the Western Pacific (Leg 129): Evidence for paleoequatorial crossings. In Larson, R.L., Lancelot, Y., et al. (Eds.), Proc. ODP, Sci. Results, 129: College Station, TX (Ocean Drilling Program), 189-201. Fisher, A.T., Abrams, L., and Busch, W.H., 1992. Comparison of laboratory and logging data from Leg 129 and the inversion of logs to determine lithologies. In Larson, R., Lancelot, Y., et al., Proc. ODP, Sci. Results, 129: College Station, TX (Ocean Drilling Program), 507-528. France-Lanord, C., Michard., A., and Karpoff, A.M., 1992. Major element and Sr isotope composition of interstitial waters in sediments from Leg 129: the role of diagenetic reactions. In Larson, R., Lancelot, Y., et al., Proc. ODP, Sci. Results, 129: College Station, TX (Ocean Drilling Program), 267-282. Francheteau, J., Armijo, R., Cheminee, J.L., Hekinian, R., Lonsdale, P., and Blum, N., 1992. Dyke complex of the East Pacific Rise exposed in the walls of Hess Deep and the structure of the upper oceanic crust. Earth Planet. Sci. Lett., 111:109-121. Fryer, P., 1992. A synthesis of Leg 125 drilling of serpentinite seamounts on the Mariana and Izu- Bonin forearcs. In Fryer, P., Pearce, J.A., Stokking, L.B., et al., Proc. ODP, Sci. Res., 125: College Station, TX (Ocean Drilling Program), 593-614. Gill, J.B., Hiscott, R.N., and Vidal, P.H., 1994. Turbidite geochemistry and evolution of the Izu- Bonin Arc and continents. Lithos, 33:135-168. Gradstein, F.M., Agterberg, F.P., Ogg, J.G., Hardenbol, J., van Veen, P., Thierry, J., and Huang, Z., 1994. A Mesozoic time scale. J. Geophys. Res., 99:24051-24074. Gust, D.A., Arculus, R.J., and Kersting, A.B., 1997. Aspects of magma sources and processes in the Honshu arc. Can. Mineral., 35:347-365. Harland, W.B., Armstrong, R.L., Cox, A.V., Craig, L.E., Smith, A.G., and Smith, D.G., 1990. A Geologic Time Scale, 1989, Cambridge University Press, Cambridge, U.K. Ikeda, Y., and Yuasa, M., 1989. Volcanism in nascent back-arc basins behind the Shichito ridge. Contrib. Min. Petrol., 101:377-393. Karig, D.E., and Kay, R.W., 1981. Fate of sediments on the descending plate at convergent margins. Phil. Trans. R. Soc. Lond., 301:233-251. Karl, S.M., Wandless, G.A., and Karpoff, A.M., 1992. Sedimentological and geochemical characteristics of ODP Leg 129 siliceous deposits. In Larson, R., Lancelot, Y., et al., Proc. ODP, Sci. Results, 129: College Station, TX (Ocean Drilling Program), 31-80. Karpoff, A., 1992. Cenozoic and Mesozoic sediment from the Pigafetta Basin. In Larson, R., Lancelot, Y., et al., Proc. ODP, Sci. Results, 129: College Station, TX (Ocean Drilling Program), 3-30. Lancelot, Y., Larson, R., et al., 1990. Proc. ODP, Init. Repts., 129: College Station, TX (Ocean Drilling Program). Larson, R.L., and, Sager, W.W., 1992. Skewness of magnetic anomalies M0 to M29 in the northwestern Pacific. In Larson, R.L., Lancelot, Y., et al, Proc. ODP, Sci. Results, 129: College Station, TX (Ocean Drilling Program), 471-481. Larson, R.L., Fisher, A.T., Jarrard, R.D., Becker, K., and ODP Leg 144 Shipboard Scientific Party, 1993. Highly permeable and layered Jurassic oceanic crust in the Western Pacific, Earth and Planet. Sci. Lett., 199:71-83. Lees, G.J., Rowbotham, G., and Floyd, P.A., 1992. Petrography and geochemistry of graded volcaniclastic sediments and their clasts, Leg 129. In Larson, R., Lancelot, Y., et al., Proc. ODP, Sci. Results, 129: College Station, TX (Ocean Drilling Program), 137-152. Lin, P-N., Stern, R.J., and Bloomer, S.H., 1989. Shoshonitic volcanism in the northern Mariana Arc 2. Large ion lithophile and rare earth element abundances: evidence for the source of incompatible element enrichments in intraoceanic arcs. J. Geophys. Res., 94:4497-4514. Mottl, M.J., 1992. Pore waters from serpentine seamounts in the Mariana and Izu-Bonin forearcs, Leg 125: Evidence for volatiles from the subducting slab. In Fryer, P., Pearce, J.A., Stokking, L.B., et al., Proc. ODP, Sci. Results, 125: College Station, TX (Ocean Drilling Program), 373- 385. Nakanishi, M., Tamaki, K., and Kobayashi, K., 1988. Mesozoic magnetic anomaly lineations and seafloor spreading history of the northwestern Pacific, J. Geophys. Res., 94:15,437-15,462. Peacock, S.M., 1990. Fluid Processes in subduction zones. Science, 248:329-337. Pick, T., and Tauxe, L., 1993. Geomagnetic paleointensities during the Cretaceous normal superchron measured using submarine basaltic glass. Nature, 366:238-242. Plank, T., and Langmuir, C.H., 1993. Tracing trace elements from sediment input to volcanic output at subduction zones. Nature, 362:739-743. _____, 1998. The chemical composition of subducting sediment: implications for the crust and mantle. Chem. Geol., 145:325-394. Plank, T., Morris, J., and Abers, G., 1994. Sediment water fluxes at subduction zones. Proc. SUBCON Meeting, Catalina, CA (Abstract). Pratson, E.L., Broglia, C., Molinie, A., and Abrams, L., 1992. Geochemical well logs through Cenozoic and Mesozoic sediment from Sites 800, 801, and 802. In Larson, R., Lancelot, Y., et al., Proc. ODP, Sci. Results, 129: College Station, TX (Ocean Drilling Program), 635-651. Roth, P.H., 1981. Mid-Cretaceous calcareous nannoplankton from the central Pacific: Implication for paleoceanography. In Tiede, J., Vallier, T.L., et al., Init. Repts. DSDP, 62: Washington (U.S. Govt. Printing Office), 471-489. Sager, W.W., and Pringle, M.S., 1988. Mid-Cretaceous to early Tertiary apparent polar wander path for the Pacific plate, J. Geophys. Res., 93:11,753-11,771. Scholl, D.W., Plank, T., Morris, J., von Huene, R., and Mottl, M., 1996. Scientific Opportunities in Ocean Drilling to Investigate Recycling Processes and Material Fluxes at Subduction Zones, Joint Oceanog. Instit. Workshop Report. Seno, T., Stein, S., and Gripp, A.E., 1993. A model for the motion of the Philippine sea plate consistent with NUVEL-1 and geological data. J. Geophys. Res., 98:17,941-17,948. Staudigel, H., Davies, G.R., Hart, S.R., Marchant, K.M., and Smith, B.M., 1995. Large scale Sr, Nd, and O isotopic anatomy of altered oceanic crust: DSDP sites 417/418. Earth Planet. Sci. Lett., 130:169-185. Stern, R.J., Lin, P-N, Morris, J.D., Jackson, M.C., Fryer, P., Bloomer, S.H., and Ito, E., 1990. Enriched back-arc basin basalts from the northern Mariana Trough: Implications for the magmatic evolution of back-arc basins. Earth Planet. Sci. Lett., 100:210-225. Stolper, E., and Newman, S., 1994. The role of water in the petrogenesis of Mariana Trough magmas. Earth Planet. Sci. Lett., 121:293-325. Tatsumi, Y., Murasaki, M., and Nodha, S., et al., 1992. Across-arc variations of lava chemistry in the Izu-Bonin Arc. J. Volc. Geotherm. Res., 49:179-190. Taylor, B., 1992. Rifting and the volcanic-tectonic evolution of the Izu-Bonin-Mariana arc. In Taylor, B., Fujioka, K., et al., Proc. ODP, Sci. Results, 126: College Station, TX (Ocean Drilling Program), 627-651. Tera, F., Brown, L., Morris, J., Sacks, I.S., Klein, J., and Middleton, R., 1986. Sediment incorporation in island-arc magmas: Inferences from 10Be. Geochim. Cosmochim. Acta, 50:535- 550. Theirstein, H.R., 1979. Paleoceanographic implications of organic carbon and carbonate distribution in Mesozoic deep-sea sediments. In Talwani, M., Hay, W.W., Ryan, W.B.F. (Eds.), Deep Drilling Results in the Atlantic Ocean: Continental Margins and Paleoenvironment, Amer. Geophys. Union, Maurice Ewing Series, 3:249-274. von Huene, R., and Scholl, D.W., 1991. Observations at convergent margins concerning sediment subduction, subduction erosion, and the growth of continental crust. Rev. Geophys., 29:279-316. Woodhead, J.D., and Fraser, D.G., 1985. Pb, Sr, and 10Be isotopic studies of volcanic rocks from the Northern Mariana Islands. Implication for magma genesis and crustal recycling in the Western Pacific. Geochim. Cosmochim. Acta, 49:1925-1930. Zheng, S.-H., Morris, J., Tera, F., Klein, J., and Middleton, R., 1994. Beryllium isotopic investigation of sedimentary columns outboard of subduction zones. USGS Circular, 366 (Abstract). FIGURE CAPTIONS 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. SITE SUMMARIES Site: BON-8A Priority: 1 Position: 31¡18.5«N, 142¡57.5«E Water Depth: 6000 m Sediment Thickness: 600 m Target Drilling Depth: 900 mbsf Approved Maximum Penetration: 900 mbsf Seismic Coverage: Conrad 2005, Line 39, shotpoint #2936 at 1613Z on 10/13/76 Objectives: The objectives of BON-8A are the following 1.Provide estimates of the sediment inputs and altered basalt inputs (geochemical fluxes) into the Izu-Bonin subduction zone. 2.Contrast crustal budgets here with those for the Marianas to test whether along-strike differences in the volcanics can be explained by along-strike variations in the crustal inputs. 3.Compare basement alteration characteristics with those in Hole 801C (also in old Pacific crust). 4.Provide constraints on the Early Cretaceous paleomagnetic time scale. 5.Provide constraints on mid-Cretaceous carbonate compensation depth (CCD) and equatorial circulation fluctuations. Drilling Program: APC, XCB, MDCB, RCB Logging and Downhole Operations: Triple combo, GLT, FMS/Sonic, GHMT, ARI, Permeability Nature of Rock Anticipated: Pelagic clay with volcanic arc ash (150 m); cherty porcellanites and chalks (450 m); basaltic pillows, flows, breccia, and possibly dikes (>300 m) Site: BON-9 Priority: 2 Position: 31¡18.5«N, 143¡2.5«E Water Depth: 5875 m Sediment Thickness: 600 mbsf Target Drilling Depth: 900 mbsf Approved Maximum Penetration: 900 mbsf Seismic Coverage: Conrad 2005, Line 39 Objectives: The objectives of Site BON-9 are the same objectives as the primary site, BON-8A. Drilling Program: Triple APC/XCB, MDCB, RCB Logging and Downhole: Same as Site BON-8A Nature of Rock Anticipated: Same as Site BON-8A Site: 801 Priority: 1 Position: 18¡38.52«N, 156¡21.6«E Water Depth: 5674 m Sediment Thickness: 460 m Target Drilling Depth: >850 mbsf Approved Maximum Penetration: 950 mbsf Seismic Coverage: MESOPAC II, Line 10 at 0600, 8/26/89 Objectives: The objectives of Site 801 are to: 1.Characterize the geochemical fluxes and geophysical aging attending the upper oxidative alteration of the oceanic crust in Hole 801C. 2.Compare igneous compositions, structure, and alteration with other drilled sections of in situ oceanic crust (in particular Hole 504B, contrasting a young site in Pacific crust with the oldest site in Pacific crust). 3.Help constrain models for seafloor alteration that depend on spreading rate and age (Hole 801C contains the world's oldest oceanic crust drilled to date, which was formed at a fast-spreading ridge, so it embodies several end-member characteristics). 4.Test models for the Jurassic Magnetic "Quiet" Zone. Drilling Program: RCB Logging and Downhole Operations: Triple Combo, GLT, FMS/Sonic, ARI, Permeability Nature of Rock Anticipated: Basaltic pillows, flows, breccia, and possibly dikes (>350 m) Site: PIG-3B Priority: 2 Position: 18¡39.78«N, 157¡5.7«E Water Depth: 5700 m Sediment Thickness: 460 m Target Drilling Depth: 1000 mbsf Approved Maximum Penetration: mbsf Seismic Coverage: MESOPAC II, Line 9 Objectives: The objectives of Site PIG-3B are the same objectives as the primary site, Site 801. Drilling Program: Triple APC/XCB to <350 m Logging and Downhole: Same as Site 801 Nature of Rock Anticipated: Same as Site 801 SCIENTIFIC PARTICIPANTS* Co-Chief John N. Ludden Centre de Recherches Pétrographiques et Géochimiques (UPR 9046) CNRS 15 Rue Notre Dame des Pauvres - B.P. 20 Vandoeuvre-les-Nancy 54501 France Internet: ludden@crpg.cnrs-nancy.fr Work: (33) 3-83-59-42-13 Fax: (33) 3-83-51-17-98 Co-Chief Terry Plank Department of Geology-Geophysics Program University of Kansas 120 Lindley Hall Lawrence, KS 66045 U.S.A. Internet: tplank@kuhub.cc.ukans.edu Work: (785) 864-2725 Fax: (785) 864-5276 Staff Scientist Carlota Escutia Ocean Drilling Program Texas A&M University 1000 Discovery Drive College Station, TX 77845-9547 U.S.A. Internet: carlota_escutia@odp.tamu.edu Work: (409) 845-0506 Fax: (409) 845-0876 Inorganic Geochemist Robin N. Armstrong Southampton Oceanography Centre University of Southampton Empress Dock European Way Southampton SO14 3ZH United Kingdom Internet: rxa@soc.soton.ac.uk Work: (44) 1703-596133 Fax: (44) 1703-593052 Inorganic Geochemist Richard W. Murray Department of Earth Sciences Boston University 685 Commonwealth Avenue Boston, MA 02215 U.S.A. Internet: rickm@bu.edu Work: (617) 353-6532 Fax: (617) 353-3290 Inorganic Geochemist Arthur J. Spivack Department of Earth Sciences University of North Carolina at Wilmington 601 South College Road Wilmington, NC 28403-3297 U.S.A. Internet: Spivack@uncwil.edu Work: (910) 256-3721, ext 225 Fax: (910) 256-8856 JOIDES Logging Scientist Roger L. Larson Graduate School of Oceanography University of Rhode Island Narragansett Bay Campus Narragansett, RI 02882-1197 U.S.A. Internet: rlar@gsosun1.gso.uri.edu Work: (401) 874-6165 Fax: (401) 874-6811 JOIDES Logging Scientist Robert A. Pockalny Graduate School of Oceanography University of Rhode Island Narragansett, RI 02882-1197 U.S.A. Internet: robp@gsosun1.gso.uri.edu Work: (401) 874-6926 Fax: (401) 874-6811 Microbiologist, Igneous Petrologist Martin R. Fisk College of Oceanic and Atmospheric Sciences Oregon State University 104 Ocean Admin Bldg. Corvallis, OR 97331-5503 U.S.A. Internet: mfisk@oce.orst.edu Work: (541) 737-5208 Fax: (541) 737-2064 Microbiologist Shelley A. Haveman Institute for Cell and Molecular Biology, Microbiology Göteborgs Universitet Box 462 Göteborg 405 30 Sweden Internet: shelley.haveman@gmm.gu.se Work: (46) 31-773-2586 Fax: (46) 31-773-2599 Microbiologist David C. Smith Graduate School of Oceanography University of Rhode Island Narragansett, RI 02882-1197 U.S.A. Internet: dcsmith@gso.uri.edu Work: (401) 874-6172 Fax: (401) 874-6240 Paleomagnetist Maureen B. Steiner Department of Geology and Geophysics University of Wyoming P.O. Box 3006, Univ. Sta. Laramie, WY 82071 U.S.A. Internet: magnetic@uwyo.edu Work: (307) 766-6584 Fax: (307) 766-6679 Paleontologist (Nannofossil) Francesca Lozar Centro di studi sulla Geodinamica delle catene collisionali Consiglio Nazionale della Ricerche via Accademia delle Scienze 5 Torino 10123 Italy Internet: f.lozar@csg.to.cnr.it Work: (39) 11-562-8928 Fax: (39) 11-541-755 Paleontologist (Radiolaria) Annachiara Bartolini Institut de Géologie et Paléontologié Université de Lausanne BFSH-2 UNIL Lausanne 1015 Switzerland Internet: abartoli@igp.unil.ch Work: (41) 21-692-4343 Fax: (41) 21-692-4305 Igneous Petrologist Jeffrey C. Alt Department of Geological Sciences University of Michigan 2534 C. C. Little Building Ann Arbor, MI 48109-1063 U.S.A. Internet: jalt@umich.edu Work: (313) 764-8380 Fax: (313) 763-4690 Igneous Petrologist Samantha R. Barr Department of Geology University of Leicester University Road Leicester LE1 7RH United Kingdom Internet: srb7@leicester.ac.uk Work: (44) 116-252-3608 Fax: (44) 116-252-3918 Igneous Petrologist Katherine A. Kelley Department of Geology University of Kansas 120 Lindley Hall Lawrence, KS 66045 U.S.A. Internet: kelley@eagle.cc.ukans.edu Work: (785) 864-7714 Fax: Igneous Petrologist Olivier Rouxel Centre de Recherches Pétrographiques et Géochimiques (UPR 9046) CNRS 15, rue Notre-Dame des Pauvres BP 20 Vandoeuvre-lés-Nancy Cedex 54501 France Internet: rouxel@crpg.cnrs-nancy.fr Work: (33) 03-83-59-42-23 Fax: (33) 03-83-51-17-98 Igneous Petrologist Angelika Schmidt GEOMAR Christian-Albrechts-Universität zu Kiel Wischhofstr. 1-3 Kiel 24148 Federal Republic of Germany Internet: aschmidt@geomar.de Work: (49) 431-600-2644 Fax: (49) 431-600-2949 Igneous Petrologist Hubert Staudigel Scripps Institution of Oceanography University of California, San Diego Institute of Geophysics and Planetary Physics UCSD-0225 La Jolla, CA 92093-0225 U.S.A. Internet: hstaudigel@ucsd.edu Work: (619) 534-8764 Fax: (619) 534-8090 Metamorphic Petrologist JosÉ J. Honnorez Institut de Géologie Université Louis Pasteur 1 rue Blessig Strasbourg Cedex 67084 France Internet: honnorez@illite.u-strasbg.fr Work: (33) 3-88-35-85-38 Fax: (33) 3-88-36-72-35 Physical Properties Specialist Lewis J. Abrams Department of Earth Sciences University of North Carolina at Wilmington 601 South College Road Wilmington, NC 28403-3297 U.S.A. Internet: abramsl@uncwil.edu Work: (910) 962-7422 Fax: (910) 962-7077 Physical Properties Specialist Tetsuro Hirono Department of Earth and Planetary Sciences Tokyo Institute of Technology Okayama 2-12-1 O Meguro-ku Tokyo 152-8551 Japan Internet: t-hirono@spa.att.ne.jp Work: (81) 11-816-5099 Fax: (81) 3-5734-3543 Sedimentologist Thomas K. Pletsch Geologisch-Paläontologisches Institut Christian-Albrechts-Universität zu Kiel Olshausenstraþe 40-60 Kiel 24118 Federal Republic of Germany Internet: tp@zaphod.gpi.uni-kiel.de Work: (49) 431-880-2938 Fax: (49) 431-880-4376 Sedimentologist Robert B. Valentine Department of Earth and Planetary Sciences Washington University One Brookings Drive Campus Box 1169 St. Louis, MO 63130 U.S.A. Internet: rbvalent@artsci.wustl.edu Work: (314) 935-7292 Fax: (314) 935-5610 LDEO Logging Scientist Gilles Guèrin Lamont-Doherty Earth Observatory Columbia University Borehole Research Group Palisades, NY 10964 U.S.A. Internet: guerin@ldeo.columbia.edu Work: (914) 365-8671 Fax: (914) 365-3182 LDEO Logging Trainee Graeme Cairns Laboratoire de Mesures en Forage ODP/Naturalia et Biologia (NEB) BP 72 Aix-en-Provence Cedex 4 13545 France Internet c/o: louvel@lmf-aix.gulliver.fr Work: (33) 4-42-97-11-22 Fax: (33) 4-42-97-11-21 Schlumberger Engineer Steve Kittredge Schlumberger Offshore Services 369 Tristar Drive Webster, TX 77598 U.S.A. Internet: kittredge@webster.wireline.slb.com Work: (281) 480-2000 Fax: (281) 480-9550 Operations Manager Glen N. Foss Ocean Drilling Program Texas A&M University 1000 Discovery Drive College Station, TX 77845-9547 U.S.A. Work: (409) 845-8482 Fax: (409) 845-2380 ODP Drilling Engineer Eddie Wright Ocean Drilling Program Texas A&M University 1000 Discovery Drive College Station, TX 77845-9547 U.S.A. Internet: eddie_wright@odp.tamu.edu Work: (409) 845-3207 Fax: (409) 845-2308 Laboratory Officer Burney Hamlin Ocean Drilling Program Texas A&M University 1000 Discovery Drive College Station, TX 77845-9547 U.S.A. Internet: burney_hamlin@odp.tamu.edu Work: (409) 845-2496 Fax: (409) 845-0876 Marine Lab Specialist: Yeoperson Jo Ribbens Ocean Drilling Program Texas A&M University 1000 Discovery Drive College Station, TX 77845-9547 U.S.A. Internet: jo_ribbens@odp.tamu.edu Work: (409) 845-8482 Fax: (409) 845-0876 Marine Lab Specialist: Chemistry Erik Moortgat Ocean Drilling Program Texas A&M University 1000 Discovery Drive College Station, TX 77845-9547 U.S.A. Internet: erik_moortgat@odp.tamu.edu Work: (409) 845-7716 Fax: (409) 845-0876 Marine Lab Specialist: Chemistry Anne Pimmel Ocean Drilling Program Texas A&M University 1000 Discovery Drive College Station, TX 77845-9547 U.S.A. Internet: anne_pimmel@odp.tamu.edu Work: (409) 845-8482 Fax: (409) 845-0876 Marine Lab Specialist: Core Johanna M. Suhonen Ocean Drilling Program Texas A&M University 1000 Discovery Drive College Station, TX 77845-9547 U.S.A. Internet: johanna_suhonen@odp.tamu.edu Work: (409) 845-9186 Fax: (409) 845-0876 Marine Lab Specialist: Curator Erinn McCarty Ocean Drilling Program Texas A&M University 1000 Discovery Drive College Station, TX 77845-9547 U.S.A. Internet: erinn_mccarty@odp.tamu.edu Work: (409) 845-8482 Fax: (409) 845-0876 Marine Lab Specialist: Downhole Tools, Marine Lab Specialist: Thin Sections Sandy Dillard Ocean Drilling Program Texas A&M University 1000 Discovery Drive College Station, TX 77845-9547 U.S.A. Internet: edgar_dillard@odp.tamu.edu Work: (409) 845-2481 Fax: (409) 845-0876 Marine Lab Specialist: Paleomagnetics Matt O'Regan Ocean Drilling Program Texas A&M University 1000 Discovery Drive College Station, TX 77845-9547 U.S.A. Internet: matthew_o'regan@odp.tamu.edu Work: (409) 845-2483 Fax: (409) 845-0876 Marine Lab Specialist: Photographer Roy Davis Ocean Drilling Program Texas A&M University 1000 Discovery Drive College Station, TX 77845-9547 U.S.A. Internet: roy_davis@odp.tamu.edu Work: (409) 845-8482 Fax: (409) 845-4857 Marine Lab Specialist: Physical Properties Patrick Riley Ocean Drilling Program Texas A&M University 1000 Discovery Drive College Station, TX 77845-9547 U.S.A. Internet: patrick_riley@odp.tamu.edu Work: (409) 845-2480 Fax: (409) 845-0876 Marine Lab Specialist: Temporary Pablo Cervantes Ocean Drilling Program Texas A&M University 1000 Discovery Drive College Station, TX 77845-9547 U.S.A. Internet: pablo_cervantes@odp.tamu.edu Work: (409) 845-8482 Fax: (409) 845-0876 Marine Lab Specialist: Underway Geophysics Steve Prinz Scripps Institution of Oceanography University of California, San Diego Ocean Drilling Program West Coast Repository La Jolla, CA 92093-0231 U.S.A. Work: Fax: Marine Lab Specialist: X-Ray Jaquelyn Ledbetter Ocean Drilling Program Texas A&M University 1000 Discovery Drive College Station, TX 77845-9547 U.S.A. Internet: jaque_ledbetter@odp.tamu.edu Work: (409) 845-8482 Fax: (409) 845-0876 Marine Lab Specialist: Marine Logistics Coordinator John Dyke Ocean Drilling Program Texas A&M University 1000 Discovery Drive College Station, TX 77845-9547 U.S.A. Internet: john_dyke@odp.tamu.edu Work: (409) 845-2424 Fax: (409) 845-2380 Marine Electronics Specialist John Pretorius Ocean Drilling Program Texas A&M University 1000 Discovery Drive College Station, TX 77845-9547 U.S.A. Internet: john_pretorius@odp.tamu.edu Work: (409) 845-5135 Fax: (409) 845-0876 Marine Electronics Specialist Pieter Pretorius Ocean Drilling Program Texas A&M University 1000 Discovery drive College Station, TX 77845-9547 U.S.A. Internrt: pieter_pretorius@odp.tamu.edu Work: (409) 845-5135 Fax: (409) 845-0876 Marine Computer Specialist Margaret Hastedt Ocean Drilling Program Texas A&M University 1000 Discovery Drive College Station, TX 77845-9547 U.S.A. Internet: margaret_hastedt@odp.tamu.edu Work: (409) 845-2480 Fax: (409) 845-4857 Marine Computer Specialist Chris Stephens Ocean Drilling Program Texas A&M University 1000 Discovery Drive College Station, TX 77845-9547 U.S.A. Internet: chris_stephens@odp.tamu.edu Work: (409) 862-4849 Fax: (409) 845-4857 ODP/TAMU Information Services Rep. Dwight Hornbacher Ocean Drilling Program Texas A&M University 1000 Discovery Drive College Station, TX 77845-9547 U.S.A. Internet: dwight_hornbacher@odp.tamu.edu Work: (409) 845-1927 Fax: (409) 845-4857 *Subject to change