Von Huene and Scholl (1991) calculated a large global flux of subducted sedimentas 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 (around 1%), based on isotopic mixing arguments, which constrain only the proportion of sediment to 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 variablesaging of the oceanic lithosphere, flow of material through accretionary prisms, and fluid circulation at active marginsthat are linked across a convergent margin and can be explored in combination through a drilling transect (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. This is largely because the current approach is a piecemeal one, with various parts of the problem 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 on most fronts by a focused drilling effort.
Exisiting Crustal Inventory at the Izu-Mariana Margin
Of the eight holes drilled in the seafloor immediately east of the Izu-Bonin Trench (not including the Shatsky Rise sites), only one penetrated basement, Site 197 (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); the recovered material being dominantly pelagic clays above resistant cherts. Although more was recovered at Sites 52 (45 m) and 578 (165 m), drilling again was stopped by the cherts, leaving hundreds of meters of unsampled sediment below. Prior drilling has provided us with many samples of the upper 50- to 150-m pelagic clay and ash unit, but almost nothing of the units below, including the oceanic crust.
The main goal of Izu-Bonin drilling 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
(Ocean Drilling Program [ODP] Sites 800-802) were sampled through the cherts to
"basement." Sedimentary units were well sampled during Leg 129, 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 Marianas 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.
Based on Leg 129 drilling and prior Deep Sea Drilling Project (DSDP) efforts, we have
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, in press]). However, our 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 at Hole 801C (of the ~135 m total penetration into basement at Holes
801B and C, only the lower 63 m of drilling recovered 43 m of normal, Jurassic tholeiitic
Thus the main goal of Mariana drilling 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.
Exisiting 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 drilling 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 at 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 (Alt et al., 1986) and DSDP Sites 417/418 (Staudigel et al., 1995), and assuming 10% interpillow material at 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 at Hole 504B) to the crust seaward of the Marianas 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 Marianas 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, and 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. Current estimates are poor, and the actual flux balance could still go either way. Drilling through the upper oceanic crust subducting beneath the Marianas, however, can dramatically improve one key flux in the mass balance equationthe alteration flux.
Exisiting Crustal Mass Balance for Izu-Bonin
Because we have yet to sample either the sediments or altered oceanic crust seaward of the Izu Trench to any significant extent, we are much more limited in our ability to determine the mass balance. 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 Marianas 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 oceanthe 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 fairly 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 Mariana and Izu arc volcanics could also be related to the chemical composition of the altered oceanic crust. 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 of the adjacent Volcano 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.
The primary motivation for Site BON-8A, a site ~60 km seaward of the Izu Trench (Fig. 2), is to provide the first complete section of sediment and altered oceanic crust entering 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. We propose to drill and core the entire sedimentary sequence (~600 m) at Site BON-8A, as well as the upper oxidative alteration zone of the basaltic basement (~300 m). The scientific objectives are to
1.provide estimates of the sediment inputs and altered basalt inputs (geochemical fluxes) into
the Izu 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 at Hole 801C (also in old Pacific crust);
4.provide constraints on the Early Cretaceous paleomagnetic time scale; and
5.provide constraints on mid-Cretaceous carbonate compensation depth (CCD) and equatorial circulation fluctuations.
In addition to serving as an important reference site for crustal inputs to the Izu-Bonin Trench, Site BON-8A can also address additional paleomagnetic and paleoceanographic problems. Because the subduction cycling objectives have already been discussed in some detail above ("Background" section), we elaborate more below on the paleomagnetic and paleoceanographic objectives.
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).
Based on its theoretical Cretaceous paleolatitude history, Site BON-8A may have formed at about 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 (Fig. 5). A site such as Site 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 of the paleoequatorial upwelling zone. Using these nannoflora, potential fluctuations in the mid-Cretaceous equatorial circulation system could be studied at Site BON-8A when it was nearly stationary near the paleoequator (especially from 115 to 95 Ma).
The primary motivation for returning to ODP Hole 801C, seaward of the Marianas Trench (Fig. 2), is to sample the upper oxidative zone of alteration, and possibly the entire extrusive layer (layer 2b), 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. The transition from volcanics to dikes may not lie much deeper (500-600 m at Hole 504B; only a few 100 m at Hess Deep). We propose to drill at least 350 m farther into basement at Hole 801C to accomplish the following scientific objectives:
1.Characterize the geochemical fluxes and geophysical aging attending the upper oxidative
alteration of the oceanic crust at 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 general models for seafloor alteration that depend on spreading rate and age (Hole 801C is the world's oldest oceanic crust and was formed at a fast-spreading ridge, so it embodies several end-member characteristics); and
4.Test models for the Jurassic Magnetic "Quiet" Zone.
In addition to serving as an important reference site for crustal inputs to the Mariana trench, Hole 801C can also address additional paleomagnetic problems. Because the subduction cycling objectives have already been discussed in some detail above ("Background" section), we elaborate more below on the paleomagnetic objective.
Site 801C is located in an area of very low-amplitude magnetic anomalies, usually called the Jurassic Magnetic "Quiet" Zone (JQZ). The JQZ has been suggested to result from (1) oceanic crust of a single polarity with small anomalies from 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 will allow testing of the above hypothesesparticularly the third hypothesis of magnetic reversals during a period of anomalously low field intensity if fresh, unaltered volcanic glass can 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.
The site objective is to penetrate through the upper, oxidative alteration zone in basaltic basement, deepening Hole 801C at least another 350 m (to ~940 meters below seafloor [mbsf] or ~480 m sub-basement, with a total drill string length of 6630 m. If drilling problems are encountered, several other sites in the Pigafetta Basin near Site 801 show the Jurassic basement reflector on seismic records and have at least 50 m of pelagic clay overlying the shallowest cherts to laterally support the drill string during initial chert penetration.
Downhole Measurements at Site BON-8A
The oceanic crust subducted in the Izu-Bonin Trench has never been sampled nor logged. To compare the sedimentary sequence and the upper oceanic crust at Site BON-8A with the those at Hole 801C, the geochemical and geophysical tools as well as the formation microscanner (FMS) will be used. Moreover, to satisfy the time-scale objective (i.e., to determine the age of the basement), the magnetic susceptibility and total magnetic field measurements could provide a paleomagnetic reversal sequence of the overlying sediment. The azimuthal resistivity tool (ARI) will also be used in the basement section to measure resistivity. Because determining the geochemical budgets in sediment and basement columns is central to the objectives of Leg 185, geochemical logging (GLT) will be extremely valuable. Leg 129 geochemical logging served as an excellent proxy for actual recovery of sediments similar to those expected at Site BON-8A (Fisher et al., 1992).
Downhole Measurements in Hole 801C
Downhole measurements were conducted in the upper 100 m of basement in Hole 801C 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 the 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. Additional permeability experiments will be carried out deeper in the hole to characterize the hydrologic properties of this end-member oceanic crust, away from the perturbations of off-axis lavas. To further characterize the petrology, hydrogeology, structure, and physical properties of this old oceanic crust, the hole will be logged using the triple combo, geochemical, and the FMS tool strings. The ARI tool will also be used to measure resistivity in basement sections.
Hole 801C (Table 1)
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 Basins (Fig. 2), "basement" consists of Cretaceous flows and sills that overlie the "normal" Jurassic crust. Because these Cretaceous units have already been sampled during Leg 129 drilling, the remaining goal is the MORB section. Hole 801C is the only location where Jurassic-aged material has been 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. 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 it is ready for more extensive basement drilling. Further background on Hole 801C can be found in Lancelot and Larson, et al. (1990). Specific site objectives for Leg 185 are listed in the "Scientific Objectives" section.
To 185 references
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