Proceedings ODP - Initial Reports, Leg 185.

Subduction zones are the primary regions on Earth today where crust recycling takes place, and through geologic time they have been sites of continent formation. Many key chemical elements important for understanding crustal growth are subducted in the sedimentary column (e.g., Th, rare earth elements, Ba, and Be) and in the uppermost oxidized portions of the volcanic section of oceanic basement (e.g., K, B, U, CO2, and H2O). The principal objective of Leg 185 was to core two sites in Mesozoic crust in the west Pacific, which is being subducted into the Mariana and Izu-Bonin subduction systems, in order to determine the inputs into the "west Pacific subduction factory." Hole 801C was first drilled in the oldest (~170 Ma) crust in the Pacific Ocean during Ocean Drilling Program (ODP) Leg 129. During Leg 185 the hole was deepened to 936 meters below seafloor (mbsf), and at Site 1149, located on magnetic Anomaly M11 (~132 Ma) 100 km east of the Izu-Bonin Trench, the complete sedimentary sequence (410 m) and an additional 133 m of highly altered volcanic basement were drilled. Using the recovered core and the logging results, it is possible to reconstruct the volcanic section for Hole 801C. Eight volcanic sequences have been defined by the combined efforts of Legs 129 and 185, some with massive lava flows up to 20 m thick and others with thin pillows and sheet flows of <1 m. The uppermost unit is a ~60-m sequence of alkali basalts drilled during Leg 129 and radiometrically dated at ~157 Ma. This unit is separated from the underlying tholeiites of normal oceanic crust by an ~20-m-thick ocherous Si-Fe-rich hydrothermal deposit. A similar deposit is present 100 m lower in the hole. These hydrothermal deposits and numerous interpillow sediments observed in the upper volcanic sequences define the alteration character of the basement, which is confined to three zones downcore and appears to be controlled by local permeability structures. The pattern of alteration for basement at Site 801 contrasts with that from other deeply drilled sections in oceanic crust where oxidative alteration decreases continuously with depth. The estimated seafloor spreading rate for Site 801 is 160 km/m.y. Both the alteration and lava sequences may be typical of fast-spreading environments, such as the present-day East Pacific Rise. Preliminary estimates of the geochemical budget for K were made for volcanic sections of ocean basement at Site 801 using gamma-ray intensities from downhole logs and multisensor track measurements, in addition to chemical analyses of core samples and estimates of the volume percentages of veins and alteration types. The K content of the entire core indicates a three- to four-fold enrichment as a result of low-temperature alteration. Similar estimations will be possible for other key elements following shore-based analyses of a unique suite of 120 community samples taken shipboard. The deep basement penetration in Hole 801C provided ideal samples for probing the causes of the Jurassic Quiet Zone (JQZ). From paleomagnetic measurements on cores and geophysical logs we discovered as many as six reversals downhole. Given the spreading rates estimated for the region, the reversals must relate to rapid fluctuations in field polarity. Therefore, at Site 801 the JQZ may result from a canceling out of normal and reversed polarities associated with an unstable and relatively weak magnetic field. Site 1149 basement is dramatically different in character from Site 801. It is pervasively altered at low temperatures to red dusky brown and preserves multicolored halos around veins and fractures. Dominated by thin flows, hyaloclastite, and flow breccia, the volcanic facies are normal-mid-ocean-ridge basalt in the upper section and enriched mid-ocean-ridge basalt at the base of Hole 1149B. The sediments being subducted into trenches must, in part, control geochemical differences in the composition of arc magmas. Both the Mariana and Izu-Bonin margins are characterized by complete subduction of the sedimentary section on the downgoing plate, thus simplifying the dynamics of the subduction problem. Although the subducting sediments have been reasonably well sampled in the Pigafetta and East Mariana Basins (Mariana region), earlier drilling attempts to recover the sedimentary section in the Nadezhda Basin, seaward of the Izu-Bonin Trench, had largely been thwarted by difficult drilling conditions. Thus, an important objective of Leg 185 was met by continuously coring and logging the ~400 m sedimentary section at Site 1149. The uppermost sediments consist of pelagic clays with admixtures of volcanic ash and biosiliceous material for which paleomagnetic data define an excellent record of 6 m.y. of sedimentation in the west Pacific. These are underlain by barren diagenetically altered pelagic clays (characterized by low sedimentation rates), which overlie radiolarian cherts and clays and a lower unit of chert intercalated with marl and chalk above basement. The basal sediments have been dated from nannofossil assemblages (Tubodiscus verenae and Rucinolithus wisei) as lowermost Hauterivian to uppermost Valanginian, in accord with the assigned M-11 magnetic lineation. The lower sedimentary units preserve a record of high rates of biogenic sedimentation (~18 m/m.y.) as the site passed beneath the equatorial zone of high biological productivity. Shipboard sediment chemistry also documents a well-preserved record of hydrothermal input into the basal sediments. The sedimentary sequence at Site 1149 is substantially different from that being subducted at the Mariana Trench, the latter being characterized by an extensive mid-Cretaceous volcaniclastic sequence derived from the local seamounts and having a lower carbonate content, which may explain some of the geochemical differences between the two arc systems. In an attempt to establish the JOIDES Resolution as a platform for microbiological studies, Leg 185 was the first ODP leg to conduct a series of on-site contamination tests while undertaking a systematic study of the deep biosphere in oceanic sediments and basement. These contamination tests involved adding highly sensitive tracers (i.e., perfluorocarbons and fluorescent microspheres) to the drilling fluids and the core barrel to evaluate the potential extent of contamination of the cores by the drilling process. The particulate tracer was not found in the samples used for microbiological experiments, suggesting these samples were uncontaminated. Some thin sections contained the particulate tracer, probably introduced during preparation of thin sections. The results of the chemical tracer experiments indicate that, on average, ~0.3 µL of drilling fluid/gram cored material was present. In addition, samples of sediments and basalts were placed in cultures aboard ship for further shore-based study. Possible microbial tracks observed in the 170-Ma volcanic glass are intriguing evidence for a deep biosphere active at the extreme depths (>930 mbsf) sampled during Leg 185.