Subduction zones are the primary regions on Earth today where crust recycling takes place, and through geological time they have been the sites of continent formation. Many of the key elements (e.g., Th, rare earth elements, Ba, and Be) that are important in understanding crustal growth are sequestered in the sedimentary column and in the uppermost oxidized portions of the volcanic section of oceanic basement (K, B, U, CO2, 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 (~165 Ma) crust in the Pacific Ocean during Ocean Drilling Program (ODP) Leg 129. During Leg 185 the hole was deepened to nearly 500 meters below seafloor (mbsf), and at Site 1149, located on magnetic Anomaly M11 (~132 Ma) 100 km east of the Izu-Bonin Trench, the entire sedimentary sequence (410 m) and an additional 133 m of highly altered volcanic basement was drilled.
Using the recovered core and the logging results, it is possible to reconstruct the volcanic section for Hole 801C. Seven volcanic sequences have been defined, 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 series of alkali basalts drilled during Leg 129 and dated at ~155 Ma. These are separated from the underlying tholeiites of normal oceanic crust by an ocherous Si-Fe-rich hydrothermal deposit. A similar deposit is 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. Thus, both the alteration and lava sequences may be typical of fast-spreading environments, such as the present-day East Pacific Rise.
Site 1149 basement is dramatically different in character. It is pervasively altered at low temperatures to red dusky brown and preserves multicolored halos around veins and fractures. The volcanic facies are dominated by thin flows, hyaloclastite, and flow breccia.
Preliminary estimates of the geochemical budget for K were made for Site 801 volcanic sections of ocean basement using gamma-ray intensities from downhole logs and multisensor track (MST) 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 fourfold enrichment as a result of low-temperature alteration. Similar estimations will be possible for other key elements following shore-based analyses.
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 a series of reversals downhole. Given the spreading rates estimated for the region, the reversals must relate to rapid fluctuations in field polarity. Thus, at Site 801 the JQZ may represent a canceling out of normal and reversed polarities associated with an unstable and relatively weak magnetic field.
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 Basin (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 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 wesei) 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 equatorial zones of high biological productivity.
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 as being carbonate free, which may explain some of the geochemical differences between the two arc systems.
Leg 185 was the first ODP leg to conduct a series of in-hole contamination tests while undertaking a systematic study of the deep biosphere in oceanic sediments and basement in an attempt to establish the JOIDES Resolution as a platform for microbiological studies. Deep biosphere contamination tests involved adding highly sensitive tracers (i.e., perfluorocarbons and fluorescent microspheres) to the drilling fluids and the core barrel to evaluate the extent of contamination of the cores by microbes introduced by the drilling process. Results of the tests revealed that the centers of advanced hydraulic piston corer (APC) cores are essentially uncontaminated during coring, whereas rotary core barrel (RCB) cores in sediment and basement contain variable amounts of introduced tracer. In addition, samples of sediments and basalts were placed in cultures aboard ship for shore-based study. Nonetheless, possible microbial tracks observed in 170-Ma volcanic glass are intriguing evidence for a deep biosphere still active at the extreme depths (>930 mbsf) sampled during Leg 185.


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