3. Site 8011

Shipboard Scientific Party2

BACKGROUND AND OBJECTIVES

Ocean Drilling Program (ODP) Site 801 was first drilled during Leg 129 in December 1989 as part of a series of drill sites aimed at recovering Jurassic sediment and oceanic crust in the Pacific Ocean (Lancelot, Larson, et al., 1990). The objectives of the leg were related to understanding the paleoenvironment of the Jurassic super ocean, which covered a large part of Earth at that time. Earlier attempts to recover Jurassic sediments and basement in the western Pacific had been hampered by the presence of overlying Cretaceous volcaniclastic turbidites, sills, flows, and chalk-chert horizons. Leg 129 was the first to succeed in recovering Jurassic oceanic basement in the Pacific Ocean, and the Hole 801C rocks are still the oldest (~170 Ma) sampled in the ocean basins (Pringle, 1992). The hole was reentered and logged during Leg 144, and a drill-string packer experiment was carried out to determine basement permeability (Haggerty, Silva, Rack, et al., 1995; Larson et al., 1993).

During Leg 129, ODP Hole 801C was cased to ~481 meters below seafloor (mbsf) and reached a total depth of 594 mbsf, ~130 m into the oceanic crust. Leg 185 drilling operations involved deepening the basement section of Hole 801C by an additional 341.4 m. The principal scientific objective involved definition of the primary and secondary mineralogy and geochemistry, as well as the physical properties of the Jurassic crust. The oceanic crust represented in Hole 801C is assumed to be representative of that subducted into the Mariana Trench. Although further from the trench than other potential reference sites, Site 801 offers the advantage that it is an established ODP basement hole in which deep penetration of the upper 450 m of normal Jurassic oceanic crust was considered possible in about three weeks of drilling and logging operations. Comparisons with other deep sites in oceanic basement indicated that a total penetration in excess of 400 m was considered adequate to traverse the upper oxidative zone of altered crust. This upper zone is enriched in elements such as K, U, and B and in CO2 and H2O, all of which are important in the geochemical budgets in the subduction factory.

The deep penetration into Jurassic oceanic crust also provided an opportunity to compare the igneous compositions, alteration styles, and physical properties of old, fast-spreading Pacific crust with other deeply drilled sections of oceanic crust of different ages and spreading rates. Site 801 was also unique in providing an opportunity to examine the magnetic nature of the basement in the Jurassic magnetic quiet zone.

Bacteria have been located in association with ridge axis hydrothermal systems and within the seafloor sediment column as deep as 800 m (Thorseth et al., 1992; Parks et al., 1994; Fisk et al., 1998; Furnes and Staudigel, 1999; Shipboard Scientific Party, 1999). The possibility that bacterial activity may occur in oceanic crust as old as that at Site 801 provided the motivation for sampling the basement for culturing and DNA extraction in the search for extremophile life. To control the extent of contamination from surface waters, drilling mud, and drilling tools, we undertook a series of contamination tests as part of the operations at Site 801.

Prior to reentering Hole 801C, a sediment core was taken with the rotary core barrel (RCB) in the top 20 mbsf, designated as Hole 801D. This core was taken as a control for microbiological tests and sampling techniques to be conducted at Site 1149. Hole 801D is described herein for sediment character, biostratigraphy, and selected physical properties.

Hole 801C is an ODP legacy hole and a geochemical reference site in the oldest in situ oceanic crust drilled on Earth.

Previous Results from Basement Drilling at Site 801

Hole 801C is located at 1838.538N, 15621.588E, in 5674 m water depth in the Pigafetta Basin (Fig. F1). A seismic section showing the total penetration at the site on both Legs 129 and 185 is given in Figure F2. The sedimentary section is characterized by an upper (56 m) pelagic clay unit, which overlies a 63-m chert-porcelanite unit. These are underlain by thick (192 m) volcaniclastic turbidites of probable Albian age and represent redeposited material from the Magellan Seamount chain. A second unit, chert radiolarite (125 m), underlies the volcaniclastics and gives way to 20 m of Callovian red radiolarites and claystones. These overlie basement located at 461.6 mbsf in Hole 801B.

The basement cores from Leg 129 were brought on board the JOIDES Resolution during Leg 185 and were examined, scanned for digital photographs, scanned on the multisensor track (MST), and resampled. In this chapter, some of the data from Leg 129 basement cores has been incorporated with that for the newly cored intervals.

A simplified section of the basement recovered during Leg 129, radiometric ages, Zr concentrations, and the bulk porosity from downhole measurements are given in Figure F3. The uppermost igneous units are alkaline in character and are probably best interpreted as basaltic to doleritic sills (Floyd et al., 1992; Floyd and Castillo, 1992). Ar/Ar radiometric ages on laser-fused samples (Pringle, 1992) give a weighted mean age of 157 Ma for this alkaline unit. The igneous units are intercalated with chert-rich sediments, which are often baked at the contact with the basalt. This alkaline sequence is 60.2 m thick and overlies a Si and Fe oxyhydroxide-rich hydrothermal unit, for which logging results (Larson et al., 1993) indicate a thickness of ~20 m. Fluid temperatures of formation calculated for the deposit give temperatures of ~16-60C (Alt et al., 1992). These fluids controlled the alteration budget of the underlying pillow basalt, which contains celadonite, glauconite, carbonate, smectite, K-feldspar, and silica as secondary minerals.

Approximately 63 m of volcanic rock was drilled below the hydrothermal deposit. The alteration intensity is highly variable in these rocks, and their colors vary from gray black to green gray to light brown. Parts of this core were clearly altered under a high fluid flux regime. The lowermost flows, in Cores 129-801C-9R and 10R, are relatively unaltered and are highly resistive with high sonic velocities. These characteristics are further evaluated in "Physical Properties" and "Downhole Measurements". Ar/Ar fusion dates on two samples from the section below the hydrothermal deposit define the age of these lavas as <171 Ma and >162 Ma (Pringle, 1992).

Geochemical studies (Floyd and Castillo, 1992) indicate that these volcanics are normal mid-ocean-ridge basalt (MORB) tholeiites. They are olivine and plagioclase phyric and relatively primitive in composition with MgO as high as 10 wt% and Zr as low as 50 ppm. A trend to more evolved compositions, defined by higher Zr content, is observed toward the base of the hole.

Hole 801C was logged during Legs 129 and 144 and was relogged during Leg 185. The results are combined in this chapter for the entire logged section below the casing. Porosity measurements obtained during Leg 144 (Fig. F3) show the high porosities measured in the hydrothermal zone. The exceptionally high permeability of this zone was measured by Larson et al. (1993) in a drill-string packer test.

1Examples of how to reference the whole or part of this volume can be found under "Citations" in the preliminary pages of the volume.
2Shipboard Scientific Party addresses can be found under "Shipboard Scientific Party" in the preliminary pages of the volume.

Ms 185IR-103

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