ODP Hole 801C 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). Rocks from Hole 801C are the oldest sampled in the ocean basins (at ~170 Ma) (Pringle, 1992). During Leg 144, the hole was reentered and logged, and a drill-string packer experiment was conducted (Haggerty, Silva, Rack, et al., 1995; Larson et al., 1993). Leg 185 succeeded in deepening Hole 801C by an additional 359 m, to a total depth in basement of 474 m (Table T2), placing this site as the DSDP and ODP drill hole with the sixth greatest penetration into normal oceanic crust. Recovery was good (47%), and a complete suite of ODP downhole logs was run to 850 mbsf. Although the reentry cone needs to be cleared of cuttings, which hampered logging by flowing back into the hole, it is in good condition and remains as ODP's legacy site in the Earth's oldest oceanic crust.
The basaltic section at Site 801 is overlain by a sedimentary section characterized by an upper (56 m) pelagic clay unit, which overlies a 63-m-thick chert-porcelanite unit. These sedimentary units are underlain by thick (192 m) volcaniclastic turbidites of probable Albian age, which represent redeposited material from the Magellan Seamounts. A second chert-radiolarite unit (125 m) underlies the volcaniclastics and gives way to 20 m of Callovian red radiolarites and claystones. These overlie basement at 461.6 mbsf at Site 801.
A stratigraphic column of the entire basement section is given in Figure F10. This section includes a composite of the rocks drilled on both Leg 129 and Leg 185 in Hole 801C. On the basis of flow morphology, geochemistry, and mineralogy, the basement section has been divided into eight major sequences (I-VIII). These sequences have been subdivided into 18 geochemical units, 60 lithologic units, and ~250 individual cooling units. The cooling units (pillows, flows, breccia, etc.) are organized into eruptive packages of similar textures and mineralogy; these are the lithologic units. The geochemical units require distinct batches of magma from the magma chamber or melting regime. We discuss here the features of the main sequences only.
The uppermost basement (Sequence I) is alkaline and composed of 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. The igneous units are intercalated with chert-rich sediments, which are often baked at the contact with the basalt. The sediments contain siliceous microfossils that define ages of early Bathonian to late Bajocian (~170 Ma) (Channell et al., 1995) and confirm the intrusive nature of the alkaline suite. This alkaline sequence is 60.2 m thick and overlies a Si and Fe oxyhydroxide-rich hydrothermal horizon (Sequence II) for which logging results (see resistivity log in Fig. F11) indicate a thickness of ~20 m. During Leg 129, ~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 and to light brown. Parts of these cores clearly were altered under a high fluid-flux regime (see "Basement Alteration"). These lavas are thin flows and pillows, but they lie above a series of thick flows; both are included as part of Sequence III, the Upper Massive Flows (Fig. F10). These thick flows have an exceptionally high resistivity (Fig. F11), although they appear to be similar in lithologic and geochemical character to other thick flows lower in the stratigraphy. Ar/Ar fusion dates on two samples from Sequence III define an age for these lavas from 162 to 171 Ma (Pringle, 1992).
Sequence III also defines a clear magmatic evolutionary trend toward more mafic, MgO-rich, and Zr-poor lavas from its base at ~580 mbsf to the overlying hydrothermal deposit (Fig. F12). The MgO-rich lavas contain abundant olivine phenocrysts, which are only rarely observed deeper in the section. The most evolved lavas, those with the lowest MgO and highest Zr, are commonly saturated in olivine, plagioclase, and clinopyroxene. In general, phenocrysts are scarce in the entire section. Plagioclase is the most common phase, but most of the lavas are classified as aphyric. Figure F12, in addition to highlighting alteration characteristics, shows the typical occurrence of plagioclase phenocrysts. Notwithstanding the paucity of phenocrysts, mineralogical examination and X-ray fluorescence (XRF) analyses for major and trace elements permitted definition of five geochemical units in Sequence III, which probably correspond to discrete magmatic episodes.
Between 600 and 720 mbsf the section is characterized by a pillow-dominated zone with well-developed interpillow horizons (Sequence IV—Upper Pillows and Flows). The amount of interpillow material of probable sedimentary origin decreases significantly downsection in this sequence. This is evident in the gamma-ray log (Fig. F11), which is smooth and decreases in intensity throughout Sequence IV. A second Si-rich hydrothermal unit similar to the larger one uphole is present within Sequence IV pillows.
From 720 mbsf to the bottom of Hole 801C at 936 mbsf, a tectonic breccia (VII) separates the Lower Massive Flows (VI, 720-890 mbsf) and a series of thin, generally <1-m-thick sheet flows and pillows, the Lower Pillows and Flows (VIII, 890-934 mbsf). The thickest flow in the former exceeds 20 m. Once more, the breccia zone also coincides with a change in geochemistry (Fig. F12), indicating that the lithologic breaks in eruption style or tectonism also correlate with the evolution in the magma composition.
The major changes in the resistivity and the natural gamma logs correspond to the major sequence divisions defined by lithology and geochemistry (Fig. F11). The logging data along with the FMS images will be integrated with the core descriptions to create as complete a section as possible from which the bulk geochemical composition of the upper oceanic crust at this site will be calculated.
This igneous sequence represents a key section recovered from fast-spreading crust (total basement penetration = 470 m) and will thus serve as an important type section with which to compare to the modern East Pacific Rise. The entire section between 530 and 890 mbsf is tholeiitic and extrusive in character. The tholeiites are all normal (N) mid-ocean-ridge basalt (MORB) (Fig. F12), with most falling on the same crystal fractionation trend from 7.5% MgO to 6% MgO (1200°-1130°C). Although it is highly altered, the capping tholeiite is very primitive, with abundant chrome spinel and >8 wt% MgO. Overall, from the base to the top of the section there is a decrease in MgO, until the lower hydrothermal unit, and then an increase in MgO until the upper hydrothermal unit (Fig. F12). During Leg 185 we recovered abundant, fresh basaltic glass in >20 cores, which represent the oldest volcanic glass sampled in oceanic crust and will be critical in assessing the primary magma compositions and possible changes in MORB melting parameters during the Jurassic. The presence of two Fe-Si hydrothermal deposits and associated intense basalt alteration within the Hole 801C section is consistent with fast-spreading crust, where hydrothermal activity is common among the spreading axis.
The ocherous hydrothermal units are a significant characteristic of Site 801. Although similar types of deposits exist near the modern East Pacific Rise, they have never been drilled in Pacific oceanic basement elsewhere. Fluid temperatures of formation calculated for the upper hydrothermal deposit (Sequence II) are ~16°-60°C (Alt et al., 1992). These fluids controlled the alteration budget of the underlying pillow basalt.
A primary objective for Leg 185 was to quantify the chemical alteration of Jurassic basement in the west Pacific in order to calculate geochemical fluxes to the Mariana subduction factory. Detailed work was done aboard ship logging vein types, breccia, hyaloclastite, interpillow units, and alteration color, as well as using the continuous multisensor track (MST) data and downhole logs to identify K- and U-rich zones from the natural gamma emission. Figure F13 provides examples of the major alteration types observed in the cores, as well as the interpillow sediments, which are clearly evident on the gamma logs (Fig. F11) and must contribute in a significant way to the alkali budget of the site. The dominant alteration minerals are calcite, smectite, pyrite, silica, celadonite, and Fe oxyhydroxides; different mixtures of these minerals define the alteration color of the cores (Fig. F13).
In tandem with the major change in the igneous units at ~720 mbsf, there is a change in the style of alteration. It is marked by a higher frequency of veins (27/m) and silica-rich interpillow material and sediment above 720 mbsf to less frequent veins (20/m) and more hyaloclastites below 720 mbsf. No interpillow sediments are found below 720 mbsf. The most extreme alteration is in the alkalic unit at the top of the basement and adjacent to the ocherous hydrothermal zones (Fig. F14). This is characterized by pervasive alteration of the igneous material to bleached pale green and buff-colored rocks, with significant concentrations of calcite, smectite, and celadonite, resulting in increases in K, CO2, and H2O contents. In the pale green and buff-colored rocks all of the ferromagnesian minerals have been destroyed, and there are gains in alkalis and losses of Mg, Fe, and trace metals. Hole 801C includes four distinct oxidative alteration zones: at the top of the basement in the brown alteration zone of the alkalic unit, adjacent to the upper hydrothermal zone (462-550 mbsf), adjacent to the lower hydrothermal zone at 610-630 m, and deep in the hole at ~750-900 mbsf (Fig. F15). These oxidative zones are flanked by gray basalt minimally altered in anoxic conditions (pyrite, calcite, and saponite) and still containing glass. The alternating oxidative zones with high fluid/rock ratios and anoxic alteration assemblages of lower fluid/rock ratios is unusual given what is more typically a general decrease downhole in oxidative alteration, as found at some other drill sites into oceanic crust. The pattern of alteration found in Hole 801C was controlled by the local permeability structure, which may have been influenced by clogging of circulation pathways with secondary minerals as the result of early low-temperature hydrothermal activity associated with the formation of the Fe-Si-hydrothermal deposits. This may be typical of fast-spreading oceanic crust.
Reconstructing the geochemistry of an incomplete sequence is challenging, particularly when most of the elements of interest in subduction fluxes (e.g., K, U, Ba, CO2, and H2O) are distributed heterogeneously. The logging data allow reconstruction of the section, particularly for quantifying brecciated and massive flow units. The gamma log provides K, U, and Th concentration data. Quantifying the number of veins and breccia intervals and integrating these data with geochemical information over the section also provides a means of calculating a mathematical average of the chemical composition of the cored section. Several scientists will work on a suite of 118 samples taken downhole to provide a comprehensive geochemical and isotope data base. Some of these samples will also be mixed to provide composite samples of the different sequences and geochemical units identified in the core (see "Preparation of Composites" in the "Explanatory Notes" chapter).
Two preliminary attempts were made shipboard to quantify the potassium content of the core. They both underscore the difficulties involved in making these estimations.
Firstly, shipboard measurements of K2O and the estimation of vol% alteration (1.7% halos, 0.05% celadonite veins, 1.5% breccia and hyaloclastite, and 96.75% relatively unaltered rock) indicate that the total section cored during Leg 185 has experienced ~17% increase in K2O and Rb contents as the result of seawater alteration. Adding interpillow sediment to this estimate increases the bulk K2O content of the Leg 185 section to 60% greater than in fresh basalt alone (from 0.08 to 0.13 wt% K2O), with 27% of the total alkali budget residing in interpillow sediment.
Secondly, a K2O budget for the core can be calculated after calibrating the natural gamma-ray signal from the MST using core analyses (see "Using Natural Gamma Ray to Calculate Potassium Budgets" in the "Site 801" chapter and Fig. F15). The MST-NGR signal agrees well with patent alteration features in the core, as well as with K2O determined from the natural gamma spectrometry logging tool (NGT). The bulk K2O calculated from the MST-NGR data for the entire tholeiitic section is 0.31 ± 0.1 wt%, which is much higher than the estimate above, based on the vol% of alteration, but agrees well with the average from the downhole logging data (0.36 wt%). The MST estimate would require that the 97% of the core that did not contain patent alteration features has 0.27 wt% K2O. An average K2O of 0.31 wt% is lower than that calculated for DSDP Site 417 (0.56 wt%; Staudigel et al., 1995) drilled in old Atlantic crust. The technique laid out in this report could be used to calculate bulk K2O at the few other ODP sites drilled deeply into basement (Holes 504B, 765C, 332) to start to form a better understanding of the controls on seafloor alteration fluxes.
Hole 801C was also unique in providing the opportunity to examine the causes for the low-amplitude uncorrelated magnetic anomalies observed within the so-called Jurassic Quiet Zone. The JQZ has been variously hypothesized as a time of no geomagnetic field reversals, of anomalously low geomagnetic field intensity, or of numerous rapid reversals. In combination with the previous results from Leg 129, the basement in Hole 801C shows a series of polarity reversals downhole. Figure F11 compares results from shipboard paleomagnetic measurements on cores with the magnetic signature from the continuous downhole geophysical logs. The cores analyzed with the shipboard magnetometer show a gradual change in the magnetic field direction from one polarity interval to the other. In both the magnetic logs and the shipboard analyses, numerous flows between those of opposite polarities display zero inclination values. These results indicate that the lavas were erupted in a period of rapid polarity fluctuations of the Earth's magnetic field. Although analysis of the core and logging data is incomplete, there appears to be a correlation between polarity changes and the different volcanic sequences. From the bottom of the hole upward, the first reversal occurs within the thin pillows and sheet flows of Sequence VIII (the major sequence change could possibly occur at the magnetic polarity change at 870 mbsf rather than at the breccia at 850 mbsf). The second polarity change is at the transition from the Lower Massive Flows (Sequence VI) to the Upper Pillows and Flows (Sequence IV). Given the spreading rate estimate of 160 km/m.y. (and, therefore, the rate at which the volcanic sequences must have formed), the lavas appear to be recording extremely rapid polarity reversals of the magnetic field. The fields may be canceling each other out, resulting in the low-amplitude uncorrelated magnetic anomalies that characterize the JQZ. The causes and rates of these rapid reversals require further research. The volcanic glass, which is preserved in small amounts down the hole, will be used to evaluate the hypothesized low intensity of the magnetic field in the Jurassic, a phenomenon that is perhaps related to the rapid reversals.