SITE 801 PRINCIPAL RESULTSSite 801: A Jurassic Basement Reference Site
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, 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 ocean crust.
Basement Stratigraphy and Geochemistry
The basaltic section in Hole 801C is overlain by a sedimentary section characterized by an upper (56 m) pelagic clay unit, which overlies a 63 m-thick chert-porcellanite unit. These 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 in Hole 801C.
A stratigraphic column of the entire basement section is given in Figure 10. This section includes a composite of the rocks drilled on both Leg 129 and Leg 185. The uppermost basement (Sequence I) is alkaline in character and is 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 division 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. 11) 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 section, "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. 10). These thick flows have an exceptionally high resistivity (Fig. 11), 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 as <171 Ma, >157 Ma (Pringle, 1992).
The Upper Massive Unit 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. 12). 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 triple saturated in olivine, plagioclase, and clinopyroxene. In general, phenocrysts are scarce in the entire section drilled during Leg 185. Plagioclase is the most common phase, but most of the lavas are classified as aphyric. Figure 12, although intended to highlight the alteration characteristics in Hole 801C, 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 18 geochemical units, which probably correspond to discrete magmatic episodes.
A second Si-rich hydrothermal unit is present deeper in the section at 630 mbsf. This unit marks a change in magma composition and, thus, probably represents a significant hiatus in the volcanic evolution. Between 600 and 720 mbsf the section is characterized by a pillow dominated zone with well-developed interpillow horizons (Sequence IVUpper 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. 11), which is smooth and of low intensity throughout Sequence IV. Although not as distinct as for Division III, there is again a trend toward more mafic lavas of increasingly younger age through Sequence IV.
Below 720 mbsf, to the end of the hole at 934 m, a tectonic breccia separates the Lower Massive Flows (720-890 mbsf) and a series of thin, generally <1-m-thick sheet flows and pillows, the Lower Pillows and Flows (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. 12), indicating that the lithologic breaks in eruption style also correlate with the evolution in the magma composition.
In Figure 11, the major changes in the resistivity and the natural gamma logs correspond to the major sequence divisions defined by lithology and geochemistry. 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) MORB (Fig. 12), 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 up to 10% MgO. Overall, from the base to the top of the section there is a decrease in MgO (Fig. 12). During Leg 185, we recovered abundant, fresh basaltic glass in more than >20 cores, which represent the oldest volcanic glass in the oceans, and will be critical in assessing the primary magma compositions and possible changes in MORB melting parameters during the Jurassic. The presence of frequent sediment intervals in the upper volcanic section and low-temperature hydrothermal units (see below) may not be unusual for fast-spreading crust, whereas high-temperature focused hydrothermal deposits and low-temperature diffuse interval areas are very common along 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 oceanic basement elsewhere. Fluid temperatures of formation calculated for the upper hydrothermal deposit (Sequence II) give temperatures of ~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." Thus, detailed work was done aboard ship logging vein types, breccia, hyaloclastite, interpillow units, and alteration color, as well as using the continuous MST data and downhole logs to identify K- and U-rich zones from the natural gamma emission. Figure 13 provides a good overview, although not exhaustive, of the major alteration types observed in the cores. In addition, this core shows one of the interpillow sediments, which are clearly evident on the gamma logs (Fig. 11) and must contribute in a significant way to the alkali budget of the hole. The dominant alteration minerals are calcite, smectite, pyrite, silica, celadonite, and Fe oxyhydroxides; and, as shown in Figure 13, different mixtures of these minerals define the alteration color of the cores.
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 at <720 mbsf, to less frequent veins (20/m) and more hyaloclastites at >720 mbsf. The most extreme alteration is in the alkalic unit at the top of the basement and adjacent to the ocherous hydrothermal zones. This is characterized by pervasive alteration of the igneous material to bleached pale green and buff-colored rocks, with significant concentration 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 losses of Mg, Fe, and trace metals and gains in alkalis. Four oxidative alteration zones are in Hole 801C: at the top of the basement in the brown alteration 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. These oxidative zones are flanked by gray basalt minimally altered at anoxic conditions (pyrite, calcite, and saponite). 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 (Staudigel et al., 1995; Alt et al., 1986). 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 very fast spreading oceanic crust.
Calculating Element Budgets
Reconstructing the geochemistry of an incomplete sequence, which is heterogeneous in particular for most of the elements of interest in subduction fluxes (e.g., K, U, Ba, CO2, and H2O), is a challenging procedure. The logging information allows reconstruction of the section, particularly for quantifying brecciated and massive flow units. The gamma log provides quantitative information for K, U, and Th. 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 geochemistry of the cored section. The geochemical and isotope data will be obtained from a suite of 118 samples taken downhole on which several scientists will work to provide a data base. Some of these samples will also be mixed together to provide composite samples of the different sequences identified in the core.
Two preliminary attempts were made shipboard to quantify the potassium content of the core. They both underline the difficulties involved in making these estimations.
Firstly, using shipboard measurements for K2O and the estimation of vol% alteration (halos, 1.7%; breccia and hyaloclastite, 1.5%; celadonite veins, 0.05%), and 96.75% relatively unaltered rock, indicates 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, with 27% of the total alkali budget residing in interpillow sediment.
Secondly, by calibrating the MST results for the natural-gamma spectrometry tool (NGT) signal with respect to K2O analyses in altered and fresh rocks, it was shown to be possible to integrate the K2O signal for a section of core. Intercalibration of the MST-NGT signal and the gamma log are in reasonable agreement, with a cutoff in the detection limit at ~0.45 wt% K2O. Whereas this cutoff is higher than much of the background value in the least-altered rock, it provides an effective way of integrating the signal for all of the K-enriched zones downhole. The bulk K2O calculated from the MST-NGR data for the entire tholeiitic section is 0.31 wt%, and from the logged interval it is 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). 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.
Character of the Jurassic Quiet Zone
Hole 801C was also unique in providing the opportunity to examine the causes for an absence of magnetic anomalies, a characteristic of Jurassic basementthe so-called Jurassic Quiet Zone. The JQZ has been hypothesized variously as a time of no geomagnetic field reversals, of anomalous low geomagnetic field intensity, or numerous rapid reversals. In combination with the previous results from Leg 129, the basement in Hole 801C shows a series of polarity reversals downhole. The results from the continuous shipboard measurement downhole and the magnetic signature in the geophysical logs are shown in Figure 11 relative to the different basement sequences. 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 corresponds to a change from thin pillows and sheet flows to the Lower Massive Flows (from the stratigraphic column this change may be best placed at ~870 mbsf rather than at the breccia at 850 mbsf). The second polarity change is at the transition from the Lower Massive Flow sequence to the Upper Pillows and Flows. Given the spreading rate estimate for the ridge axis of 160 km/m.y, and therefore, the rate at which the volcanic sequences must have formed, the lavas must be recording rapid fluctuations of the magnetic field. The fields have the effect of canceling each other out and registering an average zero polarity. The reason for and the rate of these rapid fluctuations require further research. The volcanic glass that is preserved in small amounts down the hole will be used to evaluate the intensity of the magnetic field in the Jurassic, a parameter that is perhaps related to the rapid fluctuations.
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