Summary of Objectives
The primary objectives of ODP Leg 168 are focused on exploring the causes and consequences of ridge-flank hydrothermal circulation through drilling a suite of relatively shallow holes that will allow observations of lateral gradients of temperature, pressure, fluid composition, and rock alteration. Stated in general terms, the objectives are (1) to determine the thermo-physical characteristics of hydrothermal circulation in the upper oceanic crust in off-axis settings as influenced by crustal topography, sediment cover, and permeability; (2) to determine the sensitivity of crustal fluid composition to the age, temperature, and degree of sediment burial of the igneous crust; (3) to examine the nature and fundamental causes of physical consolidation and of mineralogical and chemical alteration of the igneous crust as functions of age and degree of sediment burial; and (4) to improve constraints on the fluxes of heat and elements between the permeable igneous crust and the overlying ocean. The program will address these questions in the true spirit of a field experiment, in which several hypotheses will be critically tested with observations that can be made by drilling in each of the type-example areas.
Transition from Open to Sediment-sealed Hydrothermal Circulation
About 20 km east of the axis of the Endeavour segment of the Juan de Fuca Ridge, the elevated relief of the ridge crest plunges beneath the western edge of Cascadia Basin (Fig. 4). A fundamental change in the nature of hydrothermal circulation occurs in this area. West of the edge of the abyssal plain turbidites, basement is covered by a relatively thin (less than 1 m to a few tens of meters), discontinuous veneer of hemipelagic sediment through which fluids can pass with little hydrologic impedance, whereas to the east, the igneous crust is blanketed continuously by turbidite sediments that create a hydrologic barrier. Heat flow and estimated upper crustal temperatures, seismic velocities in the upper crust, and estimates of basement pore-fluid compositions all show clear lateral gradients that are probably associated with the transition from open to sealed hydrothermal circulation (Fig. 5). Heat flow and estimated basement temperatures increase systematically to the east, away from the area of exposed basement (Davis et al., 1992). Estimated temperatures in the upper igneous crust increase from less than 10°C near where basement rocks outcrop to about 40°-50°C 20 km to the east. Over the same distance the average heat flow increases from less than 15% to more than 80% of the value expected for the underlying lithosphere. Basement pore-fluid compositions estimated from sediment pore-fluid studies (Wheat and Mottl, 1994) change from close to that of seawater near the outcrop to strongly depleted in magnesium and enriched in calcium at a location 20 km to the east. Inferred chlorinities of the fluids in basement 20 km east of the point of burial are considerably higher than seawater. At least part of the increased chlorinity must be due to ongoing hydration reactions in the crust. This is consistent with the increase in basement temperature and with seismic data, which also reveal a systematic change. Interval velocities determined for the upper crustal seismic Layer 2A increase from values that range from 3000-3500 m/s to values exceeding 5000 m/s over the same spatial interval of about 20 km (Rohr, 1994). While these velocities have been determined for a layer known to have strong vertical velocity gradients, they probably indicate a significant increase in velocities throughout Layer 2A. The increase is believed to indicate a decrease in porosity from alteration (e.g., Wilkens et al., 1991). A similar magnitude of change occurs on other ridge flanks (e.g., Houtz and Ewing, 1976), although at a much slower rate, probably because of the much more gradual hydrologic isolation of the upper igneous crust. The rapid burial of the crust on the eastern Juan de Fuca flank has accelerated the process and made it possible to relate unambiguously the change in seismic velocities to a measured change in the hydrothermal regime.
Drilling in this area of "Hydrothermal Transition" will address numerous fundamental questions about lateral fluid and heat transport and about the physical and chemical alteration of the crust that results from water-rock interaction. Specific questions include:
2. What is the source and magnitude of the pressure gradient that drives the flow?
3. How do the changes in fluid chemistry and temperature with distance from sediment-free areas affect the nature of rock alteration?
4. What is the dominant factor responsible for the increase in upper crustal velocity?
5. Is there an accompanying decrease in permeability?
6. At what rate does the alteration take place?
Topographically Enhanced Fluid Flow
At a point roughly 100 km from the ridge axis, there is an abrupt change from the region of relatively smooth basement, typified by the "Lithospheric Heat Flow" drilling site discussed below, to a region where the basement surface is much more rugged. In this region, basement topography consists primarily of linear ridges and troughs produced by block faulting and by variations in volcanic supply at the time the seafloor was created. Local relief between ridges and troughs of 300 to 500 m is common, and major ridges are separated typically by 3-7 km (e.g., Fig. 6A, 6B, 6C). All of this relief is now buried by the turbidites of Cascadia Basin. The tops of two ridges lie a few tens to a few hundreds of meters below the sediment surface. At three locations along these ridges, small volcanic edifices rise above the sediment surface to form small, isolated basement outcrops (Figs. 7, 8).
It has long been suggested that basement topography and local basement outcrops play a key role in seafloor hydrogeology by serving to enhance buoyancy-driven flow (e.g., Fisher et al., 1990; Fisher and Becker, 1995; Hartline and Lister, 1981; Lowell, 1980) and to focus flow from the igneous crust into the oceans (e.g., Lister, 1972; Davis and Becker, 1994). The simplicity of the local structure and the small size of the volcanic outcrops in this part of the east flank of the Juan de Fuca Ridge make the area an ideal target for seafloor studies. Results of a series of systematic surveys have confirmed several things about fluid flow in the crust and discharge through the seafloor in this environment:
2. In the few locations where samples could be obtained, basement-fluid compositions also appear locally homogeneous (Mottl and Wheat, 1994).
3. Fluids leak through the sediment "seal" above buried basement ridges at geochemically detectable rates (<1 to tens of mm/y) that are inversely proportional to the local sediment thickness (Davis et al., 1992).
4. Fluids flow through the basement outcrops at rates sufficient to generate detectable thermal, chemical, and light transmissivity anomalies in the water column (Thomson et al., 1995).
5. Discharge through the outcrop has been sufficiently long-lived to allow significant hydrothermal precipitates to accumulate. Cores from the outcrop have recovered green clay, semilithified black ferromanganese crusts and layers, and reddish iron oxides in indurated sediment (Mottl et al., 1993).
6. The thermal structures of all three basement outcrops are fundamentally the same, allowing us to conclude that upflow and discharge not only are stable and long-lived, but also are a general consequence of the permeability and temperature structure of what we referred to as "Permeable Penetrators" (Figs.7, 8).
Examples of topographically influenced circulation within the upper igneous crust occur on the Costa Rica Rift flank (holes in the vicinity of and including Hole 504B; e.g., Langseth et al., 1988; Mottl et al., 1989), and possibly on the northern flank of the Galapagos spreading center (Green et al., 1981). Again, the high degree of burial makes the Juan de Fuca Ridge flank an exaggerated example of topographically influenced fluid flow, but one that is ideally suited to the study planned for Leg 168.
Whereas the general characteristics of fluid flow within the crust and through the seafloor in this environment are reasonably well constrained, proper quantification of the hydrologic regime and elucidation of the consequences of fluid flow require drilling. With the array of holes planned, the following questions can be addressed:
2. What implications can be drawn about the bulk hydrologic transport properties of the upper crust from constraints gained from observations of lateral temperature, pressure, and compositional gradients?
3. How is permeability distributed within upper basement?
4. To what degree has hydrothermal alteration of the crust proceeded at this 3.5-Ma site?
5. How have physical properties, namely velocity and permeability, changed with alteration?
6. What are the nature and magnitude of the forces that drive fluid flow through the sediment section above basement ridges and through "Permeable Penetrators," where basement is exposed at the seafloor?
7. What is the source of the fluids that vent through the seafloor at the outcrops? Do they simply come from the inferred homogeneous basement "reservoir" regionally sealed beneath the sediments or is there a component from a deeper source?
8. How are sediments chemically and physically affected by fluid seepage? Can a declining rate of seepage through the sediment section be tracked through the history of burial?
9. What is the nature of recharge? Is seawater supplied to the crust solely by regional diffuse flow through the sediments away from basement highs, or do some fluids enter the crust via locally focused pathways?
Lithospheric Heat Flow
In a region spanning ~60 to 100 km from the ridge crest, igneous basement is relatively smooth and continuously sedimented (Fig. 9). A suite of parallel seismic lines running perpendicular to the ridge demonstrates that this character is continuous for at least 30 km along strike. Because the sediment layer covering basement is so uniform and regionally extensive, the site provides an ideal target for determining accurately the level of total heat loss from young oceanic lithosphere. Local heat-flow variability normally associated with sediment thickness variations should be small, and the closest basement outcrops through which undetected heat could be lost advectively are sufficiently far away that they should have an insignificant effect.
Accurate determination of the "normal" total rate of heat loss from oceanic lithosphere has been pursued for as long as marine heat-flow measurements have been collected. Obtaining high-quality lithospheric heat-flow "calibration" data has been extremely difficult, however, because of the perturbing effects of hydrothermal circulation that are usually very difficult to avoid (e.g., Sclater et al., 1976). Many studies critically depend on knowing the relationship between lithospheric heat flow and age. The heat flow vs. age relationship allows determination of estimates of deep-rock thermal properties (e.g., Lister, 1977). It allows accurate estimation of global heat loss using the known global distribution of seafloor age (e.g., Williams and Von Herzen, 1974). An accurate background "reference" heat flow vs. age relationship allows the thermal anomalies associated with mantle plumes, asthenospheric convection, and widespread reheating events to be determined (e.g., Von Herzen et al., 1982). And, of course, it allows estimation of local and regional hydrothermal fluid budgets (see Fig. 5).
In the past, heat-flow measurements were attempted in this otherwise ideal study area, but shallow sandy layers prohibited probe penetration. As an alternative, temperature and thermal conductivity data from a single borehole could provide an accurate estimate of the regional basement temperature. This, combined with the well-determined depth to basement defined by the seismic data and the knowledge that efficient lateral hydrothermal heat transport maintains an effectively isothermal sediment/basement surface will allow an excellent determination of the regional lithospheric flux to be made. In addition to addressing this objective, drilling at this 2.8-Ma site, which is located between the two principal sites (1.0 and 3.5 Ma), will also provide intermediate samples of basement rocks and water along the overall transect from relatively fresh to hydrothermally altered oceanic crust.
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