Eruption of volcanic rocks at midocean ridges is the major mechanism by which heat is lost from the interior of the Earth. Approximately one-third of the heat delivered by the emplacement of magma at the spreading centers is removed by circulation of cold seawater through the hot volcanic rocks that make up the oceanic crust. Seawater interacts with the volcanic rocks at temperatures near 400ºC resulting in substantial chemical exchange between seawater and the igneous basement. The flux of elements between seawater and the oceanic crust makes an important contribution to buffering the composition of some elements in seawater (Edmond and Von Damm, 1983; Elderfield and Schultz, 1996). Discharge of hydrothermal fluids onto the seafloor also results in the deposition of metallic sulfides at hydrothermal vents, forming deposits similar to volcanic-associated massive sulfide (VMS) ore deposits (Hannington et al., 1995).
Basaltic rocks are exposed directly at the seafloor over nearly the entire length of the midocean ridge system, but locally, where ridges are near the continental margins, the spreading centers can be covered by sediment. Hydrothermal circulation is still an important process at sediment-covered spreading centers, but the presence of a low permeability sediment blanket over the more permeable volcanic basement changes the fluid and heat flux of the hydrothermal system (Davis and Fisher, 1994).
Leg 169 was the second leg of a planned two-leg program to investigate the geological, geophysical, geochemical, and biological processes at sediment-covered spreading centers in the northeast Pacific Ocean (Fig. F1). Building on the highly successful Leg 139 drilling (Davis, Mottl, Fisher, et al., 1992), our primary goal was to investigate the genesis of massive sulfide deposits by drilling two deposits at different stages of maturity: Middle Valley at the northern end of the Juan de Fuca Ridge and Escanaba Trough at the southern end of the Gorda Ridge. If our objectives were achieved, we expected Middle Valley to reveal details about a mature system, whereas the Escanaba Trough deposits were thought to be representative of the genesis of a massive sulfide-bearing seafloor hydrothermal system. The four primary topics investigated during this leg were (1) the mechanism of formation of massive sulfide deposits at sediment-covered ridges, (2) the tectonics of sedimented rifts and controls on fluid flow, (3) the sedimentation history and diagenesis at sedimented rifts, and (4) the extent and importance of microbial activity in these environments.
This report reviews the geological setting of the sites investigated during Leg 169 and provides an overview of the scientific results from shipboard observation and postcruise studies on samples recovered by drilling. At the time this report was written, much of the postcruise research was still in progress, so this overview should be considered to be preliminary.
Middle Valley is a medium-rate spreading center (58 mm/yr) that forms one leg of a ridge-transform-transform unstable triple junction with the Sovanco Fracture Zone and the Nootka fault (Davis and Villinger, 1992). The proximity of Middle Valley to the cold Explorer plate results in a reduced magma supply and a slow-spreading ridge morphology with a deep and wide axial trough. Current magmatic activity is mostly confined to the adjacent West Valley spreading center, indicative of a recent jump in the location of the plate boundary. Proximity of the Middle Valley spreading center to an abundant supply of terrigenous sediment during the Pleistocene sea-level lowstand has resulted in burial of the spreading center by 200 to >1000 m of turbiditic and hemipelagic sediment, with sediment thickness increasing to the north. Two areas of hydrothermal activity, separated by ~4 km, were initially drilled during Leg 139 and were further investigated during Leg 169. The Dead Dog vent field (Sites 858 and 1036) is the area of most active venting in Middle Valley (Fig. F2). The Bent Hill area (Site 856 and 1035) at present has limited hydrothermal activity, but was formerly the site of vigorous high-temperature hydrothermal discharge. Two large mounds of massive sulfide were investigated by drilling in the Bent Hill area.
Bent Hill is one of a string of small mounds that runs parallel to the eastern rift-bounding normal fault scarp. These features lie close to an inferred normal fault that appears to offset basement reflectors, but near-surface sediment layering imaged in seismic reflection profiles appears to be continuous across this fault. Bent Hill is a roughly circular feature 400 m in diameter that has been recently uplifted ~50 m (Fig. F3). It is bounded on the west by a steep scarp that parallels the rift-bounding faults and exposes semiconsolidated turbiditic sediment. Bright, reverse polarity seismic reflections that are limited in extent to the area under Bent Hill are interpreted to be generated at the interface between the base of basaltic sills and the underlying sediments (Rohr and Schmidt, 1994).
The 35-m-high Bent Hill Massive Sulfide (BHMS) deposit is located 100 m south of Bent Hill. A second twin-peaked massive sulfide deposit (ODP Mound) is located ~330 m farther south. These mounds are aligned parallel to the north-south-trending scarp that constitutes the western side of Bent Hill. The BHMS deposit is extensively weathered to Fe oxyhydroxides and partially buried by sediment. During Leg 139, Hole 856H penetrated 94 m of massive sulfide before the hole had to be abandoned because of an inflow of heavy sulfide sand from the upper weathered section of the borehole wall. A strong magnetic anomaly across this mound, related to the occurrence of magnetite, had been modeled to suggest that mineralization continued at least another 30 m below the level drilled and possibly much deeper (Tivey, 1994).
The morphology, degree of oxidation, and the lack of sediment cover on the mound south of the BHMS deposit indicate that these deposits are younger than the Bent Hill deposit. A local heat-flow anomaly occurs around the BHMS because of the high thermal conductivity of the massive sulfide body (Fisher et al., 1997). A more pronounced heat-flow high occurs at ODP Mound (Davis and Villinger, 1992; Fisher et al., 1997; Stein et al., 1998). Prior to drilling, a single 264°C hydrothermal vent had been located on the northern mound. The composition of the vent fluid is generally similar to those from the Dead Dog vent field, but this vent has lower salinity and only half as much dissolved Ca (Butterfield et al., 1994). Hole 1035H was spudded ~30 m away from a second area of hydrothermal venting that occurs near the saddle of ODP Mound. This area of hydrothermal venting was observed during the camera survey to spud Hole 1035H and was subsequently visited and sampled using the Alvin submersible. The maximum vent temperature measured was also 264°C (R. Zierenberg, unpubl. data).
The principal center of hydrothermal activity in Middle Valley is the Dead Dog vent field. Contoured heat-flow values show a concentric high (Davis and Villinger, 1992; Fisher et al., 1997; Stein et al., 1998), which is coincident with a sidescan acoustic anomaly (Fig. F4) that outlines the 800-m-long and 400-m-wide vent field. Sediment thickness over the fault block in the area surrounding the vent field is ~450 m and overlies a sill-sediment complex that forms the transition to oceanic crust (Davis, Mottl, Fisher, et al., 1992). Hard acoustic reflectors recorded immediately beneath the vent field during site surveys were determined by drilling during Leg 139 to be the top of a volcanic edifice at only 250 m depth. The presence of more permeable volcanic basement penetrating up into the sediment cover acts as a conduit that focuses flow of hydrothermal fluid to the seafloor (Davis and Fisher, 1994). The vent field contains at least 20 active vents with fluid temperatures ranging up to 276°C (Ames et al., 1993). Active vents are predominantly on top of 5- to 15-m-high sediment-covered mounds a few tens of meters in diameter. Available data from piston cores and Hole 858B suggested that subsurface deposition of anhydrite, Mg-rich smectite, and sulfide minerals contribute to the growth of the mounds. Because the high-temperature hydrothermal fluid is strongly depleted in both Mg and SO4, the abundance of these minerals in the subsurface requires that cold seawater (with abundant Mg and SO4) is drawn into the subsurface by the vigorous upflow at the active vent sites. In situ pore-pressure measurements (Schultheiss, 1997) and hydrologic modeling (Stein and Fisher, in press) indicate that seawater is being recharged into the uppermost part of the hydrothermal system. Modeling of pore-fluid compositional gradients in piston cores collected from the Dead Dog vent field also indicates local drawdown of seawater into the sediments (Glenn, 1998).
A major step toward the establishment of seafloor observatories was taken during Leg 139 by instrumentation of two sealed boreholes in the Middle Valley hydrothermal field using a circulation obviation retrofit kit (CORK) system (Davis and Becker, 1994a). The thermistor strings in these sealed boreholes eventually failed because of high formation temperatures. Part of the science plan for Leg 169 was to reinstrument these sealed boreholes to allow active experimentation on induced seismicity in a seafloor hydrothermal system and hole-to-hole hydrologic experimentation designed to constrain the physical and hydrologic properties that control hydrothermal flow on the scale of an entire vent field. An additional objective of Leg 169 was sampling of hydrothermal fluids from these sealed boreholes.
The Gorda Ridge spreading center is located offshore of Oregon and northern California and is bounded by the Mendocino Fracture Zone on the south and the Blanco Fracture Zone on the north (Fig. F1). A small offset in the spreading axis at 41°40´N latitude marks the northern boundary of Escanaba Trough, which forms the southernmost part of Gorda Ridge. Escanaba Trough is opening at a total rate of ~24 mm/yr and has a morphology consistent with its slow-spreading rate. The axial valley, which is at a depth of 3300 m, increases in width from ~5 km at the north end to >15 km near the intersection with the Mendocino Fracture Zone.
South of 41°17'(S)N latitude, the axial valley of Escanaba Trough is filled with several hundred meters (Fig. F5) of turbiditic sediment. The sedimentary cover thickens southward and is a kilometer or more in thickness near the Mendocino Fracture Zone. Turbiditic sediment enters the trough at the southern end and is channeled northward by the axial valley walls (Vallier et al., 1973; Normark et al., 1994; Brunner et al., 1999). Sedimentation was relatively rapid (up to 10 m/1000 yr) during sea-level lowstands in the Pleistocene, and the entire sediment fill of the trough probably was deposited within the last 100 k.y. (Normark et al., 1994; Davis and Becker, 1994b; Brunner et al., 1999). A reference hole through this sedimentary package and into basaltic basement was drilled to provide background information to evaluate the sedimentary and thermal history in an area away from the hydrothermal upflow zone.
Seismic reflection surveys show that the floor of Escanaba Trough is generally a smooth, flat plain underlain by continuous and relatively undisturbed turbidites. However, local areas along the axis of spreading have irregular seafloor topography characterized by circular hills 0.5 to 1.2 km in diameter that are uplifted 50 to 120 m above the surrounding seafloor. The areas of sediment disruption are sites of recent axial-rift igneous activity. Geologic and geophysical evidence suggests that axial-rift igneous activity at these sites is manifested by the intrusion of dikes, sills, and laccoliths into the sediment with less abundant volcanic flows (Morton and Fox, 1994; Zierenberg et al., 1993, 1994). Sulfide mineralization has been sampled by dredging, sediment coring, or submersible at four igneous centers within the sediment-covered part of Escanaba Trough. The dominant morphologic feature in the area of operations for Leg 169 was Central Hill (Fig. F6).
The western, sediment-covered part of Central Hill contains the most extensive sulfide deposits observed in Escanaba Trough. The massive sulfide deposits on the west and southeast flanks of Central Hill are actively venting hydrothermal fluid, and the area on the northern flank shows indications of very recent hydrothermal activity, suggesting that these deposits are all part of the same hydrothermal system. The best explored and most hydrothermally active area of sulfide mineralization on Central Hill extends west and north from the northern end of the sediment-covered hilltop. On the northern flank of the hill, massive sulfide extends >270 m from north to south and >100 m from east to west, but the western edge of the deposit has not been defined with certainty. All of the active hydrothermal vents are in an area with abundant sulfide mounds along the northwestern flank of the hill. The major element composition of the end-member hydrothermal fluid of two actively discharging vents 275 m apart is identical (Campbell et al., 1994), a result that is consistent with the hypothesis that this large mineralized area is a single hydrothermal system hydrologically interconnected at depth. The highest vent-fluid temperature measured in this field is 217°C, and the hydrothermal fluids venting in 1988 had a chloride content ~24% higher than bottom seawater (Campbell et al., 1994).
Sulfide samples collected at the surface of the deposit are predominantly pyrrhotite with variable amounts of sphalerite, isocubanite, and chalcopyrite, and minor galena, lollingite, arsenopyrite, and boulangerite. Sulfate is present as barite crusts and chimneys on massive sulfide and intergrown barite-anhydrite in active vents. When compared to Middle Valley, the abundance of barite and enrichment of metals such as lead, arsenic, antimony, and bismuth indicate extensive contribution from sediment source rocks (Koski et al., 1994). Lead isotope ratios of massive sulfide indicate that most of the lead in the deposits is derived from leaching of sediments (Zierenberg et al., 1993). Precious metals are significantly enriched relative to Middle Valley massive sulfide (Koski et al., 1994). Sediment alteration associated with formation of massive sulfides is dominated by Mg-rich chlorite, talc, or Mg-rich smectite (Zierenberg et al. 1994).