168 Preliminary Report


Ocean Drilling Program Leg 168 investigated three hydrothermal regimes located on the sediment-covered eastern flank of the Juan de Fuca Ridge. The three regimes are (1) a transition from very young (<0.8 Ma) unsedimented igneous crust to sediment-buried crust (0.8-1.2 Ma), (2) an older (~3.5 Ma) area with large topographic relief (>300 m) of igneous crust under a thick (250-600 m) sediment cover, and (3) an intermediate age (1.4-2.6 Ma) area of low relief igneous crust buried continuously over tens of kilometers by sediments averaging about 300 m thick. Ten sites were cored and roughly 1500 m of sediment and basalt was recovered. A total of four CORKs were emplaced, with thermistor strings and osmotic water samplers, in Holes 1024C, 1025C, 1026B, and 1027C. Temperature, pressure, and water chemistry data will be recorded from these holes over the next 3 to 5 years. The first direct sampling of basement fluids in Deep Sea Drilling Project/Ocean Drilling Program history was achieved in Hole 1026B, using the Water-Sampling Temperature Probe (WSTP).

Sites 1023 to 1025 span the hydrothermal transition and show a progressive change in basal sediment pore-water chemistry from normal seawater values with distance from the Juan de Fuca Ridge axis and the area of unsedimented basement. Alteration of basement basalts and upper basement temperatures also increase with distance from the area of outcrop. Slow upflow of pore fluids through the sediment section was detected at Site 1025. Pore-water chlorinity profiles at all three sites suggest that seawater is flowing laterally from the bare igneous crust of the ridge axis eastward through the upper crust, with residence times in basement of perhaps only a few thousand years.

Sites 1026 and 1027 are located only 2 km apart, on a buried basement ridge and basement trough, respectively. Sediment pore-water compositions and upper basement temperatures are similar at both sites (61°-63°C), despite the large difference in sediment thickness (250 vs. 600 m). Fluid flow in basement is, thus, inferred to be sufficiently vigorous to maintain relatively uniform temperatures at the sediment/basement interface. Direct sampling of basement pore water from Hole 1026B shows that some elemental concentrations are quite different from those measured in the basal sediment interstitial water, indicating significant reaction in the basal sediments. The basement pore waters are similar to those expelled from nearby basement outcrops, indicating a high degree of compositional homogeneity, which is consistent with the temperature data. Basement pore waters flowing up through Hole 1026B were about 2°-3°C warmer than the temperature at the sediment/basement interface. This indicates that the first zone of high permeability is somewhat deeper than the sediment/basement interface.

Sites 1028, 1029, and 1032 form a transect perpendicular to the ridge axis, across a broad area with low-relief ocean crust that was thought to be relatively isolated from the overlying ocean by a continuous blanket of sediments 200-300 m thick. Sites 1028, 1029, and 1032 were designed to measure basement temperatures in order to calculate regional lithospheric heat flux and to allow determination of basement fluid compositions over a range of temperatures. Basement basalts at these sites all show an initial stage of oxidative alteration, requiring significant open seawater circulation, followed by subsequent alteration representing relatively closed hydrothermal circulation. Pore-water geochemistry shows evidence for incompletely evolved seawater in the basement. Basement temperatures at these sites are much lower than would be expected (51°-59°C), if the crust was fully sealed from exchange of hydrothermal fluids with the ocean. This, and the basement-water compositions, indicate that there is significant lateral transport of basement fluids, and advective heat loss, over much greater distances than previously thought.

Sites 1030 and 1031 were drilled to investigate a local geochemical anomaly on a shallow (<50-m sediment) buried ridge. High chlorinity values and other elemental concentrations in the sediment pore waters may indicate a deeper reaction zone and/or a longer residence time in basement for these fluids, in contrast to sites drilled to the east and west. Pore-fluid chemical profiles indicate an upward flow of pore waters through the sediment section at these thinly sedimented sites.

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