168 Preliminary Report


While the most spectacular manifestation of oceanic crustal fluid circulation is high-temperature (350°-400°C) venting along mid-ocean-ridge axes, a much greater flux of hydrothermal heat and seawater occurs within the igneous crust along ridge flanks, well away from spreading centers. Modeling and observations of heat flow indicate that advective heat loss through ridge flanks is globally more than triple that at ridge axes, and because this heat is advected at lower temperatures, the volumetric flux of seawater through the flanks is proportionately even greater, perhaps more than 10 times that at the axes. Significant hydrothermal heat loss and fluid exchange between the crust and ocean typically continue to an age of several tens of millions of years and, thus, effect more than one-third of the ocean floor. This process plays an important role in the alteration of oceanic crust, which includes changes in its chemistry, mineralogy, and physical properties such as seismic velocity and attenuation. However, because of the wide range of conditions on ridge flanks and the limited amount of work done there to date, we know little about these processes in detail. Major questions remain: what mechanisms drive fluid flow through the crust and seafloor, what is the magnitude of elemental chemical exchange between the crust and water column, and what factors are most important in influencing water/rock interactions and, thus, in controlling fluid chemistry and the chemical and physical alteration of the igneous crust and its sediment cover?

Over the past two decades, several sites of hydrothermal circulation on mid-ocean-ridge flanks have been investigated. These include the Galapagos mounds, the southern flank of the Costa Rica Rift, the equatorial East Pacific Rise flank, the western flank of the East Pacific Rise near 20°S, the western flank of the Mid-Atlantic Ridge, and the flank of the Mariana Trough spreading axis. These sites represent a wide range in sediment thickness and continuity, sediment type, crustal age, and basement topography. Although these localities display a correspondingly wide range of hydrothermal conditions and processes, our understanding of the processes remains only semiquantitative.

As a result of a series of coordinated surface-ship and submersible studies that began with a reconnaissance survey in 1988, the eastern flank of the Juan de Fuca Ridge has become one of the most thoroughly studied ridge flanks. By providing critical hydrologic, geophysical, and geochemical samples and observations of three characteristic types of subseafloor fluid-flow systems (Hydrothermal Transition [HT] system from bare crust to sedimented crust; Rough Basement [RB] system with basement topography influencing fluid flow; and Buried Basement [BB] system with "flat" basement covered by sediment) that occur in simple form on this ridge flank, work conducted during Ocean Drilling Program (ODP) Leg 168 will greatly improve our understanding of ridge-flank hydrothermal circulation and crustal evolution.

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