While the most spectacular manifestation of oceanic crustal fluid circulation is found along mid-ocean ridge axes in the form of high-temperature (350°-400°C) springs that deposit metal-sulfide minerals, a far greater flux of both heat and seawater occurs via hydrothermal circulation in the igneous crust of mid-ocean ridge flanks. Modelling and observations of heat flow indicate that advective heat loss globally through ridge flanks is more than triple that at ridge axes, and because this heat is lost at lower temperatures the volumetric flux of seawater through the flanks is proportionately even greater, 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 (Anderson et al., 1977; Sclater et al., 1976). 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 (Alt et al., 1986; Jacobson, 1992; Purdy, 1987). 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 the most important influence on water/rock interactions and thus control fluid chemistry and the chemical and physical alteration of the crust?
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 flanks, the western flank of the East Pacific Rise near 20°S, the western flank of the Mid-Atlantic Ridge, and the flanks 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 semi-quantitative.
As a result of several surface-ship and submersible programs completed during the past seven years, the eastern flank of the Juan de Fuca Ridge has become one of the most thoroughly studied ridge flanks. Representative examples of many of the hydrothermal environments found elsewhere have been discovered and examined in detail, and the crustal seismic structure has been imaged particularly well. By providing critical hydrologic, geophysical, and geochemical samples and observations of three subseafloor fluid-flow "type-example" systems that occur in remarkably simple form on this ridge flank; Leg 168 wll reveal more information on the nature of crustal evolution. The leg will take advantage of recent ODP technological advances including improvements to several tools that will greatly enhance the chance of success and the efficiency of operations of the program.