Leg 168-Juan de Fuca Hydrothermal

Although the most spectacular manifestation of oceanic crustal fluid circulation can be 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. Modeling 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. 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, little is known 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? 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?

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 sub-seafloor, fluid-flow, "type-example" systems that occur in remarkably simple form on this ridge flank, Leg 168 will 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.

Leg Objectives

The primary objectives of Leg 168 are focused on exploring the causes and consequences of ridge-flank hydrothermal circulation by drilling a suite of relatively shallow holes that will allow observation of lateral gradients of temperature, pressure, fluid composition, and rock alteration. Stated in general terms, the leg objectives are:

• To determine the thermophysical characteristics of hydrothermal circulation in the upper oceanic crust in off-axis settings as influenced by crustal topography, sediment cover, and permeability;
• To determine the sensitivity of crustal fluid composition to the age, temperature, and degree of sediment burial of the igneous crust;
• To examine the nature and fundamental causes of physical consolidation and mineralogical and chemical alteration of the igneous crust as functions of age and degree of sediment burial; and
• To improve constraints on the fluxes of heat and elements between the permeable igneous crust and the overlying ocean.

Operations Update

Although virtually all individual holes planned for Leg 168 are separated by greater distances than normal for single-site specifications (i.e., as defined by the range for single-beacon acoustic navigation), the holes have been grouped into three operational "super sites" according to the geographic area and the primary objectives: (1) the Hydrothermal Transition site, (2) the Permeable Penetrators site, and (3) the Lithospheric Heat Flow site.

The specific drilling strategy to be adopted during the leg involves a mixture of conventional drilling, coring, and reentry operations arranged in a sequence of single holes at each site. Although this represents a departure from traditional ODP operations, it provides an efficient means by which the primary objectives, including the safe establishment of four cased reentry holes sealed with CORK hydrologic observatory installations, can be met during the 56 days allotted to Leg 168.

Long-Term Borehole Observatories (CORKs)

CORKs were deployed during ODP Legs 139, 146, and 156. The CORK system comprises a modified reentry cone, a hydrologic seal that fits inside the throat of the cone, a data logger with sufficient power and memory to record data for several years, a valve mechanism by which the sealed hole can be vented to the overlying ocean, and a sensor cable to monitor formation pressure and temperature as a function of depth. A continuous fluid sampler will also be installed as part of each sensor cable. The samplers will provide a time-series sample of basement water as the perturbations associated with drilling dissipate.

The CORK deployments during Leg 168 are intended to quantify accurately the temperatures and pressures in uppermost basement that are responsible for vertical and lateral fluid, heat, and solute transport and any resulting gradients in fluid geochemistry. The Leg 168 operations strategy was designed to maximize the chances for four CORK deployments during a single cruise.

To Semiannual Report Contents

To Program Updates, Leg Highlights Contents