SCIENTIFIC OBJECTIVES

The general goal of Leg 193 was to delineate the subsurface characteristics of the PACMANUS hydrothermal system, particularly the volcanic architecture of Pual Ridge on which it is situated, deep-seated mineralization and alteration patterns, and structural and other characteristics that dictate the nature and flow of hydrothermal fluids. Testing for microbial life within the system and collecting subseafloor hydrothermal fluids were additional objectives.

Specific objectives included the following:

1. Establish whether Pual Ridge, the edifice hosting PACMANUS, was constructed as a "layer cake" sequence of lavas or, alternatively, by inflation of large lava domes or shallow intrusions.
2. Assess the temporal interplay between volcanic and hydrothermal activity, using any older sedimentary, ferruginous oxide, or massive sulfide horizons concealed within the lava sequence.
3. Compare and contrast alteration and mineralization characteristics and processes below sites of focused high-temperature venting and diffuse low-temperature venting, respectively.
4. Sample and delineate the characteristics and sources of hydrothermal fluids within the PACMANUS system.
5. Assess the source or sources of base metals and precious metals and seek explanations for the elevated contents of copper, zinc, silver, and gold in massive sulfide chimneys at the PACMANUS seafloor.
6. Delineate fluid pathways within the system, and establish a hydrologic model from structural patterns, physical properties, and isotope geochemical evidence in fresh and altered bedrocks.
7. Establish the nature and extent of microbial activity and the potential for biomineralization within the hydrothermal system.

Our ability to achieve these objectives was constrained by a number of factors, including poor core recovery, the small number of deep penetrations achieved relative to the evident complexity of the hydrothermal system, and absence or scarcity of certain anticipated phenomena—especially subsurface massive sulfides.