Investigating the Formation of the Active Tag Hydrothermal Mound Using Observational, Petrographic and Geochemical Data

Abstract

During Leg 158 of the Ocean Drilling Program in October-November 1994, the active TAG hydrothermal mound (~26~ on the Mid-Atlantic Ridge) will be drilled to investigate the structure and processes involved in the formation of a large, mature hydrothermal deposit on a slow-spreading mid-ocean ridge. It is likely that these drilling operations will significantly impact the subseafloor circulation system and result in major changes in the fluid flow pathways. These in turn will affect the morphology of the hydrothermal deposits, and the distribution of different types of venting activity. With appropriate characterization of the structure before, during, and after drilling, this perturbation and the subsequent response and recovery can be used to better constrain some of the physical properties of the hydrothermal system. An international effort is underway to conduct such studies, with participation from U.S., British, and Japanese investigators. During two submersible programs in 1986 and 1990, we documented the surficial distribution of the various types of venting activity on the active TAG hydrothermal mound. In addition, we have completed detailed petrographic and geochemical studies of a comprehensive suite of solid samples collected during a number of submersible dives. From these two sets of data, we have deduced patterns of flow and seawater/hydrothermal fluid interactions within the mound, thereby characterizing the hydrothermal system prior to drilling.