摘要：

In April-May 2003, the Lost City Hydrothermal Field was investigated during 19 Alvin dives and 17 missions with the autonomous vehicle ABE to examine the linkages among geological, chemical and biological processes associated with a submarine hydrothermal system hosted on mantle material. In concert, these two programs resulted in 1) delineation of the geologic features that control hydrothermal flow in this area; 2) an extremely high-resolution bathymetric map (meter scale) of the field and adjacent areas of the Atlantis Massif; 3) interdisciplinary sampling of 10 individual venting sites within the field; and 4) documentation of a nearly continuous zone of deformation at the top of the massif that is very likely the surface expression of a long-lived detachment fault that caps the massif. This hydrothermal system, which is driven by exothermic serpentinization reactions beneath the Atlantis Massif, is unlike any known field examined to date. It is hosted on 1-2 my old variably altered mantle material, it contains more than 30 carbonate chimneys that reach up to 60 m in height, and generation of diffusely venting 40-90C fluids with pH 9-11 that are enriched in methane, hydrogen and other hydrocarbons support dense microbial communities. ABE bathymetry shows that a linear array of the largest structures within the field is controlled by an E-W trending, 200 m long lineament intersected by a N-S trending fault. Mapping of the near vertical cliffs adjacent to the field indicates that much of the subsurface flow within this area is controlled by very gently west-dipping faults that result in a nearly horizontal, sheet-like style of flow. Venting of diffuse fluids directly from the near vertical walls forms perpendicular growths of carbonate flanges, and results in the formation of vertical spires, and massive, shingled deposits that cascade down the cliff faces. The plumbing system within this area is very different from the vertical conduits that typify black smoker environments. The large surface areas exposed to hydrothermal fluids along the gently dipping faults may provide important environments for microbiological communities within the subsurface. Stockwork systems and variably cemented breccias preserved along the steep walls immediately adjacent to the field are reminiscent of ancient ophicalcite deposits preserved in ophiolitic rocks since the Archean. The Lost City Field is an astounding, intensely beautiful area that hosts numerous composite chimneys that extend over an area >350 m in length. Many structures contain an array of delicate flanges, multiple pinnacles, and beehive deposits. The complex structure Poseidon dominates the field: it is over 60 m in height, >40 m in length and it hosts multiple active and inactive towers, smaller pinnacles, and flanges. It is unlikely that hydrothermal systems like Lost City are unique along the global mid-ocean ridge spreading network; where massifs similar to those at the Atlantis Fracture Zone are common. In these environments, intense long-lived faulting and seismic activity, coupled with serpentinization reactions act depth serve to promote hydrothermal flow.

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