Newly acquired seismic refraction and microearthquake data from the TransAtlantic Geotraverse (TAG) segment of the Mid-Atlantic Ridge at 26°N reveal for the fi rst time the geometry and seismic character of an active oceanic detachment fault. Hypocenters from 19,232 microearthquakes observed during an eight month ocean bottom seismometer deployment form an ~15-km-long, dome-shaped fault surface that penetrates to depths >7 km below the seafl oor on a steeply dipping (~70°) interface. A tomographic model of compressional-wave velocities demonstrates that lower crustal rocks are being exhumed in the detachment footwall, which appears to roll over to a shallow dip of 20° ± 5° and become aseismic at a depth of ~3 km. Outboard of the detachment the exhumed lithosphere is deformed by ridge-parallel, antithetical normal faulting. Our results suggest that hydrothermal fl uids at the TAG fi eld exploit the detachment fault to extract heat from a region near the crust-mantle interface over long periods of time.

Within the endemic invertebrate faunas of hydrothermal vents, five biogeographic provinces are recognized. Invertebrates at two Indian Ocean vent fields (Kairei and Edmond) belong to a sixth province, despite ecological settings and invertebrate-bacterial symbioses similar to those of both western Pacific and Atlantic vents. Most organisms found at these Indian Ocean vent fields have evolutionary affinities with western Pacific vent faunas, but a shrimp that ecologically dominates Indian Ocean vents closely resembles its Mid-Atlantic counterpart. These findings contribute to a global assessment of the biogeography of chemosynthetic faunas and indicate that the Indian Ocean vent community follows asymmetric assembly rules biased toward Pacific evolutionary alliances.

[1] The results of detailed textural, mineral chemical, and petrophysical studies shed new light on the poorly constrained fluid-rock reaction pathways during retrograde serpentinization at mid-ocean ridges. Uniformly depleted harzburgites and dunites from the Mid-Atlantic Ridge at 15°N show variable extents of static serpentinization. They reveal a simple sequence of reactions: serpentinization of olivine and development of a typical mesh texture with serpentine-brucite mesh rims, followed by replacement of olivine mesh centers by serpentine and brucite. The serpentine mesh rims on relic olivine are devoid of magnetite. Conversely, domains in the rock that are completely serpentinized show abundant magnetite. We propose that low-fluid-flux serpentinization of olivine to serpentine and ferroan brucite is followed by later stages of serpentinization under more open-system conditions and formation of magnetite by the breakdown of ferroan brucite. Modeling of this sequence of reactions can account for covariations in magnetic susceptibility and grain density of the rocks.

The TAG active hydrothermal mound, located 2.4 km east of the neovolcanic zone at 26oN, Mid-Atlantic Ridge, is -200 rn in diameter, exhibits 50 rn of relief, and is covered entirely by hydrothermal precipitates. Eight different types of vent solids were recovered from the mound by the submersibles Alvin and Mir in 1986, 1990, and 1991. Detailed petrographic and geochemical studies of samples and their distribution are used to deduce patterns of fluid flowand seawater/hydrothermal fluid interaction. Geochemical modeling calculations using fluid composition data corroborate these interpretations. Current activity includes highly focused flow of 363øC fluid from a chimney cluster on the top of the mound and deposition of a high fS2-jD 2 mineral assemblage that reflects low concentrations of H2S in black smoker fluid. Slow percolation of black smoker fluid pooled beneath the black smoker cluster and entrainment of seawater result in formation of massive sulfide crusts and massive anhydrite. These three sample types are enriched in Co and Se. Blocks of sulfide and white smoker chimneys, enriched in Zn, Au, Ag, $b, Cd, and Pb, are forming on the surface of the mound from black smoker fluid that has been modified by mixing with entrained seawater, precipitation of sulfides and anhydrite, and dissolution of sphalerite within the mound. This is the first time that on-going remobilization, zone refinement, and significant modification of high-temperature fluid in the near surface has been documented in a seafloor hydrothermal system. Deposits of ocherous material and massive sulfide with outer oxidized layers that formed during previous hydrothermal episodes are exposed on the steep outer walls of the mound. Studies of the full range of samples demonstrate that highly focused fluid flow, consequent seawater entrainment, and mixing within the mound can result in formation of a large seafloor hydrothermal deposit exhibiting sample types similar to those observed in Cyprus-type ore bodies. Introduction Detailed studies of active seafloor vent sites allow investigation of the interrelationships among geologic setting, fluid composition, styles of seawater/hydrothermal fluid interaction, mineralogy and texture, metal remobilization, zone refinement, and the overall structure and development of massive sulfide deposits. The TAG active mound, located at 26øN, Mid-Atlantic Ridge (MAR), is particularly useful for investigating such interrelationships. The mound itself is large (-200 m in diameter), distinctly circular in plan view, with all black smoker activity concentrated in one area [Thompson et al., 1988]. This is in contrast to most other active seafloor vent sites where the size of individual mounds is considerably smaller (<10-30 m diameter), multiple Copyright 1995 by the American Geophysical Union. Paper number 95JB00610. 0148-0227/95/95JB-006 ! 0505.00 small deposits are aligned parallel to the ridge axis, and black smoker chimneys are discrete features dispersed on top of small mounds, instead of clustered in one area [...

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