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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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Basalts cored on legs 2 and 3 of the Deep‐Sea Drilling Project (DSDP) range in sea floor spreading age from 18 to 67×106 yr. Although many of the basalts are highly altered, fresh glass is usually present. Except for site 2–10 the fresh glasses are petrographically and geochemically similar to mid‐Atlantic ridge (MAR) axial basalts. There are no systematic compositional differences as a function of distance from the MAR axis. Two sites contain basalts with olivine (Fo90) phenocrysts, high Mg/Mg + ΣFe, high Ni and Cr abundances, and very low large ion lithophile (LIL) element abundances. These basalts are the best candidates for primary magma recovered from the sea floor; fractional crystallization of such basalt may yield the more evolved basalts typical of the MAR. More fractionated basalts with clinopyroxene phenocrysts occur at twp other sites, but they retain low LIL element abundances. Site 2‐10 contains titaniferous augite and is relatively enriched in LIL elements. It is unlikely that this basalt was derived by fractional crystallization from LIL element depleted tholeiites; instead, the site 2‐10 basalt requires a different mantle source. These results imply that the upper Atlantic Ocean basement is dominantly LIL element depleted tholeiite.

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