Results of ongoing interdisciplinary investigation of the TAG Hydrothermal Field since discovery in 1972 by the NOAA Trans-Atlantic Geotraverse (TAG) Project are synthesized to document the first and only known active submarine hydrothermal field on a slow-spreading oceanic ridge. The structural setting of the field is the E wall of the rift valley of the Mid-Atlantic Ridge at lat. 26°N where high-intensity hydrothermal activity is favoured by exceptionally close spacing of faults (tens of metres) that enhances permeability and facilitates voluminous circulation, and by projection of the wall directly over intrusive heat sources beneath the rift valley that increases thermal gradients which vigorously drive the upwelling limb of a sub-seafloor hydrothermal convection cell discharging through faults in the wall. A low in residual magnetic intensity coincides with the field and is attributed to hydrothermal alteration of the magnetic mineral component of basalt in the discharge zone. Hydrothermal precipitates in the form of manganese oxide crusts of extreme purity (40% Mn) and rapid deposition rate (200 mm 10-6 yr) cover about 10% of the sea-floor within the field, distributed along faults sub-parallel to the rift valley. The composition and mineralogy of basalts exposed at the sea-floor is indicative of low-temperature zeolite metamorphic facies. Thin sediments of the field exhibit average metal-to-aluminum ratios and non-detrital metal accumulation rates (Fe, Mn, Ni, Co, Cu, Cr) that are high relative to other areas of the Mid-Atlantic Ridge. Present hydrothermal activity is evidenced in the near-bottom water by temperature anomalies and excesses of the primordial isotope 3He, and in the surrounding water column by increases in weak-acid soluble suspended particulate matter enriched in certain metals (Fe × 10; Mn 10%). The observed distribution of hydrothermal products indicates that the special structural and thermal conditions that have concentrated hydrothermal activity in the TAG Hydrothermal Field have persisted for at least 1.4 × 106yr.

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