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Geochronological studies of a large number of precipitates from the TAG hydrothermal field and of few samples from Snakepit hydrothermal field of the Mid-Atlantic Ridge show intermittent repeated hydrothermal events at both sites. 210Pb/Pb and 230Th/234U measurements of sulfides, iron and manganese oxides, and 14C measurements of carbonates combined with observations of hydrothermal events recorded as discrete layers in sediment cores provide the basis for unravelling the temporal history of the fields. The TAG field shows intermittent activity over the past 120,000 years as evidenced by ages of low-temperature Mn oxides. The presently active black smoker mound first formed about 40,000-50,000 years ago with precipitation of massive sulfides. It has had intermittent, pulsed high-temperature activity every 5000-6000 years over the past 20,000 years which may reflect renewed magmatic activity at the ridge axis. Fluid flow is focussed at the mound site by structural and tectonic control suggested by the intersection of N-S ridge parallel lystric normal faults and an E-W transform fault. Periods of inactivity are marked by covering of the mound with pelagic carbonate ooze which is probably partially dissolved and reprecipitated as aragonite at the end of each high-temperature event. The Snakepit field had an initial event about 4000 years ago, probably shortly after the eruption of the volcanic ridge on which it sits. A recent renewal, still presently active, was probably initiated by recent fissuring of the volcanic pile. © 1990.

Spectroscopic analysis of polymetallic oxides/oxyhydroxide crusts from the Atlantic Ocean allows identification of the oxidation states of Mn and Fe. High Mn:Fe ratios, greater than or equal to 34.4 indicate presence of manganese in the form of todorokite. Occurrence of Fe(III) and Mn(IV) in the forms of goethite and delta-MnO2, respectively, suggest that Mn:Fe ratios are less than or equal to 1. X-ray diffraction shows that Fe(III) substitutes Mn(III) ions in goethites. Unit-cell dimensions for Mn-goethite and the hydrogenous/hydrothermal origin of the crusts are discussed.