Abstract
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.
Highlights
The hydrothermal phenomena that take place near or at mid-ocean ridges are directly linked to the tectonic and magmatic activity associated with seafloor spreading
This ratio can be established by measurement of the particulate matter issuing from the active black smokers. The use of this instantaneously measured ratio assumes that it is constant in all the vents of a given field and remains constant for the life time of the field. This has been partly verified on a field of the East Pacific Rise (EPR) at 12°50'N where the same ratio was measured in four different vents along the active area, and on a time scale of 6 years; this ratio is not the same as that in other fields of the EPR such as 21°N, 18°S, 20°S or on the Mid-Atlantic Ridge (MAR) (Lalou and Brichet, unpublished data)
No black smoker particles were available for analysis, so the 2~°pb/Pb ratio at to was not directly obtainable
Summary
The hydrothermal phenomena that take place near or at mid-ocean ridges are directly linked to the tectonic and magmatic activity associated with seafloor spreading. They are characterized by different kinds of deposits: hydrothermal sediments, found sporadically along the mid-ocean ridges, which provide a background on which are superimposed episodic flashes represented either by high-temperature (up to 350°C) polymetallic sulfide deposits, or by lower-temperature iron and manganese oxide precipitates. To understand the evolution with time of such episodic systems, we have used radiochronological methods to study the TAG hydrothermal field on the Mid-Atlantic Ridge (MAR) at about 26 ° N, and the Snakepit field at 23°N (Fig. 1).
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