Evelyne Brichet

New age data for Mid‐Atlantic Ridge hydrothermal sites: TAG and Snakepit chronology revisited

The chronologies of TAG and Snakepit hydrothermal fields have been established using 210Pb/Pb, 230Th/234U and 14C dating. At the TAG field, a Mn-oxide record, indicative of low temperature events, began at least 125,000 years and possibly 140,000 years ago with maximum intensities at 15,000, 7000 and 4000 years before present. High temperature events, giving rise to sulfide deposits, began about 100,000 years ago and have been intermittent to the present day. A presently active site has experienced intermittent pulses of activity every 4000 to 6000 years over the past 20,000 years. Decrease in activity is often marked by low temperature aragonite precipitation in chimney conduits at 4000, 7000 and 9000 years ago. After a period of quiescence lasting about 4000 years this site was reactivated about 50 years ago. The Snakepit field is much younger and no sulfides older than 4000 years have been recovered. Relict sulfide deposits are dated between 2000 and 4000 years old indicating this site was active during a quiescent period at TAG. Reactivation of Snakepit. took place about 80 years ago, and is presently concurrent with that of TAG. Comparison with hydrothermal sites on the East Pacific Rise suggests that on slow spreading ridges the major fracture systems focussing the hydrothermal discharge can be reactivated at intervals and new deposits precipitated on top of older ones, while on faster spreading ridges each pulse of activity is separated in space and time resulting in discrete deposits.

Hydrothermal activity on a 105-year scale at a slow-spreading ridge, TAG hydrothermal field, Mid-Atlantic Ridge 26°N

New radiometric dates are presented for the hydrothermal deposits in the Alvin and Mir relict hydrothermal zones located on the lower east wall of the rift valley within the TAG hydrothermal field and placed in context of the evolution of the TAG field as a whole over the past 105 years. The field includes an active high-temperature sulfide mound situated near the base of the east wall and a low-temperature zone at middepth on the east wall all within a 5 by 5 kilometer area. The oldest hydrothermal deposits sampled in the Mir relict zone, the low-temperature zone, and an inactive sulfide mound in the Alvin relict zone are low-temperature manganese crusts dated at 140,000, 125,000 and 74,000 years, respectively. High-temperature activity began about 100,000 years ago in the Mir zone. A 50,000 year-high-temperature event took place at the active sulfide mound, in the Mir zone, and at the inactive sulfide mound in the Alvin zone. This event is followed by sporadic activity at the Mir zone and the active sulfide mound, until 10,000 and 5000 years ago, respectively. A reactivation of the system affected the Mir zone from about 2000 to 600 years ago. The presently active sulfide mound was reactivated only 60 years ago, after a quiescence of about 4000 years. The radiochronologic record reveals that the hydrothermal activity has been episodic with a major high-temperature event throughout the TAG field at 50,000 years and other more localized events.

Geochronology of TAG and Snakepit hydrothermal fields, Mid-Atlantic Ridge: witness to a long and complex hydrothermal history

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.

Age dating of sulfide deposits from axial and off-axial structures on the East Pacific Rise near 12°50′N

Abstract During several cruises (Clipperton, 1981; Cyatherm, 1982 and Geocyarise Leg 3, 1984), conducted between 12°40′N and 12°54′N on the East Pacific Rise, massive sulfide deposits have been dredged and recovered by the diving saucer “Cyana”. These well localized and documented samples have been analyzed for their 210 Pb/Pb ratio and uranium series disequilibrium in view of dating the hydrothermal phenomenon. Using samples which are presently forming, the initial 210 Pb/Pb ratio has been established as 0.60 ± 0.06. Ten sulfide samples from the central graben (near 12°50′N) have ages less than about 10 years and are younger than most of the deposits found at 21°N on the East Pacific Rise. One sample from the eastern rim of the graben presents an age of about 150 years. Two off-axis structures with hydrothermal sulfide deposits were studied. The first is a semi-circular volcanic structure where low-temperature hydrothermal vents were observed at 500 m and 1000 m from the rise axis; sulfide samples collected from these sites yielded ages of about 2000 years. Further east, about 6 km from the rise axis, the second off-axis structure, called South Eastern (SE) Seamount presents thick inactive sulfide deposits as well as Mn-rich and nontronite deposits. An age around 20,000 years has been found for these deposits. These ages are discussed, and, taking into account the asymmetrical structure of the ridge near 12°43′N, it is concluded that the formation of the SE Seamount occurred 20,000 years ago, on a basement of about 100,000 years old, accompanied by an extensive hydrothermal event. This indicates that an important eastward shift (4–5 km from the graben) of the volcano-hydrothermal activity took place in this area (12°43′N).

On the isotopic chronology of submarine hydrothermal deposits

Abstract During the last 10 years, a large number of submarine hydrothermal deposits has been found in the Pacific Ocean as well as in the Atlantic. The chronology of such hydrothermal deposits may help in the evaluation of the importance and frequency of the hydrothermal events, which, in turn, may influence the chemistry of the oceans. These deposits are generally metallic sulfides or Mn- and Fe-oxides. In the natural radioactive series, different disequilibria are created during the deposit formation because of different geochemical behaviour of the elements. These disequilibria can act as good chronometers because they cover different time spans. The applications of 230 Th/ 234 U, 210 Pb/Pb and 228 Th/ 228 Ra methods covering 1,000–350,000, 0–150 and 0–15 a, respectively, are discussed. The different assumptions needed to employ these radionuclides as chronometers as well as the problems inherent in the methods are explained, and some examples of their application are discussed: (1) In the Galapagos hydrothermal mounds area, it will be shown that a thick Mn-oxide deposit has been formed by pulses of a Mn-rich solution which has seeped through the carbonaceous sediment from ∼ 90 to ∼ 20 ka ago. (2) On the Mid-Atlantic Ridge, hydrothermal activity is essentially represented by Mn- and Fe-oxide deposits. At 37°N (FAMOUS area), though apparent ages may be obtained, the results are difficult to interpret because no pure Mn deposit was obtained. In the TAG area ( ∼ 26° N), Mn-oxide deposits yield ages ∼ 5 and ∼ 10 ka. (3) On the East Pacific Rise at 21°N, the sulfides of black smokers have been dated between zero and ∼ 50 a, and a fossil sulfide deposit, found off axis, has been dated at ∼ 4 ka. (4) On the East Pacific Rise at 13°N, three episodes of hydrothermal activity can be described: (a) the present-day activity in the central graben which is dated between zero and 130 a; (b) slightly east of the graben, an older, but presently active deposit is dated ∼ 2 ka; and (c) 6 km east of the axis, fossil sulfide deposits as well as Mn-oxide deposits and nontronite deposits yield an age of ∼ 20 ka.

Hydrothermal manganese oxide deposits from Galapagos mounds, DSDP Leg 70, hole 509B and “Alvin” dives 729 and 721

Abstract During DSDP Leg 70, a 1.60 m thick manganese oxide layer was sampled in hole 509B. This deposit is formed of alternating layers of hard plates of pure todorokite, about 2 mm thick, and of a more powdery material deeply impregnated with manganese oxide, about 3 mm thick. A SEM study of the plates and the associated powder shows that the powdery material is a transformation of a pre-existing sediment, while the plates are a direct precipitation from a hydrothermal solution. The uranium series disequilibrium method was used to determine the ages of the plates. They are found to be in good chronological sequence and in accordance with the sedimentation rate of the area (4.9 cm/103 years) which implies that they have been formed at the sediment-seawater interface during a pulsed injection of hydrothermal solution. The powder presents systematically an “older age” which is explained by a slowing down of the injection while the normal sediment settles; the older age is due to the230Th excess of the sediment.

Chronology of selected hydrothermal Mn oxide deposits from the transatlantic geotraverse “TAG” area, Mid-Atlantic Ridge 26°N

Abstract Radiometric ages on discrete layers in some hydrothermally deposited Mn-oxide (birnessite) samples from the TAG hydrothermal field on the Mid-Atlantic Ridge, range from 16,000 to 4,000 years, with marked increased activity noted about 15,000 and 7,000 years ago. These observations, along with previously published data and inferences on different kinds of hydrothermal activity in the TAG field, suggest episodic, multistage hydrothermal events ranging from high temperature black smokers precipitating sulfides to low temperature events precipitating Fe and Mn oxides over this entire time range. Black smokers are presently active. The location of the various deposits on the eastern wall of the median valley at this latitude suggests that the high temperature events occur at greater depths.

Initial chronology of a recently discovered hydrothermal field at 14°45′N, Mid-Atlantic Ridge

Two expeditions of the ‘Sevmorgeologija’ association (1991–1994) led to the discovery of two new hydrothermal sites on the Mid Atlantic Ridge (MAR), south of the 15°20′ North Fracture Zone, one around 14°45′N and the other around 14°42′N. The northern one, between 14°45′ and 14°45.3′N has been studied in detail. About 12 mounds have been mapped and 3 of them have been sampled using a large hydraulic grab sampler. The largest one is about 200 m long and 200 m wide. When progressively moving up on the slope of an uplifted block of the rift valley floor, the sulphide samples have revealed ages ranging from about 10 ka to 60 ka. The ages were obtained using the 230Th/234U dating method used for chronological studies of diverse hydrothermal fields. The general picture of this lateral location of the samples of different ages provides evidence of a shift in the focus of hydrothermal activity with time. Moreover, there were rejuvenation stages of hydrothermal activity, including black and white smokers.

Radiochemical, scanning electron microscope (SEM) and X-ray dispersive energy (EDAX) studies of a FAMOUS hydrothermal deposit

Abstract During the French-American Mid-Oceanic Undersea Survey (FAMOUS) the French team participating in the mission explored the active A transform fault with the diving saucer CYANA, and discovered a characteristic hydrothermal deposit. The results presented here concern the structural and radiochemical study of three samples: two metalenriched samples and one sample of consolidated calcareous sediment recovered from the area. It is shown that there is a chemical segregation between silica and iron on one hand, and manganese in the other hand, and that on such deposits the layer-by-layer radio-chemical analysis of the uranium series is not applicable. Three different age estimations may be made: In the managanese encrustation, thorium 230 is depleted towards equilibrium with its parent uranium 234, suggesting a maximum age of 45,000 years to the deposit. On the contrary, the outermost layer, 0.1 cm thick, formed essentially with loose sediment stained with oxides, presents an excess of thorium 230. The Σ230Th(excess)/cm2 may be used to calculate a minimum age of 20 years. Such a sediment thickness, in this area, corresponds to a time span of 50 years.

Actinide-series disequilibrium as a tool to establish the chronology of deep-sea hydrothermal activity

This paper describes the different radiochronological methods used to date geologically recent (i.e., <400,000 years) deep-sea hydrothermal deposits as well as the basic conditions necessary to obtain reliable dates. The limitations of the different techniques also are described. Using measurements of 210PbPb, 228Th228Ra, 230Th234U, 231Pa235U and 228Ra226Ra, the authors have undertaken an exhaustive chronological study of the hydrothermal deposits along the East Pacific Rise, the Mid-Atlantic Ridge, and in some back-arc basins. The objectives of this study were to obtain regional chronologies and to establish a general synthesis on the evolution of the hydrothermal processes at the scale of the mid-oceanic ridge system. Some results obtained by other authors are included in this synthesis. The dependence of the general trends of temporal development of the hydrothermal chimneys, edifices, and fields on their tectonic settings is discussed. This study demonstrates that hydrothermal activity does not represent a regular input of matter to the ocean, and that its pulsed character must be taken into account in all modeling attempts (chemical, biological and tectonic) affected by hydrothermal processes. In areas of rapid spreading, like the East Pacific Rise, recent and fossil deposits are spatially separated. By contrast, at the slow spreading Mid-Atlantic Ridge, fossil and present activity are found in the same location.