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 230Th/ 234U, 210Pb/Pb and 228Th/ 228Ra 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.