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 Pb-210 / pb, Th-228 /Ra-228, Th-230/U-234, Pa-231/U-235, and Ra-228/ Ra-226, 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.