We discuss observations of hydrothermal activity from sites along the Juan de Fuca Ridge and report the ages and radium isotope ratios of samples from eight sites collected on a series of 11 Alvin dives in September 1988. This was a pilot program to assess whether uranium and thorium series isotopes could provide age information on the duration of active venting. The ages are based on the ingrowth of 228Th from 228Ra in both active and inactive chimneys and chimney talus composed of barite, anhydrite, chalcopyrite, wurtzite, and pyrite. Samples were obtained from (from north to south) the Fairycastle Site (northern Endeavour Ridge), the Endeavour Hydrothermal Site, and the southern Juan de Fuca Sites (named vent 3, vent 1, and Plume areas). Samples from active chimneys (20) yielded 228Th/228Ra ages in the range 0.2 to 1.2 years. We recorded no ages in the range 1.2 to 12 years for inactive chimneys or rubble. Our tentative conclusions are that the individual chimneys form rapidly and that the presently active chimney structure are <2 years old. In the Fairycastle Site there is at least one episode of hydrothermal activity recorded in the rubble which occurred at least 20 years earlier. Large sulfide edifices in the Endeavour Hydrothermal Site probably record a minimum of 20 years of discharge for each particular structure.

Chimney structures in the southern Juan de Fuca Ridge may be older, they show no evidence of significant recent growth except for an outer barite-rich layer. The results of this study suggest that hydrothermal chimneys of diverse mineralogy can be dated by measuring the ingrowth of 228Th from 228Ra decay. The technique is most accurate for chimneys that are less than 10 years old. The technique, in conjunction with detailed petrological analyses, could be used to assess the growth rates of individual chimney walls. Variations in the 228Ra/226Ra activity ratios among the three areas reflect either differences in fluid residence time, depth of generation of hydrothermal fluids, or interaction with basaltic rocks previously altered at low temperatures. ¿ American Geophysical Union 1991