Insights into hydrothermal processes along mid-ocean ridges from U-series dating of hydrothermal sulfide deposits

High-temperature hydrothermal vent sites along mid-ocean ridges are marked by the accumulation of sulfide minerals and the biological communities that harness the chemical energy from the hydrothermal fluids. The location and size of the sulfide deposits, together with their composition, are a record of the history of hydrothermal venting on the seafloor. However, the life-spans and evolution of individual hydrothermal systems, and the rate at which these sulfide deposits form, remains poorly constrained. Due to the high concentrations of Cu, Zn, Au and Ag within the contained sulfide minerals, as well as relatively high current metal prices and advances in marine technology, economic interest in exploiting these deposits is growing, driving renewed interest in the formation of these deposits. For sites of active venting, sulfide accumulation rates cannot simply be estimated from the composition and flow rate of the vent fluids because the proportion of sulfide material mobilized by hydrothermal fluids in the crust that is deposited as sulfide material at the vent site, relative to what is ejected into the water column or precipitated in the subsurface, remains unclear. Growth rates of individual chimneys have been calculated from short-lived U-series disequilibrium dating techniques (e.g., 210Pb/Pb, 228Th/228Ra). However, over the life-span of a single vent, fluid composition and discharge rates can vary, and measurements of vent fluid properties and localized chimney growth rates provide only a present-day or recent snapshot of the growth of a sulfide deposit. Longer-lived U-series isotope systems (e.g. 230Th/234U, 226Ra/Ba), from samples collected from the perceived oldest, and most weathered parts of deposits, are required to determine minimum ages of deposits on the mid-ocean ridges. In this talk I will discuss the application of U-series geochronology for dating sulfide deposition, and the development of techniques to constrain the mass accumulation rates and hydrothermal mass balance of seafloor hydrothermal sulfide deposits. Results from the Juan de Fuca Ridge, in the Northeast Pacific Ocean, and the central Mid-Atlantic Ridge, including data collected on the recent IPGP-led ODEMAR cruise, will be discussed.