Hydrothermal sulfides record the history of high-T venting along ridge segments, arcs and back-arcs.  The sulfides record evidence of changes in fluid temperature and composition with time, spatial and temporal shifts in venting locations and responses to seismic and volcanic events.  However, understanding the full context of this information requires that the relative or absolute ages of the sulfides are known.

We will present new geochronological studies of the hydrothermal sulfides within the Endeavour ISS.  Results from Ra-226/Ba geochronology in sulfide-associated barite indicate that high-T venting within the current axial valley was likely initiated <5,000 years ago.  Previous geochronological studies of hydrothermal sulfides from the Juan de Fuca Ridge have focused on rates of chimney growth and sulfide accumulation using short-lived isotopes (e.g. Pb-210 (half-life of 22.3 yrs.), Ra-228 (half-life of 5.7 yrs.) and Th-228 (half-life of 1.9 yrs.)) (Kadko et al., 1985; Grasty et al, 1988; Kim and McMurtry, 1991; Reyes et al., 1995).  These studies represent snapshots of venting activity over short timescales (<10 years).  The examination of processes that occur over the lifespan of an entire vent field require the use of radioisotopes with longer half-lives, and this long-term history is the focus of our work.  Radium-226 (half-life of 1,600 years) occurs in high abundances in hydrothermal barite, which precipitates in conjunction with sulfide minerals on the seafloor.  Measurements of the Ra-226/Ba ratios of hydrothermal barite provide a chronometer that allows for the determination of the absolute ages of sulfides between ~300 to 20,000 years old.  Age calculations of 63 sulfides from Mothra to Sasquatch show a range of ages from present to a maximum of 3,022 years.  This maximum calculated age represents a minimum age of venting at Endeavour.

Information on the ages of hydrothermal sulfides and longevity of venting at Endeavour may have profound implications for numerous topics of investigation within the ISS.  Specific areas where these data may be applicable for integration and synthesis include, but are not limited to:

• Temporal changes in vent fluid chemistry and fluxes
• Accumulation rates of sulfide
• Episodicity of venting and spatial constraints on past venting activity
• Temporal constraints on biological colonization and evolution
• Correlation between hydrothermal venting and the volcanic/tectonic history of the current axial valley

I envisage that these data will be of particular interest to participants in the following Thematic Working Groups:

• Spatial and temporal variation in chemistry and heat fluxes in hydrothermal systems, including chronic plumes
• Seafloor tectonic and volcanic events and responses, including event plumes
• Biogeochemical processes in deep-sea hydrothermal systems
• Crustal controls on magma reservoirs and ocean spreading center magmatism

A poster containing details of this study, including results and methodology, as well as some interpretation, will (hopefully) be on display during the poster session.