A model for growth of steep-sided vent structures on the Endeavour Segmentof the Juan de Fuca Ridge: Results of a petrologic and geochemical study

Petrologic and geochemical studies of vent solids from the Main Endeavour F ield (MEF) and the High Rise Field (HRF), Juan de Fuca Ridge, demonstrate t hat the steep-sided vent structures characteristic of these sites form domi nantly by flange growth, combined with diffuse flow through sealed portions of structures, and incorporation of flanges into structures. Geochemical c alculations suggest that the prevalence of amorphous silica and flanges in Endeavour deposits is the result of conductive cooling of vent fluids that have high concentrations of ammonia. At Endeavour, as the temperature of ve nt fluid decreases, ammonia-ammonium equilibrium buffers pH and allows more efficient deposition of sulfide minerals and silica from fluids that have a higher pH than conductively cooled ammonia-poor fluids present at most ot her unsedimented mid-ocean ridge vent sites. Deposition of silica stabilize s flanges and allows structures to attain large size. It also leads to diff use flow and further conductive cooling by reducing the permeability and po rosity of the structures and of feeder zones, thus decreasing entrainment o f seawater. Most inactive vent samples recovered from areas peripheral to t he HRF and MEF are similar to barite + silica rich samples from the Explore r Ridge and Axial Seamount and likely formed from precipitation of silica a nd barite on a biological substrate. Active white smoker chimneys from the Clam Bed Field, located south of the HRF are pyrrhotite rich and likely for med from vent fluids that are depleted in Zn and Cd and enriched in Pb and Ba relative to fluids exiting trans-Atlantic geotraverse (TAG) and Cleft Se gment white smoker chimneys.