Lead, Sulfur, and Oxygen Isotope Systematics in Hydrothermal Precipitates from the 14°S Hydrothermal Field, South Mid-Atlantic Ridge

At the inside corner between the South Mid-Atlantic Ridge (SMAR) and the Cardno fracture zone, the 14 degrees S hydrothermal field, with its abundant silica-rich sulfides, was identified as a volcanic massif, which provided the first opportunity to study inside corner-related hydrothermal mineralization. S, Pb, and O isotopes were measured in hydrothermal sulfide samples from the two sites (volcanic top and slope sites) in the 14 degrees S field. The homogeneous Pb isotope values (Pb-207/Pb-204=15.466-15.472; Pb-206/Pb-204=18.242-18.252) and the S-34 values (top site, +6.3 to +6.85 parts per thousand; slope site, +2.37 to +3.36 parts per thousand) suggest that the source of metals and most of the sulfur is the upper crust, whereas some sulfur is sourced from downward-penetrating seawater, especially in the top site. The calculated oxygen isotope equilibration temperatures from quartz and seawater are between 94 and 144 degrees C for the top site and between 179 and 196 degrees C for the slope site. These isotopic proxies, together with geological background survey data, have an important implication: Discrepancies between the sites may result from variations in crust permeability, which also affect the mixing level between downward-penetrating seawater and upward hydrothermal end-member fluids and determine the various mineralization types.