Abstract:
The TAG hydrothermal area, located in the Mid-Atlantic Ridge at 26.08 degrees N, is one of the largest sea-floor massive sulfide deposits in the sediment-free mid-ocean ridge. New lead and sulfur isotope data are reported in this paper for nine hydrothermal sulfides, two near-seafloor Fe-Mn oxides, and one footwall basalt collected from ODP Leg 158 drill cores. The hydrothermal sulfides show a narrow Pb-isotope variation for (super 206) Pb/ (super 204) Pb (18.2343-18.3181), (super 207) Pb/ (super 204) Pb (15.4717-15.5061), and (super 208) Pb/ (super 204) Pb (37.7372-37.8417). These ratios are between the Pb-isotopic compositions of the footwall basalt ( (super 206) Pb/ (super 204) Pb = 18.1454, (super 207) Pb/ (super 204) Pb = 15.4572, (super 208) Pb/ (super 204) Pb = 37.6534) and the near-seafloor Fe-Mn oxides ( (super 206) Pb/ (super 204) Pb = 18.6907-18.9264, (super 207) Pb/ (super 204) Pb = 15.5615-15.6279, (super 208) Pb/ (super 204) Pb = 38.1164-38.3687). All of the Pb isotope data fall in a linear array, suggesting that Pb from hydrothermal sulfides came from the mixing of mantle-derived Pb (in basalts) and seawater-derived Pb (in Fe-Mn oxides). The sulfides have a delta (super 34) S of 6.2 ppm-9.5 ppm, which is obviously higher than that of the mantle basalts ( (super 34) S = + or -0 ppm). These data are also higher than the delta (super 34) S values of sulfides from many mid-ocean ridge hydrothermal sites such as the EPR21 degrees N (0.9 ppm-4.0 ppm) and MAR23 degrees N (1.2 ppm-2.8 ppm). A two component mixing between seawater sulfate reduction and basalt-derived sulfur is suggested for the sulfur source in the TAG hydrothermal system. Hence, the study of lead and sulfur isotopes is of great importance in tracing the ore materials and fluid mixing processes in sea-floor massive sulfide deposits.