LEAD AND SULFUR ISOTOPIC COMPOSITIONS OF SULFIDES FROM THE TAG HYDROTHERMAL FIELD, MID-ATLANTIC RIDGE

Abstract

TAG hydrothermal area, located in the Mid-Atlantic Ridge 26.08°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-158 drill cores. The hydrothermal sulfides show a narrow Pb-isotope variation for 206Pb/204Pb (18.2343~18.3181), 207Pb/204Pb (15.4717~15.5061), and 208Pb/204Pb (37.7372~37.8417). These ratios are between the Pb-isotopic compositions of the footwall basalt (206Pb/204Pb = 18.1454, 207Pb/204Pb = 15.4572, 208Pb/ 204Pb = 37.6534) and the near-seafloor Fe-Mn oxides (206 Pb/204 Pb = 18.6907~18.9264, 207Pb/204Pb = 15.5615~15.6279, 208Pb/204Pb = 38.1164~38.3687). All of the Pb-isotope data fall a linear array, suggesting the Pb for hydrothermal sulfides was from mixing of mantle-derived (basalts) and seawater-derived (Fe-Mn oxides) Pb. The sulfides have a δ34S of 6.2‰ ~ 9.5‰, which is obviously higher than the mantle basalts (δ 34S = ±0‰). These data are also higher than the δ34S values of sulfides from many mid-ocean ridge hydrothermal sites such as the EPR21°N (0.9‰~4.0‰) and MAR23° N (1.2‰~2.8‰). 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 important for tracing the ore materials and fluid mixing processes in sea-floor massive sulfide deposits.