大西洋洋中脊TAG热液区硫化物铅和硫同位素研究

Lead and sulfur isotopic compositions of sulfides from the TAG hydrothermal field,Mid-Atlantic Ridge.

位于大西洋洋中脊26.08°N的 TAG 热液区是目前己知的赋存在无沉积物覆盖的洋中脊区的一个最大的海底热液硫化物矿床。新测得来自 ODP-158航次钻孔的9件热液硫化物的铅、硫同位素组成;2件铁锰氧化物和1件底盘玄武岩的铅同位素组成。结果表明,矿石硫化物的铅同位素组成^(206)Pb/^(204)Pb 为18.2343～18.3181,^(207)pb/^(204)Ph 为15.4717～15.5061,^(208)Pb/^(204)Pb 为37.7371～37.8417;它们位于该区底盘玄武岩(^(206)Pb/^(204)Pb=18.1454,^(207)Pb/^(204)Pb=15.4572,^(208)Pb/^(204)Pb=37.6534)和近洋底铁锰氧化物(^(206)Pb/^(204)Pb,^(207)Pb/^(204)Pb,^(208)Pb/^(204)Pb 分别为18.6907～18.9264,15.5615～15.6279,38.1164～38.3687)的铅同位素组成之间。三者呈线性相关关系,说明硫化物中铅来源于地幔(玄武岩)与海水(铁锰氧化物)的两端元混合。硫化物的硫同位素组成δ^(34)S 为6.2‰～9.5‰,它明显高于地幔玄武岩的硫同位素组成(δ^(34)S=±0‰),也高于东太平洋海隆 EPR21°N(δ^(34)S=0.9‰～4.0‰)和大西洋洋中脊 MAR23°N(δ^(34)S=1.2‰～2.8‰)等热液活动区硫化物的硫同位素组成,这一特征反映了 TAG 热液体系中硫来源于地幔玄武岩硫与海水硫酸盐无机还原作用产生的硫的两端元混合。此,铅硫同位素研究为现代大洋底热液硫化物矿床形成过程中矿质来源及流体混合作用提供了十分有益的信息。

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 ） ,^207 pb/^204 pb （ 15. 4717 - 15. 5061 ）, and ^206Pb/^204 Pb （37. 7372 - 37. 8417）. These ratios are between the Pb-isotopic compositions of the footwall basalt （^206 pb/^204 pb = 18. 1454, ^207pb/^204pb = 15.4572, ^208 pb/^204 Pb = 37. 6534） and the near-seafloor Fe-Mn oxides （ ^206 pb/^204 Pb = 18. 6907 -18. 9264, ^207 pb/^204 Pb = 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 δ^34S of 6.2‰ - 9.5‰, which is obviously higher than the mantle basalts （δ^34 S = ± 0‰）. These data are also highcr than thc δ^34S 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.