Chemical characteristics of hydrothermal fluids from the TAG mound of the mid-Atlantic Ridge in August 1994: Implications for spatial and temporal variability of hydrothermal activity

The TAG hydrothermal mound on the Mid-Atlantic Ridge (26°08'N, 44°50'W) was revisited in August 1994 with the submersible Shinkai 6500 in order to characterize time-series fluid chemistry prior to the ODP drilling. Fluid samples were taken from both black smokers and white smokers. Si, pH, alkalinity, H₂S, major cations (Na⁺, K⁺, Ca²⁺, Mg²⁺), major anions (Cl, SO₄²), and minor elements (Li, Sr, B, Fe, Mn, Cu, Zn, Br) as well as Sr isotope ratios were measured. We report the first Br/Cl ratios for the TAG hydrothermal fluids, showing no fractionation between Br and Cl during the fluid-rock interaction. This study shows small changes in composition of the black smoker fluids from the 1990 data (Edmond et al., 1995). Changes of pH, alkalinity, Fe, K, and ⁸⁷Sr/⁸⁶Sr values are suggestive of subsurface FeS precipitation and a decrease of water/rock ratio at a deeper reaction zone. Differences in chemical characteristics between the black and white smoker fluids were similarly observed as in 1990.