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

Abstract

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.