Stable isotope study of anhydrite and sulfide minerals at the TAG hydrothermal mound, Mid-Atlantic Ridge, 26°N

Abstract

The active mound of the Trans-Atlantic Geotraverse (TAG) hydrothermal system at the Mid-Atlantic ridge at 26°N was drilled during Ocean Drilling Program (ODP) Leg 158. One of the most important findings of the drilling is the discovery of an anhydrite-rich zone beneath the active mound. Stable isotope ratios of anhydrite and sulfide minerals are measured to investigate the formation conditions and mechanisms of the active mound and the anhydrite-rich zone. δ34S values of sulfide minerals are heavier than those of other sediment-starved ridge hydrothermal systems. The heavy sulfide sulfur may be derived from the anhydrite in the reaction zone, which was precipitated in the waning stage of the former high-temperature activity of this system. Anhydrite δ18O and δ34S are out of equilibrium with the hydrothermal fluid. Disequilibrium of the oxygen-isotope exchange reaction suggests a very short residence time of aqueous sulfate in a high-temperature environment. Anhydrite precipitation seems to have occurred instantaneously at the mixing interface where Ca2+ was supplied from the hydrothermal solution and SO2-4 the seawater. Anhydrite 87Sr/86Sr values suggest the preheating of seawater sulfate before it enters the anhydrite formation zone. The chemical behavior of seawater upon heating and interaction with basalt indicate that the preheating of seawater takes place at the environment where basalt is absent. Seawater must invade from the surface of the mound into the anhydrite-rich zone.