11. DISSECTING AN ACTIVE HYDROTHERMAL DEPOSIT: THE STRONTIUM AND OXYGEN ISOTOPIC ANATOMY OF THE TAG HYDROTHERMAL MOUND—ANHYDRITE1Damon A.H. Teagle,2 Jeffrey C. Alt,2 Hitoshi Chiba,3 and Alex N. Halliday2 |
ABSTRACTDrilling at the Trans-Atlantic Geotraverse (TAG) active hydrothermal deposit on the Mid-Atlantic Ridge during Leg 158 of the Ocean Drilling Program recovered significant quantities of anhydrite from both the mound and the underlying mineralized stockwork zone. Anhydrite is present as a cement in disseminated sulfide breccias and in complex, multigeneration veins, up to 45 cm wide, at the upper levels of the deposit. It is a common late-stage vein and vug-filling mineral throughout the rest of the deposit, apart from a zone of anomalously low conductive heat flow that has been interpreted as a region of seawater entrainment into the mound. The chemical and strontium- and oxygen-isotopic compositions of anhydrite are broadly similar throughout the mound, but are heterogeneous at a centimeter scale, with closely spaced samples displaying almost the full range of compositions. Values of 87Sr/86Sr indicate that most of the anhydrite formed from near-equal mixtures of seawater and black smoker fluids (65% ± 15% seawater). Assuming isotopic equilibrium, oxygen-isotopic compositions indicate that anhydrite precipitated at temperatures between 147°C and 270°C and require that seawater was conductively heated to between 100° and 180°C before mixing and precipitation occurred. Anhydrite from the TAG mound has an average Sr-Ca partition coefficient Kd nearly equal to 0.60 ± 0.28 (2 sigma). This value is in agreement with the range of experimentally determined partition coefficients (Kd nearly equal to 0.27 -0.73) and is similar to those calculated for anhydrite from active black smoker chimneys from 21°N on the East Pacific Rise. The range of delta18O of TAG anhydrite brackets the value of seawater sulfate oxygen (~9.34‰), which is known to be out of isotopic equilibrium with ocean-water oxygen at ocean temperatures. Anhydrite sulfate from active hydrothermal deposits, in isotopic equilibrium with fluids at approximately 200°C, will dissolve back into the oceans during episodes of hydrothermal quiescence, without further modification because of the sluggish kinetics of sulfate-water oxygen-isotopic exchange at low temperatures. This provides an additional reservoir of isotopically light sulfate oxygen to the anhydrite that precipitates within the oceanic basement during hydrothermal recharge at the ridge. |
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