Damon A.H. Teagle,2 Jeffrey C. Alt,2 Hitoshi Chiba,3 and Alex N. Halliday2


Drilling 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.

1Herzig, P.M., Humphris, S.E., Miller, D.J., and Zierenberg, R.A. (Eds.), 1998. Proc. ODP, Sci. Results, 158: College Station, TX (Ocean Drilling Program).
2Department of Geological Sciences, 2534 C.C. Little Building, The University of Michigan, Ann Arbor, MI 48109-1063, U.S.A. Teagle: teagle@umich.edu
Department of Earth and Planetary Sciences, Kyushu University 33, Hakozaki, Fukuoka 812, Japan.