14. TEMPERATURE AND SALINITY OF FLUID INCLUSIONS IN ANHYDRITE AS INDICATORS OF SEAWATER ENTRAINMENT AND HEATING IN THE TAG ACTIVE MOUND1

Margaret Kingston Tivey,2 Rachel A. Mills,3 and Damon A.H. Teagle4

  ABSTRACT

Microthermometric analyses of fluid inclusions, carried out on individual anhydrite crystals from samples recovered at depths from 0 to >120 mbsf within the Trans-Atlantic Geotraverse (TAG) active hydrothermal mound, indicate high temperatures (>337C) throughout the TAG-1 and TAG-2 areas and suggest that temperatures at depths greater than 100 mbsf are in excess of 380 to 390C. Samples from the TAG-5 area indicate a wider and lower range of temperatures (187-337C). Salinities of fluids in all inclusions analyzed fall well within the range of measured salinities in mid-ocean ridge vent fluids. Coupling data from fluid inclusion analyses with Sr-isotopic analyses of anhydrite crystals from the same locations allows determination of both the proportions of hydrothermal fluid and seawater comprising the fluids that formed the anhydrite crystals and the temperature of the fluid mixtures. These data provide evidence for seawater entrainment and significant conductive heating of seawater/hydrothermal fluid mixtures within the mound. Conductive heat gain can occur as fluids are transported through veins bounded by conductive sulfide-rich breccias. Estimated flow rates to allow for conductive heating are on the order of 0.02 to 0.08 kg/s, and geochemical calculations indicate that on the order of 10-2 mol of anhydrite should precipitate per kg of fluid. By assuming that between 1% and 10% of the 225 MW convective heat flux from the Black Smoker Complex is used to heat seawater, it is estimated that the existing ~2 104 m3 of anhydrite present within the TAG active mound could have been deposited in 80 to 800 yr. The convective process of entraining and heating seawater may be responsible for cooling black smoker fluids from >380C to the temperature of 366C currently measured in orifices of chimneys that compose the Black Smoker Complex.

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 Marine Chemistry and Geochemistry, Woods Hole Oceanographic Institution, Woods Hole, MA 02543, U.S.A. mktivey@whoi.edu
3Southampton Oceanography Centre, University of Southampton, Southampton SO14 3ZH, United Kingdom.
4Department of Geological Sciences, University of Michigan, Ann Arbor, MI 48109-1063, U.S.A.