14. TEMPERATURE AND SALINITY OF FLUID INCLUSIONS IN ANHYDRITE AS INDICATORS OF SEAWATER ENTRAINME...Margaret Kingston Tivey, Rachel A. Mills, and Damon A.H. TeagleABSTRACTINTRODUCTIONTHE TAG ACTIVE HYDROTHERMAL MOUNDMETHODSRESULTSDISCUSSIONCONCLUSIONSACKNOWLEDGMENTSREFERENCESFIGURESFigure 1. Diagram of the TAG active hydrothermal mound showing the distribution of venting at the...Figure 2. Photomicrographs of fluid inclusions in anhydrite from the TAG active mound. A. Primary...Figure 3. Frequency histograms of temperatures of final ice melting, Tm, for fluid inclusions in ...Figure 4. Frequency histograms of homogenization temperatures for fluid inclusions in anhydrite f...Figure 5. Trapping temperatures (pressure corrected) and corresponding salinities (wt% NaCl equiv...Figure 6. Trapping temperatures (pressure corrected) for fluid inclusions in anhydrite and corres...Figure 7. Salinities for fluid inclusions in anhydrite and corresponding depths of host samples f...TABLESTable 1. Origin of anhydrite crystals used in fluid inclusion study.Table 2. Summary of fluid inclusion analyses from anhydrite samples, TAG active hydrothermal mound.Table 3. Ratios of hydrothermal fluid and seawater in fluids that formed anhydrite crystals and c...Table 4. Flow rates, distances, and times required to achieve sufficient conductive heating.Herzig, P.M., Humphris, S.E., Miller, D.J., and Zierenberg, R.A. (Eds.), 1998Proceedings of the Ocean Drilling Program, Scientific Results, Vol. 15817914. TEMPERATURE AND SALINITY OF FLUID INCLUSIONS IN ANHYDRITE AS INDICATORS OF SEAWATER ENTRAINMENT AND HEATING IN THE TAG ACTIVE MOUND1Margaret Kingston Tivey,2 Rachel A. Mills,3 and Damon A.H. Teagle4ABSTRACTMicrothermometric analyses of fluid inclusions, carried out on individual anhydrite crystals from samples recovered atdepths from 0 to >120 mbsf within the Trans-Atlantic Geotraverse (TAG) active hydrothermal mound, indicate high tempera-tures (>337°C) throughout the TAG-1 and TAG-2 areas and suggest that temperatures at depths greater than 100 mbsf are inexcess of 380° to 390°C. Samples from the TAG-5 area indicate a wider and lower range of temperatures (187°−337°C). Salin-ities of fluids in all inclusions analyzed fall well within the range of measured salinities in mid-ocean ridge vent fluids. Cou-pling data from fluid inclusion analyses with Sr-isotopic analyses of anhydrite crystals from the same locations allowsdetermination of both the proportions of hydrothermal fluid and seawater comprising the fluids that formed the anhydrite crys-tals and the temperature of the fluid mixtures. These data provide evidence for seawater entrainment and significant conductiveheating of seawater/hydrothermal fluid mixtures within the mound. Conductive heat gain can occur as fluids are transportedthrough veins bounded by conductive sulfide-rich breccias. Estimated flow rates to allow for conductive heating are on theorder of 0.02 to 0.08 kg/s, and geochemical calculations indicate that on the order of 10−2 mol of anhydrite should precipitateper kg of fluid. By assuming that between 1% and 10% of the 225 MW convective heat flux from the Black Smoker Complexis used to heat seawater, it is estimated that the existing ~2 × 104 m3 of anhydrite present within the TAG active mound couldhave been deposited in 80 to 800 yr. The convective process of entraining and heating seawater may be responsible for coolingblack smoker fluids from >380°C to the temperature of 366°C currently measured in orifices of chimneys that compose theBlack Smoker Complex.INTRODUCTIONThe Trans-Atlantic Geotraverse (TAG) active hydrothermalmound is located 2.5 km east of the neovolcanic zone at 26°08′N inthe middle of a 40-km-long ridge segment on the Mid-Atlantic Ridge.The mound measures 150 to 200 m in diameter, exhibits 50 m of re-lief, and is covered entirely by hydrothermal precipitates. Geochem-ical studies of solids and fluids recovered from the surface of themound have provided crucial evidence for entrainment of seawaterinto the mound and for ongoing zone refinement within the deposit(Tivey et al., 1995; Edmond et al., 1995). In the fall of 1994, 17 holeswere drilled in the TAG active hydrothermal mound during Leg 158of the Ocean Drilling Program in an effort to investigate subsurfaceportions of the mound. Recovered material, including substantialamounts of anhydrite, provide further evidence for significant en-trainment of seawater into the mound.In order to better understand this entrainment process, detailedstudies of anhydrite within the mound have been carried out. Theseinclude fluid inclusion analyses to determine the temperature and sa-linity of the fluids responsible for formation of anhydrite, Sr-isotopicanalyses to deduce relative proportions of seawater and hydrothermalfluid involved in the formation of anhydrite, and examination of rare-earth element (REE) concentrations of anhydrite to investigate theevolution of fluids within the mound. Here we report on the fluid in-clusion analyses. Sr-isotope and REE analyses from a similar samplesuite are discussed in detail by Mills et al. (Chap. 10, this volume) andHumphris (Chap. 12, this volume). Sulfur and O-isotope, O- and Sr-isotope, and fluid inclusion analyses of alternative suites of anhydritesamples from within the mound are reported by Chiba et al. (Chap. 6,this volume), Teagle et al. (Chap. 22, this volume), and Petersen et al.(Chap. 13, this volume).THE TAG ACTIVE HYDROTHERMAL MOUNDThe TAG active hydrothermal mound is located at a depth of 3670meters below sea level (mbsl) within the larger TAG hydrothermalfield on 100 ka crust, based on spreading rates (Rona et al., 1993).The distribution of high- and low-temperature venting on the surfaceof the mound, and of sample types, is asymmetrical (Tivey et al.,1995). High-temperature (363°C) black smoker activity is stronglyfocused and localized northwest of the center of the mound (at theBlack Smoker Complex). Fluids exhibiting lower temperatures(260°−300°C) and less vigorous flow rates emanate from many small(1−2 m) chimneys concentrated in the southeastern quadrant of themound: the “Kremlin” area (Thompson et al., 1988). Low-tempera-ture fluids percolate through the hydrothermal precipitates overpatchy areas of the top and sides of the mound and through areas onthe apron surrounding the mound. Results of heat-flow studies in-clude that conductive heat-flow values are extremely variable; thereis very