TRACING THE CHEMICAL EVOLUTION OF FLUIDS DURING HYDROTHERMAL RECHARGE- CONSTRAINTS FROM ANHYDRITE RECOVERED IN ODP HOLE 504B

A unique record of the chemical evolution of seawater during hydrother mal recharge into oceanic crust is preserved by anhydrite from the vol canic sequences and sheeted dike complex in ODP Hole 504B. Chemical an d isotopic analyses (87Sr/86Sr, delta 18O, delta 34S) of anhydrite con strain the changing composition of fluids due to reaction with basalt. There is a general trend of decreasing Sr-87/Sr-86 of anhydrite, corr esponding to the minor incorporation of basaltic strontium with Sr-87/ Sr-S6 ratios decrease rapidly with depth in the dikes to values identi cal to host basalt depth in the volcanic rocks. (0.7029), Sr/Ca ratios (< 0.1 mmol/mol) suggest that recharge fluids have very low Sr concen trations and fluids evolve by first precipitating Sr-bearing phases be fore extensive exchange of Sr with the host basalt. There is a backgro und trend of decreasing sulfate delta(18)O with depth from + 12-13 par ts per thousand in the lower volcanics to + 7 parts per thousand in th e lower sheeted dikes recording an increase in recharge fluid temperat ure from approximate to 150 degrees to approximate to 250 degrees C, a nd confirming the presence of sulfate in hydrothermal fluids at elevat ed temperatures. From the amount of anhydrite recovered from Hole 504B and the amount of seawater sulfur that has been reduced to sulfide, a minimum seawater recharge flux can be calculated. This value is appro ximate to 4-25 times lower than estimates of high-temperature fluid fl uxes based on either thermal constraints or global chemical budgets an d suggests that there is significant deficit of seawater-derived sulfu r in the oceanic crust. Only a minor proportion of the seawater that p ercolates into the crust near the axis is heated to high temperatures and exits as black smoker-type fluids. A significant proportion of the axial heat loss must be advected at 200-250 degrees C by sulfate-bear ing hydrothermal solutions that egress diffusely from the crust. These fluids penetrate into the dikes and exchange both heat and chemical t racers without the extensive clogging of porosity by anhydrite precipi tation, which would halt hydrothermal circulation for any reasonable f luid flux. The heating of the major proportion of hydrothermal fluids to only moderate temperatures (approximate to 250 degrees C) reconcile s estimates of hydrothermal fluxes derived from thermal models and glo bal geochemical budgets. The flux of hydrothermal sulfate would be of a magnitude similar to the riverine input, and oxygen-isotopic exchang e at 200-250 degrees C between dissolved sulfate and recharge fluids d uring hydrothermal circulation provides a mechanism to continuously bu ffer seawater sulfate oxygen to the light isotopic composition observe d. (C) 1998 Elsevier Science B.V.