DETERMINATION OF FE-CL COMPLEXING IN THE LOW-PRESSURE SUPERCRITICAL REGION (NACL FLUID) - IRON SOLUBILITY CONSTRAINTS ON PH OF SUBSEAFLOOR HYDROTHERMAL FLUIDS

Experiments were performed to determine Fe-Cl speciation in aqueous fluids at temperatures from 200 to 450-degrees-C, pressures from 300 to 500 bars, and total dissolved Cl concentrations from 0.03 to 1.24 molal. The experiments made use of the assemblage hematite-magnetite to buffer key aspects of fluid chemistry. FeCl+ and FeCl20 species were found to best fit the experimental data. At relatively low temperatures (e.g., 200-degrees-C), our data agree with recent spectroscopic data, which show FeCl+ to be the dominant form of dissolved Fe in low to moderate Cl-bearing aqueous fluids. At higher temperatures, however, FeCl20 becomes increasingly significant. Taking explicit account of recent revisions to the HKF equation of state for aqueous species, we determined association constants for FeCl+ and FeCl20 as a function of temperature and pressure from measured pH, as well as dissolved Fe and Cl concentrations. At 300, 350, and 400-degrees-C, 500 bars, for example, log K(FeCl20) values for the following reaction: Fe2+ + 2 Cl- = FeCl20 are 4.77 +/- 0.05, 6.00 +/- 0.07, and 7.51 +/- 0.07, respectively. At 400-degrees-C, 400 and 300 bars, log K(FecL20) increases from 8.21 +/- 0.12 to 10.89 +/- 0.12. The results indicate the degree of association of FeCl20 increases with increasing temperature and decreasing pressure. As conditions approach the supercritical region (NaCl fluid), the relative stability of the complex increases dramatically. Regression analysis of the association constant for FeCl20 against the density of water results in the following relationship: log K(FeCl20) = 16.042 (+/- 0.188) - 14.579 (+/- 0.311)d, where d is the density of water. This empirical relation can be applied to hydrothermal systems to constrain Fe mobility provided d falls within the range of 0.777 to 0.358 g/cm3. Fe-Cl complex data, along with our previously determined data for the effect of dissolved NaCl on activity coefficients of H-2(aq) and H2S(aq) at elevated temperatures and pressures, permit us to constrain subseafloor hydrothermal alteration processes from dissolved Cl, Fe, H-2(aq), and/or H2S(aq) concentrations of hot spring fluids. For example, assuming vent fluids at mid-ocean ridges equilibrate with pyrite-magnetite, subseafloor temperatures and pH(in situ) values of 372-394-degrees-C and 4.6-5.2, respectively, are indicated. These results are in good agreement with phase equilibria constraints involving plagioclase-epidote-quartz coexisting with Na-Ca-Cl fluids.