Equilibrium Thermodynamic Modeling

Phase Diagrams

Simple phase diagrams of the system Fe-CO₂-S-Cl were useful in the initial phases of this project when considering whether or not even simple cooling of the hydrothermal fluid at Finn would result in altered speciation. As shown to the left, the Finn hydrothermal fluid begins in the FeCl₂ field at 300 ° C. Upon cooling, the fluid shifts into the Fe⁺² field at 2 ° C. Using phase diagrams in this way, however, is limiting and another method of analysis, equilibrium modeling using computer codes such as SUPCRT92 and "The Geochemist's Workbench", was initiated.

Equilibrium modeling
The chemical composition of hydrothermal fluid at Finn, obtained from David Butterfield (University of Washington), was input into "The Geochemist's Workbench" thermodynamic modeling program. Because fO2 (g) was not measured in the Finn fluids, a value was chosen that was consistent with the PPM (pyrrhotite-pyrite-magnetite) mineral buffer and the presence of NH₄⁺ in the vent samples. An average composition was assumed for 2 ° C seawater [see Millero (1996) Chemical Oceanography; Drever (1988) The geochemistry of natural waters]. Log K's for various inorganic and organic iron species were calculated at elevated temperatures and pressures using SUPCRT92 [Johnson et al. (1992)] and various databases [Shock et al. (1997); Sverjensky et al. (1997); Prapaipong et al. (1999)].

The goal of the modeling was to determine what affects isotope fractionation in a vent environment.
Possibilities include:

Changes in iron speciation and, thus, the availability of certain species to react and precipitate sulfide minerals
Temperature gradients and the effect of temperature on the relative stability of various sulfide precipitation reactions
Chemical gradients and the microorganisms utilizing these gradients to live

High-temperature hydrothermal fluid was subjected to a variety of reaction paths, incuding:

Results of the modeling are shown to the right and below. The basic observations from this analysis are:

Temperature
Initially, small amounts of 2 ° C seawater have a significant effect on the temperature of the hydrothermal fluid-seawater mixture.

fO₂ or O₂ (aq)
In all the situations considered, oxygen fugacity decreased along the reaction path (or with decreasing temperature). This is due primarily to precipitation and speciation reactions which utilize oxygen in solution as a reactant. As Hannington et al. (1995) note, not until large quantities of seawater (about 1000 kg, calculated in this study) mix with hydrothermal fluid does the oxygen fugacity begin to approach ambient seawater values.

Speciation
Initially dominated by FeCl₂, iron speciation changes to a Fe⁺²/FeCl⁺/FeOH⁺ assemblege dominant at lower temperatures and higher pH.

Mineral precipitation
Chalcopyrite precipitates mainly as the hydrothermal fluid cools, but not as it mixes with 2 ° C seawater. Both sphalerite and pyrite will precipitate when vent fluid mixes with cold seawater. Thus the logical precipitation sequence is: CPY (cooling in vent) - PYR/SPH (cooling/mixing with seawater seeping through cracks in chimney structure)

Precipitation reactions
Calculated log K's indicate how free energies of reaction change with temperature. The thermodynamically-favored reactions for the precipitation of both pyrite and chalcopyrite involve Fe(OH)₃^- and HS^-. While Fe(OH)₃^- is a minor species in all of the model runs, it may still provide an important pathway through which isotope fractionation may occur. Certain cross-overs also occur at temperatures of about 200 ° C between major species such as Fe⁺²/FeCl⁺ and FeCl₂/Fe(OH)₂ which, while not the most favorable reactions, may still influence sulfide precipitation.

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