Determination of Fe-Cl complexing in the low pressure supercritical region (NaCl fluid): Iron solubility constraints on pH of subseafloor hydrothermal fluids

Journal Article · Thu Oct 01 04:00:00 EDT 1992 · Geochimica et Cosmochimica Acta; (United States)

DOI:https://doi.org/10.1016/0016-7037(92)90161-B· OSTI ID:7170479

Ding, K; Seyfried, Jr, W E ^[1]

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1. Univ. of Minnesota, Minneapolis (United States)

Experiments were performed to determine Fe-Cl speciation in aqueous fluids at temperatures from 200 to 450C, 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[sup +] and FeCl[sub 2][sup 0] species were found to best fit the experimental data. At relatively low temperatures (e.g., 200C), the authors' data agree with recent spectroscopic data, which show FeCl[sup +] to be the dominant form of dissolved Fe in low to moderate Cl-bearing aqueous fluids. At higher temperatures, however, FeCl[sub 2][sup 0] becomes increasingly significant. Taking explicit account of recent revisions to the HKF equation of state for aqueous species, they determined association constants for FeCl[sup +] and FeCl[sub 2][sup 0] as a function of temperature and pressure from measured pH, as well as dissolved Fe and Cl concentrations. Fe-Cl complex data, along with their previously determined data for the effect of dissolved NaCl on activity coefficients of H[sub 2(aq)] and H[sub 2]S[sub (aq)] at elevated temperatures and pressures, permit one to constrain subseafloor hydrothermal alteration processes from dissolved Cl, Fe, H[sub 2(aq)], and/or H[sub 2]S[sub (aq)] concentrations of hot spring fluids. For example, assuming vent fluids at mid-ocean ridges equilibrate with pyrite-magnetite, subseafloor temperatures and pH[sub (in situ)] values of 372-394C 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.

• OSTI ID: 7170479
• Journal Information: Geochimica et Cosmochimica Acta; (United States), Journal Name: Geochimica et Cosmochimica Acta; (United States) Vol. 56:10; ISSN GCACAK; ISSN 0016-7037
• Country of Publication: United States
• Language: English

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15 GEOTHERMAL ENERGY
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58 GEOSCIENCES
580000 -- Geosciences
CHLORIDES
CHLORINE COMPLEXES
CHLORINE COMPOUNDS
COMPLEXES
COMPLEXOMETRY
CRITICAL PRESSURE
ELEMENTS
FLUIDS
GEOTHERMAL FLUIDS
GEOTHERMAL SYSTEMS
HALIDES
HALOGEN COMPOUNDS
HEMATITE
HYDROTHERMAL SYSTEMS
IRON
IRON CHLORIDES
IRON COMPLEXES
IRON COMPOUNDS
IRON ORES
KINETICS
MAGNETITE
METALS
MINERALS
ORES
OXIDE MINERALS
PH VALUE
PHYSICAL PROPERTIES
REACTION KINETICS
ROCK-FLUID INTERACTIONS
SOLUBILITY
SUPERCRITICAL STATE
THERMODYNAMIC ACTIVITY
THERMODYNAMIC PROPERTIES
TRANSITION ELEMENT COMPLEXES
TRANSITION ELEMENT COMPOUNDS
TRANSITION ELEMENTS