Kump et al EPSL 2005.pdf - Bryn Mawr College

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L.R. Kump, W.E. Seyfried Jr. / Earth and Planetary Science Letters 235 (2005) 654–662 657 80 70 Fe a. Concentration (mmol/kg) 60 50 40 30 20 10 0 12 11 10 H 2 S (aq) 5.0 5.2 5.4 5.6 5.8 6.0 pH H 2 S b. Concentration (mmol/kg) 70 60 50 40 30 20 10 0 a. Fe H 2 S 0 50 100 150 200 250 300 350 400 Temperature °C Concentration (mmol/kg) 9 8 7 6 5 4 3 2 1 Fe Concentration (mmol/kg) 10 8 6 4 2 b. H 2 S Fe 5.0 5.2 5.4 5.6 5.8 6.0 pH 0 Fig. 2. The effect of pH on dissolved Fe for fayalite–magnetite– pyrrhotite–(quartz)–fluid equilibria (a) and anhydrite–magnetite– (quartz)–plagioclase (An80) [8]. (b). Calculations were performed at 400 8C, 400 bars. At a pH of 5, which is typical of modern hot spring vent fluids at mid-ocean ridges [15], high Fe and relatively low H 2 S are predicted (high Fe/H 2 S ratio) assuming equilibria with the more reducing assemblage (a), while the opposite is true for the anhydrite-bearing bmodernQ system (b). The relative absence of sulfate in ancient oceans (Archean, Neoproterozoic) would permit more reducing assemblages in host rocks to persist, enhancing Fe solubility. Model calculations were performed using EQ3/6 [45] with thermodynamic data generated using SUPCRT92 [41,46], assuming dissolved chloride of 0.55 mol/kg. See Fig. 1 for additional sources of thermodynamic data. into the ancient ocean. Most of the loss in Fe is by dilution, although minor Fe-mineralization (pyrite, pyrrhotite and at sufficiently low temperature, hematite) also occurs (Fig. 3a). Because of the high Fe/H 2 S ratio, however, complete removal of H 2 S from the 0 50 100 150 200 250 300 350 400 Temperature °C Fig. 3. Reaction path model depicting the effect of mixing (cooling) on dissolved Fe and H 2 S initially set assuming fayalite–magnetite– pyrrhotite–(quartz)–fluid equilibria (a) and anhydrite–pyrite–magnetite–(quartz)–plagioclase (An80) (b) (see Fig. 2). Concentrations of Fe and H 2 S at 400 8C, 400 bars were calculated assuming pH=5, and 0.55 mol/kg dissolved chloride. Temperature change was calculated assuming mixing with a NaCl fluid (0.55 mol/kg) at 25 8C (a), while modern seawater was the low temperature mix fluid for second simulation (b). Dilution effects and temperature dependent changes in sulfide mineral solubility (pyrite, pyrrhotite, hematite) and homogeneous equilibria (pH change) cause the predicted changes in Fe and H 2 S (a). These effects can result in the delivery of relatively high Fe and high Fe/H 2 S ratio fluids to the ancient ocean affecting BIF deposition. This is not the case for modern sulfate-bearing systems due to the initially low Fe/H 2 S ratio of the predicted source fluid (b). Model calculations were performed using EQ3/6 [45] with thermodynamic data generated using SUPCRT92 [41,46], assuming dissolved chloride of 0.55 mol/kg. See Fig. 1 for additional sources of thermodynamic data.
658 L.R. Kump, W.E. Seyfried Jr. / Earth and Planetary Science Letters 235 (2005) 654–662 hydrothermal fluid is predicted (Fig. 3a). In contrast, the low Fe/H 2 S ratio of the more oxidizing modern vent fluids results in complete removal of Fe when mixed with modern sulfate-bearing seawater, a result largely due to pyrite precipitation (Fig. 3b). In actuality, vent mineralogy is undoubtedly dominated by a complex series of kinetically mediated reactions [2], although fundamental redox constraints will still dominate the temperature dependent abundance and sequence of mineral precipitates and corresponding changes in vent fluid chemistry. 2.2. Effect of pressure on vent fluid composition concentration (mmol/kg) 80 500 bars 400 bars 70 60 50 40 30 20 10 It has long been known that in addition to redox, temperature and pressure play critical roles in controlling Fe solubility in hydrothermal systems. At elevated temperatures, relatively small changes in pressure can result in large changes in dissolved Fe. Thermodynamic calculations for the fayalite–pyrrhotite–magnetite–(quartz)-bearing system at pH=5 show that a change in pressure from 500 to 400 bars results in a doubling of Fe, a modest change in H 2 S, and correspondingly a substantial increase in Fe/H 2 S (Fig. 4). Owing to the lack of thermodynamic data, it is not possible to perform similar calculations at lower pressures, although it can be inferred from experimental data [20] that at temperatures in excess of 350 8C, a further decrease in pressure would result in additional increases in dissolved Fe, particularly for hydrothermal systems lacking sulfate. This sensitivity largely results from the reduction in pH that occurs as the two-phase boundary of seawater is approached, but is also due to the enhanced stability of aqueous Fe-chlorocomplexes [19,20]. Recent analyses of fluids from modern vent systems further indicate that phase separation in response to pressure and/or temperature change can result in significant corresponding changes in dissolved Fe concentrations. For example, in spite of the low salinity of the vapor phase fluids released from subseafloor reaction zones undergoing phase separation in connection with subseafloor magmatic activity at EPR 9–108 N, Fe concentrations are unusually high on an absolute and chloride normalized basis [11]. The high Fe concentrations of the vapors likely result from aqueous complexing effects, as noted earlier. Fe H 2 S Fe/H 2 S Fig. 4. The effect of pressure on dissolved Fe, H 2 S and Fe/H 2 S for a chemical system buffered by fayalite–quartz–magnetite–pyrrhotite coexisting with 0.55 mol/kg chloride at a pH=5 and 400 8C (see text). The increase in dissolved Fe and Fe/H 2 S ratio predicted for the 100 bar pressure drop likely continues at still lower pressures owing to the effect of pressure on the stability of aqueous ferrous chloride complexes and mineral phase relations, as suggested by earlier experimental data [20]. These data suggest that the decrease in hydrostatic pressure associated with global glaciations in the Neoproterozoic may enhance the flux of hydrothermal Fe to the ancient ocean. Relatively shallow ridges in the Archean together with a sulfate-free ocean would similarly enhance Fe flux for BIF deposition at that time. Pressure sufficiently low that the two-phase boundary of the fluid (ancient or modern) is intersected would result in formation of Fe-bearing vapors and brines. Assuming both phases ultimately reach the seafloor, the Fe flux would be significant. Thermodynamic data sources are as discussed in earlier figures. The coexisting brine phase at depth is likely exceedingly rich in Fe, owing to its high chloride concentration and relatively low pH [8,10,21]. Although it is commonly believed that vapor loss causes an increase in the pH of the residual brine phase [22], this is not the case, however, in the presence of silicate minerals that can buffer pH at relatively low values in response to the increase in chloride and charge balance constraints [23], especially at high temperatures where such reactions occur rapidly. The Fe-bearing brines likely reside in the crust for some time, but ultimately mix with hydrothermal seawater and vent as well, as indicated, for example, by the composition of vent fluids at the southern Cleft Segment of the Juan de Fuca Ridge [24–26]. If phase separation/segregation takes place very near the seafloor, as has been suggested for vent
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