Oxygen fugacity, calculation

Sverjensky (1984) calculated the dependency of Eu +/Eu + in hydrothermal solution on /oj (oxygen fugacity), pH and temperature. According to his calculations and assuming temperature, pH and /oj for epidote-stage alteration of basalt and Kuroko ores (Shikazono, 1976), divalent Eu is considered to be dominant in the rocks and hydrothermal solution. Thus, it is reasonable to consider that Eu in the rocks was removed to hydrothermal solution under the relatively reduced condition more easily than the other REE which are all tiivalent state in hydrothermal solution. Thus, it is hkely that Eu is enriched in epidote-rich altered volcanic rocks. Probably Eu was taken up by the rocks from Eu-enriched hydrothermal solution which was generated by seawater-volcanic rock interaction at relatively low water/rock ratio. 

How would the reaction have proceeded if the oxygen fugacity had been fixed by equilibrium with the atmosphere To find out, we repeat the calculation, this time holding the oxygen fugacity constant... 

An interesting aspect of the calculation is that when oxidizing seawater mixes into the reduced hydrothermal fluid, the oxygen fugacity decreases (Fig. 22.4). The capacity of seawater to oxidize the large amount of hydrothermal H2S(aq) is limited by the supply of C>2(aq) in seawater, which is small. Given the reaction,... 

To see how contact with atmospheric oxygen might affect the reaction, we repeat the calculation, assuming this time that oxygen fugacity is fixed at its atmospheric level... 

In the two calculations (one including, the other excluding calcite), the resulting fluids differ considerably in composition. After reaction of 1 cm3 of pyrite at atmospheric oxygen fugacity, the compositions are... 

Fig. 3. Simulations calculated with the PHREEQC geochemical code (Parkhust Appelo 1999) (a) time-dependent diagram for the pH evolution of the Aspo ground water/bentonite interaction (b) time-dependent diagram for the pe evolution of the Aspo groundwater/bentonite interaction. Curves correspond to different initial partial oxygen pressures. Initial calcite and pyrite contents are 0.3 wt% and 0.01 wt% respectively, except for the curve of log/02 = —0.22 where calcite and pyrite contents are 1.4 wt% and 0.3 wt%, respectively, pe calculated stands for the cases where the oxygen fugacity is obtained from the groundwater redox potential (Bruno et at. 1999).

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