Principles of geothermometry

With the dump command, we cause the program to discard the minerals present in the initial system before beginning the reaction path. In this way, we simulate the separation of the fluid from reservoir minerals as it flows into the wellbore. The precip = off command prevents the program from allowing minerals to precipitate as the fluid cools. In practice, samples are acidified immediately after they have been sampled and their pH determined. Preservation by this procedure helps to prevent solutes from precipitating, which would alter the fluid s composition before it is analyzed. 

Since we have provided initial and final temperatures but have not specified any reactants, the program traces a polythermal path for a closed system (see Chapter 12). The fluid s pH (Fig. 17.1) changes with temperature from its initial value of 5 at 250°C to less than 4 at 25°C. The change is entirely due to variation in the stabilities of the aqueous species in solution. As shown in Fig. 17.2, the H+ concentration increases in response to the dissociation of the HC1 ion pair 

The CO2 fugacity decreases sharply during cooling (Fig. 17.3), as would be expected, since gas solubility increases as temperature decreases. In the calculation, the fugacity decrease results almost entirely from variation in the equilibrium constant for the reaction 

1 Variation in pH in a computer simulation of sampling, cooling, and then reheating a hypothetical geothermal fluid. Bold line shows path followed when system is held closed fine lines show variations in pH when fluid is allowed to degas CO2 as it cools. 

2 Changes in concentration of aqueous species H+, CO2(aq), and HC1 with temperature during cooling of a geothermal fluid as a closed system. A positive value indicates an increase in concentration relative to 250°C a negative value represents a decrease. 

1 free cm3 Albite 1 free cm3 Maximum Microcline 1 free cm3 Muscovite 1 free cm3 Quartz 1 free cm3 Calcite 

The log K for this reaction increases from —2.12 at 250°C to —1.45 at 25 °C. The final C02 fugacity is about 15, corresponding to a partial pressure considerably in excess of atmospheric pressure. We would certainly need to take extraordinary measures to prevent the fluid from effervescing, if we were actually performing this experiment instead of simulating it. 

Now that we have simulated sampling the fluid and letting it cool, let us predict the fluid s original temperature (which we already know to be 250 °C). The REACT commands 

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Although the scope of this book does not allow an appropriate treatment of stable isotope compositions of earth s materials (excellent monographs on this subject can be found in the literature—e.g., Hoefs, 1980 Faure, 1986), we must nevertheless introduce the significance of the various compositional parameters adopted in the literature before presenting the principles behind stable isotope geothermometry. 

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