Mathematics – Logic
Scientific paper
Sep 1981
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1981gecoa..45.1479b&link_type=abstract
Geochimica et Cosmochimica Acta, vol. 45, Issue 9, pp.1479-1494
Mathematics
Logic
7
Scientific paper
Thermodynamic calculations of compositional relations among aqueous solutions and minerals in the system Na 2 O-K 2 O-CaO-MgO-Fe 2 O 3 Al 2 O 3 -SiO 2 -H 2 O-CO 2 HCl at pressures and temperatures corresponding to liquid-vapor equilibrium of H 2 O permit quantitative description and interpretation of phase relations among rock forming minerals and aqueous solutions in magma-hydrothermal systems. The extensive data base on the Salton Sea geothermal system provides an exemplary case for predicting the chemical characteristics of geothermal fluids associated with metasomatic mineral zones observed in deep drillhole samples. Near the Elmore No. 1 well aqueous species activity ratios of a Na + / a H + and a K + / a H + vary several tenths of a log unit with increasing depth and temperature from ~ 0.6 km and ~250°C to ~ 2.2 km and ~350°C, whereas a Ca 2+ / a 2 H + decreases ~ 2 orders of magnitude for a comparable range in depth and temperature. The fugacity of CO 2 gas is ~1.5-6 bars at 310°C. Calculated values of a Sio 2(aq) , a Na + / a K + , a Ca 2+ / a Mg 2+ and f CO 2(g) , in the fluid phase coexisting with observed mineralogic phase relations are in remarkably close agreement with measured solute concentrations in geothermal fluids produced from deep drillholes near the Salton Sea. Hydrolysis reactions representing observed phase relations and written with alkali and alkaline earth cations as products have negative standard molal enthalpies ( H 0 P , T , r ) and volumes ( V 0 P , T , r ) of reactions; consequently, a Na + / a H + , a K + / a H + , a Ca 2+ / a 2 H + , and a Mg 2+ / a 2 H + decrease with increasing temperature at constant pressure, but increase with increasing pressure at constant temperature. An approximate linear relationship exists among these activity ratios and the reciprocal of absolute temperatures because H 0 P , T , r varies only slightly with increasing temperature at 250 to 300°C. However, at 300°C, H 0 P , T , r and V 0 P,T,R decrease dramatically as a consequence of extrema in the thermodynamic and electrostatic properties of the solvent near the critical region of H 2 O. Hence, empirical solute geothermometers based on linear regressions of geothermal fluid compositions at 250-300°C cannot be confidently extrapolated to higher temperature systems.
Bird Dennis K.
Norton Denis L.
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