Dissolution experiments on albite and basalt glasses at various temperatures and their application to hydrothermal alteration in geothermal fields

抄録

Albite and basalt glasses were dissolved in distilled water and 0.01 and 0.0001M HCI solutions at 47, 80 and 120°C. The compositional change of solutions was traced. At an initial stage of dissolution, elemental ratios in solution differ from those in the starting materials, thus showing the glasses dissolve incongruently. With increasing duration of experiments the release rate of elements becomes congruent, and the incongruency of the glass and the solution decreases. At the later stage, the dissolution is again incongruent, mainly owing to precipitation of secondary minerals. The release rate of each element is proportional to aⁿ_H+, where n is from 0.5 to -0.2. The activation energy for the release of elements from the glasses is from 15 to 40kJ/mol. The value is smaller than that reported for crystalline silicates. Reaction pathways on activity diagrams show that the solutions after the longest experimental duration of each series are saturated or slightly supersaturated with secondary minerals. The concentrations of Al, Ca, Mg and Fe in such solutions for the same glass series are either constant or decrease with increasing temperature at the approximately same pH, while those of Si and Na increase. Concentrations of all elements decrease with increasing pH. The pathways showed a trend approaching to an equilibrium state. The typical sequences of precipitation of secondary minerals were suggested to be amorphous silica → stable polymorphs of clay minerals (e.g., kaolinite) in strongly acid solutions, and metastable polymorphs of clay minerals (e.g., allophane and halloysite) → quartz in weakly acid solutions. We conducted two step experiments. Solutions which dissolved the glasses at high temperature were let stand or reacted with fresh glasses at low temperatures. The compositional difference between the solutions obtained at high and low temperatures suggests formation of some secondary minerals, and also dissolution of the glass during the experiment. The compositional change of solution caused by precipitation and dissolution of minerals as a result of change in the solution is explained by the solubility product of a mineral, defined in terms of total molality of related element at fixed temperature, pressure, pH and in case of need, at fixed P_O2, P_CO2, m_cl and m_s. Various alteration patterns in geothermal fields were discussed for a model of hydrothermal convection system on the assumption that groundwater flows nearly in equilibrium with rocks, at least with secondary minerals. The alteration was investigated on the basis of compositional change of hydrothermal solution by combining the experimental results obtained after long isothermal runs at different pH's and temperatures. The results of the two step experiments are also considered. This approach is systematized by different trends of the solubility products as described above for silica and other oxides, hydroxides and silicates in function of temperature and pH, and used to clarify processes observed in the field, such as silicification, argillization, quartz vein formation and zonation of wall rock alteration products.