Host: The Japanese Association of Mineralogists, Petrologists and Economic Geologists
1.Introduction Various types of alteration zone characterizing chemically geothermal phenomena have been formed in geothermal area. Alteration type is specified by characteristic chemical composition and temperature conditions of geothermal fluid, which was generated by hydrothermal fluid-rock interactions in geothermal area. However, it has not been clear how to generate the geothermal fluid enriched for selected elements and how to form the secondary minerals. These phenomena could not be explained by equilibrium relationship of rock-hydrothermal water composition. We need to consider hydrothermal water composition in the system changed by time and space. Purpose of this research is estimation of the formation mechanism of alteration zone based on experimental approaches attracting dynamics of reaction.
2.Methodology We conducted two types of hydrothermal experiments. One is batch type experiment to estimate apparent dissolution and precipitation rate and dissolution behavior, and the other is flow type experiment to examine the mass transport phenomena on hydrothermal water/rock interface. Hydrothermal experiments for pyroclastic rocks were conducted under saturated vapor pressures at given temperatures which are appropriate for ordinary geothermal environments in Japan.
3.Kinetics of water/rock interaction The pyroclastic rock did not proceed stoichiometrically with respect to chemical composition of rock forming minerals and whole rock, because dissolution rates were different for each element. The chemical composition of reacted solution is characterized by thus difference of dissolution rate and the composition of secondary minerals. Particularly, alkali and alkali earth elements are well matched with its solution chemistry. Dissolution and precipitation rate model also consisted with experimental results.
4.Mass transport coefficients Mass transport coefficients at interface between fluid and rocks in reaction term were also different for each element and an outstanding factor, for determination of the mass transport coefficient is change of temperature, flow velocity condition and the distance from water input. Specifically, δNa is greater at lower temperature conditions and δCa is greater at higher temperature conditions. The relationship between major elements depends on temperature conditions, but δvalues most strongly depend on flow rate condition. Bulk solution chemistry of reacted solution generated hydrothermal water/rock interactions could be possible to interpret using diffusion-advection-reaction rate equation introducing kinetic and mass transport parameter, which was estimated quantitatively in this experiment.
5.Summary Leaching and enrichment processes of selected elements was controlled by kinetic and mass transport processes, which is an important part in formation process of alteration zone. Dynamic approach described in this study is effective to understand coupled hydrothermal-chemical processes of geothermal alteration.
