Kinetic Evolution of Alkali-feldspars Dissolution Reactions by Using Controlled Water-Rock Interaction Experiments at Low-to-Medium Geothermal Temperature Conditions

Abstract

Water-rock (W/R) interaction experiments and geochemometrical modelling studies (based on the application of rational polynomial and logarithmic transformation regression models) have been conducted for the study the evolution of alkali-feldspars dissolution reactions under geothermal conditions. Such laboratory experiments allow mineral-water interactions and alteration processes occurring during geothermal fluid migration to be understood. Exchange reactions between Na-K and alkaline-feldspar minerals were monitored at low-to-medium temperatures for a period of nearly 24 months. The concentrations of dissolved elements, and especially the Na/K ratios were used for estimating the most probable quasi-steady state conditions of these W/R experiments. Geochemometrical modelling was performed in order to predict reliably the reaction times required to achieve quasi-steady state conditions during the experiments. The geochemometrical modelling developed in this study was previously calibrated and validated with W/R experimental data reported in the literature, for which quasi-steady state conditions were known with accuracy. Laboratory experiments were performed using batch reactors under stable P-T conditions. Volcanic rock crushed samples characterize by basaltic and dacitic compositions were set to react with distilled water at 90 oC and 150 oC using a W/R mass ratio of 5. Fluid and rock samples were carefully collected and analyzed for major composition, before and after each W/R experiment. Experimental results were used to calculate log (Na/K) values and to describe the kinetic behavior of involved reactions. Furthermore, log(Na/K) values at steady-state and reaction times were estimated by using rational polynomial and logarithmic transformation regression models. Results are compared against geochemical data inferred from well-known Na-K geothermometers [log(Na/K)GEO] (assuming theoretical equilibrium conditions). Details of the experimental and numerical studies are described in the present work.