DETERMINATION OF DISSOLUTION RATES OF ROCKS/MINERALS BY LABORATORY EXPERIMENTS

Abstract

Rocks and minerals on the Earth's surface weather at differential rates and are primarily responsible for the geochemistry of natural waters. Rock weathering is important in causing weakened surfaces of rock strata, thereby causing landslides and associated hazards. Qualitative understanding of rates of rock/mineral weathering is essential for many aspects of geologic phenomena including crust evolution, soil formation, river bank stabilisation etc. Determination of dissolution rates of rocks, minerals and soils in laboratory simulated conditions help in quantifying the data, which may be used in weathering models. In the present study, organic and inorganic buffer solutions have been used in batch experiments (rocks and minerals) and column dissolution experiments (soils) at 5° and 25°C. Afew representative rock samples and soil columns were collected from the Alaknanda-Bhagirathi river basins in the Himalayas. The samples invariably underwent differential dissolution rates controlled primarily by pH and temperature. Water samples (sampled weekly) collected over one year at two locations, one from the Alaknanda river and the other from the Bhagirathi river were analyzed to observe possible temporal and spatial variations in concentration of dissolved phases and elemental flux rates were estimated. High Ca/Si flux ratio in both Alaknanda and Bhagirathi rivers indicates dominance of carbonate weathering. Laboratory derived dissolution rates of representative lithologies at pH 8.4 and temperature 25°C is much lower compared to the average Ca - and Si02 -flux rates, whereas carbonate dissolution (~10~12 molmV) is higher compared to the dissolution rates of rock types determined in the laboratory. At lower pH (2.2 and 4.2), acidity dominates over temperature in regulating dissolution rates of rocks whereas, in alkaline condition (pH = 8.4), temperature is the dominant factor over pH. At pH 8.4 and temperature 25°C, dissolution rate of soils is >20 times slower compared to carbonate dissolution rate. At pH 8.4, activation energy (Ea) is highly controlled by change in temperature, whereas at pH 2.2 and 4.2, it has negligible effect on temperature change. The thesis is presented in five chapters. Chapter 1 presents the background of the chemical weathering process with special emphasis on dissolution reactions of carbonate and silicate rocks/minerals, major dissolution reactions occurring on the earth's surface, their mechanism and controlling factors, and its significance on watershed to global scale. It also highlights major work done by various workers on laboratory simulated dissolution rates of rocks and minerals and also on the chemical weathering of Alaknanda and Bhagirathi river basins. The primary objectives of the thesis are also enumerated. Chapter 2 deals with the geological details and sampling locations of the rocks, soil and water undertaken in the present study. Sample characteristics in terms of its mineralogical and chemical compositions have also been discussed using various parameters. Chapter 3 deals with the methodology followed for sample collection of rocks, soil and water in the field; preparation of buffers (pH of 8.4, 4.2 and 2.2) using different standard salts and preparation of samples (rocks/soil) and experimental set-up (batch and column) for carrying out dissolution experiment in the laboratory. The various instruments which have been used during the course of experimentation have also been discussed in this chapter. Chapter 4 presents the results and discussion segment of the thesis. Temporal and spatial variations in the geochemistry of the Alaknanda and Bhagirathi river water at two locations have been discussed and elemental flux rates calculated. ii Laboratory simulated dissolution rates of representative samples of carbonates, silicate rocks and soil are calculated and the role of temperature, pH, ligands and surface dislocation sites regulating dissolution rates have been discussed. Chapter 5 outlines the major conclusions derived from the present study. The thesis is endowed with 32 figures, 11 tables, references and appendices.