Kinetics of Anorthite and Quartz Dissolution in Siliicate Melts.

Abstract

Almost a century’s study on thermodynamics of igneous systems has laid a mature foundation for the equilibrium relations among different phases of minerals and silicate melts. However, this ‘static’ view does not account for various kinetic processes that drive changes in igneous systems. With the additional temporal angle, the kinetic approach focuses on mineral dissolution and growth that contribute to changes in igneous systems, and aims to quantitatively model them. Taking a kinetic approach, this dissertation presents experimental results on the dissolution kinetics of anorthite - the major mineral in the lunar highland crust, and quartz – the major mineral in Earth’s continental crust, and quantitatively models the convective dissolution or growth of anorthite and quartz in silicate melts. In the anorthite study, the non-convective dissolution of anorthite was shown to be controlled by the diffusion of Al2O3. The Al2O3 concentration profiles conform well to the constant-diffusivity solution to the diffusive dissolution. Though noticeable variation exists across published Al2O3 diffusivity data, such difference was reasonably accounted for by considering compositional effect on diffusivity. Based on Al2O3 diffusivity model and anorhite-melt equilibrium model, both diffusive and convective dissolution rates of anorthite in the basaltic melt were modeled. In the quartz study, the composition-dependent SiO2 diffusivity complicates the diffusive dissolution of quartz. Boltzmann analysis was developed to characterize the compositional dependence of SiO2 diffusivity. Combined with functional fitting approach, the two methods showed that lnDSiO2 linearly depends on the cation mole fraction of Si+Al (XSi+Al). Especially, the effect of H2O on SiO2 diffusivity is simply dilution of XSi+Al by including H as cation. The diffusive dissolution of quartz still follows the parabolic relation, but with DSiO2 interpreted roughly as the geometric average of diffusivities at interface melts and far-field melts. Based on this, convective dissolution or growth of quartz was modeled to provide constraints on the evolution rate of some igneous systems. Put together, the kinetic studies complement the existing understanding of mineral dissolution and growth, with new insights to account for composition-dependent diffusion behavior. Via kinetic modeling, the work provides important temporal constraints on the evolution rate of natural igneous systems.