During the last decade, insightful applications of thermodynamic calculations embodied within the MELTS thermodynamic model of Ghiorso and Sacks (1995) have greatly enhanced our understanding of the evolution of magmas. MELTS does the isobaric and isothermal constrained calculations by minimizing the Gibbs free energy for the system; temperature, pressure, oxygen fugacity constrained calculations by minimization of the Korzhinski potential for systems open to oxygen transfer. The adiabatic calculations are done by minimization of the enthalpy subject to fixed pressure and entropy (e.g. heat content). Applications of the MELTS calculations demonstrated in several publications have shown how complex petrologic hypotheses can be tested to yield quantitative and occasionally suprising results. This doctorate research focuses on developing quantitative models for crystal fractionation, magma recharge, magma mixing, compositional zoning in plagioclase by applying the MELTS algorithm to natural solid-liquid equilibria, and the application of Crystal Size Distribution (CSD) theory to compute crystal residence time in magma. This dissertation is composed of three chapters.
In Chapter 1, the initial system parameters of parental magma (pressure, temperature, water content, and oxygen fugacity) of the Small Hasandag volcano in Central Turkey were constrained using the MELTS algorithm and then using the same algorithm, the feasibility of isobaric fractional crystallization, magma recharge, and isobaric-isenthalpic magma mixing was tested as the controlling process in the evolution of the parental magma.
In Chapter 2, the consequences of two different physical models of magma dynamics on plagioclase zoning were determined. The consequences of magma pooling at several levels within the crust before eruption (decompressional and isobaric fractional crystallization) were explored. This process can produce normal, reverse and even oscillatory zoning in plagioclase. In another model, the effect of convection within a shallow magma chamber on plagioclase zoning was explored and was demonstrated that oscillatory zoning develops under these conditions.
In Chapter 3, the CSD theory was used to calculate the crystal nucleation rate, crystal growth rate and crystal residence time using the plagioclase and clinopyroxene present in a single basaltic lava flow at the Small Hasandag volcano. Although crystal residence times using a single mineral in a single lava flow has been studied using the CSD theory, it is not clear if the same crystal residence times can be obtained if more than one mineral is used. The results of this part of my research show that residence times calculated from the CSD theory gives the same crystal residence times whether a single mineral or more than one mineral is used in the calculations. The results show that plagioclase and clinopyroxene residence times overlap within the limits of error implying that crystal residence times calculated by using the CSD theory either clinopyroxene or plagioclase of the Small Hasandag volcano can be used in the calculation of residence time.