Trace elements in silicate melts and the thermal conductivity of the earth’s deep mantle ; insights from atomistic modeling of geomaterials ; Spurenelemente in Silikatschmelzen und die Wärmeleitfähigkeit des tiefen Erdmantels ; Ergebnisse atomistischer Modellierung von Geomaterialien

The present thesis deals with two aspects which are related to the present Earth's chemical and thermal heterogeneity: trace element partitioning and the lattice thermal conductivity of the lower mantle. We use atomistic computer simulations, first, to elucidate the microscopic mechanisms governing the incorporation of trace elements into silicate melts and their partitioning behavior in the presence of silicate melts. Second, the atomistic modeling approach is employed to obtain the lattice thermal conductivity of lower- mantle minerals at high pressures and temperatures. In chapter 1, the structure of aluminosilicate melts and glasses with 76 mol% SiO2 and varying amounts of Y and La is studied by means of ab-initio and classical molecular dynamics (MD) simulations as well as x-ray and neutron diffraction experiments. The following structural trends are found: the average coordination numbers of Y and La decrease with increasing REE content, and so does the average coordination number of Al. Furthermore, the distribution of Al coordination numbers is shifted to lower values in La-bearing melts, as compared to Y-bearing melts. These trends are rationalized in terms of cation field strengths. The ab-initio MD simulations also show that the Al avoidance rule is not valid for the studied REE-bearing aluminosilicate melts. Chapter 2 is devoted to the incorporation of Y as a trace element into calcium aluminosilicate melts. The aim is to understand how the melt composition, in particular the Ca content and the degree of melt polymerization, influences the partitioning behavior of Y between minerals and melts or between different melts. The local environment of Y in different melts is studied by means of classical MD and EXAFS spectroscopy. Using thermodynamic integration, we determine the equilibrium constant for an exchange reaction of Y and Al between two different melts. The results are consistent with experimental data and provide an atomic-scale explanation of the observed partitioning trends. Chapter 3 deals ...