An experimental investigation of metal-silicate segregation in partially molten peridotites

Core formation is a significant event in a planet's geophysical and geochemical evolution. The mechanisms of core formation are topics of active research. This dissertation is an experimental investigation of the segregation of metals from silicates to form a core.; We describe the segregation of metallic melts from a peridotite matrix in Chapter 2. Plastic deformation experiments on olivine + Fe-FeS (Fe-S) reveal that metallic melts are able to segregate by percolation when the Fe-S melt fraction exceeds 0.05. Metallic melt segregation by percolation may be a core formation mechanism in bodies that did not experience a magma ocean phase.; Chapter 3 is an attempted study of oxygen fugacity's role in the segregation of metallic melts from peridotites. The dihedral angle, theta, of Fe-S in contact with olivine changes with metallic melt's oxygen content. It was necessary to replace the thoriated tungsten (ThO2-W) pistons used previously to remove their effect on the oxygen fugacity of the sample-sleeve system. The ThO2-W pistons were replaced with olivine single crystal or natural large-grained dunite simple-shear pistons. We demonstrate that natural materials can be used as simple-shear pistons for future studies.; The influence of Fe-S on peridotite viscosity is the subject of Chapter 4. Peridotite viscosity depends exponentially on melt fraction, and melt's effect on viscosity is expressed as an empirically determined melt fraction factor, alpha. The alpha value for mid-ocean ridge basalt (MORB) in olivine aggregates is 21, and we have determined alpha = 6 for Fe-S in olivine aggregates. The difference between the alpha values of Fe-S and MORB is attributed to differences in solubility of olivine in the melts and the different dihedral angles.; In Chapter 5 we report MORB's effect on the distribution of Fe-S in olivine aggregates deformed in simple shear. MORB forms an interconnected network within peridotite hosts at lower melt fractions than Fe-S, and disrupts the Fe-S network that would otherwise form. MORB prevents the Fe-S segregation by percolation but the migration of the MORB can coarsen Fe-S blebs and possibly allow metallic melt to sink as diapirs to form a core.