Experimental approaches to evaluating silicate melt properties and trace element fractionation during crystallization at high pressures and high temperatures

Current understanding of the evolution and behavior of silicate materials that form in planetary interiors at high-pressures and high-temperatures largely come from experimental work as natural samples are either rare, or physically inaccessible. Laboratory experiments provide a comprehensive way to constrain the crystallization history, elemental partitioning, and viscosity of different silicate materials at planetary mantle pressure and temperature conditions. This work utilizes two high-pressure experimental techniques, the Paris-Edinburgh apparatus, and the piston cylinder apparatus, to measure physical and chemical properties of silicate materials. The viscosity of reduced, Fe-free silicate liquids, with and without sulfur (S-free and S-bearing), were measured to predict the viscosity of Mercury’s magma ocean (Chapter 1). These viscosity measurements were used to create predictive viscosity models. The viscosity models were combined with crystallization models to predict mantle structures for Mercury’s solidifying magma ocean. The density of the resulting mantle structures were modeled to predict possible physical processes that could have occurred in Mercury’s interior, such as mantle overturn, that could produce the proper source regions for the igneous provinces seen on the surface of Mercury today (Chapter 2). The results of the viscosity measurements revealed differences in the behavior of the S-free and S-bearing liquids, where the sulfur-bearing exhibited lower viscosities than the S-free. Energy-dispersive X-ray Diffraction and Raman spectroscopy measurements were taken on in situ liquids and recovered glasses, respectively, to analyze the liquid and glass amorphous structures between the S-free and S-bearing compositions. The results revealed that sulfur has a depolymerization effect on the silicate structure and that Al causes inherent pressure sensitivity in silicate liquids by affecting the bond length and angles of the silicate structure (Chapter 3). The final project (Chapter 4) includes measuring the trace element partitioning in mafic systems with variable Fe and Al content. This work revealed that while Al does affect the partitioning of 1+, 3+, and 4+ cations into the clinopyroxene structure, in Fe-rich systems, Fe appears to play a role in the partitioning of heavy rare earth elements (3+) onto the M1 site in 6-fold coordination that is not observed in Mg-rich clinopyroxenes.