This project is an investigation of the molecular structure of magma, also referred to as "silicate liquid". Magma is produced by melting of rocks in the Earth's interior, and also forms inside other planets. The objective of the study is to use a new approach to determine whether silicate liquids are chemically similar to organic polymers, or instead are more similar to liquid metals. This information is needed to understand how volcanoes form and how planets change over billions of years. The results also have applications in ceramics and materials engineering, and will be a significant contribution to both geology and high temperature materials science.
The approach is to use diffusive isotopic fractionation of major chemical elements (Ca, Mg, Fe, K) in silicate liquids to probe melt structure and the mechanisms of diffusion. The thesis is that the degree of discrimination between isotopes relates directly to the bonding of the elements to the alumino-silicate melt structure, and that the fractionation behavior of the cations will vary with liquid composition as melt structure changes. Previous work has demonstrated that isotopic discrimination exists, and that it causes easily measurable effects in diffusion experiments involving specific basalt and rhyolite compositions. What has not been done is to systematically vary the liquid compositions to elucidate the dependence of isotopic effects on composition, and to evaluate diffusive coupling between components of the liquids. This can be done both with natural liquid compositions, as well as on simpler systems where it will be easier to relate the isotopic effects to specifics of the liquid structure and thermodynamic properties. Diffusion-induced fractionation effects need to be understood in order to interpret isotopic variations in magmatic systems and to better understand the dynamic behavior of silicate liquids at the molecular level. A second complementary aspect of the experimental approach will be to use thermal diffusion effects to probe speciation in simple silicate liquids, and to compare these results to models based on diffusion experiments.
New experiments offer a molecular view of isotope separation This study used laboratory experiments and mathematical descriptions of atomic motions to understand how isotopes and trace elements get "printed onto" growing minerals. The study takes advantage of subtle differences in the random motions of isotopes to learn what molecular structures exist in liquids and how they affect the transport of isotopes to the surfaces of growing crystals. Information such as this helps geochemists understand environmental conditions in the distant past or in regions of the Earth that are inaccessible to direct observation. In the laboratory, rock is melted to form lava, and two lavas of different chemical composition are pressed together and allowed to slowly mix. Although different in their chemistry, the two rock types initially have the same isotopic composition. (The only significant difference between isotopes is mass. Heavy isotopes of an element have more neutrons in the atomic nucleus.) During the mixing process, isotope separation occurs because lighter isotopes move faster than heavier isotopes. By measuring the amount of isotope separation, and how it varies between different liquids, researchers are learning how to predict the magnitude of non-equilibrium isotope effects in different natural environments. Development of early career scientists This project provided support to a young scientist for graduate studies resulting in a PhD thesis and extended postdoctoral research. This early career scientist starts an academic tenure-line position as assistant professor in 2013. In addition this grant provided opportunities to two undergraduate student assistants to work in laboratories, acquiring more advanced technical skills in both cleanroom methods and in use of mass spectrometers. One has gone on to work in a professional research support role at a national lab. The other is writing an honors thesis, and will graduate end of 2012. He now plans to attend graduate school in earth sciences.
