Diffusive Isotopic Fractionation and the Structure of Silicate Liquids

University Of California-Berkeley, Berkeley CA

Investigators

Abstract

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.