This project is aimed at using new analytical and experimental methods to determine the rates of chemical processes in magmas and in geothermal waters. This kind of information is important for understanding how magmas move under volcanoes and eventually erupt, and for estimating the fluid flow rates and lifetimes of geothermal systems. A second part of the research is directed toward understanding the material properties of silicate liquids and hydrothermal solutions, using novel measurements of isotopes of common chemical elements. The new methods will be used to determine whether minerals that crystallize from magma and hydrothermal solutions do so at chemical equilibrium. If not, as is likely according to preliminary data, the objective is to use measurements of isotopic abundances of Ca, Mg, and K to determine how fast such minerals grow, which will then provide other information about the speed of related natural processes.
The proposed method for measuring mineral growth rates is based on isotopic measurements of the elements Ca, Mg, and K and will also include trace element measurements. Non-equilibrium isotopic effects will provide information about trace element fractionation processes, which are important for interpreting mineral chemistry. The research on liquid material properties uses mass-dependent isotopic changes that occur as elements and molecules diffuse through liquids. These subtle isotopic changes can now be monitored in key elements including Ca and Mg, as well as other chemical species like Ar and CO2. All of these species can exhibit mass-dependent isotopic changes related to diffusion. The changes depend on the viscosity, chemical composition, and chemical structure of the liquids, and the ways in which the each species is chemically bound to the liquid molecules. Consequently, diffusion-induced isotopic changes provide unique information about high temperature liquids that cannot be determined by any other method. Experiments will be augmented by study of natural rocks and minerals that formed under known conditions and at much slower rates than those of the laboratory experiments. The results of this research may have implications for materials science, volcanology, and geothermal energy production.
This project is primarily an experimental investigation aimed at better understanding how magmas and high temperature fluids in the Earth crystallize and change composition as a result of thermal and chemical gradients. The approach is to use the fractionation of Ca and Mg stable isotopes in silicate liquids, silicate liquid-mineral systems and hydrothermal fluid-mineral systems to probe the molecular level controls on chemical and thermal diffusive transport and to evaluate the role of kinetics in the formation of minerals in high temperature settings. Our work focused on thermal isotope fractionation in simple silicate systems (albite-diopside and albite-anorthite), hydrothermal crystal growth experiments to evaluate whether there are kinetic isotope effects during the precipitation of anhydrite from aqueous solutions at ca 120°C, the dependence of isotopic fractionation by diffusion in silicate liquids on liquid composition, and the partitioning of oxygen isotopes and titanium (Ti) during hydrothermal quartz precipitation. In addition, we investigated Ca isotope fractionation in volcanic phenocrysts to test for possible kinetic effects, and also initiated an investigation of the diffusion kinetics of U, Th, Pb, Hf and REEs as functions of temperature and oxygen fugacity, f(O2), in baddeleyite. Our work has led to several important results. We produced a general formulation for the diffusion of isotopic species in liquids, and showed that there are unexpected variations in the way isotopes behave during diffusion in silicate liquids. By investigating the behavior of simple silicate liquid systems in temperature gradients, we showed that thermal Ca isotope fractionations are much smaller relative to chemical isotope effects than in natural basalt liquids. We did find that the mineral anhydrite, when precipitated from hydrothermal solutions at 120°C shows kinetic Ca isotope effectsfavoring the light isotopes in the precipitated crystals. Small Ca isotope fractionation effects in volcanic phenocrysts may be useful for measuring the growth rates of crystals in lavas. Overall, this research has led to significant advances in our knowledge of the response of isotope ratios of major chemical elements to crystal growth and diffusion processes in magmas and hydrothermal fluids. This information will be useful for better understanding how volcanic and geothermal systems work, and for using this knowledge to understand how the Earth's crust formed and how to engineer geothermal systems for energy production.
