Calcium and potassium isotopic study of igneous and metamorphic transport processes

University Of California-Berkeley, Berkeley CA

Investigators

Abstract

Earth scientists measure the chemical composition and age of minerals and rocks to get information about how they formed. Using the information gained through analysis, and with knowledge of mineralogy, physics, and chemistry, it is possible to reconstruct how the Earth has evolved and changed over its 4.5 billion year history. There are many difficulties in this endeavor, because typically rocks and minerals are subjected to high temperatures and pressures at multiple times since they first formed. To account for the changes that these heating and burial processes caused, it is necessary to understand how chemical elements move between minerals and rock layers at high temperature and pressures, movement that also results in new minerals being formed. This project is aimed at using a novel approach to better define how the chemistry of rocks and minerals can change when they are subjected to metamorphism and melting at high temperature. The plan is to use the relative proportions of the isotopes of two important chemical elements – potassium (K) and calcium (Ca) – to distinguish the extent to which rock chemistry has been changed, and the processes responsible. This approach has not been employed previously to rocks in the field, but there is evidence that it will be instructive based on experiments that have been performed in various laboratories around the world over the past ten years. The approach can also help us to understand how magma forms in the Earth and produces volcanic eruptions. This project will have broader impacts in education and in chemistry and materials science. Although in detail there are many complexities, the proposed project will produce results that are keyed directly to macroscopic features that are widely observed in rock exposures globally and frequently used for teaching. To this end the project will produce teaching materials to illustrate the processes and their significance that will be posted on the web and made available for instructors. This project is aimed at developing a new level of understanding of high temperature igneous and metamorphic systems, emphasizing the isotopes of Ca and K, and where there is substantial potential to advance our fundamental understanding of chemical transport during petrogenesis. An essential aspect of the proposed work is a focus on field occurrences having sufficiently predictable starting conditions that they can be treated as natural experiments. This approach is in contrast to laboratory experiments, which have been done in abundance, where run conditions can be controlled but the applicability to nature is questionable. The goal is to better understand how the mobility of key chemical constituents is controlled during metamorphism and in magma chambers, and how that mobility relates to fractionation of Ca and K isotopes. Isotopic effects in these elements are tracers of micro- and nano-scale processes at mineral surfaces and transport within geologic media at larger scales. In particular, isotopic fractionation in these elements is primarily attributable to chemical diffusion, and hence the isotopic measurements will help to distinguish to what degree diffusion, as opposed to fluid advection, is responsible for the bulk of chemical redistribution during high temperature petrogenetic processes. The proposed research requires high precision isotopic and chemical characterization of rocks and minerals, insight about the origin of geologic features exposed in outcrop, knowledge of diffusion, thermodynamics, heat transport, petrology, and structural geology, insight into isotopic fractionation mechanisms, and an ability to model the processes with analytical and numerical approaches to extract key information from the measurements. This multifaceted and multidisciplinary effort, which is necessary for advancing geochemical and petrological sciences, provides rich educational opportunities for students and advanced researchers as well as an opportunity to develop a new level of understanding of rock-forming processes in the Earth. This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.