An experimental study on grain-size evolution during phase transformations in the mantle transition zone and its influence on rheological properties

Yale University, New Haven CT

Investigators

Abstract

It is now well appreciated that Earth's interior are vigorously convecting at the geological time scale. This "mantle convection" is the most important process that controls the way in which Earth evolves and continents move. As materials move in the deep interior of Earth, crystal structure of minerals changes to new ones caused by a large pressure inside of the deep Earth. These phase transitions likely change the deformability (viscosity) of materials and the main goal of this project is to evaluate the degree to which these phase transitions might change the deformability. Our focus is the changes in the size of minerals caused by phase transitions, that could modify the viscosity of materials by several orders of magnitude. The results of this study will provide a new insight into the role of phase transitions in the deep interior of Earth in modifying the nature of mantle convection. Phase transitions that occur in Earth's transition zone likely have a large influence on mantle convection. A phase transition could affect convection through its effect on density and viscosity. Among the various mechanisms by which viscosity of materials change, we focus on the change in grain-size that could change the viscosity up to ~10 orders of magnitude. We will conduct systematic experiments on the olivine to wadsleyite transitions following various P-T-t paths (P: pressure, T: temperature, t: time) and determine the size and spatial distribution of new grains. We will interpret these results in terms of a model of microstructural evolution during a phase transition, and apply these results to evaluate the change in viscosity of a subducting slab in the transition zone.