Calibrating olivine crystallographic preferred orientation as a mantle water detector

The Earth is distinguished from other planets by the occurrence of plate tectonics and the presence of liquid water on its surface. Flow of the solid mantle drives the plate tectonic cycle and deep Earth water cycle. The investigators will study the interaction between water stored in olivine, the most abundant upper mantle mineral, and flow in the mantle.

The occurrence of tiny amounts of water, in the form of hydrogen substituted into the olivine lattice, can modify the way in which the olivine crystal lattice aligns during flow. The conditions for this transition in crystal lattice alignment are within the expected water content and stress conditions of the upper mantle, but almost no laboratory experiments on polycrystalline olivine have recreated these fabrics.

To address this, the investigators will combine laboratory experiments on olivine aggregates with analysis of mantle rocks from the Talkeetna intraoceanic arc in Alaska. The process of conducting laboratory experiments will be documented through a video and slideshow designed for university-level courses. The project will involve the advanced training of a postdoctoral researcher and a summer research project for an undergraduate student that will apply machine learning algorithms to rocks.

Small amounts of water in nominally anhydrous minerals such as olivine can greatly influence the viscosity, melting temperature, and anisotropy structure of the mantle and its overall dynamics. Interpreting measurements of water abundances in mantle minerals has proven challenging as hydrogen diffusion is so fast that water contents can be significantly modified during exhumation of the mantle.

Previous experiments suggest that deformation-induced fabric development is sensitive to olivine water content and therefore olivine fabrics may provide an independent constraint on the water content of mantle minerals. This project will constrain the water content and stress conditions associated with E-type fabric formation through deformation experiments on olivine aggregates in a recently updated high-pressure experimental apparatus.

Detailed microstructural analysis of experimental samples and Talkeetna samples will be performed to evaluate their similarities, which will be used to support the extrapolation of experimental results to Earth conditions. This project will address the following questions: (1) What are the conditions favorable to producing an E-type olivine fabric? (2) Can olivine E-type fabrics be used as a mantle water detector in natural samples?

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.