OH in Rutile: An Oxygen and Water Barometer

University Of California-Los Angeles, Los Angeles CA

Investigators

Abstract

Dehydration of the subducting lithosphere induces oxidation and partial melting in the mantle wedge above subduction zones, and storage of water in the form of hydroxyl in high-pressure mineral phases may be an important mechanism for transfer of water to the mantle. Recently there has been much interest in the possible existence of a hydrosphere on the very early (>4.3 Ga) Earth, highlighting the need to learn more about the fluid composition and oxygen fugacity of rocks that can provide important information about the early terrestrial oxidation state and water cycle. Oxygen fugacity and water activity are difficult to measure or infer unless one can find coexisting oxygen- or water-buffering mineral assemblages. This project will experimentally investigate the relationship between OH concentrations in rutile and the imposed oxygen fugacity, water activity, ferric iron content, temperature, and pressure, and will use a thermodynamic model to apply these results to geologic systems for which constraining oxygen fugacity and fluid composition is important, but has been difficult to evaluate with other methods. The technique is based on OH concentration measurements in rutile (TiO2), a commonly occurring accessory mineral that is nominally anhydrous but can incorporate a significant amount of hydroxyl into its structure. In pure rutile, H+ is stoichiometrically incorporated into the structure via charge-coupled reduction of Ti4+ to Ti3+ (resulting in a change in color from pale yellow to deep blue). Diffusion experiments of hydrogen in rutile will result in quantification of equilibrium boundary conditions and rates of redox-driven and intrinsic defect-driven loss of hydrogen in nominally anhydrous minerals. A new diffusion model will be constructed from this data to resolve the seemingly contradictory results from previous hydrogen diffusion experiments in nominally anhydrous minerals and measurements of OH in natural samples. Once developed, the rutile oxygen and water barometer should have widespread applications in geochemistry and petrology. Water activity or oxygen fugacity will be able to be determined even in systems lacking hydrous minerals or traditional oxygen buffering mineral assemblages. The proposed research will allow the PI, an early-career female scientist, to carry on active research in an academic setting. This grant will support undergraduate research aids over two summers.