OH in Rutile: An Oxygen and Water Barometer

俯冲岩石圈的脱水导致俯冲带上方地幔楔体的氧化和部分熔融，高压矿物相中以羟基形式储存的水可能是水向地幔转移的重要机制。最近，人们对极早期(4.3Ga)地球上可能存在水圈的可能性非常感兴趣，这突显了需要更多地了解岩石的流体成分和氧逸度，以提供关于早期陆地氧化状态和水循环的重要信息。除非能找到共存的氧或水缓冲矿物组合，否则很难测量或推断氧逸度和水的活度。该项目将通过实验研究金红石中的OH浓度与施加的氧逸度、水活度、铁含量、温度和压力之间的关系，并将使用热力学模型将这些结果应用于限制氧逸度和流体组成很重要的地质系统，但很难用其他方法进行评估。这项技术是基于金红石(二氧化钛)中的羟基浓度测量的，金红石是一种常见的副矿物，名义上是无水的，但可以将大量的羟基结合到其结构中。在纯金红石中，通过电荷耦合还原Ti4+到Ti3+(导致颜色从淡黄色变为深蓝色)，H+被化学计量地结合到结构中。氢在金红石中的扩散实验将导致在名义上无水矿物中的平衡边界条件和氧化还原驱动和本征缺陷驱动的氢损失速率的量化。根据这一数据将建立一个新的扩散模型，以解决以前在名义上无水矿物中的氢扩散实验与天然样品中的OH测量所得出的看似矛盾的结果。一旦研制成功，金红石型氧气和水压计应该会在地球化学和岩石学中有广泛的应用。即使在缺乏含水矿物或传统的氧气缓冲矿物组合的系统中，也能够测定水的活度或氧逸度。拟议的研究将允许职业生涯早期的女科学家PI在学术环境中进行积极的研究。这笔助学金将用于资助本科生两个暑期的研究助学金。

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.