Fate of Subducted Carbonates: Structure Prediction and Solid Solution Modeling

地球科学家一致认为，在地质时间尺度上对大气中二氧化碳（CO2）浓度的调节是地球是否适合生命存在的一个重要组成部分。这个长期的碳循环调节着生物圈中碳的可用性和大气中温室气体的丰度，其基本成分是火山释放的二氧化碳，以及二氧化碳在俯冲带随旧海洋地壳返回地幔的二氧化碳。在地球化学中，我们理解像二氧化碳这样的化合物在这样的环境中如何表现的能力取决于确定在每个阶段哪种矿物或液体携带碳。这方面的一些信息可以从自然观察和实验中获得，但在碳循环的关键阶段，计算是我们可以使用的最佳工具。该项目将对陆地地幔条件下碳酸盐矿物的结构、热力学和稳定性进行详细的计算研究，以限制它们在俯冲和融化过程中储存和运输碳的作用。该方法结合了计算矿物物理学、材料科学和经典热力学的工具，在高压和有限温度下对复杂的固溶体相进行预测。如此全面的方法在地球科学中是极不寻常的。在成功预测CaMg(CO3)2白云岩比任何实验观察到的更稳定的新致密结构的基础上，并在计算端元白云岩组成的相图方面，该团队对该项目有四个目标：(1)确定纯白云岩相之间的转变动力学；(2)确定白云石结构中Fe2+对Ca(Mg,Fe)(CO3)2体系相稳定性、热力学性质和相关系的影响；(3)计算CaO-MgO-FeO-SiO2-CO2体系脱碳的相图；(4)在包括还原碳和三铁的扩展体系中，计算控制C和Fe之间氧化还原交换的相图。他们的方法首先是利用进化晶体学和密度泛函理论的结合，从第一性原理预测稳定的晶体结构。稳定结构热力学的特征是静态状态方程和通过高效分子动力学方法对温度效应和振动熵的完全非调和处理。通过详细考虑排序现象和填充构型的玻尔兹曼加权来处理固体解和构型熵。最后，根据相关相中组分的化学势计算出多组分相图。这项提案将资助一名女研究生所需的津贴和计算资源，该研究生将接受实验和计算矿物物理方法的不同寻常的组合训练。他们计算新型碳酸盐相相对于游离二氧化碳的稳定性的工作流程有可能与碳封存的新材料方法联系起来。

Earth scientists agree that the regulation of carbon dioxide (CO2) concentrations in the atmosphere over geological timescales is an essential part of our planet's suitability for life. The essential ingredients of this long-term carbon cycle, which regulates the availability of carbon for the biosphere and the abundance of greenhouse gases in the atmosphere, are release of CO2 from volcanoes and return of CO2 return to the mantle along with old oceanic crust at subduction zones. In geochemistry, our ability to understand how a compound like CO2 will behave in such environments depends on identifying what mineral or fluid carries the carbon at each stage. Some information about this can be gained from natural observations and some from experiments, but there are key stages of the carbon cycle where calculation is the best tool at our disposal. This project will carry out a detailed computational study of the structure, thermodynamics, and stability of carbonate minerals under terrestrial mantle conditions, in order to constrain their role in storing and transporting carbon during subduction and melting processes. The approach combines tools from computational mineral physics, materials science, and classical thermodynamics to make predictions about complex solid solution phases at elevated pressure and finite temperature. Such a comprehensive approach is highly unusual in Earth science.Building on their success in predicting a new dense structure of CaMg(CO3)2 dolomite that is more stable than any observed experimentally, and in computing the phase diagram of end-member dolomite composition, the team has four goals for this project: (1) to determine the kinetics of transition among the pure dolomite phases; (2) to determine the influence of Fe2+ in the dolomite structure on phase stability, thermodynamic properties, and phase relations in Ca(Mg,Fe)(CO3)2 system; (3) to compute phase diagrams governing decarbonation in the CaO-MgO-FeO-SiO2-CO2 system; and (4) optimistically, to compute phase diagrams governing redox exchange between C and Fe in an extended system including reduced carbon and ferric iron. Their approach begins with prediction of stable crystal structures from first principles using a combination of evolutionary crystallography and density-functional theory. Thermodynamics of stable structures are characterized by a static equation of state and a fully anharmonic treatment of temperature effects and vibrational entropy via a highly efficient molecular dynamics approach. Solid solutions and configurational entropy are treated through detailed consideration of ordering phenomena and Boltzmann weighting of populated configurations. Finally, multicomponent phase diagrams are computed from the chemical potentials of components in relevant phases. This proposal will fund the stipend and computational resources required by a female graduate student being trained in an unusual combination of experimental and computational mineral physics approaches. Their workflow for computing the stability of novel carbonate phases relative to free CO2 has the possibility to connect to novel materials approaches to carbon sequestration.