CSEDI: Alkaline-Earth Carbonate Melts at Deep Earth Conditions

碳酸盐是一种常见的造岩矿物，其晶体结构中含有大量的碳，因此代表了地球系统中主要的碳储集层。尽管它们在长期碳循环中起着主要作用，但我们目前对它们在地球深处相关条件下的融化行为的了解非常有限。含碳酸盐岩石中的碳通过大洋板块的俯冲带进地球，在那里，它要么通过火山作用回到地表，要么被进一步拖到地幔深处，这取决于碳酸盐融化的深度。因此，碳酸盐的熔融特性直接控制了地球碳循环的长期组成部分。该项目的重点是通过结合实验、模拟和统计建模等多种研究技术，更好地了解极端压力和温度下碳酸盐融化的细节。除了提高我们对深层碳循环的基本理解外，该项目还支持开发技术和领导专业知识；该项目由一位早期职业研究科学家领导，他将与共同研究人员一起培训两名研究生，学习各种前沿研究技术，这些技术也广泛适用于未来的材料特性研究。碳酸盐的热力学熔融特性直接控制着地球碳循环的长期组成部分，然而由于碳酸盐熔融实验的实际挑战，这一关键领域仍然知之甚少。这些困难包括在低于~ 3gpa的压力下熔化前的分解，以及熔化行为对压力、温度和成分（包括水含量）的极端敏感性，这些都严重复杂化了此类实验的分析。本文的目标是结合多种实验和理论方法来表征（Ca,Mg,Sr,Ba）CO3体系中固体和熔融碱性地球碳酸盐的热力学特征，该体系包含与地球深部最相关的主要含Mg-Ca组分。该团队的实验目标是：(1)对每个端元进行高达25 GPa的基于电导率的原位熔体检测实验，(2)进行液体一巴密度和声速测量，以确定每个组件如何影响熔体密度和可压缩性，以及(3)进行激光加热的金刚石砧细胞测量，以约束固相的高压-温度（PT）状态方程。这些与理论目标相结合：(1)利用经验分子动力学模拟碱土碳酸盐岩体系，以预测熔体和矿物相的高PT性质；(2)利用贝叶斯统计分析获得（Ca,Mg,Sr,Ba）CO3体系的全局自一致热力学模型。这项工作提供的最终热力学图像代表了将碳酸盐熔化纳入各种广泛使用的热力学数据库的道路上的一个重要垫脚石，这些数据库巩固了我们对极端行星条件下地质过程的理解。在这项工作中使用的五种技术，依赖于三位主要研究人员的合作和独特的专业知识，将提供一个全面的地幔深部条件下碳酸盐融化的热力学观点。该奖项反映了美国国家科学基金会的法定使命，并通过使用基金会的知识价值和更广泛的影响审查标准进行评估，被认为值得支持。

Carbonates are common rock-forming minerals that contain significant carbon in their crystal structures, and thus represent a dominant reservoir for carbon in the Earth System. Despite their primary role in the long-term carbon cycle, we currently have a very limited understanding of their melting behavior at the relevant conditions deep inside the Earth. The carbon hosted in carbonate-bearing rocks is carried into the Earth by subducting oceanic plates, where it is either returned to the surface through volcanism or dragged further down into the deep mantle, depending on the depth of carbonate melting. The melting properties of carbonates thus directly control the long-term component of Earth's carbon cycle. This project is focused on better understanding the details of carbonate melting at extreme pressures and temperatures by combining diverse research techniques using experiments, simulations, and statistical modeling. In addition to improving our fundamental understanding of the deep carbon cycle, this project supports developing both technical and leadership expertise; the project is being led by an early career research scientist who will join the co-investigators in the training of two graduate students in a variety of cutting edge research techniques that are also broadly applicable to future material properties research.The thermodynamic melting properties of carbonates directly control the long-term component of Earth's carbon cycle, and yet this key area has remained poorly understood as a result of practical challenges for carbonate melting experiments. These difficulties include decomposition prior to melting at pressures below ~3 GPa, as well as extreme sensitivities of the melting behavior to pressure, temperature, and composition (including water content), which severely complicate analysis of such experiments. The goal of this proposal is to combine multiple experimental and theoretical methods to characterize the thermodynamics of solid and molten alkaline earth carbonates, in the (Ca,Mg,Sr,Ba)CO3 system, which encompasses the dominant Mg-Ca bearing components most relevant to the deep Earth. The team's experimental objectives are to: (1) carry out conductivity-based in situ melt detection experiments up to 25 GPa for each endmember, (2) conduct liquid one-bar density and sound speed measurements to determine how each component affects melt density and compressibility, and (3) perform laser-heated diamond anvil cell measurements to constrain the high pressure-temperature (PT) equations of state of the solidus phases. These are combined with theoretical objectives to: (1) simulate the alkaline earth carbonate system using empirical molecular dynamics to predict high PT properties for both melt and mineral phases and (2) employ Bayesian statistical analysis to obtain a global self-consistent thermodynamic model of the (Ca,Mg,Sr,Ba)CO3 system. The final thermodynamic picture provided by this work represents a major stepping-stone on the path to incorporating carbonate melting into a variety of widely-used thermodynamic databases that underpin our understanding of geologic processes at extreme planetary conditions. Together the five techniques employed in this work, which rely on the cooperation and distinct expertise of the three principal investigators, will provide a comprehensive thermodynamic view of carbonate melting at deep mantle conditions.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.