The role of light elements at the core-mantle boundary – partitioning, demixing, and transport

在核-地幔边界之下可能存在停滞层，这对地球深处提出了一些重要的地球物理和地球化学问题。在拟议的项目中，我们计划探索这一层是否可能是由一种不同的化学成分造成的，即轻元素在那里积累，最初在岩心分离过程中在较高温度下带到岩心。为此，我们将对可能导致这种轻元素富集层的形成和稳定性的不同方面进行从头计算模拟，并确定其动态重要的物理性质。对于Focus计划中提议的其他项目中的地球发电机和热演化考虑，密度、粘度和导热系数是重要的。我们已经同意与另外两个项目合作，Wicht和朱为地球发电机，Tosi等人。用于热演化模拟。作为地球发电机的额外推动力，特别是在年轻的地球上，我们将探索SiO_2从初始核心组成中可能出溶的可能性。在更多的地球化学背景下，我们将考虑在其形成过程中，通过岩浆海洋和金属之间的平衡将轻元素输送到核心，以及化学边界层与上覆地幔可能的化学平衡。在方法上，我们将使用密度泛函理论和机器学习势进行大规模的分子动力学模拟，并在分子动力学模拟的基础上计算候选反应产物的热力学势，以定量确定轻元素在相应高压和温度下在液态铁合金(核)、固体(地幔)和液态硅酸盐(岩浆海洋)之间的分配。

The possible presence of a stagnant layer below the core-mantle boundary raises a number of important geophysical and geochemical questions for the deep Earth. In the proposed project, we plan to explore the possibility of this layer being caused by a distinct chemical composition, i.e., the accumulation of light elements there, initially brought to the core at higher temperature during core segregation. To this avail we will perform ab-initio simulations on different aspects that may lead to the formation and stability of such a light element enriched layer, and determine dynamically important physical properties of it. For the geodynamo and thermal evolution considerations in other projects proposed within the focus program, density, viscosity and thermal conductivity are of significance. We have agreed to collaborate with two other projects, Wicht and Zhu for the geodynamo, Tosi et al. for thermal evolution simulations. As an additional driving force for the geodynamo, especially in the young Earth, we will explore the possible exsolution of SiO2 from the initial core composition. In a more geochemical context, we will consider the transport of light elements to the core during its formation by equilibration between the magma ocean and metal, as well as the possible chemical equilibration of the chemical boundary layer with the overlying mantle. Methodologically, we will perform large-scale molecular dynamics simulation using both density functional theory and machine learning potentials, and – based on molecular dynamics simulations – compute thermodynamic potentials of candidate reaction products to quantitatively determine partitioning of light elements between liquid iron-alloys (the core) and solid (the mantle) and liquid silicates (the magma ocean) at relevant high pressures and temperatures.