Constraining the style of magma-ocean crystallisation by present-day Earth structure: a coupled thermodynamic-geodynamic approach

地球开始时是一个全球熔融的球体，是一个“岩浆海洋”，伴随着其吸积的巨大能量释放。虽然这种岩浆海洋的证据保存在月球表面，但在地球表面没有直接的地球化学或地球物理证据保存下来。随着岩浆海洋冷却和冻结，晶体沉淀形成第一个固体地幔，不断缩小的岩浆海洋被认为逐渐富含铁等放射性微量元素。因此，岩浆海洋的最后一滴，最终稳定了地球上的第一个地壳，被认为极其富含这些元素。岩石地球在岩浆-海洋阶段的分化对地球内部的长期演化具有深远的影响，地球内部的动力学维持着地球表面适合生命的条件。在这个项目中，我们建议解决在岩浆-海洋结晶过程中一个重要但以前被忽视的过程。最近的研究证实，结晶包已经经历了固态搅拌或对流，而岩浆海仍处于逐渐冻结的过程中。这种对流导致热物质的上升，与之相关的压力下降将导致部分熔化。然而，这种融化对岩浆-海洋成分演化的影响尚未被探索。使用新耦合的热力学-地球动力学模型，我们的目标是量化晶体包部分熔化以及相关物质与岩浆海洋交换的这些后果。我们推测，这种交换完全改变了第一固体地幔的组成结构和原生地壳的化学成分。例如，它可能会调和当今地球地幔相当温和的成分分层，而以前的岩浆-海洋结晶模型根本没有解决这一问题。我们的新模型的预测将得到检验，例如，通过询问地幔深处的地震结构，那里可能存在原始地壳的残余物。

Earth began life as a globally molten ball, a "magma ocean", following the vast energy release of its accretion. While evidence for such a magma ocean is preserved on the Moon's surface, there is no direct geochemical or geophysical evidence preserved on the Earth's surface.As the magma ocean cooled and froze, crystals settled to form the first solid mantle, and the shrinking magma ocean is thought to have become progressively enriched in iron and e.g. radioactive trace elements. Accordingly, the last droplets of the magma ocean, which ultimately stabilized the first crust on Earth, are thought to be extremely enriched in these elements. The differentiation of the rocky Earth in the magma-ocean stage has far-reaching implications for the long-term evolution of the Earth interior, the dynamics of which sustain life-friendly conditions on its surface. In this project, we propose to address an important, but previously neglected process during magma-ocean crystallization. Recent work has established that the crystal package already undergoes solid-state churning, or convection, while the magma ocean is still in the process of progressive freezing. This convection leads to upwellings of hot material, and the associated pressure decrease will bring about partial melting. However, the consequences of this melting for magma-ocean compositional evolution have not yet been explored. Using newly coupled thermodynamic-geodynamic models, our goal is to quantify these consequences of partial melting of the crystal package, and of related material exchange with the magma ocean. We hypothesize that this exchange completely changes the compositional structure of the first solid mantle and the chemistry of the primary crust. For example, it may reconcile the rather moderate compositional stratification of the present-day Earth mantle, which is not addressed by previous models of magma-ocean crystallization at all. The predictions of our new models will be tested, for example by interrogating the seismic structure of the deep mantle, which may host the remnants of the primary crust.