After accretion and formation, terrestrial planets go through at least one magma ocean episode. As the magma ocean crystallises, it creates the first layer of solid rocky mantle. Two different scenarios of magma ocean crystallisation involve that the solid mantle either (1) first appears at the core–mantle boundary and grows upwards or (2) appears at mid-mantle depth and grows in both directions. Regardless of the magma ocean freezing scenario, the composition of the solid mantle and liquid reservoirs continuously change due to fractional crystallisation. This chemical fractionation has important implications for the long-term thermo-chemical evolution of the mantle as well as its present-day dynamics and composition. In this work, we use numerical models to study convection in a solid mantle bounded at one or both boundaries by magma ocean(s) and, in particular, the related consequences for large-scale chemical fractionation. We use a parameterisation of fractional crystallisation of the magma ocean(s) and (re)melting of solid material at the interface between these reservoirs. When these crystallisation and remelting processes are taken into account, convection in the solid mantle occurs readily and is dominated by large wavelengths. Related material transfer across the mantle–magma ocean boundaries promotes chemical equilibrium and prevents extreme enrichment of the last-stage magma ocean (as would otherwise occur due to pure fractional crystallisation). The timescale of equilibration depends on the convective vigour of mantle convection and on the efficiency of material transfer between the solid mantle and magma ocean(s). For Earth, this timescale is comparable to that of magma ocean crystallisation suggested in previous studies (Lebrun et al., 2013), which may explain why the Earth's mantle is rather homogeneous in composition, as supported by geophysical constraints.

中文翻译：

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对流固体幔与上层和下层岩浆海洋之间化学平衡的时标

吸积和形成后，地球行星至少经历了一次岩浆海洋事件。随着岩浆海的结晶，它形成了坚固的岩石地幔的第一层。岩浆海洋结晶的两种不同情况涉及：固体幔要么（1）首先出现在岩心—幔边界并向上生长，要么（2）出现在幔中深度并在两个方向上生长。无论岩浆海冻结情况如何，由于部分结晶作用，固体地幔和液体储层的组成都会不断变化。这种化学分馏对于地幔的长期热化学演化及其当今的动力学和组成具有重要意义。在这项工作中，我们使用数值模型研究在一个或两个边界受岩浆海洋限制的固体地幔中的对流，并且 特别是大规模化学分馏的相关后果。我们使用岩浆海洋分步结晶的参数化和这些储层之间界面处的固体物质的（重新）熔融。当考虑到这些结晶和重熔过程时，固体罩中的对流很容易发生，并以大波长为主导。跨地幔－岩浆海洋边界的相关物质转移促进了化学平衡，并阻止了最后阶段的岩浆海洋的极端富集（否则会由于纯净的部分结晶而发生）。平衡的时间尺度取决于地幔对流的对流活力以及固体地幔与岩浆海洋之间物质转移的效率。对于地球（Lebrun等人，2013），这可以解释为什么地球的地幔在地球物理约束的支持下组成相当均匀。