The nature of compositional heterogeneity in Earth's lower mantle remains a long-standing puzzle that can inform about the long-term thermochemical evolution and dynamics of our planet. Here, we use global-scale 2D models of thermochemical mantle convection to investigate the coupled evolution and mixing of (intrinsically dense) recycled and (intrinsically strong) primordial heterogeneity in the mantle. We explore the effects of ancient compositional layering of the mantle, as motivated by magma ocean solidification studies, and of the physical parameters of primordial material. Depending on these physical parameters, our models predict various regimes of mantle evolution and heterogeneity preservation over 4.5 Gyr. Over a wide parameter range, primordial and recycled heterogeneity are predicted to co-exist with each other in the lower mantle of Earth-like planets. Primordial material usually survives as medium- to large-scale blobs (or streaks) in the mid-mantle, around 1000–2000 km depth, and this preservation is largely independent of the initial primordial-material volume. In turn, recycled oceanic crust (ROC) persists as large piles at the base of the mantle and as small streaks everywhere else. In models with an additional dense FeO-rich layer initially present at the base of the mantle, the ancient dense material partially survives at the top of ROC piles, causing the piles to be compositionally stratified. Moreover, the addition of such an ancient FeO-rich basal layer significantly aids the preservation of the viscous domains in the mid-mantle. Finally, we find that primordial blobs are commonly directly underlain by thick ROC piles and aid their longevity and stability. Based on our results, we propose an integrated style of mantle heterogeneity for the Earth involving the preservation of primordial domains along with recycled piles. This style has important implications for early Earth evolution and has the potential to reconcile geophysical and geochemical discrepancies on present-day lower-mantle heterogeneity.

中文翻译：

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地球下地幔原始和再循环异质性的耦合动力学和演化

地球下地幔成分异质性的性质仍然是一个长期存在的谜题，它可以为我们星球的长期热化学演化和动力学提供信息。在这里，我们使用热化学地幔对流的全球尺度 2D 模型来研究地幔中（本征致密）再循环和（本征强）原始异质性的耦合演化和混合。在岩浆海洋凝固研究的推动下，我们探索了地幔古代成分分层的影响，以及原始物质的物理参数。根据这些物理参数，我们的模型预测了超过 4.5 Gyr的地幔演化和异质性保存的各种机制 . 在很宽的参数范围内，原始和再生的异质性预计将在类地行星的下地幔中相互共存。原始物质通常在中地幔中以中到大型团块（或条纹）的形式存在，大约 1000-2000 公里深度，并且这种保存在很大程度上独立于初始原始材料体积。反过来，再循环的海洋地壳 (ROC) 以地幔底部的大堆和其他地方的小条纹的形式持续存在。在最初存在于地幔底部的额外致密富 FeO 层的模型中，古老的致密物质在 ROC 桩的顶部部分幸存下来，导致桩在成分上分层。此外，这种古老的富含 Fe3O 的基底层的加入显着有助于中地幔粘性域的保存。最后，我们发现原始斑点通常直接位于厚 ROC 桩的下方，有助于它们的寿命和稳定性。根据我们的结果，我们为地球提出了一种综合的地幔异质性风格，包括保护原始领域和回收的桩。这种风格对早期地球演化具有重要意义，并有可能调和当今下地幔非均质性的地球物理和地球化学差异。